Webster Samantha, Moser Newell, Fezzaa Kamel, Sun Tao, Ehmann Kornel, Garboczi Edward, Cao Jian
Mechanical Engineering, Northwestern University, 633 Clark St, 60208 IL, USA.
Materials Science and Engineering Division, National Institute of Standards and Technology, 101 Bureau Dr, 20899 MD, USA.
PNAS Nexus. 2023 May 26;2(6):pgad178. doi: 10.1093/pnasnexus/pgad178. eCollection 2023 Jun.
Process defects currently limit the use of metal additive manufacturing (AM) components in industries due to shorter fatigue life, potential for catastrophic failure, and lower strength. Conditions under which these defects form, and their mechanisms, are starting to be analyzed to improve reliability and structural integrity of these highly customized parts. We use in situ, high-speed X-ray imaging in conjunction with a high throughput laser, powder-blown directed energy deposition setup to observe powder particle impact behavior within the melt pool. Through fundamental observations of the stochastic, violent powder delivery in powder-blown DED, we uncover a unique pore formation mechanism. We find that a pore can form due to air-cushioning, where vapor from the carrier gas or environment is entrapped between the solid powder particle surface and liquid melt pool surface. A critical time constant is established for the mechanism, and X-ray computed tomography is used to further analyze and categorize the new type of "air-cushioning" pores. It is shown that the air-cushioning mechanism can occur under multiple laser processing conditions, and we show that air-cushioning pores are more likely to be formed when powder particles are larger than 70 m. By quantifying the effect of powder particle impact, we identify new avenues for development of high-quality laser, powder-blown DED products. Furthermore, we deepen knowledge on defect formation in metal additive manufacturing, which is being increasingly utilized in high performance situations such as aerospace, automotive, and biomedical industries.
目前，工艺缺陷限制了金属增材制造（AM）部件在工业中的应用，原因在于其疲劳寿命较短、存在灾难性故障的可能性以及强度较低。人们开始分析这些缺陷形成的条件及其机制，以提高这些高度定制化部件的可靠性和结构完整性。我们结合高通量激光器和粉末喷射定向能量沉积装置，使用原位高速X射线成像技术来观察熔池内粉末颗粒的冲击行为。通过对粉末喷射DED中随机、剧烈的粉末输送进行基础观察，我们发现了一种独特的气孔形成机制。我们发现，由于气垫作用会形成气孔，即载气或环境中的蒸汽被困在固体粉末颗粒表面和液体熔池表面之间。为该机制确定了一个关键时间常数，并使用X射线计算机断层扫描进一步分析和分类这种新型的“气垫”气孔。结果表明，气垫机制可在多种激光加工条件下发生，并且我们发现当粉末颗粒大于70微米时，更有可能形成气垫气孔。通过量化粉末颗粒冲击的影响，我们确定了开发高质量激光粉末喷射DED产品的新途径。此外，我们加深了对金属增材制造中缺陷形成的认识，金属增材制造在航空航天、汽车和生物医学等高性能领域的应用越来越广泛。
