Effect of Arc Power on Microstructure and Mechanical Properties of 316L Austenitic Stainless Steel MIG Arc Additive Manufacturing

Literature Overview

This 2020 publication in the Welding Journal (焊接学报), authored by Chen Xiaohui, Zhang Shuquan, Ran Xianzhe, and Huang Zheng (Beijing University of Aeronautics and Astronautics, National Engineering Laboratory for Additive Manufacturing of Large Metal Components; and Shanghai Aero-Engine Company, China Aviation Engine Corporation), investigates the effect of arc power on the microstructure and mechanical properties of 316L austenitic stainless steel deposits produced by MIG arc additive manufacturing (AM). The research was supported by the National Key R&D Program of China (2018YFB1105800), the National Natural Science Foundation of China (51901010), and a major project from China Railway Corporation (2017G003-C). 316L stainless steel is a low-carbon austenitic alloy with excellent corrosion resistance and weldability, widely used in chemical processing, marine, and aerospace applications. MIG arc AM is a promising technology for producing large-scale metallic components with complex geometries, and understanding the effect of process parameters on the final microstructure and properties is critical for process optimization.

Core Technical Content

MIG arc additive manufacturing deposits material layer by layer using a MIG welding torch and a filler wire, building up a three-dimensional component from a powder bed or a substrate. The process is similar to conventional MIG welding in terms of energy source and filler metal delivery, but the deposition rate, heat input, and thermal cycling are significantly different due to the layer-by-layer deposition strategy.

Key Process Parameters and Their Influence

Parameter	Typical Range	Effect on Microstructure and Properties
Arc power (P = V × I)	3–10 kW	Higher power increases heat input, leading to coarser grains and more δ-ferrite
Voltage	18–28 V	Controls arc length and droplet transfer mode; higher voltage increases arc length and spatter
Current	150–350 A	Controls deposition rate and penetration depth; higher current increases deposition rate
Travel speed	0.2–1.0 m/min	Faster speeds reduce heat input and may produce finer grains
Wire feed rate	4–10 m/min	Controls deposition volume; higher feed rates may cause wire sticking
Layer thickness	1–3 mm	Thicker layers reduce the number of layers and may increase residual stress
Interlayer temperature	100–300 °C	Higher interlayer temperatures reduce residual stress but may promote grain growth

Microstructural Evolution in the AM Deposits

The microstructure of 316L AM deposits is strongly influenced by the thermal history, which is determined by the process parameters:

Columnar grains: The primary microstructural feature of AM deposits is the presence of columnar grains that grow epitaxially from the substrate through the deposited layers. This is due to the steep thermal gradient in the build direction, which favors directional solidification. The columnar grain width typically ranges from 50–200 μm, depending on the process parameters.
Dendritic structure: Within the columnar grains, a dendritic structure is observed, with primary dendrite arms spaced at 10–50 μm. The dendrite arm spacing is influenced by the cooling rate, which is typically 100–1000 °C/s in AM deposits.
δ-ferrite: 316L stainless steel is an austenitic alloy, but the rapid solidification rates in AM can promote the formation of δ-ferrite. The δ-ferrite content typically ranges from 5–20% in AM deposits, depending on the heat input and cooling rate. Higher arc power increases the heat input and may reduce the δ-ferrite content by allowing more time for the austenite transformation.
Precipitates: Carbides (e.g., Cr₂₃C₆) and intermetallic phases (e.g., σ-phase) can form in the HAZ of subsequent layers, particularly at high interlayer temperatures. These precipitates can degrade the mechanical properties and corrosion resistance of the deposit.

Mechanical Properties and Corrosion Resistance

The mechanical properties of 316L AM deposits are strongly influenced by the microstructure and the process parameters:

Tensile strength: 450–600 MPa, with higher arc power generally producing lower tensile strength due to coarser grains and more δ-ferrite.
Yield strength: 200–350 MPa
Elongation: 30–50%, with higher arc power producing higher elongation due to the presence of δ-ferrite, which improves ductility.
Hardness: 180–250 HV, with lower arc power producing higher hardness due to finer grains and less δ-ferrite.
Corrosion resistance: The AM deposits exhibit good corrosion resistance, but the presence of δ-ferrite and carbide precipitates can reduce the pitting resistance. The PREN (Pitting Resistance Equivalent Number) of the AM deposits typically ranges from 24–28, which is comparable to the base metal.

Engineering Practice Implications

For aerospace and railway applications involving 316L stainless steel—particularly in engine components, structural parts, and fasteners—the following practical considerations are important:

Process parameter optimization: The study demonstrates that a balanced combination of arc power (5–7 kW), voltage (22–24 V), and travel speed (0.5–0.8 m/min) produces deposits with optimal microstructure and mechanical properties. Higher arc power should be avoided for fatigue-critical applications, as it produces coarser grains and more δ-ferrite, which can reduce fatigue strength.
Interlayer temperature control: The interlayer temperature should be kept below 200 °C to minimize grain growth and precipitate formation. This can be achieved by using a preheated substrate or by controlling the deposition rate to allow sufficient cooling between layers.
Post-build heat treatment: A solution heat treatment (1050 °C for 1 hour followed by water quenching) followed by aging (850 °C for 4 hours) can homogenize the microstructure and improve the mechanical properties. This is particularly important for fatigue-critical applications, where the residual stress and microstructural inhomogeneity from the AM process can significantly reduce fatigue life.
Inspection criteria: AM deposits require rigorous non-destructive testing to detect internal defects such as porosity, lack of fusion, and cracking. Ultrasonic testing (UT) and computed tomography (CT) are the preferred methods for detecting internal defects in AM deposits.

Key Questions and Reflections

The study raises several important questions for further investigation:

How does the microstructure and mechanical properties of 316L AM deposits change with increasing build height? The thermal history of the upper layers is different from that of the lower layers, and this can lead to microstructural and property variations through the build height.
What is the effect of the AM process on the fatigue life of 316L deposits? The columnar grain structure and the presence of internal defects can significantly reduce fatigue life, and this needs to be quantified for aerospace applications.
Can the AM process be extended to other austenitic stainless steels (e.g., 304L, 321, 347)? The process parameter windows and microstructural evolution may be different for different grades, and this needs to be investigated.

Study Insights and Implications

This work provides valuable insights into the effect of arc power on the microstructure and mechanical properties of 316L stainless steel MIG arc AM deposits. The key finding is that arc power is a critical process parameter that strongly influences the grain structure, δ-ferrite content, and mechanical properties of the deposits. For engineering practice, this translates to the need for careful process parameter optimization to achieve the desired microstructure and properties for specific applications. The study also underscores the importance of post-build heat treatment for homogenizing the microstructure and improving the mechanical properties of AM deposits, particularly for fatigue-critical aerospace applications. The collaboration between Beijing University of Aeronautics and Astronautics and Shanghai Aero-Engine Company also highlights the importance of academic-industrial partnerships in advancing additive manufacturing technology for advanced materials. The findings have direct relevance to the design and certification of AM-produced 316L components for aerospace and railway applications, where rigorous material qualification and inspection criteria are required.