Effect of Ultrasonic Nanocrystal Surface Modification on Microstructure and Mechanical Properties of SLM 316L Stainless Steel
Metal powder additive manufacturing(AM)technologies,such as selective laser melting(SLM),have attracted considerable interest owing to their near-net forming characteristic and layer-by-layer building-up strategy,which allows overcoming the constraints of traditional manufacturing technology,achieving complex components in a short time of mass customization.However,the SLM process-induced micro-defects(i.e.pores,lack-of-fusion,and undesired microstructures)will result in not only poor surface finish and interior thermal cracks but also more dispersion of mechanical properties.Therefore,for a more homogenized microstructure and smaller material anisotropy,a novel surface strengthening method of severe surface plastic deformation,ultrasonic nanocrystalline surface modification(UNSM)was applied to improve the surface integrity and mechanical properties of SLM 316L stainless steel in this study.A medium size laser powder bed fusion(LPBF 271 Series device from Farsoon Technologies Tech Co.,Ltd.)was used to fabricate the plate specimens with 316L stainless steel powder.The SLM-processed samples were fabricated using the optimized processing parameters with a laser power of 400 W,hatch spacing of 0.11 μm,a laser scan speed of 1 250 mm/s,and a layer thickness of 60 μm.Bidirectional laser scanning with a scan rotation of 67° for every layer was performed during building.An annealing heat treatment at 900℃for 2 h was conducted on the as-received SLM 316L stainless steel plate,followed by furnace cooling to room temperature.The SLM 316L stainless steel plate used in this investigation was 40 mm×20 mm×4 mm in size.For improving the strengthening efficacy,optimized UNSM process parameters were used in the current work:an ultrasonic frequency of 20 kHz,a WC(tungsten carbide)tip with a diameter of 2.4 mm,a static load of 50 N,an ultrasonic amplitude of 30 µm,a scanning speed of 500 mm/min,and a feed rate of 10 µm.The surface integrity,microstructure evolution,and plastic deformation behavior of the material before and after UNSM treatment were systematically characterized and analyzed through Vickers indentation,a scanning electron microscope(SEM),a white light interferometer,electron backscatter diffraction(EBSD),and x-ray diffraction(XRD).The result showed that the micro defects of SLM 316L stainless steel were significantly reduced.SLM's initial LOF defects were diminished under the high-frequency ultrasonic load.The surface roughness Ra decreased from 5.374 μm to 0.510 μm,and the surface hardness increased from 230HV to 461.16HV.Severe plastic deformation(SPD)occurred on the surface layer of the material,which induced the transformation of the microscopic structure from γ to α phase.The crystal microstructure was also refined from the initial irregular columnar coarse crystal to fine equiaxed crystal.As a result of local uneven plastic deformation in the UNSM process,a depth 20 μm gradient nanocrystal was captured through the result of EBSD analysis.Thus,compared with the initial SLM specimen,the UNSM treatment produced a maximum residual compressive stress of 932 MPa on the surface of the material.The improvement of the surface integrity,formation of deeper grain refinement layer,and residual stress hardening layer of SLM 316L after UNSM treatment successfully demonstrates that UNSM is a promising post-processing surface treatment technology for SLM metallic materials.
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