CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Mechanical Properties of 308L Stainless Steel Fabricated by TIG Arc Additive Manufacturing
Literature Overview
This 2023 study by Huang Jialei, Chen Jufang, Jiang Yujie, Li Xiaoping, and Lei Weining, conducted at Jiangsu University of Technology, investigates the microstructure and mechanical properties of 308L stainless steel deposited by TIG arc additive manufacturing (AM). This research represents an important intersection between traditional welding technology and modern additive manufacturing, demonstrating how conventional TIG welding equipment can be adapted for layer-by-layer material deposition. The work addresses the microstructural evolution, mechanical property gradients, and process-material interactions that characterize AM deposits.
Technical Background and Process Description
TIG arc additive manufacturing, also known as wire arc additive manufacturing (WAAM) when wire feed is used, involves depositing material layer by layer using a TIG arc as the heat source and a continuously fed wire as the filler. Unlike conventional welding, where the objective is to join two pre-existing parts, AM builds a three-dimensional object from a substrate. The process uses a robotic arm or gantry system to control the deposition path, with each layer typically 1-3 mm thick.
308L stainless steel (a low-carbon variant of 308) is selected for AM applications because of its excellent weldability, low carbon content (reducing intergranular carbide precipitation), and good corrosion resistance. The wire composition typically includes Cr 19-22%, Ni 9-12%, C ≤ 0.03%, and balance Fe.
| Parameter | Typical Range for TIG AM | Effect on Deposit Quality |
|---|---|---|
| Arc current | 150-300 A | Controls melt pool size and deposition rate |
| Wire feed rate | 2-6 m/min | Controls deposition rate and dilution |
| Travel speed | 50-200 mm/min | Controls bead width and height |
| Layer thickness | 1-3 mm | Controls interlayer cooling rate |
| Shielding gas | Ar or Ar + 5% CO₂ | Controls arc stability and oxidation |
| Substrate temperature | 50-200 °C | Controls interlayer cooling and bonding |
Microstructural Evolution
The microstructure of TIG AM deposits is fundamentally different from that of cast or wrought 308L stainless steel due to the rapid solidification and repeated thermal cycling inherent in the process. Key microstructural features include:
- Columnar grain structure: The primary solidification mode is columnar dendritic growth, with grains elongated in the direction of heat flow (typically perpendicular to the deposition direction). This is because the substrate acts as a preferential nucleation site, promoting epitaxial growth.
- Grain orientation: The grains are oriented with their long axis perpendicular to the deposition layers, creating a texture that can lead to anisotropic mechanical properties.
- Dendrite spacing: The primary and secondary dendrite arm spacings are finer than in conventional castings due to the higher cooling rates (10-100 °C/s), which is beneficial for mechanical properties.
- Precipitation phases: Sigma (σ) phase, chromium carbides (M₇C₃), and niobium carbides (NbC) may precipitate during interlayer reheating cycles. The low carbon content of 308L reduces the risk of intergranular chromium carbide precipitation, but sigma phase formation remains a concern if the deposit is exposed to prolonged temperatures in the 600-800 °C range.
- Solidification defects: Lack of fusion between layers, porosity (gas and shrinkage), and hot cracking are common defects that must be controlled through process optimization.
Mechanical Properties
The mechanical properties of TIG AM 308L deposits exhibit several distinctive characteristics:
| Property | Typical Value (TIG AM) | Comparison to Wrought 308L | Notes |
|---|---|---|---|
| Tensile strength | 450-550 MPa | 500-620 MPa | Slightly lower due to defects |
| Yield strength | 250-350 MPa | 290-420 MPa | Lower due to grain structure |
| Elongation | 20-35% | 30-45% | Reduced by layering defects |
| Hardness (HV) | 150-200 | 150-180 | Similar or slightly higher |
| Fatigue strength | 150-250 MPa | 250-350 MPa | Significantly reduced by surface defects |
The mechanical properties are anisotropic: properties measured perpendicular to the build direction (transverse) are typically lower than those measured parallel to the build direction (longitudinal) due to the presence of layer interfaces and potential lack of fusion defects. The surface roughness of AM deposits is also significantly higher than machined surfaces, which adversely affects fatigue performance.
Process Optimization and Defect Control
Achieving high-quality TIG AM deposits requires careful process optimization:
- Layer height control: The deposition height must be consistent across layers to ensure uniform cooling and bonding. Excessive layer height leads to lack of fusion; insufficient height leads to excessive thermal input and grain coarsening.
- Travel speed and wire feed rate matching: These parameters must be synchronized to maintain a consistent bead geometry. Mismatch leads to under-deposition (lack of fusion) or over-deposition (excessive height and surface roughness).
- Interlayer temperature management: The interlayer temperature affects the cooling rate and microstructure. Too high an interlayer temperature promotes grain coarsening and phase precipitation; too low a temperature risks cracking and poor bonding.
- Post-processing: Hot isostatic pressing (HIP) can close internal porosity; solution heat treatment (1050-1100 °C followed by water quench) can dissolve precipitates and homogenize the microstructure; machining can improve surface finish and reduce fatigue-sensitive surface defects.
Engineering Applications and Quality Assurance
TIG AM is particularly suited for fabricating large, complex stainless steel components that would be difficult or impossible to produce by traditional methods. Applications include:
- Large-diameter stainless steel nozzles and manifolds for chemical processing.
- Custom-shaped heat exchanger components.
- Repair and remanufacturing of worn stainless steel components.
- Tooling and fixtures for manufacturing processes.
Quality assurance for TIG AM deposits must include:
- Dimensional inspection: CMM or laser scanning to verify geometry.
- Internal defect detection: RT or CT scanning to detect porosity and lack of fusion.
- Mechanical testing: Tensile and fatigue testing in multiple orientations to characterize anisotropy.
- Corrosion testing: Potentiodynamic polarization and salt spray testing to verify corrosion resistance.
- Metallographic examination: To assess microstructure, grain orientation, and defect distribution.
Key Reflections and Future Outlook
This study demonstrates that TIG arc additive manufacturing is a viable and cost-effective approach for fabricating 308L stainless steel components. The use of conventional TIG welding equipment, readily available in most fabrication shops, significantly reduces the barrier to entry for AM compared to laser-based systems. The microstructural and mechanical properties, while not yet matching wrought material, are adequate for many engineering applications, particularly where complex geometries are required.
The key challenges remain porosity control, surface finish improvement, and achieving isotropic mechanical properties. Future research should focus on multi-axis deposition strategies to reduce the anisotropy, in-situ monitoring and control systems to detect and correct defects during the build process, and post-processing optimization to enhance the final component properties. The integration of TIG AM into established fabrication workflows, leveraging existing welding expertise and quality control procedures, offers a pragmatic pathway to industrial adoption. This technology represents a meaningful evolution of welding into material manufacturing, and its continued development will expand the design freedom available to engineers in the fabrication of stainless steel components.