A review of laser welding techniques for magnesium

Journal of Materials Processing Technology 171 (2006) 188–204

A review of laser welding techniques for magnesium alloys X. Cao ∗ , M. Jahazi, J.P. Immarigeon, W. Wallace Aerospace Manufacturing Technology Centre, Institute for Aerospace Research, National Research Council Canada, 5145 Decelles Avenue, Montreal, Que., Canada H3T 2B2 Received 7 July 2004; received in revised form 12 November 2004; accepted 27 June 2005

Abstract Laser welding will be an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive, electronics and other industries. In this document the research and progress in laser welding of magnesium alloys are critically reviewed from different perspectives. To date, two types of industrial lasers, carbon dioxide (CO2 ) and neodymium-doped yttrium aluminum garnet (Nd:YAG), have been used to investigate the weldability of magnesium alloys. Some important laser processing parameters and their effects on weld quality are discussed. The microstructure and metallurgical defects encountered in laser welding of magnesium alloys, such as porosity, cracking, oxide inclusions and loss of alloying elements are described. Mechanical properties of welds such as hardness, tensile and fatigue strength, and other important structural properties are discussed. The aim of the report is to review the recent progress in laser welding of magnesium alloys and to provide a basis for follow-on research. Crown Copyright © 2005 Published by Elsevier B.V. All rights reserved. Keywords: Laser welding; Magnesium alloy; Process; Microstructure; Defect; Mechanical property

1. Introduction Magnesium is the sixth most abundant element on the Earth’s surface and represents about 2.5% of its composition. It is also the third most plentiful element dissolved in seawater, with an approximate concentration of 0.14% [1,2]. As an extremely light metal, magnesium alloys have excellent specific strength [3], excellent sound damping capabilities [4,5], good castability [6], hot formability [7], excellent machinability [8], good electromagnetic interference shielding [9], and recyclability [10]. In general, magnesium alloys have about the same corrosion resistance in common environments as mild steel, but are less corrosion-resistant than aluminum alloys [1]. The recent progress in high purity magnesium alloys has also greatly improved the corrosion resistance [6]. Contrary to common belief, magnesium ignites with difficulty in air due to its high heat capacity [3]. Magnesium alloys, however, have limited strength, fatigue and creep resistance at elevated temperatures [6,11], low stiffness [6,7], limited ductility and cold workability at room temperature due to the hexagonal close-packed (HCP) crystal structure [4,5]. They also have poor surface properties

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such as low hardness, wear and corrosion resistance [6,7,11–13], large shrinkage during solidification, high chemical reactivity in molten state, and high cost [4,5,11]. In addition, there is a lack of systematic fundamental knowledge of magnesium alloys [11]. Most magnesium alloys are ternary types. The major alloying elements are aluminum, zinc, thorium and rare earths. Aluminum is the major alloying element in the ternary Mg–Al series, which comprises AZ (Mg–Al–Zn), AM (Mg–Al–Mn) and AS (Mg–Al–Si) alloys. There are two binary systems employing manganese and zirconium [14]. It is also common to classify magnesium alloys into those for room and elevated temperature applications. Rare earth metals and thorium are the chief alloying elements for high temperature alloys. Aluminum 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 [7]. To date, no international code for designating magnesium alloys exists but the method used by the American Society for Testing Materials has been widely adopted. In this system, the first two letters indicate the principal alloying elements according to the following codes: A, aluminum; B, bismuth; C, copper; D, cadmium; E, rare earths; F, iron; G, magnesium; H, thorium; K, zirconium; L, lithium; M, manganese; N, nickel; P, lead; Q, silver; R, chromium; S, silicon; T, tin; W, yttrium; Y, antimony

0924-0136/$ – see front matter. Crown Copyright © 2005 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.06.068

X. Cao et al. / Journal of Materials Processing Technology 171 (2006) 188–204

and Z, zinc. 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. For example, AZ91 indicates the alloy Mg–9Al–1Zn with the actual composition ranges being 8.3–9.7 Al and 0.4–1.0 Zn. Suffix letters A, B, C, etc. refer to variations in composition within the specified range and X indicates that the alloy is experimental [15]. Wrought magnesium alloys can be produced by extruding, rolling or forging in the temperature range of 300–500 ◦ C [7,10]. Since wrought semi-finished products are not available at reasonable costs [16], approximately 85–90% of all magnesium components are manufactured by casting processes [15]. Though magnesium alloys can be produced by nearly all casting methods such as sand, shell, plaster, semi-permanent, permanent mould, investment, die cast, squeeze, thixocasting and thixomolding, die casting has been the dominant process [9,17], constituting approximately 70% of all magnesium castings [11]. Currently there are four systems of magnesium alloys used commercially for die castings: Mg–Al–Zn–Mn (AZ series), Mg–Al–Mn (AM series), Mg–Al–Si (AS series), and Mg–Alrare earth (AE series). It was reported that AZ91D die-castings occupied 81% of all cast magnesium alloys in 1997 [6]. Gas porosity, however, has been a main problem for magnesium die casings as a result of high filling rate and quick cooling. Compared with non-vacuum die-castings, vacuum die cast components have less gas inclusions. While die-casting is the dominant process, sand castings of magnesium alloys are also important, especially for aerospace applications. For instance, zirconiumcontaining sand-cast alloys with rare earth elements such as yttrium, silver and zinc have been used for parts operating at temperatures between 250 and 300 ◦ C for extended periods of time [9]. As the lightest structural material available so far [3,18], magnesium alloys have the potential to replace steel and aluminium in many structural applications [4,5]. Thus, magnesium alloys have already found considerable applications in aerospace, aircraft, automotive, electronics and other fields, especially magnesium die-castings in the automotive industry [17]. Magnesium

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alloys have also been employed in nuclear energy industrial equipments due to the low tendency to absorb neutrons, adequate resistance to carbon dioxide and good thermal conductivity [11]. Many magnesium alloy components are used at high rotational speeds to minimize inertial forces [8]. To date magnesium alloys have not usually been welded except for some repaired structures because of the occurrence of defects such as oxide films, cracks, and cavities [19]. However, the wider application of magnesium alloys needs reliable welding processes. Magnesium alloy components may be joined using mechanical fasteners as well as a variety of welding methods including tungsten-arc inert gas (TIG), metal-arc inert gas (MIG), plasma arc, electron, laser, friction [20], adhesive [17], explosion, stud, ultrasonic [1], and spot welding [2]. Today TIG and MIG processes are the main methods for magnesium alloys, especially for the removal and repair of casting defects. However, low welding speeds, large heat affected zone (HAZ) and fusion zone (FZ), high shrinkages, variations in microstructures and properties, evaporative loss of alloying elements, high residual stress and distortion of arc-welded joints have caused attention to be drawn towards laser welding [10] due to the low and precise heat input, small HAZ, deep and narrow FZ, low residual stress and weldment distortion, and high welding speed [3,21–23]. 2. Laser welding processes The effectiveness of laser welding depends greatly on the physical properties of the material to be welded. Table 1 compares the typical physical properties of pure magnesium, aluminum and iron [1,23–28]. Magnesium alloys possess certain inherent characteristics such as low absorptivity of laser beams, strong tendency to oxidize, high thermal conductivity, high coefficient of thermal expansion, low melting and boiling temperatures, wide solidification temperature range, high solidification shrinkage, a tendency to form low melting-point constituents, low viscosity, low surface tensions, high solubility for hydrogen in the liquid state, and absence of color change at the melting point temperature. During laser welding of magnesium alloys, therefore, some processing problems and weld defects can be

Table 1 Properties of pure magnesium, aluminum and iron at their melting points [1,23–28] Properties

Magnesium

Aluminum

Iron

Ionization energy (eV) Specific heat (J kg−1 K−1 ) Specific heat of fusion (J/kg) Melting point (◦ C) Boiling point (◦ C) Viscosity (kg m−1 s−1 ) Surface tension (N m−1 ) Thermal conductivity (W m−1 K−1 ) Thermal diffusivity (m2 s−1 ) Coefficient of thermal expansion (1/K) Density (kg m−3 ) Elastic modulus (N/m3 ) Electrical resistivity (␮m) Vapor pressure (Pa)

7.6 1360 3.7 × 105 650 1090 0.00125 0.559 78 3.73 × 10−5 25 × 10−6 1590 4.47 × 1010 0.274 360

6 1080 4 × 105 660 2520 0.0013 0.914 94.03 3.65 × 10−5 24 × 10−6 2385 7.06 × 1010 0.2425 10−6

7.8 795 2.7 × 105 1536 2860 0.0055 1.872 38 6.80 × 10−6 10 × 10−6 7015 21 × 1010 1.386 2.3

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encountered such as an unstable weld pool, substantial spatter [4,5,16,28], a strong tendency to drop-through for large weld pools [16], sag of the weld pool (especially for thick workpiece), undercut [18], porous oxide inclusions, loss of alloying elements [4,5], excessive pore formation (particularly for die castings) [8,29], liquation and solidification cracking [30]. Nonetheless, crack-free laser welded joints, with low porosity and good surface quality, can be achieved for wrought magnesium alloys using appropriate laser processing conditions [31]. The weldability of magnesium die-castings, particularly for non-vacuum die cast components, however, is critically dependent on the gas content [21–23,32,33] because the gas may form new pores or even cause explosions in the weld pool [18,31]. Magnesium alloy welding has been insufficiently documented [10] but the relative arc weldability of most magnesium alloys has been rated based largely on the susceptibility to cracking and to some extent on joint efficiency [2]. The relative laser weldability of magnesium alloys, however, has not been systematically investigated yet. Research into stable laser welding has involved identifying and controlling the processing parameters influencing process stability and reproducibility to reliably produce defect-free welds at high welding speeds. The following discussion focuses on some important processing variables and their influences on welding process and quality during the keyhole mode welding of magnesium alloys.

2.2. Laser power High power density at the workpiece is crucial to achieve keyhole welding and to control the formation of welds. Fig. 1 shows the effect of laser power on the penetration depth (Fig. 1A) and weld width (Fig. 1B) for WE43 alloy welded at a speed of 33 mm/s and a focused diameter of 0.25 mm [36,37]. High beam powers led to deep and wide beads, and reduce both ripples and crowning [38]. Lower irradiance incident upon the workpiece would reduce the spatter as well as the loss of high vapor pressure constituents [4,5]. It was reported that a lower power level and a slower speed lead to better weld quality [18]. For die cast AZ91 and AM50 alloys with a thickness of 3 or 5 mm, the optimum power level for Nd:YAG laser is between 2 and 2.5 kW. A loss of tensile strength was found with Nd:YAG laser power lower than 2 kW [18]. Fig. 1 also clearly shows that the threshold power for deep penetration mode welding of cast WE43 alloy is approximately 1 kW for a CO2 laser, i.e. a power density of approximately 2 × 106 W/cm2 . It was reported that the minimum power density to sustain a keyhole for AZ31B alloy is approximately 5 × 105 W/cm2 for CO2 laser [4,5]. The threshold power density for AM60B die-castings is about 1.2 × 105 W/cm2 for Nd:YAG

2.1. CO2 and Nd:YAG lasers Two main types of lasers, CO2 and Nd:YAG with wavelengths of 10.6 and 1.06 ␮m, respectively, have been used to investigate the weldability of magnesium alloys. The CO2 laser has high power output, high efficiency, proven reliability and safety [34]. With the recent development of high output power, the improvement of beam quality and the possibility of glass fiber delivery, the Nd:YAG laser has entered the fields dominated by the CO2 laser. The weldability of magnesium alloys was reported to be significantly better with the Nd:YAG laser due to its shorter wavelength, which in turn reduced the threshold irradiance required for keyhole mode welding and produced a more stable weld pool [4,5,28]. Compared with CO2 lasers, Nd:YAG laser beams have a higher welding efficiency [28,35]. For instance, for a 1.5 kW laser beam with a similar spot diameter and welding speed (5 m/min), a penetration depth of 2 mm was achieved for a Nd:YAG laser as compared to only 0.7 mm for the CO2 laser [22,32,35]. A similar conclusion was reached by Sanders et al. [4,5], who compared the weldability of 1.8 mm wrought AZ31B-H24 alloy using a 2 kW pulsing wave (PW) Nd:YAG and 6 kW continuous wave (CW) CO2 lasers. Sound welds were produced with a Nd:YAG laser at a power of 0.8 kW (5 ms pulse width and 120 Hz) and a travel speed of 3 cm/s, whereas for a CO2 laser sound penetration welds were only produced at 2.5 kW and 12.7 cm/s. In addition, it was possible to use shear cut edges in the Nd:YAG case whereas milled edges were required for CO2 laser [28,35]. This is because for given power and focusing conditions poor fit-up is of a more serious concern for smaller beam diameter CO2 lasers than Nd:YAG lasers [4,5,28].

Fig. 1. Effect of CO2 laser power on (A) penetration depth and (B) bead width of cast WE43 alloy joints [36,37].


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laser [8]. For ZE41A-T5 sand castings keyhole welding is reached at a threshold irradiance of 1.5 × 106 W/cm2 for the machined surface conditions using 2.5 kW Nd:YAG laser power, but the keyhole mode is obtained at 4.0 × 105 W/cm2 for the ascast surface conditions [39]. The as-cast surface requires lower power density for the formation of keyholes indicating that the as-cast surfaces have higher energy absorptivity for Nd:YAG laser beams probably due to the coarser surface conditions. 2.3. Focal plane position The position of focal points has an important influence on welding process and quality. The focal plane should be set where the maximum penetration depths or best process tolerances are produced. Dhahri et al. [37,40,41] studied 1–5 kW CO2 laser welding of 2 mm AZ91 and 4 mm WE43-T6 alloys. Their results showed that an adequate weld could be obtained for a focal position on or 1 mm under the surface of the workpiece. Focal position on the workpiece surface had the smallest weld width while the weld width became larger when the focal position deviated above or below the surface [36]. Weisheit et al. [31,42] investigated 2.5 kW CO2 laser welding of some magnesium alloys. For thin plates (2.5 and 3 mm), the best welds according to penetration depth, aspect ratio and sag were achieved when the focal point was adjusted on the surface of workpiece, whereas for thick plates (5 and 8 mm) a position of 2 mm below the surface of workpiece proved to be the best. Thus, the focal position should be moved deeper into the material for thicker workpieces. Lehner et al. [18] further researched the tolerance of focal position. For 3 mm AZ91 and AM50 die castings welded using a 3 kW CW Nd:YAG laser, the best focal position is approximately 0.8 mm below the workpiece surface, with a tolerance of ±0.5 mm. For 5 mm material, the focal position has to shift to about 1.2 ± 0.2 mm below the surface. 2.4. Welding speed Figs. 2 and 3 show the effects of welding speed on penetration depth and weld width at different levels of power for CO2 and Nd:YAG lasers, respectively. The penetration depth and weld width both decrease linearly with increasing welding speed. However, 5 kW CO2 laser welding of WE43 and ZE41 alloys indicates that a further decrease in welding speed led to little increase in penetration depth but there are increases in weld and HAZ widths [40]. Too high welding speed was reported to reduce ripples but greatly increase crowning [38], or even to increase tendency to brittleness in the fusion zone [28]. Clearly the welding speed should be adjusted to provide the penetration depth required at a given power level. Fig. 4 displays the effect of welding speed on the penetration depth as well as on the area of molten weld pool for wrought AZ21A and die cast AZ91D alloys welded using a 1.7 kW CW Nd:YAG laser [35]. Though similar welding parameters are used, various magnesium alloys exhibit different welding performance due to their different thermophysical properties. Die cast AZ91D has a lower thermal conductivity of 51 W/m K as compared with 139 W/m K for wrought AZ21A alloy. Thus,

Fig. 2. Effect of welding speed on (A) penetration depth and (B) bead width of cast WE43 alloy joints welded using a CO2 laser [36,37].

the AZ91D alloy has a higher weld depth and weld volume compared with AZ21A alloy. It was also reported that greater penetration depth could be reached in AM50 alloy compared with AZ91 alloy welded under similar conditions using a 6 kW CO2 laser [38]. The research into laser welding processes for magnesium alloys is still in its infancy. Much more work, therefore, is needed to systematically investigate the laser-welding characteristics of different magnesium alloys because of the difference in their thermal properties. 2.5. Surface preparation The surface condition of magnesium alloys may influence the energy absorption of incident laser beams as well as the threshold power density for keyhole welding. Magnesium has a low solid absorptivity relative to steel, approximately 3% at room temperature (or a slightly higher value at the melting point) for a CO2 laser beam, but nearly 100% absorptivity can be obtained after the formation of keyholes [4,5]. Haferkamp et al. [16] stated that the absorptivity for a Nd:YAG laser beam was 8–20% at room temperature. Ceramics such as Al2 O3 , MgO and SiO2 were thought to be transparent in three ranges of the electromagnetic spectra: ultraviolet (0.2–0.4 ␮m), infrared

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Fig. 3. Effect of welding speed on (A) penetration depth and (B) weld width for die-cast AM60B alloy joints welded using a Nd:YAG laser [9].

(0.7–3.0 ␮m) and radar (>103 ␮m) [43]. The Nd:YAG wavelength of 1.06 ␮m lies in the infrared transparent region. Thus, it would be expected that MgO films usually present on the surface of magnesium alloys have little influence on the energy absorption of a Nd:YAG laser beam. This presumption tends to

be further supported by some reported data, i.e. the absorptivity of magnesium oxide (MgO) for Nd:YAG laser light is approximately 20% (5–10% for Al2 O3 ) [44]. For a CO2 laser beam with a wavelength of 10.6 ␮m, however, magnesium oxide (MgO) has an energy absorptivity of approximately 93–98% (90–99% for Al2 O3 ) [44] implying that MgO surface layers on magnesium alloys can effectively increase the absorptivity for CO2 laser beams. It is assumed that the enhanced energy absorption of magnesia oxides for CO2 laser beams will favorably melt or even evaporate the oxides present in the weld pool, leading to the decrease of oxide inclusions and the purification of weld metal [39]. In addition, the chromate coating, often used for magnesium alloys, was reported to increase the absorptivity from 3 to 9% for a CO2 laser beam [4,5]. Removal of the chromate coating by abrasion or polishing will increase the threshold irradiance required for laser welding, reduce the surface quality of the weld, and cause increased spatter and a noisier weld monitor signal. For example, at a spot size of 420 ␮m and a travel speed of 127 mm/s, the threshold irradiances for CO2 laser welding of AZ31B alloy were 4.2 × 105 W/cm2 with the chromate coating, 6.1 × 105 W/cm2 with the coating scrubbed off, and 6.9 × 105 W/cm2 with a polished shiny surface [4,5]. The natural surface of magnesium alloys is usually covered with an oxide film (MgO) due to its high reactivity with oxygen. The porous oxide films absorb moisture, especially over extended periods of time in high humidity and fluctuating temperature environments. Magnesium alloys, however, are usually supplied with oil coating, acid-pickled surface, or chromate conversion coating for protection during shipping and storage [14]. These surface oxide films or contaminants will cause weld crack (non-fusion) and porosity [7,45,46]. Thus, it has been suggested that prior to welding all surfaces and edges of workpieces and welding wires be cleaned to remove such oxide and hydride layers, grease/releasing agents, surface coating as well as any dirt picked up during forming, assembling and fixturing [3,14,47]. Some methods can be employed such as mechanical cleaning with aluminium oxide abrasive cloth, stainless steel brushes, aluminum or steel wool, chemical degreasing using alkaline cleaners, pickling using chromic acid, etc. [1,47,48]. 2.6. Shielding gas

Fig. 4. Contrast of the area of molten weld pool and penetration depth for AZ21A and AZ91D alloys using a 1.7 kW CW Nd:YAG laser [35].

It is well known that magnesium is highly susceptible to oxidation and thereby elaborate protection from the atmosphere is required [1,33]. This is achieved using inert gases. Good shielding can avoid burning or porosity [30] and protect the optics from metal slag (cinders) [37,41]. The shielding gas also influences the formation of the plasma [37,41]. Helium, with a high ionization potential of 24.5 eV and with good thermal conductivity, has a high plasma formation threshold. Thus, little plasma is produced using helium as shielding gas [7,36,37,40,41,49]. To identify the best shielding gas, blind welds of AZ91 alloy were produced using helium, argon and nitrogen [31,42]. Helium gas proved to be the best according to the surface roughness, penetration depth and seam shape factor (depth/width). Shielding with helium rather than argon during CO2 laser welding results in higher tensile strength [49]. Dhahri et al. [36,40] investigated


WE43 alloy using a 5 kW CO2 laser, and showed that gas flow of helium lower than 50 l/min may cause spraying or collapse of the molten pool. Thus, a gas flow of 50 l/min or higher is required for a satisfactory seam width. Hiraga et al. [49], using a 2 kW CW CO2 and Nd:YAG lasers studied welding of the wrought AZ31B-H24 butt joints with 1.7 mm thickness. With CO2 laser welding using helium center shielding combined with argon back shielding, sound weld beads were obtained. The center shielding using argon without back shielding, however, led to the deterioration of bead shape. With Nd:YAG laser welding, center shielding with argon is sufficient to produce sound beads even though no back shielding is used. This is because a Nd:YAG laser has less tendency to form plasma than a CO2 laser [49]. The tensile strength of CO2 laser welded joints varies in relation to shielding conditions and bead shape, but Nd:YAG laser welded joints have a higher and more stable strength regardless of shielding conditions [49]. Molten magnesium alloys have a strong tendency to sag or even drop-through due to the low viscosity and surface tension. Thus, copper or stainless steel backing is usually employed during laser welding [48]. When shielding gas is used as a backing system, the sag can be reduced, the process window can be extended to lower welding speeds and a better root surface quality can be obtained [18,22]. 2.7. Filler metal The use of filler metal provides many advantages because it can • compensate for metal loss due to vaporization [3,16,21, 22,33]; • reduce burn-up and weld drop-through [16]; • reduce porosity [16,28,33]; • control seam compositions to reduce susceptibility to the fusion zone brittleness or stress corrosion cracking [28], or to avoid weld cracks [48]; • promote process stability [16]; and • lower the sensitivity to joint gaps and lead to a slightly wider fusion zone [28]. However, the use of filler metal requires a higher power and a lower weld speed [31,42]. At present, there are no commercially available filler wires developed specifically for the laser welding of magnesium alloys. Filler wires are usually designed according to the materials to be welded and welding techniques. Due to the hexagonal lattice structure of magnesium, commercially available magnesium weld wires have rather high production costs [16] and large sizes, usually greater than 2 mm [31,42]. Recent progress in filler wires has led to a minimum wire diameter of 1.2 mm [31]. These large diameter wires have been developed specially for repair welding of magnesium castings using TIG or MIG processes. The use of the large diameter wires, however, requires high powers for laser welding, slows welding speeds, and results in broad welds with reduced aspect ratio [31,42]. On some cases, the large wires may be difficult to melt by small laser beams, leading to the instability of welding processes or even

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failure to weld. In contrast with arc welding the small fusion zone during laser welding means that smaller wire diameters should be used at high welding speeds [31,42]. Therefore, different commercial filler wires with smaller diameters or even with various sectional shapes should be developed for laser welding of various magnesium alloys in the future. Some useful guidance on the selection of appropriate filler metal can be obtained from the existing arc welding processes for magnesium alloys [50–52]. Usually, the selection of filler material depends on the base metal to be welded, the kind of weld joint, and the coating on the filler wires [1]. Filler metal with a lower melting point and a wider freezing range than the base metal will provide good weldability and minimize weld cracking for arc welding [14]. Casting repairs are usually completed with a filler metal of the same composition as the base metal when good color match, minimum galvanic effects, or good response to heat treatment are required [14]. For instance, the widely used AM and AZ series alloys can be welded with AM and AZ wires [16,28]. In arc welding, four filler alloys (AZ61A, AZ92A, AZ101A and EZ33A) are used for general rods. When workpieces to be welded contain Zr, EZ33A is thought to be the proper welding rods. If the workpieces do not contain aluminum, a rod containing aluminum should be used. Alloys ZE10, M1A, and K1A are preferably welded with the aluminum-containing rods. When welding cast alloys, it was suggested that alloys AZ101A and AZ92A instead of AZ61A be used. The expensive AZ61A alloy is better for most wrought alloys [1]. However, filler wire AZ92 used for AZ61 alloy may lead to embrittlement of fusion zone due to higher amounts of intermetallic precipitates [53]. The Mg–Zn–Zr casting alloys are seldom assumed to have acceptable weldability even when using a filler material of the same composition. However, welding rods with rare-earth elements, especially lanthanum, can refine the coarse-crystalline dendrite structure and thus increase weldability. Alloying with lanthanum increases the volume fraction of the low melting-point eutectic components, which helps heal the hot cracks during welding and reduce the degree of melting of the grain boundaries of the weld metal [14]. It was reported that Mg–Zn–Zr–La filler material has been specially developed for the welding of Mg–Zn–Zr castings [54]. 2.8. Process tolerances The lower modulus of elasticity of magnesium alloys produces higher elastic strains (or greater displacement) compared with aluminum and steel under the same loads. The low modulus of elasticity combined with high thermal expansion coefficient may cause significant weld distortion. Research into residual stresses using neutron diffraction has been reported by Barrallier et al. [55] for 5 kW CO2 laser welded WE43 alloy joints (3 mm thick). Though distortion could be remedied through proper straightening actions, strict fixturing, usually similar to that of aluminum alloys, is required for magnesium alloys [47]. The laser welding process in contrast to arc welding has minimized the residual stress and distortion due to its low heat input. The laser welded seam quality, however, still depends greatly on the joint preparation and clamping technique [3]. Workpieces

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and clamping tolerances often lead to gaps. A gap width of 0.3 mm was claimed to be the maximum value for 2.5 kW CW Nd:YAG laser welding of 3 mm AZ91 die castings when filler wire was not used [18]. Welding of AZ91 and AM50 die castings using 2.5 kW CW Nd:YAG laser showed that only gaps larger than 0.15 mm for 3 mm samples and 0.3 mm for 5 mm samples reduced the tensile strength significantly when welded without filler material [18]. Large gaps may cause sag of the welds and thereby lead to notch effects. For a reliable joining process of butt joints with “zero” gap width, the maximum lateral deviation of laser beams is about half the focus diameter [18]. Klausecker et al. [56] investigated 4 kW Nd:YAG and 5 kW CO2 laser welding of dissimilar magnesium joints (sand cast AZ91 and squeezed cast AZ91 with 20% T300 J carbon or 20% Al2 O3 short fibers). Higher energy absorptivity for fibers leads to their thermal destruction before any remarkable amount of magnesium matrix can be melted. Thus, a lateral shift of the beams towards the unreinforced AZ91 alloy is essential. For CO2 laser welding the optimum lateral displacement ranges from 0.2 to 0.4 mm while the Nd:YAG laser shows a more tolerant process window, allowing a lateral shift from 0.3 to 0.9 mm. Most specimens welded within the tolerance reach the strength of the unreinforced base material [56].

alloy. This was caused by the melting of some low meltingpoint intermetallics at grain boundaries [42]. At lower welding speed (higher heat input), alloying elements and impurities may segregate to grain boundaries and further embrittle the joint [4,5]. Generally, grain growth can be minimized at high welding speeds. The microstructure of the laser welds is characteristic of a high-speed process in which heat is rapidly extracted from the molten fusion zone by surrounding base material. Grains usually grow epitaxially from the FZ-HAZ interface. At the fusion boundary, where a relatively large thermal gradient and small growth rate are established, the microstructure was predominantly cellular [30]. The fine equiaxed grains in the fusion zones formed by cellular growth were also observed by the present authors in Zr-containing ZE41A alloy [57]. Fig. 5A shows the microstructure of the FZ and HAZ while Fig. 5B displays a closeup view of the interface between the FZ and HAZ in Fig. 5A. Weisheit et al. [31,42] have also observed a cellular morphology in all joints except for the WE54 alloy which showed a more globular grain shape. The microstructure in the weld center consists of fine and randomly oriented equiaxed dendrites nucleated in the FZ. Intense and transient convection occurring within the weld pool is unlikely to allow the formation of large

3. Microstructure A narrow weld joint is an important characteristic of high power density welding. The 2.5 kW CW Nd:YAG laser welding of 2 mm ZE41A-T5 sand castings showed that the fusion zones have widths of approximately 0.8–1.3 mm [57]. The partially melted zone is rather narrow, only several grains wide (Fig. 5). The width of the HAZ is approximately 2 mm as defined according to the variations of hardness values. Haferkamp et al. [21] welded AZ91D die castings using a 2 kW CW Nd:YAG laser indicating that the HAZ only had a width of 50–160 ␮m, depending on welding speed. The 2 kW PW Nd:YAG and 6 kW CW CO2 laser welding of wrought AZ31B alloy indicated that the width of the HAZ was 50–60 ␮m, but can be doubled at substantially slower speeds [4,5]. In arc-welded joints, the excessive heat flux experienced by metal in the HAZ could have some influence on the HAZ [1]. If the base metal has been work hardened, for instance, by cold rolling, the strengthening effect due to the cold work will be lost. If the alloy is in a fully tempered state such as T6, 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 with consequently decreased mechanical properties [1]. The grain growth was observed in the HAZ of laser welded AZ31B joints, especially at slow speeds [4,5]. Weisheit et al. [31,42] investigated 2.5–8 mm butt joints of cast AZ91, AM60, ZC63, ZE41, QE22 and WE54, and wrought AZ31, AZ61, ZW3, ZC61 and ZC71 alloys using 2.5 kW CO2 laser without filler wires. A significant grain coarsening in the HAZ was only observed in wrought AZ31 alloy. In all other alloys, the grain refining elements restricted grain growth [31]. In all laser welded cast alloys, no grain coarsening occurred within the HAZ, but a liquation of the grain boundaries adjacent to the fusion boundary was observed in the HAZ of WE54

Fig. 5. 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 (A) [57].


directionally oriented dendrites. Instead, dendrite fragmentation may occur [30]. Pastor et al. [8] further observed the equiaxed morphology in AM60B alloy occurring at low welding speeds. At higher welding speeds, however, the morphology changes from equiaxed to dendritic forms [8]. Compared with the initial structure, the rapid cooling experienced during laser welding leads to a significant grain refinement in the fusion zone [8,21,22,31]. This has also clearly been demonstrated for ZE41A-T5 joints welded using CW Nd:YAG laser (Fig. 5). Though magnesium alloys can be welded in different temper conditions, the original microstructure was reported to have little influence on the fusion zone structure [42]. Further work is still needed to learn how to control the solidification structure and to investigate its influence on the formation of welding defects and mechanical properties of laser welded magnesium joints. For dissimiliar joints, the incomplete mixing of alloying elements may cause the occurrence of concentration gradients across the width of the weld joints [42]. 4. Main metallurgical defects 4.1. Porosity The porosity identified in laser welding of aluminum alloys consists mainly of four types: hydrogen pores, porosity caused by the collapse of unstable keyholes, porosity due to entrapment of gases by surface turbulence and shrinkage porosity [45,46]. During laser welding of aluminum alloys macroporosity can be largely avoided in either keyhole or conduction mode [8]. Compared with aluminum and iron, however, magnesium alloys give rise to more stable keyholes due to a much higher equilibrium vapor pressure, lower boiling temperature and lower surface tension [29]. Vapor pressures in keyholes may drop with the depletion of evaporative elements such as magnesium and zinc in aluminum alloys. Little variation of vapor pressures in magnesium-based alloys can promotes the stability of keyholes. Thus, it was observed that there was an absence of mixed mode during laser welding of die cast AM60B alloy [29]. The mixed mode of welding is characterized by the presence of two types of weld pool geometries typical of both conduction and keyhole modes in various cross sections. Deeper keyholes, however, are inherently unstable because the vapor pressure to laterally hold the molten metal must balance surface tension and greater hydrostatic pressures as the depth of the pool increases. Narrower keyholes can increase their stability. The collapse of the unstable keyholes may cause porosity in magnesium alloys [9]. The instability of keyholes, however, was not the main cause for the formation of porosity during laser welding of AM60B alloy [8]. In addition, it was pointed out that turbulent flow of molten metal in weld pool can also form gas bubbles during laser welding of magnesium alloys [47]. Hydrogen is the only gas dissolved in molten magnesium. Most liquid magnesium alloys, however, have a higher solubility for hydrogen than solid magnesium. Compared with aluminum alloys hydrogen may be less of a problem in magnesium alloys because of a comparatively higher solid solubility (average of about 30 ml/100 g) [15]. The decrease in hydrogen solubility

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at the liquidus, and relatively rapid solidification experienced during laser welding, still cause the formation of gas porosity within the weld beads [30,58]. The tolerable hydrogen gas content in welds may depend on a number of factors, including the parameters of the welding process, alloy composition, local solidification time, thermal gradient, weld structure, and inclusion concentration. However, no tolerable hydrogen content limits have ever been reported for laser welded joints of magnesium alloys. With magnesium alloys containing zirconium, hydrogen will react with zirconium to form ZrH2 [15]. In this case hydrogen porosity should not be a problem [57]. The gas porosity may originate mainly from initial preexisting pores [8,30], from the interaction of molten magnesium with surrounding air [30,48], or with moisture in entrapped surface oxide films [48]. It is speculated that hydrogen mainly results from the interaction of water vapor (H2 O) with magnesium [30]. Hydrogen porosity greatly depends on the surface preparation of parent material. Though some large pores were found in magnesium alloys containing Zn and/or Al, the porosity in the weld joints of sand cast and extruded alloys is relatively low [31,42]. As shown in Fig. 6 hydrogen porosity is the dominant pore in laser welded die cast magnesium alloys because of their extremely high initial gas contents [29,48]. The high initial pore content in die castings is usually due to surface turbulent flow and quick cooling experienced in the die casting process. Thus, the weldability of magnesium die castings greatly depends on the initial gas porosity in the base material [3,8,17,58]. Clearly, production of weldable and heat treatable die castings necessitates the reduction of gas contents in magnesium alloys. Vacuum die-castings have relatively low initial gas contents. Material with low gas and oxide contents can be welded without spatters and the weld joints can reach the strength of base material [21,22]. When die cast magnesium alloys remelt during laser welding, the gas may expand due to heating (thermal expansion) thereby leading to the release of the gas pressure because the pressure in the preexisting pores can be higher than one atmosphere [8]. The growth of porosity results from the expansion and coalescence of the pores, causing the formation of large pores in the fusion zone as shown in Fig. 7 [29]. The pressure inside a small gas pore is usually larger than that in a large pore. If several small

Fig. 6. Pores in 6 mm die-cast AM60B alloy welded using a 1.5 kW CW Nd:YAG laser [29].

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Fig. 7. Formation of large pores in the fusion zone due to the expansion and coalescence of the preexisting pores in base die-cast AM60B alloy [29].

pores coalescence to form a large pore, there will be an increase in average pore size and total pore volume, and a decrease in pore density. Zhao and DebRoy [29] studied 6 mm AM60B alloy using a 1.5 kW CW Nd:YAG at a welding speed of 106 mm/s. The pore radii in fusion zones were one order of magnitude larger than those in the base metal (Fig. 8), while the pore densities in the weld metal were nearly two orders of magnitude smaller. The area fraction of porosity in the weld metal was about 11–17 times that in the base metal depending on welding speed [29]. Vigorous flow of metal during laser welding promotes the coalescence of bubbles and gas bubbles drift with the flow of liquid metal, with a tendency to float upward. The formation of gas porosity is greatly influenced by weld processing parameters [3]. Pore area fractions go through a maximum with welding speed (Fig. 9). The time for the formation of gas porosity decreases with increasing cooling rate in the weld pool (increasing welding speed). At slow welding speeds, the interaction time (beam diameter/welding speed) is long enough for gas porosity to nucleate in large quantity, grow and escape from the molten pool, mostly as a result of buoyancy or other convective flow processes. At high welding speeds, as is usually experienced in laser welding, gas pores will have insufficient time to nucleate and grow. If gas remains trapped in a supersaturated matrix condition, relatively pore-free welds can be obtained [30]. Thus, the porosity volume fraction in the fusion zone usually decreases with welding speed, as shown in Fig. 10 [29]. The net increase in pore volume from the expansion of initial pores also decreases with decreasing power. The number density of pores increases with increasing welding speed and decreasing laser power due to the reduced time for pores to coalesce [8]. The amount of porosity does not change consistently with beam defocusing [8]. Well-controlled remelting of a fusion zone by a second run of the laser beam can significantly reduce porosity by allowing some pores formed during the first

run to be removed [8]. In summary, the technical options to minimize gas pores would include use of base metal with low pore density, removal of hydrogen sources before and during welding, and production of a hydrogen-oversaturated weld fusion

Fig. 8. Pore size distributions for (A) base die-cast AM60B alloy and (B) the fusion zone of 1.5 kW CW Nd:YAG laser welded 2 mm joints [29].


Fig. 9. Effect of welding speed on the pore fraction of die-cast AZ91 joints welded using 600 W Nd:YAG laser with a beam diameter of 1.9 mm [30].

zone by rapid solidification. Prior to welding, the oxide and contaminated surface layers should be removed from the workpiece and filler metal, and high-grade (low-dew-point) shielding gases should be used. Low laser power, high welding speed and small beam diameters may increase solidification rate and lead to suppression of nucleation and growth of gas pores [8,30,45,46]. However, the decreases in porosity at high welding speed at constant power, or at low laser power at constant welding speed are accompanied by a concomitant reduction in penetration depth. High gas contents may cause convexity or overfill joints [21,32]. Overfill was even thought to be another main defect in die cast AM60B alloy welded using 1–3 kW CW Nd:YAG laser [8]. The concavity or overfill of the top bead is mainly

Fig. 10. Porosity formed in CW Nd:YAG laser welds of die-cast AM60B alloy with thickness of (A) 2 mm and (B) 6 mm [29].

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influenced by porosity [21], the width of the weld [21], welding speed [8,30], wire feed rate and energy input [60]. Overfill decreases with lower heat input (higher welding speed or lower power) [8]. The crown (height of the bead above the surface), width, depth and cross-sectional area of weld pools also decrease with increasing welding speed [8]. It was reported that the area fraction of porosity is roughly equal to the area overfill fraction in laser welded die castings [8]. The greater the pore fraction, the larger the crown with respect to the weld depth [30]. Thus, the occurrence of overfill could be mainly attributed to gas porosity and the resulting displacement of the liquid metal over the top surface of the work-piece [8]. The absence of crown has usually been thought to be a criterion for acceptable welds [30]. Any measure that decrease gas porosity in weld pools will reduce overfill [8]. 4.2. Weld cracking Hot cracks have been one of the main welding defects for magnesium alloys [61]. In most magnesium alloys, an increase in alloying elements will generally increase the solidification temperature range. The large freezing temperature range, large solidification shrinkage, high coefficient of thermal expansion, and low melting point intermetallic constituents potentially make magnesium alloys susceptible to heat affected zone liquation cracking and solidification cracking in fusion zones [30,62]. In addition, a number of magnesium alloys are thought to be susceptible to stress corrosion cracking. Thus, the welded joints should be used after stress relieving [28]. 4.2.1. Heat affected zone liquation cracking Heat-treatable magnesium alloys such as AZ80 and ZK60 are usually produced by extrusion processes at high cooling rates, which reduce segregation and thereby decrease the tendency for liquation [7]. Die castings of Al-bearing Mg alloys, however, often exhibit low melting point intermetallic constituents such as ␥-Al12 Mg17 (melting point at 642 ◦ C) at grain boundaries and these promote liquation in the HAZ [30]. Both the Mg–Al and Mg–Zn systems have a relatively wide freezing range and a marked propensity for the presence of low melting point eutectics when cast under industrial non-equilibrium conditions [63]. Thus, nearly all the high aluminum zinc cast magnesium alloys are probably subject to incipient fusion of grain boundary constituents [7]. Baeslack III et al. [64] studied CO2 laser welding of cast alloy WE54X indicating that the alloy may also be sensitive to heat affected zone liquation cracking. Near the fusion line, they observed a liquation of the intermetallic phase and additional melting into the surrounding matrix. On cooling, the neodymium and yttrium-rich liquid solidified to a lamellar eutectic structure at the grain boundaries, and fine intergranular cracks were observed in the HAZ along some of these grain boundaries. Similarly, the HAZ liquation cracking was observed in ZE41A joints welded using 2.5 kW CW Nd:YAG laser as shown in Fig. 11A and B [57]. The liquation cracking in the HAZ results from the formation of liquid films at grain boundaries adjacent to the fusion boundaries during the weld thermal cycles, and the inability of the liquid films to accom-

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Fig. 11. Optical micrographs showing (A) the whole cross section of ZE41A-T5 alloy joint welded using a 4 kW CW Nd:YAG laser, (B) close-up view of the HAZ cracks at the top left corner and (C) weld cracks at the top FZ.

modate thermally induced stresses experienced during cooling [64]. The cracks tend to disappear if they are refilled with the surrounding eutectic material of low melting point [59]. The liquation cracking needs to be further investigated in laser welding of magnesium alloys because alloy composition, welding processing parameters, solidification rate, and weld joint geometry may all influence the tendency to crack. 4.2.2. Solidiﬁcation cracking According to Borland’s theory for weld cracking [65,66], alloying elements having high solubility with low partitioning coefficients (e.g. Mg–Zn alloys) increase solidification cracking by promoting large freezing ranges [10]. Thus, solidification cracking occurs regularly in alloys with large solidification interval, such as Mg–Zn–Zr [14], Mg–Al–Zn [20,22,48], etc. It was reported that some Mg–Zn–Zr alloys such as M18 and M22

have solidification ranges much larger than 100 ◦ C [14]. Sandcast ZE41A alloy has an equilibrium freezing range of 120 ◦ C, thus solidification cracking was observed in the joint (Fig. 11A and C). For Mg–Al–Zn alloys, solidification cracking starts to develop when the composition promotes a wide freezing range, which typically occurs at around 10% Al. Generally speaking, 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 mildly crack sensitive because of the occurrence of low melting-point constituents (Mg17 Al12 ). When the zinc exceeds 1% in high Al-containing alloys, weld cracking becomes severe, as in the case of cast alloys AZ63A and AZ92A [7,20,22]. Even without aluminum, the alloys containing over 3% Zn are highly susceptible to solidification cracking and difficult to weld [7]. Rare earth elements beneficially reduce the tendency of weld cracking and porosity in magnesium castings because they nar-


row the freezing range [14]. For some alloys containing rare earth such as QE22, however, cracking is still observed in the age-hardened condition [31]. The guidelines developed to avoid solidification cracking in arc welding can broadly be applied to laser welding. The cooling rates and heat inputs, however, are significantly different for laser welding and the situation is further complicated by pulsing capabilities in some laser welding operations. Thus, the rules developed for arc welding may need to be modified for laser welding. Due to the narrow heat affected zone and fusion zone, laser welding offers considerable advantages for magnesium alloys, especially for those that are heat treatable and susceptible to cracking as compared to conventional arc welding. Though various theories have already been developed to understand the mechanism of solidification cracking in casting and welding, little research work has been conducted for hot cracking in laser welded magnesium alloys. Hot tearing is mainly influenced by joint composition, microstructure, welding process parameters, and joint design. Further work still needs to be done to prevent the hot cracking and to understand it in terms of metallurgy, material, and mechanics. 4.3. Oxide inclusions Magnesium has a strong affinity for oxygen. Thus, it is expected that oxides are the main inclusions in magnesium alloys [67]. Solid magnesium starts to oxidize quite readily at 450 ◦ C. Oxidation is intensified at elevated temperatures, resulting in the formation of a surface oxide layer principally of magnesium oxide [48,68]. The amorphous structure of MgO films in solid pure magnesium generally plays an important role as a protective layer to prevent further oxidation at low temperature, but the films easily become porous and loose with the increase of oxidation temperature [69]. Compared to pure magnesium the amorphous oxide films on solid Mg–Ca alloy surfaces were more protective (thermally stable) indicating that the oxidation resistance of magnesium alloy at high temperature was remarkably improved by the addition of calcium. It was reported that the addition of Al and Y to the Mg–Ca alloys could further increase oxidation resistance [69]. In addition to MgO, oxides such as CaO, Y2 O3 and Al2 O3 have also been found in Mg–Ca–Y alloys [69]. Basically, there are three sources for oxide inclusions in laser welded magnesium alloys. Besides the existing oxides in the base metal originating during primary metal processing or during casting, the entrapment of surface oxides into the molten pool during welding is another source. Generally, it is recognized that the surface oxides are formed in three stages: (i) oxygen chemisorptions, (ii) formation of the oxide layer (nucleation and lateral growth) and (iii) oxide thickening [70]. Surface magnesium oxides usually contain water to form Mg(OH)2 [20] or even Ca(OH)2 in Mg–Ca–Al alloys [69]. Thus, the entrapment of surface oxides may also lead to the formation of gas porosity. During keyhole laser welding, since the vaporization of alloying elements in keyholes is not uniform and keyhole positions vary with time, the inherently unstable keyhole flow may entrap shielding gas (the shielding gas cannot be truly pure), or even air due to imperfect gas shielding. Molten magnesium

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has extremely low viscosity and surface tension and is therefore much more susceptible to the effects of the plasma-induced force than other metals [49]. Therefore, the metal vapor may oxidize to form oxide particles. Metal at the interface between the liquid metal in the weld pool and metal vapor/shielding gas may also be partly oxidized to form oxide films due to the entrapment of air or shielding gas into the pools [45,46]. Even though quick welding combined with good shielding helps to minimize oxidation reactions during laser welding, it is still expected that magnesium alloys will be laden with oxides originating from the primary processing or from the casting process. Brittle oxides in laser welded magnesium joints may exist as particles or films. If oxides are in particle form, they may have little influence on the quality of welded joints. However, oxide films may be harmful to the joint quality of magnesium alloys because of their crack-like nature [45,46]. Oxide films are usually folded dry side to dry side in liquid metal [71]. Thus, oxide films have two sides: the dry, unbonded inner surfaces, and their wetted exterior surfaces. The gap between the two dry sides of the doubled oxide films constitutes a crack because of the expected complete lack of bonding. This mechanism has explained many problems encountered in casting and joining of aluminum and magnesium alloys. The presence of naturally occurring crack-like discontinuities, in the form of folded oxide films, is likely to promote the formation of cracks during welding since they will act both as incipient cracks and as nucleation sites for new cracks [45,46]. In addition, the unwetted dry sides of oxide films are potential nucleation sites for some volume defects such as gas and shrinkage porosity [71]. If such oxide substrates were to be removed, hydrogen would probably remain supersaturated in magnesium alloys as a strengthening element. Up to date, no attempt has been made to prevent gas porosity through the removal of its nucleating substrates, such as oxide films. The oxide films may also reduce mechanical properties of magnesium joints. It is well known that oxide films are highly damaging to aluminum and magnesium castings [71]. However, little work has been conducted in this field for laser welded aluminum and magnesium joints, probably because of the technical difficulties and the abundance of some other serious defects such as excessive pores, liquation and solidification cracking and loss of alloying elements [45,46]. To avoid the formation of oxide films in magnesium joints, possible technical solutions include the use of base material with low oxide inclusion contents, the removal of surface oxide layers, effective weld shielding, and flow control of molten metal in the weld pool (to prevent the entrapment of oxides). It was also reported that the high peak power of pulsed Nd:YAG laser would be more effective than the CW CO2 laser to disperse the surface oxide layer [4,5,72]. 4.4. Loss of alloying elements Elements such as magnesium and zinc have lower boiling points and higher vapor pressures than aluminum at weld pool temperatures (typically 1000 K) [4,5,28]. It is also well known that the temperatures reached within keyholes are far greater than the boiling temperatures of magnesium, aluminum, or zinc [58]. Thus, the preferential evaporative losses during welding

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will consist primarily of zinc and magnesium. The evaporation will cause a variation of chemical composition in the fusion zone, especially at high laser power density. Marya and Edwards [30] researched 0.9 kW Nd:YAG laser welding of die cast AZ91 alloy. The vaporization of magnesium caused an increased surface concentration of aluminum. The occurrence of elemental redistribution and porosity should then be carefully controlled by manipulation of welding parameters. Higher energy densities lead to greater evaporative losses, increased spatter, and uneven weld beads [4,5]. Thus, minimizing the irradiance incident upon the workpiece would reduce the loss of high vapor pressure elements. Larger reductions of both Mg and Zn were also reported at slower travel speeds [4,5]. Evaporation during laser welding is known to be a problem for aluminum alloys but it does not appear so detrimental in Mg–Al alloys [7]. However, it is expected that evaporative loss is more problematic in low zinczirconium alloys where zinc provides strengthening and zinc evaporation might decrease the properties to unacceptable levels [58]. Thus, further work is needed to fully understand the mechanism, to investigate the main influencing factors, to build up the quantitative relationship between the element loss of evaporation and welding process parameters, and to find a remedy for evaporative losses in magnesium alloys. During laser welding, the fine particles originating from the evaporation and condensation of volatile alloy constituents will deposit on the workpiece surface, even leading to the formation of a black coating [4,5]. It was reported that magnesium and zinc were the major components in black coating deposited near welds in wrought AZ31B alloy. The quantity of black powder increased with laser beam irradiance. When full penetration was achieved, the black powder coating decreased abruptly due to the reduced evaporation and the escape of the vapor from the bottom of the keyholes. 5. Mechanical properties 5.1. Hardness Weisheit et al. [31,42] studied 2.5 kW CW CO2 laser welding of cast magnesium alloys such as AZ91 (sand cast, die cast, sand cast-T6), AM60 (die cast), ZC63 (sand cast, sand cast-T6), ZE41 (sand cast), QE22 (sand cast, sand cast-T6) and WE54 (sand cast, sand cast-T6) and wrought (as-extruded) alloys (AZ31, AZ61, ZW3 and ZC71). For as-cast alloys, there is an increase in hardness of the FZ but little variation in hardness occurs in the HAZ. The 3 kW CW Nd:YAG laser welding of die cast AM60B alloy showed that the average hardness in the FZ is approximately 63 HV as compared with a hardness of 53 HV in base metal [8]. In the narrow fusion line, however, a low average hardness of approximately 47 HV was observed due to the accumulation of pores in the zone. Compared to the base metal, the increase in hardness of the fusion zone was probably due to its finer microstructure [8,30,31,42,53] and higher volume fraction of intermetallics such as Mg17 Al12 [8]. Hardness in the fusion zone was found to increase almost linearly with welding speed [23,30], because higher welding speeds lead to a more significant refinement of the microstructure and more alloying elements into

the matrix, even though hard intermetallics are reduced and more finely distributed at high cooling rates [4,5,21–23,33]. At low welding speeds the weld structure and hardness were nearly the same as those in the base die-cast material [22,33]. For as-extruded (wrought) alloys, little variation in hardness was observed between base material (BM), HAZ and FZ [31,42]. Though significant grain coarsening occurred in the HAZ of wrought AZ31 alloy, the hardness in the HAZ was still almost the same as that in base metal [31,42]. The little variations in hardness of the BM, HAZ and FZ were further confirmed for wrought AZ31B alloy using 1.7 kW CW Nd:YAG [21,32] or 2 kW PW Nd:YAG laser [4,5]. The similar hardness levels could be due to the complete compensation for the loss in work hardening by grain refinement. However, it was also reported that there was a gradual decrease in hardness of 6 kW CW CO2 laser welded joints from the BM to the HAZ to the FZ of AZ31BH24 alloy, with a minimum value in the FZ [4,5]. The decrease in the hardness of the HAZ was due to grain growth. The average hardness of CO2 laser welded joints decreases with slower welding speeds [4,5]. Hiraga et al. [49] studied 2 kW CW CO2 and Nd:YAG laser welding of wrought AZ31B-H24 butt joints of 1.7 mm thickness. The Nd:YAG laser welded fusion zone is slightly harder than the FZ produced by CO2 laser [4,5,49]. During CO2 laser welding, the occurrence of shielding gas and metal vapor plasma provides a heat source, causing the increase in molten volume and reduction in cooling rate and, thereby, grain coarsening and reduced strength [49]. In contrast, Nd:YAG laser welding results in more rapid cooling and smaller dendrite arm spacing. Age-hardening effects in magnesium alloys are not so strong as in aluminum alloys. Thus, for magnesium alloys agehardened (T6) prior to welding, a great drop in hardness is not expected [31,42]. For instance, it was reported that cast alloys AZ91-T6, ZC63-T6 and WE54-T6 have almost the similar hardness levels in the fusion zone and base metal, probably because grain refinement compensates for the decrease in hardness due to the dissolution of precipitates [31,42]. However, there was a significant hardness decrease in the FZ for QE22-T6 alloy, although grain refinement was observed. The reason is still not clear, but might be related to a change in submicroscopic structure [31,42]. Different effects on the hardness in the HAZ were also found. No remarkable change occurred in the HAZ of ZC63T6 and WE54-T6 alloys, but a decrease in the HAZ hardness was found in AZ91-T6 and QE22-T6 alloys. The decrease of hardness could be due to the dissolution or the coarsening of the precipitates in the HAZ [31,42]. For ZE41A-T5 alloy welded using 2.5 kW CW Nd:YAG laser, HAZ softening was observed (Fig. 12). The hardness values in the fusion zone were slightly lower than those of base metal though the joints had naturally aged for about 3 or 5 months [57]. Weisheit et al. [53] investigated cast AZ91-T4 and QE22-T4 alloys using filler wires AZ92 and QE22, respectively. The FZ in AZ91-T4 alloy showed little variation in hardness but there was a slight increase in the hardness of the FZ (from 60 to 68 HV) for QE22-T4 alloy. In the HAZ, no hardening or softening effects were found [53]. After a subsequent T6 heat treatment for laser welded AZ91-T4, QE22-T4 and WE54-T6 joints using filler wires AZ92, QE22


Fig. 12. Microindentation hardness of CW Nd:YAG laser welded 2 mm ZE41A joints naturally aged for approximately (A) 5 months and (B) 3 months.

and WE54, respectively, the hardness in the fusion zone and base metal was at the same level, indicating that the loss of alloying elements was very limited during laser welding [53]. The hardness profiles in dissimilar metal joints are associated with concentration gradients of alloying elements in the welds. Clearly, the effect on hardness in the HAZ is the same as for similar metal joints [31,42]. Marya and Edwards [30] researched 0.9 kW Nd:YAG laser welding of die cast AZ91 alloy. The hardness values within the weld bead varied significantly with the depth and welding parameters. Hardness gradually decreased from the top to the bottom of the welds. Dhahri et al. [36] investigated WE43-T6 alloy using 5 kW CO2 laser. The hardness at the top and bottom of the welds was similar but the hardness in the middle of the bead was lower. These differences in hardness distribution over laser weld joints indicate the imhomogeneity of the joints. 5.2. Tensile properties For die cast AZ91 and AM50 alloys welded using 2 kW CW Nd:YAG and 6 kW CW CO2 lasers, static tensile tests of flat tensile specimens showed that tensile strength was of the same order as that of the base metal [3,23,33]. The elongation at fracture of the weld joints was lower than that of the base material. The fractures showed a brittle mode and usually occur in high porosity areas of the base material. Lehner et al. [18] further studied laser welding of die cast AZ91 and AM50 alloys with thickness

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of 3 and 5 mm using 3 kW CW Nd:YAG and 6 kW CW CO2 lasers. All tensile strengths and yield strengths were reduced by about 10% on average after welding, but the base material had highly scattered tensile strength due to gas and shrinkage cavities. Casting quality with low gas contents is important if weld joints with good mechanical properties are to be obtained. For as-extruded AZ31 and AZ61 alloys welded using 2 kW CW Nd:YAG and 6 kW CW CO2 lasers, the tensile strength was similar to the base metal [23]. The weld joints fractured at the smallest load-bearing cross-section in welding seams and fractures proceeded from the fusion zone to the base material. Weisheit et al. [53] researched 2.5 kW CO2 laser welding of asextruded AZ61 butt joints with a thickness of 5 mm using AZ92 filler wire. It was found that AZ61 joints after the stress relief anneal treatment had only 76% of initial tensile strength. The fracture strain was reduced from 13.6 to 3.3% because higher amounts of intermetallic precipitates caused embrittlement of the fusion zone. The yield strength, however, reached 90% of the base material value [53]. In an earlier investigation of 1.36 mm thick wrought AZ31B alloy using 2 kW CW Nd:YAG laser, Haferkamp et al. [21,32] reported that the fracture strain of the joints was reduced by about 64% compared to base material, and fracture occurred in the weld metal. When filler material AZ61A was used, the static tensile and yield strength was 90–100% of the base material; the fracture strain was reduced by no more than about 16% lower than the base material and fracture position was principally in the HAZ due to the larger weld cross-section and the induced notch effect on the top and underbead [21,32]. Dhahri et al. [37,41] researched WE43-T6 alloy with a thickness of 4 mm using a 5 kW CO2 laser, indicating that the tensile strength was approximately 76–90% and the yield strength was about 64–107% of the base metal and that the elongation decreased from 6% for the base metal to 2% for the bead. Weisheit et al. [53] researched 2 kW CO2 laser welding of 5 mm butt joints of cast WE54-T6, AZ91-T4 and QE22-T4 alloys using filler wires WE54, AZ92 and QE22, respectively. The yield and tensile strengths of WE54-T6 joints in both as-welded and aged (T6) conditions reach approximately 85–90% of the base metal while fracture strain was about 70 and 83% of the initial values in the as-welded or aged (T6) conditions, respectively. The tensile properties of AZ91-T4 joints in aged (T6) condition were similar to the base material. The tensile and yield strength of QE22-T4 joints in aged (T6) condition reached 92 and 95% of the base material in T6 condition, but fracture strain had only 60% of the initial value [53]. The failure location was found to be either in the base metal or in the fusion zone. Magnesium alloys usually have high notch sensitivity [60]. The mechanical properties of welds are greatly influenced by geometric notches in weld seams. These notches are usually caused by root drop-through and metal burn-up [16]. Thus, an important criterion for reliable joints is to avoid notches in weld joints [60]. For castings, pores and oxides have important effects on joint quality. It was thought that the strength of the welds could reach the properties of base material for die-castings with low gas and oxide contents [32]. High gas and shrinkage pores will led to reduced and scattered tensile strength [18]. The distribution of pore sizes also influences the mechanical properties

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of weld joints. For a given pore volume, the welds with a higher quantity of small pores show a higher strength than those with larger pores [32]. Lehner et al. [18] further investigated the influence of gap and misalignment on the mechanical properties of welded die-castings. Only gaps greater than approximately 0.15 mm for 3 mm thick samples and 0.3 mm for 5 mm samples reduce the tensile strength significantly when welded autogenously. Despite some evaporative losses of magnesium and zinc elements, laser welds almost invariably exhibited higher mechanical properties than arc welds [62]. 5.3. Fatigue properties Magnesium alloys are notch-sensitive, and this greatly affects fatigue properties [22]. Fatigue test data for laser welded magnesium joints have only been reported by Haferkamp et al. [3,23]. They studied 2 kW CW Nd:YAG and 6 kW CW CO2 laser welded wrought (AZ61A-F to AZ61A-F) and die cast (AZ91-F to AM50-F) alloys with thickness of 2.2 and 2.6 mm, respectively. The butt-joint specimens were tested at a frequency of approximately 70 Hz and a load ratio of R = 0.1. Compared to electron beam welded joints, the laser-welded specimens could tolerate higher load amplitudes for a given number of cycles to failure. This was believed to be due to lower notch sensitivity for the laser-welded specimens. The weld joints of extruded material gave lower strengths than die cast components probably because the upper bead and root of laser-welded die-castings were not as rough as for the wrought alloys [3]. Much work should be done to investigate the fatigue properties of CO2 and Nd:YAG laser welded joints for different magnesium alloys and to compare them with base metal properties. 5.4. Other properties An investigation of the deep drawing properties of laser welded wrought sheets showed that similar forming characteristics to those of base material are attainable through the adjustment of forming temperatures [22]. Haferkamp et al. [21,32] researched 2 kW CW Nd:YAG and 6 kW CW CO2 laser welding of wrought (AZ21A, AZ31B and AZ61B with a thickness of 1.4 mm) and die cast (AM50B, AM60B and AZ91D with a thickness of 2–4 mm) alloys. Transverse bending tests of the welded samples indicated that maximum loads at fracture were 58–84% of base material properties. For the torsion test, maximum torsional moments were 60–110% and the torsion angles were 60–150% of base metal values. Compressive stress for unnotched flat tensile specimens was 82–97% of base metal [21]. Weisheit et al. [53] researched 2.5 kW CO2 laser welding of 5 mm butt joints of cast AZ91-T4, QE22-T4, WE54-T6 and wrought AZ61-F alloys using filler wires AZ92, QE22, WE54 and AZ92, respectively. The notch bar impact tests showed that the impact strength values of base metal for AZ61 and AZ91 alloys are significantly higher than those of QE22 and WE54 alloys. The impact strength in AZ alloys in the as-welded condition drops to approximately 50% of base material values. WE54 and QE22 weldments in as-welded conditions showed increased

impact strength probably due to grain refinement and the precipitation of finer intermetallics. Further post-weld heat treatment (T6) has no pronounced effects on the impact strength despite significant variations in microstructure [53]. Generally speaking, magnesium has limited corrosion resistance, especially when used in dissimilar metal joints. Haferkamp et al. [19] made some preliminary investigations on the corrosion behavior of laser welded AZ91D joints in synthetic seawater. The high welding speed could improve the corrosion resistance of the welds because of fine grain sizes, and solid solutions with higher aluminum contents. Thus, the corrosion tendency of rapidly solidified welds, as in the case for high power laser welding, is relatively low. Clearly, alloy composition, metallurgical microstructure, and welding process parameters may influence the corrosion resistance. The corrosion behavior of magnesium joints and the effects of welding process parameters on corrosion resistance, however, are not well understood and significant research remains to be done. 6. Outlook To weld magnesium joints at high productivity, high quality and low cost, a predictable, repeatable, consistent and reliable welding process will need to be developed. Wider welding operating windows are also welcome for industrial applications. Thus, further efforts should concentrate on optimizing, controlling, regulating and defining laser welding parameter-operating windows for different magnesium alloys. Process specifications for laser welding should be developed to avoid the occurrence of welding defects for the reliable production of magnesium alloy joints. Research work on modeling and simulation will aid in the understanding of the welding processes involved [73–75]. Little work in this aspect, however, has been conducted to date. No quality standards for laser welded magnesium joints are available at the moment. Defect assessment procedures specific to the laser welded joints are also needed. There is also a requirement to establish the comprehensive relationships of material, welding processes and defects with mechanical properties of laser welded joints including tensile, fatigue, fracture, formability and other static and dynamic properties, as well as corrosion properties. No work has ever been reported on the control of residual stress and distortion in laser welded magnesium alloys. Dissimilar joints between different magnesium alloys, dissimilar metals (i.e. Mg–Al, Mg–steel) and composites, with different geometries (thickness, shape), will probably be laser welded in the future. Because of tight tolerances towards edge preparation, fit-up and sophisticated clamping jigs, hybrid processes, i.e. laser beams combined with MIG, TIG and plasma arc processes or even combinations of similar or different lasers (e.g. multi-beam techniques) will possibly be used in the future. The easy manipulation and control of Nd:YAG lasers through optical fiber delivery provides welding opportunities for complex geometries where 3D welding may be carried out. Castings often have complex design and large difference in section thickness. The constraints on laser welding could be quite severe. Thus, careful process control may be necessary, especially for long freezing range alloys, such as those with high


zinc contents. Castings are also needed to repair some defects such as cracks, misrun, porosity, undersize, inclusions, broken and worn sections. However, no work has been carried out in repair welding using laser beams. 7. Summary Laser welding will probably become an important joining technique for magnesium alloys and can promote their wider uses in aerospace, aircraft, automotive, electronics and other industries. To date, the two main types of industrial lasers, both CO2 and Nd:YAG, have been used to investigate the weldability of magnesium alloys. Crack-free laser welded joints with low porosity and good surface quality can be obtained for some magnesium alloys, in particular wrought material, using appropriate laser processing parameters. Due to their inherent properties, however, magnesium alloys may exhibit some processing problems and weld defects such as unstable weld pool, substantial spatter, strong tendency to drop-through, sag, undercut, porosity, liquation and solidification cracking, oxide inclusions and loss of alloying elements. In the future scientific investigation is still needed to understand and overcome these basic weldability problems of magnesium alloys. References [1] R.S. Busk, Magnesium Products Design, Marcel Dekker Inc., New York, 1987. [2] M.M. Avedesian, H. Baker, Magnesium and Magnesium Alloys, ASM Specialty Handbook, 1999. [3] H. Haferkamp, M. Niemeyer, U. Dilthey, G. Trager, Laser and electron beam welding of magnesium materials, Weld. Cutt. 52 (8) (2000) 178–180. [4] P.G. Sanders, J.S. Keske, K.H. Leong, G. Kornecki, High power Nd:YAG and CO2 laser welding of magnesium, J. Laser Appl. 11 (2) (1999) 96–103. [5] K.H. Leong, G. Kornecki, P.G. Sanders, J.S. Keske, Laser beam welding of AZ31B-H24 alloy, ICALEO 98: Laser Materials Processing Conference, Orlando, FL, 16–19 November 1998, pp. 28–36. [6] B.L. Mordike, T. Ebert, Magnesium: properties-applications-potential, Mater. Sci. Eng. A 302 (2001) 37–45. [7] M. Marya, D.L. Olson, G.R. Edwards, Welding of magnesium alloys for transportation applications, in: Proceedings from Materials Solution ’00 on Joining of Advanced and Specialty Materials, St. Louis, Missouri, 9–11 October 2000, pp. 122–128. [8] M. Pastor, H. Zhao, T. DebRoy, Continuous waveNd:yttrium–aluminium–garnet laser welding of AM60B magnesium alloys, J. Laser Appl. 12 (3) (2000) 91–100. [9] E. Aghion, B. Bronfin, Magnesium alloys development towards the 21st century, Mater. Sci. Forum 350–351 (2000) 19–28. [10] M. Marya, G. Edwards, S. Marya, D.L. Olson, Fundamentals in the fusion welding of magnesium and its alloys, in: Proceedings of the Seventh JWS International Symposium, Kobe, 20–22 November 2001, pp. 597–602. [11] F.H. Froes, D. Eliezer, E. Aghion, The science, technology, and applications of magnesium, JOM 50 (9) (1998) 30–34. [12] I. Nakatsugawa, Surface modification technology for magnesium products, IMA 53 Magnesium: A Material Advancing to the 21 Century, UBE City Yamaguichi, Japan, 2–4 June 1996, pp. 24–29. [13] R. Vilar, Laser alloying and laser cladding, Mater. Sci. Forum 301 (1999) 229–252. [14] W.R. Oates, Welding Handbook, eighth ed., American Welding Society, Miami, Florida, 1996, pp. 121–162.

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A review of laser welding techniques for magnesium

A review of laser welding techniques for magnesium

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