Multilayer hollow glass is widely used in the doors and windows of buildings, vehicles, ships, and incubators. Fully tempered vacuum glass possesses all the characteristics of vacuum glass, and has better thermal insulation performance, higher impact strength, and better resistance to rapid cooling and healing. Currently, owing to the limitations of the tempering process and edge-sealing technology, tempered glass cannot be used for the large-scale industrial production of vacuum glass doors and windows. Direct femtosecond (fs) laser welding of glass can achieve high welding strength without generating cracks. This method typically requires a gap not exceeding a few micrometers between two pieces of glass. However, the extremely low surface flatness of tempered glass renders such close contact impossible if holding methods reported in references are used. Herein, we report a self-designed glass-welding vacuum fixture to achieve large-area close contact between two pieces of stacked tempered glass. Using this device, we successfully achieve the large-area welding of tempered glass by employing fs laser pulses. A metal spray-sealing process is developed to obtain vacuum sealing. This self-developed double-layer-tempered-vaccuum-glass production equipment based on fs laser welding is both simple and affordable, which is consistent with the goals of energy saving and low-carbon environmental protection.

Figure 1 shows automatic fs laser welding equipment used for fully tempered vacuum glass welding. A self-designed special welding fixture (Fig. 2) is fixed to a two-dimensional mobile platform. The fixture makes tempered glass contact tightly with each other by properly squeezing the glass plates through adjusting the vacuum pumping. The tempered glass measures 40.0 cm×40.0 cm×0.6 cm and the tempered glass used for the middle frame layer measures 40.0 cm×1.0 cm×0.1 cm (Fig. 3). Additionally, to avoid the deformation of the top and bottom glass layers in vacuum, an array of glass pillars (diameter of 1 mm) is placed between two tempered glass plates. Before welding, the tempered glass only requires simple wiping and cleaning. The welding process is implemented twice. The bottom glass and middle-frame-layer glass are welded first, followed by the middle glass and top glass. Experimental results show that the optimal focal position should result in a symmetric distribution of the plasma luminescence region across the interface between the two pieces of glass (Fig. 4). Successful large-area welding is achieved at a power of 10 W and a welding speed of 30 mm/s.

Figure 5 shows the importance of controlling the gap between two pieces of glass. No ablation regions or cracks are observed in the welding region when no gaps are present between the two pieces of glass. Welding fails when a gap is present because ablation occurs when the plasma plume can expand freely in the gap space. Cracks typically emerge in this case. It is essential that the plasma region must continuously grow only toward the laser. Furthermore, the experimental results show that the interrupted-scanning welding process can achieve successful large-area welding more easily compared with continuous-scanning welding. In the interrupted-scanning case, we randomly cut three small samples from one welded sample and measured their shear strengths. The average shear strength is approximately 10 MPa. Results of the push-pull test show that even in the presence of a few micro-sized ablation zones and micro-sized crack zones, the fracture surface is not at the original interface between the two pieces of glass (Fig. 6). We cannot guarantee 100% perfect contact of the tempered glass; however, our results of long-term stability testing show that a small number of micro-sized ablation regions and cracks are allowable. The optical band gap of the tempered glass is approximately 3.5 eV. The decrease in transmittance of the welded sample over the entire transparent range (375?800 nm) is less than 4.4% compared with that of the tempered glass. After welding, the samples maintain their good optical transparency (Fig. 7). The fully tempered vacuum glass is sealed with indium (Fig. 8), which improves the vacuum level and alleviates the technical requirements of fs laser welding automation production line.

Herein, we report a self-designed vacuum glass-welding fixture to solve the problem of large-area close contact between two pieces of tempered glass. We successfully develop production equipment for the fs laser welding of fully tempered vacuum glass. Using this equipment, we fabricate a 40 cm×40 cm sample. Low-melting-point metal coating equipment is designed and tested. The technical solution proposed in this study for welding tempered vacuum glass using a fs laser offers the advantages of reducing the equipment cost and simplifying the production process, thus rendering it suitable for large-scale industrial production. This technical solution is consistent with the global goals of energy conservation, emission reduction, and low-carbon economy. In fact, it is one of the best options for manufacturing low-energy green doors and windows for buildings, vehicles, ships, and incubators. Currently, automated fs laser welding equipment with dimensions of 1.5 m×3.0 m is being pilot tested to prepare for its industrial production.