It is also a solid-state welding process, which means that the materials do not melt during the creation of a bond, which provides the opportunity to join dissimilar materials. With this process, it is possible to join tubular products as well as sheet materials, depending on the coil which is used.
Joining of conventionally non-weldable materials, in a quick and cost-effective manner, such as dissimilar material joints.
Development of complex workpieces or new products, which were previously not possible with the conventional joining processes. Contact-free: no marks of forming tools and processing of coated or sensitive materials is possible.
There is no heat, radiation, gas or smoke, shielding gas, which is less harmful for the operator. In hostile environments machines can perform the joining operation, avoiding supplementary investments in operator safety. It is possible to improve the work conditions of the welder or operator, since the technology is environmentally clean. One workpiece has to withstand the impact of the other, so it is possible that a mandrel or support is required to prevent deformation. The Belgian Welding Institute has acquired large experience with the electromagnetic pulse welding process.
If there is sufficient industrial interest, BWI wants to further investigate this very innovative welding process and to demonstrate its potential benefits for the industry.
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Welding of Magnesium AlloysParviz Asadi1, Kamel Kazemi?Choobi2 and Amin Elhami3[1] School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran[2] Research Center for Advanced Materials and Mineral Processing, Faculty of Materials Engineering, Sahand University of Technology, Tabriz, Iran[3] Department of Mechanical Engineering, Tehran Central Branch, Islamic Azad University, Tehran, Iran1. A typical MPW system includes a power supply, which contains a bank of capacitors, a fast switching system and a coil. In our research laboratory we proposed several new coiles instead of the solenoidal coil.
When the eddy current is flowing, Al sheet is pressed to the Fe sheet by magnetic pressure and are heated by Joule heat . The SEM image of the welded zone in a Aluminium alloy(A6016) and SPCC welded sample is shown as a exmple in the following pictuer. Comparison of the maximum tensile shearing strength for (a) the same Aluminum alloy combination and (b) different aluminum alloy combination:q the maximum tensile shearing strength of welded sample r the maximum tensile shearing strength of the based alloy without weld.
Our experimental results show that the weld joint is always stronger than the weaker metal and in all tested combination a discontinuous or continuous pocket-type, wavy transition layer was formed without any significant heat-affected zone.
Application of MPW using several types of coil :The upper shows the coil structure and the lower side shows the welded sample. The Belgian Welding Institute has started a project concerning joining of dissimilar plate materials.
Global trends force the industry to manufacture lighter, safer, more environmentally friendly, more performant and cheaper products: the machine construction sector is aiming at more performant machine components, for the consumers and construction products, increased integration of functionalities provides a competitive advantage and in the transport sector, weight reduction is pursued.
Multi-material design is however hindered by challenges in the field of joining technology. New developments for a range of welding processes has however provided the possibility to join dissimilar metals. Covering a wide range of welding technologies provided by the partnership BWI, KUL, CEWAC, SLV and LWF, as well as a wide range of materials, innovative solutions for several industrial applications are expected.
The overall innovation target is to create systematic and reliable knowledge and data about the applicability of promising joining processes for multi-material products. Resistance spot welding equipped with intermediate layers (process tape) in between the electrodes and the workpieces to be welded is a further development of the conventional resistance spot welding process. The presence of the intermediate layers (tapes), available in different alloys with different electrical and thermal conductivities, enables to gain substantially more control on the heat input the process. This process combines both thermal and mechanical joining principles, by creating a metal bond between an auxiliary joining element and the lower joining partner.
Like resistance element welding, friction element welding also combines thermal and mechanical welding principles by the use of an auxiliary joining element. A three-component tool comprising a pin, sleeve and clamping ring is used to join two or more sheets in the overlap configuration. Ultrasonic welding is a solid-state welding process in which a connection is accomplished by local generation of high frequency vibration energy.
Friction stir welding (FSW) is a solid-state joining method meaning that no melting occurs during the process. Although this process was originally developed for joining non-ferrous materials, machine and tool material developments have enabled to apply this welding method to material combinations like aluminium to steel or copper to aluminium. This joining technology uses electromagnetic forces for deformation and joining of workpieces. In laser beam, the beam is concentrated very locally making this welding method a high power density process.
There is a range of joining processes available allowing dissimilar material combinations, each with their own technical and economic merits. Up to now only very little independent application-oriented research has been carried out regarding the feasibility of the proposed processes for industrial applications.
The ICHSF 2012 will cover all major topics on impulse forming, its processes and industrial applications. The weld is stronger than the weakest base material : during material testing the crack appears outside the weld area. It is an ecological welding process, since no heat, radiation, gas or welding fumes are produced.
Nowadays, due to the increasing international competition, companies need faster and more economic production techniques and are forced to decrease the production costs to remain competitive.
Very little objective knowledge is available concerning the technical feasibility of the use of these processes for relevant industrial applications, nor concerning the possible increase of productivity or the weld quality, and therefore concerning the economic advantages that the use of these innovative processes can lead to. The research centre of Belgian Welding Institute has investigated this welding technique and has demonstrated the potential advantages for the industry.
These processes allow to join a wide variety of materials in a fast, error-free and therefore more cost-effective manner. High-velocity electromagnetic deformation and welding processes (also know as magnetic pulse forming and welding) are highly innovative automatic production techniques. Magnetic pulse crimping and welding is particularly suitable for joining tubular workpieces.
Magnetic pulse welding shows similarities with explosive welding, however the explosive forces are generated in a safer, controlled manner, by using electromagnetism. In magnetic pulse forming electromagnetic forces are used to deform flat or tubular workpieces according to the shape of a die. Magnetic pulse forming offers several advantages compared to conventional forming techniques, of which the most important is the improved formability of materials and the reduction of wrinkling and spring-back (no reduction of pre-tension), due to the lower internal stress level.
In order to increase the applicability of the electromagnetic pulse process, further theoretical and application-oriented research was required.
Demonstration that magnetic pulse crimping is a valuable process for realising strong axial and torque joints (without addition of a filler material). Acquisition of know-how on the magnetic pulse crimping process with and without filler materials. Investigation of appropriate quality control methods, best suited for this new technology (destructive and non-destructive examination).
Investigation of the joint characteristics for potential industrial applications and demonstration workpieces. Investigation of the correlation between the joint characteristics and the process parameters. Technical-economical evaluation of the use of the process for industrial applications, compared to other conventional welding processes. This project provided objective technical-scientific, as well as practical and economic knowledge in order to make it possible for individual companies to make a technically and economically justified decision on whether or not to implement this technology in their production, and which variant to choose (without filler material or the new variant). The experimental research was performed by BWI and SLV, both possessing magnetic pulse equipment. The execution of the experimental work was assisted by finite-element modelling of the process, performed by OCAS. The Flanders and German regions have a large number of metal processing companies, and due to the increasing international competition from low wage countries, they are constantly forced to decrease production costs in order to remain competitive.
This joining technology makes use of electromagnetic forces for deformation and joining of workpieces.
The discharge current induces a strong transient magnetic field in the coil, which generates a magnetic pressure, which causes one workpiece to impact with another workpiece. The explosive deformation force is however created in a safe way, by using electromagnetic forces generated by an induction coil. From the point of view of the potential applications, which are mostly factory-made parts, this is not necessarily a disadvantage. The previous projects were focused on the application of the process for joining tubular applications. Welding con?guration (a) with a shroud gas feed tube and (b) in a gas box (Zhu et al., 2005). Diagrams of fiber laser welding system and experimental arrangement, (a) IPG fiber laser installed on a KUKA robot, (b) welding head (Wang et al., 2011). Optical micrographs showing (a) microstructure near the interface between the FZ and HAZ for a ZE41A-T5 alloy joint and (b) close-up view of Fig. Schematic diagram showing the initial impact and flattening of the sheets during MPW (Berlin, 2011).7. IntroductionMagnesium is the sixth most abundant element on the Earth’s surface, with virtually inexhaustible supplies in the oceans. The parts to be joined are inserted into the coil, the capacitor bank is charged and the low inductance switch is triggered by a pulse trigger system and the current flows through the coil. The one-turn flat which is consisted of upper and lower H-shape plates which we call that the double layer H-shaped coil is design for thin or foils work-samples.
The depth of skin effect can be obtained by, where ?O is the angular frequency of changing field. As is evident from equation (1), for the materials with higher electrical conductivity , more eddy currents create at surface and as a result, stronger magnetic pressure p and a large amount of Joule heat are generated.
It shows that the wavy morphology bond interface was formed in interface layer without any significant heat-affected zone (HAZ). The hardness measurement across the interface layer clearly shows that the hardness of intermediate layer is higher than Aluminum base metal in all points. It can be pointed out that the sheet metals retain their original properties without the heat-affected zone (HAZ) problems during weld process and the welded zone is stronger than the weaker base metals so failure always occurred outside of welded zone for these combinations. The capability of our MPW method has been also examined for several other types of metals joint, such as T-joint, circular joint, long sheet sample (up to 500mm) successfully. Combining conventional metals with others or even new materials, offers designers solutions where a design consisting out of one material fails. The prerequisite for the production of such multi-material components is the availability of suitable joining technologies.
The capabilities and limitations of these joining processes need to be understood to enable companies to use these processes for realising multi-material components.
The partners also bring in their dedicated knowledge and experience concerning destructive and non-destructive characterisation, ensuring comparable, relevant and correct results.
After creating of a pre-hole, the auxiliary element, called weld rivet, is inserted in the hole.


The main difference is the generation of the necessary process heat, which is done by friction.
The clamping ring initially fixes the sheets against a backing plate while the pin and sleeve begin to rotate in the same direction while pressing on the upper surface (Figure a).
The system sends small, linear, cyclic movements to the tip of the sonotrode, called ultrasonic vibrations. A capacitor bank is charged, when the required amount of energy is stored in the capacitors, it is instantaneously released into a coil, during a very short period of time (typically 10-15 µs). The continuous laser is typically standard, while the pulsed laser makes it possible to very accurately control the heat input and weld penetration. The capabilities and limitations of these joining processes need to be understood and known to maximise joint and component performance and to enable companies to use these processes for realising components with a higher added value. This knowledge is also essential to assist companies during the decision process whether or not to implement a specific joining process, or for the selection of a particular process (and its equipment), ensuring smooth implementation.
The working principle of the welding process is based on the use of electromagnetic forces to deform and to weld workpieces.
Therefore no heat affected zone is created, and no loss of material properties takes place. The largest impact can be expected of a productivity increase by using new innovative production techniques. By participation in this research project, companies got an insight in the possibilities of these new processes.
The workpieces to be connected need to be positioned in the overlap configuration (one tube shifted over the other). The target group has clearly a need for fast, innovative and more cost-effective production processes. This process also offers the possibility to join dissimilar materials, which are difficult to weld using the conventional welding processes. This means that after welding, parts can immediately be unloaded and further processed with standard equipment. Electromagnetic pulse welding of sheet materials, which currently receives more and more attention, is considered as quite promising, due to the many advantages this process offers.
It is the third most plentiful element dissolved in seawater, with an approximate concentration of 0.14% (Busk, 1987). When current is applied to the coil, a high-density magnetic flux is created around the coil, and as a result an eddy current is created in the work-pieces.
As a result, an electromagnetic force of i ?~B acts mainly on the Al sheet and the Al sheet is accelerated away from the coil and collides rapidly with the steel sheet.
In addition, magnetic pressure will increase according to equation (2) because the depth of skin effect decreases by increasing of the electrical conductivity. The reason of this higher hardness at the intermediate can be resulted from intense plastic deformation due to high velocity collision or to fine-grain microstructure which was formed by rapid solidification of welded interface.
A multi-material design exploits the material with desired properties for each part of the component or product. It is often impossible for companies, and especially for SMEs with limited research budget or available time, to identify the optimal technology for the specific requirements of their products.
The frictional heat causes a plasticisation of the cover sheet and allows to penetrate the material without any pre-hole operation or melting.
The rotating pin and sleeve are moved in opposite direction to each other (one is plunged into the material while the other moves upwards), creating a cavity where the plasticised material through frictional heat is accommodated (Figure b). The frictional heat generated by this penetration action softens the workpiece material to a plastic condition making it flow around the pin.
The discharge current generates a very short but high magnetic pressure, which causes one workpiece to impact with the other workpiece. These process characteristics make it possible to produce precision joints with a narrow heat-affected zone and limited workpiece deformation after welding. There is no direct joint between the top sheet and the bottom sheet, but the auxiliary joining part guarantees the fixing of the top sheet onto the carrier sheet in a mainly form-fitting and partial force-fitting joint. Since this sophisticated welding process doesn't use heat to realise the weld, it offers important advantages with regard to the conventional welding techniques!
A large amount of energy is compressed and discharged in an extremely short period of time. This also means that the welded workpieces can be unclamped immediately after welding and can be further processed immediately. Otherwise they will be obliged to move the production process to countries with lower labour costs, with loss of jobs and knowledge as a result. Over recent years the industrial output of magnesium alloys has been rising by almost 20% per annum. At the moment of collision the colliding surfaces can be cleared by a large kinetic energy getting before the collision.
Consequently this method is applicable to welding metal sheets with higher electrical conductivity such as aluminium and copper to dissimilar sheet metal. After reaching the pre-set plunge depth, the pin and sleeve return to their initial position forcing the displaced material to completely refill the keyhole (Figure c).
This ensures that the materials are brought into a plastic state, causing the upper component to be joined with the lower component. A high current is discharged in the coil in a very short period of time (10 up to 30 µs). Combination of diffusion bonding with superplastic forming which is used for superplastic wrought AZ31. Magnesium and its alloys, as the lightest structural material, are about 40% lighter than aluminium and as much as about 78% lighter than steel. This force can drive the work-pieces together at an extremely high rate of speed and creates an explosive or impact type of weld. After the collision, the cleared surfaces are being pressed together by electromagnetic force and a fixture. As eddy current pass through the sheets very fast with a short duration of about 50?Es, there is negligible heating of the metal. Heat generated by electrical resistance causes a weld nugget formation in the contact zone between weld rivet and base sheet and creates the connection.
Finally, the tool rotation is stopped and the tool is withdrawn from the joint leaving a flat surface with minimum material loss (Figure d). Also other dissimilar and similar metals that have been welded successfully (see table below).
Due to the magnetic field induced in the coil, eddy currents are generated in the outer tubular workpiece.
It is demonstrated that using magnesium alloys results in a 22–70% weight reduction, compared to using alternative materials (Kulekci, 2008). Therefore, in this welding method materials will not be suffered from bad influences of heat that resulted from the conventional spot or fusion welding method. Both currents (in the coil and in the external workpiece) induce magnetic fields, which oppose each other. The magnetic field created by the eddy currents is opposing the magnetic field in the coil. The reaction forces between the opposing magnetic fields are forcing the external part towards the internal part at high velocity to cause welding. The repelling forces between the two magnetic fields cause the acceleration of the outer workpiece in the direction of the inner workpiece. Depending on the parameter settings and the geometrical configuration and dimensions, either a real weld or a crimp joint is obtained.
Additionally, these alloys have limited fatigue and creep resistance at elevated temperatures (Mordike and Ebert, 2001).
Because of the hexagonal close-packed (HCP) crystal structure, magnesium alloys also have a limited ductility and cold workability at room temperature (Sanders et al., 1999).
These alloys have about the same corrosion resistance in common environments as mild steel, but are less corrosion resistant than aluminium alloys (Busk, 1987). Thus, magnesium alloy usage has been limited due to its poor corrosion resistance and low ductility. In order to overcome these problems, new alloys, such as Mg-AZ91D, have been developed and have improved corrosion resistance (Munitz et al., 2001).
The property pro?les demanded by automobile and other large-scale potential users of magnesium have revealed the need for alloy development.
It is also common to classify magnesium alloys into those for room and elevated temperature applications. Aluminium and zinc, added either singly or in combination, are the most common alloying elements for room temperature applications because at elevated temperatures the tensile and creep properties degrade rapidly (Marya et al. The two or one letter is followed by numbers which represent the nominal compositions of these principal alloying elements in weight percentage, rounded off to the nearest whole number. Despite the good castability of some mentioned alloys (such as Mg-AZ91D alloys), it is not always possible or economically favourable to cast complex magnesium parts.
Joining of magnesium parts, which may be crucial for these applications, is still restricted. It is, thus, very desirable that joining technologies be developed and made accessible for industrial applications (American Society of Metals [ASM], 1990).Welding and joining processes are essential for the development of practically every manufactured product.
However, these processes often appear to consume greater fractions of the product cost and to create more of the production difficulties than might be expected (ASM, 1993).
Because there are many fusion welding processes, one of the greatest difficulties for the manufacturing engineer is to define which process will produce satisfactory properties at the lowest cost. Any change in the part geometry, material, value of the end product, or size of the production run, as well as the availability of joining equipment, can influence the choice of joining method. Magnesium alloy components may be joined using mechanical clasps as well as a variation of welding methods including tungsten arc inert gas (TIG), plasma arc welding, electron beam welding (EBW), laser beam welding (LBW), friction stir welding (FSW), explosion, electromagnetic welding, ultrasonic welding, and resistance spot welding (RSW).2.
The effectiveness of laser welding depends greatly on the physical properties of the material to be welded. Magnesium alloys possess certain inherent characteristics such as low absorptivity of laser beams, strong tendency to oxidize, high thermal conductivity, high coef?cient of thermal expansion, low melting and boiling temperatures, wide solidi?cation temperature range, high solidi?cation shrinkage, form low melting point constituents, low viscosity, low surface tensions, high solubility for hydrogen in the liquid state, and absence of colour change at the melting point temperature.
Therefore, some processing problems and weld defects can be observed in laser welding of magnesium alloys such as unstable weld pool, substantial spatter (Haferkamp et al., 2001), strong tendency to drop through for large weld pools, sag of the weld pool (especially for thick workpiece), undercut (Lehner and Reinhart, 1999), porous oxide inclusions, loss of alloying elements, excessive pore formation (particularly for die castings) (Zhao and DebRoy, 2001), liquation and solidi?cation cracking (Marya and Edwards, 2000).
CO2 and Nd:YAG lasersLasers have been promoted as potentially useful welding tools for a variety of applications. The availability of high-power continuous-wave (CW) carbon dioxide (CO2) and neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers and the limitations of current welding technology have promoted interest in deep-penetration welding in the past twenty years using these devices. The weldability of magnesium alloys was reported to be signi?cantly better with the Nd:YAG laser due to its shorter wave length. Leong investigated welding AZ31 alloy using both CO2 and pulsed Nd:YAG laser, and illustrated the dif?culties of CO2 welding.
He concluded that Nd:YAG laser was more suitable for magnesium alloy welding (Leong, 1998).
Laser powerThe penetration depth with laser welding is directly related to the power density of the laser beam and is a function of incident beam power and beam diameter.
For a constant beam diameter, penetration typically increases as the beam power is increased. It is well known that high power density at the workpiece is crucial to obtain keyhole welding and to control the formation of welds.
Lower irradiance incident upon the workpiece would reduce the spatter as well as the loss of high vapour pressure constituents.
It is reported that lower power level and slower speed lead to better weld quality (Cao et al., 2006).


A loss of tensile strength was found with Nd:YAG laser power lower than 2kW (Lehner and Reinhart, 1999).
Welding speedThe effects of welding speed on penetration depth and weld width at different levels of power for CO2 and Nd:YAG lasers are shown in Fig. It can be seen that the penetration depth and weld width both decrease linearly with increasing welding speed.
Though similar welding parameters are used, various magnesium alloys exhibit different welding performance due to their different thermo-physical properties. Because of the difference in their thermal properties, much more studies, therefore, is needed to systematically investigate the laser welding characteristics of different magnesium alloys.When the laser energy input was more than that required for a full penetration, large holes or weld dropout could be observed. This could be due to the long interaction time or the higher power density, while the low viscosity liquid is not able to sustain the welding pool.
The ripple phenomenon has been associated with growth rate fluctuations due to the effects of surface tension of the weld pool during solidification. 7 shows the crown increased rapidly with increasing the welding speed and decreasing the beam power. Shielding gasIt is well known that magnesium is highly susceptible to oxidation and thereby elaborate protection from the atmosphere is required (Dilthey et al., 1998). Good shielding can prevent burning or porosity and protect the laser optics from metal slag. Helium, with a high ionization potential of 24.5 eV and good thermal conductivity, has a high plasma formation threshold. To identify the best shielding gas, blind welds of AZ91 alloy were produced using helium, argon and nitrogen (Yu et al., 2009). A comparative study of two kinds of inert shielding gases He and Ar for laser welding process showed that He gas possesses a better shielding effect for the laser generated magnesium alloy pool than Ar gas, because of its higher dissociation energy and better conductivity (Dhahri et al., 2001b). For CO2 laser welding process, the satisfied seams with smooth surface, deep penetration depth, large ratio of weld depth to weld width and high weld strength, could only be produced under the protection environment of much more expensive He gas. In contrast, for the YAG laser welding process, sound seams could be readily obtained under the protective environment of much cheaper Ar gas. It can be concluded that YAG laser is the first choice for the laser welding of magnesium alloys considering weld quality and economic factors (Dhahri et al., 2001b). It was found that the shielding gas con?guration could have a signi?cant effect on the weld quality. When the gas from the feed tube above the weld is not aligned with the traverse direction, a side cut would occur as shown in Fig. Fiber laserFiber laser, as the third generation of industrial laser, is rapidly developing in recent years due to a number of advantages such as compact size, long transmission distance, high efficiency, superior beam quality and high power output (peak output is up to 50 kW). In the fields of joining steel and aluminium alloys, fiber laser has made a big progress in experimental studies and industrial applications, showing a great potential to replace conventional Nd:YAG and CO2 lasers. It is reported that the UTS of the joint decreases, but the yield strength increases after fiber laser welding of fine-grained magnesium alloy (Yu et al., 2009). On the other hand, laser power plays a big role in the characterization of fiber laser welding of AZ31B magnesium alloy. If the alloy is in a fully tempered state, the loss of precipitates may lead to the HAZ overaged.
If the alloy is susceptible to grain growth, the grains will grow in the HAZ which leads to decrease in mechanical properties (Cao et al., 2006). A signi?cant grain coarsening can be observed in the HAZ in wrought AZ31 alloy, however, in all laser welded cast alloys, no grain coarsening occurs within the HAZ zone. PorosityThe presence of porosity in the weld is one of the major concerns during laser beam welding of magnesium alloys. The high porosity is also one of the most important factors to reduce the failure strength of welded joints undergoing higher heat input, even though their grain size in the FZ is similar to or even smaller than that of the base metal (Wang et al., 2011). During the tensile test, because of stress concentration around the porosities, the micro-crakes usually initiate from these weakest positions, and then propagate to fracture the joints (Zhao and Debroy, 2001). Magnesium has significantly high solubility for hydrogen in the liquid phase and the porosity in magnesium alloys is dependent on the amount of dissolved hydrogen (Stolbov et al., 1990).
The tolerable hydrogen gas content in welds may depend on a number of factors such as the parameters of the welding process, alloy composition, local solidi?cation time, thermal gradient, weld structure, and inclusion concentration (Cao et al., 2006). However, no acceptable hydrogen content limits have ever been reported for laser welded joints of magnesium alloys.
It was recognized that the porosity in fiber laser welds of AM60, AE42 and AS41 cast magnesium alloys was mainly caused by the growth of the initial pre-existing pores. Imperfect collapse of the keyhole and turbulent ?ow in the weld pool could also been linked with porosity formation. Thus, decreasing heat input is effective to reduce the porosity in fiber laser welds of magnesium alloys.
14a hydrogen porosity is the dominant pore in laser welded die-cast magnesium alloys because of their extremely high initial gas contents. Clearly, production of weldable and heat treatable die-castings necessitates the reduction of gas contents in magnesium alloys (Cao et al., 2006). It is proposed that well-controlled remelting of the FZ leads to removal of gas bubbles and reduced porosity in the FZ.
When die cast magnesium alloys remelt during laser beam welding, the gas can be enlarge due to heating (thermal expansion) and thus leading to the release of the gas pressure.
Finally, the growth of porosity leads to expansion and coalescence of the pores, causing the formation of large pores in the fusion zone as shown in Fig. In most magnesium alloys, an increase in alloying elements will generally increase the solidi?cation temperature range. The large freezing temperature range, large solidi?cation shrinkage, high coefficient of thermal expansion, and low melting point intermetallic constituents potentially make magnesium alloys susceptible to heat affected zone liquation cracking and solidi?cation cracking in FZs (Marya and Edwards, 2002). In addition, a number of magnesium alloys are thought to be susceptible to stress corrosion cracking.
The susceptibility to solidi?cation cracking is evaluated using the circular-patch-weldability test and operating windows of welding parameters are identi?ed (Lathabai et al., 2003).
The alloys containing up to 6% Al and up to 1% Zn possess good weldability, while the alloys containing over 6% Al and up to 1% Zn are moderately crack sensitive, because of the occurrence of low melting-point constituents (Mg17Al12).
Even without Al, the alloys containing over 3% Zn are highly susceptible to solidi?cation cracking and dif?cult to weld (Marya et al., 2000). On the other hand, hot tearing is mainly in?uenced by joint composition, microstructure, welding process parameters, and joint design. Oxidation of magnesium is increased at high temperatures, resulting in the formation of a surface magnesium oxide layer (Cao et al., 2006). The amorphous structure of MgO ?lms in solid pure magnesium generally performs an important role as a protective layer to prevent further oxidation at low temperature, but the ?lms easily become porous and loose with the temperature increase. It is reported that the addition of Ca to the pure Mg as well as the addition of Al and Y to the Mg–Ca alloys can further improve oxidation resistance. The entrapment of surface oxides into the molten pool during welding is an important source for oxide inclusions in laser welded magnesium alloys. Loss of alloying elementsThe relatively high vapour pressure of liquid magnesium compared with aluminium points to the potential problem of evaporative losses, particularly if zinc is also present as an alloying addition.
The evaporation will cause a variation of chemical composition in the FZ, especially at high laser power density. Minimizing the irradiance incident upon the workpiece would decrease the loss of high vapour pressure elements (ASM, 1993). Further work is needed to fully understand the main influencing factors to build up the quantitative relationship between the element loss of evaporation and welding process parameters.3.
Gas tungsten arc weldingGas-Tungsten Arc Welding (GTAW), also known as HeliArc, Tungsten Inert Gas (TIG), and tungsten arc welding, was developed in the late 1930s when a need to weld magnesium became apparent. Russell Meredith developed a welding process using the inert gas helium and a tungsten electrode to fuse magnesium. TIG welding is used extensively for welding stainless steel, aluminium, magnesium, copper, and reactive materials (for example, titanium and tantalum). This joining method replaced riveting as a method of building aircraft with aluminium and magnesium components (ASM, 1993). This process uses the heat, generated by an electric arc struck between a non-consumable tungsten electrode and the workpiece, to fuse metal in the joint area, and produce a molten weld pool.
The arc area is shrouded in an inert or reducing gas shield to protect the weld pool and the non-consuming electrode. The process may be operated autogenously (without filler), or filler may be added by feeding a consumable wire or rod into the established weld pool.
For direct current (DC) operation, the tungsten may be connected to either output terminal, but is most often connected to the negative pole.
Shielding gas is directed into the arc area by the welding torch, and a gas lens within the torch distributes the shielding gas evenly over the weld area. Welding currentCurrent is one of the most important process parameters to control in any welding operation, because it affects the penetration depth, welding speed, deposition rate, and weld quality. Shielding gasesThe preferred gas for the AC-TIG welding of magnesium is argon, although helium and argon–helium mixtures may be used. Argon gives a wide, shallow penetration weld bead, but will leave the weld bright and silvery in appearance. For transition metal alloys, when the welding path is coated with chemical fluxes, a technique exists of flux-assisted gas tungsten arc welding (FA-TIG). Generally, fluxes allow full penetration welding at greater rates using relatively inexpensive gas tungsten arc as the heat source.
Using argon shielding and chloride fluxes, magnesium alloy welding tests showed an increase in weld penetration (Marya and Edward, 2002).
Resistance spot weldingResistance spot welding is the localized joining of two or more metal parts together by the heat, generated using resistance to the flow of electric current through workpieces, which are held together under force by electrodes. The contacting surfaces in the region of current concentration are heated by a short-time pulse of low-voltage and high-amperage current. When the flow of current ceases, the electrode force is maintained till the weld metal cools down and solidifies.
The electrodes are retracted after each weld, which usually is completed in a fraction of a second. RSW can be separated into Silicon Controlled Rectifier (SCR) AC resistance welding and inverter DC resistance welding (Hwang et al., 2010). For high electrical conductivity of magnesium and low heat generation in the weld, high welding currents are required. Weld microstructureThe joint, achieved by a technique of resistance spot welding, consists of the weld nugget and the heat affected zone. The weld nugget consists of the cellular dendritic structure at nugget edge, accompanied by equiaxed dendritic structure within its centre for AZ31 alloy (Fig.
Effect of welding parameters on joint propertiesAs weld current increases, the nugget width increases as well. This effect is proved for both SCR AC resistance welding and inverter DC resistance welding which is shown in Table 1. In both RSW welding technics cracks and pores have been found, affecting the properties of weld. In the case of magnesium alloys, the welding part rapidly expands and retracts during welding and quickly cooling down, because of high coefficient of linear expansion.



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