The Effect of Cu Powder During Friction Stir Welding on Microstructure and Mechanical Properties of AA3003-H18
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FRICTION stir welding (FSW) is a relatively new solid-state, hot-shear joining process for welding of a wide variety of metallic materials, specially for aluminum.[1,2] FSW was invented at The Welding Institute (TWI) of the United Kingdom in 1991 as a solid-state joining technique and was initially applied to aluminum alloys.[1,3] In FSW process, a non-consumable tool with a specially designed pin is rotated and slowly plunged into the joint line between two pieces of sheets in the solid phase below the melting point of the materials.[4,5] Wrought aluminum alloys are divided into non-heattreatable and heat-treatable alloys.[6] Heat-treatable alloys are strengthened by solution heat treatment and controlled aging, and non-heat-treatable alloys are nonprecipitation-hardening alloys. In other words, they are not strengthened by second-phase particles and are further strengthened by strain hardening.[3,6] FSW of wrought aluminum alloys causes significant decrease in the mechanical properties of the weld regions.[3,7] The main reason for this problem is the high heat generation during FSW. During FSW, heat input causes annealing in the weld area such as heat-affected zone B. ABNAR, M.Sc. Student, M. KAZEMINEZHAD, Associate Professor, and A.H. KOKABI, Professor, are with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, Tehran, Iran. Contact e-mail: [email protected] Manuscript submitted October 28, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
(HAZ), thermomechanical-affected zone (TMAZ), and stir zone (SZ). Phenomena that occur during annealing of these alloys include recovery and recrystallization. Both of these phenomena eliminate the effects of strain hardening in the weld area,[3,8] and the dislocation density is reduced in the stir zone, HAZ, and TMAZ of the friction stir welds compared with the base material. Subsequently, the mechanical properties are significantly decreased in the weld area.[8–10] Unlike non-heat-treatable alloys, the mechanical properties of heat-treatable aluminum alloys can be changed with time since they can be naturally age hardened after FSW. In this class of alloys, FSW initially decreases the hardness of the stir zone in comparison with that of the base material. The strength and hardness of the weld zone regions can be improved with time due to the natural aging of the material.[3,11] But the mechanical properties of non-heat-treatable alloys are not changed with time after FSW, because they are non-precipitation-hardening.[3,12] Therefore, it can be mentioned, FSW of wrought non-heat-treatable aluminum alloys has an adverse effect on their mechanical properties such as hardness and tensile strength caused by the heat input in weld regions (HAZ, TMAZ, and stir zone).[2,3,12] According to the above, the heat from the FSW process causes annealing in these materials and it leads to a drop in hardness. In this case, the reduction in the mechanical properties of the weld zone can be avoided by controlled heat input during FSW. In various
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