APPARATUS AND METHOD FOR ULTRASONIC DETECTION OF INCLUSIONS IN A MOLTEN BODY

Abstract: An apparatus and method for ultrasonically detecting inclusions in molten bodies. The apparatus includes a probe comprising an inert refractory shell, a window supported by the shell for carrying ultrasonic waves between a coupling medium and a body of molten or molten glass, and a coupling medium contained within the shell for carrying ultrasonic waves between a transducer and the window. A preferred apparatus utilizes a coupling medium comprising liquid gallium and is suitable for de­ tecting inclusions in molten aluminum and aluminum alloys.

Description: The present invention as well as the earlier application EP-A-0171820 (published 19.02.86) relate to apparatus and methods for ultrasonic detection of inclusions in molten bodies, including molten metals and glasses. Because particulate inclusions in molten metals often have a detrimental effect on metal quality, considerable effort has been devoted in recent years to developing apparatus and methods for detecting such inclusions. Some prior art devices for detecting particulate inclusions in molten metal are described in Young et al U.S. Patent No. 3,444,726 and Mansfield U.S. Patent Nos. 4,261,197 and 4,287,755. However, each of these prior art devices comprising a transducer device and a probe suffers from one or more serious limitations making it less than entirely suitable for its intended purpose. For example, the devices relying upon a probe of titanium metal have a short operating life when used to detect inclusions in molten aluminum because titanium is soluble in aluminum. In addition, solid titanium and other solid metal probes such as that disclosed in U.S. Patent No. 4,261,197 usually contain grain boundaries, gas voids, inclusions and other impurities which may attenuate ultrasonic waves. The solid probe of U.S. No. 4,261,197 also has the disadvantage that the transducer is separated from the probe merely by a thin film of coupling medium so that the transducer is not protected from the high temperature molten body. Another disadvantage of prior art devices is the need to provide a surface coating on the probe that is wettable by molten metal. Such surface coating causes an attenuation of the ultrasonic signals, thereby interfering with particle analysis. A further limitation of prior art devices is the need to focus on a reflecting surface. Under production conditions, it is difficult to focus an ultrasonic beam and to maintain a reflective surface at a constant distance from the beam. It is a principal objective of the present invention to obviate each of the above-mentioned disadvantages in the prior art. Another objective of the invention is to provide an apparatus and method that are suitable for detecting particulate inclusions in molten glasses. One advantage of the probe described herein is that it is suitable for detecting particles in molten aluminum having a smaller effective particle size than with prior art devices. Another advantage is that the apparatus and method described herein are suitable for use either with or without focusing a beam of ultrasonic waves. In addition, it is not necessary to have a reflecting surface. In accordance with the present invention, there is provided a probe for carrying ultrasonic waves between a transducer and a molten body, said body having a melting point below about 2500°C., said probe comprising (a) a hollow shell, (b) a coupling medium for carrying ultrasonic waves between a transducer and a window spaced from said transducer, said coupling medium comprising a liquid metal contained within said shell, (c) a window for carrying ultrasonic waves between said coupling medium and said body, said window comprising (1) an interior surface portion contacting said coupling medium, and (2) an exterior surface portion wettable by said body; said shell comprising tungsten, nickel, or steel. The invention further provides a process for ultrasonically inspecting a body comprising a molten metal or alloy or molten glass having a melting point below about 2500°C., said process comprising (a) generating ultrasonic waves with a piezoelectric transducer, (b) transmitting said waves from said transducer to a window spaced therefrom through a coupling medium comprising a liquid metal contained within a hollow shell, (c) transmitting said waves through a window into a body comprising a molten metal or alloy or molten glass, said window comprising (1) an interior surface portion contacting said coupling medium, and (2) an exterior surface portion wetted by said body, and (d) receiving and interpreting reflections of said waves after their emergence from said body; said shell comprising tungsten, nickel, or steel. In a preferred embodiment, the acoustic attenuation factor for ultrasonic waves between the transducer and coupling medium is less than about 3 dB. The acoustic attenuation factor for ultrasonic waves between the coupling medium and interior surface portion is less than about 6 dB. The acoustic attenuation factor for ultrasonic waves between the interior and exterior surface portions is preferably less than about 3 dB. The acoustic attenuation factor for ultrasonic waves between the exterior surface portion and the molten body is less than about 8 dB. All acoustic attenuation factors are optimally as small as possible. In a preferred probe used for analysis of aluminum or aluminum alloys, the coupling medium comprises liquid gallium. The coupling medium preferably comprises a minor proportion of a metal solute dissolved in the gallium that reduces differences in acoustic impedance between the coupling medium, transducer and interior surface portion of the window. As used herein, the term "minor proportion" means less than about 50 wt% of the metal solute. In a particularly preferred probe the coupling medium comprises about 4.8 wt% silver dissolved in liquid gallium. The liquid gallium is preferably treated to removed oxides, solid inclusions and dissolved gases. Such treatment is desirable in order to minimize reflection of acoustic waves within the coupling medium. A probe containing liquid gallium has the advantage of lacking grain boundaries, gas voids, inclusions and other impurities found in solid metal probes. The shell comprises a refractory material that is inert to the molten body. The term "refractory" refers to materials that are capable of withstanding extremely high temperatures for prolonged periods of time and includes traditional refractories such as alumina, silica and silicon carbide as well as refractory metals such as tungsten, molybdenum, and tantalum. In a preferred probe for analysis of aluminum and aluminum alloys, the shell is a hollow cylinder consisting of tungsten. A particularly preferred window is made of single crystal sapphire. The window may also be made from polycrystalline sapphire or amorphous silica. A preferred apparatus includes a piezoelectric transducer capable of generating ultrasonic waves having a frequency of about 8-25 MHz. In order to maintain effective operation of the transducer, it is spaced from the window, preferably by a distance of at least l0 cm. In addition, the preferred apparatus further comprises a cooling means for maintaining the transducer and liquid gallium adjacent thereto at a temperature below about l49°C. A probe having an inert refractory shell is capable of withstanding the caustic effects of molten aluminum and is sufficiently resistant to thermal stress for use with molten aluminum. A probe for detecting inclusions in molten ferrous alloys comprises a tungsten shell, a liquid gallium-tin alloy coupling medium, and a sapphire window. Coupling media comprising a major proportion (greater than 50 wt%) of gallium and a minor proportion (less than 50 wt%) of tin are preferred. A particularly preferred coupling medium comprises about 55 wt% gallium and about 45 wt% tin. A probe for detecting inclusions in molten glass has a coupling medium comprising a gallium-tin alloy. The shell may be steel, nickel, cobalt, chromium, silicon, or carbon with steel being particularly preferred. The window may be nickel, steel, cobalt, chromium, or carbon with nickel being particularly preferred. Brief Description of the Drawings Figure l is a schematic view of an apparatus for ultrasonic inspection of molten aluminum, including the probe of the present invention. Figure 2 is a schematic diagram of the apparatus and method of the invention. Detailed Description of a Preferred Embodiment The probe l0 shown in Figure l is used for detecting particulate inclusions in a body l2 comprising a molten metal or alloy or molten glass. While the particular apparatus and method described herein are intended for detection of particulate inclusions in molten aluminum and aluminum alloys, the invention is not limited to use solely with aluminum or for detection of only particulate inclusions. The present apparatus and method are also probably suitable for ultrasonic testing of other metals and alloys having melting points below about l000°C., including but not limited to bismuth, gallium, indium, lead, lithium, magnesium, mercury, potassium, sodium, tin, zinc and their alloys. With appropriate modifications, the invention may also be applicable to ultrasonic testing of higher melting metals such as cobalt, copper, gold, iron, manganese, nickel, silver and their alloys. Such metals generally have melting points below about 2500°C., usually less than about 2000°C. The invention may also be applicable to ultrasonic testing of molten glasses, including silica glass, leaded silica glass, borosilicate glass, and flint glass. The greatest need for inclusion detection in glasses lies in optical quality glass manufacture. Referring now to Figure 2, the probe l0 comprises a hollow tubular tungsten shell or envelope l4 containing a liquid metal coupling medium l6. The shell l4 is provided with an upwardly extending riser tube l8 in order to allow for expansion and contraction of the coupling medium l6 at different temperatures. The shell l4 must be chemically inert both to the body l2 and coupling medium l6 at elevated temperatures. A preferred shell l4 is made from tungsten. This material is inert to aluminum alloys in the body l2 and to liquid gallium in the coupling medium l6 at elevated temperatures up to at least 800°C. The shell l4 may also be made from other inert refractories such as aluminum, silica, silicon carbide and mullite. When the shell l4 is made from tungsten, it may be provided with an outer alumina frit jacket (not shown) to retard heat transfer and thereby protect against thermal shock. A piezoelectric transducer 20 adjacent an upper end of the probe l0 sends ultrasonic waves through the coupling medium l6 to a window 22 spaced downwardly of the transducer 20. The preferred transducer 20 shown is a piezoelectric transducer. This transducer generates ultrasonic waves having a frequency of about 8-25 MHz in response to electrical stimuli. A particularly preferred operating frequency is about l5 MHz. The transducer 20 is situated adjacent an amorphous silica upper wall or upper window 23 of the shell l4. The transducer 20 is attached to the upper wall 23 by means of a thin layer of silicone mounting material (not shown). This resilient silicone layer relaxes stress between the transducer 20 and upper wall 23 resulting from expansion and contraction of the coupling medium l6. Such stress relaxation avoids breakage of the fragile transducer 20 and minimizes frequency changes of ultrasonic waves generated therein. Because the transducer 20 is sensitive to high temperatures, the probe l0 is provided with a cooling means or cooling jacket 24 for maintaining the transducer 20 and adjacent coupling medium l6 at a temperature below about l49°C. The preferred probe l0 utilizes water as a coolant in the cooling means 24. The cooling means may be omitted entirely from the probe l0 if the apparatus is used for detection of inclusions in metals having low melting points such as bismuth, lithium or their alloys, or if a sufficiently large distance is maintained between the window and transducer 20 to insulate the transducer 20 from the generally hotter body l2. In a preferred probe l0 used for detecting inclusions in a molten aluminum alloy body l2 and having a liquid gallium coupling medium l6, the transducer 20 and window 22 should be separated by a distance of at least l0 cm. While it is desirable to maintain this distance as small as possible consistent with an acceptable thermal gradient, the distance must be increased in certain cases to avoid damaging the transducer 20. In the particularly preferred probe l0 described herein, the transducer 20 and window 22 are separated by a distance of about l5 cm. The window 22 includes an interior surface portion 22a contacting the coupling medium l6 and an exterior surface portion 22b that is wettable by the body l2. It is contemplated that the interior surface portion 22a and exterior surface portion 22b will generally comprise a unitary structure, although composite structures are also within the scope of the invention. A unitary structure is preferred in order to minimize acoustic attenuation between the interior surface portion 22a and exterior surface portion 22b. A particularly preferred window 22 is made from single crystal sapphire and has a cylindrical or disc shape. Sapphire has the advantage of being readily wetted by molten aluminum and its alloys as well as being inert at elevated temperatures. The coupling medium l6 and window 22 are each made from materials which are selected to reduce acoustic attenuation between the transducer 20 and coupling medium l6, between the coupling medium l6 and window 22, and between the window 22 and body l2. As used herein, the term "acoustic attenuation factor" is defined by the following expression: wherein B is the acoustic intensity level in decibels and I is acoustic intensity of the sound waves, expressed in W/m². The acoustic attenuation factors for ultrasonic waves between the transducer 20 and coupling medium l6, between the coupling medium l6 and window 22, and between the window 22 and body l2 should each be maintained as low as possible. In order to minimize acoustic attenuation between the transducer 20 and coupling medium l6 and between the coupling medium l6 and window 22, a minor proportion of a metal solute is dissolved in liquid gallium. The coupling medium l6 preferably comprises about 2.5-5 wt% silver dissolved in liquid gallium. The concentration of dissolved silver in the coupling medium l6 should generally be maintained below about 5 wt% in order to avoid precipitation of phases that might interfere with transmission of sound waves. The gallium should be treated to remove oxides, solid inclusions and dissolved gases so that sound waves will be transmitted efficiently between transducer 20 and window 22. The preferred coupling medium containing about 4.8 wt% silver dissolved in gallium has an acoustic impedance that is close to the acoustic impedance of the window and approximately equal to the acoustic impedance of the transducer 20. Accordingly, acoustic attenuation at interfaces between the transducer 20 and coupling medium l6 is essentially zero and acoustic attentuation between the coupling medium l6 and interior surface portion 22a is only about 4 dB. In addition, when the window 22 is made from sapphire, acoustic attenuation between the outer surface portion 22b and a body l2 of molten aluminum is calculated to be only about 5.8 dB. Accordingly, the probe of the invention is highly efficient for carrying ultrasonic waves between the transducer 20 and body l2 without thermal damage to the transducer 20. The coupling medium l6 preferably has a low vapor pressure at the temperature of the body l2. Liquids having significant vapor pressure at elevated temperatures may form bubbles that would interfere with transmission of ultrasonic wave trains through the coupling medium. An advantage of liquid gallium is that its boiling point is 2403°C. Accordingly, a coupling medium comprising a principal proportion of liquid gallium has low vapor pressure even at temperatures up to about 732°C. (l350°F.), the highest temperature at which aluminum alloys are usually maintained. Referring now to Figure 2, the apparatus further comprises a voltage pulse oscillator 30, a high frequency oscillator 32 generating alternating voltage of ultrasonic frequency, and a modulator 34 receiving electrical pulses from the voltage pulse oscillator 30 and a high frequency oscillator 32. The modulator 34 generates a direct current voltage train having a voltage of about 300 volts by passing voltage of ultrasonic frequency only during the period that the modulator 34 is receiving a pulse from the voltage pulse oscillator 30. This direct current voltage train energizes the transducer 20. In the preferred apparatus shown, the transducer 20 also acts as a receiving transducer for receiving wave trains from the body l2 through the window 22 and coupling medium l6. The receiving transducer 20 then generates voltage trains in response to wave trains received from the body l2. The voltage trains so generated are sent to an amplifier 40, rectifier 42 and oscilloscope 44. A linear sweep circuit 46 synchronized with the voltage pulse oscillator 30 is connected to the oscilloscope 44. The oscilloscope 44 has a spot that is deflected in one direction by the sweep circuit 46 and deflected in a different direction by the voltage output from the receiving transducer 20. A permanent record of the oscilloscope display is maintained in a recorder 50. In Figure l, the voltage pulse oscillator 30, high frequency oscillator 32 and modulator 34 are referred to collectively as a "pulser". The amplifier 40, rectifier 42, oscilloscope 44 and sweep circuit 46 are similarly called "receiver". A description of the operation of a similar system for sending and receiving pulses to and from the transducer 20 is described in Firestone U.S. Patent No. 2,280,226, which patent is incorporated by reference to the extent not inconsistent with the present invention. As shown in Figure 2, ultrasonic waves generated by the transducer 20 are transmitted through the coupling medium l6 and window 22 into a body l2 of molten aluminum. When these ultrasonic waves strike a suspended particle or particulate inclusion 60, extra vibration wave trains are sent back to the transducer 20 and voltage trains are generated in response to such ultrasonic wave trains. The voltage trains are then displayed on the oscilloscope 44. An advantage of the probe l0 described herein is that it is so efficient at transmitting ultrasonic wave trains that the claimed apparatus detects smaller particles 60 than prior art apparatus. The probe l0 consistently detects solid particulate inclusions having an effective diameter of less than about 400 microns, and particles as small as about 50 microns have been detected in molten aluminum. The probe l0 is theoretically capable of detecting particles in about the l-l0 microns diameter range. While the present invention has been described in terms of preferred embodiments, it will be apparent to persons skilled in the art that modifications and adaptations can be made within the scope of the present invention without departing from the scope of the following claims.

Claims: A probe for carrying ultrasonic waves between a transducer (20) and a molten body (12), said body having a melting point below about 2500°C., said probe comprising (a) a hollow shell (14), (b) a coupling medium (16) for carrying ultrasonic waves between a transducer and a window (22) spaced from said transducer said coupling medium comprising a liquid metal contained within the hollow space of said shell, (c) a window (22) for carrying ultrasonic waves between said coupling medium and said body, said window comprising (1) an interior surface portion (22a) contacting said coupling medium, and (2) an exterior surface portion (22b) wettable by said body; said shell comprising tungsten, nickel, or steel. A probe according to claim 1, characterized by the fact that the window (22) comprises sapphire. A probe according to claim 1 or 2, characterized by the fact that the coupling medium (16) comprises a liquid gallium alloy. A probe according to claim 3, characterized by the fact that the coupling medium (16) comprises a gallium-tin alloy. A probe according to claim 1, characterized by the fact that the shell (14) and the window (22) each comprise nickel or steel. A process for ultrasonically inspecting a body comprising a molten metal or alloy or molten glass having a melting point below about 2500°C., said process comprising (a) generating ultrasonic waves with a piezoelectric transducer (20), (b) transmitting said waves from said transducer to a window (22) spaced therefrom through a coupling medium (16) comprising a liquid metal contained within a hollow shell (14), (c) transmitting said waves through a window (22) into a body (12) comprising a molten metal or alloy or molten glass, said window comprising (1) an interior surface portion (22a) contacting said coupling medium, and (2) an exterior surface portion (22b) wetted by said body, and (d) receiving and interpreting reflections of said waves after their emergence from said body; said shell comprising tungsten, nickel, or steel. A process according to claim 6, characterized by the fact that the body (12) comprises a molten metal or alloy and the shell comprises tungsten. A process according to claim 7, characterized by the fact that the body (12) comprises molten aluminum or an aluminum alloy. A process according to claim 8, characterized by the fact that the window (22) comprises sapphire. A process according to claim 6, characterized by the fact that the body (12) comprises molten glass, the coupling medium (16) comprises a liquid gallium alloy and the shell (14) comprises nickel or steel. A process of claim 10, characterized by the fact that the window (22) comprises nickel or steel.

Inventors: ECKERT, CHARLES EDWARD
Applicants: ALUMINUM COMPANY OF AMERICA
Application year: 1987
Main classification: G01N29
IPC codes: G01N29