Effect of Temperature on Microstructure and Fracture Mechanisms in Friction Stir Welded Al6061 Joints
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JMEPEG DOI: 10.1007/s11665-017-2704-9
Effect of Temperature on Microstructure and Fracture Mechanisms in Friction Stir Welded Al6061 Joints A. Dorbane, G. Ayoub, B. Mansoor, R.F. Hamade, and A. Imad (Submitted May 25, 2016; in revised form March 20, 2017) Aluminum and its alloys are widely used in different industries due to such attractive properties as adequate strength, ductility, and low density. It is desirable to characterize welds of aluminum alloys obtained using ‘‘friction stir welding’’ at high temperatures. Al-to-Al (both 6061-T6) butt joints are produced by friction stir welding at tool rotation speed of 1600 rpm and four levels of tool advancing speeds: 250, 500, 750, and 1000 mm/min. Microstructural properties of the different welds are investigated. Observed are noticeable differences in microstructure characteristics between the various weld zones. Mechanical properties of these welded joints are characterized under tensile tests at temperatures of 25, 100, 200, and 300 °C, at a constant strain rate of 1023/s. The optimum microstructural and mechanical properties were obtained for the samples FS welded with 1600 rpm tool rotation speed at 1000 mm/min tool advancing speed. The studied welds exhibited yield strength, ultimate tensile strength, and strain to failure with values inferior of those of the base material. Observations of postmortem samples revealed that in the temperature range of 25-200 °C the locus of failure originates at the region between the thermo-mechanically affected zone and the heat-affected zones. However, at higher temperatures (300 °C), the failure occurs in the stir zone. A change in the crack initiation mechanism with temperature is suggested to explain this observation. Keywords
fracture, friction stir welding, high-temperature testing, mechanical properties, microstructure
1. Introduction Aluminum is the most abundant metal in the earths crust. Furthermore, owing to their corrosion resistance, lightweight, formability and toughness, aluminum and its alloys are the second most important structural metal used in the automotive, aircraft and watercraft industries, after steel and its alloys (Ref 1). Because of aluminums high thermal conductivity and low melting point, welding aluminum and its alloys can be challenging while by using conventional welding techniques such as fusion welding. The solidification process following the overheating of the base material leads to the creation of defects and consequently the deterioration of the mechanical properties and metallurgical aspects. Friction stir welding (FSW) is a solid-state material joining technique, introduced in 1991 (Ref 2). The resulting temperA. Dorbane, Mechanics Laboratory of Lille, CNRS UMR 8107, Ecole PolytechLille, University of Lille-North of France, Cite Scientifique, Boulevard Paul Langevin, 59655 Villeneuve dAscq Cedex, France; and Department of Mechanical Engineering, Texas A&M University at Qatar, Doha, Qatar; G. Ayoub, Industrial and Manufacturing Systems Engineering, University of Michigan Dearborn, D
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