Many applications in mechanical engineering today place high and very specific demands on the materials used. To meet these demands, their properties must increasingly be specifically adapted to the respective application. If the property profile of a single material is no longer sufficient for this purpose, composite materials can be used. They combine the advantageous properties of different materials, thereby opening up new fields of application and enabling a more efficient use of resources.
In the case of sheet metal materials, cladding processes are frequently employed. In this process, one or more additional layers of a different metallic material are applied to a sheet metal substrate. One industrially established process is roll cladding, in which the layered sheet materials are bonded by one or more rolling passes. The process can be carried out either above or below the hot forming temperature of the materials, which is why a distinction is made between hot and cold roll cladding.
Cold rolling cladding in an oxygen-free atmosphere
Cold rolling cladding offers significant advantages in this context. The low process temperatures prevent the formation of brittle intermetallic phases. These phases weaken not only the structural but also the functional properties of the composites, such as thermal and electrical conductivity, which significantly complicates their use in current- and heat-conducting components. However, as the composite is produced during cold roll cladding solely through the mechanical contact of the joining partners without thermal assistance, the requirements for contact surface preparation and process control are particularly demanding.
Researchers at the IW – Institute of Materials Science at Leibniz University Hannover are now investigating whether the process can be improved if it is carried out in an oxygen-free atmosphere.
The aim of sub-project A05 within the SFB 1368 ‘Oxygen-Free Production’ is to develop a process control system under conditions resembling an extreme high vacuum (XHV-equivalent). To this end, the process is carried out on a laboratory scale inside a glove box. By introducing the reactive gas monosilane (SiH₄), the argon protective gas atmosphere inside the chamber is purged of any residual oxygen until the residual oxygen content matches that of interstellar space. This prevents oxidation of the metal joining partners – a problem that particularly affects highly reactive metals such as aluminium or titanium.
Research objective: Improving properties and understanding relationships
In the conventional process carried out in an oxygen-containing atmosphere, re-oxidation takes place immediately after pre-treatment, resulting in a native, thin oxide layer during rolling. The oxide layer prevents bonding and is broken up during rolling; however, this requires significant deformation and consequently high rolling force. Only the oxide-free surfaces thus exposed subsequently contribute to bonding. Under conditions suitable for XHV, reoxidation of the contact surfaces of the materials can be prevented, and only minimal deformation – and consequently minimal rolling force – is required for the purely metallic surfaces to come into contact. The reduced rolling force contributes to a less energy-intensive production of the composites. By processing in an XHV-compatible atmosphere, researchers at the IW have already succeeded in significantly increasing the composite strengths compared to conventionally joined composites.
A number of factors influence the resulting properties of oxygen-free roll-bonded composites. These include, in addition to the degree of plastic deformation, the topography of the joint contact surfaces, the type of surface treatment and the flow properties of the materials involved.
Research at the IW should therefore aim not only to improve the macroscopic properties of the composites, but also to develop a comprehensive understanding of the relationships between the process parameters, the surface treatment and the properties of the manufactured composites. A focus of this research is on changes in the microstructure near the composite zone, as existing models have so far provided few approaches to explain the actual mechanisms of composite formation. The long-term goal is to specifically control the properties of the composites through a detailed understanding of the correlations between process parameters and the resulting microstructural properties, thereby achieving independence from the materials to be combined.
Looking ahead: From the laboratory scale to industrial relevance
For the second half of the current funding period, the IW sub-project team is investigating the scalability of the XHV process from laboratory scale to industrially relevant scales. The aim is to pre-treat and pre-roll composites within the glove box as an initial process step. The bond achieved in this way should be sufficient to seal the joint zone against further oxygen ingress. Subsequently, the pre-rolled composites are to be transferred from the box to a rolling mill of industrial dimensions and finished rolled. This strategy allows the plant and handling costs for industrial applications to be minimised without the need to extend the XHV conditions to an entire production line.
In the upcoming third funding period, the concept will be extended to cover a wider range of materials. As part of this, preliminary investigations are already underway into the roll cladding of materials with vastly different mechanical properties – a challenge that affects both the XHV process and the conventional process. Here, too, the aim is to capitalise on positive synergies with oxygen-free process control. In parallel, the mechanisms of bond formation in connection with the microstructural developments in the joint zone are being investigated. The research is characterised in particular by interdisciplinary collaboration within the Collaborative Research Centre, which is supported by researchers from Leibniz University Hannover, Clausthal University of Technology, the Laser Zentrum Hannover e.V. and the University of Paderborn.
