Microstructure and residual stress distributions in friction stir welding of dissimilar aluminium alloys

The aim of this investigation was to study the effect of welding heat input and postweld natural aging on residual stress, microstructure, and precipitation distribution in different zones of dissimilar friction stir welding of 8 mm thick plates of AA6082-T6 and AA7075-T6. It was found that atomic diffusion occurs at the interface of the materials in the stir zone of the joints. Transmission electron microscopic investigations showed that reprecipitation of fine Guinier–Preston zone, β′, and η′ precipitates resulted in increased micro-hardness in the SZ after natural aging. An increase in welding heat input resulted in decreased maximum tensile residual stress and increased size of the tensile residual stress region. Natural aging within the SZ and thermo-mechanical affected zone resulted in 15–20 MPa reduction of the residual stress in these zones.     

Effect of shot peening on the residual stress distribution in two commercial titanium alloys

The residual stress distribution in two commercial titanium alloys, the near-alpha IMI-685 and the alpha-beta IMI-318, have been determined using the method of drilling holes for the machined and polished as well as the machined, polished and shot-peened conditions. The residual stresses were always compressive and their peak values for both alloys were similar. However, the stress distribution patterns were different and this could have implications for the fatigue behaviour of the alloys.     

Measurement of residual stress in quenched 1045 steel by the nanoindentation method

In this paper, the residual stress in quenched AISI 1045 steel was measured by a recently developed nanoindentation technique. Depth control mode was adopted to measure the residual stress. It was found that residual compressive stress was generated in the quenched steel. The material around nanoindents exhibits significant pile-up deformation. A new method was proposed to determine the real contact area for pile-up material on the basis of invariant pile-up morphology of the loaded or unloaded states. The results obtained by the new method were in good agreement with the residual stresses measured by the classical X-ray diffraction (XRD) method.     

Prediction of machining quality due to the initial residual stress redistribution of aerospace structural parts made of low-density aluminium alloy rolled plates

During the machining of thick, large and complex aluminium parts, the redistribution of initial residual stresses is the main reason for machining errors such as dimensional variations and the post-machining distortions. These errors can lead to the rejection of the parts or to additional conforming operations increasing production costs. It is therefore a requirement to predict potential geometrical and dimensional errors resulting from a given machining process plan and in taking into consideration the redistribution of the residual stresses. A specific finite element tool which allows to predict the behaviour of the workpiece during machining due to its changing geometry and to fixture-workpiece contacts has been developed. This numerical tool uses a material removal approach which enables to simulate the machining of parts with complex geometries. In order to deal with industrial problems this numerical tool has been developed for parallel computing, allowing the study of parts with large dimensions. In this paper, the approach developed to predict the machining quality is presented. First, the layer removal method used to determine the initial residual stress profiles of an AIRWARE^? 2050-T84 alloy rolled plate is introduced. Experimental results obtained are analysed and the same layer removal method is simulated to validate the residual stress profiles and to test the accuracy of the developed numerical tool. The machining of a part taken from this rolled plate is then performed (experimentally and numerically). The machining quality obtained is compared, showing a good agreement, thus validating the numerical tool and the developed approach. This study also demonstrates the importance of taking into account the mechanical behaviour of the workpiece due to the redistribution of the initial residual stresses during machining when defining a machining process plan.     

The influence of residual gas pressure on the stress in aluminium films

The paper presents the results of an experimental study of the influence of residual gas pressure on the internal stress in vapour-deposited thin Al films. The results of specific resistivity measurements were used for a quantitative evaluation of the degree of Al film oxidation. The curve of stress versus degree of oxidation is interpreted as due to relaxation forces generated on the crystallite boundaries.     

应用纳米压痕法测量电沉积镍镀层残余应力的研究

应用纳米压痕法测量残余应力的2种理论模型对5种电沉积镍镀层中的残余应力在不同压痕深度处进行了测量,并与X射线衍射法的测量结果进行了比较.结果表明,压深位于薄膜/基底界面处的2种压痕法测量结果与X射线衍射法的测量结果相近,且Yun-Hee模型与其符合得更好.     

Microstructure and nanoindentation measurement of residual stress in Fe-based coating by laser cladding

Laser cladding is a surface modification technique for improving surface properties. However, high residual tensile stress is always originated, which can create cracks in cladding coatings. The through-thickness residual stresses in Fe-based coating prepared by laser cladding were measured using a non-destructive nanoindentation technique. The cladding coating with a thickness of 1 mm exhibited a uniform appearance, and no crack or defect was observed. An excellent metallurgic bond was obtained between the coating and the substrate. The XRD pattern implied that the coating was mainly composed of α(Fe, Cr) solid solution, Cr₇C₃ hard phase and Fe₂B stable phase. The residual tensile stress in the cladding coating was found to increase as the increasing of the distance from coating surface. Although the tensile residual stress reaches the highest magnitude of 700 MPa up to the depth of 600 μm, no dehiscence happened. The prepared cladding coating has good plasticity and toughness, as well as low crack sensitivity.     

Effect of hydrogen in aluminium and aluminium alloys: A review

Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.     

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.     

Influence of the microstructure of aluminium alloys on their residual impact properties after a fatigue loading program

Consecutive loadings of fatigue and impact have been carried out on aluminium alloys. The aim of this study is to quantify the influence of the microstructure on the residual impact behavior after a prior fatigue loading. Two alloys with different chemical compositions and hardening modes have been investigated: 2017A-T3 used in the aircraft industry and 5454-O used in automotive applications.The fatigue pre-loadings were carried out under fully reversed tensile-compression with several pairs (stress level, number of cycles) in the high cycle fatigue zone (10⁵– 10⁶cycles). The residual impact behavior was determined under tensile loading, in the range of medium strain rates (about ). To assess the prior fatigue damage and to follow its evolution during the impact loading, observation of the specimens (surface and fracture surfaces) were made.From this study, two conclusions have been highlighted: (1) there is no direct correlation between a given prior loading and residual behavior, whatever the material; (2) the material aspect is fundamental. At the mechanical (macroscopic) scale, the Al–Mg alloy (5454-O) remains insensitive to the prior fatigue loading whereas the Al–Cu alloy (2017A-T3) undergoes a large modification in its residual performance. At a lower scale, the pre-damage signature appears for the insensitive as well as for the sensitive material. The prior damage and its contribution to the process fracture appear to be strongly linked with the material’s microstructure.     

Three-dimensional stress distribution around cold expanded holes in aluminium alloys

Aluminium alloy 6082 (HE30) and aluminium-lithium alloys 8090 and 2091 were examined after holes were cut and cold-expanded by means of an oversized mandrel. The extent of cold working was analysed by two techniques, namely neutron diffraction (ND) and the Sachs cutting method (SM). While the former requires neutron diffraction facilities and is a non-destructive measurement technique, the latter is based on the removal of small layers of material from the inside of the expanded hole while recording the apparent change in strain by the removal of the layer. The stress distributions analysed by ND are compared with the method of Sachs and the results are discussed with respect to distances over which the state of stress changes from compression to tension. These changes are significant in explaining the improvement of service life in mechanical fastener holes from the process of cold expansion.

In all alloys examined it was found that the improved fatigue performance was a function of the degree of expansion. All alloys exhibited a maximum in expansion beyond which fatigue life deteriorated. This was due to crack initiation during excessive hole enlargement exceeding the yield strength of the alloys. Crack initiation usually started from inside the hole of the fastener but always perpendicular to the applied load. The initial stages of growth occurred perpendicularly over a short distance but further growth occurred on a plane with normals inclined at about 55–60° to the tensile axis. This behaviour was due to the presence of texture development in the alloys which is beneficial to the resistance of both fatigue crack initiation and propagation.     

Residual stress magnitudes and related properties in quenched aluminium alloys

Abstract

The surface and through thickness residual stress magnitudes present in heat treated high strength aluminium alloy components are frequently reported to exceed the uniaxial yield stress of small specimens of the same alloy measured immediately after quenching. In thick section plate and forgings, it is proposed that these high residual stress magnitudes are a consequence of hardening precipitation that occurs during quenching which allows a greater elastic stress to be supported. To investigate this theory, a Jominy end quench technique is used to determine the hardness of aluminium alloys 7010, 7175 and 5083 as a function of distance from the quenched end. Cooling curves have been measured for Jominy end quench specimens using deeply buried thermocouples and are compared with finite element model predictions. Tensile properties are also determined for small specimens quenched into cold and boiling water. Vickers hardness and X-ray diffraction residual stress measurements are undertaken on specimens of varying size acting as a comparison with the Jominy results. These results in combination with optical and electron microscopy data do suggest that low temperature rather than high temperature precipitation during the quench leads to increased as quenched mechanical strengths, with the consequence that less quench sensitive alloys will support higher residual stress magnitudes as section thicknesses increase.     

Influence of aqueous salt environments on fatigue response of SiCp reinforced aluminium alloys

Abstract

The low cycle fatigue response of aluminium alloy 2124 reinforced with silicon carbide particulates produced by a commercial powder metallurgy route has been evaluated. This assessment includes measurements of fatigue crack growth rates and ‘total life to failure’ experiments. The relative performance under aqueous saline environments, namely total immersion in a 3.5% salt solution and exposure to a salt fog of similar concentration, were then characterised. While the alloy essentially demonstrates an insensitivity to saline environments in terms of its crack growth characteristics, a process of surface corrosion and pitting is highlighted and deemed responsible for significant reductions in the total fatigue life. The implications for engineering designs attempting to make use of this class of metal matrix composite are discussed.     

Effect of mean stress on residual stress relaxation in steel specimens

Abstract

Residual stress relaxation was investigated by subjecting specimens with various mean stress levels to strain controlled cyclic loading. The material studied was mild steel in three different conditions. The mean stress levels ranged from 100 to 200 MPa, and two strain amplitudes, 0·05 and 0·06%, were studied in detail. The residual stresses in the specimens were measured before and after mean stress relaxation experiments. It was found that experimental factors such as temperature variations and crack growth have a significant influence on the results. Based on the experimental results, it is proposed that the mean stress relaxation exponent should be divided into two parts: mean stress dependent and mean stress independent. The first includes the contribution of quasi-static relaxation, i.e. mean stress dependent plastic deformation. The second part includes the contribution of cycle dependent mean stress relaxation, which does not depend on the mean stress.     

Change of the ultrasonic characteristics with stress in some steels and aluminium alloys

Under application of tensile stress to specimens of several steels and aluminium alloys up to a stress close to their yielding stress, a continuous ultrasonic wave of approximately 5 MHz was propagated through the specimens in directions parallel and perpendicular to the tensile axis. The change of ultrasonic resonance frequency with applied stress was measured. The results of the measurement of the ultrasonic resonance frequency showed that the frequency ratio varied with the magnitude of applied stress, the material of the specimen, and ultrasonic wave propagating direction with respect to the direction of the stress. They also showed that the ultrasonic wave resonance frequency measuring method is useful in finding the ultrasonic characteristics of materials under applied stress.List of symbols density - , Lame's constants - l, m Murnaghan's constants - stress - wavelength - n order number of resonance frequency - f _n resonance frequency ofnth order - elastic strain - v Poisson's ratio - C ultrasonic velocity - L length of specimen     

Modification of residual stress and microstructure in aluminium alloy by cryogenic treatment

The effect of cryogenic treatment (CT) combined with solution and ageing treatment on the residual stress and microstructure of 7050 aluminium alloy was investigated. The relationship between residual stress and microstructure was discussed. The results showed that compared to solution–ageing treatment (SA), CT after solution and before ageing (SCA) exhibited lower level and higher uniformity of residual stress. The highest dimensional stability was also obtained by the process of SCA. It was found that CT induced the fine precipitates through the lattice contraction under cryogenic temperature. Furthermore, the execution of CT in advance would also promote the uniform distribution of precipitates in the subsequent ageing by releasing and homogenising the residual stress.