Researchers Investigate How Tensile Strength of 3D Printed Inconel 718 is Effected by Cooling Rates

Nickel-chromium alloy Inconel 718 is often used to 3D print parts for various fields, including automotive, aerospace, and medical. Resistant to corrosion, the material can withstand high temperatures, but cools very slowly. According to Doug Hedges, the president and COO of metal 3D printing service provider Sintavia LLC, the material is not easy to machine with, but 3D printing makes it possible to create complex interior channels within builds, which saves time. A team of researchers from the Department of Mechanical Engineering at Northwestern University have been studying the material.

This superalloy is perfect for 3D printing airframe and jet engine parts, due to its high tensile, yield, and creep-rupture properties at higher temperatures. It’s perfect for 3D printing, as it can make products more lightweight and reduce the amount of necessary machining, as it can fabricate parts at near net shape.

The research team recently published a paper, titled “Cooling rate effect on tensile strength of laser deposited Inconel 718,” in the Procedia Manufacturing journal. Co-authors include Jennifer L. Bennett, Orion L. Kafka, Haiguang Liao, Sarah J. Wolff, Cheng Yu, Puikei Cheng, Gregory Hyatt with DMG MORI, Kornel Ehmann, and Jian Cao.

The abstract reads, “The thermal history generated by the additive manufacturing process influences the resulting material properties. Although trends exist between solidification rate and microstructure, solidification rate is not enough to predict final microstructure and thus mechanical properties. The purpose of this study is to relate the combined effects of solidification time and cooling time of the built material to its final ultimate tensile strength. Cooling time was defined as the time from when the location of interest last passes through 1,200 °C to when it reaches 400 °C. Nine locations on a laser deposited IN718 thin wall were studied in detail to understand the effect of cooling rate on tensile strength. Tensile samples were machined at these locations. The thermal histories of the locations of interest were compared with build geometry and the ultimate tensile strength of that location. An inverse proportional relationship was seen between the distance of the location of interest from the substrate and the cooling time. A trend was also seen linking increased surface temperature and increased solidification time. Weighted Cooling And Solidification Time (WCAST) was defined as the sum of weighted normalized solidification time and the normalized cooling time. Ultimate tensile strength was seen to decrease as WCAST increased. Optical microscopy images of the build microstructure confirm that longer cooling and solidification times lead to coarser microstructures, which may cause the lower tensile strengths measured.”

The purpose of the Northwestern study is to, as the paper puts it, “relate the combined effects of solidification time and cooling time of the build material to its final ultimate tensile strength (UTS).” The team used a 5-axis hybrid machining and 3D printing tool to deposit a thin wall, in a zig-zag pattern, of gas-atomized Inconel 718 onto a stainless steel substrate.

During the laser deposition process, a digital infrared camera was used to capture the temperature measurements of spots on the wall that the researchers had deemed were of interest. All in all, the team studied nine locations on the thin Inconel 718 wall in order to better understand the combined effect on tensile strength from both cooling and solidification times. They determined that coarser microstructures result from longer cooling and solidification times, and observed a trend that links higher surface temperatures with increase solidification times.

Optical microscopy images showing microstructures at different locations of the sample.

The paper concluded, “By understanding the thermal conditions that result in certain mechanical properties, tool path can be planned or thermal control can be used to maintain the thermal conditions to produce a component with desired mechanical properties. This research presents the potential to create uniform or gradient mechanical properties by varying thermal conditions.”

The researchers wrote that they’ll investigate the links between thermal history and optimization of laser-deposited materials’ final properties in any studies they conduct in the future.

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