Study On Ultrasonic Inspection Of Discontinuous Defects In Magnesium Alloys

Ultrasonic defect detection is an efficient and convenient method to detect the discontinuous faults in the metallic materials, such as crack, inclusion and air bubble and so on. Discontinuous defects in the as-cast and deformed Magnesium alloys are generally observed, due to its high oxidability, difficulty to purify and weak deformation ability. As a result, quality testing becomes an important issue in the manufacture and application of Magnesium alloy. Thus, Nondestructive detection of Magnesium alloy is necessary to ensure the quality of the product and decrease the risk in the application, provide feedback for manufacture as well, which can also help improve the production process and the efficiency.Ultrasonic NDT has been applied in the industry field widely. Phased array ultrasonic testing, as a new technology of UT, has been used widely in medical treatment and industry field because of its high accuracy, high sensitivity and high efficiency. But it is not taken into widely use in the Magnesium alloy, more systematic and deepgoing study should be required on the qualifying and quantify the defects in the magnesium alloys. The gain compensating is the key to measuring the size of defects, so this study is subject to AZ80 and AZ31, aimed at the rule of microstructure’s influence (the location and size of defects, boundary, etc.) on the gain compensating, obtaining the way to test defects in different materials and magnesium alloys in various states. The conclusions are as follows:(1) The depth of defects H are different and the gain compensating G are different. There is strict linear relationship between them, G=k1·H+b1(k1>0). And b1 is intrinsic compensating amount reflecting the material types and they are about 13 and 11 for AZ80 and AZ31 magnesium alloy; k1 is the compensating reflecting material’s grain size and orientation, and the k1 of as-cast material is larger than as-rolled material because of its coarse grain.(2) When it comes to the detection of defects near to the free surface of the object, the boundary lays a serious influence on the gain compensation. The gain compensation should be decreased as a level of 1-3dB, taking boundary effect into consideration. While the overlap of faults occurs, the accuracy of the detection will be distracted by the defects nearby.(3) The compensation G for the instrument varies from different defects’ size D. D and G have a strict linear correlation, as the equation goes:G= k2·D+b2 (k2<0), b2 is the feature compensation representing the burial depth of the defect, which grows with the burial depth; k2 is a sensitivity index standing for the required gain compensation as to different size of defects. k2 becomes larger as the defect lay further to the free surface. That is, difference of size results in the growing changes on the gain compensation.(4) As defect is orthotropic to the detecting surface, the deeper the depth is, the more the gain compensation needed, with a correlation of nonlinear. Sensitivity of the gain compensation on the defects’ size become tremendous at a small burial depth.(5) Comparing result of AZ80 to that of AZ31, burial depth, boundary distance and overlap of defects obey a similar rule to the gain compensation. As it comes to the absolute value of gain compensation, it differs from material to material. In the detection of as-cast AZ31 and rolled AZ80, both of them need small gain compensation, due to the small grain size for AZ31 and recrystallization refined grains for AZ80, respectively.