Electron Beam Melting
Electron beam melting is distinguished by its superior refining capacity and offers a high degree of flexibility of the heat source and the distribution of power. Thus it is ideal for remelting and refining of metals and alloys under high vacuum in water cooled, ceramic free copper molds. Today the process is mainly employed for the production of refractory and reactive metals (tantalum, niobium, molybdenum, tungsten, vanadium, hafnium, zirconium and titanium) and their alloys. It plays an important role in manufacturing of ultra-pure sputtering target materials and alloys for the electronic industry and the recycling of titanium scrap.
ALD is supplier of complete furnace solutions including gun System for a variety of process configurations:
- EB Drip-Melt Production Furnaces
- EB Cold Hearth Refining Furnaces
- EB Pilot Production Furnaces
- EB Laboratory Furnaces
- EB Button Melting Furnaces
- EB Floating Zone Melting Furnace
ALD scope of supply is not limited to delivery of the system only but can also include supervision up to turn key installation as well as process support by ALD specialist as well as training and process development at the Inhouse ALD EB-melting service.
Metallurgy of the Electron Beam Melting Process
Electron beam guns represent high power heat sources which are able to exceed at their beam spot the melting and even evaporation temperatures of all materials. By magnetic deflection and rapid scanning at high frequencies the electron beam can be effectively directed at targets of multiple shapes and is thus the most flexible heat source in remelting technology.
The electron beam impinges on the target with typical power densities of 100 W/cm². Depending on the properties of the material which is melted, the power transfer efficiency into the material ranges from approx. 50 to 80%.
Since EB-melting is a surface heating method it produces only a shallow pool at acceptable melt rates which positively effects the ingot structure regarding porosity, segregation, etc.
The exposure of the superheated metal pool surface to the high vacuum environment at levels of 1-0.0001 Pa results in excellent degassing of the molten material.
Metallic and non-metallic constituents with vapour pressures higher than the base material can be selectively evaporated by overheating of the pool thus generating the desired high purity of the ingot material.
Rapid scanning of the beam spot along the melt surface avoids local overheating and allows today consistent production of alloys by EB-melting, especially in the titanium industry.
The high degree of flexibility of the EB heat source did lead to the development of several remelting and refining methods used from laboratory scale throughout to industrial production.
Horizontal and vertical dripmelting
Both are the classical method for processing refractory metals such as Tantalum and Niobium. Raw material in form of bars is usually fed horizontally and dripmelted directly into a withdrawl mold. The liquid pool is maintained at the upper edge of the mold by withdrawing the bottom of the growing ingot. Refining is based on degassing and selective evaporation as described above. Mostly repeated remelting of the first ingot is required to achieve the final quality. For repeated remelting, vertical feeding of the rotating electrode is applied.
Electron beam cold hearth refining (EBCHR)
This process is of great importance for processing and recycling of reactive metals, especially Titanium. The feedstock is drip melted in the melting zone of a water cooled copper hearth system from where it flows into a refining zone and then finally overflows into the withdrawal mold.
By properly sizing the hearth system the liquefied material flows within a defined dwell time through the hearth system allowing the efficient gravity separation of high density inclusions (HDI) and the removal of low density inclusions (LDI) from the material additional to the refining mechanisms as described above.
The size of the hearth system and the mold as well as melt rate are defining the number of EB guns providing the melting power and the energy distribution along hearth and mold area.
Is utilized for cleanliness evaluation of superalloy samples regarding type and quantity of low-density, non-metallic inclusions. The equipment features programmed automatic sample melting and controlled directional solidification. Low density inclusions like oxides float to the surface of the pool and are concentrated in the centre, on top of the solidifying button.
Floating Zone melting
Floating Zone melting is one used for the production of metals with highest purity.
Impurities are segregating into a molten pool and transported with the melt front to the end of the electrode.
EB furnaces operate in a semi-automatic control mode.
Process automation includes:
- Vacuum pump system
- Vacuum pressure control
- Cooling water system
- Material feed rate and ingot withdrawal rate
- Processor-based high voltage and emission current control
- PC-based automatic beam power distribution, data acquisition and archiving
For process-specific power distributions, the beam deflection has to be controlled with respect to location and dwell time.
ALD has developed the PC-based electron beam scan and control system “ESCOSYS” for simultaneous control of several EB-guns. This system fulfils the highest requirements for complex beam power distribution which is defined in melt recipes by selecting suitable deflection patterns from a variety of predefined pattern shapes.
As part of furnace commissioning a special teach-in procedure is performed during which melt geometry and dependency on the deflection frequency for the EB-gun system is adjusted. This way electron beam movement beyond the crucible boundaries are recognized and automatically limited when editing deflection patterns.
ALD Electron Beam Guns
ALD is offering three EB-gun systems for power ranges up to 60 kW, 300 kW and 800 kW beam power at 25 to 50 kV with highly advanced beam deflection controls.