Experimental and numerical analysis of intermetallics in Al–Mg friction stir welds

In this research work, it was aimed to analyse the thermal behaviour during FSW in order to understand the diffusion behaviour of Al (AA6061)-Mg (AZ31B) dissimilar joints. Three heat input levels at different weld pitch ratios (WPR) of 0.087, 0.068 and 0.051 are accounted for the analysis. Finite element modelling (FEM) is employed to predict temperature evolutions. From the FEM results and fundamental diffusion equations, the intermetallic thickness and the diffusion behaviour between the Al and Mg material were analyzed and found that the Al-rich intermetallic phases Al3Mg2 grow faster and wider than the Mg-rich phase Al12Mg17. Tensile test demonstrates that a lower welding pitch ratio (WPR) leads to the formation of thicker intermetallic layers, resulting in reduced tensile strength and joint efficiency. In contrast, a higher WPR (0.087) minimizes intermetallic thickness, yielding superior tensile properties (138mpa). Microhardness measurements at the stir zone reveal a broad range from 70 to 164 HV, signifying mechanical heterogeneity. Microstructural reveals that a complex interplay between Al and Mg materials, resulting in fine equiaxed grains, intermetallic compounds, and distinct flow patterns in the stir zone.


Introduction
Owing to the muffled-density property of Al and Mg, both are applied in many engineering applications, particularly in transportation.However, structures or components made of Mg alloys encounter problems due to their deteriorated properties like low strength and less corrosion resistance.So, replacing Mg with Al alloy is a better strategy since it has a greater strength and good resistance to corrosive environments.Hence, the joining of Al and Mg material is demanded to fulfill the design needs in various applications [1,2].
The employment of a fusion welding techniques to weld Al to Mg materials may result in poor joint properties.The melting and solidification phase of the fusion welding process is crucial due to chemical and mechanical incompatibilities between the materials that may create various welding defects [3,4].Irrespective of its similarities in physical properties, the difference in properties like thermal conductivity, and metallurgical characteristics like phase formations, crystal structure, etc make challenging to accomplish sound joints by fusion welding processes.The appropriate filler material to weld the dissimilar materials is complicated since incompatible filler materials result in hot tearing in the fusion zone or heat-affected zone [5].Halim et al attempted to join Al-Mg dissimilar weld using a laser welding process and observed the presence of greater intensity of intermetallic phases namely, Mg 17 Al 12 and Mg 2 Al 3 in the weld zone [6,7].The influence of intermetallic formation was analyzed by Liu et al and concluded that the formation of intermetallic phases results an adverse outcome on the joint properties [8].Hence fusion welding techniques cannot be considered as a potential candidate to weld Al and Mg alloys.
Friction stir welding (FSW) does not exhibit the melting and solidification phase of joining the materials and so the problems encountered during the fusion welding process will be overcome [9][10][11].Demonstrations of the joining of dissimilar combinations like Al-Fe [12], Al-Cu [13], Al-Brass [14], and dissimilar Al-Al [15] joints were attempted by various researchers.These studies detailed that the development of brittle intermetallic phases is limited in the stir zone due to low heat generation.The possibilities of further thinning of the intermetallic layer should be explored and further reduced to increase the joint strength.This can be achieved by understanding the temperature evolutions and reducing the heat generation during FSW.Amin Abdollahzadeh et al performed friction stir vibration welding (FSVW) combined with water cooling resulted in the most substantial grain refinement and the highest joint efficiency, reaching around 87% and also led to increment hardness values (78Hv) [16].Yang et al presented evidence of material migration in FSW of Al-Mg materials.The study concludes that the material properties have significant role on material flow, in which material properties of Mg dominate the region around the tool, while the material properties of Al dominate the zone far away from the tool [17].Zhao et al adapted response surface methodology to improve the process variables of Al-Mg joints and conclude that the tool rotation speed increased the heat generation in the stir zone that, results to a rise in IMC content in the intermixing zones [18].The tool revolving velocity and tool feed rate are the significant factors controlling the thermal evolutions in the stir region [19,20].
The friction stir welding of aluminum and magnesium alloys in inclined butt joints leads to two intermetallic formation mechanisms: constitutional liquation and diffusion reaction.A high weld pitch of 1.2 mm rev −1 significantly increased yield strength to 170 MPa, compared to around 110 MPa at lower pitches.Weld pitch, along with other factors like rotational rate, played a crucial role in determining the joint quality [21,22].Intermetallic compounds, Al3Mg2, and Al12Mg17, emerged upon the solidification of liquated material near 710-723 K (436 °C-450 °C), closely aligned with eutectic temperatures.The formation was linked to the solidification process rather than solid-state diffusion, implying potential strategies to mitigate intermetallic issues in Al-to-Mg FSW [23].Dissimilar friction stir welding of aluminum and magnesium alloys revealed the formation of intermetallic compounds (IMCs) in a banded structure (BS) zone.Varying tool rotational speeds produced distinctive IMC band patterns, influencing mechanical properties.Welds predominantly exhibited brittle fractures along IMCs [24].The weld pitch ratio (WPR) is the parameter that includes the contributary influence of both parameters and it the proportion of tool travel speed to tool gyratory rate.The significance of WPR on the diffusion characteristics and weld properties was not completely explored by any of the researchers.Analyzing the intermetallic compounds on the Al-Mg welded zone and investigating the influence of Weld Pitch Ratio (WPR) on joint efficiency is a novel approach.Thus, a detailed study is aimed in this study to explore the influence of WPR on diffusion behavior and intermetallics formation.In addition, its influences on the heat input, microstructural and mechanical characteristics are correlated using experimental and numerical methods.

Experimental procedure
Two dissimilar alloys, Al AA6061, and Mg AZ31B were used in this investigation.AZ31B Mg was clamped on the advancing side and AA6061 Al alloy was clamped on the retreating side.Rolled plates of 200 mm × 150 mm × 6 mm of Al and Mg were used for the experimentation.The base material properties are shown in tables 1 and 2 respectively.Three different WPR were used for the fabrication of joints that are presented in table 3. The welded samples are subjected to microstructural examination using an optical microscope.The samples required for the tensile testing and microstructural examination were machined out using wire cut electrical discharge machining (WEDM) process from the weld plated at perpendicular to the weld line.All the weld samples are roughly polished using the abrasive emery inorder to achieve smooth and flat surfaces.The samples are then intensively polished by an emery sheet of grit sequence according to ASTM-E407 standards.Special care is taken while applying chemical etchant for the dissimilar welds since the Mg side is highly reactive to the chemicals.Picral etchant is used to etch the Mg side and Keller's reagent is used to reveal the microstructures of the Al side in the dissimilar weld which is according to the procedure described by Jayaraj et al [25].The specimens required to evaluate the tensile properties were extracted from the welded samples and prepared according to ASTM E8 standard [26].The Vickers microhardness measurements were made in the sample taken for metallographic study in which the measurements were made at an interval of 0.5 mm.The tensile fractured samples were characterized by XRD technique to find the phases in it.

Numerical analysis
The FSW process is modeled using the COMSOL software package.A 3-dimensional non-linear thermochemical model was used to simulate the process.A simplified stationary model was used since it is only aimed to measure the temperature distribution of the weld joints.A transient meshed model has been used in which the element sizes are finer in the region subjected to stirring and increasing gradually towards the outer region (figure 1).This strategy is used with the objective of reducing computational time and achieving d accuracy of temperature measurement.
Considering the heat source modelling, constant heat fluxes are assumed for the shoulder and pin separately.The heat creation at the contact face of the shoulder and workpiece is due to the vertical axial pressure, contact area undergoing friction, and tool rotational speed.Thus, the heat flux is adapted from heat source model reported by sree et al [27] and it is expressed as Where F mentions to the axial force, A s mentions to the surface area of the tool shoulder, μ refers to the friction coefficient, R refers to the range toward the tool's centre and ω is the angular velocity of the tool (rad/s).
A separate heat flux was assigned to model the heat generation at the connection area of the pin and the sample as a surface heat source pin-workpiece interface and it is expressed a.s.
Where μ denotes to the friction coefficient, r p denotes to the radius of the tool pin, ω denotes to the angular velocity (rad/s), and Ybar(T) refers to the temperature dependent shear stress of the Al and Mg material.The temperature dependent properties required for the simulations are considered from the previous report [28,29].

Tensile properties
Table 4 portrayed the tensile properties of the weld joints fabricated using three weld pitch ratios.A extreme strength value of 138 MPa was recorded while using WPR 0.087.The extreme value of yield strength is 122 MPa and joint efficiency is 63.88% for this WPR condition.As the weld pitch ratio decreased to 0.068, the tensile strength decreased to 112 MPa.The yield strength and joint efficiency also reduced to 112 MPa and 57.40%.A minimum tensile strength of 116 MPa was achieved for a lower WPR of 0.051.The yield strength and joint efficiency were 96 MPa and 53.70 respectively.Figure 2 shows the tensile-tested samples for the three WPR.The tensile fracture befell in the Mg side of boundary between the intermixing zone and thermo-mechanical zone, in all three joints.Further, the magnified cross-sections clearly show the fracture path follows the Mg side followed immediately after the mixing zone.

Microhardness
Figure 3 shows the macrostructure and the respective microhardness mapping.The microhardness values ranges from 70 to 164 HV were represented using colour contour maps.The red colour shows the lower hardness values and the dark green shows the higher hardness values.A thick, soft band was observed at the Mg side interface zone in all the WPR samples.Similarly lower hardness was observed in the Al side interface-area.Both peak and lower diverse hardness were pragmatic along the stir zone for all the cases.There is no significant variation of peak hardness value for all the cases, but the location of peak hardness was varied based upon the material flow shown in the macrostructure.

Macro and microstructure
In figure 4 the microstructure of the aluminium and magnesium base material's microstructures are depicted.Figure 4(a) shows the microstructure of Mg AZ31B composed of coarse grains.Figure 4(b) depicts the microstructural features of Al AA6061 that is characterized by the existence of lineated grains affiliated in the track of rolling process of base material.Figures 5-7 show the macro and microstructure of weld joints fabricated using WPR 0.087, 0.068, and 0.051 respectively.The macrostructure is composed of various regions like parent materials, TMAZ, HAZ, and stir zone.Distinct TMAZ region was not presented in all Al-Mg dissimilar weld joints made of different WPR values.The level of blending of dissimilar material in the stir zone is characterized by a feature called interpenetrating features (IPF).At WPR, 0.087, the size of IPF is lower representing that the mixing of dissimilar material in the shoulder-influenced region was low (figure 5(a)).The increase in heat input at WPR 0.068, makes the  intermixing of Al and Mg materials throught the entire stir zone with an increase in the size of IPF.Further increase of heat input at WPR 0.051 makes further improved mixing of Al and Mg material at both shoulder and pin influenced region which in turn increase the size of IPF.   Figure 6(b) shows the mid-thickness microstructure at 0.068 WPR.The downward material flow in the shoulder influence region gets altered owing to the fraternization of the tool pin.A similar material flow was also observed for the 0.051 WPR at the mid-thickness region (figure 6(c)).Severe mixing of dissimilar material following a complex flow pattern called intercalated microstructure was noted in the pin-influenced region (figure 6(b)).Figure 6(c) shows the stacking of alternate arrangements of Al and Mg materials to result in a lamellar microstructure [31].At lowest WPR value of 0.051, the downward material flow in the shoulderinfluenced region gets more widespread as it approaches the pin-influenced region (figure 7(b)).Figure 7(d) shows the material swirling by the rotation of material as the rotating tool advances.

Temperature measurements
The FEM is utilised to predict the temperature distributions in the weld joints for the various WPR ratios.The figure 8 shows the thermal contours recorded for the welds of WPR of 0.067.It is clearly observed that the temperature distribution is not symmetric, and a wider distribution was pragmatic on the aluminium side.The temperature values are extracted and presented in the figure 9 for a better understanding of the temperature evolutions.Figure 9(a) shows the plot representing the peak temperature values recorded at the mid-section of weld seam line.It is inferred that the evidence for temperature differences in the stir zone on both sides is not noted however the outer regions experience a slight difference.To verify further, the temperature values were recorded at 10 mm far from the weld seam line and presented in figure 9(b).At the magnesium side, a slightly higher temperature is noted in the aluminium side.The peak temperature at the stir zone of each WPR was measured and found as 451 °C, 473 °C and 514 °C for WPR 0.087, 0.067 and 0.051 respectively (figure 9(c)).
The tensile fracture was seen exactly at the boundary separating the TMAZ and stirring region and so the fractured samples were subjected for the XRD analysis, and the phases present in both the fractured sample sides In order to understand the intermetallic formation and its diffusion dynamics between Mg and Al atoms, analytic approached is followed here and it can be explored from the following expression.
K0, Q, and R are the constants of proportionality, activation energy, and gas constant, respectively and T refers to absolute temperature.The temperature values predicted from the FEM analysis were applied in the above expression.The other parameters required for the calculations were considered from the literatures made by Liu et al and Rothman et al [18,19].From the equation, the relationship between intermetallic thickness and diffusion time was predicted and presented in the figure.It is evident that the diffusion time and temperature are directly proportional to the intermetallic thickness.Comparatively, the Al-rich intermetallic phase Al 3 Mg 2 exhibits a greater diffusion rate than the Mg-rich intermetallic phase Al 12 Mg 17 and Al 3 Mg 2 intermetallic recorded a thickness twice than the Al 12 Mg 17 intermetallic thickness as shown in figure 11.

Diffusion behaviour of Al and Mg materials
The eutectic temperatures, 436 °C and 451 °C are observed to be critical in the Al-Mg phase diagram due to formation of Al 12 Mg 17 , and Al 3 Mg 2 phases respectively.From FEM results, it is obvious that the stir zone temperature recorded in the stir zone is greater than the indicated eutectic temperatures and thereby, it promotes the formation and thickening of intermetallic between Al and Mg materials.FEM simulations showed that the temperature in the stir zone exceeds the eutectic temperatures for both Al 12 Mg 17 and Al 3 Mg 2 .This implies that the circumstances are favourable for the production of intermetallics, and the higher the temperature above the eutectic point, the more favourable the development of these phases.
At a higher WPR of 0.087, the frictional rubbing between the shoulder and pin surface with the materials are low due to low tool rotational speed.The lower heat input impacts the diffusion process, leading to thinner intermetallic formation.Conversely, at lower WPR 0.051, the frictional rubbing is high which leads to greater frictional heat generation and so it supports the diffusion process to form thicker intermetallic at the interfaces.On the other hand, the lower tool travel speed increases the availability to tool to create friction and stir the material which increases the heat generation and thereby supports the diffusion process which is also according to the previous reports [1].

Macro and Microstructure of Al-Mg dissimilar material
The stir zone microstructure of Al-Mg dissimilar material was characterized by intermetallic compounds, fine recrystallized grains, intercalated flow patterns, and demarcation bands.At three heat input conditions, all the above features are present, but the extent of their presence gets differed.At high WPR, the interpenetrating feature is present in the pin-influenced region.Since the flow strength of aluminum and Mg at elevated temperatures are more or less the same.Thus, the placement of dissimilar material either on the advancing side or retreating side does not have any significant effect.However, this observation converse to the reports made for  other dissimilar combinations, because, for example, the combinations like Al and copper exhibits different flow strengths and so the material placements have stronger influence [13].But few authors confirmed that the placement of dissimilar materials in both cases will result in sound FSW joints [1].During tool advances, the Mg material present in the advancing side gets transported to the retreating side.But due to low heat generation at 0.087 WPR, the material at the shoulder influenced region does not get adequate softening.Thus, the material on the advancing side does not get contributed to forming the IPF.While the tool rotational speed is increased which means lowering the WPR, the heat generated gets increased.Thus, the material gets thoroughly softened and contributed mixing of dissimilar at the shoulder-influenced region.The rotary rubbing of the tool's shoulder surface creates a downward rotation of the material.This fact was confirmed by many researchers during friction stir processing using a pinless FSP tool.Thus, the tool rotational speed has a greater impact on the formation of IPF in the stir zone.
The frictional heat and the intense deformation create fine recrystallized grains of both Al and Mg material which are equi-axially oriented.In addition to this, band like structures are also formed in the stir zone.This band is otherwise called a demarcation band or adiabatic shear band.During material, the localized high strained material results in heat generation.Despite the high thermal conductivity, in Al, the heat generated due to localized deformation remains inside the deformed system.This creates a thermal softening in the localized flow region and seems to be a band like structure.
The material flow at the shoulder influenced region follows a rotary downward flow but on the other hand the pin influenced undergoes rotary and upward way of flow based on the pin profile.This difference in flow can be separated in the mid thickness region.Below this mid thickness region, the intercalated microstructure is formed.Intercalated microstructure is defined as the complex meshing of dissimilar material in the stir zone.The complexity in the material flow is created by the conflicting of two regular flow patterns induced by tool shoulder and tool in the stir zone.In addition, the difference in the flowability of dissimilar material creates further complexity in the material flow.

Tensile and microhardness of Al-Mg dissimilar material
Joining of Al and Mg materials during FSW was achieved for two reasons.The metallurgical bond results from the diffusion of dissimilar materials and the mechanical bonding results from the complex meshing of dissimilar materials.If the diffusion gets increased, the metallurgical bond gets increased which in turn increases the intermetallic.A poor weak bond strength exhibits between the intermetallic and the matrixes.Thus, an optimum metallurgical reaction is preferred for a good bond or tensile strength.In addition, the ductility of FSW dissimilar was drastically decreased due to the presence of hard brittle intermetallics.By this way, the high 0.087 WPR results in high tensile strength over other cases.Due to low heat input and reduced intermetallic formation and thereby improved joint efficiency of 63.88% was attained.
If the intermetallic formation is fine and evenly distributed over the Al and Mg matrix, there may be strengthening of the weld region due to dispersion strengthening.From the macrostructure, it can be noted that discontinuous intermetallics were distributed in the weld region.The hard brittle nature of intermetallics acts as reinforcement and the Al-Mg weld region acts as the matrix which forms a composite structure.On tensile loading, the movement of dislocation is hindered by the intermetallic particles which strengthen the weld region.Intense stirring during FSW makes intermetallic particles settle along the grain boundary.This condition reduces the boundary energy of the grains since the intermetallics share the boundary of the grains.These grains should need more energy to grow.Thus, the grain growth occurs heating and cooling cycle of FSW can be controlled.This fact adds a reason for the improved tensile strength of FSW joints.However, the continuous thick intermetallics have more prevailed in the Al-Mg stir zone than the discontinuous fine intermetallics.The tensile fracture follows along the thick intermetallics formed at the Mg side interface between the stir zone and TMAZ (figure 2).Thus, the reduced intermetallic formation at 0.087 WPR results in maximum tensile strength compared to its counterparts.
The dynamic recrystallization results in fine equiaxed grains in the stir region.The resistance to dislocation movement offered by the grain boundaries of finer grains increases the tensile strength and hardness values.A strong variation in the hardness values of the stir region was observed in all the cases of WPR.The lower hardness values are ascribed to the local thermal softened shear bands and the moderate hardness value is attributed to the presence of fine grains and the greater hardness value is attributed to the presence of intermetallics in the stir region.Due to the diffusion of dissimilar material, the material incompatibilities form intermetallic which is hard and brittle in nature.Thus, the peak hardness values are observed at the interdiffusion Al-Mg region.This is one of the factors contributing to the greater peak hardness at the stir region than the parent material Mg alloy.

Conclusions
From this study the diffusion behaviour and intermetallic formation aspects were analyzed, and the following important findings are derived.
(i) From the FEM analysis, the peak temperatures are predicted and identified that the heat generation have larger influence on the intermetallic formation.Due to high heat generation at lowest WPR, the joint exhibit larger Al 3 Mg 2 and Al 12 Mg 17 intermetallics in the stir region.
(ii) The investigation concludes that the higher joint performance can be attained by the selection of WPR that results in the presence of thinner intermetallic formation, finer grain size and better mechanical interlocking particularly for the dissimilar joining of Al and Mg alloys.
(iv) Microhardness measurements within the stir zone showed a broad range from 70 HV to 164 HV.The complex microstructural features due to presence of material mixing and intermetallic formation is the reason for the uneven distribution of hardness values in the stir zone.

Figure 2 .
Figure 2. Tensile tested specimen and the cross-sectional view.

Figure 5 (
c) shows the interface microstructure of Mg side which holds coarsened grains of varying size.The formation of the adiabatic shear band was shown in figure5(d).The size of microhardness indentation marks at the shear band is larger than the neighbor stir region.

Figure 9 .
Figure 9. Thermal analysis.(a) Peak temperature values at midlength cross section of WPR 0.051 joint.(b) Thermal histories at 10 mm away from weld center of WPR 0.051 joint.(c) Peak temperature values of various WPR.

Figure 11 .
Figure 11.Relationships between the diffusion time and intermetallic thickness.

Table 2 .
Mechanical properties of base metals.