The strength of adhesive joints is influenced by the surface of the adherends, which is often treated before bonding to prevent interfacial (adhesive) failure. Laser Powder Bed Fusion (LPBF) offers promising potential for bonding without time-consuming surface treatments, since LPBF parts have an inherently rough surface, which is usually associated with good adhesion strength. Here we study the effect of the printing parameters on the mode I fracture toughness of co-bonded joints between untreated LPBF Ti6Al4V and Carbon Fiber Reinforced Polymer (CFRP) substrates. A factorial Design of Experiment (DoE) was set varying the laser scan speed and the build angle of the Ti6Al4V substrates, which were co-bonded with a CFRP woven laminate to form Double Cantilever Beam (DCB) joints. The results showed that increasing the scan speed from 500 mm/s to 2000 mm/s led to higher titanium surface roughness (+125% on average). On the other hand, the mode I fracture toughness was mainly affected by the build angle: the joints with vertically printed (90° with respect to the build platform) titanium adherends exhibited, on average, a 200% increase in toughness compared to the samples with titanium printed at an angle. This behavior was due to the higher number of partially melted particles on the surface of the vertical joints. A particle counting method was introduced to quantify the partially fused particles and their correlation with the mode I fracture toughness was demonstrated. Moreover, to the authors’ knowledge, for the first time an original approach was proposed to assess their interlocking contribution to joint toughness.

Parts manufactured with Laser Powder Bed Fusion (LPBF) are drawing interest in the adhesive joints research because of their high surface roughness, which is usually associated with good adhesion. This work aims to assess the adhesion strength of the inherent surface morphology of LPBF manufactured titanium.

Double Cantilever Beam (DCB) tests were carried out to determine the mode I fracture toughness of joints comprising as-printed titanium (Ti6Al4V) adherends, namely titanium-titanium secondary bonded and titanium-Carbon Fibre Reinforced Polymer (CFRP) co-bonded joints. The effect of high-temperature oxidation on the fracture toughness was also evaluated by testing a batch of joints in which the titanium underwent a post-printing thermal treatment. The as-printed specimens were compared to the same type of joints but with sandblasted titanium adherends to evaluate the effect of this surface pre-treatment on the value of fracture toughness.

The results indicate that non-oxidised titanium joints with untreated adherends had an average of 11% higher fracture toughness than their sandblasted counterparts. On the other hand, sandblasting proved beneficial for oxidised joints, increasing the fracture toughness by 64% on average over the untreated samples.

Here we investigate the adhesive properties of Selective Laser Melted (SLM) titanium surfaces in metal-composite cobonded joints without any prior surface treatment, to explore the inherent surface roughness of SLM parts to potentially create strong adhesive bonds. Double Cantilever Beam (DCB) tests were carried out to assess the joints mode I fracture toughness involving untreated SLM titanium and woven Carbon Fibre Reinforced Polymer (CFRP) adherends, co-bonded with an epoxy film adhesive. The same type of DCB joints, but now with sandblasted SLM titanium adherends, were tested to compare the adhesion strength of the untreated SLM surface versus the sandblasted surface. The findings reveal that joints with untreated SLM titanium adherends exhibit similar toughness to those with sandblasted SLM titanium adherends, indicating that the surface morphology of as-printed SLM titanium is suitable for manufacturing robust adhesive joints. The elimination of the surface treatment in the manufacturing of adhesive joints with SLM adherends could increase the interest of many industrial fields towards the use of adhesive bonding over other joining techniques.