Study On Process Of Hybrid Laser-arc Weld With TC4 Titanium Alloy

Based on the study on Laser-arc hybrid welding of medium thick titanium alloy is relatively scarce the problems of surface oxidation and porosity during welding with titanium alloy,in this paper,8mm thick TC4 titanium alloy was welded with Laser-arc hybrid welding,exploreing appropriate technology to ensure stable welding process and ideal weld forming,the formation mechanism of pore was analyzed by combining with the welding stability,characteristics observation and variation law of pores.The droplet force model and the plasma-arc coupling model are established.The interaction between plasma and arc,the necessary conditions of droplet transition are analyzed.The electrons in the photoplasma can attract and stabilize the electric arc.The necessary condition for droplet transition is that the force promoting droplet transition along the wire is greater than the force hindering droplet transition.The study of welding stability shows,the main reason why droplet transition form and arc,plasma dynamic behavior can make a different is the change of technological parameters.The change of droplet transition form,equency,critical diameter and diatance between droplet and laser beam are closely connected with welding stability."Climbing" and "drifting" of the arc,the separation of arc from the photoinduced plasma and condensation of arc impact coupling state of arc and plasma,which has the characteristics of instability.When technological parameter is appropriate,droplet smoothly and uniformly transit,the coupling effect of two heat sources is great,which make for to the promotion of welding stability.The study of joint weld formation and microstructure shows that the oxidation degree of weld surface can be reduced by decreasing heat input and increasing the proportion of helium in shielding gas.With the increase of heat input,the weld morphology transitions from "T" type to "X" type.Too large or too small welding speed will bring about formation of undercut defects.The increase of welding current and the proportion of helium in shielding gas is beneficial to improve the undercut defect.The weld of TC4 titanium alloy consist of martensite ? ',grain boundary ? and transformed ? columnar crystal,which has the characteristics of simultaneous crystallization and epitaxis.The amount of martensite ? ',the size of grain boundary ? in weld is in connection with heat input.When heat input is larger,the existence of blocky ? was found in the weld.Welding speed has no evident impact on amount and arrangement of martensite ? ' and grain boundary ? in the weld.The size and growth trend of ?-columnar crystals in laser zone and arc zone are affected by the change of welding speed.There are two kinds of pores in TC4 titanium alloy weld.The shape of Process characteristics pore is variable and its size is large,the trace of metal flow on the hole wall,whose formation originate from instability of the welding process.The pores with round shape and small size are metallurgical characteristic pores,most of them show an aggregation trend,which is formed by the accumulation and growth of the supersaturated hydrogen and the protective gas from the back of the weld that enters the molten pool and adheres to the "pore core" formed by the oxide which locate on the surface of titanium alloy.The study on the influence about technological parameters on weld porosity shows that weld porosity reduces but its size largen when laser power magnifies.The impact of other technological parameters on weld porosity originates from the change of welding stability.