1. Part VI Joining Processes and Equipment

2. FIGURE VI.1 Various parts in a typical automobile that are assembled by the processes described in Part VI.

3. TABLE VI.1 Comparison of Various Joining Methods

4. FIGURE VI.4 Examples of joints that can be made through the various joining processes described in Chapters 30 through 32.

5. Chapter 30 Fusion Welding Processes

6. Two pieces are joined together by the application of heat Heat melts the pieces and fuses their interface The operation is sometimes assisted be a filler metal

7. TABLE 30.1 General Characteristics of Fusion-welding Processes

8. OXY-FUEL GAS WELDING A fuel gas is combined with oxygen to produce the heat The most common gas is acetylene (oxyacetylene welding) TYPES OF FLAMES flame temperature may reach 3000  C 1. Neutral flame : If the two gases are in the ratio 1:1 2. Oxidizing flame: oxygen is more (not suitable for steel) 3. Carburizing (reducing) flame: oxygen is less

9. FIGURE Three basic types of oxyacetylene flames used in oxyfuel–gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; and (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel–gas welding process.

10. WELDING TORCH It is connected by hoses to the high pressure gas cylinders The cylinders have different threads, so the hoses cannot be interchanged SAFETY EQUIPMENT Goggles with shaded lenses face shields gloves protective clothing

11. FIGURE (a) General view of, and (b) cross-section of, a torch used in oxyacetylene welding. The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light. Then the oxygen valve is opened and the flame adjusted. (c) Basic equipment used in oxyfuel–gas welding. To ensure correct connections, all threads on acetylene fittings are left handed, whereas those for oxygen are right handed. Oxygen regulators usually are painted green and acetylene regulators red.

12. FIGURE 30.4 Schematic illustration of thermit welding.

13. ARC WELDING (non consumable electrode) Heat is obtained from electricity Temperature may reach 30,000C straight polarity : workpiece is +, electrode is – used in narrow and deep welds reverse polarity: workpiece is -,electrode is +, weld zone is shallower and wider AC method :arc pulsates rapidly, suitable for welding thick sections

14. FIGURE The effect of polarity and current type on weld beads: (a) DC current with straight polarity; (b) DC current with reverse polarity; and (c) AC current.

15. ELECTRODE COATING (consumable arc welding) Electrodes are coated with clay like materials that include silicate binders and powdered materials, such as oxides, carbonates and metal alloys FUNCTIONS OF COATING 1. Stabilizes the arc 2. Controls the rate at which the arc melts 3. Acts as a flux to protect the weld against oxygen The deposited coating (slag) must be removed after each pass

16. FIGURE (a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert-gas) welding. (b) Basic equipment used in gas metal-arc welding operations.

17. LASER BEAM WELDING Uses a high-power laser beam as the source of heat Beam has high energy density and deep penetrating capability Beam can be directed, shaped and focused precisely on the work piece Produces welds with good quality, and with minimum shrinkage and distortion Laser welds have good strength and are free of porosity

18. FIGURE 30.16 Detail of Gillette Sensor razor cartridge, showing laser spot welds.

19. CUTTING Materials can be cut by mechanical means or by using a heat source The sources of heat can be torches, electric arcs or lasers

20. OXY-FUEL GAS CUTTING Heat source is used to remove a narrow zone from a metal plate or sheet The flame leaves drag lines on the cut face resulting in a rougher surface than that produced by mechanical cutting ARC CUTTING similar to arc welding produces heat affected zones

21. FIGURE (a) Flame cutting of a steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flame-cut plate, showing drag lines.

22. THE WELD JOINT Three zones are clear 1. base metal 2
THE WELD JOINT Three zones are clear 1. base metal 2. heat affected zone 3. weld metal

23. FIGURE 30.19 Characteristics of a typical fusion-weld zone in oxyfuel–gas and arc welding.

24. weld metal dendrites are formed parallel to the heat flow, in deep welds grains lie parallel to the plane of the two components being welded, grains in shallow welds are perpendicular heat affected zone is within the base metal itself has a different microstructure from that of the base metal prior to its welding its properties depend upon rate of heat input and cooling and the temperature attained

25. FIGURE Grain structure in (a) a deep weld and (b) a shallow weld; note that the grains in the solidified weld metal are perpendicular to their interface with the base metal. (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead.

26. WELD QUALITY a welded joint may develop several discontinuities due to 1. thermal cycling 2. inadequate application of welding technologies 3. poor training of operator

27. MAJOR DISCONTINUITIES THAT AFFECT WELD PROPERTIES 1. POROSITY 2
MAJOR DISCONTINUITIES THAT AFFECT WELD PROPERTIES 1. POROSITY 2. SLAG INCLUSIONS 3. INCOMPLETE FUSION AND PENETRATION 4. WELD PROFILE 5. CRACKS 6. LAMELLAR TEARS 7. SURFACE DAMAGE

28. POROSITY It is caused by: 1. Gases 2. chemical reactions 3
POROSITY It is caused by: 1. Gases 2. chemical reactions 3. Contaminants METHODS OF REDUCING POROSITY IN WELDS 1. proper selection of electrodes and filler metals 2. Improved welding techniques 3. Proper cleaning 4. Prevention of contaminants from entering the weld zone 5. reduced welding speed to allow time for the gas to escape

29. SLAG INCLUSIONS Are compounds such as oxides, fluxes and electrode coating materials that are trapped in the weld zone METHODS OF PREVENTING SLAG INCLUSIONS 1. Cleaning the weld bead surface by a wire brush before the second layer is deposited 2. Providing sufficient shielding gas 3. Redesigning the joint to facilitate manipulation of the molten weld metal

30. INCOMPLETE FUSION It produces poor welds Methods OF OBTAINING BETTER WELDS 1. Raising the temperature of the base metal 2. Cleaning the weld area prior to welding 3. Modifying the joint design 4. Changing the type of electrode used 5. Providing sufficient shielding gas

31. FIGURE 30.21 Examples of various discontinuities in fusion welds.

32. INCOMPLETE PENETRATION It occurs when the depth of the welded joint is insufficient METHODS OF IMPROVING PENETRATION 1. Increasing the heat input 2. Reducing the travel speed during welding 3. Modifying the joint design 4. Ensuring that the surfaces to be joined fit each other properly

33. WELD PROFILE It can indicate incomplete fusion or the presence of slag inclusions in multiple layer welds -underfilling results -undercutting results - overlap

34. FIGURE 30.22 Examples of various defects in fusion welds and the cross-section of a good weld.

35. CRACKS result from temperature gradients causing thermal stresses in weld zone, and the variation in composition of the weld zone causing different rates of contraction during cooling Types of cracks 1. Longitudinal crack 2. Transverse crack 3. Crater crack 4. Underbead crack 5. Toe crack

36. FIGURE Types of cracks developed in welded joints; the cracks are caused by thermal stresses, similar to the development of hot tears in castings, as shown in Fig

37. FIGURE Crack in a weld bead; the two welded components did not contract freely after the weld was completed.

38. RESIDUAL STRESSES Due to localized heating and cooling, contraction and expansion of weld area causes residual stresses DEFECTS CAUSED BY RESIDUAL STRESSES 1. Distortion, warping and buckling of welded part 2. Stress corrosion cracking 3. Reduced fatigue life 4. Further distortion if a portion of the welded structure is removed by machining

39. FIGURE Distortion of parts after welding; distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly.

40. FIGURE Residual stresses developed in a straight-butt joint; note that the residual stresses shown must be balanced internally since there are no external forces. (See also Fig )

41. FIGURE 30. 27 Distortion of a welded structure. Source: After J. A
FIGURE Distortion of a welded structure. Source: After J.A. Schey.

42. WELDABILITY 0F METALS Capability of a metal to e welded
WELDABILITY 0F METALS Capability of a metal to e welded. It is affected by: Mechanical properties 2. Physical properties 3. Shielding gas 4. fluxes 5. Welding speed 6. cooling rate

43. JOINT DESIGN
testing techniques

44. FIGURE 30.29 Examples of welded joints and their terminology.

45. PROCESS SELECTION CONSIDERATIONS 1. Types of materials used 2
PROCESS SELECTION CONSIDERATIONS 1. Types of materials used 2. Costs of equipment 3. Effects of distortion and discoloration 4. Costs involved in edge preparation, joining, machining and finishing operations 5. Ease of joining 6. Type of loading
testing techniques

46. FIGURE 30. 31 Some design guidelines for welds. Source: Bralla, J. G
FIGURE Some design guidelines for welds. Source: Bralla, J.G., Design for Manufacturability Handbook, 2nd ed., McGraw-Hill, 1999, ISBN No X.