Advanced Materials for Emerging Applications Innovations, Improvements, Inclusion and Impact

Recent Advances in Friction Stir Welding of Magnesium Alloys for Use in Performance-Specific Applications

Divyanshu¹, Kunal Chauhan¹, Jimmy Karloopia¹, *, N. M. Suri¹, T. S. Srivatsan²

¹ Department of Production and Industrial Engineering, Punjab Engineering College (Deemed to be University), Chandigarh,160012, India

² Department of Mechanical Engineering, Auburn Science and Engineering Center, The University of Akron, Akron, Ohio 44325-3903, USA

Abstract

Magnesium is the sixth most abundant material in the earth’s crust that finds its applications in the fields of automobiles, aerospace, and biomedical. With noticeable advances in the domain enveloping engineering and technology, there does exist a growing need for new and improved materials to meet the demands put forth by the industries spanning the aerospace and automobile sectors. One of the important requirements for a material is light in weight. Magnesium is one such promising material, which is lighter than aluminum making it an ideal candidate for selection and use in both performance-critical and non-performance critical applications in the domains specific to automobile, aerospace and even biomedical. There are various processing routes for the manufacturing of magnesium alloys, and there exists a need for the joining of the magnesium alloys. The conventional joining processes possess defects, such as porosity, which are detrimental to achieving acceptable to good mechanical properties. Friction Stir welding is one method of solid-state joining, which offers good properties of the weld. The technique of friction stir welding (FSW) operates by rotating and plunging a non-consumable tool into the interface of two workpieces that require to be joined. Promising advantages that are offered by friction stir welding (FSW) are eco-friendly, versatile, and energy efficient. This manuscript highlights (i) the friction stir welding processing technique, as well as recent and observable advances, (ii) the classification of the magnesium alloys, (iii) the welding tool and its influence on welding, microstructural development and mechanical properties of the friction stir welded magnesium alloy, (iv) welding parameters and its influence on governing the relationships between the weld and the workpiece, and (v) typical practical applications and the variants of friction stir welding (FSW).

Keywords: Alloys of magnesium, Base metal (BM), Dwelling, Fixture, Friction stir welding, Magnesium alloy, Magnesium composite, Stir zone (SZ), Tool, Tool probe, Thermomechanical affected zone (TMAZ), Welding.

* Corresponding author Jimmy Karloopia: Department of Production and Industrial Engineering, Punjab Engineering College (Deemed to be University), Chandigarh,160012, India; E-mail:jimmykarloopia@pec.edu.in

INTRODUCTION

With sustained and noticeable advances in technology, the world is gradually moving forward towards the selection and use of materials that are light in weight, have an excellent combination of mechanical properties and tribological qualities to offer coupled with other desirable characteristics. Magnesium is one such material, having acceptable mechanical properties and tribological qualities, coupled with chemical and biological capabilities and is low in weight [1]. Magnesium is often chosen for use in a variety of fields to include the following: (i) electrical industry, (ii) the aerospace industry, (iii) the vehicle industry, (iv) biomedical applications, and (v) industry that caters to the domain of sports, i.e., sporting goods [2-4].

Magnesium makes up around 2.7% of the earth's crust and stands as the sixth most abundant element in the earth's crust [5]. The density of magnesium is 1.74 g/cm³, which is two-thirds that of aluminum and one-fourth that of steel [6]. The properties of magnesium are summarized in Table 1. Magnesium-based materials are extensively sought by firms for use in weight-critical applications essentially because of their low density coupled with high specific mechanical properties.

Table 1 Properties of magnesium [Reference 7].

There are a variety of solid-state processing and liquid-state processing techniques, such as Additive Manufacturing (AM), Stir Casting, Melt Infiltration method, Spray forming, Friction Stir Processing, and Powder Metallurgy for the purpose of manufacturing magnesium alloys and magnesium composites. For the joining of a magnesium alloy and a magnesium alloy-based composite material with both comparable materials and different materials, friction stir welding, resistance spot welding, laser welding, and diffusion bonding are all viable options [8-17].

Thomas and co-workers were the ones who initially created friction welding in 1991 [18]. When welding magnesium alloys and magnesium-based composites, Friction Stir Welding (FSW) offers a number of benefits that are not easily available with the other welding techniques. These benefits essentially include the following [19-22]:

(a) A fine microstructure,

(b) An absence of microscopic cracking,

(c) No loss of alloying elements during processing,

(d) Good dimensional stability and repeatability,

(d) Shielding gas is not required,

(e) High weld strength and toughness, and

(f) Capability of the weld to resist fatigue stress.

The basic idea behind friction stir welding is the same as that behind friction welding. During this process, heat is produced at the contact surface by the application of friction. The heat initiates the diffusion process at the surface where the two materials are to be joined. The application of a high-pressure force to these mating surfaces expedites the metal diffusion process and forms a metal- to- metal junction. This is the fundamental concept of friction welding [23-26].

Stages in Friction Stir Welding

The friction stir welding (FSW) process is broken up into three stages as shown in Fig. (1). The first stage is known as the Plunging phase. The second stage is known as the Dwelling phase, and the Third stage is known as the Welding phase [27].

The friction stir welding (FSW) technique essentially consists of only three stages, namely: (i) plunging phase, (ii) dwelling phase, and (iii) welding phase. Despite its seeming complexity, it is overall a very simple technique. A non-consumable revolving tool is used in the plunging process. The non-consumable revolving tool is composed of material that is stronger than the workpiece and has a shoulder that is bigger in diameter as well as a pin and will plunge into the workpiece to a depth that has been pre-programmed. This causes the generation of heat by penetrating the abutting edges of the workpieces that are clamped together. While the tool is being dwelled, it continues to spin in its place; as a consequence, the temperature directly underneath the rotating tool is gradually raised. An additional source of heat contribution comes from the adiabatic heat that is generated when material of the workpiece experiences plastic deformation around the revolving tool. The material surrounding the tool pin gradually softens as a result of plastic deformation, which occurs when the temperature is high. In the last step, i.e., welding phase, a travel velocity is assigned to the tool along the joining line so that it can agitate the material and create a junction. The wider diameter of the tool shoulder aids in confining the hot material, which would otherwise flow out readily to generate a flash and can result in a loss of material and a resultant poor weld if the loss of material is not contained [27].

Fig. (1))

The friction Stir Welding (FSW) process: (a) Plunging phase; (b) Dwelling phase, and (c) Welding phase.

Classification of Magnesium alloys

When alloying elements are added to pure magnesium, it contributes to altering the characteristics of pure magnesium. Because magnesium is a chemically active element, it is capable of reacting with other elements used in the process of alloying to produce intermetallic compounds. For magnesium alloys, there is no designation system that is recognized universally. The trade names of the firms that were pioneers in the production of magnesium alloys have given way to chemical and numerical naming systems.

The industry relies heavily on the Standard Alloy Designation System developed by ASTM International. In accordance with ASTM B275, every alloy will have a label consisting of letters and numbers. The letters will designate the primary alloying elements, and the numbers are a reflection of the percentages of the alloying elements [28]. The usual abbreviations that are used to indicate magnesium alloys are summarized in Table 2. These abbreviations are letters that stand for alloying elements according to ASTM.

Table 2 The ASTM designation for Magnesium alloys [Reference 29].

There are three components that make up the designation of a common magnesium alloy.

(i) Part 1 starting alphabet of the two principal alloying elements creates two abbreviation letters see Table 3, which indicate the components present in the order of decreasing proportion. The letters are arranged in alphabetical order if the percentages of the various alloying elements are the same.

(ii) In Part 2, the percentage (weight) of the two primary alloying elements is provided, expressed as a percentage of the total weight. Two whole numbers represent the two alphabets.

(iii) In Part 3, it differentiates between the numerous alloys that include identical proportions of the two principal alloying constituents. It consists of a letter of the alphabet that is allotted in sequence as compositions become standard. This is shown in Fig. (2).

Fig. (2))

ASTM designation for a Magnesium alloy [Reference 29].

The ASTM B296-03 standard was used to determine the temper designations. For the designation AZ91C-T4, a dash is used to distinguish between the alloy identification and the temper designation. The temper designations for the magnesium alloys are shown in Fig. (3).

The commonly used magnesium alloys in which aluminium is the primary or key alloying element are as follows: (i) AJ52A, (ii) AJ62A, (iii) AM50A, (iv) AM60B, (v) AS41B, (vi) AZ31B, (vii) AZ61A, (viii) AZ80A, (ix) AZ81A, (x) AZ91D, and (xi) AZ91E. The classification of magnesium alloys based on manufacturing route is shown in Fig. (4). It may be possible to manufacture an alloy having the same major alloying constituents by using more than one manufacturing technique. This is dependent on the precise composition of the specific alloy as well as the requirements imposed by alloy design. The WE43 magnesium alloy is often chosen for use in biomedical applications. The AZ91 A, B, and D alloys are chosen for use in both automobile applications and aerospace applications.

PROCESS PARAMETERS FOR FRICTION STIR WELDING (FSW)

The chosen material undergoes or experiences a significant amount of movement during friction stir welding (FSW), which could result in plastic deformation. Tool geometry, welding parameters, and joint design exert a substantial impact on both the material flow pattern and temperature distribution, which exerts an influence on the microstructure development of the material.

Fig. (3))

Temper designations for the magnesium alloys [Reference 29].

Friction Stir Welding: Joint Design

When it comes to friction stir welding, a large majority of the joints can be categorized into three primary categories: (a) butt joint, (b) lap joint, and (c) fillet joint. Several other joints may be conceptualized as a mixture of these two types of joints and is shown in Fig. (5) (a-c) Butt Joint, (d-f) Lap Joint, g) Fillet Joint.

The Butt joints and lap joints are the easiest joint configurations to work with while working with friction stir welding (FSW). For the butt joint, a backing plate is often used, and then two plates or sheets having the same thickness, are put on top of it and securely held in order to prevent the abutting joint faces from being driven apart. Since the pressure that is exerted during the first plunge of the rotating tool is very significant, further caution is essential to ensure that the plates in a butt configuration do not get separated. In order to produce a weld along the abutting line, a rotating tool is first inserted into the joint line. The rotating tool is then traversed slowly along this line while maintaining close contact between the shoulder of the tool and surface of the plates. On the other hand, in order to create a basic lap junction, it is necessary to attach two lapped plates, or sheets, onto a backing plate. In order to link the two plates, a rotating tool is vertically inserted through the top plate and then into the lower plate. The tool is then traversed along the appropriate path. The combination of butt joints and lap joints enables the production of a wide variety of additional joints. Other kinds of joint designs, such as the fillet joints in addition to the butt joint and lap joint configurations, are also feasible depending on requirements of the particular engineering application at hand.

Fig. (4))

Classification of the Mg alloys based on manufacturing route.(Data ASTM Handbook).

Friction Stir Welding (FSW) Tool

The introduction of new welding tools has been responsible for many of the advances that have been achieved in the technique of friction stir welding. Design of the welding tool, which must account for geometry of the tool as well as the material from which it is constructed, is essential to ensure an efficient execution of the operation [30]. The friction stir welding (FSW) tool often comprises a spinning round shoulder and a threaded cylindrical pin in most cases. The moving shoulder warms the workpiece mostly due to friction, while the threaded pin pushes the pliable alloy around the workpiece to essentially create the joint.

The design of the tool does exert an influence on the following:

(a) The amount of heat that is generated,

(b) The flow of plastic,

(c) The amount of power that is needed, and

(d) Overall uniformity of the welded seam.

Fig. (5))

FSW Joint Configuration (a) Square Butt, (b) Edge Butt, (c) T Butt, (d) Lap Joint, (e) Multiple Lap Joint, (f) T Lap Joint, and (g) Fillet Joint [Reference 19].

The shoulder is responsible for producing the majority of the heat and concurrently preventing the plasticized material from escaping from the workpiece. Both the shoulder and the tool-pin are responsible for influencing the flow of material.

In recent years, a number of novel aspects have been considered and included in the construction of tool design. The tools that have been developed at TWI are shown in Fig. (6). For butt welding, a tool that is designed to be cylindrical shape, Whorl shape, and MX triflute shape is chosen for use. For the lap joint, a tool designed of Flared triflute and A-skew is chosen for use. When minimum asymmetry in weld property is desired, the Re-stir shape tool is used according to TWI [31].

Fig. (6))

FSW tools designed at TWI (a) Cylindrical, (b) Whorl™, (c) MX triflute™, (d) Flared triflute™, (e) A-skew™, and (f) Re-stir™ [Reference 32].

The tool shoulders are constructed in such a way that they provide the required downward forging action that facilitates in the following:

(a) Welding consolidation,

(b) Heating the surface regions of the workpiece by frictional heating, and

(c) Restricting the flow of hot metal under the bottom shoulder surface.

The usual outer surfaces of the shoulder, the bottom end surfaces and end characteristics, and the tool probes used during Friction Stir Welding (FSW) are shown in Fig. (7). In most cases, the outside surface of the shoulder is shaped like a cylindrical cylinder. However, on occasions, a conical surface is also employed or used [19]. Since the shoulder plunge depth is typically very small (i.e., 1–5% of the gauge thickness), it is generally anticipated that the shape of the shoulder outer surface (cylindrical or conical) will have a negligible impact on overall quality of the weld.

The tool materials that are often chosen for use for the welding of magnesium alloys and composites are the following: (i) tool steel, (ii) H13 steel, (iii) High speed steel (HSS), (iv) Stainless steel (SS), (v) Mild Steel, and (vi) High carbon high chromium steel [34-36]. In Table 3 is provided a summary of the tool material, workpiece, tool shape, tool size for the magnesium alloys and magnesium-based composites.

Table 3 Summary of tool material, workpiece, tool shape, tool size for the magnesium alloys and composites.

Abbreviations: SD: Shoulder diameter (mm), SL: Shoulder Length (mm),

PL: Pin Length(mm), TA: Tool tilting angle(º),

PH: Pin height(mm), PD: Pin diameter(mm)

Fig. (7))

Friction stir welding (FSW) tool [Reference 33]: (a) Morphology of the Shoulder, and (b) The different probes.

Welding Parameters of Friction Stir Welding (FSW)

The following are the key factors that are involved in Friction Stir Welding (FSW): (i) traverse speed, (ii) tool rotation speed, (iii) thickness of the workpiece, (iv) applied pressure, (v) profile of the pin, (vi) height of the pin, (vii) angle of deviation, (viii) pin direction, and (ix) the presence of both obstruction and inhibitor forces [48, 49]. With friction stir welding for a magnesium alloy, the tool rotation speed and tool traverse speed are the two welding parameters that are considered to be of utmost importance.

The main welding parameters and their effects are neatly summarized in Table 4. The tool rotational speed is a highly essential characteristic because it has a significant influence on both the flow of the material and the formation of heat, which in turn allows for a modification of both the mechanical properties and microstructure of the joint [50]. A sound weld now depends upon the regulation of rotating speed during welding. The chances of mixing the deformed materials in the weld zone will increase as the tool rotation speed increases. This is a positive variable since it enhances the likelihood of mixing the chosen materials. Since the shoulder is responsible for about 95% of the total heat that is created by the pin, an increase in the pin rotation speed will also result in an increase in shoulder speed. This results in an observable increase in the amount of heat that is generated in the joint region.

Table 4 A summary of the main welding parameters and their effect.

A summary of the friction stir welding (FSW) parameters used in the welding of magnesium alloy and composites is shown in Table 5. There was a minimum welding speed of 120 mm/min and a maximum welding speed of 2000 mm/min. The rotational speed ranged from 100 to 2000 revolutions per minute (rpm), with the average being about 800 to 1600 rpm.

Table 5 Summary of friction stir welding parameters used in the welding of magnesium alloys and composites.

The rate of heat transmission between the weld zone and the workpiece decreases or reduces as the traverse speed increases. As a result, an impact of the welding process will be seen in the more condensed area of the welding edges [51]. On the other side, with an increase in the traverse speed, there will be a concurrent increase in the processing speed as well as a noticeable reduction in distortion. In addition, once the traverse speed is increased, the temperature gradient that is formed between the various weld zones will be reduced. It is important to note that any adjustments made to the traverse speed must also be followed by corresponding changes in the rotation speed. This is essential whenever the traverse speed is altered. In the event that this does not occur, microscopic flaws in the weld zone, such as cavities and pores, will be developed.

MICROSTRUCTURAL EVOLUTION DURING FRICTION STIR WELDING (FSW) OF MAGNESIUM ALLOY/COMPOSITE

Because of the one-of-a-kind characteristics of the Friction Stir Welding (FSW) process, the material in the weld zone goes through extreme thermo-mechanical excursions. These excursions are what drive the recrystallization and recovery processes to occur [19]. The spinning of the tool causes material surrounding the revolving pin to be stirred and mixed by plastic flow, but the primary contributor to the formation of heat is the friction that occurs between the tool and the workpiece. In addition, about eighty percent of the effort associated with plastic movement is lost in the form of heat, which results in the formation of local adiabatic heating [59].

The thermo-mechanically-affected zone (TMAZ), the nugget zone (NZ), and the heat-affected zone (HAZ) are the three unique microstructural zones that are produced as a result of the friction stir welding (FSW) process [60]. In the weld nugget zone, plastic deformation and frictional heat yield an equiaxed fine-grained recrystallized microstructure. The transition zone between the parent material and the nuggets that have been subjected to temperature and plastic deformation during friction stir welding (FSW) is referred to as the thermomechanical affected zone (TMAZ). The HAZ is the zone that lies between the TMAZ and the base material (BM). This region does not encounter any observable plastic deformation but rather goes through thermal cycling.

In a general sense, the weld zone can be separated into four distinct zones distinguished by their unique thermomechanical characteristics (Fig. 8a). These zones are referred to as the stir zone (SZ), the thermo-mechanically affected zone (TMAZ), the heat-affected zone (HAZ), and the base material zone (BM). The perimeters of the TMAZ and HAZ are denoted by a solid line and a dotted line, respectively. The temperature, strain, and strain rate all change gradually as they go from low (purple) to high (red) levels, as shown by the arrows of varying colors. The thermomechanical affected zone’s (TMAZ) outermost limit may be identified by the dashed line (Fig. 8b). The temperature, strain, and strain rate all change gradually as they go from a low level (purple) to a high level (red), as shown by the arrows of varying color.

Fig. (8))

(a) An example of a standard optical cross-section displaying the stir zone (SZ), thermomechanical affected zone (TMAZ), heat-affected zone (HAZ), and the base metal (BM). (b) An optical metallograph of the longitudinal cross-section that was acquired by the stop-action method (pin was moving from left to right [Reference 59].

The temperature is quite different from one end to the other of the thermomechanical affected zone (TMAZ). This is essentially because it is a transition zone. From the thermomechanical affected zone (TMAZ) to the base metal (BM), the peak temperature in the heat affected zone (HAZ) gradually falls or decreases, all the way down to the ambient temperature or room temperature, from a high of 0.55 TM. It is possible that the pre-weld heating will cause the heat-affected zone (HAZ) to become even larger and move further away from the base metal [BM] [59].

Xie and co-workers [61] studied friction stir welding of the two alloys (i) ZK60, and (ii) Mg-4.6Al-1.2Sn-0.7Zn (AT511). In Fig. (9a), ZK60 alloy is on the advancing side (AS), while AT511 is on the retrieving side (RS) and vice versa in Fig. (9b). There was no sign of welding-related flaws, such as heat fractures and air holes, in any of the joints that were thoroughly inspected. It is possible to recognize the weld zone that has features similar to a basin in relation to the two chosen materials.

Subsequent to metallographic etching, it is possible, in most cases, to detect multiple separate zones inside or within a friction stir welded (FSW) joint owing to intrinsic differences in plastic deformation and resultant grain size that exists within the joint. The stir zone (SZ), thermomechanical affected zone (TMAZ), and heat-affected zone (HAZ) are primary components of the weld zone. However, it is worth noting, that within the stir zone (SZ) of each joint, there are really three separate sub-zones, which are identified to be the SZ1, SZ2, and SZ3. A distinct uneven interface exists between the two chosen alloys, and mixing of the two alloys occurs, or happens, only at the bottom, as shown in Fig. (8).

Fig. (9))

Optical micrographs of the two welded joints at the cross-section: (a) ZK60/ATZ511 joint, and (b) ATZ511/ZK60 joint [Reference 61].

Henni and co-workers [62] studied the friction stir welding of the magnesium alloy AZ91D. The typical grain sizes in the base metal (BM) varied anywhere from 50 to 250 µm in size. There was a slight decrease in the number of grains in the HAZ as shown in Fig. (10). The grain size that predominates in this region is around 140 µm. There are certain grains that have a grain size that is comparable to that of the base metal (Fig. 10a). The statistical distribution of the grains, which can be seen in Fig. (9b), reveals the principal grain size to be approximately 16 µm and 14 µm in the thermomechanical affected zone (TMAZ) on both the advancing side (AS) and retreating side (RS), while it is only 2 µm in the nugget zone (NZ). This confirms a significant reduction in the number of grains that occurs in the weld zone.

MECHANICAL PROPERTIES OF FRICTION STIR WELDED (FSW) MAGNESIUM ALLOY / MAGNESIUM COMPOSITE

According to the published research on friction stir welding (FSW), this type of welding leads to considerable microstructural growth not only in the stir zone (SZ) but also in the heat-affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ). When it comes to the development of friction stir welding (FSW), the primary mechanical parameters that are taken into consideration are as follows: (i) microhardness, (ii) tensile strength, (iii) joint efficiency, and (iv) yield strength.

Fig. (10))

(a) Grain size distribution in the base metal (BM) and heat-affected zone. (HAZ), and (b) Distribution of grain size in the nugget zone (NZ) and thermo-mechanical affected zone (TMAZ) [Reference 62].

A vast majority of magnesium alloys that have been chosen for use in commercial applications are ternary in nature and are composed of aluminum, zinc, thorium, and rare earth elements. There are three primary outcomes that might occur with regard to hardness distribution over the weld bead in a magnesium alloy: (i) better (higher), (ii) the same as base hardness, and (iii) lower than the base hardness.

Joint efficiency, defined as the ratio of the strength of the welded structure to the strength of the base material, is used to quantify either an observable increase or decrease in load bearing capacity of a welded structure. In theory, achieving a joint efficiency of 100 percent should be the goal. However, in practice, the joint efficiencies are almost never that high. The heat-affected zone (HAZ) is the most vulnerable part of a structure when using traditional welding methods, such as arc welding. It is widely believed that the primary cause of this decline in productivity is the high temperatures [far and above the point of melting point] that are reached during the welding process. The mechanical properties of magnesium alloy and composite, particularly (i) Ultimate tensile strength (UTS), (ii) Yield strength (YS), (iii) Joint efficiency, and (iv) hardness at both the base material and friction stir weld region are summarized in Table 6.

Table 6 Summary of mechanical properties of the magnesium alloy and composite.

DEFECTS IN FRICTION STIR WELDING

When performing friction stir welding (FSW), the quantity of heat that is introduced into the joint plays a significant role in deciding whether a sound weld will be made and whether flaws will be present in the joint. Collapse of the nugget, surface galling, and excessive weld flash are some of the faults that tend to occur and get easily detected when there is a high level of heat input. On the other hand, should the heat input be minimal, then the weld will contain defects, such as (i) Kissing Bond (KB), (ii) Tunnelling Defect, (iii) Joint-Line Remnant (JLR), and (iv) Hook-Like defects.

The lack of heat input is what causes the tunneling defect. The production of Kissing Bond (KB) defects occurs as a result of ineffective plastic deformation brought on by the mutually interactive influences of inadequate heat input and insufficient stirring action. This eventually results in the surfaces contacting one another without any metallurgical link being formed between them. The presence of oxide layers on the interface, can potentially result in the Kissing Bond (KB) defect if it is not adequately cleaned prior to welding. An insufficient amount of tool plunge, an inadequately long pin, or an inappropriately designed tool might all be the potential causes for incomplete penetration of the tool. The various defects that occur in friction stir welding (FSW), their location in the welded structure and possible causes are summarized in Table 7.

Table 7 A summary of the various defects that occur during Friction Stir Welding (FSW) [Reference 31, 68].

APPLICATION OF FRICTION STIR WELDING TO MAGNESIUM ALLOYS

Magnesium alloy and its composites have found widespread use in a spectrum of industries including electronics, medicine, the automotive industry, and even aerospace. The following are a few examples that highlight the selection and use of magnesium.

(a) Magnesium-based materials have been used in the production of vehicle components by leading industry automakers to include (i) AUDI, (ii) DaimlerChrysler (Mercedes-Benz), (iii) Volkswagen, (iv) Toyota, (v) Hyundai Motor Company, (vi) Jaguar, (vii) Ford, (viii) BMW, (ix) FIAT, and (x) KIA Motors Corporation.

(b) Over the course of aviation history, materials based on both magnesium alloys and magnesium-based composites have seen a significant number of applications in both civilian aircraft and military aircraft. The thrust reverser (used on BOEING 747, 737, 767, and 757 models), Gearbox (used by Rolls-Royce), engines, and helicopter transmission casings are a few examples of use for this material.

(c) Magnesium alloys has been chosen for use in several biomedical applications, such as screws made of pure magnesium and magnesium alloy (WE41, AZ31) have been put to effective use.

(d) The grips of golf clubs, tennis racquets, and archery bows are all made from magnesium-based materials.

(e) The housings of electronic devices, such as mobile phones, PCs, laptops, and portable media players are made from materials based on either a magnesium alloy or composite. An example is the Apple iPod Nano case which is made from magnesium.

The few applications that have been mentioned above demonstrate that the magnesium alloys have been chosen for use in a wide variety of sectors. Nevertheless, their demand is expected to expand in the immediate to near future. There are several viable processing routes available for magnesium alloy and composites. However, there should be a standard way for joining magnesium, and friction stir welding (FSW) might be one such method.

MODIFIED FRICTION STIR WELDING PROCESSES

The different variants of friction stir welding (FSW) are as follows:

(a) Underwater Friction-Stir Welding (UFSW),

(b) Vibration-assisted Friction-Stir Welding (VFSW),

(c) Laser-assisted Friction-Stir Welding (LFSW),

(d) Electrical current-aided Friction-Stir Welding (EFSW),

(e) Ultrasonic Vibration-assisted Friction-Stir Welding (UVFSW).

Underwater Friction-Stir Welding (UFSW)

The term Underwater Friction-Stir Welding (UFSW) refers to a modification of FSW in which the base material is welded on while it is immersed in water. During the underwater friction stir welding process, the spinning tool imparts adequate amount of force to the surface of the workpiece, which often results in plastic deformation that ultimately welds the joints together. Underwater Friction Stir Welding (UFSW) is a method that is often used in joint designs, such as lap joints and butt joints. The use of Underwater Friction Stir Welding (UFSW) has the potential to decrease the number of weld-related flaws, such as (i) shrinkage, (ii) porosity, (iii) microscopic cracking, (iv) splattering, and (v) solidification [69].

Vibration-assisted Friction-Stir Welding (VFSW)

Vibration-assisted Friction Stir Welding (VFSW) is a technique of solid-state welding in which two materials are welded together in the presence of an inert environment and without the need for any filament material or an external heat source [70]. Vibration-assisted Friction Stir Welding (VFSW) offers several benefits to include the following [71]:

(a) A softening of the weldment without using a substantial amount of heat,

(b) Enhancing plastic flow of the chosen material and mechanical interlocking,

(c) Lowering the risk associated with the development and presence of defects, and

(d) Increasing joint strength.

Electrical Current-Aided Friction-Stir Welding (EFSW)

The term Electrical current-aided Friction-Stir Welding (EFSW) refers to a hybrid form of friction stir welding (FSW) process that uses the application of an electrical current through the joint interface. As a direct result of this, the chosen specimen is welded [72].

Ultrasonic Vibration-Assisted Friction-Stir Welding (UVFSW)

The term Ultrasonic Vibration-Assisted Friction-Stir Welding refers to a kind of friction stir welding (FSW) technique that is aided by ultrasonic energy. A sonotrode is positioned in front of the welding tool to ensure that the ultrasonic energy is delivered precisely where it is needed before the welding process begins.

Laser-assisted Friction-Stir Welding (LFSW)

The laser beam is used in Laser-assisted Friction-Stir Welding to generate extra heat at specific spots in front of the weld zone. This results in a noticeable reduction in the applied compressive force that and makes it possible to weld at greater rates [73]. A summary of challenges faced by modified friction stir welding is provided in Table 8.

FUTURE TRENDS IN FRICTION STIR WELDING

1. Various researchers have used primarily diverse tools, and information in the published literature provides limited information. A cylindrical threaded pin with a concave shoulder is extensively utilized in FSW, according to the published literature. But specific profile tools require justification for the purpose of selection and use.

2. Friction stir welding process parameters [to include the following: axial force, tool rotational speed, tool tilt angle, tool traverse speed, and target depth] provide a sound, defect-free weld connection. Parameter selection must be generalized.

3. The flow of material at the fine microscopic level is an extremely complicated process that needs more research to be fully understood.

4. There is a need for a deeper comprehension of the influence of alloy composition and FSW parameters on overall stability of fine-grained microstructure of the magnesium alloys that have been friction stir welded.

5. The level of residual stresses caused by friction stir welding (FSW) may be kept to a minimum by carefully adjusting the welding parameters. The mechanical properties of the welded structure can be enhanced by the use of a post-weld heat treatment (PWHT). Therefore, future research must be focused or concentrated in this area.

Table 8 Summary of challenges faced by modified friction stir welding [74].

CONCLUSION

The advancement in the automobile and aerospace engineering has led to the development of materials that are light in weight. That is why Aluminum is being replaced with magnesium, the latter being light in weight. Magnesium and its alloys are being widely employed in automobile and aerospace sectors by technique of friction stir welding (FSW). This manuscript highlights (i) the friction stir welding processing technique, as well as recent and observable advances, (ii) the classification of the magnesium alloys, (iii) the welding tool and its influence on welding, microstructural development and mechanical properties of the friction stir welded magnesium alloy, (iv) welding parameters and its influence on governing the relationships between the weld and the workpiece, and (v) typical practical applications and the variants of friction stir welding (FSW).

REFERENCES

Suitability of Nickel-base Shape Memory Alloys for Selection and use in Sensing Applications

Sachin Oak¹, *, Kedarnath Rane², Vinod Belwanshi³, ⁴, Kiran Bhole¹, T. S. Srivatsan⁵

¹ Department of Mechanical Engineering, Sardar Patel College of Engineering, Mumbai, 400058, India

² National Manufacturing Institute Scotland, Glasgow, Scotland, United Kingdom

³ School of Physics and Astronomy, University of Glasgow, Glasgow, Scotland, United Kingdom

⁴ CSIR - National Metallurgical Laboratory, Jamshedpur, 831007, India

⁵ Department of Mechanical Engineering, Auburn Science and Engineering Center, The University of Akron, Akron, Ohio 44325-3903, USA

Abstract

In the prevailing era, an influential shape memory alloy (SMA) nitinol has emerged as a potentially viable and economically affordable material that is capable of playing a significant role in both existing and emerging technological applications spanning the domains of aircraft and aerospace, biomaterials in bioengineering, sensors in health monitoring, advanced manufacturing, and microelectromechanical systems (MEMS), to name a few. A high strain recovering capability coupled with super-elasticity are two key and essential characteristics of a smart material that distinguish it easily from its conventional counterparts. The phase transformation behavior shown by nitinol (NiTi) was found to be governed by intrinsic variations in temperature. In order to obtain the desired application-based functionality of this high performing material, potentially viable approaches include the following: (i) an alteration of its chemical composition, (ii) the addition of ternary elements and quaternary elements, and (iii) the use different processing treatments. These approaches are being constantly studied, carefully and systematically examined and frequently reported in the published literature. In this manuscript, an effort is made to present and discuss several of the recent advances specific to the NiTi-based shape memory alloy applications and its phase transformation behaviour when subject to processing treatments. The influence of compositional variation of the NiTi-based shape memory alloys (SMAs) and even its ternary variants and quaternary variants, coupled with the role and/or influence of different processing treatments on both macroscopic properties and microscopic properties is the focus. The emphasis on increasing the suitability of shape memory alloys (SMSs) for selection and use in a spectrum of sensing-related or sensing specific applications is highlighted and briefly discussed.

Keywords: Compositional variation, NiTi-based shape memory alloy (SMA), Phase transformation behaviour, Processing treatments, Shape memory alloy (SMA).

* Corresponding author Sachin Oak: Department of Mechanical Engineering, Sardar Patel College of Engineering, Mumbai, 400058, India; E-mail: soak2489@gmail.com

INTRODUCTION

Shape memory alloys (SMAs) are in the prevailing and/or ongoing time period emerging as materials having a remarkable capability to recover higher strains while concurrently remembering their shape. These smart materials exhibit pseudo-elasticity properties, namely a combination of shape memory effect (SME) and super-elasticity, which are governed to an extent by their martensite to austenite phase transformation or austenite to martensite phase transformation.

Nickel-Titanium (Ni-Ti), referred to as nitinol, is a popular shape memory alloy (SMA) that has been researched during the last few decades, starting way back in the early 1950s. Though the shape memory effect (SME) was first discovered in Au-Cd alloy in 1951, Buehler and co-workers [1] in 1963, invented the shape memory effect (SME) in a Ti-Ni alloy. The titanium-nickel (Ti-Ni) alloy was having an equiatomic composition of titanium and nickel. Equiatomic or near-equiatomic NiTi shape memory alloy (SMA) can exist either in the austenite phase, martensite phase or the R-phase and concurrently reveal a dissimilar microstructure in these phases [2]. The R-phase normally forms as an intermediate phase for the austenite to martensite phase transformation and for the martensite to austenite phase transformation. Both of these transformations essentially involve two-stages for a specific composition. The R-phase is similar to the martensite phase, but does not provide the shape memory effect, or super-elasticity. At temperatures lower than the room temperature, i.e., martensite start (Ms) temperature, a softer and deformable twinned martensite phase exists. This phase has a Young’s modulus (E) and yield strength (σ YS) that is lower than that of the austenite phase [2]. When the temperature of NiTi in the austenite form decreases gradually below the martensite start temperature (Ms), then the austenite NiTi starts to transform to martensite NiTi and the phase transformation is completed well below the martensite finish (Mf) temperature. The deformable twinned martensite, when subject to pressure gets easily distorted to deformed detwinned martensite. Upon heating the NiTi having martensite beyond the austenite start temperature (As), the NiTi begins to gradually transform to austenite and beyond the austenite finish (Af) temperature, the entire austenite phase is formed. During phase transformation, the proportion of these phases tends to vary and the