Vacuum Precision Casting

In most instances the product from a VIM is further processed under vacuum. One such application is vacuum precision investment casting for the production of advanced high quality cast components.

Using the principles of the ‘lost wax’ process, a ceramic mould is prepared and heated prior to casting. A measured quantity of pre-alloyed metal is then rapidly re-melted and poured into the heated ceramic mould under vacuum.

Conventional processing produces castings with equiaxed or randomly orientated grain structures. However, Directionally Solidified and Single Crystal castings have now also become a common requirement for turbine technology as equipment advances permit the production of larger castings by these methods.

The modern vacuum precision casting furnace incorporates the latest PC SCADA controls to accurately monitor and control melting and casting parameters thereby consistently producing the optimum metallurgical properties in the cast component.

For melt capacities of 15-200 kg Consarc offers a two-chamber horizontal or vertical design for semi continuous operation. Facilities for equiax, directional solidification (DS) and single crystal (SC) castings are available in this size range.

The two chamber design features a melting chamber containing the induction melting coil and mould heating zone in DS/SC applications. A vacuum isolation valve separates the melting chamber from the mould chamber that incorporates the mould transfer mechanism. This system allows melting down of the charge to continue independently of the ceramic mould handling and transfer operations.

Directionally Solidified and Single Crystal castings

Directionally solidified (DS) and single crystal (SC) castings are required for use in aerospace and industrial gas turbine applications due to their improved mechanical properties at very high operating temperatures. Furnaces to cast these components contain features to control the solidification process in the casting. The upper melting chamber contains a mold-heating zone to allow heating the ceramic mold to temperatures higher than the alloy liquid’s temperature prior to pouring. The mould-heating zone is induction heated to provide optimum control of the thermal gradient in the mold.

The molds are placed on a water-cooled chill plate, and a baffled cooling zone is located directly below the heating zone to create a high thermal gradient for solidification in the cast component. A precision electromechanical drive system controls withdrawal of the mold from the heating zone through cooling zone to create the optimum thermal gradient and, hence, grain orientation or single crystal in the cast product.

To produce a DS cast the mould temperature must be greater than the melting temperature of the alloy during the initial pouring sequence, this is achieved by the mould heater hot zone and then progressively cooled in one direction to initiate the solidification process as shown in figure below.

Figure 1 - DS Casting

DS casting

An SC cast is produced similarly to that of a DS cast with the exception of a grain selector being added to the bottom of the mould as shown in figure below: -

SC casting

In vertical furnaces for DS/SC applications the mould raising and lowering mechanism incorporates an electro-mechanical or hydraulic drive system which is controlled to a precise specification. This level of control is essential during withdrawal from the mould-heating zone to create the optimum grain orientation in the cast product. Consarc can offer both induction and resistance-heated mould heating zones with either single or multiple zones of control depending on the application and/or customer’s preference.

Baffle Exchange DS/SC

At the bottom of the mould heater in the DS / SC furnaces a thermally insulation baffle helps to create a sharp cooling gradient in the solidification zone. This baffle is critical to the process. The baffle is normally positioned in place during the heater set up with the furnace open and heater cold.

As an option, Consarc can provide the means to accurately locate and remove / exchange the baffle with the melt chamber under vacuum and without turning off the mould heater. This feature greatly enhances the productivity of the furnace.

Large Equiax Vacuum Precision Casting Furnaces

To augment the range of vacuum precision casting furnaces, Consarc have available a series of furnaces to accommodate very large single or multiple castings for the Industrial Gas Turbine and other applications.

Utilising a two-chamber vertical or horizontal design, this range of larger furnaces can produce equiax castings from a melt capacity of 150-500 kg.

Induction Power Equipment

Power is supplied from a solid state Inductotherm induction power supply. It is taken through the vacuum chamber by means of a sealed water-cooled coaxial rotary power feedthrough or power port assembly to the melting coil.

Controls

All Consarc installations are supplied with a fully integrated control system for safe and efficient operation of the furnace.

In addition to operator initiated/remote actuated controls, Consarc furnaces are all available with PLC and the option for PC SCADA based control systems that promote automation to maintain consistency between casts.

In the computer control mode the operator has the facility to select previously defined recipes or alternatively enter a new set of recipe parameters. 'Teach in' facilities have been developed to allow successful manual operations, such as speed of crucible tilting, position of crucible (see below - Tilt / Pouring Control) and mould location, to be saved by the computer for future reproduction in the automatic mode. Comprehensive Data acquisition facilities are included along with the feature of SPC analysis.

Tilt / Pouring Control

The control of the pouring operation is critical in Vacuum Precision Casting applications. To meet the most precise requirements Consarc have developed a rage of tilting and traverse power port assemblies for fast and accurate pouring of the metal from the induction coil into the moulds. The induction coil can be tilted around its pouring axis and simultaneously traversed to move the pour lip of the crucible as close a possible to the mould. Control of both the tilt angle and the longitudinal traverse of the induction coil is achieved by the PC/PLC control system to a pre-defined recipe automatically thus ensuring consistency between each cast without manual operator variations.

Quick Change Induction Coils

A unique Induction coil connection system, specially developed by Consarc and Inductotherm, allows a wide range of quick change melting coils to be used in the same furnace thereby increasing the versatility and application of the system.