Fatigue Crack Growth Behaviour of Cobalt-based and Nickel-based Superalloy Processed by Selective Laser Melting
The production of cobalt-based and nickel-based superalloys via Selective Laser Melting (SLM) has been successfully applied in low-mass production, particularly in producing components/equipment for the spacecraft and medical industries. SLM technique allows complex and customised parts to be produced rapidly, replacing the traditional process of investment casting and machining. The major concern in terms of trustworthiness and durability of SLM parts is its performance under fatigue loading (cyclic loading), which is the most common mode of mechanical failure in engineering structures. Although significant research has been conducted in optimising SLM process parameters, changes in building direction (BD) and laser power (P) can cause variation in microstructure which consequently affecting its mechanical properties. BD orientations of an SLM part significantly affect its columnar grain structure, while P settings influence the shape/size of its meltpool boundary.
Currently, there is a lack of research reported on the Fatigue Crack Growth Rate (FCGR) behaviour of SLM superalloys. FCGR studies provide a relationship between stress intensity factor (ΔK) and crack growth rate (da/dN), which can be used to predict the life of a structure that experiences cyclic loading. Most available literatures focus on discovering the differences between the FCGR parameters (FCGR threshold: ΔKth and Paris constants: C and, m) for samples with different BD orientations, or between SLM build samples and traditionally build samples. Literature has not critically investigated the crack growth pathway in SLM’s columnar grain structure, which influences the FCGR resistance, thus affecting the FCGR parameters. Since there is no FCGR experiment reported for SLM Co-Cr-Mo and Inconel 738 superalloys, these two materials will be used to fill the research gap. Therefore, by conducting FCGR experiment, which allows the monitoring of a controlled crack movement, the aim of this PhD research aims to investigate the effect of BD and P on the crack growth pathway of SLM superalloys.
The FCGR experiments in this study were conducted in accordance with ASTM E647 standard. FCGR graphs were generated by using standard Compact tension (CT) specimens, processed at selected BD orientations and P settings. In addition, interrupted FCGR tests were performed to produce microstructure samples with visible crack growth history. The mode of fracture and detail observations of the crack growth pathway was attained through SEM and EBSD analysis.
Based on the findings in this study, the effect of BD in relation to crack growth direction (C), either CBD or C//BD on the FCGR parameter of SLM Co-Cr-Mo is contrary to the FCGR study that was reported in the literature for SLM Co-Cr-W. In this study, the FCGR threshold value (Kth) is higher, and the values of C and m in Paris equation are slightly lower for C//BD samples than the values for CBD samples, respectively. Failure analysis has revealed that the effects of the commonly known defect, lack of fusion (LOF), on both Kth and FCG rate are weak. It has been identified that crack has mainly propagated in a transgranular manner, consistent with the observation of the crack path being more tortuous, with a higher FCGR resistance determined in C//BD samples than in CBD samples. This is due to the difference in the size of crack segment, which is BD- and thus grain length-dependent.
Fractographic analysis on SLM Inconel 738’s samples revealed that majority of the cracks propagated transgranularly. Nevertheless, the minor occurrence of intergranular fracture (crack travelling through the grain boundaries) has deviated the crack pathway, especially for the C45BD sample. The occurrence of intergranular fracture is mainly dependent on the orientation of the grain boundary, i.e. crack does not propagate through a grain boundary that is perpendicular to the crack growth direction. The C45BD sample (crack growth direction 45° to BD) in SLM Inconel 738 have the highest threshold value and the roughest fractured surface compared to the C//BD and CꓕBD samples. The rough fractured surface in C45BD sample is caused by the continuous crack branching triggered by the constant encounter of the crack with grain boundaries that is orientated 45° to the crack growth direction. Consequently, a continuously high crack retardation effect is maintained throughout the threshold region which contributed to a higher FCGR resistance. On the other hand, because of the BD-dependent grain orientation, the cracks in the C//BD and CꓕBD have significantly lower chance of encountering a grain boundary that is orientated 45° to the crack growth direction, which resulted in a smooth surface fracture for both sample types. In addition, FCGR experiment of SLM Inconel 718 was also conducted in this thesis. Fractographic analysis revealed that the grain boundaries in SLM 718 are more brittle compared to the grain boundaries in SLM Inconel 738. Therefore, intergranular fracture is more dominant in SLM 718 compared to SLM 738 which resulted in a different crack growth pathway behaviour.
This thesis provides new understanding on the crack growth behaviour of SLM superalloy, highlighting the importance of the BD orientation in SLM superalloys and how it can influence the crack growth behaviour.