By Isamu Miyamoto, Yasuhiro Okamoto, Kristian Cvecek, Michael Schmidt, Henry Helvajian
While glass is widely used in different industrial field due to its excellent physical and chemical properties, there exist no reliable joining procedures of glass at the moment. We have developed a novel fusion welding procedure of glass that can weld glass even with high coefficient of thermal expansion (CTE) using ultrashort laser pulse (USLP).
Recently, ultrashort laser pulse (USLP) has brought a new wave of laser welding that enables crack-free welding of dielectric material like glass without pre- and post-heating. The advantages of USLP welding of glass are provided by embedded molten pool due to the unique laser absorption mechanism of nonlinear process. It has been shown that the stress due to the thermal shrinkage of the weld bead can be in principle prevented in USLP welding of glass where molten pool is embedded in bulk glass. The embedded molten pool also provides advantage of local melting selectively only at the joint interface.
In this study, USLP with pulse duration of 10 ps is used for welding sample pairs of Foturan glass (Schott), which are widely used in 3D structuring ranging from medical to miniaturized satellites utilizing its photosensitive property. It should be noted that Foturan cannot be welded without cracks by CO2 laser, since the molten pool has free surface and hence the shrinkage stress cannot be avoided due to the large CTE (CTEFoturan=8.2×10-6/K). Foturan samples with 1 mm in thickness with optical contact are welded using a focusing optics having a numerical aperture of 0.55.
The paper consists of two topics for characterizing welding of Foturan dealing with nonlinear absorptivity and mechanical strength of the weld joint. The nonlinear absorptivity is evaluated by experimental measurement and the simulation model at different parameters including pulse energy, pulse repetition rate and welding speed. Excellent agreement is found between the experimental and the simulated values. The nonlinear absorptivity decreases as the pulse repetition rate and the welding speed increase.
The mechanical strength is evaluated by two procedures. One is shear strength of overlap weld joint, and the other is the bending strength of the internally melted sample (single piece) where the internally melted region is exposed to the surface by lapping and polishing for three-point-bending test. It is shown that the shear strength is as high as the bending strength of the internally melted sample. The joint strength as high as the base material is obtained at average absorbed laser power Wab lower than 1.2 W at welding speeds up to 100 mm/s within conditions tested. The joint strength tends to decreases when Wab exceeds 1.2 W. Higher welding speeds are possible without upper limit in principle using higher pulse repetition rates, showing that USLP is an excellent heat source for joining of glass even having high CTE.