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.
