| Abradable seal coating(hereinafter referred to as "seal coating")is normally composed of metal phase,non-metallic lubricating phase and a certain proportion of pores.In order to balance the wear resistance and machinability of the seal coating,and to achieve precise control of performance,it is necessary to obtain the uniformity of its phase distribution of the seal coating.Using the non-destructive method to characterize the uniformity of the distribution of the seal coating is critical to the preparation and use of the coating.At present,the ultrasonic characterization of the pore distribution uniformity of the seal coating mainly faces two difficulties:on the one hand,the seal coating has a heterogeneous heterogeneous feature,and the non-metallic phase and pore morphology and distribution on the metal skeleton are random,quantitatively describing the uniformity of the pore distribution is difficult;on the other hand,the multiphase honeycomb structure causes the propagation behavior of ultrasonic waves in the coating to be very complicated.It is difficult to select the ultrasonic characteristic parameters that can reflect the uniformity of the pore distribution.In order to solve the above problems,this study is based on the Multi-Scale Analysis of Area Fractions(MSAAF)method,combining experimental testing and numerical calculations to study the quantitative description method of the pore distribution uniformity of the seal coating,and on this basis ultrasonic characterization of pore distribution uniformity is carried out.The main content and results are as follows:(1)Aiming at the characteristics of irregular and random distribution of non-metallic phases and pore morphology in the metal matrix of the seal coating,based on the area fraction multi-scale analysis method,the multi-scale grid is used to cover the entire range of the composition phase size in coatings.And the area fraction calculation reduces the measurement error caused by the irregular morphology of the constituent phases.Then a quantitative calculation method for the uniformity of the tissue distribution of the seal coating is derived.Based on the principle of this method,the poorer the pore distribution uniformity of the seal coating,the smaller the absolute value of the slope of the calculated MSAAF extrapolation fitting line k and the greater the pore uniformity length LH.The porosity of the AlSi-PHB seal coating in this study is less than 10%,which is suitable for describing the uniformity of pore distribution in the coating with LH.(2)Observation and statistics of the microstructure of the AlSi-PHB seal coating with a thickness of 1 mm are as follows:the content metal phase AlSi in the seal coating is 51.5%?52.5%,which is distributed in a network;the content of non-metallic lubricating phase PHB is 40.5%?47.5%,the average size is 40?80 ?m;and the porosity is 1%?7%,the average pore size is 10?20 ?m.Based on random medium theory and statistical principle,a random multiphase medium model of seal coating is established.With the help of this model,the effects of average pore size and porosity on the uniformity of pore distribution are studied.The results show that with the increase of porosity and the increase of the average pore size,the pore uniformity length LH increases.Compared with the results of metallographic photos,the maximum relative error of the theoretically calculated value of the pore uniformity length LH less than 9.8%.(3)Using the ultrasonic testing numerical simulation method,the relationship between the pore distribution uniformity of the seal coating and the ultrasonic time-domain and frequency-domain characteristic parameters is studied.The results show that as the pore uniformity length LH increases,the longitudinal wave sound velocity v decreases,the time-domain ultrasonic attenuation coefficient ? increases,and there is a non-unique relationship between LH and ?.The relationship between LH and main frequency offset ?f.attenuation spectrum slope K and frequency domain attenuation coefficient ?f in different frequency bands is further discussed.The results show that:?f and K both increase with the increase of LH,and the dispersion of them becomes more obvious with increasing LH.Different ?f shows different changes with the increase of LH.?0-2MHz,?2-4MHz and ?8-10MHz basically remain unchanged with the increase of LH,?4-6MHz and ?6-8MHz with the increase of LH increase,but the dispersion of ?6-8MHz is stronger.Under the detection conditions of center frequency 5 MHz and-6dB effective band 2.6?7.3 MHz,compared with other frequency bands,the sound wave energy in the 4?6 MHz band is higher,and the correlation between frequency domain attenuation coefficient ?4-6MHz and LH is also more significant.In addition,due to the stong attenuation of sound waves in 6?10 MHz frequency band,the attenuation changes caused by LH are difficult to fully reflect,resulting in significant nonlinearity and randomness in ?6-8MHz and ?8-10MHz.Experimental verification shows that the correlation obtained by numerical simulation are consistent with experimental results. |
PDF Full Text Request