Abstract: In order to improve the detection rate of internal defects in titanium alloy castings, starting from the principle, types, and characteristics of internal defects in titanium alloy castings, combined with their surface condition, coarse microscopic grains, and the unique process of hot isostatic pressing of castings, priority is given to using radiographic testing methods for internal defect detection. In the selection of X-ray machine parameters, priority should be given to fixed X-ray machines with tube currents greater than 10 mA. During transillumination, it is convenient to use large exposure amounts and lower tube voltages to obtain high contrast and clarity films. Discover smaller defects, improve defect detection rate, and better control the internal quality of titanium alloy castings.
Titanium alloy has the characteristics of low density, high specific strength, corrosion resistance, good mechanical properties at medium and low temperatures, and good biocompatibility. With the development and improvement of modern casting technology of titanium alloys, titanium alloy castings have been widely used in marine engineering, aerospace, petrochemical, medical prosthetics and other fields due to their high material utilization rate and the technological advantages of forming complex structures. Most titanium alloy castings are used in pressure structures with high quality requirements, which have high limitations on the allowable defect levels in castings. Non destructive testing must be used to qualitatively and quantitatively evaluate defects in products to ensure quality.
There are two main methods for detecting internal defects in castings: ultrasonic testing and radiographic testing. Ultrasonic testing uses sound waves to penetrate the interior of an object, and when the sound waves penetrate the object, they reflect at different interfaces (defect interfaces). The probe then receives the reflected waves to locate the defects. Radiographic testing is the use of X-rays or gamma rays to penetrate objects, and the intensity of the rays attenuates when penetrating the casting. When the internal material of the casting is uniform (without defects), the intensity of the rays attenuates uniformly. When there are defects, the intensity of the rays received by the film or imaging board changes due to the change in the attenuation coefficient. The shape and position of the defects are displayed by the difference in film blackness or the strength of the light signal.
Due to the following characteristics of titanium alloy castings, it has been decided to prioritize radiographic testing for the internal quality inspection of titanium alloy castings. The analysis is as follows:
(1) Irregular shape and rough surface make it difficult for the surface to couple during ultrasonic testing, and sound waves cannot enter the interior of the casting. Probes cannot be placed in irregular areas for testing;
(2) The uneven internal structure of titanium alloy castings can cause attenuation and scattering of ultrasonic waves, making it difficult for sound waves to penetrate;
(3) The casting of titanium alloys generally adopts the hot isostatic pressing process. Hot isostatic pressing causes diffusion creep and compaction of titanium alloys under high temperature and high pressure, resulting in sealing and vacuum defects such as shrinkage holes inside titanium alloy castings. Due to the fact that the temperature does not reach the melting point temperature of titanium alloys during hot isostatic pressing, although the shrinkage holes and vacuum defects inside are compacted, they do not reach atomic bonding. During ultrasonic testing, reflection occurs at the defect location and cannot penetrate.
When conducting radiographic testing on castings, it is common for defects to be missed due to the complex structure of the castings themselves, inappropriate parameter selection or radiographic method selection, and other reasons that result in low clarity of the obtained film. Especially for some important military and aerospace castings with extremely strict internal quality requirements, the omission of defects often leads to the scrapping of machined castings in the finished product stage, and even threatens the safety of spacecraft.
In order to obtain high contrast and clarity of the film, discover smaller defects, and improve the detection rate of defects, the selection of radiographic testing parameters should follow the following principles: ① In terms of energy, low tube voltage X-rays should be selected when they can penetrate the workpiece; ② The selection of focal length should meet the small focal length requirements on the nomogram; ③ In terms of radiographic methods, priority should be given to single wall testing, etc.; ④ In terms of exposure, try to choose a larger tube current and a longer exposure time.
1. Common defects in titanium alloy castings
1.1 Types of casting defects
The defects of titanium alloy castings are divided into internal defects and surface defects. Internal defects mainly include porosity, shrinkage, porosity, cracks, high-density inclusions, low-density inclusions, etc; Surface defects mainly include hot cracks, cold cracks, and cold shuts; The radiographic images of typical defects are shown in Figure 1.

Figure 1 Typical defects in X-ray inspection of titanium alloy castings
The causes of casting defects are diverse, and pores are formed by the sudden change in gas solubility in titanium liquid during the transition of metal from liquid to solid, resulting in the failure to escape in a timely manner during the solidification process. The direct cause of shrinkage and porosity is the occurrence of solid shrinkage during metal solidification without corresponding liquid metal supplementation. During solidification shrinkage of castings, if the internal stress exceeds the strength limit of the material, hot and cold cracks may occur.
Titanium alloy castings usually improve their internal quality by using hot isostatic pressing technology to remove defects such as shrinkage and porosity inside the castings. For near vacuum casting defects that are not connected to the outside world and within a certain volume range, hot isostatic pressing treatment can be used to transfer the volume change of internal hole defects to the surface of the casting through the diffusion creep of solid metal at high temperature and high pressure, forming surface depressions. These defects can be eliminated through subsequent polishing, machining, or welding.
At the same time, it should be noted that some defects cannot be eliminated by hot isostatic pressing, mainly divided into several categories: first, high-density inclusion and low-density inclusion defects; Secondly, pores, shrinkage pores, looseness, and cracks that are connected to the outside world; Thirdly, there are casting defects with large shrinkage and porosity that are not connected to the outside world. Therefore, X-ray inspection is required before hot isostatic pressing treatment of titanium alloy castings, and measures such as polishing and welding repair are taken to eliminate excessive defects.
1.2 Types of Welding Defects in Casting Repair
Defects on the surface and inside of castings can be eliminated through repair welding. Generally, the technical conditions of products allow 1-2 repair welding, and after repair welding, it is required to be tested according to the original radiographic testing process.
After casting repair welding, new repair welding defects may appear in the repair welding area. The assessment of welding defects in the repair welding area should be carried out in accordance with relevant technical documents, or in accordance with the radiographic testing standards for welded joints. The types of defects in the repair welding area of castings mainly include the following.
(1) Common welding defects: cracks, lack of fusion between layers, porosity, inclusions, tungsten inclusions, etc;
(2) Residual casting defects: The casting defects were not completely removed, and there were original defects such as shrinkage holes left at the edges of the repair welding area;
(3) Boundary defect: When the thickness of the casting is large, the repair welding area is small, and the depth is large, if the welding process is improper, non fusion defects will form at the edge of the repair welding area.
It is worth noting that even if there are no defects in the repair welding area, there is a significant difference in the blackness between the repair welding area and the casting area when the surface of the workpiece is polished flat. In most cases, the blackness is lower than the surrounding area. The main reason for the difference in blackness between the repair welding area and the casting area is the difference in composition and structure between the repair welding area and the casting area of the workpiece, which causes changes in the radiation absorption coefficient. Therefore, it cannot be simply determined as a defect in the repair welding area. Figure 2 shows the radiographic inspection film of the repair welding defects of a large casting made of ZTA15 material, with a wall thickness of 8 mm. After the defect repair welding, the surface of the workpiece is ground flat for radiographic re inspection. The blackness of the repair welding area is significantly lower than that of the surrounding area, and the image features of the repair welding defects are the same as those of the welded joint defects.

Figure 2 Image of Welding Defects in Casting Repair
2. Factors affecting the detection rate of casting defects
The factors that affect the defect detection rate of titanium alloy castings include the selection of radiation source, focal length, radiation energy, division of transmission range, and other factors. Due to the irregular structure of most castings, the inspection process of castings has its own particularity.
2.1 Ray sources and energy factors
The X-ray machine is mainly used as the X-ray source for the detection of titanium alloy castings, and sometimes gamma sources are also used for thick walled castings. Comparative test Table 1 shows that when using gamma sources such as Se75 and Ir192 to irradiate titanium alloy castings, their sensitivity and blackness meet the standard requirements and can evaluate defects in that area. However, compared with the negative film obtained by X-ray exposure, its defect contrast and clarity are poor. Especially when using high-energy gamma sources such as Co-60 for transparency of thick walled castings, the sensitivity of the film decreases and the clarity becomes worse. Therefore, Co-60 is generally not used for radiographic testing of titanium alloy castings, and Se75 and Ir192 should not be used as radiation sources when the wall thickness is thin.
Table 1 X-ray and Gamma Ray Comparison Tests

Note: Se75, Ir192, and Co-60 were used for radiographic examination of different plate thicknesses. Compared with X-rays, the clarity of the film was generally 1-2 wire diameters lower, with - indicating no radiographic examination of that thickness.
When using an X-ray machine for radiography, the selection of tube voltage is also limited by the allowable tube voltage. According to the requirements of Figure 3, when detecting castings with thinner wall thickness, it is advisable to use a fixed X-ray machine with a larger tube current (greater than 10 mA) to achieve the standard film blackness value. Using a lower tube voltage results in a longer radiation wavelength and a larger attenuation coefficient. According to formula (1), the larger the attenuation coefficient μ, while keeping other parameters constant, the greater the blackness difference, and larger contrast can be obtained on the X-ray film to discover smaller defects, thereby improving the defect detection rate. When detecting areas with significant changes in casting thickness, considering the issue of thickness tolerance, the selected tube voltage is usually slightly higher than the allowable value, but * usually cannot exceed the allowable value of 40 kV.

Figure 3: Permissible X-ray tube voltage for different radiographic thicknesses

　　In the formula: Δ D - Blackness difference;
μ - attenuation coefficient of radiation on matter;
G - Film gradient;
Δ T - defect size in the direction of ray incidence, mm;
N - Scattering ratio.
2.2 Factors related to transparency methods and parameters
Single wall radiographic testing is usually used for titanium alloy castings, and double wall radiographic testing is only allowed when the structure is special and single wall radiographic testing cannot be performed. During penetration, the ray beam is vertically incident on the surface of the casting, and the ray window should be aligned with the center position of the detection area. The selection of radiographic focal length is consistent with the requirements for welding joint detection, and the small focal length is determined using formulas (2), (3), or Figure 4 based on the requirements of geometric opacity.
The distance f from the selected radiation source to the surface of the workpiece should meet the requirements of formulas (2) and (3):
A-level radiographic testing technology:

　　In the formula: d - focal size of the radiation source, mm;
B - Distance from the workpiece source side to the film, mm.
B-level radiographic testing technology:

　　In the formula: d - focal size of the radiation source, mm;
B - Distance from the workpiece source side to the film, mm.

Figure 4 Nomogram of the Small Value of the Surface * of the Ray Source and the Workpiece Source Side
2.3 Factors for dividing the translucent area
The radiographic testing of titanium alloy castings does not have strict restrictions on the thickness ratio of the radiographic inspection compared to welding joints. Therefore, the division of the radiographic inspection range is usually based on the structure of the casting, the distribution characteristics of defects, and the blackness requirements of the film. When the structure of the casting is complex, the transparency range needs to be divided according to the characteristics of the transition section, such as dividing the flat and curved parts into different detection areas; When castings are prone to defects in certain specific areas, that area should be treated as a separate inspection area; For crack like defects, attention should also be paid to the angle of incidence of the rays to improve the defect detection rate. The rationality of dividing the transparency range should be measured based on the range of changes in the film density, and it is necessary to ensure that the film density range is within the standard requirements. Structures with significant thickness changes need to be more finely divided into single effective transparency areas.
When dividing the radiographic area, it is necessary to fully understand the structural parameters of the inspected object, calculate the radiographic thickness at different positions based on the incident angle of the ray, and thus select the exposure specifications for different radiographic areas. If necessary, it is necessary to repeatedly verify through comparative experiments to determine the transmission zone of *. At present, computer process simulation technology has gradually been applied to casting inspection. Through the 1:1 structure of radiographic inspection process simulation, not only can ideal radiographic zoning be achieved, but also important reference can be provided for the selection of process parameters (see Figure 5).
The product in Figure 5 is a machined titanium alloy circular casting with a simple structure, which is divided into 6 radiographic zones, A~F, based on the diameter and thickness of the casting.

Figure 5 Example of dividing the translucent area
2.4 Other factors
2.4.1 Film layout
The radiographic testing of castings uses a variety of film specifications and sizes, usually preparing films of different specifications and sizes based on the size of the single radiographic area. Regardless of the size and specification of the film used, it is necessary to ensure a tight fit between the film and the casting, ensuring the clarity of the image.
2.4.2. Scatter ray control
For casting inspection, scattering line shielding is also an essential measure to improve film quality, and lead foil intensifying screen is a commonly used method. For cavity type castings with multiple internal transition structures, when using the single wall external penetration method for inspection, the rays penetrating the single-layer wall thickness will undergo complex scattering in the workpiece cavity, and additional scattering line shielding measures need to be taken* A convenient method is to use a certain thickness of lead film to cover the back of the film, or to use a lead cover or aperture to control the radiation field range within a range close to the transparent area.
2.4.3 Film density requirements
When inspecting investment casting precision parts, it is recommended to increase the blackness of the film to 2.3 or above, in order to facilitate the detection of high-density inclusions. The defects of high-density inclusions are displayed in white on the film, as shown in Figure 1 (b); If the film has a low blackness and the defect location is very close to the blackness of the casting body, it is not easy to be detected. When conducting radiographic testing on graphite castings, attention should be paid to the inspection of the appearance of the castings, because the graphite cavity is rougher than the precision cavity of the investment mold, and the fluidity of the titanium liquid during casting is poor, which is prone to the formation of surface flow marks defects. In addition, when using graphite mold to cast thin-walled castings (with a wall thickness less than 10 mm), if the casting material is a material with a tendency to crack, such as ZTC4 material, special attention should be paid to the generation of crack defects, because graphite itself conducts heat quickly and generates casting stress during casting, which can easily form crack defects. When selecting parameters for radiographic testing, the crack detection angle K value should be less than 1.02 in order to improve the detection rate of crack defects.
2.4.4 Inspection of repair welding area
For the detection of repair welding positions, in the direction of penetration, the ray beam is perpendicular to the welding point position, which is conducive to the detection of area type defects that have not been fused.
Because drilling is often used to eliminate casting defects, especially for some thick walled castings, defects often occur in the middle of the casting. After the defects are eliminated, the repair welding position is often a cylindrical deep hole, which is prone to incomplete fusion welding defects during the repair welding operation.
3. Conclusion
Based on the structural characteristics, defect generation principles, and defect characteristics of titanium alloy castings, the preferred method for internal quality inspection of titanium alloy castings is radiographic testing. The clarity of radiographic testing is affected by the following factors.
(1) Exposure factor. When the exposure increases, the ratio of the contrast of the detail image to the graininess of the film image will increase, making the detail image easy to recognize. In addition, exposure indirectly affects the contrast of detailed images by affecting the film density. To ensure that defects can be detected, the exposure must reach a certain size.
(2) Factors related to the method of translucency. The basic principle of determining the radiographic method is beneficial for defect inspection, which is to choose a radiographic method that is suitable for defect inspection. Therefore, when determining the radiographic method, it is necessary to have a certain understanding of the defect characteristics (nature, extension, position, etc.) in the workpiece.
(3) Focus factor. The basis for determining the focal length is: ① The selected focal length must meet the requirements of geometric opacity; ② The selected focal length should provide a relatively uniform and appropriately sized primary radiographic area. The former limits the * small value of the focal length, while the latter guides how to determine the actual focal length value used.
(4) Scattering radiation protection factors. In the radiographic inspection of titanium alloy castings, the main measures to reduce the scattered lines reaching the film are: shielding, backing plate, filtering, aperture, etc. In addition, a metal intensifying screen of appropriate thickness can be used.
(5) Translucency angle factor. Try to make the direction of the main ray perpendicular to the tangent direction of the workpiece surface as much as possible. For example, for circular structural components, the circumferential radiographic method is preferred. The thickness ratio of the radiographic method is 1 throughout the week, which can complete all the radiographic work in one go, improving work efficiency. More importantly, the detection rate of harmful defects such as cracks is greatly improved compared to other radiographic methods.
Taking into account the above factors, selecting appropriate radiographic parameters can obtain high-definition X-ray films and improve the detection rate of defects in titanium alloy castings.