Advancing high-strength steel

Daolun Chen (centre) leads the AUTO21 laser welding and duraibility of high strength steels project. PHOTO: RYERSON UNIVERSITY

Demand for safer and more fuel-efficient transportation has automotive manufacturers searching for alternative materials that increase vehicle integrity and decrease weight. The answer may lie in durable and lighter tailor-welded blank components made of advanced high-strength steel (AHSS). This will lead to reduced fuel consumption (6% to 8% for every 10% reduction in vehicle weight) and carbon emissions without risking the integrity of the ductility, formability and crashworthiness of the vehicle.

Several types of AHSS developed through research funded by the US departments of energy, defense and the Canadian government are high-strength with good formability, but there are welding issues. Pieces are heated to different peak temperatures across the weld and then quickly cooled to an ambient temperature, which causes a change in the internal microstructure of the steel. This deteriorates weld performance.

AHSS used for automotive applications is coated with zinc for corrosion resistance, but this complicates lap welds. Zinc melts and gets trapped inside the molten steel, solidifying at the grain boundaries of the fusion zone, causing embrittlement. And it evaporates during the butt-welding process, increasing the vapour pressure in the fusion zone. This creates defects such as concavity and porosity, which affect fatigue resistance and durability.

High-strength steel’s fusion zone is harder than the base material, which reduces the weld’s malleability. Another issue is softening, which occurs when carbon atoms react with iron atoms, causing the accumulation of strains in the area. This contributes to the weld’s premature failure.

Welding defects also depend on the types of AHSS used. For example, softening occurs in DP980 dual-phase steel, but isn’t present in high-strength low-alloy steel. Both types show superior tensile strength, but fatigue strength is higher in the DP980 steel.

Advanced technique
Softening is minimized with higher welding speeds and smaller laser-beam sizes, which form narrower heat-affected and fusion zones. However, AUTO21 researchers discovered too high a speed causes defects in the weld, or incomplete penetration. They did find a way to work through these complications using fibre laser welding, the industry’s most advanced technique. Optimizing the welding speed and laser power has achieved defect-free, industrially acceptable welds.

The research in collaboration with the Waterloo, Manitoba and Ryerson universities continues to investigate reducing heat-affected zone softening. The focus has shifted to fibre laser welded materials and their response to monotonic and fatigue loading, which will help auto-body parts manufacturers design lighter, stronger parts.

Achieving repeatability for consistently strong, durable welds will improve the safety and fuel efficiency of vehicles, but it will also make Canadian automotive companies more globally competitive.

Daolun Chen is a professor at Ryerson University and an AUTO21 project leader. AUTO 21 is a national research initiative supported by the Government of Canada through the Networks of Centres of Excellence Secretariat.

Comments? E-mail jterrett@plant.ca.