Structure And Defects of Glass Materials Welded by Ultrashort Pulse Laser

01 Introduction

In the field of modern manufacturing and material processing, welding technology has always played a crucial role. With the rapid advancement of science and technology, the requirements for welding quality, precision, and efficiency have been continuously increasing. Traditional welding methods have shown many limitations when facing special materials, small components, and high-precision welding demands, such as large heat-affected zones, severe welding deformation, and difficulty achieving fine welding.

Ultrashort pulse laser welding, as an emerging welding technology, has attracted extensive attention and research in recent years due to its unique advantages. The extremely short pulse duration allows laser energy to be highly concentrated on the material surface in an instant. Compared to traditional welding methods, ultrashort pulse laser welding offers significant advantages such as minimal heat-affected zones and high processing precision, providing a new solution to the challenges faced by traditional welding techniques.
Glass materials, known for their high chemical inertness and low absorption rate, make joining glass pieces complex. Ultrashort pulse laser welding of glass is a method that provides high-strength joints with hermetic sealing. This technique utilizes the nonlinear absorption properties of glass under ultrashort laser pulses, achieving extremely high peak power to generate plasma, which is emitted from the material through Coulomb explosion. The processing stops before heat can diffuse through lattice coupling, resulting in a very small heat-affected zone.
The temperature during the process reaches about 1000°C, but the heat-affected zone remains extremely small, only a few tens of microns. Figure 1 shows the ultrashort pulse laser welding of glass.

Figure 1. Ultrashort pulse laser welding of glass.

02 Structure of the Melt Zone During Welding

During welding, the cross-section of the crack-free melt zone exhibits a dual structure composed of an elliptical outer structure and a teardrop-shaped inner structure. The teardrop-shaped structure is caused by plasma; once the plasma reaches a critical density, the refraction and reflection of the incident laser disrupt the longitudinal symmetry of the laser, causing the laser intensity to diverge along the incident direction.
Thus, the plasma continues to grow longitudinally while absorbing the energy of the incident laser, creating an irradiation region characterized by an asymmetric structure, as shown in Figure 2.

Figure 2. Causes of teardrop-shaped structures.

Figure 3 shows the cross-sections at different repetition rates in FOTURAN glass, where the elliptical outer structure and the teardrop-shaped inner structure can be observed.

Figure 3. Weld cross-sections in FOTURAN glass at different laser repetition rates.

03 Causes of Defects During Welding

During welding, various causes can lead to defect formation. Cracks oriented along the laser beam path and at the bottom of the melt zone were observed. In borosilicate glass blocks (D236), cracks were clearly visible at repetition rates of 50 kHz and 100 kHz, with no elliptical outer or teardrop-shaped inner structures.
This is because at lower pulse repetition rates, the high aspect ratio plasma regions exert internal pressure on the brittle material, forming cracks.

Figure 4. Structures or defects formed in D236 glass at different repetition frequencies.

In addition to cracks formed along the axis of the laser beam, other types of cracks perpendicular to the welding direction were also observed. Tunnel-shaped and transverse cracks were found in all types of tested glasses except for fused silica.
The formation of these defects is attributed to the thermal expansion and contraction of the glass material and the corresponding thermal stress.

Figure 5. Top view of the weld of D263 glass, showing cracks perpendicular to the welding direction.

Additionally, in the welding of quartz glass, bubbles were found along the entire length of the melt zone. The capture of bubbles is due to the buoyancy force inside the plasma being insufficient to overcome the rapidly increasing viscosity of the molten glass caused by fast cooling, as shown in Figure 6(b).

Figure 6. Melt flow experiments on sample cross sections were performed in different glass materials. Defects are indicated by red arrows. (a) D236. (b) Quartz glass. (c) FOTURAN.

04 Conclusion

Through nonlinear absorption properties, plasma is formed on the glass surface by the laser, achieving high-strength, hermetic welding while controlling the heat-affected zone within tens of microns.
During the welding process, the structure of the melt zone presents a dual structure with an elliptical outer part and a teardrop-shaped inner part, the latter caused by the asymmetric energy absorption of plasma.
Welding defects such as longitudinal and transverse cracks and bubbles are mainly caused by plasma pressure, thermal stress of the material, and rapid cooling. Different glass materials and laser parameters significantly impact welding quality and defect types.
Overall, ultrashort pulse laser welding provides a new approach for the precision processing of glass materials, but further optimization of the process is needed to reduce defects and improve welding reliability.

**--Cite the article published by 高能束加工技术 on April 11, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."