Review Opportunity and Challenges of Iron Powders for ...

ISIJ International, Vol. 61 (2021), No. 7

© 2021 ISIJ2015

ISIJ International, Vol. 61 (2021), No. 7, pp. 2015–2033

https://doi.org/10.2355/isijinternational.ISIJINT-2021-050

* Corresponding author: E-mail: [email protected]

© 2021 The Iron and Steel Institute of Japan. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs license (https://creativecommons.org/licenses/by-nc-nd/4.0/).CCBYNCND

Review

1. Introduction

Iron and steel is one of the key segment areas for the metal powder market and global iron powder market is close to 155 million USD. The iron powder production segment not only dominated the market in the recent past, but also anticipated to maintain its dominance in the coming years.1–3) The global iron powder production values by the key manu-facturers for the year 2012 and 2017 is presented in the Table 1. The automotive application segment remains one of the dominant sector for iron powders and is expected to grow at a rapid pace during the upcoming years.4) Besides, new developments in the production of insulated powders for soft and hard magnetic applications are anticipated to transform the fundamental design of motors.5) Armatures and stators of various shapes can be molded or pressed using advance manufacturing technologies such as Metal injection molding (MIM) without sacrificing the electrical performance. Hence, the electrical and electromagnetic industry is expected to create lucrative opportunities in the iron and steel powder market in future.4,5) Also, powder metal manufacturing companies can diversify their business by providing functional material products, which include electrical contacts, electrodes, and heat spreaders produced by blending, molding, and sintering with elemental pow-

Opportunity and Challenges of Iron Powders for Metal Injection Molding

Abhijeet Premkumar MOON,1)* Srinivas DWARAPUDI,2) Kameswara Srikar SISTA,1) Deepak KUMAR3) and Gourav Ranjan SINHA3)

1) Principal Researcher, R&D Tata Steel, Jamshedpur, Jharkhand, 831001 India.2) Principal Scientist, R&D Tata Steel, Jamshedpur, Jharkhand, 831001 India.3) Researcher, R&D Tata Steel, Jamshedpur, Jharkhand, 831001 India.

(Received on February 3, 2021; accepted on March 22, 2021)

Design flexibility, mass customization, waste reduction and the ability to manufacture near-net complex-shape structures, as well as rapid prototyping, are the main advantages of Metal Injection Molding (MIM). A brief review of the MIM technique, materials and their development to trending applications using iron powders was delineated. The ground-breaking applications of MIM in automotive, medical, and magnetic materials were discussed. The current-status of iron powder product development prepared for MIM was reviewed. In addition, this paper discussed the main processing challenges considering the MIM technol-ogy for producing high-end applications. Overall, this paper gives a summary of MIM, including a study on its benefits and opportunities and as a roadmap for future research and development in iron and steel powder manufacturing technique.

KEY WORDS: metal injection molding; iron powders; mass customization; microstructure.

ders such as copper, molybdenum, chromium, and nickel.6) Currently, estimate share of MIM in metal powder industry is minimal, close to 6 000 TPA for iron powders (exclud-ing stainless steel and high alloy steels).7) The demand is

Table 1. Global pure iron powder production (in MT) of key manufacturers in 2017 (Source: QYR Chemical & Mate-rial Research Centre, April 2017).

Iron Powder Manufacturer 2012 2017 % Growth

BASF 7 778 9 524 18.3

Sintez-CIP 633 786 19.5

JFE 2 106 2 558 17.7

Jiangsu Tianyi 909 1 148 20.8

Jilin Jien 1 202 1 439 16.5

Jiangxi Yuean 1 660 2 029 18.2

Shanxi Xinghua 160 186 14.0

Jiangyou Hebao 149 181 17.7

Jinchuan Group 136 1 109 87.7

Grimp 683 841 18.8

CNPC Powder 445 635 29.9

Global Other 2 624 3 424 23.4

Global Total 18 485 23 860 22.5

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© 2021 ISIJ 2016

expected to grow due to new application areas like electri-cal and electromagnetic, and alternative energy industries. Hence, rise in demand for MIM characteristic powders is forecasted to grow in future and is driving the demand for iron powder production in near future.4)

Superiority of MIM process over other conventional and competitive processes to produce parts with medium to high geometrical intricacy at comparatively high quantities is vividly showcased in the Fig. 1.8) With regard with the conventional subtractive manufacturing technique like CNC (Computer numerical control) machining, it has numer-ous benefits. Firstly, it is expected to automate the MIM process completely from component design to fabrication in a CAD/CAM (Computer-aided design/Computer-aided manufacturing) setting. This shortens both the production time and the amount of human involvement and efforts require for manufacturing every new component. While the programme for CNC machining can be produced from CAD models robotically as well, for designing complex shapes many re-fixturing is essential, resulting in inefficient and expensive re-fixturing and calibration actions. Secondly, MIM is a low-cost method for manufacturing components made of classy costly material.

Research efforts on the development of MIM from the parent process of powder injection molding falls back to 1970’s. After the initial incubation for a decade, improve-ments in terms of powder loading capacity, multi feed

stocks, accuracy optimization and increase in component weight evolved the MIM industry to the current form. Sche-matic representation of the progressive research and devel-opment of MIM process along with three other spinoffs from the parent MIM such as micro-injection molding (μ-MIM), bi-material injection molding, and bio-medical injection molding is presented in Fig. 2.9)

As seen in Fig. 3(a), between 1970 to 1980, MIM research popularity was lower and thereafter has gained enormous research interest in the global powder metallurgical frater-nity with the passing decades. Significant growth in the sci-entific publications since its inception is showcased in Fig. 3(a).10) Earlier in 1970, there were only 10 research papers on MIM process and then the numbers have improved in the year 1980 to 100 and further increased to 1 000 articles at the end of year 1990.11) Till 2007, this number crossed 3 600 mark.10) With higher number of publications encom-passing this filed in the last decade, some dedicated journals addressing MIM related research progress have been intro-duced, such as, Metal Injection Molding, Powder Injection Moulding International, Advance in Metal Injection Mold-ing, etc. On the other hand, Fig. 3(b) shows the survey for the number of patents published on MIM technology from 1990 to 2013 by various companies. Also, 400 patents have been recorded in the USA from 1990s.10,11) Besides, Metal Powder Industries Federation (MPIF) in North America issued its recent Standard number 35 on materials for metal injection molded components in 2007, and in the year 2012. The International Standard Organization (ISO) issued ISO

Fig. 1. Qualitative state of Metal Injection Molding (MIM) manu-facturing techniques relative to the conventional manufac-turing technologies (redrawn).15)

Fig. 2. Research overview on the development of MIM process (redrawn).17)

Fig. 3. (a): Number of scientific research paper on MIM with subsequent years, (b): MIM patent filed from 1990–2013 (redrawn).17,19)


© 2021 ISIJ2017

22068:2012 which defines the metallurgical composition, physical and mechanical properties of MIM materials. In 2004 the MPIF, European Powder Metallurgy Association (EPMA) and Japan Powder Metallurgy Association (JPMA) jointly launch the online Global Powder Metallurgy Prop-erty Database (www.pmdatabase.com) which has become free standard reference source for Design Engineers/researchers considering the use in powder metallurgy prod-ucts. Some data encompassed on fatigue properties of MIM materials which are used by Finite Element analysis (FEA) and other design software.

Thus, it is evident that, MIM technology stands unique as compared to the conventional manufacturing processes and along with its foreseen growth, it also demands huge quantities of unique metallic powders, iron powders being one of them, to cater novel application segments like electric-automotive, electromagnetic and alternative energy.1,3–5) Hence, there is a need for intensive study highlighting the importance of MIM technology, its prospects and future demands. This review, therefore, emphasis on basics of MIM technology, major aspects and attributes of feedstock influencing its functioning, market overview and some prospects usage of iron powder. The aim of the present review is to showcase the utility of iron powders in MIM technology and associated challenges with the suitability of the powders.

2. MIM Technology

2.1. MIM ProcessMIM technology was developed by Raymond Wiech of

US in 1970’s and considered the inventor of MIM process but was not successfully commercialized until 1980’s.12) Figure 4 shows the MIM process which is broadly classified into four major steps and as follows:8)

Step 1: Feedstock- Fine rounded metallic powders are usually combined with wax and thermoplastic binders in a suitable precise recipe. A proprietary compounding method results in a homogenous pelletized feedstock that can be injection molded just like plastic. This gives ultra-high den-sity and close tolerances mass production runs.

Step 2: Molding- The feedstock is heated till it become viscous mass and then injected into a desired mold cav-ity under high pressure, allowing for extremely complex

desired shapes. Once the component is removed from the mold it is known as a “Green part”.

Step 3: Debinding- The “Green part” obtained from Step 2 put through a controlled processing condition known as Debinding that removes the binder from the part and pre-pares for final step. After Debinding step, the part is named as “brown”.

Step 4: Sintering- The “brown part” is still contain residual amount of binder and is brittle and lack mechani-cal strength. During sintering step, the part is exposed close to the melting temperature of the metal which removes the residual left binder and imparts the part its final density and improves the mechanical strength. For special applications, such as automotive, medical and aerospace sectors, Hot Isostatic Pressing (HIP) is used to remove completely any residual porosity.

MIM process usually used to manufacture smaller com-ponents with complex shapes.10,12) However, it has rec-ognized itself as a competitive manufacturing process for small precision parts that would be costly to produce by alternative manufacturing methods.8)

2.2. MIM IngredientsThe MIM process requires some important ingredients

which is briefly discussed in the following subsections:

2.2.1. Metal PowdersMetallic powder is the major ingredient which goes in the

MIM manufacturing and finally becomes the single metallic molded component. Iron and steel are the most widely con-sumed metal powders in various industries.2) Among steels, stainless steel powders are extensively used, especially to cater orthodontic and biomedical uses.4) Although, there are some newly developed Ti feedstocks which are being used for making watches, surgical tools, aerospace and biomedi-cal implants. Their sales remain lower as compared to the iron and steels.3,4) Besides, some developments have also focused on super alloy compacts such as Inconel 718 to achieve high corrosion, high oxidation resistance and high temperature strength.

There are three approaches of using metallic powders in MIM (a) Prealloy powder (b) Mixed powder (c) Master alloy powder.13) Prealloyed powder signifies that each pow-

Fig. 4. Schematic diagram of MIM process. (Online version in color.)


© 2021 ISIJ 2018

der particle is a micro-casting with the desired composition. Mixed powder implies a combination of different elemental powders for example in case of stainless steel, combina-tion of iron, chromium, nickel and other additive particles, each of it used as single element. Master alloy is a hybrid metallic grade where little quantity of carbonyl iron powder is combined with 33% alloy powder.13) The alloy powder is enriched with additive elements such as Cr, Cu, Ni, Fe, Nb with trace elements such as Si and Mn.9,13) Prealloyed powder is preferably favoured for high performance, while mater alloy is preferred for low cost parts. In elemental mixed powders, prolonged sintering is needed to impart competitive properties in the parts. Prealloyed powder are usually produced using gas or water atomization technique.2)

2.2.2. Binders and AdditivesBinder is the primary ingredient used in MIM process. It

permits the flow of the particles into the mold die cavity, wets the powder surface, help in mixing and molding, so to promote different chemicals phenomena that modify the wetting behaviour. The binder plays three important func-tions, 1) it acts as a backbone polymer to offer strength, 2) acts as a filler phase which can be easily extracted in debinding, 3) it acts as a surfactant to bridge gap between the binder and metal powder.9)

Binder is undesired in the final component, but it is important in the MIM process as it transports the powder (metal) particles in the mold cavity during the injection stage and holds the required shape of the components during the ejection, debinding and the beginning of the sintering stage by solid state diffusion. For this reason, many binder systems also perform following functions; (a) promote poly-meric mechanisms which improves the viscosity to retain the final component shape on cooling. Besides, (b) some additives polymers like surfactants are commonly used in the binder system to coat the powder.14)

Commonly polypropylene and polyethylene have been used as the primary binder system to maintain the com-ponent shape after injection molding and debinding, while waxes have been used as the secondary binder in order to decrease the feedstock viscosity and to increase the replica-tion ability of the feedstock. Additive surfactant like stearic acid and oleic acid are usually added to facilitate powder

wetting by decreasing the surface energy of the binder-metal powder interface.9,14) The amount of surfactant addition is usually restricted to a relatively lower value (<5%) to reduce the possibility of powder-binder separation due to an excess low viscosity of the binder system. The binder system should deliver adequate fluidity when mixed with the metallic powder and after debinding process leaves with the minimum amount of oxygen and carbon residues in the final component.14)

2.2.3. FeedstockMIM uses a mixture of metal powders that is combined

with a polymer and wax binders. This is known as feed-stock. The quality of feedstock injected into the mold cav-ity is one of the crucial features in the MIM technology. It principally influences the final properties of the compo-nents. The determination of correct feedstock formulation is important to satisfy (i) mixing torque during the process, (ii) critical powder loading to mix the powder and the binder (iii) viscosity of the mixture, (iv) influence the homogeneity of the mixture. All these four points strongly influence the success of the MIM process.13,14)

During feedstock preparation, the selected binder and metal powder are uniformly mixed and kneaded at a temperature slightly higher than the melting point of the binder (usually in the temperature range of 140–170°C). The term “solid loading” is the ratio of metal powder to the binder, should be selected to confirm good flowability without the excess usage of binder. This is followed by mixing process, which ensures a thin layer coating of the binder on every metal powder par-ticle to impart the feedstock with a good flowability charac-teristic. Therefore, for mixing usually high shear mixers such as Sigma or Z-blade kneaders are preferred.14)

3. MIM Advantages

MIM is considered as a cost-effective solution as com-pared to the conventional metal forming techniques like machining, investment casting, and powder metallurgy. Many design, and economic limitations of conventional forming technologies can be readily overcome by MIM. Some key advantages of the MIM process is showcased in the Fig. 5 and are described below.15,16)

Fig. 5. MIM process salient features and advantages.


© 2021 ISIJ2019

3.1. Component Design FlexibilityMIM offers design flexibility and produce geometrical

complex shapes in single step that cannot be produced using conventional metal manufacturing processes without sec-ondary machining operations. The mass production scalabil-ity that allows from thousands to millions of components, rapidly and efficiently. MIM can produce precise features (tolerance limit ±5% nominal dimension) that cannot be achieved by investment casting such as small holes, thin walls (0.5 mm–50 mm) and fine surface details.4,6,17)

3.2. Achieve Critical Design DetailsMIM offers possibilities for intricate features such as

slots, dovetails, undercuts, threads, and intricate curved surfaces. It can produce cylindrical parts with greater length-to-diameter ratios. Moreover, all these benefits are accompanied with the lower unit cost.

A comparison of the unit cost of the component produced from different manufacturing and MIM processes is pre-sented in Fig. 6.17) MIM process is more suitable for mass scale production of component with complex shape and can provide cost savings through effective material utilization.

3.3. Improved PropertiesMIM components density are typically in the range of

95%–98% (vary from different materials), approaching wrought metal/alloy properties.14) Components produced using MIM have usually greater strength as compared with that of the conventional powder metallurgy processes. MIM has successfully employed for producing even high melting point metals/alloys such as tungsten and tantalum alloys.4)

3.4. Reduced Wastage/MachiningMIM ability to produce near net shape parts and removes

many secondary machining operations and thereby reduced machining wastes.14,17)

3.5. Reduced AssembliesMIM process can be used to combine two or more sim-

pler shapes into a single part (produce near net shape), more complex component design to minimize assembly cost.17)

4. Influencing Characteristics of MIM Process

In regard with the literature,18,19) the mechanical proper-ties of the end MIM-manufactured parts are effectively influenced by the MIM fabrication stages. One of the promi-nent influencers is the right selection of the starting powder and binder constituents. Various characteristics of feed stock powders and end components influencing the MIM process is illustrated in Fig. 7.

Some of the major influencers include powder size, powder purity, powder shape and porosity. Fine powders, typically D90 < 22 microns, for most alloys is required for better packing density, good powder loading and best particle to particle interactions per unit volume.20) Coarser particles can lead to distortion. Powder with higher purities > 99% are preferred to promote good sintering and contact with polymer.19,20) Spherical morphology of powders is more suitable for MIM applications owing to their excellent flowability. However, powders with non-spherical or nearly spherical morphologies are used with lower solid loading.20) Non-porous and void free powders promote high sintered density and better integrity of the MIM components.14,20) Apart from the powder properties, feedstock rheology and final component properties like density, microstructure and thermal properties stands important influencing the overall MIM process.

Hence, the characteristics of powders going into the MIM process and their understanding is critical for the success of the MIM technology. Various characterization techniques are available to evaluate the physical and chemical attributes of the powders. Some of the common characterization tech-niques adopted to test the suitability of powder for usage in MIM process are briefly discussed in Table 2. An in-depth understanding of the powder properties and their influence on MIM process and application is discussed below.

4.1. Base Powder Characteristics4.1.1. Chemical Characteristics

MIM demands high purity metallic powders, as small amount of unwanted impurities may negatively affect the performance of the component.14) The main drive of fabri-cating ferrous materials by MIM is to improve the mechani-cal properties, wear-resistance, and corrosion properties of the steel grades, combined with the exceptional shaping advantage of MIM.20)

In MIM, controlling carbon composition during sintering

Fig. 6. Comparison of unit cost and shape complexity for MIM manufacturing process with other different conventional manufacturing techniques (redrawn).17)

Fig. 7. Different metal powder characteristics influencing the MIM Process.


© 2021 ISIJ 2020

is a challenge, an excess amount of carbon can arise due to the binder system.14) For steel grades, with a very low or high carbon content, the problem could be overcome by varying the sintering atmosphere (either to reactive or neutral atmospheres, respectively).14,20) Special care is required when processing medium carbon steels, e.g. with a controlled carbon content of 0.1–0.5%.

In stainless steels, commonly, austenitic alloys such as 304 and 316 are used in their low-carbon forms, i.e., 304 L and 316 L. The purpose for the injection molding process works well with minimal carbon composition, which also lowered the susceptibility to sensitization and expected to improve corrosion properties.14) However, martensitic alloys such as 420 and 440 grade that need relatively higher carbon can be exactly controlled in MIM process for better hardness.21) Stainless steel grades such as 17–4 PH, 420 and 440 are the most widely used for medical tools and devices

due to a combination of properties such as high strength, good wear resistance, biocompatibility, and ease in manu-facturability with relatively reasonable price.21,22) The first application of stainless steel (SS) as surgical implants was 316 SS grade (have 18% Cr and 8% Ni) in 1926.14) This SS grade showed higher strength and resistance to body fluid as compared with vanadium containing alloy steels. Thereafter, stainless steel is the most preferred material used for inter-nal fracture fixation, joint replacement and segmental bone replacement. Recently, a new range of Ni-free SS grades have been developed due to the toxicity result of Ni ions on the human body.

4.1.2. Physical Characteristics(1) Powder Size, Particle Size Distribution and ShapePowder sizes and its distribution19) play a crucial role in

comprehending a high sintered density part without under-going excess shrinkage.23,24) Finer particle sizes reduce the risk of molding defects, increase viscosity of the melt, improves sintering kinetics and results in high surface finish. However, some of the shortcomings of finer size powders include poor homogeneity due to agglomeration, longer debinding times, higher risk of handling and higher cost of procurement. In contrast, coarser powders, provide higher packing efficiency, minimum shrinkage on sinter-ing, shorten debinding time and considerably cheaper and simple handling of powders, however the powder quality is normally inferior.20)

The mouldability of the feedstock is also an important factor in the MIM process and have a strong bearing on the particle size distribution.18,25) The feedstock mouldability is envisaged with the help of two parameters, 1) distribution slope parameter (Sw) and 2) flow behavior index (n). The former parameter measure is defined by the Eq. (1)18)

Sw ��

��

2 569010

.

logD

D

............................ (1)

where, the numerator signifies that D10 and D90 are 2.56 standard deviations separately on a Gaussian distribution; but, powders typically follow a log-normal distribution and the logarithmic size is suitable for gaining Gaussian behav-ior. The average particle size, D50 and Sw are significant parameters of a powder. The value of Sw is the slope of the log-normal cumulative distribution and is like a variation coefficient or standard deviation. The narrow size particle distribution indicates larger values of Sw and a broader distribution corresponds to minor values of Sw. Powders that shows value of Sw of 2 implies very broad distributions which specify simple to mould, while Sw values between 4 and 5 are difficult powders to mould. A very narrow particle size distribution range are the most difficult to mould and usually show Sw greater than 7.

Generally, MIM industry used metal powders largely in the range of 3–20 μm mean particle size and reported to improve the density, mechanical properties of the final sin-tered parts.20) Hence, the use of finer grade powders brings a finest quality in MIM industry where it demands better mechanical properties for the high-end applications. There are applications where in minimum density and mechanical properties are of interest in that case the addition of coarser

Table 2. Common characterization technique for testing MIM powders and standards.

Testing Methods and Standards Purpose

Density (Pycnometer, Archimedes)

(i) To determine true density of the powder and can also provide an indication of issues with internal voids within a powder(ii) To determine the proper solids loading/feedstock (powder/polymer mixture) and to predict shrinkage for tool design.

MFIF 63, ASTM D 2638, ASTM D 4892

Apparent Density (AD)

MPIF 28 and 48, ASTM B 417 and B 703, ISO 3923-1 and 3953

To determine the mass per unit volume of powderGeneral Indicators from AD mea-surements

Low AD Fine particles

High AD Coarser Particles

Variation in AD Change in surface roughness

Flow rate

ASTM B 213-17 (using Hall flowmeter)

ASTM B964 – 16 (using Carney funnel for non -flowing and fine powder)

To determine the flow characteristics of the powder. Powder with better flow characteristics expected to produce parts of more uniform wall thickness than powders that are sticky, tacky or that tend to bridge.(Flowability is affected by various powder characteristics like stiffness, porosity, surface texture, density and electrostatic charge)

Tap density

MPIF 46, ASTM B 527, ISO 3953

To determine the packing density of the powders on tapping and a first indicator of the powder packing efficiency that will load into a feedstock

Particle Size Distribution (PSD)

ASTM B 822 - 10, ISO 13320-1(typically measured using laser scattering/diffraction method)

To determine the particle size distribution. Maximum packing density is reached with an optimum distribution that includes both coarse and fine particles, with finer particles increasing density by filling the interstices

Specific Surface Area

BET (Brunauer–Emmett–Teller)

To determine the specific surface area of the powder. Generally, higher specific surface area is an indicative of fine powders and vice versa


© 2021 ISIJ2021

powders provides cost benefits. Mukunda et al.26) noted that the coarser powders show unstable performance in con-stant shear rate along with higher deformation and inferior mechanical properties. But with the addition of 25 wt.% of coarser irregular powder to the 75 wt.% of finer powder improves the steadiness of the feedstock.

The particle size distribution of powder varies from 0.1 to 60 μm, with D50 between 10–20 μm as shown in Table 3. There is a rising trend in sales for relatively coarser MIM powders of the powder size-32 μm, 80% −22 μm, and of finer powder grades like 90% −16 μm and finer as shown in Fig. 8. The initiatives for finer powder sizes comprises demand for miniaturization, advanced part precision and high surface finish. In the past context, medical and advanced automotive applications require high tolerances better than the 0.3–0.5% classically quoted for MIM, improved surface finish can add price and reduce finishing costs.

When the particle is spherical consistently, it promotes higher dimensional accuracy. Hence, the spherical or rounded shapes are favorable for MIM, and the particle

size should be preferably between 0.5 and 20 μm. Figure 9 shows the general trend on the influence of particle shape on the apparent density (AD). The apparent density usually decreases as the particle shape becomes less spherical. The low apparent density for non-spherical particles is due to the higher frictional surface area and non-uniformity of powder particles during packing. Spherical powders normally have high apparent densities, close to 50% of the density of the wrought metal. While sphere particles are most likely to pack without bridging or arching to generate unfilled spaces; they tend to move effortlessly with each other due to smooth surfaces.27,28) On the other hand, flake particle shaped powders often have apparent densities lower than 10% of the wrought density. Thus, by reducing surface area/volume ratios, less rough surfaces and less frictional forces act between the settling particles resulting in higher apparent density due to effective filling of the free spaces between formerly settled particles.

Most literature available puts significant importance on the influence of powder shape, controlled particle size distribution and powder loading on the flowability of MIM powder feedstock and the end mechanical properties of the sintered parts.19,29–31) Powders manufactured using gas atomization route are normally spherical in shape, charac-teristically have the highest critical solids loading (60–67% by volume) and undergo least shrinkage. While elemental powders produced using chemical reduction or precipitation route undergo more shrinkage due to the least solids loading (50–62% by volume).

With regards to shape, gas atomized particles are con-siderably spherical but may exhibit any of the features as presented in Fig. 10. During powder manufacturing, par-ticularly produced by gas atomization route make use of inert gas environment such as argon or helium to protect from oxidation and to facilitate convective cooling. Dur-ing powder formation, molten metal interacts with the gas environment, leading to the entrainment of gas within the particles which results in porosity, voids and impairs the physical characteristics of powder (flowability, for example, comprises stiffness, porosity, surface texture, density and electrostatic charge).

(2) Powder FlowabilityPowder flowability is the measure of displacement of

Table 3. Different powder characteristics obtained with different powder manufacturing method.20)

Manufacturing Method

Particle size (μm) Particle Shape Cost

Gas Atomization 5–40 Spherical High

Water Atomization 6–40 Semi-spherical/

Irregular Moderate

Centrifugal Atomization 25–60 Spherical Moderate to

High

Plasma Atomization 2–40 Spherical High

Oxide Reduction 1–10 Polygonal to

Rounded Low

Carbonyl Decomposition 0.2–10 Rounded to Spiky Moderate

Precipitation 0.1–3 Polygonal Low to Moderate

Milling 1–40 Angular/Irregular Moderate

Fine Grinding 0.1–2 Irregular Moderate

Fig. 8. Growth in popularity of different MIM powder sizes (courtesy Martin Kearns). (Online version in color.)

Fig. 9. Influence of particle shape on Apparent Density (redrawn).27)


© 2021 ISIJ 2022

powder particles in relation to each other, under the result of some directional force such as gravity, external force, vibra-tor or agitator, mixer etc. and seldom electrostatic forces. The inherit resistance of the powder encounter to such displacement is called powder strength. Powder strength arises due to inter-particle forces majorly arises due to sur-face roughness of the powder and geometrical shape of the powders. The key types of adhesive forces between powder particles (or between particles and solid surfaces) are:

(a) Van der Waals forces, (b) Liquid bridge forces, and (c) Electrostatic forces32)

Besides, flowability is also affected with the internal porosity which originates from the “satellites” (welded small powder particles to larger powder particle) that appear otherwise mostly spherical but hinder continuous and homo-geneous flowability of powder as shown in Fig. 10. Satellites welded to nearly spherical particles obstruct homogeneous powder feeding and impede homogeneous consolidation and reduced parts densification which results in poor mechanical properties.

(3) Specific Surface AreaThe metal powders with high specific surface area is

likely to agglomerate and demand more binder (low solid loading) to produce a moldable feedstock.12) Relatively finer particle size promotes interparticle friction and results in poor powder flow and packing. Such finer powders normally demand intense mixing, and produces feeding, flowing, packing, and cracking problems during molding. While, a powder with low interparticle friction are problematic and are considered inferior in shape retaining on debinding and

sintering. Therefore, both coarser and finer particles are selected based on the application and requirements, the feedstock (mixing coarser and finer particles) is prepared in a desired proportion in order to get better densification dur-ing the sintering process. However, very fine powder par-ticles with high specific surface area susceptible to oxidation and result in high production cost which in turn adversely affect the quality of manufacturing process.

(4) Powder Packing DensitySelection of suitable powder morphology along with its

purity play a crucial role in powder packing density.20) For instance, gas atomization can be packed to maximum den-sities with suitable melt viscosity during injection molding process. Also, lower oxygen in the gas atomized powder with higher packing help in final densification on sintering. In contrast, rounded or irregularly water atomized powders result in lower packing density due to the presence of oxides which trigger porosity, and result in poor sintered density.14)

In recent few years, Compressibility index (CI) which is defined as (Tapped Density-Bulk Density)/Tapped density ×100%. CI is influenced by bulk density, size and shape, surface area, moisture content, and cohesive-ness of powder and considered as an indirect measure for all the above properties in powder.

(a) Hausner Ratio (HR)HR is the ratio between tapped density and bulk density33)

of the powder and the most common technique and widely used in powder characterization. CI is closely associated with the HR and gaining popularity for predicting the flow characteristics in powder. The physical measurement of powder density (apparent/bulk and tap density) is defined in ASTM D7481-09. Yu et al.34) suggested that the HR can be used to understand the packing behavior of powders on tapping. It is regarded that powders with HR ≤ 1.25 are con-sidered as freely flowing while cohesive and non-flowing powders show a HR > 1.40.35) Figure 11 shows a relation-ship between the HR vs Melt Flow Rate, which displays the expected connection between better particles packing of powders (low Hausner Ratio) and higher melt flow rates. The requirement of HR on powder particle shape was also accepted by Zou and Yu.36) Many researchers studied that HR reduced for large non-cohesive non-spherical powders with an increase in sphericity.36,37)

(b) Angle of ReposeThe angle of repose is used as a simple physical param-

eter to relate and estimate the interparticle friction of differ-

Table 4. Flow character relationship with the compressibility index and Hausner ratio.37)

Compressibility Index (%)

Flow Character

Hausner Ratio Example

≤10 Excellent 1.00–1.11 Free -flowing granules

11–15 Good 1.12–1.18 Powdered granules

16–20 Fair 1.19–1.25 Coarse powders

21–25 Passable 1.26–1.34 Fine powders

26–31 Poor 1.35–1.45 Fluidizable powders

32–37 Very Poor 1.46–1.59 Cohesive Powders

>38 Poorest >1.60 Very Cohesive Powders

Table 5. Relationship of flow characteristics behavior with the angle of repose.

Angle of Repose (Degree) Flow Properties

25–30 Excellent

31–35 Good

36–40 Fair

41–45 Passable

46–55 Poor

56–65 Very Poor

More than 66 Poorest

Fig. 10. Schematic illustration of different spherical metal parti-cles produced during gas atomization process.


© 2021 ISIJ2023

ent powders. The angle of resistance to shear is measured by tilting the compacted powder from horizontal to cause shear. Large spherical particles will exhibit an angle of repose near 30°, and it ranges up to 38° for free-flowing powder. When the angle of repose exceeds approximately 45°, the powder is qualified as cohesive. The angle of repose is well-defined in ISO-449038)/ASTM B213,39) where powder flows freely through a funnel and collected on a plate. The slope angle of the settled cone powder collected to the base plate is the angle of repose and considered as a powder flowability. In the second method, the time required to discharge the pow-der can be used as a measure for flowability.

(c) Avalanche AngleThe avalanche angle (αP) is defined as the angle of a lin-

ear regression of the free-falling powder surface just before an avalanche begins measured to a horizontal line.35) It con-tains rotating, transparent drum filled with a known amount of powder and a camera in front of a backlight. The drum was set to rotate at 0.3 rpm and a digital camera was used to monitor the flow behavior of the powder. The avalanche angle was determined by measuring the angle where the powder reached to the maximum position of the drum as shown in the schematic of Fig. 12. Lower avalanche angle indicates that the powder has better flowability.35,40) The camera captures pictures of the free powder surface and the powder cross-sectional area inside the drum. All these pictures are studied for different values and related with the powder flowability.

4.2. Feedstock RheologyIn MIM process, the molding stage is a crucial step

for the manufacturing of components without undergoing cracks and distortions.9,10) This step demands for the detailed rheological studies, so that can be used as a guideline for maintaining suitable rheological characteristics in the MIM feedstocks. Powder-binder separation and inconsistent flow in the mold can produce defects, and eventually result in poor physical and mechanical properties of the final compo-nent.14) Viscosity of the melt, density, thermal coefficient of expansion, heat capacity and pyrolytic behavior of a feed-stock influence the MIM process.14,19,41) Amongst various molding parameters, viscosity is the single most important property of feedstock quality that impacts the success of molding. The viscosity is maintained within a narrow range to produce sound components.41) Therefore, the rheological behavior is considered as one among the important feature for the successful manufacturing. Rheological behavior of the MIM feedstocks is usually estimated using the following equation which is related with the viscosity and shear rate at a specified temperature:19)

� � �K n� 1 .................................. (2)

where, η is the viscosity of the feedstock, ϒ is the shear rate, K constant and n flow behavior index, which is nor-mally smaller than 1. The value of n specifies the degree of shear sensitivity, it also gives an important understanding about the rheological characteristics of MIM feedstocks. The low flow behavior index value indicates higher viscos-ity dependence to the shear rate. The variation of viscosity versus shear rate in log–log scale is almost linear for stable feedstock, while non-linear relationship is an indicative for inhomogeneity in the feedstock, agglomeration, and powder-binder separation. Dihoru et al.42) reported that using higher particle size distribution in the feedstock has the positive result on the packing density, but it is prone to separation due to the segregation of particles of different sizes. Khakbiz et al.43) studied the rheological behaviour and stability of the 316 L and TiC feedstock composite. It was reported that the addition of TiC powders has a considerable effect on the viscosity of the stainless steel feedstock at low-shear rates (500 s −1), due to the competition between higher packing and deagglomeration produced by hydrodynamic stress with increasing shear rate.43) It was reported by researchers that the incorporation of a secondary grade powder reduces the rheological behaviour and raises the instability indexes.42,43) From a moldability viewpoint, the low and constant viscos-ity from medium to high shear rate governs the potential of the feedstock to be injected without any additional pressure into the complex mold shapes.

4.3. Component Characteristics4.3.1. Component Density

MIM sintered components are anticipated to have some porosity and usually have densities ranging from 95–99% of the theoretical density.44) In powder metallurgy process, relatively higher sintered densities are usually obtained using finer starting powder particles which is expected to improve the sinterability.9) The sintering stage in MIM has significant influence on the final density of the component. MIM can be combine with Hot isostatic pressing (HIP)

Fig. 11. Influence of Hausner Ratio of metallic powders on the melt flow rate of MIM feedstock (redrawn).35)

Fig. 12. Schematic of the drum for measuring avalanche angle.


© 2021 ISIJ 2024

to achieve close theoretical density.44) Another advantage of HIP after MIM process is to decrease the dimensional variability of components.45) For example, if the compo-nents in the middle section densify to 98% density and the components in the corner section densify to 96% density, a dimensional variation exists between these two parts. After HIP, the component can reach to close theoretical density with complete removal of internal dimensional variation in the component. Weldability of the MIM components is also expected to increase with HIP as metallic components with porosity weld poorly, therefore, the elimination of porosity by HIP can improve weldability.

4.3.2. MicrostructureStructural and mechanical features of the final product

(obtained from MIM) are usually estimated through micro-structure analysis.14,44) Properties of MIM components are in par with most of the cast components and are superior to components processed from conventional Powder met-allurgy route.14) Cast and MIM components both have microstructural pores or voids as a result of the processing methods, where the cast voids can be larger and localized owing to the cooling of liquid to solid (see Fig. 13(a)) and the MIM voids are typically finer and well distributed across the microstructure after sintering as shown in Fig. 13(b). The large, localized voids of the cast material result in the inferior properties, whereas the distributed nature of the fine MIM pores provides a better microstructure and ensure higher structural integrity with enhanced properties. As seen in Fig. 13(c), low density, irregular and bigger pores formed in conventional powder metallurgy compo-nent along the grain boundaries result in lowering of the mechanical properties. Typical schematic microstructural differences produced from conventional casting, MIM, and conventional powder metallurgy route is shown in Fig. 13.

In the MIM process, debinding temperature, sintering temperature, sintering time, sintering atmosphere and heat-ing rate during sintering plays an important role to achieve excellent microstructure thus promoting good mechanical properties like tensile strength, elongation and fracture strength.45–47) Based on the applied sintering parameters, the component may undergo less sintering (or) proper sintering (or) over sintering.46,48) Effect of sintering on component density is explained by Myers and German for M2 tool steel.48) Effects of sintering temperature, sintering time and sintering ambience on microstructure is elucidated in the Fig. 14. Yoon et al.46) in their study on 316 L stainless steel (SS) revealed that the percentage volume fraction of poros-

ity reduces with increase in sintering temperature and sinter-ing time. It was reported that the porosity volume fraction in 316 L SS at 1 100°C is 17% (Fig. 13(a)), which has reduced to 8% at higher temperature 1 320°C (Fig. 13(b)), and has further decreased to 5% at 1 350°C (Fig. 13(c))46) for the same sintering time (1 h). While with the increasing in sintering time to 2 h for 1 350 °C, the porosity has reduced to 2% (Fig. 14(d)) and has further reduced to 0.7% poros-ity for 4 h sintering (Fig. 14(e)). Besides, significant grain growth was reported with decrease in porosity and the pore morphology was rounded and isolated. Thus, even though increase in sintering temperature benefits and results in higher component density, optimum sintering window needs to be identified to avoid, coarsening of grains, isolation of pores in grains which promotes the decrease in sinter density even at higher temperatures.46,48) Hu and team in their work revealed that,49) with increase in sintering time from 5 h to 7 h for MIM processed HK 30 stainless steel samples at

Fig. 13. Schematic of typical microstructure produced from (a) Casting, (b) MIM, (c) conventional powder metallurgy route.

Fig. 14. SEM micrographs showing different sintered parameters (a) 1 100°C, 1 h (17 P), (b) 1 320°C, 1 h (8 P), (c) 1 350°C, 1 h (5 P), (d) 1 350°C, 2 h (2 P), (e) 1 350°C, 4 h (0.7 P). P denotes the porosity in (%) volume fraction of speci-men.46)


© 2021 ISIJ2025

sintering temperatures of 1 280°C, the component density increase from 7.53 g/cc to 7.61 g/cc.49) But, with further increase in sintering time to 9 h, the component density decreases to 7.58 g/cc. The reason behind this decrease in density with higher holding times is attributed to the grain boundary coarsening which led to the intergranular gap and restricted the densification in the component.49)

Besides, ambience in which sintering is taking place also influences the product microstructure.50,51) Suharno et al.50) reported that vacuum sintering yields in higher component density. The reason is attributed to the entrapment of gas inside the pores of the component which hinders the sinter-ing process. Thus, the sintering process parameters influence the microstructure of components which further influence the mechanical performance. Apart from the parameters, debinding temperatures and residual carbon post debinding also influence the microstructural and mechanical properties of MIM components.51)

4.3.3. Pore Size and ShapeMechanical properties such as tensile strength and elon-

gation is found to be majorly dependent on the component porosity, while the yield strength is stated to be insensi-tive to porosity.46) This insensitive discrepancy of yield strength with the porosity is primarily attributed to the balanced effects of the lowering of porosity and simulta-neous grain coarsening.46,48) It has been reported that the porosity decreases with higher sintering temperature and increasing sintering time.49) Decrease in porosity is usually accompanied by the significant grain growth. Yoon et al.46) reported that the morphology of pores is also change with the decrease in the porosity. With the densification, the pores get isolated either in the grain interior or at the grain boundaries in the parts. It is also studied that the fatigue crack growth rate (FCGR) to be significantly affected by the pore structure.46) The FCGR found to be higher for the parts, where pores are isolated in the grain interior or trapped in the grain and FCGR noted to be lower for interconnected pores along the grain boundaries. The microstructural having similar porosity (8%) and grain sizes (45–50 μm) produced from conventional powder metallurgy (P/M) and MIM shows that the fatigue resistance is considerably higher for MIM produced samples than the conventional P/M process. This difference in properties is attributed to the wedge type pore morphology in the conventional P/M process whereas for MIM, the pores are majorly located in the grain interior or at the grain boundary maintaining their spherical shape.46) Therefore, the pore morphology and pore size are considered as important in influencing the mechani-cal properties, particularly the fatigue properties.

4.3.4. Thermal PropertiesThe thermal conductivity of MIM component is usually

affected with the impurities, porosity, and microstructural features of metallic powders.6) The sintering temperature and cooling rate can also affect the thermal conductivity of the alloy powders. With appropriate powder selection and proper control of sintering cycles and impurities, thermal properties can be improved.

The most widely used method to determine thermal conductivity (λ) is through thermal diffusivity (α) which is

measured by laser flash method (ASTM E1461) according to the following formula.52)

� � �� Cp .................................. (3)

Where, Cp is the specific heat and r is the density of the sample.

The testing requires disk-shaped samples and enough care to reduce the measurement variability. Electrical conductiv-ity measurement does not depend on the sample geometry. The electrical conductivity σ for elemental metals can be measured by Four-point probe method and later converted to thermal conductivity λ using the Wiedemann–Franz relationship52)

� �� L T .................................. (4)

Where, L is the Lorenz number of metal, and T is the abso-lute temperature in Kelvin.

With the advancement, efforts are paid to reduce the size of the engineering components and to meet high-power demands which comes with a challenge of heat dissipation. High thermal conductivity materials also required low coef-ficient of thermal expansion. Hence, new design geometries are recommended in the parts to manage high thermal loads by using aerodynamic fins, heat pipes, and microchannels. Components with these designs can be readily produced in mass quantities by MIM. It allows greater geometrical flex-ibility in design over other processes like extrusion and die casting. For example, round pin fins are easy to manufacture through MIM process, however machining operation is lim-ited to square fins.6)

5. Recent Market Outlook

Recently, MIM continued to gain popularity and estab-lished mass-volume manufacturing technology. New advances in powder manufacturing technology guaranteed satisfactory supplies of ultra-fine powders at lower costs. MIM has gained massive attention over the past seven years. The MIM market in Asia is majorly driven by the consumer electronics sector since 2013, whereas in European countries it is widely based on the automotive components and con-sumer products.4) In the US, firearm components drive the MIM market.1,4)

Lately, it is evident that close to 90% of powder injection molding (PIM) is coming from MIM. Out of 366 organiza-tion that exercise powder injection molding, the mainstream is in Asia. The foremost countries of PIM were the USA with 106 operations, China with 69 (while recently the expansion in China is fast), 41 with Germany, 38 with Japan, 17 with Taiwan, Korea with 14, and 12 with Switzerland.4) The number of operation steps is not essentially revealing the economic size, since one of the major MIM services is in India,4,53) which only has five MIM operation stations, while the USA has the maximum organizations, but they are all minor in size. Figure 15 shows the extensive global statistic survey contribution for MIM coming from different parts of the world.4)

Worldwide MIM sales were estimated close to $2.4 bil-lion in 2017, with an average annual progress of 18% since 2010. It is estimated global sales of stainless steels contribu-tion is dominant in MIM metallic powders. The worldwide


© 2021 ISIJ 2026

metal sales (value, not tonnage) are as surveys in 2010 – 53% contribution sales from stainless steels, 27% for steels, 10% for tungsten alloys, 7% for Iron–Nickel alloys (majorly for magnetic alloys), 4% for titanium alloys, 3% for copper, 3% for rest of the alloys.4)

Figure 16 shows market trend for MIM for different countries from 2010 to 2017. It is estimated that China has consumed over 8 000 tonnes of metal powders in MIM industries in 2017, of which around 55% was sourced from domestic powder producers. Advanced Technology & Mate-rials in China using China Iron and Steel Research Institute (CISRI) technology, has established high-pressure water atomization technology to produce ultra-fine spherical, high purity stainless-steel powders combined with vacuum gas and water/gas atomization technology.

In Europe, MIM sales has grew 11% by volume, and 9% in sales value to over $400 million in 2017. Germany has important market share at 29% followed by 18% in Italy and France at 14%. Another trend in Europe is gaining attention is the injection molding and co-sintering of two different component parts (2C-MIM).7) The 2C-MIM process not only removes the joining stage but opens up new perspec-tives for adding functional properties to MIM parts.54,55) Figure 17 shows the fuel injector valve sleeve manufactured using 2C-MIM technology produced by combining the mag-netic 430 stainless steel (SS) grade with the non-magnetic

314 SS grade in a single component.54) This MIM part is approximately 40 mm long, 6 mm diameter and has a wall thickness of 0.65 mm.

In Japan, MIM sales are forecast to rise to $114 million in 2019, a growth of 10.7% compared with 2017. In 2007, MIM production was stated at its highest level of $138 million. It is anticipated that by 2020, MIM production in Japan is expected to return to those levels attained before the financial crisis 2008.

MIM application that partitioned by various engineering areas based on the reported year for 2010–11 is shown in Fig. 18. This is a mixture of number of organizations and their relative attention, so an organization that only in dental manufacturing area is counted as 1, but an organization that in dental as well as in industrial sector is counted as 0.5 for each, and so on.

Figure 19(a) shows MIM firearm part produced in Mimecrisa, Spain made from low alloy steel grade (FN08).55) Recently, it is found that a large proportion of European police officers carry a pistol with MIM parts. The largest and smallest MIM parts manufactured at Mimecrisa is shown in Fig. 19(b).55) MIM sintered parts clearly demonstrate shrinkage as shown in Fig. 19(b). Cobra Golf of Carlsbad, California, USA, reported that using MIM technique it is possible to produce its new golf wedge, with the entire head made up of 304 stainless steel as showcased in Fig. 19(c). MIM technology has attracted consumers to fabricate smaller parts on a mass scale which add commercial value in the product such as tiny dental supports as illustrated in Fig. 19(d).56) Interestingly, automotive and medical are fast-growing areas in MIM. The rapid growth in the MIM segment since 2010, particularly in Asia contributing to a

Fig. 17. Fuel injector valve sleeve made of two of two different type of stainless steels using 2C-MIM technique55) (Courtesy Fraunhofer IFAM, Bremen, Germany).

Fig. 18. Global statistical market sectors available for MIM in 2011. (Online version in color.)

Fig. 15. MIM global statistics for the year 2010. (Online version in color.)

Fig. 16. MIM market trend shows high growth rate for countries like China, Taiwan, North America and Europe since 2010–2017 and steady growth for other countries (redrawn).7) (Online version in color.)


© 2021 ISIJ2027

significant demand in MIM powders sales. In 2018, Bulger estimated worldwide sales of MIM powder close to 18 910 tonnes and 20 205 tonnes sales for MIM feedstock as shown in Table 6.7) Some of the recent advancement in MIM tech-nology using iron powders are discussed in the following section.

6. Some Recent Advanced and Prospects of Iron Pow-ders in MIM

The term “pure iron” is used to indicate a minimum of 99.8% purity and the absence of any other deliberate alloy-ing elemental addition. However, iron and its alloys have a dominant share in the MIM market and expected to grow further and generate new prospects in different industries. Taking the advantage of high degree of freedom in material design and producing the parts to near net shape, this tech-nology is considered as a good candidate for designing the futuristic advanced iron powder based materials in variety of applications such as magnetic materials, medical implants, drug delivery, and in automobile industries etc.1,4)

Table 7 lists some common commercial iron powder grades from some of the selective major manufacturers for MIM.57) The differentiation in such product specifications made based on the parameters such as particle size distribu-tion and bulk density. Some of these properties are reachable through post processing by milling method and sorting to meet the specific application supplies. At present, carbonyl iron powder grades have found wide applications in MIM, magnetic cores for high-frequency coils, hard metal bind-ers, radar absorption materials (RAM), magneto-rheological fluids used for shock/vibration damping, clutch and brake in automotive systems, precision polishing (like to produce the smooth surface finish found on the Jet-Black iPhone,4,14) industrial diamond synthesis and food iron supplements.2,14) However, some recent advanced and unique application of iron powders in relation with MIM technology is discussed as follows:

6.1. Magnetic MaterialsRecently, there has been an emerging trend towards

downsizing and maximizing efficiency in electromagnetic parts. In contrast, with the conventional stacked electrical steel sheets, iron powder cores have the benefit of a high degree of choice for complex design. Employing this advan-tage, development related to downscaling of motors which exercise 3-dimensional magnetic circuits has progressed. Magnetic powders such as Fe–Si powder, Fe–Si–Al powder, Fe–Ni powder, etc. are recognized as starting materials for iron powder cores.5) However, pure iron powder has the advantage of high magnetic flux density as fabrication of high densification cores is possible using iron powders.

In magnetic materials, particularly soft magnetic parts represent an important area of application for MIM and there are motivating signs of huge requirements from the customers that will enhance their demand further.4,5) Some of the main attraction for the use of MIM in soft magnetic

Fig. 19. (a) Firearm part made up of alloy steel at Mimecrisa (b) Smallest and largest MIM components produced by Mimecrisa (green and sintered), with a paper clip for scale56) (c) the new Cobra King MIM wedge made from 304 stainless steel (d) tiny dental supports, with match stick for scale.57)

Table 6. Estimate global sales of MIM feedstocks and metallic powders.7)

Metal/Alloys Feedstock (tons) Powder (tons)

Stainless Steel 10 000 9 000

Fe-based 6 700 6 030

Tungsten 600 570

Titanium 275 220

Copper 430 390

Others 2 200 1 980

Total 20 205 18 190


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materials are as follows:(a) Higher achievable sintered density as compared to

the conventional P/M which results in low coercivity in the components

(b) Ensures high material utilization with near net shape capability

(c) MIM is considered ideal for designing soft magnetic cores with 3- dimensional shape complexity

(d) Miniaturization of electrical parts has opened up new opportunities for MIM

(e) Design of parts with 2C- MIM technology allows the integration of soft magnetic and non-magnetic within the same part.54)

Commonly iron powder produced from gas atomization, electrolytically and carbonyl process usually characterized by high magnetization saturation, and good processing per-formance. Pure iron resistance is low, eddy current losses in an alternating magnetic field is considered appropriate for static use. While pure iron is considered for manufac-turing the magnetic core, the pole piece, relays, the speaker magnetic conductor, magnetic shield, etc.58,59) Permanent magnet is considered for circuits where the magnetic excitation is provided by permanent magnets, or by DC or low-frequency (<10 Hz) pulsed current coils, with medium to high magnetic fluxes.60) In low electrical power and elec-tronic circuits, iron based soft magnets are used even at high frequencies (< 200 Hz).61)

Normally, the magnetic properties of pure iron can be enhanced as the impurity level is reduced and with the increasing component density.62,63) At room temperature, the residual non-metallic impurity in iron must be lower than the solid solubility limit. The solid solubility limit of carbon, oxygen, sulphur, nitrogen and phosphorous in iron is estimated to be 0.007, 0.01, 0.02, 0.001 and 1.0 percent, respectively at room temperature.62)

Conventional soft magnetic pure iron was manufactured by casting and machining techniques. However, mass pro-duction of the miniaturization parts with complex shape was greatly restricted due to the extended production period, low efficiency and high cost. Powder metallurgy is a near-net shaping technique, which is appropriate to produce small, complex shaped pure iron parts.64) The main disadvantage for powder metallurgy (P/M) pure iron is its low density which degrades the magnetic properties.

Figure 20 shows the improvement in saturation induc-tion with increase in sintered density obtained by P/M route and using MIM process. It is reported that the part density

obtained using MIM process are higher than the conven-tional P/M route.63) Hence, MIM technology can open-up the opportunity for mass production of high-density minia-turization magnetic parts with complex shape results in den-sities approaching theoretical density. Therefore, magnetic inductions close to saturation magnetic induction are possi-ble. This means that magnetic inductions equivalent to those of wrought alloys are possible for alloys of similar composi-tion. In addition, new iron-based alloys can be developed to take advantage of additional alloying elements that difficult to be considered using wrought fabrication technology.61,62) However, metal injection molding parts shrink more than conventional P/M parts, but dimensional control is less of a problem because shrinkage is more uniform.65)

In last few years, MIM technology is being explored for preparing pure iron parts by many researchers to control the residual oxygen content. It has been realized that using MIM technology, pure iron magnetic performance can be improved.63,66) Tasovac et al.67) investigated carbonyl iron parts using MIM to study the magnetic properties for different sintering conditions, 75% H2 + 25% N2 mixed atmosphere sintering. He noted that the sintered density of the material has been improved, however N, O impurities are higher.67) Many researchers66,68) has demonstrated with their comprehensive studies using MIM process that in pure soft magnetic iron with the increasing temperature from 900 to 1 350°C followed by sufficient soaking time at 1 350°C, high-density sintered body can be gained. Further it is also confirmed that the heat treatment results in favorable of microstructure homogenization and distribution of the pores of the sintered body.

Figure 21 depicts the improvement in relative sintering

Fig. 20. Magnetic property obtained for different density of pure iron made by powder metallurgy and MIM route.63)

Table 7. Some of the major commercial iron powder grades used in MIM technology.58)

Company Grade D50 (μm) AD (g/cm3) Tap Density (g/cm3) Femin (%) Cmax (%) Omax (%) Nmax (%)

BASF CIP OM 3.9–5.2 97.8 0.75–0.90 0.15–0.40 0.65–0.90

BASF CIP CC 3.8–5.3 99.5 0.05 0.18–0.35 0.01

Tianyi YMIM 90 <5.0 <3.0 ≥4.0 Bal. 0.76–0.90 0.60 0.9

Tianyi RMIM20 <6.0 <3.0 ≥4.0 Bal. 0.76–0.90 0.60 0.9

Jinchuan F01 1–6 1.0–3.5 97.5 0.80 0.50 1.0

Jinchuan F02 <5.5 2.5–3.2 3.5–4.5 99.0 0.10 0.30 0.1

Yuean HY1 5.5 99.5 0.03 0.3


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density from 1 200–1 450°C with increase in temperature. Tian et al.66) showed that the using carbonyl iron powder in MIM process, sintered density has improved significantly (above 1 393°C) using δ phase sintering. As seen from Fig. 21, there is a steep rise in relative sintered density above 1 393°C. MIM carbonyl iron resulted to full density after sintering at 1 450°C with a maximum permeability of 17 400, B6 000 of 1.81 T and coercive force of 20.6 A/m. It is postulated63,66) from the Fe–C phase diagram that pure iron present in the form of δ phase (more than 1 394°C) promote sintering. Hence, sintering made above this temperature is called “δ phase sintering”. The iron in δ phase usually have high self-diffusion coefficient and have Hedvall effect on the phase transformation point which encourage densifica-tion in parts.

It has been known that the purity of fine powders helps in improving structure-sensitive magnetic properties.66) With proper binder removal and sintering, relative maximum per-meability and coercive field are equal to those of the best processed conventional P/M parts. Also, magnetic induction is close to that of wrought parts because of the achievable full density. Some examples of MIM soft magnetic parts in the communications field are conventional hard disk drive parts of three miniature Fe–Ni parts (see Fig. 22), each weighing less than 1 g.69) These parts belong to the world’s first 1.8-inch hard disk drive.

Many researchers investigate the performance of the soft magnetic material produced from MIM, but still there is a wide scope to strengthen the theoretical literature work.63,66–68) There is an opportunity to study on the following points and solve the problem.

(a) Powder pre-treatment on the performance of soft magnetic materials

(b) Different powders such as microcrystalline, nanocrys-talline, amorphous, and composite soft magnetic material preparation.

(c) Rheological investigation on soft magnetic material by MIM

(d) Micro-injection, co-injection molding techniques using soft magnetic material.

6.2. Medical and Drug Delivery ApplicationMIM technology established well for manufacturing

medical tools and instruments for industrial scale compared to the biomedical implants.14) MIM manufactured medical implants is still on its premature developmental stage and still strong clinically approved implants need to be manu-factured using this technique. In medical application, dental orthodontic brackets and tissue biopsy jaws were some of the early successes of MIM technology.1,4) At present, 0.1 g stainless steel dental bracket sells for $6 000/kg, while a more conventional 7 g steel trigger sells for $140/kg. The ability to fabricate small parts and miniaturization adds much commercial value to the parts. Recently, biomedical stent has been produced using MIM which has small tubular mesh that expands a narrowed or blocked coronary artery or urinary system.70) At present, irrespective metallic or polymeric stents, still mainly fall short to the perfect clini-cal requirements due to late restenosis, thrombosis and other medical complications. However, metallic stents are favored medically due to their excellent mechanical property and radiopacity to their polymeric counterparts. The advent of bioresorbable metals opens an opportunity for better stent materials as they may have the potential to minimize or eradicate late restenosis and thrombosis. In recent few years, the progress of biodegradable cardiovascular implants made up of bare-metal corrosion has been considered as an alternative answer to avoid the drawbacks of permanent stents.70–72) The concept has been applied to metals like magnesium-based alloys,73) stainless steel,74) cast iron,72) zinc74) etc. for stents.72–74) It has been found that very high corrosion rates of magnesium-based alloys result in dis-satisfaction of the medical requirements of stents to deliver structural support over 6–12 months period during arterial remodelling and healing.75,76) While using stent made up of stainless-steel results in long-term difficulties after implanta-tion with chronic inflammation and neointimal hyperplasia leading to thrombosis and restenosis at the late stage.

Iron ion (Fe2+) is a crucial trace element and important portion of hemoglobin in the human body. It is a vital constituent for a variety of enzymes. Iron-based alloys is considered as one of the favorable candidates as biodegrad-able material implant.70,72,77) Iron has an advantage of rea-sonable and uniform degradation, which is desired for the biodegradable stents to avoid premature mechanical failure of the device in vessels.72) The mechanical properties of pure iron are also similar with those of 316 L SS and other stent materials.

Unfortunately, little attention has been paid till now on the biodegradable pure iron stent precursors using MIM pro-cess. The microstructural perspective obtained from MIM process is of great interest, because it can also offer a porous

Fig. 21. Effect of sintering temperature on the relative density using MIM-carbonyl iron (redrawn).66)

Fig. 22. MIM hard disk drive parts made up of soft magnetic Fe–Ni.69)


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microstructure, permitting accelerated degradation of iron in physiological environments and providing drug reservoir abilities. The control of micro porosity is one of the key tailoring parameters to reach a finest balance between deg-radation rate and mechanical properties. Figure 23 shows the MIM stent inserted in the blood stream to reinstate the blood flow towards the heart. The intravascular stents are expandable ring elements and connectors in longitudinal or circumferential channels, which usually coated using a polymer-plus-drug coating or which have microreservoirs of a drug, such as an anti-cancer drug to perform as a func-tional drug delivery vehicle.78,79)

It has been reported that MIM stent made up of pure iron showed exceptionally high ductility (13–50%), while in terms of strength, it falls between those of Mg alloys and 316 L stainless steel.72) Mariot et al. reported72) that in vitro corrosion rate of MIM pure iron is superior than cast iron and recommended MIM technology for further develop-ments as a new manufacturing route for biodegradable stents made up of pure iron.

6.3. Opportunity of Iron Powder for Activated Sinter-ing

Titanium powder metallurgy research has been recently dedicated to cost effective ways to manufacture titanium alloy parts.80–82) In the progress of manufacturing cheaper Ti alloys, the addition of iron (Fe) as a β -Ti stabilizer into the alloy compositions has been widely explored. Some researchers showed that the improvement in sinterability through the addition of fine pure iron powder (mean particle size = 8 μm).83) The same benefit of improved sinterabil-ity is not present when working with coarser iron powder (mean particle size = 97 μm). Prices for MIM titanium powder grade is considerably different with the particle shape, purity, and alloying, However, there is an opportunity to explore activated sintering benefits in conjunction with MIM in order to have high sintered density components without adopting additional steps or advanced sintering technique (such as Hot isostatic pressing (HIP), container-less sintering etc.).

6.4. MIM Opportunities in Automobile Industries for Iron and Steel

The automotive sector is one of the key segments of the economy noted for its global value chain and is a potential customer for MIM parts. The MIM technology have the

potential to enable the production of complex shape geom-etries, which is impossible to produce using conventional rigid die and pressing technique. The method involves hot mixing of ferrous powders with a binder and granulates to form the feedstock of the required characteristics. MIM parts are anticipated to deliver superior mechanical proper-ties with respect to yield strength at higher temperature. There is a huge demand from the automobile sector for denser and stronger components such as clutch race assem-blies, engine transmission systems, gears, and sprockets. In the ferrous group Fe, Fe–C, Fe–Cu–C, and Fe–Mo–Cu–Ni–C are used for crankshaft sprockets and camshaft, connecting rods, synchronizer rings, oil pump gears, bear-ings caps, etc.84) The automotive component parts with asymmetrical profiles and holes that are at an angle to each other can be designed using MIM technique.85) In such modern design cases and to get rid of multiple intermediate machining/joining processes, MIM process has been able to contribute in Value Engineering/Value Addition (VE/VA). For example, to produce parts like roller rocker needed heavy machining with conventional forging combined with additional machining operation, because the roller mounting groove had to be hollowed out. This resulted in low productivity and higher costs, restricting the mass pro-duction using conventional technology. To overcome this problem, roller rocker has been produced recently using MIM technology as shown in Fig. 24(a). In automotive engines, fuel injector components are crucial. These injec-tor components have been developed using MIM in order to deliver improved responsiveness, weight reduction and lowered the costs.86) MIM technology enables the design modification of the armature body which made hollow so as to have a restricted the armature’s movement and improve fuel flow control as shown in Fig. 24(b). With the help of MIM, it is possible to manufacture split component design by judiciously positioning of the split surface and pin/hole followed by sintering of each split piece separately before assembly into the final product. Designing such component is difficult using conventional methods; however, using MIM a high-quality product with precise dimensional control measurements of the split pieces can be made with ease. Figure 24(c) shows deflector tube shape split compo-nent used in automotive component manufactured by MIM technology. Besides, MIM offers different type of special threads that are impossible to be made by machining such as irregular-shape and multi- start threads. This provide a huge cost advantage to the customers. MIM Automotive component with trapezoidal octuple-thread is presented in Fig. 24(d).86)

6.5. Next Generation High Performance μ-MIM PartsThe manufacture of micro-parts is considered as one of

the challenging and leading technologies with a significant market potential.18,87,88) The very first study on μ-PIM was initiated by material scientists from Fraunhofer IFAM in Bremen and Forschungszentrum Karlsruhe (Karlruhe Research Centre) in Germany. Various micro products, like fluidic device, micro reactor, micro gears, micro-spin nozzle structures, micro-optical bench, spectrometer test structure, micro-molds for gearwheels etc., have been suc-cessfully made and replicated in Germany.89–91) Micro rod

Fig. 23. MIM stent used in the blood stream with inset detail.79)


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arrays have been attained with diameters of 80 and 100 μm, micro square arrays with dimensions of 100 μm. Lately, micro gear with 1 mm diameter and height of 1 mm have been produced by μ-PIM.92–94) Figure 25 shows scanning electron micrograph of the cutting tips on a dental end-odontic reamer used for the root canal procedure fabricated using MIM.57)

Similarly, Tong et al.95) have successfully produced micro metal gears using spherical carbonyl iron powder (pure >99% Fe content). He studied the replication qual-ity by varying MIM operating parameters such as injec-tion speed, injection pressure and temperature of the die and successful in optimizing the parameters. You et al.96) reported that mixing of micro-iron powder (75% Vol) with nano- iron powder (25% Vol) removed some the drawbacks during mixing/molding such as powder-binder disintegration, binder extraction and collapse of powders by capillary effect of liquid binder. It is further claimed96)

that mixing of micro-nano powder resulted in high sur-face finish in micro parts and planarization of surface on sintering. Similarly, You et al. and other researchers96–98) studied the μ-MIM process using nano-micro-iron pow-der feedstock and reported to have improvement in the compacting and sintering properties. Lee et al.99) reported enhanced sintering behavior at low temperature using nano iron powder and densification process along hierarchical boundaries. The optimization of nano controlled size Fe powder agglomerates provide prospective breakthrough guidelines for fabricating full density net-shaped μ-MIM parts. Recently, a new method has been studied by the researchers using micro sacrificial plastic mold insert (μ-SPi-MIM) to address the specific problems related to miniaturization of MIM parts.100) For superior quality of μ-SPi-MIM process, the nanosized metallic powder and oxymethylene-based binder feedstock used and molded into PMMA films with a fine line structures via nano-imprint lithography technique. The μ-SPi-MIM have reported to have unique advantage on producing macro-/micro-scale structures.100,101) It is also reported to design component with micro-scale open and closed porous structures which is not accomplished by recent semiconductor processes and depositions methods.101) Therefore, there is a high futuristic scope for μ-SPi-MIM process to find applications in battery manufacturing, micro-sensors for medical devices, micro reactor and micro-patterned electrodes with catalyst action for fuel cell.

7. Conclusions

This review aimed at evaluating MIM as a futuristic competitive manufacturing technology taking into consid-eration some fresh development and current state-of-the-art technology. The paper highlights the recent research activi-ties demonstrate an increasing application of the process in

Fig. 24. MIM automotive component (a) Roller rocker used in the car, (b) fuel injector parts, (c) screw tube shape split parts, (d) trapezoidal octuple- thread part.86)

Fig. 25. Scanning electron micrograph of a dental endodontic tool manufactured using MIM to fabricate 1 500 pyramidal bumps on the tapered shaft.57)


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different sectors on the global basis including commercial, medical, and some possible futuristic MIM products.

Several challenges and extensive research is further needed to increase confidence in the economic possibility of changing to MIM from another conventional manufactur-ing technology or justifying and grounding MIM process. Strategic rules and considerations, powder size, feedstock, injection moulding parameter and optimization, debinding and sintering parameters are vital areas of study. More-over, specialized testing techniques and reliable simulation models are needed for quality control deliberations. Lastly, increasing the capabilities of MIM by investigating different processes would be important for meeting rising demands of the present volatile market requirements in terms of func-tional integration and structural intricacy.

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