Paul Rometsch

Heat treatment of 7xxx series aluminium alloys—Some recent developments

Abstract The 7xxx series alloys are heat treatable wrought aluminium alloys based on the Al-Zn-Mg(-Cu) system. They are widely used in high-performance structural aerospace and transportation applications. Apart from compositional, casting and thermo-mechanical processing effects, the balance of properties is also significantly influenced by the way in which the materials are heat-treated. This paper describes the effects of homogenisation, solution treatment, quenching and ageing treatments on the evolution of the microstructure and properties of some important medium to high-strength 7xxx alloys. With a focus on recent work at Monash University, where the whole processing route from homogenisation to final ageing has been studied for thick plate products, it is reported how microstructural features such as dispersoids, coarse constituent particles, fine-scale precipitates, grain structure and grain boundary characteristics can be controlled by heat treatment to achieve improved microstructure–property combinations. In particular, the paper presents methods for dissolving unwanted coarse constituent particles by controlled high-temperature treatments, quench sensitivity evaluations based on a systematic study of continuous cooling precipitation behaviour, and ageing investigations of one-, two- and three-step ageing treatments using experimental and modelling approaches. In each case, the effects on both the microstructure and the resulting properties are discussed.

Effect of one-step and two-step homogenization treatments on distribution of Al3Zr dispersoids in commercial AA7150 aluminium alloy

Abstract Semi-quantitative electron probe microanalysis (EPMA) mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to study the effect of one-step and two-step treatments on the Zr distribution and Al 3 Zr dispersoid characteristics in as-cast commercial AA7150 aluminum alloy. It is shown that the Zr concentration in the dendrite centre regions is higher than that near the dendrite edges in the as-cast condition, and that homogenization at 460 °C for 20 h is insufficient to remove these concentration gradients. After homogenizing at 460–480 °C, a high number density of larger dispersoids can be observed in dendrite centre regions but not near dendrite edges. Furthermore, the dispersoid size increases with increasing the temperature during both one-step and two-step homogenization treatments.

Effect of S-Phase Dissolution on the Corrosion and Stress Corrosion Cracking of an As-Rolled Al-Zn-Mg-Cu Alloy

Abstract The effect of two specific solution heat treatment procedures on the corrosion and stress corrosion cracking (SCC) susceptibility of an as-rolled Al-Zn-Mg-Cu alloy was investigated. It is ...

Effect of recrystallisation on Al3Zr dispersoid behaviour in thick plates of aluminium alloy AA7150

The ageing responses of surface/centre layer materials from a commercially produced AA7150 thick plate have been studied after both water quenching (WQ) and air cooling (AC). The results show that the higher degree of recrystallisation near the plate surface decreases its age hardening response in AC condition. Quench-induced phases have been found to precipitate preferentially on Al3Zr dispersoids in recrystallised grains. Microstructural observations show that Al3Zr dispersoids in recrystallised grains exhibit the same crystal structure and orientation as those in adjacent subgrains.

Atom probe analysis of early-stage strengthening behaviour in an Al-Mg-Si-Cu alloy.

The strengthening of an Al-Mg-Si-Cu alloy during natural ageing and subsequent short artificial ageing was investigated using three-dimensional atom probe (3DAP) analysis and tensile testing. The contingency table and Markov chain analyses confirmed that non-random arrangements of atoms already exist after a natural ageing time of only 3.5h. Extensive use of particle analysis tools in the IVAS and PoSAP software packages revealed that whilst the commonly used minimum aggregate size (N(min)) of 10 is a reasonable choice, much more useful information about the system can be gained by additionally employing a wide range of larger and smaller N(min) values. In particular, it was found that the density and volume fraction of solute aggregates increased with increasing natural ageing time in the T4 condition. After a 0.5h artificial ageing treatment at 170 °C (designated as T6), the size, volume fraction and Mg/Si ratio of the aggregates were all found to decrease with increasing prior natural ageing time. These findings are used to discuss the detrimental effect of natural ageing, where the T6 strength has been observed to decrease rapidly with increasing prior natural ageing time before stabilising after several hours of natural ageing.

Precipitation of a new platelet phase during the quenching of an Al-Zn-Mg-Cu alloy

A previously undescribed high aspect ratio strengthening platelet phase, herein named the Y-phase, has been identified in a commercial Al-Zn-Mg-Cu alloy. Differential scanning calorimetry indicates that this phase only precipitates at temperature and cooling rate of about 150–250 °C and 0.05–300 K/s, respectively. This precipitate is shown to be responsible for a noticeable improvement in mechanical properties. Aberration corrected scanning transmission electron microscopy demonstrates the minimal thickness (~1.4 nm) precipitate plates are isostructural to those of the T1 (Al2CuLi) phase observed in Al-Cu-Li alloys. Low voltage chemical analysis by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy gives evidence of the spatial partitioning of the Al, Cu and Zn within the Y-phase, as well as demonstrating the incorporation of a small amount of Mg.

On the Electrochemical and Quasi In Situ Corrosion Response of the Q-Phase (AlxCuyMgzSiw) Intermetallic Particle in 6xxx Series Aluminum Alloys

The electrochemical response and corrosion associated with the Q-phase (AlxCuyMgzSiw) intermetallic compound was studied. Q-phase intermetallics are the principal strengthening phase in a number of Cu-containing 6xxx series (Al-Mg-Si) alloys, and Q-phase has not previously been uniquely studied in regard to its influence on localized corrosion in detail. Herein, quasi in situ scanning transmission electron microscopy was utilized in understanding the localized corrosion response associated with nanoscale Q-phase precipitates in a Cu-containing 6xxx series Al alloy sheet with a composition (in wt%): 97.3Al-0.9Si-0.74Mg-0.84Cu-0.08Fe-0.14Mn. Furthermore, the Q-phase compound was also produced on the microscale within a bulk-synthesized alloy. The electrochemical behavior of the microscale Q-phase was studied using a micro-electrochemical capillary cell and by potentiodynamic polarization. Quasi in situ scanning electron microscopy was also performed on the bulk alloy synthesized to contain microscale Q-phas...

Understanding the Origins of Intergranular Corrosion in Copper-Containing Al-Mg-Si Alloys

A definitive understanding of the mechanism of intergranular corrosion (IGC) in under-aged (UA) Cu-containing Al-Mg-Si alloys has not been clear to date. The grain boundary microstructure and chemistry in an UA Cu-containing Al-Mg-Si alloy were characterized by coupling atom probe tomography and scanning transmission electron microscopy. The rapid formation of an ultra-thin wetting Cu layer and discrete Q-phase (Al4Cu2Mg8Si7) precipitates along the grain boundaries, and a precipitate-free zone adjacent to the grain boundaries in the UA condition contribute to IGC.

The Influence of Copper Additions and Aging on the Microstructure and Metastable Pitting of Al-Mg-Si Alloys

The metastable pitting of 6xxx series aluminum alloys (Al-Mg-Si-(Cu)) as a function of Cu content and aging was studied. Potentiodynamic polarization and potentiostatic transients were performed to...

Effect of alloy composition and heat treatment on mechanical performance of 6xxx aluminum alloys

The effect of alloy composition and heat treatment, including natural ageing and pre-ageing, on the mechanical performance of eight 6xxx alloys designed with systematically varying Si, Mg and Cu contents was studied. The results show that not only the alloy composition and heat treatment before forming influence the formability, but also they have an effect on the paint bake response of the alloys. Increasing the alloy Si content, decreasing Mg/Si ratio and adding 0.3% Cu (mass fraction) were generally found to improve the tensile ductility and formability of the alloys studied, while pre-ageing was found to decrease these properties. A full property profile of these alloys in terms of strength, tensile ductility, work hardening, strain rate sensitivity, forming limit and paint bake response was presented.