Fundamentals of production processes Metal forming processes metal working

Material science and engineering

1. FUNDAMENTALS OF PRODUCTION PROCESSES (MSE 411) All components of machines and equipment are made from one engineering material or another. The components are subjected to one or more types of loading (tensile, compressive, shear, bending or torsional loading) Any material subjected to a load may either deform, yield or break depending upon the following: Magnitude of the load The nature of the material and Material’s cross-sectional dimensions

2. Stress-Strain Relationship Stress Strain Definition of normal strain Definition of normal stress 2

3. Stress-Strain Relationship   5

4. Stress-Strain Curves The stress-strain curve is a graphical representation of the relationship between stress derived from measuring the load applied on the sample and the strain derived from measuring the deformation of the sample i.e. elongation, compression, or distortion. 6

5. 7

6. 8 8

7. Elastic and Plastic Deformation   Plastic deformation from atomic point of view involves breaking of bonds with old neighbours and forming new bonds with new neighbours as large number of atoms or molecules move relative to one another upon removal of stress, they do not return to their original positions. The engineering stress required to continue extending the specimen varies due to strain hardening resulting from an increasing dislocation density. 9

8. At maximum strain hardening, the engineering stress required to increase the engineering strain decreases. The maximum engineering stress represent the highest load-carrying capacity of the cross sectional area and is called the ultimate tensile strength (UTS). Up to the point at which the load begins to decrease (the UTS), the plastic deformation is relatively uniform along the whole length of the sample guage section. This is referred to as homogeneous deformation. Beyond the UTS as the load decreases, localised deformation occur. Such localised deformation is called necking and the UTS may be viewed as the onset of plastic instability. Necking causes a local decrease in the cross sectional area of the test sample, reducing its load carrying capacity. Eventually, necking exhausts the plasticity in their region, causing the specimen to rupture. 10

9. Ductility An important behaviour observed during a tension test is Ductility . The extent of plastic deformation that the material undergoes before fracture. There are two common measures of ductility. The first is the total elongation of the specimen: (4) The second measure of ductility is the reduction in area (5) Where: A = Original cross sectional area and A is the cross sectional area at fracture location. 11

10. True Stress and True Strain The nominal (or engineering) stress-strain curve does not give a true indication of the deformation characteristics of a material because: It is based entirely on the original dimensions of the tensile test piece Ductile material that is pulled in tension becomes unstable and necks down during the course of the tensile test. In order to take into account the change in the original dimensions of the specimen during the tensile test, true stress-true strain curves are considered. 12

11. From equation (1) Hence, or (7) Similarly, true stress, Because plastic deformation occurs by a process of shear, there is essentially no volume change in the specimen during deformation (law of incompressibility or Volume constancy), hence A o L o = A i L i or 13

12. (8) Or (9) For a maximum, that is (10) 14

13. For most metals (11) Where a and b are material dependent constants So (12) Equating (10) and (12) and substituting ’ as ae b We have Whence (13) 15

14. Equations are valid only to the onset of necking beyond where true stress and strain should be computed from actual load, cross sectional area and guage length measurement. For some metals and alloys the region of the true stress-strain curve from the onset of plastic deformation to the point at which necking begins may be approximated by Holloman equation (14) Where K is the strength constant and n is the strain hardening exponent and they are both material dependent constant. Holloman equation is a power law relationship between the stress and the amount of plastic strain. 16

15. Deformation Work Work = Force x Displacement Work/unit volume = stress x strain It is the area under the true stress – true strain curve and it is the strain energy per unit volume required to stress a material from an unloaded state to the point of yielding. Deformation work is expressed as (15) 17

16. Using Holloman’s equation (equation14) we have (16) By integration, (17) where ’ tm is the mean true flow stress of the material. The energy per unit volume in terms of the mean flow stress is ’ tm  1 and equation (17) can now be rewritten as (18) This equation now gives the mean true flow stress as a function of the total strain  1. Equation (17) when combined with the volume of the work piece, V gives the total work required for metal deformation as

17. (19) The work calculated according to equation (17) assumes that the deformation is homogeneous throughout the deforming part. As this is not often the case, the quantity calculated from equation (19) is referred to as the ideal deformation work, W i . The ideal work of deformation always under estimates the actual work of deformation because no account is made for the work required to overcome frictional forces or redundant work required for localised internal shearing of the work piece. Practice Question For an annealed metal, the true stress-true strain relationship in a tensile test is approximated by Where k and n are positive constants.

18. Use this expression to find the true strain at the maximum load. In terms of k and n, what is the work done per unit volume in straining the metal to maximum load in a tensile test? Solution: At necking (i.e. maximum load),  = n Since at maximum load (i.e. necking), n = 

19. Practice Questions 1. A square bar is reduced in cross-section by extruding it seven times through dies of decreasing size. During each of the seven extrusion operations the reduction in the cross- sectional area is 35%. Calculate i . the total true strain applied. ii. the final length of the bar, in terms of its initial length iii. the total engineering strain applied. 2. Show that the law of volume incompressibility or volume constancy requires that  x +  y +  z = 0 where the subscripts x, y and z indicate the principal axes of an orthogonal coordinate system. 3. The relationship between true stress, σ` and true strain,  , for a sample of pure aluminium may be represented by  ` = 90  0.3 For a specimen of this material undergoing a tensile test, what is: i . the linear strain at which necking commences? ii. the tensile strength? iii. the work done per unit volume necessary to deform the specimen? 4. A metal obeys the Holloman relationship and has a UTS of 300MPa. To reach maximum, the load requires an elongation of 35%. Find k and n.

20. Metal forming Processes Forming is changing shapes of an existing solid body mechanically. Metals are extensively used due in large extent to their ability to tolerate considerable amount of permanent deformation without fracture. The products resulting from the working of metals are known as wrought products Processes used to change cast ingot into wrought forms(such as slabs, plates and billets) are called primary working processes while further working to the desired (finished or semi-finished) shapes by additional methods are known as secondary working processes . Purpose of working a metal is to produce required shape and improving the structure and properties. In the production of finished shape from wrought metal, the size of the starting material and the sequence of operations must be such that thorough working throughout the x-section of the material is given. The initial material used in forming metals is usually molten metal, cast into slabs, rods or pipes. Metal ore Extraction Metal Ingot 1  Forming Wrought Products 2  Forming

21. Cast structures are converted to wrought structures by plastic deformation processes. Raw materials used may also be powders, pressed and sintered (heated without melting) into individual products. Metal working Reduces any internal voids or cavities and make the metal denser. Impurities get elongated with the grains, get broken and dispersed throughout the metal to reduce their harmful effect and improve the mechanical strength.

22. Classifications of Metal Working Based on working temperature, metal working/forming can be classified into cold working, warm working and hot working. In terms of working temperature: Cold working is done at room temperature or 0 – 0.3T m or well below T RC Warm working is between 0.3 - 0.5T m or slightly below T RC Hot working is between 0.5 - 0.75T m or above T RC Where T RC is the material recrystallization temperature and T m melting temp. However for the course the broad classification shall be Cold Working and Hot Working COLD WORKING It is a controlled plastic deformation of metal performed at a temperature well below its recrystallization temperature (T RC ). Amount of cold work is an index of plastic deformation. Cold work is the amount of distortion resulting from the reduction of X-sectional area during deformation. (20)

23. Where A o and A f are the original and final cross sectional areas respectively. Change of Properties due to Cold Work include: Increase in strength of the metal Resistance to further deformation increases Ductility decreases (strain hardening increases) Elongation of grains in the principal direction of working Increased internal energy to make the material prone to cracking Chemical reactivity increases and the material is prone to corrosion. Excessive cold working will untimely fracture the material. Cold working is therefore done in steps with intermediate annealing to have cold work anneal cycle. With suitable cold work-anneal cycle, material can be produced with any desirable degree of strain hardening. For stronger finished product, the final operation should be cold working with proper amount of deformation. This may be followed by stress relieving annealing.

24. Advantages and Disadvantages of Cold Working Advantages: Better surface finish Close tolerance maintained Increase in hardness and strength Development of directional properties Less metal loss and tool wear from scaling. No loss due to oxidation. Desirable degree of strain hardening can be produced. Any degree of hardness can be produced by adjusting cold working and annealing. Disadvantages: High deformation forces and power requirement. Ductility is reduced at ordinary temperature Cost of production becomes high as it is required to clean the surfaces of the metal from oxides and scales before operation. There are chances of developing undesirable directional properties. Strain hardening occurs Electrical conductivity reduced Excessive CW may fracture the metal before reaching the desired shape. CW raises T RC for steel.

25. Limitations of Cold Working Metals having less ductility cannot be cold worked at room temp. e.g. carbon steels and some steel alloys. Residual stresses set up in metals may remain there and produce harmful effects on some properties. Only small sized components can be cold worked easily b/c large sections will require greater forces. The metals which can be suitably cold worked are mild steel of low carbon content, stainless steels, Al and its alloys, Cu and its alloys, Ni, Brasses, and monel metals

26. HOT WORKING When metals are worked above T RC or  0.7T m . Materials under this recrystallize and remain soft. Massive deformation are possible The upper limit of HW is the temp. at which the metal begins to melt (about 50  C < T m ) while the lower temp of HW is the temp at which the rate of recrystallization eliminates strain hardening. In HW, strain free lattice is formed as soon as deformation takes place. HW minimizes segregation and porosities Cavities are closed by pressure welding in HW.

27. Advantages and Disadvantages of Hot Working Advantages: There is less danger of cracking of the metal. Power requirement is less Intermediate annealing is eliminated. Grain refinement is possible Porosity and blowholes are eliminated. There is increased ductility, toughness, elongation and reduction in area Possibility of large deformation without fracture Economical and faster. Disadvantages: Poor surface finish Maintenance of temp. for light section is difficult. Precise control of dimensions is difficult. Surface hardening is eliminated Structure and properties are not uniform b/c the interior of piece cools slower than surface High handling cost Some metals can’t be hot worked as they are deficient in plasticity (hot shot materials) Short life of tools in HW

28. WARM WORKING Because plastic deformation properties are normally enhanced by increasing workpiece temperature, forming operations are sometimes performed at temperatures somewhat above room temperature but below the recrystallization temperature. The term warm working is applied to this second temperature range. The dividing line between cold working and warm working is often expressed in terms of the melting point for the metal. The dividing line is usually taken to be 0.3 Tm, where Tm is the melting point (absolute temperature) for the particular metal. Warm working and cold working share the same advantages and disadvantages at some different level to each other. The lower strength and strain hardening at the intermediate temperatures, as well as higher ductility, provide warm working with the following advantages over cold working: (1) lower forces and power, (2) more intricate work geometries possible, and (3) need for annealing may be reduced or eliminated.

29. RECOVERY, RECRYSTALLIZATION AND GRAIN GROWTH Plastic deformation at room temperature cause: The deformation of grains and grain boundaries General increase in strength Decrease in ductility Causes anisotroic behaviour These effects can be reversed and the properties of the metal can be brought back to their original levels by heating the metal to a specific temperature range for a period of time