Recrystallised grain size of commercial purity aluminium after hot torsion within steady state regime

Abstract

Hot deformation within the steady state regime of commercial purity aluminium has been carried out under wide ranges of deformation parameters: temperature, strain, and strain rate. The microstructure developed after deformation exhibited dynamically formed grains having volume fraction increasing with strain and of size inversely dependent on flow stress. The effects of the parameters of deformation and of the annealing temperature after deformation on statically recrystallised grain size and on the rate of grain boundary migration during recrystallisation were studied and can be satisfactorily described by equations in which each of the variables is considered to act independently. The effects of the deformation parameters on the grafnsize produced after static recrystallisation are interpreted in terms of their influence on the number of dynamically formed grains that can act as pre-existing nuclei. A decrease in the final recrystallised grain size with an increase in annealing temperature, which may be related to the number of dynamically formed grains that become viable nuclei, was observed. The growth rate during recrystallisation was found to decrease with increasing time, probably as a result of the distribution of stored energy within the deformed structure.

MST/1169     

Static recrystallisation kinetics of commercial aluminium: influence of hot deformation mode

Abstract

The kinetics of static recrystallisation and the recrystallised grain size of a commercial aluminium alloy have been determined after both hot torsion and axisymmetric compression within the steady state regime. Annealing of the specimens, quenched after hot deformation in both modes at 325 and 360°C, was carried out at 410°C. The presence of dynamically formed grains developed during deformation, which can act as pre-existing nuclei, exerts an influence on the subsequent static recrystallisation behaviour of the alloy. The increase in number of these dynamically formed grains with strain explains the significant effect of strain in the steady state. However, some differences result from the different modes of deformation. Hot axisymmetric compression leads to an important decrease of both the time to reach a recrystallised fraction of 50% and the recrystallised grain size in comparison with hot torsion. The different deformation modes, through their effect on the spatial distribution of particles, seem to affect the proportion of dynamically formed grains which become effective nuclei and therefore the recrystallisation kinetics. In addition, particle drag effects can influence greatly the static recrystallisation kinetics.

MST/1810     

Evolution of grain structure across joints in friction stir welded EN AW 5083 H111 plates during thermal exposure

Abstract

The mechanisms involved in the coarsening of the grain structure across FSW joints produced in EN AW 5083 H111 plates with different pin profiles were investigated. The grain coarsening across the FSW joints invariably starts inside the shoulder flow zones and involves both abnormal growth of the grains inside the dynamically recrystallised regions as well as recrystallisation of the deformed grains outside these regions. The elliptical nugget zones produced with the threaded circular pins appear to be more stable with respect to the basin shaped counterparts obtained with the triangular pin. It takes an annealing temperature of 550°C for the former to become fully coarse grained across the joint while the joint produced with the triangular pin is largely covered with coarse grains starting at 500°C. The higher resistance of the nugget zone obtained with a threaded circular pin to grain coarsening appears to be critical in the selection of the pin profiles when the manufacturing cycle of the welded EN AW 5083 plates involves post-welding heat treatments.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.     

Influence of dynamic recrystallisation on hot ductility of aluminium killed mild steel

Abstract

Torsion tests at strain rates of 10^?3, 0·16, and 1 s^?1 and at temperatures between 850 and 1100°C have been carried out on an aluminium killed mild steel after either heating directly to the testing temperature or soaking for 30 min at 1200°C, then cooling the specimen to the testing temperature. During direct heating, a dispersion of small AlN particles was present and after soaking a coarser distribution of AlN particles was observed. Compared with the latter, the former microstructure seriously retarded the completion of dynamic recrystallisation. The effect of the two different particle distributions was examined for the following parameters: peak and steady state stress, peak strain, and dynamically recrystallised grain size. The retardation of completion of dynamic recrystallisation has the additional effect of impairing the hot ductility.

MST/997     

Dynamic recrystallisation of as cast austenite in 18–8 stainless steel

Abstract

Dynamic recrystallisation behaviour of an as cast 0Cr18Ni9Ti stainless steel during hot deformation was investigated by hot compression test at a temperature range of 950–1200°C and strain rate of 5 × 10^-3–1 × 10^-1 s^-1. Change of austenite grain size owing to dynamic recrystallisation was also studied by microstructural observation. The experimental results showed that the hot deformation conditions, such as temperature, strain, and strain rate determine the dynamic recrystallisation behaviour for the as cast stainless steel, and the dynamically recrystallised grain size is determined by the deformation conditions and is independent of the strain.     

Microstructural evolution of AZ61 magnesium alloy during hot deformation

Abstract

The microstructural evolution of AZ61 magnesium alloy during hot compression at various temperatures was investigated. The experimental results show that dynamic recrystallisation occurs over a wide temperature range. Grains can be greatly refined through dynamic recrystallisation. The mean size of the recrystallised grains increases with a decrease of temperature or value of Z (Zener – Hollomon parameter), while the reciprocal of the recrystallised grain size has a good linear relationship with the natural logarithm of the Z value, as well as the hyperbolic term of the flow stress. Basal and non-basal segments have been found in both recrystallised grains and primary grains, whereas dislocation pileups exist only in recrystallised grains when the temperature is lower than 673 K. The occurrence of twins is dependent on temperature and strain. When the strain increases, primary twins evolve into secondary twins. However, secondary twins grow with an increase of temperature; some secondary twins evolve into subgrains.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

Recrystallisation rates in austenite measured by double compression and stress relaxation methods

Abstract

The feasibility of two mechanical testing methods, double compression and stress relaxation, for measuring the static recrystallisation rate in hot deformed austenite was studied. The results were verified by metallographic observations. The effect of recovery on the softening data in an interrupted compression test is eliminated when the reloading flow stresses are analysed at a total strain of 5% instead of the 0.2% offset strain used conventionally. A stress relaxation test provides consistent data on the recrystallisation event. In most instances, the stress present in the course of softening has no effect at all or only slightly enhances the recrystallisation rate. The accelerating effect is most pronounced, about 50-70%, at small strains and fine grain sizes.     

Static recrystallisation and recrystallisation during hot deformation of Al–1Mg–1Mn alloy

Abstract

Plane strain compression tests at 5 s^?1 and at temperatures of 270–480°C have been carried out on an Al–1Mg–1Mn alloy containing a bimodal distribution of intermetallic particles and after a prior heat treatment to coarsen all particles to greater than 1 μm in size. During the heat treatment, recrystallisation of the initially hot worked material only proceeded with coarsening of the fine particles. During subsequent hot deformation, thin foil electron microscopy revealed that identical subgrain structures were developed in the two materials by dynamic recovery at temperatures below 450°C. At higher temperatures, the initially recrystallised material showed localised particle stimulated dynamic recrystallisation. The subsequent static recrystallisation rate was more than 10³ times faster in the material free from small particles.

MST/751     

Flow stress and substructural change during transient dynamic recrystallization of nickel

Abstract

The transient behaviour of dynamically recrystallizing nickel following a change in strain rate has been studied using hot tensile testing, with microstructural observations being carried out by optical and transmission electron microscopy. After a decrease in strain rate, the flow curve displayed multiple peaks accompanied by grain coarsening, whereas after an increase in strain rate, the flow curve displayed a single peak associated with grain refinement. Grain coarsening after a strain rate decrease occurred in three distinct stages, in contrast to the monotonous grain refinement taking place after a strain rate increase. In the first stage, rapid grain boundary migration caused a sharp drop in flow stress over a strain of 0·002. In the second, the new grains grew, causing coarsening and impingement, with rapid work hardening in the softer regions. Finally, in the third stage, the grain size increased further, approaching the steady state value, while multiple peaks in flow stress were observed.

MST/372     

Recrystallisation of single crystal superalloy CMSX–4

Abstract

A series of experiments investigating the recrystallisation of single crystal superalloy CMSX-4 have been carried out. Indentation atroom temperature has been used to study the effects of annealing time and temperature, and it has been found that a very strong dependence upon temperature is evident. Annealing above the γ′ solvus temperature results in very rapid growth of recrystallised grains whereas annealing below the γ′ solvus greatly suppresses the advancing grain boundaries. Additionally experiments have been carried out using an electrothermal mechanical test (ETMT) machine, to study the effects of degree of plastic strain and the temperature at which the strain is introduced. The strain threshold for recrystallisation under various annealing conditions has been determined and it has been found that recrystallisation occurs more readily if strain is introduced above 950°C. Finally, apparent activation energies for recrystallisation have been determined by measuring the change in resistivity that occurs during recrystallisation.     

Flow stress and recrystallisation during hot deformation of Cu–9%Sn alloys

Abstract

Compression tests were carried out on two compositions of Cu–Sn bronze: Cu–9·2Sn and Cu–9·1Sn–0·26Zn (wt-%). The experiments were performed at temperatures from ambient up to 750°C and at nominal (initial) strain rates in the range 10^-3 to 10^-1 s^-1. The measured data were converted into true stress–true strain curves; these displayed yield drops as well as single peaks (or maxima) at higher temperatures and lower strain rates. The mean rate sensitivity applicable to the curves was 0·25. Optical metallography indicated that dynamic recrystallisation of the ‘grain refinement’ type was taking place at the higher temperatures and proceeded by necklace formation. Electron backscattered diffraction measurements were also carried out; these revealed that twinning plays an important role in these materials. The present results show that the progress of recrystallisation is considerably slower than in OFHC copper and that the recrystallised grain size is appreciably finer. These observations, taken together, all indicate that the high temperature flow behaviour of the tin bronzes is controlled by solute drag and is not of the conventional ‘pure metal’ type.     

Effect of strain path on recrystallisation kinetics during hot rolling of Al–Mn

Abstract

Hot rolling of an aluminium–1% manganese alloy has been carried out. Wedge shaped specimens were rolled in two pass schedules, of either two forward passes or a forward and a reverse pass to the same overall net strain. Through thickness marker pins were inserted to allow the investigation of plastic flow during the different rolling schedules. The reversed rolling technique allowed the determination of the effect of a strain path change on the recrystallisation kinetics during hot rolling. Following subsequent annealing, quantitative metallography indicated that the forward–forward specimens showed faster recrystallisation kinetics than the forward–reverse specimens, and produced a finer recrystallised grain size following equivalent thermomechanical treatments differing only in strain path. A through thickness microstructural gradient was found in all materials.     

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%.     

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     

High temperature deformation of aluminium bronze

Abstract

Hot compression tests were carried out on commercial Cu–8 wt-%Al alloy to test the effect of the deformation conditions on high temperature deformation characteristics and the final structure of the hot deformed material. Dynamic recrystallisation of the material was found to operate at deformation temperatures above ～900 K. Nucleation and growth of recrystallised grains were observed for specimens deformed at temperatures below ～1000 K. However, the flow stress peaks that usually mark the onset of dynamic recrystallisation were hardly seen on the stress–strain curves. During hot deformation of Cu–8 wt-%Al alloy above ～1000 K the interaction of →β phase transformation and deformation processes affected both the flow stress value and the structure of the material. In particular, post-deformation water quenching of the specimens resulted in martensitic transformation within pre-existing β grains. Moreover, local coherent iron particles were detected within β and neighbouring grains.     

Microstructural evolution in silicon alloyed γ-Ti–Al alloys after thermomechanical processing

Abstract

The microstructures of silicon alloyed γ-Ti–Al alloys containing silicide particles have been studied after thermomechanical treatments to investigate microstructural evolution. Important parameters including temperature, forging strain, and sequence of thermomechanical treatments were systematically studied. Isothermal forging below the eutectoid temperature resulted in inhomogeneous dynamic recrystallisation with fine equiaxed grains in recrystallised areas and residual α₂ + γ lamellae elsewhere. Eutectic silicides play an important role in destruction of the as cast structure by promoting dynamic recrystallisation during deformation and static recrystallisation on subsequent annealing. There is evidence that silicon, in solution, also enhances recrystallisation. The presence of fine silicides produced by precipitation in the solid state restricts the size of grains produced by both dynamic and static recrystallisation. Silicon also alters significantly the phase equilibrium between the α and γ phases.     

Recrystallisation kinetics of TWIP steel: interface velocity and stored energy

Abstract

In the present article, a modular phase transformation kinetics model has been employed to describe the static recrystallisation behaviour of a high Mn (25 wt-%) twinning induced plasticity steel just after hot deformation. The modular recrystallisation model is based on site saturation with preferential distribution of nuclei, interface controlled growth and impingement of growing particles. The model prediction has been validated with the experimental recrystallisation data of twinning induced plasticity steel. The experimental data were also modelled using Johnson–Mehl–Avrami kinetics based on random distribution of nuclei, which showed worse prediction as compared to the modular transformation kinetic model, thereby indicating the possibility of preferential nucleation around the defect sites. The rate of recrystallisation estimated from the measured fraction recrystallisation decreases sharply within a small recrystallised fraction. The interface migration velocity (interface between the recrystallised grains and the deformed matrix) was estimated from the analytical model. The interface velocity decreases sharply within initial small recrystallised fraction. The calculated stored energy of deformation, from the estimated interface velocity and mobility of the interface, suggests an inhomogeneous distribution within the deformed matrix.     

Dynamic recrystallisation during hot working of Zr–2·5Nb: characterisation using processing maps

Abstract

The characteristics of the hot deformation of Zr–2·5Nb (wt-%) in the temperature range 650–950°C and in the strain rate range 0·001–100 s^?1 have been studied using hot compression testing. Two different preform microstructures: equiaxed (α+β) and β transformed, have been investigated. For this study, the approach of processing maps has been adopted and their interpretation carried out using the dynamic materials model. The efficiency of power dissipation given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified in the maps of equiaxed (α+β) and β transformed preforms. In the case of equiaxed (α+β), the stress–strain curves are steady state and the dynamic recrystallisation domain in the map occurs with a peak efficiency of 45% at 850°C and 0·001 s^?1. On the other hand, the β transformed preform exhibits stress–strain curves with continuous flow softening. The corresponding processing map shows a domain of dynamic recrystallisation occurring by the shearing of α platelets followed by globularisation with a peak efficiency of 54% at 750°C and 0·001 s^?1. The characteristics of dynamic recrystallisation are analysed on the basis of a simple model which considers the rates of nucleation and growth of recrystallised grains. Calculations show that these two rates are nearly equal and that the nucleation of dynamic recrystallisation is essentially controlled by mechanical recovery involving the cross-slip of screw dislocations. Analysis of flow instabilities using a continuum criterion revealed that Zr–2·5Nb exhibits flow localisation at temperatures lower than 700°C and strain rates higher than 1 s^?1.

MST/3103