Goro Arakane

Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding

Abstract The effect of welding parameters on the distribution of wire feeding elements has been investigated during CO2 laser and pulsed gas metal arc hybrid welding process. The molten metal flow on the pool surface and inside of the samples was observed by a high speed video camera and an in situ X-ray transmission imaging system respectively. The results indicate that the fluid flow towards the inside of keyhole, namely inward flow, improves the homogeneity of weld metal. The distribution of alloying elements is more homogeneous in leading laser compared with leading arc, since both of the drag force of the plasma jet and momentum of droplet promote the inward flow in leading laser. Almost homogeneous distribution of alloying elements can be attained if the oxygen content in the shielding gas is more than 2%, since the Marangoni flow direction changes from outward to inward with increasing the oxygen content.

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.

Influence of oxygen on weld geometry in fibre laser and fibre laser-GMA hybrid welding

Abstract The effect of oxygen on weld geometry during keyhole mode welding has been investigated by adding a small amount of oxygen to the shielding gas during fibre laser and fibre laser–gas metal arc hybrid welding. The results indicate that the penetration depth increases and the weld width decreases with increasing oxygen concentration. This effect is attributed to the formation of a deeper keyhole when oxygen is present. The addition of sulphur up to 1500 ppm in the molten pool has no significant effect on the penetration depth. This behaviour indicates that the Marangoni convection and surface tension are not the main reasons for the deeper weld penetration when oxygen is added to the shielding gas. The increase in the penetration depth owing to oxygen addition is consistent with the formation of CO by reaction between dissolved carbon and oxygen. Rapid generation of CO in the keyhole expands the keyhole and results in deeper weld penetration.

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.

Fundamental study on welding phenomena in pulsed laser-gma hybrid welding

Hybrid welding phenomena such as the interaction between the arc and laser plume, bead formation and metal transfer have been investigated. CW or power modulated CO2 laser is combined with pulsed GMA process. Pulsing frequency, arc current and arc voltage have been controlled in such a way that one droplet transfers per one pulse. The critical welding speed is significantly improved in the hybrid welding compared with the GMA welding due to formation of stable molten pool. The interaction between the arc and the laser plume is clearly observed during the base period of the arc. The arc is attracted by the laser plume due to high electrical conductivity of the plume. Spatter generation is influenced by the interaction, since the direction of the electromagnetic force changes. On the other hand, the arc sometimes continues to interact with the laser plume also during the peak period. This causes a reduction in the arc current due to large arc length, resulting in unstable metal transfer and spatter generation. Laser power modulation synchronized with the arc voltage is effective in preventing the unstable metal transfer.Hybrid welding phenomena such as the interaction between the arc and laser plume, bead formation and metal transfer have been investigated. CW or power modulated CO2 laser is combined with pulsed GMA process. Pulsing frequency, arc current and arc voltage have been controlled in such a way that one droplet transfers per one pulse. The critical welding speed is significantly improved in the hybrid welding compared with the GMA welding due to formation of stable molten pool. The interaction between the arc and the laser plume is clearly observed during the base period of the arc. The arc is attracted by the laser plume due to high electrical conductivity of the plume. Spatter generation is influenced by the interaction, since the direction of the electromagnetic force changes. On the other hand, the arc sometimes continues to interact with the laser plume also during the peak period. This causes a reduction in the arc current due to large arc length, resulting in unstable metal transfer and spatter generati...

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.

Prevention of porosity by oxygen addition in fibre laser and fibre laser–GMA hybrid welding

Abstract Prevention of porosity in partial penetration fibre laser and fibre laser–gas metal arc hybrid welding is investigated. It is found that modulation of laser power prevents porosity formation in fibre laser welding, but that this method is not effective in hybrid welding. However, the addition of a small amount of oxygen to the molten pool can prevent porosity formation in both fibre laser and hybrid welding. This is attributed to stabilisation of the keyhole. During welding, oxygen reacts with dissolved carbon to form carbon monoxide (CO) in the keyhole. The CO partial pressure in the keyhole prevents the intense interaction between laser beam and molten metal, thus stabilising the keyhole. The role of CO formation is confirmed by the enhancement of porosity suppression with increased carbon content of the base metal.

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.