A Brazilian researcher has developed an innovative method of laser welding at high temperatures that enhances the properties of advanced high-strength (AHS) steel for applications in the automotive and aerospace industries
The automotive industry’s demand for high-performance AHS alloys has increased in recent years owing to increasingly stringent passenger safety, vehicle performance and fuel economy requirements. Characterized by improved formability and collision resilience compared to conventional steel grades, high-strength steels have been used in critical safety locations in car body structures to absorb energy from impacts.
However, some of these high-strength alloys tend to become brittle as a result of welding and may break when submitted to the hot stamping and forming required by many manufacturing processes.
This problem makes it impossible to use AHS steel not only in the automotive industry but also in other industries such as aerospace.
—Milton Sergio Fernandes de Lima, a researcher at the Brazilian Air Force Command’s Institute for Advanced Studies (IEAv)
To address this issue, de Lima has developed an innovative method of high-temperature laser welding for AHS steel appropriate for aerospace applications. The results, obtained while de Lima was a visiting fellow at the Colorado School of Mines in the United States with support from the Sao Paulo Research Foundation (FAPESP), have now been published in Welding Journal.
The technique developed by de Lima consists of the induction heating of sheets of 22MnB5 steel—the most promising AHS grade for hot stamping and forming—to approximately 450 °C ten minutes before laser welding to equalize the temperatures. (22MnB5 is used in automobile structural and safety components; the very high mechanical strength of the final part makes it possible to achieve weight savings of 30% to 50% compared to conventional cold forming grades.)
The sheets are kept at a high temperature for another ten minutes after welding to produce a bainitic structure. Metallurgists have discovered that bainite, a plate-like microstructure that forms in steel under certain conditions, is the best candidate to produce tough and reliable weld joints. In particular, it displays high values for yield and tensile strengths.
Analysis showed that sheets welded at this high temperature contained bainite and were far tougher than sheets welded at room temperature, which contained martensite, a microconstituent with lower yield and tensile strengths than bainite.
Stress tests also demonstrated the resilience of sheets welded at high temperature.
Among the specific findings were that the as-welded room temperature (AT20) welds used as a baseline presented a microstructure composed of primary austenite dendrites, which transformed to martensite during cooling, and interdendritic ferrite. The interdendritic region was richer in ferrite stabilizing elements such as Al, Si, and Cr. The FZ hardness was about 600 HV and attained a maximum value of the 700 HV in the HAZ. The tensile tests showed a negligible ductility (1.3%) and a high tensile strength (1200 MPa) for these coupons.
High-temperature welding caused the samples to exhibit microstructures of bainite plus austenite grains.
- The sample at 455 °C had around 66% of bainite in an austenite matrix. The hardness in the FZ was 320 HV and attained a maximum of 390 HV in HAZ, representing about half of the AT20 values. The tensile tests showed a maximum elongation of 2.7% and a tensile strength of 840 MPa.
- The sample at 529 °C had around 33% of bainite in an austenite matrix. The reduction in the bainite percentage compared to HT455 is due to the cooling procedure because the undercooling temperature is too low. Additionally, this sample presented some coalesced bainite. The hardness in the FZ was 310 HV and attained a maximum of 320 HV in HAZ, which was similar to the original base material. The tensile tests showed a maximum elongation of 3.5% and a tensile strength of 650 MPa.
The proposed methodology attained the initially established objective of producing tough welds directly in the bainitic range, without the need of extra heat treatments.
—de Lima et al.
According to de Lima, the technique can easily be applied in manufacturing to improve laser welding of high-strength and ultra-high-strength steel.
The automotive industry uses laser welding to join steel blanks and stamped structural body parts such as pillars, beams, rails, frames, tunnels and bars faster and more reliably than with conventional welding.
In the aerospace industry, laser welding is used by aircraft manufacturers such as Boeing and Airbus, as well as some smaller European firms, to enhance weldment reliability in structures for aircraft, rockets, missiles, satellites, re-entry vehicles, antennas, onboard systems and drones.
Laser-welded structures in this industry have to be able to withstand high temperature and external pressure. Hence the need for very high levels of reliability.
—Milton de Lima
Although the studies are in the early stages, bainitic steel is expected to be an excellent material for shielding and armoring because of its high capacity to absorb mechanical energy, he added.
Many materials developed by the aerospace industry have never flown because they fail to meet the industry’s necessarily high-reliability requirements. But byproducts of these materials may have applications and be easily introduced in other areas, such as the automotive industry.
—Milton de Lima
de Lima is currently engaged in a project, also supported by FAPESP, to prove the feasibility of his technique in Brazil and use it for laser welding of maraging steel, an essential ingredient in Brazilian rocket and missile engines.