Tokuhiro Nakabayashi

Prevention of welding defect by side gas flow and its monitoring method in continuous wave Nd:YAG laser welding

In the Nd:yttrium–aluminum–garnet (YAG) laser welding of thick plates, reduction of porosity and monitoring the keyhole/molten metal behavior are very important issues to assure the high quality welding. The authors applied a side gas flow to prevent the porosity in the bead on plate welding with 6 kW Nd:YAG laser equipment. At the same time, a reflected Ar+ laser in the same axis as the Nd:YAG laser beam and the light emission from the weld were measured as monitoring signals. Under the optimum side gas condition, pores in the weld metal could significantly decrease, the penetration depth increased slightly, and the bead width became narrower. Under that condition, moreover, the generation of spatters was quite few. An acceptable limit of the transverse misalignment of the side gas nozzle position was about 1 mm. The misalignment could be detected by using the above mentioned monitoring signals.

Develoment of 2D simulation model for laser welding

Laser welding includes many complicated phenomena such as absorption of laser light on material surface, phase transition from solid to liquid and from liquid to gas, laser light absorption and refraction within plasma, and so on. Two- dimensional unified simulation model was developed for laser welding of thick plate, and verification test using 4kW-YAG laser was carried out. Thermal-hydraulic phenomena of welding pool and keyhole are solved numerically using CIP (Cubic- Interpolated Propagation) and C-CUP (CIP-Combined Unified Procedure) method based on conservative equations of the multi-phase and multi-component fluids. Multiple reflection of laser light on the keyhole surface, absorption and refraction of laser light within the plasma are treated by ray tracing method. The availability of the model was confirmed to compare the results of experiments with numerical analysis.

Behaviour of monitoring signals during detection of welding defects in YAG laser welding. Study of monitoring technology for YAG laser welding (Report 2)

The previous paper in this series describes probe beam application as a monitoring technique during YAG laser welding and clarifies the effectiveness of this method through rationalisation of the probe beam irradiation conditions during weld monitoring. During monitoring of the welding process, however, it is important to detect welding defects. Monitoring of defects in CO 2 laser welding has been previously reported in a handful of papers. 3 These efforts have to some extent seen practical applications for detection of perforations and underfills in welding of thin sheets. During detection of underfills in welding of thin sheets, however, the percentage of underfills is high in relation to the sheet thickness, and it remains unclear whether monitoring can be performed with much the same accuracy during welding of thick plates. For detection of defects in YAG laser welding, welding of thin sheets of around 1 mm thickness has been investigated, 6 but these studies focus on detection of the penetration depth and lap welding defects, whereas detection of underfills and misalignments in butt welding has so far been little documented. In the area of YAG laser welding of thick plates, detection tests during full penetration welding by bead-on-plate welding have been reported. For welding of thick plates, however, the literature contains few studies of continuous high-power laser welding and welding being performed by increasing the peak value of the laser power in pulsed welding. The purpose of this paper is to describe a technology for detection of defects during continuous high-power YAG laser welding of thick plates. Full and partial penetration detection tests are conducted by bead-onplate welding. Butt weld gap and misalignment detection tests are also conducted by butt welding. The changes in the monitoring signals then used to detect these defects are investigated, and a rational detection approach is proposed.

High-power COIL and Nd:YAG laser welding

We have constructed a laser welding system, which enabled high-power laser welding by combining three laser beams of 1 µm wavelength. Its wavelength enables optical silica fibers transmission and the flexible system. The heart of this system consists of a 4 kW and a 6 kW Nd:YAG lasers and a 10 kW class Chemical Oxygen-Iodine Laser (COIL) beams of 6 kW Nd:YAG laser and COIL are combined in a coaxial beam and its maximum average power is 19 kW. The third laser beam, 4 kW Nd:YAG laser beam, is added obliquely from the same side of workpiece or oppositely from the reverse one. The effects of various welding parameters were investigated, such as the laser power, pulse modulation, and so on. As a result of the welding test with the 6 kW Nd:YAG laser, it was clarified that the pulse wave (PW) has good efficiency of deeper penetration at low welding speed. When the combined beam with CW COIL and PW Nd:YAG laser was used, 20 mm penetration on the stainless steel could be achieved at a welding speed of 1 m/min. By adding the third laser beam, the both side welding on 30mm thickness plate could be achieved.

Development of in-process monitoring technique in YAG laser welding

We have studied in-process monitoring technique in Nd:YAG laser welding. We used a CCD camera and a photodiode as the monitoring sensor, and observed laser processing coaxially with the laser beam. There were differences in the image of the CCD camera between full and partial penetration welding and the detection for full and partial welding was achieved by the image processing of the detected image data. And, it was suggested that a change in the focal position could be detected because a change in the luminescence intensity could be caught with the photodiode when the deviation of the focal position occurred.

High-power chemical oxygen-iodine laser welding

The present paper describes the welding characteristics with a 10 kW class Chemical Oxygen-Iodine Laser (COIL), whose wavelength is 1.32 micrometer. Bead-on-plate welding tests of 304 stainless steel plates were carried out at laser power 8.5 kW and 11 kW. Three different shielding gases (N2, Ar and He) were used through a coaxial conical shape nozzle under the lens. In COIL welding, the interaction between the laser beam and the laser induced plasma is very small because the wavelength of COIL is shorter than that of CO2 laser, so that the laser beam reaches on the workpiece without absorption by the plasma. As the result of the welding tests, the welded bead shapes did not depend on a type of the shielding gas. Radiograph and longitudinal section tests of the welded beads were carried out. When He and Ar gases were used as the shielding gas, there were several porosities. On the other hand, the use of N2 gas made no porosity. The full penetration on 10 mm thick plate was achieved in the high aspect ratio without the welding defects under the condition of laser power 8.5 kW and welding speed 1.5 m/min.

Thick plate welding with Nd:YAG laser and COIL

In the field of heavy industries, many products are made of thick metal parts. Nd:YAG laser has been recently developed up to 10 kW. Nd:YAG laser has the characteristics of the optical fiber transmittance and the good absorption by the metal surface, so that it is expected to apply Nd:YAG laser to thick plate welding. This study presents the thick plate welding with Nd:YAG laser and COIL (Chemical Oxygen-Iodine Laser). We have developed a coaxial beam combining system with beams of Nd:YAG laser and COIL. The maximum average power of the combined beam was 19 kW. Welding tests of 304 stainless steel plates were carried out. The effects of various welding parameters were investigated, such as the laser power and pulse modulation. As a result, it was clarified that the pulse wave has good efficiency of deeper penetration as compared to continuous wave at low welding speed. When the combined beam was used, 20 mm penetration depth on the stainless steel could be obtained in high aspect ratio at welding speed of 1m/min. When the combined beams and another Nd:YAG laser beam whose power was 4 kW were used, both side welding on 30 mm thickness plate could be achieved.

Development of monitoring method for YAG laser welding and its application. Study of monitoring technology for YAG laser welding

and applications of monitoring technologies for CO2 laser welding of sheet metal used in the automotive industry have been reported. On the other hand, monitoring methods for use in YAG laser welding are little documented, an important factor being the non-availability until now of any high-power YAG laser oscillators. Uprating of YAG laser oscillators to higher powers, however, is now progressing apace. The YAG laser, through offering the advantage of optical fibre transmission and a lower reflectance in relation to metals than the CO2 laser, continues to arouse growing interest. This paper describes an investigation of a monitoring method for YAG laser welding. Based on the monitoring method so far adopted in conventional CO2 laser welding, the proposed monitoring method involves observation of the keyhole behaviour, irradiation of a probe beam into the keyhole zone, and measurement of its reflected light. Within the context of a previous study demonstrating the importance of establishing the relationship between the keyhole behaviour, particularly the keyhole stability, and the formation of defects, such as porosities, the purpose of the proposed monitoring method is to provide a defect detection capability based on monitoring of the keyhole behaviour. On the basis of an investigation of detection of butt welding defects, such as gaps, misalignments, etc, with variation in the welding conditions, keyhole stabilisation is sought using a side gas for defect prevention. The signal behaviour of the proposed monitoring method is also examined. This paper discusses suitable conditions for monitoring the probe beam and demonstrates the effectiveness of the proposed method.

High-power COIL and YAG laser welding

We have constructed a laser welding system, which enabled high-power laser welding by combining three laser beams of 1 mm wavelength. Its wavelength enables optical silica fibers transmission and the flexible system. The heart of this system consists of a 4 kW and a 6 kW Nd:YAG lasers and a 10 kW Chemical Oxygen-Iodine Laser (COIL). The average power of the combined beam is up to over 20 kW. The effects of various welding parameters were investigated, such as the laser power, pulse modulation, and so on. The 10 kW COIL has a very good beam quality which is 64 mm.mrad. The beam spot diameter is 0.48 mm at the focal point. On the contrary the beam quality of Nd:YAG laser is worse, but it has the function of pulse modulation which the COIL dose not have. As a result of the welding test with the 6 kW Nd:YAG laser, it was clarified that the pulse wave (PW) has good efficiency of deeper penetration at low welding speed. When the combined beam with CW COIL and PW Nd:YAG laser was used, 20 mm penetration on the stainless steel could be achieved at a welding speed of 1 m/min.