Materials Science Forum Vols. 519-521

Abstract: A systematic study of the precipitation processes in high strength Al-Zn-Mg-Cu alloys has been conducted. A series of experimental alloys was prepared with varying Zn:Mg ratio, but equivalent total solute content, to be representative of those used in applications which demand a combination of strength, fracture toughness and resistance to environmentally sensitive cracking mechanisms. Artificial ageing curves were constructed for each alloy, based upon 7xxx series duplex treatments widely used in industry. Ageing kinetics were investigated beyond peak strength into the overaged condition, since this is known to promote the most favourable balance of properties. Differential scanning calorimetry (DSC) and transmission election microscopy (TEM) observations were made on the alloys throughout the ageing process to fully understand the precipitation events occuring. For a particular overageing treatment, higher Zn:Mg ratio alloys were consistently found to be at a more advanced stage of precipitation while higher strengths were retained at low Zn:Mg ratios. Grain boundary characteristics, such as particle size and precipitate free zone width, were also influenced by Zn:Mg ratio at a given strength level. This paper provides new understanding of precipitation kinetics and the control of important microstructural features which influence the balance of properties in 7xxx series aluminium alloys.

Abstract: RRA treatment was applied to a high-Zn, 7XXX alloy under development for aerospace applications. Microstructure of the alloy is studied at different stages of the 3-step ageing process, by Transmission Electron Microscopy, in order to understand the corresponding evolution of mechanical and corrosion properties. The Compression Yield Strength at the end of the high temperature step was found higher than at the end of the 1st step, contrary to the conventional RRA treatment. After re-ageing, the final CYS turned out significantly higher than at the T6 temper of the alloy, while the material remained sensitive to exfoliation corrosion.

Abstract: Conventionally produced high pressure die-cast (HPDC) components are not considered to be heat treatable because gases entrapped during the die-casting process expand during solution treatment causing unacceptable surface blistering. Components may also become dimensionally unstable. Both these effects prevent the heat treatment of die-castings as these phenomena are detrimental to the visual appearance, mechanical properties and utilisation of the component. Recent work has revealed a process window in which HPDC aluminium alloys that are capable of responding to age hardening may be successfully heat treated without encountering these problems. As a result, improvements of greater than 100% in the tensile properties are possible, when compared with the as-cast condition. The new heat treatment schedules are described for HPDC parts of different size and shape, the role of chemistry on ageing is discussed and microstructural development during heat treatment examined†.

Abstract: Multicomponent Al-Si based casting alloys are used for a variety of engineering applications. The presence of additional elements in the Al-Si alloy system allows many complex intermetallic phases to form, which make characterisation non-trivial due to the fact that some of the phases have either similar crystal structures or only subtle changes in their chemistries. A combination of electron backscatter diffraction (EBSD) and energy dispersive X-ray analysis (EDX) have therefore been used for discrimination between the various phases. It is shown that this is a powerful technique for microstructure characterisation and provides detailed information which can be related to microstructure evolution during initial casting and subsequent heat treatment. The mechanical properties of different intermetallic phases have been investigated as a function of temperature using the nanoindentation technique. In particular, the hardness and modulus of a number of phases have been established for a range of alloy compositions. Physical properties of some of the intermetallic phases are also discussed. Phase identity, composition, physical and mechanical properties are set in context to inform alloy design strategies.

Abstract: Twin-roll casting (TRC) is an advantageous substitution for Direct-Chill (DC) casting in the manufacturing of rolled aluminium products. The results of a study of the phase transformations and their interaction with recrystallization occurring during the annealing of TRC Al-Mn based alloys are reported. Four alloys with different contents of Mn, Si and Fe were investigated. Precipitation was studied by resistometric measurements in the course of a heating at linear rate. The microstructural processes responsible for the observed changes in resistivity were identified by TEM examinations of quenched specimens. The changes in the microstructure and solute content during homogenisations at 450°C and 610°C were monitored by conductivity and hardness measurements and polarised light microscopy. It was elucidated that the temperature and kinetics of phase transformations are influenced not only by the content of Mn, but also by Si content. In alloys with low Si content, the decomposition of solid solution and the transformation of primary phases occur in much larger temperature range than in the alloys with high Si content. The precipitation of Mn and Si, concurrent to recrystallization, was observed to retard the latter, especially in alloys with high Mn and Si content.

Abstract: The fine-fibrous nonrecrystallized structure determines substantial tensile property anisotropy of hot rolled 1424 alloy sheets. In the present work the possibility of producing a fully recrystallized structure in cold rolled sheets was investigated. To improve a technological plasticity a cold rolling was performed after preliminary annealing of hot rolled sheets. The technological plasticity was studied by means of determination a critical deformation degree at plane strain compression at room temperature. The phase composition of hot rolled and annealed sheets was investigated using TEM and X-ray analysis. The uniform fine-grained and fully recrystallized structure in sheets was obtained after rolling with preliminary annealing at 350-380°C and final solid solution treatment. After the indicated above annealing the maximum volume fraction of the equilibrium S1-phase precipitates was observed in the alloy. After cooling in air from the annealing temperature it was found a very small quantity of δ′-phase and after slow cooling with the furnace this phase was practically absent. The fully recrystallized structure in sheets provides a substantial decrease of the tensile property anisotropy in three d irections.

Abstract: Al-Mg-Si based alloys have been developed to produce an advanced feedstock material for thixoforming. Effects of excess Si, Mn addition and deformation on microstructure evolutions during reheating and partial remelting were studied. The homogenized microstructures of the Mnadded Al-Mg-Si alloys consist of the Al12Mn3Si2 intermetallic compound along grain boundaries of equiaxed grains with fine-spherical and needle-like morphologies. The deformation by cold-rolling has a strong effect on the semi-solid microstructures of the Mn-added Al-Mg-Si alloys compared with the balanced and excess Si alloys. The Mn-added Al-Mg-Si alloys exhibit good features to produce the feedstock material for thixoforming.

Abstract: For quenching of age hardenable aluminum alloys today predominantly aqueous quenching media are used, which can lead due to the Leidenfrost phenomenon to a non-uniform cooling of the parts and thus to distortion. In relation to the conventional quenching procedures in aqueous media, gas quenching exhibits a number of technological, ecological, and economical advantages. The quenching intensity can be adjusted by the variable parameters gas pressure, gas velocity as well as the kind of gas and thus can be adapted to the requirements of the component. By the higher uniformity and the better reproducibility, gas quenching offers a high potential to reduce distortion. Cost savings would be possible, because of reduced distortion and therefore less reworking. High-pressure gas quenching with nitrogen or helium, as well as air quenching at ambient pressure in a gas nozzle field was applied to the spray formed aluminum alloy Al-17Si-4Fe-3Cu-0.5Mg-0.4Zr (DISPAL S232). Hardness and tensile tests have been carried out to determine the mechanical properties after gas quenching and aging compared to water quenching. The distortion behavior of a forged aluminum component of the spray formed alloy was examined after gas quenching and after water quenching. Gas quenching showed remarkable advantages regarding distortion.

Abstract: Rapidly solidified (RS) Al-TM (TM = transition metal) alloys are perspective materials from scientific, as well as technological point of view. Generally, they are produced by the melt atomization or by the melt spinning. Subsequent compaction is commonly performed by the hot extrusion. Since transition metals, such as Cr, Fe, Ni, Zr, Ti, Mn and others, have low diffusion coefficients in solid aluminium (lower by several orders of magnitude than those of common alloying elements like Cu, Si, Mg, Zn etc.) the RS Al-TM alloys are characterized by a high thermal stability. In this paper, several RS Al-TM (TM = Cr, Fe, Ti, Mn, Ni) alloys prepared by the melt spinning and melt atomization are compared to commercially available 2xxx, 6xxx and 7xxx wrought alloys. The main structural features of both RS and wrought alloys are described. The RS alloys are characterized by the presence of micro and nano-scale crystalline and/or quasi-crystalline phases and supersaturated solid solutions. The elevated-temperature behaviour is compared for both groups of materials. The thermal stability of the investigated materials is determined by room temperature hardness measurements after various annealing regimes and a high thermal stability of the RS alloys is demonstrated. The microstructural changes and phase transformations occurring in the investigated materials upon heating are described. In the Al-TM alloys, very slow decomposition of the supersaturated solid solutions, precipitation and decomposition of the metastable quasi-crystalline phases occur.

Abstract: Al–Cu–Ce alloys have been studied in the Al-rich corner using the microstructural (LM and SEM), thermal (direct and DSC), X-ray diffraction and electron probe microanalysis techniques. Three cross-sections were of particular interest: (a) at 20%Cu (up to 12.5%Ce), (b) at 14%Cu (up to 10%Cu) and (c) at a ratio Cu:Ce=2:1 (up to 10%Ce and 20%Cu). As-cast alloys and those after annealing at 540 and 590°C were studied. Boundaries of appearance of the Al8CeCu4 primary crystals and ternary eutectic involving the phase Al2Cu were determined. The radial cross section (Cu:Ce=2:1) is supposed to be of quasibinary type as ternary eutectic does not form. The temperature and concentrations of the eutectic reaction L–(Al)+CeCu4Al8 were found to be 610°C, 14%Cu and 7%Ce. This binary eutectic has fine microstructure and is capable to fragmentation and spheroidization during heating. The paper considers possibility of using the Al–Cu–Ce system for developing novel Al casting alloys with significantly improved casting properties (hot tearing and fluidity) in comparison with 2xx commercial alloys.