Manufacture of laser textured steel-Carbon/PA6 hybrids using laser assisted automated tape placment

Abstract

Metal-composite hybrids provide great potential to manufacture lightweight automotive and aerospace components and combine the most desirable characteristics, such as fatigue, impact, and overall strength, especially in fibre direction, of each material. Selectively with thermoplastic composites reinforced metal parts can be manufactured using a robot-guided laser-assisted tape placement process (ATP). However, manufacturing metal-composite hybrids is challenging, largely due to the one order of magnitude difference in power required to heat the metal substrate and the composite tape to the same processing temperature. Surface pre-treatments such as grit blasting combined with a film coating have been reported to increase the processability due to an increased absorptance, diffuse reflection of the laser radiation and providing mechanical interlocking for the polymer. With limited suitability of grit blasting for automotive and aerospace production lines owing to introduced contaminants, new surface pre-treatments and their feasibility to manufacture strong metal-composite hybrids need to be investigated. Femtosecond laser ablation is applied to the steel substrate before bonding to engineer surfaces with enhanced absorptance, broader scattering and superior mechanical interlocking. Efficiency optimisation of the laser ablation process has been performed to manufacture advanced surface structures. Tape placement trials for the manufacture of selectively reinforced metal-composite hybrids were executed, in a first step, with an additional PA6 film interlayer between the composite and the metal and in a second step without an interlayer, directly on the steel. The impact of distinct surface structures on the manufacturing process was analysed with thermal imaging and thermocouple measurements. Ray tracing simulation findings were in good agreement with preliminary experimental results and predicted the laser radiation distribution for various surface textures. It was shown that each surface structure required the determination of individual processing parameters for the laser assisted tape placement process. For high heat flux distributions induced by the reflected laser radiation from the substrate on the feed tape, the feed tape needed shielding with a reflector from overexposure to radiation and ultimately overheating and degradation of the polymer. For medium to low heat flux distributions, no reflector was needed. For laser textured surfaces, up to 25% less laser power was required to manufacture metal-composite hybrids compared to a grit blasted surface preparation. The bonding strengths of the metal-composite hybrids were determined with a novel application for the compression shear strength test and compared to the lap shear strength test. They yielded equivalent shear strength with less scatter for the compression shear test. Bond line characterisation showed good nesting of the fibres into the asperities of the grooves and wet out of the polymer before testing. The highest shear strength for directly bonded CF/PA6 to steel of 14.7 MPa was obtained with a 70 um deep top hat structured groove with a 300 um distance between adjacent grooves. The grit blasted reference sample exceeds the compression shear strength of the strongest laser textured sample by 57%. The specimens with laser textured surfaces showed mixed failure with adhesive and cohesive failure modes, whereas the grit blasted surface pre-treatment resulted in cohesive failure of the composite with fibre-tear failure over the whole bond line. So, the successful and efficient manufacture of strong metal-composite hybrids with standard ATP equipment is reported for the first time. Show more