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I. Aluminum alloy pipes: Aluminum alloy materials are widely used in industry today, especially in air separation cryogenic unit separation systems, where they serve as a commonly used type of process pipe. Due to the inherent properties of aluminum alloy materials and the particularities of the production processes in air separation plants, the processes and requirements for degreasing, assembly, welding, and flaw detection of aluminum alloy pipes differ from those applicable to ordinary carbon steel materials. Properties of aluminum alloy materials: The main chemical components of aluminum alloy pipes are Mg at 2.0%–2.8%, Mn at 0.4%–1.5%, with the remainder being Al. Mechanical properties: In the hot-rolled state, its tensile strength is not less than 226 Mpa; the melting point is 650°C, and there is no change in color during melting. II. Weld joint alignment structure and welding characteristics of aluminum alloy pipes: Mechanical processing is used to prepare the grooves for the fabrication and installation of aluminum alloy pipes. Butt welds with different wall thicknesses should have a 14° transition zone. Regarding pipe weld groups, the following points should be noted: 1) Forced welding should be avoided to prevent large residual stresses from being generated during welding. 2) The inner walls of the butted weld joints must be aligned; the misalignment shall not exceed 10% of the wall thickness, nor more than 2 millimeters. 3) The inner wall of the pipe opening must be smooth, free of burrs and debris. 4) The gasket on the inner wall of the pipe must be in close contact with the pipe wall. 5) For welds on the inner wall of the pipe without gaskets, the gap should be as close to zero as possible. To ensure welding quality, the gasket used for aligning the weld joints is often a dual-gasket structure consisting of an aluminum alloy gasket combined with a 1.5-mm-thick stainless steel gasket, as shown in Figure 1. The function of the stainless steel backing plate is to ensure that the groove can be welded through easily, while preventing burn-through defects in the aluminum backing plate. Welding aluminum alloys is challenging, as defects such as pores, incomplete welding, lack of fusion, and cracks are likely to occur during the welding process. Aluminum alloy welding has the following characteristics: 1) Aluminum alloys have good thermal conductivity, resulting in rapid heat dissipation during welding; high welding currents can easily lead to the formation of pores. 2) If the welding groove of aluminum pipes is contaminated, pores, slag inclusions, and lack of fusion are also likely to occur during welding. 3) The liquid melt pool of aluminum alloys easily absorbs gases, thereby forming pores. 4) During the heating of aluminum alloys, as they transition from a solid state to a liquid state, there is little change in color, which poses difficulties in welding operations and can lead to incomplete welding and lack of fusion. 5) The linear expansion coefficient of aluminum alloy is twice that of steel; during welding, excessive shrinkage stresses often lead to cracks in the welds. III. Common defects in aluminum alloy pipe welds: Welds of aluminum alloy pipes frequently suffer from welding defects such as pores, slag inclusions, lack of penetration, incomplete fusion, and cracks. 1) Porosity: Porosity is the most common defect. In the flat welding position, pores are mostly found in the center of the weld, while in the grooving welding position they are primarily located above the center of the weld, often as a cluster of small pores. Its occurrence is due to various factors, such as impure argon gas, incomplete removal of the base material and welding wire, and disruption of the argon gas shield during welding. 2) Slag inclusions: These appear mainly in the center of the weld, in the form of fine particles, and sometimes they connect together to form a line. The common causes include incomplete removal of the oxide film, excessive dust in the environment, and impure argon gas. 3) Underpenetration: The defect of underpenetration often occurs in welds without backing plates; on film, it is usually found at the center of the weld, primarily due to the oxide film preventing fusion. It can also occur in welds with shims, and the main reasons for this are improper welding techniques or incorrect operator practices. 4) Cracks: Cracks can form in longitudinal and transverse directions, as well as in the root area or as arc crater cracks. Transverse cracks are more common in welds without shims; upon inspection of the actual specimens, it can be seen that most of them are surface cracks located on the back side of the weld. The reason is that an excessive amount of time was spent welding at this location, causing the weld metal on the back side to crack as it cooled and contracted. In the welds with gussets, longitudinal cracks are common, mostly occurring in the welds of large-diameter pipes, often as a result of forced alignment. This defect appears sometimes at the center of the weld and sometimes in the heat-affected zone. Radially distributed arc crater cracks often appear at the end of welding, mainly due to improper termination of welding. IV. Radiographic inspection method for welds of aluminum alloy pipes: In accordance with the provisions of standard JB/T4730-2005, for butt welds of pipes with a large diameter that allow for the application of flanges on the inner wall, single-wall single-projection radiography should be used ； For pipe butt welds with D0 > 100 mm, double-wall single-shadow radiography should be used ； For D0≤100mm, when T (wall thickness) ≤8mm and g (weld width) ≤D0/4, double-wall double-shadow radiography should be used. For prefabricated openings as well as fixed openings where patches can be applied to the inner wall, there is no issue with conducting inspections using conventional methods. For fixed openings with an inner wall diameter of D0 > 100 mm where no patches can be applied, if the outer diameter is not too large (such as Φ159), the conventional approach is to remove the upper weld seam near the source while leaving the lower weld seam on the side of the patch. To ensure a high detection rate of longitudinal cracks, the inclination angle θ of the radiation source should not exceed 15°. In actual flaw detection, since the width of the aluminum alloy backing plates is 30–40 mm and that of the stainless steel backing plates is 22–25 mm, if θ < 15° is to be maintained, the excessive width of these backing plates means that during irradiation, the X-rays will inevitably pass through both the lower weld seam and the two backing plates below. Some of the rays will pass only through the upper base material without passing through the backing plates, some will pass through the aluminum alloy backing plates, and yet others will not only pass through the aluminum alloy backing plates but also through the stainless steel backing plates. At an X-ray tube voltage of 100 KV, the radiation absorption coefficient of aluminum alloy is 0.1 (compared to 1 for steel); as a result, 1 mm of stainless steel is equivalent to 10 mm of aluminum alloy. This leads to large variations in the thickness that can be penetrated by the X-rays, making it difficult to select appropriate penetration conditions. The images on the film thus have high contrast, with some areas of the welds appearing too white and others too dark, failing to meet the standard requirements regarding film density. If the upper weld seam together with the gasket is removed, it is necessary to increase the inclination angle of the ray, thereby reducing the detection rate of longitudinal cracks. For fixed openings with D0≤100mm, a stainless steel gasket is generally sufficient. If a conventional double-wall double-projection radiography is used, causing the upper and lower welds to appear as ellipses on the film, and if the opening gap is kept at a level comparable to the weld width, the problem of low detection rate for longitudinal defects still exists. What is worse, due to the small diameter of the welds on the pipes under inspection, the images of the upper and lower stainless steel gaskets overlap more on the film. Additionally, since the walls of such thin-walled pipes are already thin, the thickness of the aluminum alloy is not significant compared to that of stainless steel; as a result, the weld images on the film are very faint, making it difficult to assess any defects. It can be seen that, due to the structural peculiarities of aluminum alloy pipe welds during assembly, it is not possible to perform radiographic inspection on them using conventional methods. V. Practical Applications: To obtain satisfactory film results, it is necessary to ensure that the total penetration depth of the rays as they pass through the weld under inspection does not differ too greatly from the total penetration depth when they pass through the surrounding base material, especially for materials with a high absorption coefficient such as aluminum alloys. When the rays are directed perpendicularly through the weld, whether in a double-wall single-shadow or double-wall double-shadow inspection, the rays pass through both stainless steel backing plates (and also through two aluminum alloy backing plates in the case of double-wall single-shadow inspection). The total thickness that the rays traverse while passing through the weld is not significantly different from the total thickness they traverse while passing through the base material; as a result, the contrast of the weld image is relatively uniform, defects are easy to detect, and photography is also simpler. For pipe welds with D0 > 100 mm, since double-wall single-shadow radiography is required, the quality of the single-wall weld is evaluated. During radiography, the distance between the X-ray machine and the pipe (the focal length) should be reduced; by shortening the focal length, the X-rays can pass through the stainless steel backing plate of the upper weld, which helps to avoid the adverse effects resulting from the overlapping projections of the upper and lower weld backing plates when using oblique radiography. For pipe welds with D0≤100 mm, since double-wall double-shadow radiography is required, both the upper and lower welds must be evaluated. It is necessary to ensure the image quality of both welds; therefore, the focal length between the X-ray machine and the pipe should be increased to improve the clarity of the film. Since the images obtained from vertical radiography of the upper and lower welds overlap, if only two vertical radiographies are performed as specified by the standards, it will be impossible to determine whether a defect is present in the upper weld or the lower weld if such a defect exists on the film. To address this issue, an X-ray examination can be performed every 120°; by comparing the films from the three different directions, it is possible to accurately determine which weld contains the defect. Furthermore, to further improve the clarity of the film, fine-grained T2 film with high clarity can be used in place of coarse-grained T3 film with moderate clarity. Radiographic inspection method for welds of aluminum alloy pipes: 1) For prefabricated openings and fixed openings that can have patches applied to their inner walls, single-wall, single-shadow radiography should be used. 2) For prefabricated openings with an inner wall diameter of D0＞100mm where pasting is not possible, double-wall single-shadow oblique radiography should be used. 3) For prefabricated openings with D0 ≤ 100 mm, double-wall double-shadow oblique radiography should be used. 4) For fixed openings with an inner wall diameter of D0＞100mm where patching is not possible, double-wall single-shadow vertical radiography should be used. A shorter focal length can be used, or the radiographing device can be placed directly on the outer wall of the tube for imaging, but the requirements regarding geometric blur must be met. 5) For fixed openings with D0 ≤ 100 mm, double-wall double-shadow vertical radiography should be used; to ensure 100% coverage of the welds and to determine the exact location of defects, a radiograph should be taken every 120°.