Taming megacast aluminium: adhesive bonding strategies for HPDC body structures

Adhesive bonding of aluminium high-pressure die-cast (HPDC) AlSi10MnMg megacastings was used as a model system to explore how release-agent chemistry and downstream surface treatments together define process windows for structurally reliable joints. Rather than emphasising absolute strength values, the work focused on the mechanisms by which weak boundary layers, oxide conversion, and near-surface morphology governed the transition from adhesive to predominantly cohesive failure and the associated scatter in lap-shear strength. Plates cast with a siloxane-based release agent (S) or a wax-oil-based release agent (W) were subjected to industrially relevant routes (as-cast, solvent-cleaned, machined, T7 heat treated, Ti/Zr passivated), and their surfaces were interrogated by a combined toolbox of roughness measurements, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and laser-induced fluorescence (LIF) in conjunction with mechanical testing. In conceptual terms, S was shown to promote an inhomogeneous oxide-polysiloxan boundary layer that limited adhesion until it was chemically transformed into a mixed inorganic interphase by T7 heat treatment and Ti/Zr passivation, whereas W produced surfaces that were intrinsically bondable after simple cleaning, with chemical conversion acting mainly as stabilisation rather than as repair. The findings thus framed robust bonding on HPDC megacastings as a choice between two design strategies: compensating bond-inhibiting by targeted thermal and conversion treatments or selecting an adhesive-tolerant release agent that shifts the as-cast state directly into a cohesive, low-scatter regime and simplifies the admissible process window.