Commonly used in passenger cars, research is being carried out to make the laser processing of ultra-high strength steel sheets more economical. By Dr Andreas Weisheit and Dr Dirk Petring, Fraunhofer Institute for Laser Technology
Sheets made of ultra-high strength steel are commonly used for B-pillars or side door sills of passenger cars, where an extremely high absorption of kinetic energy is a key issue in the event of an impact.
However, the high strength of these materials makes it necessary to adjust the processing methods, such as cutting and joining. Conventional mechanical cutting and joining processes, such as stamping, clinching or riveting, cannot be used at a reasonable expense.
Using The Light
As an alternative, the tool “light” has proven its value as a processing method in industrial applications. Laser cutting has already proven itself many times as a separation process, while joining is essentially achieved by spot welding.
Both methods, however, lead to metallurgical effects that can adversely affect the component properties. During cutting, hardening occurs in the edge zone of the cut edge; in the case of spot welding, a heat-affected zone forms with a distinct hardening reduction around the welding spot.
These two effects lead, in the worst case, to the failure of the joint and thus, in the case of damage, to a reduced absorption of the kinetic energy in the component.
By means of local laser heat treatment in the cutting edge region and in the joining zones, this weakening can be counteracted. The hardness and ductility of the material can thus be adjusted locally to tensile elongations of 10 to 15 percent.
Welding TWIP Steel
Joining ultra-high strength materials requires a low energy per unit length. This can be achieved with laser welding at 20 to 80kJ/m. For thin plates of chromium steels with martensitic microstructure and tempering steels, suitable fusion welding parameters have been developed at Fraunhofer Institute for Laser Technology (ILT).
Twinning-induced plasticity steels containing high content of manganese have also been successfully joined by laser welding in joints with the same as well as with dissimilar materials. A loss of the strength was countered here by a suitable load control.
Laser Combi-Heads For The Processing Of Car Body Components
Last but not least, it makes sense to combine individual process steps in one tool for economic reasons. This is the starting point of the collaborative project called Klasse.
Sponsored by the German Federal Ministry of Education and Research, the project is being carried out by Fraunhofer ILT along with partners from the automotive industry and laser plant construction.
The work in Klasse has resulted in a combined processing head with a hybrid laser beam source consisting of a diode laser for heat treatment and a fibre laser for cutting. For several years now, a laser processing head developed by Fraunhofer ILT and marketed by Laserfact GmbH has been successfully used in the industry, combining different work steps.
The head incorporates the two process steps of cutting and welding into one machine without a need for retooling.
In the spring of 2016, a powder nozzle was integrated into the combi-head, enabling it to carry out additive manufacturing steps by means of laser metal deposition. Thanks to this integration, set-up times can be minimised and highly flexible production made even more efficient.
In addition, the combi-head can also be used to cut composite materials (such as carbon-fibre) and reinforced components, which are becoming increasingly popular in car body manufacturing due to their low weight and excellent mechanical properties.
Local Softening Of Cold-Rolled Steels
After cold-rolling, cold-strengthened steels are generally subjected to overall recrystallisation annealing. In the cold-strengthened state, the thickness of the steel sheets can be reduced, but the material can only be deformed to a lesser degree, and its ability to absorb energy (in the event of a crash) is reduced significantly.
A local heat treatment can improve the crash properties of the components (such as the controlled folding of a crash box) by introducing soft zones into it. In sheet-metal plates made of low-alloy steels, a combination of cold rolling and local laser heat treatment prevents cracks from occurring in the subsequent forming process at particularly stressed areas, typically at the edges of shaped components.
The project, called “Local Laser Heat Treatment of Cold Strengthened Steels for Improvement of Forming and Functional Characteristics” is being funded by the German Federation of Industrial Research Associations and carried out at Fraunhofer ILT and the Metal Forming Institute at RWTH Aachen University. Based on parameter studies, the institutes are first determining correlations between the temperature-time profiles and the resulting properties and mechanical properties.
Subsequently, deformation tests and crash tests will be carried out. The latter will be validated in forming tests under quasi-static as well as under sudden loads.
In future development steps, the institutes will investigate corrosion properties, simulate the deformation in production and operation as well as arrange the process into the production process optimally.
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