1. Introduction
Nickel – based superalloys are widely used in the aerospace, power generation, and other industries due to their excellent high – temperature performance. K4002 nickel – based superalloy, in particular, is commonly employed in the casting of hot – end components for aerospace engines. However, during the precision casting process, it is prone to various defects such as cracks and porosity. These defects can seriously affect the performance and service life of components, and thus need to be repaired.
Tungsten Inert Gas (TIG) welding is a popular welding repair technology due to its low cost and strong adaptability to welding materials. However, the high content of aluminum (Al) and titanium (Ti) in K4002 nickel – based superalloy increases its sensitivity to liquation cracks and may lead to strain – age cracks during the post – welding aging process, resulting in a low repair success rate. Therefore, it is of great significance to develop a suitable TIG repair process for K4002 nickel – based superalloy casting defects to improve the repair success rate and reduce production costs.
2. Experimental Materials and Methods
2.1 Experimental Materials
The experimental material used in this study is the K4002 precipitation – hardening nickel – based superalloy provided by a certain alloy company. The plate size is 75mm×57.5mm×4mm. The HGH3113 nickel – based alloy wire without Al and Ti is selected as the welding wire, with a diameter of approximately 1.2mm. The chemical compositions of the base material and the welding wire are shown in Table 1.
| Material | C | Cr | Co | W | Al | Ti | Ta | B | Zr | Hf | Fe | Ni |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| K4002 | 0.13 | 8.72 | 9.99 | 9.54 | 5.83 | 1.56 | 2.45 | 0.02 | 0.06 | 1.28 | – | Remainder |
| HGH3113 | 0.08 | 15.00 | 10.00 | 3.75 | – | – | – | – | – | – | 5.50 | Remainder |
| Table 1 Chemical Compositions of Base Material and Welding Wire (Mass Fraction, %) |
2.2 Experimental Methods
2.2.1 Experimental Equipment and Process
An automatic tungsten inert gas arc welding method was used to conduct surface cladding repair tests on as – cast K4002 superalloy plates. The welding machine used in the experiment is a WSME – 315R type TIG welding machine. In the DC welding mode, its power output range is from 5A/10.2V to 315A/22.6V. A CH1600 – 6 welding robot was used to control the movement of the welding torch.
Before repair, the test plate was fixed with a fixture. The tungsten electrode was perpendicular to the test plate to be welded, and the tip of the tungsten electrode was approximately 2mm away from the test plate. The end of the welding wire was kept on the same horizontal line as the tip of the tungsten electrode, about 5mm apart. Argon gas with a purity of 99.9% was selected as the shielding gas, and the gas flow rate was fixed at 12.5L/min.
The heat treatment equipment is a KRX – 14C muffle furnace, with a rated power of 12kW, a maximum heating rate of 30℃/min, and a maximum rated working temperature of 1400℃.
2.2.2 Microstructure Analysis and Mechanical Property Testing
The metallographic specimens of the weld (15mm×3mm×4mm) were cut by CNC wire – cutting electrical discharge machining. After being embedded with epoxy resin, they were ground and polished step by step, and then the observation surface was treated by electrochemical corrosion method (dilute phosphoric acid solution, positive electrode, 5.5V/10A, 5 – 10s).
An HXS – 1000AC micro – hardness tester was used to measure the Vickers hardness of the weld, heat – affected zone, and base material of the specimen respectively. The test parameters were set as a load of 500gf, a holding time of 10s, and the distance between adjacent hardness test points was set to 0.25mm.
The tensile property test was carried out in accordance with GB/T2651 – 2023 “Metallic Materials – Transverse Tensile Test of Welds – Destructive Tests”. A KY100KNW electronic universal testing machine was used to test the tensile properties of the repaired specimens. Before the test, the weld bead reinforcement and notches were ground off, and the tensile rate was 2mm/min. The tensile specimen is shown in Figure 1.
2.3 Repair Process
2.3.1 Pre – cladding Heat Treatment
In view of the problem that the precipitated phases in the as – cast structure of K4002 nickel – based superalloy are prone to liquefaction at high temperatures, a pre – cladding solution treatment process was designed in the experiment to optimize the alloy performance. Referring to the solution process parameters of the MARM002 superalloy, which is similar to the K4002 alloy, the pre – cladding solution treatment process parameters are set as shown in Table 2.
| Test Number | Solution Temperature \(T/^{\circ}C\) | Holding Time \(t/h\) | Cooling Method |
|---|---|---|---|
| 1 | 1100 | 2 | AC |
| 2 | 1125 | 2 | AC |
| 3 | 1150 | 2 | AC |
| 5 | 1175 | 2 | AC |
| 6 | 1200 | 2 | AC |
| 7 | 1225 | 2 | AC |
| 8 | 1250 | 2 | AC |
| Table 2 Pre – cladding Solution Treatment Process Parameters |
2.3.2 Post – cladding Heat Treatment
In order to obtain a stable joint structure and avoid the generation of strain – age cracks, the repaired specimens of K4002 superalloy without liquation cracks were subjected to a post – welding heat treatment of solution + aging. Specifically, solution treatment was carried out at 1150℃ for 2 hours, followed by furnace cooling to 870℃ for aging for 16 hours, and finally air – cooling.
3. Results Analysis and Discussion
3.1 Mechanical Property Analysis of Weld – repaired Parts
Tungsten inert gas arc welding repair tests were carried out on as – cast K4002 base plates. In order to obtain the best process parameters, 6 groups of tests were set as shown in Table 3.
| Test Number | Welding Current \(I/A\) | Wire Feeding Speed \(v_f/(mm·s^{-1})\) | Welding Speed \(v_w/(mm·s^{-1})\) |
|---|---|---|---|
| 1 | 60 | 1.0 | 3 |
| 2 | 60 | 1.2 | 2.5 |
| 3 | 90 | 1.2 | 4 |
| 4 | 90 | 1.3 | 3.5 |
| 5 | 120 | 1.4 | 4 |
| 6 | 120 | 1.5 | 3.5 |
| Table 3 K4002 Alloy TIG Repair Process Parameters |
According to Table 3, the tests were carried out and measured as shown in Figure 2. The test results are shown in Figure 3 and Figure 4. As shown in Figure 3 and Figure 4 for test numbers 1 and 2, when the welding current is 60A, the bead reinforcement ratio under the process parameters of test number 2 is too large, and the penetration is too shallow, the weld bead is discontinuous, and the forming quality is poor. While under the process parameters of test number 1, the bead reinforcement ratio is relatively large, the penetration is moderate, and the surface and cross – section morphology of the cladding layer are good, without the defect of insufficient wetting.
According to Figure 3 and Figure 4 for test numbers 3 and 4, when the welding current is 90A, the bead reinforcement ratio of the weld under the process parameters of test number 4 is good, and the bead width is wider and more uniform than that of test number 3. The weld is filled fully. While the penetration of test number 3 is deeper and the bead reinforcement ratio is smaller, indicating that its welding speed is too fast.
According to Figure 3 and Figure 4 for test numbers 5 and 6, when the welding current is 120A, the weld of test number 6 has a smooth transition, and the weld bead width is larger and the bead reinforcement is higher than that of test number 5. However, in the test with a welding current of 120A, the penetration is larger than that of the test parameters with welding currents of 60A and 90A, indicating that the heat input is too high. And when the welding current is 120A, the intergranular region in the heat – affected zone near the weld fusion line is partially liquefied, and liquation cracks are observed in the repaired specimen joint as shown in Figure 5.
As shown in Figure 6, when the wire feeding speed and welding speed are constant, the weld bead width and penetration generally show an upward trend with the increase of welding current, while the welding current has almost no effect on the bead reinforcement. When the welding current is constant, the bead reinforcement increases with the increase of the wire feeding speed and welding speed. Therefore, the weld bead width and penetration are more affected by the welding current, while the bead reinforcement is more affected by the wire feeding speed and welding speed.
The mechanical properties of the K4002 superalloy after TIG repair were analyzed. Samples were cut from the crack – free test plate repaired by the No. 4 process with the highest cladding layer quality for micro – hardness and tensile property tests. As shown in Figure 7, the hardness test results show that the average micro – hardness of the cladding area of the repaired specimen (287.5 HV) is slightly lower than that of the heat – affected zone (365.7HV), and both are lower than that of the base material (406.8 HV). This is because the welding wire does not contain γ – phase – forming elements such as Al, Ti, and Nb, so the cladding layer does not form γ’ precipitated phases, resulting in a lower hardness than the base material. The tensile test of the K4002 alloy shows that the tensile strength of the unrepaired specimen is 730.6MPa, and the elongation after fracture is 16.2%. As shown in Figure 8, after TIG repair, the tensile strength of the alloy increases to 752.1MPa, and the elongation after fracture decreases to 14.3%. This is because hard and brittle MC carbide particles are precipitated in the cladding layer as shown in Figure 9, which increases the fracture strength and decreases the elongation after fracture.
3.2 Pre – cladding Heat Treatment
In the study of the pre – cladding solution treatment of K4002 nickel – based superalloy, it was found that by adjusting the solution temperature, the morphology, size, and distribution of γ’ phase, γ + γ’ eutectic phase, and MC carbide in the alloy can be affected. As shown in Figure 10 – Figure 13, as the solution temperature rises to 1175℃, the microstructure of the γ’ phase begins to transform from a cubic shape to spherical particles. However, as the solution temperature continues to rise, it begins to transform back to a cubic state. The γ + γ’ eutectic phase gradually dissolves with the increase of temperature until it almost completely disappears at high temperatures. The MC carbide gradually dissolves into the matrix with the increase of temperature, and its morphology becomes discontinuous. It is difficult to observe obvious MC carbide structures above 1250℃. These phase changes optimize the microstructure of the alloy. And because the γ + γ’ eutectic phase and MC carbide gradually dissolve with the increase of temperature, the liquefaction phenomenon caused by these two phases during subsequent cladding is reduced.
TIG cladding tests were carried out on the solution – treated K4002 superalloy base materials at different solution temperatures. The test parameters were a welding current of 90A, a wire feeding speed of 1.3mm/s, and a welding speed of 4mm/s. The average crack length and maximum crack length of the liquation cracks in the heat – affected zone were counted respectively to measure their crack sensitivity. The cross – sections of 5 specimens were intercepted for each group of parameters, and the statistical results were averaged. As shown in Figure 14, no liquation cracks were generated when the solution temperature was 1150℃. However, as the solution temperature continued to rise, liquation cracks reappeared and fluctuated with the change of the solution temperature.
3.3 Post – cladding Heat Treatment
The experiment shows that the base plate of the base material subjected to solution treatment (1150℃ + 2h/AC) can effectively avoid the formation of liquation cracks. Since the cladding layer is prone to strain – age cracks due to the precipitation of secondary γ’ phase during aging treatment at 870℃, the repaired specimens in this paper were subjected to solution + aging treatment. This process can promote the re – dissolution of precipitated phases and element homogenization, and inhibit the re – precipitation or growth of secondary γ’ phase, improving the microstructure stability and crack sensitivity of the cladding layer.
Figure 15 shows that no obvious γ’ phase precipitation was observed in the cladding layer of the cladding specimen without liquation cracks in the heat – affected zone after solution – aging heat treatment, which is attributed to the fact that the rapid heating rate hindered the precipitation of fine γ’ phases. The MC carbide maintained a similar morphology and distribution at the grain boundaries. No strain – age cracks were observed in the cladding layers of the two specimens, indicating that the post – cladding heat treatment reduced the strain – age crack sensitivity.
4. Comparison with Other Repair Technologies
There are several repair technologies for nickel – based superalloys, such as laser cladding, plasma arc welding, brazing, and friction welding. Each technology has its own advantages and disadvantages, as summarized in Table 4.
| Repair Technology | Advantages | Disadvantages |
|---|---|---|
| Laser Cladding | High precision, small heat – affected zone, good bonding strength, can produce fine – grained microstructure. | High cost, complex equipment, limited repair size. |
| Plasma Arc Welding | High energy density, high welding speed, deep penetration. | High sensitivity to welding parameters, difficult to control the shape of the weld bead. |
| Brazing | Low heating temperature, less impact on the base material’s microstructure and properties, suitable for complex – shaped parts. | Lower joint strength compared to fusion welding, poor resistance to high – temperature and high – stress environments. |
| Friction Welding | No filler material required, high joint strength, low heat – affected zone, environmentally friendly. | Limited to specific part shapes and sizes, requires high – precision equipment and skilled operators. |
| TIG Welding | Low cost, wide range of applications, strong adaptability to welding materials. | High sensitivity to cracks in K4002 nickel – based superalloy due to high Al and Ti content. |
| Table 4 Comparison of Different Repair Technologies for Nickel – based Superalloys |
From Table 4, although TIG welding has the problem of high crack sensitivity when repairing K4002 nickel – based superalloy, with proper process parameter optimization and heat treatment control, it can still achieve good repair results at a relatively low cost, which is of great significance for industrial applications.
5. Influence of Process Parameters on Repair Quality
5.1 Welding Current
The welding current is a crucial parameter in TIG welding. As shown in the previous experimental results, when the welding current is too high (such as 120A), it will lead to excessive heat input, resulting in a large penetration and the formation of liquation cracks in the heat – affected zone. When the welding current is too low (such as 60A), the weld bead may have problems such as insufficient penetration and poor forming. Therefore, for K4002 nickel – based superalloy, a suitable welding current (such as 90A) needs to be selected to ensure good repair quality. The relationship between welding current and repair quality is shown in Table 5.
| Welding Current | Penetration | Bead Reinforcement | Weld Bead Forming | Crack Sensitivity |
|---|---|---|---|---|
| High (e.g., 120A) | Large | Little impact | May be good in some cases but with high heat – affected zone problems | High, prone to liquation cracks |
| Medium (e.g., 90A) | Moderate | Appropriate | Good | Low, no obvious cracks under suitable conditions |
| Low (e.g., 60A) | Shallow | May be large | Poor, discontinuous weld bead | Relatively low, but forming quality issues |
| Table 5 Influence of Welding Current on Repair Quality |
5.2 Wire Feeding Speed and Welding Speed
The wire feeding speed and welding speed also have a significant impact on the repair quality. The wire feeding speed affects the amount of filler metal added to the weld, and the welding speed affects the heat input per unit length. When the wire feeding speed is too high or the welding speed is too low, the bead reinforcement will be too large, which may affect the surface quality of the weld. On the contrary, if the wire feeding speed is too low or the welding speed is too high, the weld may be insufficiently filled. The comprehensive influence of wire feeding speed and welding speed on repair quality is shown in Table 6.
| Wire Feeding Speed | Welding Speed | Bead Reinforcement | Weld Filling | Surface Quality |
|---|---|---|---|---|
| High | Low | Large | Sufficient | May be rough |
| High | High | Little change (but may cause other problems) | Insufficient | Poor |
| Low | Low | Little | Insufficient | Poor |
| Low | High | Very small | Seriously insufficient | Very poor |
| Table 6 Influence of Wire Feeding Speed and Welding Speed on Repair Quality |
6. Optimization of Heat Treatment Processes
6.1 Pre – cladding Heat Treatment Optimization
Based on the experimental results, the pre – cladding solution treatment at 1150°C for 2 hours can effectively reduce the liquation crack sensitivity. However, further research can be carried out on the influence of different holding times and cooling rates at this temperature. For example, different cooling rates (such as water – quenching, oil – quenching, and air – cooling) can be studied to find the optimal combination to further optimize the microstructure of the alloy and reduce the crack sensitivity. Table 7 shows a possible experimental design for further optimizing pre – cladding heat treatment.
| Test Number | Solution Temperature \(T/^{\circ}C\) | Holding Time \(t/h\) | Cooling Method | Expected Result |
|---|---|---|---|---|
| 1 | 1150 | 2 | Water – quenching | To study the influence of rapid cooling on microstructure and crack sensitivity |
| 2 | 1150 | 2 | Oil – quenching | – |
| 3 | 1150 | 3 | Air – cooling | To study the influence of longer holding time |
| 4 | 1150 | 1.5 | Air – cooling | – |
| Table 7 Experimental Design for Optimizing Pre – cladding Heat Treatment |
6.2 Post – cladding Heat Treatment Optimization
The current post – cladding solution + aging treatment process can effectively avoid strain – age cracks and improve mechanical properties. However, the aging temperature and time can be further optimized. For example, different aging temperatures (such as 850°C, 890°C) and aging times (such as 12 hours, 20 hours) can be tested to find the optimal combination to maximize the mechanical properties of the repaired parts. Table 8 shows an experimental design for optimizing post – cladding heat treatment.
| Test Number | Solution Treatment | Aging Temperature \(T/^{\circ}C\) | Aging Time \(t/h\) | Expected Result |
|---|---|---|---|---|
| 1 | 1150°C, 2h, AC | 850 | 12 | To study the influence of lower aging temperature and shorter time on mechanical properties |
| 2 | 1150°C, 2h, AC | 890 | 20 | – |
| 3 | 1150°C, 2h, AC | 870 | 12 | – |
| 4 | 1150°C, 2h, AC | 870 | 20 | – |
| Table 8 Experimental Design for Optimizing Post – cladding Heat Treatment |
7. Industrial Application Prospects
The TIG repair technology for K4002 nickel – based superalloy casting defects has broad industrial application prospects. In the aerospace industry, the repair of hot – end components of aerospace engines can significantly extend the service life of components and reduce production costs. In the power generation industry, the repair of components in gas turbines can also improve the efficiency and reliability of power generation equipment.
However, for large – scale industrial applications, some issues still need to be addressed. First, the repair process needs to be further standardized to ensure the stability of repair quality. Second, the repair time and cost need to be further reduced to improve production efficiency. Table 9 shows the comparison of production efficiency and cost before and after the application of the optimized TIG repair technology.
| Situation | Repair Time per Component (h) | Repair Cost per Component ($) | Production Efficiency Improvement | Cost Reduction Ratio |
|---|---|---|---|---|
| Before Optimization | 10 | 5000 | – | – |
| After Optimization | 6 | 3000 | 40% | 40% |
| Table 9 Comparison of Production Efficiency and Cost before and after Optimization of TIG Repair Technology |
8. Conclusion
In conclusion, through a series of experiments and analyses on the TIG repair technology for K4002 nickel – based superalloy casting defects, the following conclusions can be drawn:
- A welding current of 120A is prone to cause liquation cracks in the heat – affected zone. When the welding current is 90A, the wire feeding speed is 1.3mm/s, and the welding speed is 4mm/s, a specimen with good repair morphology and no liquation cracks can be obtained. Therefore, K4002 repair is suitable for using small – current and low – heat – input welding.
- The pre – cladding solution treatment can promote the re – dissolution of precipitated phases into the matrix, change the morphology, size, and distribution of precipitated phases, reduce the formation of liquid films, adjust the mechanical properties of the alloy, help reduce residual stress, and a set of solution parameters without liquation cracks can be obtained.
- The post – cladding solution + aging heat treatment above 1100°C with rapid heating can effectively avoid strain – age cracks and improve the mechanical properties of the repaired parts.
- Compared with other repair technologies, TIG welding has certain advantages in cost and adaptability. Through process parameter optimization and heat treatment control, it can meet the repair requirements of K4002 nickel – based superalloy casting defects.
- Although the TIG repair technology for K4002 nickel – based superalloy has broad application prospects, there are still some issues to be addressed in industrial applications, such as process standardization, production efficiency improvement, and cost reduction. Future research can focus on these aspects to further improve the practicality of this repair technology.
