Increasing strength and fracture toughness of AA7075-T6 adhesively-bonded joints with laser ablation

The effects of surface topography modified by laser ablation on adhesively bonded joint strength and toughness are quantified. Model joints consisting of AA7075-T6 substrates, a high strength aluminum alloy, and a commercial structural adhesive, were investigated with both tensile-shear and double cantilever beam (DCB) specimens. Surface topography was manipulated through varying single crater morphology and crater-to-crater spacing on the substrates with a pulsed Yb-fiber laser. Relationships between single crater morphology and energy related process parameters were qualitatively established. Three regimes of crater spacing, i.e. overlapping, tangential and separated, were identified and connected to key process parameters that control roughness amplitude and spacing. For this purpose, a new energy parameter that links laser pulse energy to resulting crater geometry and spacing was developed. It was found that both joint strength and toughness increase with Sa, the 3D arithmetic mean roughness height deviation, at low and mild (< 0.31 mJ) laser pulse energies, but decrease with higher energy input. The reasons for this behavior are considered in detail. Improvements of 5.9% in tensile-shear strength and 13.5% in the toughness of DCB samples demonstrate that adhesive joints with optimized topographies via laser ablation can be substantially stronger and tougher than joints with as-received substrate surfaces.