Effect of Arc Power on Microstructure and Mechanical Properties of 316L Austenitic Stainless Steel in MIG Arc Additive Manufacturing

Literature Overview

This research by Chen Xiaohui, Zhang Shuquan, Ran Xianzhe, and Huang Zheng, published in The Journal of Welding (2020, Vol. 41, Issue 5, pp. 42-49), investigates the influence of arc power on the microstructure, mechanical properties, and fracture behavior of 316L austenitic stainless steel deposited via high-power MIG arc additive manufacturing. The work was conducted at the National Engineering Laboratory for Additive Manufacturing of Large Metal Components, Beihang University, and Shanghai Commercial Aircraft Engine Manufacturing Co., Ltd., supported by multiple national research programs. This study addresses a critical challenge in arc additive manufacturing: how to balance deposition efficiency with mechanical property requirements.

Core Technical Findings

The study systematically varies arc power from 3763 W to 8400 W and examines the resulting microstructural evolution and mechanical property changes. The key findings reveal a clear trend: increasing arc power leads to larger grain sizes, reduced delta phase content, increased sigma phase formation, and a corresponding degradation in mechanical properties.

Microstructural Analysis and Phase Evolution

The microstructure of the 316L deposits exhibits columnar grain growth throughout the build, which is characteristic of the directional solidification conditions in arc additive manufacturing. Two intermetallic phases are identified: delta ferrite and sigma phase, both distributed in a worm-like morphology within the gamma austenitic matrix.

The delta ferrite phase is found both within grains and at grain boundaries, and it serves a strengthening function through solid solution and precipitation hardening mechanisms. As arc power increases, the thermal input per unit volume rises, leading to slower cooling rates and longer time at elevated temperatures. This extended thermal exposure promotes delta ferrite dissolution and, more critically, sigma phase precipitation at grain boundaries.

The sigma phase (Cr23C6) is a brittle intermetallic compound that forms preferentially at grain boundaries during prolonged exposure to temperatures in the range of 700-900°C. Its formation at high arc power is attributed to the cumulative thermal cycling inherent in additive manufacturing, where each subsequent layer reheats previously deposited material.

Fracture Behavior Transition

A particularly important finding is the transition in fracture mode from transgranular dimple fracture at lower arc powers to intergranular dimple fracture at 8400 W. This transition is directly correlated with the increased sigma phase formation at grain boundaries. The sigma phase weakens grain boundary cohesion, providing preferential crack propagation paths. From a fracture mechanics perspective, this represents a shift from a more damage-tolerant transgranular failure mode to a more brittle intergranular failure mode, which has significant implications for component design and safety assessment.

Process Optimization Recommendations

Based on the findings of this study, the following process optimization guidelines can be derived for 316L arc additive manufacturing:

1. Arc power should be kept within the lower range (approximately 3763-5000 W) to maintain adequate delta ferrite content, minimize sigma phase formation, and preserve transgranular fracture behavior.
2. Heat input management is critical: While higher arc power improves deposition efficiency, it comes at the cost of mechanical properties. A trade-off analysis between productivity and quality is essential for each application.
3. Post-build heat treatment may be required for high-power builds to dissolve sigma phase and restore grain boundary cohesion, though this adds cost and complexity to the manufacturing process.
4. Layer thickness and travel speed should be optimized in conjunction with arc power to achieve the desired cooling rate and phase distribution.

Engineering Practice Integration

In the context of aerospace and power generation applications where 316L is commonly used for corrosion resistance and high-temperature performance, this research provides essential guidance for additive manufacturing process qualification. The data on mechanical property degradation with increasing arc power must be incorporated into process specification documents and quality assurance protocols. For applications requiring high toughness and damage tolerance, such as nuclear components or pressure vessels, the lower arc power regime should be specified as the standard process window.

Study Insights and Implications

The most valuable contribution of this work is the establishment of a clear quantitative relationship between arc power and mechanical properties in 316L arc additive manufacturing. The identification of sigma phase as the primary degradation mechanism at high arc power provides a metallurgical explanation that guides process optimization. Engineers should recognize that additive manufacturing process parameters must be selected not only for deposition efficiency but also for metallurgical integrity. The transition from transgranular to intergranular fracture at high arc power is a warning signal that should be incorporated into failure analysis frameworks for additively manufactured components. This research underscores the importance of metallurgical characterization in additive manufacturing qualification and the need for application-specific process windows rather than generic parameter settings.