AWS WHB-3 9-2007 Welding Handbook Vol 3 Welding Processes Part 2 (Ninth Edition)《焊接手册 第3卷 焊接工艺 第2部分 第9版》.pdf

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1、RESISTANCE SPOT AND SEAM WELDING Prepared by the Welding Handbook Chapter Committee on Resistance Spot and Seam Welding: J. W. Dolfi, Chair Ford Motor Company (Ret.) J. E. Gould Edison Welding Institute M. J. Karagoulis General Motors Corporation B. G. Kelly Kelly Welding Solutions, P. C . C. J. Ors

2、ette RoMan Engineering Services W. Urech Soudronic Automotive Welding Handbook Committee Member: S. P. Moran Miller Electric Manufacturing Company Contents Introduction 2 Fundamentals 2 Equipment 11 Surface Preparation 14 Resistance Spot Welding 15 Resistance Seam Welding 26 Metal Properties and Wel

3、dability 39 Economics 43 Safe Practices 43 Conclusion 45 Bibliography 45 Supplementary Reading List 46 CHAPTER 9 CHAPTER 1 Photograph courtesy of Soudronic Automotive AG AWS WELDING HANDBOOK 9.3 12 AWS WELDING HANDBOOK 9.3 INTRODUCTION Spot welding and seam welding are resistance weld- ing processes

4、. Resistance welding includes a group of processes that produce coalescence of the faying sur- faces with the heat obtained from the resistance of the workpieces to the flow of the welding current in a cir- cuit of which the workpieces are a part, and by the application of pressure. A resistance spo

5、t weld is made between or on over- lapping members (workpieces) in which coalescence may start and occur on the faying surfaces, or may pro- ceed from the outer surface of one member. The weld cross section is approximately circular. A seam weld is a continuous weld made between or on overlapping me

6、mbers, in which coalescence may start and occur on the faying surfaces, or may have pro- ceeded from the outer surface of one member. The con- tinuous weld may consist of a single weld bead or a series of overlapping spot welds. 1,2 Spot welding and seam welding adapt well to auto- mation. They are

7、mainstay processes in the automotive industry and in other industries that manufacture prod- ucts involving the welding of similar thicknesses of the same metal. Examples include automobile fuel tanks, catalytic converters, mufflers, and roof joints. Seam welding is an important process for manufact

8、urers of 1. Welding terms and definitions used throughout this chapter are from Standard Welding Terms and Definitions, AWS A3.0:2001, Miami: American Welding Society. 2. At the time of the preparation of this chapter, the referenced codes and other standards were valid. If a code or other standard

9、is cited without a date of publication, it is understood that the latest edition of the document referred to applies. If a code or other standard is cited with the date of publication, the citation refers to that edition only, and it is understood that any future revisions or amendments to the code

10、or standard are not included; however, as codes and stan- dards undergo frequent revision, the reader is encouraged to consult the most recent edition. cans and containers of all types. Other products of spot and seam welding are furnace heat exchangers and storage tanks. This chapter covers the fun

11、damental principles of these processes and their variations, equipment, and mechanical systems. The advantages and limitations of the processes are discussed, and welding conditions as applied to various metals are presented. Other topics include joint design, welding schedules, surface prepa- ratio

12、n, weld quality, and economics. The chapter con- cludes with a section on safe practices specific to the spot and seam welding processes. FUNDAMENTALS In resistance spot welding and seam welding, the coalescence of metals is produced by the heat generated in the workpieces due to resistance to the p

13、assage of electric current through each workpiece. Force, exerted on the joint through the electrodes, is always applied before, during, and after the application of current to confine the weld contact area at the faying surfaces and, in some applications, to forge the weld metal during postheating.

14、 A resistance welding electrode is the part of a resis- tance welding machine through which the welding cur- rent and (in most cases) force are applied directly to the workpiece. The electrode may be in the form of a rotat- ing wheel, rotating roll, bar, cylinder, plate, clamp, chuck, or a modificat

15、ion of one of these. Figure 1.1 illustrates the mechanisms of the two processes. RESISTANCE SPOT AND SEAM WELDING CHAPTER 1AWS WELDING HANDBOOK 9.3 CHAPTER 1RESISTANCE SPOT AND SEAM WELDING 3 In spot welding, a nugget (the weld metal that joins the workpieces) is produced at the electrode site, but

16、two or more nuggets may be made simultaneously using multiple sets of electrodes. Seam welding is a variation of spot welding in which a series of overlapping nuggets are produced to obtain a continuous, leak-tight seam. One or both electrodes generally are in the form of wheels that rotate as the w

17、orkpieces pass between them or between one wheel and a flat copper electrode. A seam weld can be produced with spot welding equip- ment, but the operation is much slower. A series of sep- arate spot welds may be made with a seam welding machine and wheel electrodes by suitably adjusting the travel s

18、peed and the time between welds. PRINCIPLES OF OPERATION Spot and seam welding operations involve the coor- dinated application of electric current and mechanical pressure of sufficient magnitude and duration. The welding current must pass from the electrodes through the workpieces. The continuity o

19、f weld current is pro- moted by force applied to the electrodes. The first requirement in the sequence of operations is to develop sufficient heat to raise a confined volume of metal to the molten state. The fused metal is then allowed to cool while under pressure until it has adequate strength to h

20、old the workpieces together. The current density and pressure must be high enough to form a nugget, but should not be so high that they cause molten metal to be expelled from the weld zone. The duration of weld current must be accurately timed to prevent excessive heating of the electrode faces that

21、 could fuse an elec- trode to a workpiece and greatly reduce the service life of the electrode. As noted, heat in a resistance welding process is pro- duced by electrical current flowing through the electri- cal resistance in the workpiece. Because metals have relatively low resistance, welding curr

22、ent must be rela- tively high to produce sufficient heat to develop welding temperatures at the desired location. Weld current also must exceed heat loss by thermal conduction in the workpiece and loss to the relatively cool electrodes in contact with the workpiece. Heat Generation The amount of hea

23、t generated in an electrical con- ductor depends on the following factors: 1. Amperage, 2. Resistance of the conductor (including interface resistance), and 3. Duration of current. These three factors affect the heat generated, as expressed in the following equation: Q = I 2 Rt (1.1) where Q = Heat

24、generated, joules (J); I = Current, amperes (A); R = Resistance of the workpieces, ohms ( ); and t = Duration of current, seconds (s). The heat generated is proportional to the square of the welding current and directly proportional to the resistance and the time. Some of the heat is used to make th

25、e weld and some is lost to the surrounding metal. Figure 1.1Simplified Diagrams of Basic Resistance Spot Welding (A), and Seam Welding (B) RESISTANCE WELDING ELECTRODES RESISTANCE WELDING ELECTRODES (A) Spot Welding (B) Seam Welding4 CHAPTER 1RESISTANCE SPOT AND SEAM WELDING AWS WELDING HANDBOOK 9.3

26、 The welding current required to produce a given weld is an approximation inversely proportional to the square root of the time. Thus, if the time is extremely short, the current required will be high. The secondary circuit of a resistance welding machine and the workpieces constitute a series elect

27、ri- cal circuit. The total electrical impedance of the current path affects the current magnitude for a given applied voltage. The current in a series circuit will be the same in all parts of the circuit. The heat generated at any location in the circuit will be directly proportional to the resistan

28、ce at that point. A very important characteristic of resistance welding is the rapidity with which welding heat can be pro- duced. The temperature distribution in the workpieces and electrodes in spot and seam welding is illustrated in Figure 1.2. In effect, there are at least seven resistances con-

29、 nected in series that account for the temperature distri- bution in a weld. The resistances numbered from 1 through 7 shown in Figure 1.2 apply to a two-thickness joint. Numbers 1 and 7 denote the electrical resistance of the electrode material. Numbers 2 and 6 represent the contact resistance betw

30、een the electrode and the base metal. The magni- tude of this resistance depends on the surface condition of the base metals and each electrode, the size and con- tour of each electrode face, and the electrode force. (Resistance is approximately inversely proportional to the contacting force.) This

31、is a point of high heat gener- ation, but the surface of the base metal does not reach fusion temperature during the passage of current due to the high thermal conductivity of the electrodes (1 and 7) and the fact that they are usually water-cooled. For free- zinc galvanized steels (when zinc is in

32、elemental form as opposed to zinc alloys such as zinc-nickel or zinc iron galvanneal), the contact and faying surface resistance is not significantly higher than the bulk resistance of the material. Therefore, bulk heating of the material con- tributes substantially to the development of the weld. P

33、lanes 3 and 5 are locations of the total resistance of the base metal, which is directly proportional to its resistivity and thickness, and inversely proportional to the cross-sectional area of the current path. Plane 4 is the location of the base metal interface resistance where the weld is to be f

34、ormed. This is the point of highest resistance and therefore the point of greatest heat generation. Since heat is also generated at Points 2 and 6, the heat generated at Plane 4 is not readily lost to the electrodes. Heat is generated at all of these locations, not at the base metal interface alone.

35、 The flow of heat to or from the base metal interface is governed by the temperature gradient established by the resistance heating of the var- ious components in the circuit. This in turn assists or retards the creation of the proper localized welding heat. Heat will be generated in each of the sev

36、en locations shown in Figure 1.2 in proportion to the resistance of Figure 1.2Relationship of Resistance and Temperature as a Function of Location in the Diagrammed Circuit 1 2 4 6 3 5 7 RESISTANCE (OHMS) TEMPERATURE (C OR F)AWS WELDING HANDBOOK 9.3 CHAPTER 1RESISTANCE SPOT AND SEAM WELDING 5 each.

37、However, welding heat is required only at the base metal interface, and the heat generated at all other locations should be minimized. Since the greatest resis- tance is located at Plane 4, heat is most rapidly devel- oped at that location. Points of next-lower resistance are Planes 2 and 6. The tem

38、perature rises rapidly at these points also, but not as fast as at Plane 4. After about 20% of the weld time has elapsed, the heat gradi- ent may conform to the profile shown in Figure 1.2. Heat generated at Planes 2 and 6 is rapidly dissipated into the adjacent water-cooled electrodes at Planes 1 a

39、nd 7. The heat at Plane 4 is dissipated much more slowly into the base metal. Therefore, while the welding current continues, the rate of temperature rise at Plane 4 will be much more rapid than at Planes 2 and 6. The welding temperature is indicated on the chart at the right in Figure 1.2 by the nu

40、mber of short horizontal lines within the drawing that lead to the matching curve. In a well-controlled weld, the welding tempera- ture is first reached at numerous contact points at the weld interface; then melting occurs, and with time, the contact points quickly grow into a nugget. The following

41、factors affect the amount of heat gen- erated in the weld joint by a given current for a unit of weld time: 1. The electrical resistances within the workpieces and the electrodes, 2. The contact resistances between the workpieces and between the electrodes and the workpieces, and 3. The heat lost to

42、 the workpieces and the electrodes. Effect of Welding Current In the formula, Q = I 2 Rt, current has a greater effect on the generation of heat than either resistance or time. Therefore, it is an important variable to be controlled. Two factors that cause variation in welding current with alternati

43、ng current (ac) welding machines are fluc- tuations in power-line voltage and variations in the impedance of the secondary circuit. Impedance varia- tions are caused by changes in circuit geometry or by the introduction of varying masses of magnetic metals into the secondary loop of the machine. In

44、addition to variations in the magnitude of welding current, current density may vary at the weld interface. This change in current density can result from the shunting of current through preceding welds and con- tact points other than those at the weld. An increase in electrode face area will decrea

45、se current density and welding heat. This may cause a measurable decrease in weld size. Minimum current density for a finite time is required to produce fusion at the interface. Sufficient heat must be generated to overcome the losses to the adjacent base metal and the electrodes. Weld nugget size a

46、nd strength increase rapidly with increasing current density. Excessive current density will cause molten metal expulsion (resulting in internal voids), weld cracking, and lower mechanical strength properties. Typical variations in the shear strength of spot welds as a function of current magnitude

47、are shown in Figure 1.3. In spot and seam welding, exces- sive current overheats the base metal and results in deep indentations in the workpieces and will cause overheat- ing and rapid deterioration of the electrodes. Effect of Weld Time The rate of heat generation must be adjusted so welds with ad

48、equate strength will be produced without excessive heating and rapid deterioration of the elec- trode. The total heat developed is proportional to weld time. Essentially, heat is lost by conduction into the sur- rounding base metal and the electrodes; a very small amount is lost by radiation. These

49、losses increase in proportion to weld time and metal temperature. Given suitable current density, some minimum time is required to reach melting temperature during a spot welding operation. If current is continued, the tempera- ture at Plane 4 (refer to Figure 1.2) in the weld nugget will far exceed the melting temperature, and the inter- nal pressure may expel molten metal from the joint. Generated gases or metal vapor and minute metal parti- cles may be expelled. If the workpiece surfaces are scaly Figure 1.3Effect of Welding Current on Shear Strength of Spot Welds SHEAR STRENGTH