Transparent glass-ceramics have attracted widespread attention as a composite material with excellent performance, and potential applications require the welding of the heat-tempered glass. Femtosecond laser, as a promising tool for processing in recent years, has been widely used in the field of processing due to its characteristics of high peak power, small heat-affected zone caused in the material, high processing accuracy, and wide range of applicable materials. However, a great deal of research work has been done based on the welding of untempered glass, and the welding of tempered glass is rarely reported. There are two ways to temper glasses, one is chemical tempering, and the other is thermal tempering. The thermally tempered glass can buffer part of the external stress and inhibit the melt from filling glass gap, thus increasing the difficulty of glass welding. To our knowledge, the non-optical contact welding of Li2O-Al2O3-SiO2 (LAS) transparent glass-ceramics with zero thermal expansion coefficient has been studied by femtosecond laser pulses for the first time. This study has potential significance for the further expansion of the applications of LAS transparent glass-ceramics.
A femtosecond laser beam with a wavelength of 1030 nm is used for experimental studies of glass welding to reduce the cracking tendency during the glass welding process. A lens with a focal length of 100 mm is used to focus the femtosecond laser beam, and the temperature gradient of the focused spot is smoother, which reduces the residual stresses induced during welding process. In order to achieve a strong heat accumulation effect of the femtosecond laser pulse inside the LAS transparent glass-ceramics, the repetition rate is kept at 500 kHz. And the spots between scanning lines are guaranteed to have certain overlap. The morphology and the shear strength of the welding region of LAS transparent glass-ceramics welded by femtosecond laser with different energies are studied, and the transmittance of the welded glass is measured. The energy window of LAS transparent glass-ceramic welding is obtained by simultaneously changing the energy of laser beam and the deviation between focus and interface. The physical phase analysis of LAS transparent glass-ceramics after welding is carried out using X-ray diffraction pattern.
Femtosecond laser beam focused by a long focal length scanning galvanometer system produced long filamentary longitudinal modification regions inside LAS transparent glass-ceramics (Fig. 2). It was caused by the self-focusing effect of the femtosecond laser beam propagating inside the LAS transparent glass-ceramics, which was balanced by the self-scattering effect formed by beam diffraction and plasma generation. With the increase in pulse energy from 2.0 μJ to 4.5 μJ, the weld width increased first and then decreased, and the maximum weld width reached 10.7 μm (Fig. 4). The transmittance decrease (Fig. 6) in the welding process is mainly caused by two mechanisms: the change of light transmission inside the glass caused by the change of refractive index at low pulse energy (Fig. 7) and the scattering loss caused by micro-nano pores at high pulse energy. The displacement of the modification region caused by the increase of pulse energy was compensated by the deviation of the focus position, which enlarged the energy welding window of LAS transparent glass-ceramics and obtained higher shear strength (Fig. 8). When the pulse energy was 3.5 μJ and the deviation between focus and interface was -500 μm, the shear strength of welded LAS transparent glass-ceramics reached (30.41±1.54)MPa. The diffraction peaks of the LAS transparent glass-ceramics before and after femtosecond laser irradiating were consistent, and the intensity of the diffraction peaks did not change significantly (Fig. 9), indicating that no new crystalline phase was generated in the irradiated area of the LAS transparent glass-ceramics.
Non-optical contact single scanning welding of LAS transparent glass-ceramics is achieved with femtosecond laser pulse for the first time. With the increasing of pulse energy, the optical transmittance of LAS transparent glass-ceramics decreases. The modification areas gradually move in the direction to the laser source as the pulse energy increases, and the widths of the weld seams at the interface increase to 10.7 μm and then decrease. When the laser pulse width is 300 fs, wavelength is 1030 nm, and single pulse energy is 3.0 μJ, the effective welding of LAS transparent glass-ceramics at the interface under the 100 mm/s high-speed scanning is achieved by using the heat accumulation effect at the high repetition rate of 500 kHz, and the shear strength after welding is as high as 23.51 MPa. The transmittance is 3% higher than that of two stacked original LAS transparent glass-ceramics. The energy window of LAS transparent glass-ceramic welding is enlarged and higher shear strength is obtained by controlling the laser beam energy and the deviation between focus and interface. The X-ray diffraction shows that the LAS transparent glass-ceramics welding is achieved without generating new crystal phase. The welding process is mainly establishing a strong connection at the glass interfaces after the melting of the SiO2 glass phase.
