Stress Analysis of Flat Plate Surfacing Solidification Based on ANSYS

Literature Overview

The paper by Hao Zilong and Shi Guanglin from the School of Mechanical Engineering, Guangxi University of Science and Technology, published in the Journal of Guangxi University of Science and Technology in 2016, presents a three-dimensional dynamic finite element simulation of the temperature field and stress field during the surfacing welding process on a flat plate made of 25 steel. The research was supported by the Guangxi Science Research and Technology Development Program (Grant No. Guike Gong 1348012-18). The study employs the ANSYS finite element analysis platform with a Gaussian surface heat source model and uses the APDL (ANSYS Parametric Design Language) function to implement the moving welding heat source loading. The core objective is to investigate the distribution characteristics of residual stresses and to evaluate the influence of welding process parameters on the hot cracking tendency of the surfacing weld.

Core Technical Methodology

The numerical model is built on a solid flat plate of 25 steel, a low-carbon structural steel commonly used in general engineering applications. The heat source is modeled using a Gaussian function distribution on the surface, which is a widely accepted approach for representing the energy input of arc welding processes. The Gaussian heat source model is expressed by the following relationship:

• Heat flux density at any point on the surface: $q(r) = \frac{6\sqrt{3}Q}{\pi r_0^2} \exp\left(-\frac{3r^2}{r_0^2}\right)$
• Where $Q$ represents the effective heat input, $r_0$ is the equivalent radius of the heat source, and $r$ is the radial distance from the heat source center.

The APDL scripting capability of ANSYS is utilized to simulate the sequential movement of the welding arc along the weld path, enabling a time-dependent thermal-mechanical coupling analysis. This approach allows the model to capture the progressive heating, melting, solidification, and cooling phases of the welding process, which is critical for accurately predicting residual stress distributions.

The material properties considered in the simulation include temperature-dependent thermal conductivity, specific heat capacity, thermal expansion coefficient, and elastic-plastic stress-strain relationships. The 25 steel material properties are defined over the full temperature range from room temperature to the melting point, ensuring that the phase transformation behavior during cooling is adequately represented.

Interpretation of Key Findings

The simulation results reveal distinct spatial distributions of longitudinal and transverse residual stresses, which is a finding of significant practical importance. The maximum longitudinal stress is concentrated in the weld zone, while the maximum transverse stress is located at the fusion zone boundary. This asymmetry can be explained by the following metallurgical and mechanical mechanisms:

The longitudinal stress concentration in the weld zone is attributed to the fact that the weld metal solidifies in a constrained state, with the surrounding base metal and previously deposited weld metal acting as mechanical restraints. As the weld metal cools from the liquidus temperature to room temperature, it undergoes significant thermal contraction, generating tensile stresses that are maximized along the weld centerline where the temperature gradient is most severe.

The transverse stress maximum at the fusion zone is a direct consequence of the thermal gradient between the weld metal and the base metal. The fusion zone experiences the most severe thermal mismatch, where the weld metal contracts significantly while the base metal remains relatively stable. This creates a stress concentration at the interface, which is particularly relevant for assessing the risk of hot cracking, as the fusion zone is where the last-to-freeze liquid films are most vulnerable to solidification cracking.

Process Parameter Influence on Hot Cracking Tendency

The study systematically examines the effects of welding current and welding speed on the hot cracking tendency, and the results are highly informative for process optimization. The following table summarizes the key findings:

The reduction of welding current leads to a lower peak temperature and a narrower weld pool, which results in a reduced thermal gradient and lower residual stress amplitudes. This directly translates to a decreased hot cracking tendency because the solidification cracking susceptibility is strongly dependent on the stress state at the solidification front. When the longitudinal stress is lower, the tensile stress required to open solidification cracks is less likely to be achieved.

The increase in welding speed has a similar beneficial effect because it reduces the total heat input per unit length of weld. A faster welding speed means that the weld metal spends less time at elevated temperatures, which reduces the extent of thermal softening of the base metal and limits the magnitude of thermal contraction stresses. However, it is important to note that excessively high welding speeds can lead to incomplete penetration, inadequate fusion, and other welding defects, so there exists an optimal welding speed window that balances crack resistance with sound weld quality.

Engineering Practice Implications

From a practical standpoint, the findings of this study have direct relevance to surfacing welding operations in industries such as power generation, petrochemical processing, and heavy machinery manufacturing. Surfacing welding is widely used to restore worn surfaces, provide corrosion-resistant overlays, and improve the tribological properties of critical components. The residual stress distribution and hot cracking tendency identified in this study are particularly relevant for surfacing applications on thick plate components where the base metal provides significant restraint to the weld metal.

The observation that longitudinal stress is maximized in the weld zone and transverse stress is maximized at the fusion zone has implications for the design of post-weld stress relief treatments. If the primary concern is hot cracking, then process parameter optimization to reduce the longitudinal stress in the weld zone should be the priority. If the concern is cold cracking or fatigue cracking at the fusion zone, then the transverse stress at the fusion boundary should be the focus of mitigation efforts.

A practical process recommendation derived from this study is to use lower welding currents and higher welding speeds for surfacing applications where hot cracking is a concern. For example, in a typical 25 steel surfacing application, reducing the welding current from 300 A to 250 A while increasing the welding speed from 80 mm/min to 100 mm/min could significantly reduce the hot cracking tendency while still maintaining adequate weld bead quality.

Key Questions and Reflections

Several questions arise from the findings of this study that warrant further investigation. First, the study focuses on a single-layer surfacing deposit on a flat plate, but in practical applications, multi-layer surfacing is often required. The residual stress state in multi-layer surfacing is significantly more complex because each subsequent layer is deposited on a pre-stressed substrate, and the stress from previous layers interacts with the stress from the current layer. Understanding the cumulative stress evolution in multi-layer surfacing is critical for predicting the final residual stress state and crack susceptibility.

Second, the study uses a Gaussian surface heat source model, which is appropriate for surface heating but may not fully capture the three-dimensional nature of the heat input in surfacing welding. A double-ellipsoidal or Goldak-type heat source model that accounts for the penetration depth and the asymmetry of the heat input between the leading and trailing edges of the weld pool might provide more accurate predictions. The choice of heat source model is a critical modeling decision that can significantly influence the accuracy of the stress predictions.

Third, the study does not explicitly consider the effect of weld metal composition on the hot cracking susceptibility. The hot cracking tendency is not solely determined by the stress state but also by the solidification behavior of the weld metal, including the solidification range, the presence of low-melting-point phases, and the strain rate sensitivity of the solidification front. A comprehensive assessment of hot cracking risk should integrate both the mechanical stress state and the metallurgical susceptibility.

Study Insights and Implications

This study provides a solid foundation for understanding the residual stress distribution and hot cracking behavior in surfacing welding of 25 steel flat plates. The use of finite element analysis with a moving Gaussian heat source model is a well-established and reliable approach for welding process simulation, and the results obtained are consistent with the general understanding of welding residual stress formation.

The key insight from this study is that process parameter optimization can be a powerful tool for reducing hot cracking tendency without requiring changes to the material composition or the welding procedure specification. By systematically varying the welding current and welding speed, engineers can identify the optimal process window that minimizes the hot cracking risk while maintaining acceptable weld quality.

For engineering practice, the recommendation is to conduct a systematic process parameter study for any critical surfacing application, using both numerical simulation and experimental validation. The simulation results should be used to guide the selection of initial process parameters, and the experimental results should be used to refine and validate the model. This iterative approach ensures that the final welding procedure is optimized for both quality and crack resistance.