Corrosion behaviour of aluminium matrix composites

Samples of aluminium alloy 2014 reinforced with 20–40 vol % of alumina or silicon carbide particles were tested by the potentiodynamic polarization technique. The chosen medium was 0.1m lithium perchlorate which tends to cause localized corrosion. The measurements revealed no impairment of the corrosion performance of the matrix alloy as a result of the presence of the reinforcement phase.     

Corrosion behaviour of squeeze-cast aluminium alloy-silicon carbide composites

The corrosion behaviour of squeeze-cast Al alloy (LM11) separately dispersed with 10 vol% SiC fibres and SiC particles was investigated in 3% aqueous NaCl solution by general corrosion as well as potentiodynamic polarization techniques. Erosion-corrosion tests were also performed on the specimens in the solution. The base alloy was also subjected to identical tests to examine the influence of the presence of SiC in the matrix. The base alloy showed a lower corrosion rate than the composites. Furthermore, the alloy containing SiC fibres showed a higher corrosion rate than the one with SiC particle dispersion. Erosioncorrosion tests indicated that the rate of material loss followed a trend similar to that in other corrosion tests. The material loss was significantly higher in the case of erosion-corrosion tests. In addition to pitting and attack at the CuAl₂ precipitate-Al interface in the matrix, dispersoid-matrix interfacial attack by the corrosion medium was also observed in the case of composites. On the other hand, erosion-corrosion revealed occasional partial removal of the dispersoid due to the impingement of the electrolyte. The tendency of the dispersoid removal by the impinging electrolyte was predominantly more in the case of the composites dispersed with SiC fibres. Results are explained in terms of the interfacial bonding as well as the shape of the dispersoid.     

Corrosion behaviour of sintered 6061 aluminium alloy-graphite particle composites

The corrosion behaviour of sintered 6061 aluminium alloy and its composite dispersed with 7.0 vol% graphite particles is described. Techniques, namely immersion, tafel and impedance, were employed to study the corrosion behaviour. Immersion tests were performed in different media, namely HCl, NaCl and lubricating oil (used and fresh) while tafel and impedance tests were performed in 0.1 N HCl and 3% NaCl solution. Corrosion behaviour was monitored by measuring the weight change in immersion studies and corrosion current, i_corr, in tafel and impedance tests. The values of corrosion potential, E _corr, and i _corr were calculated from the tafel plots obtained at a scanning rate of 1 mV s^–1. In impedance study, i _corr was calculated from the value of polarization resistance, R _p, obtained from the Nyquist plot; the latter was obtained by merging the lock-in and FFT plots. Aluminium alloy and its composite suffered from corrosion in HCl and NaCl. However, the extent of severity was greater in the case of HCl. The i _corr values obtained by the impedance method also revealed a similar trend. The higher corrosion rate of the alloy and composites in HCl was due to the dissolution of the thin oxide (protective) film in the solution. The decreased corrosion rate of the composite and the base alloy in NaCl was attributed to the formation of stable corrosion product, Al (OH)₃, on the surface, which prevented further attack on the surface of the specimen. There was practically no evidence of corrosion attack on the alloy and the composite surface in used and fresh lubricating oil.     

Annealing behaviour of dilute aluminium alloys following hot deformation

Abstract

The microstructure and texture of three dilute aluminium alloys after hot deformation and annealing was assessed; In particular, the influence of deformation temperature, strain rate, and strain on the annealed texture was examined, as well as the effect of alloy composition. The microstructures of the commercially pure materials studied (Al, Al+1%Mn and Al+1%Mg) varied in the volume fraction of coarse intermetallic particles, the type of dispersoid present, and the level and type of solute in solid solution. Furthermore, the initial stages of recovery and recrystallisation were studied in detail for one of the alloys (commercially pure Al). It was found that the main recrystallisation texture component was the cube and its strength, as well as the recrystallised grain size, depended strongly on the deformation strain. The deformation strain rate and temperature, and the alloy composition also strongly influenced the grain size and cube texture strength. These results are discussed in the context of current theories for cube nucleation within cube bands in the hot deformed microstructure. The present work was carried out as part of a wider research programme, partially supported by the European Union (Brite/Euram funded), to develop micromechanical models to describe the evolution of microstructure and texture during hot deformation and annealing of aluminium alloys.

MST/3376     

Composites of aluminium alloys: fabrication and wear behaviour

In this paper processes for fabrication of aluminium-alloy composites containing paniculate non-metals, the net shape forming of these composites, their microstructures, their friction and wear behaviours and their mechanical properties are described. Composites of two wrought (2014 and 2024) and one cast (201) aluminium alloys containing 2 to 30 wt% of Al₂O₃ and SiC particles in the size range of 1 to 142m were prepared. The non-metallic particles were added to a partially-solid vigorously-agitated matrix alloy. The particles were then retained in the matrix until interface interaction, for example, the formation of MgAl₂O₄ spinel in the case of Al₂O₃ particles, were faciliated. These composites were solidified and subsequently reheated to above their liquidus temperature and formed under high pressure in a closed-die forging type of apparatus. Composites with particulate additions of size larger than 5m possessed homogeneous structures; particles of size 1m, however, tended to cluster. The wear behaviour of the composites was studied using a pin-on-disc type machine. It was shown that composites containing large amounts of non-metals, 20 wt%, exhibit excellent wear resistance whilst those with small to moderate amounts of non-metals possess tensile properties comparable to the matrix alloy. Increasing the amount of particulate additions results in reduced ductility. Finally, a method was investigated of producing components with high weight-fractions of non-metals near their surface.     

Corrosion behaviour of some vapour deposited magnesium alloys

Abstract

Binary magnesium alloys containing chromium, manganese, or titanium were made using a physical vapour deposition technique. The corrosion resistance of the alloys was assessed in aqueous chloride solutions using total immersion tests in quiescent 600 mmol L^?1 NaCI solutions. Alloying with manganese or titanium was found to lower the corrosion rate of magnesium over most of the compositional ranges of interest, whereas addition of chromium had a detrimental effect on the corrosion resistance of magnesium. The lowest corrosion rate was recorded for a Mg–Ti alloy where the value obtained was about 80 times lower than that found for vapour deposited pure magnesium. Open circuit corrosion potential measurements conducted in 600 mmol L^?1 NaCl solution showed that additions of chromium, titanium, and manganese also produced deposits which were significantly more noble than pure magnesium, suggesting that these alloys would be less susceptible to galvanic corrosion.

MST/3064     

Tensile deformation behaviour of two aluminium alloys at elevated temperatures

Abstract

The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.     

Modelling flow stress behaviour of aluminium alloys during hot rolling

Abstract

A mathematical model is proposed to predict the flow stress behaviour of aluminium alloys under hot rolling conditions. To do so, a dislocation model for evaluating flow stress during deformation is coupled with a finite element analysis to access metal behaviour under non-isothermal and variable strain rate conditions. Then, with the aid of the proposed model, a hot strip rolling process was simulated. In order to verify modelling results, flow stress behaviour of an aluminium alloy is studied employing hot compression tests in various temperatures and strain rates and the model was examined on this material. Non-isothermal hot rolling experiments were carried out and good agreement was found between predictions and experiments.     

Electrochemical behaviour of aluminium alloys containing indium and tin in NaCl solution

The electrochemical behaviour of Al, Al–In, Al–Sn and Al–Sn–In alloys in 2 M NaCl solution has been studied using an open circuit potential, potentiodynamic polarization and ac impedance measurements as well as by optical microscopy examination. The addition of alloying components to aluminium produced in all cases a considerable activation of aluminium. The activation is manifested by shifting the open corrosion potential and the pitting potential in the negative direction (for about 0.6 V) and significant reducing of the passive potential region. The degree of activation depended on alloying element and it was found that there is an increase in the order: Al < Al–In < Al–Sn ≈ Al–Sn–In. The anodic dissolution of the Al–Sn and Al–Sn–In alloys started at open circuit potential which is only 0.45 V more positive than the thermodynamic Al³⁺/Al potential. The ac impedance measurements performed at different potentials in wide potential range (corresponding to passive and active state of each examined samples) confirmed the great activity of Al–Sn and Al–Sn–In alloys compared to aluminium.     

Corrosion behaviour,microstructure and phase transitions of Zn-based alloys

This paper is aimed at investigating the corrosion behaviour, microstructure and phase transitions of Zn-based alloys with different compositions. The corrosion tests are carried out both in acidic medium using 1 N HCl solution and in temperature dependence of thermogravimetric analysis (TGA). In the two different media, in particular, the corrosion behaviour of Zn-based alloys with respect to Al and Si contents is examined, and microstructure in acidic and TGA and phase transformations in TGA are also studied. Corrosion mechanism in TGA is also examined in terms of oxidation parameters and activation energies. The study reveals that corrosion behaviour of Zn-based alloys in acidic medium shows sometimes an increase and sometimes a decrease with time due to Al content which assists in delaying the corrosion by forming a oxide layer on the surface of Zn-based alloys. This property does not appear in temperature dependence of TGA. Further, Si content appears to remain in main matrix without being affected by acidic solution. On the other hand, it is observed that in microstructure, AlO(Al₂O₃), ZnO oxides and Zn-Cu phase precipitations are formed in main matrix, grain boundaries and partially inside the grains.     

Corrosion behaviour of aluminium in hydrochloric acid (HCl) solution containing mercaptoacetic acid

Using chemical techniques, we have examined the effect of mercaptoacetic acid (MCA), on the corrosion behaviour of aluminium alloy (3SR) in hydrochloric acid solutions. Mercaptoacetic acid acts as an accelerator of corrosion of aluminium in HCl solutions, and with mercaptoacetic acid additions, a pronounced effect was observed on the corrosion rate of aluminium and the results gave first order kinetics. The accelerating action of mercaptoacetic acid on aluminium corrosion is ascribed to a catalytic effect on the hydrogen evolution reaction.     

Comparison of hardenability and over-aging precipitation behaviour of three 7xxx aluminium alloys

The purpose of the paper is to compare hardenability and aging precipitation behaviour of three 7xxx aluminium alloys (7150, 7055 and 7056). The hardenability has been studied by Jominy test. The aging precipitation behaviour has been investigated according to their hardness and electrical conductivity after over-aging treatment. The micro-structural characterisation has been observed by scanning electron microscope and transmission electron microscope. These results show that AA7056 has the highest hardenability and sensitivity to over-aging treatment compared with AA7150 and AA7055. The highest w(Zn)/w(Mg) ratio and the lowest Cu content of AA7056 are beneficial for inhibiting quench precipitates and decreasing the stability of meta-stable η′ phase.     

Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour

Subjecting target metallic samples to a very short pulse (about 20 ns) of intense (GW cm⁻²) laser light generates, through a surface plasma, a high-pressure stress wave propagating to the first millimetre in depth, which is commonly called laser shock processing (LSP). The purpose of this work was to evaluate the role of this novel process on the cyclic properties of A356, Al12Si and 7075 aluminium alloys. Major contributors to the fatigue performance improvements were investigated in order to determine the optimum shock conditions. These were mainly compressive residual stress (RS) levels for which a large range of incident shock conditions was performed. We showed that stress levels were very sensitive to the laser fluence and the number of local impacts, and experimental RS measurements were found to be in good agreement with analytical modelling results. In comparison, a conventional shot peening (SP) treatment was found to lead to higher surface hardening and RS levels, but with a very detrimental roughening not observed after LSP. High cycle (10⁷) fatigue tests carried out on laser- processed, shot-peened and untreated notched samples illustrated the efficiency of LSP as a new, promising method to improve the fatigue limits σ_D of structures, especially in comparison with enhancements displayed by SP (+22% vs. +10%). According to crack detection electric measurements, fatigue performance improvements with LSP mainly occurred during the crack initiation stage.     

Influence of strain state in mechanical behaviour of aluminium alloys using the Small Punch Test

The Small Punch Test (SPT) basically consists of deforming a miniature specimen, whose edges are firmly gripped by a die, using a high strength punch. This test was developed in the eighties and has since been used successfully on numerous occasions in those cases where there is not sufficient material to carry out standard tests to obtain the mechanical properties of materials. In recent years, it has been used successfully to determine the yield strength evolution in any area of stamped aluminium alloy components.These stamped components are subject to different kinds of plastic deformation during the stamping process. The main objective of this paper is to analyze the influence that the type of deformation reached during the stamping process has on the mechanical behaviour of the material. Furthermore, for each kind of strain, the yield load in the SPT will be related to yield strength to provide a unique valid expression to evaluate the hardening of the material.     

Evaluation of the tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys

The tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys produced by Nuclear Metals, Inc., is determined as a function of temperature. The wrought alloy and the casting alloy are both shown to have a very high stiffness to density ratio compared with common structural materials. The wrought alloy was found to have superior fatigue strength, tensile strength and ductility relative to the casting alloy; it also maintained a greater fraction of its tensile strength as a function of temperature. The stiffness of the materials can be readily explained using standard composite theory, where the material is treated as a discontinuous beryllium-reinforced aluminium matrix composite. The strength of the casting alloy is controlled to a large extent by the strength of its aluminium alloy matrix. In contrast, strengthening increments from both dislocation-based mechanisms and load transfer appear to be operative for the wrought material. Fractographic analysis of tensile specimens showed that preferential failure of the aluminium regions or the beryllium/aluminium interfacial regions occurs under certain circumstances. Fracture analysis of fatigue samples revealed no obvious fracture initiation sites and no evidence of limited/controlled crack growth regions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of Mg on the ageing behaviour of Al-Si-Cu 319 type aluminium casting alloys

The present study was performed on primary A319.2 alloy to investigate the effect of magnesium addition as well as other melt treatment parameters such as Sr modification and grain refinement on the heat treatment behaviour of the alloy. The results show that increasing the Mg content in A319.2 up to 0.45% considerably enchances the alloy response to heat treatment in the T5 and T6 tempers, more particularly, the T6 temper. Modification of the high-Mg version of 319 alloy with Sr in amounts of 350 ppm results in a marked amount of porosity formation which counteracts the beneficial effect of the modification, leading to a noticeable decline in the alloy strength. Grain refining the Sr-modified (A319.2 + 0.45% Mg) alloy produces sounder castings and, hence, an identical ageing response to that offered by unmodified high-Mg alloys. The properties, however, are more consistent. Addition of Mg (0.45%) leads to the precipitation of coarse particles of Al₅Mg₈Si₆Cu₂. Modification with Sr tends to cause severe segregation of both Cu-containing intermetallics, i.e., Al₂Cu and Al₅Mg₈Si₆Cu₂ in areas away from the growing Al-Si eutectic regions. Thus, their dissolution rates are fairly sluggish upon solutionizing at 505 °C. Increasing the solutionizing temperature would lead to incipient melting of the phases and, hence, a catastrophic failure. Fracture of intermetallic phases in the interdendritic regions is mostly brittle, with the formation of microcracks at the Si, Cu, Fe-base intermetallics and aluminium interfaces. Fracture of the -aluminium is always ductile. Hardening during ageing occurs by cooperative precipitation of Al₂Cu and Mg₂Si phase particles.