钛合金 has a small specific gravity (about 4.5), high melting point (about 1600 ℃), good plasticity, has a high specific strength, corrosion resistance, can work at high temperatures for a long time (the current hot strength titanium alloy has been used for 500 ℃) and other advantages, and thus has been increasingly used as an important load-bearing parts of aircraft and aircraft engines, in addition to titanium alloy forgings, there are castings, plates (such as aircraft skin), fasteners, etc. . The weight ratio of titanium alloy used in modern foreign aircraft has reached about 30%, which shows that the application of titanium alloy in the aviation industry has a broad future. Of course, titanium alloys also have the following disadvantages: for example, high deformation resistance, poor thermal conductivity, large notch sensitivity (about 1.5), changes in the microstructure of the mechanical properties of the more significant impact, which leads to the complexity of smelting, forging processing and heat treatment. Therefore, the use of non-destructive testing technology to ensure the metallurgical and processing quality of titanium alloy forgings is a very important subject.
Defects that easily appear in titanium alloy forgings
1). Bias-type defects
In addition to β deviation, β spot, titanium-rich deviation and strip α deviation, the most dangerous is the interstitial type α stable deviation (type Ⅰ α deviation), which is often surrounded by tiny holes, cracks, containing oxygen, nitrogen and other gases, brittle. There is also aluminum-rich alpha stable segregation (type II alpha segregation), which is also accompanied by cracks and brittleness and constitutes a dangerous defect.
Mostly high melting point, high density of metal inclusions. By the titanium alloy composition of high melting point, high-density elements are not fully melted in the matrix formation (such as molybdenum inclusions), but also mixed in the smelting of raw materials (especially recycled materials) in the carbide tool chipping or improper electrode welding process (titanium alloy smelting generally uses vacuum self-consumption electrode remelting method), such as tungsten arc welding, leaving a high density of inclusions, such as tungsten inclusions, in addition to titania inclusions, etc. .
The presence of inclusions can easily lead to the occurrence and expansion of cracks, so it is not allowed to exist defects (for example, the Soviet Union in 1977, the Soviet Union’s information, titanium alloy X-ray radiography inspection found 0.3 – 0.5 mm in diameter of high-density inclusions must be recorded).
3). The residual shrinkage
4). The hole
Holes may not necessarily exist singly, but also may be more than one dense presence, which will accelerate the expansion of low circumferential fatigue cracks and cause early fatigue damage.
Mainly refers to forging cracks. The viscosity of titanium alloy is large, poor mobility, coupled with poor thermal conductivity, so in the forging deformation process, due to surface friction, internal deformation is not uniform and the temperature difference between inside and outside, etc., easy to produce shear zone (strain line) inside the forging, which in serious cases leads to cracking, and its orientation is generally along the direction of the maximum deformation stress.
Titanium alloy thermal conductivity is poor, in addition to improper heating in the thermal processing process caused by forgings or raw materials overheating, in the forging process is also easy to cause overheating because of the thermal effect of deformation, causing microstructural changes, resulting in overheating Weiss organization.
Several Problems in Ultrasonic Flaw Detection of Titanium Alloy Forgings
In addition to the general forging ultrasonic flaw detection method should be noted in addition to the problems, titanium alloy forgings ultrasonic flaw detection and the following issues need to be noted.
1). The metallurgical quality of raw materials
Most of the defects described in the second part are in the raw materials, combined with consideration of the actual situation of China’s titanium industry production (raw materials, processes, etc.), coupled with the expensive titanium alloy, processing difficulties, and the shape of forgings are generally complex, making the forging ultrasonic flaw detection there are certain difficulties (such as dead ends, blind spots, detection direction is unfavorable, etc.), in order to block the quality of hidden problems as early as possible in the initial In order to stop the hidden quality problems at the initial stage, the metallurgical quality of raw materials should be strictly controlled, and the ultrasonic acceptance standards should be strictly required, and the methods should be more detailed.
For example, for titanium alloy round bar, in addition to the general circumferential 360 ° radial incidence longitudinal wave detection, should also be made circumferential 360 ° chordal transverse wave detection (refraction angle is generally 45 °), to ensure that the discovery of straight probe can not be found on the surface and near-surface defects (such as radial cracks). For titanium alloy billet, pie billet, ring billet, etc. in addition to vertical incidence of longitudinal wave inspection, taking into account the possible existence of cracks along the forging deformation strain line (in the 横穿-section of the approximate 45 ° orientation) and some tilt orientation of the defect, should also be 45 ° refraction angle of the radial transverse wave inspection (some foreign standards also require 5 ° incidence of longitudinal wave inspection and refraction angle of 60 ° radial, chordal transverse wave inspection, such as the British RPS705 and the U.S. RPS705. (such as the British RPS705 and the United States DPS4.713).
Due to the high sensitivity requirements of titanium alloy flaw detection, it is appropriate to use 5MHz longitudinal wave detection, transverse wave detection with a frequency of 2.5MHz (both in the same material wavelength equivalent). In the assessment, identification of defects, and sometimes use higher frequencies (such as the Soviet Union data recommended the use of 20MHz frequency).
2). Choose the appropriate detection method
In order to ensure the quality of titanium alloy forgings, in addition to strict control of the quality of raw materials, but also must prevent defects in the subsequent thermal processing, should pay attention to the forgings of the rough and semi-finished ultrasonic flaw detection, as well as the finished stage of X-ray flaw detection, fluorescence penetrant flaw detection and anodized corrosion and other inspection means, the choice of methods in principle and general forgings are basically the same.
Titanium alloy forgings microstructure changes on its mechanical properties have a more significant impact on the ultrasonic flaw detection of the level of clutter and bottom wave loss assessment play a role in checking the uniformity of titanium alloy tissue, should be given full attention.
Ultrasonic scattering at the grain boundaries and intracrystalline phase organization may be shown on the fluorescent screen as a spurious wave, may also be manifested as acoustic energy attenuation caused by the reduction in the height of the bottom wave (bottom wave loss), both of which have a certain correspondence with the microstructure. Based on the evaluation of these two parameters, coarse crystals, juxtaposed α tissue (Weiss tissue that can cause a decrease in low circumferential cyclic fatigue performance), etc. have been found.
In terms of the work done so far, the microstructure of titanium alloys with high levels of spurious waves, mostly manifested as a complete and obvious original β grain boundaries and flat and elongated Weiss alpha organization (undeformed typical Weiss organization), or appear to have more and large lumpy alpha phase, this kind of organization in the mechanical properties of the strength index decline. In addition, some casting tissue residues may also cause a high level of stray waves. But on the general superheated Weiss organization, if the original β grain boundaries and intracrystalline phase organization is more disorderly and irregular, although such an organization is bad, even from the microstructure assessment is not qualified, the level of clutter is not necessarily high, indicating that the assessment of the level of clutter is still a large limitation.
In the assessment of the bottom wave loss, certain Weiss organizations have more obvious attenuation of the high-frequency component of the ultrasonic pulse (such as parallel alpha organization), which is easier to observe on the spectrometer (Beijing Institute of Aeronautical Materials Qian Xinyuan, etc.), but there are certain practical difficulties in the industrial production of high-volume inspection of how to use ordinary ultrasonic flaw detector, the choice of * response frequency probe for detection.
It should be noted that there is also no reliable and effective ultrasonic detection method for internal deviations of titanium alloys.
In short, how to use ultrasonic response to a variety of different microstructure to control the quality of titanium alloy properties, is currently the subject of in-depth research (such as the use of higher, even hundreds of megahertz frequency, as well as the use of electronic computers for information processing, etc.). Nevertheless, in the current ultrasonic flaw detection of titanium alloy forgings and materials, the evaluation of spurious wave level and bottom wave loss are still two very valuable indicators.
In the ultrasonic flaw detection of titanium alloy materials, sometimes the tissue reflection caused by a single large grain or local tissue inhomogeneity will appear in the form of a single reflection signal, which is easily confused with the reflection signal of real metallurgical defects (such as high-density inclusions, cracks, holes, etc.), which may be due to the phase superposition of ultrasonic reflection waves by experimental analysis. In this case, the use of small diameter probe or focus probe (reduce the beam diameter), increase the ultrasonic frequency to the same detection sensitivity (flat bottom hole diameter of the same test block) when re-evaluated, it will be found that the reflected signal amplitude significantly decreased, and sometimes even disappeared, while the true metallurgical defects of the reflected signal in this case will not have significant changes. This method can identify the true metallurgical defects in titanium alloys with tissue reflections.
Of course, in the titanium alloy ultrasonic flaw detection, and other materials, like ultrasonic flaw detection, attempts to show only A-type reflection pulse signal to determine the nature of the defect is obviously impossible, must be combined with the specific flaw detection object material composition characteristics, smelting and forging process, as well as supplemented by other non-destructive testing means (such as X-ray radiography, infiltration, ultrasonic C-scan, etc.), coupled with the flaw detection personnel’s own experience Level and other comprehensive analysis and judgment, if necessary, anatomical verification (including macro, high magnification, and even electron microscopy, electron probe and other means). Therefore, at present, in the titanium alloy forgings and raw materials ultrasonic flaw detection, its quality acceptance criteria are still basically based on the parameters of the echo signal.
Examples of defects of titanium alloy forgings and materials
1. Residual shrinkage in a Φ70mm titanium alloy forging bar
Longitudinal wave (top for the longitudinal waveform photo) and transverse wave (bottom for the transverse waveform photo) can be found, longitudinal wave detection as a strong defect echo and cause bottom wave reduction (area type defects, can be roughly judged as radial direction), transverse wave detection as a clear and strong defect echo (crack-like defects). The right figure shows the transverse low magnification photo (1x).
2. The molybdenum inclusions in the titanium alloy cake billet (high density inclusions)
This is smelting as aluminum and molybdenum intermediate alloy in the molybdenum is not completely melted and left in the matrix to form, available longitudinal wave detection, regardless of changing the ultrasonic frequency and ultrasonic beam diameter can be well found, and the location corresponds well in both sides of the detection. After dissection, it was verified as molybdenum inclusions. In the transverse low multiple more “eye”, in the cake billet orientation more parallel to the end face, but some will be oriented inclined, in the cake billet is not easy to find, to be forged into disc-shaped parts due to deformation forces to change its orientation to parallel to the end face is easy to find. The left picture is a transverse low magnification photo (2x), and the right picture is an X-ray photo taken in the direction of ultrasound beam projection (the outer circle is lead wire, and the white dot in the middle is a high density inclusions – molybdenum inclusions)
a) 45° crack on ring billet transverse low magnification x 1/2
b) High magnification 100x of the crack in the ring billet on the left
c) 45° crack on the pie billet transverse low times x 1/2
d) End angle 45° crack on the pie billet brought to the die forging plate for machining to semi-finished product exposed 1x
e) Cross cracks on the forging plate x1/2
3. 45° cracks in titanium alloy pie (ring) billets and cross cracks on forged plate parts
These cracks are caused by forging, especially when forging pie (ring) billets from titanium ingots, often due to low terminal temperature, excessive hammering force, etc. and cracking along the zui large deformation stress direction. Most of these cracks in the opening bridging tighter, or the entire crack on the gap degree is very uneven, local bridging very tight, after forging mechanical processing to semi-finished products, if the surface happens to be at the bridging tighter parts, then the corrosion or penetration method may not be detected sometimes, but its internal cracking and larger, and even the appearance of holes (such as photo b)). Using 45° refraction transverse wave is easy to detect and can be judged.
a) Transverse low magnification x 1/2
b) Surface crack coloring penetration display x1
4. Radial surface cracks on Φ70mm titanium alloy rolled bar
These cracks also belong to cracks formed in forging or rolling process, which can be found by corrosion or infiltration method. It is easy to detect by using 45° refraction transverse wave for circumferential chordal sweep, while it cannot be detected by general longitudinal wave circumferential radial incidence detection.
a) Transverse low magnification x 1/3
b）Longitudinal low magnification x1/2
c）Transverse high times x500 at the central coarse crystal
5. The central coarse crystal of Φ125mm titanium alloy forged bar: the level of clutter at the central part (compared with the same sound range) reaches Φ1.2mm-6dB with the 5P14 straight probe circumferential radial probing.
a) Transverse low times x 1
b) Longitudinal low magnification x 1
c) High multiple x250 at the central coarse crystal (with bar α)
6. Coarse crystals in Φ70mm titanium alloy rolled bar
With 5P14 straight probe circumferential radial probe, the level of clutter at the center (compared with the same sound range) reaches Φ0.8mm flat bottom hole equivalent, while the level of clutter on the normal specimen is around Φ0.8mm-10~12dB.
Mechanical property test: room temperature tensile, d=5mm specimens, all taken from the center of the bar, on the same furnace number and the same specification bar.
It can be seen that the strength and plasticity values of the specimens with high level of spurious waves (more striped α-phase in the high-frequency organization) are different from those of the specimens with low level of spurious waves.
a) Longitudinal low magnification x 1
b) Transverse high magnification at coarse grain x500
c) Transverse high magnification at normal part x500
d) High magnification at normal tissue (stray wave level Φ0.8mm-10dB)x250
7. Coarse crystals in Φ75mm titanium alloy forged rod
Using 5P14 straight probe circumferential radial probe, the clutter level at the center part (compared with the same sound range) reaches Φ0.8mm-6dB, and the clutter level at the normal part is below Φ0.8mm-12dB.
Mechanical properties test: room temperature tensile, d=5mm specimen, one specimen at the center of the coarse crystal specimen (clutter level Φ0.8mm-6dB) and one specimen at 1/4D (clutter level Φ0.8mm-12dB or less).
8. The middle coarse crystal of titanium alloy cake billet (parallel α organization)
Using 5P14 straight probe from the end face of the biscuit billet for axial probing, the clutter level reaches Φ1.2mm-6dB or so. This is the superheated Weiss organization caused by the deformation heat effect during the forging deformation.
a) Longitudinal low magnification x 1
b) High magnification at coarse grain x500
9. Middle coarse crystalline layer of titanium alloy cake billet (Weiss organization)
This case is also a kind of superheated Weiss organization caused by deformation heat effect during forging deformation, but it is not found with longitudinal wave 5, 10 or even 15MHz, and its stray wave level is lower than Φ0.8mm-12dB, and the bottom wave loss is not obvious, which is found after sampling anatomical corrosion. It is estimated that the grain boundaries and intracrystalline phase organization are arranged in an irregular orientation, which makes the longitudinal ultrasonic waves scattered from the end face of the pie billet axially cancel each other after scattering and cannot show the higher clutter level on the fluorescent screen.
a. Low magnification x 1
b. High magnification of beta spot x250
10. β spot at the transition between the spoke plate and the hub of a titanium alloy die-forged disc-shaped part
It was not found with longitudinal wave 5, 10 or even 15MHz, the bottom wave loss was not obvious, the spurious wave level was lower than Φ0.8mm-12dB, the β spot was found when dissecting the disk part.
a. End surface low magnification x 1
b. Cross section low magnification x 1
c. End surface coarse crystal high times x100
d. High magnification of deformation zone of cross section x100
e. Waveform when probing axially from the end face (equivalent to Φ1.2mm-17dB)
11. Coarse crystals and deformation band on the punch core of titanium alloy ring billet
This is the core punched down during forging of titanium alloy ring billet. Due to the rapid gravity hammering, the thermal effect of rapid deformation leads to coarse crystals and obvious deformation bands, and its clutter level reaches Φ1.2mm-18dB or more (actually it is the tissue reflection at the deformation band). It was detected axially from the end face with a West German USIP11 ultrasonic flaw detector, MB5F straight probe (5MHz frequency, wafer diameter 10mm).
12. Elongated α tissue on the spoke plate of titanium alloy die-forging disc
With 5MHz, 7°, 10mmx10mmx2 combined double crystal straight probe (contact method), large diameter water immersion focusing probe (wafer diameter 50mm, focal column diameter 3.2mm), etc. can be found as Φ0.8mm diameter flat bottom hole equivalent of a single reflected signal, but when using higher frequency and small diameter wafer probe probing, the reflected signal amplitude significantly decreased, after dissection on the low magnification performance After dissection, it appears as a bright line, about 25 mm long, and as an aggregated elongated α tissue on high magnification.
Elongated alpha tissue x100
As the application of titanium alloy continues to promote, more and more to replace the steel important load-bearing parts (such as the United States has been used in civil aircraft weighing more than one ton of large titanium alloy aircraft structural forgings), its metallurgical quality requirements will be increasingly high, especially the use of ultrasonic inspection technology to control the metallurgical quality of titanium alloy forgings, as well as ultrasonic response and titanium alloy microstructure, mechanical properties of the relationship between the three aspects of research there is still There is a lot of in-depth work to be done.
Author: Jizhen Xia