| Selective Laser Melting(SLM)forming technology is one of the most mature and representative application technologies in the field of 3D printing technology,which can form complex and precise metal components.However,SLM technology can’t meet the high standards and requirements in the manufacturing process of large-scale metal components,so combining SLM forming technology with laser welding technology is expected to improve the forming rate of large-scale complex metal components,save costs and have huge development space.Therefore,the microstructure characteristics of 420 stainless steel SLM formed parts were studied first in this study,and analyzes the differences between them and rolled parts in mechanical properties.Then laser welding is carried out between SLM formed parts(SS-type),between SLM formed parts and rolled parts(SR-type)and between rolled parts(RR-type),and the microstructure,mechanical properties and corrosion resistance of welded joints are studied.The main research results are as follows:There are some differences between the stacking surface(C-plane)and scanning surface(D-plane)of 420 stainless steel SLM formed parts.The experimental analysis on C-plane shows that the surface is composed of α-Fe phase and γ-Fe phase,and the elements are evenly distributed.The average microhardness value of SLM formed parts shows anisotropy(Cplane: 568.37 HV,D-plane: 614.93 HV),and the microhardness value of D-plane is slightly higher,are much higher than that of rolled parts(166.37HV),and its tensile strength(1584.67±45 MPa)is also much higher than that of rolled parts(459.26±2.5 MPa),but plastic toughness is poor,which is mainly related to the phase composition and grain size of the formed material.Compared with rolled parts,SLM formed parts have a better corrosion resistance.On the premise of ensuring "one-sided welding and double-sided forming",appropriate laser welding parameters are selected,and all welded joints meet the defect evaluation grade B standard of laser welded steel.Further study on the microstructure and properties of welded joints shows that the fusion lines on both sides of SS-type and SR-type welds are obvious,and the microstructure of the two welds are similar,the size is larger than that of SLM forming base material,and the grains of RR-type welds gradually increase from the base metal to the weld center.The internal phase structures of the joints that participate in the welding of SLM parts are both α-Fe phase and γ-Fe phase,while the RR-type joints show a single α-Fe phase.The average microhardness value of SS-type and SRtype welds are similar(SS-type: 572.03 HV,SR-type: 593.2 HV),and the average microhardness value of RR-type welded joints is the smallest,which is 222.5 HV.SS-type tensile specimen is brittle fracture at the center of the weld,and the tensile strength of the weld is 773.93±10.67 MPa,but the elongation is only 1.24±0.07%,while SR-type tensile specimen and RR-type tensile specimen are fractured at the rolling side,the tensile strength is 450.46±7.6 MPa and 454.25±6.39 MPa respectively.SS-type joints have higher quality and the best corrosion resistance,followed by RR-type joints and SR-type joints have the worst corrosion resistance.In order to study the effect of solution treatment process on the microstructure and properties of welded joints,solution treatment was carried out on the welded parts,the results show that the grains grow in the weld after solution treatment,the phase composition and grain boundary orientation difference change,and the γ-Fe phase almost disappears,there is only a single α-Fe phase in the three joints after solution treatment at860 ℃,and the martensite structure in the weld is partially decomposed.With the increase of temperature,the number and size of carbides precipitated have increased in the heat affected zone of the weld of the rolled.Compared with the welded state,the average microhardness value of SS-type welds all decreased after solution treatment at 740 ℃,800 ℃and 860 ℃,decreased by 35.16%,36.89% and 9.47% respectively,the hardness of the weld after solution treatment at 920 ℃ increased by 2.14%.The average microhardness value of SR-type joints decreased at four solution temperatures,decreased by 50.45%、50.08%、38.78%、15.3%respectively,and the average microhardness value of RR-type joints also decreased at four solution temperatures,decreased by 37%,35.16%,21.23%and 19.25% respectively.Generally speaking,with the increase of temperature,the decrease range of average microhardness value of three types of welds becomes smaller.The change of weld strength at 860℃solution temperature was analyzed,and it was found that the strength of SS-type and SR-type weld was improved,while the strength of RR-type weld was decreased compared with the base metal,but the elongation was improved.Compared with the welded state,the quality of passivation film after solution treatment of three kinds of joints decreased in different degrees,and the corrosion resistance also decreased slightly. |
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