Mode I fracture toughness of co-bonded metal–composite joints with additively manufactured titanium: Influence of printing parameters

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. • Co-bonded joints with CFRP and Laser Powder Bed Fused (LPBF) titanium were tested. • The LPBF laser scan speed and the build angle were varied on two levels. • The mode I fracture toughness of the co-bonded joints was evaluated. • Increasing scan speed increases the surface roughness. • Quantitative assessment of the role of partially melted particles.