The wave-front distortion of the output beam of a single-mode laser diode (LD) with a collimator lens has been measured and corrected so as to provide a very precise focal spot. A 0.1 W LD at 940 nm wavelength was combined with a specially designed collimator lens, and the wave-front error was about 1λ. This wave-front error was corrected by the laser ablative shaping (LAS) method using ArF excimer laser. The final wave-front error of an output beam was better than λ/4. This output beam was focused on to the plastic plate to make a precise marking for production tracing.
Welding process of thin stainless foil was studied using a single mode Yb-fiber laser with a CW output of 40W at maximum. The spot size of approximately 10 μm in diameter at 1/e2 power density of the value at beam center was obtained by a focusing lens, the focal length of which was 55mm. Laser-plume interaction in welding process was discussed based on the effect of assist gas on the bead width, the back-reflected laser power WR and the laser power passing through the metal foil WP. It was found that defocusing by the refraction in the plume couldn't be neglected in laser welding with such a fine beam spot at especially low welding speeds. The use of assist gas effectively improved both the bead width and the shape of bead cross section.
YAG and CO2 laser weldability of Type 304 steel in nitrogen (N2) shielding gas was evaluated by investigating melting characteristics, porosity formation tendency, N content, microstructural characteristics and cracking sensitivity. Melting characteristics of weld beads produced below 4 kW were not so much different between YAG and CO2 laser. Porosity was remarkably reduced in any welds produced with nitrogen gas in comparison with normal welds made with Ar or He gas. This was attributed to the decrease in N content in a keyhole due to the reaction with evaporated Cr vapor as well as the absorption in the keyhole molten surface. The N contents absorbed in Type 304 weld fusion zones were larger under any welding conditions with CO2 laser than with YAG laser. On the other hand, in the case of several CO2 laser weld metals, solidification cracks occurred along the grain boundaries of a fully austenitic phase. Primary solidification of delta-ferrite phase normally took place in Type 304 weld metals, but a primary austenite phase was formed owing to the N enrichment, and micro-segregation of P and S increased along the grain boundaries. Consequently, cracking was induced by enhancement of cracking sensitivity due to a wider BTR. It was concluded that a great effect of nitrogen on the weldability of stainless steel was noted more remarkably in CO2 laser weld fusion zones than in YAG laser ones. It must be attributed to the N plasma formation leading to higher temperatures and consequent generation of more active N during CO2 laser welding.
An innovative laser-beam forming technology and the steel-based-powder material have been developed by combining a conventional laser sintering and a high-speed milling method, for the purpose of the fabrication of low-cost injection molds with short lead-time. Mold fabrication, which substantially affects product development cost and lead-time, has been the subject of intense experimentation, centering around different approaches, such as rapid prototyping by solidifying liquid resin using laser light. However, none of these methods have been applicable to production processes due to insufficient accuracy and service life. The newly developed laser-beam forming technology enabled the one-process quick fabrication of low-cost injection molds.
Crack-free microfabrication of sapphire with little debris deposition and little swelling around ablated regions by laser-induced plasma-assisted ablation (LIPAA) using a second harmonic of Q-switched Nd:YAG laser is described. LIPAA process has been originally developed by ourselves in which merely a single conventional pulsed laser is used. In this process, the wavelength of laser beam must be transparent to the substrate, so that the laser beam goes through the substrate first and is then absorbed by a metal target placed behind. For laser fluence above ablation threshold for the target and below damage threshold for the substrate, the laser-induced plasma is generated from the metal target and then the species fly towards the rear surface of the substrates with very high speed. Due to the interaction of the laser beam and the plasma, ablation takes place at the rear surface of the substrate. Dependence of ablation depth and width of ablated grooves on scanning speed and target-substrate distance are investigated. Based on the obtained results, LIPAA process is applied for scribing of sapphire substrates. Double scan of laser beam with a shift of the focal point shows a great potential of high-quality scribing of sapphire substrates.