Q&A: Grain structure control during metal 3D printing by high-intensity ultrasound
Prof. Ma Qian at the Manufacturing, Materials and Mechatronics Department of RMIT, is the corresponding author on a Nature Communications publication on the ultrasound 3D printing of metals. Kristina Kareh, the editor who handled the paper, asks him some behind-the-scenes questions about this work.
1. What are the key findings of your paper?
The key finding of this work is an approach to enable the formation of small and uniform grains or crystals during the 3D printing of alloys via the use of ultrasound. This allows for the 3D printing of metallic components. Ultrasonic grain refinement of metals and alloys during conventional casting and welding processes had been shown in previous work done by scientists in Russia, the UK, and other countries, and here we figure out how to apply it during the additive manufacturing of metallic materials.
2. What was your role in this work?
This work is part of my PhD student’s thesis: the lead author Carmelo Todaro is a PhD candidate, and it is great for him to see his work peer-reviewed and published. In the beginning, the project was centered on grain refining detrimental iron-rich intermetallic compounds in aluminium alloys using ultrasound, so that these alloys could be recycled. In about May 2017, I encouraged Carmelo to also look at whether this ultrasound method could work for metal 3D printing. Because many metallic alloys tend to have columnar grains during 3D printing, trying to combine ultrasound with 3D printing for microstructure control became the rationale behind the current work, which was challenging. We basically had to think about redesigning everything in terms of experiments until we got the desired result. So it essentially all started with recycling aluminium alloys, especially for Carmelo!
I was also inspired by the ways that sonotrodes (which produce the ultrasounds) have been implemented in other industrial processes, like for example chemical processing or drug making. In those processes, there are containers that contain a few tons of liquid chemicals, and to speed up the chemical process a few hundreds of sonotrodes are attached to the walls of each huge container to make it vibrate. There is also ultrasonic welding, which is commercially utilised. This points to ultrasonication being good business!
3. Everyone has a story: how did you come to do this particular piece of research?
I published a few well-received papers on ultrasonic grain refinement of light alloys about ten years ago and have continued working on the ultrasonic treatment of liquid metals over the last seventeen years. At that time, I was in the UK, funded by a research grant from EPSRC (the Engineering and Physical Sciences Research Council) for solidification microstructure control by physical methods including ultrasonic grain refinement.
4. What was the most critical moment during the study? Any anecdotes you’d like to share with us about challenges with obtaining these results and preparing the manuscript?
The most challenging moment was how to effectively introduce the ultrasonic energy to the additive manufacturing process. We came up with what I think is a very simple or brilliant idea to demonstrate the concept: if you look at the paper, the bottom part of Fig.1 is the sonotrode, which is made out of a titanium alloy (Ti-6Al-4V), and we deposited the molten titanium alloy (Ti-6Al-4V) directly on the surface of this sonotrode. This should be the most direct way of introducing the ultrasonic energy in the liquid metal during a metal 3D-printing process. We positioned the flat surface of the sonotrode upwards – when doing ultrasonic grain refinement of liquid metals, we usually do it the other way! I think there should be other effective ways of introducing the ultrasonic energy in the liquid metal during metal 3D-printing.
The second challenge came from how to control the additive manufacturing process because the ultrasound offers extra energy. This meant we had to adjust the power of the laser, otherwise everything would overheat. It took a while for us to realise that. The ultrasonic energy was much higher than expected, and we therefore had to adjust the additive manufacturing process.
5. What do you hope will be the impact of your research?
We are very keen on commercialising this setup, and we are actively discussing this with manufacturers of sonotrode devices. This technology does have the potential to be commercialised, and the goal in the near future is to make a wide variety of real components for assessment. I will be in a better position to answer this question in about six months’ time!
We are confident that we can make a 100-mm diameter sonotrode face for this 3D-printing setup which, for us, would be really exciting. We currently have investment casting, metal injection moulding, and die casting, which are commercial processes that aim to make reasonably small parts with equiaxed grain structures. With this 100-mm diameter face, we can 3D-print a wide variety of intricate titanium, stainless steel, aluminium, and other high strength alloy parts.
6. Which is the development that you would really like to see in the next 10 years?
That would be something that was mentioned by one of the reviewers: that this invention could be used for many applications and could prompt the development of a new type of additive manufacturing machine. For example, an ultrasound-assisted laser metal 3D-printing system, or it could be implemented in a plasma-based or electron beam-based metal wire additive manufacturing setup.
People have also applied ultrasonic treatment to the solidification of liquid polymers, and now we have the chance to try this technology with polymeric 3D printing, which can also involve nucleation and growth when the polymers are semi-crystalline. I remember a few years ago, there was a paper published on the nucleation and growth of polymeric materials with ultrasonic vibration, so we may be able to do something there.
7. What would your dream conference be like?
Normally, I go to TMS (the Minerals, Metals, and Materials Society conference). It is pretty good and very informative, with about six to nine parallel additive manufacturing symposia. And every two years, I organise the Asia-Pacific International Conference on Additive Manufacturing (APICAM) in Australia: it is around four hundred people, which is large enough!
8. What is your favourite material?
Titanium. Because it has helped humanity so much and it is the premier material for bone replacement and repair as it is biocompatible.
9. If you weren’t a material scientist, what would you like to be (and why)?
A mathematician! Because mathematics has helped me a lot in addressing many engineering problems. I also love the magic of mathematics, the beauty of mathematical formulae, and the beauty of the mathematical world (this is the real reason). Mathematics allows for all the things to become formulae and one equation can have so many implications.
10. Where do you go from here?
I definitely will continue to focus on metal additive manufacturing and try to commercialise this development. As time goes on, I hope we will have more new and exciting developments.