Yousuke Kawahito

High quality welding of stainless steel with 10 kW high power fibre laser

Abstract The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in bead on plate welding of type 304 austenitic stainless steel plates with a 10 kW fibre laser beam. The penetration depth reached 18 mm at the maximum at 5 mm s−1. At 50 mm s−1 or lower welding speeds, however, porosity was generated at any fibre laser spot diameter. On the other hand, at 100 mm s−1 or higher welding speeds, underfilling and humping weld beads were formed under the conventionally and tightly focused conditions respectively. The generation of spatters was influenced mainly by a strong shear force of a laser induced plume and was greatly reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was dependent upon several dynamic or static factors, such as melt volume above the surface, strong melt flow to the rear molten pool on the top surface, solidification rate and narrow molten pool width and corresponding high surface tension. Its suppression was effective by producing a wider weld bead width under the defocused laser beam conditions or reduction of melt volume out of keyhole inlet under the full penetration welding conditions. Concerning porosity, X-ray transmission in situ observation images demonstrated that pores were formed not only from the tip of the keyhole but also at the middle part because of high power density. The keyhole behaviour was stabilised using a nitrogen shielding gas, resulting in porosity prevention. Consequently, to produce high quality welds in 10 kW high power fibre laser welding, the reduction procedures of welding defects were required on the basis of understanding their formation mechanism, and 10 kW fibre laser power could produce sound deeply penetrated welds of 18 mm depth in a nitrogen shielding gas.

Elucidation of high-power fibre laser welding phenomena of stainless steel and effect of factors on weld geometry

The fibre laser has been receiving great attention due to its advantages of high efficiency, high power and high beam quality, and is expected to be one of the most desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, in bead-on-plate welding of Type 304 stainless steel plates with 6 kW fibre laser, the effects of laser power, power density and welding speed on the formation of sound welds were investigated with four laser beams of 130, 200, 360 and 560 µm in spot diameter, and their welding phenomena were clarified with high-speed video cameras and an x-ray transmission real-time imaging system. The weld beads showed a keyhole type of penetration at any diameter, and the maximum penetration of 11 mm in depth was obtained at 130 µm spot diameter and 0.6 m min−1 welding speed. It was found that the laser power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, and sound partially penetrated welds without welding defects such as porosity, underfilling or humping could be produced at wide process windows of welding speeds between 4.5 and 10 m min−1 with fibre laser beams of 360 µm or 560 µm in spot diameter. The high-speed video observation pictures and the x-ray images of the welding phenomena at 6 m min−1 welding speed and 360 µm spot diameter show that a sound weld bead was formed owing to a long molten pool suppressing and accommodating spattering and a stable keyhole generating no bubbles from the tip, respectively.

Characterisation of plasma induced during high power fibre laser welding of stainless steel

Abstract The objective of this research is to obtain a fundamental knowledge of generation behaviour and ionised state of a plume or plasma induced during bead on plate welding of a 20 mm thick type 304 stainless steel plate with a 10 kW fibre laser beam of 0˙9 MW mm–2 power density, on the basis of 10 000 to 40 000 flames s–1 high speed video observation and spectroscopic analysis. The high power fibre laser produced a partial penetration weld of 12 mm in depth at 50 mm s–1 welding speed. According to the high speed observation pictures, the laser induced plume was repeatedly generated from a keyhole at the interval of about 0&dot5 ms period to reach 12 mm in maximum height. The spectroscopy indicated the line spectra of neutral atoms of alloying elements of type 304 such as iron (Fe), chromium (Cr) and manganese (Mn). However, ionised spectra of alloying elements and line spectra of argon (Ar) neutral atom were not apparently detected under these welding conditions. Furthermore, the temperature and the ionisation degree of the laser induced plume were calculated to be approximately 6000 K and 0&dot02 respectively, by the Bolzman plots and Sahas equation. Consequently, the plume induced with the 10 kW fibre laser beam of the ultra high power density was judged to be weakly ionised plasma from these experimental results.

Laser direct joining of carbon fibre reinforced plastic to stainless steel

Abstract This study was performed to evaluate the characteristics and strengths of dissimilar joints between carbon fibre reinforced plastics (CFRP) and stainless steel using high brightness continuous wave disc laser. Tensile shear tests demonstrated that strong CFRP steel joint with ∼4800 N could be produced. The joints were tightly bonded on atomic or molecular sized level through Cr–Fe oxide film on the metal surface, and small fragments of type 304 steel were detected on the fractured CFRP. Many bubbles of submillimetre size were formed all over the plastic melted zone in the CFRP. The gas compositions inside the bubbles were H2 and hydrocarbon series as pyrolysis gases of the CFRP as well as N2 in air. These results suggest strong chemical and physical bonding of the CFRP on the oxide film of the type 304 plate. Consequently, it was confirmed that a strong joint could be produced between CFRP and type 304 stainless steel with a laser beam.

Laser direct joining of carbon fiber reinforced plastic to aluminum alloy

Laser direct joining of carbon fiber reinforced plastic (CFRP) to aluminum alloy was performed using a high quality continuous wave diode laser with a line-shaped beam. The tensile shear test results demonstrated that a strong lap joint with about 3000 N could be produced between CFRP (of 3 mm thickness and 20 mm width) and aluminum alloy (of 2 mm thickness and 30 mm width). The joints fractured along the joint interface or in the melted zone of CFRP. It was observed from the cross sections of the joints that many bubbles of submillimeter size were irregularly formed in the wide and shallow melted zone of the CFRP. Moreover, melted polyamide plastic was tightly bonded on atomic or molecular sized level through nanometer thick aluminum oxide film (κ-Al2O3) existing on aluminum alloy surface, suggesting high possibility of chemical or physical bonding between CFRP and metal. Consequently, it was confirmed that a strong lap joint of CFRP to aluminum alloy could be produced by direct irradiation of a laser beam.

Elucidation of phenomena in high-power fiber laser welding and development of prevention procedures of welding defects

Fiber lasers have been receiving considerable attention because of their advantages of high power, high beam quality and high efficiency, and are expected as one of the desirable heat sources for high-speed and deep-penetration welding. In our researches, therefore, the effects of laser powers and their densities on the weld penetration and the formation of sound welds were investigated in welding of Type 304 austenitic stainless steel, A5052 aluminum alloy or high strength steel plates with four laser beams of about 0.12 to 1 mm in focused spot diameter, and their welding phenomena were observed with high-speed video cameras and X-ray transmission real-time imaging system. It was found that the laser power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, but on the other hand at low welding speeds deeper-penetration welds could be produced at higher power. Laser-induced plume behavior and its effect on weld penetration, and the mechanisms of spattering, underfilling, porosity and humping were elucidated, sound welds without welding defects could be produced under the improved welding conditions. In addition, importance of the development of focusing optics and the removal of a plume during remote welding will be emphasized in terms of the stable production of constant deep-penetration welds and the reduction in welding defects in high power laser welding.

Fibre laser welding of aluminium alloy

The objectives of this research are to investigate the effects of various welding conditions on penetration and defect formation, to clarify their welding phenomena and to develop the procedure of reduction of the defect. Fibre laser bead-on-plate welding was performed on several aluminium alloys, in particular A5083, at the power of 6 or 10 kW and several power densities from 0.4 kW/mm2. It was found that the weld beads were narrower and deeper with an increase in the laser power density. For example, fully penetrated weld beads in 10 mm thick plates were produced at the laser power density of 640 kW/mm2 and the welding speed of 10 m/min. However, convex–concave bead surfaces were formed. Moreover, in the case of the high power density, no porosity and many pores were present at high and low welding speeds, respectively. On the other hand, in the case of the ultra-high power density, few pores were generated in high speed welding. These reasons were interpreted by observing keyhole behaviour, bubble formation and the molten pool geometry during high power fibre laser welding with a high-speed video camera and microfocused X-ray transmission in situ observation method. Moreover, the porosity in the weld bead was reduced and prevented by the utilization of nitrogen gas instead of Ar gas, or the forward inclination angle of 40° (50° from the right angle) in Ar shielding gas.

Visualization of refraction and attenuation of near-infrared laser beam due to laser-induced plume

This study was performed to obtain a fundamental knowledge of the interaction between a near-infrared laser beam and an induced plume during welding. The plume was characterized by spectroscopy, and the effect and mechanism of the laser-induced plume on the refraction and/or attenuation were investigated by high-speed video observation of the plume and the probe laser, and power meter measurement of the fiber probe laser beam of 1090 nm wavelength which passed horizontally through the plume formed during bead-on-plate welding of an 8-mm-thick type 304 plate with a 1.5 kW yttrium-aluminum-garnet (YAG) laser beam. The plume induced by YAG laser at the focus position grew about 20 mm toward the incident laser beam and was identified to be nonionized metallic vapor of 3600 K in average temperature on the basis of the spectroscopic analyses. The high-speed observation images and the power measuring results revealed that the rapid movement and the low brightness of the probe laser beam, seen after the plume, we...

In-process monitoring and feedback control during laser microspot lap welding of copper sheets

The establishment of laser welding of copper sheets, as an electronics manufacturing process is anticipated. This study was performed with an objective of stably and consistently producing fully penetrated lap welds in two copper sheets of 0.1 mm thickness. A new procedure for in-process monitoring and feedback control was developed for microspot lap welding with combined beams of a pulsed fundamental yttrium–aluminum–garnet (YAG) laser and an AO Q-switched second harmonic YAG laser. It was found that reflected laser power and heat radiated from the weld area were effective as process monitoring signals. A database of these values corresponding to the sizes of the bottom fusion zones of laser spot welds was prepared in terms of joint strength for feedback control utilizing a neural network model. A pulse width during the irradiation of a fundamental YAG laser was controlled at 0.15 ms intervals according to the neural network prediction based upon the monitoring signals. Upon investigation of 200 samples,...

Effect of weakly ionised plasma on penetration of stainless steel weld produced with ultra high power density fibre laser

Abstract A weakly ionised plasma can be generated in stainless steel welding with a 10 kW fibre laser beam at the ultra high power density of ∼1 MW mm–2 in Ar shielding gas. The objectives of this study are to obtain a fundamental knowledge of optical interaction between a fibre laser beam and the weakly ionised plasma, and to evaluate effects of the plasma on weld penetration. The optical interaction was investigated by the high speed video observation or the power meter measurement of another probe fibre laser beam, which passed horizontally through the weakly ionised plasma induced during bead on plate welding of a 20 mm thick type 304 plate with a 10 kW fibre laser beam of 0˙9 MW mm–2 in power density. The probe laser observed was refracted at 0˙6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fibre laser beam. The attenuation of the probe laser was measured to be ∼4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering. Moreover, a stable laser welding process could be produced at such ultra high power density that 11˙5 mm deep penetration was obtained even if the laser peak power was modulated 1 ms periodically from 10 to 8˙5 kW. It was consequently considered that the optical interaction between the 10 kW fibre laser beam and the weakly ionised plasma was too small to exert the reduction in weld penetration.