Module 2 Notes

M.E.(MPDD) Module-2 Process modeling and product design BVCOE, Navi Mumbai

Page 1

Module-2 Process modeling and product design

2.1 Metal forming processes

Metal forming is a very important manufacturing operation. It enjoys industrial importance

among various production operations due to its advantages such as cost effectiveness, enhanced

mechanical properties, flexible operations, higher productivity, considerable material saving. The

objects and articles that we use in our daily life are man-made, engineered parts, which are

obtained from some raw material through some manufacturing process. All these objects are

made of a number of small components assembled into finished product. The pen that we use for

writing, for example is made of several small parts, assembled together. An automobile is

supposed to be an assembly of more than 15000 parts, produced through various manufacturing

operations. Manufacturing of finished parts and components from raw materials is one of the

most important steps in production. Production encompasses all types of manufacturing

processes. Manufacturing refers to the conversion of raw materials into finished products

employing suitable techniques. There are several methods of manufacturing such as metal

casting, metal forming, metal machining, metal joining and finishing. Some of the modern

methods of manufacturing include micro machining, nano fabrication, ultra precision

manufacturing etc. In order to fulfill the requirements of the ever-increasing demands of various

types of industries, the manufacturing engineer has to choose the right type of material and the

right type of equipment for manufacture so that the cost of production and the energy

consumption are minimum. The selection of suitable manufacturing process should also include

concerns for environmental impacts such as air pollution, waste disposal etc.

Modern concepts such as lean manufacturing, adaptive control, agile manufacturing, group

technology etc have considerable influence on cost reduction and quality improvements of

products. Computers and robots play important role in modern manufacturing techniques, today.

Modeling and simulation of the process prior to mass production helps the manufacturing

engineer fix up the best operating parameters and hence achieve the finished product to the

utmost level of quality and cost-effectiveness. The present course is focused on one of the

important methods of manufacturing, namely, metal forming.

Materials are converted into finished products though different manufacturing processes.

Manufacturing processes are classified into shaping [casting], forming, joining, and coating,

dividing, machining and modifying material property. Of these manufacturing processes,

forming is a widely used process which finds applications in automotive, aerospace, defense and

other industries. Wrought forms of materials are produced through bulk or sheet forming

operations. Cast products are made through shaping molding and casting. A typical automobile uses formed parts such as wheel rims, car body, valves, rolled shapes for chassis, stamped oil

pan, etc. In our daily life we use innumerable formed products e.g. cooking vessels, tooth paste

containers, bicycle body, chains, tube fitting, fan blades etc. Forming is the process of obtaining

the required shape and size on the raw material by subjecting the material to plastic deformation

through the application of tensile force, compressive force, bending or shear force or

combinations of these forces.


Page 2

Fig.2.1: Various manufacturing operations on materials

2.1.1 Classification of Forming:-

Fig. 2.2: Classification of metal forming processes

Typically, metal forming processes can be classified into two broad groups. One is bulk forming

and the other is sheet metal forming. Bulk deformation refers to the use of raw materials for

forming which have low surface area to volume ratio. Rolling, forging, extrusion and drawing

are bulk forming processes. In bulk deformation processing methods, the nature of force applied

may be compressive, compressive and tensile, shear or a combination of these forces. Bulk

forming is accomplished in forming presses with the help of a set of tool and die. Examples for

products produced by bulk forming are: gears, bushed, valves, engine parts such as valves,

connecting rods, hydraulic valves, etc.

Sheet metals forming involve application of tensile or shear forces predominantly. Working upon

sheets, plates and strips mainly constitutes sheet forming. Sheet metal operations are mostly

carried out in presses hydraulic or pneumatic. A set of tools called die and punch are used for the sheet working operations. Bending, drawing, shearing, blanking, punching are some of the

sheet metal operations. A new class of forming process called powder forming is gaining

importance due to its unique capabilities. One of the important merits of powder forming is its

ability to produce parts very near to final dimensions with minimum material wastage. It is called

near-net-shape forming. Material compositions can be adjusted to suit the desirable mechanical

properties. Formability of sintered metals is greater than conventional wrought materials.

However, the challenge in powder forming continues to be the complete elimination or near-

complete elimination of porosity. Porosity reduces the strength, ductility and corrosion resistance

and enhances the risk of premature failure of components. Based on the nature of deformation


Page 3

force applied on the material, during forming, metal forming processes are also classified into

several types as shown below:

Forming is also classified as cold forming, hot forming or warm forming. Hot forming is the

deformation carried out at temperatures above recrystallization temperatures. Typically,

recrystallization temperatures for materials ranges from 0.5 Tm to 0.8 Tm, where Tm is melting

temperature of material.

2.1.2 Brief description of forming operations

We discuss briefly the various forming operations in the following sections.

- Bulk forming processes:

Rolling is a compressive deformation process, which is used for producing semi-finished

products such as bars, sheets, plates and finished products such as angles, channels, sections.

Rolling can be carried out both in hot and cold conditions.

Fig.2.3: Rolling Process Fig.2.4: Forging processes

Forging is a bulk forming process in which the work piece or billet is shaped into finished part

by the application of compressive and tensile forces with the help of a pair of tools called die and

punch. Forging can be done in open dies or closed dies. Open die forging is usually used for

preliminary shaping of raw materials into a form suitable for subsequent forming or machining.

Open die forming is done using a pair of flat faced dies for operations such as drawing out,

thinning, etc.

Closed die forming is performed by squeezing the raw material called billet inside the cavity

formed between a pair of shaped dies. Formed products attain the shape of the die cavity. Valve

parts, pump parts, small gears, connecting rods, spanners, etc are produced by closed die

forming.

Coining is the process of applying compressive stress on surface of the raw material in order to

impart special shapes on to the surface from the embossing punch e.g. coins, medallions


Page 4

Fig.2.4: Direct extrusion process

Extrusion involves forcing the raw material through a narrow opening of constant cross-section

or varying cross-section in order to reduce the diameter and increase the length. Extrusion can be

done hot or cold. Extruded products include shafts, tubes, cans, cups, gears. Basically there are

two methods of extrusion, forward and backward extrusions. In forward extrusion the work and

the extrusion punch move along the same direction. In backward extrusion the punch moves

opposite to the direction of movement of the work piece.

Fig.2.5: Backward extrusion or Indirect extrusion

Wire drawing process is used for producing small diameter wires from rods by reducing their

diameter and stretching their length through the application of tensile force. Musical strings are

produced by wire drawing process. Seamless tubes can be produced by tube drawing process.

-Sheet metal operations:

Deep drawing is a sheet metal process the process in which a sheet metal is forced into cup of

hollow shape without altering its thickness using tensile and compressive forces. Complex shapes can be produced by deep drawing of blanks in stages redrawing, multiple draw deep drawing etc. Hydro mechanical deep drawing uses both punch force and hydrostatic force of a

pressurized fluid for achieving the shape. Flanges and collars are formed by flanging process.

Spinning transforms a sheet metal into a hollow shape by compressive and tensile stresses.

Spinning mandrel of given shape is used against a roll head. Embossing imparts an impression

on the work piece by means of an embossing punch. Bending of sheets includes rotary bending,

swivel bending, roll bending using rotary die. Die bending using flat die or shaped die is used for

bending of sheets, or die coining of sheets.

Fig.2.6: Wire Drawing Fig.2.7: Deep drawing


Page 5

Fig.2.8: Bending and shearing

2.2 Analysis of forming

2.2.1 Slab Method of analysis:-

Forming of materials is a complex process, involving either biaxial or triaxial state of stress on

the material being formed. Analysis of the forming process therefore is highly involved.

Prediction of forming load in a particular process is rather empirical. However, fairly accurate

methods have been developed in order to predict the forming process and process parameters.

Some of the early methods of forming analysis include slab analysis; slip line field analysis,

upper bound analysis etc. With the availability of high speed computers, we can depend on finite

element method for accurate predictions of forming loads. Numerous metal forming software

have been developed based on finite element procedures for complex shapes with more realistic

boundary conditions. In this lecture we will discuss the simple slab method of forming analysis,

with a typical example.

Slab method is a simple analytical procedure based on principles of mechanics. We can assume a

simple relation between forming load and material flow stress in the form: F = kA, where k is an

empirically determined constant which takes into account friction, redundant deformation etc.

The general methodology involved in slab method can be stated as follows: First the material

under deformation is sliced into infinitesimally small portions. Then force balance is made on the

small element. From force balance a differential equation in terms of the forming stress,

geometric parameters of the billet and friction coefficient is formulated. This differential

equation is solved with suitable boundary conditions. The solution gives us the required forming

stress. This method may involve some simplifying assumptions. Hence this method may be

considered approximate. Moreover, it may not be easy to apply this method for more complex

forming processes, such as impression die forging. Slab method is developed with the

assumption that the material flow is homogeneous during forming.

2.2.1.1 Upsetting of a ring

Let us try to understand the slab method of forming analysis with the help of a simple example.

Sliding or Columbic friction often occurs at the material tool interface. As a result of friction the

forming load is enhanced. The flow of material is also non-uniform due to friction. Another type

of friction condition, namely, shear friction or sticking friction could be convenient to consider in

the analysis. In shear friction model, we assume the frictional shear stress to be proportional to

shear yield strength of the material. Thus we have: =mk , where m is friction factor and k is shear yield strength.

The following assumptions are the basis of the slab analysis:

1. The reference axes are in the directions of the applied stresses


Page 6

2. Friction does not cause non-uniform deformation. Therefore material is assumed to deform

homogeneously a plane remains a plane after deformation.

Consider the homogeneous deformation of a ring shaped specimen subjected to upsetting force.

Let us assume shear friction at tool-material interface. The ring compression process is widely

used for finding the coefficient of friction for given condition of friction. Consider an elemental

portion of the ring specimen and the various stresses on this element. The following diagram

shows the stresses acting on the elemental part of the ring.

Consider a small sector of an elemental ring of radius r, radial thickness dr, height h and the

angle of the sector as d. The ring is subjected to upset force F, which is to be determined.

The various stresses acting on the sector are:

Fig.2.9: Stresses acting on elemental ring subjected to upsetting

There exists a neutral radius in the ring, such that the material deformation happens towards the

axis for radii less than the neutral radius. There is a decrease in diameter of the ring. For radii

greater than the neutral radius, the material flow is away from the axis-axially outward. This

condition exists because of friction. Therefore, the friction force is observed to act axially

outward within the neutral section. It acts radially inward in sections beyond the neutral section.

The force balance along the radial direction gives:

The force balance along the radial direction gives:

Dropping higher order terms, and applying =r


Page 7

We need to solve for z the axial stress for upsetting the ring We can apply Tresca yield criterion in order to replace r in the above differential equation. Let us assume that the two principal stresses acting on the ring are: z and r

Therefore, we have:


Page 8

In the above equation the bracketed term represents the factor which accounts for friction effect

during the forming. The limitation of uniform deformation assumption in slab method is

overcome in another method of analysis called slipline field analysis, which is discussed in the

next lecture. The upset force is found to vary linearly with the friction factor m, as observed from

the above equation. Further, we also note that the forming force required increases with

reduction in height of the ring. Rings of smaller height require greater forming force as compared

to rings of larger height. This is expected because the redundant deformation zone extends

towards centre for rings of smaller height. Ring compression test is a simple test for

determination of friction factor or the coefficient of friction. It can also be used for studying the

lubrication characteristics of different lubricants.

2.2.2 Slipline Field Method of analysis:-

Slab analysis of the forming process is considered approximate due to the assumption of

homogeneous deformation of material. Slipline field analysis is more accurate as it considers the

non-homogeneous deformation also. This method is widely applied for forming processes such

as rolling, strip drawing, slab extrusion etc. Slipline field analysis is based on the important

assumptions that the deformation of material is plane strain type, no strain hardening of the

material, constant shear stress at interfaces, the material is rigid plastic. The general methodology

of this analysis can be described by the following steps: First differential equations in terms of

mean stress and deviatoric stress for plane strain deformation are formulated Slipline field is

constructed graphically out of orthogonal maximum and minimum shear lines. From known

stress at some point, the integral constants are determined. From this the forming load can be

found. Before we proceed to understand the methodology of the analysis a few definitions should

be considered. What are sliplines? They are planes of maximum shear, which are oriented at 45

degrees to the axes of principal stresses. Maximum and minimum slip lines are orthogonal. What

is plane strain deformation? It is a type of plastic deformation in which the material flow in one

of the three principal directions is constrained. The material strain in the third direction is zero.

This is possible by the application of a constraint force along the third direction. All

displacements are restricted to xy plane, for example. Examples for this type of deformation

include strip rolling, strip extrusion etc. Constraint to deformation along the third axis could be

introduced either through the die wall or through the rigid material adjacent to deforming

material, which prevents the flow. The basis for slipline field analysis is the fact that the general

state of stress on a solid in plane strain deformation can be represented by the sum of two types

of stresses, namely the mean stress and the pure shear stress.

For plane strain condition we have

We can write the Tresca criterion for plane strain as: 1 3 = 2k For plane strain deformation we have the equilibrium of stresses written in differential form as:

These two differential equations will be transformed into two algebraic equations along a

changed coordinate system, namely, along two directions of maximum shear. Then they can be

solved subjected two suitable boundary conditions.

Consider the plane strain state of stress acting on x-y plane. Let x,y and xy be the stresses acting in this plane.


Page 9

Fig. 2.10: Stresses in Plane strain condition

Now substituting 3 and 4 in 5, 6 and 7 we get:

We get:


Page 10

Then the equation 11 and 12 becomes:

If the directions x and y are taken to be directions of maximum shear, denoted as directions and , Then we have:

Equations 15 and 16 represent the two differential equations transformed to the directions of

maximum shear, and . Here the directions and are called slip lines (lines of maximum shear) Therefore, we may now conclude from 15 and 16 that:

p+2k = constant along line and = f () ------ 17 Similarly, p -2k = constant along line and = f () ------18 Or we can write: p = -2k along lines And p = 2k along slip lines

The above equations mean that the pressure p changes by an amount equivalent to change in the

angle as one move along the slip lines. The following conditions are to be remembered while

establishing the slip line field: The stress normal to a free surface is a principal stress and hence

the slip lines meet the free surface at 45o.

and lines always meet at 45o on a frictionless surface. They meet at 0o and 90o on a surface with sticking friction Slip happens along the slip lines as there is maximum shear along the slip

lines. Further, along the tangent to the slip lines there is a discontinuity of velocity. The angle

between the intersections of one type of slip line with the other type of slip line remains the same

all along the slip line.

The radii of curvature of the intersecting slip lines ( lines) along one type of slip lines ( lines) change by an amount equal to their distances traversed.

2.2.3 Upper bound analysis

Limit analysis is an alternative analytical approach that is receiving increased acceptance and is

being used with increased frequency.

Two separate solutions have been developed:

1. the upper-bound solution provides a value for the power required which is equal to or greater

than the actual power, and

2. the lower-bound solution provides a value equal to or lower than the actual power

The upper-bound theorem may be stated as flows: Any estimate of the collapse load of a

structure made by equating the internal rate of energy dissipation to the rate at which external

forces do work in some assumed pattern of deformation will be greater than or equal to the

correct load. The bases of an upper-bound analysis can be summarized as follows:


Page 11

An internal flow field is assumed and must account for the required shape change. As such, the field must be geometrically self-consistent

The energy consumed internally in this deformation field is calculated using the appropriate strength properties of the work material

The external forces (or stresses) are calculated by equating the external work with the internal energy consumption

For a mathematical proof that such solutions predict loads equal to or greater than the exact load

to cause plastic deformation, various sources (Johnson and Mellor, 1973) may be consulted.

With such solutions, the assumed field can be checked for complete consistency by drawing a

velocity vector diagram, which is commonly called a hodograph.

In applying the upper-bound technique to metalworking operations, several simplifying

assumptions are invoked:

The work material is isotropic and homogeneous

The effects of strain hardening and strain rate on flow stress are neglected

Either frictionless or constant shear stress conditions prevail at the tool work piece interface

Most of the cases considered will be those where the flow is 2 dimensional (plane strain), with all deformation occurring by shear on a few discrete planes. Elsewhere the material

is considered to be rigid. If shear is assumed to occur on intersecting planes that are not

orthogonal, these planes cannot in reality, be planes of maximum shear stress. Many

such fields can be posed and the closer such a field is to the true flow field, the closer the

upper-bound prediction approaches the exact solution.

ENERGY DISSIPATION ON A PLANE OF DISCRETE SHEAR: - Figure 2.11 shows an

element of rigid metal, ABCD, moving at unit velocity, V1 and having unit width into the study.

AD is set parallel to yy'. As the element reaches the plane yy' it is forced to change direction,

shape and velocity. Thus, to the right of yy' the element has the shape A'B'CD' and velocity V2,

at an angle 2 to the horizontal. Fig. 1b is the hodograph; the absolute velocities on either side of yy' are V1 and V2 and they are drawn from the origin, O. Both V1 and V2 must have the

horizontal component. Vx; otherwise, material approaching and leaving yy' would differ in

volume; this would violate the concept of incompressibility. The velocity V*12 is the vector

difference between V1 and V2 and is the velocity discontinuity along yy'. It is assumed that

V*12 occurs along the line (or plane) yy'. The rate of energy dissipation on yy' must equal the

work per volume times the volume per time crossing yy'. Because deformation is due to shear,

the work per volume, w, equals the shear stress times the shear strain, . Here, must be the shear strength, k, of the metal and = dy/dx, thus:

w = k(dy/dx) (1)

The volume crossing yy' in an increment of time, dt, is the length of line, S, along yy' times the

depth of the plane perpendicular to yy' (unity) times Vx. Thus:

vol/time = S (1) Vx (2)

Combining Eq. (1) and (2) gives the rate at which work, W, is done to effect this shear

deformation:

dW/dt = k(dy/dx) (SVx) (3)

Comparing Fig. 1a and b, dy/dx = V*12 / Vx so:


Page 12

dW/dt = k S V*12 (4)

For deformation fields involving more than one plane of discrete shear:

dW/dt = 1 k Si V*

i (5)

where, Si and V*i pertain to each individual plane.

Equation (5) is the form used in most problems involving upper bound calculations. It implies

that an element deforms in a way that offers maximum plastic resistance.

Most of the flow fields assumed in this study consist of a number of polygons, which are viewed

as rigid blocks. This means that the velocity of all the material inside a polygon is the same and

is represented on the hodograph by the point that is common to the lines that bound the polygon

on the proposed deformation field. The polygons are separated by the lines of velocity

discontinuity and these discontinuities as well as effects of boundary friction must be considered

when summing the contributions to the total internal energy dissipation.

(a) (b)

Fig. 2.11 (a) Basis for analysis of energy dissipation along a plane of intense shear discontinuity and (b) the

hodograph or velocity vector diagram

2.3 Forging process:-

Bulk deformation processes involve shaping of materials to finished products which have small

surface area to thickness or surface area to volume ratio. Sheet metal forming produces parts

having large surface area to thickness ratio. In sheet metal forming thickness variations are not

desirable. Examples for sheet metal forming are: beverage cans, automobile body etc. Bulk

forming processes may be primary processes such as rolling of ingot to blooms or billets, in

which the cast metal is formed into semi-finished raw material. In secondary forming, the raw

materials, such as blooms, billets are converted into finished parts such as gears, wheels,

spanners etc. Rolling, forging, extrusion and drawing are bulk forming processes. The present

module describes the salient aspects of forging process.

In ancient times, people employed forging for making coins, jewelry, weapons, Forging is a

deformation processing of materials through compressive stress. It is carried out either hot or

cold. Hot forging is done at temperatures above recrystallization temperatures, typically 0.6 Tm,

or above, where Tm is melting temperature. Warm forging is done in the temperature range: 0.3

Tm to 0.5 Tm. Cold forging has advantages such as good surface finish, high strength and

greater accuracy. Hot forging requires lower loads, because flow stress gets reduced at higher

temperatures. Strain rates in hot working may be high 0.5 to 500 s-1. Strains in hot forging are also high true strains of 2 to 4 are common. Typical applications of forging include bolts, disks, gears, turbine disk, crank shaft, connecting rod, valve bodies, small components for hydraulic

circuits etc. Forging has several advantages. Closer dimensional accuracies achieved require very


Page 13

little machining after forging. Material saving is the result. Higher strength, greater productivity,

favorable grain orientation, high degree of surface finish are other merits. However, complex die

making is costly.

Forging is a metal working process in which useful shape is obtained in solid state by hammering or pressing metal.

2.3.1 Different Forging Operations

1. Upsetting: - The thickness of the work reduces and length increases

2. Edging: - The ends of the bar are shaped to requirement using edging dies.

3. Fullering: - The cross sectional area of the work reduces as metal flows outward, away from

centre.

4. Drawing: - The cross sectional area of the work is reduced with corresponding increase in

length using convex dies.

5. Swaging: The cross sectional area of the bar is reduced using concave dies.

6. Piercing: The metal flows around the die cavity as a moving die pierces the metal.

7. Punching: It is a cutting operation in which a required hole is produced using a punching die.

8. Bending: The metal is bent around a die/anvil.


Page 14

-Classification of Forging Processes

(A) Based on Temperature of the work piece:

1. Hot forging: (most widely used):- Forging is carried out at a temperature above the

recrystallization temperature of the metal.

2. Cold Forging: Forging is carried out at a temperature below the recrystallization temperature

of the metal.

(B) Based on Arrangement of Dies:

1. Open Die Forging: - Flat dies of simple shape are used.

2. Closed Die Forging: - Work piece is deformed between two dies with impressions (cavities) of

the desired final shape on them.

Fig 2.12 Open die forging Fig.2.13 closed die forging

2.3.2 Slab analysis of forging:-

In a plane strain condition, the strain in one of the principal directions is zero.

During a forging process,

the thickness of the work piece decreases

the length of the work piece decreases

The width remains unchanged

In a general condition of stress:

Let 1,2, and 3 be the principal stresses, and 1, 2 and 3 be the principal strains.

In a plane strain condition, 2=0 (as width remains constant).

In this condition, it can be shown that the principal stress 2, acting in a direction where 2=0, is the algebraic mean of the other two principal stresses.

This means

Consider Upset forging of a rectangular slab under plane strain condition, using a wide and

flat die.


Page 15

Fig.2.14 Plane strain upsetting of rectangular billet Fig. 2.15 Stresses acting on a small and the

stresses acting on the element of thickness dx elemental billet of thickness dx

Consider a rectangular slab as shown in the figure.

Let t = thickness of the work (decreases during forging)

L = length of the work (increases during forging)

W = width of the work (remains constant)

Consider an elemental volume in the work piece with length dx at a distance x from the centerline.

The stresses acting on the elemental volume are:

i) P = forging pressure

ii)x= Longitudinal stress due to lateral flow of the metal

iii) =Shear stress due to friction between work and die surfaces

Under the equilibrium conditions, the summation of forces acting in a longitudinal direction must

be zero.

Applying the above condition and conditions of plane strain, it can be shown that:

The above equation is solved for different conditions as follows:

Case i): Sliding friction at the contact surface between work and die:

Coefficient of friction is low. The height of the billet is small so that the forging pressure is

constant over the height of the billet.

Assume that x and y are principal stresses [Though y can not be assumed as principal stress as a shear stress is also acting on the plane on which the normal stress is acting]


Page 16

Here y is the forging stress necessary at any height h of the billet. Force balance on the element gives: Assuming the dimension of the billet perpendicular to the plane of the paper,

-----------------9

We have to eliminate because there are two unknowns in the above equation.

For eliminating we can apply the von Mises yield criterion for plane strain. According to this

criterion, we have:

--------------------10

From this we have dy=dx .The force balance equation now becomes:

------------------------------11

Upon integration, we get:

-----------------------------------12

To solve the constant A, we need a boundary condition.

At x = a, x = 0 [free surface] From the yield criterion we have: At x=a, y=y Substituting this in equation 12 and simplifying we get,

-----------------------13

P is the forging pressure Equation 13 can also be written as:

-------------------14

Where L = 2a is width of the billet From the above equation we find that as L/h increases, the

forging pressure increases resistance to compressive deformation increases. This fact is utilized in closed die forging where the deformation resistance of flash, being high [due to high L/h] the

die filling is effective. Note: Y is plane strain yield strength of the material If the material is work hardening type of material, we have to replace Y with Yf which is the flow stress of the material The variation of forging pressure normalized with plane strain yield strength Y is shown with respect to the billet thickness:

Fig. 2.16 Friction hill in plane strain upsetting under sliding friction


Page 17

Forging pressure variation across the billet due to friction is shown above. The pressure

distribution curve is called friction hill. Area under the friction hill represents the forging work

done. As shown in figure, as the coefficient of friction increases, the forging pressure increases

and hence the work done. Average forging pressure: The average forging pressure is given as:

------------15

Substituting for p from equation 13, we get:

We can get approximate expression for average forging load by expanding exponential function

as infinite series. We get:

----------------16

Note that the forge pressure is a function of instantaneous height of billet. As height gets

reduced, after successive plastic flow, forging pressure increases. If the rectangular billet is

subjected to plane stress compression stress acting along the height axis and the length axis, there will be material flow in the width direction. It is found that the extent of flow along width

direction is several times greater than the flow along longitudinal direction. Because of lower

friction along width, material flows freely along width direction. If a rectangular block is

compressed, due to friction and non-uniform flow, bulging and barreling take place. Bulging

refers to the non-uniform flow considered on the plane of the loading, while barreling refers to

the non-uniform deformation along the height of the specimen. The reason for bulging and

barreling is the material flow along the diagonal direction is rather sluggish, compared to the

other directions.

Case-ii) Sticking friction:-

The frictional shear stress p increases towards the axis as the forging pressure p increases. However, the maximum frictional shear stress can not exceed the shear yield strength of the

material. When the limiting condition of = k, we can say sticking exists at the interface. Generally, we can relate the friction shear stress with shear yield strength by the relation:

= mk,

m is friction factor, which can not exceed 1. Under sticking friction the friction shear stress and

shear yield strength are related as:

= k ------------------------17

Where k is shear yield strength. For sticking friction, the limit of friction shear stress is the shear

yield strength of the material m=1.

In general, with = mk, the forging pressure is given by:

---------------------------------18

As per the above equation, with sticking friction [m = 1], one can write the forging pressure as:


Page 18

-----------------------19

This is a linear relation, which is shown in figure below:

Fig. 2.17 Upset forging with sticking friction variation of forging pressure

2.4 Drawing Process: - Cup drawing or deep drawing is one of the widely used sheet metal

forming operations. Cup shaped objects, utensils, pressure vessels, gas cylinders, cans, shells;

kitchen sinks etc are some of the products of deep drawing. In this process, a sheet metal called

blank is placed on a die cavity, held in position using a holding plate or holding ring and pressed

against the die cavity using a solid punch. The sheet metal attains the shape of the die cavity with

flat bottom. Both die and punch should be provided with corner radius in order to avoid shearing

of the sheet.

Fig. 2.18 Cup drawing process sequence of operation


Page 19

During drawing of sheet into the die, there is thickening of the sheet upto 12%. Therefore,

clearance is provided between the punch and die. The radial clearance therefore is equal to the

sheet thickness plus the thickening of sheet. Punch pushes the bottom of the sheet into the die

cavity. The flat portion of the sheet under the holding plate moves towards the die axis, then

bends over the die profile. After bending over the die profile the sheet unbends to flow

downward along the side wall. The vertical portion of the sheet then slips past the die surface.

More metal is drawn towards the center of the die in order to replace the metal that has already

flown into the die wall. Friction between holding plate and blank and that between die and blank

has to be overcome by the blank during its horizontal flow.

2.4.1 Analysis of Cup Drawing:

Tensile stress is induced on the sheet at various locations

within the die cavity. Maximum tensile stress is caused near the end of punch, at the profile of

punch, because the sheet bends over the edge of the punch due to tensile stress. The sheet

unbends along the cup wall. Necking of the sheet takes place near the punch profile due to excess

tensile stress, resulting in fracture. The sheet under the holding plate, namely, the flange

undergoes compressive hoop stress, radial tensile stress and compressive stress due to blank

holding plate. Thickness of the cup wall increases from bottom to top. The die-punch clearance,

usually, taken as 1.1t, where t is thickness of the sheet. Stresses acting on the sheet at various

locations are shown in figure. The flange portion of the blank is subjected to a compressive hoop

stress due to it being drawn towards the center. It is also subjected to radial tensile stress. The

compressive stress of the hold down plate will be acting in the axial direction. If the hoop

compressive stress is high or if the metal in the flange is not restrained wrinkling of the metal in

the flange happens. To prevent wrinkling, the hold down plate is used. The material of the flange

undergoes compressive hoop strain and a radial tensile strain. The result is the metal in the

flange, as it flows towards the center, tends to thicken due to circumferential shrinking. However, due to bending under the punch and die profile, the metal undergoes thinning. The

metal at the center of the blank, which is getting pressed by the punch bottom, is subjected to

biaxial tensile stress due to the punch. The metal in the gap between die wall and punch is now

subjected to longitudinal and hoop tensile stresses. If the clearance is less than the metal

thickening on the flange side, the metal in the cup wall is squeezed. This process of thinning of

the cup wall is called ironing. In order to reduce thickness and to cause uniform thickness on the

cup, ironing is used in some drawing process, employing smaller clearances between die and

punch. The drawing force required under ideal frictionless flow conditions will increase linearly

with punch stroke due to increase in strain on the metal and also because the material gets strain hardened. Friction due to hold down pressure as well as sliding tends to increase reach a

peak value and decreases early during the drawing. This is due to the fact that after certain

amount of drawing the amount of material under the hold down plate reduces. Ironing force

operates during the later part of the process, as sufficient thickening has to occur. About 15% of

the total force is spent on bending and unbending of the blank on the die and punch profile. 70%

of the total force is required for radial drawing of the material. 10% of the energy goes for

overcoming friction. If the blank hold down force is too high or if draw beads are used under the

hold down ring, the material around the punch will begin to stretch instead of being drawn. This

may lead to localized necking or diffuse necking depending on strain rate sensitivity, lubrication,

and punch geometry. On the other hand, a lower hold down pressure makes the metal flow freely

into the die cavity.

The material which occupies the length represented by the difference between the die and punch

radii is likely to undergo wrinkling folding due to hoop compressive stress. This is due to the fact that the diameter of the blank has become sufficiently smaller. Therefore, the smaller

material is unable to support the hoop stress and hence wrinkles. This happens especially when

the hold down pressure is insufficient and the thickness of sheet is too small and the material


Page 20

flow is pure drawing mode. Sachs has given an approximate expression for total drawing force,

which is given below:

Dp is punch diameter Do is blank diameter H is hold down force B is force for bending and

unbending T is blank thickness Y is yield strength of the material In deep drawing material just

above the bottom of the punch is subjected to circumferential tensile stress and longitudinal

tensile stress. Punch force acting on the bottom of the cup is transferred to the side of the cup.

The narrow ring of metal just above the bottom of the cup is subjected to plane strain condition.

As a result, failure of the cup easily happens in this zone due to necking induced by the tensile

stress, leading to tearing. Punch force is shown to vary with the stroke of the punch. It is difficult

to predict the punch force in deep drawing. However, an expression for maximum punch force is

given by:

UTS is ultimate tensile strength of the material, to is initial thickness of blank The maximum

tensile force on the cup which causes tearing can be estimated form the plane strain condition as:

In wire drawing the strain hardening exponent n has significant influence on deformation and

draw force. Whereas in deep drawing strain hardening does not affect significantly both draw

stress and deformation. Clearance between die and punch is a critical factor in deep drawing.

Normally, radial clearances of 7 to 14% of the sheet thickness are common. Too small a

clearance may cause shear on the blank. Sharp corner on the punch could cause fracture of the

cup along the corner. Too large a radius on the corner of punch may cause wrinkles on the

flange. Similarly die corner radius, if small, can cause fracture on the flange. Corner radius is

normally 5 to 10 times the sheet thickness. Blank holder pressure is another important factor. 0.5

to 1% of the ultimate strength of the sheet material is normally taken to be the hold pressure. Too

large a hold pressure results in tearing along cup wall. Too low a value leads to wrinkling in

flange. An approximate expression for holding force is given based on the initial area of the

blank and assuming that the holding pressure is 0.015 times yield strength.

Rd is die corner radius. Thick sheets could be drawn without blank holder. In such case, the limit

on the diameter of sheet is governed by: Do-Dp< 5to

2.5 Modern materials-

Modern materials are developed through the invention of new or

improved processes, for example, as a result of 'man' made materials/ingredients or human

intervention, in other words not naturally occurring changes. They are altered to perform a

particular function. Many smart and modern materials are developed for specialized applications

but some eventually become available for general use.

2.5.1 Steel: - Steel is hard, strong, bluish gray metal alloy of iron and is one of the most widely used materials in the world; steel is Most important engineering and construction material. Steel


Page 21

has carbon content from 0.2 to 2.1 percent (by weight), depending on classification according to

composition and their physical properties.

Steels can be classified by a variety of different systems depending on:

The composition, such as carbon, alloy (high alloy and Low alloy) or stainless steel.

The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods.

The finishing method, such as hot rolling or cold rolling

The product form, such as bar plate, sheet, strip, tubing or structural shape

The deoxidation practice, such as killed, semi-killed, capped or rimmed steel

The microstructure, such as ferritic, pearlitic and martensitic

The required strength level, as specified in ASTM standards

The heat treatment, such as annealing, quenching and tempering, and thermo mechanical processing

Quality descriptors, such as forging quality and commercial quality.

2.5.1.1 High strength low alloy metals: - High-strength low-alloy (HSLA) steels are

designed to provide better mechanical properties and/or greater resistance to atmospheric

corrosion than conventional carbon steels in the normal sense because they are designed to meet

specific mechanical properties rather than a chemical composition.

HSLA Classification:

1. Weathering steels, designated to exhibit superior atmospheric corrosion resistance

2. Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed

to develop a highly deformed austenite structure that will transform to a very fine

equiaxed ferrite structure on cooling

3. Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation

hardening but with low carbon content and therefore little or no pearlite in the

microstructure

4. Microalloyed steels, with very small additions of such elements as niobium, vanadium,

and/or titanium for refinement of grain size and/or precipitation hardening

5. Acicular ferrite steel, very low carbon steels with sufficient hardenability to transform on

cooling to a very fine high-strength acicular ferrite structure rather than the usual

polygonal ferrite structure

6. Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly

distributed regions of high-carbon martensite, resulting in a product with low yield

strength and a high rate of work hardening, thus providing a high-strength steel of

superior formability.


Page 22

7. Alloy Steel: - Steel containing significant quantities of alloying elements (other than

carbon and the commonly accepted amounts of manganese, silicon etc.) to effect

changes in the mechanical or physical properties.

8. Low Alloy Steel: - Steel containing less than 3.5% of alloying elements e.g. 2.25% Cr

1% Mo.

9. Micro Alloy Steel: - Steel containing small amounts of vanadium, niobium and/or

titanium. Individual elements generally less than 0.10% and total microalloying

elements generally less than 0.15%. Also known as HSLA steels.

10. Micro alloyed:- Micro alloyed steel is a type of alloy steel that contains small amounts

of alloying elements (0.05 to 0.15%), including niobium, vanadium, titanium,

molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain

microstructure or facilitate precipitation hardening.

11. The steel grades are usually required for the production of body parts of vehicles, trucks,

and machineries.

12. In forgings, micro alloy steels are able to develop higher mechanical properties (yield

strengths greater than say 60,000 psi) and higher toughness as forged by just cooling in

air or with a light mist water spray.

2.5.1.2 Micro Alloyed Steel :-Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements (0.05 to 0.15%), including niobium, vanadium, titanium,

molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain

microstructure or facilitate precipitation hardening. These steels lie, in terms of performance and

cost, between carbon steel and low alloy steel. Yield strength is between 500 and 750 MPa

(73,000 and 109,000 psi) without heat treatment. Weldability is good, and can even be improved

by reducing carbon content while maintaining strength. Fatigue life and wear resistance are

superior to similar heat-treated steels. The disadvantages are that ductility and toughness are not

as good as quenched and tempered (Q&T) steels. They must also be heated hot enough for all of

the alloys to be in solution; after forming, the material must be quickly cooled to 540 to 600 C

(1,004 to 1,112 F).

Cold-worked microalloyed steels do not require as much cold working to achieve the same

strength as other carbon steel; this also leads to greater ductility. Hot-worked microalloyed steels

can be used from the air-cooled state. If controlled cooling is used, the material can produce

mechanical properties similar to Q&T steels. Machinability is better than Q&T steels because of

their more uniform hardness and their ferrite-pearlite microstructure.

Because microalloyed steels are not quenched and tempered, they are not susceptible to quench

cracking, nor do they need to be straightened or stress relieved. However, because of this, they

are through-hardened and do not have a softer and tougher core like quench and tempered steels.


Page 23

The Difference between Microalloy and Regular Alloy Steels

1. Microalloy steel is manufactured like any other, but the chemical ingredients added at the initial melt of the steel to make it a microalloy include elements like Vanadium,

Titanium, and higher amounts of Manganese and perhaps Molybdenum or Nickel.

2. Vanadium, Niobium, and Titanium are also grain refiners and aggressive Oxygen scavengers, so these steels tend to also have a very fine austenitic grain size.

3. In forgings, microalloy steels are able to develop higher mechanical properties (yield strengths greater than say 60,000 psi) and higher toughness as forged by just cooling in

air or with a light mist water spray.

4. Normal alloy steels require a full austenitize, quench and temper heat treatment to develop properties greater than as rolled or cold worked.

Since microalloyed steels are able to get higher properties using forging process heat- rather

than an additional heating quenching tempering cycle- they can be less expensive to process to

get improved mechanical properties.

The developed microstructure ultimately makes the difference. The microstructure developed in

the steel depends on the grade and type.

Normal alloy steels require a transformation to martensite that is then tempered in order

to achieve higher properties.

Microalloy steel precipitates out various nitrides or carbides and may result in either a

very fine ferrite- pearlite microstructure or may transform to Bainite.

For machinists, if the steel is already at its hardest condition, the microalloyed microstructure of

either ferrite pearlite or Bainite is less abrasive than that of a fully quench and tempered alloy

steel.

2.5.1.3 Dual phase steel:-

Dual-phase steels (DP steels) consist of ferrite and a dispersed hard martensitic second phase in

the form of islands. Usually they are low-carbon low-alloy materials with 10-40

vol.% hard martensite or martensite-austenite particles embedded in a ductile ferrite matrix. As

they combine high strength and good formability at low production costs they are widely used

for automotive applications. Sometimes the martensite regions tend to percolate or appear in the

form of elongated bands which is not desirable. Increasing the volume fraction of the hard

second phase martensite generally increases the strength but sometimes reduces ductility. Such

microstructures enable achieving ultimate tensile strength values in the regime of 400-

1200 MPA. For some alloys also baintic portions are used in DP steel microstructures. Dual

phase steel microstructure can exhibit a number of advantageous properties compared to other

advanced high strength steels. For example the strength of the DP steel microstructure can be

designed by the volume fraction of martensite and the ductility by its dispersion. They do not

reveal a pronounced yield point elongation and show a modest ratio between the ultimate tensile

strength and the yield strength low of around. DP steels undergo high strain hardening especially

at the beginning of plastic deformation. Also, they can be strengthened by static or dynamic

strain ageing through the so called bake hardening effect.


Page 24

DP steels with low carbon content exhibit excellent resistance to fatigue crack propagation at

growth rates close to fatigue threshold. The alloying elements used in DP steels have different

types of effects Carbon, used in the range between 0.060.15 wt.% acts as an austenite stabilizer, strengthens the martensite and determines the phase distribution. Mn , used between

1.53 wt.% also stabilizes the austenite, is a ferrite solid solution strengthener and retards ferrite formation. Si promotes ferritic transformation while Cr

and Mo, used up to 0.4 wt%, can retard pearlite and Bainite formation. Additionally

microalloying elements such as V or Nb can be used as precipitation strengtheners and to refines

the microstructure

DP ferrite plus martensite steels are produced by controlled cooling from the austenite phase (in

the case of hot band products) or from the two-phase ferrite plus austenite phase during an

intercritical annealing treatment step (in the case of continuously annealed cold-rolled and hot-

dip coated products) to transform some austenite to ferrite before a rapid cooling transforms the

remaining austenite to martensite. The microstructures of DP steels are typically not good

candidates for applications that require high drawability. They usually exhibit rather poor hole

expansion ratio values. This drawback, however, can be eliminated by adding Ti with the aim of

inducing precipitation strengthening in ferrite to reduce the differences in hardness between

the two phases. Alternatively, the martensite - ferrite constituents may be replaced or aided by

introducing also a Bainite phase. This means that depending on the composition and process

route, hot-rolled steels requiring enhanced capability to resist stretching on a blanked edge (as

typically measured by whole expansion capacity) can have a microstructure containing

significant quantities of Bainite. In response to the increasing demand for fuel efficiency, CO2

reduction, and occupant safety, it was shown that grain refinement is an effective tool to

strengthen dual phase steels without raising alloying costs or allowing a decrease in ductility.

Fig. 2.19

Dual-phase (DP) steels comprise a soft, continuous phase (a mechanically separable or visually

identifiable portion of a structure) known as ferrite surrounding islands of a hard phase known as

martensite.

The ferrite phase is mainly pure iron, just like the extra-deep-drawing steel grades used to make

automotive fenders; this phase gives this steel grade its high ductility. The martensite phasewhich has extremely high strength, like the quenched grades that are used for springs and cutting

toolsis responsible for the high tensile strength (TS).


Page 25

Dual-phase steel (DPS) is a high-strength steel that has a ferrite and martensitic microstructure.

DPS starts as a low or medium carbon steel and is quenched from a temperature above A1 but

below A3 on a continuous cooling transformation diagram. This results in a microstructure

consisting of a soft ferrite matrix containing islands of martensite as the secondary phase

(martensite increases the tensile strength). Therefore, the overall behavior of DPS is governed by

the volume fraction, morphology (size, aspect ratio, interconnectivity, etc.), the grain size and the

carbon content.[1]

For achieving these microstructures, DPS typically contain 0.060.15 wt.% C and 1.5-3% Mn (the former strengthens the martensite, and the latter causes solid solution

strengthening in ferrite, while both stabilize the austenite), Cr & Mo (to retard pearlite or Bainite

formation), Si (to promote ferrite transformation), V and Nb (for precipitation strengthening and

microstructure refinement). The desire to produce high strength steels with formability greater

than microalloyed steel led the development of DPS in the 1970s.

DPS have high ultimate tensile strength (UTS, enabled by the martensite) combined with low

initial yielding stress (provided by the ferrite phase), high early-stage strain hardening and

macroscopically homogeneous plastic flow (enabled through the absence of Lders effects).

These features render DPS ideal materials for automotive-related sheet forming operations.

The steel melt is produced in an oxygen top blowing process in the converter, and undergoes an

alloy treatment in the secondary metallurgy phase. The product is aluminum-killed steel, with

high tensile strength achieved by the composition with manganese, chromium and silicon.

Their advantages are as follows: Low yield strength

Low yield to tensile strength ratio (yield strength / tensile strength = 0.5)

High initial strain hardening rates

Good uniform elongation

A high strain rate sensitivity (the faster it is crushed the more energy it absorbs)[4]

Good fatigue resistance

Due to these properties DPS is often used for automotive body panels, wheels, and bumpers

Types and Availability

Like high-strength, low-alloy (HSLA) steels, DP steels are available in different strength levels,

which increase with the percentage of martensite. However, producing martensite is not as easy

as simply changing the chemistry; controlled chemistry, controlled cooling, and tight process

controls are required to change the microstructure to the desired balance of ferrite and

martensite.

Whereas most sheet steel mills have the necessary equipment and process capabilities to produce

HSLA steels, only certain mills are equipped to produce DP steels.

Tensile Properties

Another significant difference between HSLA and DP steels is the consistency of the material

from steelmaker to steelmaker. The tensile properties and chemical make-up of a given grade of

HSLA steel are essentially the same from any steelmaker. However, the production of the DP

microstructure depends heavily on the equipment used and the steelmakers capabilities, so the steel melt chemistry is different among producers.


Page 26

The tensile properties will meet the pertinent specification, but since the steelmakers get there

using different chemistry, the carbon equivalent--and therefore weldability--can be different. If

you switch steel suppliers during the life of the part or between die development and production,

be aware of this potential for different manufacturing performance on a seemingly identical

grade.

Formability

The ratio of yield strength (YS) to TS can indicate the relative formability of a particular grade.

When YS is close to TS, only limited deformation can occur before the metal cracks. A larger

gap means the material can be formed into more complex shapes before it fails. While HSLA

grades have a typical YS-to-TS ratio of 0.8, DP steels are closer to a ratio of 0.6, indicating that

DP steels are more formable than HSLA grades at a similar strength level.

The best indicator of sheet metal formability is the strain-hardening exponent, also known as the

n-value. Higher n-values represent an increased ability to distribute strains across a part, as

opposed to concentrating strains in a specific area that can lead to failure. A distinguishing

feature of DP steels is a higher n-value when calculated over a lower strain range than the typical

10 percent to uniform elongation strain. This allows for a more uniform strain distribution early

in the press stroke. The higher n-value also indicates a greater work-hardening capability.

Unlike HSLA grades, DP steels can be bake hardened. This means their YS increases from

work-hardening during forming and again after being processed through a paint curing (baking)

cycle. A flat sheet with a YS of 350 MPa can increase in strength to more than 500 MPa from the

combination of work hardening and bake hardening.

The most common TS levels for DP steels are 590, 780, and 980 MPa. Rather than using these

TS levels, some steelmakers specify these grades as 600, 800, and 1000 MPa. They essentially

are the same grades, although the processing differs somewhat to attain the slightly higher

minimum TS levels.

Automotive Applications

The choice of a material for an automotive application depends on the function, environment,

and requirements of the part being produced. The microstructure of advanced high-strength

steels (AHSS) can be tailored to produce specific properties that are engineered for each

application. This is why usage of AHSS grades is growing in the automotive industry, and DP

steels are leading the way.

The auto body structure must protect the passenger compartment in the event of a crash;

significant impact energy much be dissipated in less than 100 milliseconds before it reaches the

occupants. The front and rear crumple zones require steel grades and structures that can absorb

this crash energy, making DP steels ideal candidates for automotive parts.

At the same YS, DP steels have greater TS and elongation than a comparable HSLA steel. In

addition, because more complex shapes can be formed from DP steels, the part geometry can be

engineered to have the best crash response. With the right section geometry, sufficient crash

performance may be achieved even with a thickness reduction.


Page 27

At the higher strength levels, DP steels are used in the passenger safety cage, where anti-

intrusion is critical. DP steels also can be found in such applications as front and rear

longitudinal rails, rockers, pillars, pillar reinforcements, roof rails, and cross members.

2.5.3 Smart materials: - 'Intelligent' or 'Smart' materials may be defined as 'Those materials

which sense any environmental change and respond to it in an optimal manner'.

2.5.3.1 Functional requirements:- following mechanisms may be essential for any material to

be made intelligent:

(i) A sensing device to perceive the external stimuli (skin which senses thermal gradients,

an eye that senses optical signals, etc.), termed as 'sensor' function.

(ii) A communication network by which the sensed signal would be transmitted to a

decision-making mechanism (e.g. the nervous system in humans and animals), termed as

'memory' function.

(iii) A decision-making device which has the capability of reasoning (e.g. the brain),

termed as 'processor' function.

(iv) An actuating device, which could be inherent in the material or externally coupled

with it (e.g. stiffening of muscles in humans and animals to resist deformation due to

external loading), termed as 'actuator' function

2.5.3.2 Concept of smart or intelligent materials

Fig. 2.20 concept of an intelligent material

Designing a material with sensor, processor and actuator functions is the fundamental step in the

evolution of an intelligent material for achieving a desired response adaptable to the

environment.

a. Sensor function: - The concept of a sensor function in a smart material is defined as the

ability of the material to sense the response characteristics of self with respect to environmental

factors such as mechanical loading, temperature, humidity and electrical inputs.


Page 28

An example of this function is that of a piezoelectric sensor embedded in a composite material.

This sensor diagnoses the mechanical disturbance imposed on the material by generating a

voltage which can be further measured and analyzed.

b. Memory and processor function: - This mechanism stores the signals sensed and

transmitted by the sensor function.

The characteristics of these signals are then compared with prestored acceptable values acquired

during the 'training' process of the processor.

The training process may be carried out using an artificial intelligence technique, e.g. pattern

recognition method.

Typically, this function is in the form of executable artificial intelligence software that could

produce a logical output in the form of an electrical voltage that could further be amplified and

used to activate an actuator mechanism.

c. Actuator function: - This mechanism is coupled with the material.

It produces an output corresponding to the signal received from the processor function.

This output is usually in the form of restoring stress, strain or change in temperature, or stiffness

of the actuator mechanism that is coupled with the material.

This change would be designed to neutralize the effect of the change in environment on the

material, thereby adapting the material continuously to its environment.

A typical intelligent composite cantilever beam, which consists of sensor, processor and actuator

functions.

2.5.3.3 Classification of smart materials: - Smart materials can also be classified into two

categories i.e., either active or passive.

Active smart materials as those materials which possess the capacity to modify their geometric or

material properties under the application of electric, thermal or magnetic fields, thereby

acquiring an inherent capacity to transduce energy. Piezoelectric materials, SMAs, ER fluids and

magneto-strictive materials are considered to be the active smart materials and therefore, they

can be used as force transducers and actuators. Piezoelectric materials, which convert electric

energy into mechanical force, are also active.

On the other part, the materials, which are not active, are called passive smart materials.

Although smart, they lack the inherent capability to transduce energy.

Fiber optic material is a good example of a passive smart material.

Such materials can act as sensors but not as actuators or transducers.


Page 29

2.5.4 Shape memory metals:- A shape-memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy) is an alloy that "remembers" its original shape and that

when deformed returns to its pre-deformed shape when heated.

This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic,

pneumatic, and motor-based systems. Shape-memory alloys have applications iThe two main

types of shape-memory alloys are copper-aluminium-nickel, and nickel-titanium (NiTi) alloys

but SMAs can also be created by alloying zinc, copper, gold and iron.

Although iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are

commercially available and cheaper than NiTi.

NiTi based SMAs are more preferable for most applications due to their stability, practicability

and superior thermo-mechanic performance.

SMAs can exist in two different phases, with three different crystal structures (i.e. twinned

martensite, detwinned martensite and austenite) and six possible transformations in industries

including automotive, aerospace, biomedical and robotics.

The two main types of shape-memory alloys are copper-aluminium-nickel, and nickel-titanium

(NiTi) alloys but SMAs can also be created by alloying zinc, copper, gold and iron. Although

iron-based and copper-based SMAs, such as Fe-Mn-Si, Cu-Zn-Al and Cu-Al-Ni, are

commercially available and cheaper than NiTi, NiTi based SMAs are preferable for most

applications due to their stability, practicability and superior thermo-mechanic performance.

SMAs can exist in two different phases, with three different crystal structures (i.e. twinned

martensite, detwinned martensite and austenite) and six possible transformations.

NiTi alloys change from austenite to martensite upon cooling; Mf is the temperature at which the

transition to martensite completes upon cooling. Accordingly, during heating As and Af are the

temperatures at which the transformation from martensite to austenite starts and finishes.

Repeated use of the shape-memory effect may lead to a shift of the characteristic transformation

temperatures (this effect is known as functional fatigue, as it is closely related with a change of

microstructural and functional properties of the material).The maximum temperature at which

SMAs can no longer be stress induced is called Md, where the SMAs are permanently deformed.

The transition from the martensite phase to the austenite phase is only dependent on temperature

and stress, not time, as most phase changes are, as there is no diffusion involved. Similarly, the

austenite structure receives its name from steel alloys of a similar structure. It is the reversible

diffusionless transition between these two phases that results in special properties. While

martensite can be formed from austenite by rapidly cooling carbon-steel, this process is not

reversible, so steel does not have shape-memory properties .In the fig. 2.21, (T) represents the

martensite fraction. The difference between the heating transition and the cooling transition gives

rise to hysteresis where some of the mechanical energy is lost in the process. The shape of the

curve depends on the material properties of the shape-memory alloy, such as the alloying and

work hardening.


Page 30

Fig. 2.21 Typical transformation versus temperature curve for a specimen under constant load (stress) as it is

cooled and heated. Ms martensite starts, Mf martensite finish, As austenite starts and Af austenite finish.

-One-way vs. two-way shape memory

Shape-memory alloys have different shape-memory effects. Two common effects are one-way

and two-way shape memory. A schematic of the effects is shown below.

Fig. 2.22 The procedures are very similar: starting from martensite (a), adding a reversible deformation for

the one-way effect or severe deformation with an irreversible amount for the two-way (b), heating the sample

(c) and cooling it again (d).

-One-way memory effect: - When a shape-memory alloy is in its cold state (below As), the

metal can be bent or stretched and will hold those shapes until heated above the transition

temperature. Upon heating, the shape changes to its original. When the metal cools again it will

remain in the hot shape, until deformed again.

With the one-way effect, cooling from high temperatures does not cause a macroscopic shape

change. A deformation is necessary to create the low-temperature shape. On heating,


Page 31

transformation starts at As and is completed at Af (typically 2 to 20 C or hotter, depending on

the alloy or the loading conditions). As is determined by the alloy type and composition and can

vary between 150 C and 200 C.

-Two-way memory effect: - The two-way shape-memory effect is the effect that the material

remembers two different shapes: one at low temperatures, and one at the high-temperature shape.

A material that shows a shape-memory effect during both heating and cooling is said to have

two-way shape memory. This can also be obtained without the application of an external force

(intrinsic two-way effect). The reason the material behaves so differently in these situations lies

in training. Training implies that a shape memory can "learn" to behave in a certain way. Under

normal circumstances, a shape-memory alloy "remembers" its low-temperature shape, but upon

heating to recover the high-temperature shape, immediately "forgets" the low-temperature shape.

However, it can be "trained" to "remember" to leave some reminders of the deformed low-

temperature condition in the high-temperature phases. There are several ways of doing this.[11]

A shaped, trained object heated beyond a certain point will lose the two-way memory effect.

Manufacture:- Shape-memory alloys are typically made by casting, using vacuum arc melting or

induction melting. These are specialist techniques used to keep impurities in the alloy to a

minimum and ensure the metals are well mixed. The ingot is then hot rolled into longer sections

and then drawn to turn it into wire.

The way in which the alloys are "trained" depends on the properties wanted. The "training"

dictates the shape that the alloy will remember when it is heated. This occurs by heating the alloy

so that the dislocations re-order into stable positions, but not so hot that the material

recrystallizes. They are heated to between 400 C and 500 C for 30 minutes, shaped while hot,

and then are cooled rapidly by quenching in water or by cooling with air.

-Properties: - The copper-based and NiTi-based shape-memory alloys are considered to be

engineering materials. These compositions can be manufactured to almost any shape and size.

The yield strength of shape-memory alloys is lower than that of conventional steel, but some

compositions have a higher yield strength than plastic or aluminum. The yield stress for Ni Ti

can reach 500 MPa. The high cost of the metal itself and the processing requirements make it

difficult and expensive to implement SMAs into a design. As a result, these materials are used in

applications where the super elastic properties or the shape-memory effect can be exploited. The

most common application is in actuation.

One of the advantages to using shape-memory alloys is the high level of recoverable plastic

strain that can be induced. The maximum recoverable strain these materials can hold without

permanent damage is up to 8% for some alloys. This compares with a maximum strain 0.5% for

conventional steels.

Applications:-

1. Aircraft and spacecraft:- Boeing, General Electric Aircraft Engines, Goodrich

Corporation, NASA, Texas A&M University and All Nippon Airways developed the Variable

Geometry Chevron using a NiTi SMA. Such a variable area fan nozzle (VAFN) design would


Page 32

allow for quieter and more efficient jet engines in the future. In 2005 and 2006, Boeing

conducted successful flight testing of this technology.

SMAs are being explored as vibration dampers for launch vehicles and commercial jet engines.

The large amount of hysteresis observed during the superplastic effect allows SMAs to dissipate

energy and dampen vibrations. These materials show promise for reducing the high vibration

loads on payloads during launch as well as on fan blades in commercial jet engines, allowing for

more lightweight and efficient designs. SMAs also exhibit potential for other high shock

applications such as ball bearings and landing gear.

There is also strong interest in using SMAs for a variety of actuator applications in commercial

jet engines, which would significantly reduce their weight and boost efficiency. Further research

needs to be conducted in this area, however, to increase the transformation temperatures and

improve the mechanical properties of these materials before they can be successfully

implemented

2. Automotive: - The first high-volume product (> 5Mio actuators / year) is an automotive

valve used to control low pressure pneumatic bladders in a car seat that adjust the contour of the

lumbar support / bolsters. The overall benefits of SMA over traditionally-used solenoids in this

application (lower noise/EMC/weight/form factor/power consumption) were the crucial factor in

the decision to replace the old standard technology with SMA.

The 2014 Chevrolet Corvette became the first vehicle to incorporate SMA actuators, which

replaced heavier motorized actuators to open and close the hatch vent that releases air from the

trunk, making it easier to close. A variety of other applications are also being targeted, including

electric generators to generate electricity from exhaust heat and on-demand air dams to optimize

aerodynamics at various speeds.

3. Robotics: - There have also been limited studies on using these materials in robotics, for

example the hobbyist robot Stiquito , as they make it possible to create very lightweight robots.

Recently, a prosthetic hand was introduced by Loh et al. that can almost replicate the motions of

a human hand [Loh2005]. Other biomimetic applications are also being explored. Weak points of

the technology are energy inefficiency, slow response times, and large hysteresis.

4. Civil Structures: - SMAs find a variety of applications in civil structures such as bridges

and buildings. One such application is Intelligent Reinforced Concrete (IRC), which incorporates

SMA wires embedded within the concrete. These wires can sense cracks and contract to heal

macro-sized cracks. Another application is active tuning of structural natural frequency using

SMA wires to dampen vibrations.[24]

5. Piping: - The first consumer commercial application was a shape-memory coupling for

piping, e.g. oil line pipes for industrial applications, water pipes and similar types of piping for

consumer/commercial applications.


Page 33

6. Telecommunication: - The second high volume application was an autofocus (AF)

actuator for a smart phone. There are currently several companies working on an optical image

stabilization (OIS) module driven by SMA wires.[citation needed]

7. Medicine: - Shape-memory alloys are applied in medicine, for example, as fixation

devices for osteotomies in orthopedics surgery, in dental braces to exert constant tooth-moving

forces on the teeth.

The late 1980s saw the commercial introduction of Nitinol as an enabling technology in a

number of minimally invasive endovascular medical applications. While more costly than

stainless steel, the self expanding properties of Nitinol alloys manufactured to BTR (Body

Temperature Response), have provided an attractive alternative to balloon expandable devices in

stent grafts where it gives the ability to adapt to the shape of certain blood vessels when exposed

to body temperature. On average, 50% of all peripheral vascular stents currently available on the

worldwide market are manufactured with Nitinol.

8. Optometry: - Eyeglass frames made from titanium-containing SMAs are marketed under

the trademarks Flexon and TITANflex. These frames are usually made out of shape-memory

alloys that have their transition temperature set below the expected room temperature. This

allows the frames to undergo large deformation under stress, yet regain their intended shape once

the metal is unloaded again. The very large apparently elastic strains are due to the stress-

induced martensitic effect, where the crystal structure can transform under loading, allowing the

shape to change temporarily under load. This means that eyeglasses made of shape-memory

alloys are more robust against being accidentally damaged.

9. Orthopedic surgery:-Memory metal has been utilized in orthopedic surgery as a

fixation-compression device for osteotomies, typically for lower extremity procedures. The

device, usually in the form of a large staple, is stored in a refrigerator in its malleable form and is

implanted into pre-drilled holes in the bone across an osteotomy. As the staple warms it returns

to its non-malleable state and compresses the bony surfaces together to promote bone union.

10. Dentistry:-The range of applications for SMAs has grown over the years, a major area of

development being dentistry. One example is the prevalence of dental braces using SMA

technology to exert constant tooth-moving forces on the teeth; the nitinolarch wire was

developed in 1972 by orthodontist George Andreasen. This revolutionized clinical orthodontics.

Andreasen's alloy has a patterned shape memory, expanding and contracting within given

temperature ranges because of its geometric programming.

11. Engines:- Experimental solid state heat engines, operating from the relatively small

temperature differences in cold and hot water reservoirs, have been developed since the 1970s,

including the Banks Engine, developed by Ridgway Banks.

Materials:- A variety of alloys exhibit the shape-memory effect. Alloying constituents can be

adjusted to control the transformation temperatures of the SMA. Some common systems include

the following (by no means an exhaustive list):


Page 34

Ag-Cd 44/49 at.% Cd, Au-Cd 46.5/50 at.% Cd ,Cu-Al-Ni 14/14.5 wt% Al and 3/4.5 wt% Ni ,Cu-

Sn approx. 15 at% Sn, Cu-Zn 38.5/41.5 wt.% Zn ,Cu-Zn-X (X = Si, Al, Sn) ,Fe-Pt approx. 25

at.%Pt ,Mn-Cu 5/35 at% Cu , Fe-Mn-Si, Co-Ni-Al[27], Co-Ni-Ga, Ni-Fe-Ga, Ti-Nb, Ni-Ti

approx. 5560 wt% Ni, Ni-Ti-Hf, Ni-Ti-Pd, Ni-Mn-Ga.

2.5.5 Metallic glasses: - Glass is a uniform amorphous solid material, usually produced

when the viscous molten material cools very rapidly to below its glass transition temperature,

without sufficient time for a regular crystal lattice to form. The t

Module 2 Notes

Documents

Transcript of Module 2 Notes