Abstract: The air radiator’s end cover was found to be leaking oil. After disassembling the product, a ventilation test was conducted, revealing cracks in the weld of the LF21-M aluminum alloy end cover. To determine the nature and cause of the air radiator’s failure, a macroscopic examination of the radiator was performed, along with macro-and micro-analysis of the weld area. Energy spectrum analysis was conducted on the fracture surface, and the microstructure and hardness of the material near the crack were systematically characterized. The results indicate that the cracking at the end cover weld of the air radiator is due to high-cycle fatigue. The incomplete penetration at the butt weld joint between the cover plate and baffle plate,combining with the weld penetration depth being less than design requirements result in increasing the stress concentration at the weld root location, which adversely affects the fatigue resistance at the root of the cover plate weld. Through problem investigation and fault replication, it is identified that welding torch angles exceeding 100° causing the incomplete weld penetration. The experimental verification demonstrates that when employing a 75° welding torch angle, the complete weld penetration is achieved with excellent fatigue resistance performance.