Ductilization and grain refinement of AA7075-T651 alloy via stationary shoulder friction stir processing

This study investigates the microstructural evolution, mechanical properties, and fracture behavior of AA7075-T651 aluminium alloy subjected to stationary shoulder friction stir processing (SSFSP). SSFSP samples were produced at three different rotational speeds in a range of 600 – 1000 rpm. The results reveal that SSFSP leads to a uniform grain refinement within the Stir Zone (SZ), reducing the grain size to approximately 2 – 3 μ m from the initial 15 μ m in the base material (BM) irrespective of the probe rotational speeds. After SSFSP, the elongation increased by over 50 % at the cost of 10 % reduction in the ultimate tensile strength for all samples. It was worth to note that variations in tool rotational speed exhibited minimal influence on the microstructure and mechanical properties, offering wide range of probe rotational speeds. This could be attributed to the use of non-rotating shoulder with prob dominated microstructure in the SZ. Fractographic analysis confirmed the ductile nature of fractures, revealing development of fine dimples due to grain refinement. This work underscores the effectiveness of SSFSP in achieving significant grain refinement followed by drastic increase in ductility, which offers valuable insights for using stationary shoulder at wider range of rotational speed.


Introduction
Aluminium alloys have been widely used in various applications in automotive, aerospace, and marine industries [1,2].Aluminium 7xxx series alloys are a family of high-strength aluminium alloys which are typically composed of aluminium, zinc, and magnesium.Aluminium 7075 is one of the common grades of Al 7xxx which with high strength, excellent strength-to-weight ratios, good toughness and corrosion resistance is an outstanding candidate to be utilized in high-performance applications.However, this alloy has relatively low ductility compared to other aluminium alloys [3].
In recent years, some material processing techniques have been employed to improve or modify the properties of conventional alloys due to demanding of light weight structural materials with superior mechanical properties [4].Friction stir processing (FSP) as a solid-state technique is able to tune the microstructure and properties of materials.In this technique, a rotating tool with pin and shoulder is plunged into the base material (BM) and has a transverse movement in a specific direction.These rotation and movement caused a severe plastic deformation or thermo-mechanical deformation in the BM.As a result, a stir zone (SZ) is created which contains fine grain microstructure due to dynamic recrystallization (DRX) [5].In FSP, both the heat input and the mechanical loading affect the microstructure and subsequently the mechanical properties.
In FSP of Al 7075, controlling heat input is of vital importance since the strengthening precipitates in the alloy tend to dissolve under high heat input, leading to reduced hardness and strength.The heat input, moreover, affects the grain growth with degrades the mechanical properties [6].The higher heat input, the more grain growth and more deterioration of grain refinement.The heat input in FSW is governed by tool rotational speed, transverse speed, and the tool geometry.
Stationary shoulder friction stir processing (SSFSP) is a subset of FSP in which the shoulder does not rotate and it only moves over the plate inducing the force.In conventional FSP, the friction between the shoulder and the BM generates more heat, resulting in a relatively wide heat affected zone (HAZ).The HAZ softening, therefore, which is a common phenomenon in FSP of Al alloys can degrade mechanical properties.However, the use of a stationary shoulder during the welding of 7xxx series alloys has already shown promising results.It results in sound weld quality due to low heat input and small temperature gradient and reduced residual stress.The size of the thermomechanically affected zone (TMAZ) and HAZ, as the result, decrease significantly.These features make this technique desirable for achieving uniform grain refinement and producing a microstructure with uniform grain size in the stir zone.Furthermore, the use of stationary shoulder tooling is expected to reduce the dissolution of strengthening phase particles during processing.
In recent years several investigations have been conducted on the utilization of FSP for AA7075 aluminium alloys.Patel et al. [7] demonstrated that FSP can enhance the ductility of AA7075 by 227 % through grain refinement.In another study [8], they revealed that the implementation of CO2 cooling could lead to even higher elongation, resulting in an impressive increase of up to 572 %, alongside notable grain refinement on the order of 1.96 μm.Tonelli et al. [9] reported equiaxed grain refinement with an average size of 5 μm in AA7075 aluminium alloys achieved through four passes of FSP.Various alternative processes have been also utilized for the purpose of grain refinement in AA7075 aluminium alloy.Avery et al. [10] employed additive friction stir deposition to achieve grain refinement in wrought AA7075, reducing the average grain size from 100 μm to equiaxed grains with an average size of 5.05 μm.In another study, Lee et al. [11] successfully attained significantly finer grains with an average size of approximately 250 nm utilizing high-pressure sliding technique, leading to notable enhancements in both strength and elongation.
While significant grain refinement is achievable in the stir zone through FSP of AA7075 aluminium alloys, utilizing FSP to enhance ductility is challenging due to the propensity of conventional FSP to induce grain growth and precipitate dissolution in HAZ and TMAZ in this alloy [12].As previously mentioned, stationary shoulder FSP offers a solution or mitigation for these issues.However, SSFSP of aluminium alloys has not been comprehensively explored yet.Few researchers indicated that it is possible to improve surface quality and decreasing HAZ width implementing stationary shoulder in friction stir welding (FSW) of AA7075 aluminium alloys.Wu et al. [13] indicated that SSFSW is able not only generating improved surface quality for AA7075-T7651 aluminium alloys, but more leading to a reduction in the width of the HAZ.Another study focusing on SSFSW of AA7075 also underscored the high-quality welds achieved through the process [14].Recently, a study conducted by Vivek et al. showcased the attainability of uniform properties throughout the thickness in SSFSP of AA7075 aluminium [15].The study demonstrated that SSFSP has the capability to enhance the ductility of AA7075 by 48 % when compared to the original untreated material.We found research gap in FSP, especially using advanced FSP tooling system in form of stationary shoulder tool.Therefore, the present investigation is to delve deeper into the microstructure evolution during SSFSP and identify the influence of probe rotational speed on the resulting microstructure and properties.

Materials and method
In this study, a 6.35 mm AA7075-T651 aluminium sheet was used as the base material (BM).Three different tool rotational speeds of 600, 800, and 1000 rpm were used for SSFSP.For all SSFSP samples, the travel speed and the tilt angle of the axis were 50 mm/min and 2.5 • , respectively.The SSFSP was done in one pass without any additional cooling system.The SSFSP tool was composed of two main parts: i) rotating probe or pin and ii) non-rotating shoulder which both pin and shoulder made from H13 tool steel.The pin was a threaded probe with diameters of 6.35 mm and 3.85 mm in the root and the tip, respectively.The schematic illustrations of the process and the pin are shown in Fig. 1.
A 3D super depth digital microscope (Keyence VHX-600) was used to quantitatively measure the surface topography of SSFSP region.After SSFSP, the samples were cut transversely for microstructure examinations.
After FSP, the cross-sections of all samples were mounted, ground, polished, and etching using Keller's reagent.A Scanning Electron Microscope (SEM, ZEISS SUPPEA5) was used for studying the microstructures of the distinct zones.The SEM was equipped with EBSD detector for grain refinement analysis.For EBSD analysis, the acceleration voltage, sample tilt angle, and working distance were 20 kV, 70 • , and 9 mm, respectively.Due to difference in grain size, step sizes were 0.5 μm and 2.0 μm for the SZ and BM, respectively.The AZtecCrystal 1.1 software from Oxford Instruments was used to analyze the EBSD results.Grain size measurements were carried out using the linear intercept method, and the distinction between low and high angle grain boundaries was made based on the 10 • criterion.A recrystallized grain can be distinguished within a group of plastically deformed grains through the application of the grain orientation spread (GOS) method.In crystal orientation mapping, a grain is defined as a cluster of pixels exhibiting a misorientation below a specific threshold angle as defined by the user.In the current GOS method, grains are designated using a grain tolerance angle greater than 5 • .This determination was executed through the application of AZtecCrystal software, utilizing its built-in 'recrystallized fraction component' function.
Recrystallized grain identification was based on GOS values: GOS <2 • for recrystallized grains, 2 • < GOS <5 • for sub structured grains, and GOS >5 • for deformed grains, respectively, facilitating the measurement of recrystallized fraction.The Vickers microhardness across the centre of SZ thickness under the loading conditions of 200 g load and 10 s dwell time.The dog bone shape tensile specimens with gage dimensions of 20 mm length, 2 mm width and 1.5 mm thickness were extracted in processing direction from the SZ.The room temperature tensile testing was conducted at the strain rate of 1 × 10 − 3 s − 1 .

Surface morphology and cross-section analysis
The surface topography of stationary shoulder friction stir processed (SSFSPed) zones of samples produced at various tool rotational speeds and the surface roughness profiles are illustrated in Fig. 2. As can be seen, thanks to the sliding motion by the stationary shoulder, the surface is smooth and the stationary shoulder only produced negligible flashes.Additionally, it is evident that the sliding motion induces an ironing effect along the direction of the process, further enhancing the overall smoothness of the surface.
The cross-sections of SSFSPed samples including stir zone (SZ), thermomechanically affected zone (TMAZ), heat affected zone (HAZ), and base material (BM) are shown in Fig. 3. Compared to conventional FSP, where the SZ is wider close to the surface, SSFSP exhibits a notably uniform SZ width throughout the thickness.In conventional FSP, the rotation of shoulder results in the broader SZ near the surface.Conversely, in SSFSW, it is the pin's rotation that primarily influences the SZ formation, resulting in a consistent SZ width across the entire thickness.Moreover, it is noticeable that the increment of tool rotational speed from 600 rpm to 1000 rpm results in a slight enlargement of the SZ width and a broadening of the TMAZ.At lower tool rotational speeds, the SZ resembles a probe-like structure.However, with higher tool rotational speeds, the SZ takes on a cylindrical shape.Evidently, SSFSP exhibits a narrower TMAZ in comparison to conventional FSP, as this particular area is formed solely due to the rotation of the pin, distinct from the shoulder's influence.The stationary nature of the shoulder, which does not engage in rotational movement, contributes to reduced friction and subsequent heat input.The heat input, therefore, is significantly diminished in SSFSP.

Microstructure evolutions
The microstructure at the interface of the SZ and BM are illustrated in Fig. 4. The initial region adjacent to the interface on the base sheets is referred to as the TMAZ.In this zone, it is observed that the grains exhibit a coarse structure like the BM.Nevertheless, they have  undergone severe deformation.It is important to note that the elongated grains oriented along the rolling direction have been further elongated through the material's thickness.This phenomenon is attributed to the unique design of the pin, which induces material flow towards the surface.Moreover, enhancing the tool rotational speed from 600 rpm to 1000 rpm results in a remarkable increase in the extent of grain deformation.In all samples, it is noteworthy that no void or pore observed at the interface between the SZ and the BM.
The microstructure of the center of SZ, as marked in Fig. 3, containing of the inverse pole figure (IPF) map and grain boundary map of SSFSPed samples are shown in Fig. 5.The SZ in all samples is characterized by the presence of finely equiaxed grains as a result of dynamic recrystallization (DRX), which is a prevalent occurrence in FSP.Similar kind of DRX have been documented in previous studies of FSP in aluminium alloys [6,17].DRX is widely recognized as a principal factor contributing to grain refinement, and it is more prominent in case of severely plastically deformed material.As it can seen from Fig. 5(e and  f), SZ is found of large fraction of high angular grain boundaries (HAGBs) in all three samples.Irrespective of probe rotational speeds, all three samples exhibited >85 % HAGBs.HAGBs often impede dislocation motion and reduce plastic deformation.This effect can lead to grain boundary strengthening, where the presence of large fraction of HAGBs hinders the movement of dislocations, enhancing the overall strength of the material.On the other hand, large fraction of HAGBs can also promote ductility and prevent brittle fracture.In a refined grain structure with well-distributed HAGBs, energy absorption during deformation can occur at the grain boundaries, allowing for greater plastic deformation and preventing premature failure.
The average grain size from EBSD analysis and the grain distribution corresponding to the EBSD map (Fig. 5) are presented in Fig. 6.After SSFSP, the grain size within the SZ reduces to approximately 2-3 μm, a considerable reduction compared to the base material (BM) grains, which are around 15 μm.The significant grain refinement happened in SSFSP of AA7075-T651 aluminium sheets.In Fig. 6-b, it is apparent that the size distribution of both large and small grains within the SZ remains similar, regardless of the tool rotational speed utilized in SSFSP.Moreover, this phenomenon demonstrates a consistent pattern across a range of tool rotational speeds, increasing from 600 rpm to 1000 rpm.Furthermore, it is noteworthy that across all tool rotational speeds, a majority of the grains possess a diameter of less than 6 μm.This underscores the effective capability of SSFSP in achieving significant grain refinement in aluminium alloys.This finding emphasizes the versatile  range of tool rotational speeds that can be proficiently employed for successful SSFSP of aluminium alloys.Fig. 7 presents the percentage of recrystalized grains based on EBSD analysis.During FSP, an initial stage involves the substantial deformation of grains, leading to a notable escalation in the density of dislocations.This increase of dislocation density, coupled with sustained stress induced by friction and material flow, acts as a driving force for the occurrence of dynamic recrystallization.Following the recrystallization stage, the resultant recrystallized grains undergo coarsening, primarily attributed to the heat generated by friction.Notably, in these processes, a higher tool rotational speed effectively accelerates both dynamic recrystallization and subsequent grain growth.

Mechanical properties
The hardness profile of SZ and the adjacent area are illustrated in Fig. 8. Observing the horizontal hardness line, the hardness of the SZ measures around 120 HV, while the BM contains a hardness of 140-150 HV.Within this hardness line, there is an apparent occurrence of HAZ softening, a typical phenomenon in the FSP of aluminium alloys.In fact, AA 7075-T651 aluminium alloys undergo precipitation hardening [15].The exposure to additional heat, as inherent in FSP, disrupts this hardening mechanism, leading to a decline in hardness.In the SZ, the significant grain refinement attributed to recrystallization relatively compensate the adverse impact of precipitate dissolution, owing to the smaller grain size.The TMAZ also experiences enhanced dislocation density due to severe deformation, which serves to mitigate the effects of precipitate dissolution.However, within the HAZ, the localized heat generated during the process singularly contributes to a detrimental effect, ultimately leading to softening.According to the vertical hardness line, it is important to note that the hardness remains relatively consistent throughout the thickness, showing uniform grain refinement through the thickness.
The tensile behaviors of the SSFSPed samples, subjected to varying tool rotational speeds, and the BM are depicted in Fig. 9. Notably, the strain has experienced an increase, ranging from approximately 0.07 to 0.11.Despite the significate increase in strain, the ultimate tensile strength (UTS) has only decreased from around 500 MPa to 450 MPa.In AA7075-T651 aluminium alloy, precipitation hardening is the main strengthening mechanism.During FSP, the heat derived from the process detrimentally affect the precipitates and cause partially dissolution and coarsening of precipitates, leading to the reduction in UTS [18].On the other hand, grain refinement increases both strength and ductility in aluminium alloys.Here, in SSFSP, the absence of rotational movement in the shoulder results in a lower heat input, effectively mitigating the extent of precipitate dissolution and coarsening.However, the grain refinement happens properly.Therefore, substantial increase in ductility is achieved, while the UTS experiences a marginal reduction.The combine effect of grain refinement along with presence of large fraction of HAGBs are the result of enhancing the ductilization without losing significant strength.
This observation signifies that SSFSP brings about a considerable enhancement in the ductility and formability of AA7075, without sacrificing the strength.Furthermore, it is evident that variations in tool rotational speed do not exert a substantial influence on the tensile properties.This observation highlights the increased flexibility in   A. Baghdadchi et al.

Fig. 1 .
Fig. 1. a) Schematic of SSFSP process and dimensions of the tensile specimen, and b) the schematic illustration of the stationary shoulder and the pin used in SSFSP.

Fig. 2 .
Fig. 2. The high-resolution image and the surface roughness map of the SSFSPed samples produced at a) 600 rpm, b) 800 rpm, and c) 1000 rpm [16].

Fig. 4 .
Fig. 4. The macrostructure of the SZ and TMAZ at top side (a, b, c) and bottom (d, e, f) of the SZ.

Fig. 6 .
Fig. 6.EBSD analysis including a) the average grain size, and b) the grain distribution in SZ of SSFSPed samples.