The investment casting process draws a lot of attention because of its high-value products, but the technologies involved in the process are quite impressive in their own ways. Now a pilot-scale melting system is offering some promise for increasing the volumes and range of alloys for those high-value products.
The FastCast concept developed by ALD Vacuum Technologies GmbH address the issue of reactive metals (titanium, aluminum, and superalloys) that are quite standard for investment casting and the risk of contamination in ceramic crucibles. Crucibles are fine for non-reactive materials, but the higher-value alloys are more commonly melted in cold-wall crucibles, with low superheat and high power consumption as the accepted trade-off for lower levels of impurity in the metal.
ALD is proposing non-contact levitation melting as an industrial-scale alternative, increasing the limited volume of that lab-scale process from about 50 g to 500 g. Using numerical modelling, the ALD developers have designed a system that works with two alternating magnetic fields, so that higher volumes of molten material can be kept in levitation.
As explained by Dr. Sergejs Spitans, R&D process engineer, physicist, and simulation expert at ALD Vacuum Technologies, “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 (the combination of electric and magnetic force on a point charge due to electromagnetic fields) 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.”
Spitans, working with his dissertation supervisor Prof. Dr.-Ing. Egbert Baake from the Institute of Electrotechnology at Leibniz University Hannover, 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 wherein numerous aluminum-, nickel-, and titanium- (Ti-6Al-4V) alloys up to 500 g were melted in high purity without inclusions.
Now, various reactive metals often selected for investment casting can be melted reliably in a non-contact levitation process without contamination, and subsequently cast in a mold.
Two horizontal and orthogonal electromagnetic fields of different frequencies are applied to exert Lorentz force at the bottom of the levitated sample too, so that 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, so much higher superheat temperatures to be achieved.
The high superheat indicates greater varieties for mold design are possible, such as thin-walled and complex castings – e.g., medical or aerospace components – and an integrated mold centrifuge also helps in producing highly complex investment castings. Levitation melting also contributes to less turbulent mold-filling … “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,” according to ALD.
The patented process design also allows robust superheating with comparatively low power input, reducing mold-preheating requirements and promoting defect-free casting. An integrated mold centrifuge coupled with the high superheat makes it possible to cast highly complex investment castings in reactive materials, like titanium alloys.
After many simulation-aided design iterations, the optimized process was transferred to a functional plant including feeder, preheating furnace, and casing. “The final scale-up configuration has a modular levitation assembly group that consists of four ferrite poles and four inductors, each is water-cooled and protected by heat shields,” according to Spitans.
“Levitation melting is only slightly more efficient than the cold wall crucible, however, the advantages like predefined melt purity, absence of the skull scrap, fast melting speed and tremendous superheat up to 250 C at the moment of mold filling makes the process extremely attractive for complex castings,” Spitans said.
The pilot plant offers a semi-automated process chain with up to 10 molds, and almost all relevant titanium- and aluminum-based alloys as well as super alloys have been cast successfully. It is available for test runs, and ALD is committed to developing a production-scale FastCast system in cooperation with interested users, customized (feeding, mold numbers) to their own production programs.