Microstructure, mechanical and corrosion behavior of high strength AA7075 aluminium alloy friction stir welds – Effect of post weld heat treatment

Abstract

High strength aluminium alloy AA7075 (AleZneMgeCu) is a precipitate hardenable alloy widely used in the aerospace, defense, marine and automobile industries. Use of the heat treatable aluminium alloys in all these sectors is ever-increasing owing to their excellent strength-toweight ratio and reasonably good corrosion resistance. The shortage in corrosion resistance, however, usually poses negative concern about their reliability and lifetime when they service in the variable marine environments. These alloys also exhibit low weldability due to poor solidification microstructure, porosity in fusion zone and lose their mechanical properties when they are welded by fusion welding techniques. Friction stir welding (FSW) is a reliable technique to retain the properties of the alloy as the joining takes place in the solid state. The welds are susceptible to corrosion due to the microstructural changes in the weld nugget during FSW. In this work, the effect of post weld treatments, viz., peak aging (T6) and retrogression & reaging (RRA), on the microstructure, mechanical properties and pitting corrosion has been studied. Friction stir welding of 8 mm-thick AA7075 alloy was carried out. The microstructural changes of base metal and nugget zone of friction stir welds were studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. Tensile and hardness test of base metal and welds has been carried out. Pitting corrosion resistance was determined through dynamic polarization test.It was observed that the hardness and strength of weld were observed to be comparatively high in peak aged (T6) condition but the welds showed poor corrosion resistance. The resistance to pitting corrosion was improved and the mechanical properties were maintained by RRA treatment. The resistance to pitting corrosion was improved in RRA condition with the minimum loss of weld strength.

A review on friction stir welding, parameters, microstructure, mechanical properties, post weld heat treatment and defects

“…A great deal of research has shown that mechanical properties, including the hardness of the HAZ, and sometimes rarely the hardness of the weld zone of FSW, are greatly reduced. 61 For example, Cavaliere's and his colleague's by study the area of the two FSW cross-section micro hardness of Al2024 and Al7075 alloys showed that the hardness of the weld center is 150Hv and the hardness of both sides of the joint after reaching the peak, distance 2mm from the weld center, is decreased. 58 The results of Al-Badrawy and his colleagues 6 on the weld cross-sectional area of Al7075 alloy FSW showed that the bond strength is reduced compared with the base metal.…”

Characterization of corrosion behavior of aluminum welds reinforced with copper powder

“…However, it has been established that all welding processes have influence on the final properties of the welded structures, which often leads to deterioration of its corrosion properties. The poor corrosion resistance of aluminum in the as-welded condition tends to limit its application range [2]. Thus, for aluminum to suit engineering applications in a corrosive environment and/or at elevated temperatures, it often requires some degree of reinforcement.…”

Effect of Friction Stir Welding Process Parameters and Post-Weld Heat Treatment on the Corrosion Behaviour of Aa6061-O Aluminum Alloys

“…It has been reported that the corrosion resistance of FS welds can be improved by several treatments after or during the welding [12][13]. Vincent Proton et al [12] investigated the corrosion behavior of the nugget of a FSW joint employing a 2050 Al-Cu-Li alloy.…”