Study of the Effects of Hot Forging on the Additively Manufactured Stainless Steel Preforms

Abstract

The production of wrought stainless steel components in power generators can involve a combination of many manufacturing processes. These are expensive in tooling costs and number of operations, as in the Hot Forging (HF) of stainless steel turbine blades. Additive Manufacturing (AM) techniques provide a valuable opportunity to produce near-net-shaped preforms, thus avoiding the material wastage and high tooling costs associated with the intermediate stages of HF processes. This study focuses on the proposed hybrid AM and HF method, in which AM is used to produce near-net–shape preforms which are subsequently formed into net-shaped parts by HF. The HF process is used to significantly reduce the material defects introduced by, and intrinsic to, AM processes. To understand the mechanical and microstructure changes during various AM and HF conditions, single-phase 316 L stainless steel was used as the test material. Samples were produced by AM using a laser powder-bed fusion AM machine. Two different AM build directions were used to produce samples, so, as to allow evaluation of the anisotropic properties induced by AM. These samples subsequently underwent a HF process, in which various processing conditions of plastic strain and forging temperature were applied, to study the general effects of thermal plasticity on the AM microstructure. Tensile testing, optical microscopy (OM), scanning electron microscope (SEM) together with electron backscatter diffraction (EBSD) techniques were used to characterise the evolution of mechanical properties, porosity and grain size. The HF technique was found to remove defects from the AM material, resulting in enhanced mechanical strength, ductility, and isotropy. The technique therefore offers a potential alternative to conventional forging while retaining the required level of material performance.