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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. EHF can form hollow shapes with much ease and at less cost compared to other forming techniques. EHF is more adaptable to automatic production compared to other high energy rate forming techniques.
EHF can produce small – to intermediate sized parts that don't have excessive energy requirements. 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.
Magnetic pulse forming is a technology that has gained interest in the last decades – thanks to the increased formability it offers for high-resistance-low weight ratio materials such as aluminum and magnesium alloys. Hard X-rays can penetrate solid objects, and their largest use is to take images of the inside of objects in diagnostic radiography and crystallography.
The roentgen (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one electrostatic unit of charge of each polarity in one cubic centimeter of dry air. The rad is the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram. The sievert (Sv) is the SI unit of equivalent dose, which for X-rays is numerically equal to the gray (Gy). X-rays are generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem.
X-ray fluorescence: If the electron has enough energy it can knock an orbital electron out of the inner electron shell of a metal atom, and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted.
So the resulting output of a tube consists of a continuous bremsstrahlung spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines.
In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the dose. To generate an image of the cardiovascular system, including the arteries and veins (angiography) an initial image is taken of the anatomical region of interest. A specialized source of X-rays which is becoming widely used in research is synchrotron radiation, which is generated by particle accelerators. The most commonly known methods are photographic plates, photographic film in cassettes, and rare earth screens.
Before the advent of the digital computer and before invention of digital imaging, photographic plates were used to produce most radiographic images. Since photographic plates are sensitive to X-rays, they provide a means of recording the image, but they also required much X-ray exposure (to the patient), hence intensifying screens were devised.
Areas where the X-rays strike darken when developed, causing bones to appear lighter than the surrounding soft tissue.
Contrast compounds containing barium or iodine, which are radiopaque, can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels. An increasingly common method is the use of photostimulated luminescence (PSL), pioneered by Fuji in the 1980s. The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. For many applications, counters are not sealed but are constantly fed with purified gas, thus reducing problems of contamination or gas aging. Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier. Chemical milling or industrial etching is the subtractive manufacturing process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape.[1][2] It is mostly used on metals, though other materials are increasingly important. In 1782, the discovery was made by John Senebier that certain resins lost their solubility to turpentine when exposed to light; that is, they hardened. Etching has applications in the printed circuit board and semiconductor fabrication industries. Cleaning is the preparatory process of ensuring that the surface to be etched is free of contaminants which could negatively impact the quality of the finished part.[2][11] An improperly cleaned surface could result in poor adhesion of the maskant, causing areas to be etched erroneously, or a non-uniform etch rate which could result in inaccurate final dimensions. Masking is the process of applying the maskant material to the surface to ensure that only desired areas are etched.[2][3] Liquid maskants may be applied via dip-masking, in which the part is dipped into an open tank of maskant and then the maskant dried. The maskant to be used is determined primarily by the chemical used to etch the material, and the material itself. Maskants to be used in photochemical machining processes must also possess the necessary light-reactive properties. Where E is the rate of etching (usually abbreviated to etch rate), s is the depth of the cut required, and t is the total immersion time.[10] [15] Etch rate varies based on a number of factors, including the concentration and composition of the etchant, the material to be etched, and temperature conditions.
Demasking is the combined process of clearing the part of etchant and maskant.[2][17] Etchant is generally removed with a wash of clear, cold water (although other substances may be used in specialized processes). Nital (a mixture of nitric acid and ethanol, methanol, or methylated spirits for mild steels. Flux-cored arc welding, and check out Flux-cored arc welding on Wikipedia, Youtube, Google News, Google Books, and Twitter on Digplanet. FCAW was first developed in the early 1950s as an alternative to shielded metal arc welding (SMAW).
Of course, all of the usual issues that occur in welding can occur in FCAW such as incomplete fusion between base metals, slag inclusion (non-metallic inclusions), and cracks in the welds. There are a range of factors to consider when weighing up choices for wire-feed welding on a particular job.
Justin Scott, Freshman in Gering, practices shielded metal arc welding on April 19 in WNCC's mobile welding station. Flux-cored wires are used for shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW) applications in various end-user industries. Culick teaches a variety of welding techniques including gas tungsten arc welding, flux cored arc welding, shielded metal arc welding (stick) and gas metal arc welding. Tungsten, inert gas (TIG) welding uses a non-consumable tungsten electrode to produce the weld arc. 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.
Many metals tend to deform more readily under extra – fast application of load which make these processes useful to form large size parts out of most metals including those which are otherwise difficult – to – form. There are several advantages of using these forming processes, like die costs are low, easy maintenance of tolerances, possibility of forming most metals, and material does not show spring-back effect. The explosives used are generally high – explosive chemicals, gaseous mixtures, or propellants.
The sheet metal work piece blank is clamped over a die and the assembly is lowered into a tank filled with water. The process is versatile – a large variety of shapes can be formed, there is virtually no limit to the size of the work piece, and it is suitable for low – quantity production as well. Explosive forming is mainly used in the aerospace industries but has also found successful applications in the production of automotive related components. Since the repelling force acts on the coil as well the work, the coil itself and the insulation on it must be capable of withstanding the force, or else they will be destroyed. When a tube – like part x is to fit over another part y (shown as insert in Fig 9.3(i)), coil is designed to surround x so that when energized, would force the material of x tightly around y to obtain necessary fit. From energy considerations, the ideal pressure pulse should be of just enough magnitude that accelerates the part material to some maximum velocity and then let the part come to zero velocity by the time it covers the full fly distance. It has found extensive applications in the fabrication of hollow, non – circular, or asymmetrical shapes from tubular stock.
A bank of capacitors is first charged to a high voltage and then discharged across a gap between two electrodes, causing explosions inside the hollow work piece, which is filled with some suitable medium, generally water.
With the modern equipment, it is now possible to precisely control the energy within specified limits, therefore the primary factor is the dimensional accuracy of the die. All materials that can be formed by conventional forming processes can be formed by EHF also.
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. One major complexity of the process lies in the design and study at the work piece level and the interaction between the several physical aspects involved: the electromagnetic waves as source of energy, the thermo-mechanics controlling the strain and stress evolution, as well as the study of fracture and damage under high-speed loading conditions. This process produces an emission spectrum of X-ray frequencies, sometimes referred to as the spectral lines. The intensity of the X-rays increases linearly with decreasing frequency, from zero at the energy of the incident electrons, the voltage on the X-ray tube.
A second image is then taken of the same region after iodinated contrast material has been injected into the blood vessels within this area.
The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen.
In modern hospitals a photostimulable phosphor plate (PSP plate) is used in place of the photographic plate. It was developed from armor-decorating and printing etching processes developed during the Renaissance as alternatives to engraving on metal. Etchants mixed from salt, charcoal, and vinegar were applied to plate armor that had been painted with a maskant of linseed-oil paint.
This allowed the development of photochemical milling, where a liquid maskant is applied to the entire surface of a material, and the outline of the area to be masked created by exposing it to UV light.[7] Photo-chemical milling was extensively used in the development of photography methods, allowing light to create impressions on metal plates. It is also used in the aerospace industry[10] to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes. The surface must be kept free from oils, grease, primer coatings, markings and other residue from the marking out process, scale (oxidation), and any other foreign contaminants.
Maskant may also be applied by flow coating: liquid maskant is flowed over the surface of the part. Due to its inconstant nature, etch rate is often determined experimentally immediately prior to the etching process.


A de-oxidizing bath may also be required in the common case that the etching process left a film of oxide on the surface of the material.
But FML is formed in a mold tool, so there is no need for stretching, rolling or mechanical or chemical milling.
Tom Wolf said Friday Tech Met Inc., a custom and standard chemical milling company, will expand operations in the Donora Industrial Park, a move that will result in 41 new, full-time jobs. According to its website, its work is focused on high-temperature, titanium and ferrous alloys, and primarily serves the aerospace engine fabricating market in the eastern United States. FCAW requires a continuously-fed consumable tubular electrode containing a flux and a constant-voltage or, less commonly, a constant-current welding power supply. The advantage of FCAW over SMAW is that the use of the stick electrodes used in SMAW is unnecessary.
The weld area is protected from atmospheric contamination by an inert shielding gas such as Argon or Helium.
It is the third most plentiful element dissolved in seawater, with an approximate concentration of 0.14% (Busk, 1987). There are two techniques of high – explosive forming: stand – off technique and the contact technique. The process has the greatest potential in limited – production prototype forming and for forming large size components for which conventional tooling costs are prohibitively high. The work coils needed for different applications vary although the same power source may be used. A high charging voltage is supplied for a short time to a bank of capacitors connected in parallel.
The expandable coils are less costly and are also preferred when high energy level is needed. The magnetic coil is placed inside the tube – like part, so that when energized would expand the material of the part into the collar. All forming coils fail, expendable coils fail sooner than durable coils, and because extremely high voltages and currents are involved, it is essential that proper safety precautions are observed by the production and maintenance personnel.
The compression applications involve swaging to produce compression, tensile, and torque joints or sealed pressure joints, and swaging to apply compression bands or shrink rings for fastening components together.
These explosions produce shock waves that travel radially in all directions at high velocity until they meet some obstruction. These materials are aluminum alloys, nickel alloys, stainless steels, titanium, and Inconel 718.
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.
This work is dedicated to the development of a predictive model and computational tool able to deal with the interaction between the electromagnetism and the thermo-mechanics in a 3D finite elements frame work. The spectral lines generated depend on the target (anode) element used and thus are called characteristic lines. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. Photographic film largely replaced these plates, and it was used in X-ray laboratories to produce medical images.
In the pursuit of a non-toxic contrast material, many types of high atomic number elements were evaluated.
After the plate is X-rayed, excited electrons in the phosphor material remain "trapped" in "colour centres" in the crystal lattice until stimulated by a laser beam passed over the plate surface. Electrons accelerate toward the anode, in the process causing further ionization along their trajectory.
Etching is used widely to manufacture integrated circuits and Microelectromechanical systems.[10] In addition to the standard, liquid-based techniques, the semiconductor industry commonly uses plasma etching. For most metals, this step can be performed by applying a solvent substance to the surface to be etched, washing away foreign contaminants.
Certain conductive maskants may also be applied by electrostatic deposition, where electrical charges are applied to particles of maskant as it is sprayed onto the surface of the material. Various methods may be used to remove the maskant, the most common being simple hand removal using scraping tools. The company, which is retaining 35 existing positions as well, will buy the former PolyOne Corp. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere, producing both gaseous protection and liquid slag protecting the weld. However, this core contains more than just flux, it also contains various ingredients that when exposed to the high temperatures of welding generate a shielding gas for protecting the arc. Over recent years the industrial output of magnesium alloys has been rising by almost 20% per annum. The explosive charge is placed at some predetermined distance from the work piece, see Fig 9.1. Flat coils have been used on flat sheets to produce stretch (internal) and shrink (external) flanges on ring and disc – shaped work pieces. We introduce the computational aspects of the electromagnetism, from the selected approach to include the geometry of the parts down to the coupling with the electric machinery behind the process. The radiologist or surgeon then compares the image obtained to normal anatomical images to determine if there is any damage or blockage of the vessel. In more recent years, computerized and digital radiography has been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g. For example, the first time the forefathers used contrast it was chalk, and was used on a cadaver's vessels.
This process, known as a Townsend avalanche, is detected as a sudden current, called a "count" or "event". Often such information (normally ranging marks) was etched onto equipment such as stiletto daggers or shovels.
The material may also be immersed in alkaline cleaners or specialized de-oxidizing solutions. Most industrial chemical milling facilities use maskants based upon neoprene elastomers or isobutylene-isoprene copolymers. The process is widely used in construction because of its high welding speed and portability.
This type of FCAW is attractive because it is portable and generally has good penetration into the base metal.
In fact, since it uses both a flux-cored electrode and an external shielding gas, one might say that it is a combination of gas metal (GMAW) and flux-cored arc welding (FCAW). 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. When the charging is complete, which takes very little time, a high voltage switch triggers the stored electrical energy through the coil. The deformation can be controlled by applying external restraints in the form of die or by varying the amount of energy released, Fig 9.4. This is followed by the computational techniques needed to couple the electromagnetic computation to the thermo-mechanical one with a special focus on the problem of tracking the displacement of the deformable part within the electromagnetic module. When the film is developed, the parts of the image corresponding to higher X-ray exposure are dark, leaving a white shadow of bones on the film.
This particular style of FCAW is preferable for welding thicker and out-of-position metals. It is demonstrated that using magnesium alloys results in a 22–70% weight reduction, compared to using alternative materials (Kulekci, 2008). A high – intensity magnetic field is established which induces eddy currents into the conductive work piece, resulting in the establishment of another magnetic field. We also introduce some aspects more related to the physics of the process such as the phenomena of elastic spring-back elimination and surface bonding (welding). Photographic plates are mostly things of history, and their replacement, the "intensifying screen", is also fading into history.
Some disadvantages are that this process can produce excessive, noxious smoke (making it difficult to see the weld pool); As with all welding processes, the proper electrode must be chosen to obtain the required mechanical properties. The forces produced by the two magnetic fields oppose each other with the consequence that there is a repelling force between the coil and the tubular work piece that causes permanent deformation of the work piece.
The metal silver (formerly necessary to the radiographic & photographic industries) is a non-renewable resource. Operator skill is a major factor as improper electrode manipulation or machine setup can cause porosity.
The main advantages of this process is that in a closed shop environment, it generally produces welds of better and more consistent mechanical properties, with fewer weld defects than either the SMAW or GMAW processes. A methodology for identification of the electric parameters defining the machinery and needed to perform the simulation is introduced. Thus it is beneficial that this is now being replaced by digital (DR) and computed (CR) technology. In practice it also allows a higher production rate, since the operator does not need to stop periodically to fetch a new electrode, as is the case in SMAW.
Where photographic films required wet processing facilities, these new technologies do not.
However, like GMAW, it cannot be used in a windy environment as the loss of the shielding gas from air flow will produce porosity in the weld. 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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