1. The metallurgical quality of raw materials Most of t […]
1. The metallurgical quality of raw materials
Most of the defects of titanium alloy forgings exist in the raw materials. Considering the actual production situation of my country's titanium industry (raw materials, processes, etc.), titanium alloys are expensive and difficult to process, and the shapes of forgings are generally more complicated. There are certain difficulties in ultrasonic detection of forgings (such as dead ends, blind areas, unfavorable detection directions, etc.). In order to prevent hidden quality hazards at the initial stage, the metallurgical quality of raw materials should be strictly controlled, and the ultrasonic acceptance standards should be strictly required. The method should also be more detailed.
For example, for titanium alloy rods, in addition to the 360° radial incident longitudinal wave flaw detection on the general circumferential surface, a 360° chord transverse wave flaw detection on the circumferential surface (generally 45° refraction angle) should be performed to ensure that straight probes cannot be detected. Surface and near-surface defects found (such as radial cracks). For titanium alloy billets, cake billets, ring billets, etc., in addition to vertical incident longitudinal wave flaw detection, it is considered that there may be cracks along the forging deformation strain line (mostly approximately 45° orientation in the cross section) and some oblique orientations For defects, the radial transverse wave inspection with a refraction angle of 45° should also be done (some foreign standards also require the inspection of 5° incident longitudinal waves in the water and the radial and chordal transverse wave inspection with a refraction angle of 60°, such as RPS705 in the United Kingdom and DPS4 in the United States. .713).
Because of the high sensitivity requirements of titanium alloy flaw detection, it is appropriate to use 5MHz for longitudinal wave detection and 2.5MHz for transverse wave detection (the two have the same wavelength in the same material). When assessing and identifying defects, sometimes higher frequencies are used (for example, the Soviet data recommends using a frequency of 20MHz).
2. Choose the appropriate 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 the subsequent thermal processing. Attention should be paid to the ultrasonic inspection of the blank and semi-finished products of the forgings, as well as the X-ray inspection, fluorescent penetrant inspection and inspection at the finished product stage. In principle, the selection of inspection methods such as anodized corrosion is basically the same as that of general forgings.
3. Several parameters to be evaluated
1. The ultrasonic acceptance standards for titanium alloy forgings and forgings are very strict, and there are many parameters required for evaluation. For the current acceptance standards for ultrasonic inspection of foreign aviation titanium alloy forgings, please refer to the list of foreign aviation titanium alloy forging ultrasonic inspection acceptance standards (the highest level).
It can be seen from Table 1 that to achieve such a high acceptance standard, not only the technical level of flaw detectors is required, but also ultrasonic flaw detectors and probes with good performance, such as high sensitivity, signal-to-noise ratio and dynamic range Larger, better linearity, lower electrical noise level, higher resolution, etc.
2. The microstructure changes of titanium alloy forgings have a significant impact on its mechanical properties. The evaluation of the clutter level and bottom wave loss in ultrasonic testing plays a role in checking the uniformity of the titanium alloy structure, and full attention should be paid . The scattering of ultrasonic on the grain boundary and intragranular phase structure may be displayed as clutter on the phosphor screen, or it may be manifested as a reduction in the height of the bottom wave caused by the attenuation of sound energy (bottom wave loss), both of which are related to the microstructure. Correspondence. According to the evaluation of these two parameters, it has been found that coarse grains, side-by-side α structure (Widmanstatten structure that can cause low cycle fatigue performance), etc. have been found. Judging from the work done so far, the microstructure of titanium alloys with high levels of clutter is mostly manifested as a complete and obvious original β grain boundary and a straight and slender Widmanstatten α structure (undeformed typical Widmanstatten structure), or Obviously there are many and large block-like alpha phases, and this kind of organization shows a decrease in strength index in terms of mechanical properties. In addition, some cast structure residues may also cause higher levels of clutter. But for the general superheated Widmanstatten structure, if the original β grain boundary and intragranular phase structure orientation is disordered and irregular, even though such a structure is not good, it is even unqualified from the microstructure evaluation, and its clutter However, the level is not necessarily high, indicating that the evaluation of the clutter level currently has greater limitations. In the evaluation of bottom wave loss, certain Widmanstatten tissues have obvious attenuation of the high-frequency components of the ultrasonic pulse (such as juxtaposed α tissue), which is easier to observe on the spectrum analyzer (Xinyuan Qian, Beijing Institute of Aeronautical Materials) Etc.), but there are certain practical difficulties in how to use ordinary ultrasonic flaw detectors for large-scale inspections in industrial production, and to select the probe with the best response frequency for inspection. It should be noted that there is currently no reliable and effective ultrasonic detection method for internal segregation in titanium alloys. In short, how to use ultrasonic response to various microstructures to control the performance and quality of titanium alloys is currently a topic that needs in-depth research (such as the use of higher frequencies, or even hundreds of MHz, and the use of electronic computers for information processing. Wait). Nevertheless, 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 flaw detection of titanium alloy materials, sometimes the structure reflection caused by a single large crystal grain or local uneven structure will appear in the form of a single reflected signal, which is easy to cause and true metallurgical defects (such as high-density inclusions, cracks, The reflected signals of holes, etc.) are confused. Through experimental analysis, it is believed that this reflected signal may be caused by the phase superposition of ultrasonic reflected waves. In this case, when using a small diameter probe or focusing probe (reducing the beam diameter), increasing the ultrasonic frequency, and reassessing with the same detection sensitivity (test block with the same flat-bottom hole diameter), you will find that the reflected signal amplitude is significantly reduced. Sometimes it even disappears, and the reflected signal of the real metallurgical defect will not change significantly in this case. This method can identify true metallurgical defects and structural reflections in titanium alloys. Of course, in the ultrasonic flaw detection of titanium alloy, it is also the same as the ultrasonic flaw detection of other materials. It is obviously impossible to judge the nature of the flaw only with the reflected pulse signal displayed by the A-type. It must be combined with the material composition characteristics and smelting of the specific flaw detection object. And forging processing technology, and supplemented by other non-destructive testing methods (such as X-ray photography, penetration, ultrasonic C-scan, etc.), plus the inspection personnel’s own experience level, etc. for comprehensive analysis and judgment, and if necessary, anatomical verification (including Macro, high magnification, even electron microscope, electron probe and other means). Therefore, at present, in the ultrasonic inspection of titanium alloy forgings and raw materials, the quality acceptance standards are basically still based on the parameters of the echo signal.