Category Archives: Laser Arc Welding

Influence of Oxygen on Weld Geometry in Fibre Laser Welding

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Abstract? By: Lin Zhao1, Susumu Tsukamoto1, Goro Arakane1, Tomohiro Sugino2

Consortium of JRCM (The Japan Research and Development Center for Metals)

1 National Institute for Materials science, 1-2-1, Sengen, Tsukuba 304-0047, Japan

2 IHI Corp.,1, Shin-Nakahara-Cho, Isogo-ku, Yokohama 235-8501, Japan

It is well known that oxygen can significantly increase the penetration depth in arc welding. It is caused by change in the Marangoni convection direction from outward to inward with increasing the oxygen content.  Increase in the penetration depth by oxygen was reported also in laser welding. Some researchers suggested that it was caused by the same mechanism as arc welding. However, narrow and deep keyhole is formed during welding and the penetration depth should be mainly determined by the keyhole depth. Thus, the mechanism of increase in the penetration depth by oxygen has not been clarified yet. In the present study, the effect of oxygen on the keyhole and fluid flow behaviour has been investigated to understand the mechanism of different weld geometries for various oxygen contents in fibre laser welding.

Partial penetration bead on plate welding was carried out on 20 mm thickness 0.10C-0.30Si-1.33Mn steel under the constant laser power of 7 kW and welding speed of 1.0m/min. To elucidate the effect of oxygen content on the penetration depth, the oxygen content in He-O2 shielding gas was varied from 0 to 20%.

The penetration depth increases and the weld width decreases with increasing the oxygen content as shown in Fig. 1. In high oxygen content, the fluid flows from the rear pool end to the keyhole on the pool surface and it flows down just behind the keyhole. This means the inward Marangoni convection is promoted by oxygen. However, the keyhole depth observed from the X-ray transmission image coincides well with the penetration depth in the transverse section both in high and low oxygen contents as shown in Fig. 2. This indicates that increase in the penetration depth by oxygen is not caused by the fluid flow. Deeper keyhole formed in high oxygen shielding increases the penetration depth.

The most possible mechanism is formation of CO. There are a lot of oxide films on the pool surface in 0% O2 shielding, whereas few oxide films exist in 10% O2 shielding. This indicates that some oxides enter into the keyhole by inward Marangoni convection. As the keyhole wall temperature is quite high, these oxides can be decomposed. CO is more stable than the other oxides such as FeO, MnO and SiO2, at high temperature. Then, CO formation is possible in the keyhole. If CO is formed in the keyhole, it can expand the keyhole, resulting in reduction in the keyhole surface temperature. As a result, the keyhole depth increases by adding oxygen.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper 1802

Fundamental Phenomenon of Remote Welding

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By: Shinpei Oiwa

Remote welding has many advantages; very fast welding, flexibility of process designs, efficient production and so on. Now remote welding attracts much attention due to such advantages. In fact, remote welding has already been applied to many industries, especially automobile industries. Meanwhile, remote welding requires high beam quality. Although CO2 laser has been used for remote welding, fiber laser has also been increasing recently. Fiber laser is one of the most promising lasers. CO2 laser beam can only travel within a mirror system; however, fiber laser can be transmitted through an optical fiber. Therefore, remote welding with fiber laser is more flexible and can weld more complex objects compared with CO2 laser.

On the other hand, it is well known that a laser-induced plume comes out from a keyhole affects welding results greatly during laser welding. Generally, this plume is removed with a shielding gas. In remote welding, however, it is often difficult to remove the plume using such a gas due to its long focal length. Until now, the interaction between fiber laser and its plume has been revealed. However, phenomenon of the plume affecting welding results during remote welding are not fully understood.

Our research purpose is to reveal the phenomena during remote welding of zinc-coated steel sheets with fiber laser. This material is often used in automobile bodies. Fig. 1 shows the top and bottom appearances of a weld bead made with fiber laser remote welding under certain conditions. The appearances of bottom surface shows that a weld bead changed from full to partial penetration. Then we observed the plume during remote laser welding with a high-speed camera. As a result, the plume ascended to high as the welding proceeds. An observed plume is shown in Fig.2. From the observation results, the cause of the transition was interpreted by considering that the high plume heated the space above a specimen and then refractive index distributions were formed in the beam path due to its high temperature. This distribution affected laser beam to cause the change of weld bead geometry. In addition, it was found that air blowing between the laser head and specimen by a fan improved welding results by removing the high plume and heated air. Fig. 3 shows the appearances of a weld bead produced by using the fan. It is seen that full penetration bead was formed all over.

Fan blowing is very easy and low-cost improvement for remote welding bead and this method is efficient for real welding situation. Now we have succeeded in observing accurate refractive index distribution during welding by using an interferometer with a fiber laser.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Fibre Laser welding for Lightweight Design

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By: Jan Karlsson1, Alexander Kaplan1

1Luleå University of Technology, SE-971 87 Luleå, Sweden; www.ltu.se/tfm/produktion


This project aims at weight reduction by laser welding of high strength steel for certain applications, and at the same time aims at initiating a knowledge platform for lightweight structures, including, besides other issues, the optimization of welding technology. To enable welding information to be transferred and used in a larger perspective, creating a broad knowledge platform may be a solution. A problem with most studies today is that the information presented by them is not directly transferable to different applications, although the same solutions for suppressing defects might be applicable. By producing a broad knowledge platform from a weld situation (even if not all is usable) other similar, but different, applications may use parts of it to facilitate an increase the weld quality.

When building a knowledge platform, a wide spectrum of methods can be applied to analyze the welding process and its result, ultimately linking them together by conclusions and making generalizations. Intermittent sampling and analysis are the main steps when analysing welding in the present project. By comparing different methods, more comprehensive information is obtained and a better understanding of defects may be achieved. The figure below shows the context between different stages of the weld “lifecycle” (horizontal axis) and different suitable analysis methods and resulting data (vertical axis), to be combined in a comprehensive knowledge platform.

Some suitable analysis methods for each stage are being developed and in the future are going to be used in context with each other at present department.  Among the included methods are:

As part of this work, present paper has been produced in a line of applying the method. However, some stages have yet to bee applied. In the experiments, which are part of a weight reducing program, two different high strength steel grades have been welded by a fibre laser to create a fillet corner joint. To make the story short, the following conclusions could be drawn from the analysis of the experiments:

  • Undercut and root sagging defects can be prevented or suppressed by
    • Changing weld position from flat to horizontal
    • Sufficiently large beam inclination – again against sagging
    • Adjusting the focal point depth (below the surface)
    • Adjusting the welding speed (too slow leads to wide melt and sagging)
    • Adjusting the lateral beam position (particularly sensitive when having an inclination angle)
  • Spatter becomes more frequent and more intense for lack of penetration
  • The weld joint interface acts as an insulator, to be thermally overcome by suitable inclination angle, position and joint separation
  • The combined use of analysis methods (e.g. high speed imaging) provides more information and facilitates analysis, understanding and parameter optimisation
  • Rules for suppressing defects can be stated and documented, to be proven for other cases with respect to their limits of generalisation

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Paper # P128

Diffusible Hydrogen Analysis of Hybrid Laser Arc Welding

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By:  Paul A. Blomquist, Carl Chretien, Applied Thermal Sciences (ATS); Stan Ferree, ESAB Welding and Cutting Products; Dale Anderson and Brian Marx, Concurrent Technologies Corporation

Applied Thermal Sciences has been working to develop the equipment and process technology for Hybrid Laser Arc Welding (HLAW), with the goal of improving the productivity and reducing the cost of shipbuilding in the United States.  Recently, as part of a project team led by the Navy Metalworking Center, ATS developed procedures and received qualification approval for the HLAW process as applied to high strength steels for primary structural components of naval surface combatant vessels.  For these steels, control of weld metal diffusible hydrogen is critical.   The addition of the laser to the traditional Gas Metal Arc Welding (GMAW) process results in a higher total melt volume than that of the GMAW process alone, which could affect hydrogen absorption into the weld pool.  A literature search revealed no information relating to diffusible hydrogen characteristics of HLAW.  For this reason, comparative measurements were made using HLAW, autogenous Laser Beam Welding (LBW) and conventional GMAW, to determine the diffusible hydrogen content.  All testing was performed in accordance with AWS A4.3, the standard for evaluation of diffusible hydrogen in weld metal.  Additional work was performed to evaluate diffusible hydrogen results in the context of total melt volume versus merely added weight of weld metal.  Results show that the HLAW process results in low levels of diffusible hydrogen in the welds and that the addition of the LBW process contributes very little to the diffusible hydrogen in the welds.  These results demonstrate that the HLAW process can be applied to hydrogen-sensitive steels, keeping in mind that all typical “low-hydrogen” practices, such as cleanliness of parts, control of environment, etc., are maintained.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Prevention of porosity by oxygen in partial penetration laser and laser-gma hybrid welding

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By: Susumu Tsukamoto1, Lin Zhao1, Goro Arakane1, Tomohiro Sugino2

1 National Institute for Materials Science, 2IHI Corp.

High power and high brightness solid state lasers such as fibre and disc lasers have a great potential to weld heavy section plate members.  In this case, one of the major problems is formation of some weld defects such as porosity and hot cracking.  Especially, the porosity is easily formed in deep partial penetration laser welds.  From this point of view, prevention of the porosity in partial penetration fibre laser and fibre laser-GMA hybrid welding has been attempted in the present study.

Most attractive characteristic of the laser welding is narrow and deep weld penetration. It is attributed to formation of narrow and long keyhole by evaporation of the materials as shown in Fig.1.  However, the keyhole is basically unstable, if the length is larger than the circumference.  Then, the keyhole tends to be closed during deep penetration laser welding.  If the keyhole is closed near the root, the separated keyhole tip forms the bubble and remains as a porosity.  Thus, the porosity is formed by keyhole instability.  Stabilisation of the keyhole should be effective to prevent the porosity.

In the previous paper, we revealed that the porosity was successfully prevented by periodical change in the laser power (laser power modulation) in deep penetration CO2 laser welding.  It is because the laser power modulation can stabilise the keyhole at optimum frequency and waveform.  Then, we applied this technique to fibre laser and hybrid welding at first.  The results indicate that the power modulation can effectively prevent the porosity in fibre laser welding, but a lot of porosities are still remained in the hybrid welds.  On the other hand, the porosity is effectively suppressed with a small amount of oxygen addition in the shielding gas both in fibre laser and hybrid welding as shown in the x-ray radiographs of Fig.2. Observation of dynamic keyhole behaviour using an in-situ x-ray transmission imaging system confirmed that the keyhole was stabilised by a small amount of oxygen addition.  Spatter generation can also be reduced by oxygen in fibre laser welding due to stabilisation of the keyhole.  Thus, oxygen can stabilise the laser welding process and then prevent the weld defects.

Fig.1 Formation of keyhole in laser welding.

Fig.2  X-ray radiographs of hybrid welds with or without oxygen in the shielding gas.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

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