Advanced high-strength steels are moving into the fast lane of automotive applications, thanks to the growing recognition that can help engineers reduce vehicle weight and improve crash worthiness. But the extra strength comes at a price: The strength gains that make these steels so desirable also compel designers and manufacturers to consider new welding techniques.
The reason why largely boils down to the pinchforce required to hold sheets together for in spot welding. Whereas a pinch of 400 pounds will accommodate a typical low-carbon steel, a pinch of 1,400 lbs or more would be needed for the higher strength steels. Dave Anderson, staff manager of the AISI Automotive Technical Panel, says that spot welding can successfully fabricate high-strength steel parts, but the process does require higher heat, pinch pressure, frequency and hold times. Shifting these parameters allows good welds to be made but often with weld cycles that differ from those found in industry today. Lincoln Electric Co., a global supplier of metal welding and cutting systems, is just one of the companies that has recognized the need for all-new welding systems for advanced high strength steels. Veteran Lincoln engineers, like Jim Nicklas, consumable research and development leader for gas metal arc welding (GMAW) products, are part of an ongoing research project to develop next-generation welding systems. Lincoln officials meet with original equipment manufacturers, like Ford Motor Co., and various Tier 1 companies on a weekly basis to explore new approaches to welding high-strength steel.While Nicklas declined to reveal too much about Lincoln’s research efforts, he says the developments will explore improved system automation, new electrode designs, and software to control the GMAW welding process. Nicklas explains that most troublesome area to address will be the heat-affected zone of the thinner gauge, higher strength steels because these are more susceptible to loss of mechanical properties than thicker sheets. Lincoln is currently exploring GMAW techniques that reduce heat levels from the typical current level of roughly 30 kJ per inch, Nicklas says. And he adds while some new technology is needed, spot welding will remain a viable fabrication technique for the foreseeable future.Ron Krupitzer, senior director of automotive applications for the American Iron and Steel Institute (AISI), Washington, agrees with Nicklas that current high-strength steels are compatible with existing spot welding technologies. However, like Lincoln technicians, Krupitzer acknowledged the pressing need to cultivate new technologies to meet the demands of future automotive programs. Krupitzer, who also serves as the AISI staff participant in the Auto/Steel Partnership, also notes that ongoing development work at the partnership will likely help a GMAW/metal inert gas (MIG) welding find a strong niche in the production of engine cradles, chassis frames and bumper frames made from advanced high-strength steels. Together these advanced-high-strength steel applications promise up to a 25-percent reduction in chassis weight. Krupitzer said the primary challenge for GMAW/MIG in the near term will be to control heat input in order to limit the heat-affected zone of the weld as well as to demonstrate its ability to perform in a robust, automated automotive production environment.Redefining Failure
The move to advanced high strength steel may also require some changes in the philosophy of joint design. As Nicklas explains, weld strength has traditionally exceeded the strength of the base material. “Previously you would design a part around the lower strength of the steel,” he said. “The weld would never fail.” With advanced high-strength steels, though, the base-metal strength approaches or surpasses the weld metal strength, which means the weld has become the weak spot and the margin of error associated with weld quality tightens. “This means we need a near-perfect weld every time,” Nicklas says. He goes on to advise design engineers to fully investigate weldability issues associated with the new steels. “Designers may need to rethink traditional failure modes and design parts to divert stress away from welded areas,” he suggests.
While spot welding remains a viable fabrication technique for high-strength steel, new technologies are under development. In one project, the Auto/Steel Partnership is developing system automation to weld dual-phase structural steel components in a tensile strength range of 800 to 1,000 MPa. (Photo: AISI)
Even if high-strength steels does require new thinking on the part of designers, not everything will change. Joseph Defilippi, acting general manager of research for U.S. Steel Corp., offers reassurance that many of the basic rules will remain the same. If weld fatigue is the critical design criteria for design engineers, then part geometry rules, regardless of the steel grade specified for the application, Defilippi says. The U.S. Steel executive pointed out that there are extensive computer modeling tools available to help engineers optimize part and weld design of high-strength steels.Additional challenges for welders as cited by Lincoln involve dealing with the wide alloy variations in high-strength steels. Coatings, which can result in porosity problems in welds, are another concern, Nicklas says. Defilippi acknowledges there will be a manageable learning curve with regard to existing and emerging welding and bonding techniques to support high-strength steel designs. “Certainly there will be different approaches to problems that occur in the interaction between the new steel compositions and manufacturing capabilities,” Defilippi says “There is no same solution for different materials and different part designs. This is a fact of life. Yes, there are differences in the various grades and alloy blends of high-strength steels, but we can learn to weld all of them.”Research Programs Accelerate
U.S. Steel--at its regional development center in Monroeville, Pa., and its automotive applications center in Troy, Mich.--has been looking at MIG welding for high-strength steels. According to Difilippi, the company has comfortably met automotive specifications in weld fatigue, cross tension, and shear tension tests. He also revealed that the integrated steel giant is developing super-strong 1,400 MPa hot-rolled grades for future applications in vehicle frame pillars. On a separate track, design teams at the Auto/Steel Partnership currently are focused on welding techniques and system automation to produce truck frames, front rails, bumper beams and engine cradles using dual-phase steel sheet with a tensile strength range of 800 to 1,000 MPa. One team recently completed its work on frames while a new team is examining cradles and subframes. There is also a AISI design team working specifically on incorporating high-strength steel components into bumper systems. Anderson says that, in addition to spot and MIG welding, the partnership is exploring alternative, leading-edge joining technologies, some of which have been borrowed and adapted from the aerospace industry. One technique is hybrid laser-assisted arc welding. Another hybrid process under study involves the use of an advanced epoxy adhesive applied as a continuous bead along a steel flange, combined with spot welding. A similar system is used to bond aluminum aerospace structures. Anderson said this hybrid process has the promise of delivering a 10-percent improvement in stiffness compared with standard spot welding, while maintaining desired mass levels.
A Guide to High Strength Steels
While metal compositions vary, high-strength steels are defined as an alloy recipe that incorporates manganese, columbium, vanadium, titanium, chromium, molybdenum and silicon to achieve tensile strengths greater than 300 MPa, which represents the tensile strength of conventional sheet steel. In the automotive world, “advanced high-strength steel” refers to the strongest steels currently available in production quantities, while while “ultra-high-strength steel” defines future strength levels.According to Joseph Defilippi, acting general manager of research for U.S. Steel Corp., the current focus for high-strength steels in the automotive arena is dual-phase sheet with a tensile strength of 780 MPa as well as “transformation induced plasticity” or “TRIP” sheet with a similar tensile strength. Dual-phase steel combines high work hardening and ductility steel characteristics, while TRIP incorporates a third retained-austinite phase in the metallurgy, which imparts more ductility than dual-phase steel. Several steel sheet producers--U.S. Steel; AK Steel Corp., Middletown, Ohio; Mittal Steel USA, East Chicago, Ind.; and Kobe Steel Ltd., Kobe, Japan--offer commercial quantities of these materials, and car builders have incorporated structural applications using the 780 MPa steels for current platforms.
The next step up on the high-strength steel development ladder would be dual-phase and TRIP sheet with a tensile strength of 980 MPa. This strength level is being reviewed for 2007 models and beyond, Defilippi reports.And stronger is better when it comes to automotive steels. Defilippi points out that improved vehicle crash worthiness directly correlates with higher tensile strengths offered by the new steels.There are currently over 100,000 vehicles in North America that employ 500 to 600 MPa high-strength steel components. And the steel industry estimates that there will be 140 to 400 pounds of high-strength steel per U.S.-built vehicle as of this year, growing at an annual rate of 5 percent. Applications include an array of chassis structural components, inner frames and pillars, engine cradles and bumper frames. The next hurdle for applications will be to specify high-strength steel in exposed body panels.
Design engineers can find information on high-strength steel metallurgy and welding technology on the Web. Site to explore include:
AISI auto/steel partnership, www.autosteel.org
International Iron and Steel Institute (IISI), www.worldsteel.org
Association for Iron and Steel Technology (AIST), www.aist.org