Study On Friction Stir Welding Of Aluminum Alloy Thick Plate

Thick aluminum alloy plate has been widely used in defense industry.But when the thick plate is welded by traditional fusion welding,welding defects such as lack of fusion and greater residual stress are found in the joint.As a new solid-state welding method,friction stir welding has been shown to be suitable for joining aluminum alloys. In this paper,the friction stir welded thick aluminum alloy was carried out.Microstructure was analyzed by OM,SEM and EDS and the tensile test and measurement of microhardness were conducted to evaluate the properties of joint.The influence of welding parameters on microstructure and properties of joints was also studied.In addition,the simulation of temperature field during friction stir welding was conducted with the ANSYS software based on the experimental results.The results show that due to the great temperature gradient along the thickness the nugget zone is divided into top nugget zone,middle nugget zone and bottom nugget zone.The microstructure in bottom nugget zone on retreating side is uneven with grains of different sizes.The sheet ?(Al2Cu) phases and rod-shaped S(Al2CuMg) phases are found in the joint.In the nugget zone,part of ? phases are dissolved and small S phases are dispersely distributed at the grain boundary.The strengthening phases in thermal mechanical affected zone show the same direction and the phases in heat affected zone are bigger.The results of stratified tensile tests show that the strength of top joint is 348 MPa,up to 76.5% of base metal and the minimum strength locates at the bottom joint.The top and middle joints break at the boundary between the nugget zone and thermal mechanical affected zone on retreating side and the fracture of the bottom joint is found in the nugget zone on treating side. The microhardness curve shows a “W” shape, in which the maximum value lies in the nugget zone and the minimum value lies in heat affected zone.Through thickness the microhardness at the top is bigger than that at the bottom.With increasing the rotation speeds,the heat input is bigger and the matrix and strengthening phases are growing.When the rotation speed is 1000 rpm,the weld surface is over-melted,resulting in skin raising and serious flash.Coarse structure and hole defect appear in the nugget zone.When the rotation speed is increasing the strengths of the top and middle joints are reduced,but the strength of the bottom joint is enhanced.With the improvement of the welding speeds,the heat input is lower and the grains are smaller.When the welding speed is 200mm/min, groove defect occurs on the weld surface on advancing side.The strength of the top joint increases first and then descends when the welding speeds increases.The strength of the middle joint is higher and the strength of the bottom joint is lower,with increasing the welding speeds.The simulated results show that the peak temperature is lower than melting point of the base metal during the friction stir welding.The temperature gradient before the rotation tool is big and the temperature gradient behind the rotation tool is small.The peak temperature perpendicular to the weld direction is lower when the feature point is farther from the center line and the peak temperature decreases gradually along the thickness from top to bottom. With the increase of the rotation speeds,decrease of the welding speeds,heat input and the peak temperature improve.