Microstructure properties and fatigue behaviour of multiple materials fabricated by Laser Powder Bed Fusion process

Abstract

Multiple material additive manufacturing (MMAM) is a manufacturing technique that allows the fabrication in a single operation of components with different materials integrated into specific design locations. This technique allows the targeting of specific properties of materials and placing them into functional areas of a component, while reducing the number of manufacturing operations, the weight and costs of assemblies. The ability to produce components made of multiple materials in a single operation raises some challenges around the structural integrity of such components and the repeatability of the process. Considerable effort is made by the scientific community to understand and control the microstructure and mechanical properties of components fabricated by AM technologies. Standards are evolving and the transfer from “conventional” post treatment is also being investigated. The consequences on the microstructure properties and the effect on the mechanical response of these combinations will vary drastically from one combination couple to another. The overall objective of this work is to understand and try to enhance the structural integrity of multiple metallic materials produced by laser powder bed fusion (L-PBF) in a layered architecture. The alloys used in these studies are 316L Stainless Steel (SS) and Inconel 718 (IN718), both are widely employed in very demanding service environments where their oxidation resistance or ability to sustain high stress levels at elevated temperature are sought after. The fabrication of layered components has been chosen for crack propagation mitigation purposes, extending the fail-safe service lives of these components, a critical topic in structural integrity of key components. Layered architectures combining IN718 and 316L have been fabricated using the L-PBF process, and the effect of Heat Treatment (HT) on the microstructure and mechanical properties of these specimens has been studied. Single edge notched bend (SENB) specimens of bi-layer and four alternating layers were built horizontally, both IN718 and 316L layers were fabricated using the same set of process parameters (with an Energy ratio of 140J/mm2). A specific HT strategy including a solution treatment and a single stage ageing (1050°C/45min/FC + 620°C/4h/AC) was applied in order to allow precipitation strengthening to occur within the IN718 layer, while limiting the formation of detrimental phases within the 316L material and at the transition zone between the two alloys. The microstructure and mechanical properties were studied by optical microscopy (OM), scanning electron microscopy (SEM), tensile testing, micro and nanohardness testing. The fatigue tests were performed on single edge notched bending (SENB) specimens to determine the crack propagation process through multi-layer specimens under 3 point bending cyclic loading. In the as-built (AB) condition, the yield strength of 316L alloy was 15% lower than that of IN718, but still 51 % superior to the value recommended by standard for conventionally manufactured 316L. This is an effect of the specific microstructure obtained from AM in the AB condition, where each grain is composed of subcellular structures (~0.5-1μm), that are usually formed due to the rapid cooling of the AM process and is due to the segregation of heavier elements also causing multiple dislocations to form around these cells. Whereas, IN718 produced by L-PBF in AB condition has a yield strength 34.3% lower compared to values expected from standard for conventionally manufactured and heat treated Inconel alloys, and displays an ultimate tensile strength (UTS) value 36.7 % lower than expected from standard for conventionally manufactured and heat treated Inconel alloys. These differences in strength are due to the lack of precipitation strengthening when the Inconel alloy is not thermally post treated. The crack propagation resistance mechanisms were differentiated in each alloy layer, and it was observed that grain misorientation has an effect on crack tortuosity within the 316L layer in the AB condition, while the fatigue crack propagation (FCP) in IN718 was observed to be transgranular and mostly dependent on the intrinsic strength of the alloy. The softer 316L layer (in AB condition) has been shown to release crack tip stress via substantial secondary crack opening, effectively shielding the crack propagation within the 4-layer specimen tested in AB condition, before the transition to the IN718 layer. The heat treatment of 316L/IN718 specimen caused a local inter-diffusion between the FCC(Fe) and the FCC(Ni) at the interface, leading to a region of 140μm depth where the local formation of Laves phases, locally enhanced decohesion with the matrix when subjected to FCP. The HT has improved tensile properties and supported the strengthening within the IN718 alloy leading to an increase of 11% in yield strength, 22% in UTS and 50% in Elongation. Meanwhile the 316L alloy saw a loss of 31% in YS and 5.2% UTS, although its tensile properties after HT remain superior to the values quoted for conventionally manufactured 316L. In the FCP test the heat treatment produced differences in da/dN versus ΔK, significantly improving IN718 resistance to crack propagation, whereas 316L has displayed a decrease in its crack propagation resistance. The effect of shielding/antishielding was also diminished by the effect of a weaker interface, where the local precipitation of Laves phases created an area more prone to decohesion. However, it was demonstrated that the shielding/antishielding effect of the interface transition is mostly effective when the ΔK at the crack tip is below 20MPa√m. Overall promising observations of the potential ability to produce multi-material components and heat treat them in order to create crack propagation resistant components have been made, however the design of such component should take into account the placement of such interfaces in order to take advantage of the shielding this transition in material can offer.

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ORCID for Nong Gao: orcid.org/0000-0002-7430-0319

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