Session: 16-01-01: Government Agency Student Poster Competition

Paper Number: 150174

150174 - Effects of Spark Plasma Sintering Heat Treatment on the Microstructure and Mechanical Properties of Metallic Alloys Fabricated by Laser Powder Bed Fusion. 

To eliminate the defects with irregular shapes inside metallic parts made by L-PBF, heat treatments are necessary. Hot isostatic pressing (HIP) is the most prevalent heat treatment strategy used by NASA currently to alleviate the detrimental effects of defects on the parts performance. However, HIP has its drawbacks, such as extended cycle time, elevated costs, and safety concerns. In comparison, spark plasma sintering (SPS) heat treatment shows the promising features of both time and energy savings. SPS is commonly used to sinter metallic powders into consolidated parts to achieve rapid heating and cooling rates along with a brief temperature-pressure holding time. This technique utilizes a combination of electrical current and uniaxial compression pressure to facilitate swift densification, leading to the production of high-quality and high-performance materials. The concept of SPS for heat treatment to eliminate defects stems from the typical process involved in powder consolidation, where when current passes through the sintering die, the die is heated up, which simultaneously heats up the powders. When a spark discharge appears in a gap or at the contact point between the powders, a local high-temperature state (up to 10,000 °C) is generated, causing evaporation and consolidation of the surfaces of the powders by accelerating the mass transfer process to form sintering necks between the powders. Drawing from this concept, when a pore or microcrack is subjected to a SPS process, sparks would be generated in the gap regions of the pores or microcracks. Subsequently, this would create extremely localized high temperatures, which would accelerate mass transfer to fill the defects’ sharp edges, analogous to the formation of sintering necks to aid in powder consolidation.

Thus, in this present study, spark plasma heat treatment was utilized for heat-treating metallic components produced using LPBF for improved mechanical performance and with the objective of achieving properties comparable to those of samples treated using the HIP method. The preliminary investigation was carried out as follows: spark plasma heat treatments were performed on a high-chromium nickel-based superalloy made with LPBF with the following profile: the sample was heated from room temperature to a high temperature of 1100°C (with the material's melting point being approximately 1360 °C) at a rate of 100 ℃/min. After maintaining this temperature for 5 minutes, the sample was then cooled down to room temperature within the furnace. The microstructures and mechanical behaviors of the L-PBF sample before and after heat treatment were analyzed. From transmission electron microscopy (TEM) results, there were significant changes in the microstructures, including the transformation of irregular-shaped pores to spherical-shaped ones and more defined grain boundaries. Positron annihilation lifetime spectroscopy (PALS) tests revealed a considerable reduction in atomic-level defects after spark plasma heat treatment. Lastly, tensile stress-strain curves highlighted the changes in mechanical behaviors post-heat treatment, specifically a decrease in alloy strength and a substantial improvement in ductility, likely due to defect removal (atomic level) and defect shape alteration (micro-scale). The preliminary study has shown conclusively that spark plasma heat treatment can be utilized to tailor and improve the properties of L-PBF components by modifying their microstructures.

Currently, ongoing investigations involve the study of the microstructure and mechanical properties of  SPS-heat-treated Ti-6AL-4V (Ti64) alloys fabricated by LPBF. In the study, Ti64 alloy samples were heated from room temperature to the sintering (hold) temperature at a rate of 100 ℃/min. The sintering temperatures and their corresponding holding times in parenthesis were as follows: 900 ℃ (5 mins), 1000 ℃ (5mins), 1000 °C (10 mins), 1000 ℃ (15 mins), 1100 ℃ (5 mins). Subsequent studies would involve utilizing advanced material characterization tools, such as scanning electron microscopy (SEM), TEM, and X-ray diffraction (XRD) to assess the evolution of defects and the micrsocstructure of SPS heat-treated samples. Mechanical tensile testing would also be performed to assess the mechanical performance of heat-treated samples. Lastly, thermodynamic modeling will be utilized to explore the optimal spark plasma heat treatment processing windows for heat treatments.

Presenting Author: Edem Honu Southern University and A & M College

Presenting Author Biography: •Edem is an aerospace structural and materials enthusiast with strong research interests in metal powder-based AM technology, heat treatment, hydrogen embrittlement, and machine learning.

•Currently, a Research Assistant in the CAM-Q group (Consortium for Additive Manufacturing Qualification) at Southern University and A & M College under the supervision of Dr. Congyuan Zeng and a student researcher for LAMDA (Louisiana Materials Design Alliance) at Louisiana State University.

•His specific focus is on the rapid qualification, optimization, and spark plasma heat treatment of additive manufactured parts exploring Laser Beam-Powder Bed Fusion AM process for safety critical applications in NASA’s liquid rocket engine, leveraging materials science, and thermodynamic modeling.

•Additionally, he reviews and validates hydrogen-compatible materials for elevated hydrogen gas operations through phase field modeling and machine learning.

Authors:

Edem Honu Southern University and A & M College
Congyuan Zeng Southern Univeristy and A & M College
Patrick Mensah Southern University and A & M College