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Investigation of the microstructure and mechanical properties of Mg–Al–Zn alloy joints formed by different laser welding processes

  • Qi Luo , Shaogang Wang EMAIL logo and Yingying Guo
Published/Copyright: October 18, 2023
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High Temperature Materials and Processes
From the journal High Temperature Materials and Processes Volume 42 Issue 1

Abstract

The microstructure and mechanical properties of Mg–Al–Zn alloy joints by using autogenous laser beam welding (LBW) and laser-MIG hybrid welding, respectively, are investigated. The results show that the weld formation of the hybrid welding joint is relatively good, and there are mainly α-Mg matrix phases and β-Mg17Al12 strengthening phases in the weld metal. The microstructure in the fusion zone (FZ) of the two joints is different. The LBW joint is composed of columnar crystal and equiaxed dendrite. The hybrid welding joint consists of fine equiaxed grains, and the grain size in the laser zone is larger than that in the arc zone. The microhardness in FZ is higher due to the precipitation of the β-Mg17Al12 phase in this region. Under the optimized welding procedure, the strength coefficient of the two joints is >90%. There are many dimples on the tensile fracture surface of the hybrid welding joint, which is characterized by the pattern of the ductile fracture.

Keywords: Mg–Al–Zn alloy; laser beam welding; laser-MIG hybrid welding; microstructure; mechanical properties

1 Introduction

Magnesium alloys have excellent comprehensive properties, such as low density, high specific strength and specific stiffness, good electromagnetic shielding effectiveness, and impact absorption properties. They are widely used in many industrial fields such as aerospace, automobile, nuclear power, electronic communication, etc. However, due to their low melting and boiling points, large coefficient of thermal expansion, and strong affinity to elements O and N, the weldability of magnesium alloys is poor. During the fusion welding process, welding defects such as coarser grains in the weldment, evaporation, and burning loss of alloying elements, inclusions, gas pores, and hot cracking were easily generated [1].

Laser beam welding (LBW) has the advantages of energy concentration, rapid welding speed, and small welding deformation, which is suitable for the welding of magnesium alloys. Some researchers had carried out LBW of magnesium alloys, and great progress was made. Shen et al. [2] investigated the effect of welding speed on the microstructure and mechanical properties of the AZ61 magnesium alloy joint by LBW. Harooni et al. [3,4] concluded that the main reason for the generation of weld porosity was the oxide layer on the magnesium alloy surface. The weld porosity could be greatly reduced by preheating.

The partially melted zone (PMZ) of the ZK60 magnesium alloy joint by LBW was investigated, and the results showed that the eutectoid phase distributed along the grain boundary in PMZ would be liquefied during the welding of the as-cast magnesium alloy [5]. There were mainly two kinds of liquation processes. A liquation network distributed along the grain boundary was induced by the melted base metal (BM) and alloying element segregation. Another liquation process was generated by the residual secondary phase at the grain boundary to form liquid metal [6]. Yang et al. [7] studied the liquation behavior in weldment of the fine grain AZ91 magnesium alloy sheet, and the β-Mg17Al12 phase with a low melting point had a great effect.

Zhou et al. [8] investigated the crack behavior in laser weldment of the AZ91D magnesium alloy, and there existed many eutectic phases near the crack. By combining the cryogenic and heat treatment, the tendency to crack was decreased in the laser-welded AZ91D; thus, the mechanical properties of the welded joint were improved [9]. Usually, the content of the Al element is different in different kinds of magnesium alloys, and the cooling rate of the molten pool is different during the LBW process. It had a certain influence on the precipitation of strengthening phases in weldment and the generation of welding defects [10].

In order to obtain a high-quality magnesium alloy joint and avoid welding defects, the LBW procedure is further improved by some researchers. The AZ31B magnesium alloy was welded using LBW with beam oscillation [11]. During welding, laser beam oscillation could break up the columnar crystal near the fusion line (FL), which decreased the grain size, and the equiaxed grains were formed in the weldment. Lei et al. [12] carried out the ultrasonic-assisted LBW of magnesium alloy. The results showed that due to the cavitation and acoustic streaming effect of ultrasonic vibration on the molten pool, the microstructure and morphology of the welded joint could be significantly improved, and the susceptibility to welding defects was decreased.

Compared to the autogenous LBW process, laser-arc hybrid welding is an advanced and efficient welding technology and has the advantages of both LBW and arc welding. Hu and den Ouden [13] investigated the synergetic effect between the laser and arc during the hybrid welding process. The results of laser-arc hybrid welding behavior showed that the probable spatter in the autogenous LBW process could be avoided by the introduction of an arc [14]. In the process of laser-MIG hybrid welding, coupled discharge occurred between the arc plasma and keyhole plasma to form the hybrid arc plasma with complex characteristics [15]. Because the ionization energy of Mg (7.6 eV) is lower, magnesium ions participated in the conduction process by the replacement of some Ar ions [16], and thus the conversion of electric energy into thermal energy was promoted. In addition, the stability of the arc was improved, and the quenching of the arc and drift of the arc root was suppressed [17].

In the present study, Mg–Al–Zn alloys are welded using autogenous LBW and laser-MIG hybrid welding. The weld morphology, microstructure, and mechanical properties of the two joints are investigated and compared. The welding procedure is optimized under the experimental conditions. The mechanism of weld formation is explored during the hybrid welding process. Some pieces of evidence can be provided for the welding of magnesium alloys in engineering applications.

2 Materials and procedures

The BM is a Mg–Al–Zn alloy plate with a dimension of 150 mm × 100 mm × 4 mm. Autogenous LBW and laser-MIG hybrid welding are carried out, respectively, using an IPG fiber laser and MIG welding robot. During the hybrid welding process, the magnesium alloy filler wire with a diameter of 1.2 mm is used. The chemical compositions of BM and the filler wire are given in Table 1. The diagrams of the two welding processes are shown in Figure 1.

Table 1

Chemical compositions of the BM and filler wire (wt%)

Material Al Zn Mn Si Fe Cu Ni Mg
BM 2.5–3.5 0.6–1.4 0.2–1.0 ≤0.08 ≤0.003 ≤0.01 ≤0.001 Bal.
Filler wire 2.8 1.0 0.3 0.02 0.003 ≤0.01 ≤0.001 Bal.
Figure 1 Diagram of the welding process: (a) LBW and (b) laser-MIG hybrid welding.
Figure 1

Diagram of the welding process: (a) LBW and (b) laser-MIG hybrid welding.

Before welding, the specimen is ground with sandpaper to remove the oxide layer on the alloy surface, and then it is cleaned with acetone. The butt joint is employed, and it is welded along the longitudinal direction of the specimen. The shielding gas is 99.99% Ar, and its flow rate is 20 L·min−1. The defocus amount of the laser beam is 0 mm. During the hybrid welding process, the distance between the two heat sources is 0 mm, and the angle between the welding torch and the horizontal plane is 60°. After some attempt welding, the filler wire feeding speed of 2.1 m·min−1 is determined. The other welding parameters are listed in Table 2, where L1, L2, and L3 represent the LBW joints, while H1, H2, and H3 represent the hybrid welding joints.

Table 2

Welding parameters of the Mg–Al–Zn alloy

Sample no. Laser power (kW) Arc current/Arc voltage (A/V) Welding speed (m·min−1)
L1 3.5 — 2.0
L2 3.5 — 2.4
L3 4.0 — 2.4
H1 3.5 70/16 2.0
H2 3.5 70/16 2.4
H3 4.0 70/16 2.4

After welding, the microstructure and mechanical properties of the welded joint are investigated. First, the metallographic observation specimen is extracted from the joint, which is perpendicular to the welding direction. It is corroded by the reagent (6 g picric acid + 10 mL acetic acid + 10 mL H2O + 70 mL alcohol) after grinding and polishing. The microstructure of the welded joint is observed using a HU1200 type optical microscope and JSM-6360LV type scanning electron microscope (SEM). A D8 Advance X-ray diffractometer (XRD) is used to identify the phase constituent in the fusion zone (FZ). The FZ of the LBW joint is analyzed using an energy dispersive spectrometer (EDS). Map scanning analysis of the hybrid welding joint is carried out to ascertain the distribution of Al in the weldment. The substructure of the weldment is analyzed using a JEM-2100F type transmission electron microscope (TEM). Subsequently, the microhardness in the weld zone is measured using a HXS-1000AC microhardness tester, with a load of 200 g and a duration time of 15 s. The tensile specimen of the welded joint is prepared, and its dimension is shown in Figure 2. The tensile tests are carried out at room temperature using a KY-100KNW universal electronic tensile testing machine with a loading rate of 1.5 mm·min−1. The joint fracture morphology is observed using SEM, and the fracture characteristics are analyzed.

Figure 2 Dimensions of the tensile specimen.
Figure 2

Dimensions of the tensile specimen.

3 Results and discussion

3.1 Weld morphology

Figure 3 shows the macrograph and the cross section of joints L2 and H2, respectively. The LBW joint is narrow and it has a large weld depth-to-width ratio. As shown in Figure 3(a)–(c), the appearance of the weld is continuous and uniform but it has slightly sunken in the local region. It is mainly caused by the lower surface tension of the molten pool and the instability of the keyhole during the LBW of magnesium alloys. The weld morphology of the hybrid welding joint presents the typical “wine cup” shape, as shown in Figure 3(f). During the hybrid welding process, the upper part of the weld is affected by the two heat sources of laser and arc. It has the feature of arc welding, which is defined as an “arc zone.” The lower part of the weld is defined as a “laser zone,” which displays the characteristics of laser deep penetration welding. As shown in Figure 3(d) and (e), the top surface of the hybrid welding joint is better, and the root surface is continuous and uniform. No welding defects such as spatter and undercut are generated. In the hybrid welding process, the probable surface depression in the weldment can be improved by the addition of the filler wire. The width of the hybrid welding joint is slightly increased compared with that of the LBW joint, as shown in Figure 3(d) and (f).

Figure 3 Macrograph and the cross section of welded joints: (a) top surface of joint L2, (b) root surface of joint L2, (c) cross section of joint L2, (d) top surface of joint H2, (e) root surface of joint H2, and (f) cross section of joint H2.
Figure 3

Macrograph and the cross section of welded joints: (a) top surface of joint L2, (b) root surface of joint L2, (c) cross section of joint L2, (d) top surface of joint H2, (e) root surface of joint H2, and (f) cross section of joint H2.

3.2 Microstructure

The microstructure of the LBW joint is shown in Figure 4. As shown in Figure 4(a), the FL is clear and the PMZ is not obvious. The microstructure of FZ is mainly composed of columnar crystals near FL and equiaxed dendrites in the weld center, as shown in Figure 4(b). During the LBW process, the heterogeneous nucleation of the liquid metal is attached to the unmelted BM. Due to the high thermal conductivity of magnesium alloys, the temperature gradient perpendicular to the FL is the largest. Grains grow along the direction of heat dissipation in the cooling process. As a result, the columnar crystal is formed near the FL.

Figure 4 Microstructure of the LBW joint: (a) the transition zone and (b) FZ.
Figure 4

Microstructure of the LBW joint: (a) the transition zone and (b) FZ.

The temperature gradient in the weld center is decreased, and the equiaxed dendrite is formed. Solute elements tend to aggregate at the dendrite arms during the solidification of the molten pool. The relationship between the curvature of the dendrite root and the equilibrium temperature can be described as follows [18]:

(1) T − T 0 = − 2 ρ V s γ T 0 / Δ H 0 ,

where T is the temperature of the liquid metal, ρ is the curvature, T 0 is the equilibrium temperature, V s is the molar volume of the solid phase, ΔH 0 is the standard molar enthalpy, and γ is the interfacial energy. Solute elements are enriched at the dendrite root, and the growth of dendrite is inhibited. Because of the rapid cooling rate of the molten pool during the LBW process, a large number of fine equiaxed dendrites are formed in the weld center.

The microstructure of the hybrid welding joint, at a distance from the weld surface of about 2 mm, is shown in Figure 5. It consists of a heat-affected zone (HAZ), PMZ, and FZ. The PMZ in the arc zone is wider than that in the laser zone. During the hybrid welding process, the heat input in the laser zone is larger due to the interaction between the arc and laser beam, and the arc is attracted and compressed by the laser beam. The energy of the laser beam tends to concentrate, while the arc heat is relatively dispersed when it acts on the workpiece. In addition, the vapor pressure of Al is lower than that of Mg, resulting in the evaporation loss of Al being more serious in the molten pool. Al can be supplemented to a certain extent by the addition of the filler wire. The content of Al in the arc zone is increased, which increases the liquid–solid temperature range. Consequently, the PMZ in the arc zone is widened to a certain extent, as shown in Figure 5(a). The PMZ is relatively narrow in the laser zone, as shown in Figure 5(c), because it is mainly affected by the laser beam with an energy concentration. The grain size near the FL is finer because the supercooling is larger and the number of nuclei is more during the cooling process.

Figure 5 Microstructure of the hybrid welding joint: (a) transition zone in the arc zone, (b) weld center in the arc zone, (c) transition zone in the laser zone, and (d) weld center in the laser zone.
Figure 5

Microstructure of the hybrid welding joint: (a) transition zone in the arc zone, (b) weld center in the arc zone, (c) transition zone in the laser zone, and (d) weld center in the laser zone.

As shown in Figure 5(b) and (d), the weld center is composed of fine equiaxed grains. The sizes of equiaxed grains in the arc zone and laser zone are calculated and fitted, respectively, as shown in Figure 6. The average diameter of grains in the laser zone (13.6 ± 6.0 μm) is larger than that in the arc zone (6.7 ± 4.0 μm). The detailed discussion is given in Section 3.5.

Figure 6 Grain size in the FZ of the hybrid welding joint: (a) arc zone and (b) laser zone.
Figure 6

Grain size in the FZ of the hybrid welding joint: (a) arc zone and (b) laser zone.

3.3 Phase constituent and distribution of alloying elements

The β-Mg17Al12 phase is the main strengthening phase in the Mg–Al–Zn alloy. The XRD analyses of the two weldments are carried out, as shown in Figure 7, and mainly the β-Mg17Al12 strengthening phase and α-Mg matrix phase were found in the FZ of the two joints.

Figure 7 X-ray diffraction pattern in the FZ: (a) LBW joint and (b) hybrid welding joint.
Figure 7

X-ray diffraction pattern in the FZ: (a) LBW joint and (b) hybrid welding joint.

During the LBW process, the solidification of the molten pool presents the non-equilibrium feature due to the rapid cooling rate. Solute atoms tend to aggregate at the equiaxed dendrite, and the secondary phase particles are precipitated in the weldment. As shown in Figure 8(a), EDS analysis is carried out to the secondary phase (P1) in the equiaxed dendrite center. Figure 8(b) shows that the content of Mn is higher. Mn can combine with Al to form Al8Mn5 or other compound phases in the weldment, which can act as the nucleus during the solidification process. EDS analysis of the secondary phase at the dendrite arm (P2) was also conducted. As shown in Figure 8(c), Al and Zn are present. Combining with the results of XRD analysis in Figure 7(a), it is speculated that the intermetallic phase Mg17(Al, Zn)12 is formed in the weldment, which was consistent with the results of Wahba et al. [19].

Figure 8 SEM image and EDS analysis in the FZ of the LBW joint: (a) SEM image of the FZ, (b) EDS analysis at P1, and (c) EDS analysis at P2.
Figure 8

SEM image and EDS analysis in the FZ of the LBW joint: (a) SEM image of the FZ, (b) EDS analysis at P1, and (c) EDS analysis at P2.

Figure 9 shows the SEM image in the FZ of the hybrid welding joint and mapping distribution of Al. Due to the rapid cooling rate of the molten pool, the solubility of Al in the Mg matrix is greatly decreased under the non-equilibrium solidification process, which results in insufficient diffusion of Al atoms. As shown in Figure 9(a), the precipitation of the β-Mg17Al12 phase is mainly at the grain boundary in the arc zone. As shown in Figure 9(b), the distribution of Al is less in the α-Mg matrix phase, and most of Al is concentrated in front of the solid–liquid interface. The content of Al is higher at the grain boundary in the arc zone, and thus the β-Mg17Al12 phase nucleates and grows preferentially. The atomic arrangement on both sides of the grain boundary has a larger degree of misfit, and the interface between the β phase and Mg matrix presents a non-coherent relationship. The growth of the β phase will consume a large amount of Al, which leads to the formation of an Al-deficient region. The driving force of precipitation is reduced, and the quantity of the β phase within grains is decreased. The distribution of the β phase is relatively uniform within grains, as shown in Figure 9(c). During the hybrid welding process, more energy is absorbed in the laser zone, and the holding time for the molten pool is relatively long at high temperatures. Consequently, the atom diffusion is sufficient and the distribution of Al is homogeneous, as shown in Figure 9(d).

Figure 9 SEM image and distribution of Al in the hybrid welding joint: (a) SEM image in the arc zone, (b) distribution of Al in (a), (c) SEM image in the laser zone, and (d) the distribution of Al element in (c).
Figure 9

SEM image and distribution of Al in the hybrid welding joint: (a) SEM image in the arc zone, (b) distribution of Al in (a), (c) SEM image in the laser zone, and (d) the distribution of Al element in (c).

In order to explore the substructure of the FZ in the hybrid welding joint, the secondary phase is observed and analyzed using TEM. The TEM images are shown in Figure 10. The morphology of the secondary phase in the α-Mg matrix is shown in Figure 10(a), and its size is about 300 nm. Figure 10(b) displays the selected area electron diffraction pattern of the secondary phase, and it is calibrated to be the β-Mg17Al12 phase. The β phase is precipitated in the α-Mg solid solution. As shown in Figure 10(c), the β phase is mainly distributed at the grain boundary. There are dislocations in the weldment, as shown in Figure 10(d). Under the action of an external stress, the movement of dislocations is inhibited to a certain extent by the β phase.

Figure 10 TEM images in the FZ of the hybrid welding joint: (a) morphology of the secondary phase, (b) selected area electron diffraction pattern, (c) β phase at the grain boundary, and (d) β phase and pile up of dislocations.
Figure 10

TEM images in the FZ of the hybrid welding joint: (a) morphology of the secondary phase, (b) selected area electron diffraction pattern, (c) β phase at the grain boundary, and (d) β phase and pile up of dislocations.

3.4 Mechanical properties

3.4.1 Microhardness distribution

In general, the hardness of the material is mainly related to its chemical composition and microstructure. As shown in Figures 4 and 5, the microstructure of different zones in the welded joint is different. Figure 11 shows the microhardness distribution curve of the LBW joint. The hardness in the FZ is significantly higher than that in the HAZ and BM. The increase of hardness in the FZ is due to the precipitation of the β-Mg17Al12 phase and grain refinement. The hardness in the HAZ is slightly higher than that in the BM. During the LBW process, the β phase in the HAZ is dissolved into the α-Mg matrix under the action of the weld thermal cycle. The metastable supersaturated solid solution was formed in this region due to the rapid cooling rate [20], and the subsequent aging hardening effect was generated. Consequently, compared to that of the BM, the hardness in the HAZ is increased to a certain extent.

Figure 11 Microhardness distribution curve of the LBW joint.
Figure 11

Microhardness distribution curve of the LBW joint.

Figure 12 shows the microhardness distribution curve of the hybrid welding joint. The hardness in the FZ is the maximum. As mentioned above, the grain size in the arc zone is relatively fine. During the hybrid welding process, the arc has a stirring effect on the molten pool, and the shielding gas can enhance the cooling of the liquid metal. The content of Al in the molten pool is supplemented by the filler metal, which promotes nucleation and grain refinement. As a result, the hardness in FZ is higher. Although the grain size in the laser zone is coarser than that in the arc zone, the hardness in the laser zone can be improved to a certain extent by the precipitation of the β phase.

Figure 12 Microhardness distribution curves of the hybrid welding joint: (a) arc zone and (b) laser zone.
Figure 12

Microhardness distribution curves of the hybrid welding joint: (a) arc zone and (b) laser zone.

The hardness of HAZ in the laser zone is slightly higher than that of BM. The distribution curve of microhardness is similar to that of the LBW joint. This is because the laser zone is mainly affected by the laser beam. However, the hardness of HAZ in the arc zone is slightly lower than that of the BM. During welding, the arc is attracted by the laser beam, which increases the heat input in the arc zone. The recrystallization of grains occurs in the HAZ. The effect of work hardening in the original BM caused by rolling deformation is weakened [21], thus the hardness in this region is decreased to a certain extent. With the distance far away from the weld center, the heat effect is gradually decreased, and the complete recrystallization of grains will not occur. The dislocation density near the BM is higher, which leads to the increase of hardness.

3.4.2 Tensile properties

The tensile strength and elongation of BM are 255.4 MPa and 12.3%, respectively. The results of tensile tests for welded joints under different welding procedures are given in Table 3. The corresponding stress–strain curves of welded joints are shown in Figure 13. All of the welded joints fractured at the weld metal near the FL during the tensile process. Compared to that of the BM, the tensile strength and elongation of welded joints decreased to a certain extent. The tensile strengths of joints L2 and H2 are 230.0 and 234.8 MPa, being the highest, which are 90.1 and 91.9% of that of the BM, respectively. Their corresponding welding parameters are determined as the optimized welding parameters. Both of the welded joints have good mechanical performance. The precipitation of the β-Mg17Al12 phase in the FZ has the effect of dispersion strengthening. Moreover, the solution-strengthening effect generated by Al and Zn in the weldment can improve the mechanical properties of the welded joint to a certain extent.

Table 3

Results of tensile tests for welded joints under different welding procedures

Sample no. Tensile strength (MPa) Strength coefficient (%) Elongation (%)
L1 222.9 87.3 9.8
L2 230.0 90.1 11.2
L3 228.5 89.5 11.0
H1 226.3 88.6 8.9
H2 234.8 91.9 11.5
H3 223.4 87.5 10.4
Figure 13 Tensile stress–strain curves of welded joints.
Figure 13

Tensile stress–strain curves of welded joints.

As shown in Figure 10(d), the β phase precipitated at the grain boundary can inhibit the dislocation motion during stretching, which results in a large amount of dislocations pile up. Stress concentration is induced at the interface between the β phase and the Mg matrix. Under the action of an external stress, the β phase departs from the Mg matrix, and micro-cracks are generated at the grain boundary. In the continuous tensile process, the crack gradually propagates. Finally, the joint fails in the weldment.

3.4.3 Fracture analysis

In order to understand the characteristics of tensile fracture, the fracture surface of the joint is observed by SEM. Figure 14 shows the fracture morphologies of the two joints. There are mainly equiaxed dimples on the fracture surface of the LBW joint, as shown in Figure 14(a). The formation of dimples involves the following stages: generation of micropores, growth of micropores, and connection to each other. The β-Mg17Al12 phase is the main strengthening phase in the weldment, which has a non-coherent relationship with the Mg matrix. During stretching, the stress concentration is induced by the pile-up of dislocations, which results in the uncoordinated plastic deformation between the β phase and Mg matrix. The micropores are generated, and the microcracks appear subsequently. The ductility of the Mg matrix is relatively good. The β phase particles are pulled out during plastic deformation, and dimples are formed on the fracture surface. In addition, a few cleavage steps are found on the fracture surface of the LBW joint, as shown in Figure 14(b). It has a certain tendency to brittle fracture and is characterized by the mixed mode of the ductile–brittle fracture. As shown in Figure 14(c), there are many dimples on the fracture surface of the hybrid welding joint. The quantity of dimples is relatively more and its distribution is uniform. The cleavage step is not obvious, as shown in Figure 14(d). The fracture morphology of the joint presents the characteristic of ductile fracture.

Figure 14 SEM images of the joint tensile fracture: (a) LBW joint, (b) magnification of red rectangle in (a), (c) hybrid welding joint, and (d) magnification of red rectangle in (c).
Figure 14

SEM images of the joint tensile fracture: (a) LBW joint, (b) magnification of red rectangle in (a), (c) hybrid welding joint, and (d) magnification of red rectangle in (c).

3.5 Mechanism of weld formation in the hybrid welding process

As described above, there is a difference in the grain size between the arc zone and laser zone. The formation of the laser zone is mainly affected by the laser beam energy during welding. Because the reflectivity of magnesium alloys to the laser beam is higher, the laser energy transfers to the BM through Fresnel absorption in the keyhole. The Fresnel effect is characterized by the multiple reflection and absorption of the laser beam on the keyhole wall. In order to explore the energy distribution of the laser beam in the keyhole, the Fresnel absorptivity (A) can be calculated by the following equation [22]:

(2) A = 1 − 1 2 1 + ( 1 − ε cos φ ) 2 1 + ( 1 + ε cos φ ) 2 + ε 2 − 2 ε cos φ + 2 cos 2 φ ε 2 + 2 ε cos φ + 2 cos 2 φ ,

where the value of ε is related to the conductivity per unit depth of the metal, and φ is the angle between the incident laser beam and the normal of the material surface. The lower the φ value is, the higher the absorptivity is. The formation of a keyhole can enhance the absorption of laser energy during welding.

In the welding process, the centerline of the keyhole lags behind the laser beam axis. At the same time, the keyhole bends toward the opposite welding direction and mainly occurs in the lower part of the keyhole. Consequently, the value of φ is the minimum at the bottom of the keyhole. According to Eq. (2), the laser energy is relatively low in the upper part of the keyhole and is mainly distributed in the lower part of the keyhole. Jin et al. [23] analyzed the Fresnel absorption and multiple reflections of the laser beam in the keyhole, and the absorption on the keyhole wall was calculated. Results revealed that the distribution of laser energy on the keyhole wall was non-uniform. The absorption of laser energy in the lower part of the keyhole was more than that in the upper part of the keyhole. During welding, the material is melted to form the keyhole, and a large amount of plasma is formed in the keyhole. Some laser energy is absorbed, and it is transferred to the workpiece, which results in the inverse bremsstrahlung absorption of the laser beam. Cheng et al. [24] studied the effect of Fresnel absorption and inverse bremsstrahlung absorption on the laser power distribution of the keyhole wall during CO2 laser welding of aluminum alloy. Results demonstrated that the laser energy absorbed by inverse bremsstrahlung absorption of the keyhole plasma was higher than that by Fresnel absorption.

The wavelength of the fiber laser (1.064 μm) is short, and its inverse bremsstrahlung absorption coefficient is lower [22]. During the laser-arc hybrid welding, the loss of energy is low when the laser beam passes through the arc plasma. The temperature of the liquid magnesium alloy was related to the absorption of laser energy [25]. The formation of the molten pool in the lower part of the hybrid welding joint mainly depends on the laser energy and heat conduction. A large amount of laser energy is absorbed by the keyhole wall during welding.

As shown in Figure 15, owing to the low-temperature difference within the keyhole (microregion B), the influence of surface tension on the flow behavior of the liquid metal is small. There is a relatively long time to the molten pool at high temperatures, and enough driving force is provided for the growth of grains. As a result, the coarser equiaxed grains are formed in the laser zone. In the continuous welding process, under the action of gravity, the liquid metal at the rear of the keyhole wall would make the keyhole close by the pattern of bottom backfilling [26]. Because the temperature of the keyhole is higher, it has a certain temperature difference at the junction of the molten pool between the inside and outside keyhole. The temperature gradient is larger near the keyhole (microregion A), under the action of surface tension, and the gas–liquid interface moves toward the rear of the molten pool. In addition, the arc force has a certain stirring effect on the molten pool. During welding, the arc zone is protected by Ar gas, and the cooling rate of the molten pool is rapid, which is advantageous to grain refinement.

Figure 15 Schematic diagram of the molten pool during laser-MIG hybrid welding.
Figure 15

Schematic diagram of the molten pool during laser-MIG hybrid welding.

4 Conclusions

  1. Compared with that of the autogenous LBW joint, the weld morphology of the Mg–Al–Zn alloy joint by hybrid welding is relatively good, and no welding defects such as undercut and gas pores are generated. During the hybrid welding process, the weld formation and joint mechanical performance can be improved by the addition of filler wires.

  2. The FZ of the LBW joint consists of columnar crystals and equiaxed dendrites, and the PMZ is not obvious. The FZ of the hybrid welding joint is composed of fine equiaxed grains, and the size of equiaxed grains in the laser zone is larger than that in the arc zone. The weld metal is mainly composed of the α-Mg matrix phase and β-Mg17Al12 strengthening phase.

  3. The microstructure of different zones in the welded joint is different, which results in the variation of microhardness in the weld zone. The hardness in the FZ is the maximum, while the hardness in the HAZ decreased to a certain extent. The precipitated β-Mg17Al12 phase is beneficial to increase the hardness in FZ.

  4. The LBW has the feature of energy concentration, which is advantageous to the mechanical properties of the welded joint. The tensile strengths of the LBW joint and hybrid welding joint are 90.1 and 91.9% of the BM, respectively. There are many dimples distributed on the tensile fracture surface of the hybrid welding joint, and it is characterized by the pattern of ductile fracture.

  1. Funding information: This project was supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), which is gratefully acknowledged.

  2. Author contributions: Qi Luo: Investigation, formal analysis, writing-original draft; Shaogang Wang: methodology, supervision, writing-review and editing; Yingying Guo: resources, experimental preparation, formal analysis.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2023-06-07
Revised: 2023-08-31
Accepted: 2023-09-18
Published Online: 2023-10-18

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/htmp-2022-0292
Keywords for this article
Mg–Al–Zn alloy; laser beam welding; laser-MIG hybrid welding; microstructure; mechanical properties
Creative Commons
BY 4.0

Articles in the same Issue

  1. Research Articles
  2. First-principles investigation of phase stability and elastic properties of Laves phase TaCr2 by ruthenium alloying
  3. Improvement and prediction on high temperature melting characteristics of coal ash
  4. First-principles calculations to investigate the thermal response of the ZrC(1−x)Nx ceramics at extreme conditions
  5. Study on the cladding path during the solidification process of multi-layer cladding of large steel ingots
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  7. Comparison of data-driven prediction methods for comprehensive coke ratio of blast furnace
  8. Effect of different isothermal times on the microstructure and mechanical properties of high-strength rebar
  9. Analysis of the evolution law of oxide inclusions in U75V heavy rail steel during the LF–RH refining process
  10. Simultaneous extraction of uranium and niobium from a low-grade natural betafite ore
  11. Transfer and transformation mechanism of chromium in stainless steel slag in pedosphere
  12. Effect of tool traverse speed on joint line remnant and mechanical properties of friction stir welded 2195-T8 Al–Li alloy joints
  13. Technology and analysis of 08Cr9W3Co3VNbCuBN steel large diameter thick wall pipe welding process
  14. Influence of shielding gas on machining and wear aspects of AISI 310–AISI 2205 dissimilar stainless steel joints
  15. Effect of post-weld heat treatment on 6156 aluminum alloy joint formed by electron beam welding
  16. Ash melting behavior and mechanism of high-calcium bituminous coal in the process of blast furnace pulverized coal injection
  17. Effect of high temperature tempering on the phase composition and structure of steelmaking slag
  18. Numerical simulation of shrinkage porosity defect in billet continuous casting
  19. Influence of submerged entry nozzle on funnel mold surface velocity
  20. Effect of cold-rolling deformation and rare earth yttrium on microstructure and texture of oriented silicon steel
  21. Investigation of microstructure, machinability, and mechanical properties of new-generation hybrid lead-free brass alloys
  22. Soft sensor method of multimode BOF steelmaking endpoint carbon content and temperature based on vMF-WSAE dynamic deep learning
  23. Mechanical properties and nugget evolution in resistance spot welding of Zn–Al–Mg galvanized DC51D steel
  24. Research on the behaviour and mechanism of void welding based on multiple scales
  25. Preparation of CaO–SiO2–Al2O3 inorganic fibers from melting-separated red mud
  26. Study on diffusion kinetics of chromium and nickel electrochemical co-deposition in a NaCl–KCl–NaF–Cr2O3–NiO molten salt
  27. Enhancing the efficiency of polytetrafluoroethylene-modified silica hydrosols coated solar panels by using artificial neural network and response surface methodology
  28. High-temperature corrosion behaviours of nickel–iron-based alloys with different molybdenum and tungsten contents in a coal ash/flue gas environment
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  35. Evaluating thermal properties and activation energy of phthalonitrile using sulfur-containing curing agents
  36. Investigation of the liquidus temperature calculation method for medium manganese steel
  37. High-temperature corrosion model of Incoloy 800H alloy connected with Ni-201 in MgCl2–KCl heat transfer fluid
  38. Investigation of the microstructure and mechanical properties of Mg–Al–Zn alloy joints formed by different laser welding processes
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  40. Effect of P and Ti on the agglomeration behavior of Al2O3 inclusions in Fe–P–Ti alloys
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  45. Round-robin measurement of surface tension of high-temperature liquid platinum free of oxygen adsorption by oscillating droplet method using levitation techniques
  46. High-temperature production of AlN in Mg alloys with ammonia gas
  47. Review Article
  48. Advances in ultrasonic welding of lightweight alloys: A review
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  51. Corrosion behavior of a Co−Cr−Mo−Si alloy in pure Al and Al−Si melt
  52. Al–Si–Fe alloy-based phase change material for high-temperature thermal energy storage
  53. Density and surface tension measurements of molten Al–Si based alloys
  54. Graphite crucible interaction with Fe–Si–B phase change material in pilot-scale experiments
  55. Topical Issue on Nuclear Energy Application Materials
  56. Dry synthesis of brannerite (UTi2O6) by mechanochemical treatment
  57. Special Issue on Polymer and Composite Materials (PCM) and Graphene and Novel Nanomaterials - Part I
  58. Heat management of LED-based Cu2O deposits on the optimal structure of heat sink
  59. Special Issue on Recent Developments in 3D Printed Carbon Materials - Part I
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