Ageing and work-hardening behaviour of a commercial AA7108 aluminium alloy

In the 7xxx aluminium alloying system several mechanisms influence the hardening behaviour of the alloys, e.g. particle size and distribution, dislocation density and alloying elements in solid solution. This work is an experimental study of ageing and work-hardening considering a commercial AA7108 alloy in the as-cast and homogenized condition. Tensile specimens have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The tensile data for the different tempers have been evaluated in elucidation of already existing models based on a one-parameter framework. The precipitate size and distribution have been further investigated in the transmission electron microscope for a selection of tempers, and the influence of these parameters on the work-hardening behaviour has been discussed.     

Crystal plasticity finite element simulations of pure bending of aluminium alloy AA7108

The crystal plasticity finite element method (CP-FEM) is used to investigate the influence of microstructure on the bending behaviour of the heat treatable aluminium alloy AA7108. The study comprises two materials obtained from the AA7108 aluminium alloy by different thermo-mechanical treatments. The first one is an as-cast and homogenized material consisting of large grains with random texture, while the second one is a rolled and recrystallized material having refined grains with weak deformation texture. The behaviour of the two materials in plane-strain bending is investigated numerically and compared qualitatively to existing experimental data. The crystallographic texture and grain morphology of the materials are explicitly represented in the finite element models. The numerical results display a strong effect of the grain morphology on the bending behaviour, the surface waviness and the development of shear bands. These results are consistent with the experimental observations. The simulations further indicate that crystallographic texture affects the bending behaviour of the rolled and recrystallized material.     

A study of precipitation and recrystallization behaviour of aluminium alloy AA1235

The recrystallization behaviour of 92% cold rolled commercial pure aluminium has been studied. Annealing was done at different conditions to evaluate the effect of recrystallization temperature and time on the microstructure and texture of the alloy along with a study of subsequent precipitation. Variation of orientation between grains has been studied by the orientation imaging microscopy (OIM). During precipitation, cube component {001}<100> has dropped and rolling texture component has increased comparatively. Recrystallization texture is the combination of cube, rolling and random texture. However, during grain growth strong cube grains have formed. A significant number of dislocations are present during grain growth owing to the pinning effect of Al₃Fe particles.     

Nature of the interface between AA7072 alloy explosively clad to AA8090 aluminium alloy

Aluminium-lithium based alloy plates were explosively clad with Al-1 wt% Zn alloy sheets. Clad plates were evaluated for bond continuity, interface shape, microstructure, variation of elemental concentrations across the bond interface, and bond strength. Comparisons of selected characteristics were made with roll clad sheets developed earlier.Ultrasonic tests revealed the bond to be continuous at all locations except over 50 mm wide edges of the plates. Both straight and wavy shaped interfaces were observed, often alternating arbitrarily. Microstructures on each side of the interface were distinct and characteristic of the individual alloys bonded. No localized melting was observed in the interface regions. Elemental concentration varied sharply across the bond line in the as-clad condition, later changing to a smooth profile after heat treatment. The diffusion widths, when expressed as a percentage of the cladding thickness, were much smaller than the corresponding values of previously studied roll clad sheets.'Tensile shear strength' of the clad samples exceeded the shear strength of monolithic Al-1%Zn alloy, thus indicating good bonding. The bond strength values were marginally lower than those of roll clad sheets. These differences could, perhaps, be due to the differences in the extent of elemental diffusion across the bond interface between the two techniques.     

Control of microstructure and earing behaviour in aluminium alloy AA 3004 hot bands

Abstract

Control of earing behaviour at the hot band stage is a critical requirement for successful manufacture of aluminium alloy sheet for beverage cans. The present study has combined production scale experiments with laboratory examinations to investigate the effect of various material and process parameters on microstructure, texture, and earing of the resulting products. It is shown that optimisation of the product is strongly dependent on (i) iron content of the alloy, (ii) ingot homogenisation temperature, (iii) finish hot rolling temperature, and (iv) heating rate during hot band annealing. Earing level after annealing is shown to depend on the balance between cube (+ Goss) texture intensity and the volume of material having almost randomly spread orientations. Pronounced 0/90° earing tendency is usually associated with coarse and elongated grain structures. A model is shown which represents the microstructure–texture evolution as a competition between cube/Goss grains, which nucleate systematically within transition bands, and randomly oriented grains, which nucleate in the vicinity of coarse second phase particles.

MST/1032     

On the origins of the anisotropic mechanical behaviour of extruded AA2017 aluminium alloy

This paper presents some experimental investigations about the origins of the anisotropic behaviour in cyclic loadings of AA2017 aluminium alloy. In the first step, fatigue damage evolutions were quantified for controlled proportional cyclic loadings in axial and shear directions. In this stage, the aim was to confirm the anisotropic mechanical behaviour, which has recently been revealed. To this end, several models of fatigue damage quantification were used. After a comparative study between the obtained results we confirmed the anisotropic nature of the used material. In the second step, microstructural investigations were performed in order to understand the origins of the anisotropic mechanical behaviour. We used scanning electron microscopy to analyse phases and precipitates in the transversal and the longitudinal sections. It was deduced that the structure and the morphology of these entities are responsible for the anisotropic behaviour of the used aluminium alloy. Moreover, the results obtained using Kikushi diagrams, poles figure and inverse poles figures have also confirmed these conclusions. Indeed, these results have shown great differences in crystallographic texture of the material.     

Comparison of corrosion behaviour of friction stir processed and laser melted AA 2219 aluminium alloy

Dissolution of second phase particles (CuAl₂) present in AA 2219 aluminium improves the corrosion resistance of the alloy. Two surface treatment techniques, viz., solid state friction stir processing and fusion based laser melting lead to the reduction in CuAl₂ content and the effect of these processes on the corrosion behaviour of the alloy is compared in this study. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were carried out to compare corrosion behaviour. The corrosion resistance achieved by friction stir processing is comparable to that obtained by the laser melting technique.     

Fatigue behaviour of post weld heat treated electron beam welded AA2219 aluminium alloy joints

This paper reports the effect of post weld heat treatment on fatigue behaviour of electron beam welded AA2219 aluminium alloy. An attempt has been made to enhance the fatigue strength of the electron beam welded joints through post weld heat treatment methods such as solution treatment, artificial aging, solution treatment and artificial aging. Electron beam welding machine with 100 kV capacity has been used to fabricate the square butt joints. Servo hydraulic controlled fatigue testing machine with a capacity of 100 kN has been used to evaluate the fatigue life of the welded joints. Of the three post weld heat treated joints, the solution treated and aged joints are enduring higher number of cycles under the action of cyclic loads.     

Analysis of crack in aluminium alloy AA2219 weldment

Aluminium alloy AA2219 is used for fabrication of propellant tanks of launch vehicles. After welding of one of the heat treated plate (T87 temper condition) with a rolled ring (T851 temper condition), crack has been noticed near the weldment. A thorough metallographic analysis was carried out to investigate the cause for development of crack, using optical and scanning electron microscope. Morphology of cracks alongwith the attributable factors are discussed and the remedial measures are suggested.     

Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion–compression

Polycrystalline pure aluminium (99.99%) has been deformed at room temperature by the Cyclic-Extrusion–Compression (CEC)-method to strains in the range 0.9–60 (1–67 cycles). At different strains, the microstructure and local crystallography have been characterised in particular by transmission electron microscopy. It has been found that the microstructure develops from a cell block structure into an almost equiaxed structure of cells and subgrains, that the spacing between the boundaries subdividing the structure is almost unaffected by the strain and that the misorientation across these boundaries increases with the strain over the whole strain range. At the largest strain, the average misorientation across the deformation induced boundaries is 25°. The flow stress in compression is measured after the cyclic deformation and it is found that the flow stress increases with strain towards a saturation level which is reached at a relatively low strain. The discussion comprises the effect of deformation mode and plastic strain over a large strain range on the microstructural evolution and mechanical behaviour of aluminium.     

Effect of recrystallisation on Al3Zr dispersoid behaviour in thick plates of aluminium alloy AA7150

The ageing responses of surface/centre layer materials from a commercially produced AA7150 thick plate have been studied after both water quenching (WQ) and air cooling (AC). The results show that the higher degree of recrystallisation near the plate surface decreases its age hardening response in AC condition. Quench-induced phases have been found to precipitate preferentially on Al₃Zr dispersoids in recrystallised grains. Microstructural observations show that Al₃Zr dispersoids in recrystallised grains exhibit the same crystal structure and orientation as those in adjacent subgrains.     

Superplastic behaviour of annealed AA 8090 Al-Li alloy

Abstract

High temperature flow behaviour and microstructural evolution were investigated in an annealed AA 8090 Al - Li alloy over the temperature range 623 - 803 K and strain rate range ~ 6 × 10⁶ - 3 × 10² s^-1. Stress - strain rate data, obtained using a differential strain rate test technique and plotted in log - log scale, exhibited three regions I, II, and III, with increasing strain rate. In these regions, the values of strain rate sensitivity index m and the activation energy for deformation were determined to be 0.17, 0.43, and 0.17; and 758.8, 93.3, and 184.3 kJ mol^-1, respectively. The stress - strain curves obtained from constant strain rate tests exhibited flow hardening at lower strain rates and higher temperatures whereas flow softening occurred at higher strain rates and lower temperatures. The microstructural evolution revealed the dominance of grain growth under the former conditions and dynamic recrystallisation under the latter conditions. Ductility and m were found to increase with temperature, with the maximum values of 402% and 0.55, respectively, at a temperature 803 K and strain rate 2 × 10^-4 s^-1.     

Creep behaviour of AA 6061 metal matrix composite alloy and AA 6061

Abstract

The tensile creep properties of a pure AA 6061 matrix and an AA 6061 matrix reinforced with 22% of irregularly shaped Al₂O₃ particles (metal matrix composite) are presented for a temperature of 573 K and initial stresses between 15 and 70 MPa (where 70 MPa is about one-half of the yield stress). The metal matrix composite (MMC) was fabricated by a stir casting process and both materials were extruded. All the specimens were overaged before testing. The MMC exhibits a higher secondary creep rate for the whole range of loads. A stress exponent of n ≈ 1 for stresses from 15 to 25 MPa for the unreinforced material indicates the dominating diffusional creep mechanism. A stress exponent of n ≈ 3 is found from 25 to 50 MPa concluding dominating dislocation creep for the unreinforced material. This mechanism is found to be dominating for the MMC from as low as 15 MPa to 50 MPa (n ≈ 3). Although the secondary creep rate of the reinforced samples is higher than that of the unreinforced, the exposure time is longer for the MMC at stress levels below 20 MPa. The transition between the secondary and the tertiary creep stage occurs earlier in the unreinforced material. Thus, the 1% creep limit of the unreinforced alloy is reached only in the tertiary creep stage, whereas it can be applied as a conservative design criterion for the composite in the whole stress range. Furthermore, the MMC promises at low stress levels higher creep lifetime than the unreinforced alloy. Creep damage in the tertiary stage of the MMC was found to be as a result of void nucleation resulting in particle decohesion from the matrix. Relatively high tertiary creep strains are produced by necking of the unreinforced samples.     

Study of fatigue behaviour of 7475 aluminium alloy

Fatigue properties of a thermomechanically treated 7475 aluminium alloy have been studied in the present investigation. The alloy exhibited superior fatigue life compared to conventional structural aluminium alloys and comparable stage II crack growth rate. It was also noticed that the fatigue crack initiated from a surface grain and the crack extension was dominated by ductile striations. Analysis also revealed that this alloy possessed fracture toughness and tensile properties superior to that noticed with other structural aluminium alloys. Therefore the use of this alloy can safely reduce the overall weight of the aircraft.     

Modelling recovery and recrystallisation during annealing of AA 5754 aluminium alloy

Abstract

A microstructure model taking into account recovery and recrystallisation has been developed to predict the yield stress and the recrystallised grain size during continuous annealing of cold rolled AA 5754 sheet alloy. Using isothermal annealing tests, recovery and recrystallisation kinetics were quantified as a function of temperature and cold reduction. The model was formulated employing the internal state variable approach with the following three state variables: dislocation density, volume fraction recrystallised, and grain size. A rule of mixtures is adopted to separate the effect of recovery and recrystallisation in the overall softening. Model validation has been carried out by comparing the predicted softening curves with those obtained in continuous heating tests replicating heating rates of industrial continuous annealing lines. The model can be applied to non-isothermal processing routes of industrial cold rolled AA 5754 with thickness reduction in the range 40-80%.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

Mechanical behaviour and texture of annealed AZ31 Mg alloy deformed by ECAP

Abstract

AZ31 Mg alloy samples were processed by equal channel angular pressing (ECAP) at 220°C for four passes. An average grain size of ～1·9 μm with reasonable homogeneity was obtained. The ECAP process imparted large plastic shear strains and strong deformation textures to the material. Subsequent annealing of the equal channel angular pressed samples produced interesting mechanical behaviours. While yield strength increased and ductility decreased immediately after undergoing ECAP, annealing at temperatures <250°C restored ductility significantly at a small decrease in of yield strength. Annealing at temperatures >250°C reduced yield strength without additional improvement in ductility. It is believed that the combination of stress relief via dislocation elimination, refined microstructure and the retention of a strong ECAP texture at low annealing temperatures enhance ductility. High temperature annealing breaks down the ECAP texture resulting in no further improvement in ductility. The results show that the mechanical properties of the alloy can be positively influenced by annealing after ECAP to achieve a combination of strength and ductility.