Experimental Investigation of Dry and Cryogenic Friction Stir Welding of AA7075 Aluminium Alloy

An attempt has been made to investigate dry and cryogenic friction stir welding of AA 7075 aluminium alloy, which is pre-dominantly availed in aerospace and defence component industries. These industries avail friction stir welding for joining two nonferrous materials, and minimal deviations and maximum strength are the preliminary and long time goal. A cryogenic friction stir welding setup was developed to conduct the joining of two aluminium alloy pipes. An increase of 0.76–42.93% and 3.79–31.24% in microhardness and tensile strength, respectively, is ascertained in cryogenic friction stir welding in correlation to dry friction stir welding of aluminium alloys. TOPSIS evaluation for the experimental run indicated tool proﬁle stepped type, pipe rotation speed of 1000rpm, welding speed of 50mm/min, and axial force of 8 kN as close to unity ideal solution for dry and cryogenic friction stir welding of AA 7075 aluminium alloys. The friction stir-welded component under the cryogenic environment showcased drop in temperature, curtailed surface roughness, and ﬁne grain structure owing to reduction in temperature diﬀerential occurring at the weld zone. A curtailment of 50.84% is ascertained in the roughness value for cryogenic friction stir welding in correlation to dry friction stir welding of AA 7075 alloy. A decrement of 21.68% is observed in the grain size in the cryogenic condition with correlation to the dry FSW process, indicating a drop in the coarse structure. With the curtailment of grain size and drop in temperature diﬀerential, compressive residual factor and corrosion resistance attenuated by 40.14% and 67.17% in the cryogenic FSW process in correlation to the dry FSW process, respectively.


Introduction
Aluminium alloy 7075 has been extensively availed in the areas of military vehicle armour materials, Earth excavating machines, bridges, aerospace components, and majority of the defence high-stressed applications [1]. e salient importance of this material is limited due to its welding properties as the fusion problem is predominant in 7075 aluminium series, i.e., porosity formation, solidification of hot cracked areas in the welded zone, and increase in the microfissuring aspect in the semipartially melted zone (SPMZ) [2][3][4][5][6][7]. e composition of AA 7075 aluminium alloy possesses copper, which does not have a specific melting point, and this creates a wide melting range for the alloy with a very low solidus temperature making it susceptible to hot cracks (welded zone) during fusion welding [1,5,8]. Friction stir welding (FSW) is a solid state welding method, availed to bring about joining of nonferrous materials such as aluminium, magnesium, copper, and steels [3-7, 9, 10]. FSW is the preferred joining process for high-strength aluminium alloys particularly 2xxx and 7xxx series as they are generally considered difficult-to-weld or unweldable [2]. Major benefits of availing the FSW process is (i) limited weld defects, (ii) addressing aluminium welding capability (difficult to weld material classification for aluminium), and (iii) way better dimensional capability if the process occurs in a controlled manner. All these above are specific to FSW if the whole process is well maintained and run in an optimized condition [3][4][5][6][7][8]10]. In this method of joining materials, a nonconsumable tool pin profile generates friction heat by rotating between the parent material, as indicated in Figure 1. e rotation of the tool ensures proper stirring effect and mixing of the materials in and around the tool pin; thus, with a combination of mechanical and thermal treatment, the welding process is completed [3-7, 9, 11].
Even though the parent material remains in its solid state, drawbacks are ascertained in the line or region of welding as it undergoes high levels of deformation [6,10], causing an upsurge of unavoidable changes in the microstructure of the component leading to curtailment of mechanical properties [3,4], coupled with aggrandizement of tensile residual stress [3,[5][6][7]. To address these, postweld treatments were carried out, by aging, solution treatment, and room temperature welding (long range of time) [2,3,8,10]. ough posttreatments lead to a drop in the volume fraction of the residual tensile phase, microstructure changes are limited to the boundary lines; also, in terms of cost, time, and labour, it was considered to be laborious procedure physically and economically. An alternative method involved performing friction stir welding in a soluble oil coolant to drop the temperature differential, but an antiwelding property increase was ascertained due to the formation of sulphide aggregates curtailing the strength (shear) of the component [12]. Another major factor of oil coolants is in the area of formation of fumes and gases which when exposed in the welding environment causes long-term health deteriorations [13]. So, given the scenario, dry FSW was concluded to be a cleaner operation along with being cost effective in nature [5,14].
Another area that has been unexplored is in the area of cryogenic friction stir welding (subzero welding) of aluminium alloys. Corresponding to enhanced surface properties, nil change to the composition and high strength of the tool profile are added factors to explore the application of cryogenics in the welding operation [12,13,15]. With a clear abundance availability of liquid helium, hydrogen, oxygen, and nitrogen, subzero liquid comprising of nitrogen is given the preference owing to the ratio of expansion and abundant supply present in the atmosphere [12,13,[16][17][18].
Cryogenic welding finds a substantial drop in the tool workpiece interface owing to the subzero nature of the liquid, thus in turn causing a drop in temperature and retention of the lubricating medium [12,13]. Also, having a high evaporation rate and causing no external or internal changes to the parent material highlights the importance of green manufacturing techniques in the welding scenario involving cryogenic liquid nitrogen [15,19]. In the defence and aerospace sector, a number of components are mass produced; thus, minimal deviations and high strength welds are expected from the manufacturing sector.
is creates more importance especially with the involvement of nonferrous alloys. In this work, an investigation is carried out for dry and cryogenic friction stir welding of AA 7075 aluminium alloy in which attributes of the welding operation are tool profile, pipe rotation speed, welding speed, and axial force in correlation to tensile strength and microhardness. e surface morphology, residual stress component, topographical analysis, and corrosion analysis of the welded surfaces are conducted to validate the welding (FSW) operation. e welding method parameters of the operation are highlighted in Figure 2.

Development of Cryogenic-Assisted Friction Stir Welding
Operational Setup. A friction stir welding tool is placed on a tool holder (FSW setup), and the workpiece is held by the centre of the lathe. Elaborated experimental details of dry and cryogenic-assisted friction stir welding are depicted in Figure 3.
For conducting cryogenic-aided friction stir welding, a cryogenic setup was developed which is depicted in Figure 4 and tabulated in Table 1.
TA 55 liquid nitrogen container of 51.5 litres capacity is availed for the FSW cryogenic system. To retain the subzero fluid, a completely sealed aluminium container is availed with steel closure. Inlet of the pipes of size 6 mm and 4 mm is placed inside through the steel cap enclosure for the flow of compressed air and the outlet towards the welding interface. To ensure a fail-safe approach, a relief valve is placed. A flexible hose made of very low thermal conductivity stainless steel (braided type) is attached to the nozzle at one end and the stainless steel pipe at the other end, in order to supply the liquid nitrogen. Careful thermal insulation is done with help of foam (polymer in nature) in order to prevent being affected by external heat during the transfer. e nozzle is availed to point the fluid towards the friction stir welding zone [12,[14][15][16][17][18][20][21][22]. An AWM720P1 flow sensor is availed to ensure a constant volume flow rate for liquid nitrogen at 1 bar pressure atmospheric pressure which is 1.977 litres/minute, by taking the total head loss in consideration, elevation point of the FSW setup, and having initial velocity and exit pressure to be zero.

Tool and Workpiece for Friction Stir Welding.
e FSW operation was availed using three types of tools, i.e., involute, step, and thread tool, as indicated in Figure 5.
Each of the friction stir welding tools is distinctive and is availed to enhance the operation of welding in dry and cryogenic conditions. e tool being nonconsumable is the major factor for deciding the temperature differential in the operation. Variable weld speed and pipe speed (workpiece) are chosen based on preliminary experiments [6,10,14]. e tool is made of AISI M2 tool steel, whose dimensional details are listed in Figure 5. e tool holds distinctive hardness and is highly resistant to wear properties in correlation to the workpiece taken under consideration. e workpiece under consideration is AA 7075 aluminium alloy, whose chemical and physical properties are tabulated in Tables 2 and 3, respectively. e AA 7075 aluminium alloy constitutes zinc as its major alloying element. Due to its internal resistance to high stress and fatigue strength, also possessing better machinability, it is availed for transport applications in automotive, marine, and aviation firms. Also, these are availed as brazed/ welded barrels in defence rifles. Airframes and bicycle frames are made of AA 7075 aluminium alloy, and welding plays a significant role in each of them [3,4,7,11]. e dimensions of the workpiece (pipe tube) for the FSW cycle are depicted in Figure 6, and the workpiece placement is depicted in Figure 7.

Temperature Measurement (Monitoring Perspective).
A K-type (diameter: 3 mm) thermocouple is availed to measure the temperature and keep a close online monitoring of the temperature around the weld region. e specifications of the same are mentioned in Figure 8 and Table 4. e concept of novelty exists in the monitoring of temperature values as it is mentioned with a drop in differential without compromising the welding process which       Advances in Materials Science and Engineering improves the efficacy of the parent material to be welded [12,13,15,19]. e temperature was converted using a data acquisition system (DAQ). e range of availing a K-type thermocouple is its sensitivity factor being 39-43 µV/°C and thus has a working temperature of −199°C to +1270°C [12,13,15].

Microhardness Measurement (Response: Attribute).
e microhardness of the friction stir-welded component is significant, as the quality of the operation in disparate environments is dependent on the final strength of the welded material. is is evaluated by Vickers (scale: microns) illustrated in Figure 9, with dead weight loads of 0.01-1 kg and 2 kg options [15].

Tensile Strength (Response: Attribute).
Tensile testing specimens were machined according to the dimensions of ASTM standards. Specimens for the tensile test were prepared using a wire electro discharge machine (WEDM) as per ASTM-E8 standard, and the ultimate tensile strength was measured using an electromechanical universal testing machine, as depicted in Figure 10.

Friction Stir Welding in Disparate Environmental
Conditions. For performing friction stir welding of AA 7075 aluminium alloys in the dry and cryogenic environment, orthogonal (L 27 ) array is availed [5,15,19,[21][22][23][24][25] after running an in-depth preliminary testing before finalizing on the working range for the input factors. e parameters taken in consideration are microhardness and tensile strength in correlation to tool pin profile, pipe rotation speed, welding speed, and axial force applied on the parent material.
e experimental values after conduction of friction stir welding of aluminium alloy are tabulated in Table 5.
Analysing and applying multiattribute algorithms, researchers have established disparate methods of solving the multiple attribute-response problem. e commonly availed technique is ELECTRE, GREY, TOPSIS, and VIKOR [12,13,15,19,[21][22][23][24][25] in which TOPSIS holds a higher ground due to its simplicity in adapting, minimal variation, and drop in complexity with involvement of multiple vectors (provides a common vector plane solution). e method adopted for analyzing the attributes and responses is indicated in Figure 11. e criteria of importance for factors under consideration are listed out by Simos weighting criteria method which is tabulated in Table 6 [19]. e valuation obtained using the TOPSIS method is listed out in Tables 7 and 8 for dry and cryogenic friction stir welding, respectively.
From the criteria analysis (preference to unity factor), tool profile stepped type, pipe rotation speed of 1000 rpm, welding speed of 50 mm/min, and axial force of 8 kN are close to unity, i.e., ideal solution for dry and cryogenic friction stir welding of AA 7075 aluminium alloys [12,15,19,[22][23][24][25]. ese close to ideal samples are taken to study the individual microstructural properties affecting the welding zone in disparate environmental conditions.

Effect of Responses on Factors of Friction Stir
Welding. An increase of 0.76-42.93% and 3.79-31.24% in microhardness and tensile strength, respectively, is ascertained in cryogenic friction stir welding in correlation to dry friction stir welding of aluminium alloys, which is tabulated in Table 9 and depicted in Figure 12.

Ramification of Microhardness in FSW of Aluminium
Alloy.
e hardness test was conducted on 27 specimens for disparate friction stir-welded conditions for AA 7075 aluminium alloy, as depicted in Figure 13. For a clear understanding, the micro-Vickers hardness test was conducted on different zones of the welded material, as depicted in Figure 13, and corresponding values are tabulated in Table 10.
e important zones are parent zone, heat affected zone (HAZ), and weld zone. From the analysis, it was found that the heat affected zone showcases the least hardness value as maximum heat is absorbed in this region creating the coarse and fine granular irregular structure. From Table 10, we can observe an increment of 42.05%, 11.11%, and 14.73% in the hardness value for the cryogenic FSW process for AA 7075 aluminium alloys in correlation to the dry FSW process in HAZ, TMZ, and NZ areas. e hardness values were obtained at 1.5 mm below the weld surface [11]. e presence of precipitates of magnesium and zinc (MgZn 2 ) leads to the curtailing of the hardness in the FSW process [2,3].
Excessive temperature differential occurring in the dry FSW process leads to metastable precipitates that cause a drop in the hardness in the weld region [3,4,10]. An immediate remedy is found in the cryogenic FSW process due to the fine structures occurring in the region, thus creating a brittle transition region along the weld area [5]. In the cryogenic FSW process, the dissolved precipitates are curtailed due to the drop in temperature differential [12,13,15].

Ramification of Tensile Strength in FSW of Aluminium
Alloy.
e tensile samples taken for consideration are depicted in Figure 14, and the tensile properties are dependent on the microstructure and friction stir welding environment. e tensile plot for the friction stir-welded joints for the dry and cryogenic condition for AA 7075 alloy is shown in Figure 15.
A steady tensile strength is ascertained in the cryogenic condition owing to fine precipitate formation, which gets distributed in a fine homogeneous pattern having a definite    [5]. e strengthening of these alloys is attributed to the metastable phase formed in cryogenic friction stir welding [3,7]. e coarse structures, ascertained in dry friction stir welding, create a drop in the tensile strength [6][7][8].

Microstructure Study.
e friction stir-welded condition is analyzed to understand the surface of AA 7075 aluminium alloy, with a magnification of 25x availed in a optical microscope to determine the microstructure of the welded component, as indicated in Figure 16.       Advances in Materials Science and Engineering e need for microstructure analysis is important as this indicates the hardness, strength, and corrosion resistance of the welded surface as these form the crux for applications in the industrial perspective [15,19]. Detailed microstructure analysis and SEM images are shown in Figure 17-20 for dry and cryogenic FSW of AA 7075 aluminium alloy.
Friction stir-welded aluminium alloy consists of precipitates in the form magnesium zinc (MgZn 2 ) which are predominant in dry condition welding, as the materials' grain size in original that was observed was 92 microns before the friction stir welding process was carried out in two different mediums [6]. A decrement of 21.68% is observed in the grain size in the cryogenic condition in correlation to the dry FSW process, indicating a drop in the coarse structure, as tabulated in Table 11 [7,[12][13][14][15].
In the cryogenic condition, equiaxed and fine grains are ascertained in the weld zone, which indicates a drop in the grain size owing to smaller intermetallics formed in the zone of welding [12]. With a drop in temperature in the cryogenic condition, a continuous evaporation pad is formed which removes the temperature differential occurring at the zone of welding [12][13][14]19]. e presence of wide craters/microvoids   is substantially present in dry condition welding due to the colossal temperature differential and increased friction factor (nonavailability of lubrication) [18]. Additionally, the component of hard residual factor and flowability (stress) on the workpiece is substantial when it comes to welding. With a drop in temperature differential, cryogenic friction stir welding causes an indirect curtailment of flow stress and drop in the tensile factor (residual) [5,22]. e rough coarse grains in dry FSW is owing to the precipitate formation (intermetallic fusion) [2]. Also, having induced stresses along the line of weld causes colossal surface roughness leading to uneven and irregular distribution of the material along the welded region. An abatement of microcraters and fine-grain structures is obtained in the cryogenic friction stir-welded condition owing to the homogeneity of the welding process in correlation to the dry FSW process for AA 7075 aluminium alloy [22,25]. Dry FSW induces the tensile residual factor in AA 7075 alloy, which leads to attenuation of microcracks leaving short of applications in the area of high stress loading conditions [6,15].   Advances in Materials Science and Engineering

AFM Analysis.
e study of the surface (topographical) is carried out by atomic force microscopy (AFM) for dry and cryogenic friction stir welding of AA 7075 aluminium alloy. e profile image is depicted in Figure 21, whose profile (average) roughness is tabulated in Table 12.
From the microstructure study, high craters were observed in dry friction stir welding, and also from Figure 21, extensive waviness pattern is ascertained indicating the irregular weld zone (HAZ) due to presence of coarse grains [5,6,19,24,25].
A curtailment of 50.84% is ascertained in the roughness value in cryogenic friction stir welding in correlation to dry friction stir welding of AA 7075 alloy. e surface roughness decrease can be attributed to the curtailment of the HAZ zone, as decrease in the roughness parameter is dependent on the temperature differential drop (lower amount of heat conducted to the HAZ zone) [24]. Also, a drop in the surface roughness factor indicates grain refinement occurring in the welding surface [11,19]. Along with a drop in temperature differential, curtailment of surface roughness results in higher grain refinement, as ascertained in the cryogenic friction stir welding condition. e waviness pattern is ascertained in both the welding conditions due to the irregularity in the surface (disorientation of grains). e analysis (topographical) plays a key role in analyzing the boundary edges, corrosion phenomenon, and residual factor (compressive stress) [5,23].

XRD Analysis.
e residual factor (stress) plays a key role in applications of welding. A welded material undergoes high compressive and tensile loading at the point of the welding area. e residual factor is calculated by availing sin square technique [13,22]. While performing the friction stir welding, induced stresses occur due to axial force and   temperature differential (colossal) occurring in disparate regions of the welding material, which plays a key role in determining the surface attributes of the operation [3,25]. e factor of stress is tabulated in Table 13, and the correlated XRD plots are depicted in Figure 22 and further analyzed availing sin 2 ψ technique [5,12].
From the analysis, it is found that the compressive residual factor increases by 40.14% in cryogenic friction stir welding in correlation to dry friction stir welding of AA7075 aluminium alloy. e attenuation in the residual factor indicates higher weld life integrity manufactured under cryogenic conditions. In the XRD plot, a shift in the peak is ascertained in the cryogenic friction stir welding condition, indicating higher refinement, a reduction in grain boundaries, thus inducing colossal amount of internal stress in the weld region [4,8,23]. e refinement indicates a drop in the grain (average) size along the weld area surface. With a compressive factor, an increase in wear resistance is ascertained along the region. Broadening of peak is ascertained in cryogenic friction stir welding, enhancing the component of strain (lateral factor) along with curtailed dislocations occurring in the weld region due to the subzero welding condition (drop in differential: brittle transition, leaving smaller microstrains devoid of microcracks, and reduction of wear). Width of XRD peak shifts by 39%, indicating a curtailed grain (average) size. ese microstrains, dislocations, and drop in

18
Advances in Materials Science and Engineering surface roughness bring the factor of corrosion resistance for the welded material under the cryogenic condition [15,22,24].

Corrosion Analysis.
A oxidation factor is expected for Al-Al welded joints. In dry FSW of aluminium alloy, frictional heat generation and humidity (atmospheric) of 55% speed up the oxidation rates of aluminium during the welding cycle [6,19]. e formation of intermetallic compounds consisting of carbon increases the surface roughness factor and induced tensile stresses, thus curtailing the corrosion resistance factor in the dry FSW condition [6]. is phenomenon is devoid in the cryogenic FSW process as the absence of microcracks and void-related problems in the welded region aids in the corrosion resistance factor [6,8]. A component of weld undergoes extensive cycles of friction, wear, tear, and load factor. e worn out material exposed to environmental conditions in the areas of oil transportation and automotive parts undergoes corrosion with exposure to disparate fluids or air-based application [6,13,22,24]. e anodic polarization changes in the surface of the friction stirwelded component are depicted in Figure 23, and the corresponding corrosion rate is tabulated in Table 14.
e rate of corrosion resistance aggregates by 67.97% in cryogenic FSW in correlation to dry FSW of aluminium alloys, owing to reduction in surface roughness factor, drop in grain size, and surge in the compressive residual factor [6,14]. When all of these occur in the weld region, an attribute of the passive barrier occurs in the cryo FSW component, indicating higher resistance to corrosion [22,24]. Presence of secondary phases, indicating grain boundary precipitates, enhances the factor of corrosion resistance and mechanical strength of the welded region, aiding in retainment and augmentation of the barrier (passive region) formed [8,15].

Conclusions
Friction stir welding operation was adopted for joining AA 7075 aluminium alloy (application: oil industries, chemical pipe transportation, and defence barrels) in dry and cryogenic environmental conditions. e major findings of the experimental investigation are (1) An increase of 0.76-42.93%, and 3.79-31.24% in microhardness and tensile strength, respectively, is ascertained in cryogenic friction stir welding in correlation to dry friction stir welding of aluminium AA7075 alloys. (2) From the criteria analysis (TOPSIS: preference to unity factor), tool profile stepped type, pipe rotation speed of 1000 rpm, welding speed of 50 mm/min, and axial force of 8 kN are close to unity ideal solution for dry and cryogenic friction stir welding of AA 7075 aluminium alloys. (3) In cryogenic friction stir welding, a drop in temperature differential is experienced which aids in the curtailment of welding temperature, thereby reducing the internal stresses (tensile) induced on the components surface. ese factors alleviate surface morphology by curtailing the coarse structures (precipitates in the form magnesium zinc (MgZn 2 )) formed in the dry FSW condition, and a decrement of 21.68% is observed in the grain size in the cryogenic condition in correlation to the dry FSW process. (4) e friction stir-welded component under the cryogenic environment showcased drop in temperature, curtailed surface roughness, and fine grain structure owing to reduction in temperature differential occurring at the weld zone. A curtailment of 50.84% is ascertained in the roughness value in cryogenic friction stir welding in correlation to dry friction stir welding of AA 7075 alloy. (5) e compressive residual factor increases by 40.14% in cryogenic friction stir welding in correlation to dry friction stir welding of AA7075 aluminium alloy. e attenuation in the residual factor indicates higher weld life integrity manufactured under cryogenic conditions, owing to higher refinement of grain size and a reduction in the grain boundary phenomenon. (6) Width of XRD peak shifts by 39%, and broadening of peak indicates a curtailed grain (average) size. ese microstrains, reduction in dislocations, and drop in surface roughness bring the factor of passivation barrier, aiding in corrosion resistance and high internal stresses opposing the wear component. (7) e rate of corrosion resistance aggregates by 67.97% in cryogenic FSW in correlation to dry FSW of aluminium alloys, owing to reduction in temperature differential, surface roughness factor, drop in grain size, and surge in the compressive residual factor.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.