Papers by Author: Olaf Engler

Abstract: During processing of age-hardenable AA 6xxx series alloys for automotive applications the sheets may experience significant time spans between solution heat treatment at the aluminium supplier and age hardening upon the final paint bake cycle at the carmaker. Natural ageing during these pause times is known to greatly affect materials properties of autobody sheet. In the present study we explore the impact of natural ageing on the tensile properties and the in-plane anisotropy of alloy AA 6005C. Materials properties at various degrees of natural ageing are modelled with the help of a nanoscale material model NaMo, which consists of a precipitation model simulating the formation of clusters and phases upon natural ageing as input to a mechanical model simulating the evolution of yield strength and work hardening. Plastic anisotropy is modelled from the materials crystallographic texture by a visco-plastic self-consistent polycrystal-plasticity code VPSC.

Abstract: In this study we aim at combining the results from transmission electron microscopy (TEM) and atom probe tomography (APT) to study the early stages of phase decomposition in the age hardening alloy AA 6016. Samples are subjected to different periods of natural ageing or artificial pre-ageing at elevated temperature in order to produce different types of clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their compositions, whereas TEM is applied to analyse and quantify number density and sizes of the particles during artificial ageing at 185°C. It is shown that the two techniques, TEM and APT, are complementary and a combined approach yields more detailed insight into the early stages of phase decomposition in age hardening 6xxx series alloys than possible by the sole use of either technique individually.

Abstract: In the present paper, an extended age hardening model for Al-Mg-Si alloys is presented. In this new approach the combined precipitation, yield strength and work hardening model, known as NaMo Version 1, has been further developed to account for the effects of room temperature storage and cold deformation on the resulting age hardening behaviour. Incorporation of these two new stages in NaMo largely increases the versatility of the model by allowing simulations of complex multi-stage industrial processing involving thermomechanical treatment as well. Part 1 of this work deals with the theoretical background and experimental validation of the extended version of NaMo, while Part 2 focuses on the new applications of the model by showing some numerical examples related to production of automotive body panels.

Abstract: Aluminium foil is rolled double-layered during the final rolling pass. When the sheets are later separated, the inside surface is dull and the outside surface is shiny. The matt inner side is characterized by significant surface corrugations which are believed to be a precursor for the initiation of fracture upon a subsequent forming operation. Therefore, understanding of the development of the matt side of Al foil will help to control and, eventually, improve the properties of Al foil. It was the goal of the present study to correlate the development of the matt side with the spatial arrangement of the crystallographic orientations of the foil rolling texture. This approach builds on a recent project to correlate the phenomenon of roping in AA 6xxx alloy sheet for car body applications to the occurrence of band-like clusters of grains with similar crystallographic orientation. Large-scale orientation maps obtained by electron back-scattered diffraction (EBSD) were input into a visco-plastic self-consistent crystal-plasticity model to analyse the strain anisotropy caused by the spatial distribution of the various rolling texture components. The new model is applied to several Al foils with different characteristics of the matt side.

Abstract: Resistivity against intergranular corrosion (IGC) is one of the major requirements for AlMgMn 5xxx-series alloys for automotive chassis applications. In 5xxx alloys IGC is caused by the formation of β-Al₅Mg₃ precipitates along the grain boundaries. Todays 5xxx alloys for chassis applications have been developed such that under specific test conditions they will not exceed a certain mass loss by IGC. However, current developments in the automotive industry will lead to an increased temperature load on chassis parts, in particular for front axle applications in the vicinity of the car engine. Therefore it is to be expected that the properties of the existing 5xxx series alloys will not be sufficient any more. Certain alloy elements, including Mn, Cr, Cu and Zn, alter type and morphology of the Mg-bearing precipitates and, hence, reduce the sensitivity against IGC. The present study was aimed at developing a series of Zn-containing Al alloys which are free of IGC, while maintaining mechanical properties of current 5xxx chassis alloys. Alloy development was performed by micro-chemistry simulation with the aim to avoid the formation of the detrimental β-Al₅Mg₃ precipitates. Eventually a series of three alloys was cast and processed on lab scale and tested for mechanical properties and resistivity against IGC after application of various critical time/temperature scenarios in order to validate that the newly developed alloys are free of IGC.

Abstract: A modular ductile failure model is presented and applied to the forming of an AA5182 aluminium alloy sheet. A detailed description of the failure model and its calibration is provided. The final application of the calibrated failure model to the deep drawing of a cruciform cup reveals a good correlation with the experimental findings. Finally, a study on the influence of the r-value on formability is conducted.

Abstract: The basic equations and mathematical framework of a mean-field model for recovery and recrystallization, the latter based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach, capable of handling time-dependent nucleation of recrystallization, is presented. Different approaches to account for time-dependent nucleation are discussed. A physically-based nucleation model where “nucleation” of recrystallization is brought about by “abnormal” subgrain growth seems most appealing, in terms of realism and mathematical convenience. Its implementation and effects on the recrystallization behavior are demonstrated through an example of back-annealing after cold deformation of a generic aluminium alloy case

Abstract: The control of the plastic anisotropy during forming of a metallic sheet requires detailed knowledge on its microstructure and, especially, crystallographic texture. During the thermo-mechanical processing of aluminium sheet products in commercial production lines the material experiences a complex history of temperature, time and strain paths, which result in alternating cycles of deformation and recrystallization with the associated changes in texture and microstructure. Thus, computer-based alloy and process development requires integration of models for simulating the evolution of microstructure, microchemistry and crystallographic texture into process models of the thermo-mechanical production of Al sheet. The present study focuses on recent developments in linking softening modules that simulate the progress of recovery and recrystallization with the following texture changes to deformation and microchemistry models.

Abstract: Even anisotropic superplastic flow, which is a result of an elongated grain shape and texture, can lead to extreme elongations to fracture (superplasticity). Therefore, to identify the mechanisms of deformation present during superplastic flow alone, the effects of the microstructure should be eliminated first. Using an Al 5083 alloy, in which an equi-axed microstructure is present from the beginning, it is shown that grain boundary sliding, accompanied by grain rotations, is the rate controlling mechanism.

Abstract: During the thermo-mechanical processing of aluminium sheet products in commercial production lines the material experiences a complex history of temperature, time and strain paths, which result in alternating cycles of deformation and recrystallization with the associated changes in microstructure and, especially, crystallographic texture. Thus, computer-based alloy and process development requires integration of models for simulat¬ing the evolution of microstructure, microchemistry and crystallographic texture into process models of the thermo-mechanical production of Al sheet. In the present paper the influence of texture on the anisotropic properties is explored for various industrially processed aluminium alloy sheets for packaging applications. Besides the use of experimentally measured sheet textures as an input for the anisotropy calculations, particular attention is given to the use of modelled textures. Here, results from a comprehensive through-process modelling of the texture evolution during the thermo-mechanical production of aluminium sheet are utilized. Eventually, this will enable us to predict the evolution of texture and the resulting properties along the entire process chain and hence to improve product quality of aluminium sheet products avoiding laborious and expensive plant trials.