High temperature fracture behaviour of laser-powder bed fusion produced nickel superalloy

As aerospace technology continues to advance, the demand for more complex and intricate components has outpaced traditional manufacturing capabilities. Additive manufacturing (AM) techniques, such as laser-powder bed fusion (L-PBF), have emerged as promising alternatives for producing such parts. L-PBF has can create near fully dense components, making them viable for service. The process however produces a unique microstructure not present in wrought counterparts and the high temperature performance of L-PBF parts remains insufficiently documented. Understanding these characteristics is critical, given the extreme operating conditions encountered by aerospace components. Inconel 718® (IN718) is among the most widely used alloys for high temperature aerospace applications. This research investigates the high temperature fracture toughness of L-PBF produced IN718, focusing on identifying microstructural contributors at room and elevated temperatures. Specimens were produced in three build orientations using an L-PBF machine, stress-relieved at 950 °C for 2 hours, air cooled, and then heat treated according to one of two schemes. The first heat treatment uses a high temperature homogenisation at 1110 °C for 2 hours followed by water quenching, while the second used a lower homogenisation at 1020 °C for 1 hour followed by water quenching. Both heat treatments were subsequently aged at 720 °C for 24 hours. Specimen quality and heat treatment effects were confirmed through density testing and microstructural analysis. The high homogenisation treatment resulted in recrystallisation, annealing twins and eliminated the cellular sub-grain microstructure characteristic of AM, while the lower homogenisation treatment was unable to remove the columnar dendrites. Fracture toughness tests were conducted at room temperature and at 650 °C using a fatigue tester equipped with a furnace. A modified normalisation based on ASTM E1820 was used to generate a J–R curve and determine the 𝐽𝐼𝐶 of the specimens. These L-PBF produced specimen fracture toughnesses were compared to those reported in the literature for both wrought and L-PBF produced IN718. The results showed that, at 650 °C, laser-powder bed fusion IN718 specimens from the first heat treatment exhibited higher fracture toughness than those from the second, though still lower than wrought IN718 (434 kJ/m²). The high homogenisation heat treatment was able to better retain its fracture toughness at 650 °C. The material properties displayed significant anisotropy, with clear dependence on build orientation. These findings highlight the relationship between heat treatment induced microstructural changes and the anisotropic fracture behaviour of L-PBF produced IN718. The results provide guidance for optimising post processing conditions to balance between strength and fracture toughness in AM superalloys.

Grantor dc:publisher

Stellenbosch : Stellenbosch University

Year dc:date.issued

2026

Author dc:creator

• Giliomee, Francois

Advisors dc:contributor.advisor

• Neaves, Melody
• Becker, Thorsten

Language dc:language.iso

en

Repository record dc:identifier.uri

https://scholar.sun.ac.za/handle/10019.1/136028

OAI identifier oai:identifier

oai:scholar.sun.ac.za:10019.1/136028