1. Introduction
In the automotive industry, environmental regulations and fuel efficiency are becoming increasingly important, and in response to these changes, body parts made of non-ferrous metals such as aluminum are increasingly being used to lighten the weight of vehicles. In particular, aluminum alloys are increasingly being used in automotive bodies and components due to their high specific strength and excellent corrosion resistance1). However, the welding process for fabricating parts using aluminum alloys presents challenges due to the weaknesses of both arc welding and laser welding, which can lead to poor weld quality and productivity. Arc welding is characterized by high heat input and low energy density, which can lead to thermal distortion, degradation of mechanical properties, and multiple pores in the weld. Laser welding enables high- quality joints with precise welds and minimal thermal effects, but high costs and limitations for large-scale production hamper its widespread use. Laser welding can also produce internal porosity or defects, which can lead to poor quality2-5). Laser-arc hybrid welding is a prospective technology that combines the advantages of arc welding and laser welding to achieve high efficiency, high quality, and deep penetration at the same time6,7). The process combines the precision of the laser with the high heat input of the arc to achieve strong welded joints with minimal thermal deformation and pore formation. However, there are currently no clear criteria for the hybrid welding process for aluminum alloys, which has led to many studies on the optimal welding conditions8). In laser-arc hybrid welding, X. Zhan et al.9) analyzed the effect of pore formation on hardness as a function of welding speed, and L. Huang et al.10) studied the porosity, microstructure, and mechanical properties as a function of configuration of laser and arc. In addition, I. Bunaziv et al.11) analyzed the effects of porosity, microstructure, and tensile strength on the distance between the laser and the arc, configuration of laser and arc, and S. Yan et al.12) studied the effect of porosity formed by laser power and arc current on fatigue strength. J. Kim et al.13) analyzed the bead welding characteristics according to the shielding gas and the distance between the laser and the arc, and G. Xu et al.14) reported that the porosity decreased as the arc current increased. J. Kim et al.15) showed the differences in microstructure, porosity, and mechanical strength according to wire diameter in wire arc additive manufacturing (WAAM). In this study, aluminum alloys were welded by the laser-arc hybrid welding method, and the differences in mechanical properties and microstructure according to wire diameter (Φ 1.2, Φ 1.6) were analyzed to suggest wire selection criteria for laser-arc hybrid welding and to derive optimal conditions from hybrid welding of aluminum alloys. The results of this study are expected to open up application possibilities in the automotive industry using non-ferrous metals as well as in various fields utilizing laser-arc hybrid technology.
2. Materials and Methods
2.1 Experimental equipment
The Laser-Arc Hybrid Welding (LAHW) system is configured as shown in Fig. 1, and the experiments were conducted using this system. The laser-arc hybrid welding system consists of a welding head (HighLight, BIMO), a MIG welding system (FRONIUS TPS5000), and a 6-axis robot (KUKA KR 60 HA robot). A separate bracket was used to mount the MIG welding torch on the laser head.
The fiber laser used is the FL-ARM 6000 fiber laser from Coherent. The laser beam is divided into a center beam and a ring beam and is a fiber laser of up to 6 kW with a wavelength of 1064 nm. The fiber laser center beam has a diameter of 70 ㎛ and the ring beam has a diameter of 180 ㎛. The size of the diameters can be changed by the collimation system of the laser head (Precitec, YW50). The angle between the arc torch and the specimen is 60 degrees, and the angle between the laser head and the specimen is 83 degrees. The distance between the laser and the arc wire (DLA) is 3 mm, and the contact tip to work distance (CTWD) is set to 15 mm. In Fig. The experimental setup of the variable beam laser-arc hybrid welding is shown in Fig. 2, and the experimental parameters of the bead-on-plate (BOP) and butt welding experiments are shown in Table 1 and Table 2. Depending on the condition variables, the heat input (QH) of laser-arc hybrid welding was calculated as the sum of laser beam energy (QL) and arc energy (QA), which is shown in Eq. 116) .
PL is the laser power and PA is the arc power. ηL and ηA are the laser absorption coefficient and arc efficiency factor, respectively, which are 0.7 and 0.8. νt is the welding speed. The amount of heat input according to the experimental conditions is shown in Table 3.
Table 1
BOP test weld parameter
Table 2
Butt weld parameter
Table 3
Energy input of laser-arc hybrid welding
2.2 Experiment Materials
The materials used in this study were A356-T6 castings and AA6N01-T5 extrusions, which are increasingly used in automotive applications, with the casting measuring 200 × 100 × 12 mm3 and the extrusions measuring 200 × 100 × 5 mm3. The casting was machined so that it could be butt welded flush with the extrusion with a thickness of 5 mm. ER4043 1.2 mm and 1.6 mm wires were used. Although a variety of wires are used for conventional MIG welding, the purpose was to select a more suitable wire for the hybrid. The shielding gas was Ar 99.999% with a flow rate of 20 l/min. The chemical composition of each material is shown in Table 4.
Table 4
Chemical composition of materials
2.3 Analysis methods
After the welding experiment, an optical microscope (OM) was used to observe the cross-sectional geometry and microstructure according to the wire diameter (Φ 1.2, Φ 1.6), and polishing and etching were performed for observation. Keller’s solution (Modified keller reagent; 175 ml distilled water + 20 ml nitric acid + 3 ml hydrogen chloride + 2 ml hydrogen fluoride) was used as the etching solution. To analyze the mechanical properties according to the wire diameter (Φ 1.2, Φ 1.6), the hardness was tested by Vickers hardness tester at equal intervals with a test load of 500 gf and a pressurization time of 15 seconds. Tensile strength was measured using a universal mechanical INSTRON machine (Zwick Z100), and subsize tensile specimens were tested according to ASTM-E8 at a speed of 1 mm/min. Fig. 3 is a schematic of the butt weld experiment and tensile specimen fabrication. After the tensile test of the butt welded specimens, the fracture surfaces of each condition were observed and analyzed by Scanning Electron Microscope (SEM). Finally, the defects inside the laser-arc hybrid weld were analyzed by 3D-CT.
3. Results & Discussion
3.1 BOP weld geometry based on wire diameter
A basic study was conducted to compare the welding characteristics of laser-arc hybrid welding on cast and extruded materials. BOP pre-welds were performed using extruded material for the laser-arc hybrid welding parameters of welding speed and wire diameter. After welding, the bead and cross-section were checked to derive the BOP welding conditions that could achieve a weld depth up to the thickness of the extruded material. Fig. 4 shows a cross-sectional view of a BOP weld using a wire diameter of 1.2 mm, and Fig. 5 shows a cross-sectional view of a BOP weld using a wire diameter of 1.6 mm. In both conditions, the bead width and weld depth became wider and deeper as the heat input increased (welding speed decreased, arc current increased), and the keyhole shape became more pronounced at higher welding speeds. The weld with 1.2 mm wire had a wider bead width and the weld with 1.6 mm wire had a higher overall height. In Fig. 4 (a), (b) and Fig. 5 (a), (b) with a welding speed of 1.5 m/min, the cross-section of the 1.2 mm wire showed more micropores, while the cross-section of the 1.6 mm wire showed a shape with macropores along with micropores. In Fig. 4 (c), (d) and Fig. 5 (c), (d) with a welding speed of 2.0 m/min, the distribution of micropores did not show much difference, but macropores were more present in the cross-section of 1.2 mm wire. Although the internal pores showed differences in each condition, it was determined that it is necessary to analyze the pores distributed throughout the weld, rather than the results observed in the cross-section at a specific location.
3.2 Butt weld geometry based on wire diameter
In order to form a stable keyhole weld during butt welding, the butt weld was performed based on BOP welding conditions with full penetration. Fig. 6 shows the cross-sectional geometry of the laser-arc hybrid butt weld. It is a cross-sectional view of the weld using laser power (Center Beam: 2 kW, Ring Beam: 3 kW), arc current (150 A), and welding speed (2.0 m/min) conditions. As shown in Figs. 4 and 5, the weld was secured as thick as the extruded material (5 mm), but the laser power was increased to obtain a deeper weld in the butt weld, and the arc current of 150 A and welding speed of 2.0 m/min were selected to reduce the melt area and secure a clear keyhole shape. Fig. 6 shows that the keyhole shape was clearly observed in both conditions of butt welding. The bead width of the weld with the 1.6 mm wire was wider, and a deeper penetration depth was observed. This difference in geometry is believed to be due to the difference in feed rate and wire feed volume depending on the wire diameter. The presence of internal pores in the cross-section of the weld shows that the weld with 1.2 mm wire has fewer pores than the weld with 1.6 mm wire. The internal porosity of the entire weld was analyzed using 3D-CT to compare it.
3.3 Comparing microstructure by wire diameter
The dominant microstructure in the center of the specimens with a wire diameter of 1.2 mm and 1.6 mm was observed to be equiaxed grains, as shown in Fig. 7 (b), (c), (e), and (f). It can be seen that the laser-dominated region in Fig. 7(c) and (f) has a smaller grain size than the arc-dominated region in Fig. 7(b) and (e) in both conditions. It is judged that the laser dominant zone was cooled quickly by rapid heat dissipation to the base material as the welding progressed in front of the laser. It can also be seen that the microstructure of the arc-dominated region, Fig. 7(b), is denser than that of Fig. 7(e), and the laser-dominated region, Fig. 7(c), is denser than that of Fig. 7(f). Since the feed volume of the 1.6 mm wire is 724.18 mm3 and the feed volume of the 1.2 mm wire is larger than 690.12 mm3, it is believed that the wider bead width, deeper penetration, and larger heat input resulted in a denser microstructure than the 1.2 mm wire.
Fig. 7
Macro images of the butt-weld results using wire diameters of 1.2 and 1.6 mm are shown in (a) and (d). And micro images of the arc dominant central region are shown in (b) and (e), and of the laser dominant central region are in (c) and (f), respectively. The magnification of the micro images was set as × 700
Fig. 8 (a), (b) are pictures of the bead surface of a weld with 1.2 mm wire and 1.6 mm wire. As you can see, the bead surface is very sound. No cracks, deformations, defects, etc. were found. However, in Fig. 8 (b) and (e), internal pores were observed in both conditions. The total volume of pores in the weld using 1.2 mm wire is 118.37 mm3 and the volume fraction of pores to weld volume is 2.3%. The total volume of pores in the weld using 1.6 mm wire was 204.01 mm3 and the volume fraction of pores to weld volume was 4.19%. The volume difference between the two conditions was 85.64 mm3, and the volume fraction of pores was about 1.89 times different. The volume distribution of pores in Fig. 8 (c) and (f) shows that there are large volume pores in Fig. 8 (c), but the absolute number of pores in Fig. 8 (f) is highly distributed. The difference in porosity according to wire diameter is judged to be due to the difference in feeding speed and the difference in wire feeding volume according to wire diameter. The 1.2 mm wire has a feed rate of 6.8 m/min and the 1.6 mm wire has a feed rate of 4.0 m/min, making the 1.2 mm wire 1.7 times faster. The feed volume was larger for the 1.6 mm wire, as mentioned earlier, which likely had a greater impact on pore formation.
Fig. 8
The images of two-dimensional hybrid welding product is shown in wire diameter (a) 1.2 mm and (d) 1.6 mm. And the 3D-CT analysis results is shown in (b) and (e), respectively. The internal porosities were highlighted with color depending on their size. And the defect volume distribution is shown in (c) and (f)
3.4 Comparing mechanical properties based on wire diameter
Fig. 9 is a graph showing the tensile strength and elongation for the 1.2 mm and 1.6 mm wire diameter conditions. The average tensile strength values for the 1.2 mm and 1.6 mm wires are 190.84 ± 3.61 MPa and 168.99 ± 2.19 MPa, respectively. The 1.2 mm wire has a 21.85 MPa higher tensile strength. The elongation is 8.58 ± 1.4% and 4.18 ± 0.11% for the 1.2 mm and 1.6 mm wires, respectively. Similarly, we can see that the 1.2 mm wire forms a 4.4% higher value. This is because the 1.6 mm wire was more affected by internal defects (pores) than the 1.2 mm wire, and the 1.2 mm wire formed fewer internal defects.
Fig. 9
Tensile strength and elongation results are plotted. The graph of wire diameter 1.2 mm and 1.6 mm
Fig. 10 shows the results of SEM analysis of the fracture modes of the tensile test for the two wire types. Fig. 10 (a), (c) are fracture surfaces of welded specimens using 1.2 mm wire, and Fig. 10 (b), (d) are fracture surfaces of welded specimens using 1.6 mm wire. In both wires, the fracture occurred at the weld, suggesting that brittle fracture due to pores in the weld is the dominant fracture mechanism19). In particular, the specimen with 1.6 mm wire had significantly more pores than the specimen with 1.2 mm wire, and the fracture surface in Fig. 10 (d) was mainly dominated by cleavage fracture rather than regular dimple shape. On the other hand, in Fig. 10 (c), the specimen with 1.2 mm wire showed a relatively ductile fracture with mostly dimpled fracture and some cleavage fracture. These fracture surface analysis results were in good agreement with the experimental results, which showed that the welds with 1.2 mm wire had superior tensile strength and elongation compared to the welds with 1.6 mm wire. It is believed that the smaller diameter of the welding wire reduces the occurrence of pores in the weld, which improves the fracture toughness17,18) and consequently improves the mechanical performance.
Fig. 10
SEM fractographs of tensile testing specimens (a) 1.2 mm and (b) 1.6 mm diameter wire were shown. And the SEM images of fracture surface with higher magnification were shown in (c) and (d), respectively. The magnification of (a), (b) and (c), (d) was set as x 30 and x 250 in each
Fig. 11 is a graph showing the hardness values of the center line of the weld for wire diameters of 1.2 mm and 1.6 mm. The average hardness values for 1.2 mm wire and 1.6 mm wire are 76.9 ± 32.06 HV and 68.41 ± 0.6 HV in the laser-dominated region and 65.25 ± 0.66 HV and 60.57 ± 1.03 HV in the arc-dominated region, respectively. The 1.2 mm wire showed 8.52 HV and 4.68 HV higher values in the laser-dominated and arcdominated regions, respectively. This is due to the denser size of the equiaxed grains in the weld center line of the 1.2 mm wire than the 1.6 mm wire.
4. Conclusions
In this study, a comparative analysis of weld geometry and tensile strength, hardness, porosity, and grain size as a function of wire diameter in hybrid welding of aluminum alloys was conducted. The study of the microstructure and mechanical properties of welds made by wires of different diameters for the same input heat input led to the following conclusions.
1) Welding with a 1.2 mm wire diameter was found to be more favorable for laser-arc hybrid welding. The smaller diameter wire resulted in a denser microstructure, lower porosity, and better mechanical properties than the thicker wire.
2) The tensile strength and elongation values were higher for the 1.2 mm diameter wire than for the 1.6 mm diameter wire. This is due to the fact that the 1.2 mm diameter wire has a smaller distribution of pore volume than the 1.6 mm wire, as confirmed by the 3D-CT results.
3) Hardness values were measured to be higher for the 1.2 mm diameter wire than for the 1.6 mm wire in both the arc-dominated and laser-dominated regions. These results are attributed to the smaller grain size of the 1.2 mm diameter wire due to its finer equiaxed texture.
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