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     

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     

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.     

Dislocation substructures developed during dynamic recrystallisation in polycrystalline nickel

Abstract

The dislocation substructures developed during dynamic recrystallisation (DRX) were studied by means of tensile tests and metallographic observations on polycrystalline nickel. The average cell size in the homogeneous substructures decreased rapidly with straining to about half the peak strain, whereupon DRX nuclei began to form. The average cell size then approached a constant value for increasing strain. Full DRX substructures were distributed heterogeneously throughout all areas, and were classifiable into three categories: (i) DRX nuclei, (ii) growing DRX grains containing a dislocation density gradient, and (iii) large DRX grains with a fairly homogeneous substructure. The average cell size in region (iii) could be expressed as a function of either the peak flow stress or the DRX grain size. The relationship between these microstructures and flow behaviour under DRX are discussed in detail.

MST/1285     

Effect of multipeak dynamic recrystallisation on structure of deformed and annealed copper

Abstract

Compression tests were carried out on fine grained copper at 870 K and at a constant true strain rate of 1·4×10^?3 s^?1. Under these conditions, well defined flow stress oscillations followed by steady state flow stress are obtained. Grain size measurements of as deformed material revealed non-monotonic grain coarsening when stress oscillations appear. It was found that grain coarsening is most effective when the flow stress decreases after the first flow stress peak. Periodic flow stress is accompanied by periodic grain coarsening until the latter becomes practically independent of strain when the steady state flow stress region is attained. The structural effects of static processes (recovery and recrystallisation) in dynamically recrystallised material were examined closely. According to the model of periodic dynamic recrystallisation, one would expect periodic changes of the driving force for static restoration processes (mainly metadynamic and/or static recrystallisation). From the present work, conclusions are drawn that are contrary to this concept of structural softening. The critical strain leading to grain coarsening during post-deformation annealing of hot deformed copper was found to be significantly less than the strain corresponding to the first flow stress peak. For higher strains, the grain size of dynamically recrystallised copper was found to be highly stable during annealing for 7 h at 870 K.

MST/978     

Microstructural characterisation of friction stir processed aluminium

Abstract

Friction stir processing was carried out on commercially pure aluminium, and a detailed microstructural characterisation was performed by electron backscattered diffraction and transmission electron microscopy. Friction stir processing resulted in significant grain refinement with narrow grain size distribution. The microstructure showed fine and equiaxed grains, with some ultrafine grains being also observed. Electron backscattered diffraction studies showed majority of the boundaries to be high angle, confirming the occurrence of dynamic recrystallisation (DRX). Transmission electron microscopy observations revealed dislocation arrangement into subgrain boundaries, grains having different dislocation densities and in different stages/degrees of recovery. Electron backscattered diffraction analysis also revealed a progressive transformation of sub-grain boundaries into high angle grain boundaries. A multimechanism of dynamic recovery, continuous DRX and discontinuous DRX seems to be operating during the process. The microstructure is not affected by changing the rotation speed from 640 to 800 rev min^?1, except that the grain size was marginally larger for higher rotational speed.     

Acceleration of regeneration treatment for nanostructured bainitic steel welding by static recrystallisation

Abstract

In order to accelerate bainite transformation during regeneration treatment following nanostructured bainite steel welding, static recrystallisation mechanism is adopted to refine the coarse grain size during two-pass welding. Large mechanical deformation was introduced into the coarse grain zone of the first weld pass, where static recrystallisation would occur with thermal effect of the second weld pass. Static recrystallisation kinetics of nanostructure bainite was studied by two-pass hot deformation experiments. The explicit temperature function of two-pass welding was presented, and effect of welding parameters on static recrystallisation of the deformed coarse grain zone was analysed. Experimental results showed that grains in original coarse grain zone were greatly refined and that nanostructured bainite regeneration was remarkably accelerated.     

Effect of nish rolling temperature on static recrystallisation in hot bands of electrical steel containing 1·3% silicon

Abstract

The effect of the finish rolling temperature (FRT) on recrystallisation behaviour in hot bands of an electrical steel containing 1·3%Si was investigated. Four sequential passes of hot rolling were carried out on the 1·3%Si electrical steel, with finish rolling temperatures ranging from 980 to 700°C, followed by isothermal annealing at 720°C. The experimental results showed that when Ar ₁ <FRT <Ar ₃, fine equiaxed subgrains formed at the boundaries between deformed and non-deformed grains in a necklacelike arrangement, and strain induced boundary migration (SIBM) was the main mechanism corresponding to the formation of recrystallisation nuclei for steels finish rolled below Ar ₁. However, the study also demonstrated that when FRT <(Ar ₁­100 K), a second nucleation mechanism, i.e. subgrain growth, became active in recrystallisation, this resulted in an increase of nucleus density. Steels in which SIBM was the dominant mechanism of recrystallisation possessed the largest grain size, and strongest textures with major component {100}〈110〉.     

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     

Hot working and crystallographic texture analysis of magnesium AZ alloys

Abstract

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10^?3 and 5 s^?1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol^?1 and Z values range from 10 to 10 s^?1. Textural examinations show basal texture as the predominant orientation.     

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     

Microstructural modelling of dynamic recrystallisation using an extended cellular automaton approach

Dynamic recrystallisation (DRX) governs the plastic flow behaviour and the final microstructure of many crystalline materials during thermomechanical processing. Understanding the recrystallisation process is the key to linking dislocation activities at the mesoscopic scale to mechanical properties at the macroscopic scale. A modelling methodology coupling fundamental metallurgical principles with the cellular automaton (CA) technique is here derived to simulate the dynamic recrystallisation process. Experimental findings of a titanium alloy are considered for comparison with theory. The model takes into account practical experimental parameters and predicts the nucleation and the growth kinetics of dynamically recrystallised grains. Hence it can simulate different stages of microstructural evolution during thermomechanical processing. The effects of hot working temperature and strain rate on microstructure were studied, and the results compared with experimental findings.     

Recrystallisation and texture in hot deformed aluminium alloy 7010 thick plates

Abstract

The recrystallisation behaviour and its effect on the crystallographic texture during solution heat treatment of thick aluminium alloy 7010 plates has been studied both on commercially hot rolled plates and on laboratory specimens deformed in plane strain compression. In addition to X-ray macrotexture analysis, individual grain orientation measurements within the recovered subgrains and the recrystallised new grains were performed by means of electron backscattering diffraction. Significant recrystallisation takes place during the solution treatment with almost no change in the macrotexture. This behaviour is explained by the balancing effect of randomisation of the texture during recrystallisation and texture sharpening within the recovered matrix regions.

MST/3433     

Processing map for hot working of Ni–16Cr–8Fe alloy (IN 600)

Abstract

The hot deformation characteristics of IN 600 nickel alloy are studied using hot compression testing in the temperature range 850–1200°C and strain rate range 0·001–100 s^?l. A processing map for hot working is developed on the basis of the data obtained, using the principles of dynamic materials modelling. The map exhibits a single domain with a peak efficiency of power dissipation of 48% occurring at 1200°C and 0·2 s^?1, at which the material undergoes dynamic recrystallisation (DRX). These are the optimum conditions for hot working of IN 600. At strain rates higher than 1 s^?1, the material exhibits flow localisation and its microstructure consists of localised bands of fine recrystallised grains. The presence of iron in the Ni–Cr alloy narrows the DRX domain owing to a higher temperature required for carbide dissolution, which is essential for the occurrence of DRX. The efficiency of DRX in Ni–Cr is, however, enhanced by iron addition.

MST/1856     

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 control by secondary processing of strip cast low carbon steel

Abstract

The secondary processing of low carbon steel strip produced by twin roll casting was investigated to examine its effect on microstructural development and mechanical properties. The as cast microstructure is predominantly acicular ferrite with regions of bainitepearlite and polygonal ferrite. Deformation at temperatures below Ar1 produces a heterogeneous microstructure with regions of moderately deformed acicular ferrite adjacent to highly deformed regions containing shear bands. Cold rolled and warm rolled steels show similar behaviour to conventional hot band in that dynamic recovery during warm rolling results in sluggish recrystallisation and produces a coarse final grain size. However, the initial as cast microstructure recrystallises at a slower rate than conventional hot band and produces a weaker recrystallisation texture. This can be attributed to the heterogeneous microstructure of the as cast strip such that, after rolling, nucleation occurs within shear bands and more ill defined sites, which results in nucleation of randomly oriented grains thereby producing a weak final texture. It was found that austenitising the as cast strip followed by rolling in the vicinity of Ar3 produces a uniform distribution of equiaxed, ultrafine ferrite UFF grains throughout the thickness of the strip. The production of UFF by twin roll casting and subsequent rolling represents a simple processing route for the production of fine grained low carbon sheet steel products.     

Predicting recrystallised volume fraction in aluminium alloy 7050 hot rolled plate

Abstract

An investigation into the partial recrystallisation that occurs during solution treatment of hot rolled thick aluminum alloy 7050 plate has been performed for three alloy variants with different zirconium concentrations. Electron backscattered diffraction has been used to study the grain and subgrain structure in the as rolled condition and after solution treatment. Localised recrystallisation in the grain boundary regions during solution treatment has been largely attributed to the stimulating effect of large intermetallic particles on the boundaries, combined with the low number density of pinning dispersoids close to the grain edges. A model that predicts the dispersoid distribution across a grain has been applied to predict the fraction of recrystallisation after solution treatment. It has been demonstrated that the recrystallised fractions predicted by the model show good agreement with those measured experimentally. Example calculations are presented showing the predicted effect of the zirconium level, homogenisation conditions, and subgrain size on the recrystallised fraction.     

Microstructure characterisation of Fe–21Cr–15Ni–Nb–V non-magnetic austenitic stainless steel during hot deformation

Hot deformation behaviour of Fe–21Cr–15Ni–Nb–V stainless steel was investigated by isothermal compression in the temperature range of 950–1150°C with a strain rate of 0.01–10?s^?1. The results showed that complete recrystallisation occurred beyond 1050°C, resulting from the pinning effect of (Nb, V)(C, N). The nucleation of dynamic recrystallisation (DRX) was performed by the bulging, sub-grain swallowing and twinning mechanism. With increasing strain rate, new twinning was transformed into the Σ3 regeneration mechanism in the partial DRX region, while an opposite transformation was observed in the complete DRX region. In the partial recrystallisation region, grain rotation resulted in the formation of 110 orientation. In the complete recrystallisation region, the texture tended to distribute randomly at a high strain rate, and the grain growth was accompanied by the emergence of stable 100 orientation.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

High temperature,high strain forging behaviour of binary Al–2Li alloy

Abstract

This investigation has demonstrated the utility of coupled computer simulation and constant strain rate, isothermal compression of double cone wedge tests within the dynamic recrystallisation regime, for predicting the influence of strain, strain rate and temperature on the high temperature, high strain forging behaviour of an annealed binary Al–2 wt-%Li alloy. Initially, experimentally determined true stress–true strain compression data were used to simulate isothermal forging of double cone compression specimens. At intermediate temperatures (673–773 K) and strain rates (0.01–0.001 s^-1), simulations predicted large gradients in strain across the specimen, the microstructural features in this case corresponding to both dynamic recovery and dynamic recrystallisation (DRX) within the specimen. At higher temperatures (773–823 K) and lower strain rates (0.0005–0.001 s^-1), simulations predicted a uniform strain distribution over the cross-section of the specimen, the microstructural observations correlating to DRX at lower strains and dynamic grain growth at high strain levels. Two models, one statistical, and the other phenomenological, were utilised to predict the grain size variation in the specimen as a function of strain. Both models showed excellent correlation with the experimentally measured grain size data.