Lasers Meet the Challenge of Welding Aluminum

Jürgen Held and Stan Koczera

Aluminum has always been a challenge to weld, primarily because of its low melting point. But its weight advantage over materials such as steel has increasingly put the metal in the spotlight.

Forty years ago, apprentice welders had to develop their own tricks and techniques to weld aluminum conventionally. These included identifying which shielding gas to use and determining how to reduce the size of the weld path. In those days, when the aluminum absorbed the plasma, the water vaporized. The problem was always the same: If the molten material became too hot, it exploded, producing pores or even cavities.

Today, the issues related to the low melting point can be overcome by using high-beam-quality lasers. Although there are competitive welding processes, the laser advantage lies in the capability for precise beam focus and, hence, energy delivery to the workpiece. The key to successful processing, though, lies in combining the laser with a precision machining system.

Compared with other materials, aluminum is far more demanding of the laser, posing special problems for engineers. The material’s sensitivity to heat can lead to problems with heat-affected zones. One way to resolve subsequent cracking is to weld with wire, which can change the mixture of the molten material. The best wire choices combine nonaluminum metals that are easily weldable, although the selection will also depend on the type of aluminum alloy being joined.

Engineers process aluminum door modules for Audi on a laser welding system in which each of the two laser welding heads, mounted on its own independently traversing shuttle bridge, covers a working area of 3 x 1.5 m and welds two doors.

Other critical components of the welding system include precision control, as well as a system designed to hold work in position with absolute clamping consistency regardless of material tolerances. It is only with such features that manufacturers can prevent high scrap rates.

Nd:YAG or CO₂?

Using lasers to microweld aluminum probably began in the 1970s, and early applications relied primarily on Nd:YAG systems. Relatively low power, pulsed Nd:YAG systems were used mainly for butt and overlap welding. During the 1980s, applications expanded to include simple, noncritical tasks such as window frames and doors.

Large-scale applications didn’t emerge until the 1990s, driven by the automotive industry’s reaction to rising fuel costs. This major push to reduce vehicle weight focused on the use of aluminum in place of steel. Aluminum also offered such significant advantages as resistance to corrosion, formability and ease of painting.

The common belief that the Nd:YAG is the most suitable laser for this application is supported by physics, because this laser’s shorter wavelength provides a higher absorption rate. On the other hand, there is evidence that CO₂ lasers of high beam quality can do as good a job at lower cost. One aluminum manufacturer’s technical staff and some European technical institutes have acknowledged that, in keyhole welding, once the keyhole is made, neither source offers a significant advantage. The primary issue is to maintain high beam quality when piercing the material. Preliminary tests are usually advisable to determine which laser technology is the better option.

Laser advances that have contributed to efficient aluminum welding include not only higher power, but also refined beam quality. Only five or six years ago, the k-factor — which indicates the quality of mode structure — of 3-kW lasers was 0.4. Welding aluminum with those lasers was tricky and required proper shield gases. Today’s Nd:YAG or CO₂ slab or high-Q lasers have k-factors of 0.8 or 0.9, making the welding process easier.

Double-focus welding

The development of double-focus beam technology has made wider molded seams possible. Double-focus welding uses two foci as close as 0.4 mm to each other. In tailored blank welding, engineers use this strategy to overcome the disadvantage of a small gap in the joint. If a laser beam with the highest energy in the middle blows through the gap, it can melt the sides, lose energy and result in a bad weld. With a double focus, no energy is lost in the gap, and the system can concentrate 100 percent of the beam energy into the joint. The melted material flows together. Good root protection prevents the molten aluminum from dropping away. For instance, with a butt joint, solid material is placed underneath the butt.

Laser welding systems have become increasingly flexible. Engineers can weld steel and other materials with the same welding head used for processing aluminum. With some machines, switching from welding to cutting involves no more than a few minutes to exchange processing heads.

With automated laser welding of door modules, engineers can program the system to weld up to 110 complete door sets (440 doors) per shift at a rate of four minutes per set.

Automotive applications

The aluminum welding battle between Nd:YAG and CO₂ lasers is, as expected, being fought first in the automotive industry. One example of this is a large-scale welding system that Schuler Held Lasertechnik GmbH & Co. KG created for Dura Automotive Karosseriekomponenten GmbH in Herne, Germany, to process automobile door modules. Studies conducted early in the design phase indicated that CO2 lasers with high beam quality could do the job. In fact, the CO2 proved even more suitable for this aluminum alloy and application than the Nd:YAG.

Dura Automotive Karosseriekomponenten welds aluminum automotive body parts using a dual-head CO₂ laser system.

The three-axis system at Dura simultaneously fabricates a set of four doors on two mirror-image revolving tables with clamp fixtures for holding the workpieces. Each of the two welding heads, mounted on its own independently traversing shuttle bridge, covers a working area of 3 x 1.5 m and welds two doors. The system can weld up to 110 complete door sets (440 doors) per shift with a cycle time of four minutes per set, or one door per minute. The equipment design ensures constant production through a beam-switching system. If one of the laser units is not operative, the remaining unit welds the complete door set. The laser employed in the Dura equipment is a 2.5-kW CO₂ slab laser, the DC 025 from Rofin-Sinar.

Throughout the automotive industry, Audi may be the most experienced and innovative user of aluminum laser welding. The body of its Audi A2 vehicle, for example, is fabricated completely of aluminum. The company robotically welds the chassis, as well as the roof and floor of the car. The use of robots is made possible by a spring-loaded element that mechanically maintains the laser focus on the top of the surface being welded.

Future view

Aluminum processing also is gaining acceptance within the aerospace and shipbuilding industries. One reason involves the potential cost savings. Consider a single aerospace application — wing construction. Aluminum welding is perhaps the only way to reduce the time and cost involved. Eliminating hundreds of rivets will reduce weight, and joints should have equal or better integrity. However, because of the wing size of modern airliners, aluminum welding using robots would be unwieldy. Processing will most likely require specially designed and constructed laser machining systems.

The first application of laser welding to airplane construction involves joining stringers, or supports, to fuselages. This process is being tested for Airbus.

Other industries that are expected to engage in laser welding of aluminum are those concerned about component weight and corrosion, as well as whether or not welding structural elements or tailored blanks with complex profiles can reduce production costs.

Meet the authors

ürgen Held is a founder of Held Lasertechnik, a company acquired in 1999 by Schuler AG (The Schuler Group). He is now president of Schuler Held Lasertechnik, whose main facility is in Heusenstamm, Germany, near Frankfurt.

Stan Koczera is manager of US laser sales at the company’s US headquarters in Canton, Mich.