Due to their chemical properties, refractory metals have unique performance characteristics, which allow their use under extreme conditions, for instance in heat exchangers or another chemical apparatus engineering. Their material characteristics make refractory metals essential for electronic industry applications as well.
However, only limited melting processes are suitable for refining such materials. The high melting point and chemical reactivity exclude utilising ceramic crucibles, atmospheric melting conditions as well as most melting techniques. Electron Beam Melting (EBM) meets the higher temperature requirements as well as provides inert melting conditions.
Adapting to cleaning requirements
Nevertheless, furnace design and process need to be adapted for such dedicated requirements to match material purity and economical requirements. ALD Vacuum Technologies provides a specialised EBM service with its electron beam gun design that allows for precise power distribution, process-optimised material feeding, and refining, as well as selected superheating above 3.500°C. Thanks to an independent parameter control including power profile, melt rate, or vacuum level, the process is highly flexible and cost-efficient.
All parts of the plant in contact with the melt itself are removable or easy to reach, so cleaning can be carried out quickly during material changes. In addition, the company operates two furnaces for melting refractory metals for customers who do not have internal capacity available.
Despite their rarity, refractory metals are key materials for innovative high-tech industries that require chemical-resistant and highly conductive parts. “However, such metals like hafnium, niobium, and tantalum have very high melting points of even over 3.000 °C and, therefore, require correspondingly high temperatures, which many conventional induction furnaces and refractory crucibles cannot provide,” says Jochen Flinspach, Operational Manager Electron Beam Melting service, ALD Vacuum Technologies. He further explains, “Reaction of the melt pool with the crucible made of refractory material like zirconium oxide can lead to inclusions of foreign particles in the melt, which can have problematic consequences for subsequent processing, like worse conductive properties or lower resistance of the fabricated parts.”
Moreover, these metals are reactive in a higher temperature range, i.e., above the liquidus temperature. Therefore, even minimal oxygen input leads to severe impairment of the material quality and melting under vacuum or inert gas conditions is required. In addition, with conventional melting methods, the gas value inside the melting chamber can often be too high, causing the material to become brittle.
Electron beam melting
With electron beam melting, not only required temperatures for the melting process are achievable. The flexible power distribution and high vacuum conditions feature unique refining capabilities. Typically, one or multiple electron beam guns are connected to the melt chamber, depending on the required power level and melt surface area to be scanned. The company features in-house process expertise, furnace design and own gun development resulting in a furnace design matching the requirements of this specific industry. The gun control is fully integrated into the furnace control concept and is not an add-on function as offered by other vendors.
Thus, interactions between individual furnace functions and implementation of process-related logic are ensured. Modern melting furnaces offer semi-automated operation where human interaction is requested for corrective functions only. High reproducibility on process results as well as reliable furnace operation are the benefits of such a development.
ALD Technologies GmbH has developed unique EBM plants for various applications over the years with their gun and furnace design. Thanks to their expertise and R&D capabilities, the plant types are suitable for processing high purity, chemical, or thermal resistant metals. As a specific application, the refining of tantalum or other reactive refractory metals as alloying agents for superalloys may be mentioned. “The EB-melting has been established as a variable process capable of producing material with highest purity and gas contents lower than 10 ppm. It is also suitable for mass production of reactive metals or alloys required for aerospace or chemical applications,” Flinspach adds.
Ensuring higher beam efficiency
For maximum beam efficiency, the company offers the automated beam control system ECOSYS, which ensures optimal energy distribution. The handling is practice-oriented, processes are largely automated and the operator only performs a corrective task. Process visualisation provides relevant data arranged on a screen, while details can be called up on corresponding pages if required.
According to Ulrich Biebricher, Remelting & Melting Specialist, ALD Vacuum Technologies, “Simple and safe cleaning of the equipment is as important as flexible control, since the metal to be melted can change frequently during production schedule.” Therefore, particular attention is paid to easy maintenance of the equipment. Robust design, good accessibility, and quick replacement of components in touch with the melt are features of the plant design. Thus, maintenance of equipment and EB guns can be carried out by the operator alone.
Refining high-purity metals
“Our 60 kW EB plant is suitable for ingots up to 100 mm in diameter and 500 mm in length. It is used for smaller material quantities or laboratory and research applications as well as precious metals melting,” Biebricher explains. “Our 600 kW plant was designed for ingots starting at 42 mm up to 300 mm in diameter and 2,000 mm in length.”
The range of melting materials at ALD includes refining high-purity metals for the electronics industry, preparing refractory or reactive metals for chemical usage, as well as consolidating return material from a wide variety of applications. Test melts for customers to qualify new process steps are also part of the portfolio, as is the preparation of metal samples. “In a recent project, tantalum was recovered from used capacitors. We processed it from the scraps into a semi-finished product and conditioned it further for the customer. In the end, the product quality was good enough for reuse in further processing or selling it as a precursor,” concludes Flinspach.
Courtesy: ALD Vacuum Technologies