The unweldables

© Photo | Gernot Walter

Joining materials that have long been considered unweldable is one of the biggest challenges in lightweight engineering. The laser offers some promising solutions.

The shift toward lightweight construction is presenting industry with a growing challenge. The material mix is becoming more diverse while, at the same time, there is increasing pressure to develop mass-production joining processes that are fast, efficient, and add no extra weight.

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About the author

Seiji Katayama is the general director of the renowned Joining and Welding Research Institut at Osaka University, where he has spent most of his career, starting from the late 1970s. His work at the JWRI revolutionized the aluminum laser welding as well as metal-plastic bonds.

That is why we are seeing the emergence of a growing number of welding and soldering methods for joining lightweight materials such as aluminum, titanium, magnesium, and fiber-reinforced plastics.

Laser methods are proving to be particularly flexible here. As well as being easy to integrate into existing production setups, they also obtain good results with many material combinations that had previously been considered very awkward or impossible to weld.

Aluminum is playing an increasingly important role as one of the materials in these new welding endeavors.

Welding aluminum and steel

Steel is one of the most commonly used metals. Experiments have succeeded in welding thin (less than 2 mm), low-carbon steel sheets to group A6XXX aluminum alloys (containing magnesium and silicon) in resilient, durable lap joints produced by diode or YAG lasers.

In this process, the laser is focused for the most part on the steel, but without melting it. Heat conduction and a small amount of absorbed beam energy then melt the aluminum. This produces a thin layer of metallic iron and aluminum compounds at the interface between the aluminum and the steel. If this layer is thinner than 10 microns and if the iron-aluminum compounds with low aluminum content predominate, then the result is good fatigue characteristics and high tensile strength.

The experiments have also shown that flux and filler materials such as aluminum-silicon wire further improve the mechanical characteristics of the welds. In a variation on the process, only the steel sheet is irradiated. The aluminum is melted via heat conduction, and the weld is subsequently rolled.

Lap joint and welding

A completely different approach involves welding the lap joint through the steel or stainless steel sheet and anchoring the roots of the weld around 0.2 millimeters deep in the aluminum sheet. This method produces high resistance against shearing. On the other hand, the resistance against stripping is low, although it changes when the welded surface area increases — with three parallel welds, for example. In this case, the specimen fails in the aluminum base material.

Butt welding is also possible. Again, only the steel is irradiated and the aluminum is melted via heat conduction. The joint comes apart even at relatively low tensile loads or as a result of external shocks. An interesting approach compensates for this weakness.

Here the steel component is fitted to a flange on the aluminum component. This creates a combination of lap and butt welding, where the laser melts the steel close to the butt. While doing so, it also melts, by heat conduction, a track in the aluminum in the flange below the steel, creating a lap joint. At the same time, however, the molten steel also melts the aluminum in the butt.

This creates a double seam, which embeds the steel in the aluminum and anchors it in the aluminum base material via the roots for the lap joint, significantly improving the weld’s mechanical characteristics.

Welding aluminum and titanium

Aluminum and titanium alloys can be welded with lap joints. Generally, titanium is welded onto aluminum alloys. The laser beam heats up the titanium and melts the aluminum material by means of heat conduction. As with the welding of steel and aluminum, a thin layer of a titanium-aluminum compound is created on the border between the titanium and aluminum alloys.

The joint exhibits hightensile strength and the samples ultimately fail in the alumin um base material. Examination of the welds created using pure aluminum and titanium also showed that the molten aluminum erodes and thereby roughens the surface of the titanium, which contributes to the strength of the joint.

Welding Aluminum and plastics

The laser makes it possible for the first time to weld metallic assemblies and plastic parts. Although such joints are still in the experimental stage, the results are very promising. In particular, ther moplastic materials such as PET, PA, and PC form durable joints with metals.

Lap joints are used, whereby the metal is irradiated either directly or through the plastic. Then the metal melts the plastic by means of heat conduction. The plastic forms little bubbles close to the surface of the metal. These expand, exert pressure on the surrounding plastic, and press it into the irregularities on the surface of the metal.

In addition, van der Waals forces arise due to the pressure, and chemical reactions take place between the plastic and aluminum oxides. They create a chemical bond in addition to the mechanical one.

Solid-state lasers are used mainly for these welds — primarily diode lasers, but also disk and fiber lasers. Experiments were carried out for instance with three millimeter thick sheets of aluminum alloy A5052 and two-millimeter plates of amorphous PET.

The specimens were welded across the full width of the lap joint. Tensile testing revealed that when the weld surface is sufficiently large, the plastic workpiece stretches at the joint with the metal and eventually tears. This happened at a tensile load of around 3,000 newtons.

Mix for the future of light construction

None of the above examples are welded joints in the classical sense, where parts are made of similar materials and those materials mix in a melt in the seam and create a joint. With such disparate materials, this would lead to weak, brittle seams that are very susceptible to hot cracking. Instead, all the above methods build on the possibility of treating the two parts differently.

One of the parts is melted and the other merely heated, in order to transport heat and facilitate the distribution and adhesion of the pool. The only tool that permits such selective heating reliably and with reproducible results is the laser. While low-intensity diode lasers are also suited to the task, the ideal lasers for most processes are pulsed solid-state lasers, which allow extremely fine control of heat input.

Lasers achieve joints that welders had long thought to be either extremely difficult or entirely impossible. It is no great leap to conclude that the continued development of lightweight construction and of miniaturization will be closely associated with further developments for these methods. Although the number of manufacturing applications is still small, we are certain that continuing research work and the needs of industry will change this in future.

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