Kunimitsu Takahashi

Polarization effects of a high‐power CO2 laser beam on aluminum alloy weldability

Linearly polarized CO2 laser beams have been used to investigate the effects of polarization on aluminum alloy weldability. Bead‐on‐plate welding tests have been performed on A5083 (AlMg4.5Mn0.7) alloy plates with quasi‐TEM00‐mode beams whose polarization direction is parallel or perpendicular to the welding direction, referred to as the parallel or the perpendicular polarization beam, respectively. Comparison of the fusion zone shapes between the two cases shows that deeper penetration, and hence more efficient optical energy coupling to the work can be obtained with the parallel polarization beam. This effect can be interpreted as being due to the higher Fresnel absorption at the keyhole front wall for the parallel polarization beam than for the perpendicular polarization beam. It is also found that the polarization effect is much more evident in the case of He gas shielding than in the case of Ar gas shielding. The weaker polarization effects observed in the case of Ar may be due to more densely formed...

Direct measurement of reflectance from aluminum alloys during CO 2 laser welding

A simple method for measuring the reflected amount of a CO(2) laser beam from the surface of a welded aluminum alloy is described. This device can be used for cases in which an integrating sphere is impractical to use. A pyroelectric detector measures the amount of reflected laser radiation at prespecified locations that define a semisphere on top of the weld spot. The spatial distribution of a reflected laser beam is obtained. It is found that ~80% of the total incident power is reflected by the alloy until a welding keyhole is developed. However, once the keyhole is formed, the reflected amount is as low as 4-8%.

Laser ablation of silicon wafer with a water microdrop

We have developed a new laser ablation process using a water-microdrop with a diameter of 70u2002μm. The shapes of the debris were investigated at various delay times. A Q-switched Nd:yttrium-aluminium-garnet laser of 25u2002ns pulse width and 532u2002nm wavelength was used. The spattering of debris was remarkably reduced by using the water microdrop. The debris pattern was dependent on the shape of the microdrop on the wafer, which in turn was dependent on the surface condition of the wafer and the delay time. The use of a smaller microdrop would result in laser dicing with a small ridge at the rim of the ablation point and no debris. The flexural stress of 720u2002MPa was obtained near the blade dicing.

Comparison of aluminum alloy welding characteristics with 1 kW CO and CO2 lasers

Welding characteristics of various aluminum alloys (A1100, A2017, A5083, A6063, A7N01) are investigated with CW CO and CO2 lasers near 1 kW power level. The beam characteristics of both lasers at the welding point are similar, so that a comparison can be made of the effect of the wavelength difference. The welding characteristics are assessed by measuring the size of the weld bead. It is found that the threshold power between heat‐conduction and deep‐penetration type welds is lower for the CO laser. This effect is attributed to the higher Fresnel absorption on the material surface at shorter wavelength. Simple reflectivity experiments which measured the strength of the reflected laser beam from the material surface confirmed this. For both laser types, the threshold power also depends upon the thermal conductivity of the alloy. The operating power level is relatively low compared to most common welding conditions, so no difference between gas types is observed because of low‐temperature plasma. The lower threshold power needed for deep‐penetration welds with the CO laser may be exploited for high‐speed welding of thin‐sheet metals with high reflectivity and high thermal conductivity, such as aluminum or copper alloys.

Laser ablation with water micro-drop for dicing of silicon wafer

The purpose of the research reported here was to obtain laser dicing of silicon wafers without deposition of ablated particles. This was done by using micro-drops of water in laser ablation process. The micro-drops of water 70u2005µm in diameter used in these experiments were made by a micro-nozzle with a piezoelectric actuator. Micro-drops were continuously put on a silicon wafer moving at a constant speed while the wafer was irradiated with laser pulses (wavelength of 532u2005nm) reaching the wafer surface through the micro-drops. Using micro-drop reduced the deposition of ablated particles. The form of the ablated part strongly depends on the surface condition of the substrate and the delay time between the micro-drop and the laser pulse. This result indicates that the laser ablation using 180-pl micro-drops of water produces the same effect as underwater laser ablation. A flexural stress as much as 720u2005MPa was measured. This stress is close to that generated when a blade is used for dicing.The purpose of the research reported here was to obtain laser dicing of silicon wafers without deposition of ablated particles. This was done by using micro-drops of water in laser ablation process. The micro-drops of water 70u2005µm in diameter used in these experiments were made by a micro-nozzle with a piezoelectric actuator. Micro-drops were continuously put on a silicon wafer moving at a constant speed while the wafer was irradiated with laser pulses (wavelength of 532u2005nm) reaching the wafer surface through the micro-drops. Using micro-drop reduced the deposition of ablated particles. The form of the ablated part strongly depends on the surface condition of the substrate and the delay time between the micro-drop and the laser pulse. This result indicates that the laser ablation using 180-pl micro-drops of water produces the same effect as underwater laser ablation. A flexural stress as much as 720u2005MPa was measured. This stress is close to that generated when a blade is used for dicing.

High-power unstable resonator CO laser

The operating characteristics of a multikilowatt unstable resonator CO laser are described. A positive-branch confocal unstable resonator was applied to a large-volume, transverse, self-sustained dc glow discharge-excited cw CO laser with a closed-cycle, subsonic gas flow. Output power and near-field beam properties were measured for various geometrical output couplings. An output of 5.0 kW has been obtained for a coupling of 0.25 (magnification M=1.16) with an electrical conversion efficiency of 16.9%. It has been revealed that the output from the unstable resonator is comparably high to that from the stable resonator under the same gas and discharge conditions. The output beam patterns and the sensitivity of power to angular misalignment of the mirrors are also reported.

Cutting of steels by high-power CO laser beam

The cutting characteristics of a 5- μm CO laser have been measured for austenitic stainless steel and mild steel under O2 assist conditions at a laser power of up to 3 kW. The results have been compared with data for 10.6- μm CO2 laser. Although there remains an uncertain factor relating to different beam qualities of the two lasers used, it has been shown that maximum cutting speeds for the CO laser are considerably higher than those for the CO2 laser at the same laser power.The cutting characteristics of a 5- μm CO laser have been measured for austenitic stainless steel and mild steel under O2 assist conditions at a laser power of up to 3 kW. The results have been compared with data for 10.6- μm CO2 laser. Although there remains an uncertain factor relating to different beam qualities of the two lasers used, it has been shown that maximum cutting speeds for the CO laser are considerably higher than those for the CO2 laser at the same laser power.

Ultrafast laser processing and metrology

Precisely controlled blind via-holes were formed in multilayer substrates by an ultrafast laser. A LIBS detector integrated into the laser processing machine realizes the full potential of the ultrafast laser.

Evaluation of secondary contamination in laser cutting of surface-contaminated metals

We investigated the mixing of simulated surface contamination in the cut surfaces of laser-cut stainless steel. A 10 micrometers layer of molybdenum was coated by wire explosion spraying of 8mm-thick 304L stainless steel to serve as the simulated contaminated metal plate. The cutting was done with a CO2 laser using a nitrogen and oxygen gas mixture as cutting assist gas. The mixing of the molybdenum into the cut walls was evaluated by EPMA surface analysis and fluorescent x-ray analysis. The result revealed that the degree to which the molybdenum was mixed into the cut surfaces decreased as the concentration of oxygen in the assist gas increased. Also, the result of contaminant removal from the cut piece by chemical oxidation reduction decontamination processing was that the greater the oxidation, the less the molybdenum that remained after the processing. Thus, to reduce the mixing of simulated surface contaminants in the cut surfaces, a cutting assist gas that has a high concentration of oxygen should be used.

Application of slip-band visualization technique to tensile analysis of laser-welded aluminum alloy

Recently we have developed a new optical interferometric technique capable of visualizing slip band occurring in a deforming solid-state object. In this work we applied this technique to a tensile analysis of laser-welded aluminum plate samples, and successfully revealed stress concentration that shows strong relationships with the tensile strength and the fracture mechanism. We believe that this method is a new, convenient way to analyze the deformation characteristics of welded objects and evaluate the quality of welding. The analysis has been made for several types of aluminum alloys under various welding conditions, and has shown the following general results. When the penetration is deep, a slip band starts appearing at the fusion zone in an early stage of the elastic region of the strain-stress curve and stays there till the sample fractures at that point. When the penetration is shallow, a slip band appears only after the yield point and moves vigorously over the whole surface of the sample till a late stage of plastic deformation when the slip band stays at the fusion zone where the sample eventually fractures. When the penetration depth is medium, some intermediate situation of the above two extreme cases is observed.