Hybrid laser arc welding: Has its time arrived?

PAUL DENNEY

During 2010's FABTECH show, two related events occurred that address the status of laser hybrid welding. First, ESAB (www.esabna.com) added a new section to its corporate web site related to its hybrid laser arc welding (HLAW) systems (see Fig. 1).

The same week, Lincoln Electric (www.lincolnelectric.com) announced a strategic partnering with IPG Photonics (www.ipgphotonics.com) for the development of HLAW welding systems. To have two of the world's largest arc welding companies promoting HLAW says something about how the process is being viewed by industries.

History of HLAW

While HLAW has been investigated for a number of years, there were a lot of reasons for its limited utilization. "I felt that a big disadvantage of the work that we were doing was that we were using a big clunky old laser that didn't focus all that tightly," says Vivian Merchant, an independent consultant, of his research efforts in the early 1990s at the Canadian Defence Research Establishment. He went on to say that their interest in using HLAW was to achieve higher production rates for military applications such as welding HY-80 material for submarine fabrication as well as for the welding of high strength materials for cross country pipelines. Vivian goes on to say that they recognized that "without the laser, it will take 10 passes to weld this one-inch-thick steel. With the laser, we can narrow the groove, and weld this one-inch-thick steel with only two passes!"

Even with "clunky" lasers, some applications did transition from the laboratory to the factory floor. Efforts in Germany with HLAW continued to be very active in the 1990s. Researchers such as The ISF - Welding and Joining Institute of RWTH Aachen University worked with companies such as Meyer Werft Shipbuilding, Papenburg, Germany, to develop and assist in the implementation of the technology. The result was the opening of a new panel line at Meyer Werft in 2000 for the welding of deck and bulkhead panels with stiffeners using HLAW with CO₂ lasers. Not only did this represent a major acceptance of the technology, but it required the development and acceptance of new specifications for shipbuilding by organizations such as DNV and Lloyds.

A major change

The late 1990s and early 2000s saw a major change in laser technologies that would greatly impact the HLAW process. With advances in diode technology, greater power, better beam quality, and lower cost allowed these lasers to be used alone or as pumping sources for advanced laser systems. You no longer had to choose CO₂ lasers (with hard optics) for high power or Nd:YAG for the flexibility of fiber delivery (but limited to 4 kW). Diode lasers were being used to produce fiber and disk lasers that were not only fiber deliverable and equal to or higher power than CO₂ lasers, but which were easier to operate and maintain.

"The recent appearance of the new high-brightness laser technologies has by far had the greatest impact," said Brian Victor of the EWI, Columbus, Ohio. "These new lasers are much less intimidating for potential HLAW users. They're easier to operate/maintain, more robust than previous laser technologies, fiber-delivered for flexible manufacturing, capable of higher powers than possible with previous industrial fiber-delivered lasers, and all at a relatively low cost per kW."

Dr. Simon Olschok, chief engineer at ISF - Welding and Joining Institute, RWTH Aachen University, states that he is aware that hybrid is used in "…ship building (in use), automotive (in use), vessel fabrication, pipe welding orbital up to 6 mm, and pipe welding J-Lay up to 15 mm." He states that since efforts at Meyer Werft Shipbuilding, he is aware of a number of companies that have or are considering hybrid welding. These efforts are being fueled because of "new laser sources which allow robot usages and that there are now many institutes around the world that do hybrid welding (that can assist in development and implementation)."

While hybrid processing has occurred in European shipyards since 2000, efforts have also been ongoing in the United States. Applied Thermal Sciences (ATS), Sanford, Maine, has worked with the US Navy to qualify the welding of structural HSLA 65 components as well as lightweight sandwich panels (know as LASCOR) out of duplex stainless steel. The LASCOR panels are targeted for applications on the new DDG 1000 ship. To echo Dr. Olschok's earlier comments, ATS's original efforts in hybrid welding were with high power CO₂, while recent efforts have been accomplished with high power fiber and disk lasers.

Hybrid welding is not only for shipyard applications. In Sweden, the first industrial application of HLAW was initiated in spring 2005 at Duroc Rail AB in Luleå. This application consisted of using a 20 kW CO₂ laser at Duroc combined with a GMAW source for welding together large, thick, high strength steel sheets for rail car applications. This application was developed from the Nordic Network on Hybrid Welding (NORHYB) that had the goal of disseminating this technology to the Nordic industries.

"Non-arc" hybrid processes

While HLAW implies an "arc," there are many other "hybrid" processes that are being developed and implemented that combine laser with other "electrical" processes. In these other cases, the laser is being used as a precision heating source and/or the electrical process is supplying an inexpensive heat source to the application.

An example of one of these "non-arc" hybrid processes is hybrid laser brazing. This process uses resistance heating between the part and the tip of the wire feeding system to increase the temperature of the wire. The laser then is used to take the brazing alloy, usually a bronze alloy, to a melting temperature while at the same time heating the substrate to a high enough temperature to allow for wetting without flux. This can occur at very high speeds (> 5 meters/minute) and result in joint quality that can be painted over. A number of car companies have implemented this process, including VW, Mercedes Benz, and Chrysler. Examples of some of the uses include truck lids and roof ditch welds.

Another "borderline" HLAW process is laser cladding. This can be accomplished like laser brazing without an arc and using simply resistance heating in conjunction with laser. Or the laser can be combined with the GMAW process for the deposition of a consumable wire. Usually these processes are used to repair a worn or damaged surface and match the chemistry of the substrate or the material being deposited to tailor the surface of the part for improved corrosion and/or wear resistance.

For most "hybrid" applications, the GMAW is assisting the laser process by:

• Adding filler material at an elevated temperature with little or no additional energy from the laser,
• Permitting welds to be made in joints with greater fit-up issues and gaps than can be normally welded by autogenous laser welding, and
• Altering the chemistry of the weld metal.

However, in some cases, the laser assists the GMAW process. As reported by Brandon Shinn at FABTECH 2005 under the AWS Technical Sessions, combining a laser with pulsed GMAW welding of titanium resulted in a higher quality weld. Normally, welding titanium by the GMAW process is difficult because the arc cathode is not "stable" and drifts around on the weld bead. The result is an irregular weld bead and spatter. However, Shinn found that focusing as little as 200 W of laser power on the weld puddle "locked" the cathode location, resulting in a very regular weld bead. This "laser-assisted GMAW" hybrid process could achieve very high welding speeds and small weld beads for use in welding of thin titanium structures such as those used in aerospace applications.

These increases in applications are also driving examination of what is an acceptable hybrid weld. As stated earlier, DNV and Lloyds have already developed standards for hybrid welding for shipbuilding applications. But with the recent activities for HLAW in other industries, there have been additional interest/needs for the development of other standards/specifications by other organizations. The American Society of Mechanical Engineers (ASME) and the American Welding Society (AWS), both of which have had laser specifications for years, are active in the development of standards and/or recommended practices for hybrid processing. Both organizations are reacting to requests for specifications by companies that are using their specifications to develop their procedure qualification records (PQRs) and their welding procedure specifications (WPS). These standards will address what parameters need to be documented and how much variations or changes will be allowed before a process must be partially or fully re-qualified.

Obstacles to greater utilization

While there seems to be an increase in the implementation of hybrid processes, there still remain obstacles to greater utilization. When asked what "obstacles" for implementation they see, Stan Ream and Brian Victor of EWI responded, "The main limitation is equipment cost. Most of the key HLAW markets are going to be in industries that currently use conventional arc welding. Buying a laser and the associated equipment will be a significant cost increase. To make a large capital investment of a HLAW system, the productivity and other ROI benefits of the HLAW process have to be well understood upfront."

While cost is a factor, others believe there are physical limitations to the process. Dr. Olschok said he thought that as the weld thickness increases, a point will be reached where the ability to produce a "free-formed root bead" will be reached. He said that today this limit is between 12 mm and 15 mm, depending on the laser used. Physical limitations were also voiced by EWI representatives, who commented that while the HLAW process does allow for greater fit-up variation than normally possible for autogenous laser welding for thinner material, as the thickness increases, the limitation of the HLAW process will require industries to improve their fit-up tolerances over what is presently achieved for the arc process. The cost for this improvement will have to be factored against the other benefits of the HLAW process.

Further R&D

While implementation of the HLAW process is occurring, further research and development is being accomplished for other applications. While it may not be practical to achieve 25 mm thick welds in a single pass with an HLAW process today, it may be possible to achieve this with multiple pass welds. Researchers are investigating this approach for pipeline and other thick section applications by using HLAW for the root pass and HLAW or conventional arc welding for the subsequent pass. There is also work being accomplished in the use of multiple wires for greater fill or cladding applications and also efforts to have the laser and the GMAW power suppliers working in "concert" together to achieve even greater performance and control.

As has been seen, the "hybrid age" may have been slow in coming, but has accelerated in the last few years. This acceleration in implementation has been driven by improvements in lasers, especially high brightness lasers. These new lasers have allowed for easier integration into systems and lower ownership cost, which have improved the ROI of the process. In addition, as new standards become available, there is a potential for the technology to be accepted and implemented. Further implementation will be driven by additional advances in the lasers and processes.

Acknowledgments

The author would like to thank Stan Ream and Brian Victor of EWI, Dr. Simon Olschok of Aachen University, and Vivian Merchant for their extensive comments and input that made this article possible.

Paul Denney is senior laser applications engineer for Lincoln Electric Corp., Cleveland, Ohio, email [email protected].