Isao Kawaguchi

Keyhole behavior in high-power laser welding

Dynamic keyhole behavior has been observed to elucidate the formation and suppression mechanism of the porosity in 20 kW CO2 laser welding with the depth of 20 mm. The results indicate that the bubble is formed by capillary instability of the cylindrical keyhole. The tip of the keyhole is broken up by instability during rapid decrease in the depth, so called spiking phenomenon. Spontaneous fluctuation in the keyhole depth and spontaneous keyhole perturbation during welding promotes the bubble formation. Pulse modulation of the laser power is effective in stabilizing the keyhole and hereby suppressing the porosity if the frequency coincides with the eigenfrequency of the molten pool oscillation. The suppression effect is enhanced if the waveform is controlled appropriately.

Development of high power CO2 laser welding process

Abstract A deep penetration CO2 laser welding process has been developed as a low heat input, high efficiency fabrication route for advanced structural steels. To improve fundamental understanding of the process and facilitate the elimination of welding defects, the laser–plasma interaction has been characterised using a power probe, infrared spectroscopy, and high speed photography of monochlomatic images. The extent of the laser induced plasma was found to increase on raising the laser beam focal point from below the surface to the surface position. Plasma absorptivity was strongly affected by plasma composition, and hence by vapour generated by inhomogeneous melting of the keyhole wall; this effect exerts an important influence on keyhole instability. Variation in the keyhole wall profile as a result of inhomogeneous melting has been found to lead to porosity through the generation of bubbles that fail to escape from the weld pool. It is shown that appropriate pulse modulation of the laser beam can improve keyhole stability, thereby greatly reducing porosity in the welded joint.

Suppression of welding defects in deep penetration CO2 laser welding

Characteristics of deep penetration welding have been investigated using 20 kW CO2 laser facility. Formation mechanism of the porosity has also been discussed. The porosity forms most significantly for upper focussing conditions, under which the low power density of the laser beam tends to incline the keyhole front wall at the root. The laser beam reflected at the inclined keyhole front superheats the molten metal, resulting in forming the porosity at the root. The porosity is not observed in N2 shield welding, although large N-plasma is formed during welding. The suppression might be caused by periodical change in the heat input due to periodical plasma formation and dissolution of the shielding gas in the molten pool. Pulse welding of low frequency is effective in suppressing the porosity due to promoting the periodical molten metal flow.Characteristics of deep penetration welding have been investigated using 20 kW CO2 laser facility. Formation mechanism of the porosity has also been discussed. The porosity forms most significantly for upper focussing conditions, under which the low power density of the laser beam tends to incline the keyhole front wall at the root. The laser beam reflected at the inclined keyhole front superheats the molten metal, resulting in forming the porosity at the root. The porosity is not observed in N2 shield welding, although large N-plasma is formed during welding. The suppression might be caused by periodical change in the heat input due to periodical plasma formation and dissolution of the shielding gas in the molten pool. Pulse welding of low frequency is effective in suppressing the porosity due to promoting the periodical molten metal flow.

Keyhole behavior in deep penetration CO2 laser welding

A slender keyhole formed by deep penetration laser welding fluctuates violently and is apt to form porosity. To reveal the keyhole depth behavior, the authors observed the keyhole images simultaneously by using a microfocus x-ray transmission imaging system and plasma plume images in deep penetration CO2 laser welding at various welding speeds. Plasma light emission was also measured by using photodiodes. The upper part of the keyhole was found to fluctuate largely and its motion corresponds to the large plasma plume generation. The keyhole depth fluctuation corresponding to the light emission coefficient of the plasma inside the keyhole was also observed.A slender keyhole formed by deep penetration laser welding fluctuates violently and is apt to form porosity. To reveal the keyhole depth behavior, the authors observed the keyhole images simultaneously by using a microfocus x-ray transmission imaging system and plasma plume images in deep penetration CO2 laser welding at various welding speeds. Plasma light emission was also measured by using photodiodes. The upper part of the keyhole was found to fluctuate largely and its motion corresponds to the large plasma plume generation. The keyhole depth fluctuation corresponding to the light emission coefficient of the plasma inside the keyhole was also observed.

Power modulation in deep penetration laser welding - Optimization of frequency and waveform to prevent the porosity

High power laser is a promising tool to weld heavy section plate members efficiently. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. In the present paper, laser power modulation is attempted to suppress the porosity in 20 kW CO2 laser welding with the depth of 20 mm. Dynamic keyhole behaviour is analysed using a micro-focused X-ray transmission imaging system to evaluate keyhole stability. Waveform control of power modulation is effective in stabilising the keyhole and leads to not only enhancing the porosity suppression effect but also expanding the optimum frequency range to prevent the porosity. Monitoring of plasma signal is also tried using a photo diode to determine the optimum frequency. The keyhole is stabilised at the eigenfrequency of the molten pool oscillation. The narrow optimum frequency can be successfully determined by plasma monitoring.High power laser is a promising tool to weld heavy section plate members efficiently. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. In the present paper, laser power modulation is attempted to suppress the porosity in 20 kW CO2 laser welding with the depth of 20 mm. Dynamic keyhole behaviour is analysed using a micro-focused X-ray transmission imaging system to evaluate keyhole stability. Waveform control of power modulation is effective in stabilising the keyhole and leads to not only enhancing the porosity suppression effect but also expanding the optimum frequency range to prevent the porosity. Monitoring of plasma signal is also tried using a photo diode to determine the optimum frequency. The keyhole is stabilised at the eigenfrequency of the molten pool oscillation. The narrow optimum frequency can be successfully determined by plasma monitoring.

Suppression of porosity using pulse modulation of laser power in 20 kW CO2 laser welding

Formation mechanism of the porosity has been investigated in the deep penetration laser welding with the depth of about 20 mm using a 20 kW CO2 laser facility. Dynamic keyhole behaviour has been observed using a micro-focused X-ray transmission imaging system developed by Matsunawa et al. The results indicate that the porosity is formed by instability of the capillary keyhole. The tip of the keyhole is broken up by instability during an abrupt decrease in the depth by so called spiking phenomenon. Laser power modulation with a square wave can effectively reduce the porosity formation by matching the frequency with that of the molten pool oscillation. On the other hand, a lot of large porosities are formed at the frequency of 100 Hz. This may be caused by the resonant effect of the keyhole oscillation. The suppression effect of the porosity can be enhanced by power modulation with the modified waveform.Formation mechanism of the porosity has been investigated in the deep penetration laser welding with the depth of about 20 mm using a 20 kW CO2 laser facility. Dynamic keyhole behaviour has been observed using a micro-focused X-ray transmission imaging system developed by Matsunawa et al. The results indicate that the porosity is formed by instability of the capillary keyhole. The tip of the keyhole is broken up by instability during an abrupt decrease in the depth by so called spiking phenomenon. Laser power modulation with a square wave can effectively reduce the porosity formation by matching the frequency with that of the molten pool oscillation. On the other hand, a lot of large porosities are formed at the frequency of 100 Hz. This may be caused by the resonant effect of the keyhole oscillation. The suppression effect of the porosity can be enhanced by power modulation with the modified waveform.

Keyhole behaviour in high power laser welding of thick steel plates - Formation mechanism and suppression of weld defects

Full penetration laser welding of thick steel plates has been carried out using a 20 kW CO2 laser facility. Dynamic keyhole behaviour has been observed using an x-ray transmission imaging system to elucidate the formation mechanism of the porosity and hot cracking for various welding conditions. Relatively wide optimum power range to prevent the porosity can be attained when the back surface is shielded by the inert gas. However, the convex shaped weld cross-section, which is susceptible to hot cracking, is formed due to significant perturbation of the keyhole caused by the back shielding gas flow. The keyhole is stabilised without using the back shielding. However, large porosities are likely to occur near the bottom surface, since the plasma formed just below the keyhole supplies monatomic nitrogen, which can easily dissolves into the molten pool. Supersaturated nitrogen forms bubbles in the molten pool during welding and causes the porosities. A small amount of aluminium addition in the base metal is effective to prevent this type of porosity due to denitrification effect of aluminium.Full penetration laser welding of thick steel plates has been carried out using a 20 kW CO2 laser facility. Dynamic keyhole behaviour has been observed using an x-ray transmission imaging system to elucidate the formation mechanism of the porosity and hot cracking for various welding conditions. Relatively wide optimum power range to prevent the porosity can be attained when the back surface is shielded by the inert gas. However, the convex shaped weld cross-section, which is susceptible to hot cracking, is formed due to significant perturbation of the keyhole caused by the back shielding gas flow. The keyhole is stabilised without using the back shielding. However, large porosities are likely to occur near the bottom surface, since the plasma formed just below the keyhole supplies monatomic nitrogen, which can easily dissolves into the molten pool. Supersaturated nitrogen forms bubbles in the molten pool during welding and causes the porosities. A small amount of aluminium addition in the base metal is e...

Monitoring of keyhole behavior in deep penetration laser welding

In high power laser welding, the slender keyhole is unstable and fluctuates. It is apt to form weld defects. Therefore, many types of monitoring system have been developed to judge the welding state. Especially, monitoring systems of plasma light emission is easy to use. However, the relationship between the plasma light emission and the keyhole behavior is not clear. Understanding of the relationship would be useful for monitoring and controlling the welding phenomena.We observed keyhole behavior by a micro-focused X-ray transmission imaging system and plasma light emission by photodiodes in deep penetration laser welding simultaneously. It was found that the plasma light emission corresponds to the keyhole behavior and large plume plasma is generated by the interaction between the laser light and the molten metal at the top of the keyhole.In high power laser welding, the slender keyhole is unstable and fluctuates. It is apt to form weld defects. Therefore, many types of monitoring system have been developed to judge the welding state. Especially, monitoring systems of plasma light emission is easy to use. However, the relationship between the plasma light emission and the keyhole behavior is not clear. Understanding of the relationship would be useful for monitoring and controlling the welding phenomena.We observed keyhole behavior by a micro-focused X-ray transmission imaging system and plasma light emission by photodiodes in deep penetration laser welding simultaneously. It was found that the plasma light emission corresponds to the keyhole behavior and large plume plasma is generated by the interaction between the laser light and the molten metal at the top of the keyhole.