Effect Of Cryogenic Treatment On Properties Of Selective Laser Melting Formed Dental Co-Cr Alloy

Compared with the traditional casting method,selective laser melting（SLM）technology has the characteristics of rapid forming of complex parts and short molding cycle,which can meet the requirements of personalized medical design and small batch production,and has a wide application prospect in the manufacturing of medical implants.However,due to the rapid melting,cooling and solidification crystallization of SLM shaped samples,they tend to accumulate a lot of residual stress and produce defects,which have a negative impact on the performance.Machining parameters optimization and heat treatment can effectively weaken or eliminate defects and improve mechanical properties to a certain extent.Nevertheless,the improvement performance of the above methods is limited.Cryogenic treatment technology can further improve the mechanical properties of alloy,such as toughness and strength,and explore the influence of cryogenic treatment process on the microstructure and properties of selected laser melting forming samples,which has well research significance and application value to broaden the application of SLM technology in the medical field.In this paper,for the dental Co-Cr alloy powder material,the SLM molding sample was printed,the heat treatment and cryogenic treatment were carried out.Optical microscope（OM）,scanning electron microscope（SEM）,X-ray diffractometer（XRD）and other equipment were used to characterize the microstructure,and nano indentation instrument and tensile equipment were used to test the mechanical properties.The specific heat treatment method is to heat the printed sample to 1150℃for 0.5h,1h,2h annealing(labeled as FC_0.5,FC₁,FC₂),and heat the printed sample to1150℃for 1h,2h annealing to 900℃water-cooled quenching（labeled as WC₁,WC₂）.There were 5 groups of heat-treated samples with different microstructure and morphology.Cryogenic treatment is to cool the sample to-196℃for 3h and then return to room temperature.Firstly,the optimization of process parameters was mainly carried out in the paper.Next,the heat treatment samples of different structure forms were formed after different heat treatment of the printed samples,and the heat treatment samples were cryogenic.The changes of microstructure and mechanical properties of Co-Cr alloy before and after cryogenic treatment were observed,and the strengthening mechanism of Co-Cr alloy after cryogenic treatment was obtained.The main conclusions are as follows:1.For annealed samples,grain size grows gradually with the extension of holding time;After holding for 0.5h,short strip precipitates were precipitated inside the grain,and the precipitates gradually disappeared after holding for 1h.After annealing,the strength of the printed sample decreases and the elongation increases significantly.After cryogenic treatment,austenite is decomposed into martensite,and a large number of fine carbides are precipitated inside the grain,which plays a significant role in dispersion strengthening of the metal matrix.Among them,the elongation of the FC_0.5sample is 20%,which is significantly improved to 32%after cryogenic treatment,indicating that cryogenic treatment can increase the elongation of the alloy.2.For quenched samples,martensite transformation occurs under rapid cooling,with more martensite than annealed sample,and a large amount of granular carbides andσphases precipitate inside the grain.With the extension of holding time,σphase and carbide grow and coarser gradually.Quenched samples after cryogenic treatment,martensitic transformation occurs to the residual austenite,and fine carbides precipitate inside the grain.The tensile strength of WC₁sample increased by 16.3%,and the elongation increased from 10.16%to 28%.The strength and elongation are obviously enhanced.3.After cryogenic treatment,the yield strength,elongation and microhardness of samples are improved;The coarse austenite is easier to be"split"by martensite during cryogenic treatment,and the grains can be refined.The mechanical properties of long time heat preservation heat treatment samples are improved more obviously after cryogenic treatment.The strengthening mechanism changes from solution strengthening and precipitation strengthening before cryogenic treatment to dispersion strengthening and fine grain strengthening after cryogenic treatment.