SLM成形钛合金薄壁结构边缘挂渣及中心凹陷研究

目的 探讨激光功率和扫描速度对选区激光熔化(Selective Laser Melting，SLM)成形钛合金Ti-6Al-4V薄壁结构边缘挂渣及尺寸精度的影响规律，分析薄壁中心凹陷的主要原因。方法 设计两因素三水平试验，对比不同工艺参数下成形薄壁上表面微观形貌及壁厚，提出影响薄壁边缘挂渣和中心凹陷的机理，并得到较优的成形工艺参数。结果 挂渣现象对薄壁尺寸精度的影响较大，薄壁的厚度越小，影响越大。当薄壁厚度达到2 mm时，边缘挂渣区域厚度明显减小。相较于扫描速度，激光功率对挂渣区域厚度的影响更大。采用低激光功率、较高扫描速度有助于减小挂渣区域厚度。激光能量密度越大，SLM成形薄壁尺寸误差越大，二者存在三次非线性关系。结论 在低激光功率对应的低能量密度下，成形薄壁尺寸误差较小，当能量密度为44.44 J/mm³时，薄壁获得了最小壁厚(529.37 mm)。试验获得的成形薄壁结构误差最小的工艺参数为P=200 W，v=900 mm/s，h=50 mm。SLM成形过程中的勾边扫描策略及残余应力导致的翘曲变形是造成薄壁中心凹陷的主要原因。

Research on Edge Slagging and Center Depression of Thin Walled Titanium Alloy Structures Formed by SLM

To investigate the accuracy of thin-walled specimens formed by Selective Laser Melting (SLM), the effects of laser power and scanning speed on the edge dross dropout and dimensional accuracy of thin-walled structures formed by SLM of titanium alloy Ti-6Al-4V were investigated, and the main factors causing the concave phenomenon in the center of thin walls were discussed. By adjusting the laser power and scanning speed, a total of nine specimens were prepared in a two-factor, three-level full-factor test to observe and compare the slagging phenomena and thin-wall dimensions on the microscopic surfaces of thin walls formed under different process parameters. In this process, the influence mechanism of different process parameters on the thin-wall forming accuracy was proposed, and the optimal forming process parameters for the forming of thin-wall structures were obtained. It was found that the slag hanging phenomenon on the side surface of the thin wall was the main reason affecting its accuracy, and the laser power was the main factor causing large area slag dropping. Low laser power and low scanning speed were beneficial to reduce the slag hanging thickness, but too high scanning speed was likely to cause surface defects. Compared with the scanning speed caused by the hanging slag and sticky powder phenomenon, the laser power had a greater impact. Under the same process parameters, with the increase of thin-wall thickness, the slagging area became smaller and smaller. The wall width of SLM thin wall had a three-dimensional nonlinear relationship with laser energy density, and the wall width of SLM thin wall increased with the increase of energy degree. When having the same laser energy density at different process parameters, the specimens still exhibited large differences. The problem of diffusion of residual energy would lead to serious dimensional error problems for thin-walled structures with small thickness. Low energy density corresponding to low laser power could obtain a thin-walled structure with less error, while a reasonable scanning speed should be selected to avoid excessive defects on the upper surface of the thin wall. In this study, experimental data were compared, and the specimens all exhibited large dimensional errors when the laser power was 300 W. When the energy density is 44.44 J/mm3, the minimum wall width value obtained is 529.37 mm, the final test yields the minimum process parameters for forming thin-walled structures with errors of P=200 W, v=900 mm/s, and h=50 mm. The degree of concavity on the surface of thin walls of different thicknesses in SLM forming varies. With the increase of thin wall thickness, the height difference between the edge of thin wall and the center also increases. Especially at the thin wall edge, the height of the thin wall shape decreases sharply. The sharp decrease in the height of the thin wall edge caused by the edge scanning strategy, and the edge warping and thermal shrinkage caused by residual stresses combined to cause the concave phenomenon in the center of the thin wall.