Steve Lin’s research while affiliated with Purdue University West Lafayette and other places

The processing of high temperture thermoplastic polymers as a hot melt adhesive is one means of joining thermoplastics to metals. The presetn study establishes the role of processing in thermoplastic joining of a titanium alloy (Ti-6A1-4V) using thin flim poly(etherketoneetherketoneketone) (PEKEKK). A design of expriments (DOE) was performed to determine the effects of the soak above melt temperature (Tm = 380°C), hold at the isothermal recrystallizztion temeprature (Tiso = ∼ 340°C) and cooling rates on the joint strength, Extended soak times are required to remove the residual spherulitic crystallinity in the polymeric adhesive. Whit the elimination of the residual crystallinity, high-strength transcrystalline regions are enhanced at the polymer-metal interface during the hold time at Tiso and during cooling to ambient temperature. Average tensile strengths of 137 MPa were achieved, exceeding the bulk polymer tensile strength of 118 MPa by 16%, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) show that extended soak times above Tm led to excessive polymer squeeze-out, resulting int he creation of strength-reducing shrink voids or unwetted regions. As a result, the failue mode of the joint, which was normally cohesive for high-strength joints, became a mix of cohesive and adhesive in parts with excessive polymer squeeze out.