Effects of Initial Temper Condition and Postweld Heat Treatment on the Properties of Dissimilar Friction-Stir-Welded Joi
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NTRODUCTION
A couple of problems are encountered in the fusion welding of aluminum alloys (Al alloys), such as, porosity formation and loss of low melting point elements in the fusion zone.[1–6] Cracking may also occur in joining of age-hardened grades. Moreover, fusion joining of dissimilar Al alloys is further complicated. As a solid joining technique, friction stir welding (FSW) is an alternative method for joining Al alloys,[7] particularly in dissimilar combinations. The problems experienced in fusion joining of these alloys are usually not encountered in FSW.[8–20] However, the loss of strength in the weld region also occurs in the FSW of these alloys, particularly in age-hardened grades, although to an extent that is much lower than those taking place in fusion welds.[3–6,8–10] The degree of the strength loss taking place in the weld region during FSW of age-hardened Al alloys depends on the weld parameters, and it takes place even though optimum weld parameters are used.[21] Welding of dissimilar materials is important and needed quite often in several structural applications for optimization of properties such as strength, weight, and corrosion resistance.[22] Fusion welding of dissimilar materials is problematic, as mentioned previously. FSW, on the other hand, offers potential for use in the joining of dissimilar materials combinations, and defect-free joints can be produced by this solid-state joining technique.[7] Numerous studies were recently conducted on FSW of dissimilar materials combinations.[12,23–31] However, strength loss also occurs in the weld area of _ ˘ LU, Research Assistant, and GU¨REL C¸AM, GU¨VEN IPEKOG Professor, are with the Faculty of Engineering, Mustafa Kemal University, _ 31200, Iskenderun/Hatay, Turkey. Contact e-mail: [email protected] Manuscript submitted December 23, 2013. Article published online March 18, 2014 3074—VOLUME 45A, JUNE 2014
friction-stir-welded dissimilar Al-alloy joints as well as in similar welds, particularly in age-hardened grades, although the degree of it is much lower than those encountered in fusion welding.[15,32–44] Therefore, there is still a need for further work in order to understand microstructure-property relationships of dissimilar Al-alloy welds and to improve joint performance. It has been well demonstrated by various researchers that the strength loss in the weld area of similar frictionstir-welded Al-alloy joints can be restored by various postweld heat treatments (PWHTs). For instance, several researchers[33–38,45] reported that the strength of the weld zone was improved in friction-stir-welded similar AA6063, AA2219-O, AA2024-T4, AA2024-T4, AA6061-T6, AA6061-O/T6, and AA7075-O/T6 joints, respectively. Similarly, PWHT may also be employed to restore the mechanical properties of dissimilar joints in the joint area. However, the study on the effect of PWHT in dissimilar friction-stir-welded joints is very limited in the open literature.[46,47] Both of these studies showed that a PWHT consisting of solutionizing followed by aging restored the
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