By: Silke Pflueger
Laserline Inc., Santa Clara, CA
Why is Aluminum Welding such a Hot Topic?
New CAFE standards demanding an average fleet gas mileage of 54.5 mpg by 2025 will not only require radical engine improvements, but also drastically weight reduced cars. Using aluminum instead of steel can decrease the weight of a car body up to 50%, as shown in the Audi A8, which was 239 kg lighter than its steel predecessor when it was introduced in 1994.
Joining aluminum initially represented quite a challenge, but is now mainly solved with riveting, MIG welding, and to a large extent by laser welding, enabled by new laser technologies.
Laser welding of aluminum
Diode lasers offer a distinct advantage for aluminum joining: At 900 to 1060 nm, the wavelength of the diodes is closer to the absorption peak of aluminum than the 1030 nm of disk or 1070-1080 nm of fiber lasers. Several car manufacturers have carried out extensive development and testing programs, and now use diode lasers for structural welds as well as outer skin joints.
In general, welding 6000 series aluminum which is commonly used in car bodies requires an AlSi filler material to prevent hot cracking of the weld. The laser welding equipment is largely the same as the industrially proven components used in the brazing process, where the filler wire is added using tactile seam tracking optics.
After a multi-year development project, Audi decided to use a 4 kW diode laser with a 400 µm fiber to weld the aluminum tailgate of several vehicles, and is running this system on their production floor. The finish of the zero-gap weld on the tailgates meets the highest specifications for the optical quality of a visible joint without further post-processing (see Figure 1).
Kuka North America has successfully used both a 4 kW / 400 µm laser and a 6 kW / 1000 µm laser for aluminum welding, depending on the joint requirements. Typical weld speeds are 3 to 4 m/min. One more unusual application was to weld structural reinforcements under truck beds. This was achieved by using a low brightness diode laser with a 1 µm fiber. It yields an edge lap weld with good weld strength yet very little to no show on the backside (see Figure 2a).
A different type of edge lap welds is used in structural applications where no optical requirements exist. Here, it is important to get as much strength as possible. Figure 2b shows a structural edge lap weld using a 4 kW diode laser, 0.8 mm spot size, at a 3 m/min processing speed. Notice the much wider area of the joint and the bump on the underside of the material.
Beyond car bodies
New hybrid and electric engine technology involves large amounts of batteries. Li-Ion battery cans mostly are made from aluminum, requiring a hermetic seal where the cap is attached. The weld process cannot damage any of the components inside the can.
A heat conduction weld is the solution to this welding problem, resulting in an extremely stable process with a large process window. One example is a 1 mm thick housing out of 3000 series aluminum, with the weld depth tightly controlled between 0.3 and 0.6 mm, not requiring any pre-cleaning process if at all possible.
Using a 0.6 mm focus diameter, a 200 mm focusing lens and a weld speed of 4.5 m/min, the process is stable between 1800 and 2800 W, yielding weld depths between 0.35 and 0.55 mm, well within the process window despite the large variation in output power.
Laserline’s applications lab in Germany has performed extensive multi stitch testing which proved the diode laser weld process robust against several millimeters of changes in focal position and several degrees of variation of the weld angle. It also demonstrated the robustness of the fiber coupled diode lasers against back reflections.
Excellent process stability, reliability and cost advantages have resulted in diode lasers being utilized in many new additional aluminum welding applications, giving the weld engineers another tool for a difficult joint.