Effects of Thermal Stress on the Formation and Cracking Behavior of Nickel-Based Superalloys by Selective Laser Melting Based on a Coupled Thermo-Mechanical Model

Abstract

Complex thermal cycles and stress fields commonly occur in the selective laser melting process for nickel-based superalloys, which are prone to generating cracks and decreasing the performance of forming parts. In this paper, the reasons for cracking were analyzed by combining the experiment with the evolution behavior of the temperature field/stress field during the solidification process of a nickel-based superalloy (FGH96) via a three-dimensional finite element thermo-mechanical coupling model. It showed that a radial temperature distribution of the melting pool led to a similar distributed stress; as a result, the value declined slowly along the scanning direction but declined quickly along the direction perpendicular to the scanning direction. A stress concentration with maximum stress up to 339 MPa was found at the center of the molten pool, easily causing a crack in SLM. It was found that both the initiation and propagation of the cracks were along the grain growth direction and were affected by the epitaxial growth of columnar crystals. For the case of process parameters with relatively high power or low scanning speed, the stress value of the molten pool during solidification was more than 370 MPa so as to form a large area of cracks. The adjustment of the rotation angle between the adjacent layers was effective at avoiding stress accumulation in the building direction and prevent the formation of long grain boundaries, thus avoiding crack propagation. The present study lays a foundation for the wide applications of selective laser melting technologies in nickel-based superalloys.

Keywords

• selective laser melting
• thermal behavior
• stress field
• cracking