Fatigue Behaviour of Functionally Graded Steels Produced by Additive Manufacturing

Abstract

Wire arc additive manufacturing (WAAM) can create large parts, due to its high deposition rate. WAAM can be used to create functionally graded materials (FGMs) such as a strong core of high strength low alloy (HSLA) steel and corrosion-resistant cladding of austenitic stainless (AUS) steel. However, HSLA and AUS steel are dissimilar metals with different microstructure and mechanical properties. Joining dissimilar metals with WAAM creates a complex interface with a different set of properties compared to parent materials. The current state of the art covers the microstructure, tensile properties, and hardness of such FGM. However, there is a lack of understanding about the fatigue properties at the interface. This study investigates the fatigue crack growth behaviour at the interface of ER70S-6 (HSLA steel) and ER316L (AUS steel). Initially, a benchmark study of the HSLA and AUS steel was done to obtain Paris parameters, threshold stress intensity range (∆Kth), and hardness. HSLA steel showed 43 to 77% lower Paris slope parameters than AUS steel, which suggests a better fatigue resistance for HSLA steel. However, no significant difference in ∆Kth values was observed. The hardness of HSLA and AUS steel were 247.0±6.1 HV and 227.7±5.5 HV respectively, which suggests a correlation between hardness and fatigue resistance. Further analysis showed a power law relation between the Paris slope parameter and hardness for HSLA steel. This was attributed to the grain size. Smaller grain sizes improve fatigue resistance and increase hardness. Hence, hardness can be an indirect measure of fatigue resistance. Under constant ∆K experiments, AUS steel showed up to a 61.7% difference in fatigue crack growth rate (FCGR). The anisotropy in AUS steels is due to the large grain size in the range of 100 μm and the structure of the weld beads, resulting in heterogeneity. After the benchmark study, the graded materials were tested under similar conditions. The crack in the graded material grows from AUS steel to a 2 mm wide interface and then to HSLA steel. A variation of FCGR at the interface region was measured, notably 1) a decrease of 5.1E-05 to 6.1E-05 mm/cycle before the interface region, 2) a gradual increase of 5.5E-05 to 1.32E-04 mm/cycle within the interface region, and 3) a sudden increase of up to 3E-04 mm/cycle. To understand these behaviour microscopy, fractography, and electron backscatter diffraction (EBSD) analyses were utilised. The sudden increase of FCGR in HSLA steel was attributed to process-induced porous defects. The decrease of FCGR before the interface region was due to the increase in grain boundaries, secondary cracks, an increased crack closure effect, and martensite formation through transformation-induced plasticity (TRIP) in the interface region. The gradual acceleration of FCGR within the interface region was due to the martensite phases formed during solidification. The martensite formed during solidification had twice the hardness in comparison to HSLA steel. The martensite formed by the TRIP effect also resulted in 25-50% higher hardness near the crack flanks compared to the regions away from it. For the application of this FGM, the interface can act as a protective layer by reducing the FCGR. However, the martensite formed during solidification in the interface should be mitigated.