TIG Arc Additive Manufacturing of 308L Stainless Steel Microstructure and Mechanical Properties

Literature Overview

The paper by Huang Jialei and colleagues from Jiangsu University of Technology, published in Hot Working Technology in 2023, presents a systematic study of the microstructure and mechanical properties of 308L austenitic stainless steel components produced via TIG arc additive manufacturing (TIG-AM). The research focuses on straight-wall specimens fabricated layer-by-layer using TIG welding as the deposition process. The study integrates solidification microstructure prediction with experimental verification through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis, providing a comprehensive understanding of the relationship between processing parameters and final material properties.

Additive Manufacturing Process Characteristics

TIG arc additive manufacturing differs fundamentally from conventional welding in several respects:

• Layer-by-layer deposition: Each weld pass creates a new layer that must bond metallurgically with the previously deposited layer, creating a complex thermal history that varies significantly from layer to layer.
• Repeated thermal cycling: The material near the base experiences multiple heating and cooling cycles as subsequent layers are deposited above, leading to grain coarsening and phase transformations that do not occur in single-pass welding.
• No consumable electrode: The TIG process uses a tungsten electrode with separate filler wire feeding, providing clean, contamination-free deposition suitable for high-purity materials.

The 308L grade selected for this study is a low-carbon austenitic stainless steel specifically designed for welding applications, with carbon content limited to 0.03% maximum to minimize sensitization risk. This makes it an excellent choice for additive manufacturing where the repeated thermal cycling could otherwise promote chromium carbide precipitation at grain boundaries.

Microstructural Analysis and Solidification Prediction

The study employed the solidification mode prediction method for austenitic stainless steels to forecast the expected microstructure, then verified the predictions experimentally. The predicted and actual microstructures both consist of austenite (γ) as the primary phase with a small amount of delta ferrite (δ), which is consistent with the well-established solidification behavior of 308L stainless steel under TIG welding conditions.

The microstructural characterization revealed important spatial variations within the deposited component:

The bottom layer exhibits the highest hardness due to the cumulative thermal effects of multiple subsequent deposition cycles. Each new layer acts as a heat source that reheats the underlying layers, potentially causing grain growth and precipitation coarsening. The top layer, experiencing only a single thermal cycle, retains a finer grain structure with higher dislocation density, contributing to its elevated hardness.

Mechanical Performance Assessment

The tensile testing results demonstrate that the additively manufactured 308L stainless steel achieves mechanical properties comparable to or exceeding those of conventionally wrought material:

The ductile fracture mode is particularly significant, as it confirms that the additive manufacturing process did not introduce embrittling features such as excessive delta ferrite, carbide precipitation, or porosity that would promote brittle fracture. The elongation value of 41.7% is notably high and indicates excellent formability potential, which is important for downstream forming operations.

Quality Control and Process Monitoring Considerations

From a manufacturing quality assurance perspective, several critical monitoring points must be established for TIG-AM production:

1. Layer-to-layer bonding quality. The metallurgical bond between consecutive layers is the weakest link in additively manufactured components. Incomplete melting or cold laps between layers would create internal defects that are difficult to detect by conventional NDT methods.
2. Porosity control. Gas porosity and lack-of-fusion porosity are the most common defects in TIG-AM. The shielding gas flow rate, wire feed speed, and travel speed must be tightly controlled to minimize porosity formation.
3. Dimensional accuracy and distortion. Cumulative thermal distortion across multiple layers can lead to significant dimensional deviations. Real-time monitoring of layer geometry and in-process correction strategies are essential for maintaining dimensional tolerance.
4. Residual stress management. The sequential deposition of layers creates complex residual stress fields that can lead to warping or cracking, particularly in thick components. Stress-relief heat treatment after fabrication may be necessary.

Study Insights and Practical Implications

This research demonstrates that TIG arc additive manufacturing can produce 308L stainless steel components with mechanical properties that rival or exceed those of conventionally manufactured material. The agreement between predicted and actual microstructures validates the use of solidification theory for process design and optimization in additive manufacturing. For engineering applications, the high ductility achieved suggests that TIG-AM components could be used in applications requiring both strength and formability, such as pipe fittings, pressure vessel components, and complex structural parts. The key challenge moving forward is scaling this process from laboratory specimens to production-scale components while maintaining consistent quality across large build volumes. The spatial variation in hardness and microstructure observed in this study highlights the need for thorough material characterization at multiple locations within any production component.