Prof. Bruno, Tatiana Mishurova, Tobias Thiede and Alexander Ulbricht

Prof. Giovanni Bruno with Ph.D. student Tatiana Mishurova and Ph.D. students Tobias Thiede and Alexander Ulbricht in front of the Selective Laser Melting (SLM) device

Source: BAM

Currently there is a rapid development in additive manufacturing techniques, as they promise many advantages in the automobile industry, aviation and medicine to name just a few. The layer-by-layer manufacture of objects based on virtual models enables completely new geometrical forms and high-degree material utilisation. However, the new techniques have so far hardly been used for safety components and many questions remain unanswered. One of them is: how do material properties and defects of additively manufactured objects affect their service life?

The AGIL project "Microstructure Development in Additively Produced Metallic Components: from Powder to Mechanical Failure" managed by Prof. Giovanni Bruno wants to find answers.

Professor Bruno, everyone is talking about additive manufacturing. What makes this method so attractive?

The excitement itself is promising. There is hope that many products can be manufactured faster and cheaper using additive manufacturing. I can tell you that's not yet the case. There are two big advantages: there is less waste and very complicated shapes can be produced that cannot be produced by other methods.

… and the disadvantages?

The industry is entering a new world: the world of speed and cost. That the quality of additively manufactured parts is good must yet be confirmed. If a part looks nice, this does not mean that it's good inside.

Is your objective to improve the quality of additively manufactured parts?

Yes, this is where BAM enters the scene. Our mission is to guarantee safety solutions, therefore it’s quality and sustainability that count for us. In the project AGIL we investigate how additively manufactured components and materials behave under stress over time and how they change during their service life. The quality must be adequate, especially for components relevant to safety, otherwise it may become dangerous and expensive.

We also look at whether the process itself can be problematic. Think about the powder used. How good are logistics and supply chain? Are they sustainable? How pure is the powder? Many powders contain pores. How do you produce pore-free manufactured components when the powder already contains pores? Of course, our industrial partners also ask these questions. But we look at the safety aspects and this makes the difference.

Picture of a BAM employee with an additively manufactured grid structure

Project researcher Alexander Ulbricht with additively manufactured grid structure that was produced by Siemens AG, Power and Gas, Berlin, in cooperation with BAM.

Source: BAM

How can one actually imagine that?

Characterisation so far carried out in industry, is fine: I have a component. Oh, that has pores. They are dangerous. Now I am trying to eliminate these pores. Alright, I have done it. Oh, there must now be a lot of tension because some of my parts are already breaking during manufacturing. OK, then I'm going to do a heat treatment to relax the stresses. I did that. Very good. Is the microstructure now perfect? Are the dimensions correct? Are there any distortions? Only when all these aspects are considered satisfactory, then my part is completed. – But that's just the beginning. That would be like saying: my child is healthy and he will always be a healthy person. No. We must keep watching. Unfortunately, the microstructures in the materials become unstable due to repeated melting and solidification. And this instability can cause various unexpected phenomena under external influence such as loads, temperature cycles, etc. This means we may discover something tomorrow that we do not expect today.

How can the quality of additively manufactured components be improved?

By giving them more time and taking more care. That's exactly the point: basic research. We at BAM know the properties of materials very well. But manufacturing processes will be at least as important in the future. We also need to know them, especially with respect to scientific aspects and component safety.

You deal with additive manufacturing processes of metallic materials. What are the similarities and differences to conventional welding processes?

There are several additive manufacturing processes. When we look at laser metal deposition or LMD, it actually works like a weld seam that you repeatedly place over a sample. Of course, the length scale in additive manufacturing is smaller than in conventional welding. The width of the laser spot is only a few tenths of a millimetre in some processes. The temperatures are different too. A laser induces very high temperatures, and this, of course, is followed by a correspondingly sudden cooling. A material that is additively fused will freeze within milliseconds. In welding processes, however, materials have much more time to solidify. Local phenomena, faster phenomena, more unstable microstructures – this is typical of additive manufacturing processes.

Where are additive methods better than other methods?

The issue is the complexity of form. Weld seams per se cannot be so complicated. Object sizes also play a role, and of course the processes. It's about physics, diffusion, relaxation.

Are then additive manufacturing methods not THE future’s production technology?

No, of course they aren’t. It does not make any sense to do everything using additive manufacturing. Not even if the methods are mature, because then they will not be very cost-effective. Simple objects must be produced conventionally in the future because it is faster, cheaper, and perhaps safer.