| The composite structure composed of copper and steel has the dual advantages of performance and cost.It is widely used in military industry,electric power,shipbuilding,automobile,aerospace and other fields.However,the physical and chemical properties of copper/stainless steel are quite different,which leads to the metallurgical challenge of copper/stainless steel composite components in molten state.Wire-feeding laser welding was conducted with high energy density,small post-welding deformation,high welding efficiency,as well as no vacuum requirements.Furthermore,it also has the advantages of reduced assembly accuracy,convenient metallurgical adjustment,and unlimited joint forms.It is used to solve the problems of serious metal burnout,high requirements for beam accuracy,and large gap sensitivity in copper/stainless steel laser welding.Wire-feeding laser welding was adopted to weld T2 copper and 301 stainless steel dissimilar metals with 600 alloy welding wire(Ni-based welding wire),S201 welding wire(Cu-based welding wire),and ERNiCu-7 welding wire(Ni-Cu-based welding wire),the effects of welding parameters on weld formation,the microstructure characteristics at different positions of joints and mechanical properties were studied respectively.The test results show that when 600 alloy welding wire was used,as the welding power increased,the weld width tended to increase,and the weld depth and reinforcement tended to decrease.As the welding speed increased,the weld width,depth and reinforcement all tended to decrease.Under the combined action of the various forces existing in the molten pool,the copper/weld interface produced an uneven shape,with the increase of welding power,the cross-section of the joint changed from an X shape to T shape.Cu-HAZ had coarse copper grains,and the average size of the grains in Cu-HAZ increased with the increase of heat input.There was unmixed zone(UZ)at the copper/weld interface,and the microcracks in the weld of copper side disappeared with the increase of welding power.The solidification mode of the stainless steel side weld was A mode,and the composition was austenite.The copper and stainless steel base metal zone had a high proportion of small-angle grain boundaries,while the weld zone,stainless steel/weld interface and copper/weld interface all had a high proportion of large-angle grain boundaries.The weld zone had a relatively uniform hardness distribution,and the hardness in Cu-HAZ decreased.Through orthogonal test,it is concluded that the tensile strength is most affected by welding power,followed by welding speed and wire filling speed.The optimized tensile strength of the joint was253.75 Mpa,which reaches 96.29% of the tensile strength of the copper base metal.The strain in the weld zone was low and there was a stress concentration at the copper/weld interface,both of which leaded to low elongation of the joints,and the fracture mode of the joints was quasi-cleavage fracture.When S201 welding wire was used,the butt gap,the offset position of the laser beam was changed and welding heat input was increased at the same time to obtain a well-formed joint.As the welding power increased,the weld width tended to increase,and the weld depth and reinforcement tended to decrease,as the welding speed increased,the weld width,depth and reinforcement all tended to decrease.Under the combined action of various forces existing in the molten pool,the stainless steel/weld interface produced a strip of UZ,and there were many possibilities for the mixing of stainless steel and copper in the weld.The width of UZ on the stainless steel side decreased with the increase of welding power,and increased with the increase of welding speed,the increase in welding power leaded to more complete mixing of the molten pool metals.The structure in the weld was composed of fine ?-Fe and ?-Cu,along the direction of weld of copper side to weld of stainless steel side,the size of ?-Fe phase and ?-Cu phase increased,and as the welding speed increased,the size of each component phase reduced,and the ?-Fe phase was more evenly distributed in the weld.The tensile strength of the joints had reached more than 90% of the base metal,and the fracture mode was ductile fracture,the elongation of joints filled with S201 wire significantly increased,reaching more than 20%.The tensile strength of the joints decreases with the increase of the average area of grains in Cu-HAZ,there is no obvious relationship between the tensile strength of the joints and the width of Cu-HAZ.When ERNiCu-7 welding wire was used,as the welding power increased,the weld width increased,the weld depth decreased,and the weld reinforcement first increased and then decreased.Increasing welding power can effectively suppress the appearance of UZ and element transition platforms on the steel side.Under the effect of supercooling,there was also a UZ at the copper/weld interface,and there was grain refinement at the copper/weld interface.As the welding power increased,the area of the UZ at the copper side decreases.Compared with the copper base material,the grains in Cu-HAZ were not significantly coarsened.The copper/weld interface had a high proportion of low-angle grain boundaries,while the steel/weld seam had a high proportion of high-angle grain boundaries.The average grain size at the stainless steel/weld interface was large,but the average grain size at the copper/weld interface was small.There was an area with a higher hardness value in the UZ on the copper side under different welding power.In the tensile test,the tensile strength and elongation of the joints increased with the increase of welding power.When the welding power was1500 W,the tensile strength was 241.45 MPa,which reached 91.64% of the tensile strength of the copper base metal,the elongation of the joint was greater than 22.5%.When the welding power was lower than 1300 W,the fracture mode of the joints was quasi-cleavage fracture,and when the welding power was higher than 1300 W,the fracture mode of the joints was ductile fracture. |
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