A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—I. Haz temperature and strain rate distribution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part I the different components of the model are outlined and analytical solutions presented which provide quantitative information about the HAZ temperature distribution for a wide range of operational conditions. Moreover, a general procedure for modelling the HAZ strain rate distribution has been developed by introducing a series of kinematically admissible velocity equations which describe the material flow fields in the radial, the rotational, and the axial direction, respectively. Calculations performed for both types of materials show that the effective strain rate may exceed 1000 s⁻¹ in positions close to the contact section due to the high rotational velocities involved. Application of the model for evaluation of the response of AlMgSi alloys and AlSiC metal matrix composites to the imposed heating and plastic deformation is described in an accompanying paper (Part II).     

Microstructure–yield strength models for heat treatment of Al–Si–Mg casting alloys II: modelling microstructure and yield strength evolution

A microstructure–strength mathematical model for the heat treatment for commercially significant Al–Si–Mg casting alloys was developed and validated. As part of the model development, the evolution of microstructure and mechanical properties during heat treatment of an industrially cast A356 aluminium alloy was studied in an experimental investigation. A model framework to predict the evolution of microstructure and yield strength during heat treatment was proposed for hypoeutectic Al–Si–Mg casting alloys and validated against experimental measurements. In part II of this paper, the model framework to predict the evolution of microstructure and yield strength during the ageing process is described. The framework is focussed on the development of strength during natural and artificial ageing and includes the relative influences of alloy chemistry and the prior solution treatment and natural ageing history of the material. Model validation has been done using independent experimental and literature data, and the model has been applied to industrial heat treatment scenarios in order to determine the potential for process optimisation.

On a développé et validé un modèle mathématique de la microstructure-résistance du traitement thermique des alliages de moulage d’Al-Si-Mg ayant une importance commerciale. Faisant partie du développement du modèle, on a étudié, par examen expérimental, l’évolution de la microstructure et des propriétés mécaniques lors du traitement thermique d’un alliage d’aluminium A356 moulé industriellement. On a proposé une structure de modèle prédisant l’évolution de la microstructure et de la limite d’élasticité lors du traitement thermique des alliages de moulage hypo-eutectiques d’Al-Si-Mg et on l’a validé par des mesures expérimentales. Dans la seconde partie de cet article, on décrit la structure du modèle prédisant l’évolution de la microstructure et de la limite d’élasticité lors du traitement de vieillissement. La structure se concentre sur le développement de la résistance lors du vieillissement naturel et artificiel et comprend les influences relatives de la composition chimique de l’alliage et du traitement antérieur de mise en solution et de l’histoire du vieillissement naturel du matériau. On a effectué la validation du modèle en utilisant des données expérimentales indépendantes et des données de la littérature, et on a appliqué le modèle à des scénarios de traitement thermique industriel afin de déterminer le potentiel d’optimisation du procédé.     

A process model for age hardening of aluminium alloys—II. Applications of the model

The process model for ageing of aluminium alloys, developed in Part I [H. R. Shercliff and M. F. Ashby, Acta metall. mater.38, 1789 (1990)], is applied to a number of heat treatments, establishing a basis for such problems as the prediction of the strength loss in the heat-affected zone of welds. First, the reheating of previously aged material is considered. Heat treatment using a parabolic thermal cycle is then modelled in terms of an equivalent isothermal treatment, and extension to weld thermal cycles is considered. Finally the isothermal models are presented as novel “iso-yield diagrams”, which are useful for evaluating the data from thermal cycles and have potential as process diagrams.     

Microstructure–strength models for heat treatment of Al–Si–Mg casting alloys I: microstructure evolution and precipitation kinetics

A comprehensive microstructure–strength mathematical model for the heat treatment of Al–Si–Mg casting alloys is presented. As part of the model development, the evolution of microstructure and mechanical properties during heat treatment of an industrially cast A356 aluminium alloy was studied in an extensive experimental investigation. For the solution treatment process, the changes in dendritic composition and eutectic morphology in the temperature range 773–833 K (500–560°C) were quantified using microprobe and image analysis techniques. For natural and artificial ageing, the kinetics of precipitation/clustering was determined using an isothermal calorimetry technique in conjunction with hardness and mechanical property measurements. Two other Al–Si–Mg model alloy compositions were used to study the effects of alloy chemistry on microstructure response during heat treatment. The overall aim of the experimental work presented here is to facilitate the development of a comprehensive microstructure–strength model for the heat treatment of Al–Si–Mg casting alloys that will be presented in part II of this paper.

On présente un modèle mathématique détaillé de la microstructure-résistance du traitement thermique des alliages de moulage d'Al–Si–Mg. Faisant partie du développement du modèle, on a étudié l'évolution de la microstructure et des propriétés mécaniques lors du traitement thermique d'un alliage d'aluminium A356 moulé industriellement lors d'un examen expérimental de grande envergure. Pour le traitement de mise en solution, on a quantifié les changements de la composition dendritique et la morphologie de l'eutectique dans la gamme de température de 773 à 833 K (500 à 560 °C) en utilisant les techniques de la microsonde et de l'analyse d'image. Pour le vieillissement naturel et artificiel, on a déterminé la cinétique de précipitation/agrégation en utilisant une technique de calorimétrie isotherme en conjonction avec les mesures de dureté et de propriétés mécaniques. On a utilisé deux autres compositions de l'alliage modèle d'Al–Si–Mg pour étudier les effets de la chimie de l'alliage sur la réponse de la microstructure lors du traitement thermique. Le but global du travail expérimental présenté ici est de faciliter le développement d'un modèle détaillé de microstructure-résistance du traitement thermique des alliages de moulage d'Al–Si–Mg qui sera présenté dans la seconde partie de cet article.     

Growth and microstructure of Al2O3SiCSi(Al) composites prepared by reactive infiltration of silicon carbide preforms

Alumina matrix composites have been grown by directed melt oxidation of an AlSiZnMg alloy into sintered SiC preforms. Reaction between melt and the silica layer on the pre-oxidised SiC yields a matrix in which the residual alloy is principally silicon. However, there is no evidence of the formation of Al₄C₃. The coarsest particles display the least reaction while porosity increases as the particle size decreases .Thus, owing to the enrichment of silicon, growth rates are retarded compared to those in free space. The microstructure of the oxide is monocrystalline over distances of the order of the inter-particle spacing, however, a new type of growth fault that constitutes an inversion boundary in the Al₂O₃, has been found. Owing to the substantial reaction between the particle and the melt, the volume fraction of the alloy constituent in the final composite depends sensitively on particle size, in contrast to its relative invariance in directed melt oxidation into inert preforms.     

A process model for age hardening of aluminium alloys—I. The model

Process modelling techniques are used to describe the changes in yield strength due to age hardening of heat-treatable aluminium alloys. A model for the isothermal ageing curve is developed. This is demonstrated for a number of alloys and the success of the approach is assessed. Applications and a new diagram, showing the variation of strength with temperature and time, are described in an accompanying paper.     

The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—II. Tensile properties

The effect of chromium and iron additions and of annealing and working on the microstructure and tensile properties of vapour quenched AlCr and AlCrFe alloys has been determined. Tensile strengths of the worked AlCrFe alloys were in the range 568–831 MPa. Chromium in solid solution or iron present as iron-rich precipitates increased the yield stress by 44.7 MPa/at.%Cr and 333 MPa/at.%Fe respectively. The contributions to the yield strength of AlCr alloys were solid solution 40% and dislocation density/cell size 60% and to the yield strength of AlCrFe alloys were solid solution 25%, iron-rich precipitates 42% and dislocation density/cell size 33%. Vapour quenching may allow the more efficient use of alloying elements in the strengthening of Al-alloys and greater flexibility in obtaining the desired combination of solute concentration, particle volume fraction and particle size.     

Overview no. 89 Thermoelasticity and plasticity of composites—II. A model system

A theory of the workhardening and Bauschinger effect in two-phase materials, combining dislocation mechanisms with a continuum model, is extended to high volume fraction of the hard phase by using the mean field theory of Paper I [Acta metall.31, 1795 (1983)]. Application of the extended model to available workhardening data for the simple experimental model system of copper with continuous tungsten fibres reveals a novel workhardening contribution: “elastic friction”. The contribution arises from the interaction of gliding dislocations with the complex spatially fluctuating pattern of internal stresses induced by the applied stress as a result of elastic heterogeneity. Elastic friction is taken into account in a simple model of the Bauschinger effect, the “modified Orowan-Wilson model”, which is substantiated by a new set of experiments on copper-tungsten with large tungsten volume fractions.     

Morphology and chemical nanoanalysis of discontinuous precipitation in MgAl alloys—I. Regular growth

Discontinuous precipitation in MgAl alloys has been studied both by conventional TEM (morphology) and STEM with local chemical analysis (concentration profiles). Numerous growth defects have been identified. In the case of regular growth, a detailed analysis of concentration profiles using Cahn's solution has been performed, proving the necessity to introduce three velocity scales in order to describe the overall kinetics. The thermodynamical balance of the phenomenon has been derived by applying Hillert's model.     

Rapidly solidified AlCu alloys—I. experimental determination of the microstructure selection map

Laser rapid solidification experiments have been performed on AlCu alloys of hypereutectic composition 36, 40 and 44 wt% Cu. By taking thin foils from the surface of the laser traces it has been possible to study the resulting growth morphologies using TEM; the microstructural orientation allowing the morphologies to be correlated with their local growth velocities. Microstructural orientation allowing velocity range 0.01–2.0 ms⁻¹ have been studied, and eutectic (both with and without oscillatory) instabilities), dendritic, cellular, banded and planar front growth have all been observed. By combining these results with earlier observations made on alloys with lower Cu concentrations, it has been possible to produce a microstructure selection map for AlCu alloys. The map correlates microstructure to growth velocity and composition in the ranges 0.01–2.0 ms⁻¹ and 0–44 wt% Cu, i.e. the major part of the binary AlAl₂Cu eutectic (0–54 wt% Cu). As well as providing an interesting overview of solidification structures, several features of this map can be used to gain information on the AlCu phase diagram.     

A transformation kinetic model and its application to CuZnAl shape memory alloys—I. Isothermal conditions

By generalizing the impingement factor, an isothermal transformation kinetic model dy/dt = nk(1 − y)^{c + 1} (kt)^{n − 1} with a kinetic parameter of the impingement exponent, c, has been developed. The correlation between the existing isothermal kinetic functions, such as the Johnson-Mehl-Avrami equation and the Austin-Rickett equation, has been explained in terms of difference in the impingement exponent c. This paper describes a procedure for the simultaneous determination of the kinetic exponents, n and c, and their effects on the shape of the (y vs 1n t) kinetic curves. It has shown in the isothermal case that the use of the Arrhenius type equation of activation can be justified when activation energy is independent of the fraction transformed y. This modified kinetic equation has been applied to the analyses of the isothermal aging kinetics of three CuZnAl shape memory alloys with different aging products. These experimental results provide a further demonstration of validity of this modified kinetic equation for analyzing the heterogeneous solid state transformation kinetics.     

Microstructure and microchemistry of an AlZnMgCu alloy matrix-20 vol.% SiC composite

The microstructure and microchemistry of an underaged and overaged AlZnMgCu alloy matrix composite reinforced with 20 vol.% SiC was examined uusing analytical electron microscopy. The presence of MgO particles at the Al/SiC interfaces and Mg₂Si in the matrix of the composites suggests that Mg could be depleted in the matrix. The precipitates in the unreinforced control alloys were found to be consistently larger than those of the composites following the same heat treatment conditions, suggesting that the aging kinetics of the unreinforced control alloy are faster than those in the composite. Data on the aging acceleration in other Al-alloy matrix composites has been analyzed to study aging kinetics. Aging acceleration was found to be more pronounced with increasing average absolute values of the atomic size misfit parameters of the constituent elements of the major precipitates. This result suggests that the preferential segregation of solute atoms and easier nucleation of precipitates from segregated solutes is responsible for aging acceleration in other composites. On the other hand, the depletion of Mg atoms in the matrix and the lack of interaction between Zn atoms and dislocations seems to be responsible for the aging deceleration in the reinforced AlZnMgCu alloy composites.     

Microstructure and mechanical properties of a 2091 AlLi alloy—III. Quantitative analysis of portevin le chatelier instabilities and relation to toughness in AlLi,AlCuMg and AlLiCuMg (2091) alloys

The microstructure and mechanical properties of a 2091 alloy are studied and compared to simpler AlLi and CuMg alloys. For ageing times between 6 and 24 h at 150°C, the 2091 alloy exhibits a toughness drop and a simultaneous change in PLC characteristics (as evidenced by a combination of local and total strain measurements), but no significant change in microstructure, except for the size of δ′ precipitation. SEM in situ tests show that plastic instabilities are always related to extra damage. A quantitative model accounts for the toughness drop, based on plastic dissipation by PLC active bands.     

Morphology and chemical nanoanalysis of discontinuous precipitation in MgAl allyos—II. Irregular growth

Loacl chemical analysis of discontinuous precipitation in MgAl alloys has been performed in various cases where the lamellae exhibit a change in habit plane. It is shown that such an event is always preceded by the appearance of an asymmetry in the solute concentration profile in the α lamellae, and that it results in a decrease of this asymmetry. From a stability analysis of the grain boundary shape, a scenario is proposed for explaining the occurrence of these changes in habit planes.     

The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—I. Young's modulus

AlCr and AlCrFe alloys were quenched from the vapour phase onto substrates held at 230–370°C. Up to 5.6at.%Cr (10.7 wt%) was retained in solid solution. The effect of chromium content (C_Cr) on Young's modulus (E) was measured; E increased at a rate of 5.7 GPa/at.%Cr. The addition of iron up to 0.5at.% (1.0 wt%) had no detectable effect on Young's modulus of the AlCr binary alloy. Conversion of chromium in solid solution to intermetallic particles did not affect the modulus. The specific modulus (E/ρ where ρ = density) increased with chromium content and E/ρC_Cr, was 1.35 MNm/kg at.%Cr. Vapour quenched AlCr alloys have greater potential for improving the modulus of Al-alloys compared with AlLi or Al powder alloys containing iron, manganese or chromium.     

Analyses of slice compression tests for aligned ceramic matrix composites—II. Type II boundary condition

Slice compression tests performed on aligned ceramic matrix composites have been analyzed. A composite cylinder has been adopted to model the composite. Analytical solutions have previously been obtained using Type I boundary condition, in which the outer cylindrical surface of the composite cylinder is assumed to be free. This boundary condition represents a lower bound constraint for the composite cylinder during interfacial debonding. An upper bound constraint, i.e. Type II boundary condition, is adopted in the present study to reanalyze slice compression tests. Limitations exist for Type I and II boundary conditions, and for the existing methodology in evaluating interfacial properties. Hence, a more realistic constraint, i.e. Type III boundary condition, and alternate methodology are also proposed.