Microstructural engineering for enhanced fatigue performance using laser powder bed fusion.

Abstract

The static mechanical strength of LPBF manufactured materials can rival that of the equivalent cast and wrought materials, but are more susceptible to fatigue failures due to stress concentrating roughness and porosity defects. Fatigue performance can be enhanced by spatial heterogeneous microstructure by improving the resistance of crack growth rate due to the yield stress gradient effect. In this thesis, two strategies (varying rescanning parameters and varying material compositions) were applied to fabricate spatial heterogeneous microstructure by LPBF. The microstructure was firstly studied, and then the mechanical properties and fatigue propagation performance of the heterogenous microstructure were evaluated.

Firstly, cooling rate during LPBF process was predicted by an analytical modelling based on Wilson-Rosenthal equation and therefore a relationship between linear energy density and cell size are established. Rescanning strategy with a varying rescanning parameter resulted in heterogeneous microstructure for 316L SS with a coarse cell size of 0.84 μm in initial scanned region and a locally refined cell size of 0.35 μm in rescanned region, while maintaining a high level of densification (99.96%). Such variation in properties may be useful for applications requiring parts with hardened surfaces, or localized strengthening at stress concentrations and sites of expected failure. However, hardness in both initial II scanned region and rescanned region appeared to be similar. Without plasticity mismatch, no yield stress gradient effect on fatigue was expected and so fatigue testing of rescanned specimens was not carried out. Secondly, 316L stainless steel (SS) and precipitation hardening 15-5PH martensitic SS was joined by LPBF. Good apparent bonding was observed at the interface without any visible cracks or defects. Coarser grains are located above the interface in the 316L SS layer, and finer grains are observed below the interface in the 15-5PH SS layer. Each material's distinct microstructure and properties was achieved at far-interface regions, with a narrow wavy region (∼115 μm) at the interface that exhibits high densification and a sharp transition in microstructure and properties.

Due to a yield strength ratio of 1.56 between 316L SS and 15-5PH SS, fatigue propagation performance of bimaterials were evaluated to investigate the effect of Paris Law, residual stress distribution, and yield stress gradient effect. The results showed that a baseline crack growth rate was provided by Paris Law of each material and there are some local variations in crack growth rate caused by shielding effect (due to mismatch of plasticity) and residual stress effect (due to mismatch of thermal expansion coefficient). Paris law showed that the crack growth rate of 15-5PH SS was found to be five times faster than that of 316L SS with the same stress intensity factor range, ΔK. A tensile residual stress with a magnitude of 119 MPa was measured on the top 316L SS side about 2 mm above interface, while a compressive residual stress with a magnitude of about 305 MPa was measured on the bottom 15-5PH SS side about 3 mm below interface. Compared to the effect of yield stress gradient effect, high residual stress (magnitude of tensile residual stress > 35 MPa and the magnitude of compressive residual stress > 87MPa) existed in bimaterials dominated the crack growth rate. Therefore, to improve the fatigue crack propagation resistance by using a bimaterial fabricated by LPBF, compared with the effect of yield stress gradient effect, residual stress is a more significant factor and an architectural design with taking advantage of compressive residual stress can potentially prolong fatigue lifetime.

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Identifiers

ORCID for Anqi Liang: orcid.org/0000-0001-6574-5220

ORCID for Andrew Hamilton: orcid.org/0000-0003-4627-849X

ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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