The Evolution of Steel in the Automotive Industry

The resistance spot welding (RSW) process has played an outsized role in automotive welding for over 100 years. Despite the many advances made with other joining processes (including mechanical fasteners) in this competitive arena, it is tough to beat the flexibility, robustness, and economics of a resistance spot weld.  

The same can be said for steel, the primary material used in automotive body structures (steel.org/steel-markets/automotive). Despite the increased usage of aluminum and other materials, steel is still the material of choice for many applications.  

The last 40 years have brought incredible changes to the automotive industry. For example, my 1984 Ford Mustang GT had a carburetor on a 302 small-block V8. Now, it is common to see direct fuel injection and turbocharging on three-cylinder engines. Care to guess which one produces more horsepower, has lower emissions, and gets way better mileage?  

These changes can also be seen in how the body structures of yesterday have morphed into modern incarnations. These modern designs are stronger and lighter than their earlier brethren and provide much better crash protection. And they are still being welded with RSW, as they are mostly still steel. But how have steels changed in the last few decades? 

The modern automotive material playbook is almost breathtaking in its breadth. If we look at the automotive grades of carbon steel available, we see a range of properties that was not possible 40 years ago: ultimate tensile strength (UTS) that ranges from 200 to 2000 MPa, with corresponding elongation ranging from 50 to 5%.  

Do you need a brace to prevent intrusion into the passenger compartment? Try press-hardened steel with a UTS upward of 2000 MPa. Are you concerned you need a high UTS and a high-yield strength material for a chassis component? There is a complex phase material ready to go. This is a far cry from the days when most body components were either mild, bake-hardening, or high-strength low-alloy steel. Just like everything else, the steel we use has evolved. 

The resistance welding community has also responded with advances in weld controls, process monitoring, electrode alloys, scheduling methodologies, power supplies, etc. No one is standing still. So, where does this all come together? 

AWS can help. The AWS D8D Subcommittee on Automotive Resistance Spot Welding has been developing standards for the automotive industry for many years and can provide direction in this regard. 

How should we characterize the weldability of these different materials? The AWS D8.9M:2022, Test Methods for Evaluating the Resistance Spot Welding Behavior of Automotive Sheet Steel Materials, helps in this respect. This standard’s industry consensus evaluations can help determine the weldability of new materials and the robustness of new processes. 

Once welded, how does one determine acceptability? AWS D8.1M:2021, Specification for Automotive Weld Quality — Resistance Spot Welding of Steel, is another industry consensus standard written specifically with the new generations of steel in mind. 

Both AWS D8.1 and D8.9 are essential references within the automotive community and are integral to many quality programs. Both of these documents (and others) are under the purview of the AWS D8D Subcommittee. Our goal is to bring the best welding minds together to help the industry. While our work is neverending, it is appropriate to take time to acknowledge these efforts now and then. As chair of D8D, I am both the beneficiary of those before me and custodian of their legacy. As such, I, along with the other committee members, will continue to address the hard issues as they come to the attention of this all-volunteer group and as the industry evolves. 

This article was written by DONALD F. MAATZ JR. (laboratory manager at R&E Automated Systems) for the American Welding Society.