Fe/polymer joining via Fe/TiB2 composite structures via in-situ laser-induced reaction of Fe-Ti-B system: Effect of powder composition

Achieving strong direct joining between steel and polymers through mechanical interlocking is crucial for developing multi-material structures, particularly in the automotive and aerospace industries. This study synthesized micro-scale structures on a pure Fe substrate (simulating interstitial-free (IF) steel) for mechanical interlocking with thermoplastic parts. Numerous submillimeter-scale Fe/TiB₂ composite particles were in-situ synthesized by laser scanning on the Fe-Ti-B powder mixture and well-bonded with the Fe substrate. The effects of powder composition (TiB₂ volume fraction) on the morphology, microstructure, and joint strength with PA6 were investigated. A TiB₂ volume fraction over 60 % was essential for the formation of the composite particles promoted by a TiB₂ skeletal structure. Higher TiB₂ volume fractions increased the area fraction of the composite particles and decreased the bonding ratio (adhesive) of the particles with the substrate due to poor adhesiveness at the edge of the laser-scanning line. A wider high-temperature region was generated at a higher TiB₂ volume fraction, suggesting that the reaction heat to form TiB₂ assisted the bonding of the particles with the substrate at the edge of the laser scanning line. The Fe/PA6 joint strength increased to approximately 30 MPa with increasing the TiB₂ volume fraction to 100 % and showed a linear correlation with the product of particle area fraction and bonding ratio. A higher TiB₂ volume fraction was preferable for enhancing the joint strength via the micro-scale structures synthesized by laser scanning on the Fe-Ti-B powder mixture. A combination of the micro-structuring process using a high fraction of TiB₂ with advanced joining technologies will contribute to manufacturing high-strength Fe/polymer hybrid parts.