Rakshith M.,Seenuvasaperumal P.

School of Mechanical Engineering,Vellore Institute of Technology,Vellore,632014,India

Abstract Magnesium(Mg)alloys despite being the ideal candidate for structural applications,owing to their high specifi strength and low density,are not widely used due to lack of active slip systems at room temperature in their hexagonal close-packed crystal structure,eliciting poor ductility and formability.Amongst the various series of Mg alloys,the AZ and ZK series alloys have been standouts,as they inherit better room temperature strength and fl w characteristics through their solute elements.Grain refinement as well as eliminating casting defects through metal processing techniques are vital for the commercial viability of these alloys since they play a key role in controlling the mechanical behaviour.As such,this review highlights the effect of different Bulk-deformation and Severe Plastic Deformation techniques on the crystal orientation and the corresponding mechanical behaviours of the AZ31 alloy.However,every process parameter surrounding these techniques must be well thought of,as they require specially designed tools.With the advent of finit element analysis,these processes could be computationally realized for different parameters and optimized in an economically viable manner.Hence,this article also covers the developments made in finit element methods towards these techniques.? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:AZ31 magnesium alloy;Severe plastic deformation process;Rolling;Extrusion;Finite element simulation.

1.Introduction

Vehicles are the inevitable mode of transportation.As time progresses,demand and usage of crude oil have always been increasing.However,fluctuatio in crude oil prices and its limited availability is threatening to the automobile and aerospace domain.Also,it is proved that increased usage of these vehicles contributes to the greenhouse effect due to its exhaust emission gas.Thus,reducing fuel usage and the rate of exhaust gas emissions is a challenging task[1,2].In this regard,engineers and scientists are performing research on alternate fuels and weight reduction methods for vehicle structures.By using lightweight metals,a structure could be optimized for reduced fuel usage.

Mg is believed to be the right alternative for conventional metals(Aluminium,Titanium,iron,etc.)used in automotive and aerospace structural applications,with desirable properties such as low density,high specifi strength,easy recyclability,and yet economical.However,limitations such as poor ductility,corrosion,and wear resistance are restricting its usage in real-world applications[3,4].As per Von-Mises criteria,5 independent slip systems are required to enable uniform plastic deformation[3,5].This is unfulfille for pure Mg,as plastic fl w is only realized through basal slip at room temperature.To overcome these limitations,Mg-based alloys are widely found in different series(AZ,AM,AXE,ZE,ZK,etc.)with varieties of soluble elements[1,2].Amongst these,AZ31 alloy is widely used due to its abundant commercial availability and good tensile characteristics,AZ31 alloy’s chemical composition is shown in Table.1.However,AZ31 alloy usages are limited in as-cast condition due to low yield strength and the casting defects[2].

Table 1Chemical composition of AZ31 Mg Alloy[2].

Hall-Petch relation demonstrates that grain refinemen is a promising method for improving yield strength[6].Thus,these alloys are formed with secondary processes like rolling and extrusion processes,which results in enhanced yield strength through refine grains.Also,it was accepted that severe plastic deformation(SPD)processes could also be performed on Mg alloys to achieve grain sizes in the ultra-fin grain(UFG)range or even in the nano range[7].With this level of grain refinemen in AZ31 alloy,grain boundary sliding can be activated at room temperature leading to quasisuperplastic elongations.The current author has experience in this discipline,by performing the High-Pressure Torsion(HPT)process to achieve grain size about nanoscale for AZ31 alloy[4].

Nonetheless,the above-mentioned techniques are controlled by their respective processing parameters,which subsequently influenc the grain size morphology and basal plane orientation[4].Secondary processes such as these are vital for making AZ31 alloys compatible for real-world applications,as long as it is economically feasible.And this is where Finite Element Analysis(FEA)is useful,by simulating the thermo-mechanics behind these techniques.This is essential for cost-effective process optimization[8].Various secondary processing techniques of AZ31 alloy and the related FEA behind them will be elucidated in this review.

2.Bulk deformation processing techniques

Metal forming operations performed to transform billets of one shape into another is commonly known as Bulk Deformation Processing Techniques.This is accomplished through the use of metal forming tools,where materials are plastically deformed to achieve the desired shape and mechanical properties.There are two major bulk deformation techniques used to process AZ31 alloy:Rolling,and Extrusion.These processes as schematically illustrated in Fig.1.

2.1.Rolling

Rolling as a metal forming operation is done to produce metals in the form of sheet,bars,sections,and tubes[9].Here,one or more rolling mills are placed in an arrangement,such that the work piece can be inserted between these mills,as schematically shown in Fig.1(a).The friction between the rolls and the work piece is the driving force that pushes the material forward,and the loads exerted by the mills compress the work piece to a lower thickness.Thus,the desired mechanical property is achieved through the resulting microstructure refinement However,this is influence by processing parameters such as rolling speed,number of passes,thickness reduction ratio,and most importantly,the working temperature.Thus,the rolling process can be classifie broadly by the working temperature:hot rolling,cold rolling,and cryogenic rolling.The effect of these respective processing parameters over the material property of AZ31 alloy is discussed broadly in sub-sections.

2.1.1.Hot rolling

Hot rolling process is usually carried out above the recrystallization temperature of a particular metal[9].Generally,the number of passes,rolling speed,and initial texture are the important microstructure refinin parameters in hot rolling.However,the number of passes crucially depends on the desired reduction of thickness.The reduction can be achieved through multiple passes with a small reduction% per pass.But it has proven to be less efficien and expensive for AZ31 alloy,as intermediate annealing is required to maintain the pass-to-pass workability and suppress edge cracks or material fracture[10].Thus,high thickness reduction and fewer number of passes are preferred for the pronounced effect of dynamic recrystallization(DRX).The effect of variation in reduction% in AZ31 alloy can be observed through the microstructure orientation maps shown in Fig.2[11].

Also,DRX behaviour can be controlled by rolling speed[12].This is because deformation energy in the rolling process is consumed in different ways at different rolling speeds.At low speed,part of deformation energy is absorbed and stored by lattice defects such as twinning.While the rest is released by edge cracking[10,12].The presence of twinning behaviour promotes strain hardening of the material,and the subsequent grain refinemen increases the strength of the AZ31 alloy.However,at higher speeds,the heat energy transferred to rollers is less,which promotes DRX and grain growth.Thus,high-speed rolling is preferred to enhance the stretch formability of the AZ31 alloy at room temperature,as it extends DRX[12,13,15].Additionally,the role of a pre-annealing process towards enhancing ductility was revealed[13]:AZ31 alloy sheet with weak basal texture had poor stretch formability upon rolling,but the opposite was observed when the sheets with strong basal textures.Therefore,the initial texture of the AZ31 alloy influence ductility.

Differential speed rolling(DSR)/Asymmetric rolling[14,15]is a rolling process where further increase in grain refinemen can be achieved by inducing shear deformation during the rolling process.On the same basis,calibre rolling(also referred to as groove rolling or stretch pass rolling)could also be performed,where an AZ31 billet is rolled and reduced through a series of grooves[16-18].During the processing,the reduction direction is changed over each pass by rotating the billet 90°,facilitating the formation of an equiaxed microstructure.When AZ31 alloy was calibre rolled at high temperatures such as 723K,exceptional ductility was observed due to texture strengthening[18].Thus,it is believed to be an economical and efficien process for the production of Mg alloys in the form of bars.

Fig.1.Schematic illustration of a).Rolling Process,and b).Extrusion Process.

Fig.2.Orientation image maps illustrate the angles between the c-axis and Normal Direction of the sheet AZ31 alloy rolled at different reductions rates,in Normal Direction(a-d)and Transverse Direction(e-h):(a)and(b)10%;(c)and(d)20%;(e)and(f)30%;(g)and(h)40%.[11].(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

2.1.2.Cold rolling

Cold rolling process is usually carried out below the recrystallization temperature of a particular metal[9].The thermo-mechanics involved behind cold rolling is different compared to hot rolling.In the same way,controlling parameters such as the number of passes,and rolling speed have a different effect on the material property[6,19,20].As discussed earlier,Mg deforms only by basal slip,which consequently increases with the applied loads.This is achieved through an increase in the reduction% of the rolled sheet.However,Mg alloys develop failures such as edge cracks and even fractures at reduction rates as low as 15%[19-21].So,the number of passes and the reduction% need to be tailored carefully for a particular temperature.

The rolling speed,however,proved to have a considerable effect in grain refinemen[10].It was evident that highspeed rolling involves less heat transfer from the work piece to the roller.And higher temperatures imparted in AZ31 alloy through plastic strain subsequently decrease the requisite strain for DRX to occur.This induced more grain refinemen and activation of〈c+a〉slip systems[19,22,23].In low-speed rolling,however,deformation energy is accumulated and strong basal texture is formed at the surface.And an inhomogeneous deformation microstructure was developed in AZ31 alloy,leading to poor ductility characteristics and defects.Thus,a cross-rolling process could be performed at adequate conditions,before cold rolling[15].The intense basal textures are then weakened,as a result of the interaction of deformation mechanisms and DRX.Such interactions can also be produced by shear deformation through asymmetric cold rolling.A weak basal texture and with significantl improved tensile-ductility is achieved[13,24].

Similarly,DSR with a roll speed ratio of 3:1 resulted in good isotropy of strength and excellent elongation characteristics[24].These texture weakening attributes were also observed when AZ31 bars are calibre-rolled at cold working temperatures[16,25].And studies have been conducted in modifying these grooves to ensure uniform deformation and isotropy across the cross-section of a rolled bar[26,27].However,increasing calibre rolling strain gives rise to a unique split of basal poles owing to the alternating loading axis[25].This split texture is unique to calibre-rolled Mg alloys[28].Nonetheless,these bars are usually subjected to annealing,as it is crucial to modulate the strength-ductility balance.As such,Kong et al.[25],investigated the effects of subsequent annealing process at different temperature regimes on the mechanical behaviour of calibre-rolled AZ31 alloy.When annealed at low temperatures(373-473K),a decrease in strength and hardness was observed but ductility of the initial calibre-rolled alloy was preserved.Conversely,annealing at high temperatures(573-673K),promotes grain growth but deteriorates all mechanical properties,thereby degrading the strength-ductility balance.This reduction in strength and hardness could be explained in light of the Hall-Petch relation.And the decrease in ductility could be justifie from the activation of mechanical twinning that induced a highly localized shear deformation and strain incompatibility.

Furthermore,the application of three-high skew rolling process(also referred to as radial-shear rolling or cross-screw rolling)allows the production of AZ31 alloy bars with higher elongation in a single pass.Also,this process is capable of producing a fina product with high straightness,low ovality and fewer surface defects,when compared to the calibrerolling process[26,29,30].In this process,the work piece is deformed plastically by drive rollers rotating in the same manner,whose axes are inclined to the rolling axis(18° commonly preferred for AZ31 alloy).Hence,the metal undergoes helical fl w along a spiral trajectory,causing densificatio across the entire cross-section of the rolled bar and forming a tangential stress component along the periphery[31].As such,microstructure refinemen is primarily influence by increasing rolling speed as they activate additional deformation mechanisms.Doing so leads to higher band twist in the deformation zone,which intensifie the tangential stress component and redundant strain,causing microstructure refinement Here,a bimodal grain structure was observed between the periphery(high strain intensity)and the axial zone(low strain intensity)of the rolled bar,thus leading to a non-uniform microhardness distribution[26].And,it could be inferred that the periphery is composed of fin grains relative to the axial zone.The constructive interplay between these fin grain structures and activation of non-basal slip systems,enables three-high skew rolled AZ31 bars to have significan tensile strength and yield strength.However,both calibre rolling[16,17]and three-high skew rolling processes[30,31]have proven to be effective methods for obtaining fin grain sizes comparable to those obtained from SPD techniques.

2.1.3.Cryogenic rolling

Rolling process performed when the work piece has undergone cryogenic treatment at temperatures below 83K[32,33]is commonly referred to as cryogenic or cryo rolling(CR).Since dynamic recovery is suppressed at cryogenic temperatures,grain refinemen is further enhanced[34].The effectiveness of CR on the strengthening of material is remarkably influence by its stacking fault energy(SFE)[35,36].With AZ31 alloy having an intermediate SFE,it deforms by twinning during CR,but dislocation slip during room temperature rolling.This makes CR the most effective over room temperature rolling in improving the strength.

As such,cryogenic treatment changes grain orientation in AZ31 alloy,attributing to frame-like twinning[33].Further,the tensile strength and the hardness were higher than the untreated samples.This can be ascribed to the increased content of precipitated phases,like Mg17Al12,which strengthens the material[37,38].Similar to other types of rolling,the initial texture of AZ31 alloy influence the twinning,microstructural and textural evolutions in the CR process[39].After CR,UFGs were formed and the strength of the sample significantl increased.Although anisotropy is developed,where tensile strength along the rolling direction was higher than of the transverse direction.Unlike other deformation processes,the reduction percentage for AZ31 alloy in CR is only around 8%[40],as micro cracks and defects begin to develop along the grain boundaries.The variation of grain size,ultimate tensile strength,and elongation of AZ31 alloy,with respect to temperature and rolling reduction from various literature,are shown in Fig.3(a-c).

2.1.4.Extrusion

Extrusion is a metal forming technique performed to produce metals in various fi ed cross-sectional profile that cannot be achieved by rolling[9].Here,a ram is made to push the billet through a die of the desired cross-section,as illustrated in Fig.1(b).The die can be designed in such a way to produce complex profile in a material without any draft.As a processing technique,extrusion can be used to tailor a billet material to have desired mechanical properties.Processing parameters such as temperature,extrusion rate(the rate at which the work piece is being extruded),and extrusion ratio(the ratio of the cross-sectional area of the work piece before and after extrusion)can greatly influenc microstructure refinemen[41-43].Similar to the rolling process,the extrusion process is also generally classifie by the materials working temperature:hot extrusion,and cold extrusion.However,cryogenic extrusion isn’t feasible due to the limited workability of Mg towards the magnitude of extrusion forces.The following sub-sections will discuss how the respective processing parameters will influenc the mechanical behaviour of AZ31 alloy upon extrusion.

2.1.5.Hot extrusion

Hot Extrusion process is carried out above the recrystallization temperature of a particular metal[9].Here,parameters such as extrusion ratio,billet temperature,and ram speed have a great influenc over microstructure refinemen[41].The effect of varying processing temperatures in the extrusion of AZ31 alloy is shown in Fig.4(a-c)[42].It is thus inferred that extrusion is easily performed at higher temperatures with the absence of work hardening effect,promoting DRX.Although grain growth is enhanced,an upper limit for billet temperature must be established to avoid effects such as oxidation,sintering,and hot shortness.This is done by observing the development of surface cracks.To address this,Hsiang et al.[43],stated that oxidation prevention methods must be considered at extruding at 673K.

Fig.3.3D scatter plot showing the effect of rolling temperature and reduction percentage on AZ31 Magnesium alloys,a).Grain size,b).Ultimate tensile strength and,c).% of elongation.(Spheres-Symmetric Rolling;and Stars-Asymmetric Rolling).(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

However,grain refinemen seems to be influence by the initial texture[40,41],as more elongation was observed in the AZ31 extrudate when the billet was initially homogenised at 673K.This might be attributed to the dissolution of intermetallic compounds such as Mg17Al12and Al8Mn5in the billet at the time of cast.This also explains why a homogenised billet can be extruded faster than an as-cast billet.Nevertheless,the extrusion speed directly corresponds to the extrusion load,and a fi ed speed throughout the process dramatically increases these loads and yield serious defects in AZ31 alloy[43].With a multi-speed approach,even with a high extrusion ratio,loads are moderated within a shorter period,and a defect-free AZ31 alloy is obtained.Although only an extrudate processed at lower speeds obtain the fines grain size[41],which was parallel to the observation in Fig.4(c-d)[42].

Further grain refinemen can be attained by inducing more deformation by varying extrusion ratios and die designs[41,43-47].As mentioned before,AZ31 alloy has good workability in high temperatures,and thus,high extrusion ratios(≥39)[48]can be achieved easily[41].In the die,various features can be implemented to promote shear deformation:particularly asymmetric extrusion feeders with an angled chamfer in one face[47],and asymmetrically tapered feeders[45].This produces grain refinemen and a tilted weak basal texture,which enhances room temperature strength and plasticity.

2.1.6.Cold extrusion

Cold or Warm Extrusion process is carried out below the recrystallization temperature of a particular metal[9].In the absence of high temperatures,oxidation effects are less pronounced,and cold working induces greater strength in the extrudate.Only fewer studies have been carried out in cold extrusion of AZ31 alloy,as higher forces are required for deformation and the process is more expensive[49,50].All related process parameters such as ram speed and extrusion ratio need to be chosen appropriately for a given temperature as they all have a direct influenc over the load acting on the material.

A pre-extruded AZ31 billet with an oil-based graphite lubricant was used in a cold extrusion study which yielded grain sizes fine than of hot extruded AZ31 alloy[49].Although at higher extrusion rates,and lower temperatures,surface cracks and defects are more common[50].This provokes the usage of back pressure(BP)and a hydrostatic medium to extrude the metal.The application of BP allows the usage of higher extrusion ratios and extends the formability of AZ31 alloy.As such,strong elongated or even fibrou grains were produced.The adiabatic heating effect generated during deformation can be as high as 0.4-0.65 of the absolute melting temperatures,stimulating recovery,and DRX.The variation of grain size,ultimate tensile strength,and elongation of AZ31 alloy,with respect to temperature and extrusion ratio from various literature,are shown in Fig.5(a-c).

Fig.4.Transmission Kikuchi Diffraction orientation maps and inverse pole figure of AZ31 along extrusion direction,extruded at(a)175 °C,Electron Back Scatter Diffraction orientation maps and inverse pole figure along extrusion direction of AZ31 extruded at(b)300 °C and(c)400 °C with ram speed of 0.1mm.s?1,and(d)400 °C with ram speed of 3mm/s.[42].

3.Severe plastic deformation

As established earlier,materials produced through the application of SPD offer the possibility of refinin the grain size in the sub-micrometre range of 100-1000nm or even in the true nanometre range of<100nm,which are drastically smaller than those produced using conventional bulk-deformation techniques[51,52].The techniques that fall under SPD can process a specimen without changing its cross-sectional dimensions.And the process introduces a high dislocation density in a material,which then rearrange into arrays of high-angle grain boundaries.This results in excellent homogeneity and mechanical properties which are in conjunction with the Hall-Petch relation[6,51].Several different SPD techniques may exist[53]but for AZ31 alloy,but the most attention has been given to the procedures involving equal-channel angular pressing(ECAP)[54],Accumulative Roll Bonding(ARB)[55],and HPT[4],as schematically illustrated in Fig.6.

3.1.Equal channel angular pressing

ECAP is a commonly used SPD technique producing materials with grains in the sub-micron range[51].The working principles of ECAP are schematically depicted in Fig.6(a).This shows that the process is operated in a die containing a channel bent by an angle near the centre of the die.The billet is machined to fi within the channel and a plunger is used to drive it through the die under applied pressure.And grain refinemen occurs at the channel bend due to shear deformation,inducing exceptional mechanical properties.

In practice,the deformation introduced in AZ31 alloy is controlled by various parameters such as the working temperature,channel angle,number of passes,and the application of back-pressure[51,54,56-58].Apart from this,experimental evidence shows that microstructural evolution depends upon how the sample is rotated between each pass:Route A-rotate 0°,Route BA-rotate±90°,Route BC-rotate 90°,and Route C-rotate 180°[59-61].Amongst them,pressing through route BCis commonly preferred,since a regular and periodic restoration of the equiaxed structure is achieved and deformation is observed on all three orthogonal planes.

The underlying principle of ECAP is a sample being subjected to repetitive pressing to induce high strains.However,the imposed plastic strain is primarily decided by the number of passes.Also,it was found that a gradual decrease in grain size occurs with the increasing number of ECAP passes[62].However,it was commonly observed that the material cannot undergo further grain refinemen once DRX is completed.This could explain why grain size mostly remains unchanged after 4 passes for Mg alloys[62,63].This number,however,changes with the type of process[64].In AZ31 alloy,grain refinemen is characterized by nucleation of a necklace-like pattern of fin grains surrounding a coarse grain in the initial pass,which will later be homogenised through further passes,irrespective of the billet being cast or wrought[54,58,65-67].

Thus,it was inferred that,by increasing the imposed plastic strain,UFG can be achieved which induces better mechanical properties.Nevertheless,as with any other processing technique,the temperature does play a vital role in ECAP.When performed at higher temperatures,higher activity of DRX is observed[62].Despite better processing speed and workability,fin grain sizes couldn’t be obtained.The specimens processed at elevated temperatures produce yield stress values lower than their as-cast form due to DRX and subsequent grain growth[58].On the contrary,a significan increase in the yield stress was observed when the specimen was processed at room temperature.With grain refinemen being primarily determined by DRX,it was informed that fine grain sizes can be obtained at lower temperatures[61,68].

Anyhow the threshold working temperature depends on the workability of the material to be processed[61].Due to the lack of available slip systems in AZ31 alloy,the specimens easily form cracks or fail below 473K during ECAP[68,69].However,the usage of back-pressure provides the advantage of pressing AZ31 alloy at lower temperatures,resulting in remarkable grain refinement Though a bi-modal grain structure is formed initially,with the aid of multiple passes,it rapidly becomes homogeneous and grains reduce to the submicron range.In addition to that,grain refinemen with the application of back-pressure has led to the development of superplastic capabilities when AZ31 alloy is tested in tension at elevated temperatures[54,70,71].This can be attributed to the formation of bimodal microstructures,as larger grains can easily accommodate grain boundary sliding through intragranular slip and twinning[72-76].Thedislocations and〈c+a〉non-basal slip was prevalent with the use of back-pressure.This contributes to the strain hardening of AZ31 alloy and thus suppressing necking.This confirm that the highest level of super plasticity corresponds with the decrease in processing temperature and a concurrent increase in back-pressure.

Fig.5.3D scatter plot showing the effect of Extrusion temperature and extrusion ratio on AZ31 Magnesium alloys,a).Grain size,b).Ultimate tensile strength and,c).% of elongation.(Spheres-Symmetric Extrusion;and Stars-Asymmetric Extrusion).

The material behaviour could further be enhanced by varying the channel angle[51,61].After considering the amount of deformation imposed,it could be inferred that the lesser the channel angle,the fine the grain size produced.But considering the workability of AZ31 alloy,various studies concluded that a 90°angled die induced maximum grain refinemen with the least amount of defects.The arc of curvature or corner angle is also an important parameter since a sharp turn in the channel produces friction between the die and the material[54,65].To avoid the formation of dead zones within the material fl w,a corner angle of 20-30°is given while processing the AZ31 alloy.

Nonetheless,ECAP still won’t be able to satisfy the demands of the manufacturing sector as it can’t produce continuous billets[77,78].This is because the friction developed in the channel is proportional to the length of the billet.After extensive FEA,a cyclic process named Incremental-ECAP(I-ECAP)was proposed[79].This is done by separating the feeding and the plastic deformation stages of the conventional ECAP.Despite being a continuous SPD process and operating at temperatures around 500K,I-ECAP was capable of producing large amounts of strains in AZ31 alloy,and mechanical properties similar to that of conventional ECAP[78-80].Additionally,the working temperatures and processing routes are the most influentia process parameters in I-ECAP for the resulting mechanical behaviour.The effect of processing AZ31 alloy through various routes in I-ECAP is shown in Fig.7[78].

3.2.High pressure torsion

HPT technique has a longer history than ECAP,as the outlying principle behind it was firs proposed more than 70 years ago[81,82].Although it was recognized as a tool for exceptional grain refinemen only in the last two decades.It is capable of producing grains of the UFG range and sometimes even the nanocrystalline range(grain sizes less than 100nm)making it the most effective of all SPD techniques[83].In this process,the material is subjected to torsional shear straining with high hydrostatic pressure of 0-7GPa[53,81-85].Generally,the specimen is in the form of a disk located within a cavity between two anvils,as schematically illustrated in Fig.6(b).And the process is usually conducted under quasiconstrained conditions[82],where a limited fl w of material is allowed in between the upper and lower anvils.In practice,the HPT process begins with the disk being subjected to hydrostatic pressure for 30 s,which is commonly referred to as theN=0 condition.The torsional straining is then induced by rotating one of the anvils.This causes shear deformation in the material,which initiates microstructure recrystallisation.However,the resulting grain size is influence by processing parameters such as the working temperature,the pressure applied,number of rotations,and speed of rotation.

Fig.6.Schematic illustration of SPD Processes,a).ECAP,b).HPT and,c).ARB.

Amongst the various SPD processes,HPT is highly preferred for AZ31 alloy,as it can be performed at room temperature without any segmentation like in the ECAP process[68].This is ascribed to the hydrostatic stress applied,which prevents the formation of these defects and increase workability.The torsional strain on the work piece is estimated using the number of anvil rotations N,which starts with 0,?,?and extends up to 20 turns[86-89].The current author performed HPT on AZ31 alloy for the above-mentioned strains under 5GPa pressure with 1rpm speed,as shown in Fig.8[4].It was observed that atN=?the grain refinemen starts by sub grain formation and shear bands.This is parallel with other works of literature,where equilibrium grain size at the UFG range was achieved by 1 pass[4,86,90,91].The presence of shear bands however indicates the presence of inhomogeneity of deformation,where grain sizes vary through the radii and thickness.This variation of grain sizes through the radii of the AZ31 disk is shown in Fig.9[86].

This,however,is mendable with an increasing number of turns,as microstructure becomes increasingly homogenous with higher dislocation densities and grain growth due to the heat generated through torsional straining.And as discussed earlier,the amount of hydrostatic pressure also influence the degree of microstructural refinement With a pressure of 2.5GPa,complete homogeneity of AZ31 alloy was achieved afterN=15[87].However,the authors’work achieved the same amount of homogeneity with 5GPa pressure andN=5[4].In addition,the working temperature is also an influen tial parameter for the resulting grain size.High-temperature processing results in grain growth and elongation due to pronounced DRX,while low temperatures processing induces significan grain refinemen and hardness[85].

Moreover,processing with high hydrostatic pressure attributes to the superplastic behaviour in AZ31 alloy atN=10[89].This behaviour is prevalent as a result of grain boundary sliding controlled by grain boundary diffusion[89,92-95].Maximum elongation of 520% was observed when the superplastic AZ31 specimen was tensile tested at 623K[89].Apart from this,the anvil’s surface roughness also plays a vital role in sample homogenization.Due to limited slippage between the work and anvil,more strain was induced[89].Further,Pauline et al.[96]performed HPT on AZ31 alloy at various strain rates.Through this study,it was observed that increasing strain rate causes a reduction in the grain size and consequently an increase in hardness.This is attributed to the decrease in〈a〉type prismatic and pyramidal fraction of dislocations[87,93].

Fig.7.Electron Back Scatter Diffraction maps and corresponding micro-textures of AZ31 alloy processed by I-ECAP at,a).250 °(route A),b).200°(route A),c).250 °(route Bc),d).200°(route Bc).[78].

3.3.Accumulative roll bonding

Accumulative Roll Bonding(ARB)is an SPD process,which can be accomplished by using the same facility as the rolling process[53,55].It is a multi-stage process involving a series of rolling,cutting,brushing,and stacking,as schematically illustrated in Fig.6(c).Commonly the sheet is reduced in thickness by 50%upon rolling and then bisected span wise.These two halves are then evenly stacked one over the other to be rolled again.This process is repeated till large strains are accumulated in the sheet and UFGs are produced.

The principle behind ARB is the Interface bonding,which is achieved by DRX at the sheets interface[97,98].Here the fin grains are recrystallized and replaced until the interface is eliminated.Using ARB,AZ31 alloy sheets can be roll bonded to sheet metals of 3 types:similar materials[55,98-106],dissimilar materials[107-112]and,similar/dissimilar materials with reinforcing agent[113].For all these types,however,the processing parameters exert their influenc over the resulting material properties.These include the number of layers,reduction%,working temperature,rolling speed,and rolling friction.

The number of passes in ARB is crucial for homogenous UFG formation as revealed by the EBSD microstructure in Fig.10[99].The grain size was observed to decrease drastically after one pass.Even though only bimodal structures were obtained at this point,AZ31 alloy showed a superior combination of high strength and good elongation.The ductility being attributed to non-basal slip and the strong back stress offered[104,106,114].With further passes,more homogeneity is met in grain size distribution.This leads to the suppression of twin generation and reduction in mechanical anisotropy.Even while rolling dissimilar metals to AZ31 alloy,the diffusion layer thickens with the increasing number of passes[109,110].

Fig.8.Optical Microscope image of AZ31 Mg Alloy(a)as-cast and(b)HPT N=0 samples,and Transmission Electron Microscope bright fiel images of HPT(c)N=?and(d)N=5 samples[4].

Fig.9.Microstructures of AZ31 alloy after HPT processing at 473K for N?1 turn at(a)the centre,(b)the half-radius,and(c)the edge of the disk[86].

Fig.10.Electron Back Scatter Diffraction microstructure of(a)Homogenised AZ31 alloy;and alloy processed through ARB after(b)firs pass;and(c)second pass[99].

This effect is also observed when the reduction percentage of AZ31 alloy is increased from 50%,resulting in an enhanced bonding area by the effect of high strain accumulation[107,108,110].However,the fines grain sizes aren’t produced with the highest reduction percentage[102].This might occur due to inadequate preheat temperature,attributing incomplete DRX,and the residual accumulated strain energy.Fatemi et al.[55]took this into account,and mean grain size of 0.9μm was achieved with an 85% reduction in a single pass.

Nevertheless,In ARB,the onset of DRX decides the level of grain refinemen and forms shear bands in the direction of shear stress[98,112].The best degree of refinemen is achieved when the process temperature is low.High values of tensile strength,yield strength,and microhardness are also obtained.Although,increasing working temperatures have a direct effect on elongation,and formability index[55,100,111].This is due to the higher number of nuclei created,which facilitates grain growth and formability.Although not many studies have been conducted on the effect of varying rolling speed,it is understood from a few studies that decreasing rolling speed results in stronger interface bonding[97-100].This is attributed to the prolonged period of loads applied,to cause more DRX and diffusion.Bonding is also dependant on the friction between the rolls and the specimen,and between the interfaces[101].Thus,wire-brushing operation between every ARB pass is done to remove oxides/any other surface layers,which improves the bonding quality.

Grain boundary subdivisions through the transition of low angle to high angle grain boundaries induced by the accumulated strain are one of the mechanisms that result in excellent great grain refinemen with super plasticity[92,94,101].Thus,super plasticity can be observed in the AZ31 alloy during hot tension tests.At 573K and a slow strain rate of 10?4s?1,superplastic elongations of about 562% were obtained[101].Though testing the same specimen at a higher strain rate of 10?2s?1,superplastic elongation was reduced to 316%.Taking into account the strain rate sensitivity exponent of 0.34,it can be inferred that grain boundary sliding plays a primary role in superplastic deformation.The variation of grain size in AZ31 alloy,with respect to working temperature from the literature of the above-discussed SPD techniques,are shown in Fig.11.And the mechanical behaviour elicited through various process parameters of those techniques are given in Tables.2-4.

Table 2(continued)

Table 2(continued)

Table 2Effect of ECAP processing parameters on the mechanical behaviour of AZ31 Mg Alloy.

Table 3(continued)

Table 3Effect of HPT processing parameters on the mechanical behaviour of AZ31 Mg Alloy.

Table 4Effect of ARB processing parameters on the mechanical behaviour of AZ31 Mg Alloy.

3.4.Numerical methods and finit element analysis

An ideal processing technique must achieve the required refinemen efficientl and reliably[2].However,this is easier said than done,as a tremendous amount of energy and money must be spent in bulk processing AZ31 alloy.As such,Industries must rely on numerical methods/processing maps in choosing suitable process parameters.And with the advent of FEA,a more structured approach has been established[115,116].By computationally simulating a processing technique,one can assess the thermo-mechanical behaviour of the material at any given condition.Further,a process optimization diagram can be generated through FEA without having to process any material[8].Apart from obtaining an optimum set of process parameters,this data is crucial to establish threshold factors while processing to maintain workability and avoid any failure.

Fig.11.Effect of processing temperature to the grain refinemen of various SPD processes.(Sphere-ECAP after 4 passes;Star-HPT after 5 turns;Cube-ARB after 3 passes).

The metal is continuous throughout the process,for deformation techniques such as rolling and extrusion.Thus,the true strain experienced is based on the initial and fina cross-sections of the work piece.This is represented in the Eq.(1)and(2)[117].

where,?=true strain;t0=initial sheet thickness,mm;tf=reduced sheet thickness,mm;A0=initial billet crosssectional area,mm2;and Af=extruded billet cross-sectional area,mm2.This is very useful in calculating the average fl w stress of the material as it is the minimum stress a material requires to sustain plastic deformation.It is also useful to keep track of forming defects,as crack propagation is inversely proportional to the fl w stress.The average fl w stress is formulated in the Eq.(3)[117]given below,

where Yf=average fl w stress,MPa;K=strength coefficien(MPa);andn=strain hardening exponent(varies with material and working temperature).Thus,these values could be used to calculate the required rolling force,and the total power needed to rotate each roll mill using the Eq.(4)and(5)[117]respectively.

whereF=rolling force,N;w=the width of work to be rolled,mm;L=length of contact between the rolls and work,m;P=total power,J/s;andN=rotational speed,rpm.But unlike rolling,the extrusion process can be used to produce various cross-sections of an initial AZ31 billet.Thus,the effect of die shape is usually considered while calculating the total ram pressure required for extrusion to occur.The shape factor for a given design can be calculated by the Eq.(6)[117],and for a requisite value of fl w stress,the extrusion pressure can be calculated by Eq.(7)[117].

where Kx=shape factor;Cx=perimeter of the extruded cross-section,mm;Cc=perimeter of a circle of the same the extruded cross-section,mm;p=extrusion pressure,MPa;L=length of billet needed to be extruded,mm;and D0=initial diameter of the billet,mm.The above-discussed parameters are essential for making finit element models for a given deformation process.

Using FEA,cases such as roll wear damage[118],and frictional behaviour[119-121]could be evaluated by monitoring local contact pressure and local sliding.Similarly,various simulations have been performed in extrusion for a range of conditions[122,123].Using DEFORM-3D software,process optimization diagrams can be generated for a desired property in the AZ31 extrudate.The velocity fl w field obtained from the simulation are useful in detecting any possible network of micro-cracks[123].However,for evaluating microstructural behaviour crystal plasticity models are required.Walde et al.[124],implemented a model(including the effects of crystallographic slip,recrystallization,and twinning[125,126])in the ABAQUS software illustrating texture evolution of AZ31 alloy during hot rolling.At present,the cellular automata(CA)model[127]has overcome all shortcomings in evaluating grain size morphology,where material properties are mapped from finit element nodes to a number of CA arrays.It was understood from Fig.12[127],that the established CA model can accurately predict the microstructural evolution of AZ31 alloy upon hot deformation.

However,the initial boundary conditions for SPD techniques cannot be evaluated by the change in cross-sectional dimensions since they always remain uniform.Therefore,numerical methods surrounding its processing parameters rely on the accumulated strain in the work.For the ECAP process,the effective strainεafter N passes is calculated using the Eq.(8)[128,129].

whereφ=channel angle;andψ=corner angle.Using the fi nite element code in DEFORM-3D software,FEA has been performed to study the deformation behaviour for applied strains associated with various channel angle and die angle configuration[130,131].With AZ31 alloy being hypothesized as a rigid plastic element,friction between the work and tool interface plays an important role in the plastic deformation[132].This was inferred by varying the amount of lubrication,where the required extrusion force increased proportionally with the apparent friction coefficient Similar studies related to frictional behaviour have also been performed for I-ECAP[133-134].The die design must also be thought of by simulating pressing at various temperatures and fille angles.This results in better precision,stiffness,and tool life[135].

Unlike other SPD processes,HPT permits a define continuous variation of strain in a single cycle.Since the applied torque exerted can be easily computed through the angle of rotation,the effective strainεcan be estimated by Eq.(9)[87,126],

whereN=number of turns;r=radius of the disk;andh=thickness of the disk.Even though no FEA studies of AZ31 alloy has been performed yet,simulations performed on other materials could still draw various conclusions[136-139].With the DEFORM-2D and 3D software,fl w behaviour,temperature evolution,stress,and strain distributions can be evaluated for various sets of loading conditions under unconstrained,quasi-constrained,and constrained conditions.It was also noted that the effect of friction was more pronounced compared to that of ECAP processes,and dissimilar friction coefficient at the top and bottom anvils have a prominent effect on strain gradients through the work piece thickness[140].

Analogous to ECAP,the accumulated strain during every pass is essential to model ARB[99,126,136].The effective strain for N passes in ARB can be calculated by Eq.(10)[99].

wherer=reduction ratio per pass.However,not many studies[141-143]have been conducted in ARB for AZ31 alloy since intermediate stages of cutting,stacking,and roughening is difficul to model.Although a single cycle ARB process was simulated in ABAQUS,considering the effect of friction,and the corresponding heat generation[136].The influenc of working temperature and friction conditions was observed through the strain distribution through the AZ31 sheet thickness.Nevertheless,a new experimentally validated model was developed for AA1050 alloy where ARB simulations were performed for 5 cycles[142].Here,cutting and stacking operation has been realised using the mesh mapping solution in ABAQUS.The deformed mesh of a single passed sheet was mapped towards two different sheet geometries,which is then integrated into a single sheet before the next cycle.

Fig.12.The CA predicted grain size morphology of AZ31 Mg alloy after hot deformation at(a)573K-0.001 s?1;(b)673K-0.1 s?1;(c)723K-1 s?1.[127].(For interpretation of the references to colour in this figur legend,the reader is referred to the web version of this article.)

4.Summary

The discussion above elaborates on various bulkprocessing techniques and its respective process parameters.And these fine-graine or UFG materials with critical mechanical properties such as high specifi strength,and good fatigue life,are vital to the advancement of the automobile and aerospace industry.As such,these techniques can be used to achieve significan grain refinement homogeneity,and hardening of AZ31 alloy.

Under bulk deformation techniques,the highest levels of grain refinemen and mechanical properties were achieved by performing asymmetric rolling,calibre rolling and three-high skew rolling.Whereas amongst SPD techniques,HPT presented the prospect of processing AZ31 alloy at room temperatures,and grains refine to the nano-range.However,only a few studies have explored the effects of combining multiple processing techniques in AZ31 alloy:enhanced tensile strength and ductility upon undergoing both ECAP and rolling[144,145],and the production of high-strength micro-gears by combining ECAP and extrusion[146].

Nevertheless,all SPD techniques are capable of producing an equiaxed UFG microstructure and superplastic elongation in AZ31 alloy.Thus,there is more attention is towards viable continuous production methods of UFG materials.Using Finite-Element methods,the machinery behind these techniques could be designed and optimum processing parameters can be obtained for the desired mechanical property in AZ31 alloy.As I-ECAP was computationally verifie and performed for AZ31 alloy,more feasibility studies can be conducted for various continuous processing techniques such as Incremental-HPT,Single-Task Incremental HPT,and Accumulative Extrusion[128].

Amongst Monte Carlo,Sphere growth,Inverse analysis,and various other methods used to represent microstructure evolution,the CA finit element method has been very successful in modelling Static and Dynamic recrystallization phenomenon[147].With only limited studies being performed,there is still of potential with the CA finit element method,to predict grain size morphology of AZ31 alloy using SPD techniques.