The following is an excerpt.
One year ago, SFI-CASA started some preliminary activities in the field of additively manufactured metals.
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A truss structure in 3d-printed aluminium before subjected to a shockwave in the shock tube. (Photo: Vegard Aune)
These activities were aimed to answer two questions:
- How do 3D-printed metals behave under extreme loads?
- Can we simulate their mechanical behaviour with the models we commonly use at the Centre?
Selective Laser Melting (SLM)
Nowadays, we find a large variety of additive manufacturing techniques and materials. Thus, it was necessary to restrict the initial investigations to a single material and manufacturing process: an AlSi10Mg aluminium alloy built by Selective Laser Melting (SLM). Parts built using this technique are fabricated layer by layer using a high power-density laser beam on a powder bed. The laser melts and fuses each layer of the part in a closed system with an inert gas atmosphere, producing robust and well-bonded parts.
However, this is a costly process due to the high costs of the machinery. So far, SLM is only commercially suitable for the production of specific parts with complex geometries or spare parts and tools for maintenance in off-shore facilities and spacecraft.
Do the Constitutive Models Work for SLM Alloys as Well?
The first activity run at the Centre intended to analyse the behaviour of AlSi10Mg components under quasi-static loads, and also to see if the constitutive models we use every day for extrusions or rolled sheets of aluminium work for SLM alloys as well. A preliminary microstructural analysis and some mechanical tests revealed a material with a complex microstructure but with a quite isotropic mechanical behaviour.
The curves on its right are the stress-strain curves of the alloy, in three directions.
We used the data from the mechanical tests to calibrate some of the material models that we commonly use for aluminium, and we were able to simulate the component tests with a good degree of accuracy.
A Leap Into Ballistic Tests
Once we saw that we could easily simulate this material with our current tools for quasi-static loads, we leapt into the ballistic tests, without being 100 percent sure if the ductility of the alloy was high enough.
Right: An image of the simulations, where the colour map represents the damage variable.
For this activity, we got the help from Tim Koenis, a master student from the Eindhoven University of Technology.
After shooting the first plates of the SLM material at velocities between 300 and 1000 m/s we saw that not only did the plates withstand the tests in one piece, but they also showed some petalling which is common to observe in more ductile materials.
A High Degree of Accuracy.
Careful calibration of a constitutive model for this material enabled us to reproduce the ballistic tests using numerical simulations with a high degree of accuracy. The following image shows a 3D-scanned plate after the tests and the corresponding numerical simulation. Which one is which, is left to the reader to decide.
3D-scan of one of the AlSi10Mg plates after the ballistic tests and the corresponding finite element simulation.
It was also interesting to compare the results of the experimental penetration tests in the SLM alloy with some additional tests on plates made of a die-cast alloy with the same chemical composition. We saw that the behaviour of both materials was almost identical, even though the cast showed a much more scattered failure strain in the tensile tests compared to the SLM alloy.
A Flexible Technology
The SLM technology is also being used at the Centre to investigate the effects of blast loads on structures. The flexibility of the technology makes it possible to manufacture scaled structural models in a few hours with excellent precision. Some preliminary tests at the shock tube showed how blast waves interact with structures using these structural models.
In a few weeks, we have two upcoming activities in additive manufacturing. A student from the University of Edinburgh spend a few weeks at CASA this summer to investigate the behaviour of SLM high-strength steel under ballistic penetration.
In August we start a collaboration with the Dutch company MX3D and Multiconsult to look into the mechanical properties and behaviour of the structures manufactured by MX3D.
ALSO READ: A Robot-printed Steel Bridge can Lead CASA Deeper into 3D-printing
The author of this report, researcher Miguel Costas Piñó joined SFICASA in January 2017. He earned his PhD at the University of A Coruña (Spain) in January 2016 with his thesis Crashworthiness analysis and design optimization of hybrid impact energy absorbers, being awarded the Cum Laude and international distinctions, and the extraordinary doctorate award. His research topics include material mechanics, crashworthiness, structural optimization, metal plasticity, impact engineering and finite element simulations of impact problems using explicit codes.
