Crack Forming Mechanism and Anisotropy of Inconel 738 Alloy by Selective Laser Melting

Abstract:

Inconel 738 alloy is a precipitation strengthened nickel-base superalloy mainly used in today’s heavy-duty gas turbines for hot gas path components such as blades, vanes and heat shields due to its possesses an exceptional combination of high-temperature strength and oxidation resistance. However, the high Al and Ti contents are highly hot cracking susceptibility during selective laser melting (SLM) forming, easily formed the microcracks, leading to the result in premature failure of the alloy during service, causing major accidents. In this work, the crack formation mechanism, microstructure anisotropy and their effect on mechanical property by SLM forming was investigated by means of SEM, EBSD, DSC, XRD and Universal Tensile Testing Machine. The results show that austenite in Inconel 738 alloy mainly precipitates phase and MC carbide during SLM forming, where the solidification process is L→ → +MC→ + +MC. The low melting point eutectic structure liquefies due to reheat to form crack source, and microcracks are formed under residual tensile stress. Meanwhile, the residual stress around the microcrack distributes uniformly, and the grain misorientation near the initiation of microcrack is higher than that without microcrack zone. In addition, the direction of microcrack on XY plane is perpendicular to the direction of laser scanning, and the direction of microcrack on XZ plane is parallel to Z axis. The preferred orientation of Inconel 738 alloy formed by SLM is related to the maximum temperature gradient. The mechanical properties of SLM formed specimens along XY and XZ directions are higher than those of precision casting specimens, and the strength in XZ direction is higher than that in XY direction, while the elongation in XZ direction is lower than that in XY direction.

Key words:selective laser melting, Inconel 738 alloy, crack forming mechanism, anisotropy, mechanical property