Microstructural evolution of 6063 aluminum during friction-stir welding
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I. INTRODUCTION
FRICTION stir welding is a new, solid-state welding technique which was invented by The Welding Institute (TWI) in 1991.[1] It has enabled us to butt-weld aluminum alloys, which are often difficult to fusion weld, without voids, cracking, or distortion. Recent studies have shown the basic characterization of weld formation and the mechanical properties of the welds.[2–9] Microstructural changes during friction-stir welding have been the subject of some research papers recently.[10–15] Concerning age-hardenable aluminum alloys, microstructural studies in the welds of 7075-T651 Al by Rhodes and coworkers[10,11] and of 6061-T6 by Murr and co-workers[12,13] are represented. They have demonstrated that the weld zone consists of fine equiaxed grains characterized by recrystallization and that mechanical properties are deteriorated in the weld zone and heat-affected zone, depending on the density of strengthening precipitates. The precipitate distribution is strongly influenced by the thermal hysteresis. Localized frictional heating during friction-stir process produces significant microstructural change, which leads to local variation in mechanical properties in the weld. So, the local relationship between microstructure and mechanical properties is very important, but the details of this relationship are not yet known. The local temperature is dominant in determining the microstructure, but is very difficult to measure during welding. The maximum temperatures were estimated from the microstructures in previous studies.[10–13] Rhodes and coworkers[10,11] have shown that larger precipitates might have gone into solution and reprecipitation in the weld center, YUTAKA S. SATO, Research Associate, and HIROYUKI KOKAWA, Professor, are with the Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. MASATOSHI ENOMOTO, Leader, is with the SAD Project, Showa Aluminum Corporation, Oyama Works 480, Ihuzuka, Oyama City, Tochigi 3238678, Japan. SHIGETOSHI JOGAN, Welding Manager, is with the R&D Department, Showa Aluminum Corporation, 6-224, Kaisan-Cho, Sakai City, Osaka 590-8576, Japan. Manuscript submitted January 4, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
and they concluded that process temperatures are between about 400 8C and 480 8C in friction-stir-welded 7075 Al. On the other hand, Murr and co-workers[12,13] have indicated that some of the precipitates are not dissolved during welding and stated that the temperature rises to roughly 400 8C in a friction-stir-welded 6061 Al. The difference is due to differences in the solubilities of various precipitates in 7075 and 6061 Al, but the behavior of the precipitate phenomenon and the temperatures are not yet well known. The objective of the present study is to examine the microstructure formation and distribution, especially the precipitation sequence, in friction-stir-welded 6063 Al, correlating the local thermal hysteresis and hardness. In order to make the precipitation phenomena simple, 6063 Al was
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