Kurzzeit-Wärmebehandlung von AlMgSi-Legierungen: Phasenumwandlungen und mechanische Eigenschaften

Durch lokal unterschiedliche mechanische Eigenschaften kann der Materialfluss während einer Kaltumformung verbessert, die kritische Umformzone entlastet und somit die Umformbarkeit erhöht werden. Mit einer lokalen Kurzzeit-Wärmebehandlung können somit gezielt verfestigte und entfestigte Bereiche eingestellt werden, wodurch sich eine nachfolgende Umformung verbessern lässt. Tailored Heat Treated Profiles (THTP) sind bisher wenig erforscht. In dieser Arbeit werden die mikrostrukturellen Veränderungen während unterschiedlicher KZWB der aushärtbaren Legierungen 6060 T4 sowie 6082 T4 untersucht

A Phenomenological Mechanical Material Model for Precipitation Hardening Aluminium Alloys

Age hardening aluminium alloys obtain their strength by forming precipitates. This precipitation-hardened state is often the initial condition for short-term heat treatments, like welding processes or local laser heat treatment to produce tailored heat-treated profiles (THTP). During these heat treatments, the strength-increasing precipitates are dissolved depending on the maximum temperature and the material is softened in these areas. Depending on the temperature path, the mechanical properties differ between heating and cooling at the same temperature. To model this behavior, a phenomenological material model was developed based on the dissolution characteristics and experimental flow curves were developed depending on the current temperature and the maximum temperature. The dissolution characteristics were analyzed by calorimetry. The mechanical properties at different temperatures and peak temperatures were recorded by thermomechanical analysis. The usual phase transformation equations in the Finite Element Method (FEM) code, which were developed for phase transformation in steels, were used to develop a phenomenological model for the mechanical properties as a function of the relevant heat treatment parameters. This material model was implemented for aluminium alloy 6060 T4 in the finite element software LS-DYNA (Livermore Software Technology Corporation)

Scanning Rate Extension of Conventional DSCs through Indirect Measurements

In this work, a method is presented which allows the determination of calorimetric information, and thus, information about the precipitation and dissolution behavior of aluminum alloys during heating rates that could not be previously measured. Differential scanning calorimetry (DSC) is an established method for in-situ recording of dissolution and precipitation reactions in various aluminum alloys. Diverse types of DSC devices are suitable for different ranges of scanning rates. A combination of the various available commercial devices enables heating and cooling rates from 10−4 to 5 Ks−1 to be covered. However, in some manufacturing steps of aluminum alloys, heating rates up to several 100 Ks−1 are important. Currently, conventional DSC cannot achieve these high heating rates and they are still too slow for the chip-sensor based fast scanning calorimetry. In order to fill the gap, an indirect measurement method has been developed, which allows the determination of qualitative information, regarding the precipitation state, at various points of any heat treatment. Different rapid heat treatments were carried out on samples of an alloy EN AW-6082 in a quenching dilatometer and terminated at defined temperatures. Subsequent reheating of the samples in the DSC enables analysis of the precipitation state of the heat-treated samples. This method allows for previously un-measurable heat treatments to get information about the occurring precipitation and dissolution reactions during short-term heat treatments

Effects of Aging under Stress on Mechanical Properties and Microstructure of EN AW 7075 Alloy

In the present study, microstructural and mechanical properties of EN AW 7075 following stress-aging were assessed. For this purpose, properties of stress-aged samples were compared with values obtained for conventionally aged counterparts. It is revealed that the strength and hardness of EN AW 7075 can be increased by the presence of external stresses during aging. Precipitation kinetics were found to be accelerated. The effects of conventional and stress-aging on the microstructure were analyzed using synergetic techniques: the differently aged samples were probed by differential scanning calorimetry (DSC) in order to characterize the precipitation processes. DSC was found to be an excellent screening tool for the analysis of precipitation processes during aging of this alloy with and without the presence of external stresses. Furthermore, using electron microscopy it was revealed that an improvement in mechanical properties can be correlated to changes in the morphologies and sizes of precipitates formed