Lecture 2 (1).pptx
- 1. Plastic Deformation • Most of the metal forming processes involve large plastic deformation of the work piece. • So it is important to understand the behavior of common metals and alloys when they undergo plastic deformation. • The simplest method to understand mechanical a uniaxial behavior of a metal is to carry out tension test. • Standard method: ASTM E8M
- 2. Tensile test on a round bar
- 3. Specimens before and after testing
- 4. Engineering stress strain curve
- 5. Different stages in a tensile test
- 6. Tensile test • The shape of the stress strain curve depends on composition, prior history of plastic deformation, strain rate and temperature during the test. • Important parameters used to describe the curve are: • Yield strength and Ultimate tensile strength (measures of strength) • Percentage elongation and percentage reduction in area (measures of ductility)
- 7. Strength • Yield strength: Stress required produce a small specified amount of plastic deformation. Usually measured as stress at 0.2% off set. (i.e. e = 0.002). • UTS: The maximum stress that a material can withstand in a uniaxial tension test. Given by Maximum load/Original area of cross section.
- 8. Strength • Al, Mg, Zn, Sn and Pb alloys are examples of low strength materials (UTS: 10-200 MPa). • Steels, Ni, Ti, Mo and W alloys are examples of high strength materials (UTS: 200- 2000MPa). • High YS is important for load bearing and structural parts.
- 9. Ductility • It is an indication of the ability of a material to deform plastically before fracture. It determines to what extent a material can be deformed in metal forming processes without failure. • Elongation in a tensile test is the total change in gauge length with respect to initial gauge length (expressed as %). • % elongation = (lf-lo)/lo x 100 • If cross sectional area is measured at the location of fracture after the test, then • % reduction in area, r = (A0-Af)/A0 x 100
- 10. Ductility • Total usually elongation is 10-60% for most of the common engineering materials at RT. • Brittle materials have very low elongations or zero ductility.
- 11. Comparison of different materials
- 12. Malleability
- 13. Toughness • Toughness depends on both strength and ductility.
- 14. Strain hardening • It is a phenomenon by which metals harden due to deformation at room temperature (cold working). • Movement of dislocations is one of the chief mechanisms by which plastic deformation occurs. • Grain boundaries, impurities, precipitates act as barriers or obstacles to the movement of dislocations. Dislocations get piled up at the grain boundaries.
- 15. Strain Hardening • To overcome the barriers and cause continuous plastic flow, higher energy is required. So the stress required to cause further plastic flow increases. • As a result the strength and hardness increase with increase in strain.
- 17. Specific energy or ideal plastic work (per unit volume)
- 18. Uniform and Non-uniform Elongation
- 19. Necking/instability in a tensile test Necking or localized deformation begins at maximum load when the increase in stress due to decrease in cross sectional area becomes greater than the increase in load-carrying capacity of the metal due to strain hardening.
- 20. Necking and fracture • True strain up to point of necking/instability is called true uniform strain and it is equal to strain hardening exponent. • Beyond this point, deformation outside the neck region ceases and further deformation of the neck requires decreasing load. • Diffuse necking may terminate in fracture but is often followed in sheet metals by localized necking, which takes place at an angle to the specimen axis.
- 21. Summary
- 22. Effect of temperature on flow stress
- 23. Effect of temperature on flow stress • Above a certain temperature, strain hardening is nullified by softening mechanisms like recovery and recrystallisation that occur simultaneously with deformation (dynamic recrystallization). • Flow stress decreases with temperature and above recrystallisation temperature, flow stress remains almost constant as the deformation proceeds.
- 24. Superplasticity • It is the phenomenon by which some alloys exhibit very large elongations (more than 1000%) under controlled conditions of tensile deformation. • Examples are some Al based and Ti based alloys. • Used in aerospace applications. • Often used in combination with diffusion bonding.
- 25. Optimum conditions for superplasticity • Very low strain rate (10-8 to 10-5 /sec) • Uniform and equiaxed grain structure • Very fine grain size (less than 10 microns) • Sufficiently high temperature • High value of strain rate sensitivity index (m > 0.5)
- 27. Hardness Resistance offered by a material to permanent indentation or abrasion. A measure of wear resistance. Usually measured by making a small indentation on the work piece with a harder tool (indenter).
- 28. Hardness testing Does not have units, expressed by a number. Rockwell hardness test Vickers test Brinnels test. hardness hardness
- 29. Hardness Important for: Cutting tools Abrasives for grinding and other finishing operations Tools for casting, forming and shaping Parts subjected to wear and abrasion.
- 30. Modification of Properties • Alloying • Mechanical Working • Heat Treatment
- 31. Alloying • The properties of metals can be improved by alloying additions. • Tool steel (High Speed Steel) 18%W, 4%Cr, 1% V, 0.8% C • Stainless steel 17-19%Cr, 8-10% Ni, 2%Mn, 1%Si, 0.15% C • As % Zn increases, strength and hardness of brass increase. Ductility increases and then decreases.
- 32. Mechanical Working Cold working Hot Working
- 33. Heat Treatment Subjecting the material to a controlled cycle of heating and cooling to obtain desired properties. • Annealing: Reduces strength and hardness, improves ductility and machinability. • Normalising: Hardness and strength higher than annealed material. • Hardening: Done by either oil or water quenching. Hardness and brittleness increase and toughness reduces.
- 35. Surface hardening or case hardening • Used in situations where only alteration of surface properties is desirable. • Improves surface hardness and retains the toughness of the core. • Gear teeth, fasteners, clutch bearings, plates, cutting tools, dies etc. Carburising, Nitriding, Carbonitriding, Cyaniding, Boronising