4.1 Preliminary experiments for deposition conditions
The current and travel speeds were determined as the deposition parameters. The currents were 100 A, 180 A, and 260 A, and the travel speeds were 5 mm/s and 6 mm/s, respectively. Figure 6 shows the results from the preliminary experiments for determining the parameters. At a current of 100 A , the deposition parameters are unsuitable regardless of the travel speed, as shown in Fig. 6a and b. In this case, the amount of wire supplied is insufficient. In addition, the wire feed speed is too low, so the deposition beads do not overlap in the zigzag method. This means that the relationship between the metal wire used in the deposition and parameters is not appropriate. The metal wire is completely molten owing to a suitable current at 180 A regardless of the travel speed, as shown in Fig. 6c and d. When the current is 180 A , the spatter depends on the travel speed, as shown in Fig. 6c. The spatter occurs only at 5 mm/s , i.e., a slow deposition speed, indicating the formation of excessive melting pools and increased heat inputs, thereby resulting in spatters. When the travel speed is 6 mm/s , the deposition speed is adequate. Therefore, in this case, spatters are not caused by this parameter. When the current is 260 A , the metal wire is completely molten, as shown in Fig. 6e and f. However, spatter occurs. This is because the wire feed speed is too high, and the current is high. Through experiments to determine the parameters, the best deposition parameters are as follows: the current (I), voltage (V), wire feed speed (Fwire), and travel speed (S) are 180 A , 14.6 V, 5.2 m/min, and 6 mm/s , respectively. Table 3 presents the results of the preliminary experiments based on the deposition parameters. The deposition beads do not overlap in the zig-zag method at 100 A . Therefore, the case when the current is 100 A is excluded from the experiments with test specimens. Ultimately, the conditions used in the repair experiments were 180 A and 260 A , respectively.
Thus, experiments with the test specimens were performed under the determined conditions. Figure 7 shows the repaired test specimens. Figure 7a and b show the results of experiments at 180 A and 260 A , respectively, at the same travel speed of 6 mm/s . When the current is 260 A, many spatters occur. When the current is 180A , it is possible to verify that the wires are completely molten without spatter formation. Therefore, this is considered the most suitable parameter value for the current.
Table 3
Results of preliminary experiments
No
|
I
(A)
|
V
(V)
|
Fwire (m/min)
|
S
(mm/s)
|
Spatter
|
Overlap
|
Class
|
a
|
100
|
12.2
|
2.6
|
5
|
×
|
△
|
Normal
|
b
|
100
|
12.2
|
2.6
|
6
|
×
|
×
|
Bad
|
c
|
180
|
14.6
|
5.2
|
5
|
△
|
○
|
Normal
|
d
|
180
|
14.6
|
5.2
|
6
|
×
|
○
|
Good
|
e
|
260
|
16.5
|
8.2
|
5
|
○
|
○
|
Bad
|
f
|
260
|
16.5
|
8.2
|
6
|
△
|
○
|
Normal
|
4.2 Mechanical property and microstructure
After performing preliminary experiments, the deposition cross-sections were confirmed for the cases with 180 A and 260 A, where the wire was completely molten. Figure 8 shows the digital microscopy results from the preliminary experiments for parameter determination. The pores occur under both 180 A and 260 A, but the pores occur less often at 180A than at 260 A. Repairs were performed on the test specimens with the two determined parameters.
The mechanical property, microstructure, and chemical composition of the repaired specimens were analyzed. Figure 9 shows the results of a digital microscope analysis of the cross-sections of the repaired specimen. Porosity occurs more often when there is a large amount of heat input. The sizes of the pores are also large under conditions with a high heat input (i.e., of 260 A).
Figure 10 shows the results regarding the microstructure and chemical composition. Figure 10a and b show the results regarding the microstructure and chemical composition of the test specimen repaired using WAAM when the currents are 180 A and 260 A, respectively. A comparison of Fig. 10a and b shows that micro-pores occur in the substrate when the current is 260 A. The Ni value change from 96.09–70.24% among the chemical components of the wire when the current is 180 A. Fe, one of the chemical components of the substrate, changes from 94.32–91.59% when the current is 180 A. In addition, Ni changes from 96.09–70.78% among the chemical components of the wire when the current is 260 A. Fe changes from 94.32–91.3% when the current is 260 A.
Figure 11 shows the results from the Vickers hardness measurements. Generally, the hardness is higher in the deposition zone than in the substrate. In addition, the hardness increases as the nickel content increases. The hardness gradually increases as the substrate and nickel are combined at the interface, and the hardness is the highest in the deposition zone. However, there is little difference in the hardness between the deposition zone and substrate. The hardness shows a 17% increase in the repaired specimen at 180 A relative to the gray cast iron. Moreover, the hardness shows a 15% increase in the repaired specimen at 260 A relative to the gray cast iron. Thus, the hardness values of test specimens repaired at 180 A and 260 A are not significantly different.
4.3 Verification experiment
A verification experiment was performed based on experimental results. Figure 12 shows a schematic of the specimen for verification. The specimens for the verification experiment were produced in parts of the cross slide where breakage occurred frequently. Figure 13 shows the workflow of the repair of the damaged part with surface defects using machining and WAAM. The surface was flattened through slot machining to ensure that the deposition material was well-bonded to the surface. In addition, slot machining was applied to make the deposition height approximately constant. The surface flattened through slot processing was filled with the deposition material using WAAM. Post-processing was then performed to obtain the same specifications as the existing parts. The verification experiment was performed at 180 A, owing to the sizes of the pores and mechanical property. Figure 14 shows the results of the verification experiment.