Written by Peggy Jones & Daisy Martlew
NSIRC PhD student Gowtham Soundarapandiyan has won one of three runner up places in the TWI Industrial Impact Awards. The Awards, which recognise students’ research that makes a significant contribution or takes an innovative approach to solve an industrial problem, were given at the NSIRC 2019 Annual Conference.
Gowtham’s research, titled ‘Effect of Powder Recycling on Structural Integrity of Additively Manufactured Ti6Al4V Material’, was commended as offering an outstanding solution to a problem facing the aerospace, automobile, medical and nuclear industries.
Fabricating near net shaped parts is an outcome increasingly demanded by the aerospace, automobile, medical and nuclear industries. Naturally, these industries demand high structural integrity of manufactured parts, as well as cost effectiveness and a decreased energy footprint.
Undertaken at TWI, the research aims to draw these demands together, and aid industrial understanding of how the additively manufactured production of high value engineering parts can be made more reliable and efficient. His PhD intends to strike a balance between making this process material efficient without compromising the structural integrity of produced parts.
Gowtham said “it is a pleasure to undertake my PhD in an industrial environment. The constant guidance and support given by the industrial experts enabled me to direct my work towards addressing the real industrial needs. I also have the opportunity to interact with other major industrial stakeholders and scientists during different TWI events which helps me establish useful collaborations. After my PhD, I am keen to work on different additive manufacturing projects within TWI or in one of their innovation centres”.
Ti6Al4V which is one of the most widely-used titanium alloys for manufacturing near-net shaped parts, is favoured by industry because of its excellent strength-to-weight ratio, and corrosion resistance properties. However, it is difficult to forge and machine, and is also very costly.
What is additive manufacturing...?
"Additive manufacturing (AM) is the industrial production name for 3D printing, a computer controlled process that creates three dimensional objects by depositing materials, usually in layers." - TWI Global
As additive manufacturing (AM) is increasingly being used to produce high value engineering components, it is important to consider how AM can be used to produce parts made from the Ti6Al4V alloy which meet the high levels of structural integrity standards.
Gowtham’s research seeks to understand the hurdles faced when recycling titanium powders during laser powder bed AM process. Whilst laser powder bed fusion (L-PBF) is capable of producing high-integrity, near net parts, the process inherently leaves large amounts of titanium feed stock powder unconsumed. Due to the high cost of this powder, there is a need to recycle it; however, the unconsumed powder deteriorates during the harsh conditions created by the L-PBF process.
Manufacturers are unsure about the consequences of using recycled powder on the part properties due to the knowledge gap on the changes in powder characteristics with recycling, and the lack of standards. In order to avoid any risk, they employ virgin powder/powder recycled only for few runs to manufacture safety critical parts. This knowledge gap resulted in hundreds of kilograms of ‘out-of-spec’ powders that are either stored or disposed-off resulting in increased energy footprint and reduced process efficiency.
Gowtham began by characterising virgin and recycled Ti6Al4V powder in order to understand how it degrades. He identified that physical properties such as flowability, density, particle size distribution and powder morphology were less affected. However, the oxygen content of the powder increased enough to move it out of specification after multiple recycling.
As oxygen is one of the primary limiting factors in effective recycling of Ti6Al4V powder, blending recycled powder with virgin powder reduced the overall oxygen content. The next step within the project was to investigate the effect of blending recycled and virgin powder on the build properties.
Argon gas atomised (AGA) Ti6Al4V powder was artificially oxidised up to 3000ppm. This oxidised powder was then blended with virgin powder to produce blends where oxygen content registered 1500, 1900 and 2500ppm.
Test coupons were fabricated using these blends, which then underwent microstructure and mechanical testing, looking at its hardness, tensile strength, Charpy impact and fatigue resistance. A number of in-situ studies were also undertaken to determine the effect of oxidised powder particles in the blend on the laser-powder interaction using the process replicator and synchrotron X-ray imaging techniques.
As a result of this research, Gowtham anticipates multiple ways in which manufacturers will benefit. First, they will better understand the challenges and nature of change in Ti6Al4V powder characteristics due to recycling. Second, it will be possible to correlate the effect of powder degradation against the structural integrity of parts. Third, better understanding on the effect of oxidised powder particles in recycled-virgin powder blend on the build properties. Fourth, upon further in-situ study, suitable process parameters for recycled powders could be established. Fifth, recycled powder can be used with greater confidence when manufacturing safety-critical parts. Finally, suitable ratios of blending recycled and virgin powder can be determined.
Gowtham is an NSIRC PhD student with Coventry University, and sponsored by Lloyd's Register Foundation.
For more information, or to speak to Gowtham about his research, please contact email@example.com.