【摘要】 低熔点Mg元素在激光粉末床熔融成形过程中易蒸发产生金属蒸气，影响激光成形过程的稳定性，从而影响镁合金的成形质量和力学性能。针对此问题，通过在镁合金中添加TiC质量分数为2%的陶瓷增强相制备镁基复合粉末，采用激光粉末床熔融成形TiC/AZ91D镁基复合材料，研究了工艺参数对激光粉末床熔融成形镁基复合材料成形质量及力学性能的影响。当激光功率为140 W、扫描速率为300 mm/s时，激光能量输入适中，粉末熔化充分，熔道清晰且熔池稳定。激光成形样品中的孔隙较少，成形质量较佳，致密度达到～99.4%。激光成形TiC/AZ91D镁基复合材料由未熔TiC增强相、网络和胞状β-Mg₁₇Al₁₂析出相以及α-Mg基体组成。激光成形镁基复合材料的抗拉强度和延伸率分别为～335.5 MPa和～3.45%，这主要是β-Mg₁₇Al₁₂析出相阻碍位错运动产生的Orowan强化效应和较高的成形质量引起的。研究结果为激光粉末床熔融成形高致密度高性能的镁基复合材料提供了参考。更多还原

【Abstract】 Objective Owing to the low melting point of Mg, it tends to evaporate and form metallic vapor during the laser powder bed fusion（LPBF） process, affecting the stability of the laser forming process, forming quality, and mechanical properties of as-built magnesium alloys. Ceramic reinforcements added to the magnesium alloys offer an effective approach to enhance the forming quality and mechanical properties of the magnesium alloys. However, there remains limited work on the LPBF-fabricated TiC/AZ91D magnesium matrix composites, and the effect of laser processing parameters on the forming quality and mechanical properties of these composites remains unclear. This study investigates the effect of processing parameters on the forming quality and mechanical properties of LPBF-fabricated 2%TiC/AZ91D（2% is mass fraction） magnesium matrix composites. The findings provide valuable insights for the development of high-densification, high-performance magnesium matrix composites via laser additive manufacturing.Methods TiC/AZ91D powder mixture was prepared by ball milling. The laser powder bed fusion equipment was used for the fabrication of TiC/AZ91D magnesium matrix composites using the powder mixture. The Archimedes method was employed to measure the relative density of the LPBF-fabricated samples. Surface roughness was measured using a laser spectral confocal microscope. Optical microscope（OM） was used to observe the distribution of defects and the morphology of melt pool of the LPBFfabricated samples. Microstructural characterization was conducted using a scanning electron microscope（SEM） equipped with an energy dispersive spectrometer（EDS）. The hardness was measured using a microhardness tester, and tensile testing was performed using a universal testing machine.Results and Discussions The surface roughness of the LPBF-fabricated TiC/AZ91D magnesium matrix composites decreases by ～41% as the laser energy density increases（Fig. 3）. This improvement can be attributed to sufficient energy input at a higher laser energy density, enhancing powder melting and melt spreading and thus reducing the surface roughness. The relative density of the TiC/AZ91D magnesium matrix composites increases as the laser power increases and decreases as the scanning speed increases（Table 1）. A maximum relative density of ～99.4% is achieved at a laser power of 140 W and scanning speeds of 200 mm/s and 300 mm/s. Under high energy input, relatively long existence time for melt pool and small cooling rate are conducive to the escape of residual gas and reduction of porosity. At a laser power of 140 W and scanning speed of 300 mm/s, only few spherical pores are observed, well correlating with the high relative density of the fabricated samples（Fig. 4）. The depth-to-width ratio of melt pool increases with the laser power（Fig. 5）, as high energy input at a high laser power can enlarge the melt pool. The microstructure of the LPBF-fabricated TiC/AZ91D magnesium matrix composites comprises white unmelted TiC particles, light gray network and cellular β-Mg₁₇Al₁₂ precipitates, and dark gray α-Mg matrix [Fig. 6（a）]. TiC particles with a high melting point, are partially melted and can serve as heterogeneous nucleation sites to refine grains of matrix and improve the mechanical properties of the material. In the LPBF process, the slow cooling rate allows more time for Al to diffuse into the matrix, resulting in large amounts of β-Mg₁₇Al₁₂ precipitated in the matrix. At a laser power of 140 W and scanning speeds of 200 mm/s and 300 mm/s, the hardness distribution of the LPBFfabricated TiC/AZ91D magnesium matrix composites is relatively uniform（Fig. 7）. The average microhardness decreases from（114.0±2.5） HV to（102.9±0.5） HV as the laser power increases, and increases from（101.4±0.3） HV to（104.1±0.5） HV as the scanning speed increases（Fig. 8）. These changes in hardness are attributed to a reduction in residual stresses and grain refinement of the composites. The ultimate tensile strength of the LPBF-fabricated TiC/AZ91D magnesium matrix composites increases from ～296.5 MPa to ～335.5 MPa as the laser power increases, representing a 13% increase（Fig. 9）. This enhancement is attributed to increased Orowan strengthening in the composites. The ultimate tensile strength of the samples increases with the scanning speed up to a peak of ～335.5 MPa at 300 mm/s. However, the elongation decreases with the increase in scanning speed, reaching a maximum of ～3.47% at 200 mm/s（Fig. 10）. This is owing to the reduced energy input at higher scanning speeds, which increases the metallurgical defects that can act as crack initiation sites, resulting in a reduced elongation. Fracture analysis of the LPBFfabricated samples reveals un-melted TiC particles and no cracks or apparent porosity observed at the surface（Fig. 11）. The uniform distribution of β-Mg₁₇Al₁₂ phases effectively hinders the propagation of microcracks, enhancing the strength of the material and improving its overall mechanical properties.Conclusions In this work, the 2%TiC/AZ91D magnesium matrix composites are fabricated at different laser processing parameters. The effect of the laser power and scanning speed on the manufacturing quality and mechanical properties is investigated. The surface roughness of LPBF-fabricated TiC/AZ91D magnesium matrix composites reaches ～12.4 μm at optimal processing parameters, which is attributed to sufficient laser energy input, enhancing the melt flowability and spreading. At a laser power of 140 W and scanning speed of 300 mm/s, the energy input ensures complete powder melting, clear melt tracks, and stable melt pools. The LPBF-fabricated samples have few pores, achieving good forming quality with a relative density of ～99.4%. The microstructure of LPBF-fabricated TiC/AZ91D magnesium matrix composites is composed of unmelted TiC reinforcements, a network-like distribution of β-Mg₁₇Al₁₂ precipitates, and α-Mg matrix. The average microhardness of the LPBF-fabricated TiC/AZ91D magnesium matrix composites reaches（114.0±2.5） HV at optimal processing parameters. At a laser power of 140 W and scanning speed of 300 mm/s, the tensile strength and elongation of the magnesium matrix composites reach ～335.5 MPa and ～3.45%, respectively. This improvement is primarily owing to the Orowan strengthening caused by fine β-Mg₁₇Al₁₂ precipitates that can hinder the dislocation motion.更多还原