1 Introduction

Present era also cognized as technological epoch strives to provide competitive and cutting edge technical solutions to real life problems related to fuel economy and human comfort. Numerous industries are putting a massive wealth and exertions in finding out the alternative lightweight materials to substantially bring down the structure weight as well as overall manufacturing cost in conjunction with enhanced portability and human ease. In such framework of digging out, magnesium (Mg) alloys drew special attention due to its light weight, low density (1.74 g/cm³) and remarkable characteristics. Mg is known to be the lightest metal found on the earth thereby being extensively sought after by a number of well renowned industries. Its specific gravity is equivalent to 2/3rd of aluminum (Al), 2/5th of titanium (Ti), and 1/4th of steel^[1](Fig. 1). Its specific strength and specific stiffness are quite impressive. On equivalent circumstances, the strength/weight ratio for 1 kg Mg alloy could be approximately equivalent to 2.1 kg steel, and 1.8 kg Al. Moreover by replacing engineering plastic from Mg alloys, the thickness and the weight can be reduced upto 64% and 36% respectively^[2]. Mg is seen to be an attractive substitute of Al, owing to its overwhelming performance and unique properties such as excellent fatigue resistance, splendid specific strength^[3], highest known damping capacity^[4], good ductility, comparatively superior noise and vibration dampening characteristics, excellent castability^[5], hot formability^[6], excellent machinability^[7], good electromagnetic interference shielding^[8] and so on.

Possessing these astonishing properties, denser metals not even only steel, but others such as cast iron, copper alloys and even Al alloys can be substituted easily with Mg and its alloys^[9-10]. Some of the notable properties of commonly used metals in manufacturing processes are given in Table 1.

Mg is known to be ninth and eighth most abundant element in the universe^[12] and in the Earth's crust^[13], respectively, and fourth most common element came up in the Earth (after Fe, oxygen and silicon), making up 13% of the planet's mass. It is the third most abundant element dissolved in seawater, after sodium and chlorine^[14]. Besides, it is the third most commonly used structural metal, following Fe and Al. Based on an average estimate, more than 60 different minerals have Mg content of or over 20%. However, from commercial application point of view, only dolomite, magnesite, carnallite, brucite, talc, and olivine are crucial ones. Mg and its alloys claim their significant admiration over other metals (Al and steel) on the ground of being lesser dense (two-thirds the density of Al), lighter and having higher strength. From its existence point of view, naturally Mg is found only in combination with other elements. Therefore, extraction of Mg is of prime concern to cast it employable for manufacturing applications. Mg is extracted primarily by electrolysis of its salts incurred from brine, and is commonly employed as a constituent in alloys of Al-Mg, also named as magnalium or magnelium. China has emerged as one of most dominant producer and supplier of Mg, and is almost totally dependent on the silicothermic Pidgeon extraction process^[15] (oxide reduction takes place at very high temperature in presence of silicon, mostly obtained by a ferrosilicon alloy where Fe is presented as reaction spectator) to extract the metal. Several extraction processes have been dealt in details in past literatures^[16-18], thus authors limit themselves from its discussion. Pure Mg suffers some disadvantages limiting its application in pure form such as it is extremely prone to corrosion. However, when alloyed, its corrosion resistance properties can be improved. Mg is referred as Mg alloy when mixed with commonly used alloying elements such as Al, Zn, Manganese, silicon, copper, rare earth metals and zirconium etc. Therefore, depending upon the percentage of mixing elements, the Mg alloys are obtained in different forms and known by different names accordingly. Some of the commonly used Mg alloys are listed in the Table 2.

As there is not any internationally adopted system to designate Mg alloys, thereof the basic nomenclature delimitated by American Society for Testing Materials (ASTM) is taken into consideration. According to ASTM, first two characters will represent the chief constituents in Mg alloys such as A, Al; B, bismuth; C, copper; D, cadmium; E, rare earths; F, Fe; G, Mg; H, thorium; K, zirconium; L, lithium; M, manganese; N, nickel; P, lead; Q, silver; R, chromium; S, silicon; T, tin; W, yttrium; Y, antimony and Z, zinc. Further, it is followed by numbers which dictate the nominal compositions of these chief constituents' elements in weight percentage. For example AZ31 represents Mg-3Al-1Zn with the actual composition ranges being 2.5-3.5Al and 0.7-1.3 Zn^[20].

As Mg and its alloys have high potential to replace conventional materials, they can be used in applications where considerable mass reduction and high specific properties are expected. It is observed that on the ground of mechanical properties and weight reduction, Mg alloys stand out an effective substitute of Al as well as many other commercial metals (Fig. 2).

Overviewing splendid and unique properties of Mg alloys with its future prospects in several industries, there is an urgent need to compile and evaluate the critical findings for quick and complete reference from its perception to processing, machining to joining, application as well as economics point of view.This paper is an attempt to elaborate a pertinent review that comprehensively covers advantages, limitations, forthcoming barriers and recent technological wakeups in context to Mg and its alloys specifically addressed in manufacturing domains.

2 Magnesium Reserve

Mg is discovered in seawater, brines and in deposits of the earth crusts. Primarily three types of Mg ores are pointed out namely magnesite, dolomite and carnallite (Fig. 3). A large amount of dolomite and magnesium-bearing minerals is globally widespread, therefore possible exploration sites of Mg are very vast and even limitless. In Canada, Gossan resources have vast deposits of high-grade dolomite and silica sand that are primarily used in Mg extraction. China, ranking 2nd in magnesite resources claims 27 proven deposits mainly located in Liaoning, Shandong, Xinjiang, Tibet and Gansu. Amidst them, Liaoning accounts largest reserve equivalent to 85.6% of total reserve in China and renowned as a world's high quality magnesite site.

China is dependent on magnesite to a large extent and is the primary source of Mg extraction. A general estimation states that Mg-bearing brines are estimated to be present in the billions of tons, mostly along the world's coastlines (Table 3).

Russia, China and Korea represent domination in context to Mg reserves than others countries. An estimation spilled out during 2013 stated that, Russia as one of the biggest leader in Mg reserve accounted 27% (650 million tons), followed by China 21% (500 million tons), and North Korea 18.8% (450 million tons) of the total 12 billion tones world Mg reserve^[21]. Some of the famous Mg rich sites in the world are Dead Sea and the Great Salt Lake in Utah United States, South Australia, Queensland and Tasmania etc. Recently, Albanian Minerals seemed to have the world's largest and richest Mg ore mines and reserves with over 20 billion tons, with its present worth equivalent to trillions of dollars. This Mg site in Albanian Minerals mine's is known to be the finest in the world, with over 54% rich in Mg content.

3 Mg Production and Consumption

A tremendous rise in demand of Mg alloys is foreseen in last few decades after evolvement of novel and cost effective techniques of its processing, followed by rapid adoption by several automobile, aerospace and mobile applications companies. Consequently to cater monolithic industrial demand, a subsequent rise in Mg production is also evident in consecutive years leaded by China than the other nations (Fig. 4). It was observed that till the end of 2014, globalized Mg production reached to a level of 900 kpty which was only 600 kpty in late 2009. Admitting an example of automobile industry, the average share of Mg and its alloys in some of the models was drastically increased up to 26 kg per vehicle by the end of 2014, which was just 2.3 kg per vehicle till 2012^[24]. Furthermore, basic mobile phones, smart phones, tablets, computers, notebooks and other electronic devices comprise another large scale Mg alloys market shares.

China has come out to be a dominant share holder in Mg production, reaching to near about 4 000 tons per year out of the total 5 850 tons globalized production, which represents 68.38% of total Mg production market share. Fig. 5 shows the globalized Mg production divided in country wise where China is at apex followed by Russia and Turkey.

Overlooking the statistics of global Mg alloys consumption during year 2008 and onwards (Fig. 6(a)), it is evident that the Mg consumption graph is continuously rising owing to its foreseen sharp demand. Studies have revealed that during 2012, the globalized automotive Mg alloy consumption was 214 000 tons which reached up to 340 000 tons till the end of 2015, showing an increment of close together 4.9% each year^[26]. Automotive industry is looked to have major thrust area of Mg application covering 62% followed by electronics components near about 29% of its total consumptions (Fig. 6(b)).

Till 2007, the consumption of Mg worldwide including its production, import and export was estimated to 1 050 thousand tons which further declined by 11% and 15% in 2008 and 2009 respectively due to recession in global market^[24]. The volume of Mg consumption recovered to pre-crisis levels and reached a new peak of 1.1 million tons by 2013^[24]. China being a major producer and consumer of Mg has become a hot spot for global investments hub since last consecutive years.

Besides being a hub of manufacturing industry, China is also a top consumer of Mg approaching to 400 000 tons per year, which could be estimated roughly more than 40% of global Mg Consumption. During 2005, China had a share accounted to only 17.7%, which drastically gone up to 41.5% by 2013^[24], meanwhile in other countries such as EU and U.S, the trend was declining. A rough estimation on global scale states that, North America has near about 23% of Mg consumption while Europe accounts only 18%. Meanwhile Japan and Russia can also be termed as major players in this field with their aggregate accounts equal to 12%^[24].

Increased production rate to cater overwhelmed industrial demands with inclusion of cutting edge cost effective technologies has substantially lower down the Mg production cost in last few years. Beyond that novel methods of Mg processing and economical production solutions have further attempted to minimize the difference in cost of Mg alloy and Al to a greater extent (Fig. 7). It leads to an open and viable choice to industries to replace other metals with Mg without a brink of enhanced production cost of finished products. The cost of Mg alloy intimately depends on the price of Mg. In context to China market, typically the difference in price of Mg alloy to pure Mg for each ton comes out to be 10 000 yuan. In the period of 2004-2006, Mg price hovered in between 13 400-21 500 yuan while in 2007, it rose rapidly with an increment of 138.7% till the end of May 2008. Since April 2009, the difference in price of Al and Mg was lowering down unceasingly and both of them were reaching at approximately same price level (Fig. 7(a)). In context to global market, Mg price also seemed to have negative slope in most of the subsequent years (Fig. 7(b)) except during end of 2007 and early 2009, approaching to 1.94 USD per kg in the first month of 2017 from 3.4 USD per kg in 2012. This implies that advent of new technologies and economical production solutions of Mg processing have substantially lower down its price to a great extent and presented as a cost effective and viable substitute over other metals for application in numerous structural parts.

The International Magnesium Association (IMA), has forecasted about future Mg use expansion in its report, primarily because of a rise in production of rolled Al for automobile frames and bodies and Mg alloy components for the automotive industry. IMA's report depicted that the volume of Mg consumption may cross the limit of 1.37 million tons globally till 2020.

4 Application Areas

Mg and its alloys represent their widespread application and have been integrated in a number of structural parts in automobile, aerospace, mobile, sports and others. In last few years, a massive amount of Mg inclusion is evident owing to its splendid characteristics and comparatively lower overall costs, resulted due to technological wakeups in its processing. Furthermore application of Mg and its alloys is also vital on the point of significant weight reduction with enhanced fuel economy. Some of the major application areas of Mg and its alloys are grouped and discoursed as follow.

4.1 Aerospace

Mg has been employed on a large scale in aerospace industry for manufacturing a number of structural parts for both in civil and military purposes aircrafts for a long time. Mg and its alloys significantly reduce the aircraft weight, substantially bring down the fuel consumption and emissions as well as the operations cost too. Mg has already been implemented in manufacturing of several aircrafts parts such as frames, engines, and thrust reverser in the models of Boeing 737, 747, 757, and 767 as well as in helicopters. Besides, missiles and spacecrafts have also implemented Mg in its different structural parts overviewing its properties of lift off weight reduction, capability to withstand in high temperature environment and safety against impact of high energy particles. It has also been used in massive quantities in intercontinental ballistic missiles (ICBM's) such as Atlas, Titan, and Agena. With the invention of cutting edge Mg alloys, its application is drastically enhanced covering more volume in aircraft manufacturing such as gearbox casts, gearbox and engine components, wings, fuselage skin, door, wheels and undercarriage, cockpit instrument panel to dashboard and service panels, seat components and rudder pedals etc. (Fig. 8). A rough estimation states that, Boeing 727 aircraft generally employs Mg and its alloys in its more than 1 200 parts. Airbus claimed that replacing other metals with Mg to make cabin furniture in Airbus A330 substantially decreased the payload thus allowing another 3 more passengers to be boarded on a 295 seated aircraft than before.

A Japanese scientist, Professor Yoshihito Kawamura working in materials science at Kumamoto University together with his research team has invented a new Mg alloy which is nonflammable and has comparatively higher strength that could be implemented in aircraft construction and engines. Its ignition temperature is claimed above 1 117 ℃ which is higher than boiling point of pure Mg (1 091 ℃), and tensile strength is equal to that of super-duralumin^[37].

Next achievement was knocked by a research group in University of Manchester who developed a new species of Mg alloy claiming higher performance and comparatively lower density. This alloy was implemented by Magnesium Elektron (ME), to manufacture a number of aerospace and automotive parts with the aim to achieve enhanced product performance with reduced fuel consumption.

Several new Mg alloys have been developed or in verge of development to provide safeguard to the metal structure against corrosion and being implemented in large scale in latest models of helicopters and fixed wing military aircrafts, for example McDonnell Douglas MD500, Westland Lynx, F22 Raptor and Apache Mark-3 attack helicopter and the F35 Joint Strike Fighter (JSF). These alloys reduce aircrafts weight on a massive level up to 35% by just replacing to Al, not even so they also fulfill rigorous aircraft design guidelines to a greater extent. Manufacturing of these aircrafts on large scale is proposed and approximately 3100 F35 JSF aircraft are anticipated to be in service by 2035.

4.2 Automobile

It was 1920, when automobile industry began to adopt Mg in some of its structural parts, such as early age commercial vehicles Volkswagen Beetle containing an average of 20 kg of Mg alloys. Envisioning its overwhelming attributes with eye-opening strength to weight ratio, fuel efficiency, increased performance and sustainability and increased fulfillment environmental and legislative influences guidelines, led to its tremendous and widespread integration in numerous automotive parts as prevised in last few decade. A rough estimation suggests that 0.4 liter of fuel per 100 km is saved for each 100 kg weight reduction of vehicle^[32]. A number of steel components can be easily substituted by a single cast piece of Mg thereby increasing the component strength as well as reducing the manufacturing costs. Several renowned automotive giants such as Audi, DaimlerChrysler (Mercedes-Benz), Ford, Jaguar, Fiat and Kia Motors Corporation have already began the process of replacing steel and Al with Mg in various structural parts of their products such as gearboxes, steering columns, driver's air bag housings, steering and driving wheels, seat frames, fuel tank covers, engine and body components, cylinder head, sunroofs, pedal brackets etc.^[38]

Mg and its alloys have further enhanced their popularity as they can be easily processed by conventional metal forming processes^[39-40]. They can be cut, machined and reshaped easily by conventional metal working processes such as extrusion, rolling, press forming, drawing, pressing and forging^{[38, 41-44]}. Fig. 9 shows the innovative process developed by General Motors (GM) to manufacture light weight and corrosion resistant Mg sheets as metal structure panels which are being integrated into new aged vehicles to obtain weight reduction and fuel economy. In recent decades, trend to implement Mg alloys in several die castings parts has also in course.

Historically, Al alloys seemed to have top berth in Mg consumption, however during 2012 Mg end application was related to specifically in Mg cast alloys—reaching to a point of 33% of the total volume consumed globally (Fig. 10). The automobile industry, where Mg cast alloys are being used extensively for a number of structural parts, is the largest consumer of Mg cast components (Fig. 11). As an estimate during 2012, average share of Mg came out to be 2.3 kg/vehicle, trending up to 26 kg in some automotive models^[24]. Next onwards Ti sponges and steel desulfurization come in picture covering a volume equivalent to 123 000 tons or 11% and 119 000 tons or ~ 11% respectively of the total Mg consumed globally by volume during 2012^[24]. Mg is also been extensively used by other sectors such as Fe modularization and electrochemical protection. During 2012 the volume of Mg consumed by these two areas estimated to be approximately 65 000 and 60 000 tons, respectively.

GM's has taken on a novel thermal-forming process especially for Mg and its alloys that facilitates production of thin structural panels using lightweight materials. Such a process demands heating Mg blocks up to 450 ℃ before its molding^[50]. It is notified that low-cost and high-efficiency thermal forming significantly cut down the overall manufacturing cost also well-compensate the increased raw material cost.

Mercedes-Benz used Mg alloys to frame car bodies of its early age model Mercedes-Benz 300 SLR and has successfully participated in several world-class car ace events at Le Mans, the Mille Miglia, and other places. Porsche applied Mg alloys in the frames of its renowned car models 917/053 that has won Le Mans in 1970, besides company also integrated it into mainframe of engine block to obtain considerable weight reduction. Volkswagen has been implementing Mg made components into its car models since last many years. Volkswagen XL1, developed in 2014 was well known fuel-efficient car in the world used Mg alloys to cast its body frames. Mg built this car claimed to do a staggering 313 miles per gallon.

Mitsubishi Motors employed Mg alloy to cast its paddle shifters. BMW, another automobile giant put on high temperature Mg alloys (AJ62A) for its N52 engine which has been included in several 1, 3, 5, 6, and 7 and in Z4, X1, X3, and X5 series models worldwide (Fig. 12). First of its kind plug-in hybrid vehicle technology developed by a German automaker, BMW i8 coupe is a revolutionary replica of BMW (MILAN:BMW)'s hallmark which is proven to be excellent in rigorous vehicle safety standards and provides comforts riding employed Mg for a number of components making itone of the lightest vehicle in the world.

Renault SA (PARIS: RENA) expanded Mg application in car ceiling which got its weighs only 4 kilograms, besides body aerodynamic efficiency was bettered by 30% compared to the Clio. A mixture of steel, light alloy and Mg has also been implemented in Mercedes-AMG GT S model to cast its body frames.

Referencing a report of United States Automotive Materials Partnership (USAMP)^[54], where it is stated that by the end of 2020, 350 lbs of Mg will replace 500 lbs of steel and 130 lbs of Al per vehicle, consequently gaining an overall weight reduction equivalent to 15%. It is also mentioned that Mg integration would lead to considerable fuel savings approaching to 9%-12% with a significant drop in CO₂ emission, thus reducing harmful impact on global warming without any major alterations in design. As a matter of fact, Mg is refereed as a green metal of the 21st century in several texts.

4.3 Electronic

The current trend of electronics market strives to get small, lightweight, portable and cost effective electronics gadgets. As Mg is a unique metal fulfilling most of current market expectations is thereof chosen as a key element in electronics goods manufacturing and thus making Mg-made products sought after in global electronic market. Mg also delivers better heat transfer and dissipation characteristics as well as electromagnetic and radio frequency interference shielding, thereby provides an alternative to replace plastics and Al components with Mg on a large scale to achieve enhanced durability, lightness and portability. Several electronics components/casings require casting complex shapes which is quite less troublesome with Mg. Cameras, cell phones, laptops and portable media device housings etc. are common illustrations of Mg applications (Figs. 13-14).

Recently Apple has implemented Mg in manufacturing of iBook laptop frames with the aim to reduce its weight. Dell notebook 14 Rugged Extreme has casted a number of its components using Mg alloys such as chassis and upper casings. Microsoft has developed Pro 3 as a 2-in-1 hybrid device by using a premium Mg-alloy material which houses a sublime 12in display (2 160 ×1 440) with a density of 216 pixels-per-inch (ppi). Lenovo LaVie laptops bring NEC's super-light laptop designs using super light Mg lithium alloys in throughout its chassis. Samsung launches a new revolutionary NX1 smartphone model employing cutting edge technology integrated with Mg making it dust and water resistant with substantial weight reduction.

4.4 Others

Application areas of Mg alloys are not limited only up to aerospace, automotive and electronic domains even it has proven its viability going beyond that and therefore now a days countless products are using Mg as a chief constituents by leaps and bounds, for example in sports goods, two wheelers, safety instruments, fireworks sparklers, primary and secondary batteries, dry-cell battery walls, hand held instruments and spectacles etc. (Fig. 15). Among them, sports industry is ranked top as one of the major consumer of Mg alloys attributing the reason of its high in demand, light in weight and astonishing impact resistance property. Mg metal is very suitable to form into intricate shapes, which is an extremely vital virtue to golf clubs, tennis rackets and the handles of archery bows.

Furthermore its excellent damping and vibrations absorption characteristics make it a unique metal to be applicable in bicycle, motor cycles frames and the chassis of in-line skates. Shock and vibrations absorption are vital properties to have a comfortable and smooth riding for riders. Mg is also used for several acoustic and optical instruments such as frames of spectacles and lenses due to lower density, stands greater column loading per unit weight and has a higher modulus.

A group of German researchers leaded by Maximilian Fichtner and Zhirong Zhao-Karger has invented a new promising electrolyte using Mg and its alloys, which are supposed to unfold the pathways to develop an entirely new generation of batteries. Korea Institute of Science and Technology (KIST) has successfully invented and tested the Mg-Air battery technology claiming its charging time as just 10 min which is currently being implemented on large scale in a number of car models. Toyota has announced to employ Mg-ion batteries for its vehicles developed by KIST, claiming energy density 5 times higher than lithium-Ion batteries. Doron Aurbach Bar Ilan University has developed Mg ion based new technology device which renders more power compared to lithium-ion batteries. Recently formulated Mg based batteries would be comparatively lighter, and last for 100% longer than currently used ones. Toyota (NYSE:TM) and Toyota's Prius have implemented Mg-sulfur batteries replacing lithium-ion batteries for its electric cars anticipating to deliver more power than other one.

5 Processing of Mg Alloys

In last few decades, a tremendous rise in demand of Mg and its alloys is foreseen by quite a large number of industries promoting a thrust to develop superior, more advanced and cost effective Mg alloys to furnish a better substitute of Al, Fe or other metals. Consequently it has drawn a great attention from research community to evolve cutting edge and cost effective solutions fulfilling industrial needs. A rapid rise in unlimber of research articles on Mg and its alloys is evident in last few years (Fig. 16) proving its widespread implications. However, still a significant research gap exists in some of vital areas associated with its processing, alloy development, joining, surface treatment, corrosion resistance, and mechanical properties improvement etc.

5.1 Alloying

As of the mid of 2013, consumption of Mg and its alloy was less than one million tons per year, compared with 50 million tons of Al alloys. Its application in several structural parts was not favorable due to its tendency to corrode, combust and creep at high temperature^[58]. Therefore attempts were made to integrate some alloying elements to improve its properties and mitigate its limitations. Alloying is the process of mixing some foreign elements (alloying elements) into the parent metal to enhance its properties.

Two primary causes of its poor corrosion resistance are foreseen^[59] firstly, second phases or impurities presented in Mg cause internal galvanic corrosion^[60] and secondly, the hydroxide film on the Mg surface is quite less stable compared to the passive films which are usually formed in stainless steels and Al. Physical properties of Mg can be altered depending on the type and amount of added constituents which are of paramount vital for its integration in several automobile, aerospace and electronic structural components. Table 4 shows a comprehensive description of each Mg alloying constituent and their outcomes. Several automotive components require good amount of ductility as well as strength especially for those items who are attached to head on collision during an accident. The alloy development abides by a series of requirements therefore accordingly grouped to provide certain desired properties (Fig. 17).

5.2 Casting

Broadly speaking, Mg alloys represent satisfactory amount of castability however when integrated with Al as a chief alloying component exhibits superior casting properties thus making unique and implacable in diversified casting processes for example in pressure assisted casting, warm and cold chamber high pressure die casting, gravity casting, squeeze casting, thixo-casting and thixomolding etc.^[66] Amidst all, die casting is most favorable method subjected to Mg alloys and is widely adopted by aerospace, automotive, mobile and electronic goods industries. Die casting poses the virtue of substantial weight reduction with impressive damps in the noise and vibration, enhances service life of finished part and improves percentage elongation which are predominantly crucial for several engineering applications, however higher cost associated with the rare earth metals restricts its enhanced application^[67]. Recently a new species of dispersed Mg alloy is developed within reasonable cost limit having additional casting properties thereby making it viable for elevated temperature applications^[68]. Mg alloys debased with zirconium, rare earth metals and Zn are best suited for applications with temperature range 250-300 ℃^[69-72].

Castability increases with rise in Al content owing the reason of improved fluidity in context to Mg alloys, besides excess Al alleviates the probability of shrinkage micro porosity due to an increase in freezing range. Zn is another optional element to enhance fluidity, however, excess Zn contents also lead to hot cracking and porosity. The diecastability of the Mg-Al-Zn is evinced in Fig. 18. It is evident that Mg alloys with 10% Al and less than 2% Zn favor diecastability.

5.3 Forming

At room temperature, formability of Mg is not found up to the desired level thereby restricting its applicability in numerous industrial applications. Lower formability is attributed because of hexagonal crystal structure as well as low tendency to twinning only which permits only minor deformations. The crystallites orientated in different directions represent the distortion on separate base slip plane which is the main cause of mutual slip hindrance^[74-75], besides at elevated temperatures (230 ℃-370 ℃), it can be utilized in most of conventional processes^[69] because of the thermal activation of pyramid sliding planes in the hexagonal structure^[76]. Several authors have confirmed that the formability (drawability) of Mg alloys is drastically improved at elevated temperatures^[77-78]. Lee et al.^[79] concentrated their research on a gas forming process to look into the formability behavior of Mg at various interval of elevated temperatures. They concluded that 200-300 ℃ is optimum temperature range for deep drawing of Mg alloy. The rise in ductility while working at elevated temperature can be attributed to the fact of increased slip system and deformation twining^[80]. Hsu et al.^[81] adopted Hill's quadratic yield criterion associated with a power-law hardening model to predict and confirm the forming failure during hydroforming process. A far-flung use of forming technologies for the processing of Mg alloys is limited because of insufficient knowledge about the forming technologies and suitable optimized process parameters to have a beneficial finished product^[82]. To evaluate formability study of Mg alloy, its deformation properties and material's characteristic such as anisotropy or flow curves need to be analyzed^[83].

Fig. 19 shows the flow curves for Mg alloy AZ31B at different temperatures obtained by exercising uniaxial tensile test as per the test standard EN 10002. It is evident that the forming temperature plays an important function to determine stresses and possible strains. The cause of reduction of the flow stresses for a temperature range above 200 ℃ could be imputed to temperature-dependent relaxation^[84].

A detailed formability study of Mg alloys suggests that there is very limited experimental and theoretical research work on the elaborated look into the forming limiting diagram (FLD) of Mg alloys especially at elevated temperatures. While FLD is nowadays seen to be a commonly used tool to predict formability index for evaluation of the material formability in the industry^[81], could be hotspot for the researchers and requires a greater dig out.

6 Welding and Joining of Mg and Its Alloy

Welding/joining of Mg alloys exerts an unfathomed consequences on its far-flung diligences, expansion, especially for transportations vehicles where a number of large-size, complex components are integrated to form a structure. The joining is profound to make between different grades of Mg as well as with other metals. Besides reason to have outstanding properties such as anti-knocking, high conductivity, electromagnetic shielding and non-toxicity^{[39, 85]} with improved strength to weight ratio than other metals in addition to that its geometrically increased demand in several industries made it extremely prominent to employ a reliable and robust method to join these reactive metals together as well with others. As Mg alloy has a great affinity toward oxygen, therefore it may require a protection cover of neutral gases to safeguard the weld pool. Oxygen also forms a thermodynamically stable oxide layer on Mg surface that is prone to make the weld to suffer from severe anomalies^[86]. A successful and defect free joining with Mg alloys requires well defined optimized parameters under proven environmental conditions. In due course of time, both fusion welding and solid state welding techniques are employed to join varieties of Mg alloys with similar and other alloys to satisfy industrial standards such as laser welding^{[7, 87-91]}, linear friction welding^[92], metal inert gas (MIG) and tungsten inert gas (TIG) welding^[93-96], hybrid welding^[97-99], diffusion welding^[100-103], friction stir welding, etc.^[104-109]

Welding of Mg and Al is critical as well as vital to build a compound structure which can be applied to reduce the weight and cost of the component^[110-111]. However it was noted that welding of Mg with other metals is quite difficult and suffers from several shortcomings (Table 5), such as lower strength, defects especially for Al and Mg, attributing the chief cause of inter metallic layer (IMC). Several attempts are made to counter the IMC layer to improve mechanical properties of weldment^{[105, 108, 113]}. In this section, authors limit themselves discussing only authoritative and widely adopted joining techniques out of a large pool of joining methods followed for Mg alloys.

6.1 Fusion Welding

Fusion welding is a category of joining processes that relies on melting of the base metals up to their melting point temperatures^[114]. In general, fusion welds show a wide heat affected zone (HAZ) due to the high-temperature phase transitions. However, some techniques such as laser or electron beam welding minimize the HAZ due to their high density of power and fast welding speed. Though a number of fusion welding techniques are in use to join Mg alloys, the authors limit themselves discussing only prominent, effective and industry adopted welding methods in detail.

6.1.1 TIG and MIG welding

Various automotive industries are applying TIG and MIG welding processes to join similar and dissimilar Mg alloys, besides they are also employed for the removal and repair of casting defects. Both being gas shielded arc welding processes, TIG uses non-consumable tungsten electrodes while MIG employs consumable electrodes (wires) during welding.

Wang et al.^[94] studied about Al-Mg dissimilar welding produced by cold metal transfer (CMT) MIG welding method. They did not observe any obvious weld defects in the weld bead. Addition of Si to the weldment and super low heat input during welding process substantially inhibited the formation of the brittle intermetallic compound. The micro-hardness in the vicinity of the fusion zone at Mg side was higher than the weld metal and the Mg substrate (Fig. 20) owing to the influence of thermal cycle on HAZ formation. It showed substantial increase in the strength and the deterioration of the ductility of weld bead^[94]. It was noticed that the MIG welding was not only suitable for a better gap bridging but also had the ability to make up for the lost alloying agents. Controlling of process parameters in MIG welding is quite easy. Moreover, it claims higher efficiency compared to other fusion welding processes. Additionally, MIG welding is proven to be very suitable to weld thick metallic plates.

Wohlfahrt and Rethmeier^[115-117] applied a triggered short arc to obtain weld beads of Mg alloys. They concluded that the strength of AZ31 weld beads was near about 81%-93% of the parent metal. Moreover, the fatigue strength seemed to be of the order of 50%-75% compared to base metal. Ueyama et al.^[118] practiced pulsed MIG welding to obtain Mg alloys weld beads and observed that the tensile strength of the welded section was approximately 91% of the parent metal. Zhang et al^[119]. used MIG process with Zn foil as an interlayer to obtain Al/Mg lap joints. They observed that presence of Zn foil as the interlayer restrained the reaction between aluminum and magnesium and the macroscopic cracking. After using Zn foil, the presence of Al-Mg compounds is not found besides the tensile strength of the lap joint is also improved.

Besides the MIG welding, TIG welding of AZ31 Mg alloy is widely utilized for joining metals due to its easiness in operation and low cost^{[96, 120]}. Liu et al.^[95] evaluated the microstructure characteristics of Mg/Al joint using TIG welding. They observed an obvious fusion zone lying between weld metal and Mg substrate. Mg substrate near to the fusion zone seemed to be predominately affected by the welding thermal cycle. A close view of the micro-hardness reveals that it is of the order of HM 275-300 near the fusion zone of Mg side, HM 60-100 at the weld metal and HM 160-200 at the fusion zone of Al side (Fig. 21), which represents a clear evidence of the brittle phase near fusion zone having higher hardness. Liu et al.^[121] conducted TIG welding of AZ31 Mg alloy with filler, and concluded that the grain size in HAZ had significant variety as compared to no filler TIG welded joint, due to which the fracture location during tensile testing got shifted, besides the ultimate tensile strength of welded joint was also quite improved.

TIG and MIG seem to have some issues such as low welding speed and high heat input which result in severe defects in weldment such as high residual stresses & distortion and wide HAZ, which creates a thrust to search alternates of them lead to development of laser welding^[88]. Some of the major advantages of laser welding over TIG and MIG welding are high welding speeds, much increased energy density, narrow HAZ, comparatively less distortion, and no slag or spatter etc.^{[3, 122-123]}

6.1.2 Laser welding

Laser welding is a non-conventional technique to join similar as well as dissimilar metal parts by using a laser beam as heat source. The concentrated laser beam permits deep and narrow welds with considerably high welding speed. This technique can be used to join wide variety of metals. CO₂ or Nd:YAG laser beam has been primarily used to join similar and dissimilar Mg alloys^{[90, 124]}, and appeared to be a better option than other fusion welding methods. Quan et al.^[125] applied continuous wave CO₂ laser to weld AM60 alloys and observed a narrow HAZ without any significant grain coarsening. High density of Mg₁₇Al₁₂ precipitates with fine grains are found in fusion zone with comparatively higher hardness^[125]. During CO₂ laser welding of AZ31 wrought alloy, pore formations took place in the weld across the fusion boundary^[124]. This can be attributed due to the hydrogen rejection and surface taints from the solid phase during solidification of weldment. It also state that the collapse of the keyhole and turbulent flow in the weld region is primary cause of porosity formation. Fig. 22 show some typical defects occurred in laser welding of AZ31 due to non-optimized process parameters. Weisheit et al.^[89] also carried out the CO₂laser welding of several commercial Mg alloys. They depicted that most of the Mg alloys were easily welded without suffering any serious defects, and the joints had sufficient strength and narrow fusion zone and HAZ. Only high pressure die cast AZ91 and AM60 alloys showed quite high porosity and cracks due to gas escaping that was entrapped into material during die casting process. Leong^[126] looked into the feasibility of CO₂ and pulsed Nd:YAG laser welding of AZ31 alloy, and pointed out the troubles of CO₂ welding with respect to Nd:YAG laser. He made a statement that CO₂ laser was not proved effective to weld Mg alloy because of its high beam irradiance and high oxidation potential, but no such problem was evident in case of large size Nd:YAG laser beam.

During investigation of porosity in AM60, Zhao et al.^[127] found that main cause of porosity in the weldment was the presence of hydrogen in the parent material which could be remedied by its re-melting. Abderrazak et al.^[128] proposed an analytical thermal model for welding WE43, which was used to determine the relation among the bead width/penetration depth, the input laser power, and welding speed. Experimental verification for the predicted keyhole formation and the weld bead geometry was performed^[90]. The large reduction in laser power leads to lower surface power density and change of the welding mode from partially penetrated keyhole to the conduction mode. Nonetheless, by using the optimized parameters as well as following the appropriate process conditions, defect free welds with required surface quality and low porosity can be achieved^[129].

Laser welding of Mg and its alloys seemed to have some shortcomings in terms of unstable weld pool, extensive spatter^{[4, 91, 126]}, far dropping from weld pool^[91], sag of the weld pool (for thick metals), undercuts^[130], inclusions of porous oxide, noticeable loss of alloying elements^{[4, 126]}, enlarged pore formation (particularly for die castings)^{[7, 127]}, cracking during liquation and solidification^[131]. Past literatures manifest that weldability of Mg die-castings, especially non-vacuum die cast parts, was influenced by gas content^{[3, 91, 122-123]}, since the presence of gas had been primarily responsible for creating several pores in weldment and could also lead to explosions in some worst cases^[129-130].

Mg alloys were reported to have higher absorptivity to the near-infrared radiation of the Nd:YAG laser than that of far-infrared CO₂ laser beams. As a consequence, there is a lower threshold irradiance requirement to make a keyhole with better process stability^[4]. Nd:YAG laser seemed to have improved consistency in weld strengths regardless of the nature of shielding gas employed^[132]. Besides lamp-pumped Nd:YAG lasers also seemed to have limitations in terms of lower wall plug efficiency (1%-4%) and poor beam quality.

6.1.3 Hybrid welding

In context to welding of Mg alloys, several techniques such as MIG, TIG, LBW, etc., have already been discussed in past literatures^{[89-91, 95-96, 115, 118, 133]}. However, it is evident that none of them is ideal, and most of them have their own drawbacks such as the energy efficiency of LBW is quite less because of high reflectivity of Mg alloy, while in case of TIG welding, higher heat exchange ratio leads to spread the weld bead, resulting into grains coarsening. The trend of integration of multi-welding heat source, named as hybrid welding, is quite in vogue and seemed to accomplish Mg welding requirements up to a greater extent. Consequently, quite a large number of research articles based on the implications of hybrid welding techniques on Mg alloys have appeared in last few decades. Some of very popular hybrid welding techniques employed to join Mg alloys are laser-TIG (LATIG) techniques, MIG-CO₂ laser hybrid welding, and MIG-Nd:YAG laser hybrid welding, etc.^{[97-98, 134-139]}. Hybrid techniques claimed to have superior outcomes over conventional techniques in terms of increased joint penetration, greater arc stability and economically cost effective^{[99, 140]}. Wang et al.^[99] employed LATIG adhesive bonding technique for joining Mg and Fe. They summarized that Mg and Fe were successfully joined via a Ni transition layer. The adhesive provided a conductive joining mode, which enhanced the diffusion among Mg, Ni and Fe. Fig. 23 evidences a transition area between Mg and Fe consists of Mg, Fe and Ni elements without any Al content. It depicts that adhesive and Ni elements primarily influence the interface between Mg and Fe. To evaluate elemental diffusion, EPMA analysis is performed (Fig. 23, (b)-(d) which reveals the average spreading of Ni elements covered by Fe elements at center. Further it is seemed that the penetration depth and arc stability are improved particularly at high rates of travel and low TIG current^[141].

It is observed that the electric arc changes the electron density of laser plasma, which in turn reduces the absorption and reflection ability, thereby heat efficiency of incident beam is quite increased^[142]. Researchers also implemented the neural network modelling to predict the shape of the weld pool and evidenced that the penetration was a sensitive function of the laser power^[143-145]. MIG welding integrated with laser arc also claims several advantages in terms of arc stabilization at weld pool surface, burning arc stability at low currents and increment of coefficient of absorption etc. These characteristics substantially bettered the productivity and arc welding stability, reduced the overall application cost. In addition, filler metal is also allowed for filling of seam gap^[146]. Sun et al.^[147] studied about TIG-laser (CO₂ or pulsed Nd:YAG) hybrid welding of AZ31 alloy. They observed that sound welds can be produced with TIG welding alone, but such welds suffered from coarser grains thereby affecting the welds properties. Fine grains weldment structure is evident in case of laser welding, but welds in Nd:YAG laser welding seemed to have higher oxygen content and more number of cracks. In a similar attempt on Mg alloys, Liu et al.^[97] investigated about the susceptibility to solidification cracking. They resulted that in case of laser welding the welds did not suffer from hot cracking for a wide range of parameters, while TIG welding suffered to have some metal loss which was not associated with laser welding. Simultaneous integration of both TIG and laser beam seemed to have α-Mg and Mg₁₇Al₁₂ in fusion zone.

Fusion welding is prone to generate porosity in welds and thermal distortion. Thereby, many alternative methods are being attempted these days such as adhesive bonding, brazing and soldering and mechanical fastening. These methods encountered various shortcomings, such as holes are left during mechanical fastening, deep surface preparation is vital for adhesive bonding etc. allowing their very limited industrial application. Besides, solid state welding methods are widely adopted by a number of industrial operations owing to their virtue to join difficult to weld metals and alloys with considerable enhanced strength. They are proved to be beneficial in minimizing the formation of brittle intermetallic compounds that were responsible for deteriorating joint properties.

6.2 Solid State Welding

Solid state welding is termed as a group of welding processes which produce coalescence at a temperature quite below the melting point temperature of joining metals without addition of any filler metal. Some of commonly used solid state welding processes are diffusion bonding, cold welding, ultrasonic welding, hot pressure welding, roll welding, explosion welding, friction welding and friction stir welding etc. In these processes, pressure, temperature, and holding time alone or in combinations bring out coalescence at joining metals faying surfaces without any significant melting. In subsequent sections, authors limit themselves to discuss only most prominent solid state welding methods from industrial application point of view.

6.2.1 Diffusion bonding

Diffusion bonding/welding (DB/W) is a solid-state joining process employed to join similar as well as dissimilar metals. The principle of DB/W is based on the solid state diffusion, where atoms of two similar or dissimilar surfaces diffuse into each other at elevated temperature (nearly 0.5 to 0.7 times the melting point temperature of the metals) with sufficiently high pressure (Fig. 24).

Initially this process was intended to join thin metal foil/sheets, metal wires and filaments. Further it was advanced to widen its applicability for joining high strength and refractory metals primarily employed in aerospace and nuclear industries. DB/W technique has also been applied successfully for joining of dissimilar metals such as Mg and Al alloys without suffering any serious defects as foreseen in fusion welding such as weld inclusion of refractory oxides, severe thermal cracking, formation of brittle intermetallic etc.^[101] During DB/W, selection of optimum parameters such as bonding pressure, bonding temperature and holding time etc.^[149] is extremely vital to achieve high quality welds^[150]. Literature analysis indicates quite less number of research articles on DB/W of Mg alloys depicting a wide research gaps seeking prompt attention^{[102-103, 151-156]}. Available research articles seemed to focus only on some basic areas such as phase formation analysis, microstructure prediction, interfacial micro-hardness as well as their subsequent impact on bonding strengths^[157].

6.2.2 Friction stir welding

Since the invention of friction stir welding (FSW) process (in 1991 at The Welding Institute UK), it has been applied in a number of diversified fields on metal processing in a very short time and globally adopted by numerous branches of manufacturing and engineering. Being a solid-state welding technique, it is proved to be a milestone to weld similar as well as dissimilar metals and even the nonmetals.

The mechanism of FSW is quite simple. A non-consumable rotating tool with a specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be joined and traversed along the line of joint. The heating is accomplished by friction between the tool and the workpiece and plastic deformation of workpiece. The localized heating softens the material around the pin and combination of tool rotation and translation lead to movement of material from the front of the pin to its back, thus making a solid state joint.

Due to its overwhelming and outstanding outcomes over conventional welding processes such as easy to use, no hazardous fumes, energy saving, less distortion, fewer or no defects, enhanced mechanical properties, environment friendly, improved aesthetics in weld appearance, and still counting, it has been widely acquired in numerous commercial applications in aerospace, railway, maritime, automotive, electronics and many more other sectors. According to a market research report recently brought out by Credence Research, Inc., globalized FSW market growth is expected to rise during the period of 2016-2024^[158], reaching up to 887.8 US$ Mn revenue till the end of year 2024. Such market growth of FSW on year to year basis is attributed to the fact that a number of industries have replaced or on the verge of replacing conventional welding methods with FSW processes to cater current technological market challenges^[159]. Fig. 25 shows the predicted revenue on year to year basis from 2014 onwards and the FSW market by segregating it in terms of equipment type, end-use application, and geography. Fig. 26 depicts some of the primary application areas where FSW technology has been adopted to manufacture a number of components by several industrial giants such as Apple, Ford, and NASA etc.

During FSW of similar and dissimilar metals, weldment seemed to have severe plastic deformation creating dynamic recrystallization (DRX) which permits the material to flow in the solid-state by sliding between recrystallized, equiaxed, and refined grains in the weldment^{[104-105, 165-167]}. Through conventional means, joining of Mg alloys is quite difficult due to chances of cracking, expulsion and void in the weldment^{[121, 168]}. Being a solid state welding technique, FSW is proved to be more efficient and effective in joining of Al, Mg^[169-171] and other alloys as well as in their combinations.

Wang et al.^[172] has reported improved hardness and grain refinement during FSW of AZ31 Mg alloy, while Hirano et al.^[173] claimed comparatively reduced hardness of the FSW weldment and smaller grain size than that of the parent metal for the same alloy. Yan et al.^[174] investigated FSW of AZ31Mg/1060Al where they did not observe any crack in the stir zone (SZ). Moreover, no metallurgical defects are evident for a wide range of tool rotating speed ranging from 1 000 to 1 400 r/min. They did not report any pattern of onion rings in SZ which was sometimes reported by other researchers in FSW of similar alloys^{[169, 175-176]}.Rao et al.^[107] studied about effect of process parameters on friction stir linear welding (FSLW) of Al to Mg. They stated that weld integrity and lap-shear strength are function of tool rotation speed as well as traverse speed. Optimum parameters, 1 500 r/min tool rotation speed and 50 mm/min or 75 mm/min tool traverse speed yielded good weld strength (Fig. 27). Similar results have also been reported by same authors during FSSW of Al to Mg^[177].

Firouzdor et al.^[178] investigated about joint configuration effects on weld strength during FSW of AA6061 and AZ31, and proposed that butt weld configuration resulted in higher weld strength than that in lap configuration. In addition, the weld strength can be further improved when Mg and Al are placed at advancing side and retreating side respectively as it provides enough frictional heat input to inhibit the formation of IMC at the interface and defects are reduced. Several other researcher have also documented defect-free welds with improved mechanical properties during FSW of different Al and Mg alloys^{[108, 113, 179-180]}. Fu et al.^[108] obtained sound FSW welds of AA6061T6 and AZ31B Mg alloy with tensile strength of weldment reaching up to 70% to that of Mg base metal under optimized parameters. They also summarized that the welding speed has a greater impact on x-axis torque than that of rotational rate (Fig. 28). As evident, spindle torque decreases when tool rotation speed increases and it seems to increase a little, when traverse speed increases due to the varied heat input. Ahmad et al.^[181]studied thermal history and microstructure analysis of Al-Mg alloy and resulted grain refinement in the SZ attributing the continuous dynamic recrystallization. Further peak temperature atSZ seems to decrease with increase in welding speed as a consequence of reduction of grain size at SZ. Thermal plots show the asymmetry of temperature distribution with respect to the weld line posing higher temperature fields at the advancing side (Fig. 29). It is also visible that as welding speed increases, this asymmetry also increases. Asymmetric temperature distribution in FSW has also been reported by other researchers through numerical solutions^[182-183].

Some researchers have also been valuated friction stir spot welding (FSSW) technique, apart from FSW, to join Al-Mg alloys, and they indicated about the IMC formation in the SZ seriously affecting joint properties^{[109, 184-185]}. Furthermore, some of the studies are also reported on increasing the bonded area of the nugget by employing enhanced FSSW processes such as bonding-FSSW^[186] and preheating FSSW^[187]. To counter effects of IMC layer, addition of a separate metal layer (such as Zn, Cu, Ni, or Ti) is also in vogue between Mg-Al surfaces during FSSW^[188-191], which seems to have a promising result to mitigate its undesirable effects^[188-191]. The typical stages of microstructure evaluation during FSSW are divided and named into subsequent steps (Fig. 30(a)) as plunge, rotation, and dwelling further categorized in zone Ⅰ, zone Ⅱ and zone Ⅲ (Fig. 30(b)). Zone Ⅰ determines the keyhole formation at interface due to destruction of bonding between Mg and Al substrates. In zone Ⅱ, most Zn gets diffused into Mg substrate with small quantity at Al substrates. In zone Ⅲ, diffusion of Zn coating and Mg substrate occur due to shoulder effects, as a consequence brazed bonding zone seems out (Fig. 30(c)). Finally Zn coating makes reaction with Mg-Al substrates, thereby reducing chances of Mg-Al brittle IMC (Fig. 30(d)).

Sato et al.^[192] pointed out constitutional liquation that took place in the intermetallic compound Mg₁₇Al₁₂in the weld center. During lap shear strength and fatigue analysis for Al-Mg alloys, Chowdhury et al.^[193] noted that Al/Mg dissimilar weld has lesser lap shear strength, failure energy and fatigue life than both Mg/Mg and Al/Al similar welds due to hard IMCs facilitating an easy fracture path. Further, they studied about the effect of tool pin thread orientation (right-hand thread (RHT) and left-hand thread (LHT) during FSW of AZ31B-H24 Mg butt joints^[194]. They observed improved fatigue strength with LHT pin tool in the clockwise rotation. Reason is attributed to the fact of reduction in weld porosity. Fatigue fracture taking place between TMAZ and SZ (independent on AS or RS) also depends on pin tool thread orientation. They observed sound FSW joints with improved mechanical properties for LHT pin tool with clockwise rotation^[195]. Cao et al.^[196] also reported some defects at the bottom of the welded joints while using a RHT pin having clockwise rotation. They noted twice the hardening capacity of the FSWed Mg alloys than that of the base metal^[197]. Nagasawa et al.^[198] stated considerably smaller grain size during FSW of AZ31B without any notable difference in hardness profiles at SZ and base metal. However, notable reduction in SZ hardness because of bigger grain size during FSW of AZ31B-H24 alloy is also reported by Lee et al.^[199]. Tensile strength seemed to decrease with an increase in the welding speed^[200] and increase with an increase in rotational speed for same alloy^[201]. Researchers have affirmed that the tool rotation speed as well as welding speed have a significant effect on amount of heat generation, heat dissipation and cooling rate^[202-203]. Forcellese et al.^[170] correlated forces and temperatures to predict mechanical properties of AZ31 Mg alloy. They proposed that ultimate tensile strength (UTS) and ultimate elongation (UE) exhibited peak values when vertical force was minimum and welding temperature was maximum. It was reported that UTS and UE were decreased with an increase in welding speed for a constant rotation speed while both were increased with an increase in rotation speeds for a constant welding speed (Fig. 31). This behavior can be ascribed as the enhanced material flow at the SZ while at higher rotation speed and vice versa for welding speed.

Luo et al.^[204] attempted dissimilar FSW of Mg-Zn-Gd and Mg-Al-Zn alloys. They observed that the materials positions, grain size and crystallographic orientation highly affect the mechanical properties of weldment. Micro-hardness as well as tensile strength of ZG61/AZ91D weldment seemed to be considerably higher than AZ91D/ZG61 (Fig. 32). However, they did not find any IMCs layer at the weld interface of both alloys. Table 6 gives important outlines of past literatures reported on FSW of Al-Mg.

Benefits of ultrasonic in metal workings and joining are well known and have been sufficient documented in past literatures^[233-236]. As a novel technique, ultrasonic vibrations are being implemented in conjunction with the friction stir welding/processing (FSW/P) to enhance material flow across the pin to obtain enhanced mechanical properties and sound weldments. A good number of research articles have been reported and stacked on ultrasonic vibration integration in FSW/P on similar and dissimilar Al and other alloys^[237-242]. Ji et al.^[231] studied about ultrasonic vibrations on FSSW (UAFSSW) of Al to Mg alloys and stated that enhanced material fluidity due to ultrasonic vibrations increased the dynamic recrystallization, as a result the average grain size at SZ was decreased from 8 to 4 μm. However, effect of ultrasonic vibrations on FSW of Mg alloys with similar and dissimilar metals is very few in open literature^[231], thus leaving a wide area of spectrum for forthcoming researchers.

7 Summary

Magnesium and its alloys acknowledged as metals of 21st century are the one of the major breakthroughs of this era. Rigorous environmental guidelines to cut CO₂ emission, fuel economy, ease to cast and form conventionally, reduced vehicle weight and cost effects are principal and chief majors of adoption of Mg alloys, making it stand out of the crowd of the remanufacturing metals. There is tremendous scope for application of Mg and its alloys for establishing cost effective manufacturing solutions for relatively all types of industrial applications from automotive, electronics, aerospace, and sports etc. Reports predict that by 2020, Mg consumption may cross the limit of 1.37 million tons in global scale. Recently developed Mg based alloys are renowned to be World's strongest and lightest metal and have potential to transform the manufacturing scenario of the world.

It was evident that open literature lacks in several aspects in context to Mg from its processing, alloy development, joining, surface treatment, corrosion resistance and mechanical properties improvement. It is observed that these domains do not get proper and in-depth consideration by a large community of researchers and therefore needed to be amplified further on urgent footing by suitable research and developments methodologies.The non-conventional techniques for processing, machining and welding of Mg are yet its initial stage of exploration and demanded further research and development. Novel and innovative methodologies are extremely vital from its production to application to make it cost effective and economically viable alternative metal for enhanced and globalized application in automotive, aerospace, electronic as well as other industries.