Welding With Diode Lasers

High-power diode lasers can be used for both conduction welding and keyhole welding.

In keyhole welding, the laser is focused to achieve a high power density on the workpiece. Once the metal starts to melt, it can absorb much more heat from the laser. This raises the metal’s temperature above the boiling point, generating metal vapor. The pressure of this vapor opens a channel around the laser beam, forming the so-called keyhole. The keyhole traps almost all of the laser power and converts it into heat, allowing the laser to produce welds that are deep and narrow.

A 1-kilowatt, fiber-coupled diode laser can keyhole-weld 304 stainless and mild steel up to 0.125 inch thick at a rate of 72 inches per minute.

In conduction welding, the laser energy is absorbed mostly at the surface of the metal, and heat transfer into the bulk material occurs by conduction. Conduction welds are shallow with a bowl-shaped profile, and they tend to be wider than they are deep. The heat-affected zone is large, and the transition from the fusion zone to the base metal is smooth and gradual. Welds made with a diode laser do not require as much surface preparation as those made with CO₂ or Nd:YAG lasers.

With conduction welding, diode lasers can be used for:

• seam welding thin sheets.
• welding parts with differing thicknesses.
• welding materials prone to cracking.
• welding parts with complex, 3D joining faces.

Diode lasers are good for conduction welding medium- and high-carbon steels, which tend to form an undesirable martensitic fusion zone when subjected to high temperatures and fast temperature cycles. Galvanized steels can also be conduction welded with diode lasers. The low-temperature process creates a fusion zone with a uniform dilution of zinc and steel with no porosity.

Stainless steel is another candidate for conduction welding with diode lasers. Stainless absorbs near-infrared wavelengths better than longer wavelengths. And, because conduction welding is a lower-temperature process, it doesn’t remove volatile alloying elements from the fusion zone, preserving the material’s corrosion resistance.

Diode lasers offer a distinct advantage for welding aluminum. The wavelength of diode lasers is closer to the absorption peak of aluminum than the 1,030-nanometer wavelength of disk lasers or the 1,080-nanometer wavelength of fiber lasers.

Several car manufacturers are using diode lasers to weld structural components and outer skin joints.

In general, welding 6000 series aluminum, which is commonly used in car bodies, requires an aluminum-silicon filler wire to prevent hot cracking of the weld.
The filler wire is applied during the weld with the aid of optical seam-tracking technology.

Audi uses a 4-kilowatt diode laser guided with a 400-micron optical fiber to weld the aluminum tailgates of several vehicle models. The finish of the tailgate welds meet the highest specifications for visible joint quality and does not require further processing.

Kuka Systems Corp. North America has successfully welded aluminum with both 4-kilowatt and 6-kilowatt fiber-coupled diode lasers. Typical weld speeds are 3 to 4 meters per minute. One application was to weld structural reinforcements under truck beds. This was achieved with a low-brightness diode laser guided by a 1-micron fiber. It yielded an edge-lap weld with good strength yet little or nothing showing on the back side.

Beyond Car Bodies

Hybrid and electric vehicles require a lot of lithium-ion batteries. The battery cans are mostly made from aluminum and require a hermetic seal where the cap is attached. The weld process cannot damage any of the components inside the can.

Conduction welding with a diode laser solves the problem. For example, a diode laser was used to weld a housing made from 3000 series aluminum 1 millimeter thick. The laser enabled the manufacturer to tightly control weld depth to between 0.3 and 0.6 millimeter. Better still, the metal did not need to be cleaned prior to welding. The beam spot was 0.6 millimeter in diameter, and the weld speed was 4.5 meters per minute.

Diode lasers can even be used for micro-welding thin stainless steel foils. In the electronics industry, parts such as pressure sensors with diaphragms are made by welding thin metal sheets together. A 2-kilowatt fiber-coupled diode laser can weld metal sheets ranging in thickness from 150 microns to 3 millimeters.

Without an optical fiber, a 500-watt diode laser can weld foils less than 100 microns thick. (Without the fiber, the shape of the beam is elliptical rather than circular, making for a narrower weld.)

Besides metal welding, diode lasers can be used for a host of other joining applications. For example, diode lasers can be used for noncontact selective soldering of sensors and other heat-sensitive electronic components. The advantages of laser soldering are limited heat load and a highly defined, localized energy input. In the photovoltaic industry, diode lasers solder the conductive ribbons connecting individual solar cells in a panel.

Similarly, diode lasers can be used for brazing. Compared with spot welding, laser brazing produces a more aesthetic joint. At the Volkswagen assembly plant in Wolfsburg, Germany, a 6-kilowatt diode laser and a silicon bronze filler metal are used to braze the zinc-coated steel roof of the Touran minivan. The joint is brazed at a rate 4.4 meters per minute.

Diode lasers can also weld thermoplastics, as long as at least one of the parts is transparent to the laser.