CC BY
18iSHS 2019
Moscow, Russia
VIBROACOUSTIC MONITORING OF THE INTERMETALLIC PHASES FORMATION IN SURFACE ALLOYING USING ELECTRON-BEAM
TECHNOLOGY
S. V. Fedorov*", M. P. Kozochkin", Them Htoo Maung", and Min Htet Swe"
aMoscow State Technical University STANKIN, Moscow, 127994 Russia
*e-mail: sv.fedorov@icloud.ru
DOI: 10.24411/9999-0014A-2019-10039
Among the many ways to modify the surface of machine parts and tools the most common methods of chemical and thermal treatment, involving in most cases, the diffusion saturation of alloying elements with the possibility of formation of reinforcing phases in the near-surface layer. The same methods include technologies of self-propagating high-temperature synthesis (SHS) based on the use of internal energy of the chemical interaction of the initial reagents [1].
This paper presents experiments on the hardening of the aluminum alloy (1.3% Cu, 0.26% Fe) surface by intermetallic phases of the NiAl system. The aluminum plate was coated with a thickness of about 0.2 |im of heat-resistant nickel alloy NiCr20TiAl. The use of various fillers or diluents instead of pure nickel, even those involved in the synthesis as reaction byproducts, is not a limitation for the creation of a SHS system.
Subsequently, the plate with a metal coating applied to it was exposed to a series of pulses of a wide-aperture low-energy high-current electron beam with a duration of about 5 |is. Chemical reactions between the metal film and the aluminum base were initiated.
Processing was carried out in the installation RITM-SP, which is a combination of a source of low-energy high-current electron beams RITM and two magnetron sputtering systems on a single vacuum chamber [2]. The installation allows to obtain a beam of microsecond duration with a current density of up to 10.000 A/cm2 at an accelerating voltage of 15-30 kV. The area of one-time processing is about 50 cm2. The operating pulses were given at the generator charging voltage values from 18 to 24 kV, which corresponded to the irradiation energy density from 4.2 to 6 J/cm2.
The formation of the structure in the near-surface layer of the material is largely due to the pulsed nature of the action in the microsecond range. Here, the main factors of the surface alloying process are the energy of the electron beam, which depends on the accelerating voltage and the thickness of the deposited thin film. The dependence of the modified layer thickness on the accelerating voltage has a pronounced extreme character. Irradiation with insufficient energy in the beam is not able to initiate the SHS process, and its excess leads to evaporation of most of the film.
However, the control of the surface electron-beam alloying process of significantly complicates the instability of the electron beam pulse parameters and the process of its interaction with the processed material, which leads to some random changes in quality indicators that occur spontaneously, regardless of the control system. In this situation, it is proposed to use the acoustic emission method to monitor the process.
The pulsed action of the electron beam on the sample causes a vibroacoustic wave associated with the effect of thermoelasticity in a thin surface layer heated to the evaporation temperature of the substance in the vacuum. The recoil impulse due to the evaporation of the material in the irradiation zone also contributes. The high vibroacoustic activity of the resulting process is shown experimentally.
To register the vibroacoustic signal generated during the technological process, the plate was connected to the accelerometer using a waveguide in the form of a copper wire with a cross section of 2.5 mm2. The channel diagram is shown in Fig. 1. The use of a wire waveguide made it possible to have the recording equipment at a distance from sources of electromagnetic interference.
■^npWipf'P"
Fig. 1. Experimental setup: the sample placed in the RITM-SP unit for surface alloying and the scheme for vibroacoustic signals recording; 1 sample; 2 wire waveguide; 3 receiving plate; 4 KD-35 accelerometer; 5 PM-3 preamplifier; 6 VSV-003 amplifier; 7 E440 ADC; 8 registering computer.
The initial signal, passing through the elastic system, undergoes transformations in accordance with the amplitude-frequency characteristic of the observation channel. Experiments have shown that in this case, the components in the frequency range up to 40 kHz stand out against the background of noise. Low-frequency components up to 1 kHz, including all kinds of noise caused by the operation of the units of the RITM-SP unit, were also excluded from consideration. To obtain information about the processes occurring on the irradiated sample, the recorded vibroacoustic signal was subjected to time and frequency analysis. Effective (mean square) values of the amplitude of these signals filtered in different frequency ranges were taken as parameters of the vibroacoustic signal reflecting the kinetics of the processes on the workpiece surface.
The width of the analyzed frequency bands approximately corresponded to the width of the octave band. Due to the discreteness of the spectra obtained, the analysis in narrower frequency ranges could have a significant spread associated with energy fluctuations in the electron beam and some variability of the amplitude-frequency characteristic of the elastic system.
It is observed that the amplitude of the vibroacoustic signal for the case of a coated sample increases significantly, starting from the third-fourth pulse. Analysis of optical images confirmed that the formation of intermetallic phases on the surface of the aluminum plate under irradiation by the first pulse, as a rule, does not occur. The film is partly evaporated and partly mixed with the base and crystallizes in the form of long dendrite crystals (Fig. 2b). With further irradiation, in place of these crystals appear intermetallic phase inclusions up to several microns in size (Fig. 2c).
Figure 3 shows the recording of vibroacoustic signal and the effective values of its amplitude in octaves 32, 16, and 2-4 kHz, the irradiation of plates of pure aluminum (Figs. 3a, 3c) and coated (Figs. 3b, 3d). The main signal energy was released in the first 40 ms. In Fig. 4, the signal spectra for (a) low and (b) high frequencies are shown.
It is seen that in the presence of an alloying coating in the processes of changing the structure of the sample surface, short discrete pulses prevail, giving a contribution to energy at higher frequencies, which is in good agreement with [3, 4]. The growth of high-frequency energy in the spectrum of the vibroacoustic signal is accompanied by an increase in the content of the intermetallic phase.
ISHS 2019 Moscow, Russia
(a) (b) (c)
Fig. 2. The structure of the metal film of NiCr20TiAl alloy: (a) before the action of the electron beam; (b) after the action of the first pulse of the electron beam; (c) after irradiation with five pulses.
—-fC. h-
—V
(c) (d)
Fig. 3. Records of vibroacoustic signal and effective values of its amplitude in octaves 1 32, 2 16, and 3 2-4 kHz at irradiation of pure aluminum plates (a, c) and plates coated with NiCr20TiAl alloy (b, d) at the time of intermetallic phase formation.
15 17 19 21 23 f, [kHz]
(a) (b)
Fig. 4. The comparison of the amplitude spectra of the vibroacoustic signal during the pulse exposure in the range (a) 1-15 kHz and (b) 15-25 kHz for 1 coated plate and 2 uncoated inserts.
When irradiating an aluminum alloy without coating, there is a clear predominance of low-frequency energy. On records of effective values of signal amplitude in the range of 2-4 kHz (Figs. 3 c, 3d) it can be seen that the irradiation of the plate without coating gives an amplitude
eight times greater. This suggests that the process of formation of the intermetallic phase takes place without the "rough" pulses that accompany the processes of melting and recrystallization in aluminum. Vibroacoustic signals accompanying the processes in materials subjected to electron beam effects have a complex varies spectrum. To effectively control the results of irradiation of various materials in an automated mode, a deterministic algorithm for processing vibroacoustic information is required, which allows to evaluate the quality of the results in real time. In this case, quite a limited set of diagnostic parameters, the digital values of which allow you to monitor the quality of the process and decide on its repetition or change of the initial parameters.
For example, the analysis of vibroacoustic signals during irradiation of coated plates and a clean plate in the coordinates E (coefficient of kurtosis) - Alf/Ahf (the ratio of amplitudes in the bands of low and high frequencies in the neighborhood of the maximum signal in an octave of 16 kHz) makes it possible to unambiguously judge the passage of the reaction of intermetallic compounds (Fig. 5). Due to the change in the nature of the vibroacoustic signal on the chart, clusters of points are formed that are distinguishable in these coordinates. The oval shows a cluster of pulsed electron-beam effects, where the successful passage of the synthesis was recorded.
(Aimm_
1 1 1
O -Al+Ni+Cr - A. - pureAl
>1
Sio •n
2
5 L
i
< '4 <
k' y
0 2 4 6 8 s
Fig. 5. Graphical representation of the results of the analysis of vibroacoustic signals in the coordinates E (coefficient of kurtosis)/Alf/Ahf (the ratio of the amplitudes in the low frequency band to the amplitudes in the high frequency band in the neighborhood of the maximum signal in octave 16 kHz); number near the point means a sequence of pulses with a charging voltage of 20 kV.
The financial support provided by the Ministry of Education and Science of the Russian
Federation, in the framework of the state task in the field of scientific activity of MSTU
"STANKIN" (no. 11.1817.2017/4.6).
1. A.S. Mukasyan, C.E. Shuck, Kinetics of SHS Reactions: A Review, Int. J. Self-Propag. High-Temp. Synth., 2017, vol. 26, no. 3, pp. 145-165.
2. A.B. Markov, E.V. Yakovlev, V.I. Petrov, Formation of surface alloys with a low-energy high-current electron beam for improving high-voltage hold-off of copper electrodes IEEE Tran. Plasma Sci., 2013, vol. 41, pp. 2177-2182.
3. R.M. Gabbasov, A.I. Kirdyashkin, V.G. Salamatov, Acoustic emission during combustion of Ni-Al composites, IOP Conf Series: J. Phys.,2018, vol. 1115, 042025.
4. P. Gulyaev, Temperature hysteresis in the unstable combustion mode of SHS: experiment with high-speed micro-pyrometry, IOP Conf. Series: J. Phys., 2018, vol. 1115, 042025.
