In high-end equipment such as high-power laser systems, aerospace sensors, and precision medical devices, it is often necessary to reliably integrate transparent/glass components with metallic structures. However, the pronounced differences between transparent materials and metals in optical, thermal, and mechanical properties mean that conventional adhesive bonding or thermally assisted joining is frequently limited in strength, heat resistance, and environmental durability. In this regard, femtosecond laser can induce nonlinear absorption inside transparent materials, while metals exhibit linear absorption. In principle, this enables localized energy deposition and bonding at the contact interface between the two materials. However, in practice, if an ideal optical contact cannot be formed between the materials, the coupling of laser energy becomes unstable, leading to a pronounced deterioration in welding strength and consistency.
In a recent paper published in Light: Advanced Manufacturing , a research team led by Professor Ji’an Duan at the State Key Laboratory of Precision Manufacturing for Extreme Service Performance, Central South University, together with colleagues, proposed an engineering-oriented solution for rough metal surfaces. By employing a burst-mode femtosecond laser, they achieved stable welding between sapphire and Invar (Fe–36Ni) under non-optical-contact conditions. The study also provides the first systematic elucidation of the coupled energy-deposition behavior arising from linear absorption and nonlinear absorption at transparent–metal heterogeneous interfaces, offering a new mechanistic framework and practical processing guidance for high-performance dissimilar-material joining.
Most previous studies on ultrafast laser welding of transparent materials to metals have relied on highly polished metal surfaces to achieve “optical contact.” In contrast, this work directly targets real-world engineering conditions: even with an Invar surface roughness of Sa = 2.128 μm, the research team successfully produced sapphire–Invar joints with a maximum shear strength of 11.73 MPa. Achieving stable welding at this level of roughness represents a clear advantage over previously reported non-optical-contact ultrafast laser welding of transparent materials to metals, and highlights the engineering potential of transparent/metal dissimilar material joining.
Using high-speed imaging techniques, the study continuously monitored plasma emission during welding, providing process-level evidence of how linear absorption and nonlinear absorption temporally couple under a non-optical-contact interface, and how this coupling sustains the plasma and associated energy deposition.It also observed a dynamic transition during scan welding in which the interface gradually evolves from a “free space” condition to a “confined space” condition. In addition, joint characterization shows that failure occurs predominantly on the sapphire side as brittle fracture caused by cracks induced by welding-related thermal stress. This constitutes the primary bottleneck preventing further strength improvement and offers practical guidance for subsequent enhancement of joint performance.
Overall, this work achieved stable ultrafast laser joining of sapphire and Invar alloy under engineering-relevant rough-surface conditions, and, through high-speed in situ techniques, clarified the key mechanisms governing non-optical-contact welding of transparent materials to metals. It provides new mechanistic insights and methodological guidance for realizing high-strength, high-stability welding of dissimilar materials.
This research was strongly supported by Wuhan Huaray Precision Laser Co., Ltd. Guided by its vision of “changing lives with laser tools,” Huaray is committed to providing global users with stable, reliable high-end laser products and laser-based application solutions, serving advanced manufacturing and the life sciences. The company also hopes to offer more comprehensive equipment support for ultrafast laser welding of dissimilar materials.
Light: Advanced Manufacturing
Tailoring Sapphire–Invar Welds Using Burst Femtosecond Laser