Introduction

The 2319-Al alloy is a high-strength aluminum alloy. It has a high specific strength, excellent fatigue property, and good weldability, which is widely used in aerospace and defense (Ref 1, 2). Conventional manufacturing of 2319-Al alloy structural parts is smelting-casting or plastic deformation methods, which are not suitable for producing complicated structures (Ref 3). Additive manufacturing (AM) technology is considered to be a promising intelligent manufacturing technology because of its high efficiency and low cost (Ref 4, 5). Compared with traditional processing technology, it can reduce the production process, shorten the processing cycle, which is especially suitable for low cost and small batch of complicated structures (Ref 6). The traditional AM heat source for metal structural parts can be divided into three categories: laser, electron beam, and arc (Ref 7). The laser is widely used in AM as a heat source, which conveniently manufactures components with complex shapes (Ref 8,9,10). The electron beam as a heat source of AM also can fabricate high precision parts with complex shapes (Ref 11, 12). The aluminum alloy deposited process is very challenging because it is sensitive to pores and cracks (Ref 13). Karg et al. (Ref 14) reported that the crack-free Al-Cu specimens were printed by SLM. However, the ultimate tensile strength was only 235 MPa.

The wire and arc additive manufacturing (WAAM) technology uses metal wire as the filling material and arc as the heat source to fabricate metal structures by layer-by-layer welding (Ref 15, 16). Compared with laser and electron beam additive manufacturing, WAAM has the advantages of high production efficiency and low equipment cost. It is very suitable for fabricating large structures with low precision requirements (Ref 17). Some problems in WAAM also need to be solved, such as heat accumulation leading to excessive grain size. Bai et al. found that the grain size of the 2219-Al sample deposited by the TIG method was approximately 50 μm (Ref 18). Cong et al. deposited samples by CMT process and eliminated pores effectively, and so the tensile strength of Al-Cu alloy reached 248 MPa (Ref 19). Lei et al. reported that the GMAW-based AM technology has a high deposition rate and is more suitable for manufacturing large metal structural parts than GTAW and PAW (Ref 20).

Laser-MIG hybrid additive manufacturing method combines the advantages of laser and arc, it can realize high-precision and high-efficiency additive manufacturing. Because of the laser-arc synergy effects, the stability of arc improves, which has a great prospect in the manufacture of high-precision and crack-free parts (Ref 21). Zhang et al. (Ref 22) optimized process parameters to fabricate 5356-Al samples by laser-MIG and improved the forming quality of structures. However, researchers mainly focused on the macrostructure of the deposited samples, and there are few studies on the microstructure evolution or the relationship between microstructure and mechanical properties.

Laser plays a very important role in laser-MIG additive manufacturing. However, few studies are focusing on how laser power affects the microstructure and properties of structures. To prepare specimens with excellent mechanical properties, this paper focused on 2319-Al alloy samples fabricated by different laser power with laser-MIG hybrid additive manufacturing. In this study, the forming quality of the deposited sample is studied. The grain size and precipitates are systematically characterized. The microhardness and mechanical properties are analyzed in correlation with the changes in microstructure. And the most suitable laser power is found.

Experimental Details

Experimental System

This experiment uses a self-built low-power pulsed laser-induced MIG arc additive manufacturing system. This system consists of a welding platform, a pulsed laser generator with a maximum power of 800 W(type Riton LWS-800FK), and a MIG welder (type OTC WB-P500L). The schematic diagram of the system is shown in Fig. 1. The laser generator and the MIG welder are combined by a welding robot (type OTC FD-V20), and the movement of the heat source is driven by the movement of the welding robot to complete the deposition of each layer.

Fig. 1.
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Schematic diagram of the additive manufacturing system

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The angle α of the welding torch is fixed at 70°, and the angle β between the laser beam and the welding torch is maintained at 45°. This experiment adopts the method of back-and-forth depositing, and the depositing direction is changed whenever a stack of walls is completed to avoid inconsistency between the height of the arcing position and the arc extinction position. The AM process of this experiment is shown in Fig. 2.

Fig. 2.
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Schematic diagram of the process of additive manufacturing

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Arrangement of the Experiments

In this experiment, ER2319 is deposited on a 2219-Al substrate. The wire has a diameter of 1.2 mm. Table 1 shows the chemical composition of the wire and substrate. Ar with a purity of 99.99% is used as shielding gas, and its flow rate is 20 L/min. The substrate and the wire with sandpaper are polished before welding. Then, it’s washed with acetone to remove the oil on the surface. The welding parameters of each layer remain the same during welding. To study the effects of different laser powers on deposited samples, laser powers of 0, 100, 200, 300, and 400 W are used. The scan speeds are 400 and 450 mm/min. The welding current is 120 A, and the wire feeding speed is 720mm/min. And the residence time between adjacent layers is 60 s.

Table 1 Chemical composition of materials in weight percentage
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The electronic universal testing machine CSS-44100 is used for the tensile test, the test pieces of the tensile test are processed in accordance with the position shown in Fig. 3, and deposited specimens are processed into metallographic samples.

Fig. 3.
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Sampling position and dimensions of tensile sample

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The microstructure is observed using an optical microscope (type GDM-82980) and a scanning electron microscope (type ZEISS-SUPRA 55). The hardness at different locations of the wall is tested using a model hardness tester with a load and time of 0.2 kg and 10 s, respectively.

Results and Discussions

Macro Morphology

Figure 4 shows the morphology of single-layer deposited samples without laser; Fig. 4(a) shows the sample morphology with the scanning speed of 400 mm/min; and Fig. 4(b) shows the sample morphology with the scanning speed of 450mm/min. It can be found that the deposited layer isn’t uniform in Fig. 4(b). When the scanning speed reduces to 400mm/min, the deposited layer becomes more uniform, and the forming quality improves. When the scanning speed is high, thermal input is less and the liquid metal solidifies without enough time to spread around (Ref 23), so it’s difficult to deposit more layers. When the scanning speed reduces, the heat input of the molten pool increases, the molten pool spreads around, and the deposited layer becomes uniform. Therefore, the scanning speed is 400 mm/min without the addition of a laser.

Fig. 4.
figure 4

Morphology of single layer without the addition of laser: (a) 400mm/min;(b) 450 mm/min

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Figure 5 shows the morphology of deposited samples with different laser power. Each sample is 30 layers, and the dimensions are shown in Table 2, LE is the effective length of samples, WE is the effective width of samples, and HE is the effective height of samples. Figure 6 is side views of the marked position in Fig. 5, and we observe the sidewall from the same position in Fig. 5. Figure 6(a) shows a strong molten pool overflow. There exists a distinct stair step in Fig. 6(b)-(e), which means lower surface roughness due to the stair-stepping effect (Ref 24).

Fig. 5.
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The deposited sample morphology under different laser power: (a) 0 W; (b) 100 W; (c) 200 W; (d) 300W; (e) 400 W

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Table 2. Effective size corresponding to different parameters
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Fig. 6.
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Side walls quality under different laser powers: (a) 0 W; (b) 100 W; (c) 200 W; (d) 300 W; (e) 400 W

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The utilization of materials is also named the deposition efficiency in the AM (Ref 25). It has been an evaluation index of AM technology. The effective area of the sample is obtained by cutting off the excess portion. As shown in Fig. 7, this is a schematic representation of the effective area of a sample. The ratio of the effective volume(EV) to the original volume of the sample(OV) is the material utilization of the sample (Ref 25). The expression of material utilization is shown in Eqs 1-5:

$$ MU = EV/OV \times 100\% $$
(1)
$$ EV = L_{E} \times W_{E} \times H_{E} $$
(2)
$$ OV = S_{W} \times V_{W} \times L/V $$
(3)
$$ S_{W} = \pi d^{2} /4 $$
(4)
$$ L = 180n $$
(5)
Fig. 7.
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Schematic diagram of the effective area of a deposited sample

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where MU is the material utilization rate, SW is the cross-sectional area of the wire, VW is the wire feed speed, L is the total length of the wire used in the deposition process, V is the deposition speed, d is the diameter of the wire, and n is the number of layers of the deposited sample.

After calculation, the original volume of the sample is 157,782.09 mm3. The effective size measured by the sample of different parameters is shown in Table 2.

After calculation, the material utilization rates under different parameters are: 74.52, 80.24, 82.51, 87.81, 79.59%. When the laser power is 300 W, the value is 17.83% higher than that without laser.

After adding the laser, it can attract and compress the arc to reduce the overflow of the molten pool, and the laser can provide a stable spot for the arc (Ref 26, 27), which makes the deposited process more stable, and the surface forming quality is better than the single arc, so the deposited sample has a larger effective volume after adding the laser. However, when the laser power is too large, the heat input of the deposition process becomes large, which causes the molten pool to overflow, the surface forming quality deteriorates, and the effective volume of the sample becomes small.

Microstructure

The microstructure of deposited samples perpendicular to the scanning direction is shown in Fig. 8. It can be observed that the microstructure morphology in this zone is a typical equiaxed shape. Grain size can be recognized. Figure 8(a) is the microstructure without adding laser, the average grain size is 48.5 μm. Figure 8(b) shows the microstructure when the laser power is 100 W. The average grain size is 40.4 μm. Figure 8(c) shows the microstructure when the laser power is 200 W. The average grain size is 35.0μm. Figure 8(d) shows the microstructure when the laser power is 300 W. The average grain size is 30.8 μm, which is approximately 36.5 % smaller than that without laser. Figure 8(e) shows the microstructure when the laser power is 400 W. The average grain size is 37.5 μm.

Fig. 8.
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Microstructure in different laser power under the OM: (a) 0 W; (b) 100 W; (c) 200 W; (d) 300W; (e) 400 W

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The laser acted on the deposited metal to evaporate the metal particles, and the formed metal vapor produced an oscillating and stirring effect on the molten pool (Ref 28). And the peak laser power is larger when the laser just enters the molten pool, and there is an impact effect on the molten pool. This effect changes the surface tension of the molten pool, and the molten metal flows backwards in the molten pool and then back again (Ref 29), as Fig. 9 shows, which contributes to the fracture of dendrites and increase the nucleation points. The nucleation rate increments make the grain refined (Ref 30]. When the laser power is around 300 W, there is a balance between the stirring effect of the laser and the heat input. At this time, the grain refinement effect is better. When the laser power is increased to 400 W, the thermal input to the molten pool increases. As a result, the grain size is larger than that of the laser power of 300 W.

Fig. 9.
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The illustration of grain refinement: (a) Molten pool without laser. (b) Molten pool with laser-MIG

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Tensile Tests

Figure 10 shows the tensile strength of the deposited samples under different laser power. With the increase of laser power, the tensile strength of samples increases first and then decreases in the vertical and horizontal directions. When the laser power is 300 W, the maximum ultimate strength is 268 ± 5 and 279 ± 4 MPa, which are 19 and 20.1% higher than that without laser. When the laser power is 400W, grain size becomes larger, and the tensile strength decreases in both horizontal and vertical directions. The elongation in vertical and horizontal directions is 8.89 and 9.98%, respectively, without laser. When the laser power is 300 W, the maximum elongation in vertical and horizontal directions is 10.17 and 11.71%, respectively.

Fig. 10
figure 10

Tensile properties under different laser power: (a) tensile strength; (b) elongation

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The fracture surface investigated by SEM is exhibited in Fig. 11. Figure 11(a) shows the fracture surface in the horizontal direction without laser. Figure 11(b) shows the fracture surface of the tensile specimens in the horizontal direction when the laser power is 300 W. There are a large number of dimples in Fig. 11(a) and (b), and small dimples distributed around large dimples. Therefore, the fracture mode is microvoid coalescence fracture, which is a typical ductile fracture. There are pores in Fig. 11(a) and (b). These regular spherical pores’ inner walls are smooth which can be judged to be hydrogen pores (Ref 28). The pores in Fig. 11(a) are approximately 20-50 μm in size, and the pores in Fig. 11(b) are approximately 10-30 μm in size. It is consistent with the increase in elongation after adding laser. The reason is the pulse laser can stir the molten pool, which increases the flow of the molten pool and promotes the escape of gas. It effectively reduces the internal porosity of samples (Ref 31, 32).

Fig. 11.
figure 11

Fracture morphology of the deposited samples: (a) sample without laser; (b) sample with laser power of 300 W

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Micro-Hardness

Figure 12 shows the micro-hardness distribution of the samples with different laser power along the deposition direction. The red dotted lines in Fig. 13 represent the measuring position (A test point is every 0.1 mm). The hardness values of the sample change periodically along the deposition direction, which is related to the periodic distribution of the microstructure along the deposition direction (Ref 33, 34). Table 3 is the average hardness of different laser power. And micro-hardness of the sample with 300 W laser is 88.8 HV0.2, with an increase of 11.2% compared to that without laser. It is concluded that the refined grains is the main reason for the increase in microhardness. According to Hall–Petch theory, the decrease of grain size leads to the increase of grain boundary. This phenomenon is beneficial to hinder the dislocation movement, which improves the microhardness and strength (Ref 35). When the laser power is 300 W, the grain size is smaller than other samples, and the hardness of the sample is the maximum. On the other hand, some eutectic particles are dispersed inside the grain and play a role of dispersing strengthening.

Fig. 12.
figure 12

Hardness distribution curve under different laser powers

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Fig. 13.
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Schematic diagram of the cross section of a deposited sample

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Table 3 Average hardness of samples under different laser power
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The Precipitated Phase of the Additive Wall

Figure 14 shows the distribution of eutectics under SEM. It shows that a large amount of eutectics distributes on the black matrix. When there is no laser, the distribution of eutectics shows a network structure, and few eutectic particles are inside the grains. Compared with the grain morphology in Fig. 8(a), it can be concluded that these reticulated eutectics distribute along grain boundaries. The eutectic structures are refined after adding the laser (300W), and the network structures are replaced by the long striped eutectic structures.

Fig. 14
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The distribution of eutectic: (a) without laser; (b) with laser at 300 W power

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In the interior of the deposited samples, the Cu mainly solubilizes in Al matrix (Ref 28). Figure 15 shows the results of component analysis of the sample with a laser power of 300 W. As shown in Fig. 15(a) and (b), the result shows that the composition of eutectics is Al69.88Cu30.12, which is relatively close to the eutectic composition ratio of 67/33. It can be concluded that these phases are eutectic Al2Cu, and the black matrix is Al (Ref 28).

Fig. 15.
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Results of component analysis. (a) The test position; (b) Test results of point A

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Conclusions

In this study, deposited samples of different laser power are fabricated by laser-MIG hybrid additive manufacturing. The microstructure and mechanical properties of samples are investigated. The following conclusions can be obtained:

  1. 1.

    The utilization of materials reaches 87.81%, and the value is 17.83% higher than that without laser when the laser power is 300 W.

  2. 2.

    The addition of pulsed laser can refine grains and reduce porosity defects, thus improving the mechanical properties of deposited samples.

  3. 3.

    As the laser power increases, the hardness and tensile strength of deposited samples increase continuously, reaching the maximum values at the laser power of 300 W.