Friction stir welding of AZ31 magnesium alloy: A review

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Introduction
Magnesium (Mg) is 6th most plentiful constituent on the earth's exterior surface and embodies roughly 2.5 percentage of its total composition [1].Magnesium alloys owes excellent specific strength [1], good castability [2], hot formability [3], and recyclability due to its less-weight.Magnesium alloys have confined strength, creep and fatigue resistance at higher temperatures [4], less stiffness [1], low ductility [5] and limited cold workability at normal temperature [6] owing to hexagonal lattice structure.Magnesium alloys possess poor hardness [7], low melting point, large thermal expansion [4], and great chemical reactivity in molten condition [8].Mg alloys tender immense capacity to decrease weight by replacing the ordinarily utilized materials, due to its density is around 1/4 of steel and 2/3 of aluminum [9].
The research and advancement of Mg alloys have amplified tremendously for engineering application in the course of the last twenty years.In terms of manufacturing process significance, Mg alloys can be broadly classified as cast Mg alloys and wrought Mg alloys [10].Most commercially available Mg alloys are ternary type and formed by zinc, aluminium, and rare earth elements.Aluminium is the chief alloying constituent in this ternary Mg-Al alloy that comprises AM (Mg/Al-Mn), AZ (Mg/Al-Zn), and AS (Mg/ Al-Si) alloys [11][12][13].Magnesium alloy recent investigation interests are primarily focused on enhancing ductility, specific strength, and creep resistance [14].AZ series of magnesium alloy is the most widely used alloy series [15,16].Particularly, the AZ31 that comprises of aluminium 3% and zinc 1% is mostly utilized in industry, and considered as the appropriate magnesium alloy for the structural applications related to structure [17,18].The earlier publication reported the application of AZ31 in the automotive industry [19,20], aerospace industry [21], railway industry [22], construction Industry [23], and other industries.
A tool rotating with shoulder and a pin / probe is plunged into the adjoining sides of plates and moved along the joint line to form the joint with friction stir welding technique [26].The weld side in which the welding path is similar to the tool revolving direction is recognized as the advancing direction and the side in which the welding path is reverse to the tool revolving direction is recognized as the retreating direction [27].The Schematic representation of FSW is as shown in Fig. 1.The main elements of the welding tool are a probe/pin having length slightly less than thickness of the workpiece/plate and protrude from the base/ shoulder of the tool.The heat is generated due to the friction among the workpiece and the rotating tool, and along with plastic deformation of workpiece material [28].The workpiece material in vicinity of the pin softens before it attains its melting point due to this localized heating [26].The combined rotation and translation of FSW tool move this soften material from the front side of probe to the back side of probe [29].A joint is formed in solid state only due to the result of this process [30,31].For welding of magnesium alloy / lightweight alloys; FSW has several advantages in comparison to the other welding technique [8].The research and advancement in FSW area and accompanying technologies such as friction stir processing [32][33][34][35], friction stir additive manufacturing [36,37] etc. have been emerging quickly, with many research organisations, industries and universities.Research publication numbers has also increased significantly.In this article, the contemporary state of acquaintance and progress of FSWed AZ 31 Mg alloys are reviewed.

FSW factors
The FSW technique comprises a material movement with complex plastic deformation in the course of the welding.There are three principal factors influencing the FSW process; (i) Tool Configuration, (ii) Welding variables, (iii) Joint shape.Temperature distribution and material flow pattern have vital effect by this factors [38][39][40].

Tool configuration
Tool geometry substantially influences the FSW process.Prior investigators reported that the tool configuration acts an important role in material movement and regulates the traverse velocity of FSW process [41].Mishra et al. [26] reported two principal functions of tool during FSW process, (i) material flow, and (ii) localized heating.Bahari et al. [42] and Patel et al. [43] described that at the early stage of tool plunge the heat is produced due to the friction among the workpiece and the rotating tool, along the plastic deformation of workpiece material.The tool pin is pushed into material until the shoulder physically contacts the workpiece [26].
Padmanaban et al. [25] examined the influence of tool shoulder diameter, tool material and pin profile on the FSW of AZ31B Mg alloy.Tool comprises of a shoulder and a pin utilized for examination is as shown in Fig. 2. They summarised that the weld joints produced by tool steel of 66 HRC and high carbon with threaded profile pin and 18 mm shoulder diameter exhibited good tensile properties.Tool material having greater hardness may produce more heat attributable to the high coefficient of friction.Fig. 3 and Fig. 4 shows the schematic representation of the shoulder shapes and features, FSW tool pin designed at TWI [25,44].Yang et al. [45] investigated influences of tool geometry with features on shear strength of welds on AZ31 mg plates with different welding tools.They found that the triangular pin as compared to the cylindrical tool successfully confines the hook and reduces the hook depth on the retreating side.Othman et al. [46] examined the effect of shoulder diameter / pin diameter ratio on FSW of mg alloy AZ31.The shoulder/ pin diameter ratio considered in this experiment are from 2.25 to 5.5 mm with an increment of 0.25 mm.They revealed that shoulder diameter / pin diameter ratio 5.5 exhibits least tensile strength whereas shoulder / pin diameter ratio 3.33 exhibits maximum tensile strength with 91% joint efficiency from based metal.
Bruni et al. [48] have conducted a series of experiments and numerical analysis to study the influence of the tool geometry on FSWed AZ31 mg alloy sheet's mechanical properties.They witnessed that the tensile strength and ductility of the FSW joints increase with an increase in the shoulder diameter and also confirmed results by the FEM simulations.
Yuan et al. [49] studied the effect of tool features and FSW welding parameters on top sheet thinning of AZ31 mg alloy during FS lap welding.They witnessed that the tool feature has a substantial effect on the hooking effect, cold lap features, morphology.Best mixing result for material flow that leads to intermittent and comparatively flat cold lap features are realized with concave shoulder.Yin et al. [50] investigated the influence of FSW tool shapes on material properties of FS spot welded AZ31 Mg joint.They Fig. 1.FSW process graphical representation [24].
Fig. 2. FSW tool nomenclature [25] Fig. 3. FSW tool probes [25] Fig. 4. FSW tool Shoulder shapes and surface features [47] observed that for all tool rotational speed settings the friction stir spot welded AZ31 joint made by three-flat/threaded tools exhibits superior mechanical properties than joints prepared with a threaded tool.Patel et al. [51,52] demonstrated FS processing of AZ31B Mg alloy with the stationary shoulder tool.Malarvizhi et al. [53] examined the influence of the tool feature design with various shoulder diameters for the FSW of AZ31 alloy.They found that the weld joint made with 21 mm tool shoulder diameter exhibited maximum tensile strength out of all joints made with various tool shoulder diameters.Buffa et al. [54] and Forcellese et al. [55] reported that good weld joints and higher mechanical properties can be obtained using a pinless shoulder tool for FSW of AZ31 alloy.The pinless tool design causes a more uniform microstructure than that achieved using the pin tool.Here, the tool shoulder shape, size, exterior surface, and features become more vital.The surface appearance and mechanical properties of the FSWed AZ31 thin sheet joint for the effect of tool configuration were investigated [56].Researchers found that among two variations of tool the ''pinless'' tool result in the higher values of the ductility and tensile strength in comparison with the ''pin'' tool.
During FSW, the tool shoulder is slightly introduced in the workpiece face.Forcellese et al. [55], padmanaban [25] and Mishra and Ma [24] concluded that the foremost portion of the heating is due to the friction amongst the workpiece and tool shoulder.Therefore, the pin & shoulder dimensions are crucial related to the other tool configuration [57].A curved-in shoulder with cylindrical threaded pins are utilized broadly for FSW process [58,59].The economical tool with long life is still a challenge for research in the FSW of soft alloys [60,61].Various material properties for tool such as hardness, strength, CTE, fracture toughness, and heat conductivity affect the tool performance, tool wear, and weld quality [62,63].Table 1 list out the tool geometries, tool materials and welding parameters utilized to weld AZ31 mg alloys.

Welding variables
Tool rotation speed in the anticlockwise or clockwise side and tool traverse rate in the weld direction are two major parameters in the FSW process [64].Tool rotation causes the stirring and blending of substance nearby the tool probe where as tool traverse conveys the stirred matter from the facade to the rear of the probe and completes the welding procedure [42].Lee et al. [65] investigated the influence of various FSW factors on the AZ-31B Mg alloy weld material properties and concluded that joint efficency raised with higher tool revolution rate and lesser weld speed.Rana et al [66][67][68] investigated the influence of tool rotation rate and speed of traverse on the material flow encountered during FSP of AA 7075 and suggested lower traverse speed for uniform material mixing and plastic deformation.Whereas Prins et al. [69] examined the influences of additional heating and cooling during FSW on the joint efficiency in the dissimilar material joining and corroborated the substantial improvement in that with water cooling assisted FSW attributed to grain refinement and strengthening.That is to say that the heat plays a vital role during FSW governs the material flow led by plastic deformations.Lim et al. [70] observed that the FSW process parameters not significantly influence the tensile properties of FSWed AZ-31B H24 grade alloy.Bruni et al. [71] experimentally and analytically investigated the influence of welding factors on properties of mg alloy FSWed AZ31 sheets.They concluded that the finest results have been achieved with the conical pin tool with rotation rate 2000 rpm and traverse velocity of 80 mm/min.Abdollahzadeh et al. [72] examined FSW parameters influence like tool rotation speeds and transverse rate on the grain size, surface appearance, and hardness & tensile properties of FSWed AZ31 thin sheets joint.They obtained fully recrystallized grains in the nugget zone near the weld axis.Padmanaban et al. [31] investigated the axial force to check its influence on the material properties of FSW fabricated AZ-31B Mg alloy joints.They revealed that with low axial force, there was possibly an inadequate stirring at the lower most region of plate and with greater axial load, the sound weld joint was achieved.They concluded that adequate axial load was essential to fabricate good quality joint as the temperature in the course of FSW govern the quantity of material plasticized and the temperature yielded was extremely reliant on the axial force.Apart from the tool revolution speed and rate of traverse, some other process factors like the tool tilt or spindle angle regarding the workpiece face, axial force and target depth also affect the weld quality [73,74].Proper tool tilt in the direction of trailing side ascertain that the shoulder of tool embraces the stirred metal and transfer it effectively from the facade direction of the probe to the rear of the tool probe [75].

Joint shape
Butt and lap joints are most suitable joint configuration for FSW.Fig. 5 (a) and (d) shows the square butt-joint and simple lap-joint in sequence.Two workpieces in the form of sheets with alike thickness are put altogether and clamped accurately to evade the joint surfaces from being separated throughout the process [26].Padmanaban et al. [25] reported that the tool forces are higher at the time of the preliminary plunge of the FSW tool, so extra precaution is necessary to ensure the location of plates.During FSW, the rotating tool is forced into the weld line until the tool shoulder touches the plates surface and then moves along the weld line to form the weld [ 41 76].Two overlapped sheets are fastened together on a backing plate in lap joint configuration.In lap join, a revolving tool is dig across the top sheet & into the bottom sheet then move along the weld line.[77].
Some other arrangements can be formed by the blend of butt joints and lap weld joints.Several additional configuration as shown in Fig. 5, apart from lap and butt joints are also suitable for various usages as conveyed by Khaled [78] and Öchsner et al. [79].

Microstructure evolution
Earlier researchers [80][81][82][83] stated that during FSW the base metal microstructure gets modified and lead to the development of heat-affected zone (HAZ), nugget zone / stir-zone (SZ), and thermo-mechanically affected zone (TMAZ) as shown in Fig. 6.Shen et al. [84] considered the effects of tool probe diameter on the mechanical properties and microstructure of FS spot-welded AZ31B Mg alloy joints.They revealed that the normal grain size of a-Mg in SZ, TMAZ and HAZ intensified with the rise in pin diameter respectively.Different joint properties are substantial influenced by the microstructural changes in different weld zones after welding.Hence, it is essential for researchers to examine the microstructure of the FSW joints.

Stir zone
During FSW, fine-grained recrystallized microstructure is generated in the stir/nugget zone due to plastic deformation and frictional heating.Some investigator witnessed the onion ring shapes in the nugget zone during FSW [41,128].Chen and Zhang [129] studied the effect of the improved cooling with air and water on the mechanical material properties and microstructure of the FSWed AZ31 Mg alloy joint.For water cooling condition the normal grain size in SZ was 1.3 lm, which is very small as compared to the joint fabricated in air cooling environment.Some investigators [25,76,83,[130][131][132] reported that the recrys-tallized small grains contains more dislocations, sub-grains, and sub-boundaries.The recrystallized boundary among the stir zone / nugget & the base material is comparatively strewn on the weld retreating direction, though relatively clear on the weld advancing direction.
Chowdhury et al. investigated the fatigue life and lap shear strength of friction stir spot welded AZ31 Mg and found that the high temperatures and intense plastic deformation encouraged vigorous recrystallization inducing the presence of refined and uniform grains in the SZ.However, due to the major consequence of the friction heating, the grains in the HAZ and TMAZ became coarser.Table 2 presents the workpiece materials, welding parameters, tool geometries and the grain sizes described in the earlier researech for FSW of AZ 31 mg alloy.

Thermo-mechanically affected zone
Researcher [30,64,80,82,141,142] reported that TMAZ is the transition zone formed among the base material & the stir zone / nugget in the course of the creation of FSW weld.Liu [27] & Czerwinski [16] reported that the TMAZ encounters both deformation and temperature throughout FSW.The TMAZ is categorized as an extremely deformed microstructure zone.Grains in this particular zone are elongated and aligned towards SZ owing to the stresses induced on account of the extreme plastic deformation in the SZ.In this zone, there is no recrystallization occurred due to inadequate deformation strain.Kouadri-Henni et al. [143] presented that in the TMAZ of AZ31 Mg grains showed a stretched shape because of plastic deformation in FSW.They witnessed that in the TMAZ just outside the stir zone a deformed grain structure is formed which comprises sub-grains.As the distance from the SZ decrease the deformation of grains increased.Due to insufficient thermal exposure, deformation and as per the grain size gradient TMAZ contains courser grains than that the stir region.These remarks were verified with the study of the grains distribution in the microstructure.

Heat-affected zone
There is a HAZ past the TMAZ in the weld zone which encounters a thermal effect but does not undergo plastic deformation.Mishra et al. [26] stated that in the HAZ region the material does not experience any plastic deformation during FSW.The metal in this region was not rubbed by the shoulder or stirred by the pin but was affected by the temperature during FSW, leads to certain microstructural variations.

Residual stresses
FSW is a solid phase welding technique where weld joints of good quality produces with low-distortion, even though a considerable amount of residual stresses exist in the weld joint postfabrication.[76,144].
Commin et al. [130] examined the effect of the induced residual-stresses and microstructural changes on tensile properties of AZ31 using FSW and reported that the maximum residual tensile stresses were found at the TMAZ.In FSW lesser residual stresses would be anticipated because of the small thermal gradients seen as related to fusion welding techniques like laser welding.In FSW residual stresses were generated during cooling as this technique needs very rigid clamping.Residual stresses reduced with increase in tool shoulder diameter.Actually, larger shoulder diameter results in greater heat input, and consequently discrepancy in thermal expansion reduced during cooling.Kouadri-Henni et al. [76] examined the residual stresses generated during FSW welding AZ31mg alloys.An average value of the transversal and longitudinal residual stresses was calculated in the different regions of FSW weld joint.The base material has a residual compressive stress.FSW technique extremely varies the distribution of residual stress in the different regions.They found feeble residual compressive stresses in HAZ.The center of the weld zone has quite low residual tensile stresses whereas; TMAZ and FZ residual stress profile indicated two conspicuous crests in the TMAZ.Moreover, the weld profile was somewhat asymmetrical in nature and the TMAZ did not behave the similarly on the weld advancing direction and the weld retreating direction.The weld retreating side has lower level of residual stress as compared to the weld advancing side in the FSW joint.
During FSW, residual compressive stress crests were observed within the nugget zone/fusion zone (FZ) in a transversal direction whereas, tensile residual stress crests were observed within TMAZ in the longitudinal direction.Mironov et al. [97] examined the tensile performance of FSWed magnesium alloy AZ31 using digital image correlation and electron backscatter diffraction (EBSD).An extensive strain gradient was realized along the joint in the stir zone.They concluded that considering the common nature of this strain ramp from the facade side to the backside, it is tough to describe stochastic variations of residual stress through the weld path.Thus, the source of this phenomenon is absolutely unclear and needs advance study.
Yan et al. [145] reported that the proper selection of the welding method, suitable welding parameters, good penetration ratio   control, and appropriate post-weld heat treatment (PWHT) are different ways to enhance the mechanical characteristics of AZ31 FSW joint.Wang et al. [131] evaluated microstructure and mechanical properties of FSWed AZ31 based on different postweld heat treatment temperatures.They concluded that the PWHT is an efficient way to reduce/abolish the residual stress, improve joint performance and to regain the FSW welded joint property of AZ 31 Mg alloys.

Mechanical properties
The present research on FSW describes that the FSW results in substantial microstructural alteration in the SZ along within TMAZ & HAZ.This alteration affects mechanical properties after welding.For that reason, the mechanical properties of FSW of AZ31 Mg alloy are reviewed here and are encapsulated in Table 3.

Tensile properties
Earlier, Lee et al. [65] and Sun et al. [112] studied the influence of various FSW factors on the AZ-31B mg alloy mechanical properties and concluded that tensile strength of joint improved with intensifying tool spin rate and reducing welding rate.Pareek et al. [155] also investigated FSW for AZ-31B Mg alloy and found that the joint strength rises with an increase in rotational rate.But, Lim et al. [70] observed that the processing parameters has no significantly effect on the tensile strength of FSWed AZ-31B-H24.Padmanaban et al. [25] investigated the influence of various tool material on the FSW of AZ-31B Mg alloy.It was found that the weld produced with high carbon tool steel of with HRC 66; shoulder diameter of 18 mm and pin profile with threads evidenced higher tensile properties.
Shen et al. [84] studied tool pin diameter influence on the microstructure and material properties of FS spot welded AZ-31B mg alloy joints.For FS spot welded AZ31B Mg alloy joints with the increase in tool pin diameter the tensile shear force increased as the width and height of the curved interface governed the specimen failure.Sheng et al. [156] examined the microstructure & material property of the AZ31 mg alloy FSWed joint.The results indicate that the mechanical properties of the considered crosssection decrease from the upper to lower direction of the joint and the initial portion is the weakest unit, whereas the central portion is the strongest one.
Chen and Zhang [129] examined the influence of the enriched cooling with air and water on the microstructure and material tensile properties of the FSWed AZ31 mg alloy joint.They revealed that under enhanced cooling environment the ultimate tensile load improved by 15.7%, and the tensile deformation value amplified by 62.2%.Forcellese et al. [55] reported that good weld joints and higher mechanical properties can be obtained using a pinless shoulder tool for FSW of AZ 31 Mg alloy.Dorbane et al. [102] studied mechanical, texture and microstructure response of the FSWed AZ31 mg alloy joints.Test samples with FSW perpendicular direction as major axis shown higher strength levels than base material.The highest tensile elongation was exhibited by the FSW parallel samples.

Hardness
In different magnesium alloy, the hardness varies with the variation of aluminum content in it.Researchers also revealed that for any particular Mg alloy the hardness profile influenced by grain size and/or precipitate distribution in the weld.Fukumoto et al. [157] studied the influence of PWHT on microstructures and mechanical properties of FSWed AZ31 mg alloy.The distribution of micro-Vickers hardness along the tool rotational axis in the vicinity of the weld interface shows that the hardness near the weld interface is greater than that of the base metal.The maximum value of hardness is found located roughly at 3 to 6 mm distance from the weld interface at the work-hardened region.Grain refinement and/or work hardening are responsible for the additional hardness.Since, recrystallized fine grains in FSW at the weld boundary contain some dislocations; the extra hardness at the weld boundary is due to the grain refinement.Conversely, the grain size in the work hardened section is alike to the base material.Hence, the extra hardness here is due to the work hardening.
Researcher [158] stated that hardness contour influenced by precipitate dispersal and grain size or both in the FSW joint.The hardness value reduced progressively from about 73 HV through the TMAZ and HAZ to around 63 HV at the mid of the SZ of the weld.The occurrence of the lowermost hardness within the SZ accredited to the grain growth and vigorous recrystallization.Bigger grain size at the greater tool rotational speed leads to the reduction in the hardness [158].Singarapu et al. [159] concluded that with an initial increase in rotational rate increases the microhardness up to a certain level and then reduce slowly.They specified two causes for the hardness improvement in stir zone.(i) Tiny intermetallic compounds particles helps in hardness improvement.(ii) Stir zone grain dimension is much smaller than the parent material; here grain modification plays an important part in metal strengthening.Further increase in tool rotational rate generates elevated heat that leads to softening of material and in turn results in a reduction in the micro hardness.Researchers described that the nugget zone /stir zone softening resulted due to the dissolution of strengthening and/or coarsening of precipitates.Shen et al. [84] investigated the influence of diameter of tool pin on the microstructure & material mechanical properties of FS spot welded joint of mg alloy AZ-31B.They concluded that the smaller the pin, higher the micro hardness of SZ, TMAZ and HAZ.Chen and Zhang [129] examined the influence of the improved cooling with air and water on the microstructure and material properties of the FSWed AZ31 mg alloy joint.They revealed that the micro hardness in SZ considerably increased.Chowdhury et al. [160] studied work hardening performance the tensile strength, and microstructures of FSWed AZ31B half-hardened H24 temper Mg alloy sheet at various strain rates.They realized that the hardness declined progressively from around 70 HV in base material to around 50 HV at the middle of the joint.This is owing to the development of nonuniform cast structures in the friction zone, in combination with a bigger size grain in the FZ as compared to that in the HAZ and BM.
Sevvel et al. [30] investigated the effect of the process variables such as tool rotating rate and welding path at the fixed axial force with a cylindrical taper pin profile HSS tool on properties of FSWed AZ31B Mg alloy lap joints.Ugender et al. [80] considered the influence of tool material & rotational rate on microstructure & mechanical properties of FSW lap joint of AZ31B mg alloy.They found that the joint produced with 1120 rpm rotational rate and 40 mm/min welding velocity presented greater hardness of 75Hv in the stir zone.These are two key causes for the enhanced hardness of the stir region.(i) grain refinement in stir region (very fine grin in SZ than base metal) plays a vital part in strengthening of material, (ii) the small intermetallic compound particles are also an advantage in hardness enhancement [80,159].

Fracture behavior of FSWed AZ 31 Mg alloys
Some researchers have stated that in FSW of AZ-31 Mg the UTS and tensile elongation are considerably decreased which was principally due to the discrepancies in texture, nugget shape and grain size.Some investigations on fracture performance concluded that when the joint is loaded in a perpendicular direction, the fracture may take place in the TMAZ or HAZ on the AS or RS of the weld joint, according to the FSW conditions [161][162][163].
Commin et al. [130] revealed that the variance in grain size is not the key factor influencing the fracture location of the FSWed joint.Park Liu et al. [164] studied the impact of textural change and twinning on fracture performance of AZ 31 Mg alloy.They found that different twinning action amongst the SZ and TZ-side and sudden texture change at the SZ /TZ interface influences fracture behavior of bending.They also revealed that both, base and surface test workpieces are fractured near to the SZ /TZ boundary on AS.Yang et al. [165] inspected the effects of rotation rates on fracture behavior of FSWed AZ31 Mg Alloy.They concluded that at 800 & 2000 rpm, fracture locations were the center of NZ and the boundary of TMAZ/NZ.Dorbane et al. [102] studied the tensile fracture behavior and microstructure of the FSW twin roll AZ31 Mg alloy.They concluded that FSW right angle samples exhibited the least strain to fracture and fracture taking place among the SZ and the TMAZ in the AS.Zhang et al. [96] experimentally investigated the fatigue fracture behavior of the FSWed AZ31 joint.They found that at applied stress amplitudes, the fatigue specimens fractured along the HAZ at the AS.

Defect formation during FSW of AZ 31 Mg alloys
Researchers witnessed some kind of defects when the improper parameters were selected during FSW experiments.Some researchers focused on varying the shape and size of hook defects through optimizing the FSW parameters [113,121,166].Cao and Jahazi [167] investigated the effects of probe length and tool rotational rate on the quality of the FSWed AZ-31B Mg alloy lap joint in terms of welding defects.They concluded that hooking can be regard as mainly harmful defects in FSWed lap joints.They also revealed that the cavity defects look primarily at small heat input.Liu et al. [128] and Yuan et al. [168] investigated the defects formed with different FSW conditions.They concluded that the burr defect occurred due to the large heat input Mironov et al. [38] during their study on the effect of FSW temperature on flow of material for AZ31, found that the defects were correlated with the material vortex flow generated by the tool tip.Campanelli et al. [169] during preliminary investigation and microstructure observations on FSW of AZ31 Mg alloy observed the presence of a hook defect at the interface of overlapped sheets.Chowdhury et al. [160] during their investigation on FSW of AZ31B Mg alloy observed the sub-surface porosity and notch defect near the base surface of FSWed workpiece with a pin tool having righthand thread [170].

Concluding statements
In this paper recent advancement in FSW technique, mechanical properties, residual stresses, microstructure, and applications of FSW of AZ31 mg alloys have been reviewed.Following are some conclusions drawn from this article: The tool pin diameter & profile, tool material and shoulder diameter significantly affected the joint properties.The friction induced among the workpiece & tool shoulder is the prime factor responsible for heat produced in FSW.Therefore pin & shoulder dimensions are vital than the design aspect of the FSW tool.Apart from the parameters like the tool rotation rate & traverse velocity; tilt angle of tool regarding the specimen surface, axial force and the target height play a vital role in fabricating good welds.Axial force governs the welding temperature in FSW.Tool pin geometry, axial force flow stress of material and welding temperature greatly affects the material flow patterns.Apart from the butt and lap configuration, there is a lot of research opportunity for T butt joint, T lap joint, edge butt joint, multiple lap weld joint, and fillet weld joint design for different applications.During FSW process, base material microstructure is modified and lead to the formation of SZ, TMAZ and HAZ.Every zone contain different microstructure, grain size, precipitate size and distribution, and dislocation density, During FSW process significant amount of residual stresses induced because of the large deformation.TMAZ and SZ have tensile residual stresses where HAZ has feeble residual compressive stresses.The weld advancing edge has a greater intensity of residual stress as compared to the retreating edge.The mechanical properties of material after weld are greatly affected by the FSW variables and circumstances.The higher hardness in the stir region / nugget is due to the grain modification and solid solution strengthening.

Future viewpoint
Previously published literature reported that various tools are utilized by individual researchers but a tool with cylindrical pin and concave shaped shoulder is rarely utilized for FSW of AZ31 Mg alloy.Specific profile pin tools were also built and there is a vast research scope for special profile tools development and application but needs justification.

Table 1
Review of tool materials, tool dimensions and welding process parameters utilized for FSW of AZ 31 Magnesium alloys.

Table 2
Summary of workpiece materials, tool dimensions, welding process variables and grain-size for FSW AZ 31 Magnesium alloys. [115]

Table 3
Summary of mechanical properties of FSWed AZ 31 Magnesium alloys.