1. Metallurgical quality of raw materials
Most of the defects of titanium alloy forgings exist in the raw materials. Considering the actual situation of China’s titanium industry production (raw materials, processes, etc.), coupled with the expensive and difficult processing of titanium alloys, the shape of forgings is usually more complicated, which makes certain difficulties in ultrasonic inspection. Forgings (such as dead corners, blind spots and unfavorable detection directions, etc.). In order to prevent potential quality hazards as early as possible, the metallurgical quality of the raw materials should be strictly controlled, and ultrasonic acceptance standards should be strictly required. The method should also be more detailed.
For example, for a titanium alloy round bar, in addition to longitudinal longitudinal wave inspection on the entire circumferential surface of 360 °, 360 ° circumferential transverse wave inspection (usually 45 ° refraction angle) should also be performed to ensure that no straight line can be found. The needle found surface and near-surface defects (for example, radial cracks). In addition to normal incidence longitudinal wave flaw detection, for titanium alloy billets, cake billets, ring billets, etc.,
Due to the high sensitivity of titanium alloy flaw detection, it is best to use 5MHz for longitudinal wave flaw detection and 2.5MHz for transverse wave flaw detection (two wavelengths in the same material are the same). When evaluating and identifying defects, higher frequencies are sometimes used (for example, the 20 MHz frequency recommended by Soviet sources).
2. Choose the right detection method
In order to ensure the quality of titanium alloy forgings, in addition to strictly controlling the quality of raw materials, it is also necessary to prevent defects in subsequent hot processing. Attention should be paid to ultrasonic flaw detection, X-ray flaw detection, fluorescence penetration flaw detection and principle inspection of forged blanks and semi-finished products. In principle, the selection of inspection methods such as anodizing corrosion and general forgings.
3. Several parameters need to be evaluated
- 1. The ultrasonic acceptance standards for titanium alloy forged blanks and forgings are very strict, and many parameters are required for evaluation. At present, the acceptance standards for ultrasonic flaw detection of foreign aviation titanium alloy forgings have been listed in the list of foreign aviation ultrasonic flaw detection standards (the highest level).
It can be seen from Table 1 that to achieve such a high acceptance standard, not only the technical requirements for flaw detection personnel are very high, but also the sensitivity, signal-to-noise ratio, dynamic range and other properties of ultrasonic flaw detectors and probes are better. Larger, better linearity, lower electrical noise level, higher resolution, etc.
- 2. The microstructure changes of titanium alloy forgings have an important influence on their mechanical properties. The evaluation of clutter level and bottom wave loss in ultrasonic flaw detection plays an important role in checking the uniformity of the structure of the titanium alloy, and should be given sufficient attention. The scattering of ultrasonic waves on the grain boundaries and the intragranular phase structure may appear as chaos on the fluorescent screen, or may appear as a decrease in the height of the bottom wave due to attenuation of sound energy (bottom wave loss). . correspond. Based on the evaluation of these two parameters, coarse grains and side-by-side α-structures (Webster structures that may lead to low-cycle fatigue properties) were found. From the work done so far, the microstructure of the titanium alloy with high clutter level mainly manifests as a complete and obvious original β grain boundary and an elongated Weissmann α structure (undeformed typical Weissmann structure) , Or there are many large α-phase bands, the performance of this type of structure appears as a decrease in strength index. In addition, some casting tissue residues may also cause higher levels of confusion. However, for the general superheated Wei’s structure, if the original β grain boundary and intragranular phase structure are more disordered and irregular, although this structure is not good, even if the microstructure evaluation is not qualified, its clutter level may not be very high High, indicating that the evaluation of clutter level still has relatively large limitations. When evaluating the bottom wave loss, some Wei’s tissues have more obvious attenuation of the high-frequency components of ultrasonic pulses (such as parallel α tissues), which is more easily observed on the spectrum analyzer (Beijing Institute of Aeronautical Materials Qian Xinyuan, etc.) ), But there are some practical difficulties in how to use ordinary ultrasonic flaw detectors and choose the probe with the best response frequency for large-scale inspection in industrial production. It should be noted that there is currently no reliable and effective ultrasonic testing method to detect internal segregation of titanium alloys. In short, how to use ultrasound to respond to various microstructures of titanium alloys to control the performance and quality of titanium alloys is currently the subject of in-depth research (such as the use of higher frequencies, even hundreds of megahertz, and the use of electronics The computer is acquiring information) Processing is waiting). However, in the current ultrasonic flaw detection of titanium alloy forgings and materials, the evaluation of clutter level and bottom wave loss are still two very valuable indicators.
- 3. In the ultrasonic inspection of titanium alloy materials, sometimes the tissue reflection due to a single large grain or local tissue unevenness will appear in the form of a single reflected signal, which is easy and true metallurgical defects (such as high density inclusions) ), Cracks, cracks, reflected signals from holes, etc.), and through experimental analysis, it is believed that the reflected signal may be caused by the superposition of the phase of the ultrasonic reflected wave. In this case, when using a small-diameter probe or a focusing probe (reducing the beam diameter) to increase the ultrasonic frequency and re-evaluate with the same detection sensitivity (a test block with a flat bottom hole of the same diameter), the signal is reflected The range will be greatly reduced. Sometimes it even disappears. In this case, the reflected signal of real metallurgical defects will not change significantly. This method can identify true metallurgical defects and structural reflections in titanium alloys. Of course, in the ultrasonic inspection of titanium alloys, as with the ultrasonic inspection of other materials, it is obviously impossible to use only the reflected pulse signal displayed by the A type to judge the nature of the defect, and it must be combined with the material composition for the specific inspection object and forging process Characteristics and melting, supplemented by other non-destructive testing methods (such as X-ray photography, penetration, ultrasonic C-scanning, etc.), and the flaw detector ’s own experience level, etc., for comprehensive analysis and judgment, if necessary, anatomical verification (including Macro, high magnification, even electron microscope, electron probe, etc.). Therefore, at present, in the ultrasonic inspection of titanium alloy forgings and raw materials, the quality acceptance standard is still basically based on the parameters of the echo signal.