MIG Arc Additive Manufacturing of 316L Stainless Steel and Inconel 625 Functionally Graded Materials

Literature Overview and Research Context

The 2024 study published in Precision Forming Engineering (Vol. 16, No. 10, pp. 208-216) by Wen Chunjie and colleagues from Guangdong Polytechnic Normal University and South China University of Technology presents a significant advancement in arc additive manufacturing (AM) technology for producing functionally graded materials (FGMs). The work addresses the critical challenge of joining dissimilar materials — specifically austenitic stainless steel (316L) and nickel-based superalloy (Inconel 625) — in a single continuous build without introducing interfacial defects.

Functionally graded materials offer a unique advantage in engineering applications by providing a smooth transition in composition and properties between two distinct materials, eliminating the sharp interfaces that typically serve as crack initiation sites in welded dissimilar joints. This approach is particularly relevant for high-temperature components in aerospace and power generation, where thermal expansion mismatch between steel and nickel-based alloys can cause premature failure.

Core Technical Findings

Additive Manufacturing Process Configuration

The authors employed a dual-pulse MIG arc additive manufacturing process to deposit 50 layers of alternating 316L and Inconel 625 wire on a steel substrate. Two gradient configurations were investigated: a 25% transition ratio and a 5% transition ratio. The dual-pulse arc mode was selected to provide improved arc stability, reduced spatter, and better melt pool control — all critical factors for achieving layer-to-layer consistency in additive manufacturing.

The 50-layer deposition represents a substantial build height, demonstrating the process's capability for producing thick-section components. The reciprocating deposition pattern ensures uniform thermal distribution and minimizes residual stress concentration in any single direction.

Deposition Quality and Defect Analysis

Both specimens successfully completed the 50-layer build with good surface morphology and no visible defects such as lack of fusion, sagging, or cracking on the exterior surface. Cross-sectional examination revealed no macroscopic porosity or hot cracking, which is a notable achievement given the significant differences in thermal conductivity, melting point, and solidification behavior between 316L stainless steel and Inconel 625.

However, a critical finding emerged from the detailed examination of the 25% gradient specimen. In the region containing 75% 316L and 25% Inconel 625 by mass fraction, microcracks were observed, accompanied by reduced hardness and degraded tensile properties. This localized failure zone significantly compromised the overall quality of the 25% gradient specimen. In contrast, the 5% gradient specimen exhibited no such degradation zone and maintained consistent mechanical properties throughout the entire build height.

Mechanical Properties and Hardness Distribution

Hardness measurements revealed a clear trend of increasing hardness with increasing nickel-based alloy content, which is consistent with the known precipitation hardening capability of Inconel 625. The 5% gradient configuration demonstrated superior overall quality, with higher strength and ductility characteristics that make it more suitable for industrial applications.

The Significance of the 5% Gradient Ratio

The superiority of the 5% gradient ratio over the 25% ratio is a critical finding with important implications for FGM design. The 5% transition provides a more gradual compositional change between layers, which reduces the thermal stress concentration at the interface between dissimilar materials. The 25% ratio creates a more abrupt transition, leading to localized regions where the thermal expansion mismatch and metallurgical incompatibility generate sufficient residual stress to initiate cracking.

This finding suggests that the optimal gradient design for FGM production depends on finding the right balance between compositional transition rate and build efficiency. A very gradual gradient (such as 5%) may require more layers and longer build times, but it produces superior mechanical integrity. The engineering trade-off between build time and component quality must be carefully evaluated for each specific application.

Engineering Practice Implications and Reflections

Process Optimization Considerations

From a manufacturing perspective, the dual-pulse MIG arc AM process demonstrated in this study offers several advantages over other AM technologies such as laser powder bed fusion (LPBF) or directed energy deposition (DED) with laser sources. Arc AM typically has higher deposition rates, lower equipment costs, and greater flexibility in wire feedstock selection. The dual-pulse mode further enhances process stability, which is essential for multi-layer builds where consistency is paramount.

The 50-layer build height achieved in this study demonstrates the scalability of the process for producing thick-section components. However, several practical challenges remain for industrial implementation. Thermal management of the substrate and previously deposited layers is critical to prevent excessive residual stress accumulation. The steel substrate used in this study may not represent the optimal base material for all applications, and substrate selection should be considered as part of the overall process design.

Quality Control and Inspection Strategies

The multi-level evaluation system employed by the authors — encompassing surface morphology, deposition efficiency, microstructure, hardness, and tensile properties — represents a comprehensive approach to AM quality assurance. For industrial deployment, non-destructive testing (NDT) methods such as ultrasonic testing (UT) and computed tomography (CT) would need to be developed to detect internal defects without damaging the component. The crack formation observed in the 25% gradient specimen highlights the importance of process parameter optimization and the need for thorough validation before production use.

Application Potential

The successful production of defect-free FGM components using arc AM opens new possibilities for several industrial applications. In the power generation sector, turbine components that experience significant thermal gradients could benefit from FGM construction that provides thermal expansion compatibility between the hot and cold sections. In the aerospace industry, engine components and structural brackets that require the corrosion resistance of stainless steel and the high-temperature strength of nickel alloys could be manufactured as single-piece FGM components rather than welded assemblies.

Study Insights and Outlook

This research makes a meaningful contribution to the growing field of arc additive manufacturing by demonstrating that functionally graded materials can be produced with industrial relevance through MIG arc AM. The identification of the 5% gradient ratio as the optimal transition parameter provides actionable guidance for process development, while the failure analysis of the 25% gradient specimen offers valuable insights into the metallurgical mechanisms governing FGM quality.

The study also highlights an important principle in FGM design: the compositional transition rate must be carefully controlled to prevent localized property degradation. This finding has implications beyond additive manufacturing and extends to any manufacturing process that produces gradient materials, including thermal spray, diffusion bonding, and selective laser melting with powder blending.

Future research should investigate the effects of build orientation, interlayer dwell time, and substrate preheating on FGM quality. Additionally, the mechanical behavior of the 5% gradient FGM under cyclic loading, elevated temperature, and corrosion conditions should be characterized to establish a complete performance database for design applications. The integration of in-situ monitoring and real-time feedback control into the arc AM process would further enhance the reliability and repeatability of FGM production for industrial use.