Automotive body engineers won't have it easy over the coming years as they struggle to reduce the weight of their designs while simultaneously improving crash performance and cutting costs. Tailored steel blanks can play a role in meeting all three of these sometimes-conflicting goals. The blanks allow different grades and thicknesses of steel to be combined into flat blank assemblies or, increasingly, tubes and profiles. Tailored blanks can help save cost and weight--often 20 percent or more of each--by allowing the selective use of lighter, cheaper steels in combination with very high strength materials. They also allow parts consolidation in many cases. "The power of the technology is the ability put the strongest steels where they are most needed, " says Peter Mould, program director of the American Iron & Steel Institute's newly formed Tailored Steel Product Alliance (TPSA). In particular, the blanks support the use of advanced high strength steels (AHSS), including newer multi-phase grades whose yield strengths typically exceed 550 MPa. All of this explains the recent upswing in tailored products, some of which have been available for more than 20 years. According to Mould, global use of tailored welded blanks (TWB), the most popular type of tailored product, grew from 120 million parts/year in 2001 to 250 million in 2005. "We're heading to 300 million over the next few years," he adds. Here's a look at some of the recent tailored blank technology developments that will help fuel that growth:
Blanks On A Roll
One of the more recent developments in tailored steel products does away with need to weld blanks in the first place. Mubea, a German automotive supplier, has developed a new "flexible rolling" process that can create thickness variations within a continuous coil. Mubea's tightly-controlled cold rolling process allows for localized thickness reductions up to 50 percent of the nominal sheet thickness with thickness tolerances of +/-50 micrometers. The transitions between regions of different thickness can take place very smoothly, with slopes between 1/3000 and 1/100. According to Michael Rehse, a Mubea engineering manger who gave a presentation on flexible rolling at the recent Great Designs In Steel Conference, the "tailor rolled blanks" produced by the flexible rolling process allow engineers to better match load requirements to localized sheet thicknesses--and do so without the cost, seams, and microstructure changes associated with laser welding. Forming properties of the tailor rolled blanks remain similar to conventionally rolled metal, Rehse reports. Applications for the technology include a wide variety of chassis parts. Typical weight reductions versus conventional parts have come to at least 20 percent. For a more detailed look at the technology and all the possible applications, download Rehse's presentation.
Non-linear Welding Opens Doors
In terms of popularity, door inners are the most popular application for tailor welded blanks (TWB) today. The reasons why come down to performance and economics, according to TSPA's Mould. "The laser welding of a thick and/or higher strength portion into the door inner blank allows reinforcements to be eliminated and mass to be reduced," Mould says. What's new, he adds, is that the laser welding process used to make the blanks has improved to the point where curvilinear joints can be produced more easily, which in turn allows more design flexibility and optimization of the materials mix on components such as door inners as well as body side structures. As an example of the latter, Mould points to the the door-opening panel on the Ford F-150 crew cab. Non-linear laser welding joins a 1.1 mm electro-galvanized sheet to a 2.0 mm hot-dip-galvanized dent resistant steel. Aside from getting a part with higher stiffness where needed, this application also featured parts consolidations that eliminated 20 end-item stampings and 88 stamping dies, Mould reports. Get more information on the TSPA.
Stronger Steels In The Mix
While tailored blanks got their start with conventional steels, they are increasingly seen as a way to incorporate advanced high-strength steels (AHSS) into automotive components. Rather than creating a monolithic structure out of the costly advanced steels, tailored blanks allow engineers to apply the highest-performing steels only where the loads demand them.At the Great Designs In Steel Conference, Frank Mei of TWB Co. LLC, a tailored blank supplier, detailed some of new materials combinations that have started to turn up in tailored blanks--both welded varieties as well as emerging forms like tailored tubes and strips. Among the newer developments include the use of boron steels in conjunction with HSLA steels to provide lightweight, crash resistant structures. Other promising materials combinations are in development (see chart). For more information, download Mei's presentation.