Fraunhofer ILT refines laser welding of martensitic chromium steels

14 Oct 2020

Alloys are suited to electric vehicle battery cases; development is part of German AiF research project FAAM.

Dissimilar welded joint between martensitic Cr steel (top) and high-Mn steel.

Martensitic chromium steels are one of the alloy grades suited to automotive applications since they are both lightweight and corrosion resistant. These materials are now in demand for the design of collision-safe battery boxes for electric cars, for example.

For this reason, the Fraunhofer Institute for Laser Technology (ILT) in Aachen, Germany, uses these sophisticated components as demonstrators for laser welding and heat treatment process development.

As part of the AiF research project FAAM, supported by FOSTA (Forschungsvereinigung Stahlanwendungen e.V), industry experts have been investigating these metals.

The latest online conference in summer 2020 focused on new lightweight construction solutions, joining technology and end face seams, among other things, for martensitic chromium steels. The ILT investigated how suitable it is to weld a press-hardened chromium steel with martensitic microstructure X46Cr13 (1.4034) in similar and dissimilar joints for assembly applications.

Martin Dahmen, in the Macro Joining and Cutting Group at Fraunhofer ILT explained, “The main focus was on the mixing of the different materials, on the metallurgy and the resulting property profiles.”

Heat treatment

Joining quality can be improved by heat treatment. For this purpose, linear seams of a 1.4034 joint of the same type were heat-treated in the lap joint from 300 to 700 °C outside the process (ex-situ); the seams had to prove their quality in the subsequent shear tensile test.

“At 400 to 500 °C, the highest strengths and lowest hardnesses were obtained,” said Dahmen. “What is remarkable is the high proportion of ductile failure on the fracture surface by 400 °C.” The researchers targeted achieving short holding times in order to use laser radiation for heat treatment.

So how do the results compare with dissimilar compounds? Since the tempering behaviors differ, the results vary. Investigations on the combination 1.4034 with dual- phase and fine-grained structural steel showed that a tempering temperature of 400 °C is also best, the researchers conclude. ILT states, “The situation is different with other materials: Care should be taken with press-hardened manganese-boron steels as they lose their strength already at 300 °C, whereas this temperature does not affect the 1.4034 to a large extent.”

In a downstream process with a diode laser, the iILT has also demonstrated an effective way to temper the weld zone. The measured hardness values showed that temperatures of up to 650 °C can be reached. This corresponds to the maximum temperature at which the material can be tempered without loss of strength.

Laser heat treatment allows selective heat treatment at the lap joint so that only the critical material is processed. The optical properties of the surface can be used specifically for heat treatment. Dahmen said, ”The increased degree of absorption of the weld seam leads to tempering of the weld and the fusion line, while the heat- affected zone experiences less heat input. With an adapted intensity distribution, a significant increase in efficiency is possible.“

Test pieces: ILT uses lightweight steel alloy e-vehicle battery boxes.

Battery boxes demonstrate welding quality

The tests showed that work-hardened austenite and cold-rolled fine-grained structural steel cannot be heat treated by laser. At 400 °C laser tempering worked for the material combination 1.4034/dual phase steel DP980. The ILT says it intends to use the results to further develop laser-based processes in a future project. These findings should serve as the basis for calculating and designing a battery box with a crash frame fitted to it.

The module carrier consists of a material mix of ultra-high strength and supraductile steels. In the case of the crash frame made of non-work-hardened high-manganese steel, the ILT uses the high specific energy absorption to absorb an impact. The high specific energy absorption is due to the formation of twins. Thanks to this combination, the unladen weight of around 70 kg is significantly less than conventional steel battery boxes, which weigh up to 150 kg in an integral design.