Scripta Materialia,Vol.36,No.I,pp.69-75,1997 Pergamon Elsevier Science Ltd Copyright199 Acta Metallurgica Inc. Printed in the USA.All rights reserved 1359-6462/97$17.00+.00 PIⅡS1359-6462(96)00344-2 EFFECTS OF FRICTION STIR WELDING ON MICROSTRUCTURE OF 7075 ALUMINUM C.G.Rhodes,M.W.Mahoney,W.H.Bingel, R.A.Spurling and C.C.Bampton Rockwell Science Center,Thousand Oaks,CA 91360 (Received April 15,1996) (Accepted July 30,1996) Introduction Friction stir welding is a relatively new technique developed by The Welding Institute (TWI)for the joining of aluminum alloys(1).The technique,based on friction heating at the faying surfaces of two pieces to be joined,results in a joint created by interface deformation,heat,and solid-state diffusion (2-4). Fusion welding of commercial aluminum alloys is difficult at best.For instance,using arc welding techniques,an alloy like 2024 has only limited weldability and the 7XXX series are not recommended for welding(5).Some Al alloys can be resistance welded,but the surface preparation is extensive,with surface oxide being a major problem.Friction stir welding,on the other hand,can be used to join most Al alloys,and surface oxide is no deterrent to the process.No special cleaning techniques are required prior to welding. Although the work piece does heat up during friction stir welding,the temperature does not reach the melting point.The friction-stir-welded joint,then,does not have the dendritic structure typical of a fusion-weld joint,with its attendant serious degradation of mechanical properties. Details of the friction stir welding process are available in the literature(1,2).Basically,a non- consumable tool with a specially designed rotating probe is entered into the abutting edges of the sheets or plates to be welded.Once entered,the rotating tool produces frictional heat and plastic de- formation in the workpiece.The tool,or the part,is then translated along the joint to complete the joining process. In evaluating friction stir welding,critical issues (beyond a sound joint)include microstructure control and localized mechanical property variations.A serious problem with fusion welding,even when a sound weld can be made,is the complete alteration of microstructure and the attendant loss of mechanical properties.Being a solid-state process,friction stir welding has the potential to avoid sig- nificant changes in microstructure and mechanical properties. The objective of this study was to evaluate the microstructural changes effected by friction stir welding of 7075 Al.The effects on tensile properties are reported in a separate paper(6). 69ScriDta Materialia. Vol. 36. No.]. DD. 69-15. 1997 Perga.mon PI1 S1359-6462(96)00344-2 , . . . Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/97 $17.00 + .OO IEFFECTS OF FRICTION STIR WELDING ON MICROSTRUCTURE OF 7075 ALUMINUM C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling and C.6. Bampton Rockwell Science Center, Thousand Oaks, CA (Received April 15, 1996) (Accepted July 30, 1996) 91360 Introduction Friction stir welding is a relatively new technique developed by The Welding Institute (TWI) for the joining of aluminum alloys( 1). The technique, based on friction heating at the faying surfaces of two pieces to be joined, results in a joint created by interface deformation, heat, and solid-state diffusion (2-4). Fusion welding of commercial aluminum alloys is difficult at best. For instance, using arc welding techniques, an alloy like 2024 has only limited weldability and the 7XXX series are not recommended for welding(5). Some Al alloys can be resistance welded, but the surface preparation is extensive, with surface oxide being a major problem. Friction stir welding, on the other hand, can be used to join most Al alloys, and surface oxide is no deterrent to the process. No special cleaning techniques are required prior to welding. Although the work piece does heat up during friction stir welding, the temperature does not reach the melting point. The friction-stir-welded joint, then, does not have the dendritic structure typical of a fusion-weld joint, with its attendant serious degradation of mechanical properties. Details of the friction stir welding process are available in the literature(l,2). Basically, a non￾consumable tool with a specially designed rotating probe is entered into the abutting edges of the sheets or plates to be welded. Once entered, the rotating tool produces frictional heat and plastic de￾formation in the workpiece. The tool, or the part, is then translated along the joint to complete the joining process. In evaluating friction stir welding, critical issues (beyond a sound joint) include microstructure control and localized mechanical property variations. A serious problem with fusion welding, even when a sound weld can be made, is the complete alteration of microstructure and the attendant loss of mechanical properties. Being a solid-state process, friction stir welding has the potential to avoid sig￾nificant changes in microstructure and mechanical properties. The objective of this study was to evaluate the microstructural changes effected by friction stir welding of 707fi Al. The effects on tensile properties are reported in a separate paper(6). 69