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Large structural components are the backbone of construction machinery products, and they reflect the design capabilities as well as the manufacturing standards of manufacturers. Due to the important role of structural components in terms of load-bearing capacity, appearance design, and enabling product functionality, they should be given special attention in the design and manufacturing of construction machinery products. Large structural components pose challenges due to their massive size and numerous welding areas; some of them require high precision in terms of dimensions and positioning, making it difficult to support them. In particular, the issue of overall deformation has always been a major problem for process engineers. This paper mainly analyzes the welding deformation of large structural components and its causes, based on the actual manufacturing processes of typical structural parts for graders and loaders produced by China State Construction Engineering Corporation, and proposes some process strategies to control such deformation. 1 Typical examples of deformation in large structural components and their hazards 1.1 Deformation of the rear frame of the PY160C type grader The rear frame is the largest structural component of the PY160C type grader (see Figure 1); its dimensions are 3,300 mm × 1,100 mm × 800 mm. It consists of 72 individual parts and is a large structural component with a frame-like structure. It is manufactured by bringing together these parts, aligning them, and then welding them together. During the assembly of the complete machine, this component must be connected to components such as the engine, transmission, driver’s cab, and drive axle. The required specifications for its fabrication are: flatness of the base surface ≤ 3 mm, distortion ≤ 5 mm, and perpendicularity ≤ 2.5 mm. After welding in the conventional manner, the typical deformations are as follows: twist of 10–30 mm, flatness of 5–8 mm, and perpendicularity of 5–10 mm. The center distance between the lateral hole groups is also out of specification; for example, the installation distance for the rear axle is 720 mm ± 0.5 mm, but after welding it becomes 720 mm + 2 mm. Although a shaping process is carried out after welding, the degree of deformation remains relatively high compared to the assembly requirements, resulting in a high rate of rework for this component; as a result, backward techniques such as \"matching welding\" and \"shim adjustment\" were later employed for correction. 1.2 Deformation of the boom of the ZL50C type loader This component is a large H-shaped structural part (see Figure 2); its external dimensions are 2,800 mm × 1,200 mm × 800 mm. It is manufactured by welding the joint seams at the large cross-sections after assembling the support components with the two boom plates together. Its welding method involves continuous welding with 3 layers and 6 weld passes. This component is a key supporting element for the movement of the bucket during loader operations, and the manufacturing specifications require that the symmetry of dimensions a and b with respect to the center line be 1.5 mm ； The parallelism of the centerlines of the holes in groups A, B, C, and D is Φ1.0 mm ； The coaxiality of the holes in each group is Φ0.5 mm. After welding in the aforementioned manner, the main deformation is a lateral bending of the support, which usually results in dimension b being 8–15 mm smaller than required. The symmetry between dimensions a and b is off by 1.5–2 mm, and the center lines of dimensions a and b are not parallel to each other; there is an angle of 2°–3° between these two center lines. This leads to deformation of the boom, and during prolonged operation, the bucket may become unstable. 2 Theoretical analysis of the deformation of structural components Construction machinery components are mainly assembled and welded from cold-rolled and hot-rolled steel plates, section steels, and their formed products, with low-alloy structural steel being the primary material used. From the perspective of structural component manufacturing processes, the causes of deformation in large-scale structural components mainly stem from three aspects: welding thermal stress, residual stress, and external forces. 2.1 Welding thermal stress deformation During the welding process, the workpiece undergoes uneven heating and cooling, which affects the metal material. During welding, the heat source for heating is a moving high-temperature arc; the temperature of the weld and the heat-affected zone metals is very high. The metals expand due to the heat, but this expansion is restricted and suppressed by the metals at normal temperature, resulting in compressive plastic deformation. The degree of welding deformation in structural members is proportional to the energy input by the heat source during welding. 2.2 Residual stress deformation Residual stresses mainly include welding residual stresses and forming process residual stresses. When welding is completed in a certain part of a workpiece, the weld metal shifts from expansion to contraction; however, it is constrained by the metal at room temperature, and this results in welding residual stresses. Residual stresses in forming processes are mainly caused by external forces acting on the workpiece during manufacturing; for example, improper methods are used when the workpiece is bent freely ； Few passes are required for leveling the steel plate ； Excessive cutting depth in machining, among other factors, can cause residual stresses in the formed part. 2.3 Deformation caused by external forces Mainly refers to abnormal deformation resulting from knocks, impacts, drops, collisions, or overloading during assembly and welding processes. Combined with the theoretical analysis of the stresses causing deformation, it can be seen that the deformation of the rear frame of the aforementioned PY160C leveler is the result of the combined deformation from hundreds of welds. The deformation of the boom of the ZL50C loader is a typical case of welding thermal stress deformation caused by large heat fields in the welds. The thickness of the boom plate in this component is 50 mm; during welding, the temperature in the heat-affected zone can reach 850°C. As the metal in that heated area undergoes phase changes, residual stresses are generated. These residues remain in the base material, not only causing deformation of the component but also affecting its performance and quality. 3 Process measures to overcome deformation of structural components 3.1 Correction of deformation in the rear frame of the grader Traditional methods involve using heat and external forces to address the twisting deformation of the rear frame of the PY160C type grader. Place the item flat on the workbench, prop up two or three of its corners to secure it to the bench, heat the stress concentration areas with a flame, and then use mechanical means to pull on the elevated corners in order to correct the distortion. But when we correct other secondary deformations, the warping deformation reappears. This method of repeated correction not only consumes a great deal of manpower and resources but also generates new residual stresses in the workpiece, posing a potential risk of deformation in the product later on. Through continuous practical exploration, we have re-evaluated the manufacturing process for the rear frame, adopting a fundamental approach of \"components first, then assemblies.\" The entire rear frame is divided into three components: the bearing housing, the left beam, and the right beam. Each of these components is further split into sub-components, with parts being the lowest level. The sequence for assembly, welding, and shaping is: parts → sub-components → components → rear frame assembly. This method essentially distributes all the heat and deformation incurred during the single welding of the rear frame over successive stages, correcting the deformation in separate parts, thereby reducing the deformation after final assembly. The rear frame manufactured using this process suffers from very little deformation; by eliminating the most severe warping, it was possible to easily meet the requirements specified in the drawings during its shaping. 3.2 Deformation of the clothing-carrying boom assembly We take measures from three aspects to overcome the welding deformation of the boom: First, we use additional restraints to limit the deformation of the boom; that is, two adjustable struts are installed at a distance of 200 mm from the support, and these struts press against the two boom plates from both sides in order to prevent the boom plates from deforming inward during welding ； Next is to reduce the welding line energy; for welds with a cross-sectional area of 200 mm2, a welding process of 4 layers and 12 passes in an alternating manner is adopted, as shown in Figure 3, where the numbers in the figure indicate the sequence of welding passes. Manual CO2 gas shielded arc welding was employed, and this approach proved effective in reducing the deformation of the boom ； Third, adopt the economical and simple flame correction method. The method of operation is as follows: Use an oxy-acetylene torch with a carbonizing flame to gradually heat the outer side of the boom plate, at the location corresponding to the welds with the support and the boom plate, from top to bottom. The heating rate should be 3–5 mm/min, and the heating temperature should be 750–800°C. For cooling, use clean water, applying it from bottom to top. When the deformation amount of dimension b exceeds 10 mm, the heating width should be 10–20 mm, and the heating depth should be 15–20 mm ； When the deformation amount is less than 10 mm, the heating width is 10–15 mm, and the heating depth is also 10–15 mm. By using the above methods, the form and position tolerances as well as the dimensions of the boom can meet the standard requirements. 4 Process strategies to reduce deformation of large structural components Large structural components in construction machinery are much more complex than those described in ordinary textbooks or technical materials, and the performance requirements for different such components also vary. Based on practical experience, we have identified several process strategies to overcome deformation in large structural components of construction machinery. (1) The structural design of the structural components themselves must be reasonable, that is, the three principles of weld design should be followed as much as possible: minimize the number of welds, minimize the cross-section of the welds, and ensure that the weld positions are symmetrical. (2) Choose welding methods with lower wire energy, including multi-pass welding and using CO2 automatic welding in place of gas welding or manual arc welding. (3) Select a reasonable welding sequence to ensure even heating of the workpiece. (4) For structural components with complex shapes and numerous components, the multi-layer component assembly and welding method is employed, involving multiple steps of assembly, welding, and finishing. (5) Eliminate residual stresses to ensure the long-term stability of structural components: a. Take measures for parts that undergo forming and leveling processes to ensure uniformity in the metal structure at the areas under stress. b. Heating above 800°C should be approached with caution to avoid inducing phase changes in the material. c. It is strictly prohibited to use the flame forming method to process parts or any part that has been formed by welding. d. Structural components should be tempered or naturally aged before processing. (6) Mechanical correction should be used as much as possible for simple parts. (7) For the deformation between components with constraints at both ends, mechanical means should be used to adjust it, and a certain application time of force is required. In summary, overcoming the deformation of large structural components in construction machinery is a topic with strong theoretical and practical aspects. The measures to address this deformation involve ensuring a balanced thermal field in the structural components and minimizing the generation of stress. By adhering to this guiding principle, we can continuously develop more effective process solutions in practical manufacturing scenarios. References: 1 Edited by Xi Tang. Welded Structures. Beijing: Machinery Industry Press, 1996. 2 Edited by Fu Rongbai. Flame Straightening of Steel. Beijing: Machinery Industry Press, 1996