Automotive

New Study Finds Limits of Spot Welding of Boron Steel

Research provided by: Institut Laue-Langevin

Automotive companies may soon need to rethink the use of spot welding on their factory floors.

A recent study conducted by researchers at Institut Laue–Langevin (ILL), WMG at the University of Warwick, Tata Steel, and the Engineering and Physical Science Research Council (EPSRC) has found evidence that spot welding on boron steel may be more damaging than other welding processes.

The study investigated correlations between boron steel hardness and residual stress. The boron sheets were provided by Tata Steel, while researchers at WMG carried out the experiments on the ILL’s SALSA (Stress Analyser for Large-Scale Applications) beamline. The researchers used neutron diffraction to measure residual steet because it has the ability to penetrate heavy materials such as boron steel.

Dr Thilo Pirling, ILL scientist leading the SALSA team, says: “The SALSA beamline is a well-suited instrument for this study, as it specialises in determining residual stresses in a broad range of engineering materials, including steels. It also allows larger structures to be placed within the beamline. In this case, the non-destructive nature of the technique allowed the correlation of interest to be analysed effectively, as hardness profiles could be determined on the same weld following the tests for residual stress.”

The results showed a strong correlation between reduced hardness in heat-affected zones of boron steel spot welds and increased residual stress. Researchers suggest that a new welding method needs to be developed to prevent the same damaging impact as spot welding, particularly because nothing can be done to avoid the reaction of reduced hardness when spot welding is used on boron steel.

Several industries utilize press-hardened boron steel because of it’s high-strength and weight-saving profile. The automotive industry typically uses boron steel for structural components and anti-intrusion systems. At the same time, auto manufacturers commonly used resistance spot welding, with several thousand welds being made on a single car. Spot welding exposes the sheet directly underneath to very high temperatures, causing the metal to exceed melting temperature and then rapidly solidify upon cooling.

The study shows a new method of welding boron steel may need to be utilized in the future.

“Our future work will look into two methods that can evade this issue: magnetic pulse welding, which does not use heat and as such does not cause a heat-affected zone, and post-welding heat treatment, which reverses the reduction in hardness caused by spot welding,” said Dr Neill Raath, Research Fellow at WMG, and principal researcher of this study. “This will be of particular importance to industries that use boron steel, namely the automotive and farming industries, as well as materials developers who can use the data for modelling and destructive simulations in their own work.”

Dr Thilo Pirling, ILL scientist leading the SALSA team, says: “The SALSA beamline is a well-suited instrument for this study, as it specialises in determining residual stresses in a broad range of engineering materials, including steels. It also allows larger structures to be placed within the beamline. In this case, the non-destructive nature of the technique allowed the correlation of interest to be analysed effectively, as hardness profiles could be determined on the same weld following the tests for residual stress.”

With more research, it’s possible the automotive industry can develop alternative welding methods that can lengthen the lifetime of boron steel and bring stronger, safer, and lighter vehicles to the public.

 

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