Materials Science Forum Vols. 706-709

Abstract: Hot torsion tests to fracture to simulate thermomechanical processing were carried out on a solution-treated Al-Cu-Mg alloy (Al 2024-T351) at constant temperature. Torsion tests were conducted to failure in the range 270 to 470°C, between 2 and 26 s^-1. A peak ductility of the 2024 alloy was found at about 410°C. The high temperature data was analyzed by means of a Garofalo equation, obtaining a stress exponent of 6.1 and an activation energy for deformation of 180 kJ/mol. These high temperature deformation parameters correspond to an underlying deformation mechanism of constant substructure (n=8) but experiencing increasing microstructure coarsening with increasing temperature. The workability of the alloy was characterized by maximum energy efficiency and stability maps constructed from the torsion tests data to determine optimal conditions for the forming process, which depend on applied strain rate. A forming temperature of about 400°C is recommended.

Abstract: In the present study, several Al-Mg-Si alloys have been studied with respect to microstructure characteristics, i.e. particle statistics, and resulting mechanical properties. The alloys and tempers represents a wide range of type of hardening particles stretching from pre-β’’, via β’’, to post-β’’ particles such as β’, U1, U2 and B’, and various sizes, number densities and volume fractions of these particles. The correlation between volume fraction of hardening precipitates and mechanical strength is strong within alloys with pre-β’’ and β’’ as the main hardening precipitates, but this correlation does not fit for alloys with post-β’’ precipitates. However, a strong correlation between mechanical strength and both number density and cross-section area of the hardening precipitates is found, independent of type of precipitate. The consequences of these correlations are discussed with respect to proposed hardening models found in the literature.

Abstract: A model for the heat treatment of randomly distributed metal parts processed in multi-zone continuous mesh-belt furnaces has been developed. The model accounts for the heat transfer by convection and radiation to the load and the belt. The effect of gas radiation due to the presence of CO₂ and/or H₂O gases in the furnace atmosphere has been accounted for. The effect of conduction, convection, and radiation within the parts has been considered. The effective thermal properties of the load have been calculated using a new model developed for randomly distributed parts. The effective thermal properties model has been developed using experimental data obtained from transient experiments carried out at the Thermal Processing Laboratory (TPL) of McMaster University. The continuous furnace model is capable of predicting temperature distribution within the load and the belt. It has been validated using real-life data obtained from test runs carried out at two heat treatment facilities in Ontario, Canada. The effects of load density, load emissivity and belt speed on furnace productivity have been investigated using the present continuous furnace model.

Abstract: Hydrogen gas evolution behaviour during deformation and fracture in Al-Zn-Mg alloys with and without copper additions was examined by using a testing machine equipped with a quadrupole mass spectrometer in an ultrahigh vacuum chamber (QMS-UHV) and by a hydrogen microprint technique (HMT). The QMS-UHV testing revealed that hydrogen gas was evolved at the moment of grain boundary fracture, in particular. This suggested that hydrogen atoms primarily dissolved were trapped at the grain boundaries before the fracture. It was also revealed that hydrogen gas evolution behaviour was changed according to the testing strain rate. The HMT also revealed that silver particles, which represented the emission sites of hydrogen, were observed mainly around the second phase inclusions and the grain boundaries.

Abstract: Microstructure of rapidly solidified (RS) pure Al and Al-0.8; 1.0 Cr alloys (at %) foils was examined with reference to near surface composition, solute/microstructure interactions, and surface topography. The hydrogen desorption from Al-Cr alloys was investigated in order to clarify effect of rapid solidification processing (RSP) on hydrogen trapping in specimens. It is suggested that hydrogen occupancy for vacancy traps in the alloys is decreased in contrast to that reported for pure Al foils and is related to the formation of vacancy-alloying element atom complexes.

Abstract: The effect of heat treatment conditions and small addition of Fe on occurrence of serration in Al-Zn alloys was investigated. Specimens were aged for various times up to 5Ms at 293K or 273K after quenching from various temperatures (T_Q), 398K to 853K, and tensile-tested at room temperature. Serration occurred more easily according as T_Q became lower and the aging time became shorter: in the case that T_Q=473K serration was observed even after aging for 2.6Ms, while in the case that T_Q=773K serration did not occur irrespective of aging conditions. Serration was also recognized when the specimens were furnace-cooled from 773K to room temperature. Thus, for the binary alloy serration was observed only when the aging period was short enough, but addition of Fe to the binary alloy prolonged the aging period where serration could be recognized. Aging rate measured by hardness was remarkably retarded with the increase of Fe addition. These results together with those obtained by the electrical resistometry suggest that the serration in Al-Zn alloys occurs in the early stage of aging where small GP zones or solute clusters are formed.

Abstract: Aluminum alloys are commonly used as a material for heat exchangers due to their higher thermal conductivity and specific strength among various metallic materials. Twin roll strip casting process is considered to produce the high quality and low manufacturing cost aluminum alloy fin stock for automobile heat exchangers. Thermomechamical treatment has carried out to obtain optimum processes for initial cold rolling, intermediate annealing and final cold rolling, which can meet the requirements for high strength and high thermal conductivity after brazing heat treatment. Additionally the effect of copper element variation was considered to determine the optimum content of copper element in Al-Zn-Mn-Si-Fe-Cu based alloys produced by twin roll strip casting process. Mechanical properties and thermal condutivity have been evaluated before and after simulated brazing process. The nuclei of recrystallization might be formed along shear deformation bands during initial cold rolling and should be grown during intermediate annealing to enhance the permeation of molten brazings for the following brazing process. Final cold rolling has allowed strain hardening and controlling of sagging amount as fin stock materials of heat exchanger. In the present study the suitable thermomechnical treatment and optimum copper content was suggested to balance the properties of strength, thermal conductivity, brazing behaviour, corrosion resistance and sagging resistance in Al-Zn-Mn-Si-Fe-Cu based alloys produced by twin roll strip casting process.

Abstract: 6xxx Al alloys owe their superior mechanical properties to the precipitation of finely dispersed metastable β´´ precipitates. These particles are formed in the course of optimized heat treatments, where the desired microstructure is generated in a sequence of precipitation processes going from MgSi co-clusters and GP zones to β´´ and β´ precipitates and finally to the stable β and Si diamond phases. The entire precipitation sequence occurs at relatively low temperatures (RT to approx. 200 °C) and is mainly controlled by the excess amount of quenched-in vacancies, which drive the diffusional processes at these low temperatures. Very recently a novel model for the prediction of the excess vacancy evolution controlled by the annihilation and generation of vacancies at dislocation jogs, grain boundaries and Frank loops was developed and implemented in the thermo-kinetic software MatCalc. In the present work, we explore the basic features of this model in the simulation of the excess vacancy evolution during technological heat treatments. The focus of this article lies on the effect of vacancy supersaturation during different heat treatment steps, such as quenching, heating, natural and artificial aging.

Abstract: In commercial 8xxx alloys several material characteristics such as softening behaviour, strength and technological ductility are influenced by alloying elements in their solute and precipitated form (i.e. microchemistry). Depending on the thermo-mechanical processing conditions a spectrum of microchemistry states (including variations of solute level, phase volume fraction, phase morphology) and therefore a spectrum of possible microchemistry-microstructure interactions can be obtained and used for a set up of sheet properties. In the present paper, the impact of homogenization conditions on the microchemistry development in selected 8xxx alloys is discussed with a view of controlling final properties. Effect of the processing route on the microchemistry evolution is demonstrated using the statistical Classical Nucleation and Growth (ClaNG) model.

Abstract: Mechanical properties, corrosion resistance, and microstructure of Al alloys with trace element erbium were studied. Systemic studies in the pure aluminum and 5xxx series Al alloys showed that the tensile strength was significantly improved by above 20% with little loss of elongation by Er alloying. The 5xxx series Al alloys with erbium also exhibited excellent corrosion resistance. Erbium improved the aging hardness response of 7xxx series Al alloys and the addition of 0.4%Er to Al-Zn-Mg alloys increases the hardness by 35MPa. In all experiment Al alloys, a small addition of 0.1wt% Er induced a quick increase of the tensile strength and the amount of 0.4%Er shows optimized balance of the strength and ductility. The Er addition improved the thermal stability, with increasing the starting Rex temperature about 50°C in all investigated Al alloys. With regard to the microstructure mechanisms, in all experimental Al alloys Er addition has significantly refined the microstructure, which mainly attributed to presence the Al₃Er particles. In the 7xxx series Al alloys, no observable PFZ after addition of 0.4% Er was found. The fatigue property, the fracture toughness and the thermal stability of microstructure and properties are on the way.