Study On Weld Forming,Microstructure And Mechanical Properties Of Titanium Alloy Pipeline Produced By All Position Plasma Arc Welding

Due to its excellent corrosion resistance,high specific strength and high impact and fatigue resistance,titanium alloys are widely used in petrochemical,ocean engineering,aerospace and other fields.Since most of them are thick-walled pipes,and the pipe is difficult to be deformed during welding,the all position welding is increasingly used in engineering.High-energy beam welding is widely used in the welding of titanium alloys due to its strong penetration ability,fast welding speed and good joint performance.As a kind of high-energy beam welding,plasma arc welding has the advantages of low cost,simple operation and low working environment requirements,and is an ideal choice for welding titanium alloy.Therefore,for the high-quality welding of titanium alloy thick-walled pipes,the research on all position plasma arc welding has broad development prospects.This paper selects TA2 and TC4 titanium alloy pipes of?219×8.8mm as the test materials,and adopts the newly developed all position plasma arc welding automatic welding equipment to carry out butt welding test of I-type groove,no gap and no filling.The backside gas protection shield was optimized.The effect of welding parameters on weld forming,the forming mechanism of all position plasma arc welding and the microstructure and mechanical properties of welded joints were studied.The following results were obtained:(1)The double-layer gas screen and liquid cooling are used to improve the design of back protection shield,so that the shielding gas reaches a good flow state at the outlet,the protection effect and service life of the optimized back protection shield is significantly improved.(2)A single factor test was carried out at the position of the overhead welding,and a good specification range of TA2 weld forming was obtained,which provided the basis for the process test design.The process test was carried out by the response surface method,and the quadratic regression equation model between weld forming parameters and welding parameters were established.Through the single factor and multi-factor interaction analysis,the main factors affecting the weld front and back melt width are the pulse peak current I_p,the plasma arc gas flow Q and the welding speed v,and the main factors affecting the weld front and back residual height are the welding position P and its composite factors.Through the software prediction and experimental verification,the TA2 welding parameters with good weld forming were obtained,and the TC4 welding parameters with good weld forming were obtained by further experiment optimization.(3)By analyzing the force of the perforated molten pool at all positions,it is concluded that gravity is the main cause of the instability of the all position weld pool.According to the force balance criterion of different welding positions,The welding parameter control measures for good weld forming are obtained.(4)After X-ray inspection,all position welds of TC4 have no defects.The transition zone of each characteristic position weld joints are mixture of?phases during the phase change process.The weld zone and the heat-affected zone phase composition of each characteristic position weld joints are basically the same,and mainly consist of basket-shaped?'phase,needle-like?phase and coarse?phase.The tensile properties and the impact properties of the welded joint after the process optimization are improved.The tensile properties and the impact properties are good.Both the tensile specimen and the impact specimen are microporous aggregate plastic fracture,and the joint toughness is good.The average value of the microhardness of the weld zone and the heat affected zone is higher than that of the base metal of each feature welded joints.It shows that the all position PAW welding process of titanium alloy pipeline can meet the practical application requirements of the project.