采用选区激光熔化(SLM)技术制备304L不锈钢样品，通过改变扫描道次(T)和打印层数(L)实现不同的温度梯度与凝固速率，研究尺寸效应对微观组织与力学性能的影响。具有不同T × L组合的SLM 304L不锈钢样品沿打印构建方向形成柱状晶组织，并且尺寸效应影响柱状晶结构。随尺寸增大，凝固组织沿散热方向择优生长的程度高，柱状晶的连续性更好，由低长径比的“米粒形”向近等轴的“短窄形”、进而向高长径比的“长条形”过渡，晶粒粗化现象明显。在较大尺寸样品中，随着不断远离基板也观察到了类似现象。尺寸效应对力学性能的影响主要体现在随着打印尺寸增加，材料的屈服强度下降而塑性延伸率上升，但当尺寸增加至一定程度后力学性能变化趋于稳定。尺寸效应对SLM 304L不锈钢的析出相组成和含量无明显影响。结果表明，影响力学性能的主要影响因素为柱状晶的尺寸和分布，在较大尺寸的样品中，“长条形”柱状晶占比高，导致材料强度降低而塑性增加。结合ANSYS ADDITIVE对凝固速率和温度梯度的模拟仿真结果，对尺寸效应通过影响凝固过程进而对材料微观组织和力学性能产生影响的机理进行了阐述。

As one of the most promising metal additive manufacturing methods, selective laser melting (SLM) is very attractive for fabricating complex-shaped structure components of austenitic stainless steels in the nuclear field. SLMed austenitic 304L stainless steel has been demonstrated to have excellent mechanical properties and superior corrosion resistance due to the unique hierarchical microstructure produced by the ultrafast cooling rate and high-thermal gradient. Scanning tracks (T) and depositing layers (L) are key factors for geometry design by affecting the processing efficiency and the solidification structure of the components. Hence, it is essential to clarify the effecting mechanism of sample size and geometry on material performance. In this work, samples of different sizes are designed to study the influence of geometry on the microstructure and mechanical properties of 304L stainless steel components made via SLM. Various solidification conditions are achieved by varying the temperature gradients and cooling rates by adjusting T and L. Metallographic microscopic observations in samples with various T × L combinations demonstrate that a columnar structure is formed along the build direction, which is significantly impacted by the geometry effect. The columnar grains grow preferentially along the heat dissipation direction with an increase in the sample size. The columnar grains gradually change from having a low length-diameter ratio (LDR) with a rice grain shape to a higher LDR with a short rod-like shape and then a long strip shape. Grain coarsening could also be identified along with the formation of “long strip” columnar grains. Moreover, consistent microstructure evolution behavior is observed in large-sized samples. The influence of geometry on the mechanical properties is examined via tensile testing to demonstrate the decrease in yield strength and increased plastic elongation with the rise in sample size. As the sample size increases, the mechanical properties become consistent. The comprehensive analysis concludes that grain size and columnar grains play critical roles in determining the mechanical properties according to the Hall-Petch relationship. In larger-sized samples, “long strip” columnar grains with a high proportion could lead to a decrease in material strength and an increase in plasticity. The geometry mechanism affecting the solidification process, microstructure formation, and mechanical properties of 304L stainless steel processed by SLM is explored by combining the solidification rate and thermal gradient simulation results using ANSYS ADDITIVE.