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

On the Monotonic and Cyclic Behavior of an Al‐Mg‐Zn‐Cu‐Si Compositionally Complex Alloy

In present work, the monotonic and cyclic properties of a novel Al‐Mg‐Zn‐Cu‐Si compositionally complex alloy (CCA) are investigated. Microstructural studies reveal that a eutectic phase and fishbone‐type structures are embedded in the Al‐matrix. The mechanical properties of this CCA obtained under compressive loading are found to be promising. However, low ultimate strength and brittle behavior are seen under tension. The fatigue performance of the alloy in the low‐cycle fatigue (LCF) regime is poor. Microstructural features and fracture surface analysis point to the fact that the presence of brittle intermetallic phases and shrinkage defects are responsible for the poor tensile properties and inferior LCF behavior. Homogenization at 460 and 465 °C for 3 h, 6 h, and 24 h results in spheroidization of all coarse precipitates. However, the heat treatment used is unsuitable to enhance the mechanical properties of this CCA

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

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)

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

In situ high-energy X-ray diffraction of precipitation and dissolution reactions during heating of Al alloys

During heating of Al alloys, typically a sequence of precipitation and dissolutionreactions occurs and the single (partly opposing) reactions superimpose. Differentialscanning calorimetry (DSC) is one common technique to analyse thekinetic development of precipitation and dissolution in Al alloys, but thesuperposition of the exothermic precipitation and endothermic dissolutionreactions complicates the DSC signal interpretation, as DSC measures the sum ofany heat effect. Synchrotron high-energy X-ray diffraction (HEXRD) allows thekinetic development of phase transformations to be obtained and can supportthe separation of superimposed DSC signals. HEXRD results from this workoffer a new approach to separate part of the superimposed reactions and theirkinetic development for the equilibrium phases b-Mg2Si in EN AW-6082 andg-Mg(Zn,Cu,Al)2 in EN AW-7150. Comparing DSC and HEXRD results confirmsserious overlap issues. Common DSC evaluation methods alone, usingzero crossing between endothermic and exothermic heat flow or peak positionscan be misleading regarding individual reaction start and finish temperatures aswell as regarding reaction intensities, which can be unambiguously determinedby in situ HEXRD

In-situ analysis of continuous cooling precipitation in Al alloys by wide-angle X-ray scattering

The aim of this work is to investigate quench induced precipitation during continuous coolingin aluminium wrought alloys EN AW-7150 and EN AW-6082 using in situ synchrotron wideangleX-ray scattering (WAXS). While X-ray diffraction is usually an ex situ method, a variety ofdiffraction patterns were recorded during the cooling process, allowing in situ analysis of theprecipitation process. The high beam energy of about 100 keV allows the beam to penetratea bulk sample with a 4 mm diameter in a quenching dilatometer. Additionally, the highintensity of a synchrotron source enables sufficiently high time resolution for fast in situ coolingexperiments. Reaction peaks could be detected and compared with results from differentialscanning calorimetry (DSC) by this method. A methodology is presented in this paper toevaluate WAXS data in a way that is directly comparable to DSC-experiments. The resultsshow a high correlation between both techniques, DSC and WAXS, and can significantlyimprove continuous cooling precipitation diagrams

Influence of Solution-Annealing Parameters on the Continuous Cooling Precipitation of Aluminum Alloy 6082

We use a systematic approach to investigate the influence of the specific solution condition on quench-induced precipitation of coarse secondary phase particles during subsequent cooling for a wide range of cooling rates. Commercially produced plate material of aluminum alloy EN AW-6082 was investigated and the applied solution treatment conditions were chosen based on heating differential scanning calorimetry experiments of the initial T651 condition. The kinetics of the quench-induced precipitation were investigated by in situ cooling differential scanning calorimetry for a wide range of cooling rates. The nature of those quench-induced precipitates was analyzed by electron microscopy. The experimental data was evaluated with respect to the detrimental effect of incomplete dissolution on the age-hardening potential. We show that if the chosen solution temperature and soaking duration are too low or short, the solution treatment results in an incomplete dissolution of secondary phase particles. This involves precipitation during subsequent cooling to start concurrently with the onset of cooling, which increases the quench sensitivity. However, if the solution conditions allow the formation of a complete solid solution, precipitation will start after a certain degree of undercooling, thus keeping the upper critical cooling rate at the usual alloy-specific level

Achieving ultra-high strength of Al-Cu-Li alloys by the combination of high pressure torsion and age-hardening

The combined strengthening effects of high pressure torsion (HPT) and age hardening on a recently developed 3rd generation Al-Cu-Li alloy was investigated. Solution treated samples were processed through HPT at room temperature, followed by low temperature artificial ageing (i.e. T4-HPT-AA). A micro-hardness of ∼ 240 Hv was achieved on ageing at 110 oC/60h after HPT. A further improvement in the hardness to ∼ 260 Hv was accomplished by a pre-ageing 110 oC/24h before HPT combined with a post-HPT ageing process at 110 oC for 180h (i.e. T6-HPT-AA). These novel multi-stage processes give rise to an increase in hardness by a factor of 2 as compared to the T4 condition (∼ 120 Hv). After HPT the grain size was dramatically refined to the ultrafine-grained (UFG) structure, accompanied by a large amount of dislocations. No long-range ordered precipitates were observed after HPT and subsequent ageing treatments. Instead, atom probe tomography (APT) provided clear evidence that Cu-Mg co-clusters were homogeneously distributed in the matrix of T4 and T6 processed samples and they segregate strongly to the grain boundaries (GBs) during HPT. Further ageing treatment after HPT leads to the segregation of clusters to dislocations. A strengthening model that incorporates dislocation hardening, grain boundary hardening, solid solution strengthening and a new short-range order strengthening mechanisms was used to predict the yield strength of the alloy. This model indicates that the combined effect due to all three types of Cu-Mg clusters (clustering in matrix, clustering at GBs and at dislocations) is dominant for the strength in all conditions.</p