Effect of Welding Wires Containing ZrB 2 Particles on Microstructure and Mechanical Properties of Spray-Formed 7055 Aluminum Alloy TIG Welded Joints

In this paper, a series of welding wires with ZrB 2 particles were developed by the in situ reaction, and the ZrB 2 particles were successfully introduced into the spray-formed 7055 aluminum alloy welded joints by TIG welding. The microstructure, hardness, and tensile strength of the welded joints were tested by metallographic microscope, SEM, EDS, Vickers hardness tester, and tensile testing machine. The results show that the microstructure of ZrB 2 /7055 welded joints is ﬁne dendrites. The hardness of the weld zone of the ZrB 2 /7055 welded joint is higher than that of the 7055 welded joint. With the increase of ZrB 2 particles’ content, the tensile strength of welded joints increases ﬁrst and then decreases. When the content of ZrB 2 particles is 1.5wt.%, the tensile strength reaches the maximum value of 280MPa. The tensile strength of the welded joint containing 1.5 wt.% ZrB 2 particles is 14% higher than that of the pure 7055 aluminum alloy welded joint.


Introduction
Aluminum alloy is one of the most widely used lightweight materials in industry, and its usage is only inferior to that of steel. It has high strength, good plasticity, excellent conductivity, thermal conductivity, and corrosion resistance. 7XXX series aluminum alloy is called the high-strength aluminum alloy, while the 7055 aluminum alloy is called the super high-strength aluminum alloy, which is widely used in aerospace, rail transit, shipbuilding, and other fields [1][2][3][4].
e properties of the spray-formed 7055 high strength aluminum alloy are obviously better than those of the cast aluminum alloy because of its fine grain size, uniform structure, small segregation, and high density. Mg and Zn elements are easy to burn during the fusion welding process, which leads to uneven chemical composition of the weld metal of the spray-formed 7055 aluminum alloy, reduces precipitation of strengthening phases, and makes the strength of the welded joint low [5]. In addition, the welding zone will experience a higher thermal cycle during welding, which also leads to the softening of the welded joint [6,7].
Many studies show that friction stir welding has the advantages of small heat input, small weld grain, no need to add welding wire, and small residual stress and deformation after welding. erefore, friction stir welding is the main method used to weld the 7XXX series aluminum alloy [8][9][10][11][12]. However, the friction stir welding process is fully mechanized, and the workpiece needs to be constrained in well-designed fixtures; as a result, it is difficult to weld complex components. erefore, the special complex components still have to be connected by fusion welding. e aluminum alloy welding wire is the necessary filling material for fusion welding, which is one of the important factors to determine the welding quality [13]. e commonly used welding wire of the aluminum alloy is the Al-Mg welding wire [14]. Under the correct welding process parameters, although traditional aluminum alloy welding wires can overcome the defects of welding cracks and pores, it is difficult to solve the key welding problem of low strength of the welded joint [14]. Ceramic particle-reinforced aluminum matrix composites have excellent properties such as high specific strength, good wear resistance, high hardness, and good thermal stability, which integrate the advantages of the matrix and reinforcement phase. Many studies show that 7series aluminum matrix composites with hard ceramic particles have excellent mechanical properties [15][16][17]. Many studies have shown that the mechanical properties of welded joints can be improved by using the welding wire containing ceramic particles in the argon arc welding process [18][19][20][21][22][23][24]. At present, there are few reports about the effect of welding wires containing ZrB 2 particles on microstructure and mechanical properties of the spray-formed 7055 aluminum alloy TIG welded joint.
In this paper, ZrB 2 /Al-Mg and ZrB 2 /7055 welding wires were prepared by the in situ reaction method. e effects of ZrB 2 particles on microstructure, hardness, and mechanical properties of spray-formed 7055 aluminum alloy TIG welded joints were studied. e base metal was used as welding wire for the comparative test (corresponding to the pure 7055 welded joint).
When the pure aluminum was heated to 850°C in the graphite crucible of the resistance furnace and completely melted, the prepared salt was added into the melt and ZrB 2 particles were generated by the in situ reaction. During the process of reaction, the electromagnetic agitator was turned on, and the time for the reaction was 20 min. When the temperature dropped to 750°C, pure copper and pure zinc were added in proportion of the 7055 aluminum alloy, and the time for the reaction was 10 min. e pure magnesium was added when the temperature dropped to 720°C. After the magnesium was melted completely, the melting liquid was poured into the copper mold. Subsequently, cast was acquired. Wire cutting is then adopted for the aluminum ingot to obtain the welding wire. e specific processes are as follows: firstly, the parts with casting defects on four sides of the aluminum ingot are removed. Secondly, the remaining aluminum ingots were cut into aluminum wires with a cross section of 4 × 4 mm 2 . Finally, after cleaning the aluminum wires before welding, they are connected to the required length by spot welding. e aluminum ingots, welding wires before spot welding, and welding wires after spot welding are shown in Figure 1. As shown in Table 1, the main elements (tested by the direct reading spectrometer) of the welding wire are given. e MAGIC WAVE 2600 TIG welder was used in this experiment. e weld parameters are shown in Table 2. e extruded spray-formed 7055 aluminum alloy (length 300 mm, width 150 mm, and thickness 5 mm) was chosen to weld with a V-shaped groove (60°) in the center. e blunted edge is 2 mm, and the gap is 3-5 mm. e form of the weld joint is butt. Before welding, the oil and the oxide film on the surface of the welding wire and base metal (BM) should be treated, and the cleaned weld wire and BM should be welded within 8 hours. After the weld is completed, the weld roots located in the back sides need to be removed. Figure 2 shows the weld bead morphology after welding with different welding wires. e element content of the welding wire was detected by the direct reading spectrometer. Taking the center line of the weld as the axis of symmetry, the metallographic samples (length 40 mm, width 4 mm, and thickness 5 mm) were cut by wire cutting. Keller's reagent was chosen as the etchant, and the time for etching is 12 s. e metallography was analyzed by the Axioskop2-MAT optical microscope (OM), and the microstructure of the weld and the fracture morphology of the tensile sample were examined by the Merlin Compac field emission scanning electron microscope (FESEM). Hardness was measured by the MVS-1000QZD Vickers hardness tester at the loading force of 100 g, and the loading time is 10 s. Tensile strength was measured by the CMT5205 electron universal testing machine at the speed of 12 mm/min. e tensile sample was designed according to GB-T2651-2008. e performance of the weld joint for each condition was the average value of three samples. e SEM surface morphology and elemental surface scanning of the crack of 1.5 wt.% ZrB 2 /Al-Mg welded joint are shown in Figure 4. Figure 4(a) shows that 1.5 wt.% ZrB 2 / Al-Mg welded joint is the intergranular cracking, and there are some inclusions at the crack of the welded joint. Figure 4(b) shows that Cu elements are mainly distributed on the grain boundary. Al, Mg, and Zn elements are uniformly distributed on the matrix. e inclusions in the crack contain more oxygen; it can be inferred that the inclusions are oxides. It can be seen from Figures 4(a) and 4(b) that the precipitates on the grain boundary contain Cu, Zn, and Mg elements, but the precipitates are not compact and have poor bonding with the matrix. is is because the composition of the 1.5 wt.% ZrB 2 /Al-Mg welding wire is different from that of the base metal, and the expansion coefficient is also different.

Experimental Results and Discussion
erefore, under the action of solidification shrinkage force, the dense precipitates with good bonding with the matrix cannot be formed, and the fusion line cracks. In addition, the inclusion at the fusion line will produce stress concentration and become the crack source. erefore, the welding wires with the same or similar composition to the base metal can avoid cracking in the fusion zone of the welded joint and obtain a good welded joint. Figure 5 shows the metallographic structure of ZrB 2 / 7055 welded joints. Figure 5(a) shows that the microstructure of the pure 7055 welded joint is a coarse structure with grain size of 70-120 μm. Figure 5(b) shows that the 1.5 wt.% ZrB 2 /7055 welded joint is cellular dendrites with grain size of 20-70 μm. Figure 5(c) shows that 3.0 wt.% ZrB 2 / 7055 welded joint is a fine dendrite grain with the size of 20-50 μm. As the ZrB 2 particles' content increases from 0 wt.% to 3.0 wt.%, the size of the grain becomes more and more smaller. Compared with the pure 7055 welded joint, the grain size of ZrB 2 /7055 welded joints is obviously refined, which is due to the ZrB 2 particles increasing the nucleation point and hindering the grain growth during the solidification of the weld metal. Figure 6 shows the XRD patterns of ZrB 2 /7055 welded joints, SEM surface morphology, and elemental surface scanning of ZrB 2 /7055 welded joints. It can be seen from Figure 6(a) that the pure 7055 welded joint is composed of α-Al, MgZn 2 , and Al 2 Cu phases.
e ZrB 2 /7055 welded joints are composed of α-Al, ZrB 2 , MgZn 2 , and Al 2 Cu phases. With the increasing of ZrB 2 particles' content, the intensity of the ZrB 2 diffraction peak increases gradually. It can be seen from Figures 6(b) and 6(d) that there are Al, Mg, Zn, and Cu elements in the grain boundary and in the grain of pure 7055 weld. According to XRD, the precipitates in the welded joint are MgZn 2 phases, while Cu elements are in MgZn 2 phases in the form of the solid solution. e shape of precipitates is regular. It can be seen from Figures 6(c) and 6(e) that the irregular flocculents in the 1.5 wt.% ZrB 2 /7055 welded joint contain a large amount of Zr elements, so it can be inferred that the irregular flocculents are ZrB 2 particles. MgZn 2 phases with regular shape also precipitate in the 1.5 wt.% ZrB 2 /7055 welded joint. Figure 7 shows the SEM surface morphology of ZrB 2 / 7055 welded joints. Figures 7(a)-7(c) show that, with the increase of ZrB 2 particles' content, the flocculent ZrB 2 particles in ZrB 2 /7055 welded joints gradually increase. Figure 7(d) shows that the flocculent ZrB 2 particles in the        as an effective substrate for ɑ-Al phase nucleation during solidification to promote heterogeneous nucleation. Second, the existence of ZrB 2 particles reduces the dendrite growth rate at the liquid-solid interface, increases the undercooling degree near the liquid-solid interface, further promotes the heterogeneous nucleation, and refines the grains. Figure 8 shows the hardness of 7055 and 1.5 wt.% ZrB 2 /7055 welded joints. It can be seen from Figure 8 that the hardness of the weld zone of the 1.5 wt.% ZrB 2 /7055 welded joint is higher than that of the 7055 welded joint. e hardness of the heat affected zone of 7055 and 1.5 wt.% ZrB 2 /7055 welded joints are similar. e hardness of the base metal is about 95 HV because the base metal is extrusion 7055 aluminum alloy (without heat treatment). According to Figure 7, the grain size of the 1.5 wt.% ZrB 2 /7055 weld zone is smaller than that of the 7055 weld zone, so the enhancement of hardness of the 1.5 wt.% ZrB 2 /7055 weld zone is mainly due to grain refinement. Jiang et al. [25] and Patel et al. [26] analyzed the reasons for the increase of hardness of aluminum alloy welded joints, and the results showed that the grain refinement can improve the hardness of the welded joint. In addition, the ZrB 2 particles in the weld zone also contribute to the increase of hardness. Table 3 shows the values of tensile strength of ZrB 2 /7055 welded joints with different contents of ZrB 2 particles. Table 3 shows that, with the increase of ZrB 2 particles' content, the tensile strength of ZrB 2 -7055 welded joints first increases and then decreases. When the content of ZrB 2 particles is 1.5 wt.%, the tensile strength of the ZrB 2 /7055 welded joint reaches the maximum value of 280 MPa. When the content of ZrB 2 particles is 3.0 wt.%, the tensile strength of the ZrB 2 /7055 welded joint decreases to 260 MPa. It can be seen that the tensile strength of the spray-formed 7055 TIG welded joints can be improved by introducing ZrB 2 particles. Figure 9 shows the SEM morphology of the fracture surface of ZrB 2 /7055 welded joints with different contents of ZrB 2 particles. Figure 9(a) shows that the pure 7055 welded joint is a typical brittle fracture. Figures 9(b) and 9(c) show that the 1.5 wt.% ZrB 2 /7055 and 3.0 wt.% ZrB 2 /7055 welded joints are also brittle fracture.

Tensile Properties of Welded Joints.
Combined with the microstructure of the welded joints, MgZn 2 strengthening phases are found in all three kinds of welds, and MgZn 2 phases containing Cu elements also play a role in solution strengthening. Compared with the welded joints with ZrB 2 particles, the pure 7055 welded joint is coarse dendrite, so its tensile strength is the lowest, and it is brittle fracture. For the welded joints containing ZrB 2 particles, there are two main reasons for the improvement of the tensile strength. First, the ZrB 2 particles in the welded joints can act as the second phase pinning dislocation during tensile deformation and play the second phase strengthening role. Second, ZrB 2 particles can refine the grain size of welded joints and play the role of fine grain strengthening.
e results of the literature [27] also show that the grain refinement is the important reason for the tensile strength improvement of the Al-Zn-Mg-Cu alloy. Based on the above two points, it can be inferred that the tensile strength of the welded joint with 3.0 wt.% ZrB 2 particles should be the maximum. However, it can be seen from Table 3 that the tensile strength of the 1.5 wt.% ZrB 2 /7055 welded joint is higher than that of the 3.0 wt.% ZrB 2 /7055 welded joint. is is because there are more flocculent ZrB 2 particles in the 3.0 wt.% ZrB 2 /7055 welded joint than those in the 1.5 wt.% ZrB 2 /7055 welded joint. A large number of flocculent particles will decrease the toughness of the aluminum matrix. e combination of flocculent particles and matrix will be gradually destroyed during the tensile process, resulting in cracks and the reduction of tensile strength.
From the studies of particle-reinforced metal matrix composites, we found that the distribution of particles has a great influence on the microstructure and properties of the materials. Ceramic particles with good wettability can be evenly distributed on the matrix, and the matrix structure changes from dendrite to fine equiaxed crystal. Fine equiaxed crystal can improve the strength and plasticity of the matrix at the same time. But, in this experiment, the distribution of ZrB 2 particles in the welded joints is nonuniform, which is due to the bad wettability between ZrB 2 and liquid Al. erefore, in order to prepare the welded joint with uniform dispersion of ceramic particles, it is necessary to select the particles with good wettability with the base metal of the molten pool. We think that the spray-formed 7055 aluminum alloy welded joint with high strength and

Conclusion
In this paper, a series of welding wires containing ZrB 2 particles were developed and used for TIG welding of the spray-formed 7055 aluminum alloy. is study provides a theoretical basis for further expanding the application of the spray-formed 7055 aluminum alloy. e results are as follows.
(1) With the increase of ZrB 2 particles' content, the grain size of ZrB 2 /7055 welded joints decreases gradually. ZrB 2 particles are mainly flocculent in the weld and distributed in the grain boundary and in the grain. (2) With the increase of ZrB 2 particles' content, the tensile strength of ZrB 2 /7055 welded joints first increases and then decreases. e tensile strength of 1.5 wt.% ZrB 2 / 7055 welded joint is the highest, which is 280 MPa. e tensile strength of the pure 7055 welded joint is only 245 MPa. Compared with the pure 7055 welded joint, the tensile strength of the 1.5 wt.% ZrB 2 /7055 welded joint is increased by about 14%.

Data Availability
No data were used to support the findings of this study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.