Laser-assisted hot-melt bonding is a promising joining technique for tailored multi-material components such as thermoplastic-metal hybrids that meet the high demands of today’s industrial lightweight construction. In laser-assisted hot-melt bonding, the thermoplastic joining partner is melted by heat conduction at the interface between plastic and metal and acts as an adhesive that wets the metallic surface and enables bonding without the use of additional adhesives. However, for each material combination, the generation of high quality joints requires an adjustment of the joining time between 0.3 and 1.6 s and the maximum allowable processing temperature to prevent from decomposition of the thermoplastic material or metal corrosion. To enable this, an in-depth process understanding of the thermoplastic melting rate and the joining time of the metal surface during laser joining is required, which currently leads to restrictions in industrial applications.
For this purpose, we developed a method for process design and optimization which allows the laser-assisted generation of thermoplastic-metal joints with a small number of experiments. By using a thermal imaging camera and a laser source in the NIR region for heating of the thermoplastic material, the interaction between laser process parameters, process temperature and joining time could be revealed for a wide spectrum of metal and thermoplastic materials. Our test equipment enables the adaption of suitable laser processing parameters on material-specific and temperature-dependent properties for the generation of bubble-free hot-melt joints with strong adhesion. By means of metal surface pretreatment using short-pulse laser structuring, a significant improvement of the joint strength is possible. Moreover, laser-assisted adjustment on the surface properties enables replacing the metallic joining partner with fiber-reinforced thermoplastic or thermoset and expands the range of applicable materials for laser-assisted hot-melt bonding.
In comparison to reaction bonding, laser-assisted hot-melt bonding requires no additional adhesives and allows a significantly shorter processing time due to short heating and cooling times. In addition, for laser-structured metals, high joint strengths which exceed the cohesive strength of thermoplastic material can be achieved. Due to the local energy input, the process is well suited for the use of temperature-sensitive material combinations. Laser-based hot-melt bonding is a great alternative to reaction bonding if a production and load oriented design is used that takes the absence of an additional adhesive into account. Based on our longtime experimental research knowledge in laser-assisted hot-melt bonding and our equipment, manufacturer can profit not only from a reliable and efficient joining technique of thermoplastic-metal hybrids, but also from a fast determination of adequate laser joining parameters for a large variety of material combinations.
Metal surface pretreatment using short-pulse laser structuring (left) and laser-assisted hot-melt bonding of thermoplastic-metal hybrids (right).
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