Experimental Study on the Role of Tool Geometry in Determining the Strength & Soundness of Wrought Az 80 a Mg Alloy Joints During Fsw Process

An investigation was conducted to find out the influencing role of the geometry of the FSW tool on the tensile strength, corrosion resistance, wear rate of the wrought AZ80A Mg alloy joints during the FSW process. FSW tools with two uniquely different geometries, namely stepped cylindrical shoulder tool and stepless cylindrical shoulder tool were employed at different combinations of speeds of rotation and traversing speed. Comprehensive observations & investigations were also carried out regarding the tensile strength, wear losses & corrosion resistance of the fabricated joints. The joints fabricated using stepped cylindrical shoulder tool exhibited appreciable tensile strength values at combinations of higher traversing feeds and low values of rotational speeds of the FSW tool. At the same time, the joints possessing fine sized refined grains in their nugget zone have been found to exhibit higher wear losses.


INTRODUCTION
Today's current demand & challenge related with automobile manufacturing units aims for enhancing the fuel economy along with reduction of emission levels of greenhouse gases, and this can be eventually obtained by reducing the weight of the automobile [1][2][3][4].Reducing the weight of the automobiles without any compromise in reliability & safety is a quite bit challenging task, which can be obtained with the help of designing suitable innovative structures & by employing light weight metals for manufacturing various automobile components [5,6].Various alloys of magnesium have attracted many automobile manufacturing engineers & researchers working in the domain of fabricating light weight transportation units, as they belong to the category of light weight metals [7][8][9][10].Further, features of alloys of magnesium, namely high stiffness, reasonable specific strength, ability to conduct heat in a faster rate, easy castability & machinability, tremendous electro magnetic interference damping and shielding capability etc also have made them to be one of the most inevitable material for use in present automobile manufacturing sectors [11][12][13][14] .
At the same time, the tribological properties and formability features of the majority of the Mg alloys are not meritorious.Another factor which plays a dominant role in hindering a wide usage of Mg alloys, is their poor formability at room temperature, resulting from their hexagonal close-packed (HCP) crystal.Hence, a large scale usage of Mg alloys entails the choice for suitable joining methodology to fabricate high quality, sound joints with predominant mechanical properties [15][16][17][18][19]. Friction stir welding (FSW), an energy efficient & eco friendly solid state joining method, with the ability for fabricating joints without melting the base materials, has gained the attraction of various experimental researchers working with the objective of joining light weight metals [20][21][22].FSW have been proven to possess various merits over the conventional fusion joining process like completely eliminating the inherent defects due to casting and refining the structure of grains in a adequate manner, thereby increasing the strength of the fabricated joints, their corrosion resistance properties, formability, ductility etc [23][24][25][26][27].
For example, Masayuki Aonuma et al.
[42] carried out an experimental work to understand the role of the alloying elements during joining of ZK60 Mg alloy with Ti metal using FSW process.Joints were obtained successfully and a thin Zr and Zn layer thickness -1mm) was generated at the interface of the joint surfaces, which leads to a fracture at the joint interface during tensile test and revealed the fact that, the alloying elements have appreciable beneficary effect in improving the strength of the dissimilar FSW joints.Even though, microstructures of wrought & cast Mg alloys can be naturally, unquestionably refined after FSW, the strength of the joint was often to decrease due to dissolution of precipitates and redistribution of textures.Zhe et al. [46] experimentally proved that the strength of the ZK60 Mg alloys fabricated using FSW process can be improved through adoption of aging treatment and post-weld compression.It was found during the investigation that the post-weld compression has increased the precipitates number in the fabricated ZK60 Mg alloy joints during the consequent aging.Another researcher Dhanapal et al [49] investigated the corrosion performance of the wrought AZ61A Mg alloy joints fabricated using the technique of FSW.The corrosion performance of the fabricated joints was validated by immersing the joints in solutions of NaCl with different values of pH, different concentrations of chloride ion & at various time intervals of immersion.The investigation helped us to understand that there is a notable improvement in the resistance towards to corrosion of the AZ61A joints w.r.t alkaline solutions when compared to that of the neutral and acidic solutions.Further, it was observed that the β-phase distribution places a significant role in influencing the corrosion performance.
Kim et al. [51] carried out an experimental work on understanding the microtexture evolution, in the SZ (stir zone) of the fabricated AZ31 Mg alloy joints through advanced techniques namely polycrystal modeling and microtexture analysis.It was revealed by the employment of the analysis using the EBSD (electron back scatter diffraction) that, the process of FSW has induced to a greater extent, the formation of strong fibered textures in the SZ and this development of fibered textures were dependent on the moving track of the material point w.r.t the pin of the rotating of the FSW tool.
Relatively poor ductility and formability is inevitable in AZ80A Mg alloy joints fabricated using conventional fusion joining techniques, and the existence of hcp structure with systems of slip at temperature of ambience, is one of the major reasons for this poor ductility, which hinders the successful fabrication of sound quality AZ80A Mg alloy joints.In this experimental research work, an investigational attempt was made to understand in detail, the important role of the FSW tool design & its geometry in influencing the tensile strength, losses due to wear, rate of corrosion of the AZ80A joints during their fabrication at optimized values of FSW process parameters namely traversing speed of the FSW tool and speeds of rotation of the FSW tool, with others parameters (including axial force, depth of penetration by the pin profile of the FSW tools into the joint surface etc) being constant.

Machine and Parent Material
Flat 5mm thickness wrought AZ80A Mg alloy plates of 125 mm length and 55mm width composed of 7.82% aluminium, 0.31% manganese, 0.69% zinc, 0.051% of copper, 0.049% of iron, o.11% of silicon, 0.052% of nickel and balance being magnesium [52, 53] is used as the metal of base of investigation.
The entire experimental process of joining flat plates of wrought AZ80A Mg alloy using the technique of FSW was done using a self ingeniously fabricated & developed FSW machine (semi automatic in nature).A maximum spindle speed of 1500 rpm can be achieved in the above mentioned FSW machine and this FSW machine houses a 5kW power spindle motor, which can provide a maximum of 3 ton axial force from the upward direction.This FSW machine houses an adjustable fixture which has provision for holding individual flat work plates with a maximum length of 200 mm and width of 75mm.A photographic illustration of the wrought AZ80A Mg alloy flat plates being firmly held in their respective position in the FSW machine's fixture (before friction stir welding is being carried out on those plates) can be seen in the Figure 1.

Material & Geometry of the FSW tool
Usually, the tool for carrying out an FSW is selected depending upon the materials to be joined together and fabricating a FSW tool with appropriate, suitable material plays a significant role in successfully fabricating the FSW joints, at different preferred levels & values of FSW process parameters [54][55][56].HSS (high speed steel) of grade M35 was chosen as the raw material for the FSW tool in this experimental investigation.The hardness of the above mentioned HSS FSW tool was around 61 -62 HRC.
Designing an FSW tool with suitable geometry includes the design of the tool shoulder as well as the design of the tool pin profile.The FSW tool shoulder executes the function of preventing the plasticized metal from escaping the surface of the workpiece, being welded.It also plays a vital role in generating the required amount of frictional heat in ideal volumes during the joining process [57][58][59].Likewise, the careful consideration & care must be taken while designing the FSW tool pin profile, as it regulates the uniform flow of the plasticized material along the entire NZ (nugget zone), during the friction stir welding process [60,61].
In this experimental investigation, two FSW tools (made up of M35 grade HSS material) designed with two different shoulder profiles, namely stepped cylindrical shoulder tool and stepless cylindrical shoulder tool are employed for the purpose of fabricating joints between two flat plates of wrought AZ80A Mg alloy.The photographic of the stepless cylindrical shoulder tool is shown in the Figure 2 (a).This tool was designed and fabricated in such a way that this tool has a shoulder diameter of 30 mm and a tapered pin profile.The major pin diameter of this FSW tool is 7 mm and it tapers down to 4 mm along a total pin length of 4.75 mm.Similarly, the FSW tool with stepped cylindrical shoulders is designed in such a way that, it has a 20mm outside tool shoulder diameter and the inner diameter of the tool shoulder is 15 mm, as seen in the Figure 2 (b).The geometry of this tool pin profile is exactly as that of the stepless cylindrical shoulder tool, i.e., with the tapered shaped pin as its pin profile.

Parameters of the FSW process
The experimental investigations were conducted using the concept of the 2 factors with 3 levels full factorial design with two types of replicants for both the types of the FSW tools, i.e., for the stepless cylindrical shoulder tool & for the stepped cylindrical shoulder tool.The different values of the speeds of rotation of the FSW tool and the traversing speed of the FSW tool employed in this experimental work are clearly described in the Table 1.The other FSW process parameters namely axial force, depth of penetration of the FSW tool pin profile etc are kept as constant values throughout the entire experimental work.An axial force of 3kN was maintained throughout the entire joining process and the total depth of penetration of the pin profile of both the FSW tools were about 4.75mm for the entire set of the experiments.Different stages of fabrication of flat wrought AZ80A Mg alloy plates into sound quality joints by employing the stepped cylindrical shoulder FSW tool with tapered pin profile are illustrated in a detailed manner in the Figure 3.It can be observed that, after placing the flat plates firmly in the specially designed fixture, the rotating FSW tool was slowly made to penetrate into the joint surface of the two flat AZ80A Mg alloy flat plates.Once the pin profile has made its full penetration depth of 4.75mm, the shoulder surface of the stepped FSW tool touches the joint surface, then the automatic traversing mechanism of the FSW machine is engaged in full fledged manner, which permits the FSW tool to make its further travel with the preset speed of rotation and traversing speed for the remaining entire length of the workpiece surface as seen in the Figure 3.   From the Figure 5, it can be observed clearly that, for the traversing speed of 0.5 mm/min, the strength of the fabricated joints seems to decrease when the speeds of rotation of the FSW tool is increased.For the 1.5 mm/min value of tool traversing speed, there is no significant change in the tensile strength of the joints, when the speed is increased from 500 rpm to 750 rpm.But, further increas e in the speed of the FSW tool (to 1000 rpm) decreases the tensile strength of the fabricated join ts in a drastic manner.In the case of the 3 mm/min traversing speed, the joint strength increases when the speed is raised form 500 rpm to 750 rpm, but decreases with further increase (1000 rpm) in the rotational speeds of the FSW tool.At a 500 rpm FSW tool rotational speed, the tensile strength of the fabricated AZ80A Mg alloy joints are found to decrease at 1.5mm/min & then again increases with increase in the traversing speed (i.e., at 3.0 mm/min).At the same time, the joints produced at combination of 0.5 mm/min and 500 rpm exhibits the highest tensile strength values when compared with that of the other two combinations.At a 750 rpm FSW tool rotational speed, the tensile strength of the fabricated AZ80A Mg alloy joints are found to continuously increase with the increase in the traversing speeds and the lowest tensile strength for this set is exhibited by the 750 rpm -0.5 mm/min combination.For 1000 rpm FSW tool rotational speed, the tensile strength of the joints are found to decrease with the increase in the traversing speed (1.5 mm/min), but further increase in the traversing speed (3.0 mm/min) increases the tensile strength of the fabricated joints.And the combination of 1000 rpm -0.5 mm.min exhibits the highest values if the tensile strength, when compared with that of other two 1000 rpm & feed rate combinations.

Analysis of the optical micrographs of the fabricated joints
The above observations, adds weightage to the fact that the strain hardening effects are common phenomenon encountered at low speeds of rotation of the FSW tool, due to the generation of dislocations in grain structures, leading to the piling up of the slipped planes [62][63][64].
The optical micrograph image of the AZ80A Mg alloy joint fabricated at a combination of traversing speed of 0.5 mm.min and speed of rotation of the FSW tool of 500 rpm is illustrated in the Figure 7(a).The twinned grains presence is clearly visible in this optical micrographical image.But at the same time, it can be seen that this effect of grain twinning is considerably reduced in the case of the joints fabricated at a FSW process parameter combination of 0.5 mm/min and 1000 rpm, as seen in the Figure 7 (b).The readings & observations illustrated in the Figure 6 describes us that, the stepped cylindrical shoulder tool has produced sound quality joints with appreciable tensile strength values at combinations of higher traversing feeds and low values of rotational speeds of the FSW tool.It can be seen that, the tensile strength of the AZ80A Mg alloy joints fabricated using the stepped cylindrical shoulder tool increases with the simultaneous raise in the values of the traversing speed along with the combination of low rotational speeds.On the other hand, at 1000 rpm (i.e., at larger FSW tool rotational speeds), the tensile strength of the AZ80A Mg alloy joints fabricated using the stepped cylindrical shoulder tool seems to decrease with simultaneous increase in the traversing speed.
These observations are due to the fact that, at low values of speeds of rotation of the FSW tool, the amount of heat produced due to the frictional heat would not be sufficient enough to create uniform flow of the materials and during faster traversing speeds (i.e., at higher rates of feed), the time period required for grain deformation is not enough [65][66][67].On the contrary, at larger values of speeds of rotation of the FSW tool, increase in rates of feed, will lead to the fine refinement of grains in the nugget zone.This is due to the fact that, higher tool rotational speeds generates the ideal amount of frictional heat leading to uniform flow of metals and will eventually result in rapid & uniform solidification of the grains in the nugget zone at higher traversing feeds , which is evidently illustrated in the Figure 7 (c)  and (d).
It can be understood that, the size of the grains in the nugget zone have played a vital role in determining the strength of the AZ80A joints fabricated using the stepped cylindrical shoulder tool.The internment time taken for the plasticized stirred material created by the stirring action of the stepped cylindrical shoulder seems to have influenced the softening rate of the plasticized material, in the nugget zone.This rate of softening followed by eventual cooling has led to the recrystallization process and formation of fine refined grains in the nugget zone.This internment time period is very much less with that of the stirring action created by the stepless cylindrical shoulder tool, due to the reason that, the stepped cylindrical shoulder has performed the function of preventing the plasticized material, being getting escaped from the joint surface area and the stepless cylindrical shoulder tool, due to its design geometry cannot perform this function in a required manner.This is one of the most important reason for formation of fine refined grains in the nugget zone of the AZ80A Mg alloy joints fabricated using the stepped cylindrical shoulder tool.

Losses due to wear
The combined effect & influence of the two different process parameters of the FSW technique on the wear losses of the wrought AZ80A Mg alloy joints fabricated using the stepless cylindrical shoulder tool and the stepped cylindrical shoulder tool are graphically illustrated in the Figure 8 &   From the Figure 8, it can be seen that the losses due to wear were found to be obtained at higher values of the tensile strength.This can be understood clearly by comparing the Figure 5 and 8.It can be found that, at smaller speeds of rotation of the FSW tool (i.e., 500 rpm), larger tensile strength values were obtained.Likewise, losses due to wear were also found to be minimum at this FSW tool rotational speed (500 rpm).The losses due to wear were found to increase with increase in speeds of rotation of the FSW tool (i.e., from 500 rpm to 750 rpm and 1000 rpm).
From the Figure 9, it is evident that the traversing speed (i.e., rate of feed) is found to dominate the losses due to wear on the AZ80A Mg alloy joints fabricated using the stepped cylindrical shoulder tool.At 500 rpm, i.e., at smaller values of the speeds of rotation of the FSW tool, larger values of the traversing speed (3.0 mm/min) have yielded the minimum losses due to wear and smaller values of the traversing speed (0.5 mm/min) have yielded the maximum losses due to wear.This phenomenon can be understood clearly by analysing the nugget zones of the wrought AZ80A Mg alloy joints fabricated at different combinations of speeds of rotation of the tool and traversing speed using two different tool geometries, namely, stepless cylindrical shoulder tool and stepped cylindrical shoulder tool.The joints possessing fine sized refined grains in their nugget zone have been found to exhibit higher losses due to wear when compared to that of the joints having large sized grains in their nugget zone.The reason for this is very much identical to that of the reasons discussed for the tensile strength values.Figure 10 shows in detail, the optical micrograph images of the nugget zone of the joints fabricated using stepped cylindrical shoulder tool at different combinations of the speeds of rotation of the tool and traversing speed, which are advocating this observation.Yet, the losses due to wear is found to be more for the AZ80A Mg alloy joints fabricated by the stepped cylindrical shoulder tool when compared with that of the joints generated using the stepless cylindrical shoulder tool.

Rate of Corrosion
The rates of corrosion of the wrought AZ80A Mg alloy joints fabricated at different combinations of speeds of rotation of the tool and traversing speed using two different tool geometries, namely, stepless cylindrical shoulder tool and stepped cylindrical shoulder tool are graphically illustrated in the Figure 11 and 12 respectively.
In majority of the AZ80A Mg alloy joints fabricated using FSW process, the rate of corrosion is found to be larger, when compared with that of the parent metal.The major reason for the decline in the resistance towards of the fabricated AZ80A Mg alloy joints is the refinement of the grains, which have taken place in the nugget zone, as a result of FSW process.It is a well known fact that, the grain boundaries have an unstable type of configuration when compared to that of their respective lattice structure.
Moreover, the chemical reactivitiy of these grain boundaries is very much higher and they are regions of higher energy [68,69].As a result, these grain boundaries are the common spots susceptible to the reactions of corrosion, when compared to that of the individual faces of grain.Similarly, during the etching of a pure metal, the grain boundaries are very much invulnerable to the attack related to corrosion, when compared with that of the individual grains [70].
This low resistance of the grain boundaries to the corrosion attack is mainly due to the weaker binding of the atoms, making them more dissolvable in nature, than the atoms present in the interior surface of the individual grains.Boundaries of twin nature, emanating dislocations, boundaries of subgrains etc are examples of regions having very poor resistance to corrosion attacks [71,72].In other words, it can be understood that, if there is generation of fine refined grain boundaries & structures in the nugget zone of the joints fabricated using the FSW process, the joints would be prone to the corrosion attacks very easily, when compared to that of the base metal.By comparing the datas illustrated in the Figure 11 & 12, we can observe that, the AZ80A Mg alloy joints fabricated using a stepped cylindrical shoulder tool exhibits higher rates of corrosion when compared to that of the AZ80A Mg alloy joints fabricated using a stepless cylindrical shoulder tool.• The size of the grains in the nugget zone and the formation of the fine refined grain structures have played a vital role in determining the strength of the AZ80A joints fabricated using the stepped cylindrical shoulder tool.
• The joints possessing fine sized refined grains in their nugget zone have been found to exhibit higher losses due to wear when compared to that of the joints having large sized grains in their nugget zone • The losses due to wear are found to be more for the AZ80A Mg alloy joints fabricated by the stepped cylindrical shoulder tool when compared with that of the joints generated using the stepless cylindrical shoulder tool • In majority of the AZ80A Mg alloy joints fabricated using FSW process, irrespective of the employed different tool geometry, the rate of corrosion is found to be larger, when compared with that of the parent metal.
• AZ80A Mg alloy joints fabricated using a stepped cylindrical shoulder tool exhibits higher rates of corrosion when compared to that of the AZ80A Mg alloy joints fabricated using a stepless cylindrical shoulder tool

Figure 1 .
Figure 1.Photograph of the wrought AZ80A Mg alloy flat plates being firmly held in their respective position in the specially designed fixture of the FSW machine

Figure 2 .
Figure 2. Photographs of the different designs of the FSW tools employed in this investigation (a) stepless cylindrical shoulder tool shoulder and (b) stepped tool shoulder

Table 1 .
Details of the FSW process parameters and their corresponding values employed during this investigation S. No. Speed of rotation of the FSW tool (rpm) Traversing speed of the FSW tool (mm/min)

Figure 3 . 4 . 1
Figure 3. Different stages of fabrication of flat wrought AZ80A Mg alloy plates into sound quality joints by employing the stepped cylindrical shoulder FSW tool with tapered pin profile

Figure 4 .
Figure 4. Photographs of a part of the joints fabricated during this investigation under combination of various process parameters using (a) Stepless cylindrical shoulder tool and (b) Stepped cylindrical shoulder tool 4.2 Tensile strength of the fabricated joints The combined effect & influence of the two different process parameters of the FSW technique on the tensile strength of the wrought AZ80A Mg alloy joints fabricated using the stepless cylindrical shoulder tool and the stepped cylindrical shoulder tool are graphically illustrated in the Figures 5 & 6.

Figure 5 .
Figure 5. Graphical illustration of the tensile strength of the wrought AZ80A Mg alloy joints fabricated using stepless cylindrical shoulder tool at different combinations of traversing speed and rotational speeds of the FSW tool

Figure 6 .
Figure 6.Graphical illustration of the tensile strength of the wrought AZ80A Mg alloy joints fabricated using stepped cylindrical shoulder tool at different combinations of traversing speed and rotational speeds of the FSW tool

Figure 7 .
Figure 7. Optical micrographical images of the AZ80A Mg alloy joints fabricated using the stepless cylindrical shoulder tool at a process parameter combination of (a) 0.5 mm/min -500 rpm; (b) 0.5 mm/min -1000 rpm and using the stepped cylindrical shoulder tool at a process parameter combination of (c) 3.0 mm/min -500 rpm (with less than 8 µm grain sizes) and (d) 3.0 mm /min -1000 rpm (with less than 5 µm grain sizes) 9.

Figure 8 .
Figure 8. Graphical illustration of the wear losses of the wrought AZ80A Mg alloy joints fabricated using stepless cylindrical shoulder tool at different combinations of traversing speed and rotational speeds of the FSW tool

Figure 9 .
Figure 9. Graphical illustration of the wear losses of the wrought AZ80A Mg alloy joints fabricated using stepped cylindrical shoulder tool at different combinations of traversing speed and rotational speeds of the FSW toolAt the same time, this impact is completely inverse in the case of higher values of the speeds of rotation of the FSW tool (i.3., at 1000 rpm).At this 1000 rpm, the smaller traversing speed values (0.5 mm/min) have yielded the minimum losses due to wear and higher traversing speed values (3.0 mm/min) have yielded the maximum losses due to wear.

Figure 11 .
Figure 11.Graphical illustration of the rate of corrosion of the wrought AZ80A Mg alloy joints fabricated using stepless cylindrical shoulder tool at different combinations of traversing speed and rotational speeds of the FSW tool

Figure 12 .
Figure 12.Graphical illustration of the rate of corrosion of the wrought AZ80A Mg alloy joints fabricated using stepped cylindrical shoulder tool at different combinations of traversing speed and rotational speeds of the FSW tool 5. CONCLUSION In this experimental work, a detailed investigation was carried out to understand the role of the geometry of the FSW tool in determining the tensile strength, losses due to wear, rates of corrosion of the fabricated AZ80A Mg alloy joints, by employing two different FSW tool geometries namely stepless cylindrical shoulder tool and stepped cylindrical shoulder tool, under different combinations of FSW process parameters, namely speeds of rotation of the tool and traversing speed.The recorded observations, experimental results generated in the present investigation allow us to derive the below mentioned conclusions: • Stepped cylindrical shoulder tool has produced sound quality joints with appreciable tensile strength values at combinations of higher traversing feeds and low values of rotational speeds of the FSW tool.•The size of the grains in the nugget zone and the for- Истраживање је обухватило и затезну чврстоћу, губитак услед трења и отпорност на корозију код произведених спојева.Утврђено је да спојеви са степенстим цилиндричним наставком имају веће вредности затезне чврстоће код комбинације већег броја пролаза алата и мањих вредности ротирајуће брзине.Истовремено је нађено да спојеви који имају ситнија и финија зрна у зони зрна већих димензија (грумена) имају веће губитке услед трења.