Experimental Analysis of the Aluminium Melting Process in Industrial Cold Crucible Furnaces

Most of industrial sector desires high-purity metals and alloys. Morita et al. [1] underlined that due to the outstanding properties of high-quality titanium alloys such as the corrosion resistance or the biocompatibility, these alloys are more often used as the material for the medical implants. Moreover, Fadeev et al. [2] also pointed out the importance of the high quality alloy for the gas turbine blades. Moreover, the metals used for these state of the art technologies, such as titanium or aluminium, are highly reactive. In consequence, levitation or semi-levitation melting techniques are desirable to guarantee the required quality of the processed metals or alloys. One of the semi-levitation methods is the application of a cold crucible furnace (CCF). As the authors of [3] stated the CCF melting or smelting is usually conducted in a vacuum or neutral atmosphere, e.g., argon. The study presented in [4] showed that a method of metal processing is also interesting from the economic point of view in case of the relatively expensive titanium alloys. Due to an electromagnetic field, the processed metal is pushed away from the crucible walls. Additionally, the crucible walls and the crucible bottom are water-cooled. The high temperature difference between the molten metal and the crucible bottom results in the formation of so-called skull.

Song et al. [5] suggested that the skull works as a protective layer that prevents the contamination of the processed metal from the crucible. Moreover, Sugilal [6] processed various highly reactive metals inside the cold crucible furnace. The authors of the aforementioned study noted that the skull prevents the liquid metal from superheating. On the other hand, the skull can be considered as a loss of the crucible load. Therefore, the crucible construction should be carefully designed to minimise the skull mass. Moreover, the melting procedures should be adjusted for a selected mass and shape of the load.

The general design of the cold crucible furnace is the same regardless of the applied melting method. The typical construction of CCF is an assembly of the segmented crucible walls and the coil that surrounds the crucible. The crucible segments are separated from each other by thin slits. In consequence, the crucible is, to some extent, transparent for the electromagnetic field. Therefore, the crucible operates as a secondary inductor. As previously mentioned, the crucible walls, the bottom and the coil are cooled by the water flowing through the internal ducts of these components. Therefore, the segments of the crucible and its bottom remain significantly colder than the metal or alloy inside the crucible. In the case of the investigated crucible, the cooling channels of the crucible walls were separated from the crucible bottom channel. A more detailed description of the investigated crucible is provided in Sect. 2 of this paper. Despite the relatively simple crucible assembly, the recent studies demonstrated that the crucible performance is strongly related to the crucible design. The relation between the crucible design and its performance was investigated by numerous authors. The authors of [7] reported that the number of the crucible segments, as well as the width of the slits between the segments, affects the electromagnetic field significantly and, in consequence, the crucible performance. Similar analysis was presented in [8]. The authors of that work investigated the crucible wall arrangements on the efficiency of the melting in CCF. In addition, the influence of the number of the walls segments as well as the slits thickness was investigated by these authors. According to the presented results, the design of the crucible walls significantly affects the electrical performance of the device. Dumont et al. [9] showed that depending on the crucible design the efficiency of the crucible can vary from 26% to 53%. The analysis of the cold crucible used for melting and direct solidification of titanium alloys was showed in [10]. The results presented by these authors showed how the crucible performance vary with the change of the coil position, a ratio of the metal height to length or magnetic Reynolds number. The results gathered by [10] showed that the proper crucible design and melting process are beneficial for the crystallization of the alloy during the process. Moreover, the results presented in [10] showed the good agreement with the cold crucible experimental analysis presented in the papers such as [11]. Accounting for the importance of the crucible design and the influence of the melting procedure on the performance of the melting method, the mathematical models of the CCF have recently been employed to investigate the device performance.

The metal melting is a relatively complex phenomenon. Therefore, the numerous authors investigate that process both experimentally and numerically. In particular, the free surface shape, the convection flow and the temperature field of the molten metal are the scope of the various studies. Keneda et al. [12] investigated the natural convection flow under the uniform magnetic field. The authors of that work underlined the relation between the convection, Lorentzen forces and the heat rate under various electric current. It is worth mentioning that Kenda et al. investigated the process both experimentally and numerically. The free surface characteristics of the molten metal were also investigated in [13]. Li et al. [13] conducted the number of tests on the dedicated experimental stand to analyse the free surface structure and convective heat transfer. However, the considered liquid metal flow was laminar in that case.

Taking into account the complexity of the phase change and the multiphase flow during metal melting and the difficulties related to the experimental investigation of that process, the mathematical models are more often used to simulate the flow and the phase change of the metal [14]. In particular, mathematical models that are applied during the CCF analysis should include phenomena such as the phase change of the metal during the melting, free surface fluctuations, heat transfer in the load and the crucible and the mixing of the liquid metal. Therefore, i.e. Spitans et al. [7] proposed the coupling procedure for the electromagnetic and heat and fluid transfer models. In such an approach, each sub-model should be carefully developed and validated to guarantee the satisfying results. The analogical approach for the induction melter and analysed the influence of the turbulence models on the velocity and the free surface shape prediction was used in [15]. The results gathered by the authors of that study showed the significant difference between the velocity fields and vortices obtained by utilisation of the \(k-\epsilon\), \(k-\omega\) and transition SST model. According to those results, the SST model predicted the highest velocity of the molten metal. The \(k -\omega\) model was also used by the authors of [16, 17]. That group of the researchers formulated numerical model for the dynamic simulations of the melting process of titanium and titanium alloys. In particular, the authors of [16] proposed axisymmetric pseudo-spectral mathematical formulation for the prediction of the temperature inside the load. In latter paper [17], the authors numerically investigated the influence of the crucible design parameters on the heat losses melting process in the cold crucible. The numerical model together with the experimental results presented in that paper was used to optimise the melting process in the cold crucible. Both experimental and the numerical results showed that the process should be carefully optimised for various loads. In addition, the authors of [17] underlined the relation between the free surface shape and turbulence on the temperature of the molten load. In particular, the authors of [16, 17] concluded that heat losses related to the evaporation of the aluminium from the free surface were significantly higher comparing to the heat loses caused by the radiation. Nevertheless, Bulinski et al. [15] presented that the effects of the turbulence modelling on the Lorentzen force field were negligible. In the other work of these authors [18] the validation of the developed model for the vacuum induction furnace was presented. In the validation procedure, the free surface shape as well as the temperature field were compared. According to the reported results, the acceptable agreement between the experimental and numerical results was achieved. Recently, Bulinski et al. [19] employed the coupled CFD-EMAG model for the cold crucible furnace. That study was mostly devoted to an identification of the free surface shape and free surface area. Moreover, he compared the obtained numerical results with the experimental data showing a good agreement in terms of the temperature and the free surface shape prediction. Above that, the authors of [20] performed further investigation of the process. In particular, the previously developed numerical model was used for the investigation of the evaporation process in vacuum induction furnace. The mathematical model results were compared with the experimental results in terms of mass evaporation prediction. Next, Bulinski et al. [20] used that CFD-EMAG model to analyse the evaporation process for various crucibles position and metal charges. Considering that the CCF technology is already present in commercial installations, the data used for the validation should be collected during the operation of industrial-type cold crucible furnaces. Unfortunately, the literature dealing with the mathematical modelling of the melting process in the mentioned industrial-type CCFs is fairly limited.

Song et al. [21] experimentally investigated the impurities evaporation from aluminium alloys. The results presented in that paper primarily included the evaporation kinetics, while neglecting the temperature measurements. In addition, Matsuzawa et al. [22] collected pictures of the deformation of the processed aluminium at different times. Next, these pictures were compared with the simulation results. The authors of that study pointed out the number of difficulties that were encountered during that comparison. In consequence, the experimental results presented by these authors are unsuitable for the further validation of the numerical model, especially in case of the industrial-scale CCF systems. Moreover, authors of [22] did not focus on the validation of the thermal analysis that they performed. More detailed analysis of the process was presented in [23]. These authors presented various numerical results for a direct solidification cold crucible system. In that paper, the magnetic flux density and the temperature at the alloy centre were used for the model validation. The comparison of the experimental and numerical results presented by Chen et al. [23] indicated good model fidelity in terms of the temperature and magnetic flux density prediction. However, the presented results were gathered during the investigation of laboratory-scale equipment. Hence, it is difficult to use those results to analyse industrial-scale VIM applications.

As mentioned in Sect. 1, all experimental tasks were performed on a cold crucible furnace delivered by Seco-Warwick in collaboration with the Retech company. The general view of the furnace is presented in Fig. 1. That device is supplied by a Seco-Warwick generator that offers 200 kW of input power. The furnace control system integrated with the furnace offers measurements of the active power and the actual frequency. Nevertheless, the indication of the built-in control system strongly depends on the accuracy of the inverter resonant system matching. Therefore, the actual current and the frequency were measured at the inductors resonance circuit inside the inverter cabinet. The measured current and frequency are listed in Table 1. The measured values were used to define an approximation function that assessed the current and frequency for any generator load. The approximation functions for the current [Eq. (1)] and the frequency [Eq. (2)] are presented below. The R\(^2\) for both functions was higher than 0.99.

To guarantee a vacuum, the investigated furnace is equipped with a set of pumps. The pump system includes a Kaesser compressors WVC 1200 rotary vacuum pump (4 kW and 3000 RPM @50 Hz), a Busch R5 0100 F rotary vacuum pump (2.7 kW and 1500 RPM @50 Hz) and an Agilent Technologies diffusion pump (4.4 kW). Various valves and types of insulations were installed in the furnace to prevent any intake of the surrounding air. The furnace walls, the crucible segments, the bottom and the generator are cooled by water. The water is cooled within a Parker Hyperchill PWC industrial chiller, which has a cooling capacity that is rated for 183 kW. The temperature of the water is monitored at the chiller outlet and the crucible outlet. The mass flow rate of the cooling water is not controlled or monitored during the melting process. By default, the temperature of the processed medium is measured by a pyrometer installed directly above the crucible or by an immersion thermocouple that can be sank into the molten metal. However, that set up was adjusted for the titanium melting process. The lowest temperature limit for the pyrometer is slightly lower than the titanium melting temperature and significantly higher than the aluminium melting temperature. Therefore, those device readings were not considered during the study. Fortunately, the immersion thermocouple installed in the furnace can be sank inside the molten metal any time during the process without interfering with the vacuum. Alternatively, the thermocouple can be replaced with a sample collector, which is typically used for the extraction of the liquid metal samples for further chemical analysis. Additionally, the sample collector can be used for the attachment of various measuring devices that can be sank into the metal during the process.

Two inspection windows are installed on the furnace walls. These windows are equipped with industrial cameras. Hence, the melting process can be visually controlled by the operator. Inside the inspection windows, eight additional glasses are installed on a revolver-type frame to prevent the outer window from being damaged. By positioning the revolver-type frame, the glasses can be changed during the melting process. The outer glass is made of calcium fluoride (CaF\(_2\)), which is approx. 95% transmissive for the wavelength in the range of 0.13–9.00 \(\upmu\)m depending on the glass thickness. Inside the mentioned revolver-type frame, additional CaF\(_2\) glass and germanium glass were installed. The germanium glass guarantees approximately 100% transmission of wavelengths from 8.00 to 12.00 8.00 to 12.00 \(\upmu\)m [24, 25]. This configuration is suitable for the middle range infrared (IR) cameras.

As was briefly mentioned in the introduction, the conducted experimental campaign was focused on the molten metal temperature, the meniscus shape measurements and the skull shape. For the temperature field measurements, an advanced IR camera was used, namely, a Flir A655sc, which is equipped with an uncooled microbolometer that operates in the spectral range from 7.5 to 14.0 \(\upmu\)m. The maximum frame rate available with the full window (640 \(\times\) 480 pix) recording is 50 Hz. The 41.3 mm Flir lens, which offers a field of view (FOV) of 15\(^{\circ }\)\(\times\) 11.3\(^{\circ }\), was mounted on the camera. The camera together with the lens was calibrated for the temperature range of \(-40^{\circ }\)C to 150\(^{\circ }\)C and the alternative range of 300\(^{\circ }\)C to 2000\(^{\circ }\)C. The accuracy of that camera, as declared by the manufacturer, was rated as ± 2 \(^{\circ }\)C or ± 2% of the reading. The camera was installed on the furnace wall above the inspection window equipped with the glass suitable for the applied camera. The IR camera temperature reading was verified by the immersion thermocouple. Therefore, the emissivity of the molten metal was assessed.

The meniscus shape of the molten metal was measured by a simple but robust in-house constructed tool. The general assessment of the meniscus shape and the measurement of the maximum height was possible with the developed device. The idea of the device was to fix relatively thin wires to the wooden plate. Then, the device was mounted directly above the crucible. Next, the position of the device was set, namely, the tips of the wires touched the top of the solid load. Therefore, when the molten metal meniscus occurred, the molten metal partially stuck to the installed wires. The device scheme is presented in Fig. 4. When the melting and casting processes were performed, the device presented in Fig. 4 was removed from the furnace. Next, the distance between the needles covered with the aluminium alloy and the device base was assessed. Therefore, the approximation of the meniscus shape and maximum height was performed. Moreover, the results gathered using this method are useful for the calculation of the free surface area of a molten medium.

After each experimental run, the generated skulls were collected and weighed to evaluate the loss of molten medium. The shape of the skull was also assessed. The collected material was cut along the diameter to examine the shape and height of the skull near the crucible walls, since the skull shape is especially important from the electromagnetic forces point of view. At the crucible bottom, the generated skull was near the slits between the crucible wall segments. Consequently, the Lorentz force, as well as the velocity field of the liquid alloy, change. The maximum height of the skull is the point where the meniscus adhered to the crucible walls, and thus, that information is useful for assessing the meniscus shape. To better illustrate the relation between the meniscus and the skull, Fig. 5 is presented. The shape of the meniscus tip marked in that figure was measured with the device presented in Fig. 4. The mentioned device was positioned, each time, to collect the points that were then used to reconstruct the shape of the tip. As shown in Fig. 5, the point where the meniscus slope is attached to the crucible walls corresponds to the maximal skull height. Therefore, for a known meniscus tip shape and skull height at the crucible walls, the meniscus slope can be approximated.

The melting process was conducted for various currents and frequencies and consequently various power inputs. The quasi-steady conditions were obtained for the considered variants. For the temperature measurements, an IR camera and an immersion thermocouple were used. In addition, the molten metal meniscus shape was investigated within the in-house developed device. The last part of the investigation was the analysis of the not molten part of the metal, the so-called skulls, that are typical for that process. The skull thickness and the skull height were analysed during the measurements.

During the temperature measurements, the alloy temperature and emissivity were analysed. The comparison of the IR camera results and the immersion thermocouple showed that the emissivity of the considered alloy was evaluated to be the very low value of 0.03 which was significantly higher than the emissivity of the liquid aluminium presented in literature. Moreover, the emissivity of the crucible walls was evaluated. According to the gathered results, the emissivity of the crucible walls was 0.99. That very high value was mostly caused by the layer of the oxides that cover the crucible walls and increase the roughness of the walls.

The skull weight for the various power inputs and the average skull weight were defined. The average mass of the skulls collected during the measurements campaign was 178 g, which was approximately 13% of the initial mass of the processed alloy. Additionally, the presented results show that the skull thickness is strongly related to the power input applied before the metal casting. In general, a higher power input reduced the skull weight. The skull surface profile analysis showed that the skull thickness is more or less constant along the skull diameter.

Finally, the device that was developed in-house was used to capture the shape of the meniscus tip surface. During that analysis, the points describing the shape of the meniscus tip were gathered. Next, these points were used to create the surface plots of the meniscus tip and to calculate the free surface area of the meniscus. The results recorded for that investigation show that the process was more energy efficient in the cases with higher amounts of the processed metal.

The proposed methodology of the experimental investigation can be useful for analysis of different metals and alloys in CCFs. Due to the industrial characteristics of the investigated melter, a higher accuracy of the measurements, especially in terms of the meniscus shape measurements, is difficult to achieve. Nevertheless, further research will be focused on improving the measurement instruments, and an analysis of titanium-based alloys will be conducted to improve the casting of that metal.