Papers by Author: Uwe Vogt

Abstract: High strength aluminum blanks can be obtained by grain-refinement due to an Accumulativ Roll Bonding (ARB) process, in which two sheets are iteratively brushed, stacked on top of each other and subsequently rolled together. The high shear stresses during the rolling cycles result in an ultrafine-grained microstructure with an average grain size ranging between 200 to 1000 nm. Whereas the grain-refinement causes a drastically increased strength of the aluminum material, the formability of the ARB-blanks made of industrially used aluminum alloys such as the AA6016 deteriorates to the same degree as the strength rises. In this context, a local heat treatment of the ARB-blank reducing the material’s strength and increasing its ductility in specific zones will allow to recover the blank’s formability again. The research work presented in this paper studies the microstructural effects of a short-term heat treatment on the mechanical properties of ARB-blanks made of AA6016. Experimental investigations including hardness measurements, tensile tests as well as microscopic analyses show that heat treatments of only several seconds already result in significant increases of the material’s ductility and decreases of the material’s strength. By applying these microstructural mechanisms in terms of a specific heat treatment layout, functional gradients of strength and ductility adapted to the succeeding forming operation can be setup significantly enhancing the ARB-blank’s formability.

Abstract: Tailored Heat Treated Blanks (THTB) are blanks that exhibit locally different strength specifically optimized for the succeeding forming process. The strength distribution is set by a local, short-term heat treatment modifying the mechanical properties of the material. Hence, THTB allow enhancing forming limits significantly leading to shorter and more robust manufacture process chains. In order to qualify the use of THTB under quasi series conditions, the interdependencies of the blank’s local heat treatment and the entire process chain of the car body manufacture have to be analyzed. In this respect, the impact of a short-term heat treatment on the mechanical properties of AA6181PX, a commonly used aluminum alloy in today’s car bodies, was studied. Also the influence of a short-term heat treatment on the coil lubricant, usually already applied by the material supplier, was given a closer look. Based on these experiments process restrictions for the application of THTB in an industrial automotive environment were derived and a process window for the THTB design was set up. In conclusion, strategies were defined how to enhance the found process boundaries leading to a more robust process window.