Microstructure and mechanical properties of nanosecond laser-welded sapphire joints with a titanium interlayer

In this work, sapphire substrates were bonded together via a 355 nm ns laser with titanium interlayer. The microstructural evolution, phase constitution and bonding mechanism of the sapphire joint were systematically investigated. The bonding area of the sapphire joint is composed of two distinct zones: the laser directly irradiated zone (DIZ) and the heat affected zone (HAZ). The DIZ consists of a resolidified alumina layer and a reaction layer containing titanium oxide (TiO) and the ternary compound Al₃Ti₅O₂, while the HAZ is dominated by titanium interlayer bonded directly to sapphire substrates. Therefore, the bonding mechanism is attributed to two distinct processes: metallurgical reactions induced by the laser irradiation within DIZ and the direct interfacial bonding driven by the synergistic effect of heat conduction from DIZ and clamping pressure within HAZ. The effects of processing parameters on the microstructure and mechanical properties were studied in details. The modified regions of the joints fabricated with low scanning speed distribute symmetrically along the interface, in contrast to the expanded modified regions in upper substrate for joints prepared with high laser power. This is attributed to the energy accumulation from multiple pulses, which enables laser energy to diffuse to the lower sapphires. With the increase of the laser power or the decrease of scanning speed, the shear strength demonstrates an initial increase followed by a gradually decrease. The sapphire joint achieved its maximum shear strength of approximately 127.4 ± 18.5 MPa under laser power of 0.5W and scanning speed of 50 mm/s. Fracture behavior analysis reveals a transition from interfacial separation to sapphire substrate cracking with varying process parameters, closely related to phase composition of the joint. We expect this study provides a further understanding, and promotes the practical application of nanosecond laser technology in joining of transparent ceramic materials.