Ultrashort pulsed laser has been expected to be a powerful and reliable tool for micro-welding of glass. Picosecond pulsed laser with high pulse repetition rate leads to melting of glass at the vicinity of focal region by the heat accumulation, in which the absorption point of laser energy moves periodically in beam axis. Thus, the control of focusing situation in beam axis is very important to investigate the mechanical strength of weld part. In the microwelding of borosilicate glass by picosecond pulsed laser of 1064 nm, characteristics of molten area creation and weld joint were discussed. Numerical aperture (N.A.) greatly affects characteristics of molten area formation, and superior focusing characteristics, such as N.A. 0.65 enable a long region of high power density in beam axis, which can create a large molten area without cracks even under high energy condition. An appropriately large molten area inside glass has high mechanical strength, when continuous and large molten areas were formed. In addition, high density and large size of molten area without crack led to increasing Young’s modulus, and uniform and high Young’s modulus of molten area results in higher breaking stress.
It has been known that wavelength, power density, interaction time and material properties have great influence on processing characteristics in laser material processing, in which materials with higher reflectivity classify into difficultto- weld materials. In electronic industry, aluminum alloy is widely used as structural components due to its high specific strength, and copper became an important material because of its excellent electrical conductivity. These materials have high reflectivity and high thermal conductivity, which results in instability of energy absorption and processing results. Therefore, welding defects might be noticed in the micro-joining of aluminum alloy and copper. In this paper, the smart laser micro-welding of difficult-to-weld materials such as aluminum alloy and copper were discussed. The combination of a pulsed Nd:YAG laser and a continuous diode laser could perform high-performance micro-welding of aluminum alloy. A pulsed Nd:YAG laser was absorbed effectively from the beginning of laser scanning by pre-heating Nd:YAG laser pulse with the superposition of continuous diode laser, and wide and deep weld bead could be obtained with better surface integrity. As for micro-welding of copper material, stable absorption state could be achieved using a pulsed green Nd:YAG laser, since its absorptivity showed almost constant values with change of power density. A longer pulse duration was effective to achieve not only high absorptivity but also low deviation of absorptivity. The pulse waveform with maximum peak at the early period and a long pulse duration led to stabilizing the penetration depth with less porosity.
Recently, since the size of component becomes smaller, then the welding of thin metal sheet has been required. Besides, the flexibility of process is important according to the accessibility especially for small components. Fraunhofer Institute for Laser Technology had developed the SHADOW® welding technology, in which the high speed joining with small distortion is possible using pulsed Nd:YAG laser. The possibility of high speed and high quality welding had been reported by using single-mode fiber laser. The combination of micro beam and high speed laser scanning has the advantages for thin metal sheet welding. Therefore, the characteristics of micro-welding for thin metal sheet were investigated by high speed laser scanning, in which the welding was carried out by high speed scanner system with single-mode fiber laser and pulsed Nd:YAG laser. The proper welding region was narrow by the laser beam with a large focus diameter of 160 μm without pulse control, while a small focus diameter of 22 μm can control the welding state widely. A small focus diameter can perform the excellent welding seam from the extreme beginning without pulse control. The penetration depth can be controlled by the energy density with a small focus diameter of 22 μm at the energy densities less than 1 J/mm2. Besides, the unique periodic structure appeared at the high velocity of beam scanning with a small focus diameter. Moreover, the overlap welding of 25 μm thickness sheet can be performed regardless of small gap distance between two sheets by the laser beam with a small focus diameter of 22 μm.
Plastic material can be also bent by laser forming in the same way as metal material, and larger bending angle can be obtained as compared to metal material. However, its deformatio nmechanism has not been yet clarified. Therefore, in this study, the machining characteristics and the deformation mechanism of plastic in YAG laser forming were made clear by experimental and numerical analysis. Small temperature gradient was obtained by thermal analysis in laser forming of high density polyethylene. The plastic material could be bent due to high viscoelasticity in spite of small temperature gradient. Specimen was first bent by the temperature gradient in thickness direction in a few seconds after the laser irradiation. Then, the temperature of specimen became almost the same in thickness direction, and the temperature gradient appeared only in side direction. Therefore, the deformation of specimen was occurred by the contraction of specimen in side direction and the effect of initial shape, which was deformed by the temperature gradient just after the laser irradiation. Besides, it was clarified that small thermal conductivity of plastic material lead to low cooling speed. Therefore, sufficient cooling time is necessary in order to control the deformation accurately, since the temperature of specimen should be kept equal before every laser irradiation.
ITO film, which is a kind of transparent conductive film, has been used for LCD, PDP and so on. This film is mostly removed by wet etching method. However, this method needs many chemicals, numbers of process and large-size equipments. On the other hand, laser beam processing can achieve the dry process without chemicals and drastically reduces the number of process. Therefore, selective removal of ITO film on glass substrate by LD pumped Q-switch SHG YAG laser is experimentally investigated. Electric insulation across machined groove was successful. Better groove shape can be obtained by accurate control of defocused distance and feed rate under a constant average power. Using SHG YAG laser makes it possible to remove only ITO film without any damage to glass material as substrate, since SHG YAG laser of 531 nm in wavelength is easy to transmit the glass material. When laser beam is irradiated from ITO film to glass material, a non-removed portion of ITO film remains at the bottom of groove under long defocused distance condition. On the other hand, backside irradiation method, in which laser beam is irradiated to ITO film through glass material, can prevent from remaining a non-removed portion, since absorption of laser beam occurs from the boundary part between ITO film and glass material.
YAG laser has widely been used for precision micro machining in many fields. However, adhesions of dross and spatter to the base material due to high energy beam machining lead to the deterioration of the surface integrity. It is important to understand the assist gas flow from the tip of a convergent nozzle in order to improve the surface integrity, since material is mostly removed away by the gas flow spouted in the same direction of the laser beam. In this paper, effects of the nozzle shape on assist gas flow and the machined results were experimentally analyzed using the Schlieren method. There exists an unstable region in which the pressure on the workpiece changes periodically, and the region becomes wider with the reduction of the exit diameter. The pressure on the workpiece increases with an increase of the exit diameter, which makes the dross height smaller. However, it is necessary to select the nozzle shape according to the demand of the surface integrity, because the consumption of the assist gas increases and the kerf width becomes wider with an increase of the exit diameter. Moreover, it was pointed out that a kind of nozzle with a convex curve on inner wall of nozzle led to better surface integrity.
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