Structural Parts

  Structural Parts Cluster of Excellence Integrative Production Technology for High-Wage Countries

Lightweight construction is a crucial challenge for today’s industrial production. It is considered to be one of the key technologies of the 21st century to counteract the advancing climate change and preserve natural resources. The concept of lightweight construction does not only involve the use of lightweight materials, it also includes the purposeful combination of different materials. The aim of these composites is to combine the specific advantages of different materials in order to achieve a functional and also lightweight design. Considering these aspects, the used joining technologies have a massive impact on target-oriented lightweight constructions. Two possible approaches for a purposeful lightweight-design are the integration between metal alloys and techno-polymers, and the combination of steel- and aluminiumalloys. The aim of using a material mix is to realize certain functions within a part or component with the most suitable material. In case of the metal-polymer-materials, the metal could be used to withstand mechanical stress, while the polymer-components, which distinguish oneself through their excellent formability and their high freedom of design, could take over the part of functional integration. In the case of steel-aluminium-hybrids, steel parts could be applied to the highly stressed areas of a product, while lesser strained areas would be manufactured from the much lighter aluminium. Both options offer new ways of target-orientated lightweight construction.

 

Practical Issues

Multi-component high pressure die-casting Cluster of Excellence Integrative Production Technology for High-Wage Countries

Polymer-metal-composite-materials have been used on an industrial basis for years. The manufacturing techniques used for those material combinations are usually divided into In-Mold and Post-Mould-Assembly techniques. The In-Mold-technique joining operation and the polymer moulding process are combined. This is done by placing the metallic component in the injection mold, where it is joined with the injected polymer. The main challenges with these processes are the adhesion between metal and polymer as well as the limited freedom of design. The Post-Mold-Assembly techniques are applied either if the desired component geometry cannot be produced by the In-Mold technique or its usage is unfeasible due to economic reasons. The most common Post-Mold-Assembly techniques are rivet- and gluing-methods. Even though the difference in the physical properties of steel and aluminium forbid a combined fusion-welding of these two metals, this material combination opens up new opportunities for lightweight-construction. In this context, joining-technologies are a key factor for success. Another challenge is the ongoing demand for shorter development-times. In most cases, the only way to comply with this demand is the use of numerical simulation processes. The issue is that there is still a lack of suitable methods to calculate compound-strengths and material transitions, especially in the multi-material-domain.

 

Approach

Friction stir welding Image: Thilo Vogel

One approach to expand the In-Mould-Assembly‘s range of application is the combination of metal die-casting and plastics injection molding. During the first step, the metallic component is injected into the die. The second step consists of the polymer being injected into a cavity which has been closed with a slide in the run-up before. The result is the formation of a composite inside one tool. Thermal joining is a promising approach for the subsequent joining of polymers and metals. It utilises the adhesion of a polymer-melt with solid metal to create a high-strength connection. Therefore, the metallic component is first heated above the polymer‘s melting temperature. After that the materials are joined under a continuous pressure. The advantages are a short cycle time and a high initial-strength without any additional weight caused by mechanical elements. The heat required can be provided by a heating plunger or conventional resistance-welding equipment, for example.

Friction stir welding is a common procedure to join aluminium. Its advantages lie in the fact, that the materials are
joined without reaching their melting-temperatures, so that the metals are stirred in a malleable condition. This principle can also be transferred to a steel-aluminiumcompound.

 

Technical Challenges

Multi-Material Cluster of Excellence Integrative Production Technology for High-Wage Countries

Due to the technical complexity of the dies used for the combined pressure-injection-casting-process, they need to be very precisely machined. To maintain a good bonding performance between the metal and the polymer, an exact control of the die temperature during the process is necessary. An accurate thermal processing is the key to a good
metal-polymer-adhesion quality. A homogenous temperature distribution should be achieved before starting the joining process. Unfortunately, a homogenous heating of complex work pieces cannot always be ensured, thus the process control needs constant adaption. Another challenge is the correct design of the joints. If the mounting
areas are too small or not well placed by constructive means, an additional surface-treatment needs to be applied to increase the composite-strength. Examples for such a treatment are CMT-Pin-welding, Electron-Beam-Structuring or Laser-structuring, used to generate undercuts. The joining-mechanism during friction stir welding is a result of temperature input and material-deformation. The task is to create a composite which can still be formed into a complex structure after being joined. Furthermore, friction stir welding tools need to be wear-resistant.