Nondestructive Evaluation Of Pinhole Defect Of Cast Aluminum Alloy By Ultrasonic Method

The cast aluminum alloy is widely used in many industries as its superior casting characteristics,such as automobile,aerospace,construction,electric power,and so on.But it's easy to produce all kinds of defects during the melting process.Pinhole is one of the most common defects of cast aluminum alloys.It is created by the gas not evacuated during the smelting process of the alloys.The existence of pinhole can reduce the intensity and plasticity of the aluminum alloy,and do harm to the gas density,corrosion resistance and anode oxidizability.It can damage the components easily.So,it is definitely put forward to restrict the pinholes' size in the acceptance standards of the cast aluminum alloy.According to the Standards of GB/T 8733-2000 and JB/T 7946.3-1999,the method of metallographic visual comparison of random sampling is used to evaluate the level of pinhole in modern industry.But it doesn't describe the pinholes' size,number and shape.This method is time/labor consuming and affected by human factors.In this paper,the ultrasonic nondestructive testing method by collecting signals from the samples with different pinhole levels,summarizing the relation and variation between ultrasonic parameters and pinhole defects,is used to evaluate the level of pinhole.The experiment results show that:(1) The ultrasonic longitudinal/transversal wave velocity decreases with the pinhole degree increasing.(2) The ultrasonic attenuation coefficient increases with the pinhole degree increasing.(3) The Power Spectrum obtained by Fourier Transform shows that the ultrasonic power losses more seriously in the high pinhole degree samples.The reasons and mechanism of the experiment phenomena are as follows:(1) The change of the ultrasonic wave velocity is related to the decline of the cast aluminum alloy performance parameter such as the density and Young's modulus caused by pinholes.It is also related to the increase of ultrasonic power loss caused by pinholes in the alloys.(2) The reasons that cause ultrasonic power loss include scattering and absorption.The scattering which is the mainly reason of the power loss belongs to Rayleigh scattering;and the absorption includes conduction absorption and viscous absorption.Both scattering and absorption are nearly related to pinholes.The aggravation of pinholes is the ultimate reason that increases the ultrasonic modulus decay.According to the experiment and the study,it is feasible that inspecting and evaluating the pinhole defect of cast aluminum alloys by ultrasonic nondestructive testing method.And this method is rapid,objective and accurate.