Effect of HIP Parameters on the Micro-Structural Evolution of a Single Crystal Ni-Based Superalloy

Abstract

Abstract. For reducing the porosity of single crystal (SX) nickel-based superalloys, Hot Isostatic Pressing (HIP) is used. High pressures of about 100-170 MPa lead to local deformation, which close the pores. However, since HIP also requires high temperatures (1000-1200°C) it has a pronounced effect on the microstructure and the local distribution of elements. This contribution analyses the effect of different HIP treatments on both the microstructure and the segregation of the SX superalloy LEK94 in the as-precipitation-hardened state. In addition, the effects of rapid or slow cooling are analyzed. To distinguish the effect of pressure from those of temperature, the HIPed samples are compared with specimens annealed at atmospheric pressure. IntroductionSecond-generation of SX nickel-based superalloys exhibit a combination of excellent strength, good creep properties and fatigue resistance at high temperatures. These materials are currently used as blades in gas turbine and aircraft engines. They owe their properties to the large amount (~70 vol.% [1]) of finely dispersed precipitates of the ordered (L1 2 ) γ´ phase, which show a very high solvus temperature (~1200°C). One of the main drawbacks of Ni-based superalloys is the lack of strength and ductility due to the formation of microporosity during casting, which also results in the scatter of the mechanical properties of cast components [2]. Furthermore, the shrinkage cavities can act as crack initiation sites and promote crack propagation, leading to premature rupture of the components. Therefore, it is an important issue to reduce the microporosity generated during casting. In order to solve this task and to obtain superior mechanical behaviours, the application of HIP on the material reduces the size and amount of porosity [3] via a combination of plastic deformation, creep and diffusion bonding [4][5][6]. Simultaneously, the alloy composition is homogenized to some extent since the HIP partially dissolves γ´ [7][8][9]. There have been attempts to apply HIP both directly after casting and after precipitation hardening. In the present work, the HIP technology was applied to the SX Ni-based superalloy LEK94 to investigate the evolution of the microstructure -mainly the homogenization of the alloy composition and the morphology of the γ´ precipitates in the dendrite core -after HIP by using scanning electron microscopy. One aim of the study is to determine the relationship between the HIP parameters, mainly pressure and temperature, with the observed microstructure.

High Pressure Heat Treatment of AM Parts—Combining HIP and Heat Treatment

“…This is a result of the finer γ/γ'microstructure obtained by the higher cooling rate in the HIP. More work from Ruhr-Universität Bochum on rapid quench HIP processing and HPHT on various alloys from can be found in [11][12][13][14][15][16][17].…”

Simulation of the External Pressure Influence on the Micro-Structural Evolution of a Single Crystal Ni-Based Superalloy

“…Its nominal composition is shown in Table 1. Figure 1 shows the typical γ/γ' structure developed for this material following a precipitation hardening treatment [12]. This is the starting microstructure of the specimens to be subjected to different treatments, and the initially defined microstructure for the simulations.…”

“…To investigate the influence of the HIP temperature at a given pressure, the parameters were set to P HIP =100 MPa, t HIP =10 h and T HIP i) 1000°C, ii) 1050°C and iii) 1180°C. To study the influence of pressure, iv) one simulation was carried out with a temperature of 1180°C, at ambient pressure (P atm = 0.1 MPa) and a holding time of 4 h. In order to evaluate the simulations, HIP and ambient pressure heat-treatments were performed on LEK94-samples [12]. One sample was HIPed at 1000°C and 100 MPa for 10 h. The HIP-facility used featured accelerated cooling which enabled a cooling time of around 1 h. A second sample was heat treated at 1180°C for 4 h at ambient pressure and subsequently quenched in water.…”