Influence of Laser Peening on the Water Droplet Erosion Behaviour of Gas Turbine Compressor Blade Material

Author:  Abdullahi K. Gujba, Lloyd Hackel, Mamoun Medraj
Source:  ICSP-13
Doc ID:  2017106
Year of Publication:  2017
Abstract:  
Introduction: One of the Achilles’ heels of advanced materials found in the power generation industry is the water droplet erosion (WDE) of the leading edge of compressor blades. This occurs due to the interaction between water droplets injected in to the compressor and rotating blades. WDE is defined as the progressive loss of material from a solid surface due to accumulated impacts by liquid droplets [1]. WDE encountered in gas turbines is a complex phenomenon that existed for considerable long period of time and the reason for this is the number of parameters involved during the erosion process. WDE damage is predominantly caused by two main factors; (1) the high pressure exerted by the water droplet on the exposed area of the solid surface and (2) the radial liquid flow (lateral jetting) along the surface at high speed, which occurs after the initial droplet pressure lessens [2]. Existing literature suggests that WDE is likened to fatigue-like damage due to the continuous liquid impacts in a cyclic fashion [3,4]. Also, crack initiation and propagation have been found to significantly influence WDE behaviour similar to fatigue [5]. It is known that induced compressive residual stresses from mechanical surface treatments such as shot peening (SP) or laser shock peening (LSP) retard crack initiation and propagation, improving fatigue life [6]. Hence, one would suggest that mechanical surface treatments might enhance WDE performance to a certain degree. For this reason, this work studies the effect of LSP surface treatment on WDE performance. LSP is a cold working process where the surface is subjected to pulses through high power intensity laser, generating shock waves. As the shock wave stress exceeds the dynamic yield strength of the material, plastic deformation occurs [6]. These waves deform the top surface and compressive residual stresses are extended into the material [6]. So far, the applications of LSP processing include improvement of fatigue life, stress corrosion cracking resistance, corrosion resistance, wear resistance [6]. Thus, exploring LSP in terms of WDE is worthwhile. The WDE performance is discussed based on the observed residual stresses,microhardness and microstructure. In order to understand the influence of induced compressive residual stresses, different surface conditions having variable compressive residual stress levels were employed which includes; stress relieved (SR) condition having near zero compressive residual stress, as-machined (As-M) condition having inherent compressive residual stress due to machining and laser peened (SR+LSP) sample after being stress relieved.