1. DR Imaging Inspection of Aluminum Alloy Castings

1.1 Composition of DR Technology

The DR system should not be limited to static imaging; it must also ensure that the workpiece operates normally throughout the inspection process. Moreover, the detector must be capable of capturing dynamic imaging during operation, which is where DR technology outperforms other inspection methods in terms of sophistication and accuracy.

DR technology enables the detection of castings by allowing X-ray penetration to project the actual substrate material thickness from any angle onto the detector, much like conventional photographic film. The maximum workpiece thickness is 230 mm. Furthermore, since the X-ray tube voltage must be at least 400 kV to accommodate the detector’s thickness requirements, an MG452-type X-ray radial source (with a maximum tube voltage of 450 kV) should be selected.

The operating principle of an industrial DR scanning imaging system can be simplified as follows: a single X-ray beam is directed at the workpiece under inspection to generate an X-ray image; the resulting digital image is captured by a digital imaging system; the image is then processed and analyzed by an analysis system; and finally, the test results are obtained.

1.2 Characteristics of X-ray DR Application in the Inspection of Aluminum Alloy Castings

For low-pressure sand-cast aluminum alloy components, the Al–Si (Si–Al) alloys ZL114A and the ZL105 series are commonly used. These two alloys exhibit superior casting performance compared with other materials; in particular, for low-pressure sand-cast parts made from ZL114A, the tensile strength σb in single tensile tests generally does not exceed 250 MPa, and the elongation at break typically reaches only about 9%. Therefore, within a certain range of applied stress, there is an inverse correlation between tensile strength and elongation.

The grain structure of casting materials is generally coarser and less uniform than that of welds, and they are more prone to various casting defects; however, their morphology is typically three-dimensional.

(1) The density of cast aluminum alloys is approximately 2.7 g/cm³, typically about one-third that of steel, which has a density of roughly 7.8 g/cm³. Aluminum’s lower density makes it significantly lighter than steel, which is precisely what is required. Moreover, X-ray inspection technology offers superior resolution compared with steel.
(2) Cast aluminum alloys exhibit relatively weak absorption of X-rays. In contrast, steel, due to its coarse grain structure, results in coarser radiographic images compared with those obtained for cast aluminum alloys, while also increasing signal noise and reducing the signal-to-noise ratio. Consequently, X-ray inspection is even more critical for cast aluminum alloys than for steel, necessitating higher-quality imaging hardware and more stringent requirements for image acquisition integration and processing algorithms.
(3) Because it is sand casting, the surface of the aluminum alloy castings is smooth, and the aluminum alloy casting (made of ZL114A material) is a relatively homogeneous block. After inspection, the resulting image shows a fairly uniform gray tone; although the wall thickness is only 1 mm, the gray regions can still be clearly distinguished. However, in actual industrial engineering—particularly in chemical production and casting operations—the as-cast surface often exhibits burrs and other surface irregularities, which can be exacerbated by impact testing. This necessitates more advanced inspection methods, and during the actual inspection process, these known sources of interference must be eliminated.
(4) Aluminum castings generally have smooth contours and geometries, resulting in imaging that is easy to identify and analyze. However, there are instances where the image quality is poor and defects are present. When the casting is not parallel to the horizontal plane of the testing equipment, a critical issue arises: during inspection, the casting must be positioned perpendicular to the testing equipment to ensure the formation of a normal inspection pattern. Currently, employing this alignment configuration has led to a substantial reduction in the number of detected defects.
(5) Single-wall DR radiographic imaging of aluminum castings generally yields good image quality; however, the image quality degrades in double-wall scans. Nevertheless, double-wall imaging can still clearly distinguish between defect images and normal images. If the casting structure includes three-layer walls or if wall structures with four or more layers are present, such conditions should be avoided as much as possible to minimize error in defect analysis. Under these circumstances, spatial resolution will decrease, leading to shadowing effects that can impair detection performance. In such cases, alternative radiographic techniques, such as film-based radiography, may be employed for inspection.
(6) For X-ray machines, those with a smaller focal spot size and higher DR image sharpness should generally be given priority. According to digital imaging theory, high-resolution imaging is achieved by using a microfocus X-ray source; however, microfocus X-ray sources have significant drawbacks, the most critical of which is the difficulty in addressing cooling issues. Under comparable spatial conditions, the spacing between aluminum castings and the distance from the gating plate should be kept as small as possible to ensure optimal DR integrated imaging quality. Furthermore, unless extremely stringent accuracy requirements are imposed, the magnification effect on defect images is not pronounced. To facilitate the production process, stepped-hole test blocks made from the same material—featuring hole sizes of 5 mm, 4 mm, 3 mm, and other variants—can be used as reference standards for measuring defect image dimensions, provided that these artificial holes are machined with very high precision.
(7) Dynamic DR scanning generally yields higher image quality than static imaging; however, in double-wall scanning, the two walls rotate in opposite directions, which allows defects to be clearly visualized. Dynamic scanning offers significantly higher efficiency and image quality. According to this study, for an aluminum cylinder casting with a diameter of 600 mm and a length of 1100 mm, a full-diameter scan takes approximately 58 minutes. When sensitivity and resolution requirements are not stringent, the dynamic scanning mode can be selected to enhance inspection efficiency.
(8) DR images are available in black-and-white; black-and-white images generally offer higher resolution than color images. In black-and-white mode, grayscale displays provide high image clarity and are effective for detecting small defects. This is likely because, with color displays, we are accustomed to viewing X-ray images on standard X-ray film.

2. Causes and Characteristics of Common Defects in Aluminum Alloy Castings

(1) Porosity in aluminum alloy castings arises during the metal solidification process, with gaseous inclusions—primarily hydrogen (H₂) or carbon monoxide—remaining in solution. These porosities typically appear on smooth surfaces, either as isolated voids or in clusters, and are predominantly composed of H₂. Hydrogen can form when oxygen is insufficiently consumed during metal melting, leading to the formation of discrete porosity. On radiographic images, these defects manifest as circular or elliptical dark areas of varying size.
(2) Pinhole defects in cast metals arise from gas-induced solidification shrinkage, resulting in a dense distribution of minute pore columns of a certain length, typically localized in specific areas of the casting. In color images, these defects appear as densely packed circular or elongated black regions, manifesting as either round pinholes or slender, linear pinholes. The length of these pinholes is significantly greater than their cross-sectional dimensions, and their detrimental effects are more severe than those of porosity.
(3) Shrinkage (or porosity): During cooling and solidification, molten metal is poured into the mold cavity, leading to irregularities such as insufficient liquid metal supply and delayed three-dimensional, three-cavity filling. This results in coarse-grained microstructures with dendritic features, often observed at locations where casting finally solidifies; when pores cluster together, they manifest as shrinkage or porosity, appearing as black bands, cloud-like patterns, and sponge-like textures.
(4) At the end of the solidification stage, when the casting has fully solidified and the temperature has dropped considerably, linear crack defects may form due to external or internal stresses, or as a result of the combined action of these two forces, leading to cracks of intermediate width. On radiographic films, these cracks appear as fine, black, curved and irregular lines, and are generally classified into two types: cold cracks and hot cracks. Hot cracks typically propagate along grain boundaries at low-melting-point regions, exhibiting a cross-cutting and branching morphology; in contrast, cold cracks usually initiate transgranularly, with two sharp tips and a slightly wider central portion. The radiographic images of cold cracks are clear, with well-defined edges and no branching. (Although cold-crack images have not yet been observed, they are expected to exhibit a similar negative appearance.)
(5) Cold-box casting: During the process, two or more streams of molten metal converge within the mold cavity. At the point of liquid junction, metallurgical discontinuities or incomplete fusion can give rise to porosity, which typically occurs on the thin surfaces of the casting. On X-ray images, these defects appear as smooth arc-shaped indications at both ends, accompanied by a black line of uniform width along the edge.
(6) Inclusions refer to impurities that remain on the surface and within the interior of a casting, with microstructural and compositional characteristics distinct from those of the surrounding matrix metal. Inclusions are generally classified into two main types: metallic inclusions and non-metallic inclusions. Metallic inclusions typically consist of high-melting-point metals, which usually appear as white areas in X-ray radiographs. Non-metallic inclusions are more complex and may include slag, sand, agglomerates formed during metallurgical reactions involving oxygen (or sulfur), silicon, lithium, and other elements. Due to their lower density compared with base metals, non-metallic inclusions often manifest in DR radiographs as irregular particles or slice-like structures, with poorly defined image contours.
(7) Detect the molten metal generated during the crystallization process, as its chemical composition does not meet the specified requirements and the composition of the molten metal is non-uniform, leading to excessive enrichment and precipitation of certain components within the crystals—commonly referred to as segregation. The precipitation of certain heavy metals typically appears as a milky white, cotton-like texture in the microstructure, and such segregation has a significant adverse impact on the material’s physical properties.

3. Conclusion
The application of X-ray DR technology in the inspection of aluminum alloy castings enables effective detection of defects, thereby playing a preventive role in reducing accidents in industrial production and significantly enhancing both product quality and production efficiency.

The application of DR technology addresses the shortcomings and limitations of conventional inspection methods, making it highly valuable as a reference for industrial engineering.

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