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Application of laser welding technology on glass

With the rapid development of laser technology, laser is widely used in welding various materials including stainless steel and carbon steel. Glass is a transparent, fragile and brittle material, and traditional laser sources cannot be easily absorbed by it, and the heat-absorbing glass is not suitable for processing by traditional laser welding because of its large thermal expansion coefficient and easy cracking during welding.

Generally, there are two main methods for laser welding transparent materials such as glass and plastic. One is to coat opaque pigments at the welding interface or add an intermediate layer to increase the laser absorption rate. The material near the interface absorbs the laser and the temperature rises, and the material melts and then solidifies to realize the connection of the transparent material. Another method is to use a special welding light source for welding. A high-power density laser source causes non-linear absorption between transparent materials to form an effective solder joint. More and more scientific researchers and engineers are turning their attention to lasers with special light sources for welding of different processing applications.

1. Research status

In recent years, the use of special light sources has successively realized the welding of a variety of glass and monocrystalline silicon. American PolaOnyx company uses special laser single-line/multi-line scanning to realize glass welding and sealing. Hélie used a laser to micro-weld a 100μm-thick glass end cap to a micro-structured optical fiber, and successfully welded the end cap for standard optical fiber and micro-structured optical fiber. Tamaki used a laser with a wavelength of 1558nm in their research to successfully achieve welding between dissimilar glasses and between glass and silicon wafers, and achieved welding strengths of 9.87MPa and 3.74MPa respectively.

However, most scholars have studied the results of laser welding glass, and the welding fusion area is in the shape of a drop, which is mainly composed of three parts, which are a linear structure composed of a circular cavity at the top, a molten area at the middle, and a tiny cavity at the bottom. The top and bottom cavities are prone to stress concentration, poor parameter control, and cracks. In addition, because of its drop-like structure, poor line spacing control may lead to intermittent and unconnected welds.

2. Test materials and methods

The material used in the test is optical glass, and the sample size is 25×25×1mm. The welding test steps are as follows:

a) The glass surface is clean. Soak the glass sheet with alcohol solution for 5~10min, then rinse the glass surface 3~5 times with distilled water, and finally use a hot air blower to dry the water stains on the glass surface;

b) Press and weld glass sheets. Place the laminated glass sheet in the fixture positioning groove, and adjust the welding fixture mechanism to press down the area around the glass sheet to make the glass sheet tightly fit. Experimental studies by former scientific researchers have shown that the glass bonding gap for glass welding is required to be less than 100nm (there is also a view that it is less than a quarter of the laser wavelength).

c) Adjust the focus to the junction of the two pieces of glass. When the laser is transferred from the air into the glass, the refraction will occur, and the focus will be shifted. Therefore, the focus will be found by laminating the glass sheet, and the height of the vertical glass surface vibrating lens will be adjusted at equal intervals. The glass sheet will be scanned with the same energy box respectively. Observe the position of the interface where the glass absorbs the laser light under the glass sheet, which is the focal position.

d) Laser welding of glass. Repeat steps (b) and (c) to press and weld the glass sheet and adjust the focus to the glass interface, adjust the appropriate laser welding process parameters (power, speed, scanning graphics, etc.) to weld the glass, and the glass interface is high when the energy of the laser exceeds a certain threshold, it will induce the multiphoton ionization of the glass material. The ionized free electrons accelerate and collide with other atoms to cause avalanche ionization. The temperature of the material rises, so that the glass temperature reaches the melting point and melts, and it is cooled after the blackout time to realizes welding.

3 Special laser glass welding process

Special laser is used for glass welding test, the welding shape is uniform in the entire welding area (4×4mm), the material deformation after welding is small, the flatness of the material has not changed greatly, and the thickness of the welding fusion zone located at the interface between the two materials is smaller, no thermal damage can be observed on the glass on both sides of the fusion area.

The slice image of the end face of the glass after welding shows that when the new laser welding process is used for welding, the welding fusion area does not appear to be water drop-shaped, and there are no welding crack source defects such as a circular cavity at the top and a small cavity with a linear structure at the bottom, and no intermittent linear crack defect that is not welded and formed occurs in the melting area in the middle. The welding strength test was carried out, and the material was broken in the base metal area, the welding joint did not fall off, and the welding seam had good welding strength.

4 Summary and outlook

Using a special laser source and adjusting the appropriate welding process parameters, the glass material will absorb the laser nonlinearly, and the material will be solidified after melting, so that the two transparent glasses can form a strong welding zone without adding solder, and successfully realize direct welding of the optical glass. The two layers of materials in the optical glass welding and fusion area are integrated, there is no obvious macro or micro cracks, no drop shape, so there is no drop-shaped round cavity at the top and linear damage zone at the bottom, and there is no linear unfusion at the interface, which effectively avoids the occurrence of crack sources, and the solder joints remain on the surface of the sample after the strength test, which has a high connection strength.

In recent years, with the rapid development of 5G, wireless charging, optical communication and chip technology, glass materials have gradually become 3C electronic structural parts due to their outstanding advantages such as low electromagnetic signal shielding, high hardness, light weight, low cost and suitability for mass production. Glass laser welding processing will usher in broad application prospects.