A study published in Nature Communications shows that high-frequency sound waves can have a positive effect on the micro-structure of 3-D printed alloys, making them “more consistent and stronger than those printed conventionally”.

Carmelo Todaro (pictured left), the lead author and PhD ‘candidate’ from RMIT University’s School of Engineering (in Melbourne, Australia) (www.mit.edu.au), said the promising results could inspire new forms of additive manufacturing.

“If you look at the microscopic structure of 3-D printed alloys, they are often made up of large and elongated crystals.

"This can make them less acceptable for engineering applications, due to their lower mechanical performance and increased tendency to crack during ‘printing’.

"The microscopic structure of the alloys to which we applied ultrasound during printing looked markedly different; the crystals were very fine and fully ‘equiaxed’, so they had formed equally in all directions through-out the part.”

Testing showed that these parts had a 12% improvement in tensile strength and yield stress compared with those made using conventional additive manufacturing, and the team demonstrated its ultrasound approach using two major commercial-grade alloys.

These were Ti-6Al-4V (a titanium alloy commonly used for aircraft parts and biomechanical implants) and Inconel 625 (a nickel-based super-alloy often used in the marine and petroleum industries).

“By simply switching the ultrasound generator on and off during printing, we also showed how specific parts of a 3-D printed object can be made with different microscopic structures and compositions — useful for what is known as functional grading.”

Professor Ma Qian, the study’s co-author and project supervisor, said he hoped that the team’s promising results would spark interest in specially designed ultrasound devices for metal 3-D printing.

“Although we used a titanium alloy and a nickel-based super-alloy, we expect that the method can be applied to other commercial metals, such as stainless steels, aluminium alloys and cobalt alloys.

"We anticipate that this technique can be scaled up to enable 3-D printing of most industrially relevant metal alloys for higher-performance structural parts or structurally graded alloys.”