Research On Selective Laser Melting Process And Properties Of 316L Stainless-Steel And Titanium Alloys

Selective laser melting(SLM),as a method of additive manufacturing,has the characteristics of simple design process,high precision of formed parts and excellent mechanical properties,creating a broad application prospect.With their respective advantages,316L stainless steel(316L SS)and titanium alloy have great application value in the aerospace and biomedical fields.For example,stainless steel crowns,surgical instruments,titanium alloy heat shields,artificial joints,etc.These parts require high precision and even be individually customized,and the processing of titanium alloys is very difficult.The technological characteristics of SLM just meet the above requirements,so in recent years,researchers have invested a lot of attention in the SLMed 316L SS and titanium alloys.However,the strength of 316L SS is limited,and it also puts forward higher requirements for corrosion resistance when serving in a special environment;in the field of biological applications,the titanium alloy has a large difference from the human bone,and the corrosion resistance needs to be further improved.Therefore,in this study,316L SS was doped with Co-Cr-Mo-W and Fe-based amorphous powder,and formed with different SLM forming parameters.The mechanical properties and corrosion resistance of 316L SS and its composites were studied.Through different laser parameters,SLM experiments were performed on the prefabricated Ti-Cu-Zr-Fe-Sn-Si-Nb titanium alloy.An amorphous coating is formed in situ on the surface of the titanium alloy,providing a certain reference value for the performance improvement of the 316L SS formed by SLM and the biomedical titanium alloy.First of all,in order to improve the corrosion resistance of SLM forming 316L SS,different laser parameters were used for selective laser melting of Co-Cr-Mo-W composite powder in this paper.The relative density and metallographic analysis show that the optimal laser power and scanning speed of 316L SS and the composite are 200 W,800 mm/s and 170 W,680 mm/s,respectively.Under optimal parameters.the relative density of both materials can reach more than 99%.The pitting potential of the composite increased by 277 mV compared with that before mixing,and the corrosion current density decreased from 1.08 × 10-5 A/cm2 to 1.70× 10-6 A/cm2.This is because the Cr element contained in the Co powder promotes the formation of a Cr-containing passivation film.Cellular sub-grains are preferentially corroded due to more grain boundaries exposed in the corrosive medium than dendritic sub-grains.At the same time,the addition of elements with large atomic radius causes lattice distortion.The strength of the composite sample has been slightly improved,the tensile strength has increased by about 50 MPa and maintains a high elongation of 40%.Secondly,the mechanical properties of amorphous reinforced SLMed composites were investigated.After strengthening,the sub-grain size becomes smaller and denser.Insufficient atom diffusion causes element segregation to eventually lead to grain boundary strengthening,providing a hindrance to dislocation movement.In the early stage of plastic deformation,the total strain mainly comes from cellular sub-grains.In the later stage of plastic deformation.the dislocations accumulate in the sub-grain boundary and are difficult to move.The contribution to the total strain is transformed into the shear movement of the dislocation along the dendritic sub-grain,which also makes the strain hardening rate drop sharply.The tensile fracture shows a mixed morphology of a large number of intergranular ductile fractures and a small number of transgranular ductile dimples.Compared with the 316L SS sample,the amorphous reinforced composite material achieved a 33%improvement in Vickers hardness.The nominal yield strength has increased by about 41%.The ultimate tensile strength increased by nearly 49%,reaching 1123.88±14.54 MPa.And the total strain has reached 0.23±0.01.Finally,the microstructure and properties of prefabricated Ti-Cu-Zr-Fe-Sn-Si-Nb alloy sheet treated with different laser parameters were studied.Under the action of laser heat,the cross section of the sample presents three regions with different morphologies.The dense amorphous coating is distributed on the surface of the material.The fish-scale heat-affected zone is composed of nanocrystals,which are grown radially and connected into Cu4Ti3 dendrites in the matrix zone.The effective elastic modulus of the amorphous region(123.22 ± 4.64 GPa)is significantly lower than that of the matrix region(149.03 ± 5.76 GPa)and the heat-affected zone(153.22±3.96 GPa).Zr,Ti and Nb elements have a strong affinity for oxygen,so they easily form ZrO2,TiO2 and Nb2O5 on the sample surface after polarization,preventing further contact between the corrosive solution and the alloy.In addition,the amorphous coating has no element segregation regions and local dissolution,which can avoid the formation of the galvanic coupling effect and reduce the corrosion current density by 15%.