Designing and producing lighter-weight vehicles is a primary goal of some automobile manufacturers. Less weight allows for improved performance, safety and fuel economy while also reducing material costs. Advanced high-strength steel (AHSS) alloys represent a strong candidate for use in structural components of automobiles. They also present new difficulties in alloy composition; OEMs should pay attention to ongoing research in their formation, properties and effective stamping of these metal parts.
Observed Advantages of AHSS
Conventional mild steels have low carbon content and minimal blending elements which result in simple microstructures. AHSS alloys use carefully controlled additions of alloying elements to produce complex multi-phase structures with unique mechanical properties. There are a variety of AHS steel compositions and their properties provides considerable versatility. Dual-phase (DP) steels and transformation-induced plasticity (TRIP) for example, already see considerable use in vehicles. DP steels are highly resistant to fractures and necking. TRIP steels demonstrate a capacity for high energy absorption under sudden strain, excellent for the “crumple zones” of vehicles.
Formability Concerns of AHSS
Formability – the ability of a piece of metal to undergo plastic deformation without sustaining damage – is a critical trait for automotive alloys to ensure that parts do not fracture within metal forming presses. AHSS alloys, however, exhibit issues with formability as it relates to springback during some more complex precision metal stamping processes. Assessing the related mechanical properties of AHSS, in turn, is difficult with usual testing because the multi-phase structure leads to non-constant strain hardening. The viscous pressure bulge (VPB) test has shown promise, though, in constructing flow stress curves for AHSS samples.
Effective Modeling of Fracture States
The forming limit curve (FLC) is a standard in predicting the forming behavior of sheet metal to illustrate stress states that result in fracture. Traditionally, this involves a series of tests at varying ratios of shear and strain to determine when fracture occurs, but the variability of AHSS responses to strain make this especially costly in time and materials. The Center for Precision Forming at Ohio State University, however, has demonstrated a “three-point technique” for approximating the FLC with just three tests: pure shear, pure strain and biaxial tension. This can provide a more time- and cost-effective method for OEMs and their progressive metal stamping companies they work with.
The advances made in newer metal forming presses allow sheet metal stampers in PA to make fine adjustments to speed and force when forming critical shapes. There is great promise for precision metal stamping in the automobile industry. Further research in controlling the properties and performing effective testing and working will help manufacturers realize goals of more efficient automotive design.