Effect of Backing Plate Thermal Property on Friction Stir Welding of 25-mm-Thick AA6061
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ND BACKGROUND
THERMAL boundary condition at the bottom of the work piece is important in determining the weld process response and resulting weld properties. For a given plate geometry, alloy type, and tool design, the choice of weld control parameters (tool rotation speed, welding speed, and forge force), which results in ‘‘good’’ weld, depends on the thermal boundary conditions (BC) at the work piece. The rate of heat flux through the bottom of the work piece mostly depends on the backing plate (BP) diffusivity and hence, as has been pointed out in literature the most important thermal BC for the process is the thermal condition at the BP.[1–3] Unfortunately, the effects of changes in thermal boundary condition at the bottom of the work piece during friction stir welding have not been thoroughly examined in the open literature. Consequently the effects of thermal management at the bottom of the work piece are poorly understood at present. Rosales et al.[4] reported welds made with AA2024 and AA6013 where steel, copper, and ceramic-coated BPs were used at three different combinations of rotational and welding speeds while the forge force PIYUSH UPADHYAY, Post-Doctoral Researcher, formerly with the Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, is now with the Pacific Northwest National Laboratory, Richland, WA. Contact e-mail: [email protected] ANTHONY REYNOLDS, Professor, is with the Department of Mechanical Engineering, University of South Carolina. Manuscript submitted June 12, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
was kept constant. The in-plate far field temperature measurements varied significantly when using different BPs indicating the importance of BP conductivity. As a part of quench sensitivity study, Nelson et al.[5] reported that the use of heated BP resulted in higher peak temperature and lower cooling rate, yielding inferior mechanical properties in AA7075 welds. Some researchers have used thermal management at the BP to optimize friction stir spot welding (FSSW) process in relatively thin sheets. Su et al.[6] for instance reported that in FSSW of 1.3-mm-thick AA 6111 a greater fraction (from 12.5 to 50 pct) of energy generated by the tool was transferred into the weld zone when mica clamp and BP was used instead of conventional steel clamp and BP. This is reasonable since the heat dissipated into the BP is reduced with greater insulation. In a similar study, in FSSW of 0.9-mmthick AA 6111, Bakavos and Prangnell[7] found that the use of ceramic BP resulted in 318 K (45 C) increase in the peak processing temperature while lap shear strength was reduced by 15 pct compared to conventional steel BP. As the application of friction stir welding widens, it will be important to understand the behavior of process variables like stir zone temperature and resulting weld properties with the change in thermal boundary condition. In this paper the authors have focused on the role of BP material on resulting weld properties for 25.4mm-thick AA 606
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