Plasma transfer arc additive manufacturing of 17-4 PH: assessment of defects

Plasma transferred arc additive manufacturing is a growing technology in the additive manufacturing world. The plasma transferred arc additive manufacturing system’s ability to produce large samples, compared with other common additive manufacturing techniques, makes it highly desirable in many industrial applications. The selected material in this additive process is 17-4 precipitation hardened stainless steel, which is widely used in numerous fields, such as the aerospace, chemical, and mining industries. However, two types of voids were found in the deposits and these voids are detrimental to the mechanical properties. The identified voids were oxide layers and porosity. The presence of oxide layers was correlated to the interaction of atmospheric oxygen with the chromium present in the stainless steel. A shielding hood was designed to provide continuous shielding with inert gas to prevent oxide layer formation. The other source of voids was attributed to the porosity in the initial powders and to the choice of welding process parameters. Changing the powder supplier and optimizing the process parameters, mainly by increasing the heat input to ensure complete melting of the powders, greatly reduced the amount of porosity in the finished part. Hardness measurements were obtained from multiple samples. Hardness was only affected by the aging process, during which copper precipitates formed within the 17-4 precipitation hardened stainless steel matrix. X-ray diffraction and transmission electron microscopy analyses were conducted to characterize the martensitic matrix before and after the heat treatment and to view copper precipitation after the heat treatment. It is demonstrated that an operating solution to avoid oxide formation is the use of 5% hydrogen in the shield, center, and powder gas feeds.