Welding
Welding is a fabrication process that joins materials by using high heat to melt the parts together and allow them to cool, causing fusion. Welding is distinct from lower temperature techniques such as brazing and soldering, which do not melt the base metal.
Dissimilar welding: uses, challenges, and limitations
Various industrial structures require the welding of dissimilar metals to achieve high strength to weight ratio. A wide variety of metals are used to impart complex shapes and enhance the loading capacity needed in different parts of the components. This also enhances the worth of these components as the materials are used per application requirements. High strength materials such as titanium and steel are used in the part of the component where the load-bearing capacity of the component should be high. In contrast, light metals such as aluminum and advanced lightweight materials are essential to better the fuel economy without compromising safety and performance, especially in the transportation industry. Therefore, it becomes a reality that the multi-material component is the future. The traditional steel components in the automatic industry are being replaced with lighter and stronger materials such titanium alloys, polymer composites, metal-reinforced composites, etc.
During the fabrication of structures, welding of dissimilar materials is essential, which could also compose dissimilar materials with different thicknesses depending on local requirements. This kind of structure not only reduces the weight of the components but also fulfills the local need. It becomes cost-effective as it distributes the weight and material properties in the final structure with a substantial reduction in the weight and cost of the hybrid components. In addition to the body structure, dissimilar welding is also used in some critical components, such as turbocharger impeller for high-efficiency gas and diesel engines. The impeller is made of carbon steel and Inconel [1]. Since dissimilar welding produces a lightweight component with superior performance aircraft industry also takes the help of dissimilar welding in advanced future aircraft, especially in airframe and propulsion systems [2]. Various highstrength metals such as titanium steel have been friction stir welded with softer metals such as Al and Magnesium to be used in the airframe structure. Considering the complexity involved in the designing aspects in aero-plane or automotive structures, a successful and reliable joining of dissimilar metals is important from industrial point of view.
Conventional vs solid-state welding
Conventionally fusion welding has been used to join two similar and dissimilar welding. Due to high temperature in fusion welding compared to that of solid-state welding, fusion weld usually suffers from porosity, residual stress, distortion, and evolution of intermetallic phases formation. Even though solidstate welding is not able to completely eradicate all problems associated with fusion welding, but it possesses a substantial advantage in regards to the comparatively less distortions and loss in base metal strength along with less fraction of intermetallics [3]. Solid-state welding and fusion welding contrast to each other in terms of the large temperature gradient, material flow characteristics, and substantial change in weld mechanical properties [4]. It is necessary to mention that welding by solid-state welding method is costly compared to the fusion welding method and these machines are seldom movable due to its requirement for high power, bulky space, and weight.
Friction Stir Welding
Friction stir welding (FSW), also referred as a “green welding” technology, was invented at The Welding Institute UK in the year of 1991. It is considered one of the most significant inventions in metal joining technology due to its ease of operation, efficiency, versatility, and eco-friendliness. Initially, FSW has been applied to weld aluminum alloy and other low temperature alloys, which are difficult to weld by conventional fusion welding method. Later on, FSW was introduced to weld other materials such as Mg, Ti and Steel.
In FSW, a non-consumable, constantly rotating tool having a profiled nib and a shoulder is fed in between the abutting edges of the metals to be joined and subsequently traversed along the line to form a continuous joint. The metal edges get plasticized and mixed each other, forming a joint after cooling down.
Detailed description-related to the FSW process has been described by several researchers in the past [1,5]. Principles of operation of the FSW process and important nomenclature have been explained in Fig.1. It is important to mention that the selection of materials positioned in the advancing side (AS) and retreating side (RS) of butt weld are very critical, especially in dissimilar FSW.
Fig.1 Schematic of Friction Stir Welding technique [6]
Due to frictional contact of tool shoulder and pin, frictional heat is generated, which deforms the processed material and consolidates the displaced materials in the stirred zone (SZ). The rate of heat generation also depends on the tool rotation and welding speed which can be optimized to have good quality joints.
The SZ material is rigorously deformed due to tool stirring, and hence, the grain size of the material gets substantially refined. As it is known that material strength is inversely proportional to grain size, FSP sample exhibits superior mechanical properties. However, the mechanical property of the FSW sample primarily depends on strength of the interfaces between titanium and aluminium along with microstructural refinement taking place in the weld nugget. FSW is considered as solid-state welding because no melting takes place as this method is based on the coefficient of friction, which becomes tends to zero at melting temperature. The reduction of the coefficient of friction leads to a reduction of heat input, which results in temperature not being able to increase above a particular value. The self-controlled nature of heat generated and temperature does not allow the processed material to melt. FSW is beneficial in regards to enhanced mechanical property and reduction of defects, commonly observed in fusion welding. However, it is essential to mention that the wide application of FSW solely depends on the wear of the non-consumable tool during welding. The tool produces the thermo-mechanical deformation and frictional heating necessary for stirring. During traversing, the tool is subjected to two types of load; normal and tangential load. In addition, the tool is also subjected to a transverse load if the two materials being welded are different. This difference in the properties of the two metals will cause this transverse load. Initially cylindrically threaded and concave shoulder tools made of tool steel were used to weld aluminum through FSW. At present, most of the tools contain features designed for specific functions and to maintain the amount of deformation, deformed region and temperature evolution during the weld. It is desirable to retain these features till the end of the weld. Further replacement of the tool, especially when it is made of high-temperature alloys, would be expensive, thus limiting the industrial viability of the process. Hence, selection of tool material and dimensional stability are the important aspects of research and development of this process.
Advantages & disadvantages of FSW
Advantages
A. Due to solid-state joining, the materials do not reach their melting point temperature thus several issues associated with fusion welding processes such as distortion, porosity, and intermetallic can be significantly suppressed.
B. It helps to retain dimensional stability due to low temperatures and low distortion.
C. It exhibits dynamically recrystallized microstructure in SZ, which is responsible for better mechanical properties of the weld region.
D. Due to solid-state and low temperature in FSW process, there is no loss in alloying elements.
E. Dissimilar FSW can replace joints comprising of multiple parts and multiple materials used in fasteners.
F. Due to the absence of any shielding gas, or additional consumables flux or electrode, FSW is an environmentfriendly technique.
G. Dissimilar FSW used in hybrid structure reduces the weight of the component, which in turn decreases the fuel consumption in lightweight vehicles and aircrafts.
H. FSW consumes significantly less energy than that of modern fusion wildings such as laser welding.
Disadvantages
A. The loads experienced during FSW are high, and thus machine used in FSW is costlier and bulkier restricting its commercial application.
B. The instrument is bulky, and hence, it cannot be used remotely. It requires wide space with a high power connection.
C. Since FSW requires special fixtures and tools, it cant be used in particular complex design directly.
D. A specially designed tool having high-temperature stability and wear resistance is essential for FSW. Tool specification varies with material thickness and welding material. Therefore, a single tool cannot be used for different materials or thicknesses like fusion welding.
E. Since the forces experienced by the workpiece are high, friction stir welding (FSW) requires a special fixture to tightly clamp the workpiece in order to hold the material during welding.
Issues in FSW of dissimilar metals
The major operational issues encountered during similar and dissimilar FSW have been summarized by Mishra et al. [9]. These include material flow, thermal behavior, friction and deformation etc. All these key concerns are dependent on material properties and process parameters. Furthermore, they finally affect heat generation and material flow. It has been proved in a number of researches that material flow, even in a similar FSW process is a very complex phenomenon. This, in the case of dissimilar metal’s FSW, it will be more complex. The important aspects of dissimilar welding need to be understood.
They are,
A. The most important material property in dissimilar welding is the differences in melting temperature. This will develop asymmetry in thermal history, flow stresses, and thus the deformation and all other output parameters that include forces and material mixing. The placement of material in AS and RS of the weld and tool offset have to be optimized due to the difference in melting temperature and flow stresses. The differences in the flow stresses play an important role in selection of tool material and life of tool.
B. In dissimilar welding, the thermal conductivity of both the materials are different, leading to asymmetrical thermal profile across the weld centerline. Such asymmetrical thermal profile affects material flow, microstructural evolution, and coefficient of friction. Such variation will develop further asymmetry in shear force across the weld leading to variation in load experienced by the tool.
C. Two materials have a different coefficients of thermal expansion. Therefore, expansion during welding and contraction during cooling lead to distortion at the weld interface. In similar welding, such expansion, and contraction develop an internal crack. In dissimilar welding, this problem will be more severe than that during similar welding.
D. The flow stress of the materials in dissimilar welding varies with temperature at a fixed strain rate and strain. This influences the flow characteristics within the weld nugget.
FSW of Al and Ti
The presence of intermetallic phases in dissimilar welding of Al to Ti is inevitable even in solid state welding. The Al/Ti phase diagram can be used to predict the possible phase evolution. Murry et al. [7] have proposed the binary Al/Ti system. The major equilibrium phases present in Al/Ti system are face center cubic (FCC) Al, solid solution of Al with Ti, a number of intermetallic phases (Al3Ti, Al/Ti, Al2Ti and AlTi3), hexagonal close packed (HCP) αTi and body center cubic (BCC) βTi. Rawers et al. [8] showed that the intermetallics Ti3Al, Al3Ti along with Al/Ti are found to exist at the interface. They also proposed two-step reaction sequences for the evolution of intermetallics: Ti+3Al=TiAl3+ΔH1
This released exothermic energy (ΔH1) was sufficient to promote the further reaction between Al3Ti and Ti in the process of formation of TiAl. 2Ti+TiAl3=3TiAl+ΔH2 These two exothermic heat reactions provide the driving force for the formation of the other two intermetallics (Al3Ti and TiAl). Researchers have agreed with the fact that the reaction between Al and Ti gives rise to the evolution of Al3Ti intermetallic at the reaction interface. More thermal exposure to that interface composed of Al3Ti and Ti leads to the evolution of AlTi. Yi et al. [9] studied the influence of the heating rate and particle size on the formation of AlTi intermetallics. The heating rate showed substantial influence on the density of the product by reducing porosity that evolves when intermetallics form. The results also suggested that solid Al-solid Ti reaction leads to the formation of single-phase Al3Ti, whereas in a reaction between liquid Al and solid Ti the evolution of a mixture of all three possible intermetallic phases occurs at a temperature lower than the melting point of Ti. The cavities formed across the Al/intermetallic interface can be blamed for reduced bond strength. Loo et al. [10] found out that formation of titanium trialuminide (Al3Ti) intermetallic is because of the diffusion reaction in between the Al and Ti substrates. Hence to better the joint quality, the suppression of Al3Ti can be thought as primary solution. To analyze the microstructure characteristics, Kar et al. [11] made Electron Beam Scattering Diffraction (EBSD) analysis of Al-Ti FSW joints (Fig.2). In IPF maps, rotational flow and dotted regions are clearly visible (Fig. 2A), which can be titanium,
Fig.2 EBSD maps of aluminum at the surface of the weld nugget show an
(A) Inverse Pole Figure (IPF) map + grain boundary with a color code of grain orientation,
(B) IPF map with grain size < 5 μm,
(C) IPF map with grain size > 5 μm,
(D) GBCD map,
(E) Grain Orientation Spread
(GOS) map with a color code of average grain misorientation,
(F) grain size distribution map [11].
intercalated, or intermetallic particles. The material flow lines show the grain size variation and their orientation. The EBSD scan confirms uniform and homogenous deformation of Al grain in the SZ. White and black boxes in (Fig. 2A) are used to represent material flow lines. Fine-grain structures with a high fraction of second phase particles are bound with white boxes, while grain structures are marked with black boxes. In (Fig. 2B) and 2C, respectively, grains of size less than and more than 5 μm are shown. It is observed that very fine grains (grain size ≤5 μm) were formed across the second phase particles. Higher fraction of second phase particles with low grain size depicts extensive grain refinement in the FSW process [12] (Fig.2D). A Grain Boundary Character Distribution (GBSD) map of Al-Ti joint is given in (Fig. 2E). Significant accumulation of Ti particles is seen across the grain boundaries. Comparatively high fraction (∼72 %) of HAGBs is seen across the material flow lines while their distribution is varied with changes in the flow lines. To quantify the fraction of recrystallized grains without remnant deformation, a GOS is constructed from the same scan, and the result is shown in Fig. 2F. The GOS map is helpful in distinguishing strain-free (recrystallized; GOS ≤ 2°) and deformed grains (GOS > 2°). The GOS table associated with the GOS map indicates that ∼70 % of the grains are recrystallized. Fig. 3A shows an IPF map represented by the [001] IPF across the Al side in Al-Ti FSW joint. Refined microstructure of at Al side (grain size = 14 ± 8 μm) in SZ as compared to the base metal Al (average grain size of 45 μm) is seen. The grain orientation in the SZ is depicted by <101> and <111> ║ normal surface. The GBCD map shown in Fig. 3B reveals that the HAGB fraction (45 %) decreases in this region compared to weld surface. Fig. 3C shows the GOS map depicting the degree of misorientation inside a grain. This region exhibits a lower fraction (48 %) of strain-free grains (GOS < 2°) in comparison to Fig. 2C. The grain size distribution map (Fig. 3D) from the same scan exhibits a higher fraction of coarse grains with a higher range of distribution in the center of the SZ. This depicts the reduced grain stability in this region compared to the weld one [13].
Fig.3 EBSD maps of aluminum in the weld nugget show an
(A) IPF map + grain boundary with a color code of grain orientation,
(B) GBCD map,
(C) GOS map with a color code of average grain misorientation,
(D) grain size distribution map [11]
Zhao et al. [14] evaluated the Al/Ti FSW interface with different pin tools. They observed that pin length has major effect on the interface characteristics. In case of pin length of 2.9 mm, clear interface of Al/Ti joint was seen without any noticeable deformation of Ti substrates. When the pin length is kept 3.1 mm, an apparent deformation in Ti matrix was seen. Consequently the hook profile feature and Ti fragments were seen in the SEM images. In case of 3.3 mm pin length, size of hook feature and Ti chips was surprisingly increased. Thus it can be concluded that higher pin length promoted higher material flow leading to more deformation of Ti. The maximum value of shear strength and elongation was reported in case of 3.1 mm pin length, which were 147.5 MPa and 4.1% respectively. For 2.9 mm pin length, Al/Ti joints show 78.0 MPa shear strength and 1.7% elongation. Better interfacial bonding due to the formation of diffusion layer and intermetallic (IMCs) at interface under the influence of promoted interfacial pressure and temperature can be reasoned for better mechanical properties in case of higher pin length.
FSW of Al and Mg
Al and its alloys have good corrosion resistance to the outside atmospheric corrosion, high specific mechanical properties, etc., and become the main substitute for steels. On the other hand, Mg and its alloys have also received special attention due to its lightweight, low density (1.74 g/cm3) and remarkable characteristics. Mg is known to be the lightest metal found on the earth and is extensively sought after by a number of well-renowned industries. On equivalent circumstances, the strength/weight ratio for 1 kg Mg alloy could be approximately equivalent to 2.1 kg steel, and 1.8 kg Al. Several structures such as gearbox assembly, trunk lid inner panel, die-cast part and mainframe of engine blocks and oil pans, etc. have already been built successfully with Mg alloys [15].
In order to combine the properties of both the metals together and gain the best fit in terms of weight reduction, cost and performance, etc., a successful joining of Al/Mg alloys is vital. Cui et al. have mentioned that by proper design consideration, the dissimilar joints of Al alloy to Mg alloy can result in the weight saving up to 27.3%, without major compromise in the overall mechanical properties [16]. A successful joining of both dissimilar alloys is a key factor in the safe and efficient operation of the vehicle. A successful joining of both dissimilar alloys is a key factor in the safe and efficient operation of the vehicle.
However, the joining of Al and Mg alloy is a difficult task due to the poor plastic flow-ability of Mg alloy and enormous IMCs formation. Various welding processes such as vacuum diffusion bonding [17], laser welding [18], gas metal arc plug welding [19] and metal-inert-gas arc welding with different filler wires [20], etc. have been attempted in past. Unfortunately, most of them have been proven unattractive as the weld joints suffered from multiple defects such as spatters, cracks, voids [15]. Besides, the massive formation of brittle IMCs, which is due to the high heat input, has further degraded the joint strength and thus restricted their application in industrial usages [21]. The IMCs layer thickness as high as 120 micron (μm) has been reported by Wang et al. during metal-inert-gas arc welding of Al and Mg alloys [22].
Recently FSW technique has been found promising to join various grades of Al and Mg alloys. The FSW is a solid-state joining process that utilizes a non-consumable tool to join the faying surfaces of two metal without their melting. The heat is produced due to the frictional contacts in between the constantly rotating FSW tool and workpiece, which results in a softened and plasticized region adjacent to the abutting edges. During the tool traversing along the joining line, it intermixes the two metals plates across their contact edges and forges the hot and plasticized metals under mechanical pressure exerted by the tool. Apart from Al and Mg alloys, the FSW technique is capable to join other metals also such as copper alloys, titanium alloys, mild steel, and stainless steel, etc. More recently, it has also been successfully used to join the polymers. The FSW has its widespread applications such as in modern shipbuilding, trains, and aerospace applications.
While heating such as in the FSW process, the Al and Mg-rich phases do react to each other in the solid-state. The diffusion of Al/Mg substrates facilitates the formation of the intermetallic across the dissimilar interface and SZ. Primarily the IMCs phases such as Al3Mg2 or Al12Mg17 have been reported across the Al and Mg interfaces [23]. The formation of these IMCs phases largely depends on the localized concentration of the substrates. Upon further heating, the eutectic reaction Mg + Al12Mg17 → L takes place at the eutectic temperature of 437 °C. While another eutectic reaction occurs at Al + Al3Mg2 → L at the eutectic temperature of 450 °C. Based on the welding conditions, thin liquid films are formed across the dissimilar interfaces. The process of liquid formation taking place far below the melting point temperature of Al and Mg metals is called constitutional liquation [23]. During cooling, these two eutectic reactions are reversed, and the IMCs phases, Al12Mg17 or Al3Mg2 are formed depending upon the concentration of each substrate. The formation of IMCs is a critical concern during the dissimilar alloy’s welding on account of the following two reasons. Firstly, the IMCs can severely degrade the weld strength [24]. Secondly, the IMCs favors crack initiation and its propagation, which further degrades the joint quality. The weld structure shows the complex intermixing and flow pattern characterized by the intercalation lamellae in the SZ [25]. The SZ replicates the dynamic recrystallization (DRX) and clear observation of grain size as compared to the BMs. In the FSW process, the recrystallization is done by frictional heat produced at the shoulder and tool pin and also the heat produced due to the mechanical stirring of adjacent material. Though the FSW process is a lower heat input process, the formation of Al3Mg2 and Al12Mg17 are inevitable during the dissimilar joining of Al/Mg alloys joints under most of the process parametric conditions [26].
The dissimilar FSWed Al and Mg alloy joint is characterized by an interfacial layer which may be comprised of brittle IMCs phases Al3Mg2 and Al12Mg17 [26]. The existence of the Al12Mg17 IMCs phase is quite preferential due to its lower eutectic temperature and has been reported in past literature [26]. Primarily the Al3Mg2 IMCs phase is observed across the AA6061 side while that of the Al12Mg17 across the AZ31B Mg region. As visualized by Firouzdor et al. via TEM studies of the SZ, the Al3Mg2 IMCs consists of fine grains of Al3Mg2 but also the embedded nano-sized Al particles in it [27]. It does also symbolize the clear evidence of constitutional liquation as reported by Sato et al. [23]. For FSW joint, when AA6061-T6 is kept at AS and tool offset is taken +0.3 mm, joint with higher strength can be obtained. The combination of high rotation and welding speed results in lower weld strength. Fu et al. [26] reported maximum weld strength of 175 MPa at 700 rpm and 50mm/min when AZ31B is placed at AS and tool offset is kept 0.3 mm.
Recently some new variants of FSW are attempted to reduce the intensity of IMCs. Liu et al. [28] minimized the IMCs by using a small plunge depth of the tool pin into the lower sheet. Mofid et al. [29] added liquid nitrogen during the FSW the Al– Mg alloys. Kumar et al. [30] used Ni interlayer to suppress the diffusion of Al and Mg and hence reducing the IMCs formation. Among all, ultrasonic assistance in FSW is proven best to reduce the IMCs and enhance the joint quality [31,32]. Kumar et al. [33] added ultrasonic vibration assisted friction stir welding (UVaFSW) process and compared the results with conventional FSW. For the FSW joints made at 600/100 (600 rpm rotation speed and 100 mm/min welding speed), a substantial wide interfacial gap can be observed across the dissimilar faying surfaces (Fig.4). It does signify the poor material flowability and lack of intermixing of both the alloys across their interfaces.
Fig. 4 Al–Mg weldment interfacial microstructure (a-f) middle region (g-l) bottom regions [33].
Besides, microvoids and tunnel defects can also be seen for the joints made at a higher rotation speed, i.e. 1000/100 (Fig. 4i,j). A substantial thickened black colored line is characterized in most of FSW joints across the dissimilar interface which may represent the possible existence of IMCs layers. UVaFSW joints made with 800/100 do not suffer from any visible defect. However, the existence of the IMCs layer is still found across the dissimilar interfaces which get thickened with an increase in the rotation speed, i.e. 1000/100. With ultrasonic assistance, the interfacial gaps of Al–Mg alloys joints are substantially reduced, metal to metal penetration is increased, the IMCs layer is thinned or diminished, the cavities are filled, and the frequency of defect formation is greatly suppressed (Fig. 4b,d,f and h,j,l) [34]. It does also signify a considerable improvement in material intermixing and the flowability of Mg alloys with that of Al alloy during acoustic assistance. As ultrasonic vibrations induce additional strain and plasticization into the weldment [35], which can increase the solid diffusion of one’s atom into the other’s matrix. It has been reported that with ultrasonic assistance, the thickness of IMCs layers, as well as IMCs bands, can be substantially reduced while their compositions remain the same [35,36]. This it can be inferred that the ultrasonic assistance has a positive influence to enhance the metal-tometal intermixing, suppression of weld defects and reduction in IMCs. A reduction in weld anomalies can bring substantial improvement in the joints strength [34].
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