TiC/Ti6Al4V Composites By Selective Laser Melting And Microstructure Evolution Following Different Heat Treatment

Titanium alloys have been increasingly utilised in aerospace,chemical and biomedical fields for their outstanding comprehensive properties,such as light weight,high strength,strong corrosion resistance and good biocompatibility.However,with the rapid development of science and technology,Ti6Al4 V alloy has been unable to meet the needs of aviation industry,military industry,national defense industry and other fields in terms of strength,toughness,wear resistance,etc.,and traditional forming methods cannot meet the preparation standards of complex components.Selective laser melting(SLM)forming technology is a new forming technology with high efficiency and low cost.In this paper,taking Ti6Al4 V alloy as the research object,introducing Graphene(Gr)as reinforcing phase,using SLM technology to form Ti6Al4 V alloy and titanium-based composites,and systematically studying the effect of in-situ formation of TiC on the micro-interface structure and mechanical properties of the composites.Effects of solution temperature and aging time on the evolution of microstructure and properties of Ti6Al4 V alloy and TiC/Ti6Al4 V composite under different heat treatment regimes.The main research contents and conclusions are as follows:(1)The micro interface of in situ synthesis TiC reinforced Ti6Al4 V composites formed by SLM was studied.In the XOY surface of the TiC/Ti6Al4 V composite,the number of TiC generated in situ in the overlapping area of the molten pool is more than that in the nonoverlapping area,and a large number of basket structures are formed;in the XOZ surface,the number of TiC in remelting zone is more than that in melting zone.Compared with pure Ti6Al4 V,the internal structure of the composite is significantly smaller,and the addition of graphene plays a role in refining the grains.The TiC particles are well bonded to the Ti matrix,and the mismatch degree of the TiC/?-Ti interface under a certain orientation relationship is6.2 % and 9.3 %,respectively,forming a semi-coherent interface,which makes the TiC/Ti6Al4 V composite produce strong interfacial bonding.The hardness of TiC/Ti6Al4 V composite is about 13.6% higher than that of pure Ti6Al4V;the lubricating effect of graphene and the formation of nano-TiC particles improve the wear resistance of the composite;without losing ductility,the limit of TiC/Ti6Al4 V composite is The tensile strength is about 32% higher than that of pure Ti6Al4 V.(2)The effect of solution temperature on the microstructure evolution of Ti6Al4 V alloy and TiC/Ti6Al4 V composite formed by SLM is as follows: With the increase of solution temperature,below the transformation point,the acicular martensite in the matrix of the alloy and composite material gradually increases.It transforms into lath ? phase with parallel toothlike colonies;when approaching the phase transition temperature,there are obvious coarsening alpha laths and overgrown colonies,and the shape of ? columnar grain boundaries tends to be irregular;above the phase transition point,the ? phase is obvious and uniformly distributed.With the increase of solution temperature,the grain size of Ti6Al4 V alloy becomes significantly larger than that of TiC/Ti6Al4 V composite material,and the existence of in-situ TiC particles in the composite material plays the role of grain refinement,making the internal structure more uniform.The ? phase nucleates along the ? phase boundary,and when the dislocations move to the grain boundary,dislocation accumulation occurs due to the hindering effect of the ? phase.The accumulation of dislocations increases the dislocation density and promotes the strength of the alloy.With the increase of solution temperature,the microhardness value of the alloy first decreased slightly and then increased greatly,and the highest value could reach 430 HV.The microhardness of TiC/Ti6Al4 V composites increased slightly at first and then increased significantly,and the highest can reach 607 HV,and the microhardness of the composites is always higher than that of the alloys at the same stage.The friction coefficient of alloy decreases to about 0.3 with the increase of solution temperature,and the friction coefficient of composite material first increases and then decreases to about 0.6.Solution treatment effectively reduces the friction coefficient of Ti6Al4 V alloy,but has little effect on the friction coefficient of TiC/Ti6Al4 V composites.(3)The effects of aging time on the microstructure evolution of Ti6Al4 V alloys and TiC/Ti6Al4 V composites formed by SLM are as follows: There is a small amount of acicular martensite ?' phase in the alloy matrix after aging for 4 h,and at the same time obvious Widmanderin structure appears.It has a significant effect on the ?+? structure,and the size of the martensite ? phase is limited by the size of the ? phase.The microstructure of the composite increases with the aging time,and the degree of ? spheroidization of the lath increases significantly compared with that of the alloy.The evolution process of alloy microstructure is from ? phase to ? phase transformation matrix,and then to lamellar martensite ? phase and equiaxed martensite ? phase.With the gradual increase of aging time,the microhardness value of the alloy sample showed a slight downward trend,however,the microhardness of the TiC/Ti6Al4 V composite sample decreased significantly from 463.2 Hv to 362.7 Hv,and the size of the indentation of the two continued to increase.More visible cracks appear around the material indentation.The internal structure of the aged Ti6Al4 V alloy is mostly equiaxed structure,and the equiaxed structure becomes thicker with the aging time,which has a weak effect on the hardness.After aging treatment,TiC/Ti6Al4 V composites formed more lamellar?,or merged into irregular ? phase.As the aging time increases,the size of the reinforced phase increases due to sufficient energy accumulated at the in-situ tip,which significantly reduces the microhardness of the composite.