PSI - Issue 53

S. Senol et al. / Procedia Structural Integrity 53 (2024) 12–28

23

12

Author name / Structural Integrity Procedia 00 (2019) 000–000

with EDM and M surface conditions, compressive residual stresses are noted. For EDM, it is stated that the residual stress state is highly dependent on several parameters, such as the workpiece material, the wire/tool material, the dielectric liquid, and the EDM pulse settings (Srinivasa Rao et al., 2016), all influencing the thermal history, the material removal mechanism, heat affected zone, and recast (white) layer characteristics (Das et al., 2003; García Navas et al., 2008; Salvati & Korsunsky, 2020). A detailed investigation on the residual stress formation during EDM, however, falls out of the scope of the current study. The authors believe that the source of compressive residual stresses observed in EDM samples, for the EDM parameters and workpiece/ material used in this study, can originate from a stress build up due to 1) the white layer formation, possibly with a compositional difference (Arunachalam et al., 2017) and oxidic-nature (Sidhu et al., 2015), and 2) the thermal expansion mismatch between the white layer and the bulk material (Salvati & Korsunsky, 2020). More straightforwardly, for samples with M surface condition, the compressive stresses are known to be induced mechanically by means of the hardening due to plastic deformation as a reaction to the force exerted on the surface by the tools during machining (Chighizola et al., 2021; Masoudi et al., 2015). Therefore, the tensile stresses stored in the L-PBF part is counterbalanced by the compressive stresses introduced during EDM and M in this study. Nevertheless, the stress data must be interpreted with caution. It is important to notice that the absolute stress values affecting the mechanical properties might differ with the changes in scan strategy, sample geometry, and/or the measurement sensitivity that can be affected by roughness, compositional (i.e. in heat-affected zone or white layer) and microstructural gradients (crystallographic texture in heat-affected and remolten zones) (Salvati & Korsunsky, 2020). Keeping this in mind, the residual stress measurement data presented here is used for comparative purposes only. Regarding the fatigue performance, tensile residual stresses are reported to adversely affect crack initiation and propagation, conversely, a compressive residual stress state leads to a reduced stress intensity factor, delayed crack initiation, and reduced crack growth rates. (Ge & Xiang, 2016; Webster & Ezeilo, 2001). Therefore, while tensile residual stresses in samples with AB and R surface conditions would have a degrading effect on their fatigue performance, samples with EDM and M surface conditions are favoured by the elimination of tensile stresses and the introduction of beneficial compressive stresses. Furthermore, no considerable difference in hardness is recorded between different surface conditions, except for the ‘white layer’ (recast layer), which exhibits a higher hardness, forming on the EDM surface (Fig. 5(c)). This ‘white layer’ is reported to form as a result of re-solidification (recast) and may lead to compositional changes due to evaporation, depositions from the EDM wire (Salvati & Korsunsky, 2020; Sidhu et al., 2015) or oxide formation (Sidhu et al., 2015). It has been demonstrated that hardness might also have an influence on the crack growth and fatigue performance, although it is minor and interrelated to the residual stress state, grain size, and/or precipitate states (Bussu & Irving, 2002; Y. H. Li et al., 2019; Ordnung et al., 2022). Also in this study, the precise effect of the increased hardness due to the presence of a very thin, hard surface recast layer in the samples with EDM surface condition, on the fatigue behaviour remains undiscovered due to the synergistic effects of additional fatigue-influencing factors such as the surface roughness and residual stress. However, (1) the high scatter in EDM data, (2) the presence of several crack initiation points within or near the porous, rough white-layer of the EDM surface, (3) the better fatigue performance of remolten (R) samples, despite the similar surface roughness and higher tensile residual stresses as compared to samples with EDM surface condition, indicate that the aforementioned hard and porous recast layer observed on the EDM samples has an adverse effect on their fatigue performance. Finally, it has been reported that re-melting (laser polishing) might affect the grain size, and that the grain size has an impact on the fatigue performance, as grain boundaries act as obstacles against dislocation motion, finer grain sizes hinder crack propagation, whereas, larger grain sizes lead to accelerated crack propagation (Ge & Xiang, 2016; Wei et al., 2020; Zhao et al., 2019). Therefore, grain size analysis will be conducted as future work. 4. Conclusions  The up-facing inclined surface quality of crack-free, dense, high-strength hybrid particle reinforced (Ti+B 4 C)/Al Cu-Mg metal matrix composite samples is successfully improved by the application of optimized in-process dL PBF surface treatment, reducing the surface roughness. Part surface smoothening is achieved by initially removing powder from the curved surface using a pulsed wave laser and subsequent re-melting using a