Welding of magnesium and its alloys: an overview of methods and process parameters and their effects on mechanical behaviour and structural integrity of the welds

An overview of welding methods and process parameters and its effects on mechanical behaviour and structural integrity of magnesium and its alloys are discussed. These alloys are less dense and beneficial structural alloys for improved energy efficiency, eco-friendliness and driver of circular economic model for sustainable design and innovative ecosystem. While the application of Mg-alloys is projected to increase, understanding the mechanical behaviour and structural integrity of welded joints are critical. Thus, fusion and solid-state welding processes of these alloys are discussed with emphasis on mechanical characterization. Laser welding is the most effective fusion welding technique for most Mg alloys whereas, the predominant solid-state method is friction stir welding. The importance of process variables such as heat inputs, welding velocity (speed) and post weld treatments on themicrostructural evolution, onmechanical and physical properties of the distinct zones of the weld joints are described. The weldment is the most susceptible to failure due to phase transformation, defects such as microporosity and relatively coarse grain sizes after solidification. The implication of the design of quality weld joints of Mg alloys are explored with areas for future research directions


Introduction
Magnesium and related alloys are great engineering alloys from research and industrial point of view. These alloys have favourable high strength-to-weight ratio [1][2][3][4], splendid electromagnetic interference (EMI) shielding resistance, strong thermal conductivity, great damping capacity [5,6] and ease to recycle [7], which is critical to circular economic model. These properties have made the alloys attractive for some engineering applications in the automobiles, aeronautical, and biomedical industries. The utilization of Mg alloys as biomaterials are not only due to excellent biocompatibility, but also by its intrinsic properties, especially the rate of dissolution in body fluids [8]. The density and yield strengths of these alloys are similar to that of human bone [9]. The closeness of elastic moduli and densities of these alloys to the human bone solves the implant-bone mismatch [9].
Magnesium is a naturally occurring element in bones and required for metabolism in human beings. It contributes to bone growth and stabilization and any deficit in the humans can lead to osteoporosis and other bone related diseases [9]. The use of Mg-based alloys as implants are beneficial as they dissolve easily into the blood stream during the healing process without causing any adverse effect when controlled [9,10]. Magnesium is a green engineering material [11]. These materials and their waste product do not emit toxic or poisonous gases into the atmosphere. They promote sustainability and drastically reduce environmental pollution and hazards to human welfare. Mg-based alloys contribute to energy conservation and recyclability [11].
It is also the sixth most abundant element and current industrial usage is increasing, estimated at ∼20% per annum. It is about 40% and 75% lighter than Al and steel, respectively. This makes these alloys the right fit for biomaterials, green engineering, and light weight structural materials.
Mg-based alloys are laced with some drawbacks especially high susceptibility to oxidation, low melting point (650°C) during welding [12,13] and poor ambient temperature formability [12][13][14][15][16][17]. Poor formability at ambient temperatures is attributed to strong basal crystallographic texture from pronounced basal slips under thermomechanical treatment [14,15]. The hexagonal closed packed (HCP) structure provides little deformation modes and pathways, accounting to the poor formability in ambient temperatures [14][15][16][17]. The low boiling point (1100°C) and low melting (650°C) are attributed to the low intermolecular attraction of the atoms, hexagonal closed packed (HCP) structure and associated crystallographic anisotropy [14][15][16][17]. The low melting temperature hinders the use of these materials at relatively high temperatures. These alloys are also temperature-sensitive as their strengths and elastic moduli decrease with increasing temperatures [18]. The percentage elongation also increases with increasing temperature up to the melting temperature and then drops drastically to near zero. This has hampered the successful applications of most Mg-based alloys in the industry.
Challenges associated with welding of Mg-based alloys are discussed. These include broadening and bulging of weld pool (particularly for thick workpieces) [13,19], inclusions and contamination of weld zone with porous oxides, undercutting, reduction in mechanical properties (strength-ductility trade-offs) and cracking in the welded joints [13,19]. Spattering and discontinuous weld beads are also observed in Mg-based materials [20]. These challenges affect the structural integrity of the Mg-based alloys thus resulting in reduced mechanical properties.
The weldability and structural integrity of Mg-based alloys can be improved using two approaches. Firstly, optimization of different welding techniques and its effects on mechanical properties of Mg-based alloys is essential. These welding methods are mainly fusion and solid-state welding. The method affects the bonding at the fusion zone, heat affected zones and its impact on microstructural and mechanical characteristics of the alloy. Typical fusion welding methods are tungsten inert gas welding (TIG) [21][22][23][24], laser or electron beam welding (EBM) [4,25] and hybrid laser-arc welding [26,27]. Friction stir welding (FSW) and resistance spot welding (RSW) [28,29] are the main solid-state welding techniques emphasized in this review for Mg-based alloys [27,[30][31][32]. The second approach is the optimisation of process parameters of the welding process to reduce defects and improve weld quality [33,34]. Structural integrity is directly related to the quality of the weldment which is driven by the process parameters [35,36]. These include arc travel speed, welding current, electrode feed speed, wire electrode extension, welding voltage, electrode orientation, quality of the shielding gases, electrode diameter and polarity [25-,29, 33-36]. Few reviews have focused on different welding methods of Mg-based alloys, the selection and effects on structural integrity mechanical properties of Mg alloys [37][38][39][40]. Thematic reviews and critical overviews of process variables parameters on the mechanical and microstructural characterization of Mg-alloys are still lacking. A concise overview of processing parameters and its effects on minimizing weld defects and its effects on mechanical properties and structural integrity are presented. This review seeks to fill in the research gap and critically review the advantages of welding metallurgy and prospects for Mg-based alloys.
2 Methodology and classification of magnesium and its alloys 2

.1 Literature survey and database
This review paper is synthesized from carefully selected peer-reviewed conferences, journal articles, authoritative thesis and dissertations. Published journal articles, conference proceedings, research reports, dissertations and theses were collected from various electronic databases. These include Google scholar, Elsevier, Cambridge Core Books and Journals, Wiley Online Library, Scopus, SpringerLink, Taylor and Francis Journals and google search. All articles and conference proceedings included in this review were mainly in English. The keywords used for the search are Mg-based alloys, mechanical properties of Mg-alloys, welding metallurgy, microstructural characterization of Mg alloys.

Physical metallurgy of Mg and its alloys
Magnesium alloys are mainly based on Al and Al-free systems [26,34,[41][42][43][44]. The Al-based system are designated as AZ (Mg-Zn-Al) and the AM (Mg-Mn-Al) sequence [12,45,46]. The low melting Mg 17 Al 12 intermetallic phase accounts for poor creep resistance with operating temperature below 120°C. Similarly, there are the AE (Mg-Al-RE) and AXJ (Mg-Al-Ca-Sr) series [45,46]. The AE series are mainly alloyed with rare earth elements with high creep resistance due to precipitates of rare element at grain boundaries [45,46]. These alloys operate at temperatures of ∼180°C. The Al-free systems are mainly Zr-containing which are designated as ZK (Mg-Zr-Zn), WE (Mg-Zr-Nd-Y) and QE (Mg-Nd-Zr-Ag), ZE (Mg-Zr-Zn-RE) and Elektron 21 (Gd ÀMg-Nd-Zr) sequence [47]. These alloys have operating temperatures approaching 300°C. Generally, the effects of various alloying elements on the mechanical and microstructural characteristics of Mg-based alloys are given in Table 1.
Magnesium alloys are produced and used for commercial purposes [26,34,[41][42][43][44]. Some of these alloys are given in Table 2 showing physical, mechanical and corrosion properties. Some potential areas of applications have been suggested.

Welding types of magnesium and its alloys
Joining of engineering materials to create intricate and complicated shape is at the heart of engineering design and fabrication of structures. This is being used extensively Great sand-casting behaviour, good ambient temperature strength and ductility ZK51, Z61 [50][51][52][53]. Phase transformation process due to excess heating leads to the formation of a fusion zone (weldment) and the heat affected zone (HAZ) (Fig. 1), adversely or positively affecting mechanical behaviour and structural integrity [52]. The fusion zone is a solidification microstructure made of the deposited metal and the portion of the melted parent or base metal during welding. The HAZ is the area close to the fusion zone and not melted but undergoes microstructural evolution from the extremely high temperatures associated with the welding process [52]. Some of the fusion welding methods include electron beam welding (EBM), arc welding, laser welding, gas metal arc welding (GMAW), and hybrid welding [54].

Tungsten Inert gas welding (TIG) gas tungsten arc welding (GTAW)
The TIG welding method requires the generation and application of heat from an electric arc. It may be referred to as gas tungsten arc welding (GTAW). Non-consumable tungsten electrodes are used in the welding process. The adjoining metals are melted together, and the molten pool is shielded in inert gases such as Ar, He and N 2 to prevent atmospheric contamination. Advantages of using GTAW are the ability to weld with or without a filler metal and one can easily switch between automatic and manual mode (no need for consumable electrodes). This yields solid, clean and aesthetic joints with minimized defects [55]. A major drawback of GTAW is the presence of pores in the welds compromising weld quality and integrity. Coarse grains are also formed in the weldment. These coarse grains are sites for the movement of dislocations hence easy propagation of cracks. This follows the Hall-Petch relation where the grain sizes are inversely proportional to strength. The process is also time consuming compared to other welding methods and require specialized training [39,56].
Grain sizes are critical to improving mechanical properties of alloys and weldments, which underpins the Hall-Petch relation [57][58][59][60]. Decreasing grain sizes contribute to enhanced weld quality and is attainable by optimizing welding parameters to control high thermal gradients for grain growth [60]. Welding speeds have significant effects on grain sizes, which affect mechanical and microstructural characteristics [61]. For typical AZ31B alloys, a speed of 135 mm min À1 resulted in a tensile strength of 188 MPa, which is relatively high for Mg-based alloy [61]. The tensile property is attributed to small grain size and evenly dispersed fine precipitates within the microstructure [43]. Similarly, the tensile strength of LA141 Mg alloy was ∼95% of the base metal after welding at a current of 60A and speed within the range of 2.8 to 3.2 mm s À1 as shown in Table 3 [62]. Postweld heat treatment (PWHT) improved the plasticity of the weld zone by 16% an improved microhardness profile shown in Figure 2. Using welding voltage of 20-22 V, welding current of 60-120 A and welding speed of 5 mm s À1 , the AZ91D alloy was welded without cracks in the butt joints [63]. The butt joints had fine grain sizes contributing to the high strength. In the HAZ, hot cracking was observed under the welding bead.
Coarse grains during GTAW are detrimental to mechanical properties [65]. Refinement of grains and other microstructural features such as precipitates, carbides and nitrides improve certain properties as yield and tensile strengths, especially at the weld zone using a magnetic arc oscillation [65]. By placing a magnetic arc oscillator in the welding direction, dendritic microstructures are refined due to the mechanical mixing process in the weld zone. For a AZ31B gas welded alloy, a magnetic oscillatory frequency from 1 to 3 Hz showed great improvement in tensile properties [66]. Generally, the tensile properties increased gradually with increasing oscillation arc frequency from 1 to 1.5Hz and then peaked at 2Hz and then a sudden decline was observed for frequencies above 2 to 3 Hz (Tab. 4). Low oscillatory frequency results in wide weld bead with decreased penetration due to low thermal and mechanical agitations. The transition of fine grains to coarse and elongated grains at relative low oscillation frequency and above 2 Hz accounts for the poor tensile properties. The highest tensile properties recorded at 2 Hz are characterized by fine grains and evenly distributed precipitates.
Dynamic grain refining using ultrasonic vibration assisted treatment is used to improve mechanical properties of weld joints [67][68][69][70]. Thus, refining the grain sizes improve bond strength of the welds which improves mechanical properties (e.g., tensile and yield strengths) [71]. Similar grain refinement was carried out on AZ31 Mgalloy under different vibration direction, vibration amplitude, and groove angle [68]. The welding was done at 20 cm/min and at 100A with the grain size decreasing significantly under the vibration conditions. Effects of vibration parameters on the phase proportion of the intermetallic b-Mg 17 Al 12 at the welding zone was marginal but with relative increased micro-hardness and tensile properties (Fig. 3) [68]. Vibration also affects the weld thickness. The impact of vibration on microstructure and mechanical properties of the weld joint are determined by the amount of energy introduced. This is also dependent on the amplitude, orientation of the vibration, depth, and groove direction.
Coarse grains, residual melting and stress cracking are detrimental to tensile properties in the weld zones. These are due to the thermal peaks and troughs welding in the welding pool and base material. The intensity of the  D.E.P. Klenam et al.: Manufacturing Rev. 8, 29 (2021) residual melting and the stress cracking close to the weld are very high [72]. Finite element modelling (FEM) are used to study the impact of residual melting and stress cracking on low residual stresses [73]. Generally, fusion of the low melting point eutectic b-Mg 17 Al 12 phase in the welded seam contribute to the overall reduction in tensile strength. These intermetallic phases promote stress cracking under very low residual stress regime. Similarly, FEM with infrared radiation is used to investigate the welding temperature and cooling curves of AZ31B alloy weld joints [74]. The emissivity of the AZ31Mg-alloy was ∼2.

Metal inert gas welding
Metal inert gas welding (MIG) is among the common form of welding metals. It is also known as gas metal arc welding (GMAW) which requires the melting and solidification of base metals together with a wire electrode [63,76,77]. Improvement initiatives for Mg-based alloys are to minimize porosity rate, improve mechanical properties of joints, understand mechanisms impacting porosity and other weld defects [76]. Minimization of pores after GMA welding of AZ31B Mg-alloy sheets was achieved by increasing welding speed above 8.3 mm s À1 and wire feeding rates above 110 mms À1 [77]. These parameters increased the rate of solidification due to lack of evaporation of Mg and Zn elements associated with low welding rates below 8.3 mm/s. There is also short time for the formation of unwanted bubbles associated relatively high welding speeds. Bubbles in the weld zone compromise structural integrity of the weld adversely affecting mechanical properties. Thus, weld speed and wire feeding rates are critical parameters which should be optimized to improve mechanical properties and structural integrity of welds of Mg alloys. The process parameters affecting weld quality and mechanical properties are presented [63,76,78]. Low welding speed improves the cracking resistance of MIG welded AZ91D Mg alloys [63]. Solidification cracking was noted in the HAZ at welding speed of ∼5 mm s À1 . Such low welding speed is generally accompanied with intense heat and high thermal stresses resulting in evaporation of vital elements with associated crack initiation [63,76]. High welding speed leads to low applied heat and low thermal stresses. This improves resistance to crack propagation during MIG welding of AZ91D Mg alloys [63]. The welding speed for typical Mg-alloys is estimated to range from 400 to 1400 mm/min for MIG, which could lead to great properties of weldments [77]. Another process parameter affecting weld quality and mechanical properties of AZ31B Mg alloy is pulse current [79]. Defects such as inclusions in the weld beads, pores, micro segregation and micro-cracks are minimized due to the pulsed rework current and a negative pulse current. Based on the pulse current, tensile strength up to ∼97% of the base metal and elongation up to ∼78% of the weld bead of AZ31B Mg-alloy was observed. These process variables can be highly optimized to improve the mechanical and microstructural features.
The impact of process parameters on Mg alloy using DC double pulse MIG welding is discussed [80]. This controls the melting pool as the time is shorten, reducing the weld width and penetration depth. Low frequencies also prevent disorderliness as heat input is moderated. The penetration of large weld beads and great appearance is observed with continuous increase of current (either low or high pulse) until maximum allowable D-value of 60A is attained. Increasing pulse current above maximum allowable D-values increase arc agitation of the molten pond, leading to spattering and disorderliness. The microstructures of the weldment is a mixture of a-Mg solid solution and b-Mg 17 Al 12 intermetallic typical of AZ31B alloy [72]. Uniform and finer equiaxed grains were in the FZ compared to the parent metal. This is due to the strong stirring effect of the double pulse MIG welding arc. The UTS of the fusion Table 3. Tensile properties of LA141 alloy welded plates and base metal before and after heat treatment [64].

Specimen
Heat zone was 234MPa (96% of the BM), whereas elongation was 11% (75% of BM). The reduction in property were due to the precipitations and volume defects in the fusion zone acting as stress concentration points and crack initiation sites. By optimizing the process parameters, mechanical properties and weld quality can be improved.

Electron beam welding
Electron beam welding (EBW) uses dense stream of high velocity focused electron beams impinging on the materials to fuse materials to be joined together until fusion [51,81,82].
An electron gun is the source of the electron beam composed of tungsten cathode and high vacuum anode. Electron beams are focused on the material in such a way that the capacity of the alloy to dissipate the electrons by conduction is overcome. This heat input then leads to vaporization, melting and ionization. The process is intricate, and the welding quality depend on the welding process parameters. The accelerating voltages range from 30 to 175 kV, whereas the beam current ranged from 50 to 1000 mA depending on the type of Mg alloy. The EBW process is generally used for metals that are difficult to weld and when there is repeatability. Other process parameters include welding speed, spot size and vacuum level (below 10 À3 torr). By   Table 5. Effect of welding speed on cooling rate, grain sizes and mechanical properties of AZ61 alloy [96].
Welding speed (mm/min) keeping the beam current below specific thresholds (dependent on alloy), vaporization is prevented and thus a keyhole is not created. Thus, the welding heat input and the beam current are significant parameters in enhancing the shape and the structural integrity of the weld and the keyhole stability [83]. Changes in process variables affect quality and geometry of weld joints [51,84,85]. Deep penetration and surface thermal effects are associated with electron beam welding for AZ Mg alloys [84]. The beam width in EBW is used to determine the force of the field depth which critical to welding process [51,84]. Sharp focused electron beam increase the welding energy density and the penetration depth [51,84,85]. The FZ has poor mechanical properties (hardness and strength) than the base metal and heat affected zones [84]. These are associated with coarse grains and porosity due to solidification of the fusion zone [51,86,87]. The variation in the electron beam spot width greatly improved the heat propagation characteristics resulting in good quality weld joint [88].
The electron beam currents have great effects on the rate of penetration [51]. This leads to a fine-equiaxed microstructure with significant improvement in the tensile properties of the weld joints.

Laser welding
Laser welding is the application of beam of lasers to join metals or thermoplastics together [12]. It is a process that requires concentration of the heat source and relatively high welding speeds. The keyhole and conduction processes are the main modes of laser welding [12]. Welding in the conduction mode is carried out at power densities less than 105 W/cm 2 . The laser beam is absorbed mainly at the surface with negligible penetration power. Thus, conduction limited weld has very high width to depth ratio. The keyhole laser welding mechanism occurs at relatively highpower densities above 105 W/cm 2 . By introducing laser beam of power densities of ∼106 W/cm 2 , the spot at which the laser is concentrated melts and vaporizes before any conduction can take place. The focused beam, by penetrating the workpiece forms a cavity, a keyhole filled with ionized vapour, forming plasma.
Effective and good weld joints from laser welding is affected by the physical and mechanical properties of the metal being joined. Mg-based alloys have strong oxidizing tendencies, low absorptivity of laser beams, low viscosity, high thermal conductivity, low melting points, high solidification shrinkage and high affinity for hydrogen in the liquid state. Laser welded Mg-alloys are prone to unstable weld pool [89,90], loss of alloying element of Zr and Al due to vapourisation and undercut [80,91,92].
A major review of welding of Mg-alloys based on this method was done in 2006 [12]. Emphasis was on the use of CO 2 and Nd-doped Y-Al garnet as laser source with wavelengths of 10.6 and 1.06 mm. The use of high powder density on the workplace has been emphasized for keyhole welding. Generally, the effect of laser power, focal plane position, welding speed and surface preparation, shielding gas, filler metal and process tolerance on the quality of laser welded Mgalloys were reviewed [12]. Filler metal was identified as the critical factor which is influenced by high power and low welding speed. The challenge is there are few filler wires developed to aid the welding process in laser welded Mg-alloys.
Optimizing process parameters are critical to improving laser welded Mg alloys mechanical properties [93][94][95]. The tensile strength of welded joints achieved 88% of the base metal at a welding speed of 10 m min À1 using a 3 kW fibre laser [93]. Generally, there is grain size reduction, Similarly, the tensile and fatigue properties and the microstructural evolution of fibre laser welded AZ31B-H24Mg alloy and the influence of diverse welding speed have been reported [95]. The welding process led to the formation of Mg 17 Al 12 eutectic particles, equiaxed and columnar dendrites around the fusion zone boundary and the recrystallization of grains in the thermal zone. The fusion zone had the least hardness, resulting in decreased yield and fatigue strengths. This is mainly due to the differential solidification, grain coarsening and defects due to entrapped gases, oxide inclusions, cracking, depletion of alloying elements and porosity within weld microstructure have effects on structural integrity [12].
Relatively high welding speeds ranging between 30 and 120 mm/s led to small melting and fusion zones in the laserwelded joints of Mg-based alloys [94,95]. This is accompanied by reduced grain size resulting in high yield strengths and good fatigue life with relatively low strain-hardening [95]. On the other hand, high welding speed (give the range) and low power levels (give the range) promote good penetration and an active flux with good effects on overall weld quality [28]. The influence of welding speed and laser power on mechanical properties and microstructure of 2.0 mm thick ZK60 sheets have been studied [95]. When welding speed was raised from 1 to 4 m min À1 , tensile strength and elongation initially increased but then decreases as welding speeds exceeded 4 m/min. The welding speed is a critical parameter for laser welded AZ61 and its effects on the cooling rate, grain sizes, volume fraction of phases and mechanical properties as shown in Table 5. Generally, the mechanical properties (UTS and yield strength) of the base metal are higher than the fusion (FZ) and HAZ. This is due to the variation in microstructure and its associated grain sizes and volume fraction. The microstructure of extruded AZ61 was uniform and equiaxed with intermediate grain size of 39mm, whereas the fusion zone had dual equiaxed dendritic a-Mg (soft) and dispersed intermetallic b-Mg 17 Al 12 particles with average grain sizes decreasing with increasing welding speed. The fusion zone after solidification is more a cast structure in contrast to the base metal, which is wrought, a contributing factor of the observed higher strength than the FZ and HAZ. The HAZ undergoes heat treatment hence coarse grain sizes are sometimes observed [96]. Similar trends have been reported for fatigue strength between FZ, HAZ and BM [95].
Volume and line defects such as precipitates, shrinkage pores and inclusions are detrimental to quality and mechanical properties of laser-welded joints. The main volume defect is pores formed, pre-existing one during differential solidification and entrapment of various gases [11,27,97]. For instance, pores formed due to the presence and entrapment of hydrogen [11,27,97]. Concentrated heat on the base metal can also lead to keyholes which are unstable and detrimental [11,[98][99][100]. Low vaporization of some alloy elements can also be detrimental to the mechanical properties and structural integrity of welds [101].
Minimization of the defects in welds is critical to structural integrity [97]. Ways to reduce pore formation in Mg-based alloys include optimizing the Mg content and processing parameters [97]. The use of high welding speed leads to narrow fusion zones and this speeds up the solidification process, thus few and small pore sizes are observed in the weld bead [102]. In some cases, the welding is preceded by preheating, a form of heat treatment that effectively reduces sizes of pre-existing pore and prevent new ones from nucleating at the interconnection of two intersected sheets of AZ31B Mg alloys [103]. Optimizing processing parameters have also reduced the porosity of fusion zones. For instance, fibre laser welding of ACM522 Mg alloy resulted in increased depth of penetration and increased weld bead with increased laser intensity [104]. The microstructure of the weldment were refined and the porosity of the weldment decreased with increasing welding speed [105].
Oscillating frequency is another process parameter that affect the quality of laser welded joints of Mg alloy. Low oscillating frequency (∼50 Hz) with welding speed of 33 mm s À1 and beam oscillating diameters of 0.35 mm improved aesthetic qualities of the weld with remarkable mechanical properties [106]. This is shown in the microstructures in Figure 4. The beam oscillation showed continuous columnar grains at the fusion, whereas non continuous fusion outline is associated with regions without the beam oscillation effect.
Also, Gao et al. [107] in their study, assessed the effect of process variables on the AZ31 Mg alloy properties, the authors observed that an increase or decrease or any change within the beam oscillation frequency immediately affected the alloy mechanical characteristics. By reducing the gradient, the beam swing splits the expansion of the columnar grains on the welding edge. The broken dendrites area unit then pass through robust convection and eddy effects into the weld centre, which act as nucleation areas for the equiaxed dendrite. In their research, Lei et al. [108] found that the ultrasonic laser welding technology used in joining AZ31B Mg with a welding velocity of 1.0 m min À1 and a focal length of 192 mm significantly reduced its weld porosity to less than 1 percent. In addition, the number of nucleation particles increases when ultrasonic vibration is used, thus increasing the number of refined grains during the weld pool crystallization process, which enhances the mechanical properties of the welded joint. The mean area of grain in the weld centre is thus reduced from 359.9 mm to 213.7 mm.  Table 4 and 5 shows that the majority of AZ31 and AZ61 Mg alloys have been investigated [84,[109][110][111]. The different welding process parameters considered were equally indicated. The parameters for these operations are similar. Researchers investigated the effect of process parameters such as welding speed, vibration frequency, welding current, welding voltage, vibration frequency, focal length of the focusing lens on the properties of the joints such as the mechanical properties (tensile properties, bend properties, fatigue strength and microhardness) and the thermal effect [25][26][27][28][29][33][34][35][36][37][38].

Summary on fusion welding of Mg alloys
The above studies show that various approaches have been considered to improve the microstructural and mechanical properties of welded Mg alloys. However, there are various process parameters affecting the structural integrity of the welds. The microstructural and mechanical properties are dependent on these process parameters ranging from the welding speed, rotation rate, travel speed, welding current and vibration frequency. The difficulty is that optimizing one could compromise the other. Thus, the effects of very few processing variables on microstructural and mechanical properties of Mgalloy welds have been investigated. There is the need to use computational modelling coupled with numerical simulations to understand the overarching effects of these parameters [85,[112][113][114]. This method will be faster, less expensive, and not lengthy as the Edisonian approach. Data collected can be validated with some experimental work.

Hybrid welding
Hybrid welding refers to procedures in which laser welding and other welding methods are consolidated in a conventional welding zone with the goal of achieving what a singular welding procedure could not accomplish by adding synergy of the advantages of welding procedures, thus removing their weakness. Due to advantages such as its capacity to produce a concentrated, deep and narrow welding pool with precision without the side effects of heat deformation, laser welding has been anticipated to offer promising outcomes in the welding of Mg alloys [115]. On the other hand, Laser welding systems are more expensive, and the electrical effectiveness of most laser welding process systems is poor. By the way of illustration, laser welding procedures displays a reduced capacity to bridge gaps thus calling for extra task in ensuring the workpiece alignment and maintaining accurate surface preparations thus adding to the overall cost of production. In contrast to this, arc welding techniques displays excellent strength in the weakness of laser welding as it displays an increased capacity to bridge gaps and an enhanced electrical efficiency [116]. For example hybrid laser beam and electrical arc welding resulted in increased welding speed and depth of penetration as well as improved gap bridging and process stability [117].
The idea of a laser/arc hybrid welding technique was first proposed in the late 1970's [118], the schematic illustration of the process is as shown in Figure 5. The quest to develop a better hybrid system was carried out by Japanese researchers where they established different techniques and correlating technologies, but there was a setback as these methods were not economically viable [119]. In the early 1990s, with the relentless effort of the researchers, there was a headway of engaging high voltage laser beam as the primary heating source while making the use of electric arc as the subsidiary power source [120,121].
New initiatives and development in TIG have some drawbacks. This includes high cost of maintenance and the occurrence of flaws such as porosity and cracks in the weld zone. These defects are not peculiar to the modified welding techniques in any way though hybrid laser welding was designed for satisfactory industrial usage [122] [174]. Notwithstanding, there is increase usage of hybrid welding of various materials in the 21st century including Laser-MIG [123], Laser-Plasma [124] and Laser Double arc welding [122]. The Laser-TIG and GMAW-GTAW hybrid welding techniques were applied to the welding of Mgalloys recently [125].

Laser-tungsten inert gas (laser-TIG) hybrid welding
The efficiency of welded joints for thin plates is directly improved by increasing welding speed [102] Hybrid welding of laser-tungsten inert gas (TIG) is a new technique for high speed welding [145]. In previous research, fusion welding methods such as arc welding and low power laser beam welding were used, but the welding speed was slow [149][150][151]. Laser tungsten inert gas hybrid welding is a modern highspeed magnesium alloy welding process. A few useful studies have been performed on laser-TIG hybrid welding of Mg alloys. Most welding rates, however, were below 2000 mm/min [152]. By increasing the welding speed from 2000 to 6,000 mm/min, Li et al. [145] sought to examine the impact of welding rates on the microstructure and mechanical characteristics of high-speed welding through hybrid welding of AZ61 Magnesium alloy with laser-TIG  [118], reuse with permission from AIP Publishing D.E.P. Klenam et al.: Manufacturing Rev. 8, 29 (2021) technique. With the optimum welding speed of approximately 6000 mm/min, faulty free welds were achieved. As welding speed increased rapidly, both the linear energy and average grain size decreased, so fully penetrated joints could be obtained at welding speed without macroscopic porosities or cracks. As the welding speed rises, the microhardness of the welded fusion zone increases exponentially. It was also observed that the mode of fracture differs between ductile and brittle fractures, and the traditional intergranular fractures.

GMAW-GTAW hybrid welding
The impacts of several welding process factors on weld production, such as wire extension, tungsten electrode distance, bypass current, and the workpiece, have been assessed [147]. The optimal forming weld parameters are 225 A current, 140 A bypass current, 21 V welding voltage, 2.8 m min À1 welding speed, and 5 mm between the end of the tungsten electrode and the workpiece. Hybrid GMAW-GTAW welding is a high-speed welding technology with high consistency and excellent forming of Mg alloys and welding bead. The application of TIG and a MIG hybrid system are stable in MIG arcs, thus these hybrid welding techniques can be turned into a high quality and efficiency new welding process. [125].

Solid state welding
Solid-state welding procedures involve joining of two metals without the application of heat [48,49]. However, heat is generated from the contact surfaces due to the movement of a fixed component part relative to a moving one. This approach is mainly driven by applied pressure and any applied temperature comparatively lesser than the melting point of the mating alloys [48]. The mechanism of bonding is by diffusion of interfacial atoms. The main advantage of SSW is lack of typical weld defects and porosity as observed in the conventional fusion welding techniques. Any phase transformation due to microstructural evolution is negligible due to little or no heat applied during welding. The surface finish and mechanical properties at the weld joints are better than most conventional fusion methods. The main disadvantage is the difficulty in joining more than two components at a time. There is also the need to fill very wide root gaps since it does not require the use of filler materials [153]. There is no melting involved in SSW and weld appearance are great with improved mechanical properties at the weld joint [49].
The SSW technique is classified under different categories. These include cold welding, ultrasonic welding, roll welding, forge welding, pressure welding, friction welding, diffusion welding, explosion welding, and friction stir welding [154]. Friction stir welding (FSW) is widely use as it is flexible and allows for all sorts of metals to be jointed. For instance, soft and difficult to weld metals like Mg, Al, Cu and Zn are welded with good mechanical properties and with low defects. Other SSW techniques such as roll welding, friction welding and explosive welding have also received some attention, but with some drawbacks. These include causing severe injuries and air pollution [155].
In this review, emphasis is on friction stir welding which is discussed in the next section. The interplay between processing parameters and microstructural features are briefly discussed. Relevant mechanical properties are explained based on the evolution of microstructural features such as grain sizes across the different zones. This is done to understand the suitability for specific engineering application.

Friction stir welding (FSW)
Friction stir welding is non-fusion solid state welding process. It uses a quasi-consuming, rotating tool with a pin, extending across the weld line from a wider shoulder to join metals together [49,156]. The weld is created by sequential spinning and forward movement of the instrument pin along the weld line, inducing friction and strong plastic deformation at temperatures below the melting point of the adjoining metals [156]. The process does not have welding defects such as shrinkage, hot fracturing and porosity, significantly reduces the thermal stresses induced in the metals [49,156]. A typical friction stir welding process is shown in Figure 6 [157].
Two main classes of input parameters affect the quality of FSW welds [158][159][160]. These are process variables and tool parameters. The process variables include the rate of rotation, welding speed (tool travel rate), plunge depth and the downward force. The tool parameters are tool pin profile, tool design, tool altitude tilting and the dimensions of the tool. The final surface morphology, mechanical and microstructural features of the joint are significantly influenced by the input parameters. Similarly, factors such as the root gap between the materials, thickness of materials, dwell time of the tool, temperature, and humidity are critical to weld integrity and appearance [161].
The effect of input parameters results in four distinct zones in a typical FSW joints [42,157,161] and is illustrated schematically in Figure 6c [157]. The zones are stirred zone (SZ) or nugget zone (NZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base metal (BM) zone. Due to difference in applied frictional heat and the flow of the material during welding, different grain structures and features are observed. This results in differences in mechanical properties at these zones. Distinct features of these zones are discussed further.

Stirred zone (SZ) or nugget zone (NZ)
The stirred zone (SZ) is recrystallized dynamic area in the middle of the weld area (Fig. 6c). Frictional heat and plastic deformation due to direct contact between the tool and the workpiece led to remelting, resulting in fine grains [42,161]. The SZ which is characterized by recrystallization can be identified by onion rings as a result of frequent rotation movement of the tool pin on the soldered surface [42,161].

Thermo-mechanically affected zone (TMAZ)
The TMAZ is the area between the stir zone and the heat affected zone. It undergoes relatively high temperature change during the welding thermal cycle, and experience minor plastic deformation with no recrystallization [42,161]. Generally, relatively coarser grains are observed in the TMAZ compared to the SZ. This is mainly attributed to the initial microstructures of the Mg alloy and the coarseness is due to the minor plastic deformation triggered by the movement and plunging of the tool shoulder (Fig. 6b) [42,161].

Heat affected zone (HAZ)
In the HAZ the tool and the metal are not linked directly. In this zone, the metal is not stirred or rubbed by the pin. There is no distortion in this area, but the metal is influenced by the thermal welding which leads to a few changes in the microstructure. The grains are near the base metal in this area, but are somewhat smaller [42,161].

Base metal (BM)
The BM is the base or parent material. Considering it is farther from SZ and TMAZ, microstructural features do not change as there is negligible effects of mechanical and thermal cyclic stresses to induce any significant plastic deformation. The starting microstructure of a typical Mg alloy defines the grain sizes in this zone. Typical coarse grains are expected in this zone compared to the SZ and TMAZ, which then decreases steadily across TMAZ and SZ [42,161]. However, there are results that suggest small grain sizes at BM with coarse grain observed for TMAZ and SZ [157]. This is generally affected by tool rotational and welding speeds.

Effect of input parameters on microstructure and mechanical properties
Input parameters lead to changes in microstructural evolution and mechanical properties of friction stir welding of Mg-alloys [162][163][164][165][166]. The use of Bobbin tool friction stir welding (BTFSW) on ZK60 Mg alloy resulted in significant grain refinement and disintegration of the Mg4Zn7 precipitate in the stir zone (SZ) [167]. Extensive thermal softening at the SZ resulted in lower the yield strength of the BTFSW joints than that of BM. This is mainly attributed to grain refinement influenced by the welding speed of 300-400 mm/ min. Similarly, carefully selected rotational speed of 300-800 rpm and constant tool speed of 500 mm/min using a displacement-controlled type of FSW on ZK60 Mg alloy [168]. The UTS of the joint was 91-95% of the BM. The input parameters resulted in refining the microstructure with Zrrich precipitates and bimodal recrystallized grains sizes ranging from 0.6 to 1.5 mm in the SZ. The dissolving of Mg-Zn precipitates at the SZ as a result of the welding speed was accompanied by extensive thermal softening [169].
The effect of the heat input and the rotational speed of FSW joints of AZ61 Mg alloy was studied [164]. The heat input was determined to be commensurate to the welding speed and indirectional proportional to the rotational speed. A welding and rotational speed of 2 is undesirable for AZ61 alloy joints due to the formation of tunnel, cavity, and flash defects in the weld region. Grain refinement of the SZ was attributed to reduction in tool rotational speed and vice versa. This is also supported for other Mg alloys [154,170].
Inadequate penetration, kissing bound, tunnel defects, worm hole, and lazy S are defects associated with FSW [171]. To limit the occurrence of these defects, hybrid FSW are being used [172]. These include using secondary energy sources such as laser beam, inductive coil [173,174]. However, these secondary energy sources do have demerits, which leads to weld defects. Ultrasonic approach, there by or which is being used as it promotes the flowability of the material, thereby or which increasing the rate of penetration with negligible increase in peak temperature with great weld joints [175]. Ultrasonic assisted FSW also contributes to continuous frequency oscillation, resulting in high-quality weld joints. The ultrasonic vibrations enabled the formation of very fine grains in the SZ thereby increasing its hardness and strength when compared to other regions [172]. The fine grain sizes increase the grain boundaries hindering the dislocation motion, thereby       D.E.P. Klenam et al.: Manufacturing Rev. 8, 29 (2021) causing an increase in the tensile strength and hardness. The effect of input parameters on weld joint of FSWed of AZ91-C Mg alloy is shown in Table 7. Mechanical properties and welds were enhanced with increasing welding speed in the FSWed joints of Al-Zn-Mg alloy [176]. The effect of the welding speed is shown in the improved strain hardening coefficient and the grain sizes in the SZ. Thus, with increased heat input and rotational speed, and decreased welding speed, the grain sizes are optimized to improve the mechanical properties and weld integrity. By defining and optimizing the input parameters, structural integrity of the weld joints and the mechanical properties can be improved. The effect of low rotation speed on improved fatigue life of AZ31 Mg FSW joints has been studied [154].
The main process varaiables such as rotation and welding speeds have serious effect on mechanical properties through grain size refinement. This has been shown for Mg-Zn-Zr alloy at for Mg-Zn-Zr alloy, a rotational speed of 600 rpm and traveling speed of 300-400 mm min À1 , results in substantially polished grains [167]. Similar results was also observed in the FSW of AZ31B-H24 Mg alloy with decreasing speed of 20mm/min and increasing rotational rate to ∼1000 rpm [176]. Thus, for effective and great weld integrity and mechanical properties close to that of the BM, welding and rotation speeds need to be optimized [157,[177][178][179][180][181].
By means of simulated spot solder of magnesium alloy sheets, the effects of increasing absorbed beam strength, processing speed and depth of penetration were shown. For welding currents above 8000A with a long slope welding current and high electrode power, blow-free pitch welding with a large nugget and high tensile shear strength was achieved [182]. Generally, increased rotation speed and welding current is necessary to get the best of friction spot welding of Mg alloys. Table 8 shows a summary of some of the research work that was carried out on solid state welding of Mg alloys. For fusion welding process (Tab. 6), AZ31 and AZ61 alloys are the most investigated due to their medium strength, good weldability, and formability at ambient temperatures.

Summary of the mechanical properties of fusion and solid-state welding types
The mechanical properties, primarily tensile and hardness, obtained from fusion welding were compared with those obtained from solid state on a zone-by-zone basis Table 9 and 10, and it was discovered that in both forms of welding, there was a depreciation in the degree of the respective properties as contrasted to the base metal

Areas for future research direction
There are some areas of concern and recommendation for the weldability of Mg-based alloys. Magnesium and its alloys have poor room (ambient) temperature ductility due to the basal slips in typical HCP crystals. Prior to welding, there is the need for heating of the Mg-based component, which is easier under laboratory scale experiment. This is expected to induce coarsening mechanism during the heating process. Scalability of this process requires investigation considering there is projected increase usage of Mg-based alloys. Thus, the application of localised heating mechanism and low time cycle can be achieved to improve the mechanical properties. There is also the need to design and develop specific tools and equipment to optimize the right temperature and processing parameters required within an acceptable time frame. Improving the structural integrity of joints of Mg-based alloys also offer opportunities for further research. Weld joints of Mg-based alloys have volume defects such as porosity, low toughness and susceptibility to cracking. There is the need to design and tailor post weld heat treatment to improve upon the mechanical properties and structural integrity. These must be balanced with cost and explored further for insight.
Magnesium-based alloys with addition of rare earth metals open opportunities for advanced cardiovascular stent materials. Combination of finite element modelling and experimental analyses of the welding activities of these alloy and optimising the process parameters are critical. The use of the two techniques concurrently will reduce the iterative nature of experiments, thus reducing overall cost and providing relative database for optimising composition and mechanical properties. Evaluation of functional and structural properties of weld joints of Mg-based alloys require further studies.

Summary and outlook
An overview of welding techniques, process variables and their effects on weld performance of Mg-alloys is presented. The naming convention and different types of Mg-based alloys and the effects of alloying elements were briefly discussed. By analysing recent procedures, essential parameters and the weld performance using various techniques, the following conclusions can be made.
The most effective fusion welding technique which improves mechanical properties and aesthetic qualities of the weld joint of Mg alloys is laser welding. In the case of solid-state welding, friction stir welding showed enhanced mechanical properties with optimized input variables such as rotational and welding speed, which affects the heat input. There is increasing interest on solid-state welding techniques in the past 10 years compared to fusion welding. This is due to less weld defects associated and low heat input of the former. There are new technologies such as laser, coil as well as ultrasonics to augment, complement and strengthen conventional welding processes of Mg-based alloys.
Weld quality and improved mechanical properties depends on the configuration of processing variables. Notable ones include welding speed, welding current, rotational speed, and dwell time. These are critical factors resulting in the tunability of mechanical properties of the weld, fusion and heat affected zones. Optimized process parameters can lead to very fine microstructures (∼2 mm grain sizes) from dynamic recrystallization at the weld interface. This results in the Hall-Petch effects with appreciable increase in yield strength, UTS and hardness to about 98% of the base Mg-alloy. Insufficient input energy generally leads to weld defects such as cracks and voids. Ways should be explored to develop more standard specifications for process parameters to guide the welding and testing of Mg-alloys for structural integrity. This can be done by developing modelling and simulation tools to reduce the Edisonian approach to welding metallurgy, which is costly and time consuming. For instance, temperature variation across the weld joint, as well as grain nucleation and growth can be estimated using simulation or finite element models. These models can produce results consistent with experiments to show how heat, temperature and strain fields are influenced by process parameters.
The constraints of restricted process configurations and the high energy required to join the Mg alloys continue to be a major challenge. Extensive research through experiments and simulations is still needed to address these challenges. This would be extremely important to ensure that Mg-alloys are deployed in various industries due to good properties they possess. Currently, AZ 31 and AZ63 are the common Mg alloys due to their great mechanical   [190] AZ31B Rotational Speed: 1500-1800 rpm; Traverse Speed: 100-120 mm min À1 ; different pin profiles: Cylindrical and truncated conical Effects: Excellent joints were achieved using the abovementioned parameters, including a controlled welding speed of 100 mm min -1 and an optimum rotational speed of 1500 rpm, which resulted in defect-free joints.
EBSD measurements [195] AZ31B-H24 Welding direction: perpendicular to the rolling direction of the workpiece; Welding speed:10 mm/s or 20 mm/s; The tool rotational rates: 1000 rpm and 1500 rpm Pin length: 2.75 mm Effects: The hook length and consequent fatigue life were severely impacted by process welding conditions. Hooking flaws were removed using the best combination of 1000 rpm and 20 mm/s welding speed, which improved the welding and joint fatigue life.

Microhardness
Fatigue Properties [178] AZ31B Welding speed: 10À20 mms À1 ; Rotational rate: 1000-1500 rpm, Pin length: 2.75 mm Effects: The best joints with the required mechanical characteristics were produced by combining a high welding speed of 20 mms À1 with a low tool rotating rate of 1000 rpm.

Microhardness
Tensile Shear Properties [157]     properties and ease of weldability. The design and development of new or improvement of existing welding technique will ensure many of Mg alloys desired in this review would become beneficial to structural and functional applications.