Joining ultra-high-strength reliably with laser technology

Incredibly light and holds up in collisions, but often impossible to weld: this characterizes ultra-high-strength chromium steels that, thanks to their high carbon content, could not be reliably bonded together by laser until now.

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AACHEN, GERMANY - Incredibly light and holds up in collisions, but often impossible to weld: this characterizes ultra-high-strength chromium steels that, thanks to their high carbon content, could not be reliably bonded together by laser until now. The Fraunhofer Institute for Laser Technology (ILT) uses a B-pillar to show how laser welding can be reliably used on press-hardened, martensitic chromium steels.

Within the scope of the research project SECOMAL, ILT has determined process parameters and process windows for the laser welding of three ultra-high-strength chromium steels—pure ferritic, ferritic-martensitic, and pure martensitic, with carbon content ranging from 0.02 to 0.46 mass percent. Hardened, they achieve a tensile strength of up to 2GPa with fracture strain of 10 percent. Their inherent resistance to corrosion makes these steels ideal for vehicle manufacturing.

The materials with the lowest and the highest carbon content—ferritic and martensitic chromium steels, respectively—can now be easily joined, even when the materials are hardened. "Only the martensitic stainless grade 1.4021, with an average carbon content of 0.21 percent, poses dfficulties," says Dipl.-Ing. Martin Dahmen, a Fraunhofer ILT researcher.

According to the textbook, preheating martensitic steel is recommended before joining and then tempering it—that is, heating the welding zone locally—to improve the toughness in the heat-affected zone. Hardened sheets can be tempered up to 450°C without causing any loss of quality. In principle, all types of laser beams are suitable for welding materials, but since the laser should produce parallel seam edges, Dahmen recommends using only so-called brilliant beam sources and CO2 lasers.

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FIGURE 1. The microstructure of a weld in 1.4034 steel. (Courtesy: Fraunhofer ILT, Aachen, Germany)

But how does the laser compare to metal active gas (MAG) welding? "With judicious heat treatment, the hardened chromium steel can be joined without difficulty, with the exception of 1.4021," says Dahmen (FIGURE 1). "On the other hand, MAG welding is problematic because of the resulting high-energy input in the joining areas, even with appropriate heat treatment."

Fraunhofer ILT has showcased what successful laser welding looks like in practice on a test specimen of a B-pillar of ultra-high-strength steel welded to a vehicle rocker panel (FIGURE 2). "This proves that welding ultra-high-strength materials by laser offers a viable alternative to manganese boron steels," says Dahmen.

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FIGURE 2. At EuroBLECH 2014, a test specimen for B-pillars demonstrated that laser welding can be used reliably on press-hardened, martensitic chromium steels. (Courtesy: Fraunhofer ILT, Aachen, Germany)

The focus of the SECOMAL joint research project is investigating how laser and MAG welding can be used for fusion welding ultra-high-strength stainless steels with a martensitic structure. SECOMAL is a collaborative research project of the Fraunhofer ILT and the Paderborn University Laboratory of Materials and Joining Technology (LWF), the Fraunhofer Institute for Structural Durability and System Reliability (LBF), and the steelmaker Outokumpu Nirosta. It is funded by the German Federal Ministry for Economic Affairs and Energy (BMWi), the German Federation of Industrial Research Associations Otto von Guericke eV (AiF), and the Research Association for Steel Application (FOSTA).

For more information, please contact Dahmen by e-mailing martin.dahmen@ilt.fraunhofer.de.

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