Effect of Longitudinal Magnetic Field on the Microstructure and Properties of A-TIG Welding with Different TiO₂ Coating Amounts

Abstract

In order to improve the poor weldability and low quality of welded joints during the welding of magnesium alloys, a longitudinal magnetic field is introduced in the welding process of A-TIG welding with a fixed magnetic field frequency (30 Hz) and magnetic field current (1.5 A). Experimental analysis is performed on the effect of the magnetic field on the microstructure and mechanical properties of welded joints under different TiO₂ active agent coating amounts. The results show that the grain size tends to decrease and then increase with the increase in the active agent coating under the magnetic field. This is mainly because the active agent changes the arc morphology, which in turn affects the melt pool motion. The Lorentz force generated by the longitudinal magnetic field acts on the molten pool and will have an agitating effect on the pool. Both the magnetic field and the active agent are convective to the melt pool, and when the magnetic field and the active agent act together will further enhance the convective effect. However, when the active agent is too thick, it will affect the fluidity of the molten pool during welding and reduce the quality of the welded joint. Under the action of magnetic field, when the active agent coating amount is 3 mg/cm², the grain size is the finest and the mechanical properties are the best. At this time, the tensile strength was 292 MPa, elongation was 11.2%, and hardness was 78.9 HV in the weld zone and 77.8 HV in the heat-affected zone. Further analysis of the melt pool change and grain refinement mechanism under the combined effect of the magnetic field and active agent revealed that the magnetic field promotes the solidification of the second phase in the weld tissue, but the effect on the heat-affected zone is not obvious. The addition of the magnetic field was found to refine the grains by EBSD testing, reducing the average grain size by 1.43 μm. This indicates that the introduction of the magnetic field in the A-TIG welding process improves the mechanical properties and microstructure of the welded joint, which is conducive to solving the problem of poor weldability in the welding process of magnesium alloys.

1. Introduction

Since the 21st century, China’s energy and environmental protection problems have been prominent. Magnesium alloys are the “21st century green engineering metal materials” and “the most important commercial lightweight materials” which gives them people’s extensive attention in the 3C industry, automotive manufacturing and the aerospace field [1]. However, because the difference between the melting point and boiling point of magnesium is too small, and the surface tension of magnesium in the molten state is small, the magnesium alloy in the welding process is prone to welding spatter, porosity cracking and a series of welding problems [2,3,4].

At present, there are mainly tungsten arc welding (TIG), melting electrode argon arc welding (MIG), resistance spot welding (RSW), electron beam welding (EBW), laser welding (LBW), friction welding (FSW), plasma arc welding and other welding methods [5,6,7,8,9] for magnesium alloy welding. Compared with other welding methods, tungsten arc welding (TIG) has a wide range of process conditions, simple equipment, the ability to be welded with or without metal filler conditions, and a series of advantages. In the 1960s, the Patton Welding Institute (PWI) in Ukraine proposed “flux-layer TIG welding”, which is the earliest sense of A-TIG welding [10]. A-TIG welding is actually the coating of an active agent on the area to be welded based on conventional TIG welding, and the quality of the welded joint is improved by selecting different types of coating agents and the amount of coating [11,12]. At present, the main types of active agents are oxide-type, metal monomer-type, halide-type and composite active agents formed by the combination of these types of active agents. The contraction force generated by oxidation-type active agents due to the ionization of the O-element during the welding process leads to the contraction of the arc, which in turn changes the movement of the molten pool and the temperature gradient of the welded joint, improving the welding efficiency [13,14]. Liu, L [15] analyzed the effect of the TiO₂ active agent on the A-TIG welding process and found that the TiO₂ active agent could increase the weld depth of fusion and affect the movement of the melt pool [16,17,18,19,20,21], but the effect on the mechanical properties was not obvious. The TiO₂ oxidation-type active agent is effective at affecting the formability of the welded joint and increasing the weld depth of fusion, but it has a limited effect on improving the performance of the welded joint. Magnetron welding technology is a new welding technology that has developed rapidly and matured in recent years [22,23]. The introduction of a magnetic field in the welding process will cause a change in the arc morphology, which will agitate the molten pool and refine the grains in the molten pool, improving the performance of the welded joint [24,25]. Therefore, the introduction of a magnetic field in the A-TIG welding process not only changes the molten pool metal mass and heat transfer process, but also refines the grain, improves the mechanical properties of the welded joint and is a simple, low-cost process with low energy consumption [26]. In this paper, the effect of different coating amounts with and without the magnetic field on the welded joints was explored, and mechanical property experiments, microstructure examination, physical phase analysis and EBSD analysis were performed on the welded joints under different parameters to study the laws and mechanisms, which are of some significance for improving the weldability of magnesium alloys and the strength of welded joints.

2. Materials and Methods

AZ91 magnesium alloy was selected as the test base material; test plate specifications were 100 mm × 100 mm × 5 mm. The main chemical composition of the base material is shown in

Table 1. Chemical composition of base material (wt%).

Al	Zn	Mn	Si	Cu	Fe	Mg
8.3–9.7	0.35–1	0.15–0.5	<0.01	<0.03	<0.005	Balance

.

The test uses the WSE-500 inverter-type welding machine to butt-weld the test plate; the entire welding process was performed without filling the wire. Welding parameters are shown in

Table 2. Welding process parameters.

Welding Current (A)	Welding Voltage (V)	Welding Speed (mm/min)	Arc Length (mm)	Argon Gas Flow (L/min)	Angle of Tungsten Tip (°)	Tungsten Pole Diameter (mm)
80	17	300	2	18	60	2.5

.

The magnetic field is generated by the self-developed coupling magnetic control equipment MCWE-10/100, the magnetic field coil is placed directly below the test plate, the center of the welding gun and the magnetic field coil are on the same axis to ensure the generation of longitudinal magnetic field, the test plate and the coil are separated by a copper plate with good thermal conductivity, the workpiece does not move during the welding process, the welding gun moves with the trolley and the test piece is welded in a single pass without wire filling. The welding device is shown in Figure 1.

According to the literature and referring to the previous experimental results of the group, it was found that the magnetic field frequency of 30 Hz and magnetic field current of 1.5 A had the most obvious effect on the performance improvement of magnesium alloy TIG welding, so the experimental process fixed the magnetic field frequency and magnetic field current of 30 Hz and 1.5 A, respectively; a magnetic field strength of about 15 mT was measured by Gauss meter. TiO₂ was used as the active agent for coating, and the coating amounts (cm²) were 1 mg, 2 mg, 3 mg, 4 mg and 5 mg. The weighed active agent was dissolved in an appropriate amount of alcohol by an electronic balance (0.001), and the active agent was evenly coated on the surface of the test plate with a brush and left for 24 h. The test plate was allowed to dry completely for welding. After welding, tensile specimens were prepared by wire cutting along the central area of the rolled product. The back of the test plate was milled to remove 2 mm and processed into 3 mm thick tensile specimens for stretching; for each group of the three tensile specimens, the measurement results took the average. The test was conducted on a universal testing machine with a loading rate of 3 mm/min and a test temperature of room temperature. For metallographic specimens, specimens were first cut in turn with 800, 1500 and 2000 water-abrasive sandpaper in accordance with conventional methods of preparation of metallographic samples for mechanical grinding, and then polished with soft tweed polishing cloth, polishing paste with a particle size of 0.25 diamond, and a polishing process of continuously pouring cool water, until they were polished into a mirror surface without scratches. After polishing, the polished surface was cleaned with water, wiped with a cotton ball dipped in alcohol and then blow-dried with a hair dryer to facilitate corrosion. The composition of the etching solution was ethylene glycol 20 mL, glacial acetic acid 60 mL, nitric acid 1 mL, distilled water 19 mL. After that, the structure and fracture morphology of the welded joints were characterized by scanning electron microscopy (SEM) using an S3400 equipped with an energy-dispersive spectrometer (EDS).

3. Experimental Results and Discussion

3.1. XRD Diffraction Analysis

The following figure shows the pictures of XRD diffraction inspection of welding under different processes, and the results are shown in Figure 2.

Figure 2a is the main spectrum. From top to bottom, respectively, are shown the heat-affected zone (HAZ) and weld zone (WZ) profiles under the action of the magnetic field, as well as the heat-affected zone (HAZ) and weld zone (WZ) profiles without the action of the magnetic field. Figure 2b shows the local enlarged view of the vertical line area. It can be seen from the figure that the physical phase composition of the welded joint is mainly composed of α-Mg, Al₂Mg, β-Al₁₂Mg₁₇, MgO and TiO, where α-Mg is the matrix, Al₂Mg and β-Al₁₂Mg₁₇ are the second phase and MgO and TiO are inclusions. Under normal circumstances, the welding process of magnesium alloy should not show MgO and TiO. The reason for the appearance of this two-phase region during this experiment is that TiO₂ reacts with Mg in the matrix to form MgO and TiO under the high temperature of the electric arc. After welding, during the solidification process, MgO and TiO did not rise to the surface in time and remained inside the weld. This eventually led to the appearance of these two-phase regions on XRD. However, the addition of the magnetic field leads to a significant enhancement of the diffraction peaks of TiO and Al₁₂Mg₁₇ as the second phase from the local magnification of Figure 2b, indicating that the introduction of the magnetic field promotes the solidification of the second phase, and this enhancement effect will be reflected in the mechanical properties. For the heat-affected zone, the diffraction peaks in the heat-affected zone do not change significantly when the magnetic field is introduced, as seen in Figure 2a, indicating that the introduction of the magnetic field has a limited effect on the heat-affected zone under active agent conditions.

3.2. Effect of Magnetic Field on Microstructure

Figure 3a–e shows the microstructure of the welded joint when the coating amount is 1 mg/cm², 2 mg/cm², 3 mg/cm², 4 mg/cm² and 5 mg/cm² under the action of magnetic field. The elemental content of points A, B and C in Figure 3c is shown in Figure 4; point A shows the primary crystal, point B is the white highlighted area in the microstructure and point C is the solidified phase at the grain boundary. From Figure 4, it can be seen that the three test points have obvious differences in composition; the main elements of point A and C are Mg and Al, and point B contains a small amount of Ti elements. Combined with the XRD analysis and the corresponding lattice constants in the database, it can be determined that point A is the magnesium alloy matrix α-Mg (lattice constants a = b = 3.209, c = 5.211; α = β = 90°,γ = 120°), in which a certain amount of Al is dissolved. Point B contains a small amount of Ti elements, so it was determined that point B is a mixture of α-Mg and TiO (lattice constant a = b = c = 4.117; α = β = γ = 90°). Point C is the precipitated phase on the grain boundary, mainly β-Al₁₂Mg₁₇ (lattice constant a = b = c = 10.560; α = β = γ = 90°). It can be seen from Figure 3 that the welded joint structure is composed of α-Mg and β-Al₁₂Mg₁₇ distributed at the grain boundaries, and the grain size tends to become smaller before increasing at the coating amount (a) to (e), and the grain size is the smallest at the coating amount of 3 mg/cm². As the secondary dendrite arm is subjected to convection from the magnetic field and the active agent, the shear stresses generated cause the secondary dendrite arm to be “broken” back into the melt pool. A portion of the unremelted grains act as a nucleation source for heterogeneous nucleation as second-phase particles, thus reducing the concentration fluctuation, reducing the nucleation work and increasing the nucleation rate, thus refining the grains. The TiO₂ active agent affects the motion of the molten pool during the welding process, promoting convective action in the molten pool. The Lorentz force generated by the applied magnetic field acts on the molten pool and the arc, tightening the arc force, and likewise changing the motion of the molten pool. Therefore, the introduction of magnetic fields under different active agent coating conditions has an effect on improving the welded joint quality of welded joints. However, the amount of active agent coating is not proportional to the quality of the welded joint. When the amount of coating is further increased, the grains do not become smaller. This is because when the active agent covering the surface of the area to be welded is too much, the welding process will affect the mobility of the liquid metal in the molten pool and affect the heat dissipation and conduction of the outer layer of the arc zone, resulting in overheating of the welded joint, making the grains coarse.

3.3. Effect of Magnetic Field on Mechanical Properties at Different Coating Amounts

In order to further verify the effect of grain refinement on the properties of welded joints under the action of a magnetic field, tensile experiments and microhardness experiments were conducted on welded joints. The tensile strength and elongation obtained from the tensile experiments with different coating amounts by the magnetic field are shown in Figure 5. It can be seen from the graph that the effect of the magnetic field on tensile strength and elongation is most obvious when the amount of active agent coating is 3 mg/cm² and 4 mg/cm². The addition of the magnetic field does not change the overall trend of tensile strength and elongation, both of which increase and then decrease, and both of which reach a maximum at an active agent coating of 3 mg/cm². The addition of the magnetic field increases the tensile strength and elongation by 10.2% and 11.4%, respectively. The analysis found that due to the joint action of the magnetic field and active agent, the melt pool is subjected to the Lorentz force generated by the magnetic field and the surface tension generated by the active agent welding process, both of which occur in convection to refine the grains. According to the Hall–Petch formula${ \mathsf{\sigma }}_{\mathrm{s}}={\mathsf{\sigma }}_0$ + K${\mathrm{d}}^{-\frac{1}{2}}$, it is known that there is a linear relationship between the yield strength${ \mathsf{\sigma }}_{\mathrm{s}}$ of the material and the grain size${ \mathrm{d}}^{-\frac{1}{2}}$. Therefore, refining the grains will improve the tensile strength of the material. When the active agent coating amount is small, this convection effect is not obvious. When too much coating is applied, it will result in a lower temperature in the outer layer of the arc area, becoming an electrical and thermal insulator. This will affect the conduction and radiation of the welding heat source. In addition, when the active agent is too thick, it will affect the fluidity of the melt pool during the welding process, which will have a negative impact on the welded joint.

Fracture morphology is an important means of analyzing the fracture performance of materials. Due to the different fracture modes of materials, different fracture morphologies are produced, which are mainly divided into toughness fracture, dissociative breaks, shear fracture, etc. From the microscopic fracture morphology, the fractures with different coating amounts under the action of the magnetic field mainly have tough nest-type and quasi-dissociative morphology, with very few holes at the fracture surface, which is one of the reasons affecting the tensile strength of the material. Figure 6 shows the macroscopic pictures of the tensile specimen after fracture as well as the fracture; from the macroscopic fracture pictures of the tensile parts, there is an obvious necking effect on the fracture of the magnesium alloy. Figure 7a shows the microscopic morphology of the specimen under the condition of no magnetic field and an active agent coating amount of 4 mg/cm². From the figure, it can be seen that the magnesium alloy welding process produces cracks belonging to the cracking along the crystal. When the temperature decreases, the eutectic makes the shrinkage and deformation at the welded joint produce downward tensile stress, which tends to fracture and form crystalline cracks. It can be seen that the generation of tensile stresses in the welded joint due to the low-melting-point eutectic β-Al₁₂Mg₁₇ is a necessary condition for the generation of crystalline cracks, which is also responsible for its poor mechanical properties. Figure 7b shows the fracture shape of the specimen coated with 3 mg/cm² under the magnetic field. The fracture is flush and bright; the fracture location occurs in the fusion zone of the welded joint, which is a brittle fracture; there are small deconstruction steps in the fracture; there are tearing ribs in the quasi-deconstruction fracture unit; and there are also a small number of round or oval craters of different sizes. Therefore, the fracture belongs to the mixed type of fracture: a tough nest-quasi-deconstruction fracture.

Microhardness is one of the important indicators reflecting the performance of welded joints. In order to study the variation of hardness in the welded joint and heat-affected zone, the hardness variation curves obtained by adding magnetic fields under different active agent coating conditions are shown in Figure 8. From the graph, we can see that the addition of the magnetic field does not change the variation pattern of hardness; it is the trend of increasing first and then decreasing, and the maximum value of hardness is obtained at 3 mg/cm². For the heat-affected zone hardness, the addition of the magnetic field results in a higher hardness in the heat-affected zone than in the absence of the magnetic field. Except for the singularity at 4 mg/cm², the addition of the magnetic field results in a welded joint with better hardness than the no-field condition. The addition of the magnetic field affects the state of motion of the liquid molten pool. The convective action makes the molten pool movement more intense, and for the weld zone, the convective action leads to the fragmentation of secondary dendrite arm within the molten pool, resulting in grain refinement and increased microhardness. For the heat-affected zone, the magnetic field has a stirring effect on the weld pool, which affects the heat transfer process, which leads to the heat-affected zone in the coarse grain area to receive less heat for grain growth, inhibiting grain coarsening, resulting in an increase in heat-affected zone hardness.

For different coating amount conditions, the introduction of the magnetic field was found to improve the joint properties, and the best properties were all obtained at 3 mg/cm², which is consistent with the experimental result of the finest grain size in the microstructure analysis at 3 mg/cm². The final determination of 3 mg/cm² under the action of the magnetic field is the best process parameter, which corresponds to the mechanical properties of 292 MPa tensile strength, 11.2% elongation, 78.9 HV hardness in the welded joint and 77.8 HV hardness in the heat-affected zone.

In order to further analyze the characteristics of the intracrystalline structure and crystal growth under the combined effect of the magnetic field and active agent, EBSD tests were performed on welded joints with a coating amount of 3 mg/cm², and the results are shown in Figure 9; and TEM analysis was used to observe the morphological distribution of twinning when the coating amount was 3 mg/cm², and the results are shown in Figure 10.

Figure 9 shows the crystal orientation and grain size as well as the twinning diagram for a titanium dioxide coating amount of 3 mg/cm². Figure 9a shows the results of EBSD detection in the welded joint under 3 mg/cm² coating without magnetic field, from which it can be seen that the composition of the material phase is the same compared to Figure 9b (EBSD results in the welded joint under 3 mg/cm² active agent when a magnetic field is introduced). Comparing Figure 9a,b, it can be found that the introduction of the magnetic field further refines the grains, and the average grain size is 16.98 μm without the magnetic field and 15.55 μm with the magnetic field, and the introduction of the magnetic field reduces the average grain size by 1.43 μm. In Figure 9b, the distribution of twin crystals under the magnetic field with a coating amount of 3 mg/cm² is shown in red. In order to better observe the morphology of the twinned crystals, TEM analysis was performed on the positions shown in blue in the figure, and the results are shown in Figure 10. From the figure, it can be found that there is a high density of dislocation accumulation near the twinning, and the number of twinnings plays a decisive role in the mechanical properties of the weld area. On the one hand, it can achieve the effect of stress relief, reduce crack nucleation, blunt the crack tip and hinder crack expansion, which has a positive effect on the prevention of thermal cracking of magnesium alloys. On the other hand, the increase in the number of twinned crystals inevitably leads to an increase in the number of twinning boundaries, which can make the original grains be divided to achieve the effect of refining the grains and enhancing the strengthening of fine crystals [27,28].

4. Conclusions

The effect of the mechanical properties and microstructure of an AZ91 magnesium alloy TIG welded joint by longitudinal magnetic field with different TiO₂ active agent coating amounts was studied. The conclusions are as follows:

(1)

The longitudinal magnetic field is introduced in the A-TIG welding process, and the grain size tends to decrease and then increase with the increase in the active agent coating under the effect of the magnetic field. When the active agent is too thick, it affects the fluidity of the molten pool during the welding process, hinders the heat dissipation in the weld zone and reduces the quality of the welded joint.

(2)

Under the conditions of magnetic field frequency 30 Hz and magnetic field current 1.5 A, the hardness, tensile strength and elongation of the welded joint reached their maximum values when the active agent coating was 3 mg/cm², which were 78.9 HV in the welded joint and 77.8 HV in the heat-affected zone, 292 MPa in the tensile strength and 11.2% in the elongation, respectively.

(3)

For the active agent coating amount of 3 mg/cm² the addition of the magnetic field is beneficial to the precipitation of the second phase in the welded joint and refines the grain, resulting in a reduction in the average grain size by 1.43 μm. TEM analysis found the magnetic field to be under the action of a 3 mg/cm² coated amount of dislocation near the twinning, which reduces welding defects and improves the quality of the joint.