Dance of the photons
The next generation of scanner optics will release the focus spot from the working plane. Here’s how users will benefit from this three-dimensional dance.
Conventional wisdom states that a gap width of 0.2 millimeters is the upper limit for remote laser welding. However, Volkswagen has now developed a solution that makes this rule of thumb a thing of the past. Remote welding works by using two mirrors inside the scan head to rapidly focus the laser beam on the workpiece with a high degree of precision.
The automaker has taken this method one step further by making the laser beam oscillate, or ‘dance’, along the gap. This new technique is called laser stir welding, and Volkswagen has already integrated the process into its production lines.
Thorge Hammer, who is responsible for technology planning and development, body shop planning, and tool and die operations at Volkswagen, explains that the mirrors in this process manipulate the beam in a circular motion as they guide it along the gap: “We call this the ‘wobble effect’, and it causes the laser to stir the melt pool, which increases the volume of molten material.
As a result, we can now bridge larger gaps than we could before.” It also means that the laser is even capable of processing components designed for MIG and MAG welding, without need for modifications. “We weld mounting blocks with gaps of up to 0.5 millimeters,” says Hammer.
There is no doubt that scanner welding using robot-guided optics has become firmly established in car body manufacturing and is increasingly pushing production engineers towards ever more exciting innovations, but there has always been one major limitation — the Z axis.
The scanner mirrors could make the focus spot dance and jump across the workpiece along the X and Y axes. To bring the Z axis into the equation, the only option was to move either the entire scan head or the lens inside the scanner optics. Now, however, new 3D scanner optics have given the focus spot a whole new freedom of movement.
Equipped with a highly dynamic drive unit, the movable lens quickly positions the spot in a precise location on the Z axis without moving the optics. This allows the laser to move around in a third dimension, eliminating the problem of working in different planes and enabling the beam to quickly reach small weld spots in previously inaccessible locations.
Agitating, welding, jumping
Returning to the example at Volkswagen, the company has incorporated this technology in the laser stir welding process it uses to join sub-assemblies for the Golf VII, which are subsequently integrated in a platform. The front seats are held in place on the four seat supports, while the two mounting blocks referred to by Hammer are used to secure the engine and the power train.
The seat supports are made from deep-drawn, medium-strength sheet steel, 0.7 mm thick. To fix the mounting blocks, Volks wagen welds a 3 mm flow-formed sheet into a shell 3 mm thick. “The ability to move in a third dimension allows us to laser weld components with undercuts,” says Hammer.
Volkswagen uses six remote welding systems equipped with 4,000-watt disk lasers to process the Golf VII sub-assemblies, including the laser welding system for welding on the fly. This is the minimum setup to achieve the cycle time required to turn out 4,500 pieces a day.
“The laser has 7.5 seconds to carry out a total of 19 weld seams per part,” says Hammer. “But at a laser power of 4,000 watts, the robot should not take more then 1.2 seconds to move between two welding spots.” A newly developed robot-based control system re-calculates the position of the scanner every millisecond and corrects the position as necessary to create the welding structures specified.
This new-found freedom of movement means the laser system can also be employed in the passenger compartment. OEM supplier Faurecia uses this process to manufacture its seating products. Remote welding already plays a key role in the company’s production operations — the technique is used to weld the frames for the seat backs of the front and rear seats, the recliners and seat tracks.
The laser beam is used to process the materials in packages 0.7 to 6 mm thick and comprising multiple layers. For materials up to four millimeters thick, the laser uses remote welding technology. Each year, Faurecia uses this technique to manufacture 18 million frames for front and rear seats and 115 million seat recliners and tracks. Geert Verhaeghe, senior expert for welding, adds: “There is a clear trend toward lightweight construction — materials that are thin yet stable — and consequently toward high-strength steels.”
This material can only be welded using a laser. Faurecia now has more than 20 systems in operation worldwide, equipped with robot-guided and fixed scanner optics, and the third dimension gives them more freedom of movement.
“The PFO 3D from TRUMPF enlarges the working area without requiring us to move the optics over the workpiece,” says Uwe Viehmann, joining technology manager. “The seat backs come in different heights. The PFO 3D programmable focussing optics do a better job with them because we can now position the laser beam in the Z axis using the same dynamic system we use for the X and Y directions.”
Verhaeghe already has one eye on the next technological innovation — he wants to use the PFO 3D for welding on the fly. “Some of the components we process are very large, and the combination of robots and 3D optics would give us an even bigger working area,” he says.
By gradually making a transition from CO2 to solid-state lasers, the company is gaining important benefits: “Disk lasers are far more efficient — which has helped us significantly reduce our costs,” says Verhaeghe. Optimizing the design of the joints and connections improved the situation for the laser and led to a positive side-effect.
Laser-welded joints are up to 30 times smaller and thinner than their MAG-welded counterparts. That saves resources and cuts down weight: the laser-welded spots take up just 0.6 millimeters of space compared to the 10 millimeters required in resistance spot welding.
Contact with the future
Lasers are making their mark deep inside the cars of today and tomorrow, with the vast flexibility of the third dimension finally making it possible to weld housings for high-voltage batteries. Each battery module consists of individual cells in rectangular aluminum enclosures.
By the time it reaches the laser processing stage, the battery is already inside the housing and fully charged. That means a laser can only be used if heat input is kept to a minimum and if welding depth is precisely controlled — penetration would damage the battery, which could have fatal consequences with a battery that is already charged.
Spatter is unacceptable and the seam must be fully sealed. Welding performed using TruDisk disk lasers with an output power of between one and five kilowatts, on battery housings ranging in thickness from 0.8 to 1.5 millimeters, has confirmed that the laser lives up to its promises: the weld seams were fully sealed and the battery sustained no damage.
Neue Anwendungen hat der Laser auch jenseits des Gehäuseschweißens im Blick: das Schweißen dünner Batteriekontakte und die Verbindung der einzelnen Batteriezellen zu kompletten Modulen. Derzeit werden sie noch geschraubt, da die Batteriezellen häufig gewechselt werden müssen.
In addition to welding housings, lasers are finding other novel applications in batteries, including welding thin battery contacts and joining individual battery cells into complete modules. Since at the moment battery cells to be changed, the current practice is to screw the modules together. But once these teething problems have been ironed out, the laser could become the preferred solution for holding the modules firmly together.
At the moment, these new applications are at an early stage, and manufacturers are only just starting to consider what form of automation would work best in each case. Nowadays, lasers supported by modern scanner optics play a crucial role in the production of car body parts and seating components. The laser is already playing a key part in the development of electro-mobility, and it will continue to help light the way toward ever more cost-effective vehicle production.
TRUMPF Laser GmbH + Co. KG
Dr. Rüdiger Brockmann
This article was first published in winter 2011.