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Project

Multi-Material Additive Manufacturing of Functionally Gradient Materials

Selective laser melting (SLM) is a powder-bed additive manufacturing process in which an object is built by selectively melting the fine powder by high-power laser in a layer-wise manner. It holds the advantage of producing complex-shaped parts with high density. For the concern of powder cross-contamination, the commercial SLM enables mostly the fabrication of single-material components in a single manufacturing operation. Recently, researchers have attempted to manufacture multi-material parts via the modified SLM methods. However, there are not yet commercially successful multi-material products fabricated by the modified SLM facilities. It thus requires not only machine modification but also the know-how to solidly join dissimilar metals under the laser-beam condition.

The thesis aims to understand interfacial behaviours when manufacturing multi-materials by using SLM. To achieve this, four multi-material pairs, divided into three parts based on different base metals of 316L, Ti-6Al-4V and AlSi7Mg, were processed via commercial SLM. The first part analyzed the influence of SLM processing parameters on producing the interface of 316L/Hastelloy X. The second part compared the interfaces resulting from adding near-α titanium alloy (Ti-6Al-2Sn-4Zr-2Mo) and γ-TiAl (Ti-48Al-2Cr-2Nb) on the top of Ti-6Al-4V, respectively. Considering that Ti-6Al-2Sn-4Zr-2Mo is fully new material for SLM, the processibility and post-heat treatment of the alloy were also investigated in part 2. The third part studied the interface formed by SLM adding A357 (AlSi7Mg0.5) on the top of cast A356 (AlSi7Mg0.3). Since the high-rough cast surface may worsen the succeeding growth of A357, different surface treatments were compared in producing a strong interface. Then, these SLM-built multi-materials were characterized to understand the grain growth and material mixing at the interface region. Mechanical testing of the multi-materials was also conducted to verify the reliability of dissimilar joints.

The study on multi-material 316L/Hastelloy X indicated that SLM parameter, particular the laser energy density, plays a critical role in forming a solid joint. Processed by the optimal parameters, the bimetal exhibited epitaxial grain growth from 316L to Hastelloy X, without deadly defects at the interface. The subsequent tensile testing also confirmed a robust bonding between 316L and Hastelloy X.

By comparing the two titanium-based bimetals in terms of their SLM processibility and interfacial formation, it was found that Ti-6Al-4V/γ-TiAl displayed a much wider interface than Ti-6Al-4V/Ti-6Al-2Sn-4Zr-2Mo, despite under the similar processing parameters. The interfacial width difference is because the chemical composition of Ti-6Al-2Sn-4Zr-2Mo is much closer to Ti-6Al-4V than that of γ-TiAl, thus requiring less convection for material changeover and then producing a narrower interface zone. The high brittleness of γ-TiAl led to the failed fabrication of Ti-6Al-4V/γ-TiAl under the fast-cooling SLM (without using baseplate pre-heating). In comparison, Ti-6Al-4V/Ti-6Al-2Sn-4Zr-2Mo was successfully built under the optimized condition. The successful manufacturing of Ti-6Al-2Sn-4Zr-2Mo also broadens the material palette of SLM.

Laser remelting in the SLM chamber, rather than the expected wire electrical discharge machining (W-EDM), is concluded as the optimal treatment to improve the original casting surface. The growth of SLMed A357 on the remelted surface led to a defect-free and well-built interface. Laser remelting will also not add extra costs for future industrialization. In comparison, although the EDM exhibited optimality in decreasing the surface roughness, massive Al2O3 was induced at the same time by electrospark. The alumina was formed in large clusters, which decreased the wettability of the EDM surface to grow SLMed A357 and in consequence caused interfacial delamination.

The established understandings on interface formation under the SLM processing could promote the development of multi-material SLM technologies. The successful multi-material cases also possess potentials in chemical (316L/Hastelloy X), aerospace (Ti-6Al-4V/Ti-6Al-2Sn-4Zr-2Mo) and automobile (cast A356/SLMed A357) applications.

Date:4 Oct 2016 →  4 Dec 2020
Keywords:Additive Manufacturing
Disciplines:Manufacturing systems
Project type:PhD project