1. Study on fatigue properties of resistance spot welding joints after salt spray corrosion
If the appearance quality of a railway passenger car is very poor, especially if the exposed structure surface has obvious wavy lines and uneven feeling, and even under the action of light, there are occasional and chaotic reflective effects, then even if the facilities in the passenger car are complete and advanced, they will not be favored by users.
In order to avoid the above problems, resistance spot welding, as an important welding application technology, is more and more favored by railway vehicle manufacturers.
This welding method can effectively solve the welding deformation and avoid the above problems.
At the same time, welding automation has laid a foundation for the rapid development of spot welding technology.
The steel plates commonly used for railway vehicles in China are mainly divided into 2mm, 2.5mm and 3mm.
The Q310NQL2 + Q345NQR2 material combination is a common design combination.
The welding method is basically resistance spot welding with small deformation.
Resistance spot welding belongs to overlapping structure, because the surface treatment after welding cannot be realized on the overlapping surface, so appropriate process must be selected for surface treatment before welding.
The sand blasting process is a single process with high efficiency and good economy.
It can not only effectively remove the oil stain, welding particles and welding oxide layer on the surface of the parts, but also clean the steel surface after sand blasting and improve the mechanical properties of the material surface.
Therefore, the general railway vehicle manufacturers conduct sand blasting on the steel before resistance spot welding.
As various working conditions and environment are complex in the vehicle operation cycle, the special lapping structure of resistance spot welding makes it impossible to carry out anti-corrosion treatment on the lapping position after welding, and corrosion has a certain impact on the fatigue performance of the welded joint, so it is particularly important to systematically carry out the research on the impact of resistance spot welding joint corrosion of the base metal after sandblasting treatment on the fatigue performance.
This research has very important practical significance for guiding the actual production, improving the welding structure and process design of railway vehicles, and improving the overall technical level of resistance spot welding.
2. Test materials and test methods
2.1 Test materials
The test materials are Q310NQL2 steel with a plate thickness of 2.5mm and Q345NQR2 steel with a plate thickness of 3mm.
The resistance spot welding test is conducted after the surface of the test materials is sand blasted.
The chemical composition and mechanical properties of the test materials are shown in Table 1 to table 4 respectively.
Table 1 chemical composition (mass fraction) of q310nql2 steel (%)
| C | Si | Mn | S | P | Cu | Cr | Ni |
| ≤0.12 | 0.25~0.75 | 0.2~0.5 | ≤0.02 | 0.07~0.12 | 0.25~0.55 | 0.30~1.25 | ≤0.65 |
Table 2 mechanical properties of Q310NQL2 steel
| Yield strength / MPa | Tensile strength / MPa | Elongation (%) |
| ≥310 | 480~670 | ≥22 |
Table 3 chemical composition of Q345NQR2 steel (mass fraction) (%)
| C | Si | Mn | S | P | Cu | Cr | Ni |
| ≤0.12 | 0.20~0.75 | ≤1.00 | ≤0.02 | 0.07~0.15 | 0.25~0.55 | 0.30~1.25 | 0.12~0.65 |
Table 4 mechanical properties of Q345NQR2 steel
| Yield strength / MPa | Tensile strength / MPa | Elongation (%) |
| ≥345 | 490~675 | ≥22 |
2.2 Test method
Conduct resistance spot welding process test on Q310NQL2 + Q345NQR2 steel plate after surface sandblasting treatment, which is the same as the manufacturing process of the factory’s existing car.
See Table 5 for its welding parameters
Table 5 resistance spot welding parameters
| Welding method | 21 | 21 | 21 |
| Electrode diameter / mm | 16 | 16 | 16 |
| Current type | DC | DC | DC |
| Pulse | preheat | Main welding | Main welding |
| Electrode pressure / kN | 10 | 10 | 10 |
| Preloading time / ms | 1000 | – | – |
| Welding current / kA | 7 | 12 | 12 |
| Slow rise time / ms | – | – | – |
| Welding time / ms | 300 | 600 | 600 |
| Slow down time / ms | – | – | – |
| Cooling time / ms | 100 | 100 | – |
| Holding time / ms | – | – | 900 |
The size of the test piece is shown in Fig. 1.
Fig. 1 size of resistance spot welding process test specimen
After welding, the appearance, smoothness and section of the test piece shall be tested according to the internal enterprise standards of CRRC.
Salt spray corrosion and pulsating tensile fatigue tests were carried out on the joints after welding.
(1) Salt spray corrosion test: acetic acid salt spray (ASS) is used to conduct salt spray corrosion test on resistance spot welding joints.
The test cycle is 48h and 1000h.
The size of the specimen used in the salt spray corrosion test is 138mm × 60mm (see Fig. 1).
(2) Pulsation tensile fatigue test the pulsation tensile fatigue test was carried out on the resistance spot welded joints without salt spray corrosion and the resistance spot welded joints with salt spray corrosion for 48h and 1000h respectively.
The pulsation tensile fatigue test shall be carried out according to the requirements of ISO14324:2003 fatigue test method for destructive inspection of resistance spot welds and spot welds, and the test shall be carried out according to the provisions of this standard.
The stress ratio R = 0.1, and the specified cycle life shall be taken as 1 × 107 times.
During the test, when the fatigue crack size is large enough that the load cannot be added, the vibration can be automatically unloaded and the number of cycles can be recorded.
The thickness of the backing plate shall be consistent with that of the corresponding base metal.
The fatigue fracture surface was analyzed by JSM-6360LV scanning electron microscope.
3. Test results and analysis
3.1 Comparison of joint fatigue performance
The comparison of F-N curves of resistance spot welding joints is shown in Fig. 2.
Fig. 2 Comparison of F-N curves of resistance spot welding joints
The F-N curve and the fatigue limit under 1×107 cycles determined by the lifting method are determined according to the conventional method.
The finally determined fatigue limit F0.1 of Q310NQL2 + Q345NQR2 resistance spot welded joints without salt spray corrosion and after 48h and 1000h salt spray corrosion is 4.55kN, 3.71kN and 3.87kN respectively.
It can be seen that the fatigue limit of joints without salt spray corrosion is higher than that of joints with salt spray corrosion;
After 48h and 1000h salt spray corrosion, the fatigue limit of the joint is similar.
3.2 Fracture analysis
The macroscopic morphology of fatigue fracture of Q310NQL2 + Q345NQR2 resistance spot welded joint fatigue test piece without salt spray corrosion and after 48h and 1000h salt spray corrosion is shown in Fig. 3-5.
The micro morphology is shown in Fig. 6 to Fig. 8.
It can be seen that there are no defects in the crack initiation area of the resistance spot welding joint specimens without salt spray corrosion and after 48h and 1000h salt spray corrosion, and there is no obvious crack initiation source.
Because the specimen is subject to shear and tensile stresses, according to the stress state of the specimen, there is stress concentration at the nugget along the length direction of the specimen, and the nugget is circular, and the structure distribution in the fusion zone is uneven, so the initial crack occurs in the fusion zone due to the stress concentration;
It can be seen from Fig. 6a that there are obvious undulations at the crack initiation of the fusion zone.
This is because the crack starts from the initial position and expands along the fusion line.
After a certain length of expansion, a new crack source is formed due to stress concentration, notch and other factors to expand toward the base metal, so that it is finally broken.
Because there are few interruptions during the fatigue fracture propagation, there are no obvious fatigue cracks in the fatigue crack propagation region;
The size of the expansion zone increases with the increase of fatigue cycles;
The final fracture zone is in the form of shear dimple + cleavage, and the dimple is large.
There is no defect at the fracture of the test piece, so it can be seen that the stress concentration is the main reason for the fracture.
4. Conclusion
1) Resistance spot welding process test was carried out on Q310NQL2 + Q345NQR2 steel after surface sandblasting treatment.
The fatigue limit F0.1 of resistance spot welding joints without salt spray corrosion and after 48h and 1000h salt spray corrosion for 1×107 times was 4.55kN, 3.71kN and 3.87kN respectively.
It can be seen that the fatigue limit of spot welded joints without salt spray corrosion is higher, and the fatigue limit of spot welded joints with salt spray corrosion is lower;
The fatigue limit of spot welded joints after 48h and 1000h salt spray corrosion is similar.
2) All the fatigue fracture specimens started in the fusion zone, the expansion zone increased with the increase of the number of cycles, and the final fracture zone showed the shape of the shear dimple.
