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Welding Deformation and Control Methods

Formation mechanism of welding deformation

The core of welding deformation is actually thermoelastic plastic deformation. During welding, the local material will be heated to a very high temperature, and the material will expand. However, the surrounding cold metal will give it a constraint to limit its expansion, which will form compressive plastic deformation. When the temperature drops after welding, the material will start to shrink, but it will be restricted by the previous constraints and cooling parts, and the shrinkage will be hindered. This will form residual stress, ultimately leading to welding deformation.

Classification of Welding Deformation

Longitudinal deformation: deformation along the length of the weld seam, approximately 0.5-0.7mm per meter;

Lateral deformation: refers to the deformation perpendicular to the length direction of the weld seam, which affects the width accuracy;

Angular deformation: Uneven heating up and down of the weld seam leads to different shrinkage up and down after cooling, resulting in angular changes in the welding process;

Wave deformation: It is generally prone to occur during thin plate welding because the welding stress causes the thin plate to lose stability;

Distortion and deformation: Improper welding sequence or insufficient rigidity of the welded parts result in inconsistent shrinkage in various areas, ultimately leading to distortion.

Core factors affecting welding deformation

Material properties: Different materials have different coefficients of expansion, and materials with higher coefficients of expansion will experience more significant welding deformation, such as aluminum alloys. The coefficient of expansion of aluminum alloys is much larger than that of ordinary steel, about (20-24) * 10 ^ -6/℃, making deformation control more difficult when welding aluminum alloys. There is also the thermal conductivity of materials. Materials with good thermal conductivity have faster heat diffusion and less deformation during welding; Materials with poor thermal conductivity are prone to heat concentration and deformation during welding. In addition, the mechanical properties of materials, such as yield strength and plasticity, can also affect their deformation; Materials with low yield strength are more prone to plastic deformation, resulting in more severe welding deformation.

Structural design: The number, position, and shape of welds can all affect deformation. For example, the more welds there are, the more heat is generated during welding, and the greater the deformation. If the welds are arranged asymmetrically, it is easy to produce uneven deformation during shrinkage. The setting of the cut is unreasonable, for example, if the bevel angle is too large, it will increase the filling amount of the weld seam, and the input heat will increase the deformation. The rigidity of the structure, structures with high rigidity have strong resistance to deformation, welding deformation is small, and deformation is large for structures with low rigidity.

Process parameters: Welding current, voltage, and welding speed all affect heat input. The larger the heat input, the more obvious the welding deformation will be. For example, when the welding current increases, the welding penetration will increase, and at the same time, the more heat input, the more deformation will occur. The welding sequence also has a significant impact on welding deformation. Incorrect welding sequence can result in uneven stress distribution in various parts of the welded component, leading to deformation. In addition, preheating and post heating can also affect deformation. Preheating can change the temperature gradient of welding, reduce residual stress, and thus reduce deformation. Post heating can accelerate the cooling of welding or keep the weld at high temperature for a period of time to reduce residual stress.

Prevention and Control of Welding Deformation

Weld seam design: Optimize weld seam design to avoid excessive welding. Some people believe that the thicker the weld seam, the stronger it is. However, excessive welding not only increases welding costs but also inputs more heat, leading to increased deformation. Therefore, in the design process, the appropriate weld seam size should be selected according to the stress situation, and the weld seam size should not be blindly increased. Reasonably design the groove angle. If the groove angle is too large, it will increase the amount of weld filling, input more heat, and increase deformation. Therefore, try to minimize the groove angle while ensuring penetration.

Structural layout: The welds should be symmetrically arranged so that during shrinkage, the shrinkage in all directions is relatively uniform and not prone to uneven deformation. It is also necessary to avoid excessive concentration of welds. In areas where welds are concentrated, heat input is concentrated, stress is also concentrated, and deformation will be relatively large. Therefore, when designing, try to disperse and arrange welds as much as possible to evenly distribute heat

Material and welding material selection: Try to choose materials with low thermal expansion and good plasticity. The welding material should match the base material to reduce stress and deformation during the welding process.

The core method of process control

Reverse deformation method: Before welding, the welded part is pre pressed or bent into a welding shape opposite to the welding direction of the welded part. This way, when the welded part shrinks after welding, the reverse deformation and welding deformation cancel each other out to obtain a flat welded part. For example, when welding thin plates, wave deformation is prone to occur. The thin plate can be pre bent into a shape opposite to wave deformation, and wave deformation can be reduced after welding.

Rigid fixation method: During welding, use fixtures or fixed supports to fix the welded parts and limit their deformation. For example, when welding frame structures, use fixtures to fix the frame to the platform, so that the welded parts are not easily deformed during welding. But the rigid fixation method has a disadvantage, which is that the residual stress after welding is relatively large, so stress relief treatment needs to be carried out after welding.

Optimization of welding sequence: A reasonable welding sequence can make the stress distribution of the welded parts uniform and reduce deformation. For example, when welding symmetrical structures, the symmetrical welding sequence should be used to weld the symmetrical welds first, which can make the shrinkage uniform. Another approach is to weld from the middle of the weld seam to both ends, or from thicker areas to thinner areas, which can reduce stress concentration and deformation during the welding process.

Heat input management: Control the heat input during the welding process, minimize heat input as much as possible to reduce deformation. For example, selecting appropriate welding parameters, reducing welding current and voltage, and increasing welding speed can reduce the heat input per unit length of weld seam. Adopting multi-layer and multi pass welding, the welding heat input of each layer is relatively small during multi-layer and multi pass welding, and the heat of the previous layer welding will be partially offset by the heat of the subsequent layer welding, which can reduce the overall heat input and thus reduce deformation.

Post weld correction and stress control

Mechanical correction: Use mechanical force to correct the welding deformation of the welded part, such as using equipment such as presses, jacks, and rolling machines to apply external force to cause plastic deformation of the welded part and correct the welding deformation. Mechanical correction is suitable for situations where the deformation is relatively small or where the rigidity of the welded parts is relatively high.

Flame correction: Heat the deformed part of the welded part with a flame, and use the principle of metal thermal expansion and contraction to generate new deformation of the welded part, thereby offsetting the deformation of the welded part. When flame straightening, attention should be paid to the temperature and position of the heating. Heating at too high a temperature can affect the performance of the material. If the heating position is not correct, it will not only fail to correct the deformation but also make it more severe. Flame straightening is suitable for situations with large deformation or low welding rigidity.

Heat treatment for stress relief: Heat the welding to a certain temperature and keep it warm for a period of time, then slowly cool it down. This can eliminate residual stress during the welding process and reduce deformation. Heat treatment has a good effect on stress relief, but it is costly and requires specialized equipment, so it is generally used for important welded parts or welded parts with high residual stress.

Hammer striking method: Use a hammer or specialized tool to strike the weld seam and surrounding metal, causing plastic deformation of the weld seam and surrounding metal to reduce residual stress and correct deformation. The hammering method is suitable for situations where the weld seam is relatively thin. When hammering, attention should be paid to the force. If the force is too small, it will not be effective, and if the force is too large, it may crack the weld seam.

Special materials and material response

For example, when welding special materials such as titanium alloys and high-temperature alloys, the methods of controlling deformation during welding are also different because the thermal expansion coefficient and thermal conductivity of these materials are different from ordinary steel. For example, when welding titanium alloys, it is necessary to weld under argon protection to prevent oxidation of the titanium alloy, while also controlling heat input to reduce deformation.

When welding large structures, due to the large size of the structure, uneven deformation is prone to occur during welding. Therefore, it is necessary to use methods such as segmented welding and overall preheating to control deformation.

Conclusion

Welding deformation control is not a matter of a single link, but a systematic engineering of the entire process. From the design phase to process development, welding process, and post weld correction, welding deformation control must be considered at every stage. For example, in the design phase, it is necessary to optimize the weld seam design and structural layout. In the process phase, appropriate welding parameters and welding sequence should be selected. During the welding process, strict adherence to process requirements should be followed, and timely correction and stress relief treatment should be carried out after welding. Only by doing every step well can welding deformation be effectively controlled.