Research On Welding Process And Heat Treatment Process Of Heavy Thickness TA15 Titanium Alloy By Electron Beam Welding

In this article,the keyhole effects of heavy thickness(≥70mm)TA15 titanium alloy with electron beam welding was researched through the study on the influence of different welding parameters on the weld shape of cross section.Meanwhile,the heterogeneity of welded joint were studied,and the change rule of microstructures and mechanical properties of welding joint along weld width and weld depth were revealed.In addition,the heat treatment strengthening process of TA15 electron beam welded joint was explored.The research results show that the weld shape of cross section do not change when using the same welding parameters on different thickness of TA15 titanium alloy by electron beam welding,but the overall weld shape size decreases with the increase of thickness.The electronic beam current has little effect on weld shape,with beam current increases,the weld depth increases rapidly at first then increases slowly,and the weld width shows slow growth then rapid growth;Change the focusing current can change the shape of weld,when the focusing current gradually close to the active zone of EB focus,the weld width on surface and weld depth increases,weld width decreases,in the active zone of EB focus change the focusing current,the depth of t weld does not change.It is found that a good parallel weld can be obtained by integrative adjusting the beam current and focusing current.The research on the microstructure and properties of welding joint along weld width and weld depth show that the weld zone composed of columnar β grains,and in which the transgranular acicular α? martensite were dispersedly distributed,with increase of the distant from near weld zone to the near base metal in HAZ,α? martensite decreases,primary α phase increased;The microhardness of weld joint show that the weld zone is the highest,the heat affected zone is the second,the base metal is the lowest,the tensile strength and yield strength at room temperature are higher than that of the parent metal,and the yield strength is more obvious,the elongation and the reduction of area of the weld joint are reduced by 37.2% and 8.9% respectively compared to the base metal.Along the weld depth direction,the microstructure of weld metal unchanged,but the size of β and α? grains decreased;the microhardness of weld metal and weld joint were almost invariable along the depth direction;with the increasing of depth,the strength of weld joint decreased slightly and the plasticity increases,the tensile properties of weld metal at room temperature did not change significantly;The impact toughness and fracture toughness of the joint decrease with the increase of depth,and the impact toughness decreased by 15.3% and fracture toughness decreased by 17.6% respectively in bottom of joint compared with the top of joint.The effects of different heat treatments temperature at 970℃,860℃ and 800℃ on the microstructure and mechanical properties of the joints were studied,the results show that the structure of parent metal was still duplex after different heat treatment process,the acicular secondary α grains lying on transformed β phase merged and grew up with the increase of the heat treatment temperature,and the heat affected zone near the weld zone and weld zone occurred different degrees of α?→α phase transition,the higher the temperature the higher the phase transition;With the increase of heat treatment temperature,the microhardness of the weld metal decreased gradually,the tensile strength at room temperature and high temperature of weld joints decreased,the elongation and the reduction of area increased,the impact toughness increases rapidly,and the impact toughness at room temperature and high temperature increases by 111.6% and 72.8% respectively.When the heat treatment temperature is 860℃,the weld joint has excellent fatigue property and its fatigue limit was 734.5MPa,increased by 5.7% and 8.2% respectively compared to 800℃ and 970℃.When the stress level is higher than 700 MPa,the weld joint after heat treatment at 970℃ has higher fatigue life than that of 800℃heat treatment,at low stress(less than 700 MPa),the weld joint after heat treatment at 800℃ has better fatigue property compared to 970℃.