Abstract
The present work demonstrates a technique for the hot forging of metal surfaces in water at 1000 °C or higher, termed energy-intensive multifunctional cavitation (EI-MFC). In this process, the energy of cavitation bubbles is maximized, following which these bubbles collide with the metal surface. This technique will be employed to improve the surface structure of CM186LC/DS, a Ni-based columnar crystalline superalloy used to manufacture the rotor blades of jet engines and gas turbines that are exposed to high-temperature oxidizing environments, with the aim of improving creep strength. EI-MFC processing induces compressive residual stress in the metal that prevents the occurrence of surface cracks and also increases surface hardness, improves corrosion resistance, and increases the coefficient of friction. The latter effect can enhance the adhesion of thermal barrier coatings applied to Ni-based superalloys by thermal spraying. The technology demonstrated herein can be applied to present-day jet engine and gas turbine components and also to the production of hydrogen combustion turbines operating at 1700 °C with higher combustion efficiency than the current 1500 °C class gas turbines. In addition, the high processing energy obtained using the EI-MFC technique has the potential to flatten rough surfaces resulting from the stacking pitches of various metals manufactured using three-dimensional printers, and so improve surface strength.
Highlights
Ni-based superalloys can be classified as either cast or forged alloys, and these metals have applications to different jet engine components
We investigated the possibility that either ultra-high-temperature and high-pressure cavitation (UTPC) or energy-intensive multifunctional cavitation (EI-multifunction cavitation (MFC)) processing could induce rafting-like phenomena on the metal surface
The surface potential data were obtained by measuring four areas for each processing condition, performing three line analyses in each area, and taking the average of the total of 12 line analyses. These results demonstrate that the corrosion potential of the UTPC specimen was not changed after processing, while that of the metal subjected to EI-MFC was increased, meaning that the corrosion resistance was improved
Summary
Ni-based superalloys can be classified as either cast or forged alloys, and these metals have applications to different jet engine components. Ni-based superalloys are used to fabricate high-pressure turbine blades employed in combustion chambers as well as the disks that support these blades These materials are used to produce lowpressure turbine blades that approach exhaust gases. These alloys are able to withstand high temperatures and pressures and; the temperature limit of the Ni-based alloy from which turbine blades are fabricated determines the temperature of the combustion chamber and is directly linked to the efficiency of the engine and gas turbine. Up to the 1950s, these materials were wrought alloys, while unidirectionally solidified conventional casting (CC) alloys were produced in the 1960s (Reed, 2006) This new process eliminated crystal grain boundaries perpendicular to the longitudinal direction of the turbine blades, and led to the development of directional solidification (DS) alloys. The volume-based proportion of the γ0 phase in these materials is not necessarily high, and the creep resistance is maximized at a proportion of approximately 70%
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