Study On Metal Transfer Behavior And Welding Formation Of 6061 Aluminum Alloy Variable Polarity CMT And CMT+P

Variable polarity cold metal transfer(VP CMT)and variable polarity cold metal transfer plus pulse(VP CMT+P)are based on CMT and CMT+P technology.The heat input of VP CMT is further decrease,and the mode of heat input of VP CMT+P is more diversify.This paper aims to investigate the variety law of metal transfer process and arc morphology,compare the characteristic of heat input mode in different polarity;and explore the effects of heat input of this two welding technologies on welding formation for different thickness 6061 aluminum alloy plate.This investigation has great academic significance and application value.Firstly,the characteristics of metal transfer behavior of VP CMT and the effect of weld parameters on metal transfer behavior were investigated.In the range of appropriate weld parameters,the metal transfer mode of VP CMT is a short circuit transfer mode,the droplet radial dimension in direct current electrode positive(DCEP)cycle is small than that in direct current electrode negative(DCEN)cycle.The arc in DCEP and DCEN cycle are bell shape and inverted platform shape,respectively.Compared to DCEN cycle,base metal absorbed more heat in DCEP cycle.With the increasing of wire-feed speed(WFS),the droplet size increased firstly,and then decreased in DCEP cycle;there was no obvious change in DCEN cycle.The arc brightness and volume increased continuously.Changing the number of DCEP and DCEP cycle which won't lead to the droplet lost its stable short circuit transfer mode.Secondly,the metal transfer behavior of VP CMT+P was investigated.The transfer mode of direct current P cycle is ?one droplet per pulse?,and VP CMT cycle is short circuit transfer.The length and brightness of P cycle are more than those of CMT cycle.Negative pole heat production and bigger arc force which made base metal was heated obviously in P cycle.When the WFS arrived at 8.0 m/min,the metal transfer process of VP CMT+P was unstable,which leading to the welding process became instability and spatter appeared.And then,the effects of VP CMT welding parameters on 2 mm thickness aluminum alloy welding formation were studied.With the increasing of WFS,the heat input increased,weld penetration and width increased.In the heat-affected zone(HAZ),the microstructure developed from that part of ?'' phase transform into ?' phase into that coarse ? phase of plate shape existed.With the increasing of the distance from the center of weld bead,the micro-hardness of weld joint firstly increased,and then decreased,finally increased.The increasing of the heat input leading to the tensile strength of weld joint decreased.The highest tensile strength was 219.8 MPa among 4 samples,which was approximately 63.0% of the base metal.The fracture was located in the HAZ.Increasing the number of DCEN cycle or decreasing the number of DCEP cycle could improve the property of weld joint.The highest tensile strength was 218.4 MPa when changing the number of DCEP or DCEN cycle.Finally,the 3 mm thickness aluminum alloy was penetrated using VP CMT+P by increasing the number of P cycle or decreasing the number of CMT cycle.The highest tensile strength was 223.6 MPa,which was approximately 64.1% of the base metal.