超声纳米晶表面改性对选区激光熔化316L不锈钢微观结构和力学性能的影响

Effect of Ultrasonic Nanocrystal Surface Modification on Microstructure and Mechanical Properties of SLM 316L Stainless Steel

目的改善选区激光熔化(Selective laser melting,SLM)316L不锈钢的表面完整性和力学性能。方法采用超声纳米晶表面改性(Ultrasonic Nanocrystal Surface Modification,UNSM)这一新兴表面塑性变形方法对SLM 316L不锈钢进行超声冲击强化,利用维氏硬度计、扫描电镜、白光干涉仪、EBSD、XRD等对处理前后材料的表面完整性、微观组织演变和塑性变形行为进行表征和分析。结果经过UNSM处理后,SLM 316L不锈钢的微观缺陷明显减少,初始未熔合孔隙发生闭合,表面粗糙度Ra由5.374μm降至0.510μm,表面硬度从230HV增至461.16HV;同时,材料表层发生了剧烈的塑性变形,形变诱导材料微观组织从γ相向α相转变,微观结构由初始不规则柱状粗晶转变为等轴状细晶。从EBSD表征结果可知,在材料表面形成了深度约为20μm的梯度纳米晶,材料内部存在明显的不均匀变形;与初始SLM试样相比,通过UNSM处理在材料表面引入了最大为932 MPa的残余压应力。结论超声纳米晶表面改性能够显著改善SLM 316L不锈钢的表面完整性,形成较深的晶粒细化层和残余应力硬化层,从而有效提高其耐腐蚀性和疲劳抗性,是一项有前景的SLM后处理技术。

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 0.11μm,a laser scan speed of 1250 mm/s,and a layer thickness of 60μm.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 30μm,a scanning speed of 500 mm/min,and a feed rate of 10μm.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 5.374μm to 0.510μm,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 fromγtoαphase.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 20μm 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.