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Automotive Welding

Automotive Welding

After more than two decades since its introduction, advanced high-strength steels (AHSS) are making major inroads into the automotive manufacturing. The sluggish acceptance of advanced high-strength steel came from the auto industry’s reluctance to making changes, some industry experts said. However, rising fuel prices, environmental issues, and increased safety concerns combined to push vehicle makers into developing lighter weight cars. At first, the automakers turned consideration to such lower-density materials as plastics and composites, aluminum and magnesium, but, in 1996 the steel industry saw the potential for its automotive market to deteriorate, and responded. “The innovation of advanced high-strength steel allows meeting of these goals (having a vehicle with reduced weight and increased safety) in an economically feasible manner with stronger, stiffer materials,”

Dr. Roger Heimbuch, director of the Auto/Steel Partnership (www.autosteel.org), said. The Auto/Steel Partnership is an organization that is dedicated to developing and using advanced steels on cars and trucks. Carl Occhialini, automotive segment coordinator for Lincoln Electric Co. (www.lincolnelectric. com), said while aluminum has taken some leaps and bounds over the last five years in use as an automotive material, it has peaked. Aluminum is lighter in weight than steel, but it also has tradeoffs in performance. One of the tradeoffs is in the way that it welds, Occhialini said. “It has a great strength-to-weight ratio, but its heat affected zone softens when welded, and may not come back to its full strength,” Occhialini said. With advanced high-strength steel, automakers can push a vehicle’s rating up a grade, when they are trying to reach the pinnacle of a 5-star crash rating.

More and more vehicle components are moving to advanced high-strength steel, including intrusion side-beams, rocker panels and B-Pillars. In addition, dual phase, advanced high-strength steel is seen as a material that is allowing vehicle manufacturers to make quantum leaps forward in terms of strength while using thinner materials that have greater rigidity. Occhialini said to get even more rigidity, manufacturers are moving from traditional “C-shaped” beams, which are open on one side, to a closed beam. Closed beams are made possible by advanced high-strength steel. Closed beam, made with the thinner materials, are providing the automakers greater rigidity, but they require different welding techniques, Occhialini said. “When they do that, it prohibits the use of resistance spot welds. There are a lot of places that can’t be reached with a resistance spot welder, so the use of the fusion (arc welding) process is continuing to rise,” Occhialini said. In addition, advanced high-strength steel presents challenges to welding processes, Occhialini said. Lincoln Electric has is conducting research to develop recommendations on welding advanced high-strength steel, he said. “There’s a great deal of learning to be done by the welding community when it comes to advanced high-strength steel. For the first time in the history of welding, we’re running into steels that are stronger than the traditional strength weld levels which makes this a real paradigm shift. “The question is: Do you want to use higher strength welding consumables? Generally, no. However, by applying good welding principles, the use of very special consumables can be avoided,” Occhialini said.

Roger Heimbuch, director of the Auto/Steel Partnership, said in a presentation at a workshop on the uses of advanced high-strength steel in the auto industry that recent advances in advanced high-strength steel offer the industry the potential of having materials with previously impossible combinations of properties and cost advantages. “That we’re able to increase steel’s properties simultaneously with advanced high-strength steel is a technological paradigm shift,” Heimbuch said. Heimbuch noted the auto industry’s move over the past two decades to metal substitutes such as polymers and composites in its effort to reduce the weight of vehicles, while maintaining vehicle safety and performance, and he noted the inroads that aluminum and magnesium made in vehicle structures. “Advanced high-strength steel allows meeting these goals in an economically feasible manner with stronger, stiffer materials, offering both performance and ductility,” Heimbuch said. As Occhialini noted, the ability to weld advanced high-strength steels is an issue, and it becomes more critical for higher megapascal (MPa) steels, those in the 980-MPa and Dual Phase (DP) 1000-MPa ranges. “There are several grades of advanced high-strength steel from very weldable to less weldable and the complexity of the steel dictates how much trouble you would have welding it,” Dr. Jerry Gould, technology leader for resistance and solid state processes at the Edison Welding Institute (www.EWI.org), said. “The leaner compositions, those below a 600-MPa grade, are reasonably weldable. Steels beyond that range – in the 780-MPa, 980-MPa and the DP 1000-MPa ranges, are progressively more challenging to weld. The biggest issue is a combination of hardenability and the hardness of the steel itself,” Gould said. The carbon content of advanced highstrength steel in the 980 and higher range grades is a minimum of 0.15 percent. Steel’s hardness is related to carbon content. Increased amounts of carbon also can cause solidification defects that can lead to porosity and the tendency to interfacial failure. Gould said the hardness of steel is a big concern in welding advanced high-strength steel. “The harder the steel weld metal the less the inherent ductility and the more prone these joints are to interfacial fracture. The higher the strength of the base metal the more problems we tend to have,” Gould said.

With resistance spot welding, there also is a concern with the strength of the base metal, weld size and joint geometry using advanced high-strength steel, and how stresses are transferred to the weld. “Smaller weld sizes and higher parent material strengths means higher stress concentrations at the weld. This can be another source of interfacial failure in these joints. Further, with advanced high-strength steel these higher stress levels can interact with both harder weld microstructures and any residual porosity, to result in interfacial fractures. There is also the problem of residual bond line defects – porosity in the weld means cracking will occur pore-to-pore,” Gould said. A report from the Auto/Steel Partnership, An Investigation of Resistance Welding Performance of Advanced High Strength Steels, said that interfacial fracture modes also cause problems in meeting existing weldability qualification standards, and may prevent a material from passing automotive welding qualifications standards, so it is essential to know which factors promote such fractures and how to avoid or minimize them. Automotive manufacturers need to know that resistance welds for advanced high-strength steel meet their standards. “The issue facing the producers using resistance welding is to determine if interfacial fracture concerns with lower grades of steel are applicable for advanced highstrength steel materials,” the report said. The result of the Auto/Steel’s investigation validated and confirmed advanced high-strength steel resistance weld effectiveness by applying welding processes developed in the tes t to an advanced high-strength steel lightweight front structure. “This confirmation test demonstrated that the advanced high-strength steel resistance welded assembly performed to engineering requirements with no remarkable weld failures,” the project conclusion said.

So, where is the automotive industry heading with advanced high-strength steel? “Most people are confident they can weld 600 megapascals of steel, and they’re growing comfortable at the 780 grades,” Gould said. “Above that, there’s a difference of opinion on weldability of these products. In terms of backing away from trying to weld these materials, while we’re using a wider variety of welding processes today to address the issues, for the most part the automotive industry is still interested in resistance spot welding,” he added. Occhialini agreed. “Aluminum is becoming more widely used for vehicle structures, especially in the European market, but I tend to see the growth of advanced high-strength steel coming on strong. “It’s more user friendly for arc welding, and steel is something that we’re more used to working with versus aluminum. We’re dealing with the issues, and use of advanced high-strength steel is growing – it’s here now,” Occhialini said.