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土耳其卡拉布克大学、古姆沙内大学、卡拉德尼兹技术大学及Coşkunöz模具机械研发中心的研究人员报道了在电阻点焊中通过区域快速冷却优化先进的高强度钢接头的研究。 相关研究成果以“Optimizing Advanced High-Strength Steel Joints via Regional Rapid Cooling in Resistance Spot Welding”为题发表在《steel research international》上。
During resistance spot welding (RSW), thermal cycling causes the microstructure of the martensitic steel to differentiate in the heat-affected zone, resulting in internal stresses. In this context, researchers have developed a new and innovative method to minimize the adverse effects of non-uniform hardness changes in the weld area on the durability of the joint. The method uses a unique prototype device based on the District Rapid Cooling (RRC) process, which is integrated into the welding machine and operates in sync with the welding machine during the welding process. With this setting, the aim is to effectively control the microstructure and hardness values of the welded area. This innovative approach is designed to optimize material properties during the welding process, leading to potential improvements in the field of welding technology. According to the microstructure results, the RRC process reduced the heat-affected zone to 1.27 mm, increased the hardness value by 9.2%, the tensile shear strength by 3%, and the transverse tensile strength by 9%, and according to the fatigue strength results, all specimens did not break when subjected to a force of 0.3 kN.
Figure 1: RRC system conception and process steps in the process.
Figure 2: 3D view of the electrode and test fixture designed for rapid cooling of the heat-affected zone.
Figure 3: Fixtures manufactured and matched to welding machines.
Figure 4: Fatigue Test Fixtures: a) Design and b) Test Equipment.
Figure 5: Macro/microstructure images of WRRC specimens: a) welded metal; b,c) Heat-affected zone of DP600; d) fine-grained heat-affected zone; e) Base metal; f) UCHAZ； g) Fine-grained heat-affected zone; h)SCHAZ； i) Base metal.
Figure 6: Macro/microstructure images of an RRC specimen, a) welded metal, b, c) heat-affected zone of DP600, d) fine-grained heat-affected zone, e) base metal, f) UCHAZ, g) fine-grained heat-affected zone, h) SCHAZ, and i) base metal.
Figure 7: Specimen hardness values obtained by a) WRRC and b) RRC.
Figure 8: Hardness map of a) WRRC and b) RRC weld zones.
图 9：拉伸剪切试验结果，a) WRRC，b) RRC，c) WRRC-RRC 平均值。
图 10：拉伸剪切测试后的破裂图像，a) RRC，b) WRRC，c) RRC。
Figure 11: SEM image of the tensile shear failure mode, a) WRRC and b) RRC.
图 12：交叉拉伸试验结果，a) WRRC，b) RRC，c) WRRCC + RCC 平均值。
Figure 13: Fracture form of a welded sample after a cross-tensile test, a, b) WRRC and c) RRC.
Figure 14: Fatigue test results (RRC and WRRC).
Figure 15: SEM image of the fractured surface (WRRC and RRC).
In this study, an innovative process was developed to improve the durability of joints made of 22MnB5 and DP600 materials, aiming at the tempered subcritical heat-affected zone, which is a key weak link in thermoforming 22MnB5 point resistance welding. The designed method significantly improves the properties of the material by increasing the hardness of the weak links around the weld. The results of the study are as follows:
1) After measuring the electrode immersion depth of the welded specimen, it was found that none of the solder joints was less than 30% of the post-weld cross-sectional thickness specified in the standard.
2) During the macroscopic inspection of the welding specimen, no fusion defects (porosity, cavities, cracks, etc.) that may affect the welding quality are found in the welding core and its vicinity.
3) When the macroscopic and microstructure of all specimens were examined in cross-sections, it was found that the melting zone had a complete martensitic microstructure. Due to their different electrical resistance and chemical composition, 22MnB600 steel has a greater effect on the core mixture that forms the weld metal than DP22 steel.
4) After the addition of the RRC process to the welding operation, the tempering softening heat-affected zone narrows slightly, which has a positive effect on the static load-bearing capacity of the connection.
5) The hardness map shows that in the WRRC series, the average width of the tempering softening heat affected zone of 22MnB5 is 1.48mm, while in the RRC series, the width of this zone is reduced to 1.27mm on average.
6) When evaluating the tensile shear test results of specimens produced with alternating current, it was found that the RRC handling performance was 3% higher.
7) After comparing the cross-tensile strength of the specimens, it was found that the strength of the RRC series was 9% higher than that of the WRRC series.
8) In the tensile-shear and cross-tensile tests, the fracture mode of all specimens is in the form of snaps, and the separation of all specimens occurs between the solder core and the 22MnB5 HAZ.
9) In the fatigue test, all specimens exceeded the limit value of 106 fatigue cycles under a load value of 0.3kN. In addition, it was found that the RRC process had a lower number of cycles at the same load value.
Paper Links:
https://doi.org/10.1002/srin.202400232
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