Heat Transfer in Induction Melting of Aluminum Oxide in Cold Crucible

This article aims to explain the mechanism of heat transfer between melt and cold crucible (CC) in processes referred to as induction skull melting (ISM). Several experiments were performed for this purpose. In these experiments, all relevant electrical and mechanical quantities that affect the heat transfer between the melt and the cold crucible were monitored. Firstly, this article deals with the description of the equipment used in the experiments. Aluminum oxide was considered as experimental material (EM). Therefore, the article also deals with the design of a suitable cold crucible for its melting. Subsequently, the article deals with the implementation of the experiment itself and its phases. Finally, based on the data obtained during melting processes, this paper deals with the calculation of the real heat transfer from the melt to the cold crucible. The main contribution of this article is a comprehensive view of the values of mechanical and electrical quantities that accompany the melting of the considered experimental material (EM) in the designed cold crucible. In conclusion, the values of thermal quantities obtained in the performed experiments are compared with numerical simulation.

Effect of Boron Content and Cooling Rate on the Microstructure and Boride Formation of β-Solidifying γ-TiAl TNM Alloy

Boron is a unique and popular grain refiner element in cast titanium aluminide (TiAl) alloys, as it helps to improve mechanical properties if properly alloyed. However, the formation mechanism of different types of borides in cast TiAl alloys is not yet clearly understood. This study seeks to correlate the chemical composition and cooling rate during solidification of cast TiAl alloys, with the type of boride precipitated and the resulting microstructure. Several β-solidifying γ-TiAl alloys of the TNM family were cast, alloying boron to a starting Ti-44.5Al-4Nb-1Mo-0.1B (at.%) alloy. The alloys were manufactured with an induction skull melting furnace and poured into a stepped 2, 4, 8 and 16 mm thickness mold to achieve different cooling rates. On one hand, the results reveal that boron contents below 0.5 at.% and cooling rates during solidification above 10 K/s promote the formation of detrimental ribbon borides. On the other hand, boron contents above 0.5 at.% and cooling rates during solidification below 10 K/s promote the formation of a refined microstructure with blocky borides. Finally, the formation mechanisms of both ribbon and blocky borides are proposed.

Deoxygenation of liquid titanium with aluminum addition

To realize radical cost reduction of titanium, a process is needed which can directly make use of low quality material such as scrap, TiO2 or titanium ore. In this work, a highly efficient process has been developed to produce low oxygen titanium alloy using aluminum to rapidly reduce oxygen during melting. In this experiment titanium was prepared including 0.8 mass% oxygen. This titanium and aluminum in the range of 0 – 60 mass% was measured, mixed and melted by PAM (plasma arc melting) or ISM (induction skull melting). After melting, a small piece was taken and the aluminum and oxygen content was analyzed by ICP emission spectrometry and inert gas fusion-infrared absorption method respectively. A sample melted with CaO-CaF2 flux was analyzed as well after flux was mechanically taken off. As aluminum content increased, oxygen content decreased. For example, when 61.9 mass% aluminum was added, the oxygen content decreased to 0.028 mass% and Al2O3 was observed in the cross-section of the sample after melting. This was produced when the aluminum content increased and the oxygen solubility decreased in the metal. Flux addition was also clearly effective for deoxygenation.

Induction Skull Melting of Ti-6Al-4V: Process Control and Efficiency Optimization

Titanium investment casting is one of the leading and most efficient near-net-shape manufacturing processes, since complex shape components are possible to obtain with a very low amount of material waste. But melting these reactive alloys implies the usage of specific melting technologies such as the Induction Skull Melting (ISM) method. In this work the ISM was extensively studied with the aim of deepening the characteristics of this specific melting method and improving the too low energy efficiency and overall process performance. A 16 segment copper crucible and 3 turns coil was employed for the melting of 1 kg of Ti-6Al-4V alloy. Through the calorimetric balance, real-time evolution of the process parameters and power losses arising from the crucible and coil sub-assemblies was displayed. Results revealed the impact of coil working conditions in the overall ISM thermal efficiency and titanium melt properties, revealing the use of these conditions as an effective optimization strategy. This unstudied melting control method allowed more heat into charge and 13% efficiency enhancement; leading to a shorter melting process, less energy consumption and increased melt superheat, which reached 49 °C. The experimental data published in this paper represent a valuable empiric reference for the development and validation of current and future induction heating models.

Microstructure and Oxidation Resistance of Y Modified Silicide Coatings Prepared on Zr-Ti-Al Alloy

Zr-20Ti-5Al (at. %) alloy used as substrates for Si-Y2O3co-deposition experiments was prepared by firstly vacuum non-consumable arc melting and then high frequency induction skull melting. The results showed that Y modified silicide coating prepared at 1250 °C for 4 h possessed a double-layer structure, mainly consisting of a thick (Zr, Ti)Si2outer layer and a 15 mm thick (Zr, Ti)Si inner layer. Meanwhile, the growth rate of ZrSi2phase changed with temperature, while the growth rate of ZrSi did not vary significantly with temperature. The growth of the coating as well as the two layers followed parabolic laws, and the co-deposition process was controlled by diffusion. ZrSi2was not appropriate as oxidation-resistant coatings to protect Nb based alloy from oxidation due to the lack of the formation of good quality glassy SiO2layer in the scale.

Development of Low Cost and Low Elastic Modulus of Ti-Al-Mo-Fe Alloys for Automotive Applications

Ti-Al-Mo-Fe alloys were developed as low cost beta Ti alloys for automotive springs, and designed based on Molybdenum equivalency and Bo-Md molecular orbital method. Low priced Mo-Fe master alloys were introduced as alloying elements for the cost and elastic modulus reduction. Eight Ti-Al-Mo-Fe alloy candidates were pre-designed according to the Bo-Md method. Primary laboratory scale ingots were thus melted by ISM (Induction Skull Melting). After primary property evaluation, the Ti-2Al-9.2Mo-2Fe alloy was optimized finally and large scale ingot was made by VAR for further property evaluation. Resultantly, it shows that the alloy has lower elastic modulus (60-70 GPa) and good tensile properties in the solution condition, and compares well with the other developed commercial beta alloys.

Aluminium Evaporation during Ceramic Crucible Induction Melting of Titanium Aluminides

Melting TiAl based alloys in ceramic crucibles often leads to chemical contamination, alloy heterogeneity and non-metallic inclusions. The severity of such phenomena usually depends on the nature of crucible materials, the melting stock composition and the melting parameters, namely superheating time and temperature and melting pressure. Among the referred drawbacks, Al loss during melting is a critical aspect, as its concentration in TiAl based alloys has a very strong effect in their mechanical properties. Although a few studies of critical factors affecting the evaporation behaviour of Al during electron beam and induction skull melting of Ti-Al alloys had been carried out, until now no information was released on this subject for the ceramic crucible induction melting process. In this work a Ti-48Al alloy was induction melted in a zircon crucible with Y2O3 inner layer, using 50 and 100 °C superheating temperatures and 0, 60 and 90 second holding times, and poured into a graphite mould. The effect of different temperature/time combinations in the alloy composition, Al loss by evaporation and extent of the metal/crucible interaction was studied for different melting pressures. Al loss was found to increase significantly for melting pressures below around 10-1 mbar, at a rate that increases as melting pressure decreases, until a maximum rate is reached, remaining constant for lower pressure levels. Metal/crucible interaction increased directly with the melting pressure and superheating time, leading to alloy contamination with yttrium and oxygen. For the experimental set-up and conditions used on this work, optimal superheating time/pressure combinations that lead to acceptable alloy composition and sanity have been identified.