Forming and Shaping

1. Forming and Shaping ISAT 430 Module 7

2. Forming and Shaping • Meanings blend • Forming means changing the shape of an existing solid body. • Shaping usually involves molding or casting • The resulting product is usually near net shape. ISAT 430 Dr. Ken Lewis

3. Forming and Shaping Processes • Rolling -- Flat • Production of flat plate, sheet and foil • Long lengths, high speeds • Good surface finish • Requires • High capital investment • Incurs low to moderate labor cost. ISAT 430 Dr. Ken Lewis

4. Forming and Shaping Processes2 • Rolling – Shaped • Production of various structural shapes • Bars • I-beams • High speeds • Requires • shaped rolls, expensive equipment • Low to moderate labor costs • Moderate operator skill ISAT 430 Dr. Ken Lewis

5. Forming and Shaping Processes3 • Forging • Production of discrete parts with a set of dies. • Some finishing operations usually necessary • Usually performed at elevated temperatures • Die and equipment costs are high • Requires • Moderate to high labor cost • Moderate to high operator skill • Similar parts can be made by casting and powder-metallurgy techniques ISAT 430 Dr. Ken Lewis

6. Forming and Shaping Processes4 • Extrusion • Production of long lengths of solid or hollow products with constant cross section • Product is cut into desired lengths • Cold extrusion has similarities to forging and is used to make discrete products. • Requires • Moderate to high die and equipment costs • Low to moderate labor costs • Low to moderate labor skill ISAT 430 Dr. Ken Lewis

7. Forming and Shaping Processes5 • Drawing • Production of long rod and wire with round or various cross sections. • Smaller cross section than extrusion • Good surface finish • Requires • Low to Moderate die and equipment costs • Low to moderate labor costs • Low to moderate labor skill ISAT 430 Dr. Ken Lewis

8. Forming and Shaping Processes6 • Sheet metal forming • Production of a wide variety of shapes with thin walls • Simple or complex geometries • Requires • Moderate to high die and equipment costs • Low to moderate labor costs • Low to moderate labor skill ISAT 430 Dr. Ken Lewis

9. Forming and Shaping Processes7 • Powder Metallurgy • Production of simple or complex shapes by compacting and sintering metal powders • Competitive with casting, forging, and machining processes • Requires • Moderate to high die and equipment costs • Low labor costs • Low labor skill ISAT 430 Dr. Ken Lewis

10. Forming and Shaping Processes8 • Processing of plastics and composite materials • Production of a variety of continuous or discrete products • Extrusion, spinning, molding, casting • Can be competitive with metal parts • Requires • Moderate to high die and equipment costs • High operator skill in composite fabrication ISAT 430 Dr. Ken Lewis

11. Forming and Shaping Processes9 • Forming and shaping of ceramics • Production of discrete ceramic products by a variety of ways • Shaping, drying , firing processes • Requires • Moderate to high die and equipment costs • Low to moderate labor costs • Moderate to high labor skill ISAT 430 Dr. Ken Lewis

12. Rolling • Rolling is a process to reduce the thickness of a long workpiece by compressive forces applied through a set of rolls. • First developed in the late 1500’s ISAT 430 Dr. Ken Lewis

13. Sequence of events • A steel ingot is cast into a rectangular mold • Placed in a furnace while just solidified and held for many hours (36) until the temperature is uniform. • This process is called soaking • Furnaces are called soaking pits. • Implies that properties will be uniform throughout the ingot and process that way. • The rolling temperature for steel is about 1200°C • From here the ingot goes to the rolling mill. ISAT 430 Dr. Ken Lewis

14. Rolling • Starting material depends upon what you are producing. • Bloom • Square cross section 6 x 6 in or larger • Slab • Rolled from an ingot or a bloom • Rectangular cross section 10 x 1.5 in or more • Billet • Rolled from a bloom • Square cross section 1.5 x 1.5 in or larger. ISAT 430 Dr. Ken Lewis

15. ISAT 430 Dr. Ken Lewis

16. ISAT 430 Dr. Ken Lewis

17. Metal behavior in forming An aside

18. Stress -- strain • Elasticity below the elastic limit • Strain hardening above it. • In the plastic region, the metal’s behavior is expressed as: Where K = strength coefficient psi (MPa) n is the strain hardening exponent. ISAT 430 Dr. Ken Lewis

19. True Stress Shear Rate Flow Stress • As the metal deforms its strength increases (strain hardening) • Thus the stress required to deform must be increased • Flow stress • Instantaneous value of the stress needed to keep the metal “flowing” Yf = flow stress MPa ISAT 430 Dr. Ken Lewis

20. True Stress Shear Rate Average Flow Stress • The average flow stress is the average stress needed over entire strain region. • Just integrate the flow stress over the strain region of interest: ISAT 430 Dr. Ken Lewis

21. Effect of Strain Rate • In theory, a metal in hot working should be perfectly plastic with n = 0. • The rate of metal deformation is directly related to the speed of deformation v. v is the velocity of the roll or ram h is the instantaneous height of the piece being deformed. ISAT 430 Dr. Ken Lewis

22. Effect of Strain Rate • Note that if v is constant, the strain rate will increase with decreasing h. ISAT 430 Dr. Ken Lewis

23. Effect of strain rate • Similar • C is strength constant • m is the slope, called the strain rate sensitivity exponent. • The effect of temperature is pronounced. ISAT 430 Dr. Ken Lewis

24. Effect of temperature on stress ISAT 430 Dr. Ken Lewis

25. Temperature • Cold working (~room temperature) • Advantages • accuracy • Surface finish • Strain hardening increases strength • Grain flow can provide directional properties • No heating required ISAT 430 Dr. Ken Lewis

26. Temperature • Cold working (~room temperature) • Disadvantages • Higher forces and power needed • Part must be dirt and scale free (stress risers) • Ductility and strain hardening limit the amount forming that can be done without part fracture or cracking. ISAT 430 Dr. Ken Lewis

27. Temperature Tm = melting T. • Warm Working (0.3Tm – 0.5Tm) • Working above room temperature but below recrystallization temperatures. • Advantages • Low forces and power • More intricate work geometries possible • Need for annealing may by reduced ISAT 430 Dr. Ken Lewis

28. Temperature Tm = melting T. • Hot working (0.5Tm – 0.75Tm) • The recrystallization temperature is about one half the melting point. • So hot working is above these temperature • Disadvantages • Deformation process causes localized heating which can cause localized melting (bad Ju Ju) • Scale formation increases as the temperature increases. • Lower dimensional accuracy • Poorer surface finish • Shorter tool life. ISAT 430 Dr. Ken Lewis

29. Temperature Tm = melting T. • Hot working (0.5Tm – 0.75Tm) • Advantages • Can produce SIGNIFICANT PLASTIC DEFORMATION of the metal. • Lower forces and power • Brittle metals can be hot worked. • Strength properties are usually isotropic • No work hardening ISAT 430 Dr. Ken Lewis

30. Back to Flat Rolling

31. Flat Rolling • A strip of thickness h0 enters the roll gap and leaves at a thickness of hf. • The initial velocity V0 increases to Vf at the exit. • Note that because the surface speed of the roll is constant, there must be relative sliding between the roll and the strip ISAT 430 Dr. Ken Lewis

32. Flat Rolling • At one point (no slip point), Vstrip = Vmill. • To the left, the roll moves faster than the strip • To the right the roll moves slower than the strip • Friction is necessary • Too much ruins the surface and costs power • Too little and nothing happens. ISAT 430 Dr. Ken Lewis

33. Flat Rolling • The draft is (h0 –hf) • The maximum draft is a function of the coefficient of friction and the big roll radius R • Higher friction and bigger roll, the greater draft. • Compare • Large tires and rough treads on tractors and off road vehicles. ISAT 430 Dr. Ken Lewis

34. Flat Rolling • The roll force F is shown as perpendicular to the strip (rather than perpendicular to the point of contact) • Because R >>> h • The roll force may be estimated as: ISAT 430 Dr. Ken Lewis

35. Flat Rolling L = roll strip contact length w = strip width Yavg = average true stress on the strip in the roll gap ISAT 430 Dr. Ken Lewis

36. Flat Rolling • Equation assumes no friction • The higher , the further off the formula (low side). • The power per roll can be estimated by assuming F acts in the middle of the arc of contact • The torque/roll is F x a so in S. I. Units (Newton, meters, seconds) the power per roll is: F is in Newtons L is in meters N is rpm ISAT 430 Dr. Ken Lewis

37. Flat Rolling • Equation assumes no friction • The higher , the further off the formula (low side). • The power per roll can be estimated by assuming F acts in the middle of the arc of contact • The torque/roll is F x a so in English Units (Pounds, feet, seconds) the power per roll is: F is in Pounds force L is in feet N is rpm ISAT 430 Dr. Ken Lewis

38. Example: An annealed copper strip, 9 in (228 mm) wide, and 1 inch (25 mm) thick is rolled to a thickness of 0.8 in (20 mm) in one pass. The roll radius is 12 in (300 mm), and the rolls rotate at 100 rpm. What is the roll force and power required? From Table 3.4 pg 51 Groover, K = 300 MPa, n = .5 ISAT 430 Dr. Ken Lewis

39. Example: An annealed copper strip, 9 in (228 mm) wide, and 1 inch (25 mm) thick is rolled to a thickness of 0.8 in (20 mm) in one pass. The roll radius is 12 in (300 mm), and the rolls rotate at 100 rpm. What is the roll force and power required? But, there are two rolls so the power is: ISAT 430 Dr. Ken Lewis

40. Effect of rolling on Structure • This is a typical ingot • The outer edges have small grains • Faster cooling • Note the large interior grains. • Slow cooling • Plenty of time to grow. ISAT 430 Dr. Ken Lewis

41. Effect of rolling on Structure • The normal forces elongate the grains • With enough energy new smaller grains grow • Structure becomes much more uniform • Better strength and ductility ISAT 430 Dr. Ken Lewis

42. Shape Rolling • Flat rolling is just the start. • Straight and long structural shapes • Bars • Channels • I – beams • The material cross section is reduced non-uniformly • The sequence and type of rolls is quite complex. ISAT 430 Dr. Ken Lewis

43. Shape rolling of an H– section part. ISAT 430 Dr. Ken Lewis

44. Ring Rolling • Ring is placed between two rolls, of which one is driven • Volume of the ring is constant to the diameter increases during the process • Ring blanks • Cut from a plate • Cutting a thick walled pipe. ISAT 430 Dr. Ken Lewis

45. Ring Rolling -- shapes • Shapes can be quite complex. • Uses • Large rings for rockets and turbines • Gearwheel rims • Ball bearing races • Flanges. ISAT 430 Dr. Ken Lewis

46. Thread Rolling • Straight or tapered threads put in round rods. • Most bolts and screws are made this way. • Production rates of up to 80 pieces per second are possible ISAT 430 Dr. Ken Lewis

47. Thread Rolling • No loss in material • Good strength (cold working) • Surface finish is very good • Process induces residual compressive stresses on surface which improves fatigue life. ISAT 430 Dr. Ken Lewis

48. Thread Rolling ISAT 430 Dr. Ken Lewis

49. Thread properties • Machining cuts through the grains • Rolling compresses them ISAT 430 Dr. Ken Lewis