A systematical work was studied to illustrate the influence of defocusing distance on the forming quality of inner structure fabricated by selective laser melting for Ti-6Al-4V alloy. The relationship between defocusing distance and dimensional accuracy, surface roughness as well as flatness was investigated, finite element analysis (FEA) was used to show the temperature distribution. The results show that defocusing distance not only had an impact on laser energy density, but also showed a significant influence on the surface pressure of the metal powder. Smaller defocusing distance (0.0 mm) accompanied with higher molten pool maximum temperature (3262.96°C), powder melting and splashing at the same time. On the contrary, larger defocusing distance leading to unmelted powder and powder bonding, which influences the forming quality of samples. Dimensional accuracy was less affected in 0.0, 1.0, 2.0 mm defocusing distance (5%), but changed dramatically when it is 3.0 mm (22%). In the same condition, similar variation trend of surface roughness (Ra) and flatness was observed, and varying from 5.1 to 27.3 μm and 0.05 to 0.26 mm, respectively. Simultaneously, the bottom surface is less affected, while the other three sides have the opposite situation. Pores and unmelted powder can be seen on the surface, it is the comprehensive effect of laser energy density and surface pressure. It proves that it is feasible to manufacture inner structure by controlling this process parameter during SLM process, the influence mechanism of defocusing distance on forming quality was also illustrated in this work.

系统研究了离焦距离对激光选区熔化Ti-6Al-4V合金内部结构成形质量的影响。研究散焦距离与尺寸精度、表面粗糙度和平面度之间的关系，并使用有限元分析（FEA）显示温度分布。结果表明，散焦距离不仅对激光能量密度有影响，而且对金属粉末的表面压力也表现出显着影响。更小的离焦距离（0.0 mm）伴随着更高的熔池最高温度（3262.96°C），粉末同时熔化和飞溅。相反，较大的离焦距离会导致粉末未熔化和粉末结合，从而影响样品的成型质量。尺寸精度在 0.0、1.0、2.0 毫米散焦距离 (5%) 中的影响较小，但在 3.0 毫米 (22%) 时发生了显着变化。在相同条件下，观察到表面粗糙度（Ra）和平整度的变化趋势相似，分别从 5.1 到 27.3 μm 和 0.05 到 0.26 mm 变化。同时，底面受到的影响较小，而其他三面则相反。表面可以看到气孔和未熔化的粉末，这是激光能量密度和表面压力的综合作用。证明了在SLM过程中通过控制该工艺参数来制造内部结构是可行的，本文还阐述了离焦距离对成形质量的影响机制。但在 3.0 mm (22%) 时发生了巨大变化。在相同条件下，观察到表面粗糙度（Ra）和平整度的变化趋势相似，分别从 5.1 到 27.3 μm 和 0.05 到 0.26 mm 变化。同时，底面受到的影响较小，而其他三面则相反。表面可以看到气孔和未熔化的粉末，这是激光能量密度和表面压力的综合作用。证明了在SLM过程中通过控制该工艺参数来制造内部结构是可行的，本文还阐述了离焦距离对成形质量的影响机制。但在 3.0 mm (22%) 时发生了巨大变化。在相同条件下，观察到表面粗糙度（Ra）和平整度的变化趋势相似，分别从 5.1 到 27.3 μm 和 0.05 到 0.26 mm 变化。同时，底面受到的影响较小，而其他三面则相反。表面可以看到气孔和未熔化的粉末，这是激光能量密度和表面压力的综合作用。证明了在SLM过程中通过控制该工艺参数来制造内部结构是可行的，本文还阐述了离焦距离对成形质量的影响机制。分别为 26 毫米。同时，底面受到的影响较小，而其他三面则相反。表面可以看到气孔和未熔化的粉末，这是激光能量密度和表面压力的综合作用。证明了在SLM过程中通过控制该工艺参数来制造内部结构是可行的，本文还阐述了离焦距离对成形质量的影响机制。分别为 26 毫米。同时，底面受到的影响较小，而其他三面则相反。表面可以看到气孔和未熔化的粉末，这是激光能量密度和表面压力的综合作用。证明了在SLM过程中通过控制该工艺参数来制造内部结构是可行的，本文还阐述了离焦距离对成形质量的影响机制。