Study On Underwater Friction Stir Welding Of Al/Mg Dissimilar Alloy

Friction stir welding is a new type of solid-state connection technology,which does not require shielding gas and filling materials,has small residual deformation after welding,low residual stress,and no pollution such as arc,noise,and radiation.Magnesium alloys and aluminum alloys have significant advantages such as low density,high specific strength,and good thermal conductivity.They are widely used in automobile manufacturing,rail transportation and shipbuilding industries.In recent years,the concept of energy conservation and environmental protection has been vigorously promoted and the processing technology of magnesium/aluminum dissimilar alloys has been continuously improved.Magnesium/aluminum alloys have more extensive applications in the industry.With the continuous development and exploration of deep-sea resources,the demand for underwater welding technology has become increasingly prominent.Therefore,it is of far-reaching significance to apply friction stir welding of magnesium/aluminum alloys to underwater environment research.Our research mainly focused on 2mm thick 6061-T6 and AZ31 B magnesiumaluminum dissimilar alloy.We designed and manufactured the stirring head and welding fixture meeting the research,and then went on the lap test under the conventional or underwater welding environment.After the exploration and improvement of the welding process,the welding parameters suitable for both welding environments were obtained.Subsequently,a series of mechanical properties,such as stretching and hardness testing,were performed on the welded plates cut from sample,and the feasibility of underwater welding of dissimilar alloys was analyzed.By analyzing the mechanical properties of the welded joints in both welding environments,the results show that in the underwater welding environment,the maximum tensile strength of joints occurs at the welding parameters n=900 r/min,v=80 mm/min,which is 181.63 MPa,about 70% of the tensile strength(260 MPa)of the base metal,the 6061 aluminum alloy.In the tensile test of conventional welding test specimens,the maximum tensile strength was found at n=750 r/min,v=60 mm/min,which is 176.23 MPa,about 67.8% of the base metal.The two materials are well combined together with no external defects such as burr grooves,the cross-section of the weld zone likes a hook,but holes are formed in the weld zone.The fracture joints of the joints are less plastic,mostly cracking at 45°,show the originally gray of base metal,and the crosssection is the weak area of the welded joints for underwater friction stir welding.Compared to welded joints in conventional environments,welded joints in the underwater environment require higher rotational speeds to provide sufficient heat input,but the joints perform slightly better.The hardness drops from the base material to the lowest value and then gradually increases toward the welding center,typically "W" type.Due to the non-uniform plastic deformation on the advancing and retreating sides of the weld,both the upper aluminum plate and the lower magnesium plate are not symmetrical about the hardness on both sides of the center of the weld.Compared with the weld in the conventional environment,the underwater welding environment has the advantages of reducing the heat input and reducing the thermoplastic deformation,and has a certain positive effect on reducing burrs and incomplete penetration defects.In conventional environment,the stirring head would be worn,broken and plastically deformed after used for a long period of time.However,water could absorbs part of the heat when the stirring head used in the underwater welding environment,thus reducing the wear of the stirring head.