Module 10 Lecture 2 Workability

NPTEL - Mechanical Engineering - Forming

Joint Initiative of IITs and IISc – Funded by MHRD of 12

Workability

R. Chandramouli

Associate Dean-Research

SASTRA University, Thanjavur-613 401



Table of Contents

1.Workability ................................................................................ 3

1.1 Workability and processing map......................................................................................................... 3

1.2 Metallurgical factors on workability: .................................................................................................. 4

1.3 Dynamic recovery and dynamic recrystallization: .............................................................................. 5

1.4 Stress state on workability: ................................................................................................................. 7

1.5 Fracture limit and workability: ............................................................................................................ 8

1.6 The hot tension test: ......................................................................................................................... 11



1.Workability

1.1 Workability and processing map Workability is a term related to bulk deformation processing of materials such as forging, extrusion, rolling etc. It refers to the ease with which a material could be formed without defects such as cracks. Formability refers to sheet metal processing. Ductility of the material is an important factor governing workability. High ductility material is expected to have good workability. Two factors are considered important while considering workability. One is the material factor – ductility, microstructure, grain size etc. The other factor is process factor – geometry of die, shape of material, friction etc.

Fracture of material during deformation is considered as the most important criterion for formability. Deformation is limited by fracture. High ductility will delay the fracture so that the material could be worked to greater levels of deformations. Fracture may occur on surface, or internally within the deforming material or it could occur at die-material interface. We have considered in earlier lecture how deformation zone geometry affects the internal fracture such as center burst.

Die contact surface cracks are primarily caused by excessive surface shear. Friction or low hydrostatic pressure can cause surface cracks. Surface cracks affect the surface finish. Extreme levels of tensile stress at center causes center burst or chevron cracks in extruded or drawn products. Sometimes we may have to change the deformation zone geometry to avoid such internal cracks. Defects in forging such as underfill, folds, laps, seams are found to be related to material flow. Improper material through narrow sections could cause defects on surface. Proper die design is necessary in order to eliminate the possibility of such defects. Flow – through defects are observed when material has difficulty in flowing through narrow sections because of chilling effect.

Metallurgical factors such as grain size, grain distribution also decide workability. In forming processes, one can control the grain flow, thereby enhancing the strength along the possible direction of loading. Fibrous structure is characteristic of most of the metal forming processes. In rolling for example grains become directional, thereby providing directional variation of properties (anisotropy). Workability is known to be reduced in case of coarse grained structure. Large grain boundary areas are more prone to cracks. Hot working increases the workability. However, hot forming may promote grain growth, which may coarsen the grains. In hot rolling of steel grain growth inhibiting elements such as titanium are added so as to promote fine grained structure after rolling. This increases the strength.



One way of evaluating workability is through processing map. A processing map specifies the regions of fracture and regions of safe forming for combinations of process parameters such as forming temperature, strain rate.

Fig. 2.1.1: Typical processing map. X-axis is temperature on degrees Celcius

From the map we understand that there is a maximum value of ductility corresponding to a given strain rate at a given temperature of working.

1.2 Metallurgical factors on workability: Some of the metallurgical factors such as grain size, texture, strain hardening etc also contribute to workability. For instance, severe working may promote crystallographic texture, which leads to anisotropy. Shear bands may also form due to excessive working, due to plastic instability in compression.



In dynamic material model, the material flow stress is correlated with strain rate, temperature of working so that one can map the safe limit of working and limits of fracture under varying strain rate or temperature conditions. Strain rate sensitivity and temperature sensitivity of material are important in this method. The DMM model integrates flow stress, temperature, microstructure, with workability.

The parameter m, called strain rate sensitivity, given by:

governs the flow stress of a material.

Similarly, temperature sensitivity of flow stress is given by the parameter s, given by:

We expect m to be between 0 and 1. A high value of m means the onset of plastic instability and necking is delayed. A value of m near to unity may promote superplastic forming. Similarly, the value of s is dictated by entropy considerations.

Therefore, we have:

High value of s means dynamic recrystallization occurs. Low value of s indicates that dynamic recovery is occurring. Dynamic recovery refers to formation of subgrain structure by cross slip of dislocations – a process of softening during hot working. Dynamic recrystallization also could cause softening.

High strain rates due to high values of s may cause adiabatic conditions which may create strain localization. This may lead to cracking.

1.3 Dynamic recovery and dynamic recrystallization: Effect of strain rate on flow stress is more pronounced in hot working. Hot working considerably increases workability. During hot working the phenomena of dynamic recovery and dynamic recrystallization contribute to softening of materials. Dynamic recovery involves dislocation cross slip, annihilation of dislocations, and thereby the formation of sub grains. As a result the flow stress is considerably reduced. Uniform equiaxed grains form as a result of dynamic recovery.



Dynamic recrystallization tends to occur in materials which have high stacking fault energy. When the dislocation density increases to high levels, recrystallization is the only possibility through which the internal strains get relieved. High dislocation density and recrystallization could lead to internal cracks during forming.

Fig. 1.3.1: Dynamic recovery and recrystallisation

Formation of dead metal zones in forming processes could lead to flow localisations. Friction and die wall chilling can also cause flow localisations. This affects the workability severely. Flow softening may also induce flow localisations. Flow softening is expressed by a parameter called flow softening rate, given as:

The parameter includes strain rate sensitivity factor and flow softening rate parameter, and is given by:

If is greater than 5 in compressive deformation, non-uniform deformation happens. Flow localization could also be caused by adiabatic heating conditions due to high strain rates of working of the material during hot working.

Workability of cast metals is generally poor. Therefore they are usually hot worked. Presence of low melting phases may cause localized melting, causing hot shortness. Wrought structure is found to enhance workability. In cold working, the material gets work hardened. Stresses are

Dynamic recovery

Dynamic recrystallization



not relieved in cold working. Therefore workability is reduced. Hot working involves recovery process. Therefore workability is higher in hot working.

1.4 Stress state on workability: It is recognized that hydrostatic state of stress could improve workability of materials. Extrusion is a process which can deform most of the materials because of the state of compressive stress involved. It is known that hydrostatic extrusion enables extrusion of brittle materials such as ceramics. Compressive stress is known to enhance workability. The mean or hydrostatic stress is given by:

A workability parameter has been defined in terms of hydrostatic stress as followed:

= 3 where is the effective stress. Variation of the workability parameter with strain to

fracture is shown in figure below. The strain to fracture is higher for compressive stress state and lower for tensile stress state. Further, the diagram shows various forming processes superimposed in the curve. Wire drawing process, being a tensile deformation process, has lower workability. Extrusion process, being a compressive deformation has higher workability parameter.



Fig. 2.4.1: Workability parameter

1.5 Fracture limit and workability: There are several simple tests being used for determination of workability. The tensile test, Hardness tests, compressive test, hot torsion test, etc are commonly employed for determination of workability.

Workability is related closely to fracture. Fracture criteria will help us in predicting workability. There are two types of fracture, namely, ductile and brittle fracture. Ductile fracture is characterized by extensive plastic deformation before fracture, formation of voids and cavities. Localised necking could also happen. Brittle fracture is sudden and is not accompanied by any plastic deformation. One criterion for ductile fracture is given in the form:

= C

This states that fracture occurs when the strain energy per unit volume reaches a critical value C. We can also write:



= C

where is the critical or the largest tensile stress locally acting, which causes the fracture, is effective stress. Fracture criteria in the form of correlation between the tensile and compressive strains corresponding to the condition of free surface cracks has been accepted as one of the easiest methods of evaluating fracture.

One of the important tests for workability is the fracture limit test, which correlates fracture with state of stress and frictional conditions. The fracture limit test is carried out using cylindrical specimen. Fracture limit line is established by conducting simple compression test on the cylinder under given condition of friction. The axial true strain is then plotted against radial true strain. For homogeneous deformation, the fracture limit line is a line with slope of -0.5. With interfacial friction between the cylinder and die the lateral surface of the cylinder undergoes bulging. Strains are measured at the instance of occurrence of cracks on the bulged surface. The height to diameter ratio of the cylindrical specimen is varied in order to obtain varied conditions of fracture. The test is done at increasing strain values in order to extend the range of strains. Flanged cylinder or tapered cylinder could also be used in the test in order to extend the strain range. Typical fracture limit lines are shown in diagram below. The curves above the -0.5 slope line correspond to bulged specimen. With bulging the curves have greater slope.



Fig. 1.5.1: Fracture limit curves

Application of fracture limit curve for workability can be understood from a simple illustration. Refer to the diagram given below: Consider the upsetting of a bolt head from a cylindrical rod of given diameter and height. To achieve a given strain given by: ln(d/D), different strain paths can be adopted depending on the condition of friction, work piece geometry etc. For two different materials the fracture limit lines are shown in figure below. The combination of material A and strain path a give the required strain without fracture. Material A with strain path b will not be able to produce the required bolt head without fracture. However Material B



with strain path b could give a fracture free bolt head. The strain path b may correspond to a condition of poor lubrication – high friction, due to which bulging of free surface occurs.

Fig. 1.5.2: Application of fracture limit criteria for upsetting a bolt head

In order to achieve the required amount of strain without fracture, we may look into other alternatives, namely, changing the die geometry, changing the preform shape, size etc.

1.6 The hot tension test: In order to establish the upper and lower limits of temperatures for hot working, as well as determining the workability of materials at elevated temperatures, the hot tension test is performed. This test is carried out in a machine called Gleeble thermal simulator. The test specimen in the form of a cylindrical rod with a reduced diameter of 6.4 mm and overall length, including the button head of 89 mm is held horizontally, gripped with water-cooled grippers, heated resistively through the copper grippers. Thermocouples are embedded in the specimen for precise measurement of temperature. The amount of heating can be controlled by adjusting the electric power input to the specimen. Temperature, displacement, load are measured with respect to time. For constant cross-head speed, the strain rate gets reduced as the specimen elongates. However, at the instance of and after necking, there is increase in strain rate. The



percent reduction in area of cross-section is calculated and is taken to be a measure of workability.

Qualitative rating of workability from the area reduction is given in table below:

Table 1.1: Workability for various forming processes

Reduction in area – hot tension test, %

Workability Application

<30 Poor 30 - 40 Marginal Rolling, forging with light

reductions 50 - 60 Good Rolling, forging with normal

reductions 60 - 70 Excellent workability, very

little cracks Rolling, forging – heavier reductions

> 70 Superior workability Rolling, forging – heavier reductions

Module 10 Lecture 2 Workability

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