Micro X-ray Computed Tomography and Machine Learning Assessment of Impregnation Efficacy of Die-Casting Defects in Metal Alloys

Al-alloy die-casting, vacuum pressure impregnation, dual-energy X-ray CT Die-cast light metal alloys in various industrial applications require precise airtightness, and vacuum pressure impregnation (VPI) is typically used to seal casting defects to ensure product reliability.Evaluating the efficacy of VPI in sealing alloy defects is crucial.In this study, laboratory-based micro X-ray computed tomography (micro-XCT) was effectively employed in conjunction with advanced direct conversion CdTe semiconductor sensors to nondestructively evaluate the efficacy of standard VPI in sealing die-casting defects of industrial Al alloys.The internal casting defects and the low-atomic-number impregnation sealant distribution were visualized by adjusting the scalar opacity mapping in 3D CT.In 2D CT, it is challenging to identify the sealant resin in the narrow leakage paths of the alloy sample due to its low grey contrast, and a machine learning approach with the Trainable Weka Segmentation (TWS) plugin was applied to segment the CT images more precisely than by the traditional intensity-based image processing technique.TWS efficiently segmented the Al alloy, air pores, and diffused sealant resin in the samples, providing an in-depth analysis of the impregnation efficacy.Dualenergy XCT (DXCT) with photon-counting sensors was utilized as a quantitative method based on the effective atomic number to identify the impregnation material in the alloys as the commercially used Super Sealant P601 polymer resin.Sensors and Materials, Vol.36, No. 1 (2024) With the rapid development of CT technology, computer performance, and software, XCT in the industrial field has become well known for its faster and more cost-effective investigation than traditional evaluation methods. (3)As a robust nondestructive testing (NDT) method, XCT is increasingly widely used in various industrial sectors, particularly the automotive, aerospace, and material industries. (4,5)(8) After penetration, multiple 2D projection images on an image sensor are captured from various angles while the sample is rotated to record the X-ray intensity distribution. (9)A model of the object is then reconstructed using dedicated image processing software, enabling a comprehensive qualitative and quantitative analysis of the object's external and internal structures.CT is now used for material analysis, internal flaw detection, dimensional metrology, assembly analysis, and reverse engineering applications. (10,11)Furthermore, its capacity to capture numerous complete tomograms per second enables the real-time examination of sample transformations resulting from reactions or applied processing operations. (12)e have been investigating the efficacy of the impregnation technique in sealing die-casting defects of light metal alloys.15)(16)(17)(18) However, the use of high-pressure die-casting (HPDC) in producing automotive light-alloy castings generates scraps due to defects such as gas pores and shrinkage cracks. (19,20)nterconnected pores and leakage paths in light alloys often result in the leakage of gases, liquids, and oil.These defects can adversely affect casting products and ultimately lead to reduced performance and reliability. (21)Despite several advances in die-casting technology, completely eliminating defects remains a significant challenge.(24)(25) Nevertheless, the current standard dipping water leak test is inadequate to ensure impregnation efficacy , and a reliable, nondestructive imaging technique is required.In this study, we investigate the efficacy of vacuum pressure impregnation (VPI) in sealing Al alloy die-casting (ADC12) defects using a laboratory-based semi-automated micro-XCT, effectively using high-performance flat panel detectors (FPDs) based on a cadmium telluride (CdTe) compound semiconductor crystal as the sensor material for qualitative and quantitative analyses.These sensors operate through direct conversion, enabling robust detection.Whereas micro-XCT has been extensively applied as an NDT method for analyzing the structure and imperfections of metal alloys, (26)(27)(28)(29)(30) its potential for visualizing and characterizing impregnation resin within sealed casting defects remains largely unexplored.This is due to the resin's low atomic number and mass density, resulting in reduced X-ray attenuation. (31)Consequently, its detection in CT images is challenging owing to increased X-ray beam hardening and decreased attenuation caused by higher-energy X-ray photons.The impregnation resin primarily remains