Lecture 7 Mechanical Properties Of Metals

1. EMT282 Principles of Engineering Materials Lecture 7Mechanical PropertiesOf Metals Dr. Rozana Aina Maulat Osman Microelectronic Engineering Block 9: Room 00-01-0A 04-9885532 6-1

2. OutlineMechanical Properties of Metal • Metal Processing • Casting • Hot & Cold Rolling • Extrusion • Forging • Drawing • Stress & Strain • Tensile Test • Hardness & Testing

3. Casting • Most metals are first melted in a furnace. • Alloying is done if required. • Large ingots are then cast. • Sheets and plates are then produced from ingots by rolling Wrought alloy products. • Channels and other shapes are produced by extrusion. • Some small parts can be cast as final product. Example :- Automobile Piston. 6-2

4. Cast Alluminum Ingot

5. Casting Casting mold Cast parts Casting Process 6-3

6. VIDEO • Slingshot Green Sand Casting Aluminium • http://www.youtube.com/watch?v=CO7QYGdshT4

7. Hot Rolling of Steel • Hot rolling Greater reduction of thickness in a single pass. • Rolling carried out at above recrystallization temperature. • Ingots preheated to about 12000C. • Ingots reheated between passes if required. • Usually, series of 4 high rolling mills are used. 6-4

8. Cold Rolling of Metal Sheet • Cold rolling is rolling performed below recrystallization temperature. • This results in strain hardening. • Hot rolled slabs have to be annealed before cold rolling. • Series of 4 high rolling mills are usually used. • Less reduction of thickness. • Needs high power. 6-5

9. Cold Rolling (Cont..) Initial metal thickness – Final metal thickness x 100 % Cold work = Initial metal thickness 6-6

10. Calculate the percent cold reduction after cold rolling 0.040-in-thick aluminum sheet to 0.025 in. Solution:

11. A 70% Cu-30% Zn brass sheet is 0.0955 cm thick and is cold-rolled with a 30 percent reduction in thickness. What must be the final thickness of the sheet? Solution:

12. Video • Hot • http://www.youtube.com/watch?v=ibhWN_qReuo • Cold • http://www.youtube.com/watch?v=3H3QhbQtsLo

13. Extrusion Die Container • Metal under high pressure is forced through opening in a die. • Common Products are cylindrical bar, hollow tubes from copper, aluminum etc. • Normally done at high temperature. • Indirect extrusion needs less power however there is limit on load applied. Metal Direct Extrusion Container Metal indirect Extrusion Figure 5.9 6-7

14. Video • Indirect • Seamless extrusion of Aluminium Tubes (indirect) • http://www.youtube.com/watch?v=OsdZ6cj3y_g

15. Forging • Metal, usually hot, is hammered or pressed into desired shape. • Types:- • Open die: Dies are flat and simple in shape * Example products: Steel shafts • Closed die: Dies have upper and lower impression * Example products: Automobile engine connection rod. • Forging increases structural properties, removes porosity and increases homogeneity. Direct Forging Metal Indirect Forging Dies 6-8

16. Drawing • Wire drawing :- Starting rod or wire is drawn through several drawing dies to reduce diameter. • Deep drawing:- Used to shape cup like articles from flats and sheets of metals Change in cross-sectional area % cold work = X 100 Original area Wire or rod Carbide nib 6-9

17. Stress and Strain in Metals • Metals undergo deformation under uniaxial tensile force. • Elastic deformation:Metal returns to its original dimension after tensile force is removed. • Plastic deformation:The metal is deformed to such an extent such that it cannot return to its original dimension 6-10

18. Engineering Stress and Strain F (Average uniaxial tensile force) Engineering stress = σ = A0 (Original cross-sectional area) Engineering stressσ = F Units of Stress are PSI or N/M2 (Pascals) Δl A0 1 PSI = 6.89 x 103 Pa Change in length Engineering strain = ε = Original length A Units of strain are in/in or m/m. F 6-11

19. Calculate the engineering stress in SI units on a 2.00-cm-diameter rod that is subjected to a load of 1300 kg.

20. Poisons Ratio Poisons ratio = . w0 w Usually poisons ratio ranges from 0.25 to 0.4. Example: Stainless steel 0.28 Copper 0.33 6-12

21. Shear Stress and Shear Strain S (Shear force) A (Area of shear force application) τ = Shear stress = Amount of shear displacement Shear strainγ = Distance ‘h’ over which shear acts. Elastic Modulus G = τ / γ 6-13

22. Mechanical Properties Obtained from Tensile Test • Modulus of elasticity • Yield strength at 0.2 percent offset • Ultimate Tensile Strength • Percent elongation at fracture • Percent reduction in area at fracture Ductility

23. Tensile test • Strength of materials can be tested by pulling the metal to failure. Load Cell Specimen Extensometer Force data is obtained from Load cell Strain data is obtained from Extensometer. 6-14

24. Tensile Test (Cont) Commonly used Test specimen Typical Stress-strain curve 6-15

25. Mechanical Properties • Modulus of elasticity (E) : Stress and strain are linearly related in elastic region. (Hooks law) • Higher the bonding strength, higher is the modulus of elasticity. • Examples: Modulus of Elasticity of steel is 207 Gpa. Modulus of elasticity of Aluminum is 76 Gpa σ (Stress) Δσ E = Strain E = Δε Δσ ε (Strain) Δε Stress Linear portion of the stress strain curve 6-17

26. Yield Strength • Yield strength is strength at which metal or alloy show significant amount of plastic deformation. • 0.2% offset yield strength is that strength at which 0.2% plastic deformation takes place. • Construction line, starting at 0.2% strain and parallel to elastic region is drawn to fiend 0.2% offset yield strength. Figure 5.23 6-18

27. Ultimate tensile strength • Ultimate tensile strength (UTS) is the maximum strength reached by the engineering stress strain curve. • Necking starts after UTS is reached. • More ductile the metal is, more is the necking before failure. • Stress increases till failure. Drop in stress strain curve is due to stress calculation based on original area. Al 2024-Tempered S T R E S S Mpa Necking Point Al 2024-Annealed Strain Stress strain curves of Al 2024 With two different heat treatments. Ductile annealed sample necks more 6-19

28. Tensile Strength Figure 7.11 Typical engineering stress-strain behavior to fracture, point F. The tensile strength is indicated at point M. • Very familiar property and widely used for identification of a material. It is used for the purposes of specifications and for quality control of a product.

29. Ductility • It is a measure of the degree of plastic deformation that has been sustained at fracture. • A material that experiences very little or no plastic deformation upon fracture is termed brittle. Figure 7.13 Schematic representations of tensile stress-strain behavior for brittle and ductile materials loaded to fracture.

30. Percent Elongation • Percent elongation is a measure of ductility of a material. • It is the elongation of the metal before fracture expressed as percentage of original length. % Elongation = • Measured using a caliper fitting the fractured metal together. • Example:- Percent elongation of pure aluminum is 35% For 7076-T6 aluminum alloy it is 11% Final length – initial Length Initial Length 6-20

31. Percent Reduction in Area • Percent reduction area is also a measure of ductility. • The diameter of fractured end of specimen is meas- ured using caliper. • Percent reduction in area in metals decreases in case of presence of porosity. % Reduction Area Initial area – Final area = Final area Stress-strain curves of different metals 6-21

32. True Stress – True Strain • True stress and true strain are based upon instantaneous cross-sectional area and length. • True Stress = σt = • True Strain = εt = F Ai (instantaneous area) 6-22

33. Hardness and Hardness Testing • Hardness is a measure of the resistance of a metal to permanent (plastic) deformation. • General procedure: Press the indenter that is harder than the metal Into metal surface. Withdraw the indenter Measure hardness by measuring depth or width of indentation. Rockwell hardness tester Figure 5.27 6-27

34. Hardness Tests Table 5.2 6-28

35. Example 1: Tensile Testing of Aluminum Alloy Convert the change in length data in Table 6-1 to engineering stress and strain and plot a stress-strain curve.

36. Example 1: SOLUTION

37. Example 2 : Young’s Modulus of Aluminum Alloy From the data in Example 1, calculate the modulus of elasticity of the aluminum alloy. Use the modulus to determine the length after deformation of a bar of initial length of 50 in. Assume that a level of stress of 30,000 psi is applied. Example :SOLUTION

38. Example 3 : Ductility of an Aluminum Alloy The aluminum alloy in Example 1 has a final length after failure of 2.195 in. and a final diameter of 0.398 in. at the fractured surface. Calculate the ductility of this alloy. Example 3 :SOLUTION

39. Example 4: True Stress and True Strain Calculation Compare engineering stress and strain with true stress and strain for the aluminum alloy in Example 6.1 at (a) the maximum load and (b) fracture. The diameter at maximum load is 0.497 in. and at fracture is 0.398 in. Example 4: SOLUTION

40. Example 4 : SOLUTION (Continued)

41. Example Problem 5 A cylindrical specimen of steel having an original diameter of 12.8 mm is tensile tested to fracture and found to have an engineering fracture strength (σf) of 460 MPa. If its cross-sectional diameter at fracture is 10.7 mm, determine: (a) The ductility in terms of percent reduction in area (b) The true stress at fracture