The following is an excerpt.

For the first time, 3D printed maraging steel has been studied under ballistic penetration.

 

To the surprise of the researchers, the heat-treated material is capable of destroying bullets in a fashion rarely seen before.
«This could propose a step forward in the application of 3D printed maraging steel in the industry, and potentially may be very effective in personal protection», says MEng and Erasmus + student Maisie Anne Edwards-Mowforth, who studies Mechanical Engineering at the University of Edinburgh.
Her summer project thesis is on the mechanical behaviour of a 3D-printed, high strength, maraging steel under ballistic penetration. This subject brought her to the laboratory facilities at SIMLab, NTNU where her supervisors are Professor Tore Børvik, and researchers Miguel Costas and Martin Kristoffersen.

The effect of heat treatment

She explains her choice of SIMLab with great enthusiasm: «NTNU stands out as best place to go to do
any practical impact testing. The standard of research at CASA is excellent, not to mention the friendly staff and students in the department.».
She is investigating the effect of heat treatment on the material’s elasticity and overall performance. This is done through experimental testing, metallurgical studies and numerical modelling using Abaqus software.

«I am studying engineering because of how all-encompassing it is in nature».

Builds on previous work with aluminium

Her work is a continuation of former MSc student Tim Koenis’ thesis on aluminium: «Ballistic testing and numerical simulation of 3D printed AlSi10Mg». Koenis studied at the Eindhoven University of Technology and visited SIMLab and SFI CASA in spring 2019.

Edwards-Mowforth brought two types of 3D printed steel plates to test in the lab, each with aslightly different processing method: As fabricated (right) and heat-treated (left). She aims to model how the material behaves during the impact of the bullets. Although there are several papers on the properties of 3D-printed maraging steels compared to traditional manufacturing methods, she tells us, this is the very first time the material has been studied under ballistic penetration.

Particularly suited to 3D printing

Why is maraging steel particularly interesting?
Maisie explains that heat-treating the material makes it more brittle rather than less brittle – the opposite effect than for regular high strength steels. «Usually, one would expect a heat treatment to relieve internal stresses and dislocations, and make the material more ductile», she says.
«However, maraging steel gets its strength from intermetallic precipitates, rather than interstitial alloying elements like high carbon steel does. Therefore, under heat treatment, these precipitates (primarily of Nickel) grow; which drastically increases the yield strength and reduces the ductility – or elasticity of the steel».

Shattered bullets

These properties also mean that maraging steel is particularly suited to 3D printing. The Intrinsic Heat Treating (IHT) in additive manufacturing naturally introduces this ageing process even before traditional heat treating.

Besides, the lack of interstitial alloying carbon also makes maraging steel easier to weld.
«When it comes to the ballistic impact, we have found what is expected», Maisie explains.
«The material with a higher yield strength possesses a higher theoretical ballistic limit velocity. What has been particularly interesting though, is the capability of the heat-treated material to not only stop the bullet, but completely shatter it» (left).

Utilizing the benefits of additive manufacturing

A positive end result of this would be utilizing the benefits of additive manufacturing in personal protective equipment, such as portability of production and complex, one-off parts.
«A comparison could be made between the testing done on 3D printed maraging steel with conventional manufacturing methods. If there is a little discrepancy, it could lead to a step forward with this goal».

READ MORE: How do 3D-printed Materials Behave Under Extreme Loads?
READ MORE: A Robot-printed Steel Bridge can Lead CASA Deeper into 3D-printing

And what kind of curiosity keeps the 20-year-old from Hull, UK going?
«I am studying engineering because of how all-encompassing it is in nature. It revolves around making the world a better place – but ‘better’ is not a scientific category. We are informed by science and philosophy, yet strive not just to study the world, but have an impact on it through implementation.»

Attended the finals of the SpaceX Hyperloop

This summer, she took a 2 week break from the internship at CASA to attend the finals of the SpaceX Hyperloop Pod Competition in Los Angeles, with the Hyperloop Team ‘HYPED’ from the University of Edinburgh. 
To get to the finals, each team passed through a rigorous selection process, writing several design and safety packages and taking part in interviews with SpaceX engineers. «For HYPED, it was a pleasure to present our pod, ‘The Flying Podsman’, to the public and to meet Elon Musk at SpaceX Headquarters», she says.

Maise Anne Edwards Mowforths involvement with HYPED over the course of the 3rd year of her degree consisted of leading the 15 member Static team to build the Pod’s composite structures.

The hunt for a model

After LA and SpaceX, she returned to the ballistics.
«Ballistics is an exciting area of mechanics because there are so many factors affecting the performance of a material. As well as different failure modes to consider, the material is under large deformation, high strain rates and high temperatures. This requires ‘elastic-thermoviscoplastic models’ to perform analysis».
She adds: «I am very much a beginner in the area, so at the minute it all seems like total chaos, but very exciting all the same. It makes it even more satisfying to see a model that you have made predict what will happen in a test! ».
Part of the process is to conduct a metallurgical study on the two materials and investigate just how the microstructure looks after fabrication and the effect that the heat treatment has on it. «As we have seen in the tests, the difference in processing can completely change the macro properties of the steel».