The effect of post-weld heat treatment on the mechanical properties of 2024-T4 friction stir-welded joints Hakan Aydın a, * , Ali Bayram a , _ Ismail Durgun b a Uludag ˘ University, Faculty of Engineering and Architecture, Department of Mechanical Engineering, 16059 Görükle-Bursa, Turkey b Tofas ß-Fiat, Türk Otomobil Fabrikası A.S ß., 16369 Bursa, Turkey article info Article history: Received 12 August 2009 Accepted 13 November 2009 Available online 18 November 2009 Keywords: C. Heat treatments D. Welding E. Mechanical abstract In this study, the effect of post-weld heat treatment (PWHT) on the mechanical properties of friction stir- welded 2024 aluminum alloys in the T4 temper state was investigated. Solution heat treatment and var- ious ageing treatments were given to the welded joints. The PWHT procedures caused abnormal coars- ening of the grains in the weld zone, which resulted in a drop in micro-hardness at the weld zone compared to the base material of the joints. T6 (190 °C – 10 h) ageing treatment after welding was found to be more beneficial than the other heat treatments in enhancing the mechanical properties of the 2024- T4 joints. However, the T6 (190 °C – 10 h) heat treatment led to significant ductility deterioration in the joint. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Aluminum alloys are increasingly used in many important man- ufacturing areas, such as the automobile industry, aeronautic and military, because of their low-density and good mechanical prop- erties [1]. However, the welding of aluminum and its alloys has always represented a great challenge for designers and technolo- gists. Many difficulties are associated with this kind of joint process [2]. It is obvious that serious problems, such as tenacious oxide layer cavities, hot cracking sensitivity, and porosity, may occur when fusion welding is applied to aluminum and its alloys. Moreover, the conventional techniques, such as fusion welding, often lead to significant strength deterioration in the joint because of a dentritic structure formed in the fusion zone [3]. Friction stir welding (FSW) is a solid state metal joining technique that was developed and patented by The Weld Institute of Cambridge, UK, in 1991 [4], and took place during the solid state phase; the above-mentioned problems were not observed [3,5–8]. FSW is well suited for joining aluminum alloys, especially those that are typically considered to be un-weldable, such as 2XXX and 7XXX series aluminum alloys [3]. 2024 Aluminum alloy is an age-hardenable alloy that possesses enhanced strength because of the precipitation of the Al 2 CuMg phase upon solutionising and artificial ageing. Precipitate strength- ened alloys show a worsening of mechanical properties in the weld zone because of the dissolution and growth of strengthening pre- cipitates during the welding thermal cycle [5,9–12]. Though FSW joints yield better joint efficiency compared to fusion welding pro- cesses, the gap between the strength values of the base metal and the weld metal is large [13]. FSW gives rise to softening of the joints and results in the degradation of mechanical properties [9,14,15]. To recover the loss of mechanical properties in the weld zone, one option is to fully post-weld, reheat, and treat welded components [16]. Few studies have been performed to determine the effect of PWHT on the mechanical properties of friction stir-welded alumi- num alloys [8,13,16,17]. Chen et al. [8] observed that the tensile strength of the friction stir-welded joints of 2219-O aluminum alloy could be significantly improved by the PWHT process. Elangovan and Balaubramanian [13] also examined the influence of PWHT on the tensile properties of friction stir-welded 6061 aluminum alloys. A 40–45% decrease in strength for the friction stir-welded joints compared to that of the base material (BM) was observed in this study. This strength loss of the friction stir-welded joints in this study has been significantly improved by the artificial ageing treatments after welding. Krishnan [17] investigated the effect of PWHT on the properties of friction stir-welded 6061 alloys. He observed that the weld (stir) region exhibited very coarse grains after the PWHT, and the samples failed after PWHT during the root bend test. Sullivan and Robson [16] also applied a PWHT to friction stir-welded 7449 aluminum alloys. They found that this joint has a dramatic effect on the nugget zone (NZ) and thermo-mechanically affected zone (TMAZ) microstructures. The hardness in the NZ and TMAZ also decreased. 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.11.030 * Corresponding author. Address: Uludag ˘ Üniversitesi, Mühendislik Mimarlık Fakültesi, Makine Mühendislig ˘i Bölümü, 16059, Görükle-Bursa, Türkiye. Tel.: +90 224 294 19 88; fax: +90 224 294 19 03. E-mail address: hakanay@uludag.edu.tr (H. Aydın). Materials and Design 31 (2010) 2568–2577 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes