Kulwnt Singh,Gurhinder Singh,Hrmeet Singh

aI.K.Gujral Punjab Technical University,Kapurthala,Punjab,India

b Guru Kashi University,Talwandi Sabo,Punjab,India

Abstract Friction Stir Welding(FSW)is considered to be the most significant development in metal joining in last two decades.FSW has many advantages when welding magnesium or lightweight alloys.The Friction stir welding of magnesium alloy has many potential applications in major industries i.e.land transportation,aerospace,railway,shipbuilding and marine,construction,and many other industrial applications.Even magnesium alloys have been used in industrial equipment of nuclear energy as magnesium alloys have low tendency to absorb neutrons,sufficient resistance to carbon dioxide and excellent thermal conductivity.Recently,the research and development in FSW field and associated technologies have been developing rapidly worldwide.In this review article,the basic principle of friction stir welding and several aspects of friction stir welded magnesium alloys have been described.The current state of friction stir welding of magnesium alloys is summarized.In spite of this,much remains to be learned about the process and opportunities for further research are identified.

Keywords:FSW;Tool Geometry;Stir Zone;Mg Alloy;Texture;Microhardness.

1.Introduction

On the Earth’s surface,Magnesium is the 6th most abundant element and characterizes approximately 2.5%of its composition[1].Magnesium is also the 3rd most plentiful element dissolved in seawater,about a concentration of 0.14%[2].Due to the lightweight properties,magnesium alloys(Mg alloys)have outstanding specific strength[3],sound damping capabilities[4],hot formability[5],good castability[6],and recyclability[7].Due to the hexagonal close-packed(HCP)crystal structure,magnesium alloys possess limited strength,fatigue and creep resistance at elevated temperatures[8]and low stiffness[3],limited ductility[9]and cold workability at room temperature[10].Magnesium alloys exhibit poor hardness,wear and corrosion resistance properties[11],large shrinkage during solidification[8],and high chemical reactivity(molten state)[12].Most commercial magnesium alloys are ternary in nature and made up of aluminium,zinc,thorium and rare earth.In this ternary Mg-Al series,aluminium is the main alloying element that involves AZ(Mg-Al-Zn),AM(Mg-Al-Mn)and AS(Mg-Al-Si)alloys[13,14].It is the part of common practice to classify Mg alloys on the basis of room and elevated temperature applications.For hightemperature application alloys,the chief alloying elements are rare earth metals and thorium whereas aluminium and zinc are used as alloying elements for room temperature applications[1].For designation of these magnesium alloys,currently,use the American Society for Testing Materials(ASTM)standard.In sequence,the first two letters indicate the principal code for major alloying elements as Table 1 shows.These two letters are followed by two numbers,indicating the concentration of the major alloying elements.The fi fth symbol is a letter,signifying the alloy modification.In some cases,this alloy code is followed by a designation of temper that is similar to the Temper designation of aluminum alloys:F-As fabricated,O-Annealed,H-Cold worked,T4-Solution treatment and natural aged,T5-Artificial aging,and T6-Solution treatment followed by artificial aging[15,16].

Table 1 Alloying element code for magnesium alloys[1,15].

Fig.1.Representation of friction stir welding process[34].

In Friction stir welding,a non-consumable rotating tool with a pin and shoulder is inserted into the abutting edges of plates to fabricate the joint and traversed along the line of the joint[35].The side of the weld where the direction of welding is in agreement with the rotational direction of the tool is known as the advancing side and the other side,where the direction of welding opposite the rotational direction of the tool is known as the retreating side[36].An important feature of the welding tool is a probe/pin which protrudes from the base of the tool i.e.the shoulder and a length only marginally less than the thickness of the plate/workpiece.In this process,the heating is accomplished by friction between the rotating tool and the workpiece and plastic deformation of workpiece material[37].This localized heating softens the workpiece material around the pin before it reaches its melting point[38].The combination of welding tool rotation and translation leads to the movement of soft material from the front of the tool pin to the back of the tool pin[39].As the result of this process,a joint is produced in solid state[40,41].

FSW has many advantages over the other welding technique when welding magnesium alloys or lightweight alloys[12].The advantages of friction stir welding process are illustrated in Table 2.The research and development in FSW field and associated technologies have been developing rapidly,with many companies,research institutes and universities.Today there are about 300 companies worldwide using FSW and 1900 patent applications had been filed relating to FSW[42-44].The number of research publications has also grown exponentially.In this paper,the current state of understanding and development of friction stir welded magnesium alloys are reviewed.

2.Friction stir welding factors

The FSW process involves a complex phenomenon related to plastic deformation and material movement during the welding process.The major factors affecting FSW process are(1)Tool Geometry,(2)Welding parameters,(3)Joint con figuration.These factors play an important role in the material flow pattern and temperature distribution[46-49].

2.1.Tool geometry

Tool geometry has the significant influence in FSW process.Earlier researchers report that the tool geometry plays a critical role in material flow and governs the traverse rate of FSW process[50].Padmanabham et al.[51]studied the effect of tool pin profile,shoulder diameter and tool material on the friction stir welding of AZ31B Mg alloy.Fig.2 shows the tool consists of a shoulder and a pin used for investigation.It was concluded that the joints fabricated by high carbon steel tool with 66 HRC with threaded pin profile and a shoulder diameter of 18mm(keeping D/d ratio=3)showed excellent tensile properties.Due to the higher coefficient of friction,the tool material which has higher hardness may generate more heat.Fig.3 represents the shoulder shapes and surface features,FSW tool probes,and friction stir welding tools designed at TWI.

W?eglowski and Pietras[53]reported that during the welding process,FSW tool has two principal functions-(i)lo-calized heating,and(ii)material flow.Colligan and Mishra[55]and Dobriyal et al.[56]described that the heat is produced in the initial stage of tool plunge due to the friction between pin and workpiece and some additional heating resulted from deformation of the material.The tool pin is being plunged into workpiece till the tool shoulder touches the workpiece surface[57].The shoulder is marginally inserted under the workpiece surface.Mishra and Ma[32],Elangovan et al.[58]and Deya et al.[59]reported that the friction between the tool shoulder and workpiece resulted in the major component of heating.From this heating feature,the size of pin and shoulder is very important than the other design features of the tool.Xu et al.[60]and Rajakumar et al.[61]stated that the shoulder of the tool provides con finement for the heated volume of workpiece material.It is reported that the second function of the FSW tool is to stir/blend and move the material during the process.The tool design governs the FSW process loads and also homogeneity in microstructure and properties of joint.Zhao et al.[62]studied the impact of the tool designed with different shoulder diameters during the friction stir welding of AZ31 Mg alloy.They analyzed the effect of these parameters on the formability of the material and its mechanical properties.They produced joints containing fine re fined grains in the joint zone.Patel et al.[63]studied the influence of tool pin profile and welding parameter on the tensile strength of magnesium alloy AZ91 during FSW process.They concluded that heat generation is important for proper welding which is produced by friction between shoulder and plates.The shoulder diameter of 18mm with threaded straight cylindrical pin profile,at the rotational speed of 710rpm and the welding speed of 28mm/min was found to be suitable to produce sound weld having high tensile strength.Tozaki et al.[64]and Bakavos and Prangne[65]reported that good weld joints can be obtained using a probe free shoulder tool with the bottom scrolled shoulder surface.This feature played a significant role in stirring/blending the material.Here,the tool shoulder outer surface shape and feature becomes more important.Generally,a concave shoulder and threaded cylindrical pins are used[66-68].The costeffective and long life tools are still challenges for researchers for the FSW of soft alloys[69,70].The Tool material properties(strength,hardness,fracture toughness,thermal expansion coefficient and thermal conductivity)affect the weld quality,tool wear and performance of tool[71-73].The hardness of materials used for FSW tools such as stainless steel,high-speed steel,armour steel,mild steel and high carbon steel is 40,73,58,30 and 66 HRC respectively[74].Table 3 lists the tool materials,tool geometries and welding operating parameters used to weld various grades of the magnesium alloys.

Table 2 Advantages/Benefits of friction stir welding over others joining processes[32,45].

Fig.2.Nomenclature of FSW tool[52].

2.2.Welding parameters

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Reference Remarks erating parameters Op size Tool shape Tool material Tool ued)Table 3(contin piece material rk Wo[63][107][102][105][104]th tensile streng ofeed e straight rical pin prof ile tational sp een e th pact e and th in with intool.ro rotation/m merical th welding good g speed perature ent betw a welding has a t,im Tensile properties th en of were due to e mechanical ne zo tained mm threaded tool 710rpmalin increased with nu toold 28 cylindat an High ob weldin increase eed of There is th reem dels lts ofand experiments in aspects of forces,tem history and grain as sp agresu mo size.Water,environm nsiderable co on properties weld.Tensile streng hardness proved im grain ref i nement the stir toto to eed-710 to28 90/0 toto eed 2540 eed-Sp eed-Sp Sp m Welding sp eed-710 toto m Welding Speed-25 Sp eed-800 tomm eed-102525 Sp Ro tational m Welding Speedin/m tational in/m tational tational m Welding Speedtational m Welding sp in/m ol rp Ro rp Ro rpm/min Ro rp Ro rp To 1400mm 56ol To 1400mm 40ol To 1400 100m ol To 00 16min ol To 25mm 1575 Pin -m mm-5.8 mm th-18m length and mm 18th m Probemm eter m Squarein3°m Pind)eters-Pin leng eter-18m Pin length-5.5 Pin leng mm 2P diam ulder en angle-8m(sho-5eter-18m 3°tilt eter-1 Pin length-2.6 Shoulder diammm eter-6diammm eter anglediam×4.25 mm diammm diam Shoulder eter-6 diam Pin diam m Shoulder 4.8m tilt Tool Shoulder 4.25 Pinm Tool 2.6m Shoulder eter-7 diam m(tip end)2m rical pin,hand lind Straight ed,cy read rical threaded ht straig nical th Cylind Threadedcy lindrical pin,Co lindrical pin left er rical pin pin prof ile Tapp lind cy nical,Co lindrical,Cy lindrical pin Cy Square Tapered cy threaded e(60 grad rking r D2 steel HCHR C)t Wo Ho 3 hot H1g steel ol HC to613 steel H12344 Steel AISI workin 3 to H1 m Magnesiu esium Alloy alloy 91Magn Mg C Magnesium D Mg AZ Alloy 91 AZ alloy 91 AZ 91 AZ alloy 91 AZ

Fig.4.Joint con figuration(a)Square butt joint[92](b)Overlapped joint[93].

In friction stir welding process,there are two important parameters:(i)tool rotation rate(rpm)in clockwise or counterclockwise direction and(ii)tool traverse speed(mm/min)along the line of joint[75].The rotation of tool results in stirring and mixing of material around the rotating pin and the translation of tool moves the stirred material from the front to the back of the pin and finishes welding process[76].Lee W.et al.[77]studied the effect of different friction stir welding parameters on the mechanical properties of AZ31B-H24 magnesium alloy joints and reported that joint strength increased with increasing tool rotation speed and decreasing welding speed,whereas Lim et al.[78]found no significant effect of processing parameters on the tensile strength of friction stir welded AZ31B-H24 alloy.In addition to the tool rotation speed and traverse speed,some another process parameters are the angle of spindle or tool tilt with respect to the workpiece surface,target depth and axial force[79-81].A suitable tool tilt towards trailing direction ensures that the tool shoulder holds the stirred material and move material efficiently from the front side of the pin to the back of the pin[82].The insertion/target depth of the tool pin into the workpieces is also an important factor for producing sound welds.The target depth of pin is related to the tool pin height.The tool shoulder face always kept in contact with the surface of workpiece to move the stirred material effectively to produce solid state joint.This can be done by proper selection of the insertion/target depth of the tool pin.Bahari[83]reported that the insertion depth should not be too shallow or too deep.In the case when it is of very little depth,the tool shoulder does not contact the original workpiece surface and cannot move the stirred material efficiently which resulted in the generation of welds with inner channel/surface groove.And when the insertion depth is too deep,the tool shoulder plunges into the workpiece producing excessive fl ash.Ouyang et al.[84]reported that during friction stir welded AA6061 alloy,the shoulder force that was directly responsible for the plunge depth of the tool pin into the surface of the workpiece during the plungement.The material flow patterns highly depended upon the geometry of the threaded tool,welding temperature,material flow stress and axial force.Krishnan et al.[85]studied the mechanism of onion ring formation in the friction stir welds of aluminium alloys and found that the material flow patterns are highly dependent upon the axial force.Padmanaban et al.[86]studied the effect of axial force on mechanical properties of AZ31B magnesium alloy joints fabricated by friction stir welding.They found that when the axial force was relatively low,there was a possibility of insufficient stirring at the bottom but with higher axial force,the weld was sound.It showed that sufficient axial force was required to form good weld because the temperature during friction stir welding defined the amount of plasticized material and the temperature was highly dependent on the axial force.

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2.3.Joint configuation

The most suitable joint con figurations for friction stir welding are butt and lap joints.A simple square butt joint and lap joint are shown in Fig.4.Two sheets of the workpiece with same thickness are placed together and properly clamped to avoid the abutting the joint faces from being forced apart during the process.During the initial plunge of the FSW tool,the forces are large so additional care is required to ensure the position of plates as reported by Padmanaban et al.[52].The rotating FSW tool is plunged into the joint line and traversed along the line when the shoulder of the tool is in intimate contact with the surface of the plates,produced a weld.[87-88].In case of lap joint,two lapped sheets are clamped on a backing plate.A rotating tool is vertically plunged through the upper plate and into the lower plate and traversed along the desired direction,joined the two plates[89].Many other con figurations can be produced by the combination of butt and lap joints.Some other types of joint designs Fig.5(edge butt joint,T butt joint,multiple lap joint,T lap joint,and fillet joint),apart from butt and lap con figurations are also useful in many applications as reported by Vila?a et al.[90]and Khaled[91].

Fig.5.Other joint con figurations for friction stir welding[32].

Fig.6.Overview of the microstructural zones in FSW joint AZ31[109].

3.Microstructural evolution

Earlier researchers[56,63,74,101,104,106,109]reported that FSW modified the microstructure of the base metal and resulted in the formation of weld nugget/stir zone(SZ),thermo-mechanically affected zone(TMAZ)and heat affected zone(HAZ)(Fig.6).Mishra and Ma[32].Sahu et al.[110]and Cam[111]cleared that several microstructural zones,e.g.,the weld nugget/stir zone(SZ),the thermomechanical affected zone(TMAZ),and the heat affected zone(HAZ),are generated during the FSW process and every zone exhibits different microstructural characteristics,including grain size,dislocation density,and residual stress as well as precipitate size and distribution.These microstructural changes in various zones have a significant effect on post-weld properties of the joint.Therefore,it is important for investigators to evaluate the microstructure of FSW joint.

3.1.Stir zone

During friction stir welding,plastic deformation and frictional heating results in the generation of a recrystallized fine-grained microstructure in the stir/nugget zone.Under some FSW conditions,onion ring structure was observed in the nugget zone[85,112,113].However,some investigators[52,97,103,104,106,114]reported that the small recrystallized grains of the nugget/stir zone contains the high density of sub-boundaries,sub-grains,and dislocations.The interface between the recrystallized nugget/stir zone and the base metal is relatively diffuse on the retreating side of the tool,however quite sharp on the advancing side of the tool.Table 4 presents the grain sizes reported in the previous literature with the substrate materials,tool geometries and welding parameters used to weld various grades of the magnesium alloys.

3.2.Thermo-mechanically affected zone

As reported in existing literature[40,56,63,74,101,104,109,115-124], there is the formation of a transition zone-thermo-mechanically affected zone(TMAZ)between the parent material and the nugget/stir zone(SZ)during the fabrication of weld joint by FSW.Liu[125]and Czerwinski[126]stated that the TMAZ experiences both temperature and deformation during FSW process.The TMAZ is characterized as a highly deformed structure zone.The elongated grains of parent metal were deformed in an upward flowing pattern around the stir zone.Due to insufficient deformation strain,the recrystallization did not occur in this zone.Kouadri-Henni et al.[114]also reported that the welded zone is composed of two parts:(i)Transition region(TMAZ)and(ii)Stir zone.In the TMAZ magnesium grains presented an elongated shape due to plastic deformation during FSW.They observed that a deformed grain structure consisting of sub-grains is formed just outside the stir zone in the TMAZ.The deformation of the grains increased with decreasing distance from the SZ.The grain size in the TMAZ is coarser than that in the stir region,following a grain size gradient,because of insufficient deformation and thermal exposure.These observations confirmed by the distribution of grains size.

3.3.Heat-affected zone

Beyond the TMAZ,there is a heat-affected zone(HAZ)which experiences a thermal cycle but does not undergo any plastic deformation.Esparza et al.[127]reported that In the FSW process,the HAZ is a zone where the material experiences no plastic deformation.The metal was neither stirred by the pin nor rubbed by the shoulder but was influenced by the heat of welding,leading to some microstructural changes.

4.Texture evolution

The strength,formability,ductility and corrosion resistance are highly influenced by texture of the material.The FSW joint consists of nugget/stir zone,TMAZ,HAZ and base material with different thermo-mechanical chronicle.During the FSW process,the plastic deformation in the material occurs at the elevated temperatures and preferential orientation or texture evolution is possible in SZ and TMAZ of the welded joint[109].As texture is strongly affect the mechanical properties of the material,the investigation of the crystallographicorientations arising from the FSW process has been becoming an emerging research area of interest.For FSW of Mg alloys,the local texture developed in the joints due to the critical material flow driven by the rotating tool which affect the mechanical performance.Some research reported that the initial texture not much affected the final microstructure and texture in the nugget zone(NZ)[168],different texture distributions formed in the thermo-mechanically affected zone(TMAZ)exerts an influence on the mechanical and fracture behavior of the joint[169].Some texture studies of FSW magnesium alloys have been reported[109,131,137,149,168-181].Mironov et al.[96]reported that the AZ31 Mg alloy base material had a bimodal microstructure consisting of relatively coarse elongated grains and a minor fraction of fine equiaxed grains.They found that some of the coarse grains contained lenticular-shaped{10ī2}twins presumably originating from the prior hot extrusion.The texture was dominated by a moderate{hkil}<1210> fiber orientation.In the stir zone,significant grain re finement took place and the{0001}B- fiber texture became predominant.The microstructure and texture of the base material and the material in the central part of the stir zone,higher-resolution EBSD maps and(0001)and(112ˉ0)pole figures are shown in Fig.7.The formation of this texture is also typically reported in frictionstirred magnesium alloys[114,131,137,149,172,176,180,181].Chowdhury et al.[109]reported that AZ31B-H24 Mg alloy contained a strong crystallographic texture with basal planes(0002)largely parallel to the rolling sheet surface and<112ˉ0>directions aligned in the rolling direction(RD).But,After FSW the basal planes in the SZ were slightly tilted toward the TD determined from the sheet normal direction(i.e.top surface)and also slightly inclined toward the RD determined from the transverse direction(i.e.cross section)due to the intense shear plastic flow near the tool pin surface.The prismatic planes(10ī0)and pyramidal planes(10ī1)formed fiber textures.Yang et al.[170]reported that during FSW of Mg-3Al-1Zn alloy,the shoulder size did not influence the texture modification induced by FSW in the nugget and weaken the{0002}texture in the TMAZ so that fracture was occurred in the nugget.Commin et al.[137]reported that AZ31 hot rolled base metal,the{0002}basal plane normal was parallel to the sample normal direction.The shoulder diameter 13mm did not induce such large modification and the fracture was located in the TMAZ but in the 10mm shoulder diameter sample,the strong{0002}texture was modified by the welding process.The basal plane progressively oriented perpendicular to the welding direction when approaching the nugget zone.Liu et al.[133]reported that sudden change of texture at the stir zone(SZ)/transition zone(TZ)interface and different twinning action between the TZ and SZ-side have an impact on fracture behavior of bending samples.The effect of textural variation on fracture behavior in bending is highly dependent on the local stress state.Park et al.[149]reported that the heterogeneous basal plane texture in friction-stir-welded AZ61 Mg alloy in the fracture region caused the preferential plastic deformation and eventually.Here,the major issue to recognize is the nucleation of new grains and continuous deformation influence the final texture of the material joint during FSW.It is essential to distinct the effect of the deformation by tool shoulder through the forging action when the tool pin has passed.The deformation under tool shoulder is expected to influence the final texture considerably and it enhances a shear deformation component to the recrystallized volume processed by the tool pin at lower temperature.

Table 4 Summary of substrate materials,tool geometries,welding parameters and grain size of FSW Mg alloys.

Fig.7.EBSD orientation maps and 0001and 1ī20 pole figures illustrating microstructure and texture in(a)base material,and(b)central part of stir zone[96].

5.Residual stresses

Although FSW is a solid-state joining process which produces low-distortion welds of high quality,significant levels of residual stresses can be present in the weld after fabrication[136].Commin et al.[97]reported that the magnesium alloy usage has been limited due to its active chemical properties,high coefficient of thermal conductivity,low melting point,hot cracking sensitivity,coarse grains and complex residual stresses in the weld seam after welding.Commin et al.[137]studied the influence of the microstructural changes and induced residual stresses on tensile properties of wrought magnesium alloy friction stir welds and showed that the highest tensile residual stresses were obtained in the TMAZ.The larger tool shoulder led to reducing residual stresses.Actually,increasing the shoulder size resulted in a higher heat input and therefore a thermal expansion mismatch reduction during cooling.Ahmed and Krishnan[138]reported that high-level residual stresses can occur in weldment due to restraint by the parent metal during weld solidification.The stresses may be as high as the yield strength of the material itself.When combined with normal load stresses these may exceed the design stresses.The removal of residual stresses takes place due to the fact that the thermal energy received by the metal allows for grain boundary sliding and removal of metallurgical defects like dislocations,vacancies and slip planes.The existing literature showed that the residual stresses induced during the welding process of materials.In FSW,residual tensile stress peaks are observed in the TMAZ in the longitudinal direction and compressive residual stress peaks occured in a transversal direction in the fusion zone(FZ)or nugget zone.Kouadri-Henni et al.[139]reported that the FSW process highly modified the distribution of residual stress in the different zones.The HAZ has weak compressive residual stresses and TMAZ and FZ the residual stresses are tensile in nature.The residual stress profile in TMAZ and FZ showed two noticeable peaks in the TMAZ whereas the centre of the FSW has very low tensile residual stresses.Additionally,the profile was partially non-symmetrical transversal to the weld and the TMAZ has not the same behaviour on the advancing side and the retreating side of the weld.The advancing side of the weld has a higher level of residual stress in comparison with the retreating side.Yan et al.[140]reported that there are many methods to improve the mechanical properties of welded joints such as to select the appropriate welding method,to select the appropriate welding parameters,and to control the penetration ratio and the correct choice of post weld heat treatment.Post-weld heat treatment is an effective way to eliminate the residual stress and to restore the property of the welded joints of magnesium alloys and improve the performance of the joints[141].

6.Mechanical properties

The existing literature of FSW reports that the friction stir welding results in significant microstructural development within the stir zone as well as in TMAZ and HAZ.This variation affects the post-welding mechanical properties.Therefore,the mechanical properties are reviewed here and are summarized in Table 5.

6.1.Tensile properties

As reported earlier that joint strength increased with increasing tool rotation speed and decreasing welding speed[77],but Lim et al.[78]found no significant effect of processing parameters on the tensile strength of friction stir welded AZ31B-H24 alloy.Pareek et al.[142]studied FSW of AZ31B-H24 magnesium alloy and showed that the tensile strength increases with increasing rotational speed.Park et al.[143]also studied FSW on AZ31Mg alloy plates at tool rotation speed 1230rpm and transverse speed 90mm/min which showed lower yield strength and elongation,and slightly lower ultimate tensile strength(UTS)of the weld.Nakata et al.[144]studied the optimal processing conditions for FSW of AZ91D Mg alloy sheet.The higher tensile strength in the weld was obtained than base material with a rotational speed between 1240rpm to 1750rpm and transverse speed of 50mm/min.Padmanabham et al.[52]studied the tool material effect on the friction stir welding of AZ31B magnesium alloy.Different tool materials such as stainless steel,highspeed steel,and armour steel,mild steel and high carbon steel were used.It was concluded that the joints fabricated by high carbon steel tool with 66 HRC,threaded pin profile and a shoulder diameter of 18mm exhibited superior tensile properties.Chowdhury et al.[109]reported the strength and ductility of the AZ31B-H24 Mg alloy after FSW decreased at all the strain rates,with a joint efficiency lying in-between about 75 and 82%.A minor increase of yield strength and ultimate tensile strength with increasing strain rate was observed,while the ductility in the base alloy decreased considerably.After FSW the strain rate dependence of the tensile properties nearly disappeared within the experimental scatter.

6.2.Hardness

As reported earlier that most commercial magnesium alloys are ternary in nature and made up of aluminium,zinc,thorium and rare earth.In ternary Mg-Al series,aluminium is the main alloying element which involves AZ(Mg-Al-Zn),AM(Mg-Al-Mn)and AS(Mg-Al-Si)alloys[13,14].Mishra and Ma[32],Dickerson et al.[145]and Uematsu et al.[146]reported in literature that aluminum alloys are of two types:(i)heat-treatable(precipitation-hardenable)alloys,(ii)non-heat-treatable(solid-solution-hardened).So the hardness of the magnesium alloy varies with the variation in aluminium fraction in it.It has been also reported that hardness profile affected by precipitate distribution and/or rather than grain size in the weld.The hardness value decreased gradually from about 73 HV to approximately 63 HV in the center of the SZ through the HAZ and TMAZ of the joints.The presence of the lowest hardness in the SZ attributed to the dynamic recrystallization and grain growth.The decrease in the hardness was due to the larger grain size at the higher rotational rate[129].Zhang et al.[147]and Xie et al.[148]reported that during FSW of Mg-Al-Ca and Mg-Zn-Y-Zr magnesium alloys,large intermetallic compounds(Al2Ca and Mg-Zn-Y phases)were broken up and dispersed in the nugget zone,resulted in the significant increase in hardness.So,the hardness in the nugget/stir zone was substantially higher than the other zones.But,Park et al.[149]reported that the uniform hardness throughout various zones of the weld of FSW AZ61.Esparza et al.[150]also found that FSW AM60 weld exhibited almost similar hardness across the whole weld.Xie et al.[151]reported that ZK60 is a precipitation strengthening magnesium alloy and the MgZn2precipitates in ZK60 magnesium alloy is much finer than Al12Mg17particles in AZ and AM system alloys.So,the hardness of the ZK60 alloy is mostly governed by the precipitates.The hardness values within the nugget zone of the as-welded joint found significantly lower than those of the base metal and other zones due to the further dissolution of MgZn2particles during FSW process.In such cases,the precipitate is the dominant factor to govern the hardness of the nugget zone rather than the grain size.Singarapu et al.[47]reported that increasing the rotational speed increases the microhardness and then gradually decreases.They stated two reasons for the improved hardness of stir zone.(i)the grain size of stir zone is much finer than the base metal,grain re finement plays a significant role in material strengthening.(ii)the small particles of intermetallic compounds are also a benefit to hardness improvement.Further increasing the rotational speed decreases the microhardness due to the high heat generation which causes material softening and resulted in a decrease in the microhardness.They reported that the softening of the nugget/stir zone was the result of coarsening and/or dissolution of strengthening precipitates.Sunil et al.[94]studied the joining of AZ31 and AZ91 Mg alloys by friction stir welding.They reported that a gradual increase in the hardness from AZ31 base material to AZ91 base material.In the nugget/stir zone,large variations in the hardness values found due to the combined effect of fine grain structure and the presence of hard Mg17Al12phase along with some regions of AZ31.Solid solution strengthening also contributed toward an increase of hardness as the nugget zone becomes a supersaturated solid solution due to the dissolution of more aluminium by reduced Mg17Al12phase.They concluded that the increased hardness in the nugget zone can be attributed to the grain re finement and the presence of Mg17Al12particles along with solid solution strengthening.Kouadri-Henni et al.[114]reported that the significant variations(increase or decrease)of microhard-ness measured in each transition region i.e.BM and HAZ,HAZ and TMAZ,TMAZ and nugget/stir.The microhardness in the HAZ,near to the TMAZ zone,is the same as the base metal and sometimes slightly higher.The presence of precipitates formed in this zone responsible for augmenting the microhardness.The microhardness in the TMAZ and in the nugget/stir is lower than in the base metal,even though the size of the grains was smaller than the base metal.They explained that microhardness decreases due to the influence of the dislocation density,residual stress variations in the weld and existence of the crystallographic texture.This difference in microstructure explained by the reduction in the number of precipitates which leads toward a reduction in the microhardness.

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7.Applications

The applicationsofFSW aswellasmagnesium alloy has been reported in earlier published literature [6,19,21,23,34,35,40,41,45,47,52,63,74,92-95,97-99,101-103,105,106,108,109,114,128,129,131,132,134-135,137,139,149,152,155-181]as summarized below:

1.Land Transportation:Engine chassis,wheel rims.Track bodies,tail lifts for tracks,mobile cranes,body frames,fuel tankers.

2.Aerospace Industry:Fuel tanks,wings,fuselages,cryogenic tank for space vehicles,aviation fuel tanks.

3.Railway Industry:Goods wagons,container bodies,underground carriages and trams.

4.Shipbuilding and Marine Industries:Panels for decks and floors,helicopter platforms,hulls and superstructures,marine and transport structures,offshore accommodation,masts and booms for sailing boats.

5.Construction Industry:Bridges,frames,pipelines,reactors for power plants.

6.Other Industry Sectors:Refrigeration panels,Motor housing.

8.Concluding remarks

In this review article current development in FSW process,microstructure,residual stresses,mechanical properties,and applications of friction stir welding of Mg alloys have been addressed.The basic conclusions that can be drawn from this review article are as follows:

1.Tool geometry is the most influential part of FSW process development.The tool pin profile,shoulder diameter and tool material highly influenced the joint quality.The friction between the tool shoulder and workpiece is the major component of heating.So,the size of pin and shoulder is more important than the design features of the tool.The scrolled shoulder surface also played a significant role in stirring/blending the material.

2.The FSW process parameter(angle of spindle or tool tilt with respect to the workpiece surface,target depth and axial force,insertion depth),in addition to the tool rotation speed and traverse speed,plays an important role in producing sound welds.The material flow patterns highly depended upon the geometry of the tool pin,welding temperature,material flow stress and axial force.The welding temperature is dependent on the axial force.

3.There is a lot of scope for research because the joint designs like-edge butt joint,T butt joint,multiple lap joint,T lap joint,and fillet joint apart from the butt and lap con figurations are also useful in many applications.

4.FSW modified the microstructure of the base metal and resulted in the formation of weld stir zone(SZ),thermomechanically affected zone(TMAZ)and heat affected zone(HAZ).Each zone exhibits different microstructural characteristics,including grain size,dislocation density,and residual stress as well as precipitate size and distribution.5.During the FSW process,the plastic deformation in the material occurs at the elevated temperatures and preferential orientation/texture evolution is possible in SZ and TMAZ.The texture affects the mechanical properties of the material.

6.The significant level of residual stresses produced due to the large deformation during FSW process.The advancing side of the weld has a higher level of residual stress in comparison with the retreating side.The HAZ has weak compressive residual stresses and the residual stresses are tensile in nature in TMAZ and FZ.

7.The mechanical properties are highly influenced by the FSW parameters and conditions.The increase in hardness in the nugget/stir zone is dependent on the grain re finement as well as the presence of Mg17Al12particles along with solid solution strengthening.

8.The friction stir welding of magnesium alloy has a wide range of applications in land transportation,aerospace,railway,shipbuilding and marine,construction and many other industries.

9.Future outlook

1.The mostly different tool is used by individual researchers and the only inadequate information is available in existing literature.Earlier published literature reported that a cylindrical threaded pin with concave shoulder tool features is widely used for FSW.But many special profile tools have also developed which needs to justify.

2.FSW process parameters(tool rotational speed,tool traverse speed,tool tilt angle,axial force,and target depth)are important to produce sound and defect-free weld joint.The selection criterion of these parameters needs to generalize.

3.Generally,butt and lap joint con figuration have been choosing for FSW as reported in published work.But,for the real implementation of the FSW,researchers need to be work on other joint designs.

4.The material flow is the very complex phenomenon and still requires more understanding.

5.More understanding required about the effect of alloy composition and FSW parameters on the thermal stability of fine-grained microstructure of friction stir welded Mg alloys.

6.The residual stresses induced during FSW can be minimized by selecting the appropriate welding parameters and the mechanical properties can be improved by post weld heat treatment(PWHT).So the future researchers should be focused in this domain.

7.The effects of the final deformation by tool shoulder through its forging action when the tool pin has passed need more understanding to elaborate the texture development.