Although remote laser welding (RLW) is commonly used in Europe for joining metals in automotive manufacturing, RLW systems are not in wide use in the North American manufacturing sector. But using remote laser welding makes complete sense for high-volume manufacturing applications where resistance spot welding is currently used.
A remote laser welding system, or cell, doesn't take up much room on a shop floor and is significantly faster than traditional welding systems. RLW systems typically produce two to five welds per second, compared to one weld every three seconds for a resistance-welding gun. In the long run, it is also more cost effective for material processing applications requiring a large amount of spot welds on one assembly.
All that is needed to increase acceptance of RLW is some education about this time-saving materials processing technology and a little help from manufacturing and design engineers willing to incorporate RLW early into their designs.
Lasers and Robots
In an RLW system, the laser beam is focused over the workpiece from about a half meter or more. A combination of mirrors and mechanical movement of the laser-delivery mechanism results in very fast beam pointing. In fact, weld-to-weld repositioning may be less than 50 milliseconds. This is more efficient than traditional spot welding or more recent laser welding involving just robot motion because the seconds needed to move the robot from one weld to another are now eliminated. In some instances, the time it takes to move the robot from one weld position to the next takes up to 90% of the production cycle time.
RLW needs only single-sided access to the joint, but the tooling must be configured so that the line of sight between the beam delivery and the workpiece is not blocked.
RLW relies on fixturing and external clamping to ensure proper joint fit-up. If weld shield gas is needed, delivery nozzles and piping must be incorporated in the tooling itself.
A Closer Look at RLW
RLW became possible only with the advent of high-powered lasers that could generate beams with sufficient quality to keyhole weld from a distance. Today both CO2 and solid-state laser resonators can produce sufficient beam quality to accomplish the joining process.
An example of a remote-weld cell concept, with three stations to accommodate four parts per cycle. The RLW system attached to a robot can make all the welds on those four parts in 84 seconds. If done over two shifts for an entire year, a company would be looking at about 29 million spot welds per year at a lower cost per weld than resistance welding. Drawing courtesy of NuTech Engineering.
The keys to converting this powerful laser beam—usually from a resonator of at least 4,000 W to 6,000 W—into a usable welding technology are the long-focal-length lens and the computer-controlled targeting mirror.
Two types of mirrors are commonly used: galvo-style mirror structures and gimbal mirrors. The galvo approach includes two single-axis mirrors, which create the fan angle beneath the RLW head. This mirror structure often is found in compact and lighter-duty welding systems.
The gimbal approach requires one heavy-duty mirror that has two rotational axes to deliver the beam and is usually favored in cells that have CO2 laser resonators.
Work area sizes for these RLW power sources and beam delivery systems range from 200 mm long by 240 mm wide by 200 mm high to more than 1 m by 1 m by 1 m in volume.
The solid-state lasers (disk or fiber lasers) usually work with optical fiber-delivery systems. This allows the process head to be mounted on a robot for improved positioning and access flexibility.
Several systems offer on-the-fly processing where the robot sweeps out a smooth path and the RLW head performs the detailed pointing and drawing of the weld stitch on the part.
Because of the flexibility of the laser beam delivery system, RLW cells have a smaller footprint than typical spot-welding cells found in North American manufacturing facilities. In many instances, the resonator is located on a mezzanine above the actual cell.
Investment and Operating Costs
The most education about RLW needs to take place in the area of operating costs. The investment cost for an RLW system is much larger than for a single-gun resistance spot-welding system. However, RLW is faster than resistance welding and has lower operational costs as production volumes grow.
An amortized cost comparison of welding systems.
The chart above provides an amortized cost comparison of spot-welding systems—a single resistance spot-welding robot, a four-gun cell using resistance welding, an RLW system with a CO2 fixed laser head, and an RLW system with a robot and a fiber-delivered beam—over a two-year period. The resistance spot-welding technologies are very competitive as long as volumes are below five million welds per year, but as production volumes grow, the speed of the RLW systems stands out. Labor costs skyrocket as the single gun is unable to keep up with the multi-gun cell and the RLW systems.
Running a CO2-powered RLW system is slightly more involved than a fiber-delivered system because you have to take into account laser gas usage. It's not as involved as high-powered, deep transmission welds for which you need to flood the metal surface with helium to ensure that the beam gets down to the surface, but it does involve some planning to coordinate replacement of this consumable.
Having said that, fiber-delivered RLW systems tend to run a little higher than CO2 RLW systems on a cost-per-watt basis. In the end, both RLW systems are similar in total operating costs.
RLW cells really shine when manufacturing engineers design them to accommodate multiple jobs. It is conceivable that a multiple-station cell with the proper tools can perform the work of four resistance-welding cells, with most having multi-gun setups, and still not be at total capacity.
Design Engineers to Be Agents of Change
Because RLW systems are limited in North America, interested manufacturers simply can't visit a facility near them doing RLW. That will change, though, as companies realize that RLW is a cost-effective means of joining metal when production volumes are large and when proper planning takes place.
Design engineers likely will have to be the agents of that change.
But they can't simply suggest that a manufacturing process be replaced. Vehicle chassis are crash-tested months in advance of model launches in the automotive industry. And welding technology is part of an official specification that will likely live on until the nameplate on the automobile is retired.
Instead, design engineers need to incorporate the use of RLW during the prototype phase, so that the technology stands a better chance of becoming part of an official manufacturing specification. Only from those low-volume origins will RLW be able to prove itself in high-volume manufacturing settings.
Continuing RLW Education
RLW is primarily used in automotive applications and predominantly in Europe. It is used in a variety of high-volume, welding-intensive assemblies, such as doors, instrument panels, seat backs, and side impact structures. In one application, an RLW system performs 46 welds on a rear seat back assembly in 12 seconds.
That type of performance should interest all automakers and their suppliers, and it slowly is in North America: A Chrysler facility is now using RLW to join rear door assemblies for the Jeep Liberty.
Robert Mueller is senior laser solutions engineer for NuTech Engineering Inc. in Toronto. His PhD in physics is from York University in Toronto. A version of this article was originally published in The Fabricator magazine.
Update 2013: Geert Verhaeghe of Faurecia Autositze (Germany) will discuss remote laser welding for automotive seat production at the LASE plenary session, part of SPIE Photonics West, on 6 February 2013 in San Francisco.
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