TOPIC 9-Extra Notes

TOPIC 9POWDER METALLURGY

Sources:1. Manufacturing Engineering & Technology 5th Edition, Serope

Kalpakjian, Steven Schmid, Prentice Hall, 20062. Fundamentals of Modern Manufacturing: Materials, Processes

and System, Mikell P. Groover, John Wiley & Sons (Asia), 20073. MYM Handout 2009

CONTENT The Characterization of Engineering Powders Methods of producing metal powders. Processes used to consolidate and sinter powders

into desired shapes. Additional processes and finishing operations to

enhance properties. Materials and Products for PM Competitive aspects of this unique process. Design Considerations in Powder Metallurgy

Powder Metallurgy (PM) Metal processing technology in which parts are produced from

metallic powders

Metal powders are compacted into desired and often complex shapes and sintered (heated without melting) to form a solid piece.

Usual PM production sequence:1. Pressing - powders are compressed into desired shape

to produce green compact Accomplished in press using punch-and-die tooling designed

for the part2. Sintering – green compacts are heated to bond the

particles into a hard, rigid mass Performed at temperatures below the melting point of the metal

Powder Metallurgy (PM) Powder metallurgy has become competitive with processes (such

as casting, forging, and machining), particularly for relatively complex parts made of high strength and hard alloys.

The most commonly used metals in P/M are iron, copper, aluminum, tin, nickel, titanium, and the refractory metals.

Figure (a) shows the examples of typical parts made by powder-metallurgy processes.

(b) Upper trip lever for a commercial irrigation sprinkler made by P/M. This part is made of an unleaded brass alloy; it replaces a die-cast part with a 60% cost savings.

(c) Main-bearing metal-powder caps for 3.8 and 3.1 liter General Motors automotive engines.

Why Powder Metallurgy is Important PM parts can be mass produced to net shape or

near net shape, eliminating or reducing the need for subsequent machining

PM process wastes very little material - ~ 97% of starting powders are converted to product

PM parts can be made with a specified level of porosity, to produce porous metal parts Examples: filters, oil‑impregnated bearings and gears

Why Powder Metallurgy is Important Certain metals that are difficult to fabricate by other methods can

be shaped by powder metallurgy Tungsten filaments for incandescent lamp bulbs are made

by PM

Certain alloy combinations and cermets made by PM cannot be produced in other ways

PM compares favorably to most casting processes in dimensional control

PM production methods can be automated for economical production

Limitations and Disadvantages High tooling and equipment costs

Metallic powders are expensive

Problems in storing and handling metal powders Degradation over time, fire hazards with certain metals

Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing

Variations in density throughout part may be a problem, especially for complex geometries

Engineering Powders

A powder can be defined as a finely divided particulate solid

Engineering powders include metals and ceramics

Geometric features of engineering powders: Particle size and distribution Particle shape and internal structure Surface area

Particle Shapes in PMSeveral of the possible (ideal) particle shapes in powder metallurgy.

Particle Shapes in PM(a) The scanning-electron microscope image of iron-powder particles made

by atomization. (b) Nickel-based superalloy (Udimet 700) powder particles made by the

rotating electrode process.

Measuring Particle Size Most common method uses screens of different

mesh sizes

Mesh count - refers to the number of openings per linear inch of screen A mesh count of 200 means there are 200 openings

per linear inch Since the mesh is square, the count is equal in both

directions, and the total number of openings per square inch is 2002 = 40,000

Higher mesh count = smaller particle size

Measuring Particle Size Screen mesh for sorting particle sizes

Production of Metal Powders The powder-metallurgy process consists of the following operations, in

sequence1. Powder production2. Blending3. Compaction4. Sintering5. Finishing operations

The choice depends on the requirements of the end product.

The microstructure, bulk and surface properties, chemical purity, porosity, shape, and size distribution of the particles depend on the particular process used.

Production of Metal Powders In general, producers of metallic powders are not the same companies

as those that make PM parts

Any metal can be made into powder form

Many methods are available to produce metallic powders1. Atomization2. Reduction3. Electrolytic deposition4. Carbonyls5. Comminution6. Etc…

In addition, mechanical methods are occasionally used to reduce powder sizes

Production of Metal Powders particle shapes in metal powders, and the processes by which they are

produced. iron powders are produced by many of these processes.

Methods of powder productionAtomization

Atomization produces a liquid-metal stream by injecting molten metal through a small orifice.

Figure show the methods of metal-powder production by atomization: (a) gas atomization; (b) water atomization; (c) atomization with a rotating consumable electrode; and (d) centrifugal atomization with a spinning disk or cup.

In centrifugal atomization, the molten-metal stream drops onto a rapidly rotating disk or cup, so that centrifugal forces break up the molten-metal stream and generate particles.

Iron powders produced by decomposition of iron pentacarbonyl (photo courtesy of GAF Chemical Corp); particle sizes range from about 0.25 ‑ 3.0 microns

(10 to 125 -in).

Methods of powder productionReduction

The reduction of metal oxides (i.e., removal of oxygen) uses gases, such as hydrogen and carbon monoxide, as reducing agents.

By this means, very fine metallic oxides are reduced to the metallic state.

The powders produced are spongy and porous and have uniformly sized spherical or angular shapes.

Methods of powder production Electrolytic deposition

Electrolytic deposition utilizes either aqueous solutions or fused salts. The powders produced are among the purest available.

Methods of powder production Carbonyls

Metal carbonyls, such as iron carbonyl and nickel carbonyl are formed by letting iron or nickel react with carbon monoxide.

The reaction products are then decomposed to iron and nickel, and they turn into small, dense, uniformly spherical particles of high purity.

Methods of powder production Comminution

Mechanical comminution (pulverization) involves crushing milling in a ball mill, or grinding of brittle or less ductile metals into small particles.

A ball mill (Figure b) is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls.

With brittle materials, the powder particles produced have angular shapes; with ductile metals, they are flaky and are not particularly suitable for powder-metallurgy applications.

Methods of powder production Mechanical alloying

In mechanical alloying, powders of two or more pure metals are mixed in a ball mill.

Under the impact of the hard balls, the powders fracture and bond together by diffusion, forming alloy powders.

The dispersed phase can result in strengthening of the particles or can impart special electrical or magnetic properties of the powder.

Methods of powder productionMiscellaneous methods

Other less commonly used methods for making powders are:

1. Precipitation from a chemical solution

2. Production of fine metal chips by machining

3. Vapor condensation More recent developments include techniques based on high-

temperature extractive metallurgical processess—based on the reaction of volatile halides (a compound of halogen and an electropositive element) with liquid metals and the controlled reduction and reduction/carburization of solid oxides.

Methods of powder productionMiscellaneous methods

Nanopowders Most recent developments include the production of nanopowders of copper,

aluminum, iron, titanium, and various other metals.

Because these powders are pyrophoric (ignite spontaneously) or are contaminated readily when exposed to air, they are shipped as thick slurries under hexane gas (which itself is highly volatile and combustible).

Microencapsulated powders These metal powders are coated completely with a binder. For electrical

applications (such as magnetic components of ignition coils and other pulsed AC and DC applications), the binder acts like an insulator, preventing electricity from flowing between particles and thus reducing eddy-current losses.

The powders are compacted by warm pressing, and they are used with the binder still in place.

Blending metal powders Blending (mixing) powders is the next step in powder-metallurgy

processing.

It is carried out for the following purposes:1. Powders of different metals and other materials can be mixed in

order to impart special physical and mechanical properties and characteristics to the P/M product.

2. Even when a single metal is used, the powders may vary significantly in size and shape, hence they must be blended to obtain uniformity from part to part.

Blending metal powders3. Lubricants can be mixed with the powders to improve their flow

characteristics. They reduce friction between the metal particles, improve flow of the powder metals into the dies, and improve die life.

4. Other additives—binders (as in sand molds) are used to develop sufficient green strength and additives also can be used to facilitate sintering.

Powder mixing must be carried out under controlled conditions in order to avoid contamination or deterioration.

Compaction of Metal Powders

Compaction is the step in which the blended powders are pressed into various shapes in dies.

The purposes of compaction are to obtain the required shape, density, and particle-to-particle contact and to make the part sufficiently strong for further processing.

Figure (a) shows the compaction of metal powder to form a bushing. The pressed powder part is called green compact. (b) Typical tool and die set for compacting a spur gear.

Compaction of Metal Powders Pressing in PM:

(1) filling die cavity with powder by automatic feeder; (2) initial and (3) final positions of upper and lower punches during pressing, (4) part ejection.

Compaction of Metal Powders The pressed powder is known as

green compact, since it has a low strength just as is seen in green parts in slip casting.

The density of the green compact depends on the pressure applied.

The higher the density of the compacted part, the higher is its strength and elastic modulus.

The reason is that the higher the density, the higher the amount of solid metal in the same volume, and hence the greater its strength (resistance to external forces).

Miscellaneous compacting and shaping processes

Powder-injection molding (PIM). In this process (also called metal-injection molding (MIM)).

Generally, metals that are suitable for powder-injection molding are those that melt at temperatures above 1000°C.

The major advantages of powder-injection molding over conventional compaction are:

1. Complex shapes having wall thicknesses as small as 5 mm can be molded and then removed easily from the dies.

2. Mechanical properties are nearly equal to those of wrought products.


3. Dimensional tolerances are good.

4. High production rates can be achieved by using multicavity dies.

5. Parts produced by the PIM process compete well against small investment-cast parts, small forgings, and complex machined parts. However, it does not compete well with zinc and aluminum die casting or with screw machining.


Rolling In powder rolling (also called roll

compaction), the metal powder is fed into the roll gap in a two-high rolling mill and is compacted into a continuous strip at speeds of up to 0.5 m/s.

The rolling process can be carried out at room or at elevated temperature.

Sheet metal for electrical and electronic components and for coins can be made by this process.


Extrusion

Powders can be compacted by extrusion, whereby the powder is encased in a metal container and hot extruded.

After sintering, preformed P/M parts may be reheated and forged in a closed die to their final shape.

Superalloy powders, for example, are hot extruded for enhanced properties.


Pressureless compaction

In pressureless compaction, the die is filled with metal powder by gravity, and the powder is sintered directly in the die.

Because of the resulting low density, pressureless compaction is

used principally for porous metal parts, such as filters.


Spray Deposition Spray deposition is a shape-generation process. The basic components of the spray-deposition process for metal powders are

(a) An atomizer, (b) a spray chamber with inert atmosphere, and (c) a mold for producing preforms.

The mold may be made in various shapes, such as billets, tubes, disks, and cylinders.

Figure shows the spray deposition (Osprey process) in which molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe.

Sintering

Heat treatment to bond the metallic particles, thereby increasing strength and hardness

Usually carried out at between 70% and 90% of the metal's melting point (absolute scale)

Generally agreed among researchers that the primary driving force for

sintering is reduction of surface energy

Part shrinkage occurs during sintering due to pore size reduction

Sintering Sequence

Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in size; and (4) grain boundaries develop between particles in place of the necked regions.

Sintering Cycle and Furnace

(a) Typical heat treatment cycle in sintering; and (b) schematic cross section of a continuous sintering furnace.

Densification and Sizing Secondary operations are performed to increase density, improve

accuracy, or accomplish additional shaping of the sintered part

Repressing - pressing sintered part in a closed die to increase density and improve properties

Sizing - pressing a sintered part to improve dimensional accuracy

Coining - pressworking operation on a sintered part to press details into its surface

Machining - creates geometric features that cannot be achieved by pressing, such as threads, side holes, and other details

Secondary operations

Powder-metal parts may be subjected to other finishing operations such as:

• Machining: for producing various geometric features by milling, drilling, and tapping (to produce threaded holes).

• Grinding: for improved dimensional accuracy and surface finish.

• Plating: for improved appearance and resistance to wear and corrosion.

• Heat treating: for improved hardness and strength.

Impregnation and Infiltration

Porosity is a unique and inherent characteristic of PM technology

It can be exploited to create special products by filling the available pore space with oils, polymers, or metals

Two categories:1. Impregnation2. Infiltration

Impregnation The term used when oil or other fluid is permeated into the pores of

a sintered PM part

Common products are oil‑impregnated bearings, gears, and similar components

Alternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part

Infiltration Operation in which the pores of the PM part are filled with a molten metal

The melting point of the filler metal must be below that of the PM part

Involves heating the filler metal in contact with the sintered component so capillary action draws the filler into the pores

Resulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength

Alternatives to Pressing and Sintering

Conventional press and sinter sequence is the most widely used shaping technology in powder metallurgy

Additional methods for processing PM parts include: Isostatic pressing Hot pressing - combined pressing and sintering

Materials and Products for PM

Raw materials for PM are more expensive than for other metalworking because of the additional energy required to reduce the metal to powder form

Accordingly, PM is competitive only in a certain range of applications

What are the materials and products that seem most suited to powder metallurgy?

PM Materials – Elemental Powders

A pure metal in particulate form

Applications where high purity is important

Common elemental powders: Iron Aluminum Copper

Elemental powders can be mixed with other metal powders to produce alloys that are difficult to formulate by conventional methods Example: tool steels

PM Materials – Pre-Alloyed Powders

Each particle is an alloy comprised of the desired chemical composition

Common pre-alloyed powders: Stainless steels Certain copper alloys High speed steel

PM Products

Gears, bearings, sprockets, fasteners, electrical contacts, cutting tools, and various machinery parts

Advantage of PM: parts can be made to near net shape or net shape

When produced in large quantities, gears and bearings are ideal for PM because: The geometry is defined in two dimensions There is a need for porosity in the part to serve as a reservoir

for lubricant

PM Parts Classification System

The Metal Powder Industries Federation (MPIF) defines four classes of powder metallurgy part designs, by level of difficulty in conventional pressing

Useful because it indicates some of the limitations on shape that can be achieved with conventional PM processing

Four Classes of PM Parts

(a) Class I Simple thin shapes, pressed from one direction; (b) Class II Simple but thicker shape requires pressing from two

directions; (c) Class III Two levels of thickness, pressed from two directions; (d) Class IV Multiple levels of thickness, pressed from two directions,

with separate controls for each level.

Design considerations

Because of the unique properties of metal powders, their flow characteristics in the die, and the brittleness of green compacts, there are certain design principles that should be followed:

1. The shape of the compact must be kept as simple and uniform as possible.

2. Provision must be made for ejection of the green compact from the die without damaging the compact.

3. P/M parts should be made with the widest acceptable dimensional tolerances.

4. Part walls generally should not be less than 1.5 mm thick; however, with special care, walls as thin as 0.34 mm can be pressed successfully on components 1 mm in length.

Design considerations4. Steps in parts can be produced if they are simple and their size doesn’t

exceed 15% of the overall part length.

5. Letters can be pressed if they are oriented perpendicular to the direction of pressing and can be raised or recessed.

6. Flanges or overhangs can be produced by a step in the die.

7. Notches and grooves can be made if they are oriented perpendicular to the pressing direction.

8. Parts produced through powder-injection molding have similar design constraints as with injection molding of polymers

9. Dimensional tolerances of sintered P/M parts are usually on the order of 0.05 to 0.1 mm.

Process Capabilities

The process capabilities of powder metallurgy may be summarized as follows:

1. It is a technique for making parts from high-melting-point refractory metals, and parts which may be difficult or uneconomical to produce by other methods.

2. High production rates are possible on relatively complex parts using automated equipment and requiring little labor.

3. Powder-metal processing offers good dimensional control and (in many instances) the elimination of machining and finishing operations; in this way, it reduces scrap and waste and saves energy.

Process Capabilities

4. The availability of a wide range of compositions makes it possible to obtain special mechanical and physical properties, such as stiffness, vibration damping, hardness, density, toughness, and specific electrical and magnetic properties. Some of the newer highly alloyed superalloys can be manufactured into parts only by P/M processing.

5. It offers the capability of impregnation and infiltration for specific applications.

Economics of powder metallurgy

Because powder metallurgy can produce parts at net or near-net shape and thus eliminate many secondary manufacturing and assembly operations, it increasingly has become competitive with casting, forging, and machining.

As in other metalworking operations, the cost of dies and tooling in powder metallurgy depends on the part shape complexity and the method of processing the metal powders.

Thus, tooling costs for processes such as hot isostatic pressing and powder-injection molding are higher than the more conventional powder processing.

Economics of powder metallurgy

Equipment costs for conventional P/M processing are somewhat similar to bulk deformation processing of metals, such as forging.

Labor costs are not as high in other processes, primarily because the individual operations (such as powder blending, compaction, and sintering) are performed on highly automated equipment.

Thus, the skills required are not as high.

TOPIC 9-Extra Notes

Documents

Transcript of TOPIC 9-Extra Notes