TECHNICAL ARTICLE—PEER-REVIEWED Effect of Changing Heat Treatment Conditions on Microstructural and Mechanical Properties of Friction Stir Welded Sheets of AA2024 with Interlayer Strip Width AA7075 Mohamed Abu-Okail . Ibrahim Sabry . Ahmed Abu-Oqail . W. M. Shewakh Submitted: 3 January 2020 / in revised form: 13 March 2020 / Published online: 7 May 2020 Ó ASM International 2020 Abstract The heat treatment of friction stir tailor-welded aluminum joints is one important key process to improve the efficiency of joint strength and ductility as well as fracture locations, since almost all of the welded parts are subjected to a forming process. Therefore, the effect of heat treatment on microstructural characterization and mechanical properties of welded joints is a very important research area to improve the nugget zone in welded structures of aircraft segments. The objective of this study is to investigate the effect of changing pre-weld heat treatment conditions (first case: solution heat treated with artificial aging (T 6 ) and second case: annealed heat treat- ment (O)) by microstructural characterization with an optical microscope, scanning electron microscope and x- ray diffraction and mechanical properties using micro- hardness, tensile and bending tests. After that, ANOVA was used in order to determine the interaction effect between the conditions of heat treatment (T 6 and O) and interlayer strip widths. The results revealed that the soft- ened zone of FSW joints in the annealed condition (O) was relatively more homogeneous than FSW joints from solu- tion heat treated and artificially aged (T 6 ). On the other hand, the strength efficiency of FSW joints from annealed (O) samples was the higher value than the joints efficiency at solution heat treated and artificially aged (T 6 ). Further- more, the annealed (O) FSW joints fractured in the base metal, while FSW joints from solution heat treated and artificially aged (T 6 ) samples fractured in the HAZ. The microstructure and mechanical properties of weld nugget zones were influenced by the pre-weld heat treatment conditions. Finally, the results of ANOVA agreed with the results of mechanical properties. Keywords Friction stir welding (FSW) Á Heat treatment conditions Á Strength Á Ductility Introduction The heat treatment of friction stir tailor-welded (FSW) aluminum joints is one important key process to improve the efficiency of joints strength and ductility as well as fracture locations, this poses challenges that limit the wide acceptance of the welded structures in aircraft industry, where almost all of the welded parts are subjected to post- weld heat treatment before the forming processes [1, 2]. FSW is a solid-state technology used for joining aluminum and its alloys [3, 4]. This technique contains several advantages over fusion welding including high joint strength, low distortion, no cracks, low power consumption and non-pollution [5–7]. Therefore, FSW is considered as a good alternative to fusion welding and has been applied in aerospace, automotive, electronics and shipbuilding [8–10]. In many industrial applications, especially in the air- plane industry, the effect of prior heat treatment on microstructure and mechanical properties of welded joints is an important research area to improve the weak part of M. Abu-Okail (&) Á I. Sabry Manufacturing Engineering and Production Technology Department, Modern Academy for Engineering and Technology, P.O. Box 11571, Cairo, Egypt e-mail: MohamedAbuOkail@gmail.com A. Abu-Oqail Á W. M. Shewakh Mechanical Department, Faculty of Industrial Education, Beni- Suef University, Beni-Suef, Egypt W. M. Shewakh Industrial Engineering Department, Faculty of Engineering, Jazan University, Jazan, Kingdom of Saudi Arabia 123 J Fail. Anal. and Preven. (2020) 20:701–722 https://doi.org/10.1007/s11668-020-00868-z