Neutron beam cast on welds

Materials World magazine
,
1 Sep 2017

A new study recommends that improvements and alternatives to spot welding should be developed for the automotive industry. Simon Frost reports.

Press-hardened boron steels have become a staple of the automotive industry in recent years, their high strength-to-weight ratio allowing manufacturers to produce lighter cars without compromising on safety. A new study by researchers at the Warwick Manufacturing Group (WMG), University of Warwick, UK and Institut Laue-Langevin (ILL), France, sheds new light on the correlation between spot welding and residual stress in these important steels, suggesting that alternative welding methods and post-treatments should be developed to increase their safe service life.  

‘Boron steels have excellent shape tolerance and can be joined by existing processes in the assembly line, which means, aside from the hot forming operation, vehicle assembly using these materials is not significantly more complex, time-consuming or expensive than employing conventional low-carbon steels,’ Sullivan Smith, Automotive Programme Manager at TWI, UK, told Materials World. ‘Resistance spot welding is the joining technology of choice for all steel vehicle manufacturers due to its high speed, reliability, low cost and easy, flexible robotic automation.’ 

While it is known that applying thermal energy or mechanical forces to a structure will result in residual stresses, the study, led by WMG research fellow Dr Neil Raath, is the first to experimentally prove a direct dependence of lower durability and induced residual stresses in heat-affected zones (HAZ) during spot welding of boron steels. ‘The study demonstrates how processing parameters, which determine the thermal and mechanical history of a microstructure, also determine the level and orientation of residual stresses,’ Smith said in response to the paper. 

The data was collected at ILL’s SALSA neutron-scattering beamline, using boron steel sheets supplied by Tata Steel. ILL scientist Dr Thilo Pirling, who leads the SALSA team, explained, ‘The SALSA beamline […] specialises in determining residual stresses in a broad range of engineering materials, including steels […] 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 neutron diffraction tests for residual stress.’ The neutron diffraction beam can penetrate heavy materials at a finer resolution than other non-destructive methods such as electron or X-ray diffraction.

Three weld configurations – adaptively welded boron steel, fixed-schedule and fixed-schedule DP600 – were examined, with compressive residual stresses evident in the HAZ of all three. In the DP600 welds, a flat residual stress was evident across the nugget, with a smooth decrease into the base material, while the fixed-schedule boron steel showed peaks on the HAZ periphery and a decrease in residual stress at the weld centre. This extreme variation of properties, the paper claims, could make a significant contribution to the weld’s overall loading response. 

Asked about the study’s recommendation that post-welding heat treatments and alternative welding methods such as magnetic pulse welding (MPW) should be developed to lengthen the life of boron steels, Smith noted that magnetic pulse welding has not been proven capable of joining multi-component structures as complex as those in a car. ‘The only way an alternative process would be used to replace resistance spot welding would be if it offered a significant performance advantage and/or improved production economics, and MPW is yet to offer this for these kinds of applications,’ he said.  

Post-weld heat treatment, he noted, is already commonly employed in the automotive industry within the spot process itself via a second, lower-intensity pulse that heats the cooled weld to temper the martensite, without the need for a secondary post-weld heat treatment process. 

This process, he explains, is evident in the WMG study. ‘The “adaptive” resistance spot welding process used in this study clearly already contained some element of post-welding heat treatment,’ Smith said. ‘Where the hard centre part of the weld contains its own tempered zone and a smaller internal hard zone, this feature is created after a first weld has been formed and cooled, then a secondary, lower current (or shorter time) pulse is applied that heats a smaller area up to a temperature where austenite is again formed.’ This centre austenite area cools to form martensite, and the surrounding area of martensite (from the original spot weld) is tempered and softens. 

To Smith, the pertinent point is the need to determine whether the magnitude and direction of the residual stresses has a measurable influence on the properties of joints within the final structure of a car body – and determine how those stresses could be controlled if so.

To read the study, Effect of Weld Schedule on the Residual Stress Distribution of Boron Steel Spot Welds, visit bit.ly/2fyIhkN