Numerical Simulation And Characteristics Research On The Weldability Of CMT Welding For TC4 Titanium Alloy

Titanium alloy has a series of excellent properties such as high specific strength,low density,strong corrosion resistance,high impact toughness and strong fatigue resistance,which are widely used in various fields.Traditional titanium alloy welding methods such as TIG welding,MIG welding,plasma welding and electron beam welding have the disadvantages of overheating,high spatter and coarsening of the grain of the welded joint.As titanium alloy has a small modulus of elasticity,the welding deformation is large,and the deformation after welding is usually difficult to correct.In response to these problems,this paper investigates the numerical simulation and characteristics on the weldability of CMT welding for TC4 titanium alloy using the CMT(Cold Metal Transfer)welding technique with low heat input,low spatter and low post-weld deformation.In this paper,a cold metal transfer(CMT)welding model based on time interval loading and dual ellipsoidal volume heat flow distribution was established using the APDL language in ANSYS software to numerically simulate the temperature field,stress field and deformation of TC4 titanium alloy.The CMT welding heat source was loaded in an intermittent mode.The results of numerical simulations showed that with the loading of the heat source,the temperature of the thermal cycle curve in the form of fluctuations gradually increased to a maximum temperature of 3950 ℃.During the cooling process,the temperature showed a smooth decreasing trend.The residual stresses were concentrated at the weld and heat affected zone.And the maximum equivalent force at the center of the weld was 730 MPa.The maximum deformation occurred in the middle of the specimen at the weld,which was 0.44 mm.At the same time,we compared the temperature field,stress field and welding deformation calculation results of MIG welding and CMT welding of TC4 titanium alloy.It could be found that the maximum temperature of CMT welding of TC4 titanium alloy was reduced by 37% compared with MIG welding.And it was also found that the residual stress distribution area and post-weld deformation of CMT welding were reduced compared with MIG welding.The results of the influence of welding parameters on the distribution characteristics of the CMT welding temperature field,stress field and deformation showed that when the wire feed speed increased from 4.5 m/min to 6.5m/min,nonpenetration was changed into full penetration.Besides,the peak temperature of the joint increased.The range of residual stress concentration area increased and the post-weld deformation increased.When the welding speed increased from 0.45 m/min to 0.6 m/min,full penetration was changed into nonpenetration.Besides,the peak joint temperature decreased,the range of residual stress concentration area decreased and the post-weld deformation decreased.The test results showed that when the wire feeding speed increased,the weld bead of the weld gradually widened.The weld width,penetration and depth-width ratio of the joint increased.The heat affected zone expanded and the β grain in the weld had a clear growth trend.The martensite coarsened.When the welding speed increased,the weld bead of the weld gradually narrowed.The weld width,penetration and depth-width ratio of the joint gradually decreased.The range of the heat-affected zone became smaller and the β grains in the weld were refined.With the increase of the arc length correction coefficient,the weld width increased first and then decreased.The depth-width ratio of the weld decreased first and then increased.The range of the heat affected zone did not change significantly.TC4 titanium alloy CMT welded joints in the weld zone formed Widmanstatten structure and basketweave structure.The coarse grain area,fine grain area and transition area of heat-affected zone formed a small amount of basketweave structure,fine needle martensite and massive α phase,respectively.And the base metal area was a typical α+β two-phase microstructure.The average hardness of the weld,heat affected zone and base metal for TC4 titanium alloy were 385 HV,369 HV and 336 HV,respectively.The microstructure of the weld and heat-affected zone consisted mainly of the α’ phase.And the β phase with a much smaller content than the α’ phase.The joint of CMT welding fractured in the base metal and the weld strength was1058.13 MPa.The fracture mode was ductile fracture.CMT welded joints of titanium alloys without additional gas shielding generate an oxygen-rich α layer on the surface with an average thickness of 3.06 μm.At the same time,the joints without the use of external gas shielding device fractured at the weld and heat-affected zone.The strength of the joint reached 94% of the base metal.The fracture mode was ductilebrittle mixed fracture.The presence of the oxygen-rich α layer slightly reduced the tensile strength and plasticity of the joint.