DT1410 - Materials and Processes in Design

1. DT1410 - Materials and Processes in Design Unit 4 – Processing of metals jeopardylabs.com/play/unit-4663

2. Hot Working of Metals • Hot Working: • Conventional wisdom says that “hot working” means that the metal is deformed or worked at an elevated temperature … however, technically “hot working” means that: • The metal is worked at a high enough temperature that no plastic deformation, strain hardening, or “cold working” takes place. • “Cold Working” discussed in Chapter 10

3. Hot Working of Metals • Hot Working – Formal Definition: • A process of forming metals while they are heated above the recrystallization or transformation temperatures.

4. Hot Working of Metals • Hot Working: • One way to check for hot working is to measure the hardness or strength of the metal before and after the working process; if no strengthening occurs, it is hot working • Hot working occurs such that before the metal cools, the temperature is high enough that is anneals the metal and removes the cold work that otherwise would have occurred

5. Hot Working of Metals • Hot Working Processes: • Hot Rolling – Forging • Swaging – Extrusion • Drawing – Spinning • Piercing – Welded pipe

6. Hot Working of Metals • Purpose of Hot Working • During the ingot casting process (molten steel being formed into useful shapes) steel cools from its surfaces toward the center • This forms large, columnar, dendritic grains • That is, the grains grow parallel to the direction of heat flow • These large grains are usually lower in strength and higher in elongation than is desired • Shrinkage voids also are formed which significantly reduce strength

7. Hot Working/Heat Treating http://www.engineeringmotion.com/videos/306/heat-treating

8. Hot Working of Metals • Purpose of Hot Working • These undesirable grains and internal shrinkage voids can be removed or altered by the processes of Hot Working: • Hot Rolling • Drawing • Extruding • Forging

9. Rolling Mill Intermediate Forms • Ingots are sufficiently plastic to be taken to a hot rolling mill to be shaped into one of the intermediate forms called: • Blooms • 6” x 6” square semi-finished steels • Slabs • 10” wide and 1.5” or more thick • Billets • 1.5” by 1.5” squares

10. Rolling Mill Intermediate Forms • Blooms (and often ingots) are further processed in structural rolling mills into other structural shapes: • Railroad rails • I-Beams • Angles • Wide-flange Beams • Zees • Tees

11. Steel Rolling http://www.engineeringmotion.com/videos/397/steel-manufacturing-rolling

12. Recrystallization • Steel ingot, with its typical grain imperfections and voids, is unsatisfactory for applications where high strength is required. • Parts made from such steel would easily fail under impact loads • The “columnar” grains in a cast ingot must go through recrystallization to give steel the required strength.

13. Recrystallization • A process in which the distorted grain structure of metals that are subjected to mechanical deformation is replaced by a new strain-free grain structure during annealing.

14. Anisotropy • A condition created by the rolling process that gives a material specific physical properties in different directions. • Rolled steel is strongest in the direction of rolling.

15. Forging

16. Forging Processes • Heating metals to forging temperatures greatly increases their plasticity and workability. • Because of this, hot forging was and still is a very useful method of forming steel articles, both large and small.

17. Forging Processes • Plasticity: • The quality of material such that it can be deformed without breaking. • Clay is a completely plastic material. • Metals exhibit plasticity in varying amounts.

18. Forging Processes • Forging: • A method of metalworking in which the metal is hammered into the desired shape, or is forced into a mold by pressure or hammering, usually after being heated to a plastic state. • Hot forging requires less force to form a given part than does cold forging, which is usually done at room temperature.

19. Forging Machines • Open-die • Impression-die drop • Press • Swaging • Upset • See pages 178 – 188 in Bruce TB

20. Open Die Forging http://www.engineeringmotion.com/videos/60/forging-open-die

21. Hot Extrusion • Extrusion: • A process of forcing metal through a die, similar to squeezing toothpaste out of a tube. • Usually under high pressure and at elevated temperatures, forcing materials through a die containing the shape desired so that a shaped product is produced. • Metal and polymer shapes are produced using this method.

22. Hot Extrusion • Extrusion: • Stronger non-ferrous metals (aluminum, copper, brass, magnesium) and ferrous metals usually need to be at a forging temperature to make them plastic enough for extrusion. • Square and round tubular products, structural shapes (tees, zees, angles) and round, square, or hexagonal solid shapes are common extruded pieces

23. Hot Extrusion • Advantages of Extrusion: • Allows for the manufacture of intricate shapes • Example: • Inexpensive bronze gears can be extruded into 20 ft or 40 ft lengths and later cut off to the desired widths. • For short runs of product, extrusion is more economical due to lower tooling costs

24. Hot Extrusion • Two basic types of Extrusion: • Direct Extrusion • Metal placed in a container • A Ram is pushed • The work billet is forced to flow through one or more openings in a die.

25. Hot Extrusion • Two basic types of Extrusion: • Indirect Extrusion • A die is mounted into a ram • As the ram is pushed against the work billet material flows through the die opening and through the ram

26. Extrusion http://www.engineeringmotion.com/videos/415/metal-extrusion-manufacturing

27. Unit 4 – Manufacturing Processes --Metals Chapter 10 Cold Working of Metals

28. Cold Working of Metals • Historically, cold working was accomplished by hammering on soft metals (gold, silver, copper, etc) for jewelry and other ornamentation. • Today, cold-formed products range from very fine hypodermic needles to huge pipeline tubes and from tiny hair-sized filaments to propeller shafts for ships. • Almost any conceivable shape can be made from cold-forming processes.

29. Cold Working of Metals • Cold Working: • Deforming a metal plastically at a temperature below its lowest recrystallization temperature. • “Cold” working usually occurs at room temperature • Strain hardening occurs as a result of this permanent deformation.

30. Cold Working of Metals • Common methods of Cold Working: • Cold rolling – Blanking • Pressing – Drawing • Forming – Extruding • Bending – Straightening • Roll forming – Spinning

31. Cold Working of Metals • Advantages of Cold Working: • Better surface finish • Closer dimensional tolerances • Better machinability • Superior mechanical properties • Better strength-to-weight ratios • Enhanced directional properties

32. Cold Working of Metals Disadvantages of Cold Working: • Metal is more brittle • Metal is less workable • Annealing is required to continue the process • The metal may contain residual stresses that can cause warping or distortion • Subsequent heating (welding) will change the cold working structure and reduce its strength

33. Factors in Cold Working Properties that enable metals to be “Cold Worked” • Plasticity: • The ability of metals to be deformed permanently in any direction without cracking or splitting. • The higher temperatures used in hot working make metals more plastic, but recrystallization at high temperatures prevents them from increasing in strength. • To achieve higher strengths and hardnesses, the metal must be deformed at normal temperatures (below the recrystallization temperature).

34. Factors in Cold Working • When metals are cold worked to a certain point, the next operation requires forces greater than those previously applied to deform the metal further. • Each operation brings the particular metal closer to its ultimate strength and point of rupture. • As the amount of cold work increases the yield strength it gets closer to the maximum stress or tensile strength, and the strain at failure decreases. • The objective in cold working, therefore, is to stop well short of failure.

35. Factors in Cold Working • The degree of deformation (amount of cold working) determines the level of toughness, strength, hardness, and remaining ductility. • If more deformation is needed, then a process anneal is used to restore plasticity. • Process anneal on the cold-worked steel is often carried out in a closed container of inert gas to avoid scaling problems – a technique called a bright anneal.

36. Factors in Cold Working Properties that enable metals to be “Cold Worked” • Ductility and Malleability: • Ductility – the property of a metal that allows it to deform permanently or to exhibit plasticity without rupture while undertension. • Steel, aluminum, gold, silver, nickel • Malleability – the ability of a metal to deform permanently when loaded in compression. • Metals that can be rolled into sheets or upset cold forged are malleable. • Most ductile metals are malleable, but some malleable metals are not very ductile (lead)

37. Factors in Cold Working Properties that enable metals to be “Cold Worked” • Ductility and Malleability: • Some metals and alloys such as stainless steel, high-manganese steel, titanium, and zirconium tend to work harden, that is, to quickly increase hardness as cold working progresses. • Work hardening is often a troublesome difficulty in machining operations.

38. Factors in Cold Working Properties that enable metals to be “Cold Worked” • Elastic Behavior: • When a metal is placed under stress within its elastic range it will return to its former shape when the load is removed. • If the metal takes on a permanent set by loading it beyond the elastic limit, it will be permanently deformed, but will “bounce back” to some extent because of its elastic properties. • This is known as “springback” or elastic recovery.

39. Factors in Cold Working Properties that enable metals to be “Cold Worked” Elastic Behavior: • The tendency for a metal part to return (or relax) somewhat from the form to which it has been bent or deformed when the forces causing the change in the form are removed.

40. Factors in Cold Working Elastic Properties of metals can also cause problems: • The vast majority of metals are “polycrystalline” – they consist of many crystals or grains that were initially formed when the metal solidified. • Cold working tends to stretch out the grain in the direction of working, yet the grains still may have atom lattices that are not aligned. • Residual stresses may be left behind after cold working.

41. Factors in Cold Working Elastic Properties of metals can also cause problems: • Residual stresses: • Stresses induced within the structure of a metal by cold working, machining, and heat treatments and remaining in the metal after the treatment is completed. • These stresses are “alive” and active within the metal. • They can cause a correctly bored hole to be oblong or a straight piece of bar to “banana” when machined on one side.

42. Factors in Cold Working Residual stresses: • The solution to residual stresses are to use a stress relief or recovery anneal Stress Relief Anneal: • The reduction of residual stress in a metal part by heating it to a given temperature and holding it there for a suitable length of time. This treatment is used to relieve elastic stresses caused by welding, cold working, machining, casting, and quenching.

43. Cold Rolling in the Steel Mill • Hot-rolled steel bars and plates must be sufficiently over-size because the cold-finishing process reduces them in cross section. • The hot-rolled bars or sheets are placed in a hydrochloric or sulfuric acid dip that removes the scale – a process called pickling. • Cold rolling then flattens and lengthens the grains in the direction of rolling.

44. Blanking and Pressing • One of the most versatile forms of metal working is that of converting flat sheet and strip metals into useful articles. • Sheet metal can be: • Pierced • Punched • Blanked

45. Blanking and Pressing • Piercing: • Cutting (usually small) holes in sheet metal. • Punching: • The operation of cutting a hole in sheet metal using a die. The hole material is scrapped. • Blanking: • The operation of cutting a shape with a die from sheet metal stock. The hole material is saved and used for further operations.

46. Drawing, Forming, Extruding Metal • Stamping Operations: • Coining and Embossing are stamping operations that form the surface of metal. • Impressions of letters, figures, and patterns are formed by pressing them onto the metal

47. Drawing, Forming, Extruding Metal • Stamping Operations: • Coining: • Shaping a piece of metal in a mold or die, often creating raised figures or numbers. • Embossing: • The raising of a pattern in relief on a metal by means of a high pressure on a die plate.

48. Progressive Metal Stamping http://www.engineeringmotion.com/videos/53/progressive-metal-stamping

49. Drawing, Forming, Extruding Metal • Bar, Tube, and Wire Drawing: • Bar Drawing: Figure 10.25 (p. 205) • Performed on a draw bench. • The drawing process hardens the metal and gives it a smooth finish.

50. Drawing, Forming, Extruding Metal • Bar, Tube, and Wire Drawing: • Seamless Tubing: Figure 10.26 (p. 205) • Also cold drawn on a draw bench • This process includes a mandrel inside the tube to thin the walls and provide internal finish.