Investigation On Process,Microstructure And Performance Of The Zero-thinning Friction Stir Weld Of 7N01 Aluminum Alloy

Friction stir welding technology of aluminum alloy is widely used in the field of aerospace and railway trains.However,there is a weld thinning phenomenon in the welding area during conventional friction stir welding,both the strength and reliability of the connected component are reduced.Therefore,a new zero-thinning friction stir welding process is needed.In this paper,7N01 aluminum alloy was welded by the zero-thinning friction welding process.The microstructure and properties of the welded workpiece were investigated in order to optimize this process parameters and the best welding process parameters were achieved.Further research is carried out on Lazy S.Under reasonable welding parameters,the good weld surface can be obtained.The thinning of the weld area was measured,there is no thinning phenomenon in the weld area,even local thickening.The welding joint tensile strength reach to 353MPa,the elongation rate is 11.7%,and the strength was 91.7%of the parent material.In comparison to conventional friction stir welding,the mechanical properties of the zero-weld-thinning friction stir welding joint are more homogeneous along thickness direction.The microhardness distribution in the weld area indicates that the core area is softened.The welding parameters were optimized based on the response surface method in order to improve the mechanical properties of the zero-weld-thinning friction stir welding joints.Variance analysis was carried out on the response surface test its accuracy,found that model the whole fitting is good,tensile strength and elongation of single factor test in the significant degree of sorting,the order of significant is welding speed v>rotation speedω>shoulder plunge depth p.The regression analysis of the response surface model shows that high rotation speed and moderate welding speed should be selected to get better welding joint without any defects.Under the condition of the rotation speed of 1000rpm,the welding speed of 200mm/min and the shoulder plunge depth of 0.25mm,the tensile strength of the joints reached to the maximum 363MPa,which was 93.8 percent of the base metal.The elongation was 11.2%.The Lazy S in the zero-thinning friction welding is mainly due to the accumulation of small holes in the plastic metal isolation by Al₂O₃ particles in the welding seam or the original oxide film.The fracture location of the weld is basically the same as the Lazy S position,It is shown that Lazy S is the main reason of welded joint fracture.With the welding speed increases,the heat input in the mixing area decreases,the oxidation degree of the base material is decreased,and the number and intensity of the cavities were significantly reduced in Lazy S.