Ultrafast laser microwelding of glass-to-glass and glass-to-opaque materials
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Techniques for joining materials, especially glass to dissimilar materials, while maintaining their surface and optical properties are essential for a wide range of industrial applications. Current techniques rely on adhesives or interlayers which can exhibit issues with creep, out-gassing or aging. Ultrafast laser welding based on nonlinear absorption in transparent material offers an attractive solution to this problem. Bringing two material surfaces into close (optical) contact and focusing the ultrafast laser onto the interface allows for localised melting and rapid resolidification, forming strong bond and welding the two surfaces together. The highly localised nature of this absorption means that welds can be created whilst avoiding significant heating of the surrounding material―important for joining materials with significantly different thermal expansion coefficients. Using a picosecond laser system (Trumpf TruMicro), a range of welds between similar material (borosilicate glass to borosilicate glass, fused silica to fused silica, borosilicate glass to fused silica) and highly dissimilar materials (sapphire to stainless steel, fused silica/borosilicate glass to silicon/aluminium/copper/stainless steel) have been demonstrated. Theoretical simulations were carried out to investigate the aberrations that occur to a laser beam focused inside material and to describe the behaviour of the generated plasma. With the guidance of theoretical work and developed experiment setup, a large range of parameters related to welding were investigated both in bulk material and welding for different materials and surface conditions. Shear strength tests on welds shows a maximum value could be obtained between parameters resulting in barely welded seams, for low power, and obvious cracking, for higher power. Optimised welding for borosilicate to borosilicate glass creates stronger bonds (108.8 N/mm2) than traditional joining methods (adhesive, typically 15~25 N/mm2). Parameter maps were made for different surface separation and surface conditions to determine a successful weld. In order to weld highly dissimilar materials, different welding patterns were designed to relax residual stress and eliminate cracks. Welding with galvo-scanner was also introduced as an alternative method for industrial applications which provides a high scan speed and flexible patterns. To increase welding strength and expand the parameter tolerance for a successful welding, focus vibration methods were proposed to reduce the residual stress. Finally, welding of example industrial parts was demonstrated for different application requirements.