Compact, Efficient Melting for High-Purity, High-Value Alloys

Specialty metal alloys represent an increasingly important part of the total market for metalcastings, even beyond high-tech and aerospace/defense manufacturing. One of the difficulties for foundries and diecasters, or metal producers of any type, is melting high-specification alloys efficiently and effectively. ALD Vacuum Technologies GmbH has developed a furnace for melting high-purity copper-titanium-copper alloys without inclusions that form as a result of air contact, excessive heat loss, or unfeasible cycle times. It’s a vacuum melting process, and by preventing oxidation and with the help of special stirring as well as degassing techniques the melt is kept as pure as possible.

Metal sampling, alloy addition, and temperature measurement are possible without interrupting the process. Melt transfer for pouring or continuous casting can be carried out under controlled conditions.

The new melting concept, which has been installed and operated commercially at two locations, was developed specifically for melting copper-titanium alloys, which are in high demand for electronic applications like conductive springs, connectors used in automotive systems; and for parts installed in cameras, smartphones, and PCs. These alloys are produced in several ways, but vacuum induction melting is a new approach.

“Most conventional induction furnaces are quite large and not very practical for the batch sizes that are common in copper-titanium alloy manufacturing,” explained Andreas Eich, melting product manager at ALD Vacuum Technologies. “Additionally, conventional induction melting usually takes place under atmospheric conditions.”

Atmospheric contact reduces melt cleanliness and deteriorates material properties, raising the production cost due to a reduced titanium yield. So, the furnace design has to be extensively updated, including a way to create and maintain an inert gas atmosphere. This explains the choice of vacuum induction melting (VIM.) However, the possible need to transport the molten metal introduces more potential exposure to ambient air.

ALD engineers developed Vacuum Induction Degassing (VID) or Vacuum Induction Degassing and Pouring (VIDP) concepts, with a compact plant design in which melting and casting can take place completely under controlled atmosphere. The design is operation at two different plants in Asia and can be adapted to existing melting infrastructure at other production sites, according to the developer.

In those installations, the copper-titanium material is melted under a controlled atmosphere in a vacuum induction furnace, and then cast in a semi-continuous vertical casting machine to produce high-purity slabs as starting material for further processing, e.g., hot forming, annealing, and cold forming. The vacuum-tight process makes the results highly reproducible and the yields predictable because the metal-purity risk factors are minimized.

In addition to the induction furnace itself, an ALD launder chamber and the tundish and caster equipment used by the plant operator are also installed in the vacuum tight housing, and work under an inert gas atmosphere.

In the related VID furnace concept, transfer of molten metal to the caster is done via an open transfer launder in a normal ambient atmosphere with inert gas shrouding. This set-up can produce batch sizes of up to 30 metric tons.

Metal purity and microstructure targets are aided by additional process and monitoring features. First, the melt is kept very homogeneous by ALD’s three-phase electromagnetic stirring before it is transferred for casting or pouring, helping to improve process kinetics for degassing and removal of non-metallic inclusions.

Furthermore, grade-specific quality requirements are achievable within reasonable process cycle times.

Sampling and temperature measurement can be performed without interrupting the vacuum. “This helps in calculating what needs to be re-alloyed and allows working within very tight analysis limits. Alloy addition can be carried out in a very controlled and reproducible manner, as erratic burn-off under air contact is avoided in this furnace concept,” according to Henrik Franz, v.p.-Research & Development at ALD.

The two optional arrangements allow ALD to offer vacuum induction melting as a flexible solution for different high-value alloy melting requirements – with melt transfer under controlled atmosphere or under inert gas shrouding.

“The proven combination of vacuum-based melting with simultaneous sampling and an economically viable continuous casting system provides excellent product quality at suitable dimensions adapted to further processing,” concluded Stefan Lemke, v.p.-Melting & Remelting at ALD Vacuum Technologies.