Papers by Keyword: Dispersoid

Abstract: AA6xxx alloys are used for various automotive and architectural applications where microstructural characteristics are critical to have acceptable final properties. Abnormal Grain Growth (AGG) in these alloys, industrially termed as Peripheral Coarse Grain (PCG) is undesirable and Mn, Cr based non-coherent dispersoids are used to control the extent of PCG. Homogenization soaking temperature and time along with heating rates determine the size, distribution with grains and volume fraction of these dispersoids. In this study the heating rates are varied in lab and industrial setting to assess the effect of aforementioned dispersoid features using SEM and digital microscopy. It is found that higher heating rates lead to coarser and lower area fraction of dispersoids which finally results in markedly large PCG in industrial extrusion. Observed dispersoid features were described based on basic kinetics and Thermo-Calc^TM predicted trends of micro-segregation, fraction of dispersoids and fraction of potential nucleating sites (β’-Mg₂Si) of dispersoids. A static recrystallization model was used to calculate the driving and retarding pressures based on substructural (EBSD analysis) and dispersoid features (SEM+ image analysis). The predicted recrystallisation response and PCG grain size was in close agreement with the observed values. This study highlights the significance of homogenization heating rates in addition to soaking time and temperature for PCG control.

Abstract: A study was conducted on the evolution of microstructure during homogenization for two Al-Mg-Si alloys with different Mn levels, i.e. 0 and 0.5wt%. The homogenization treatment was conducted over a wide range of temperatures above the Mg₂Si solvus. The holding time at the peak temperature ranged from 2 hour to one week. Microstructure evolution of the constituent particles and Mn dispersoids were characterized by means of optical microscopy and FEG-SEM. The Mn content in and out of solution was estimated using the Thermo-calc (TTAl6 database) and resistivity measurements. The micro-segregation and distribution of the main alloying elements before and after homogenization were systematically studied by electron probe micro analysis (EPMA). It was found that the Mn content together with the homogenization practice had a significant influence on the microstructure evolution. By combining all the measurements, a comprehensive quantitative dataset describing microstructure evolution during homogenization was developed.

Abstract: The influence of hot deformation on the evolution of size, shape, and fraction of dispersoids has been studied in a simple 3xxx aluminium alloy by means of hot torsion testing. It has been shown that at high strain rates, deformation leads to spheroidization of the dispersoids, an increase in number density, and an increase in volume fraction. The increase in number density and volume fraction are associated with precipitation of new particles. The enhancement of manganese diffusion is a key factor in promoting rapid dispersoid evolution during deformation. A model has been developed to estimate the effect of deformation induced vacancies and dislocations on diffusion. This predicts that an order of magnitude increase in diffusion coefficient between may occur under typical hot deformation conditions, consistent with the rapid microstructural changes measured experimentally.

Abstract: Twin roll casting (TRC) is an efficient process used to produce thin plates directly from molten metal. However, TRC has a much higher cooling rate during solidification compared with conventional direct chill casting. Accordingly, the constituent phases are refined and the added elements are supersaturated by this casting method. Supersaturation of the added elements hinders both recovery and recrystallization because fine dispersoids precipitate from the supersaturated solid solution during annealing [ and coarsen the grain size of the final plate, which leads to degradation of formability. Therefore, it is important to understand the influence of the solute element and dispersoids on recrystallization behavior to obtain an appropriate grain structure for forming. In this study, the effects of the homogenization conditions and cold rolling reduction on the recrystallization behavior of twin-roll cast 3XXX series aluminum alloy were investigated.

Abstract: Recrystallization control over the whole process route is strictly needed in order to achieve the required texture and properties. An important factor for this is the different type of particles formed in the material, their size and volume fractions. This study addresses the role of the Mn-bearing dispersoids and the Mg-Si (-Cu) particles, forming during different phases of the sheet processing, thus significantly influencing the recrystallization. It will be shown that, when present, the latter ones are playing an important role in the recrystallization control, not only in promoting the recrystallization by means of Particle Stimulated Nucleation, but also retarding it, if the particle size is sufficiently fine. Moreover, they can completely block the recrystallization, if concurrently precipitating during it. The Mn-dispersoids is comparable in its pinning effect, but takes over only when the Mg & Si atoms are in solid solution. Additionally, an update of the Diagram of Humphreys on the effect of particles on the recrystallization of 6xxx alloys is proposed and discussed.

Abstract: The development of both dispersoid and constituent particle types during high temperature deformation has been investigated. Using torsion testing, which enables good temperature and strain rate control, the development of particles in terms of individual properties and the overall population has been examined during extended high strain rate deformation. Torsion tests also allow material that has the same thermal history but different levels of strain within a single sample to be compared. Quantitative comparison of particles has been performed using high resolution SEM imaging. Strain has been shown to have an important influence on particle evolution, beyond changing the kinetics of particle evolution alone. It has been demonstrated that the shape of the dispersoids is altered when they are evolving under the action of strain compared to that obtained from a thermal effect.

Abstract: Second phase particles in wrought aluminium alloys are crucial in controlling recrystallization and texture. In Al-Mn-Fe-Si (3xxx) alloys, the size, spacing, and distribution of both large constituent particles and small dispersoids are manipulated by heat treatment to obtain the required final microstructure and texture for operations such as can-making. Understanding how these particles evolve as a function of process conditions is thus critical to optimize alloy performance. In this study, a novel 3-dimensional technique involving serial sectioning in the scanning electron microscope (SEM) has been used to analyse the intermetallic particles found in an as-cast and homogenized Al-Mn-Fe-Si alloy. This has allowed an accurate determination of the size and shape of the constituent particles and dispersoids derived from a 3-dimensional dataset. It is demonstrated that a proper consideration of the 3-dimensional microstructure reveals important features that are not obvious from 2-dimensional sections alone.

Abstract: In the present work an Al-Mn-(Fe-Si) model alloy has been subjected to different homogenization treatments, to achieve materials with different microchemistry states in terms of constituents, levels of Mn in solid solution (potential for concurrent precipitation) and dispersoid densities, followed by cold rolling and back-annealing. Characterization of the microchemistry state after homogenization and the evolution in dispersoid precipitation and its effects on the softening behavior after deformation has been performed. It is demonstrated that variations in microchemistry may have dramatic effects on the softening kinetics and the final grain structures, where both pre-existing fine and dense dispersoids before back annealing as well as precipitation concurrent with recovery and recrystallization strongly retard kinetics and generally lead to a coarse grain structure, while conditions with no or limited concurrent precipitation softens much faster and generally results in an even, fine and equi-axed grain structure. The different softening behaviors have been discussed in terms of Zener drag effects derived from the dispersoid evolutions.

Abstract: The transition metals such as chromium and manganese are usually added to 6000 series Al-Mg-Si alloys to control recrystallization and grain size and thus the properties of alloys. In Cr/Mn-addition alloys, Cr or Mn will expense Si to form the dispersoids as AlMnSi or AlCrSi and tend to decrease its aging effect. The aim of this work is to investigate the effect of transition metals (TMs) addition on the hardness and the microstructural features of Al-Mg-Si alloys. Al-Mg-Si alloys, which can be remarked as the quasi-binary alloys of Al-Mg₂Si, are prepared with Cr or Mn addition by laboratory casting. Some other transition metals, such as Co and Ni, are also added to Al-Mg-Si alloys. The grain size of four alloys decreases with TMs addition, which consequently increases the as-quenched hardness of the alloys comparing with that of the Al-Mg₂Si alloy without TMs addition. The difference between Cr/Mn-addition alloy and Co/Ni-addition alloy is that the dispersoids are formed in Co/Ni-addition alloy without expensing Si. Therefore, there is little effect on the aging effect of Si in Co/Ni-addition alloy. Keywords: transition metals, hardness, microstructural, Al-Mg₂Si, dispersoids.

Abstract: Homogenisation of aluminium alloys is the high temperature heat treatment (450-600 °C) performed after casting and consists of three distinct steps; heat-up, soak and cooldown. This review considers the metallurgical importance of homogenisation and how it impacts on the further processing and final properties of some aluminium alloys, with emphasis on homogenisation of extrusion billet. The introduction of continuous homogenisation has significantly improved the temperature uniformity of homogenisation allowing the soak time to be minimised. Batch homogenisation, however, provides flexibility in practices tailored for different aluminium alloys. Soft 6060 and 6063 alloys are best homogenised at a higher soak temperature than harder alloys such as 6061 and 6082. The homogenisation cooling rate can also impact on the behaviour of the billet during extrusion processing as well as affecting the final mechanical properties. An understanding of the microstructural changes occurring as a result of homogenisation allows the cast house to ensure that the billet processing meets the customer requirements.