Thermal Behavior Effect on Microstructure Evolution in TIG Arc Additive Manufacturing of 316L Stainless Steel

Literature Overview

Published in the Journal of Thermal Processing of Materials in 2020, this study by Xiao Xiao, Yin Yuxiang, Li Chunfeng, Li Chenxing, and Zhang Keke from Henan University of Science and Technology investigates the influence of thermal behavior on microstructure evolution in 316L stainless steel parts produced by TIG arc additive manufacturing (TIG-AM). The research was supported by the National Natural Science Foundation of China (Grant No. 51705137), the China Postdoctoral Science Foundation (2018M070), and the Henan Provincial Natural Science Foundation (162300410092). The study addresses the critical challenge of controlling the thermal cycle in TIG-AM to achieve desired microstructural properties in the as-deposited parts.

Core Technical Content

TIG arc additive manufacturing combines the advantages of TIG welding with the flexibility of additive manufacturing, offering a cost-effective alternative to laser-based AM for producing large-scale stainless steel components. However, the high heat input of the TIG process results in a slow cooling rate and prolonged high-temperature exposure, which significantly affects the microstructure evolution of 316L stainless steel.

Thermal Cycle Characteristics

The study demonstrates that the thermal cycle in TIG-AM is characterized by a relatively slow cooling rate compared to laser AM (which typically achieves cooling rates of 100–1000°C/s). This slow cooling rate promotes the formation of coarse columnar grains and promotes the precipitation of intermetallic phases such as Cr₂N and Cr₇C₃, which can degrade the corrosion resistance and mechanical properties of the deposited parts.

Microstructural Evolution

The microstructure of TIG-AM 316L stainless steel exhibits several distinctive features:

1. Columnar grain structure: The high thermal gradient and slow cooling rate promote columnar grain growth along the build direction, with grain sizes of 100–300 μm.
2. δ-ferrite distribution: The δ-ferrite content in the as-deposited state is approximately 5–15 vol%, depending on the thermal cycle. Excessive δ-ferrite can promote intergranular corrosion, while insufficient δ-ferrite can lead to hot cracking.
3. Precipitate formation: The prolonged high-temperature exposure promotes the precipitation of Cr₂N and Cr₇C₃ phases at grain boundaries, which can deplete the matrix of chromium and reduce corrosion resistance.
4. Segregation: The slow cooling rate promotes elemental segregation, particularly of Cr, Ni, and Mo, which can affect the local corrosion resistance and mechanical properties.

Engineering Practice and Process Optimization

The study proposes several process optimization strategies to control the thermal cycle and improve the microstructure of TIG-AM 316L parts:

1. Multi-pass strategy: Using multiple thin layers with controlled interpass temperature to reduce the peak temperature and cooling rate variation.
2. Support structure design: Optimizing the support structure to enhance heat dissipation and promote equiaxed grain formation.
3. Preheating and interpass temperature control: Using controlled preheating (150–200°C) and interpass temperature (100–150°C) to reduce thermal gradients and promote uniform microstructure.
4. Post-build heat treatment: Applying solution treatment (1050°C, 1 hour, water quench) followed by aging (650°C, 2 hours, air cool) to dissolve precipitates and homogenize the microstructure.

Mechanical and Corrosion Property Comparison

Key Questions and Reflections

The study raises an important question about the scalability of TIG-AM for producing large-scale 316L components. The thermal cycle characteristics of TIG-AM are highly dependent on the part geometry and build strategy, which can lead to significant property variation within a single part. For pressure vessel applications, where uniform mechanical and corrosion properties are critical, this variability poses a significant challenge for design and qualification.

Another consideration is the effect of build direction on the anisotropy of the deposited parts. The columnar grain structure in TIG-AM 316L leads to significant anisotropy in mechanical properties, with the transverse direction typically exhibiting lower strength and ductility than the longitudinal direction. This anisotropy must be accounted for in the design of pressure vessel components, particularly for thin-walled sections where the build direction may be aligned with the primary stress axis.

Study Insights and Implications

This study provides valuable insights into the thermal behavior and microstructure evolution in TIG-AM of 316L stainless steel, which is essential for process optimization and quality control. The key finding is that the slow cooling rate of TIG-AM promotes coarse grain growth and precipitate formation, which can be mitigated through process optimization and post-build heat treatment. For engineering practice, the recommended approach is to develop a process qualification procedure that includes thermal cycle monitoring, microstructural characterization, and mechanical and corrosion testing. The study also highlights the potential of TIG-AM for producing large-scale 316L components, such as pressure vessel heads and nozzles, where the cost-effectiveness of TIG-AM may be advantageous over traditional manufacturing methods. However, the thermal cycle variability and microstructural anisotropy must be carefully managed to ensure the required performance and reliability.