Improvement of Metal-To-Metal Adhesive Joint Strength by Pretreatment of Substrates With a Yb-Fiber Pulsed Laser

Abstract

Adhesive bonding is nowadays successfully employed in a wide range of engineering application and ensures greater design flexibility, more efficient production and improved performance compared to the traditional joining technologies. The most important applications of adhesive bonding include microelectronics devices, civil infrastructures, aerospace industry, etc. The strength and reliability of adhesive joints strongly rely on the establishment of intermolecular forces at the adhesive/substrate interface. As a consequence, substrate surface preparation before bonding plays an important role. Surface treatments, such as mechanical grinding or sandblasting, have been widely employed in order to increase surface roughness and joint strength. A viable alternative to mechanical treatments is represented by chemical etching, which promotes the formation of a surface morphology enabling an improved mechanical interlocking. On the other hand, anodizing processes affect adhesion and durability by mean of an electrochemically formed porous oxide film that improves chemical interaction between the adhesive and the adherends. Recent works have focused on laser irradiation carried out by mean of excimer, solid state or fiber lasers, e.g. ytterbium (Yb) fiber laser. Experimental data have shown that a pulsed laser surface pretreatment can induce a beneficial action on the strength of adhesive joints. This is due to the contaminants removal and to the favorable changes in both the chemical composition and surface morphology. The present work of thesis describes an experimental study about the shear strength and the mode I fracture toughness of adhesive joints with laser ablated metal substrates. An ytterbium-doped pulsed fiber laser was employed to perform laser ablation on AA6082-T4 aluminum alloy and on AISI304 stainless steel. Morphological and chemical modifications induced by laser irradiation were evaluated by means of surface profilometry, scanning electron microscopy (SEM) and X-Ray photoelectron spectroscopy (XPS). In addition, surface wettability was analyzed by means of the sessile drop technique. In order to assess the capability of laser ablation to improve mechanical interlocking, cross-sectional areas of the samples taken across the interfacial region were probed under an optical microscope. Single lap shear tests (SLS) and thick adherend shear tests (TAST) were carried out on AA6082-T4 and AISI304 samples in order to assess shear strength while the mode I fracture toughness was determined using the Double Cantilever Beam (DCB) test on aluminum samples only. For comparison, control samples were prepared using classical surface degreasing and grit blasting. The obtained results indicated that laser ablation has a favorable effect on both the shear strength and fracture toughness of Al/epoxy joints. Indeed, a +20% increase was recorded for the shear strength, while a remarkable three-fold enhancement for the mode I fracture toughness was observed with respect to control samples. On the other hand AISI304 TAST samples showed a +77% increase of the average shear stress at failure with respect to simple degreasing.