PSI - Issue 56

Sapam Ningthemba Singh et al. / Procedia Structural Integrity 56 (2024) 11–18 Sapam Ningthemba Singh et al./ Structural Integrity Procedia 00 (2023) 000–000

12 2

1. Introduction Additive manufacturing (AM) is a manufacturing process where the material is deposited in a layer by layer fashion according to the design data. It can process polymers, composites, ceramics, and metallic materials. Powder bed fusion (PBF) and directed energy deposition (DED) are two major types of AM processes for processing metallic materials. While the PBF process offers higher dimensional accuracies than the DED process, the DED process has the ability to fabricate parts with much higher layer thickness as compared to the PBF process (Singh and Deoghare 2022; Dávila et al. 2020). Increasing layer thickness can significantly improve the material deposition rate and hence, the manufacturing lead time. Ti6Al4V alloy is one of the widely used materials in the aerospace, biomedical, and automobile industries, having excellent mechanical properties such as strength-to-weight ratio, toughness, fatigue, and ductility properties (S. Liu and Shin 2019). However, due to poor machinability and difficulties in processing Ti6Al4V, AM of Ti6Al4V is gaining attention in the manufacturing world (Arrazola et al. 2009). The applications of Ti6Al4V alloy in the aerospace, biomedical and automotive industry has been reported in the literature (Tamayo et al. 2021; Z. Liu et al. 2021; Duraiselvam et al. 2014; Froes et al. 2004). The design freedom and ability to fabricate complex parts, the laser AM of Ti6Al4V has been investigated extensively in the literature. It is reported that post-processing of additively manufactured parts is important to obtain a smooth and regular surface as the surface roughness of the additively manufactured Ti6Al4V parts is more than those of the parts obtained from casting (de Oliveira Campos et al. 2020). It is also reported that the properties of Ti6Al4V parts manufactured by PBF and DED process are competitive to that of cast materials and other additively manufactured parts (Baufeld, Biest, and Gault 2010). However, the fatigue performance of additively manufactured parts is generally lower than that of wrought counterparts (Sterling et al. 2015). It is also reported that the as-built samples have lower fatigue performance than those of machined samples of the additively manufactured samples (Le et al. 2020). The fatigue performance of additively manufactured Ti6Al4V alloy can be improved by employing various post processing operations, including heat treatment, hot isostatic pressing (HIP), shot peening, laser shock peening (LSP), etc. (Cao et al. 2018; Li et al. 2016; Kahlin et al. 2020). After such post-processing, some fatigue performances were more than those of the wrought parts (Li et al. 2016). This has further been supported by the findings from (Günther et al. 2017). A smoother surface is favorable for a higher fatigue performance and rough surfaces should be reduced to a possible and reasonable level (Bagehorn, Wehr, and Maier 2017; Pegues et al. 2018). LSP is one of the promising post-processing methods that induce compressive residual stress (CRS) on surfaces with or without the application of an overlay layer and protective layer. Internal defects on the prepared samples were the main reasons for the part failure during the ultra-high cycle fatigue of Ti6Al4V samples manufactured by the selective laser melting (SLM) process (Jiang et al. 2021). Improvement in fatigue life after LSP was observed for different AM processes such as SLM, PBF, electron beam melting (EBM), etc. (Jin et al. 2020; Aguado-Montero et al. 2022; Kahlin et al. 2020). With all these advances, the scientific literature available in this research area is mostly focused on the powder PBF-based AM processes. Most of the available information is on the thin layer thickness process. With DED, the layer thickness can be significantly increased (more than 1 mm). The research on the fatigue performance of high layer thickness is limited, and further research is needed with regards to the high layer thickness LDED process as well as enhancing the fatigue life with post-processing methods such as LSP. The present paper is focused on the fatigue life enhancement of Ti6Al4V alloys manufactured by the LDED process using the LSP process.

Nomenclature AM

additive manufacturing ASTM American Society for Testing and Materials CRS compressive residual stress CNC computer numerical control DED directed energy deposition EDM electrical discharge machining HIP hot isostatic pressing LDED laser directed energy deposition LPBF laser powder bed fusion