High-Speed Friction Stir Welding of AA7075-T6 Sheet: Microstructure, Mechanical Properties, Micro-texture, and Thermal H
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ODUCTION
FRICTION stir welding has matured over the last two decades,[1,2] and a large body of literature has been published on related topics including evaluations of joint properties, optimization of processing parameters, and descriptions of material flow during the process.[3–13] A few derivative processes such as friction stir processing,[14–16] stationary shoulder FSW,[17] and friction stir scribe welding were also developed to expand the possible applications and benefits of FSW. However, large-scale application in mainstream industries like automotive and aerospace is still limited due to several issues associated with current state-of-the-art research. One of the factors that impedes the popularization of the technique is its relatively slow joining speed.[18] Current FSW joining speeds are around hundreds of millimeters per minute, while more production-friendly welding technologies can be operated at speeds above 6 M/min. A summary of the available welding parameters of FSW on heat-treatable aluminum alloys in open literature is presented in Figure 1 in blue
JINGYI ZHANG and DAVID P. FIELD are with the School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164. Contact e-mail: [email protected] PIYUSH UPADHYAY is with the Pacific Northwest National Laboratory, Richland, WA 99354. YURI HOVANSKI is with Brigham Young University, Provo, UT 84602. Manuscript submitted May 18, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
dots.[3,5–13,17,19–58] The majority of these studies are found within the red dash-lined triangular envelop, limited to speeds much lower than 1 M/min with only a few studies performed at medium joining speeds above 1 M/minute. In this project, we study welds made with joining speeds of meters per minute. Welding speed and tool rotation rate are two essential and parallel comparable parameters in FSW. They both control the heat input and material flow behavior in the process. In addition, the welding speed is known to be one of the key factors that control the cooling rate after the weld is formed.[21,59,60] When welding heat-treatable aluminum alloys, this cooling step can be considered as the quenching step after exposure to elevated welding temperatures. The quenching step is a defining treatment for heat-treatable aluminum alloys because their major strengthening mechanism is precipitation strengthening, and the precipitate reaction is controlled by the thermal history. A carefully controlled quenching step, which is strongly influenced by the welding speed, can determine the final microstructure and thus performance of the weld. This is because the more rapid cooling rate will allow more solute atoms to remain in solution before the precipitation reaction begins. The faster a friction stir weld is made, the faster the heat source/tool pin leaves each section of the workpiece and results in a higher cooling rate. The welding speed also determines the tool revolution per unit length of weld and therefore affects the mechanism and rate of heat generatio
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