Down-facing surfaces are one of the most challenging features in metal parts produced by laser powder bed fusion (LPBF). A combination of reasons, primary of which are residual stresses and overheating cause these features to have the worst surface finish and dimensional accuracy of all LPBF surfaces. In order to examine this phenomenon, a Design of Experiments (DoE) study is conducted for three different inclination angles, namely 45°, 35° and 25° and for two different layer thicknesses of 60 µm and 90 µm. The results from the DoE are used to establish quadratic regression equations that can be used to predict the quality marks of surface roughness and the relative dimensional error.This fundamental investigation helps to explain the reasons for the major defects in down-facing surfaces of parts produced with Ti-6AL-4 V material, namely the dross formation and attempts to improve the predictability of quality within the region. Further to the establishment of the quadratic equations, a discussion is conducted on the thermomechanical processes involved in the mechanism of dross formation and explanations are given on the reasons behind the observed physical phenomena. The trend of the propagation of (Root Mean Square) RMS Surface roughness (Sq) and the relative dimensional error with respect to the Volumetric Energy Density (VED) is discussed in detail. The respective quadratic equations are then tested by a second round of validation prints, and the results confirm the feasibility of the developed quadratic models to accurately predict process outcomes especially when operating near the suggested optimal printing zones. The high roughness of low VED printing is attributed to the formation of ‘inverse mushroom’ structures, and the low roughness of high VED surface is attributed to the formation of large flat regions formed as adjacent meltpools that can fuse together at various locations.

朝下的表面是激光粉末床融合 (LPBF) 生产的金属零件中最具挑战性的特征之一。综合原因，主要是残余应力和过热，导致这些特征的表面光洁度和尺寸精度是所有 LPBF 表面中最差的。为了检查这种现象，针对三种不同的倾角（即 45°、35° 和 25°）以及两种不同的层厚（60 µm 和 90 µm）进行了实验设计 (DoE) 研究。DoE 的结果用于建立二次回归方程，可用于预测表面粗糙度和相对尺寸误差的质量标志。这项基础调查有助于解释所生产零件朝下表面主要缺陷的原因采用 Ti-6AL-4 V 材料，即浮渣的形成和提高区域内质量的可预测性的尝试。在建立二次方程的基础上，讨论了熔渣形成机制中涉及的热机械过程，并对观察到的物理现象背后的原因进行了解释。(Root Mean Square) RMS 表面粗糙度的传播趋势（Sq ) 和相对于体积能量密度 (VED) 的相对尺寸误差进行了详细讨论。然后通过第二轮验证打印测试相应的二次方程，结果证实了开发的二次模型准确预测过程结果的可行性，尤其是在建议的最佳打印区域附近操作时。低 VED 打印的高粗糙度归因于“反向蘑菇”结构的形成，而高 VED 表面的低粗糙度归因于形成大的平坦区域，这些区域形成为相邻熔池，可以在不同位置融合在一起。