Microstructure Evolution And Process Optimization Of Additive Manufacturing TC4 Titanium Alloy Assisted With Ultrasonic Micro-Forging Treatment

Titanium alloys is widely used in aerospace and other fields due to its advantages of low density and high specific strength.Metal additive manufacturing(AM)technology can produce high performance parts accurately and quickly through the gradual accumulation of materials.It is especially suitable for the manufacture of complex parts that are difficult to process in aerospace and has great advantages in shortening the manufacturing cycle.However,the problems such as microstructure inhomogeneity and insufficient comprehensive mechanical properties seriously limit the wide application of metal AM parts.In this paper,the ultrasonic micro-forging treatment(UMFT)is used to assist the laser AM TC4 titanium alloy.During the AM,the surface of the part is forged and rolled at high frequency to achieve grain refinement,defect reduction,residual stress reduction and properties improvement.The effects of different process parameters on the macroscopic forming quality,microstructure and microhardness of TC4 alloy were researched by OM,SEM,TEM and other material analysis methods.The microstructure evolution and interfacial regulation mechanism under the action of UMFT were studied,and the defect healing mechanism and equiaxed grain regulation mechanism under the action of multi-physical coupling were revealed.The research results have positive guiding significance for the production of high performance TC4 alloy AM parts.The main research results are as follows:(1)Based on the relationship between laser process parameters and macroscopic forming quality,a mathematical model y=a*PαVβFγ+b was established for the combination of process parameters and macroscopic geometric parameters of morphology.According to the contact angle θ and dilution rate η as a good metallurgical bonding index deposition and substrate,we get a better range of parameters,combined with lateral/vertical overlap rate model and cross section morphology,the lateral overlapping rate(30%-40%)and vertical lap rate(10%～30%)was determined.(2)The study of single layer TC4 deposition layer fabricated by laser fuse shows that UMFT can effectively reduce the surface roughness,forge surface grains and improve the hardness while causing plastic deformation on the surface of the deposition layer(the maximum plastic deformation is 151 μm).With the increase of ultrasonic amplitude,the roughness and macroscopic dimension had little influence,but the proportion and average grain size of equiaxed grains in the deposition layer improved,and the hardness increased slightly.With the increase of pressure,the depth of micro-forging increases,the grain equiaxed trend is enhanced,and the surface hardness increases by more than 10%on average.(3)The results show that the coarse columnar grains are transformed into a large number of cellular grains in the multilayer TC4 with UMFT,and the α’ average grain length is reduced from 22.5 μm to 9.4 μm,the grain orientation tends to be isotropic,a certain amount of dislocations appear in the crystal,and the microhardness increases by 20%on average,especially at the top of the deposition layer,the maximum hardness value is above 440 HV.(4)Combining the solidification theory of metal and UMFT effect,the process parameters and microstructure evolution model were established,and the key parameters range of equiaxed grain formation was obtained.By optimizing the process parameters,the columnar grain was transformed into equiaxial grain to obtain excellent mechanical properties.The interface of the deposition layer is mainly colony α with a width of 100～300 μm,and the interface layer fluctuates due to UMFT.In addition,UMFT can greatly reduce internal defects and refine pores,and the mechanism has been further studied.