Study On Design And Preparation Of Aluminum And Titanium Base Light Alloys And Composites Fabricated By Selective Laser Melting

Aluminum and titanium alloys are commonly used light-weight materials for high-end equipment due to the high specific strength and toughness.Selective laser melting(SLM)is one of the typical metallic additive manufacturing technologies based on powder bed,which can realize the integral fabrication of complex components with controllable geometry.However,due to the extreme non-equilibrium solidification intrinsic in the SLM process,traditional alloys are facing chanllenges that it is difficult to restrain metallurgical defects,tailor non-equilibrium microstructure,control stress and deformation,and enhance comprehensive properties.In this paper,the SLM process of Al-Cu-Mg and Ti-6Al-4V alloys were studied,including the formation mechanism of metallurgical defects,the material composition design method specific for SLM,the strengthening and toughening mechanism,and the design and preparation methods of multiple material lattice structures.Aiming at the problem that it is hard to manufacture wrought aluminum alloy via SLM,the studies on the SLM process simulation,thermodynamic calculation,SLM and heat treatment process of Al-Cu-Mg alloy,were performed.It was found that the process optimization method could effectively suppress the generation of pores,but could not completely eliminate the cracks.The formation mechanism of cracks was that the maximum stress concentration occurring at the molten pool boundary and heat affected zone exceeded the ultimate strength,and the liquid film between the the columnar grains inside the molten pool would cause shrinkage during solidification and lead to cracks.Through the calculation of crack susceptibility index,growth-restriction factor and phase diagram,it was found that the crack initiation of Al-Cu-Mg alloy could be effectively inhibited by the modification of Ti element and Ti B₂ particles.Through the modification of 1.5 wt.%Ti,a large number of Al₃Ti nano-particles were formed during the SLM process as nucleating agent,which effectively promoted the grain refinement and columnar-to-equiaxed transition,inhibiting crack initiation.The Ti-modified Al-Cu-Mg alloy had good mechanical properties with ultimate tensile strength of 403.4 MPa,yield strength of 373.2 MPa,and elongation at break of 9.3%.The microstructure of 1-3 wt.%Ti B₂/Al-Cu-Mg-Ti composite exhibited homogeneous and refined equiaxed grains.With the increase of Ti B₂ content(1-3 wt.%),the tensile strength of the composites was 486.3 MPa,551.2 MPa and 592.2 MPa,respectively.The the yield strength was 453.7 MPa,523.4 MPa and 570.8 MPa,respectively.The ductility was 13.9%,10.1%and4.8%,respectively.After T6 heat treatment,the yield strength of 1 wt.%Ti B₂/Al-Cu-Mg-Ti composite increased to 504.1 MPa,while the elongation at break decreased to 10.8%.Aiming at the technical bottleneck that it is difficult to tailor the non-equilibrium microstructure of SLM-fabricated Ti-6Al-4V,the studies on the SLM process of Ti-6Al-4V alloy and Ti B/Ti-6Al-4V composite,the heat treatment process and the fabrication of gradient-material lattice structure were performed.It was found that the microstructure of Ti-6Al-4V alloy presented acicular?'martensite and coarse?grains,which showed high tensile strength(1149.6 MPa)and yield strength(1076.4 MPa)but low elongation at break(6.6%).Ti B₂particles added in the Ti-6Al-4V powder were in situ reacted with Ti to form acicular Ti B particles during the SLM process.With the addition of 1 wt.%Ti B₂,the compressive strength and yield strength were 1539.3 MPa and 1337.7 MPa,respectively,and the fracture strain was9.7%.When Ti B₂ content further increased,the porosity increased and mechanical properties decreased.After the solution heat treatment at 1000°C and aging heat treatment at 600°C on the Ti B/Ti-6Al-4V composite with 1 wt.%Ti B₂ addition,the matrix was transformed into homogeneous(?+?)dual-phase microstructure,with yield strength of 1146.8 MPa,ultimate compressive strength of 1680.1 MPa and plasticity of 20.1%.The SLM-fabricated Ti B/Ti-6Al-4V gradient-material lattice structures showed good metallurgical bonding at the interfaces of graded layers.Compared with the homogeneous Ti-6Al-4V lattice structure,the gradient-material lattice structure had higher yield strength and elastic modulus.Finally,the Al-Cu-Mg-Ti/Ti-6Al-4V multiple material component was prepared by SLM.It was found that there were numerous metallurgical defects at the interface.When a pure Cu interlayer was introduced between Ti-6Al-4V and Al-Cu-Mg-Ti alloy to avoid the direct contact between Al and Ti,the metallurgical bonding effect of the interface was greatly enhanced.During the compression process of the multiple material lattice structure,the Al-Cu-Mg-Ti alloy layers broke earlier than those of Ti-6Al-4V,indicating that the different regions with different material distribution had different mechanical properties.