Abstract

In an attempt to enhance the water droplet erosion (WDE) performance of Ti-6Al-4V, a typical material used for compressor blades in gas turbines, this work studies the WDE performance/behaviour of reference untreated and surface treated Ti-6Al-4V. Existing literature suggests that WDE is likened to fatigue-like damage due to the continuous liquid impacts in a cyclic fashion. Also, the crack initiation and propagation have been found to significantly influence WDE behaviour similar to fatigue. It is known that induced compressive residual stresses from mechanical surface treatments retard crack initiation and further propagation and improves fatigue life. Hence, mechanical surface treatments might enhance WDE performance. For this reason, this work employed two mechanical surface treatments, laser shock peening (LSP) and ultrasonic nanocrystalline surface modification (UNSM), for the first time. UNSM treatment induced high levels of compressive residual stresses into the material. Variation in grain size was observed across the modified layer and the microhardness of the UNSM condition was enhanced significantly compared to the As-M condition. Although, significant amount of compressive residual stress was induced via LSP, the treatment showed mild increase in microhardness and no noticeable changes in the microstructure. This was attributed to the low level of cold work (about 5 %) during LSP processing. The WDE performance tests were conducted in a rotating disc rig in accordance with ASTM G73 standard. Influence of impact speed (between 150 and 350 m/s) on WDE performance was explored on two different sample geometries (T-shaped flat and airfoil). First, to understand the WDE behaviour of the reference or as-machined (As-M) Ti-6Al-4V before applying surface treatments, its WDE behaviour was studied. In the second and third parts of this study, the WDE performance of UNSM treated versus As-M Ti-6Al-4V and LSP treated versus As-M Ti-6Al-4V conditions was investigated, respectively. WDE results showed that the T-shaped flat UNSM samples had enhanced WDE performance at speeds 250, 275 and 300 m/s compared with the As-M condition. At 350 m/s, both UNSM and As-M conditions showed similar performance due to diminished effect of the UNSM treatment. UNSM airfoils showed mild enhancement in the WDE performance at 300 m/s during the advanced stage compared with the As-M condition. At 350 m/s, no enhanced performance was observed for the UNSM airfoil condition. LSP showed little or no beneficial effect at any stage of the WDE performance at all tested impact speeds for the T-shaped flat geometry. However, LSP airfoil samples showed only mild enhancement in the WDE performance at 300 m/s during the advanced stage compared with the As-M condition. At 350 m/s, no enhanced performance was observed for the LSP airfoil condition. It was concluded that the influence of mechanical surface treatments on the WDE performance of material depends in general on the erosion test condition, sample geometry, materials’ properties and microstructure. However, for the mechanical treatment to be effective in improving WDE performance, it has to cause surface hardening and grain refinement. Compressive residual stresses alone are not sufficient to enhance WDE performance especially for the T-shaped flat geometry. For the airfoil, the induced compressive residual stresses show limited beneficial effect in mitigating erosion at the advanced erosion stage. This is due to the fact that compressive residual stresses are through the thickness of the airfoil. This is the case observed at relatively low speed of 300 m/s. However, at 350 m/s where the test condition is severe, the induced compressive residual stresses show no beneficial effect on the airfoil geometry.