Micro-welding Processes And Mechanisms Of Transparent Brittle Materials By Ultrafast Laser

In the state-of-the-art high-tech fields including optoelectronics and aerospace,brittle materials such as glass,silicon,and sapphire are widely used due to their unique and excellent properties in aspects of optics,electronics,acoustics,thermology and chemistry.Among all the applications they are most frequently employed in assembly and packaging which involve the bonding of various homogeneous or heterogeneous materials to provide reliable sealing without affecting device functionalities.Nowadays available brittle material bonding methods usually suffer poor stability,additional materials and requirements for severe surface quality and approximate expansion coefficients of the materials to be welded,therefore new joining techniques allowing for efficient and precise connections are urgently needed,and this dissertation focuses on the mechanism and technology of ultra-fast laser microwelding of brittle materials.As for the mechanism,the whole nonlinear absorption process of ultrafast laser in transparent brittle materials and the conversion of laser energy into thermal energy of the material is summarized,indicating that in micro-welding the picosecond laser is advantaged compared with the laser with shorter pulsewidth such as femtosecond laser owing to higher absorption rate and thermal effects.The micro-welding mechanism of ultra-fast lasers for transparent brittle materials is described,and it is pointed out that the gaps of the materials must be filled by reversible expansion and irreversible expansion of melting materials.Meanwhile the thermal stresses should be restricted in case that they accumulate and cause defects in the transverse and longitudinal directions of the welds.Technically,a picosecond laser scanning micro-welding method for transparent brittle materials is proposed.One of the characteristics is that scanning galvanometers can process welding areas in high speed,improving the welding efficiency.At the same time,due to the large Rayleigh length of the focused beam by the flat-field scanning focusing lens,the focus positioning accuracy and requirements for material surface roughness are relaxed,and feasible gap separation is promoted to 3?m,making this technology practical in industrial mass production.The unique interference and multiple scanning mechanisms different from existing ultrafast laser welding methods are explained,and the influence of various processing parameters on the welding quality is theoretically analyzed such as pulse repetition frequency,pulse energy,scanning times,focus height and overlap rate.The welding experiments of quartz glass and soda-lime glass with great differences in physical properties are performed considering the influence of various factors on the shear strength of the weld.Priorities of these factors and optimal combination of welding parameters that yields high searing strength are obtained through orthogonal experiments.Then a complete experiment concerning processing speed and pulse energy is performed to evaluate the sealing effect of welding seams,and a satisfactory energy-speed range is obtained.Except for tests on welding strength and sealing properties,the microscopic topography of weld seams are observed to confirm the quality of welding seams.A continuous laser is introduced to the existing picosecond processing system to further improve the existing welding methods.After verifying the feasibility of dual-beam welding with a 20?m gap in between,the preliminary analysis of the composite beam processing mechanism and the experimental verification of the shear strength and sealing performance of the composite beam welding seam are investigated.The results prove that the picosecond laser welding is improved to some extent.The shear strength of seams can reach 20 MPa or more while maintaining excellent sealing performance.