SÃO PAULO, Brazil, Jan. 2, 2018 — A technique for high-temperature laser welding has been developed for use with advanced high-strength (AHS) steel alloys. High-strength steels exhibit improved formability and collision resilience compared to conventional steel grades. However, some of these high-strength alloys become brittle as a result of welding and could break when submitted to the hot stamping and forming required by many manufacturing processes.
The technique, developed by researchers at the Brazilian Air Force Command’s Institute for Advanced Studies (IEAv), combines laser welding and induction heating to produce 22MnB5 steel welds in the bainitic range.
To develop the technique, researchers heated sheets of 22MnB5 hot pressed, AHS steel sheets to approximately 450 degrees Celsius ten minutes before laser welding to ensure an isothermal condition. The sheets were kept at a high temperature for another ten minutes after the welding to allow sufficient time to produce a bainitic structure.
To test the technique, researchers welded a steel part both in ambient and high temperature conditions. Tests showed that the sheets welded at high temperature contained bainite, and were far tougher than sheets welded at room temperature. Sheets welded at room temperature contained martensite, a micostructure with lower yield and tensile strength than bainite. Stress tests also demonstrated the resilience of sheets welded at high temperature.
“We succeeded in producing tough weldments directly in the bainitic band without any need for additional heat treatment,” said researcher Milton Sergio Fernandes de Lima.
Lima believes that induction-assisted laser welding is a technique that could be applied to automotive and aerospace manufacturing to improve laser welding of high-strength and ultra-high-strength steel.
“Laser-welded structures in this industry have to be able to withstand high temperature and external pressure,” he said. “Hence the need for very high levels of reliability.”
Although the study is in the early stage, the proposed technique attained the initial objective of producing tough welds directly in the bainitic range, without the need for extra heat treatments. The researchers believe that bainitic steel, because of its high capacity to absorb mechanical energy, could be a useful material for shielding and armoring.
“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,” said Lima.
The researchers’ current goal is to demonstrate the feasibility of their technique for the laser welding of maraging steel, which is used in Brazilian rocket and missile engines.
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