| The thesis is based on urgent demanding requirements of lightweight for Al/Steel composite structural connection in automobile and railway transportation industries.In order to resolve thick intermetallic compound(IMC)and low joint mechanical properties of fusion welding,friction stir lap welding of 6061 aluminum alloy and Q345 D steel with a thickness of 3 mm was carried out.The change of microstructure and mechanical properties of the joint under different welding process parameters was analyzed.Besides,the mechanical properties of the joints under the optimal parameters were improved by post-welding heat treatment.Effects of the change of process parameters on the interface microstructure and mechanical properties of the joint after welding heat treatment were analyzed,which provides a new idea for the application of aluminum/steel dissimilar metal joints..Under the rotation speed of 600-1400 rpm,traverse speed of 50-400 mm/min and plunge depth of 0.1-0.3 mm,there is no obvious welding defect on the surface of aluminum/steel friction stir lap weld.Different regions in the joint have different microstructure evolution due to welding thermal cycle and direct mechanical action of welding tools.According to the difference of microstructure,the different areas of the joint can be divided into the stir zone(SZ),the thermal-mechanical affected zone(TMAZ),the thermal affected zone(HAZ)and the interface of Al/Steel connection zone.When the welding process parameters were changed,different welding heat input can lead to significant changes in the interface structure.When the rotation speed was 1000 rpm,the traverse speed was 200 mm/min,and the plunge depth was 0.2 mm,a large number of amorphous structures were formed in the interface area.When the heat input is too large,a thick layer of intermetallic compounds appears at the interface.When the heat input is too small,there are no obvious intermetallic compounds at the interface,but the metallurgical bonding is poor.The joint hardness test results show that the joint hardness presents a W-shaped distribution along the horizontal direction on the aluminum side.The hardness of the stir zone is the highest,and the hardness of the advanced side is higher than that of the retreating side.The hardness distribution of the steel increases obviously along the thickness direction at the lateral interface.The joint tensile shear test shows that the maximum tensile shear load value of the joint was 6001 N with rotation speed of 1000 rpm,traverse speed of 200 mm/min and plunge depth of 0.2 mm.Through the failure analysis of the joint,under the optimal parameters,the joint fracture occurs at the interface and on both sides of the base metal.The fracture morphology analysis shows that the fracture mode of the joint is ductile-brittle mixed fracture.When the heat input is too large,the joint fails along the intermetallic compound,while the heat input is too low,the joint fails directly along the interface.The joint with rotation speed of 1000 rpm,traverse speed of 200 mm/min and plunge depth of 0.2 mm was subjected to post-weld heat treatment.The effects of different postweld heat treatment process parameters on the interfacial microstructure and mechanical properties of Al/steel friction stir lap joints were studied.The results show that the intermetallic compounds grow up obviously at high annealing temperature and long holding time,which reduces the mechanical properties of the joints.The maximum tensile and shear load under the optimal post-weld heat treatment process parameters of the joint is 8330 N,which is 38.8% higher than that of the joint without heat treatment.The fracture location and fracture morphology show that there are a lot of intermetallic compounds on the surface of the fracture under the optimal heat treatment conditions,and the crack propagation tends to be in the intermetallic compound layer on the steel side and the base metal,and the fracture form is ductile-brittle mixed fracture. |
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