At present, a great variety of materials such as metals and their alloys, ceramics, plastics, and composites are available for engineering applications with their own traits, standards, and limitations. They are used either singly or in combination for a particular application. Selection of the ideal material having expected application-dependent properties is an essential feature in every industrial application as it improves the function, quality, performance, properties, and cost of the product/component. After the selection of material, the use of an appropriate manufacturing process for the targeted application is equally important.

In this section, a broad outline of the current scenario, in reference to data available in the literature relating to the importance and use of light metal alloys, is furnished. Metals or alloys having low density, high specific strength, and low toxicity are termed light metal alloys (LMA) or simply light alloys (LA). Generally, magnesium, aluminum, and titanium alloys are considered light alloys; owing to its toxic nature, beryllium is excluded from this category (Edwards and Crouch, 2017). A plot showing relative bulk densities of commonly used alloys is presented in Figure 2. Light alloys are of great importance in aerospace, automobile, defense, and medical industries, where a high strength-/stiffness-to-weight ratio is imperative to warrant their use by reducing mass and increasing efficiency (Monteiro, 2014). The cost and blend of properties of Al alloys, viz., high specific strength, superior formability, excellent cast ability, and electrical/thermal conductivity, along with excellent corrosion resistance make these alloys appropriate for a wide range of uses. To increase fuel efficiency and reduce greenhouse gas emissions, they are utilized as a structural material in the automotive sector to produce space frames, engine brackets, engine blocks, cylinder heads, brake rotors, and interior body panels (Davis, 1993). They are also utilized in manufacturing aircraft structural components and marine engines (Spada, 2002; Dorum et al., 2009; Hatch, 1984). To serve the purpose, the current study is focused on Al alloys.

Aluminum is the most abundantly produced and utilized non-ferrous metal around the world. Compounds of aluminum have been used for several hundred years. The first commercial production of aluminum started about 170 years ago. Since then, the demand for Al and its alloys has increased and has reached approximately 29 million tons per year (Aalco, 2021). Low density (around 2.7 gm/cc), high strength (achievable through alloying and heat treatment), and excellent corrosion resistance are the three key attributes together with supplementary traits like high electrical/thermal conductivity, high ductility, non-magnetic nature, low working cost, and high recyclability of Al alloys, make them suitable for many engineering applications (Mukhopadhyay, 2010; Mukhopadhyay, 2012a). Al alloys are used as armor material for lightweight combat vehicles and form the core structure of scout vessels and the superstructure of large ships. Frigates and destroyers are also fabricated using Al alloys. Both commercial and military aircraft are manufactured using high-strength Al alloys as structural material. Because the ductile to brittle transition temperature (DBTT) of Al alloys is very low and they do not undergo embrittlement at low service temperatures, they are utilized to produce cryogenic fuel tanks for rockets and launch vehicles (Mukhopadhyay, 2012b).

Typically, Al alloys are categorized as cast alloys and wrought alloys (non-heat-treatable and heat-treatable wrought alloys). The fraction of alloying elements is higher, and the tensile strength is lower, in cast Al alloys than in wrought Al alloys. Cast Al alloys are utilized in the cast condition. Their applications include window fittings, garden tools, gearbox and axle housings, cast wheels, and farm equipment. Wrought Al alloys are worked in the solid state using specific tools to form them into various standard and nonstandard shapes. They possess better mechanical properties, structural integrity (no casting defects), and surface finish than cast Al alloys. Al alloys that cannot be strengthened by age-hardening heat treatment are called non-heat-treatable wrought Al alloys. Typically, they are strengthened by cold working through mechanical reduction. Al alloys that respond to age (precipitation) hardening heat treatment to attain reasonably high strength levels are called heat-treatable wrought Al alloys.

Alloying elements play a vital role in heat treatability and properties of Al alloys. The elements Cu, Zn, Mg, Si, Mn, and Li are the principal alloying elements used to strengthen the Al alloys. Table 1 shows a four-digit numerical designating system used to specify wrought Al alloys. Out of these four digits, the first digit denotes the chief alloying element, the second digit denotes the variation of the alloy, and particular alloys in the series are represented by the third and fourth digits (Chawla and Gupta, 1993). Al alloys are also specified by the temper designation system, which depends on the series of mechanical and/or thermal treatments given to an alloy, such as O (annealed), H (strain-hardened), F (as fabricated), or T (thermally treated) (Chawla and Gupta, 1993; Bhojak et al., 2023; Pawar et al., 2024).

Precipitation (age) hardening is one of the most extensively exercised mechanisms for the strengthening of metallic materials. The strongest Al alloys, viz., 2xxx, 6xxx, and 7xxx, are developed by precipitation hardening heat treatment. The hardness and strength of a few metallic alloys are attained by the evolution of homogeneously distributed tiny precipitates within the parent matrix through the heat treatment process, called precipitation or age hardening (Wang et al., 2006). This extremely fine precipitate hinders the motion of dislocations by obstructing their path and ultimately results in the strengthening of the heat-treated alloys (Edwards et al., 1998). Precipitation hardening depends on the variation in solid solubility with temperature to produce tiny precipitates. An alloy system can be precipitation-strengthened only if there is a formation of a terminal solid solution at elevated temperature and solid solubility decreases with a drop in temperature (Wang et al., 2006; Totik et al., 2004). Three basic steps that are involved in the process of precipitation hardening are listed below and presented using Figure 3.

Solution zing is also called solution annealing. It involves the heating of the alloy above the solvus temperature within the single-phase region. It is held at that temperature to dissolve all the alloying elements and form a homogeneous solid solution. The quenching is rapid cooling using a quenching medium to obtain a supersaturated solid solution (SSSS). The aging consists of reheating a supersaturated solid solution (SSSS) to a medium temperature (lower than the solvus temperature) to attain a finely dispersed precipitate particle. High strength in the 2xxx, 6xxx, 7xxxx, and 8xxx series Al alloys can be attained via age hardening. In the 2xxx series, the principal alloying element is Cu, which readily responds to age hardening and forms high-strength precipitates like CuAl2 and Al2CuMg. 2xxx series Al alloys possess high strength, toughness, and fatigue resistance. Mg and Si are the chief alloying elements of the 6xxx series. Formation of precipitates like Mg2Al3 and Mg2Si during age hardening results in strengthening of these alloys. Low fracture toughness of these alloys limits their use in aerospace and defense applications (Pan et al., 2016). Li is the key alloying element in the 8xxx series Al alloys. Though the addition of lithium in aluminum matrix reduces density and improves elastic modulus and tensile strength, the high cost of lithium metal and the high processing cost of Al–Li alloys limit their applicability (Robson, 2004). Zn, Cu, and Mg are the principal alloying elements in 7xxx series Al alloys (Pan et al., 2016; Robson, 2004). During precipitation hardening of these alloys, precipitates like CuAl2, Mg2Al3, Al32 (Mg, Zn) 49, and MgZn2 are formed, which contribute to strengthening, and the yield strength achieved is in the range of 450–600 MPa (Zendehdel and Hassani, 2012). 7xxx series Al alloys usually have superior strength and hardness to 2xxx alloys; therefore, they are used in defense and aircraft structural applications where higher stresses are imparted (Xu et al., 2017). Al alloy 7075 (AA7075) is the most abundantly utilized alloy in the defense and aerospace industries among all of the 7xxx series Al alloys.

AA7075 was established by the Aluminum Company of America in 1943 with Zn as the principal alloying element. Zn, Cu, and a small fraction of other alloying elements act as hardeners to strengthen the alloy by forming tiny intermetallic/precipitates during the age-hardening process (Fernandez et al., 2010). The alloy possesses exceptional strength, good ductility, and toughness together with excellent fatigue resistance (Wang et al., 2008). The corrosion resistance of AA7075 is considerably superior to that of the 2xxx series Al alloys, but it is more notch sensitive (has a higher embrittlement tendency) than most of the Al alloys (Lin et al., 2006). When annealed, it has excellent formability and machinability. The main disadvantage of AA7075 in the high-strength T6 temper condition is its high susceptibility to stress corrosion cracking (SCC). However, this can be addressed by using retrogression and re-aging (RRA) heat treatment, which increases resistance to SCC without compromising the alloy’s strength (T6 condition). AA7075 is widely used in structural elements of automobiles, armed vehicles, bridges, large earth-moving equipment, aircraft, and many other high-end applications because of its special combination of properties (Wei et al., 2004).

Metal working techniques that involve very large plastic strains and complex state of stresses resulting in very high defect density along with ultrafine grain structure are called severe plastic deformation (SPD) techniques (Vailey et al., 2000). Recently, many researchers have been exploring various SPD techniques to fabricate bulk ultrafine-grained (UFG) materials with superior properties (Valiev et al., 2016). Using SPD techniques to manufacture bulk UFG materials with high grain boundary disorientation and relatively homogenous and equiaxed microstructures has clear uses in the creation of high-strength materials. Numerous studies are being conducted to enhance SPD methods that can commercially produce UFG metals and alloys. The most crucial feature of these techniques is that the workpiece’s exterior dimensions do not change much during or after processing (Bagherpour et al., 2019). To date, there have been reports of around 120 distinct SPD techniques with a range of deformation modes for varied material geometries and forms (Zendehdel and Hassani, 2012). These include MDF (multidirectional forging), RCS (repeated corrugation and straightening), TE (twist extrusion), HPT (high pressure torsion) (Valiev and Langdon, 2006), ECAP (equal channel angular extrusion/pressing (Hong et al., 2014), and ARB (accumulative roll bonding) (Mishra et al., 2000).

Mishra et al. (2000) used projected friction stir processing (FSP) and the basic philosophy of friction stir welding (FSW) as a new SPD technique for microstructural modification in 1999. Without altering the plate’s shape or size, FSP creates a submicron-level ultra-fine-grain structure by applying enormous shear strain to the material. FSW was created in 1991 by the Welding Institute (TWI), UK, and since then, there have been multiple documented process modifications of this technology. A revolving tool with a shoulder and a joining pin is used in the solid-state metal joining process known as FSW. To join two metal sheets, the pin is inserted into their interface and passed through (Lockwood et al., 2002). A recrystallized weld stir zone microstructure is the unique feature of the resulting joint produced via FSW that typically has a finer grain structure than the base metal (Mishra and Ma, 2005). During FSP, a rotating tool is traversed across a single piece of material to modify the microstructure, properties, and, sometimes, the composition (Mishra, 2004).

FSP may be the most significant and extensively studied variety of the friction stir technique due to its unique features, which include minimal heat generation, mechanical mixing, huge material flow, high forging pressure, ultra-fine-grain structure, and random disorientation of grain boundaries (Charit and Mishra, 2005). FSP is basically a local thermomechanical metal working method, wherein the tool is plunged into the workpiece and then traversed along a desired path to alter the localized microstructure (Arbegast, 2003). Localized heating and softening of the friction between the tool and the workpiece causes the substance to form. A small volume of plasticized material flows from the front of the pin (advancing side) to the back of the pin (retreating side). This leads to intense plastic deformation of the material and results in a significant increase in defect density. Simultaneous heat generation and plastic flow of material eventually result in dynamic recrystallization.

Numerous factors, including process parameters, tool shape, and tool material, affect the flow of plasticized material during FSP, making it exceedingly complex and leaving little knowledge of the deformation process (Charit and Mishra, 2005; Arbegast, 2003; Besharati-Givi and Asadi, 2014; Grujicic et al., 2010). According to Arbegast et al. (Besharati-Givi and Asadi, 2014), the material flow during FSP can be depicted as five basic working zones for metal: (i) Preheat. (ii) Initial deformation. (iii) Extrusion. (iv) Forging. (v) Post heat/cool down, as shown in Figure 4a. Temperature increases ahead of the pin because of the frictional heat produced by the rotating tool and adiabatic heat originating from material deformation. This is called the preheat zone, and the heating rate of this zone is governed by the thermal characteristics of the material together with the tool traverse speed. As the tool begins traversing forward and the temperature of the material increases beyond the critical temperature (the temperature required to start material flow), an initial deformation zone forms and leads to material flow. Furthermore, as shown in Figure 4b, an extrusion zone with a finite width is formed, wherein material travels around the pin from the front to the back. Under hydrostatic pressure, material entering the forging zone from the tool’s front is pushed into the hole left by the pin. Finally, cooling of material takes place under either forced or passive cooling conditions.

Microstructural changes in the material occur because of material flow during FSP. As illustrated in Figure 5 (Charit and Mishra, 2005; Arbegast, 2003; Besharati-Givi and Asadi, 2014; Grujicic et al., 2010), the microstructure of the friction stir-processed (FSPed) specimen essentially exhibits four different zones: the stir zone (SZ), the transition zone (TZ), the heat-affected zone (HAZ), and the base metal (BM).

The SZ, sometimes referred to as the nugget zone (NZ), is characterized by a very fine, recrystallized, and nearly equiaxed grain structure. It represents the location of the pin during processing and hence experiences the highest strain and temperature. Often, a few concentric rings can be observed in the SZ known as an “onion-ring” structure, which is a distinctive feature of this zone. The TZ, also known as the thermomechanically affected zone (TMAZ), appears in close vicinity and on either side of the SZ. It is subjected to the thermal cycle and mechanical stress. The strain and temperature in TZ are lower than those of SZ. Mostly deformed and rotated grain structure and sometimes, mixed microstructure comprising bulky and fine grains are observed in this zone. The HAZ is commonly observed in most of the thermomechanically processed materials. In an FSP specimen, it is present between TZ and BM. Material experiences thermal cycle without any plastic deformation in the HAZ. Though the peak temperature in this zone is less than that in TZ, it still has a noteworthy impact if the microstructure is thermally unstable. The BM is the region that is unaffected by the FSP.

Tool rotation speed is also referred to as spindle speed. It is the rate at which the tool rotates during processing and is measured in terms of revolutions per minute (RPM). It has a remarkable influence on the material flow and heat input (Mishra, 2004). The ideal tool rotation speed is established on the basis of the material to be processed. Although there is a complicated relationship between rotational speed and processing heat, it would seem that increasing rotation speed will result in more heat input and potentially lower-quality processed material (Simar et al., 2012). In addition, premature tool wear, chatter, and tool breakages may occur due to excessive rotational speed. Using the correct rotation speed will significantly influence tool life and the quality of processed material (Zhang et al., 2012).

The tool traverse speed is also called the feed rate. It is the travel speed at which the tool advances against the workpiece and is measured in terms of inches per minute (ipm) or millimeters per minute. It has a strong influence on the overall thermal cycle. It has been stated that decreasing the traverse speed will lead to higher heat input and vice versa (Mishra, 2004). It is reported that lower than the required heat input causes voids or other flaws in the SZ, and, in a worst case, the tool may break, whereas excessive heat input results in poor quality of processed material because of the liquation of low-melting point phases. To facilitate the successful and defect-free processing (FSP), the ideal combination of rotation and traverse speed is necessary, which will ensure the optimum heat input required for extensive plastic flow of the material and reduce the tool’s acting forces (Mishra, 2004).

One of the most important process characteristics for good processing quality is the depth to which the shoulder penetrates inside the workpiece during FSP. The pressure below the tool increases by plunging the shoulder below the workpiece, and it leads to sound forging of the material at the back of the tool. Precise setting of the plunge depth is necessary to generate sufficient downward pressure and to ensure the complete penetration of the tool inside the workpiece. Plunge depth that is either less than or more than adequate may cause defects during FSP and deteriorate the quality of processed material (Grujicic et al., 2010).

The angle between the spindle shaft and the plane normal to the workpiece is called the tool tilt angle. Generally, it is kept between 0° and 3°. It helps to forge the material in the cavity left by the FSP tool (Mishra et al., 2000).

As shown in Figure 6, depending upon the type of pin (probe), three different types of FSP tools have been reported, viz., fixed, adjustable, and self-reacting. The tool, which is fabricated as a single piece comprising both the shoulder and the probe (pin), is called a fixed probe tool (Figure 6a). It can be used to process the workpiece with a specific thickness only because the pin length is fixed (T et al., 2010). In the case of an adjustable probe tool (Figure 6b), the shoulder and the probe are fabricated separately and then assembled to allow adjustment of the pin length. Two different materials can be used to fabricate the shoulder and the pin. Both the fixed and the adjustable tools need a backing anvil (T et al., 2010). The bobbin tool is made by assembling three separate parts—the top shoulder, probe, and bottom shoulder—as shown in Figure 6c. It does not require a backing anvil to process workpieces of different thicknesses (Mishra et al., 2000).

As the research on friction stir welding/processing (FSW/P) develops, new geometrical features are being developed and adapted concerning FSP tool design (Mishra et al., 2000). With the invention of novel tool geometries, the material flow around the pin has become more complex, and it is generally unique for a particular tool geometry (Mishra et al., 2000). The key parameters in determining the processing speed and the quality of the processing are shoulder profile, shoulder diameter, pin profile, pin length, and pin diameter (Mishra et al., 2000).

The main function of a tool shoulder is to generate the frictional heating necessary to plasticize the material of the workpiece during FSP. Figure 7 illustrates the most typical shoulder shapes and features (Rai et al., 2011). Concave shoulder geometry is the most commonly used shoulder design in FSW/P (Zhang et al., 2012). Its ability to restrict material extrusion from the sides made it more popular than other shoulder designs (Sharma et al., 2018). Typically, the concavity angle used ranges between 6° and 10°, and due to its simple shape, it is easy to fabricate. It can produce sound (without defect) and high quality FSP. Geometric features, such as concentric circles, ridges, scrolls, knurling, or grooves, can be incorporated on the shoulder surface to increase frictional heat and material flow. Other potential shoulder shapes are shoulders with a flat surface or a convex profile. Earlier convex end profile shoulders were not frequently used because they tended to push the material away from the pin (Sharma et al., 2018); however, their ability to process complex curvature makes them suitable for processing/joining different-thickness workpieces (Lockwood and Reynolds, 2002).

Material deformation and frictional heating are the two main functions of an FSW/P tool pin. This tool component is made to (i) produce enough frictional heat; (ii) shear and soften the material in front of it; (iii) transport the softened metal to the tool’s back side. The tool pin is largely responsible for controlling the depth of processing in FSP and speed (Sharma et al., 2018). Tool pin geometry has a significant impact on the flow of plasticized material and is closely correlated with the FSP quality (Liu et al., 2020; Leszczynska-Madej et al., 2024; Huang et al., 2023).

The selection of tool material for FSW/P depends upon the desired tool life and the workpiece material. Appropriate tool material selection results in good quality FSP material for the desired application. The requirement of tool materials for FSP of soft metal alloys (magnesium, aluminum, etc.) is not very stringent. However, there are challenges in developing cost effective and durable tools for FSW/P of harder alloys (nickel, titanium, and steels, etc.) (Pawar et al., 2024) that will consistently produce commercially viable and structurally sound products.

FSP tool material properties, viz., mechanical strength, wear resistance, hardness, fracture toughness, thermal conductivity, and coefficient of thermal expansion, influence the product quality, tool life, and performance (Zhang et al., 2012). An FSW/P tool should not react with either the workpiece material or atmospheric oxygen and keep its dimensional stability without losing geometrical features (Sharma et al., 2018). The material and tool compatibilities are shown in Table 2.