This paper investigates the severe warping and flatness deviation of the large shear cover, a primary load‑bearing component of the four‑wheel alignment scissor lift, after welding stiffeners. Through thermo‑mechanical coupling analysis of welding mechanisms, it is clarified that transient stress and residual stress induced by non‑uniform heat input constitute the root cause of bending deformation about the neutral axis. Based on thermo‑elasto‑plastic theory and the welding deformation prediction model, a theoretical deformation of 7.4 mm is calculated. This agrees well with the measured value of 8.5 mm, verifying the accuracy of the analytical model. A full‑process deformation suppression strategy featuring “pre‑control – in‑process regulation – post‑treatment” is proposed. Its core innovation integrates quantitative reverse deformation technology based on curvature prediction, a multi‑point and multi‑axis hydraulic rigid clamping system, and skip welding sequences with precise heat input management. Furthermore, stamping forming process optimization is introduced as an important method to enhance structural stiffness before welding. Optimized stamping die design improves the initial rigidity of the plate and reduces stress susceptibility during welding. Process tests demonstrate that the integrated scheme stabilizes welding deformation of the large shear cover within 1.8–2.5 mm, down from the original 8.5 mm. This performance exceeds the design requirement of 3 mm, effectively guaranteeing the flatness of the lift platform and the accuracy of four‑wheel alignment. The findings provide general engineering guidance for precision welding manufacturing of large thin‑walled box structural parts.

With increasing electrification of automotive chassis systems and stricter alignment accuracy requirements, integrated four‑wheel alignment lifts have become core equipment for the high‑end vehicle service market. Measurement accuracy directly relies on the macroscopic geometric precision of the lift platform (large shear cover), particularly its flatness. For the 4.6 m scissor lift manufactured by our company, the flatness tolerance of the large shear cover after stiffener welding is specified as ≤3 mm.

The original process is as follows: a 7.5 mm Q235B steel plate is bent and mechanically straightened to achieve an initial flatness of 1 mm. Stiffener plates are then welded onto its inner side by CO₂ gas metal arc welding using intermittent welds (each weld segment: 150 mm long, with a 300 mm spacing between segments). Nevertheless, this process causes pronounced upward bending warpage with a measured deformation of up to 8.5 mm, severely limiting product yield and functional performance. This work aims to fundamentally address the welding deformation control challenge of this large structural component through systematic theoretical and experimental research.

Analysis of Welding Deformation Causes

In‑depth analysis of deformation mechanisms

Welding deformation essentially arises from irreversible plastic strain accumulation driven by combined thermal strain, phase‑transformation strain and restraint stress during localized non‑uniform heating and cooling. For the large shear cover, deformation manifests mainly as a coupling of transverse shrinkage and angular distortion. Transverse shrinkage dominates bending behaviour: welds are located below the sectional neutral axis. Their transverse shrinkage during cooling is restrained by surrounding cold metal, generating asymmetric longitudinal tensile stress relative to the neutral axis and producing a bending moment that warps the component upward.

Structural stiffness and boundary effects: At a span of 4.6 m, the component has a relatively low sectional moment of inertia and insufficient bending stiffness. Welding stress is therefore readily released in the form of macroscopic bending deformation.

Preliminary estimation is carried out using the classical shrinkage force model:

where: \(A_W\) — Cross‑sectional area of the weld A — Cross‑sectional area of the component \(\Delta L\) — Shrinkage deformation L — Length of the component \(k_1\) — Correction factor; CO₂ gas metal arc welding is adopted in this company, and \(k_1\) is taken as 0.043.

Calculated by the above formula:

This theoretical value follows the same trend as the measured value of 8.5 mm (Figure 1). The deviation arises because the model does not fully account for the intermittent thermal shock effect of staggered welding and the dynamic variation of structural restraint. For more accurate prediction, the thermo‑elasto‑plastic finite element method is adopted for numerical simulation. Its governing equations include the energy equation, constitutive equation and equilibrium equation, which can dynamically reproduce the welding temperature field, stress field and final deformation, providing a digital foundation for subsequent process optimization.

Figure 1 Measured Gap after Welding

Design of Systematic Deformation Control Scheme

Based on the principle of prevention first, correction second, the following multi‑level control system is established.

Pre‑welding Active Control Strategies

(1) Optimization of stamping forming process During plate bending and forming, the initial rigidity and geometric accuracy of the plate are effectively improved by optimizing the stamping die design and adopting a multi‑step incremental forming process. By controlling forming force and forming speed during stamping, the concentration of residual stress inside the material is reduced, thus lowering the deformation susceptibility in the subsequent welding process. Moreover, the component after stamping has a more uniform stress distribution, offering a more stable structural foundation for subsequent welding.

(2) Quantitative reverse deformation technology Principle: An initial deformation opposite in direction and comparable in magnitude to the expected welding deformation is prefabricated before welding to offset post‑welding deformation. Implementation: Based on measured and simulation data, the optimal reverse deformation is determined as 10 mm (pre‑bending radius of curvature \(R \approx 265\ \text{m}\)). A dedicated hydraulic die is used for batch precision pre‑bending.

(3) Multi‑point and multi‑axis rigid clamping system

1. A modular welding fixture is designed, with the core components as follows:
2. High‑rigidity base platform (flatness ≤ 0.1 mm/m). Array hydraulic clamping mechanisms: arranged symmetrically within 150 mm on both sides of welds, providing normal clamping force ≥5 kN/m² to effectively suppress transient angular distortion during welding.
3. Lateral positioning modules: restrict the degrees of freedom of the component in the horizontal plane. The clamping constraint can only be released after the weld cools down below 80 ℃.

(4) Optimization of tack welding The tack weld spacing is reduced from 450 mm to 280 mm, and the tack weld length is increased to 30 mm. This ensures “zero‑gap” fitting between the stiffener plate and base metal and improves the overall initial rigidity.

Precise Regulation during Welding

(1) Through process tests, the welding parameters are optimized as: current \(I=(180\pm10)\ \text{A}\), voltage \(U=(22\pm1)\ \text{V}\), welding speed \(v=35\ \text{cm/min}\). The heat input is calculated accordingly:

 

The volume of the molten pool and the width of the heat‑affected zone are effectively controlled.

(2) Dynamic thermal field distribution strategy — skip welding sequence Instead of conventional sequential welding, a symmetric skip welding sequence is adopted (Figure 2). This sequence spatially disperses the instantaneous heat sources and staggers the welding in time, avoiding local heat accumulation. It enables partitioned counteraction of welding stress inside the long component and markedly reduces the tendency of bending deformation.

Figure 2 Schematic of Optimized Symmetric Skip Welding Sequence

 

Post‑welding Residual Deformation Correction

For a very small number of workpieces exceeding tolerances, the cold mechanical straightening method based on the three‑point bending principle is adopted. On a large hydraulic press, overpressure is applied to the convex region of the component with deflection as feedback. Reverse plastic elongation is generated in this area to eliminate residual bending.

Scheme Implementation and Effect Verification

A 3D scanner is used for full‑dimensional scanning of actual workpieces. The contour map of upper‑surface flatness obtained (Figure 3) visually shows the deformation under the new process. The comparison between the original and new processes is shown in Table 1.

Table 1 Comparison between Original and New Processes

	Original Process	New Process
Pre-deformation	Pre-deformation	No pre-deformation
Fixture	General Fixture	Special Fixture
Welding Sequence	Sequential Welding	Skip Welding
Deformation	8.5mm	2.2mm

 

Conclusions

This study verifies that deformation control of large thin‑walled welded structures must start from its mechanical nature and adopt a systematic engineering approach with multi‑link coordination. The integrated scheme proposed in this work, with quantitative reverse deformation and multi‑point multi‑axis rigid clamping as the core and precise heat input as well as optimized welding sequence as regulation measures, can efficiently and stably limit the welding deformation of the large shear cover within 2.2 mm from the original 8.5 mm. This scheme successfully breaks through the product quality bottleneck and brings remarkable economic benefits to the enterprise. Future work will introduce an on‑line stress monitoring system and adaptive welding robots to realize real‑time perception and active compensation of welding deformation, advancing toward intelligent and digital welding manufacturing. Meanwhile, further research will be carried out on the hybrid stamping‑welding manufacturing process. By proactive stress design during the stamping stage, a more favorable initial stress state before welding can be obtained so as to suppress deformation at the source.