Research On Process And Microstructure And Performance In Dissimilar Al-Steel Friction Stir Spot Welding

With the development of social economy and shortage of available energy,it has become a hot issue to how to reduce the structural weight to control energy consumption in the field of engineering.Aluminum-steel structure can not only reduce the structural weight,but also ensure the strength of the structure,so joining aluminum-steel has a good application prospect.As a new solid phase joining technology foundationed on Friction Stir Welding,Friction Stir Spot Welding has advantages of simple process and high quality.It shows great potential on connecting of aluminum alloy and dissimilar metal materials.In this artical,2mm 6082 Aluminum Alloy and Q345 high strength steel are welded by Friction Stir Spot Welding,the effects of rotational speed of friction tool,dwell time of friction tool and plunge depth of friction tool on macrostructure,micro-structure and mechanical properties were analyzed,and the welding parameters were optimized successfully,The formation mechanism and fracture mode of Al-steel weld joint by Friction Stir Spot Welding joint are explored.In this paper,2mm 6082 aluminum alloy and Q345 high strength steel were welded by Friction Stir Spot Welding.By orthogonal test design,the effects factor for weld quality at different weld parameters are plunge depth,dwell time,rotational speed,respectively,and the optimal combination of different welding parameters are acquired:rotational speed of friction tool is 2400rpm,dwell time of friction tool is 40s and plunge depth of friction tool is 0.4mm,the shear maximum value reach 4.4KN in all the welded joints at the same weld parameters.The joint of Friction Stir Spot Welding in aluminum-steel is symmetrical about the rotation axis of the stirring head.There is a keyhole near the center of the stirring tool.The Hook structure is formed for the layer steel stirred into aluminum alloy,and the dissimilar material is form a complete joint by mechanically mixed and metallurgical combined.The ratio of height to width(HH/FBR)of Hook shows increases first and then decreases with the increase of heat input.HH/FBR of the Hook and the shear tensile strength have same change trend with welding parameters,while HW of Hook and the cross tensile strength have same change trend with the change of the welding parameters.The results of the micro-hardness distribution curve of weld joint showes the rules of symmetry for the center line of aluminum-steel joint.The micro-hardness distribution curve of aluminum have the maximum value,the micro-hardness distribution shows inverted "V" shape of the steel,and the highest hardness emerging the stir zone.Longitudinal micro-hardness distribution shows the interface hardness is much higher than that on both sides of the parent material,which shows the hard and brittle intermetallic compound are formed in the interface area,the intermetallic compound is AlFe3 and Al13Fe4.The joint was analyzed by scanning electron microscope,and it is found that the joint was composed of stir zone and diffusion zone.The steel and aluminum matrix form a "Hook" shaped distribution on the macro level,so mechanical connection is very strong.Two kinds of metal materials are interlaced in defferent layers at microstructure,and the intermetallic compound is formed between the atomic diffusion and the metallurgy connection between the layers.The intermetallic compound of continuous and intermittent is showed in the interface.Therefore,the aluminum-steel friction stir spot welding joint has mechanical combination and diffusion combination and metallurgical bonding.After observing the fracture surface after the shear tensile tests and the cross tensile tests by scanning electron microscope,it is found that the fracture mode is the shear fracture.A cleavage step with a river like pattern on the surface of the joint,which illustrate the fracture plain is brittle fracture on the macroscopic,and the fracrure plain is cleavage fracture on the microscopic.The fracture crack originates from the defect of the hook structure,and then extends along the intermetallic compound of the edge of weld joint,finally,the weld joint happened fracture when the crack extend all the weld joint.