Low frequency relaxation effect observed on 2024 aluminium alloy

2024 Aluminium alloy has been studied by isothermal internal friction after quenching and annealing at different temperatures. Experiments were performed in a very large frequency range (10⁻⁴–50 Hz) between room temperature and 823 K. At each measuring temperature, the experiment was started after complete stabilization of the microstructure and therefore the transient effects due to θ′ and θ″ precipitation were not observed. Nevertheless, a new relaxation effect was obtained in the reversion temperature range. This effect is not thermally activated, has a maximum at about 0.1 Hz and increases with the measuring temperature. It disappears completely after annealing at 823 K and successive slow cooling; it is therefore related to the θ precipitates.     

Influence of texture and grain structure on strain localisation and formability for AlMgSi alloys

The influence of texture and grain structure on strain localisation and formability is examined by experiments and numerical simulations for the extruded aluminium alloys AA6063 and AA6082. In the as-extruded condition, the AA6063 alloy has an equiaxed, recrystallised grain structure with strong cube texture, while AA6082 has a fibrous, non-recrystallised grain structure with strong β-fibre texture. By deforming and heat treating the materials after extrusion, a recrystallised equiaxed grain structure with a close to random texture is obtained for both alloys. A comprehensive test programme is conducted to determine the work hardening, plastic anisotropy and formability of the materials. Strain localisation and failure are examined by optical microscopy. An anisotropic plasticity model is calibrated for the materials and used in calculation of forming limit curves by means of the Marciniak–Kuczynski (M–K) analysis for anisotropic materials. It is found that strong cube texture leads to superior formability properties for biaxial stretching while random texture slightly lowers the formability. The strong β-fibre texture of AA6082 in the as-extruded condition leads to reduced formability. The results of the M–K analysis are very conservative compared with the experimental results, and a parametric study is undertaken to investigate the sensitivity of the predicted forming limit curves to some parameters not well defined by the experiments.     

Vanadate post-treatments of anodised aluminium and AA 2024 T3 alloy for corrosion protection

Porous anodic films formed on aluminium in sulphuric acid were post-treated using various double-dip processes that incorporate a sparingly-soluble, vanadate, corrosion inhibitor. The processes were optimised by investigating the effects of the double-dip order, the rinsing step, the immersion time and the double-dip method. The optimised processes were applied to AA 2024-T3 alloy, which was then exposed to salt spray. V-Ce(III) and V-Zn processes provided corrosion protection similar to that of sealed, sulphuric acid-formed films. A V-Ni process provided increased protection, similar to that of hot water sealed films formed in chromic acid.     

In situ X-ray tomography of intergranular corrosion of 2024 and 7050 aluminium alloys

In situ synchrotron X-ray tomography was used to monitor the development of corrosion in service-like conditions. Deliquescence of sodium chloride contaminants formed a droplet environment that caused intergranular corrosion in 2024-T351 and 7050-T7451 alloys. For the 2024 alloy, corrosion grew at comparable rates in the longitudinal and long-transverse directions. Initiation occurred at an intermetallic particle, which was surrounded by a cluster of four other particles within a 20 μm radius. For the 7050 alloy, corrosion grew fastest in the long-transverse direction, intermediate in the longitudinal direction, and slowest in the short-transverse direction. Corrosion growth of one fissure was constrained due to corrosion growth in another fissure, which may indicate that the two fissures were competing for current in the cathodically limited conditions.     

An integrated process for modelling of precipitation hardening and springback in creep age-forming

Creep age-forming (CAF) process has been developed and used to manufacture complex-shaped panel components in aerospace applications. CAF is based on the complex combination of stress relaxation, creep and age hardening. The aim of this paper is to introduce an integrated technique to model stress–relaxation, creep deformation, precipitate hardening and springback in a CAF process. Firstly, a new set of physically-based, unified creep-ageing constitutive equations is presented, which is based on the high temperature creep and ageing kinetics, and, is determined for a solution-treated and quenched AA7010. This new material model is then implemented in the commercial FE solver ABAQUS through a user defined subroutine. An integrated FE simulation process is introduced for the simulation of CAF and springback. In addition to the stress relaxation, creep-age precipitate growth and yield stress evolution during CAF are predicted.     

Synergistic effect of pH and oxalate concentration on corrosion of aluminium alloy 2024-T3

This work aims at obtaining comprehensive information on the corrosion behaviour of AA2024-T3 in oxalic acid solutions under different concentrations (0.007–0.2?M) and pH (2–6). Various tests to characterise the corrosion behaviour are performed, including weight loss and electrochemical tests. Coupon surfaces are examined using infinite-focus microscopy, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy. Corrosion rates, anodic and cathodic reaction kinetics are detailed to provide a fundamental understanding of the electrochemical behaviour of AA2024-T3 as a function of pH and oxalate concentration. The synergistic effect of pH and oxalate concentration on corrosion of AA2024-T3 is evaluated for both AA2024-T3 matrix and main intermetallics (i.e. Al–Cu–Mg and Al–Cu–Fe–Mn) and the electrochemical behaviour of each constituent element in AA2024-T3 is systematically studied.     

Simulation and measurement of aluminium–nitride precipitation in hot rolled Al killed low carbon steel coil

Abstract

Aluminium–nitride precipitation is of great importance in the deep drawability of Al killed low carbon steels. The industrial processing of Al killed steel has some critical points at which the drawability of the final product is strongly influenced by the processing parameters (hot rolling end temperature, coiling temperature). The nitrides are dissolved at the slab reheating temperature (1250°C), and most of the nitrogen remains in solid solution until the hot rolling finishes. The precipitation of nitrides in ferrite occurs more quickly than in austenite, consequently the coiling temperature must be enough low to keep the nitrogen in solid solution. The main purpose of this paper is to describe the aluminium–nitride precipitation process in hot rolled coil. Moreover, on the basis of a phenomenological model of the precipitation process the precipitated fraction of nitrides coiling has been calculated.     

Long-term atmospheric corrosion behaviour of aluminium alloys 2024 and 7075 in urban, coastal and industrial environments

Atmospheric corrosion of alclad and extruded 2024 and 7075 were investigated by weight loss, loss in mechanical properties and depth of pitting over 20 years. The results demonstrated the inner cladding layer on alclad ones had higher corrosion resistance. After 20 years exposure, the cladding had not been penetrated by pitting and those alclads retained their mechanical properties well. Exfoliation occurred on extruded ones in coastal and industrial atmospheres. Especially in coastal atmosphere extruded 2024 suffered severe exfoliation and experienced rapid deterioration of mechanical properties. Furthermore, morphology and chemical compositions of corrosion products were analysed by SEM, XRD and EDS.     

A morphological study of filiform corrosive attack on chromated and alkaline-cleaned AA2024-T351 aluminium alloy

In order to characterise filiform corrosion on a commercial AA2024-T351 aluminium alloy, a detailed microscopical study using SEM and EDS was performed. One set of AA2024-T351 aluminium alloy samples was alkaline-cleaned and deoxidised and chromate conversion coated. Another set was alkaline-cleaned only. Both samples were similarly spray coated with a 42 μm clear polyurethane topcoat. Filaments were subjected to a range of specimen preparation techniques. Sections and top views examined by SEM revealed varying degrees of attack ranging from generalised etching without local attack to severe local attack in the form of pitting, resulting in grain etchout, grain boundary attack and subsurface etchout. EDS revealed the presence of chloride deep into the pits and the subsurface etchout.     

On the microstructure and phase identification of plasma nitrided 17-4PH precipitation hardening stainless steel

Systematic microstructure characterisation of plasma nitrided (350-500 °C for 10 to 30 h) 17-4PH alloy was carried out using SEM, XRD and TEM. Experimental results have shown that the microstructure and phase constituents of the plasma surface alloyed cases are highly treatment temperature dependent. When treated at low-temperatures (≤ 420 °C), the microstructure is dominated by nitrogen supersaturated martensite (α'_N-expanded martensite); Nitrogen S-phase grains can be formed from the pre-existent retained austenite by converting the retained austenite grains in 17-4PH but no continuous S-phase layer was found. When treated at high-temperatures (above 420 °C), a surface γ′-Fe₄N compound layer was formed, CrN precipitated and S-phase was decomposed.     

Asymmetric microstructure and fracture behaviour of friction stir welded joints of 2024 aluminium alloy under cyclical load

In this study, the effect of asymmetric microstructure on fracture behaviour of friction stir welded joints of 2024-O aluminium alloy under cyclical load was investigated via optical microscopes, electron backscattering diffraction, microhardness and infrared thermography, and digital imaging correlation (DIC). The results demonstrate that the thermo-mechanical affected zone (TMAZ) at retreating side possesses a relatively higher fraction of low-angle grain boundaries and coincidence site lattice boundaries in comparison to that at the advancing side (AS), which results in the strain localisation and fracture path along the region between nugget zone and TMAZ at the AS under cyclical load. The profile of microhardness presents the asymmetric characteristic as well. The temperature evolution of the joint obtained by infrared camera is in good agreement with strain distribution measured by DIC. The thermography proves to be generally sufficient to predict the fracture path before failure under cyclical load.     

Comparison of susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys in neutral chloride solutions using electrochemical noise analysis

The susceptibility to pitting corrosion of AA2024-T4, AA7075-T651 and AA7475-T761 aluminium alloys was investigated in aqueous neutral chloride solution for the purpose of comparison using electrochemical noise measurement. The experimentally measured electrochemical noises were analysed based upon the combined stochastic theory and shot-noise theory using the Weibull distribution function. From the occurrence of two linear regions on one Weibull probability plot, it was suggested that there existed two stochastic processes of uniform corrosion and pitting corrosion; pitting corrosion was distinguished from uniform corrosion in terms of the frequency of events in the stochastic analysis. Accordingly, the present analysis method allowed us to investigate pitting corrosion independently. The susceptibility to pitting corrosion was appropriately evaluated by determining pit embryo formation rate in the stochastic analysis. The susceptibility was decreased in the following order: AA2024-T4 (the naturally aged condition), AA7475-T761 (the overaged condition) and AA7075-T651 (the near-peak-aged condition).     

Characterisation of kinematic hardening and yield surface evolution from uniaxial to biaxial tension with continuous strain path change

With respect to multistage forming processes the material behaviour and the history of the strain path during the process is of special interest for the improvement of the numerical prediction of forming processes. While different researchers investigated the Bauschinger effect during a load reversal and biaxial loading with pre-strained specimens, in this paper the yield locus evolution in the first quadrant of the principles stress space under biaxial loading of a modified cruciform specimen without test interruption is presented. The movement of the yield surface centre caused by kinematic hardening is approximated by an alternative approach based on experimental results.     

Cold metal transfer welding of aluminium to magnesium: microstructure and mechanical properties

This paper reports the procedure to join aluminium A6061-T6 to AZ31B magnesium alloy using Al-5%Si filler by cold metal transfer and pulsed-cold metal transfer processes. The effect of process parameters on the microstructure of weld, nugget/magnesium interface and tensile strength of the joints are analysed. Microstructural analysis shows that the diffusion layer thickness increases with increase in the welding current. The diffusion layer thickness was found to vary depending on the process. Pulsed process resulted in high joint strength compared to conventional method. X-ray diffraction study revealed the formation of different phases in the weld. The welding current, welding speed and the wire-feed rate had significant effect on the joint strength. Failure at the weld/Mg interface was observed.     

Local flange buckling and its relation to elastic springback in forming of aluminium extrusions

This paper presents a numerical and experimental investigation of interactions between local buckling and elastic springback in forming of thin-walled aluminium extrusions. The cross sectional distortions that are present in typical aluminium sections prior to elastic unloading cannot be disregarded. This is the case even when these beams are bent with relative large radii. The numerical analyses presented herein have shown that both element type and formulation have to be chosen carefully in order to correctly predict elastic recovery. Full integration tri-linear brick elements in combination with the assumed strain formulation have proven to be preferable to linear thick-shell, conventional full integration brick elements as well as reduced integration elements.     

Localized coating failure of epoxy-coated aluminium alloy 2024-T3 in 0.5 M NaCl solutions: Correlation between coating degradation, blister formation and local chemistry within blisters

The role of pH on the nature and rate of the degradation of epoxy coatings on AA2024-T3 panels and subsequent corrosion of the substrate during immersion in NaCl solutions was investigated. In acidic solutions both blister formation and growth are rapid. Blisters become very large (≈1 cm) and new blisters appear to form for a certain time after exposure. Often very small (∼0.1 mm) clear blisters surround these large blisters. Enhanced blister formation is due to irreversibly increased permeability of the coating for chloride ions and protons, the formation of more defect sites within the coating, and the weakening/dissolution of the oxide layer in low pH environments. In neutral pH solutions, coatings fail by forming one, or at most two, active blisters (red in color) within a few days of immersion with the time-to-failure dependent upon coating quality and thickness. Blister growth is a very slow process, and blister diameters rarely exceed a few millimeters even after several weeks. The accumulation of corrosion product within the blister slows down the corrosion rate and blister growth. The chloride concentration in the occluded solutions within the blister is significantly increased over the bulk concentration, and the pH is often in the acidic range. From electrochemical measurements it can be concluded that the anodic and cathodic reactions are confined to the blister and its immediate surroundings, rather than involving more of the surface over which the coating is intact. Based on corrosion morphology it is concluded that replated copper contributes to the overall cathodic reaction.     

Microbially influenced corrosion of zinc and aluminium - Two-year subjection to influence of Aspergillus niger

Aspergillus niger. Tiegh., a filamentous ascomycete fungus, was isolated from the metal samples exposed to marine, rural and urban sites in Lithuania. Al and Zn samples were subjected to two-year influence of A. niger under laboratory conditions in humid atmosphere. Electrochemical impedance spectroscopy (EIS) ascertained microbially influenced corrosion acceleration (MICA) of Zn and inhibition (MICI) of Al. EIS data indicated a two-layer structure of corrosion products on Zn. The microorganisms reduced the thickness of the inner layer, whose passivating capacity was much higher when compared to that of the outer layer. An increase in aluminium oxide layer resistance but decrease in the layer thickness implied that MICI affected primarily the sites of localized corrosion of Al (pores, micro-cracks, etc.). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) studies indicated that bioproducts (i.e. organic acids) did not form crystalline phases with corrosion products of zinc. The study suggested a hypothesis that microorganisms could be used as corrosion protectors instead of toxic chemicals, application of which tends to be increasingly restricted.     

Experimental evidence of concurrent compositional and structural instabilities leading to ω precipitation in titanium-molybdenum alloys

The ω phase is commonly observed in many commercial β or near-β titanium alloys on rapidly cooling from the single β phase field and also during subsequent isothermal annealing. However, the crystallographic formation mechanism for the ω particles is hitherto unclear/under discussion. The present study primarily focuses on ω precipitation within the β (body-centered cubic (bcc)) matrix of simple model binary titanium-molybdenum (Ti-Mo) alloys. It provides direct experimental evidence of the formation of ω-like embryos from competing compositional and structural instabilities arising in the bcc lattice of Ti-Mo alloys during rapid cooling from the high-temperature single β phase field. The displacive partial collapse of the {1 1 1} planes of the parent bcc structure within compositionally phase-separated regions containing several at.% less of Mo, forming ω-like embryos, has been conclusively shown by coupling aberration-corrected high-resolution scanning transmission electron microscopy with atom probe tomography observations. Growth and coarsening of these ω-like embryos take place during subsequent isothermal annealing, accompanied with both a completion of the collapse of the {1 1 1} β planes leading to a fully developed ω structure as well as rejection of Mo from these precipitates, resulting in near-equilibrium compositions.     

Modelling thermal cycles and intermetallic growth during friction melt bonding of ULC steel to aluminium alloy 2024-T3

Abstract

Dissimilar materials, aluminium 2024-T3 and ultralow carbon steel, have been welded by a novel process called friction melt bonding. A finite element thermal model is developed to predict temperature cycles and to estimate the fusion pool geometry and the intermetallic bonding layer thickness. The total mechanical power input in pseudo-steady state is inferred from in situ measurements at the tool torque and rotational speed. Temperature dependent properties, including the latent heat of fusion, and proper contact conditions between the welded plates and the backing plate are included. Predicted temperatures are in agreement with the measurements at various distances from the weld centreline. Molten pool geometries and intermetallic thicknesses, whose control is crucial to insure good weld mechanical performances, are also in accordance with the experimental observations.     

The origin of weld seam defects related to metal flow in the hot extrusion of aluminium alloys EN AW-6060 and EN AW-6082

Longitudinal weld seams are an intrinsic feature in hollow extrusions produced with porthole dies. As these joins occur along the entire extruded length, it is desirable that these weld seams have a minimal impact on the structural integrity of the extrudate. In particular, defects associated with weld seam formation should be avoided. In this research, the occurrence of defects related to material flow inside the extrusion tooling is studied. In lab-scale experiments, EN AW-6060 and EN AW-6082 aluminium alloy billets are formed into strips by means of the direct hot extrusion process. By utilising model dies with an internal obstruction similar to the supports present in porthole dies, a strip with a central longitudinal weld seam is formed. The effects of different geometries of the weld-chamber and the processing conditions on the quality of the weld seam are investigated. Characterisation is performed through mechanical testing, focusing on the ability of the weld seam area to accommodate plastic deformation, and microstructural analysis provides insight into the defects related to unsound metal flow. Through computer simulations, conditions related to weld seam formation are modelled and correlated with the experimental results. The experimental results demonstrate that metal flow controlled by the die geometry causes defects leading to inferior mechanical performance of the extrudate. It is further argued that current weld seam formation criteria utilised in finite element modelling need enhancement to incorporate these flow related defects.