In many applications in aluminium industry, the number of inclusion-critical products increases and the quality of those products depend on the inclusion concentration and size. In order to improve the quality of aluminium products and the effectiveness of the processes, a reliable and cheaper on-line detection method is needed. Ultrasonic detection has been used in steel industry, but relatively rare in aluminium industry, although it would theoretically allow for an online non-intrusive detection of the cleanliness of the melt. In this work, the current information on ultrasonic inclusion detection was gathered and recommendations were provided on the Prerequisites for a set-up for ultrasonic detection of non-metallic inclusions in aluminium as a contribution on previous works. It has been concluded that ultrasonic waves seem promising, and should be experimented more on an industrial level to have a clear view on the potentials of the method.
Aluminium is widely present in daily life, being used in transportation, packaging, electrical engineering and many other fields. In many applications, the number of inclusion-critical products increases and the quality of those products depends on the inclusion concentration and size. The non-metallic inclusions impact the formability and surface quality crucially. This issue makes the detection of the inclusions a topic of interest [
Several inclusion detection methods exist, such as PoDFA (Porous Disc Filtration Analysis), PreFil (Pressure Filtration Melt Cleanliness Analyser) as offline methods and LiMCA (Liquid Metal Cleanliness Analyser) and Metalvision MV20/20 as online methods. The PoDFA and PreFil techniques are based on concentration of the solid inclusions by filtering the liquid sample. The filter cake is later examined by metallographically. It has the advantages of providing good information about the shapes and types but it lacks of a global knowledge of the concentration of the melt and the evaluation is a skilled and a long operation [
In order to improve the quality of aluminium products and the effectiveness of the processes, a reliable and cheaper on-line detection method is needed [
This work aims gathering the current information on ultrasonic inclusion detection and providing recommendations on the requirements for an ultrasonic detection set-up for aluminium industry as a contribution on previous works.
Ultrasounds are defined as waves with frequency higher than 20 kHz. It consists of a series of mechanical compressions and expansions in a system. They can be created by a piezoelectric transducer: an apparatus that converts electric energy into mechanical energy [
In metallurgical applications, a few parameters will be of importance. First of all, velocity, as it may be used to determine the position of inclusions in the melt by measuring the delay between emission and reception. The velocity of the wave is determined by:
where V is the velocity of the wave in cm/ms, f frequency, λ wavelength, Cij its elastic constant with i and j representing respectively the longitudinal and shear direction, and ρ density [
Secondly, the attenuation of the amplitude of the wave must be taken into account. As it will be explained in the next chapter, detection works by getting signals echoing back from the inclusions, and thus a high enough amplitude is needed for the detection of this return signal. The amplitude attenuation follows this equation:
where A0 is the initial amplitude, x the distance from initial location and α the attenuation coefficient of the
Technique | Real Time | Sample Volume | Detection Size Range (µm) | Cost | |
---|---|---|---|---|---|
PoDFA | Metallography | No | Medium | None | Low |
PreFil | Metallography | Partial | Medium | None | Medium |
LiMCA | Change in Electrical Resistivity | Semi | Small | 15 - 150 | High |
MetalVision | Ultrasound | Yes | Large | 20 - 160 | Medium |
medium in which the waves is propagating. The coefficient α is dependent on the frequency and thus can be properly determined only by experimenting in the used medium [
Thirdly, the acoustic impedance Z is used to determine the acoustic transmission and reflection at the interface between two materials. It has primary importance in an ultrasonic detection system to determine the efficiency of the system at the interface between the rods and the aluminium melt. The acoustic impedance is defined as follow [
The amount of energy reflected at the interface can be defined as a ratio R of the initial energy by the following equation:
where; Z1 and Z2 are respectively the acoustic impedance of the first and second material.
The power variation can be of great order and this must absolutely be taken into account when choosing the emitting power. It is worth noticing that as medium of propagation changes, the velocity will also change, but as the frequency is constant, the wavelength will also change in the second material [
Ultrasounds have currently many uses, such as; echography, degassing of Aluminum, filtration of chemical compounds, motion sensors.
The use of ultrasonic waves allows for a non-invasive and non-destructive treatment of the melt during processes. From the metallurgical point of view, improving the current techniques by the use of ultrasonic waves can greatly improve the efficiency of the detection processes. Ultrasonic detection of particles would allow for a fast and online detection, saving adjustment time and costs.
The MetalVision MV20/20 detector is composed of two buffer rods, one emitting and one receiving the waves, a reflector located in the melt, as shown in
MetalVision uses the principle of attenuation and reflection of a signal, correlating it with the particle size distribution and the concentration of inclusions in the melt [
The system provides the user with three plots: cleanliness value, particle size and particle count for each particle size range, as shown in
MetalVision has advantages, as it is able to detect particle up to 20 μm, but it lacks in constant results and is dependent on the concentration change of the biggest particles. This could be a direction of improvement while developing an ultrasonic detection system.
Filtration systems using ultrasonics are usually a combination of a filtering media and an ultrasonic transducer [
Use of ultrasonic waves for the filtration has two main effects. Firstly acoustic streaming can help disperse the sediments at the surface of the filter. Secondly, the cavitation induces at the entry of the capillary allows for the sonocapillarity effect, in which the melt is pushed through the filter channel due to the explosion of the cavitation bubble. This allows the melt to go through multilayered filter even if the capillarity pressure would have been too high to allow filtration [
Porosity is one of the major problems in aluminium products, occurring because of the precipitation of hydrogen due to the decrease of its solubility during the solidification. An efficient and fast method for degassing of aluminium melts is based on inducing cavitation in the melt by submitting it to ultrasonic waves. Noble gas is usually blasted through the melt and creates bubbles to remove the inclusions. However, in the case of ultrasonic applications, as the variation of pressure due to the waves occurs, numerous bubbles are created endogenously in the melt, which grow through rectified diffusion and remove the hydrogen while rising up to the surface [
The use of ultrasonic waves in the metallurgical industry is rising, mostly through the improvement of melt treatment it allows. Currently only the MetalVision system offers a solution for detection of particle in the melt using ultrasounds. The potential offered by ultrasonic waves for the detection needs to be deepened.
The techniques using ultrasonics are already being used in different fields such as medicine and conditions of ultrasonic applications are well-known. However, the applications in melts need new challenges.
The current status of inclusion detection by ultrasounds allows us to describe the prerequisite for a detection set-up. Three main areas can be described, where the most challenges happen: The piezoelectric element is responsible for the generation of the waves, whose specifications have an influence on the detection. The buffer rod, transmitting the waves to the melt, whose shape and material ensure less signal loss and the contact point between the rod and the melt. Lastly, the particle and melt movement effects the detection, with the influence of particle concentration, settling and melt velocity.
Each area should be investigated in order to optimize the efficiency and sensibility of the detection set-up.
The piezoelectric element is responsible for creating the ultrasonic waves. However, some key points must be
investigated to calibrate the detection such as the influence of the amplitude, coupling the transducer to the rod and prevention the unwanted motion (vibration) of the rods.
Although the theory behind ultrasonics is well developed and understood, no detailed theory has been specifically applied for particle detection in melts. Developing such a theory is primordial to improve the efficiency of the set-up and to find the most suitable specifications for the piezoelectric element.
It appears that the particle detection is a function of the power of the waves rather than its frequency, with a relation between the size of the detected particle and the power of the wave in logarithm:
where A and B are two constants related to the frequency and the characteristics of the emitting system, and p the size of the detectable particle [
Using high frequency allows for a more focused area of detection and a better discrimination, but it also increases the loss of power inside the melt [
The measurement procedure during the detection can be summarized as following: sending pulses in the melt, usually at a rate of 100 pulses per second at a given frequency, the amplitude is slowly increased until a reflected signal is received. This first signal gives the size of the largest particle in the melt. This means that, to be able to detect smaller particles, the sound system must be able to emit at a high power level, usually around 90 dB [
A main issue of the ultrasonic detection is the temperature range at which transducers can be operated. A piezoelectric element cannot be directly inserted in the melt to emit, so the use of a buffer rod is mandatory. One side of the rod is attached to the emitting element and the other side is inserted into the melt. Without cooling, the transducer can reach more than 160˚C due to conduction of the heat in the rod, which is beyond the working temperature of such transducers, thus air-cooling is usually used [
The sensitivity of the device is directly related to the Signal-to-Noise Ratio (SNR). It is defined as the ratio between the strength of the desired signal at the end of the probe and the strength of the noise produced inside. This noise is generated due to the mode of vibration and diffraction. It has been shown that the detectable size of the particles depends on the power of the waves rather than the frequency; therefore a higher ratio means better sensitivity of the device as more energy is transferred inside the melt [
The maximal power delivered by the piezoelectric element is of primary importance, but studies also show that using a cladding [
The signal to noise ratio is determinant in the ability of the device to differentiate between different particles when the refracted signal arrive at the same time. It is a very important specification of the rod that has to be taken into account.
The buffer rod has a primordial importance on the transmission of the ultrasonic waves between the piezoelectric element and the melt. Its shape, material and coating have influences on the sensitivity of the system.
The design has to be made between two possible configurations: the “Pulse Echo” configuration, which uses a single rod as both transmitter [
The Pulse Echo configuration has the advantage of being cheaper and easier to set up, but suffers from higher noise and echoes inside the rod, which decreases the quality of the detection due to the delay line. The Pitch Catch configuration has a better SNR and allows focusing the detection in one particular area by changing the relative position of the rods, but the set-up is more expensive [
Buffer rods are typically between 200 mm and 300 mm in length [
Using a tapered rod is a good solution to prevent echoing inside the buffer rod. Usually an angle around 1.5˚ is enough to see considerable improvement in the SNR [
A focusing lens can be used at the end of the rod to increase the power transmitted to a particular area in the melt [
Material type is especially important from the metallurgical point of view due to aggressive chemistry of aluminium which is not easily combined with many materials. The choice of material for the rod is of primary importance for the life of the probe, as well as for its effectiveness. A good material should meet the following requirements: good wetting ability by the liquid metal under ultrasonic waves, good acoustic conductivity, good resistance to corrosion, good resistance to thermal shocks, low thermal conductivity and high melting point. The durability of the material can also be improved by using coatings and cooling [
Steel rods are mainly used [
A stainless steel cladding [
Two types of positioning of the rods are possible: parallel or tilted toward another (V-shape) as shown in
The V-shape setup creates for more focused signals in the melt, thus increasing its sensitivity. The receiving rod receives a higher percentage of the reflected waves and particles down to 10 µm can be detected [
The parallel shape works on the same principle but the waves are transmitted vertically in the melt, and the receiving rod gets a smaller part of the reflected waves. This allows for a bigger volume in the melt to be analysed, but the amplitude of the initial waves has to be higher to be able to detect smaller particles.
It should be noted that one type of set-up might be preferable depending on the aim of the analysis. If the aim is to detect the smallest particle in the melt, using a V-shape with focusing lens at the end of the rod might work better. If the aim is to get a global knowledge of the particle size and concentration in the melt, using a parallel shape system is recommended.
There has been no comparative research done about the influence of the distance between the two rods on the sensitivity of the system. It should be investigated for the further works either.
The set-up for detection suffers from a few recurring problems at the interface between the rod and the melt. Some corrosion may occur and a cleaning can be needed to increase the lifespan of the rod.
First problem is the coating around the steel rod does not protect the tip from the corrosion. A typical AlFe3 alloy deposit has been found on the tip of the rod in the work of Mountford after a long period of continuous detection [
The second problem, which can be faced in both laboratory and industrial utilizations, comes from the tendency of particle to gather at the tip of the rod [
Interactions between melt and particles must be also well understood in terms of turbulence, settling and agglomeration. These phenomena affect the detectable particle size range, detectable volume and the reliability of the detection results.
The modulation of the amplitude of the received signal gives an indication of the site, and the first particles detected are the biggest ones [
In the work of Mountford, pure aluminium at about 730˚C was used for the ultrasonic detection and alumina particles were added and detected for 30 seconds with waves of 2.25 MHz at 72 dB to observe the detecting process. By addition of alumina with known concentration, the number of the detected particles was expected to increase linearly however it was not observed as it is seen in
However, the shape factor has also a direct influence on settling velocity of particles which can cause different problems during the detection [
Ultrasonic detection can potentially be used in launder, holding furnaces and crucible furnaces. Slower melt velocities such as in crucible or holding furnaces make the calibration of over counting particles easier. However, it has been reported by M. Badowski et al. that the melt velocity can disturb the particle movement even in crucible furnaces [
Stirring also affects the melt flow and the detection characteristics since it prevents the settling and keeps the big particles in the flow [
The use of water model is a good way to test theory on the minimal detectable size of the particles. The usual minimal size is around 20 µm [
The sensibility of the detection is affected by the concentration of particles in the melt. For high inclusion concentrations, the particle count is often less than the real because some of the small particles are hidden in the “shadow” of the bigger particles. A big percentage of particles are not reachable by the reflected ultrasonic waves [
The previous research in the years 1985 to 2015 on ultrasonic particle detection in aluminium melts can be summed up as follows:
・ An on-line monitoring of inclusions would allow for faster and more efficient processes.
・ The physics of ultrasonic is well-known and allows creating and arranging models. Existing water models can be used for the first tests to validate the setup.
・ MetalVision is expensive equipment, which has limits when the contrast between concentration of bigger sized particles and smaller sized particles is too high. This could be improved by developing a new on-line system.
・ A set-up for detection by ultrasonic waves might be described in 4 parts: the piezoelectric transducer in charge of producing the waves, the rods which are transmitting the waves to the melt, the interface between the rods and the melt, and finally the interface between the waves inside the melt and the particles.
・ The detection of inclusions in aluminium melts by ultrasonic waves seems promising, and should be experimented more on an industrial level to see whether the change of scale has an influence on the results.
・ This work is a review of different problematics and possibilities associated with the particle detection by ultrasonic waves. Although it is working both theoretically and on a laboratory scale, more tests are surely needed.
・ Future works should focus on combining different futures of ultrasonic detection such as detection of the size of the particles, their position in the melt and the concentration of the melt. The question still stands if it is possible to do all of these together.
・ Another area for the future research would be improving the rod. Finding a compromise between good conductivity and corrosion resistance is very important which would make the detection system cheaper than the current solutions.
The research leading to these results has been carried out within the framework of the AMAP (Advanced Metals and Processes) research cluster at RWTH Aachen University, Germany.
Mertol Gökelma,Damien Latacz,Bernd Friedrich, (2016) A Review on Prerequisites of a Set-Up for Particle Detection by Ultrasonic Waves in Aluminium Melts. Open Journal of Metal,06,13-24. doi: 10.4236/ojmetal.2016.61002