June 26, 2009
Remote laser welding is a fairly new kid on the block in terms of joining metal in North America. But it makes complete sense for those high-volume applications where resistance spot welding is used commonly.
In an RLW system (see Figure 1), the laser beam is focused over the workpiece from a distance of about 0.5 m 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 percent of the production cycle time.
Of course, RLW can't simply be dropped in to replace another technology. Best results are achieved when a design team considers RLW as a viable option for its metal joining needs.
For example, RLW needs only single-sided access to the joint, but it requires that the line of sight between the beam delivery and the workpiece remains unblocked. 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.
Although the initial investment costs for an RLW system are higher than for a traditional resistance spot welding robot, RLW is faster than resistance welding and has lower operational costs as production volumes grow. For those applications requiring a large amount of spot welds on one assembly, remote laser welding cells make a lot of sense.
Before looking at operating costs for RLW cells, it is important to understand how the technology works and where it is being used.
RLW became possible only with the advent of high-powered lasers that could generate beams strong enough to keyhole-weld from a distance. Today both CO2 and solid-state laser resonators can produce sufficient beam quality to accomplish the joining process.
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 (see Figure 2a) and gimbal mirrors (see Figure 2b). 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 (see Figure 3). In many instances, the resonator is located on a mezzanine above the actual cell.
Today RLW is primarily used in automotive applications and predominantly in Europe. It is used to join a variety of high-volume, welding-intensive assemblies, such as door assemblies, instrument panels, seat backs, and side impact structures. In one production application, an RLW system performs 46 welds on a rear seat back assembly in 12 seconds.
That type of performance interests automakers and their Tier 1 and 2 suppliers, but RLW still is not widely accepted by the North American manufacturing community. That is slowly changing—with one of the first major systems now in place at a Chrysler facility to join rear door assemblies for the Jeep Liberty®—but RLW education needs to continue.
The most education 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, but the total cost to operate an RLW system decreases significantly when compared to resistance spot welding systems.
Figure 4 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 5 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 multigun 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 multigun setups, and still not be at total capacity.
Figure 5 demonstrates such a remote weld cell concept. The three stations accommodate four parts per cycle, and 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.
Because RLW systems are still somewhat unknown in North America, interested manufacturers simply can't visit a facility near them doing RLW because installations are limited.
That will change 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.
Because vehicle chassis are crash-tested months in advance of model launches in the automotive industry, an engineer can't simply suggest that a process such as resistance spot welding be replaced in a manufacturing process. That welding technology is part of an official specification that will likely live on until the nameplate on the automobile is retired.
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.
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