Microstructure and mechanical properties of double side friction stir welded AA 7075 aluminum alloy

Abstract

Friction stir welding of aluminum alloys is, nowadays, widely used in several industries and the welded joints are expected to exhibit adequate weld characteristics. In fact, normal single side friction stir welds, particularly thick section joints, are prone to reductions in weld properties due to higher heat inputs. Double side friction stir welding (DS FSW) is considered to be one of the possible solutions to address the problem of weld property reductions. Hence, the present work aims at investigating the possibility of improving the joint properties of welds by welding from both sides. Double side friction stir welds were made using 10 mm thick AA7075 plates and the welds were investigated for mechanical properties using hardness, tensile, impact and bend tests. Microstructure, precipitate morphology and fracture surface of the samples were also studied by optical (OM), transmission electron (TEM) and scanning electron microscopies (SEM), respectively. The results showed that the weld nugget exhibits fine equiaxed grains and severe dissolution of strengthening precipitates. In addition, the hardness of the double side welds improved by about 20 HV compared with that of single side welds due to increased survival rate of strengthening precipitates in the WN and reduced PFZ's width in the HAZ. The results of tensile and impact tests showed that in DS FSW samples, the yield strength increased by about 30 MPa and impact toughness increased by 4.83 J compared with single side welds. By and large, it was observed that the better microstructural and mechanical properties can be achieved by double side friction stir welding.