Investigation into the Influence of Post-Weld Heat Treatment on the Friction Stir Welded AA6061 Al-Alloy Plates with Dif
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TRODUCTION
IT is desirable to obtain comparable mechanical properties in weld zone of a joint to those of the base plate.[1] However, this is not usually the case in joining of Al-alloys, particularly in fusion welding, although a filler wire is used, and several difficulties are encountered. These difficulties include porosity formation, cracking, and strength loss in weld zone.[1–5] On the other hand, porosity formation and cracking are not encountered in friction stir welding (FSW), owing to the fact that it is a solid state joining process, provided that suitable weld parameters are employed. Many researchers reported that Al-alloys can be successfully joined by this method.[6–18] However, the strength loss in weld zone cannot be thoroughly eliminated in FSW of these alloys, particularly in higher strength age-hardened grades, although joints are produced without the defects experienced in fusion welding stemming from melting and solidification of the material. This is due to the fact that the heat input during joining, although not high enough to melt the material, leads to solution and/or coarsening of the strengthening particles in the stir zone (dynamically recrystallized—DXZ, and thermomechanically affected zones—TMAZ) and to overaging (coarsening of the strengthening particles) in the heat affected zone (HAZ).[19] As a result of this, a strength loss in the _ ˘ LU, Research Assistant, and GU¨REL C¸AM, GU¨VEN IPEKOG Professor, are with the Faculty of Engineering, Mechanical Engineering Department, Mustafa Kemal University, 31200 Iskenderun, Hatay, _ ERIM, _ Turkey. Contact e-mail: [email protected] SEC¸IL formerly Professor with the Faculty of Engineering, Mechanical Engineering Department, Dokuz Eylu¨l University, 35160 Buca, Izmir, Turkey, is now retired. Manuscript submitted April 8, 2013. Article published online October 12, 2013 864—VOLUME 45A, FEBRUARY 2014
weld zone is experienced after FSW although the degradation of base plate properties is not as high as those occurring in fusion joining. Thus, the strength of stir zone is usually lower than that of the base plate, particularly for age-hardened grades. Post-weld heat treatment (PWHT) is usually employed to restore the properties of fusion-welded Al-alloys joints wherever possible. Thus, several studies were conducted to investigate the effect of PWHT on the joint performance of FSWed Al-alloys joints, namely AA6063,[20] AA2219O,[21] AA6061-T6,[22] AA2024-T4,[23] and AA7039.[24] However, contradicting results were reported on the effect of PWHT, for instance a restoration of strength was reported for FSWed 2024-T4[23] and a loss of strength was observed for FSWed AA7039.[24] Moreover, Elangovan and Balasubramanian[22] conducted FSW on AA6061 alloy plates only in T6-treated condition and studied the effect of different PWHTs on the joint performance. They observed that a PWHT consisting of solutionizing and quenching followed by artificial aging resulted in a loss of strength whereas only artificial aging without solutionizing led to an increase in strength. As
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