As a core component of the transmission system, the welding process of the universal joint shaft significantly impacts its structural strength throughout the entire process, from material selection and heat treatment to weld formation and stress control. It directly determines the shaft's reliability and service life under complex alternating loads. The rationality of the welding process not only affects the mechanical properties of the weld itself but also profoundly influences the overall strength of the universal joint shaft through indirect factors such as residual stress distribution and changes in material microstructure.
The selection of welding materials is the primary step in ensuring structural strength. Universal joint shafts often use high-strength alloy steels, such as 40Cr steel or HQ80C low-carbon quenched and tempered steel. These materials require a reliable connection to the shaft through welding. For example, when welding a 40Cr steel universal joint shaft, E8515 high-strength alkaline low-hydrogen welding rods must be used, with a tensile strength exceeding 850 MPa, to match the high torque transmission requirements of the shaft. Improper material matching, such as a weld metal strength lower than the base metal, can easily lead to stress concentration under alternating loads, resulting in weld cracking.
Welding process parameters play a decisive role in weld formation quality. Taking a 40Cr steel universal joint as an example, the workpiece needs to be preheated to 300℃ before welding, and then slowly cooled in asbestos ash after welding to reduce welding stress. Simultaneously, DC reverse polarity welding, combined with a U-shaped bevel design, ensures full penetration at the weld root. Insufficient preheating temperature or excessive welding current can easily lead to coarsening of the weld metal grains, reducing tensile strength and impact toughness; while an excessively small bevel angle may cause incomplete penetration defects, forming crack initiation points.
Heat treatment is a crucial step in eliminating welding stress and improving structural strength. After welding, the 40Cr steel universal joint requires tempering at 550-600℃ for 2 hours to eliminate residual stress in the weld and heat-affected zone, preventing stress corrosion cracking. If the tempering process is omitted, the shaft may deform due to stress release during long-term operation, even leading to fatigue fracture. Furthermore, heat treatment can improve the microstructure of the weld metal, making it closer to the mechanical properties of the base material.
The influence of weld geometry on structural strength cannot be ignored. During universal joint shaft welding, geometric discontinuities such as weld height increase, misalignment, and angular deformation can lead to stress concentration. For example, if a T-joint is not beveled at the plate edge, the stress concentration may be significantly higher than that of a butt joint, causing premature shaft failure under dynamic loads. Therefore, the welding process must strictly control the weld formation dimensions to ensure uniform stress distribution throughout the shaft.
Welding defects pose a potential threat to structural strength. Defects such as incomplete penetration, porosity, and slag inclusions reduce the effective load-bearing area of the weld and create stress concentration around the defects. For example, dense porosity can degrade the mechanical properties of the weld metal, leading to crack propagation under alternating loads. Cracks, as the most dangerous welding defect, exacerbate stress concentration through their tip effect, even causing shaft fracture. Therefore, the welding process must strictly control weld quality through non-destructive testing and other methods.
The welding sequence and clamping method are crucial for controlling shaft deformation. During universal joint shaft welding, failure to use a symmetrical welding sequence or improper clamping leading to shaft bending can induce additional stress and reduce structural strength. For example, an improper welding sequence in a certain model of universal joint shaft led to excessive shaft curvature, resulting in increased vibration and noise during operation, ultimately causing transmission system failure.
The synergistic effect of the welding process and subsequent machining directly affects the final strength of the universal joint shaft. After welding, the shaft needs to be machined, such as turned or ground, to correct welding deformation. Insufficient machining allowance or improper cutting parameters may damage the weld surface quality and reduce fatigue strength. Therefore, the welding process must be closely coordinated with subsequent machining processes to ensure the shaft's dimensional accuracy and surface integrity.
