Research On The Heat Treatment Process Of Laser Selective Melting Forming TC4

Ti-6Al-4V(TC4)titanium alloy has been widely used in astronavigation,ship,marine,weapon manufacturing industry and other fields because of high specific strength,high heat-resistant and strong corrosion-resistant.However,the traditional manufacturing technology of titanium alloy has many disadvantages,such as large energy consumption,long production cycle,low material utilization rate and the difficulty of forming complex parts,which restricts the application and development of titanium alloy.Selective Laser Melting(SLM)has advantages in rapid manufacturing,high material utilization and high forming precision.It is an important new method which enjoys great development prospects for manufacturing complex titanium alloy parts and this technology possesses a great advantage in technical application.TC4 formed by SLM has high strength but poor plasticity.At the same time,uneven residual stress is accumulated in the sample,which causes deformation and cracking of the parts.The appropriate heat treatment process can release the residual stress in the rapid melting process,control the α→β phase transformation,adjust the shape,size and content of the phase,and allow full play to the potential performance of metal materials.For this reason,this paper has put TC4 formed by SLM as the research object and used optical microscopy,scanning electron microscopy,X ray diffraction system,experimental test and simulation to systematically study the effect of heat treatment on micro structure and properties of titanium alloy.The main research work includes the following points:The effect of annealing treatment on TC4 titanium alloy was studied.The results showed that the internal structure coarsened,the volume fraction of β phase increased,the strength decreased,the plasticity increased first and then decreased with the increase of heating temperature.The influence of residence time and cooling rate on microstructure and properties depended on temperature,and they mainly adjusted the size of microstructure.After annealing treatment at 840℃ for 2h,followed by AC,the tensile strength of the sample was 950MPa,the ductility increased to 18%and a better comprehensive performance was obtained.By solution treatment in two phase zone or solution and aging treatment,the staggered basket-weave(α+β)microstructure or homogeneous disperse(α+β)phase was obtained with the increase of β phase content and α phase grain size,the strength and hardness of the samples decreased obviously.A worse match of strength and plasticity was obtained than annealing at 840℃.The effect of hot isostatic pressing on the TC4 formed by SLM was studied.The results showed that hot isostatic pressing at low temperature was difficult to reduce porosity,achieve thermal densification and adjust macro-properties.When the temperature was close to the βtransus temperature,TC4 consisted of the staggered basket-weave microstructure and lamellar microstructure,the width of grain increased substantially,the thinner lamellar α phase was broken and dissolved.The porosity in the sample decreased obviously and the density increased.A better match of strength and plasticity was obtained than the annealing,solid solution and solution and aging treatment.