Selective Laser Melting Of TC4 Titanium Alloy And Graphene Titanium Matrix Composites

TC4（Ti6Al4V）titanium alloy have been widely used in both the biomedical and aerospace industries,due to their high weight-to-strength ratio,high strength,high corrosion resistance,and biological compatibility.However,TC4 has poor wear resistance and heat transfer properties,so it is expected that graphene is used as a reinforcing phase to prepare graphene titanium matrix composites（GTMCs）to improve its wear resistance.TC4 and GTMCs were prepared by vacuum ball milling and Selective Laser Melt（SLM）technology.The surface of the graphene is plated with aluminum by organoaluminum chemical reduction to improve the dispersibility of graphene in the GTMCs.The microstructure and properties of SLM-formed TC4 and GTMCs have been studied.In addition,the molecular dynamics simulation of the aluminized graphene interface model was investigated,and the vacuum ball milling process parameters of the GTMCs powder were optimized.The following conclusions are drawn:（1）The influence of TC4 titanium alloy SLM forming process and heat treatment process on microstructure and properties was obtained.The microstructure and forming quality of TC4 titanium alloy are significantly affected by laser power（P）,scanning point distance（PD）and exposure time（ET）.The macroscopic structures of the TC4 processed by SLM consist of columnar grains that grow epitaxially.The columnar crystals largely comprise an extremely fine acicular martensiteα’ -phase with a certain orientation formed by rapid cooling and solidification during the SLM process.When the heat treatment temperature is below the critical temperature T₀（the temperature above which the titanium alloy forms theα’ -phase during rapid cooling,872℃）,the acicular martensiteα’ phase is converted into a lamellar mixture ofα+β;with the increase of heat treatment temperature,the size of the lamellar mixture structure gradually grows and thus the microhardness decreases.When the heat treatment temperature is above T₀,theβ-phase that formed during the holding time recrystallization transforms into a compact secondaryα-phase,and a basketweave structure can be found.The structure improves the microhardness of the titanium alloy.In addition,the holding time has little effect on the structure and microhardness.（2）The mechanism of interfacial bonding between Al element and graphene of Al-plated graphene was obtained by molecular dynamics simulation.The optimal adsorption site of Al atoms on the graphene surface is the central H point of the hexagonal honeycomb lattice,because the adsorption energy of Al atoms here is the highest.The Al/Gr single interface model and the Al/Gr/Al double interface model of Al element adsorbed on the surface of grapheme was established,both Al atoms and graphene layer exhibit strong interaction and close interface.Although the thermal stability of both models is improved,the Al/Gr single interface model exhibits heat shrinkage,while the Al/Gr/Al interface model exhibits thermal expansion.As the temperature increases,the Al atoms move toward the graphene,and the temperature has little effect on the graphene layer.（3）A GTMCs powder is obtained in which the graphene is uniformly distributed.With the increase of ball milling time,ball-to-batch weight ratio and ball milling speed,the ball milling energy is increased,and the graphene dispersion becomes more uniform.At the same time,the TC4 powder particles are more severely deformed,which is not conducive to the fluidity of the composite powder.After optimization,the vacuum ball milling process parameters are ball milling time 3h,ball to powder weight ratio of 8:1,and ball milling speed of 240r/min.The GTMCs powder prepared is consistent with the SLM process requirements..（4）GTMCs material produced by SLM was obtained.Compared with TC4,the morphology of the molten pool in the optical micrograph image can be clearly observed,and the microstructure is still mainly martensiteα’ phase.The high density GTMCs can be formed under appropriate SLM process parameters.The graphene is uniformly distributed in the GTMCs.Combined with microstructure analysis and XRD results,the lattice distortion occurred inside the GTMCs,which caused grain refinement.The in-situ synthesized TiC made the bonding strength between the graphene and the titanium matrix interface extremely high.The strengthening mechanism of graphene for GTMCs is fine-grained strengthening and Orowan strengthening,mainly.Both hardness and wear resistance of the GTMCs are higher than TC4.