【作者】 李俊杰；

【导师】 胡绳荪；

【作者基本信息】 天津大学 ， 材料加工工程， 2018， 硕士

【摘要】 变极性CMT及变极性CMT+P技术以CMT及CMT+P技术为基础,变极性CMT的热输入进一步降低;变极性CMT+P的热输入模式更丰富。本文旨在探究熔滴过渡过程以及电弧形态的变化规律,对比不同极性时热输入模式的特点;针对不同板厚的6061铝合金分别探索了两种焊接方法的热输入对接头成形的影响规律,具有重要的理论意义和应用价值。首先,探究了变极性CMT的熔滴过渡行为特点,以及焊接参数对熔滴过渡行为的影响。在合理的焊接参数范围内,变极性CMT焊接的熔滴过渡形式均为短路过渡,DCEP周期的熔滴的径向尺寸要比DCEN周期的熔滴小;DCEP和DCEN周期中电弧分别呈“钟罩形”和“倒圆台形”。相较于DCEN阶段,工件在DCEP阶段获得更多的热量。随着送丝速度的增大,DCEP周期的熔滴尺寸呈现先长大后减小的趋势,DCEN周期的熔滴变化不明显;电弧亮度和体积持续增加。改变DCEP和DCEN周期个数不会导致熔滴过渡失去稳定的短路过渡形式。其次,对变极性CMT+P熔滴过渡行为进行了研究。直流脉冲阶段的熔滴过渡形式为“一脉一滴”过渡,变极性CMT阶段则为短路过渡。脉冲阶段的电弧弧长和亮度均比CMT阶段大。脉冲阶段时,阴极产热以及较大电弧作用力使得工件受热更加明显。当送丝速度达到8.0 m/min时,变极性CMT+P熔滴过渡过程不稳定,导致焊接过程不稳定、出现飞溅现象。再次,研究了变极性CMT焊接参数对2 mm铝合金焊接成形的影响。随着送丝速度的增大,热输入增大,熔深和焊缝宽度增加;热影响区微观组织由有部分β’’相转化成β’相发展为存在粗大的盘片状β相;随着距离焊缝中心的增加,焊接接头的显微硬度呈现先上升再下降再上升的趋势。热输入增大导致焊接接头的抗拉强度降低,焊接试样的最高抗拉强度为219.8 MPa,大约为母材的63.0%,断裂位置位于热影响区。增加DCEN周期或减少DCEP周期个数能够提升焊接接头的性能,抗拉强度最大为218.4 MPa。最后,通过增加脉冲或减少CMT个数,采用变极性CMT+P实现了3 mm板的熔透;最大的抗拉强度为223.6 MPa,为母材抗拉强度的64.1%。更多还原

【Abstract】 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 β’’ phase transform into β’ phase into that coarse β 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.更多还原