Aluminium to aluminium: the long road to metallurgical bonding in high-pressure die-casting

The aim of the project is to join aluminium sheets or profiles directly to cast structures within the high-pressure die-casting (HPDC) process by means of a metallurgical bond. “The concept is not new – but to date there is no series-ready process for aluminium-to-aluminium bonding in HPDC,” explains project manager Christoph Pille from Fraunhofer IFAM. At the same time, he sees clear potential: “Metallurgical compound casting has often been written off as unfeasible. We take a different view – the demand is still there. That is why we are revisiting the topic and specifically exploring new approaches.”

Oxide layer acts as a barrier

The current state of the art is clear: form-fit joints can be realised in HPDC, for example through mechanical interlocking or tailored surface roughness. A true metallurgical bond – meaning a connection without an intermediate layer – remains the exception. The main reason lies in the material itself: aluminium immediately forms an oxide layer, which acts as a stable barrier and prevents contact between the melt and the sheet.

The Fraunhofer team is therefore pursuing an approach that addresses material, surface and process together. Two technological routes are at the core of the work: firstly, the oxide layer is deliberately removed and the surface is then protected against reoxidation by temporary protective layers. Secondly, metallic interlayers are being investigated as a means of forming the bond. The objective is to provide a highly reactive aluminium surface during casting, allowing a shared microstructure to form between the casting and the sheet.

A sensitive interplay

In addition to the surface, temperature plays a decisive role. The sheet must be heated locally to a level close to its solidus temperature in order to become diffusion-capable. At the same time, the sheet and the melt must solidify together in such a way that stresses in the joining zone are avoided. The experiments carried out so far demonstrate how sensitive this interaction is: even the positioning of the sheet within the die – whether it is overcast on one side or fully encapsulated from all sides – has a significant influence on temperature distribution and, consequently, on joint formation.

Initial results show that load-bearing metallurgical transitions are fundamentally achievable. Metallographic analyses reveal zones in which casting and sheet merge without a visible interface. At the same time, it becomes clear that the joint does not yet form across the entire surface. Instead, localised contact areas develop, which still behave mechanically more like a “perforation” – with correspondingly limited strength.

Heat-treated sheets lose strength

Material selection is a key influencing factor. Sheets from the 5xxx series largely retain their properties during the process, whereas heat-treated 6xxx alloys lose significant strength due to thermal exposure. In tests, this led to failure occurring in the sheet rather than in the joint area itself – albeit at considerably reduced strength levels. “We are significantly altering the material properties through the process – and that has to be taken into account,” Pille explains.

The joint strengths achieved so far remain below those of conventional adhesive bonds. At the same time, the results indicate that the potential is greater than the measured values suggest: even now, the joint forms locally with high stability – the challenge is to extend these areas across the entire interface.

A technology with potential

Against this backdrop, fusionCASTING is not yet a finished process but rather a systemic approach. It shifts the focus away from process parameters alone towards the interplay between surface, temperature and alloy. This is precisely where its potential lies: if these parameters can be controlled, hybrid structures could emerge in the future that combine the advantages of casting and sheet in a single process.

Which open questions remain and what the next steps in the project will be are explained by Christoph Pille in the following interview. 

You attribute this, among other things, to temperature distribution. What role does process control play in this context?

Christoph Pille: A very significant one. In the case of single-sided casting, for example, the sheet is partially cooled by the die, which means we do not reach the required temperature everywhere. Where the sheet is further away from the die surface, bonding works better because more thermal energy is available. Closer to the die, heat dissipation is too high and the energy input is insufficient. With full encapsulation, we expect better results because the sheet is exposed to the melt from both sides, resulting in higher thermal input and a more homogeneous temperature field. This is one of our key hypotheses that we are currently testing experimentally.

Material selection also plays a crucial role. What have you learned in this regard?

Christoph Pille: This has been one of the most important findings. With 6xxx alloys, we observed that the issue is not the joint itself, but the sheet. Due to the temperatures in the casting process, these heat-treated materials lose their strength. This means that even if the joint is good, the system fails elsewhere. For applications where mechanical performance is critical, these alloys are therefore currently unsuitable without subsequent heat treatment – and that is precisely the process step one aims to avoid for sustainability reasons. Naturally hard 5xxx sheets behave much more robustly in this respect.

The achieved strengths are still below those of conventional adhesive joints. How do you assess this?

Christoph Pille: This needs to be viewed in context. If we relate our results to the actual bonded area, we are closer to the target values than it might initially appear. Our target for shear strength is in the range of around 10 to 30 MPa, as we are competing with conventional overlap joints in car body construction, where structural adhesives are typically used. These adhesives generally achieve shear strengths of around 20 MPa, and that is the range we are aiming for as well.

What are the next steps in the project?

Christoph Pille: We are currently working on three aspects in parallel. First, we want to further optimise surface pre-treatment, particularly with regard to how stable these conditions are over time. Second, we are investigating temperature management in more detail, especially for fully encapsulated configurations. Third, we aim to better understand the process as a whole: why does bonding work very well in some areas but not in others? From this, we will derive targeted improvements. Our goal is to stabilise the system to a point where we can reliably achieve a fully joint interface.

Thank you for the interview.