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WELDING RESEARCH -S263WELDING JOURNAL ABSTRACT. In this work, the mash seam welding (RSEW-MS) process has been analyzed with

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Content	WELDING RESEARCH -S263WELDING JOURNAL ABSTRACT. In this work, the mash seam welding (RSEW-MS) process has been analyzed with respect to the requirements of the different mechanisms of solid-state welding, and the role of each assessed. The process was first analyzed to predict local strains along the weld interface. This was done through a simple geometric analysis of the weld stackup. Thermal cy- cles were then analyzed using a variation of the 2-D Rosenthal equation and an ap- propriate heat-generation term for mash seam welding. From this relationship, an equation was deduced defining the dura- tion of temperature excursions above crit- ical temperatures for metallurgical reac- tions. Calculations of weld interface strains suggest that for mash seam weld- ing, maximum strain levels are on the order of 110%. Further, for realistic lap conditions, the degree of strain appeared to be a function of the setdown of the joint stackup only and not the included lap. In addition, these strains were found to be relatively low compared to published strain requirements for cold pressure welding (400–500%). As a result, it was concluded that actual surface straining plays a reduced role in joining during mash seam welding of sheet steels. In the absence of sufficient strain, solid-state welding must occur by thermally assisted processes. For such processes, longer times at temperature promote such mech- anisms as oxide dissolution, diffusion, and weld area recovery/recrystallization. Analysis of thermal excursions above crit- ical metallurgical temperatures suggests that higher currents, slower travel speeds, and increased laps all should improve weld performance. These observations are all consistent with extensive manufactur- ing experience. Introduction Resistance mash seam welding is a variant of both conventional resistance seam welding (RSEW) and projection welding (PW). Mash seam welding uses equipment quite similar to that for con- ventional seam welding, including a large resistance welding frame, and rotating wheel-type electrodes to conduct the cur- rent. Mash seam welding differs from con- ventional seam welding in that the degree of overlap (between the sheets) is rela- tively small (on the order of 1–2 times the sheet thickness). During welding, this lap area is resistance heated and forged down to form a joint. A schematic representa- tion of the lap prior to welding and the final joint geometry is provided in Fig. 1. This heating and forging characteristic is similar to solid projection welding. Mash welding is essentially a heating and forging process. Resulting joints are typically of solid-state character, and, when properly formed, show no evidence of melting. Earlier work on mash seam welding (Ref. 1) has suggested that for- mation of a fusion zone was characteristic of these joints, and this is certainly possi- ble if sufficiently high currents and low travel speeds are used. However, most mash seam welds today are made as solid- state joints. Mash seam welding shows considerable advantage over conventional seam welding in a number of areas. Since a relatively narrow lap is used, this process is quite insensitive to the particular wheel geometry. As a result, the process is usu- ally conducted with relatively wide, flat wheels. These flat wheels offer excellent final surface finish and, in addition, greatly reduce sensitivity of the process to joint tracking. The final joint thickness is also generally very close to the base mate- rial thickness (typically 110–130% of that thickness). In addition, with secondary mechanical planishing, final joint thick- nesses equal to the base material thickness can be obtained. Given these advantages, it is not sur- prising that mash seam welding is widely used in industry today. Applications for mash seam welding include appliance manufacturing (e.g., dryer drums, motor shells) and fabrication of can and drum as- semblies, bicycle rims, etc. More recently, mash seam welding has been used in au- tomotive applications for the production of tailor-welded blanks (Refs. 2–4). It is somewhat surprising that, given the widespread application of this process, the underlying mechanics of joint forma- tion are not well understood. As men- tioned above, some work was done to de- fine the characteristics of these joints using metallographic inspection (Ref. 1). However, that work focused on the fusion characteristics of this process, and, as de- scribed, most of these joints are now made in the solid-state regime. As a result, this work is not applicable to most mash seam welding applications. SUPPLEMENT TO THE WELDING JOURNAL,OCTOBER 2003 Sponsored by the American Welding Society and the Welding Research Council Theoretical Analysis of Welding Characteristics during Resistance Mash Seam Welding of Sheet Steels Surface strains and approximate thermal cycles can be easily calculated for resistance mash seam welding and used to infer joint quality BY J. E. GOULD KEYWORDS Resistance Mash Seam Welding Solid-State Welding Strain Analysis Thermal Cycle AnalysisJ. E. GOULDis Chief Engineer, Resistance and Solid-State Welding, at Edison Welding Institute, Columbus, Ohio.
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