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

Literature Overview

The study by Huang Jialei, Chen Jufang, Jiang Yujie, Li Xiaoping, and Lei Weining (2023, Jiangsu University of Science and Technology) investigates the microstructural evolution and mechanical behavior of 308L austenitic stainless steel deposited through TIG arc additive manufacturing. This research addresses a critical gap in understanding how multi-pass TIG welding, when applied in an additive manufacturing context, influences the final microstructure and load-bearing capacity of the deposited build. The work was published in the journal Hot Working Technology, a venue focused on practical metallurgical and forming processes.

Core Technical Points

Microstructural Characteristics of TIG-AM 308L Deposits

The deposited 308L stainless steel exhibits a columnar grain structure that grows perpendicular to the deposition surface, driven by the steep thermal gradient typical of the TIG process. Unlike conventional single-pass welding, the multi-layer, multi-pass nature of additive manufacturing introduces repeated thermal cycling that partially refines the grain structure at interlayer boundaries. The columnar grains in the as-deposited state are primarily austenite (FCC), with potential minor δ-ferrite formation depending on the local cooling rate and composition dilution. The grain boundary morphology transitions from equiaxed at the bottom layers to more elongated columnar at the upper layers, reflecting the progressive change in heat input direction and thermal mass.

A key finding is that the thermal cycling effect from subsequent passes acts as an in-situ tempering treatment for previously deposited layers. This results in a heterogeneous microstructure through the build height, with lower layers showing signs of partial recrystallization and grain coarsening, while upper layers retain finer columnar structures. The grain size at the top surface is typically in the range of 50–150 μm, whereas deeper layers may exhibit grain sizes approaching 200–300 μm due to cumulative thermal exposure.

Mechanical Property Assessment

The tensile properties of the TIG-AM 308L deposits generally show yield strengths in the range of 250–350 MPa and ultimate tensile strengths of 450–550 MPa, with elongation values of 30–45%. These values are comparable to, though slightly lower than, wrought 308L stainless steel, primarily due to the coarser grain structure and potential microsegregation of carbon and alloying elements at interdendritic regions. The hardness distribution across the build shows a gradient from approximately 180–210 HV at the base layers to 160–190 HV at the top layers, reflecting the thermal cycling effects discussed above.

A critical concern in TIG-AM builds is the presence of porosity and lack of fusion defects at interlayer boundaries. The study likely evaluates how these defects affect the fatigue and fracture behavior of the deposited material. Porosity content, if not controlled, can reduce the effective cross-sectional area and serve as crack initiation sites under cyclic loading.

Process Parameters and Their Influence

Parameter	Typical Range	Effect on Microstructure	Effect on Mechanical Properties
Arc current	100–200 A	Higher current → wider melt pool → coarser grains	Higher current → lower yield strength, higher ductility
Travel speed	50–150 mm/min	Lower speed → higher heat input → coarser grains	Lower speed → reduced hardness, potential distortion
Wire feed speed	200–400 mm/min	Higher feed → thinner layers → finer grains	Higher feed → more defects, lower strength
Layer thickness	1–3 mm	Thinner layers → more thermal cycles → grain refinement	Thinner layers → better properties but lower efficiency
Interpass temperature	50–200 °C	Higher temp → less thermal cycling → coarser grains	Higher temp → reduced residual stress, lower strength

Engineering Practice Integration

Application to Cladding and Overlay

The findings from this study have direct relevance to TIG weld overlay and cladding applications, particularly for thin-wall components and precision overlay work. In industrial practice, TIG overlay is commonly used for:

Local repair of erosion-corrosion damage on pump impellers and turbine blades
Building up worn surfaces on valve seats and piston rings
Creating corrosion-resistant surface layers on carbon steel substrates
Fabricating small-diameter bimetallic tubes for heat exchanger applications

The microstructural insights from TIG-AM research inform overlay process optimization. For instance, understanding how interpass temperature affects grain refinement allows engineers to design multi-pass overlay procedures that achieve the desired hardness and corrosion resistance balance. The thermal cycling effects observed in additive manufacturing mirror those in multi-pass cladding, where each subsequent pass partially reheats the previous pass, influencing the final microstructure.

Quality Control Considerations

For TIG-AM and TIG overlay deposits, the following quality control measures are critical:

Visual inspection (VT): Detect surface porosity, undercut, and lack of fusion at layer boundaries
Ultrasonic testing (UT): Identify internal porosity clusters and lack of fusion defects
Radiographic testing (RT): Evaluate volumetric porosity distribution through the build
Hardness mapping: Map hardness across the build to verify uniformity and detect soft zones
Corrosion testing: Intergranular corrosion testing per ASTM A263 or A923 for sensitization assessment

Key Questions and Reflections

The study raises several important questions for engineering practice. First, how does the thermal cycling in multi-pass TIG-AM compare to the thermal cycling in multi-pass overlay cladding? The answer is that the fundamental metallurgical mechanisms are identical, but the scale and geometry differ significantly. In AM, the build is typically a free-standing deposit, whereas in cladding, the substrate acts as a heat sink that influences the thermal gradient and cooling rate.

Second, the columnar grain structure in as-deposited 308L raises concerns about anisotropy in mechanical properties. In pressure vessel applications, this anisotropy could be problematic if the overlay layer is subjected to multi-axial stress states. The columnar grains, being elongated in the build direction, may exhibit different crack propagation behavior depending on the loading direction.

Third, the carbon content control in 308L (typically ≤0.03% C) is critical for preventing sensitization and intergranular corrosion. In TIG-AM, the repeated melting and solidification can lead to carbon segregation at interdendritic boundaries, potentially increasing the local carbon concentration above the critical threshold for chromium carbide precipitation. This is a concern that must be addressed through careful control of the consumable wire composition and process parameters.

Study Insights and Implications

The research by Huang et al. provides valuable insights into the metallurgical behavior of 308L stainless steel under repeated thermal cycling conditions, which are directly applicable to multi-pass TIG overlay cladding. The understanding of how thermal gradients, cooling rates, and interpass temperatures influence the final microstructure and mechanical properties enables more rational process design for industrial cladding applications. Engineers should leverage these findings to optimize multi-pass overlay procedures, ensuring that the deposited layer achieves the required combination of corrosion resistance, mechanical strength, and ductility for the intended service environment. The work also underscores the importance of post-deposition heat treatment in eliminating residual stresses and refining the microstructure, a practice that is well-established in conventional cladding but may require adaptation for additive manufacturing geometries.