Papers by Author: Paul A. Rometsch

Abstract: In this work, Al-Mg-Si-Cu alloys for automotive body panels were designed and the related ageing behaviours were discussed in detail to help understand natural ageing and pre-ageing, as well as their influence on the subsequent paint-bake response. The clustering behavior of these Al-Mg-Si-Cu alloys in different ageing conditions was investigated by hardness / yield strength and electrical conductivity testing. The microstructure was investigated by using Electron Backscattered Diffraction (EBSD) technique, along with Scanning Electron Microscopy with Backscattered Electron Detector (BSE). The results show that the paint bake response is strongly influenced by the pre-ageing and natural ageing conditions. Both alloys show serrated yielding in a short natural ageing condition. Immediate high-temperature pre-ageing treatments were found to give a promising hardening response during the subsequent artificial ageing/ paint baking at 170^oC.

Abstract: The thermal stability of constituent particles in both as-cast and homogenised alloy AA7150 has been investigated by means of scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results indicate that the as-cast eutectic starts to melt at 474°C and that the melting onset temperatures of the η- and S-phase particles after homogenisation are 474°C and 484°C, respectively. Two types of homogenisation treatments were used to dissolve eutectic particles: a two-step homogenisation treatment (40 h at 465°C + 4 h at 480°C) can completely dissolve the η- and S-based constituents, whereas a significant amount of S-phase still exists after a single-step homogenisation treatment of 40 h at 465°C. After utilising the two-step homogenisation treatment to dissolve all the η- and S-based constituents without overheating, samples were cooled at different controlled cooling rates and analysed by SEM and DSC to understand the effect of cooling rate on the type, size, distribution and density of precipitates formed during the cooling process. Slow cooling was found to result in the precipitation of coarse η-phase particles, with both the amounts and sizes of these η-phase particles increasing with decreasing cooling rate.

Abstract: In this work, the influence of multi-step solution (MSS) treatments on the constituent particle dissolution, overheating and associated quench cracking behaviour in room temperature water-quenched 7150 Al alloy has been investigated. For comparison, the microstructure and quench cracking behaviour of single step solution treated samples water-quenched from 505°C were also investigated. Based on optical microscopy of differently quenched samples, the quench cracking mode and the influence of overheating of constituents on the quench cracking behaviour have been demonstrated. The results reveal that the constituent particles can be effectively dissolved in the MSS-505°C samples. When the quench temperature of MSS-505°C samples is equal to or higher than 485°C , macro quench cracks can be clearly observed. Moreover, the density and length of the quench cracks increase with increasing quench temperature. Etched microstructures indicate that the quench crack propagation mode is intergranular. However, for samples directly heated to 505°C , typical overheating can be observed at the triple junctions and these regions preferentially act as crack propagation routes.

Abstract: The effect of different pre-ageing treatments on the subsequent artificial ageing response of Al-Mg-Si(-Cu) alloys have been investigated using hardness, tensile and electrical conductivity testing. The microstructural evolution was characterised by differential scanning calorimetry (DSC) and 3-dimensional atom probe (3DAP) analysis. Pre-ageing treatments were carried out at 160-250°C for short times. Results show that the early stage artificial ageing response after 30 minutes at 170°C is strongly influenced by the pre-ageing and natural ageing conditions. A pre-ageing treatment performed for a short time at a high temperature and within a short delay after solution treatment and quenching was found to give a promising hardening response during subsequent artificial ageing. The mechanisms by which pre-ageing can reduce the detrimental effect of natural ageing on the artificial ageing response will be discussed in relation to the formation and distribution of clusters, GP zones and/or precipitates.