Study On The Friction Stir Welding Process In Al-Steel Dissimilar Joints

Lightweight is an important trend in the development of automotive industry,and aluminum/steel dissimilar connection is one of the important directions of automotive lightweight.At present,there are still welding defects caused by the large difference in the material properties of aluminum alloy and steel.In the thesis,the distribution law of the aluminum/steel welded joint structure was analyzed and the welding mechanism of aluminum/steel was revealed.According to the designed process,the structure and mechanical properties of the joint on the influence of rotation speeds and welding speeds were analyzed.The welding process was optimized through mathematical statistics,and the failure process and mechanism of the joint were explored.It is beneficial to provide research foundation and experimental basis for the connection and application of aluminum/steel.The distribution law of the welded joint structure was analyzed.The results showed that the joint was well formed without obvious welding defects.The structure was divided into welding nugget zone,heat-mechanical affected zone,heat-affected zone and aluminum/steel interface.The welding nugget zone was composed of fine equiaxed crystals,and the heat-mechanical affected zone was composed of deformed grains and partial equiaxed crystals.The structure of the heat-affected zone was similar to that of the base material,but the grain size was larger.The aluminum/steel interface structure was analyzed.The results showed welded joints formed connections through interface metallurgical reaction,element diffusion and mechanical occlusion,and intermetallic compounds Al5Fe2 and Al13Fe4 might be formed.The structure morphology of the joint on the influence of process parameters was revealed.The heat input and plastic deformation of the weld metal determined the microstructure characteristics of the joint.The welding tunnel defects appeared at a lower rotation speed,and the defects such as fins and hook-like structure appeared at a higher rotation speed.The fins,keyholes,steel particles and other undesirable structures appeared at a lower welding speed,and the joint surface was peeled at a higher welding speed.The average grain diameter of each area increased with the increase of rotation speed and decreased with the increase of welding speed.The structural characteristics of the aluminum/steel joint interface were analyzed,the results showed there was no aluminum/steel mixture in the joint aluminum/steel interface at a lower rotation speed or a higher welding speed,and the interface was only composed of Al(Fe),Fe(Al)solid solutions and the Fe2Al5 intermetallic compounds.There was obvious aluminum/steel mechanical mixture near the aluminum/steel interface of the joint at a higher rotation speed or a lower welding speed,and the interface was only composed of Al(Fe),Fe(Al)solid solutions and the Fe2Al5,FeAl3 intermetallic compounds.Cracks initiated on both sides of the aluminum/steel interface,and extended along the intermetallic compounds.The hardness of the joint on the influence of welding parameters was analyzed.The hardness distribution of the joint in the lap direction was symmetrical about the center of the weld with the shape of W,and the hardness of the weld nugget zone was higher,and the hardness of heat-affected zone was the lowest.The hardness of each zone of the joint surface decreased with the increase of rotation speed and the decrease of welding speed,and the hardness in the aluminum/steel interface increased,forming an extreme value.The tensile-shear strength of joints on the influence of welding parameters was analyzed.The tensile-shear strength of aluminum/steel friction stir welding lap joints was a maximum of 91 MPa at the welding tool speed 100 rad/s and welding speed 1.5 mm/s,and the optimal process parameters were the welding speed 103.71 rad/s and the welding speed 1.58 mm/s through Vogt formula.The fracture failure mode of the welded joint was analyzed,and the fracture morphology of the joint was tearing dimples and river patterns,indicating that the failure of the aluminum/steel friction stir welding lap joint was a mixed fracture.