A new welding technique for advanced high-strength (AHS) steel used in automotive and aerospace applications has been developed by a researcher at the Brazilian Air Force Command’s Institute for Advanced Studies (IEAv).
The new method, published in the Welding Journal, addresses problems faced previously when welding AHS steels, where doing so would cause them to become brittle and eventually break when later submitted to the hot stamping and forming featured in many manufacturing processes.
The presence of bainite (indicated with B) in AHS steels is shown to produce tough and reliable weld joints. (Image: Lima et al.)
AHS steels are characterised by improved formability and collision resilience compared to conventional steel grades, making them ideal for use in critical safety locations in car body structures to absorb energy from impacts. Their tendency to become brittle when welded however hinders their application in industry, which is currently high in demand because of increasingly tough passenger safety, vehicle performance and fuel economy requirements.
‘This problem makes it impossible to use AHS steel not only in the automotive industry but also in other industries such as aerospace,’ said IEAv researcher Milton Sergio Fernandes de Lima, who sought to address the issue by developing a new innovative method of high-temperature laser welding for AHS steel that would be appropriate for aerospace applications.
Lima's technique consists of heating sheets of 22MnB5 steel – the most promising AHS grade for hot stamping and forming – to approximately 450°C ten minutes before laser welding in order to equalise the temperatures involved. The sheets are then kept at a high temperature for another ten minutes after welding to produce a bainitic structure.
Metallurgists have discovered that bainite, a microconstituent 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 of the sheets welded at this high temperature showed that they 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 were then induced to demonstrate the improved resilience in the sheets containing bainite.
‘We succeeded in producing tough weldments directly in the bainitic band without any need for additional heat treatment,’ said Lima, who explained that 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 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,’ Lima remarked. ‘Hence the need for very high levels of reliability.’
Although the studies of the new technique are in the early stages, bainitic steel is expected to be an excellent material for shielding and armouring because of its high capacity to absorb mechanical energy.
‘Many materials developed by the aerospace industry have never flown because they fail to meet the industry's necessarily high-reliability requirements,’ said Lima. ‘But byproducts of these materials may have applications and be easily introduced in other areas, such as the automotive industry.’
Lima is currently engaged in a project supported by Sao Paulo Research Foundation to prove the feasibility of his technique in Brazil for use in welding maraging steel, an essential ingredient in Brazilian rocket and missile engines.