GB2397650A

GB2397650A - Controlling an annealing process using a laser ultrasonic method to monitor grain size

Info

Publication number: GB2397650A
Application number: GB0301661A
Authority: GB
Inventors: Brian Cecil Moss; Gregory Alan Baker; James Hayward Tweed
Original assignee: AEA Technology PLC; Accentus Medical PLC
Current assignee: Ricardo AEA Ltd
Priority date: 2003-01-24
Filing date: 2003-01-24
Publication date: 2004-07-28
Also published as: GB0301661D0

Abstract

A laser ultrasonic method is used to obtain a parameter Fr characteristic of grain size of a object 12 (eg aluminium) being annealed, and the annealing process is controlled in response to a change in the parameter Fr. The method uses a laser ultrasonic source 14 and a laser detector 20 incident at the same point 15 on the surface of the object 12. The analysed signals are those received in a predetermined time interval which commences between 3 and 12 žs after an initiating pulse. The parameter Fr is the ratio between the signal power at low frequency (eg 1-5 MHz) and the signal power at higher frequency (eg 15-50 MHz).

Description

Monitoring an Annealing Process This invention relates to a method and an

apparatus for monitoring and controlling an annealing process.

Metals such as aluminium may be subjected to cold rolling, and then an annealing process which decreases the density of dislocations, causes recrystallization, and then increases grain size. The annealing process involves raising the temperature of the sheet or plate of metal to an elevated temperature for sufficient time for the annealing to take place. Such a process may be controlled by monitoring the surface temperature of the material, for example using a pyrometer, and adjusting the temperature to which it is heated, the time at an elevated temperature (for example by controlling line speed), and the provision of any cooling medium. However this does not provide an entirely accurate control, because the surface temperature may not accurately indicate the bulk temperature, and because the recrystallization occurs at a temperature that depends . upon the degree of cold rolling and the type of alloy. c A.. À

WO 02/103347 describes a laser ultrasonic method for : . .. 25 measuring grain sizes using a pulsed laser to generate broadband ultrasonic waves, detecting with a broadband detector at a different location waves travailing through as. the object, analyzing the ultrasonic spectrum for those waves that arrive at arrival times after that of Rayleigh c waves, and determining a ratio between the high frequency power and the low frequency power. This ratio can be related to grain size.

According to the present invention there is provided a method of controlling an annealing process, wherein the microstructure of the material undergoing the process is - 2 monitored by a laser ultrasonic method to obtain a parameter characteristic of grain size, and the annealing process is controlled in response to detection of a change in that parameter.

The laser ultrasonic method may be similar to that described in WO 02/103347. However it may use a source and detector at substantially the same position, the ultrasonic waves that are analysed being those that have reflected to and fro between the front and back surfaces of the object. As with WO 02/103347 the analysed ultrasonic signals are those received in a predetermined time interval, which may for example be 10 to 20 As after the initiating pulse or 3 to 8 As. A comparison is made between the high frequency power and the low frequency power, this comparison being between the ultrasonic power in a high frequency region (covering a range of frequencies), compared to that in a low frequency region.

Those regions may or may not overlap. For example the high frequency region may be that above a threshold and the low frequency region be that below a threshold; the threshold might be set at 15 MHz. Alternatively the regions might partially overlap, for example, 1 to 3 MHz as the low frequency region, and 2.2 to 15 MHz as the high frequency region.

The presence of larger grain sizes leads to greater attenuation of the high frequency components of the spectrum. Ultrasonic waves that arrive later have travelled for a longer time in the grain structure of the sample, in which they were subject to scattering phenomena, and this accentuates the discrimination between different levels of scattering and so between different grain sizes.

Surprisingly it has been found that during an annealing process this parameter scarcely changes at all during the initial temperature increase, until a transition temperature is reached above which the parameter changes rapidly with temperature. This parameter is a far more appropriate parameter for controlling the annealing process than is the surface temperature.

Figure 1 shows a diagrammatic plan view of an apparatus for determining a parameter for control of an annealing process, applied to a sheet of metal; Figure 2 shows graphically how the control parameter measured using the apparatus of figure 1 varies with temperature.

Referring to figure 1, an apparatus 10 is shown for determining a parameter for control of the annealing of a metal sheet 12. The sheet 12 is conveyed into and through a heating furnace, and the annealing process may be controlled by controlling the temperature of the A,. . furnace, the line speed at which the sheet 12 passes through the furnace, and any cooling of the sheet 12 after passage through the furnace. A plurality of parameter-determining apparatuses 10 may therefore be required at different positions along the sheet 12, and this may include positions within the furnace provided with optical windows for this purpose. À

The apparatus 10 incorporates a laser 14 arranged to emit a pulsed light beam 16 focused by a cylindrical lens 17 to form a line 15 about 1 mm across and 10 mm long on the surface of the specimen 12 when energized, so as to generate ultrasonic waves in the sheet 12. Between the lens 17 and the surface of the sheet 12 the beam 16 is also reflected by a mirror 18 and a dichroic mirror 19 4 - and is incident along a normal. For example the laser 14 may produce pulses of energy 80 mJ at a wavelength of 1064 nm (infrared) and of duration 20 ns at a pulse repetition frequency of for example 20 Hz. Each laser pulse produces a very sharp ultrasonic pulse in the sheet 12 which includes frequencies above 1 MHz up to about 75 MHz. The laser 14 may be an yttrium aluminium garnet (YAG) laser.

The apparatus 10 also includes frequency-doubled YAG laser 20 that incorporates an etalon within the optical cavity to ensure it operates in a single longitudinal mode, so it emits a single frequency (at a wavelength of 532 nary). The laser 20 produces a continuous beam 22 of light which passes through a half wavelength plate 23 and then a polarising beam splitter 24. The polarizing beam splitter 24 reflects light with a vertical plane of polarization, and transmits light with a horizontal plane of polarization, and by adjusting the angle of the half wavelength plate 23 the intensity of the transmitted beam 1 can be adjusted. The transmitted beam 1 (which is I initially horizontally polarised) is incident on the À surface of the sheet 12, via a quarter wave plate 25 and À a spherical converging lens 26, being incident at the I 25 midpoint of the line 15. The incident beam 1 and À - reflected beam 2 also pass through the dichroic mirror 19, this being transparent at this wavelength. The . . reflected beam 2, after returning through the quarter À . wave plate 25, is vertically polarised, and so is reflected by the beam splitter 24. The beam 2 is then reflected by a prism 33 to pass through a confocal Fabry- Perot etalon 36, to be incident on a photo diode 38. The photo diode 38 provides electrical signals to an electronic detector 40 which analyzes the received signals.

The ultrasonic waves to be detected may be in the frequency range 1-50 MHz or even higher. Preferably the etalon 36 incorporates a piezoelectric tuning device to ensure that the peak intensity from the argon laser 20 (532 nm) is to one side of the transmission peak and about half way down the peak (so as to maximize the rate of change of transmission with frequency), and a feedback circuit is preferably provided to maintain this optimum sensitivity despite any vibrations of the apparatus 10.

Such vibrations would typically be no more than 1 kHz, which is well below the frequency of the ultrasonic waves.

Ultrasonic waves from the line 15 propagate through the thickness of the sheet 12 and are reflected back by the rear surface. Compression and shear waves are generated initially, and these will both be reflected to and fro, but also mode conversion takes place at each reflection, so that the ultrasonic signals become complex while decreasing in intensity.

. The detector 40 is arranged to determine the frequency spectrum, by Fourier analysis, of the signals received in a preset time interval, for example between 3 A. 25 ps and 8 ps after each initiating pulse from the laser 14. The detector 40 then determines the ratio Fr of the signal power in the frequency range 1-5 MHz to the signal power in the frequency range 15- 50 MHz. À a À

As with the apparatus described in WO 02/103347, this ratio Fr may possibly be related to grain size, d, in the sheet 12, the relationship depending on the nature of the metal.

It will be appreciated that the apparatus 10 may be modified in various ways, for example using a spot focus 6 - instead of a line focus 15 when generating the waves; and using a different type of laser 20 and a different detector system for detecting the waves. In particular the signal analysis may be performed in a different manner. The frequency windows used in the example described above have been found to give a simple relationship with grain size with copper, and little problem from noise. Nevertheless alternative windows may be used, for example 1-20 MHz compared to 16-60 MHz.

Where the detector is a Fabry-Perot etalon, and is hence sensitive to frequency variations due to movement of the surface, the detected light beam might instead be the light reflected from the etalon rather than the transmitted light. And it will also be appreciated that a different type of detector may be used, for example the emitted beam 22 might be split to provide a reference beam, the reference beam then being arranged to interfere with the beam reflected from the surface at a photodetector; in this case the signals would be different, as the signals would represent changes in phase corresponding to surface displacements.

. Nevertheless the frequency spectrum would be substantially the same and may be analyzed in the same manner.

Measurements have been done as described above on a sheet of cold-rolled aluminium 5083 alloy of thickness 10 mm. Referring to figure 2 the variation in the parameter Fr with temperature is shown for this sheet, the temperature of the sheet being gradually raised throughout the measurements. The parameter Fr changes very little as the temperature of the sheet is increased from about 80 C to about 450 C, and it is believed that this reflects the absence of any microstructural changes over this temperature range. Once the temperature rises above about 450 C, the parameter increases rapidly. This 7 - reflects the onset of the recrystallization processes that annealing aims to achieve. (It will be appreciated that this rapid change, above the threshold temperature of about 450 C, does not correspond to changes in the elastic properties of the aluminium, as these would change much more steadily over the temperature range.) Controlling the annealing process in accordance with measurements of this parameter Fr consequently enables the process to be controlled more accurately, as the control is in accordance with the occurrence of the recrystallization processes.

The time interval or window during which the signals were analysed in the above example was from 3-8 microseconds. If there is no time delay after the initiating pulse then the effect of grain size would be suppressed, but if the delay is too long then the high frequencies will have decayed essentially to zero. The start time for this window is preferably between 3 and 12 microseconds, and the duration of the time window may be up to about 20 microseconds. However the time window . must be selected to ensure that no reflections from the À edge of the object are received during that interval. À À..

:.' 25 À e see À 8 - 8

Claims

Claims 1. A method of controlling an annealing process, wherein the

microstructure of the material undergoing the process is monitored by a laser ultrasonic method to obtain a parameter characteristic of grain size, and the annealing process is controlled in response to detection of a change in that parameter.
2. A method as claimed in claim 1 wherein the analysed ultrasonic signals used to obtain the parameter are those received in a predetermined time interval.
3. A method as claimed in claim 2 wherein the predetermined time interval commences between 3 and 12 ps after an initiating pulse.
4. A method as claimed in any one of the preceding claims wherein the parameter is obtained by comparing the high frequency power and the low frequency power.

À. .
5. A method as claimed in any one of the preceding . claims wherein a source laser and a detector laser are I. incident at substantially the same point on the surface ' 25 of an object, the monitored ultrasonic waves being those that have been reflected to and fro between the front and back surfaces of the object. .
6. A method as claimed in claim 5 wherein the source laser is focused on to a line, and the detector optical system is focused on to a point substantially at the midpoint of the line.
7. A method of controlling an annealing process substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings. 9 -
8. An apparatus for performing a method as claimed in any one of the preceding claims. À. .e À.e r À.e Àe À i 1 Be À