Product
Solutions Levitation Melting in Investment Casting for
Enhanced Materials and Products Engineers succeed in building an industrial-scale levitation melting system: Special design allows casting of various reactive metals and alloys up to 500 g with high purity and short cycle times
Innovative FastCast concept opens up new casting applications and designs
melting in investment casting. Although widely used, they have some disadvantages. For most (non-reactive) materials ceramic crucibles are used which can result in impurities or ceramic inclusions affecting the quality of the cast part. Reactive metals, e.g. Titanium alloys, are melted in cold wall crucibles. The low superheat and high power consumption in the cold wall sealing process is a price for a cleaner melting process. For the first time, non-contact levitation melting can be used as an alternative on an industrial scale, as ALD Vacuum Technologies GmbH has succeeded in increasing the previous limited weight quantity of only 50 g to unprecedented 500 g. Using numerical modeling the melting experts have devised a new system design that works with two alternating magnetic fields, which allows these higher weight quantities to be kept in levitation. Now with FastCast various metals and alloys like
V titanium, aluminum, and or super
alloys can be melted reliable in a non- contact levitation process without contamination
subsequently
casted in a mold. The process allows a gentle, less turbulent filling process of the mold, which is favored by a high- speed take-off of the mold and therefore the low relative velocity between free-falling melt and sinking mold. The special and extensively patented design allows strong superheating with comparatively low power input which favors “defect”-free casting resulting in economic benefits and lower mould preheating temperatures. The integrated
50 ❘ October 2021 ®
arious crucible melting methods
are used for metal
mold centrifuge coupled with the high superheat enables the casting of highly complex and filigree investment castings made of reactive titanium alloys. Ceramic crucibles are widely used
for casting nickel-based or iron-based alloys. They are cost-efficient and to a certain extent allow superheating, i.e. raising the temperature above the liquidus temperature of the alloy. However, these crucibles are rather unsuitable for melting extremely reactive materials at high temperatures, as this leads to an inadmissible contamination of the melt.
This prevents the
manufacturing of high purity, near net- shape casting parts made of metals and alloys such as titanium. By comparison, the competitive cold wall casting technique is more suitable for such materials but requires high amount of power for melting the material in water- cooled copper crucibles. Furthermore, overheating is not very feasible with this method, because most of the power required for this flows directly into the cooling water of the crucible and is, so to speak, dissipated. Therefore, a melting process without the material coming into contact with the crucible (non-contact) would be desirable. The ALD Vacuum Technologies GmbH had set itself the goal of converting this principle into a fully functional plant fit for industrial use.
“One process fairly suitable to
start with was the so-called levitation melting,” reports Dr. Sergejs Spitans, R&D Process
Simulation Expert at ALD Vacuum Technologies.
Engineer, “A
Physicist & ‘conventional’
levitation melting uses an axisymmetric
coil to create a magnetic field in which metallic samples can be contact-free confined and melted. The problem is that Lorentz force confinement vanishes on the symmetry axis and the melt leakage is prevented in this lowest point of a levitated melt only by the surface tension. Therefore, only small molten metal samples up to 50 g can be levitated in this ‘conventional’ way.” As part of his dissertation work and under the supervision of Prof. Dr.-Ing. (Doctor of Engineering) Egbert Baake from the Institute of Electrotechnology (ETP) at Leibniz University Hannover, Dr. Sergejs Spitans from ALD used numerical models and various experiments to find a way to significantly increase the melt weight. Together with engineers from ALD, a pilot plant was developed where numerous aluminum-, nickel- and titanium- (Ti-6Al-4V) alloys up to 500 g were successfully melted in high
purity without inclusions. The
method applies two horizontal and orthogonal electromagnetic fields of different frequencies in order to exert a Lorentz force also at the bottom of the levitated sample. Therefore, the weight of the charge can be increased and the charge can be melted drip- and leakage-free. Levitation melting prevents contamination of the molten metal with the crucible material and results in significantly higher alloy purity. In addition, heat losses from the liquid metal are limited to radiation and evaporation only, which allows much higher superheat temperatures to be achieved. The high superheat opens up new avenues for mold design and thus for thin-walled and complex
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