ANALYSIS & OPINION

Laser structuring creates strong metal-polymer bonds

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Following a presentation at Lasys, Philipp Götze, at Fraunhofer IWS, outlines a new procedure of bonding metal to plastics using a laser

In the field of lightweight construction, load-bearing multi-material components that have the advantages of both metal and thermoplastic are becoming more significant. There is a need, therefore, for efficient process chains featuring adapted pre-treatment and joining processes capable of bonding dissimilar materials.

Post- or in-mould assembly processes, joining with screws or rivets, and especially adhesive bonding are technologies currently used for joining dissimilar materials such as metal and thermoplastics. However, these processes can have limited geometric flexibility, use additional materials and have comparably long joining times. New bonding solutions are therefore required for fast and automated processing.

Scientists at the Fraunhofer Institute for Material and Beam Technology (IWS) in Dresden have developed a new process for joining polymers to metals using lasers without adhesives.

The mechanics of non-adhesive bonding are quite simple: a pre-structured metal is pressed together with a thermoplastic polymer. At the same time, the metal is heated at the joint and the plastic is partially melted.

A particular challenge exists, however, when applying uniform heat to the metallic joining partner. Fraunhofer IWS uses laser heating to address this issue. Two-dimensional laser beam oscillation moves the beam quickly, so that the temperature field can be adjusted dynamically to compensate for specific heat dissipation conditions of the bonded parts.

Typically, however, there is not sufficient adhesion between low-polar thermoplastics and metal. For this reason, the metal joining partner has to be pre-treated. Depending on the material configuration, laser macro structuring can be used to enlarge the surface and realise a form-fit between plastics and metal.

Researchers at the Fraunhofer IWS have therefore developed a laser ablation processes that generates structure depths of 100µm or more at surface rates of up to 30cm2 per second. Remote or scanner optics focus a laser beam onto the metal in order to clean the surface from adhering oils or dirt at the boundary layer. The heated polymer can then fill the surface structures to form a strong bond between the polymer and metal. This eliminates the need to clean the surface with solvents or pickling baths.

Stiffening structure of a middle arm rest generated by thermal direct joining. The material is mild steel, organo-sheet GF-PA6. Image: Fraunhofer IWS Dresden

Both the uniformly heated joining and surface pre-treatment can be performed using continuous wave (CW) solid state lasers, such as fibre or disk lasers. The advantage of using kilowatt CW lasers for pre-treatment is that they achieve a much higher efficiency compared to pulsed laser systems. In addition, the higher amount of melt at the surface observed after CW processing will not reduce the joining performance.

The bonding process uses a large spot diameter and low intensities, while the ablation process requires very small spot diameters – less than 100µm at kilowatt powers – and very short interaction times between the material and beam.

Understanding the joining mechanism is the first step to making the process industrial. However, for designing multi-material part constructions, the basic joint behaviour at different load configurations has to be surveyed, and suitable material models developed.

As proof-of-concept, the Fraunhofer IWS researchers replaced a welded steel assembly with a multi-material component made of an organo-sheet and a metallic cover plate, enabling them to demonstrate the potential of the lightweight construction. This was achieved using both thermal direct joining and macroscopic form-fit connections in the web-slit design between the metal and organo-sheet. The concept study showed that thermal direct joining is indeed suitable for realising multi-material component designs, in particular thanks to its short processing times, robust control, and capability of being automated.

The latest development for Fraunhofer IWS’s non-adhesive joining process, first presented publicly this spring, is a metal-plastic joining gun called HeatPressCool-Integrative (HPCI) that generates robust connections within seconds. A ceramic ram is used to press both joining partners together, while a circular inductor heats the metal within one to three seconds. The thermoplastic joining partner melts and solidifies at the metal surface. This development enables point-shaped joining at flange widths of 15mm or more.

Simple to automate, the non-adhesive HPCI process is easy to implement in car body manufacturing. In addition to automotive, the technology also benefits other sectors in which metal-plastic material combinations are used, for example the aerospace industry or white and brown goods manufacturing. Currently, different material combinations are still being verified, for example joining high-tensile steel and carbon fibre reinforced polymers.

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