| Titanium and titanium alloys are used as the equipments and devices, such ashigh-speed propellers, high-speed submarines, hydrofoils, pumps, valves, turbinecomponents and engine blocks, cavitation erosion usually occurrs. Currently, there arefew studies on cavitation erosion, failure mechanism of titanium and its alloys andsurface modification for improving the cavitation erosion resistance. In this study,commercially pure titanium (TA2) and Ti-6Al-4V (TC4) alloys were used. Thecavitation erosion behavior of TA2and TC4alloy was studied using ultrasonicvibration cavitation equipment. The effect of the testing medium temperature, sandconcentration, surface roughness and microstructure on cavitation erosion behaviorwas also evaluated. Then the interaction of the cavitation erosion between thecorrosion was studied using electrochemical testing. Subsequently, in order toimprove the cavitation erosion resistance of TA2and TC4alloy, three kinds of surfacemodification treatments were performed to prepare a hard coating on surface of thesamples. Surface morphology, microstructure, hardness and cavitation erosionbehavior of the coating were studied, and cavitation erosion mechanisms was aslodetermined.The results of the cavitation testing for the TA2and TC4alloy showed that theTC4alloy has a good cavitation erosion resistance in distilled water compared withTA2due to a fine grain and hard microstructure. However, the cavitation erosionresistance of the TA2and TC4alloy samples declined in3.5wt.%NaCl solution. Itwas attributed to the causticity of the NaCl solution. The weight loss of TA2and TC4alloy caused by the interaction of cavitation ersosion between corrosion was increased1.61and1.42mg, was about12.8%and13.5%in the total weight loss, respectively.The results of the cavitation testing for the heat-treated TC4alloy showed that theeffect of the microstructure on the cavitation erosion behavior of the heat-treatedsamples was obvious. The widmanstatten microstructure obtained at1020oC has agood cavitation erosion resistance due to the larget amount of flaky α-Ti, whichincreases the crack path and reduces the crack stress field. The sample treated at950oC has a duplex microstructure, which was composed of the equiaxed primaryα-Ti andthe needle-like secondary α-Ti. The primary α-Ti has a good workhardening ability and the secondary α-Ti has a good ability for suppressing crack propagation, so thesamples has a good cavitation resistance. The sample with an equiaxed microstructuretreated at850oC has a lower cavitation resistance compared with the samples withthe Westergren and duplex microstructure.Nitriding treatment was used to prepare a uniform and dense nitride layer on thesurface of the TA2and TC4alloy samples. The results showed that the nitridingtreatment significantly improved the cavitation erosion resistance of the TA2and TC4alloy samples. For TA2samples, higher processing temperatures and longer timeresulted in a poor cavitation erosion resistance due to the formation of more oxide,cracks and micro-pit on the nitrided layer. For TC4alloy, the cavitation erosionresistance of the samples treated at lower temperatures and shorter treatmentconditions was poor. It was becaure that the β-Ti, α/β grain boundaries andprecipitates accelerated cavitation damage. Due to the compressive residual stressfield caused by nitrogen solid solution, the diffusion zone can arrest crack propagationand improve cavitation erosion resistance for the nitrided TA2samples.Pack carburizing and carbonitriding treatment were performed to prepare a TiCand Ti (C, N) coating on the surface of the TA2and TC4alloy samples. The results ofthe cavitation testing showed that the hard TiC and Ti (C, N) coating have a uniformand dense fine grain structure, which can increase the crack propagation path andreduce stress concentration, and thus significantly improved the cavitation erosionresistance for TA2and TC4alloy. High temperatures and long processing time canreduce cavitation erosion resistance. It was mainly because as prolonging thetreatment time and temperature, the grain of the TiC and Ti (C, N) growed and moreoxide formed on the surface, which weakened the compositive performance of thecoating. However, low temperature and time also reduced the cavitation erosionresistance becaure the coating is too thin. Therefore, the optimum carburizing processfor improving the cavitation erosion resistance of the TA2and TC4alloy is1000oC,90min and the optimum carbonitriding process is1100oC,60min. Cavitationerosion mechanisms of the coatings were as follows: the shock waves and micro-jetscaused by bubble collapsing resulted in shedding of the impurity phase and particlesand formation of micro-pits on the surface of the coating. Subsequently, the initiationof micro-cracks occurred around the micropores and the cracks propagated along thegrain boundaries. When the cracks propagated throughout the entire grain, the wholegrain falls off as brittle fracture and new micro-pits formed. Then the cracks connected the pores and formed the large cavitation craters, resulting in largermaterial weight loss. |
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