How to avoid local stress concentration after sheet metal welding?
Release Time : 2025-11-24
After sheet metal welding, localized stress concentration is a key issue affecting structural strength and durability. Its root causes lie in abrupt changes in weld geometry, welding defects, and inadequate process design. Stress concentration leads to stress in the weld area being significantly higher than the average stress, which, under alternating or impact loads, easily triggers crack initiation and propagation, ultimately reducing the structure's fatigue life and even causing brittle fracture. Therefore, a comprehensive approach is needed, encompassing joint design, process optimization, defect control, and post-processing, to mitigate the risk of stress concentration.
Joint design is the primary step in controlling stress concentration. Butt joints are preferred due to their uniform stress distribution, avoiding stress concentration caused by abrupt changes in cross-section at T-joints or corner joints. If T-joints must be used, beveling or deep penetration welding must be used to ensure complete penetration and eliminate stress concentration sources caused by incomplete fusion. Simultaneously, the weld reinforcement and transition fillet radius should be controlled. Excessive reinforcement creates sharp corners, exacerbating stress concentration, while a smooth transition disperses stress. For butt joints of steel plates of different thicknesses, the thicker side should be thinned to create a smooth thickness transition and avoid stress concentration caused by abrupt changes in stiffness. The proper selection of welding process parameters is crucial for reducing stress concentration. Welding current, voltage, and speed must be precisely matched according to the material thickness and properties. Excessive current leads to excessive heat input, causing coarsening of the weld metal grains and reduced toughness; insufficient current easily results in defects such as incomplete penetration or slag inclusions. Excessive welding speed may cause the molten pool to solidify too quickly, preventing gas from escaping and forming porosity, while excessively slow speed prolongs the high-temperature dwell time, increasing the width of the heat-affected zone and reducing material properties. Furthermore, using sequential welding methods such as segmented back-welding and skip welding can disperse the welding heat input, reducing local deformation and stress accumulation.
Welding defects are one of the main causes of stress concentration and must be strictly controlled. Defects such as porosity, slag inclusions, and cracks disrupt the continuity of the weld metal, forming stress concentration points. For example, the edges of porosity, due to geometric discontinuities, are prone to stress concentration and may become crack initiation points under load. Therefore, before welding, it is essential to thoroughly clean the base material surface of oil, rust, and oxide film. Low-hydrogen welding rods should be selected and strictly dried to prevent moisture introduction that could lead to hydrogen-induced cracking. During welding, a short arc operation must be maintained, and the shape and size of the molten pool must be controlled to prevent gas entrapment. Post-weld, visual inspection and non-destructive testing are necessary to promptly identify and repair defects.
The weld layout and structural rigidity design must consider stress distribution. Avoid densely intersecting welds or concentrating them in the same area to reduce stress superposition. For complex structures, welds can be distributed or a symmetrical welding sequence can be used to ensure uniform stress distribution. Simultaneously, the use of rigid fixing and reverse deformation methods should be appropriate. External constraints or pre-deformation in the opposite direction can offset welding deformation, reducing additional stress caused by deformation. For example, when welding thin-plate frames, clamps can be used to fix key parts to prevent angular deformation or twisting during welding.
Post-processing is a crucial step in eliminating residual stress and optimizing stress distribution. Vibration aging uses high-frequency vibration to induce minute plastic deformation in the weld metal, releasing residual stress and reducing stress peaks. Hawker's welding stress relief technology utilizes high-frequency impact to induce plastic flow in the weld toe area, creating a smooth transition and reducing stress concentration. For structures requiring high precision, localized heat treatment can be used, employing heating, holding, and cooling cycles to eliminate residual welding stress and improve the microstructure of the weld metal.
Material selection and pretreatment are also crucial. Using sheet metal with excellent weldability, such as low-carbon steel or low-alloy high-strength steel, can reduce crack susceptibility. Preheating the base material before welding slows the cooling rate, preventing the formation of hardened structures and reducing the risk of cold cracking. The preheating temperature needs to be determined based on the material thickness and carbon equivalent, and the preheating range should cover a certain area on both sides of the weld to ensure uniform temperature. Post-weld slow cooling or post-heat treatment can further reduce residual stress and improve structural stability.
Avoiding localized stress concentration after sheet metal welding requires consideration throughout the entire process, including design, manufacturing, defect control, and post-processing. By optimizing joint design, precisely controlling process parameters, strictly controlling defects, rationally arranging welds, adopting post-processing techniques, and scientifically selecting materials and pre-treatment, the risk of stress concentration can be significantly reduced, structural strength and durability can be improved, and the requirements for use under complex working conditions can be met.
