Casting titanium alloy is an excellent structural material with small specific gravity, high specific strength, and good mechanical properties at high and low temperatures. It is widely used in the manufacturing of key components of aerospace engines and aircraft fuselage structural parts. However, there are many defects such as pores, inclusions, and cracks in the ZTC4 titanium alloy castings, and there are often depressions that need to be repaired after hot isostatic pressing. Therefore, it is of great significance to study the argon arc welding repair technology for casting defects in titanium alloy castings. Through welding process tests, the welding process parameters and the pre-welding and post-welding heat treatment processes are optimized to adjust the microstructure and stress state of the weld, and the best welding process parameters are determined to avoid the formation of large stress concentrations during the solidification of the weld. The results show that the number of repair welds has little effect on the room temperature tensile and impact properties of the joint. Preheating before welding and post-welding heat treatment can effectively reduce the residual stress of the welded joint and prevent the occurrence of repair welding cracks.
Introduction
Casting titanium alloy, with its small specific gravity, high specific strength, high corrosion resistance, and good mechanical properties at high and low temperatures, is an excellent casting structural material. It is widely used in the manufacturing of key components of aerospace engines and aircraft fuselage structural parts. For example, the F – 22 fighter jet in the United States uses more than 70 titanium alloy precision castings, and 50% of the wing is composed of titanium alloy castings. A certain type of aircraft in China also uses a large number of titanium alloy hot isostatic pressing castings as the main load-bearing components, ensuring the lightweight, agile, and low-cost performance of the aircraft. Among the currently widely used casting titanium alloys, ZTC4 is a key titanium alloy material often selected for modern aircraft and engine structures. In the development of the ZTC4 titanium alloy integral precision cast intermediate casing for a certain thrust-to-weight ratio engine verification machine and a large aircraft engine in China, it was found that there are many defects such as pores, inclusions, and cracks in the castings, and a large number of titanium alloy castings are urgently needed for important domestic aircraft and engines. Titanium alloy castings often require repair welding after hot isostatic pressing. In addition, the use of castings in aircraft and engines often involves welding with other castings or forged parts. Therefore, studying the argon arc welding and laser welding repair technology for casting defects in titanium alloy integral thin-walled castings has important engineering significance and a clear demand background.
The main technical difficulty in the repair welding of large-scale thin-walled titanium alloy castings is the formation of large stress concentrations in the heat-affected zone after welding, which causes cracks around the repair welding joint. This problem is affected by the welding heat cycle, the composition of the welding filler material, the joint form, the welding restraint state, and the state of the base metal before welding and the post-welding heat treatment system. Therefore, this article will address the welding technology needs of the ZTC4 titanium alloy integral precision cast intermediate casing for a certain thrust-to-weight ratio engine verification machine and a large aircraft engine in China. Through multiple repair welding tests on test pieces, the weldability of the casting material and the influence law of the number of repair welds on the properties of the welding joint will be studied; through the appearance inspection of the weld formation and the non-destructive testing of the weld interior, the influence of pre-welding preparation and welding process on the quality of the weld will be studied; through the analysis of the joint structure and performance testing, the influence of welding specification parameters, post-welding heat treatment system and other factors on the structure and properties of the welding joint will be studied; through X-ray analysis, the change of welding residual stress before and after the repair welding of casting titanium alloy will be studied. Finally, a suitable repair welding process for titanium alloy castings will be formed.
Research Process and Methods
Materials
The base metal selected for this study is ZTC4 cast titanium alloy plate, in the state of casting + hot isostatic pressing; welding materials: for automatic argon arc welding, φ1.6TC4 welding wire is used, for manual argon arc welding, low-hydrogen φ1.0TC4 welding wire is used, and for laser welding, TC4 powder with a particle size of 150 – 325 mesh is used.
Repair Welding Test Methods
- Study on the Influence of the Number of Repair Welds on the Properties of the Repair Welding Joint
- Adopt the ZTC4 automatic argon arc welding repair method to study the influence of the number of repair welds on the properties of the repair welding joint. Select 6mm thick ZTC4 titanium alloy plates to study the influence law of the number of repair welds on the joint properties.
- Design the repair welding scheme, that is, first butt weld two 6mm thick plates, then machine a 3mm – 4mm deep V-shaped groove on the weld and perform repair welding; then refer to the previous step to machine the weld again and perform the second repair welding. Repeat this process for a total of 4 times of repair welding to obtain 5 states of test plates: un-repaired, 1 time of repair welding, 2 times of repair welding, 3 times of repair welding, and 4 times of repair welding.
- Perform post-welding heat treatment on the 5 test plates, with a system of 730°C × 2h, in vacuum.
- Finally, perform room temperature tensile and room temperature impact performance tests on the 5 states of test plates to obtain the influence law of the number of repair welds on the joint properties.
- Study on the Influence of Preheating before Welding and Post-welding Heat Treatment on the Properties of the Repair Welding Joint
- Adopt the ZTC4 manual argon arc welding repair method to study the influence of preheating before welding and post-welding heat treatment on the properties of the repair welding joint. For the defect situation of the actual workpiece, select two types of plates with thicknesses of 3mm and 6mm, as shown in Figure 1, to study the influence of preheating before welding and post-welding heat treatment on the residual stress and properties of the joint.
Figure 1: Forms of Repair Welding Joints for Two Thicknesses of Test PlatesThe forms of repair welding joints for the two thicknesses of plates: the 3mm thick test plate is a conical hole of φ8 × 2.5, used for the testing of room temperature tensile properties, and the 6mm thick test plate is a conical hole of φ11 × 5.5, used for the testing of room temperature impact and joint residual stress.- Design and manufacture a welding preheating tooling for heating before and during welding, with a preheating temperature of 150°C ± 10°C; determine and optimize the ZTC4 manual argon arc welding process, and use the optimized process to perform repair welding on the two thicknesses of plates to obtain 4 states of repair welding plates: welding, preheating welding, welding + post-welding heat treatment, preheating welding + post-welding heat treatment.
- Use the X-ray method to test the residual stress of the joint, and test the room temperature tensile and room temperature impact properties of the joint to obtain the influence law of preheating before welding and post-welding heat treatment on the residual stress and mechanical properties of the joint.
- Study on Preheating before Repair Welding and Post-welding Heat Treatment of ZTC4 Titanium Alloy Plates
- To prevent the occurrence of delayed cracks after welding, the selected φ1.0 TC4 welding wire is subjected to vacuum dehydrogenation treatment before welding.
- The manual argon arc welding process parameters are shown in Table 1, and the manual argon arc welding of the two types of plates is carried out using these parameters. Figure 3 shows the sample of the repair welding joint.
Table 1: Process Parameters of ZTC4 Manual Tungsten Inert Gas Arc WeldingPlate Thickness (mm)Welding Wire Diameter (mm)Electrode Diameter (mm)Arc Current (A)Arc Voltage (V)Gas Flow Rate (L/min) (Front/Back)3.01.01.5 – 1.640 – 607.0 – 9.011 – 15 (4 – 6)6.01.01.5 – 1.670 – 908.0 – 10.011 – 15 (4 – 6)
Figure 3: Sample of Repair Welding JointDesign and manufacture a welding preheating tooling, as shown in Figure 2, which can ensure a constant temperature of the test plate during the repair welding process and ensure the preheating effect; the preheating temperature is determined to be 150°C ± 10°C, and the post-welding heat treatment system is 730°C × 2h, vacuum treatment.Figure 2: Schematic Diagram of Preheating Welding Tooling
Results and Discussion
Influence of the Number of Repair Welds on the Properties of the ZTC Joint
Conduct an automatic argon arc welding repair process study on 6mm thick ZTC4 titanium alloy plates to study the influence of the number of repair welds (0 – 4 times) on the properties of the joint. Select φ1.6TC4 wire as the argon arc welding repair welding wire for the ZTC4 titanium alloy plate, and determine the repair welding process for the 6mm thick ZTC4 titanium alloy: no preheating before welding, and the parameters are shown in Table 2.
| Plate Thickness (mm) | Welding Wire Diameter (mm) | Arc Current (A) | Welding Speed (cm/min) | Wire Feed Speed (cm/min) | Gas Flow Rate (L/min) (Front/Back) |
|---|---|---|---|---|---|
| 6.0 | 1.6 | 160 – 200 | 10 – 14 | 60 – 100 | 15 – 20 (5 – 10) |
After welding, the room temperature performance of the joint is tested. The room temperature tensile and impact properties of the ZTC4 base metal are shown in Tables 3 and 4, and the room temperature tensile and impact properties of the joint under different repair welding times are shown in Figures 4 and 5. It can be seen that the tensile strength of the joint under different repair welding times reaches more than 98% or even 100% of the base metal, the elongation is lower than that of the base metal, which is 70% – 80% of the base metal, and the room temperature impact toughness exceeds that of the base metal, and the strengths are basically the same among each other. Thus, it can be concluded that the number of repair welds has little effect on the room temperature tensile and impact properties of the TC4 titanium alloy joint.
| Number | Temperature (°C) | Yield Strength (MPa) | Tensile Strength (MPa) | Elongation (%) |
|---|---|---|---|---|
| 1 – 1 | 23 | 922 | 854 | 9.2 |
| 1 – 2 | 23 | 924 | 844 | 5.4 |
| 1 – 3 | 23 | 921 | 837 | 7.9 |
| Average | – | 922 | 845 | 7.5 |
Table 4: Room Temperature Impact Properties of ZTC4 Base Metal
| Number | Temperature (°C) | Impact Toughness (J/cm²) |
|---|---|---|
| 1 – 4 | 23 | 26.9 |
| 1 – 5 | 23 | 27.3 |
| 1 – 6 | 23 | 28.5 |
| Average | – | 27.6 |
Influence of Manual Argon Arc Welding Preheating and Post-welding Heat Treatment on ZTC4 Plates
Compare the joint properties of preheating before welding and no preheating, post-welding heat treatment and no post-welding heat treatment, analyze the joint structures in 4 states: no preheating welding, preheating welding, no preheating welding + post-welding heat treatment, preheating welding + post-welding heat treatment. The macroscopic metallographic photos of different states are shown in Figure 6, and the weld structure photos of different states are shown in Figure 7. The test results of room temperature impact and tensile properties are shown in Figures 8 and 9.
It can be seen that the room temperature tensile strength in the 4 states reaches 97% or even 100% of the base metal, the joint strength slightly decreases after heat treatment, and the joint strength increases significantly after preheating welding + heat treatment. The elongation of the preheating welding joint is relatively lower than that of other states, which is 61% of the base metal, and the fracture basically occurs in the base metal, as shown in Figure 10. Figure 11 shows the joint samples in 5 states for residual stress testing, and the residual stress is tested by the X-ray method. The distribution of the test points is shown in Figure 12, and the test direction of each point is shown in Table 5.
Table 5: Measurement Direction of Residual Stress Test Points
| Number | Measurement Direction | Number | Measurement Direction |
|---|---|---|---|
| 1 | X Direction | 6 | Y Direction |
| 2 | X Direction | 7 | Y Direction |
| 3 | X, Y Direction | 8 | Y Direction |
| 4 | X Direction | 9 | Y Direction |
| 5 | X Direction | – | – |
The test results are shown in Table 6. It can be seen from Table 6 that the base metal is basically in a compressive stress state. After welding, tensile stress points appear in both the X and Y directions, and the tensile stress is the main reason for the occurrence of repair welding cracks. After preheating welding, the stress in the Y direction becomes compressive, and the individual tensile stress in the X direction is also significantly smaller than that in the welding state. After post-welding heat treatment, the stress of the joint is basically compressive, and the individual tensile stress value is also significantly smaller than that before treatment. Therefore, it can be concluded that welding preheating and post-welding heat treatment can effectively reduce the welding stress and prevent the occurrence of repair welding cracks.
| Sample Number | Sample State | Residual Stress Value (MPa) | | | | | | | | | |
| — | — | — | — | — | — | — | — | — | — | — |
| X – 1 | X – 2 | X – 3 | X – 4 | X – 5 | Y – 3 | Y – 6 | Y – 7 | Y – 8 | Y – 9 |
| Y – 1 | Base Metal | 745 | – 736 | – 190 | – 420 | – 24 | – 527 | – 608 | – 440 | – 961 | – 760 |
| Y – 2 | Welding | 112 | 784 | 386 | – 428 | – 188 | – 548 | 292 | 37 | 306 | – 430 |
| Y – 3 | Welding + Heat Treatment | – 33 | 207 | – 100 | – 411 | – 79 | – 459 | – 138 | – 523 | – 325 | – 456 |
| Y – 4 | Preheating Welding | 16 | 215 | 126 | 44 | 238 | – 217 | – 517 | – 328 | – 50 | – 421 |
| Y – 5 | Preheating Welding + Heat Treatment | – 38 | – 146 | 77 | 231 | 177 | – 313 | – 211 | – 377 | – 411 | – 128 |
Conclusions
- By using the automatic argon arc welding method to perform 4 times of repair welding tests on the ZTC4 test plate, it is concluded that the room temperature tensile and impact properties of the joint do not change much within the range of 4 times of repair welding.
The room temperature tensile properties of the argon arc welding repair joint reach more than 97% of the base metal, and the impact toughness reaches 100% of the base metal.
Factors Affecting the Quality of Repair Welding
- Welding Process Parameters
- The choice of welding current, voltage, and speed can significantly affect the quality of the weld. For example, an inappropriate welding current may lead to incomplete fusion or excessive penetration, while an improper welding speed may result in uneven welds or increased porosity.
- The wire feed speed also plays a crucial role in ensuring a stable arc and consistent deposition of the filler metal.
- Preheating and Post-welding Heat Treatment
- Preheating before welding can help reduce the temperature gradient between the weld and the base metal, thereby minimizing the residual stress and the risk of cracking.
- Post-welding heat treatment can further refine the microstructure of the weld, improve its mechanical properties, and reduce the residual stress.
- Welding Filler Material
- The composition and properties of the welding filler material should be compatible with the base metal to ensure good weldability and joint performance.
- The selection of the appropriate welding wire or powder can also affect the formation of casting defects during repair welding.
Prevention and Control of Casting Defects
- Inspection and Detection
- Prior to repair welding, a thorough inspection of the casting should be conducted to identify and locate the defects accurately. This can be achieved through various non-destructive testing methods such as X-ray inspection, ultrasonic testing, and dye penetrant testing.
- During the repair welding process, continuous monitoring and inspection of the weld are necessary to ensure the quality of the repair.
- Process Control
- Strict control of the welding process parameters, including preheating temperature, welding current, voltage, and speed, can help reduce the occurrence of casting defects.
- Proper cleaning and preparation of the welding area can also prevent the introduction of impurities or contaminants that may lead to defects.
- Operator Skills and Training
- The skills and experience of the welding operator have a significant impact on the quality of the repair welding. Operators should be properly trained and certified to ensure they can perform the welding tasks accurately and consistently.
- Regular training and updates on the latest welding techniques and processes can help operators stay informed and improve their skills.
Recommendations for Future Research
- Further Optimization of Welding Process Parameters
- Conduct more detailed studies to optimize the welding process parameters for different types and sizes of ZTC4 titanium alloy castings. This can include exploring the effects of different welding currents, voltages, speeds, and wire feed speeds on the weld quality and joint properties.
- Investigate the interaction between the welding process parameters and the preheating and post-welding heat treatment processes to achieve the best combination for minimizing casting defects and improving joint performance.
- Development of New Welding Technologies and Materials
- Explore the application of new welding technologies, such as friction stir welding or laser hybrid welding, in the repair welding of ZTC4 titanium alloy castings. These technologies may offer advantages in terms of reduced residual stress, improved weld quality, and increased productivity.
- Research and develop new welding filler materials that are specifically designed for ZTC4 titanium alloy castings to further improve the weldability and joint performance.
- Study on the Long-term Performance of Repair Welded Joints
- Conduct long-term performance tests on repair welded joints to evaluate their durability and reliability under different operating conditions. This can include fatigue testing, corrosion testing, and high-temperature testing.
- Investigate the effects of casting defects and the repair welding process on the long-term performance of the joints and develop strategies to enhance their service life.
- Integration of Numerical Simulation and Experimental Validation
- Use numerical simulation techniques, such as finite element analysis, to predict the temperature distribution, stress field, and microstructure evolution during the repair welding process. This can help optimize the welding process parameters and reduce the need for costly and time-consuming experimental trials.
- Validate the simulation results through experimental testing and continuously refine the simulation models to improve their accuracy and reliability.
Conclusion
The research on the argon arc welding repair process of casting defects for ZTC4 titanium alloy is of great significance for improving the quality and reliability of titanium alloy castings. Through the optimization of welding process parameters, preheating and post-welding heat treatment processes, and the selection of appropriate welding filler materials, the occurrence of casting defects can be minimized, and the mechanical properties of the repair welded joints can be improved. However, there is still room for further research and improvement in this field, and future studies should focus on the topics mentioned above to meet the increasing demands for high-quality titanium alloy castings in the aerospace and other industries.
