Temperature Field Numerical Simulation of MIG Welding Rapid Forming

Literature Overview

The 2009 paper by Zhao Jixun, Zhu Sheng, Meng Fanjun, and Li Chao, published in Welding Technology, presents a numerical simulation study of the temperature field distribution during MIG welding-based rapid forming (additive manufacturing) on 45 steel plates. Funded by the National Natural Science Foundation of China and the National 973 Program, this research investigates the thermal behavior of layer-by-layer deposition welding, comparing interval deposition and sequential deposition strategies.

Core Technical Analysis

Simulation Methodology

The study employed ANSYS software with APDL (ANSYS Programming Language) to simulate the temperature field during single-layer deposition welding on 45 steel plates. The key technical features of the simulation include:

1. Moving heat source model: The welding heat source was modeled as a moving Gaussian or double-ellipsoidal heat source, representing the heat input from the MIG welding arc.
2. Element birth and death technique: The "element birth and death" technique was used to simulate the progressive addition of material as each layer was deposited. This technique activates (births) new elements as the weld material is added and deactivates (kills) elements that are not yet part of the model.
3. Sequential layer deposition: The simulation tracked the temperature evolution as each layer was deposited, capturing the thermal history of previously deposited layers.

Two Deposition Strategies Compared

The study compared two deposition strategies:

Temperature Field Analysis

The simulation results revealed significant differences between the two strategies:

Interval Deposition:

• Lower peak temperature in previously deposited layers
• Smaller temperature gradient across the deposition cross-section
• Shorter time spent at high temperatures
• Reduced risk of austenite grain coarsening
• Better microstructural refinement potential

Sequential Deposition:

• Higher peak temperature in previously deposited layers
• Larger temperature gradient across the deposition cross-section
• Longer time spent at high temperatures
• Increased risk of austenite grain coarsening
• Potential for detrimental microstructural changes

Microstructural Implications

The study specifically addresses the effect of thermal history on austenite grain size. In 45 steel (a medium-carbon steel), the austenite grain size at the time of transformation is a critical factor determining the final microstructure and mechanical properties. Large austenite grains lead to coarse martensite or pearlite structures with inferior mechanical properties, while fine austenite grains produce fine microstructures with improved strength and toughness.

The interval deposition strategy, by reducing the peak temperature and high-temperature dwell time in previously deposited layers, helps maintain finer austenite grains. This is because the thermal cycles experienced by previously deposited layers are less severe, reducing the driving force for grain growth.

Heat Source Modeling

The moving heat source model used in the simulation is critical for accurately capturing the thermal behavior of the welding process. The heat source parameters—including heat input rate, heat source dimensions, and travel speed—must be calibrated against experimental data to ensure simulation accuracy. For MIG welding, the heat input is typically in the range of 1-3 kW, with travel speeds of 200-1000 mm/min depending on the application.

Engineering Practice Implications

Process Design for Welding-Based Additive Manufacturing

The findings from this study have direct implications for the design of welding-based additive manufacturing (WBAM) processes:

1. Interpass cooling: The interval deposition strategy suggests that controlled cooling between layers is beneficial for microstructural quality. Engineers should design interpass cooling strategies to reduce peak temperatures in previously deposited layers.
2. Layer thickness optimization: The thermal behavior depends on layer thickness. Thinner layers may require shorter intervals but provide better microstructural control. Thicker layers may require longer intervals but offer higher deposition rates.
3. Travel speed considerations: Higher travel speeds reduce heat input per unit length but may require more passes for the same deposition volume. The optimal travel speed balances deposition rate with thermal control.
4. Preheat management: For thick deposits, preheat may be necessary to reduce thermal gradients and prevent cracking. However, excessive preheat can increase peak temperatures and promote grain coarsening.

Comparison with Conventional Welding

Residual Stress Considerations

While the study focuses on temperature fields, the thermal history directly influences residual stress development. The interval deposition strategy, by reducing thermal gradients, should also reduce residual stresses. This is particularly important for WBAM applications where dimensional accuracy and fatigue performance are critical.

Key Reflections

This 2009 study was published at a time when welding-based additive manufacturing was an emerging technology. The findings regarding the superiority of interval deposition for microstructural quality have been validated by subsequent research and are now incorporated into WBAM process design guidelines. The use of element birth and death techniques in ANSYS for simulating additive manufacturing processes has become a standard approach in the field.

One limitation of this study is the focus on single-layer deposition. In practice, WBAM processes involve multiple layers, and the thermal history of each layer depends on the deposition sequence and strategy. The thermal coupling between layers becomes increasingly complex as the number of layers increases, and more sophisticated simulation models are needed to capture these effects accurately.

Another reflection concerns the validation of simulation results. While numerical simulation provides valuable insights, experimental validation is essential for process development. The study should be accompanied by experimental measurements of temperature fields, microstructures, and mechanical properties to validate the simulation predictions. Engineers should always cross-reference simulation results with experimental data before making process decisions.

The study also raises questions about the scalability of WBAM processes. As the part size increases, the thermal behavior becomes more complex due to the interaction between the heat source and the part geometry. The interval deposition strategy may need to be adapted for different part sizes and geometries, and the optimal parameters may vary significantly between small and large parts.

Summary

This 2009 study by Zhao Jixun and colleagues provides foundational insights into the thermal behavior of MIG welding-based rapid forming processes. The demonstration that interval deposition produces lower peak temperatures, smaller thermal gradients, and shorter high-temperature dwell times than sequential deposition offers practical guidance for process design. The implications for austenite grain size control and microstructural quality are particularly valuable for engineers developing WBAM processes for structural applications. For practitioners in welding-based additive manufacturing, this study reinforces the importance of thermal management in achieving the desired microstructure and mechanical properties, and provides a simulation methodology that can be adapted for more complex geometries and process configurations.