Metal additive manufacturing (AM) techniques typically operate using powders with limited particle size ranges. In the atomization process, where the melted metal alloys are first refined and degassed, then poured into a gas nozzle, where the liquid material is disintegrated into metal powder by a high-pressure gas stream, significant amounts of particles are produced outside of these ranges. This results in an accumulation of out-of-size specification metal powders without a clear use case – sintering methods having been found to be ineffective in the consolidation of these aluminum powders.
AM techniques are generally promoted as having reduced material wastage compared to conventional metallurgy, where extensive machining waste is generated, but the as-atomized particle size range is often overlooked as an issue. Business economics requires that alternative processes are utilised to convert these surplus powders into useful products, to ensure that the AM market is cost effective and meets sustainability targets. This key factor informs the foundation of this research.
Field assisted sintering technology (FAST) can provide an alternative solid-state processing route to consolidate these surplus powders into billets for subsequent processing. This enables the production of useful products from this feedstock, whilst also improving sustainability within the AM supply chain.
This project goes a step further, in combining FAST with hot rolling, to convert surplus aluminum alloy powders from atomization into sheet material in two solid-state steps. FAST can effectively consolidate the powder into fully dense billets, which are then hot rolled into sheets.
Through tensile testing, results revealed that the properties of materials that resulted from this process were comparable to the conventional material used in aerospace applications.
Pre-existing research, focussed on FAST of metal powders, has assisted in the development of this project. This brought about a final year student project designed by research associate Dr Simon Graham and led by Alicia Patel, a BEng aerospace engineering student, who assisted in the practical work in the early-stage development of research before the project was defined. Following the completion of this early-stage research, the project has been built upon with a more streamlined direction.
The research has also been informed by works carried out at the University of Sheffield relating to the processing of titanium powders which are oversized for laser powder bed fusion, where methods of crossover were identified to be of relevance. When reviewing existing literature, only one paper has been published which specialises on hot rolled, FAST produced, pure aluminum. Previously published research on A20X alloy has only considered AM or cast material.
It has been shown that FAST can rapidly consolidate aluminum alloy powders, including A20X, with a large particle size range into fully dense materials. The resulting 80 mm diameter material was also successfully hot rolled from its initial 15 mm thickness down to 2 mm sheet, although some later optimization is required to prevent edge defects within the sheet.
At the same time, conventionally cast A20X material, with the same starting dimensions, was also hot rolled under the same conditions. Tensile testing showed that – before and after the heat treatment – the FAST material exhibited similar properties to the cast material and was comparable to other aluminum sheets used in aerospace applications.
These findings were presented by Dr Simon Graham at the WorldPM 2022 Conference, in a keynote speech entitled ‘Solid-state processing of surplus aluminum alloy powders through a combination of field assisted sintering technology and hot rolling’.
The results were a promising step in creating high performance sheet material from surplus aluminum alloy powders, with some further optimization and scale up required.
The outcome of this project has demonstrated that there is a viable processing route to convert surplus alloy powders into sheet material with good mechanical properties. Although the long-term, positive, impacts cannot be quantified at this stage, there are clear economic impacts. These economic benefits relate to new revenue streams for atomizers, as well as a potential cost reduction of powders for AM.
The next steps involve completing further rolling trials to improve processing and produce higher quality of sheet product. These sheets can also be extended in the starting phases, in a bid to produce larger sheets as an output, which further demonstrates scale up opportunities. Superplastic forming of the sheet material could also be considered to produce near-net shape components.
The project was led by Dr Simon Graham, and supported by Professor Martin Jackson, research area lead, Royce at the University of Sheffield, Dr Beatriz Fernández Silva, research associate, STAR group; Dr Oliver Levano Blanch, STAR group; Nigel Adams, senior engineering technician, Royce at the University of Sheffield; Sam Lister, AMS CDT EngD researcher; and Dr William Pulfrey, AMP engineering lead (thermomechanical processing), Royce at the University of Sheffield.