Grundlagen zur thermo-mechanisch induzierten Eigenschaftsgradierung von ausscheidungshärtbaren Aluminiumknetlegierungen

Inspired by industrial established steel forming strategies [Ste09d, Ste07a, Ste17], this study focuses on the effect of differential cooling on mechanical properties and precipitation kinetics during hot stamping of precipitation hardenable aluminum alloys, EN AW-6082 and EN AW-7075. For this aim, comprehensive process parameter studies are firstly carried out to investigate the resulting intrinsic material properties and microstructural evolution based on the complex mutual interactions of the thermo-mechanically coupled mechanisms during the entire hot stamping process from solution heat treatment, hot forming and artificial aging, to identify the potential of property grading over a characteristic range. The experimental results demonstrate that the tool temperature, as a key element of the investigated process, affects significantly the material strength, due to a fundamental change of the microstructural phenomena and the strain hardening behavior when exceeding a certain temperature limit. It is shown that within a tool temperature range from 24 ◦C up to 350 ◦C and a holding time of 30 seconds microstructures with significantly different properties and features after artificial aging can be obtained. This behavior is linked to the change of quench-induced precipitates morphology and shape. TEM investigations reveal that the different mechanical properties as well as the different hardening behavior are directly related to a change in the type, size and morphology of the precipitates. A high concentration of very fine semi coherent η -phases is observed at low and coarse coherent η-phases at high tool temperatures, causing a fundamental shift of the strain hardening mechanisms from predominantly Kelly-Fine cutting mechanism to a combination of Orowan bypass mechanism and dislocation multiplication. In this regard, using segmented and differentially heated forming tools to provide a locally varying cooling rate allows the creation of a local mechanical property distribution from hard to soft revealing the high potential for functional gradation, attained by quench-induced precipitation processes during cooling and subsequent artificial aging. Microstructure investigations using ECCI-technique under controlled diffraction conditions revealed the formation of very fine and homogeneously distributed spherical particles at cooled zones, which are connected to the elevated mechanical properties, due to the suppression of second phase particle formation during cooling. In contrast, coarse and lath-shaped particles are observed at lower cooling rates, explaining the lower material strength in these zones. The obtained results of the hot stamping process using FE-methods reveals an inhomogeneous cooling during the forming step leading consequently to an inhomogeneous hardness distribution. Furthermore, it is shown that the change and the increase of dislocation density during hot stamping caused by plastic deformation does not significantly influence the resulting strength, due to the thermal activation of softening processes, such as dynamic recovery and dynamic recrystallization. Thus, it is summarized that the distribution of locally different strength values of the hot formed components is primarily influenced within the forming and cooling step. For the description and prediction of the hot deformation behavior of the precipitation hardenable aluminum alloys, in addition to FE-calculations, physically-based constitutive calculations are used at different deformation temperatures and strain rates. It is shown that the microstructure condition, deformation temperature as well as strain rate, significantly influence the prediction of the dislocation density evolutions, dynamic recovery, dynamic recrystallization and finally the hot deformation behavior. A systematic comparison of experimental and theoretical calculation of the flow stress maximum kf,m shows that the precipitation state plays a key role for the accuracy of the calculations.