Minor Addition with Substantial Effect - New Aluminum-Silicon Coating for a Reliable Hot-Forming Process

Abstract

The well-known aluminum-silicon coating has managed to be on everyone's mind as the standard coating for the hot forming process for many years. This is not only due to its good scaling protection properties, but also due to the fact that both the production of the sheet material and the hot forming processes are stable and well established. One of the main challenges in hot forming is to avoid component failure due to macroscopic cracking by hydrogen embrittlement. This is particularly critical because the cracking may occur with delay, i.e. during processing in the manufacturing chain subsequently to hot stamping. To avoid this, it is necessary to reduce the diffusible hydrogen content below a critical value in the hot formed part. This contribution describes the causes for hydrogen formation in the hot forming process and possible countermeasures. The addition of magnesium between 0.1 up to 0.5 mass % into the coating has proven to effectively reduce the diffusible hydrogen content in the hardened part. We present a model describing the underlying microscopic processes of this beneficial behavior. Furthermore, we examined the impact of Mg addition on the material properties and discuss resulting advantages, which can be observed in the hot stamping process. One main feature is the reduced effort to meet the requirement for low dew points in the furnace. This leads to a more economical manufacturing process. Moreover and important for the applicator to know is, that all performance benefits of the standard aluminum-silicon coating are retained.