Microstructure and Residual Stress Analysis of S355 Steel Laser-MIG Hybrid Welded Joints

Literature Overview

This study, published in the Transactions of the China Welding Institution (2020, Vol. 41, No. 6, pp. 12-18) by Yan Chunyan, Yi Si, Zhang Hao, Zhang Kezhao, and Gu Zhengjia from Hohai University, investigates the welding metallurgy and thermal stress behavior of 12 mm thick S355 low-alloy high-strength steel joints fabricated using laser-MIG hybrid welding at 9 kW laser power. Funded by the National Natural Science Foundation of China (Grant 51804097) and the Central Universities Basic Research Business Fee Special Fund (2017B17614), the research establishes a novel composite heat source model and employs SYSWELD finite element software to simulate temperature fields and residual stress distributions at three welding speeds: 1.0, 1.5, and 2.0 m/min. The work is significant because it bridges the gap between hybrid welding process design and residual stress prediction for low-alloy high-strength steels widely used in structural engineering, pressure vessels, and offshore platforms.

Core Technical Findings

Microstructure and Hardness Distribution

The most critical finding is that the coarse-grained heat-affected zone (CGHAZ) in all three welding speed conditions exhibits a martensitic microstructure with maximum hardness exceeding 350 HV. This is a direct consequence of the extremely high cooling rates associated with laser-MIG hybrid welding. The S355 base metal typically contains a ferrite-pearlite microstructure with hardness around 130-160 HV, so the CGHAZ represents a hardness increase of more than 2.2 times the base metal level. This hardening is driven by the rapid austenitization and subsequent diffusionless transformation during cooling, where the high thermal gradient suppresses ferrite nucleation and promotes martensite formation.

The hardness gradient from the CGHAZ into the fine-grained HAZ and weld metal is steep, creating a potential zone of mechanical heterogeneity that must be considered in structural integrity assessment. The martensitic CGHAZ, while providing high strength, also introduces brittleness and susceptibility to hydrogen-induced cracking, particularly in environments where residual hydrogen from the welding process has not fully diffused.

Composite Heat Source Model

The study proposes a double-ellipsoid plus three-dimensional cone composite heat source model to describe the energy distribution of the laser-MIG hybrid process. This model is a significant advancement over single-heat-source models because it accounts for the distinct energy characteristics of both the laser beam and the MIG arc:

• Laser component: Modeled as a double ellipsoid to capture the keyhole effect, with the front half representing the energy deposited ahead of the weld pool and the rear half representing the energy deposited behind the weld pool. The laser contributes high energy density (typically 10^6 to 10^7 W/cm²) with deep penetration.
• MIG arc component: Modeled as a three-dimensional cone to represent the arc's energy distribution, which is more diffuse than the laser but provides additional heat input and filler metal deposition. The arc energy density is typically 10^4 to 10^5 W/cm².

The composite model allows for more accurate prediction of weld pool geometry, temperature gradients, and solidification rates compared to simplified models. The key parameters in this model include the laser power (9 kW), laser spot diameter, arc power, arc radius, welding speed, and the relative position of the laser and arc with respect to the welding direction. The interaction zone where the laser and arc energies overlap is particularly important for understanding the weld pool dynamics.

Residual Stress Distribution

The SYSWELD simulations reveal that the equivalent residual stress levels are relatively high across all three welding speeds, with stress concentration occurring at the HAZ location. This is consistent with the fundamental principle that the maximum residual stress develops where the thermal gradient is most severe, which is typically at the fusion boundary and the CGHAZ.

A particularly noteworthy observation is the non-linear behavior when welding speed increases from 1.5 to 2.0 m/min: the tensile stress peaks increase only marginally, while the compressive stress peaks rise significantly. This phenomenon can be explained by the following mechanism: at higher welding speeds, the weld pool becomes more elongated and the cooling rate increases, leading to greater thermal contraction. However, the increased heat input per unit length is actually reduced at higher speeds, which means the overall thermal expansion is less. The net effect is that while the tensile stresses (which develop during cooling as the weld contracts) increase slightly due to the steeper thermal gradient, the compressive stresses (which develop in the surrounding material to balance the tensile stresses) increase more dramatically because the material adjacent to the weld is subjected to greater compressive strain without a proportional increase in thermal expansion to offset it.

Engineering Practice Integration

Implications for Welding Process Design

For engineers designing laser-MIG hybrid welding processes for S355 steel structures, several practical implications emerge from this study:

1. Welding speed selection: The transition from 1.5 to 2.0 m/min represents a critical threshold where the residual stress behavior changes qualitatively. If residual stress reduction is a priority, welding speeds below 1.5 m/min should be considered, even though this reduces productivity. If productivity is paramount, speeds above 2.0 m/min may be acceptable provided that post-weld stress relief measures are implemented.
2. Post-weld heat treatment: Given the martensitic CGHAZ with hardness exceeding 350 HV, post-weld heat treatment (PWHT) is strongly recommended for applications requiring toughness and resistance to cracking. A typical PWHT for S355 steel involves holding at 580-620°C for a duration proportional to the plate thickness (typically 1 hour per 25 mm of thickness). This treatment will temper the martensite, reduce hardness to 200-250 HV, and relieve a significant portion of the residual stresses.
3. Hydrogen control: The martensitic microstructure in the CGHAZ is susceptible to hydrogen-induced delayed cracking. Strict control of hydrogen content in the filler metal and shielding gas is essential. Using low-hydrogen flux-cored wires or solid wires with basic flux coatings, and ensuring proper preheating (typically 50-100°C for S355 steel) are critical measures.
4. Weld geometry optimization: The composite heat source model can be used to optimize the laser-arc spacing and overlap to achieve a desired weld geometry. A larger overlap between the laser and arc energy zones generally results in deeper penetration and a narrower weld, while a smaller overlap produces a wider, shallower weld.

Comparison with Conventional Welding

The laser-MIG hybrid process offers significant productivity advantages due to its high welding speed and deep penetration, but at the cost of higher residual stresses and harder HAZ microstructure. The engineering trade-off between productivity and joint integrity must be carefully evaluated for each application.

Key Questions and Reflections

The study raises several important questions that deserve further investigation. First, the effect of welding speed on the hydrogen embrittlement susceptibility of the martensitic CGHAZ is not addressed. Given that the cooling rate increases with welding speed, and that martensite is the most susceptible microstructure to hydrogen cracking, it is likely that the delayed cracking resistance decreases at higher welding speeds. Second, the long-term stability of the residual stress distribution under cyclic loading conditions is not examined. For structural applications subject to fatigue, the residual stress state can significantly affect fatigue life, and this warrants further research.

From a personal perspective, the composite heat source model developed in this study is a valuable tool, but its predictive accuracy depends heavily on the input parameters. In practice, the actual heat input distribution during laser-MIG hybrid welding can vary due to laser beam quality, arc stability, and gas shielding effectiveness. Engineers should validate the model predictions against experimental measurements for their specific equipment and process conditions before relying on them for process design.

Study Insights and Outlook

This research provides a solid foundation for understanding the welding metallurgy and residual stress behavior of S355 steel laser-MIG hybrid welded joints. The composite heat source model is a methodological advance that can be adapted to other hybrid welding processes and material combinations. The finding that welding speed has a non-linear effect on residual stress components is particularly important for process optimization, as it identifies a critical speed threshold beyond which the stress state changes qualitatively. Future work should focus on integrating the thermal-mechanical simulations with fracture mechanics analysis to predict the actual structural performance of hybrid welded joints under service conditions. Additionally, the development of active welding strategies, such as in-situ post-weld heat treatment or vibration-assisted welding, could potentially mitigate the high residual stresses without sacrificing productivity. The practical value of this study lies in its ability to guide engineers in making informed decisions about welding speed selection, post-weld treatment requirements, and quality control measures for laser-MIG hybrid welding of low-alloy high-strength steels.