| 300M ultra-high strength steel is an ideal material for manufacturing critical components with high performance in aviation and aerospace vehicles.However,the traditional manufacturing methods for 300M steel have the shortcomings such as the low material utilization ratio,long production cycle and low yield,which are difficult to meet the application requirements of complex structural parts.As a new high-efficiency additive manufacturing process,High power selective laser melting(HP-SLM)is expected to provide a new solution for the high-efficiency manufacturing of complex 300M steel components with high performance.But so far,the relevant fundamental researches are relatively deficient.Therefore,this paper systematically studies the formability,microstructure and mechanical properties of 300M steel produced by HP-SLM.Subsequently,the microstructure and mechanical properties of the as-deposited 300M steel samples were regulated by subsequent heat treatment and the mechanisms of strengthening and toughening were revealed.The main research contents and results are as follows:(1)The influence of process parameters on the formability of the 300M steel single-track,single-layer and bulk samples formed by HP-SLM was systematically studied and the optimization window of process parameters for forming near full-dense samples was obtained.In order to obtain the dense samples of 300M steel deposited by HP-SLM,the flexible powder laying strategy needs to be used to ensure the stability of the powder-spreading process.Meanwhile,a laser energy input greater than 77.10 J/mm~3 needs to be adopted to obtain the molten pool with the sufficient penetration depth to ensure the good lapping in the horizontal and vertical directions,combining with a scanning speed lower than 400 mm/s to ensure that the high-power laser beam has enough time to completely melt the metal powders with uneven thickness and splash particles.Moreover,the build-up rate of HP-SLM is significantly improved with the increase of laser power and reaches a maximum of～21.6 mm~3/s,which is more than 6 times higher than that of the traditional SLM.(2)The microstructure characteristics and evolution mechanisms of 300M steel formed by HP-SLM were studied,the influence of laser energy input on the microstructure and mechanical properties of the as-deposited samples was investigated,and the relationships between process parameters,microstructure and mechanical properties was established.The microstructure of the as-deposited samples is mainly a mixture of tempered martensite,bainite and a small amount of retained austenite.The columnar prior austenite crystals exhibit a symmetric growth along the center of the molten pool.Each columnar crystal has a large number of dendrites and there is element segregation between the dendrites.With the increase of the laser energy input,the size of columnar prior austenite grain,primary dendrite spacing and the content of retained austenite all increase;the hardness and strength of the as-deposited samples reduce significantly,while the elongation changes slightly.(3)The effects of quenching and tempering on the microstructure and mechanical properties of the as-deposited 300M steel samples were studied,the mechanism of strengthening and toughening was analyzed,and the corresponding heat treatment process was established.After quenching,the columnar prior austenite grains of the as-deposited samples become equiaxed and the dendritic morphologies disappear completely.Based on this,the relationship between on the austenite grain growth and austenitizing temperature of the SLMed 300M steel has been established.The microstructure of the as-quenched samples is mainly quenched martensite,with a small amount of retained austenite,bainite and grain boundary allotriomorphs.With the increase of austenitizing temperature,the hardness and strength of the as-quenched samples first increase slightly and then decrease significantly,while the elongation changes slightly.The microstructure of the as-quenched samples after tempering treatment is mainly a mixed structure of tempered martensite,a small amount of bainite and retained austenite.The tempering temperature has little effect on the size of the prior austenite and martensite,but has a significant effect on the transformation behavior of carbide precipitates.With the increase of tempering temperature,the martensite decomposes andε-carbides in the tempered martensite transform into cementite particles;the hardness and strength of the as-tempered samples decrease significantly,but the elongation changes slightly.The optimal heat treatment process of300M steel samples formed by SLM is 900℃,1 h/oil quenching+300℃,2 h/air cooling,twice.(4)On the basis of forming full-dense samples,the universality of the optimal heat treatment process for 300M steel samples formed by SLM under different laser energy input was studied.The microstructure of the as-deposited 300M steel samples modulated by the optimal heat treatment process is composed of tempered martensite,a small amount of bainite and retained austenite.After heat treatment,the hardness,strength and elongation of the samples all change slightly with the increase of laser energy input.The strength(ultimate tensile/yield strength:～2099 MPa/～1834 MPa)of the as-deposited 300M steel samples modulated by the optimal heat treatment process is higher than that of the 300M steel forgings and the elongation(～10.2%)is comparable to that of the forgings.Therefore,the optimal heat treatment system is universal for the as-deposited 300M steel samples. |
