Research On Ultrasonic Characterization Of Polycrystalline Microstructural Evolution Based On Process Of Metal Materials

Metal materials are commonly used as functional and structural materials,microstructural evolution brought by processing is the key to determine the property,timely and accurately characterizing grain information is essential for evaluating properties of metal materials.The most intutive way to characterize grain microstructure of metal materials is metallography,however,metallography is a destructive method,it is usually damage to materials.Ultrasonic nondestructive testing technology has the nondestructive feature,it can reflect the overall effect of polycrystalline microstructures on the ultrasonic wave propagation path,and it is gradually gained attention in recent years.At present,using ultrasonic technology to characterize polycrystalline microstructures of metal materials is mainly concentrated on equiaxed grains with significant different grain sizes.However,due to the influence of process technology,the actual polycrystalline morphology of metal materials is relatively complicated,there may be deformed grains or small changes in grain size.Given the prevalence of complex polycrystalline microstructures and the importance of characterization,two typical metal materials were used to research the ultrasonic characterization method of microstructural evolution under different process technologies in this paper.First,aiming at obvious grain size differences with deformed grains of high entropy alloy specimens,the experimental study was carried out,try to characterize the microstructural evolution during thermomechanical process such as rolling and recrystallization annealing based on ultrasonic method;in order to further study the ultrasonic characterization method of microstructural evolution with small grain size difference and double phases,choosing austenitic stainless steel as the research object,to explore the effect of microstructural on ultrasonic signals by experiment and simulation.The main research work is as follow:?1?Al_0.26CoCrFeNiMn high entropy alloys were prepared by vacuum melting and casting,then were processed by homogenization annealing,rolling and recrystallization annealing.The morphology of metallographic microstructures were observed,the mean grain size was counted,and used the 5 MHz water immersion probe to test high entropy alloys.The results show that the attenuation coefficient increases with the grain size,the attenuation coefficient at 5 MHz with the average grain size is corresponded in a cubic relationship,the correlation coefficient is 0.995.Experimental results show that the attenuation coefficient can characterize the recrystalline grain microstructural evolution of high entropy alloy with obvious grain size variation and deformed grain.?2?Aiming at the grain microstructural evolution of austenitic stainless steel during repeated welding heat affect zone,Gleeble was used to expand heat affect zone so as to prepare repeated welding thermal simulation specimens.The metallographic method,microhardness method and impact energy were used to perform microstructure analysis and mechanical performance testing of the heat affect zone,then used 20 MHz water immersion focusing probe to test Gleeble specimens.The results show that the attenuation coefficient variation is accordance with average grain size change,but is opposite to the trend of microhardness change,the variation rate of attenuation coefficient is 24.18%.?3?In order to analyze the influence of average grain size and?-ferrite on ultrasonic parameters,simulated the ultrasonic wave propagation in grain microstructure of heat affect zone of Gleeble specimens by time domain finite difference numerical simulation.Simulation results show that,average grain size is the main factor affecting ultrasonic signals when the content of?-ferrite is small.Based on the comprehensive experimental and simulation results,attenuation coefficient can be used to characterize the grain microstructural evolution of the repeated heat affect zone with small grain size difference.