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In-situ alloying for extreme functional gradient in additive manufacturing


Project Coordinator
Dr. Rocco Lupoi

Research Staff

Mr. Arnoldas Sasnauskas

Funding Bodies

Description

Additive manufacturing [AM] is an evolving series of disruptive technologies that enable the layer-by-layer fabrication of objects. The last three decades saw a rapid emergence of these technologies and their deployment in industry, with the first metal part printed via fusion in 1990. Powder preparation is the first step in AM, playing a massive part in dictating the quality of the finished component. While a range of pre-alloyed powders such as 304 SS, Ti64 and Inconel 625 are available on the market – they have a narrow composition range, suffer from limited availability and are expensive. In-situ alloying is a process that experienced increased interest due to the flexibility in developing low-cost and efficient alloys. Several alloying methods have been explored such as jar-mixing or simple-mixing, mechanical alloying and satelliting. While the development of a completely new alloy may be tedious, research has shown that slightly changing the composition of existing alloys can enhance properties such as strength, ductility, density and anti-bacterial properties. Marchese et al proved that even mixing 1% Ti64 into Inconel 625 via a ball-miller can increase the strength, hardness and density relative to the base alloy. However, out of the 5500 alloys in use today, only a small selection can be printed due to complex dynamics between solidification and melting causing poor microstructure. The satelliting of nanoparticles has proved to enable the printing of alloys such as AL7075 and AL6061, previously deemed unprintable.


Gradual vs. stepwise functional gradient material

In-situ alloying enables non-conventual formulations and functional gradient materials [FGM]. FGM are materials characterised by spatial variation in composition across the volume, corresponding in changes in material properties. The concept of FGM was first explored in the Japanese Space Agency in 1980 by layering composite and ceramic materials, but the emergence of AM empowered more complex FGM due to improved control over density and directionality. There are three types of FGM – a) variable densification with homogenous composition, b) heterogeneous composition with a gradual transition in material, and c) a combination of variable densification and heterogeneous composition. Controlling and varying the laser parameters across a print is a proven method of producing FGM as demonstrated by Traini et al, producing a Ti64 bone implant with a variation in density from the inner core to the outer surface. Multi-material FGM was also demonstrated in a step-wise format – by suction removing material and adding a different material during the fusion printing process. The same was also achieved by varying the laser powder. In-situ alloying has been deployed to enable cost-effective and rapid production of a FGM (Ti-Mo alloy), exhibiting variant microstructure however the research concluded that better quality control is required before these processes can be utilised in industry


Process diagram of A) PBF and B) MAPS

Traditionally, powder-bed-fusion (PBF) is limiting to the rapid and low-cost production of FGM. Only a single material can be fabricated at a time. While several solutions such as redesigned multi-material powder coaters and micro-vacuum to remove material have been explored, these solutions are slow and expensive, and do not address the issue of loose powder. Loose powder is a major health risk and can be quite prohibitive in processing, storage and cleaning. A new innovative process called Metal Additive manufacturing using Powder Sheets (MAPS) has been introduced as a ‘powder-free’ alternative, utilising composite sheets to produce metallic parts. This new technology can be a vehicle to produce new exotic FGM that were previously impossible – due to the freedom of multi-material composite sheets. This project will explore the deployment of MAPS to produce novel FGM with in-situ alloyed powders.