How to avoid solidification cracking in arc welding of aluminum alloys: a review on weld metal grain refinement approaches

Joining aluminum alloys with arc welding methods is frequently subject to literature and industrial applications. Although aluminum alloys have different difficulties in the arc welded process, the formation and elimination of solidification cracking defects is a more complex phenomenon. Since avoidance of this defect requires specific approaches and methods, special attempts and improvements have been studied frequently on this subject in recent years. Studies in the literature have clearly shown that this defect, which is often encountered in aluminum alloys, takes place along the grain boundaries. Therefore, the major approach to eliminate this defect is activating nucleation and decreasing the grain size. In this context, modification approaches in the literature, which are frequently used for arc welding of aluminum alloys, have been developed to use three different mechanisms including heterogeneous nucleation, dendrite fragmentation, and grain detachment. While it is aimed to increase heterogeneous nucleation by reinforcing filler metals with compounds in the inoculation approach; dendrite fragmentation and grain detachment are also aimed in the approaches where external effects and forces are used. Within the frame of references, it is also possible to review the external factors aiming to improve weld pool convection and thermal conditions under two headings, which are weld pool stirring and pulsed arc current approaches. The weld pool stirring approach also includes ultrasonic treatment and magnetic arc oscillation methods. In this study, solidification cracking defect that frequently occurs in the arc welding of aluminum alloys is explained fundamentally and the attempts to eliminate this defect are presented as a review paper in a comprehensive manner.


Introduction
Aluminum alloys are widely used in automotive, aircraft, and space industries with their high specific strength, good machinability and formability, low cost, and good maintainability.These engineering materials are also used for defensive structures owing to their high energy absorption capacity [1].In addition, aluminum has a density that is one-third of the density of steel and this unique property of aluminum alloys makes these materials stand out for weight reduction.Therefore, it is also expected that light metals such as aluminum will replace steel, which is currently used more due to the fuel economy regulations and weight reduction attempts of automobile manufacturers [2,3].Although aluminum alloys are expected to replace steels, the welding behavior of aluminum alloys is significantly different from conventional engineering materials such as steel [4,5].Therefore, the restrictive feature of these materials in engineering applications has been emphasized as requiring special applications for arc welding processes under some conditions [6][7][8].
The term weldability refers to the manufacturability of material by welding processes and for the welded structure to perform satisfactorily according to the base material in the intended service environment [9].When evaluating the weldability of a material, factors such as welding operation, strength, and arc stability should be considered in detail.Proper selection of welding parameters and their application during a welding process ensures arc stability and optimum mechanical properties after the welding.In addition, welding parameters (especially welding power and welding speed) affect the cooling rate and thermal gradient, and therefore the mechanical loads [10].Besides the general issues affecting weldability; high hydrogen solubility in the molten state, the oxide layer formed on their surfaces, and susceptibility to solidification cracking should be also considered for the welding processes of aluminum alloys [11,12].While methods such as alternating current or reverse polarization can be used to overcome the oxide layer formed on the surface of aluminum alloys, the material surface must be also properly cleaned before the welding process to prevent the porosity formation resulting from high hydrogen solubility in the molten state [13,14].Additionally, some physical properties of the aluminum alloys such as high thermal expansion coefficient and high heat conductivity coefficient have significant effects on weldability.These physical properties make aluminum alloys more susceptible to solidification crack formation in arc welding processes.These factors also make the occurrence of this defect complex and therefore there is a need to develop special approaches and methods to eliminate this defect [15,16].
When the studies are examined, it is clearly understood that the weldability of aluminum alloys is often defined by their susceptibility to solidification cracking.Depending on the alloy composition and welding conditions, cracks may occur in the weld bead during solidification [17,18].Stresses caused by solidification and thermal shrinkages during cooling can be carried by the solidified material and cause the rupture of the liquid film remaining at the grain boundaries.As a result of this situation, a welding defect called a solidification crack occurs in the welding process [19,20].
There are many studies describing the mechanism and phenomena of solidification cracking in detail for aluminum alloys in the literature [21][22][23][24][25]. On the other hand, there are a limited number of studies aiming to explain how to eliminate this defect.In this context, this study aims to present the approaches and methods used in aluminum alloys for eliminating the defect based on the formation mechanism as a review paper.

Solidification behavior and crack sensitivity of weld metal
Solidification cracking is a phenomenon that occurs similar to hot tearing in casting and is highly complex for the welding process [26][27][28].Cracking, which can occur during solidification in both casting and welding processes, mainly results from the solidification shrinkage that creates stresses in a material [29].As can be seen in figure 1, this welding defect involves a complex interaction between metallurgical and mechanical factors, driven by temperature gradients induced during welding process.While thermal-metallurgical interactions control the solidification microstructure, thermal-mechanical interactions control local and global stresses and strains.There is often no simple interaction between factors, but instead many complex interrelationships.Metallurgical factors are specific to phase relationships, while mechanical factors include stress and strain behavior [30].
Solidification cracking especially occurs in a semisolid region during the last stage of solidification.All alloys, except eutectic compositions, solidify over a temperature range with a structure consisting of both solid grains and liquid.Solidification begins at the zone defined as 'slurry' where solid and liquid phases coexist and can move freely in the molten pool.As the solidification continues, temperature decreases and the amount of solid phase (f s ) increases in this zone.At this zone defined as 'mushy', the solid grains grow and begin to interact with each other at the coherency point and behave like a solid forming skeleton.Therefore, it is possible to say that coherency temperature is the temperature that separates the slurry and mushy zones [31].After this point, the flow of remaining fluid becomes interdendritic, and with a further increase in f s , the permeability of the mushy zone decreases, leading to pockets of isolated liquid.During solidification, the semisolid material will be subjected to tensile loading due to shrinkages and mechanical constraints (e.g.welding fixtures).The flow of remaining interdendritic fluid cannot compensate for the solidification shrinkage and separation of grains caused by tensile deformation, a cavity is formed in the mushy zone.If the resultant cavity is larger than the critical size determined by fracture mechanics, it may propagate as a crack throughout the mushy zone [32].
As can be seen in figure 2, there are five main points along the dendrite axis that can be identified depending on the temperature and solid phase ratio.These are: (1) Coherency point: The columnar dendrites touch each other through their lateral branches to withstand stress, but there is a thin liquid film separating each dendrite from its neighbor, (2) The point where the liquid cannot penetrate: the dendrite side branches are in a position to prevent the liquid supply, and this point separates the liquid feeding zone from the cracking zone, (3) Coalescence point: the contacting side branches merge, that is, the two solid-liquid interfaces of the two contact arms have been eliminated, (4) Solidus temperature: dendritic columnar grains have begun to solidify in this region, (5) Solid fraction: the liquid phase has completely disappeared and solidification is complete [33].
There are different studies describing the mechanism and phenomena of solidification cracking in binary [34], ternary [35], and quaternary [36] aluminum alloys.In these studies, a susceptibility index was developed by summarizing three factors for this weld defect occurring along the grain boundary.Assume that two columnar dendritic grains grow side by side in their axial direction in the mushy zone.The first factor is that under tensile stress, the grains separate from each other laterally, causing cracking.The second factor is the lateral growth of grains towards each other by bonding together (called bridging) to provide resistance to cracking.The third factor is liquid feeding along the grain boundary to provide resistance to cracking.Especially when the workpiece is rigid or clamped down, stress occurs because the mushy zone is not able to shrink freely under solidification shrinkage and thermal contraction.Liu et al [35] also showed that the lateral growth rate of columnar dendritic grains during solidification was proportional to |d(f S ) 1/2 /dT|.Based on the consideration of the space in a volume element positioned between two grains when approximately (f S ) 1/2 = 1, an equation was derived that will serve as the criterion for cracking occurring during solidification.According to the criterion, if the rate of space increase due to the separation of grains minus the rate of space decrease due to the lateral grain growth exceeds the rate of space decrease due to liquid feeding, a void may form in the volume element, that is, a crack may form at the grain boundary.

Weld metal nucleation mechanisms
The microstructure around the weld metal formed by thermal cycles is shown schematically in figure 3.As can be seen in the figure, the mushy zone formed along the welding direction is together with the liquid phase between solid dendrites.Next to this zone, there is a partially melted zone between the solid grains and the intergranular liquid [37].When the structure here is examined in detail, it can be clearly observed that the grain structure is formed in the direction of maximum heat flow and with a perpendicular orientation to the fusion line [38].This grain structure is dominated by epitaxial grain growth and is characterized by an oriented dendritic grain structure that adversely affects the mechanical properties.A minimal amount of undercooling may be sufficient for epitaxial grain growth [39][40][41].
As can be seen in figure 4, the microstructure morphology in the weld seam is determined by the relationship between the solidification rate and temperature gradient [43].Therefore, it is possible to say that the weld metal grain structure formed after welding processes is affected by the solidification conditions controlled by the welding parameters [44].The parameters in arc welding affecting the weld seam microstructure are; • Welding voltage U (in V) • Welding current I (in A) • Welding speed ν (in mm/s) According to formula (1), while the heat input (H) increases with increasing welding current and welding voltage, it also decreases with increasing welding speed.
Additionally, these welding parameters also affect the • Solidification growth rate R (in mm/s) Further, the relationship between the cooling rate, the thermal gradient, and the growth rate is indicated in formula (2); It is also worth mentioning that the extent of constitutional undercooling is inversely proportional to G/R 0.5 [44].
Considering these welding parameters, there are two key solidification parameters that determine solidification microstructure, which are solidification growth rate (R) and temperature gradient (G) [45].Kou et al [46] revealed the effects of the parameters used in the welding process on the grain structure formed in the weld seam.According to this study, it is possible to say that both heat input and welding speed increase, the temperature gradient (G) tends to increase only slightly or even decrease.Since the growth rate (R) increases with increasing welding speed, the G/R ratio tends to decrease.Additionally, according to the constitutional undercooling theory, the lower the G/R ratio during the solidification of an alloy, the greater the amount of constitutional undercooling (ΔT C ).At low heat input and welding speed, the G/R ratio is high and the amount of constitutional undercooling (ΔT C ) is low; at high heat input and welding speed, the G/R ratio is low and the amount of constitutional undercooling (ΔT C ) is high.
Besides welding parameters, constitutional undercooling that has a significant effect on the solidification microstructure can also be achieved by partitioning the alloying elements through inoculation in the melt during solidification.Therefore, it is worth mentioning that the chemical composition also has a significant effect on nucleation [47].When the inoculation is not used, approaches developed to obtain a fine-grained structure in the weld metal generally aim to reduce the thermal gradient by providing an enhanced weld pool convection with the pool stirring effect.By reducing the thermal gradient, dendrite fragments and detached grains can survive in the weld pool and grow into equiaxed grains [42,48,49].

Weld metal modification approaches in aluminum arc welding
In arc welding processes, studies have been made in the literature to prevent crack formation during solidification [50,51].Factors affecting weld metal solidification cracking can be considered metallurgical and mechanical and often interact with each other, as mentioned before [52].
The susceptibility to solidification crack formation is significantly influenced by the composition of the weld metal, and therefore suitable filler metal selection is an important method to control solidification cracking [53,54].Another way to control solidification cracking is refining the grain structure in the weld bead with the effect of external factors [55][56][57][58].Coarse columnar grains are generally more prone to solidification cracking than refined and equiaxed grains.This is because the refined and equiaxed grains can be more easily deformed to accommodate strains from shrinkage.The fusion zone in the welding process typically includes coarse columnar grains resulting from the thermal conditions during the solidification of the molten weld metal.This situation results in low mechanical properties and poor resistance to solidification cracking.For this reason, different methods have been used in the literature to improve the fusion zones in the welding processes [58][59][60].
As can be seen in figure 5, weld metal modification approaches in the literature consider three types of new grain formation mechanisms, which are dendrite fragmentation, grain detachment, and heterogeneous nucleation [33,61].The new grains formed by the aforementioned mechanisms prevent the growth of columnar grains and allow the formation of equiaxed grains in a wide area.In other words, the more new grains are formed in the weld seam, the larger the area where the equiaxed grains will be found [62].
In the 'dendrite fragmentation' mechanism [63], the dendrite arms may be cut off by fluid flow and carried into the weld pool bulk.Here, the fragmented grains act as nuclei if they survive.Similarly, the 'grain detachment' [63] mechanism is explained by the separation of partially melted grains in the liquid-solid mixture during the welding process under the effect of fluid flow.This situation also has a similar mechanism to the fragmentation of dendritic grains to obtain more nuclei.If these nuclei can maintain their stable structures in the weld pool, they take an effective role in equiaxed grain formation.On the other hand, in the 'heterogeneous nucleation' mechanism [63], which has a different mechanism from the dendrite fragmentation and grain detachment mechanisms, inoculants or particles with high stability take part in the weld pool at a high temperature.These particles behave as embryos for heterogeneous nucleation during solidification [63].
Figure 5 shows the two methods for the formation of grain refinements in the welding process of aluminum alloys.The first is the growth of pre-existing nuclei created by dendrite fragmentation or grain detachment, and the second is heterogeneous nucleation at second-phase particles present in the melt [64].With the inoculation process, the weld metal is tried to be improved by increasing the amount of particles, which will act as heterogeneous nucleation sites in the structure, and increasing the amount of constitutional undercooling.On the other hand, methods such as stirring the weld pool and using pulsed arc current, which provide a lower temperature gradient and allow nucleation, promote dendrite fragmentation and grain detachment [65][66][67].
Fine-grained engineering materials generally have higher strength and ductility than coarse-grained materials.It is generally aimed to obtain a fine-grained structure in the weld metal because such a structure causes a decrease in the sensitivity of the weld metal to solidification cracking during welding and serves to improve the mechanical properties such as toughness and ductility of the weld metal [68].Therefore, the existence of aluminum alloys with improved weldability and mechanical properties by obtaining fine-grained structure has a great impact, especially in the transportation industry where the reduction of weight processes is critically important [69,70].
Modification approaches, which are frequently used for arc welding of aluminum alloys in the literature, have been developed to use the three different nucleation mechanisms mentioned before.As can be seen in figure 6, the inoculation mechanism is one of these approaches.In addition, methods using external factors have been also developed in order to activate the grain detachment and dendrite fragmentation mechanisms.Within the frame of references, it is possible to review the methods developed for the improvement of weld pool convection and thermal conditions under the headings of weld pool stirring and pulsed arc current.Ultrasonic treatment and magnetic arc oscillation approaches are the methods aimed at improving grain morphology by stirring the weld pool.

Inoculation
The weld bead is the section where high residual stress and large distortions occur during solidification.In addition, columnar grain morphology having epitaxial growth tendency occurring in the weld bead also causes inhomogeneous mechanical properties in the structure.Thus, this region is a significant point to be emphasized that also determines the performance and service life of the workpiece [71,72].Therefore, grain refining, which enhances mechanical properties, is an important issue for arc-welded joints [70].One of the methods used to obtain a grain structure with homogeneous, equiaxed, and isotropic properties is inoculation.Although this approach is generally used in casting processes, there are studies showing its effectiveness for welding processes as well.Inoculants that will provide nucleation are added to liquid metal and solidification is expected to occur.As a result of this process, the formation of fine and equiaxed grains is supported [73].
In arc welding processes of aluminum alloys, filler metals are generally modified with master alloys.The grain refinement effect of alloying elements added to the structure with master alloys is explained by the grain growth restriction factor (GRF).The growth restriction factor can be defined as a physical quantity related to the initial rate of constitutional undercooling development and can be used as a direct criterion for the effectiveness of grain refinement in Al alloys containing potential nucleation particles [74].In this factor explained by the relation m.C 0 .(k−1);m defines the liquidus gradient, C 0 defines the bulk composition, and k defines the partition coefficient between solid and liquid.Constitutional undercooling produced by solute elements provides the driving force for nucleation and restricts grain growth.The GRF values of various alloying elements in aluminum alloys are shown in table 1.It has been demonstrated that some alloying elements such as Ti, Ta, V, and Zr refine the grain structure of pure aluminum due to their relatively higher GRF values [75].The principle is to either introduce nucleants (externally added or in situ formed) to promote heterogeneous nucleation or add solutes with a high growth restriction factor (GRF) to rapidly generate sufficient constitutional undercooling for nucleation at the front of the solid-liquid interface [76].
Aluminum alloys are frequently modified using master alloys containing Al-Ti-B.The scientific fundamental of this master alloy is that Ti reacts with B forming TiB 2 particles and with Al forming Al 3 Ti intermetallic particles.If the master alloy is added to weld pool through filler metals, TiB 2 particles act as nuclei, while Al 3 Ti intermetallics dissolve into the melt on the base of the peritectic reaction forming α-Al (L+Al 3 Ti → α-Al).The Al 3 Ti layer formed on the TiB 2 surface also contributes to the nucleation of α-Al [78].
Although titanium-containing master alloys are frequently used for inoculation purposes, it is also suggested that titanium-containing compounds may lose their effectiveness in the presence of exceeding 4% Si amount.At high silicon amounts, compounds such as TiSi, TiSi 2 , and Ti 5 Si 3 occur between titanium and silicon due to 'silicon poisoning'.These compounds covering the surface of the TiB 2 particle adversely affect the nucleation process [78].In addition, it is also stated that the TiC compound is not stable at high temperatures because it can react with liquid Al to form the Al 4 C 3 compound, resulting in the refinement fading.Therefore, studies in the literature have emphasized that intermetallic particles must meet the following conditions in order to function as heterogeneous nuclei [79,80].These are; (1) high chemical stability to prevent interactions with alloying elements, (2) high melting point to prevent them from dissolving in the melt, (3) low lattice mismatch between substrates and solid to reduce the interfacial energy between substrates and solid [81].
Considering these situations, Ding et al [81] recommended niobium-containing compounds.The melting points of Al 3 Nb, NbB 2 , and NbC are 1680 °C, 3036 °C, and 3490 °C, respectively.Therefore, their melting points are high enough that they do not melt in aluminum melt.Al 3 Nb, NbB 2 , and NbC also have similar lattice structures and constants as Al 3 Ti, TiB 2 , and TiC, respectively.Therefore, these compounds may be promising for the weld metal of aluminum alloys.When these situations are evaluated, it is possible to say that although successful results have been obtained with different inoculation studies in the literature, laboratory-scale research on different elements and compounds is still proceeding.Compounds that have successfully formed fine grains in arc welding processes of aluminum alloys are presented in table 2 through literature research.
Su et al [92] also inoculated the ER5356 welding wire using Al-10Zr and Al-2Sc master alloys and then performed the arc welding process of the 5083 alloy.Figure 7 shows the effects of scandium and zirconium elements.As can be seen, the grain refining effect increased with the increase in the amount of scandium and zirconium.In this study, it was observed that zirconium was better than scandium for the mentioned alloy.This situation was explained by the fact that the compounds formed by different alloying elements in the structure had different growth restriction factors.
Schempp et al [19] also performed Tungsten Inert Gas (TIG) welding of 6082 aluminum alloy by alloying the filler wire with Al-5Ti-1B.In addition to eliminating the solidification crack, the columnar structure in the fusion zone turned into an equiaxed structure.In this study, TiB 2 and Al 3 Ti compounds were shown to aid the nucleation process, similar to the previous study.Thanks to these nucleation agents, a fine-grained structure was obtained.

External factors
It has been demonstrated by different authors [93][94][95][96] that the grain-refined microstructure not only reduces the susceptibility to solidification cracking of alloys but also enhances the mechanical properties of the weld metal such as strength, toughness, ductility, and fatigue life.In addition to the inoculation, there are also different approaches aiming to improve the weldability of aluminum alloys by using external factors.These methods, which do not require any specific filler wire production, include processes such as stirring the weld pool [97,98] and the use of pulsed current [99].

Weld pool stirring
It is a well-known fact that conventional fusion welding methods are not useful for grain refining.During the arc welding process, the grain structure of the weld metal becomes coarse and thus causes deterioration of the welding mechanical properties [100].A number of studies [101][102][103][104] have found that stirring the weld pool is beneficial for enhancing the properties of welded joints.Due to the weld pool stirring effect, dendrite fragmentation and grain detachment occur.Then, fragmented and detached grains, which are carried into the weld pool by the fluid flow, act as new nucleation sites.In addition, by stirring the weld pool, it is also aimed to reduce the temperature gradient of this region and thus maintain the stability of nuclei.On the other hand, it should be also emphasized that breaking the short and densely formed dendrites is a difficult process in the small-sized weld pool.Therefore, it is also possible to say that obtaining new nuclei by stirring the weld pool is more complex compared to the inoculation process [68,105,106].
Weld pool stirring process can be applied using ultrasonic vibration and the use of an external magnetic field.

Ultrasonic treatment
Ultrasonic grain refinement is a method used by applying high-frequency vibrations to the molten metal during the arc welding process.Obtaining a fine-grained weld seam by using ultrasonic vibrations is explained by two basic theories.The first of these is that the bubbles collapse and generate shock waves under the action of sound pressure exceeding a critical threshold.These shock waves break the crystallized and grown grains and cause the grains to become finer [107].The second theory is explained by the local melting point increase resulting from the high-pressure shock wave caused by cavitations.The amount of the melting point increasing (ΔT) can be calculated according to the Clausius-Clapeyron relation (3); In this formula (3), T m refers to the melting point, ΔV and ΔH represent the volume and enthalpy change during solidification, respectively, and ΔP refers to the change in pressure [108].In this case, a rise in the local pressure resulting from the collapsing of cavitation bubbles in the liquid metal can effectively increase the solidification temperature of the melt and hence the amount of undercooling [109,110].
As reported in aluminum alloys [111], ultrasonically induced cavitation in the molten pool helps the nucleation of the FCC solid phase.The nucleation of new grains in the molten metal promotes an increased equiaxed region in the weld joints and a narrowed columnar region.In addition, with the use of ultrasonic vibration, the temperature gradient in the welding decreases to lower values.The rate of the temperature gradient is a factor that determines the solidifying microstructure.The lower the temperature gradient rate is, the higher the possibility of the formation of equiaxed microstructures.Additionally, vibrations also cause an increase in the amount of undercooling, a shortening of the crystallization time, and the fragmentation of the columnar structure [112][113][114].
Cai et al [110] performed the TIG welding process of 2219 aluminum alloy under different ultrasonic vibration forces in their study.In figure 8, it was observed that the grain sizes were decreased resulting from the flow movement of molten liquid metal in the weld pool and cavitation occurring as mentioned before.Cavitation bubbles formed in the weld pool cause an acoustic pressure to arise while trying to expand.With the increase of pressure in the liquid metal, the crystallization temperature and undercooling amount of the molten metal increase.Thus, with ultrasonic cavitation, the radius required for critical nucleation is reduced and a higher rate of nucleation is activated.In addition, high pressure created by the cavitation bubbles forces dendrites to break.Fragmented grains, which take a finer form by breaking, behave as new nucleation particles.Thus, while nucleation particles increase, the grain size decreases.
It is also reported that although ultrasonic treatment has the advantage of obtaining a fine-grained weld seam in the welding process, it should be taken into consideration that the production of high-power ultrasonic equipment and high-temperature resistant horns requires high costs.Additionally, the weakening of ultrasonic intensity by high-temperature arc plasma is suggested as another disadvantage of this method [96,115].

Magnetic arc oscillation
In the arc oscillation method, grain refinement is carried out by breaking the dendrites and increasing nucleation points [116][117][118].In this technique shown schematically in figure 9, an alternative external magnetic field is used to deflect the welding arc in a controlled manner.As it is known, plasma flow force (F p ) and electromagnetic pinch force (F e ) occur with the electric field and electric current formed in the natural structure of the welding arc.F p has an effect that forces the charged particles to move toward the workpiece and creates deformation in the molten pool.The electromagnetic pinch force, on the other hand, has an effect that increases the stability and power density of the plasma.When an external magnetic field force (F L ) is generated in the arc region shown schematically, the F L will be a function of the magnetic flux density (B), movement distance (L) of the particles in the plasma, and the current (I).Considering the Lorentz force acting on the particles moving in an equal magnetic field towards the inside of the page, a right and downward force will appear on the particles (B) moving in the left part of the arc symmetry.A right and upward force will act on the particles (A) moving on the right side  of the arc axis.The periodic and continuous change in the direction of the external magnetic field created in this assumption causes the arc to oscillate [119].
Two main mechanisms of the magnetic field are concluded in studies.Firstly, fluid flow and heat transfer within the molten pool can be changed by the magnetic field, due to the stirring of the weld pool.Secondly, the magnetic field had a remarkable effect on welding plasma [120][121][122].
Forces resulting from the external magnetic field increase local Lorentz forces in the weld pool, thereby increasing fluid flow and reducing temperature gradients.In addition, arc oscillation constantly changes the shape of the weld pool, and therefore the direction of the maximum thermal gradient at the solidification boundary changes with time.This situation leads to the formation of new grains oriented according to the instantaneous direction of the maximum thermal gradient that provides grain refinement, instead of grains growing in a columnar structure.In addition, magnetic arc oscillation also creates dendrite fragmentation at the rear end of the weld pool, detachment of partially melted grains from both sides of the weld puddle, and hence new nucleation points [61,123].Increased fluid flow due to the magnetic field helps in transferring dendrite fragments and detached grains to the region of constitutional undercooling just ahead of the solid-liquid interface.Reducing thermal gradients due to the magnetic field also makes these nuclei more effective by prolonging their survival [61].
Arc oscillation also leads to two other important microstructural effects; a widening of the chill zone adjacent to the fusion boundary and the formation of fine-grained bands distributed along the length of the weld [61,124].Regarding the former, the reduction of thermal gradients due to the magnetic field can be expected to expand the region in which nuclei survive.Thus, nucleation on these particles occurs in a larger area.The formation of fine-grained bands in the interior of oscillatory sources can also be explained by differences in nuclei dissolution.In non-oscillatory welds, these nucleant particles may not survive the high temperatures maintained continuously throughout the center of the weld pool.In oscillatory welding, on the contrary, the weld pool expands, fluid flow increases and thermal gradients decrease.In addition, simultaneous back-andforth movements of the arc cause the formation of regions in the weld pool where the effect of overheating is less than in other regions.Nucleant particles can survive better in these regions of relatively low temperature, and the chill zone at the fusion boundary can then extend into these regions.Bands form at fairly regular intervals between them due to periodic reversal of arc motion [61].
Choosing suitable magnetic field types and parameters creates a desirable effect, such as acceleration, porosity suppression, or stirring in the molten pool.In addition, an external magnetic field is also applied to homogenize the elemental distribution and improve the microstructure [125][126][127].
Rao et al conducted a study [118] examining different arc oscillation parameters.In this study, it was emphasized that columnar grains growing in the partially melted zone could be prevented and equiaxed fine grains could be formed in the weld metal by choosing appropriate parameters.However, when working with non-ideal parameters, it was revealed that epitaxial growth of columnar grains in the weld metal might occur due to the insufficient stirring of the weld metal to break the dendrite tips.It was stated that the reason for this was the use of arc oscillation at high amplitude and high frequencies.Additionally, it was also stated that arc oscillation reduced penetration and expanded the area under the influence of heat, and it was emphasized that the amplitude and frequency parameters used in arc oscillation should be chosen as little as possible.
Wang et al [119] also examined the effect of magnetic flux density on maximum arc pressure and emphasized that this parameter was extremely important when applying the same welding parameters.Further, they revealed that the maximum arc pressure gradually decreased with increasing magnetic flux density.According to this study, it can be concluded that when the magnetic flux density increases, the arc impact force and therefore the maximum arc pressure decreases due to larger arc deflection.In addition, the uniformity of the arc pressure can be improved in the direction of the weld width with increasing magnetic flux density.Considering the results of this study, it can be also inferred that magnetic arc oscillation makes the arc pressure distribution relatively more uniform.In addition, Liu et al [128] indicated that the fluid flow in the weld pool under the influence of the external magnetic field is beneficial for the bubble to escape from the molten weld pool and suppress porosity.
In their study, Wu et al [129] performed the welding process of 6N01 aluminum alloy using the magnetic stirring technique.In the study where they performed Metal Inert Gas (MIG) welding process using an electromagnetic coil, they obtained the grain morphology shown in figure 10.As can be seen in the figure, it is possible to say that the grains were significantly refined with the process.Considering the aforementioned grain refinement mechanisms, it can be inferred that the fragmented dendrites or detached grains from the partially molted zone due to the Lorentz force applied fluid flow in the weld pool may act as active nucleation sites under changing thermal conditions.In addition, it was also revealed by the EBSD results that the chemical compositions of the grains in the weld seam were also uniformly distributed.In the weld seam formed under the conditions where no grain refinement method was applied, it can be seen that the grains were coarse-sized and the structure had a non-uniform chemical distribution.
In addition, Ram et al [61] also performed the TIG welding of 2090 and 7020 aluminum alloys using the magnetic arc oscillation method.In this study, it was shown that in welding processes where arc oscillation was not used, the weld seam contained intensely columnar and coarse-sized grains.On the other hand, in the welding process where arc oscillation was used, it was observed that mostly equiaxed grains were formed.Similarly, Yan et al [130] also performed the TIG welding of 6061 and 5052 aluminum alloys in order to examine the effect of arc oscillation.This study revealed that typical coarse columnar grains emerged when oscillation was not used and it was also observed that the microstructure transformed into a fine-grained structure after arc oscillation.
Generally, smaller grain sizes can be achieved with an oscillating arc compared to traditional welding methods.The reason for the formation of finer grains during the arc oscillation process is that when the molten pool has just solidified and the temperature is still relatively high after the previous arc period, the arc returns to the position near the previous molten pool and reduces the temperature gradient of the pool.The oscillating arc allows the temperature of the previously formed seam to drop.In this way, it prevents the growth of dendrites.In addition, the main grain refining effect is also explained by the fragmentation of dendrites and grain detachment [130].
High welding quality and efficiency can be achieved when external magnetic field parameters are properly matched with welding parameters such as arc voltage, welding current, and welding speed [103].On the other hand, it was also suggested by the researchers that, if the compatible parameters were not selected properly, arc instability might occur in the presence of magnetic fields used to make the welding arcs oscillate.These imbalances in oscillation could cause the arc to fade out, even temporarily [131].

Pulsed arc current
Pulsed current welding is a variation of constant current welding that involves changing the welding current from a high level to a low level at a selected regular frequency [132].Pulsed current welding can be used with methods such as MIG and TIG welding methods.In the MIG welding method, the base current value enables the electrode to melt by ensuring the formation and continuity of the arc [5,133,134].The peak current value also ensures the penetration of the melt droplet at the tip of the electrode into the workpiece [135].In the TIG welding method, the peak current is generally selected to provide sufficient penetration, while the base current value is set at a level sufficient to maintain a constant arc [136].Additionally, pulsed current TIG and MIG welding methods can be also used with alternating current.The main difference between AC and DC modes is introducing a negative polarity period into the current range.With this inversion, the advantages of using each polarity are combined.The AC positive cycle, where current flows from the base metal to the electrode (electrode positive), acts as a surface cleaning, breaking down the oxides that limit the quality of the weld, while the necessary weld penetration is achieved during the negative cycle [137].
In conventional welding, fusion zones typically exhibit coarse columnar grains due to the thermal conditions present during the solidification of the weld metal.This often results in poor mechanical properties of the weld and poor resistance to solidification cracking.For this reason, it is desirable to control the solidification structure in welding processes however; it is a fact that this is quite difficult due to high temperatures and high thermal gradients in welding processes [132].Unlike constant current welding, the heat input can be reduced by decreasing the average welding current used during the welding process by choosing the current and frequency parameters appropriately in the pulsed current welding processes.This situation allows the heat to dissipate into the base material, resulting in a narrower heat-affected zone [138].
In addition, it is also stated that welding with pulsed current also has a beneficial effect in reducing the amount of pores.This can be explained by the fact that the weld pool vibrates step by step with a high pulse current, resulting in gas bubbles being expelled from the weld pool.The arc force generated by the pulse current frequency causes the liquid weld pool to be stirred, thus helping gas bubbles escape more easily in the weld pool [139,140].
As shown in figure 11, I p and I b represent the peak and base currents, respectively.In addition, t p and t b show the pulse-off time and pulse duration, respectively.The variables here constitute the parameters of the pulse welding process compared to the constant current welding method [141].Improper selection of these parameters and their application in the welding process leads to the formation of welding defects such as lack of fusion, undercut, burn-backs, and stubbing-in.For this reason, the appropriate selection of these parameters constitutes an important stage of the method [142].
As it is known, it is very difficult to obtain fine and equiaxed grain structure in the weld seam in the constant current welding process [139].This difficulty arises from the high temperature in the fusion area and the melting of the structures expected to act as nuclei at this temperature [132].For this reason, it is possible to obtain a finegrained weld seam with the selection of appropriate frequency and current values.
Kumar et al [143] conducted a study examining the effects of the peak and base current values used in the process.In this study, it was stated that if the peak current was lower than the required value, insufficient melting in the filler metal caused a lack of penetration.On the other hand, if the current value was higher than the required value, the stability of the arc decreased and excessive melting in the filler metal occurred.In addition, Balasubramanian et al [58] demonstrated significant refinement in the grain sizes of the weld seam obtained by using the pulsed current process.In this study, it was emphasized that the thermal fluctuation effect increased as the peak/base current ratio increased and as a result of these fluctuations, periodic interruptions occurred in the solidification process.Similarly, it was also emphasized by Wang et al [144] that grain size could be effectively reduced at constant heat input with increasing current amplitude during the pulsed current welding process.
Manti et al [145] also made an examination including the effects of parameters such as pulse duration and pulse frequency.In this study, significant decreases in the grain size of the weld seam of the aluminum alloy were revealed by increasing the pulse frequency.In addition, it was also emphasized that using a high pulse duration also increased the grain size significantly if a high peak current was applied.Considering these results, it was stated that if the parameters caused the heat input to increase, this would cause the cooling rate to decrease and the solidification time to increase.
The effects of the frequency parameter used in this method were also investigated by Manti et al [59] performing the TIG welding of Al-Si-Mg alloy.In this study, it was revealed that an increase in frequency resulted in a significant decrease in grain size.The grain refining that occurred in this study was explained based on three reasons.The first of these was the rapid cooling that occurs with the falling heat input and the finegrained structure formed as a result.Secondly, the turbulence that occurred in the weld pool caused the growing dendrites to break down and act as nucleation.In this way, structures that would serve as a large number of nucleations were obtained.Lastly, the pulsed current flowing between peak and base currents caused undercooling because the net heat input to the weld pool was suddenly reduced during the base current step of the welding process.

Conclusion
Solidification cracking is a frequently encountered defect in arc welding processes of aluminum alloys and is often discussed in the literature.In this review paper, the formation of this defect in aluminum alloys is explained fundamentally and the attempts in the literature to eliminate this defect are presented.The inferences obtained through the literature research are as follows; • The weldability of aluminum alloys has been described by many researchers as susceptibility to solidification cracking.This welding defect generally results from the wide solidification range of aluminum alloys.The solidification process, which occurs with an insufficient number of nuclei and low undercooling, causes the formation of dendritic and columnar grain structures under a wide solidification range.In the absence of sufficient liquid feeding, along with the stresses and strains that occur during the solidification process, solidification cracks occur between the grains.For this reason, the solidification cracking is also referred to as an intergranular defect.
• Studies reveal that the equiaxed grain structure is more resistant to solidification crack formation in contrast to columnar and dendritic grain structures.Therefore, in order to eliminate the dendritic grain structure in the welding process and create an equiaxed grain structure, it is recommended to increase heterogeneous nucleation by inoculation process and perform dendrite fragmentation and grain detachment processes using external forces.In addition, obtaining high undercooling also allows nucleation to occur more effectively.
• In the inoculation process, filler metals are modified with different alloying elements and/or compounds that can maintain their stability in liquid metal.In this way, a fine-grained weld seam can be obtained with a higher number of nuclei and undercooling.In addition to the successful results obtained with this method, laboratory-scale research still proceeds for compounds that can undertake the nucleation task without being affected by other alloying elements in the composition.Recent studies focus on the use of niobium-containing compounds, revealing that previously recommended titanium-based compounds have limited effectiveness due to silicon poisoning.
• In addition to the inoculation process, studies are carried out on grain refinement processes in the weld metal using ultrasonic vibrations, external magnetic field, and pulsed current.These methods aim at dendrite fragmentation and grain detachment in the weld metal.
• With ultrasonic vibrations, fragmented and detached grains are carried from the partially melted zone to the weld pool with fluid flow.Additionally, by reducing the thermal gradient and ensuring high undercooling, these nuclei carried into the weld pool are expected to form a fine-grained structure.However, in addition to the studies carried out with samples that are not large-sized, it is also stated that there is a need for ultrasonic vibration generators for large-sized parts and horns resistant to high temperatures.
• By using an external magnetic field, fragmented and detached grains are transferred to the weld pool by fluid flow.Periodically oscillation of the arc provides undercooling and thus ensures that the nuclei in the weld pool form equiaxed grains.In the literature, there are successful examples of the use of magnetic fields in the welding process.However, research also emphasizes that the magnetic field parameters used to obtain magnetic oscillation must be compatible with the arc parameters.Incompatible selected parameters may lead to deterioration of the stability of the arc.
• In the pulsed arc current process, heat is applied to the material with different frequencies.In this way, it is aimed to obtain an equiaxed grain structure with increasing undercooling and cooling rate in the weld metal with a sudden decrease in heat input during the base current step.Although it has many advantages, it is also indicated that the use of pulsed current welding is limited because the process can only be performed with optimum pulse parameters.

Figure 1 .
Figure 1.Interactions between thermal-metallurgical and thermal-mechanical factors.Adapted from [30], with permission from Springer Nature.

Figure 2 .
Figure 2. Dendritic grain growth and the cross sections in melted zone during solidification.Adapted from [34], Copyright (2016), with permission from Elsevier.

Figure 9 .
Figure 9.The oscillation (a) and the forces (b) obtained through an external magnetic field.Adapted from [119], with permission from Springer Nature.

Figure 10 .
Figure 10.Grain morphologies and size distributions of the welds joining without (a) and with (b) the external magnetic field.Reprinted from [129], Copyright (2022), with permission from Elsevier.

Table 2 .
Nucleation compounds for arc welding of aluminum alloys.