Among titanium alloys, Ti-6Al-4V titanium (TC4) alloy is widely used in the aviation industry owing to its remarkable low density, good corrosion and fatigue resistance, high toughness and high specific strength properties. Due to the high chemical activity of titanium alloys at high temperatures, welding of such alloys results in brittle phases that weaken the overall joint performance. While vacuum electron beam welding (EBW) technology is suitable for welding of titanium alloys, the vacuum chamber limits the size of workpieces that can be welded. In contrast, laser welding with filler wire (LWFW) performs better in adaptability and is a promising welding technology in the aviation industry.
The applicability and feasibility of high-energy beam welding technologies like EBW and LWFW in welding thick plates have been extensively studied. Nevertheless, there is little attention to the fatigue performance of titanium alloy joints welded by EBW and LWFW despite its practical implications. Previous results revealed that the inhomogeneities of properties and microstructures could significantly affect the overall fatigue performance of welded joints. This is because microstructural inhomogeneity results in a microhardness gradient and stress concentration responsible for weakening the fatigue performance.
On this account, Professor Jian Long and Professor Lin-Jie Zhang from Xi’an Jiaotong University in collaboration with Mr. Li-Xu Zhang and Mr. Jun Wu from Xi’an Aerospace Power Machine Factory Co. Ltd and Mr. Ming-Xiang Zhuang from AVIC Xi’an Aircraft Industry (Group) Company Ltd., compared the high-cycle fatigue (HCF) performance of 30 mm thick TC4 alloy joints welded by EBW and LWFW. In addition, the fracture morphology, microstructure and microhardness of the two joints were analyzed. Their work is currently published in the journal, Fatigue & Fracture of Engineering Materials & Structures.
The authors showed that the HCF strength of the LWFW joint was about 65% of that of the EBW joint. After 2 × 106 cycles under the same load, the HCF strength of the LWFW and EBW joints were 307 MPa and 474 MPa, respectively. Moreover, the spacings between the fatigue striations in the fractures in the LWFW and EBW joints during HCF tests were 2.78 µm and 2.14 µm, respectively. Although the two joints exhibited lower plasticity than the base metal and the tensile strength of the LWFW joint was 96% that of EBW joint, the strength of the EBW joint was equal to the base metal.
The cause of the weakening HCF performance of the LWFW joint was examined. This was partly attributed to the presence of a larger microhardness gradient in the LWFW joint. The average microhardness of the weld metal of EBW and LWFW joints was 330 HV and 275 HV, indicating the weakening of the weld metal of the LWFW. Consequently, more slip systems and lower resistance of dislocation movement were observed when the material was subjected to external loading and deformation, a reason for lower LWFW joint hardness than that of the EBW joints.
In a nutshell, the HCF performances of LWFW and EBW joints of TC4 alloys were compared. The differences in LWFW and EBW joints were discussed in detail. While the weld metal of the EBW joint was dominated by acicular α’ phases, that of the LWFW joint was predominantly punctate β phases, a potential cause of the softening of the LWFW joint. In a statement to Advances in Engineering, Professor Lin-Jie Zhang explained their study provided important data support that would advance applying high-energy beam welding technology in the aviation industry.
Long, J., Zhang, L., Zhang, L., Wu, J., & Zhuang, M. (2022). Comparison of fatigue performance of TC4 titanium alloy welded by electron beam welding and laser welding with filler wire. Fatigue & Fracture of Engineering Materials & Structures, 45(4), 991-1004.