New Laser Welding Technique to Further Enhance Advanced High-Strength Steels

Pre- and post-heating for high-temperature laser welding produces bainite microstructures.

Bainitic zone in welding of DQSK (draw-quality semi-killed) steel (mild steel). Transmission electron microscopy image. (Image courtesy of Wikipedia.)

Bainitic zone in welding of DQSK (draw-quality semi-killed) steel (mild steel). Transmission electron microscopy image. (Image courtesy of Wikipedia.)

The automotive industry is witnessing a demand in higher standards for passenger safety, vehicle performance and fuel economy. The problem: Advanced high-strength (AHS) steels, commonly used in car body structures to absorb impacts, become brittle and sometimes break due to welding and hot stamping processes.

However, a solution is in sight and it seems to be swooping down from the aerospace industry.

Milton Sergio Fernandes de Lima, researcher at the IEAv (Brazilian Air Force Command’s Institute for Advanced Studies), has developed a new technique using high-temperature laser welding for AHS steels in aerospace application.

Laser welding is already used by aircraft manufacturers to enhance weldment reliability in structures for aircraft, rockets and missiles, for example. Likewise, the automotive industry already uses the technology to join steel blanks and stamped structural body parts. However, the crucial element to Lima’s new technique is in how the metals react and change under high temperatures.

Lima heats sheets of AHS-grade 22MnB5 steel to approximately 842°F (450 °C) 10 minutes before laser welding to equalize the temperatures. The sheets are kept at high temperatures for another 10 minutes after welding to produce a bainitic microstructure.

Bainite is a plate-like microconstituent that forms in steel under 482°F to 1,022°F (250°C to 550 °C). Bainite has been determined to be highly effective in producing reliable weld joints, displaying high yield and tensile strengths.

Lima’s analysis and stress tests showed that sheets welded at these high temperatures, containing bainite, were stronger than sheets welded at room temperature.

“We succeeded in producing tough weldments directly in the bainitic band without any need for additional heat treatment,” Lima said.

Lima’s studies are still in the early stages, but bainitic steel is expected to shine in shielding and armoring applications in the aerospace industry because of its high capacity to absorb mechanical energy. Lima anticipates the discovery to have far-reaching effects in the automotive industry as well.

“Many materials developed by the aerospace industry have never flown because they fail to meet the industry’s necessarily high reliability requirements,” Lima said. “But byproducts of these materials may have applications and may be easily introduced in other areas, such as the automotive industry.”

Lima is currently attempting to prove the feasibility of the technique in Brazil and use it for laser welding of maraging steel.

To learn more, view the results of Lima’s research published in the Welding Journal. To learn more, visit the São Paulo Research Foundation website.