Grain Refinement In Aluminum Alloys Research Articles

Finite element simulation of extremely high shear strain during a single-pass asymmetric warm rolling of Al-6.2Mg-0.7Mn alloy sheets

Abstract Shear strain and accumulated strain plays a critical role in the grain refinement of aluminium alloys processed by asymmetric rolling, when circumferential speeds of the top and bottom rolls are different. Asymmetric rolling induces larger grain refinement in the mode of severe plastic deformation, when accumulated strain with dominant shear strain is more than 3. Searching the optimal process parameters which provide such extremely high deformation characteristics is very important. This paper presents the distributions of the accumulated strain through strip thickness of Al-6.2Mg-0.7Mn alloy processed by a single-pass asymmetric warm rolling. Effects of thickness reduction per pass (10…60%), rolls speed ratio (1…60%), diameters of the rolls (50…500 mm), contact friction coefficient (0.1…0.4), initial strip thickness (1…8 mm) and ratio of contact arc length to average thickness of strip were investigated by the rigid-plastic finite-element analysis with taking into account the heat generation due to work of plastic deformation and friction. The non-linear effect of rolls speed ratio on the deformation characteristics during asymmetric warm rolling was found. Extremely high shear strain and accumulated strain up to e=3.8...4.8 were reached during a single-pass asymmetric warm rolling. Finite element analysis of the deformation characteristics, presented in this study, can be used for optimization of the asymmetric rolling process as a method of severe plastic deformation.

Effect of severe plastic deformation by ECAP on corrosion behaviour of aluminium alloy AA 7075

Purpose: The present study compares the corrosion behaviour of overaged AA 7075 before and after equal channel angular pressing ECAP in two media, containing chlorides, in order to answer the question how grain refinement of aluminium alloys influences their corrosion properties.Design/methodology/approach: The effect of equal channel angular pressing ECAP on corrosion behaviour of aluminium alloy AA 7075 was studied in two water solutions, containing chloride ions: 1) 0.01 M Na2SO4 with addition of 0.01%Cl-, and 2) 3g/l H2O2 and 57g/l NaCl. The changes in electrochemical characteristics, provoked by grain size refinement after equal channel angular pressing ECAP, were found using potentiodynamic polarisation. Steady state potential, corrosion potential, corrosion current density; breakdown (pitting) potential of overaged and deformed by equal channel angular pressing ECAP aluminium alloy AA 7075 were measured.Findings: In the environment with lower chloride concentration equal channel angular pressing ECAP process led to increase in pitting corrosion resistance and in the medium with higher chloride concentration - to decrease in pitting corrosion resistance. That way grain refinement does not demonstrate a uni-directional influence on corrosion resistance of AA 70775.Research limitations/implications: The results suggest the possibility for development of materials having the same chemical composition but with different corrosion resistance to different environments.Originality/value: The paper presents the corrosion behaviour of ultrafine-grained aluminium alloy AA 7075 and the influence of the chloride ions concentration in the corrosion medium on this behaviour.

FEM simulation of micro-crystalline materials during ECAP based on the dislocation evolution method

Based on severe plastic deformation, the equal channel angular pressing (ECAP) method has been used for producing metal materials with the ultrafine grain size and specific mechanical properties, particularly high yield strength. The grain sizes and the mechanical properties of ECAP processed materials strongly depend on the degree of plastic deformation, which is congregated by the evolution of dislocation slipping in the slipping planes. It is very important to analyze the dislocation density and strain hardening evolution in the slipping planes. In this paper, based on the crystal plastic model, the strain hardening & grain refinement of aluminum alloys were calculated with a dislocation evolution model during equal channel angular pressing. Next, the simulated strain, stress and grain size evolution were analyzed. Although the maximum value of the strain is very similar, the stress is rapidly increased when the materials pass the shear areas. Regarding the congregation of the dislocation density, the grain sizes decrease with the process continuing.

Investigation on the mechanism of grain refinement in aluminum alloy solidified under ultrasonic vibration

The effect of ultrasound, chilled radiating face and dissolved Ti on the grain refinement of 7085 Al alloys was investigated in this paper. It was found that small equiaxed grains occurred below the chilled radiating face, and the coarse dendrites formed below the preheated radiating face. However, the dissolved Ti from the eroding radiator cannot generate grain refinement under ultrasonic vibration, which is different from the previously reported result that Ti is a powerful grain refiner. Significant grain refinement can be induced when the ultrasound was introduced through a preheated radiator between 650 °C and 620 °C. The grain refinement, involved in the 7085 Al alloys under ultrasonic vibration, is mainly attributed to the heterogeneous nucleation and acoustic streaming.

Grain refinement of pure aluminium and Al–7Si with Al–3B master alloy

The potential of AlB2 particles in the grain refinement of aluminium alloys with and without Si was investigated in an effort to develop potent grain refiners for Al–Si based foundry alloys. The Al–Si alloys are grain refined with the Al–3B alloy, whereas pure aluminium is not. The cast grain size in the former decreases with increasing Si content. The addition of 200 ppm B into the Al–7Si alloy, the basis of most commercial foundry alloys, produces remarkably fine grains with no fading effect. The Al–B system relies on a eutectic reaction L→α (Al)+AlB2 at 659·7°C to offer grain refinement, which occurs predominantly via heterogeneous nucleation of α (Al) grains on AlB2 particles thus made available. The grain refinement starts at a critical Si content enough to depress the liquidus temperature of the alloy sufficiently below the eutectic reaction temperature. The grain refining efficiency improves with increasing Si content owing to a decreasing liquidus temperature, which in turn facilitates the formation of an increasing number of solid nuclei. Alloys with insufficient level of Si to displace the liquidus temperature of the alloy below the eutectic reaction temperature cannot take advantage of this reaction and thus enjoy grain refinement. This is clearly why pure aluminium with a liquidus point above the eutectic reaction temperature is not grain refined with the Al–3B alloy at all.

On the mechanism of the formation of primary intermetallics under ultrasonic melt treatment in an Al-Zr-Ti alloy

Ultrasonic melt treatment (UST) is known to induce grain refinement in aluminum alloys, especially when transition metals like Zr and Ti are present. The refinement of primary intermetallics (e.g. Al3Zr, Al3(Zr,Ti)) caused by UST may influence the subsequent solidification process when intermetallics act as nucleation sites. In this paper, an Al-Ti-Zr alloy is used to analyse the effect of different ultrasonic intensities on the formation of these primary intermetallics. The possible nucleation behaviour of Al3Zr particles during UST is also discussed and an edge to edge matching model is used to make a preliminary analysis of lattice mismatch between aluminum oxide and Al3Zr phase. The experimental results show that when UST is applied, the Al3(Zr,Ti) particles might nucleate on aluminum oxides and remain fine during solidification.

Influence of ultrasonic melt treatment on the formation of primary intermetallics and related grain refinement in aluminum alloys.

Ultrasonic melt treatment (UST) is known to induce grain refining in aluminum alloys. Previous studies have clearly shown that in Al–Zr–Ti alloys, the primary Al3Zr intermetallics were dramatically refined by cavitation-assisted fragmentation, and a good refinement effect was achieved. In this article, Al–Ti, Al–Ti–Zr alloys, and some commercial aluminum alloys are used to analyze the effect of UST on primary intermetallics and grain refinement. The addition of a small amount of Al–3Ti–B master alloy is also studied in order to compare with the addition of Ti and Zr in commercial aluminum alloys. Experimental results show that the ultrasonic grain refining effect is not only related to the size of particles which are refined and/or dispersed by UST, but also related to an undercooling available for activation of these particles in the solidification process. Athermal heterogeneous nucleation theory is considered to explain the effect of size and distribution of substrate particles on the grain structure with different undercoolings. The distribution of primary particle sizes results in the distribution of required undercoolings. Grain refining occurs when the undercooling is large enough to activate the refined primary intermetallics or dispersed inoculants.

On the mechanism of grain refinement in Al–Zr–Ti alloys

In high-strength aluminum alloys Ti and Zr are commonly present as alloying elements, mostly as anti-recrystallization agents. Grain refinement during solidification is also achieved using Ti but in the form of titanium borides. Our previous investigations showed that a combined addition of Zr and Ti enables considerable grain refinement in aluminum alloys upon cavitation treatment, much stronger than that of Zr alone. The role of titanium and ultrasonic processing remained unclear. In this paper we propose a mechanism of the grain refinement that includes structural changes in solidification sites, their refinement and initiation of heterogeneous nucleation at lower undercooling.

Criteria of Grain Refinement Induced by Ultrasonic Melt Treatment of Aluminum Alloys Containing Zr and Ti

It is well known that ultrasonic melt treatment (UST) promotes grain refinement in aluminum alloys. Cavitation-aided grain refinement has been studied for many years; however, it is still not being applied commercially. The current article summarizes the results of experimental work performed on various alloying systems at different stages of solidification. The influence of UST parameters and solidification conditions on the final grain structure is analyzed. It was found that small additions of zirconium and titanium can significantly increase the efficiency of UST, under the stipulation that grain refinement is performed in the temperature range of primary solidification of Al3Zr. The possible mechanisms for this effect are discussed.

Application of equal channel angular pressing with parallel channels for grain refinement in aluminium alloys and its effect on deformation behavior

A new SPD technique, equal channel angular pressing with parallel channels (ECAP-PC), has been recently developed. The method has a few advantages over conventional ECAP. In this work, the ECAP-PC is applied to the Al alloys 6061 and 6063. The AAl6061 is subjected to ECAP-PC at 100°C for 4 passes. The Al6063 alloy is subjected to ECAP-PC at 100°C and 160°C for 3 passes to study the effect of the temperature on the microstructure and mechanical properties after processing. It is demonstrated that an equiaxed ultra-fine grained (UFG) microstructure is formed in the Al6061 alloy after ECAP-PC for 4 passes. The similar microstructure is observed in the AA6063 pressed at 100°C for 3 passes whereas an increase of the processing temperature up to 160°C results in formation of complex microstructure consisting of ultra-fine grains and severely deformed areas. The latter effect is related to the significant recovery of the microstructure between two shearing events. Experimental study of mechanical properties and deformation behavior at room temperature is performed. It is demonstrated that ECAP-PC results in an increased strength of both Al alloys. It is outlined that both the length of the second channel of the ECAP-PC die and the ram speed might play a very important role in the microstructure formation during ECAP-PC processing. The use of short second channel and increased ram speed can be utilized to achieve a homogeneous microstructure even at elevated temperatures.

On the principles of the deformation methods of grain refinement in aluminum alloys to ultrafine size: II. Ultrafine-grained alloys

The applicability and efficiency of the principles of optimum structural heterogeneity and stage-by-stage grain refinement in relation to the formation of an ultrafine-grained (UFG) structure (with a grain size smaller than 1 μm) in bulk billets during severe plastic deformation are analyzed. The evolution of the criteria of these principles is considered when going from a fine-grained structure to a UFG structure and from conventional deformation treatment to severe deformation treatment. The dependence of the intrinsic mechanisms of grain refinement to ultrafine size on the initial structure of an alloy, including the heterogeneity of the initial structure, is analyzed.

An investigation on the effect of intensive shearing on the grain refinement of A5754 aluminium alloy

Grain refinement of aluminium alloys has been an established method of improving mechanical properties of aluminium alloys. A5754 wrought aluminium alloy offers a number of attractive superior properties, however, refinement of this alloy and other wrought alloys is still a matter of concern. A novel technique is introduced for the refinement of wrought and cast alloys, using A5754 alloy as a model material. This method not only provides fine and uniform microstructure throughout the products but it also offers possibilities of industrial applications. This technique eliminates the problems associated with chemical grain refinement and introduces physical refinement as a better option for achieving fine grain sizes.

Effect of growth restricting factor on grain refinement of aluminum alloys

Grain structure is an important and readily observable feature in aluminum alloy castings. Depending on the constitutional and heat-flow conditions in a solidified aluminum alloy, various morphologies are possible. Grain refining is one of the predominant techniques in controlling the quality of castings. It plays a vital role in improving metallurgical characteristics and mechanical properties of aluminum alloys. Fine equiaxed grains ensure remarkable benefits. There are a number of techniques to achieve fine equiaxed grain structure, but grain refinement by the addition of grain refiners referred to as inoculation is the most popular due to its simplicity. Grain refinement has been studied extensively by researchers for several decades, not only for developing efficient grain refiners but also for achieving an understanding of the mechanism of grain refinement. In spite of its commercial importance, benefits and numerous scientific studies in this area, the grain refinement of aluminum and its alloys is still a controversial subject. Solute elements like titanium segregate to the inoculants/melt interface affecting the dendrites and also affect the constitutional undercooling at the solid–liquid interface. This segregating power of an element is quantified by the growth restricting factor (GRF). In the present investigation, the effect of GRF on grain refinement of aluminum-silicon alloys was studied by the addition of Al-5Ti-1B master alloy. It is evident from this investigation that the growth rate of grains is inversely proportional to the GRF.

Numerical Simulation for Grain Refinement of Aluminum Alloy by Multi-phase-field Model Coupled with CALPHAD

The multi-phase field method and CALPHAD (calculation of phase diagram) are applied to the study of the equiaxed solidifications in the Al-Ti-B and Al-Si-Ti-B systems by coupled with thermodynamic and diffusion databases. In these calculations, recalescence phenomenon is considered on the basis of the latent heat, and a plot of the seed density-radius distribution is obtained by calibration with experimental data. The calculated solidified-grain sizes are in quantitative agreement with the experimental measurements for various hypoperitectic compositions.

Homogeneity of Grain Refinement of Aluminum Alloy with Compressive Torsion Processing

Compressive torsion combined loading that uses relatively low compressive pressure has a great advantage of microstructure refinement of cylindrical metal blocks without changing their shape while processing. In the present work, effects of processing temperature and rotation times on homogeneity of the refined microstructure were investigated for Al-5%Mg alloy. Although lower processing temperature was effective to obtain fine grains, it was difficult to obtain homogeneous refinement at lower temperature. Higher processing temperature was favorable to obtain homogeneous microstructure, for instance, in the cylindrical specimen of φ25×10 mm the homogeneous refinement could be obtained at higher temperatures than 373K. Increasing rotation times was also effective to obtain homogeneous refined microstructure for thicker specimens.

Effect of adding Sc and Zr on grain refinement and ductility of AZ31 magnesium alloy

Abstract Grain size in the microstructure dominates the ductility of Mg–Al–Zn magnesium alloy. The strengthening and grain refinement of aluminum alloys by Sc have been extensively studied recently. As is well known, Zr refines grains in magnesium alloys. This work presents a novel approach for enhancing the grain refinement of magnesium alloy by adding Sc and Zr. Rolling and equal channel angular extrusion (ECAE) are performed to refine the grains. Experimental results indicate that adding 0.03–0.06 wt% of Sc and 0.05–0.15 wt% of Zr reduced the mean grain size of AZ31 to 2.47 μm. The ductility of AZ31–0.15Zr–0.06Sc modified alloy can be increased to 324% at 300 °C. Results of this study demonstrate that both Sc and Zr are important in controlling grain size and ductility.

Grain refinement in aluminum alloy 2219 during ECAP at 250 °C

Grain refinement taking place in a coarse grained 2219 aluminum alloy deformed by equal channel angular pressing (ECAP) at a temperature of 250 °C to a strain of 12 was investigated. The microstructural changes are mainly characterized by the evolution of high density dislocation substructures with relatively low misorientations (∼3–5°) at strains below 2 and deformation bands (DBs) with moderate misorientation angle, which are frequently formed at strains above 3. Repeated ECAP leads to increasing the number and misorientation angle of the boundaries of DBs, so that the misorientation distribution for newly evolved dislocation subboundaries changes from single peak type at low strains to bimodal one at high strains. The fraction of high angle boundaries starts to increase at strain ɛ = 3, then it rapidly rises to about 80% at moderate to high strains and a new fine grains with almost random crystal orientation are fully developed in a whole material. The main mechanism of new fine grained structure formation during ECAP is discussed.

Thermodynamic evidence for a poisoning mechanism in the Al–Si–Ti system

The poisoning of grain refinement in aluminium alloys owing to the presence of silicon is investigated using CALPHAD based thermodynamic modelling software. The existence of primary phases other than α-aluminium, namely TiSi2 and TiSi, which are stable at typical melt holding temperatures, is consistent with microstructural observations of other researchers. The compositional and temperature range of these phases are calculated. The software is also used to determine the effect of silicon and titanium additions on the growth rates of α-aluminium grains via the growth restriction parameter Q. These results are used to explain poisoning in silicon containing foundry alloys via a thorough review and discussion of the relevant literature.

Heterogeneous grain initiation in solidification

Grain refinement in casting of aluminium alloys can be quantitatively modelled by assuming that the barrier for grain initiation on an inoculant particle is that for free growth from the particle rather than for the initial formation of the solid. This leads to deterministic nucleation behaviour in which the number of grains is dependent on undercooling and not on time. Taking grain refinement of aluminium alloys as an example, the conditions for such athermal nucleation are analysed for a variety of nucleant particle shapes. The relevance for other cases, for example the action of ice-nucleating agents in living systems, is explored. In general, it is shown that for potent heterogeneous catalysis of solidification, the finite size of the nucleation sites renders classical nucleation theory inapplicable. The concept of nucleation in such a case is briefly discussed.

The Effect of Dispersoids and Processing Variables on Grain Refinement of Aluminium Alloys Deformed by ECAE

The effect of fine, non-shearable, dispersoids on the grain refinement of aluminium alloys, severely deformed by ECAE, has been studied using a model Al-0.2%Sc alloy, during deformation by Routes A and BC (+90degrees rotation) using a 120°, or 90°, die. The microstructure in the dispersoid-containing alloy was compared to a single-phase alloy using high resolution EBSD analysis. With Route A the presence of fine dispersoids was found to significantly retard the development of new high-angle grain boundaries and the formation of a fine grain structure, by homogenising slip and inhibiting microshear bands. This difference was greatly enhanced, using Route BC. Similar results were obtained with the two die angles, but with Route A refinement tended to be slightly less effective when using the 90° die.