Arc melting and tilt casting apparatus

Abstract

An arc melting and tilt casting apparatus having a casing provided with a vacuum chamber for housing a hearth having a melting trough and pouring means, arc-melting electrode means passing through the casing in the chamber, a mold having a melt receiving orifice, vacuum generating means, sealing means for maintaining the vacuum in the chamber and tilting means for tilting the apparatus to cause the melt to flow from the melting trough via the pouring means in the mold through the mold orifice. The hearth and mold are connected together and moveable as a single unit in the vacuum chamber and out from the chamber.

Claims

1. An arc melting and tilt casting apparatus capable of being tilted, the apparatus comprising: a casing provided with a vacuum chamber, a sample manipulator arm, and an arc-melting electrode passing through the chamber, where the vacuum chamber has a seal for selectively maintaining a vacuum in the vacuum chamber, and where the casing has an opening in a bottom portion of the casing; a sliding unit having an upper surface, the sliding unit capable of being selectively raised and lowered through said opening, the sliding unit defining: a hearth having a melting trough and a pouring nozzle, each formed on the upper surface of the sliding unit; and a slot opening to said upper surface and extending into said sliding unit, the slot holding a mold having a melt-receiving orifice oriented with respect to said melting trough and said nozzle so as to cause a melt to flow from the melting trough via the nozzle and through the mold orifice, when said apparatus is tilted, such that the hearth and mold are moveable as a single unit into the vacuum chamber and out from the chamber from below and through the opening in the bottom portion of the casing.

2. The apparatus of claim 1, including a piston for cap casting.

3. The apparatus of claim 2 characterized in that the piston for cap casting is connected to the unit to be moved together with the unit.

4. The apparatus of claim 2 characterized in that it further includes an aperture for suction casting.

5. The apparatus of claim 1, characterized in that it further includes an aperture for suction casting.

6. The apparatus of claim 5 characterized in that the aperture for suction casting is connected to the unit to be moved together with the unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an exemplary Arc Melting and Tilt Casting Apparatus.

DETAILED DESCRIPTION

(2) The aim of the present invention is to create a versatile instrument, in which high purity conditions can be maintained throughout the process, even when melting alloys with high affinity for oxygen. To this end the arc melting and tilt casting apparatus having a casing provided with a vacuum chamber for housing a hearth having a melting trough and pouring means, arc-melting electrode means passing through the casing in the chamber, a mould having a melt receiving orifice, vacuum generating means, sealing means for maintaining the vacuum in the chamber and tilting means for tilting the apparatus to cause the melt to flow from the melting trough via the pouring means in the mold through the mold orifice, is characterized in that the hearth and mold are connected together and moveable as a single unit in the vacuum chamber and out from the chamber.

(3) The design of the present invention provides a high-vacuum chamber to be filled with a low-oxygen atmosphere, and takes special care to keep the system hermetically sealed throughout the process. In particular, movements of the arc-melting electrode and sample manipulator arm are accommodated by deformable metal bellows, rather than sliding O-ring seals, and the whole furnace is tilted for tilt casting.

(4) It is known that in high vacuum systems each feedthrough and each seal produces a measurable leak. Sliding O-ring seals, where a moving surface slides against the O-ring that provides the vacuum seal, are particularly prone to leaking. Also, when a vacuum chamber has been opened to atmosphere, it takes a long time to evacuate moisture adsorbed from the air onto the inside surfaces of the vacuum chamber 7 9. To provide the high-purity conditions desired for BMG processing, the main process chamber should therefore have as few feedthroughs and as little surface area as possible. Nevertheless, the apparatus should allow the full range of motions and manipulations needed to carry out each step of the process, preferably without opening the chamber to atmosphere.

(5) The apparatus is equipped with a manipulator arm so that, for pre-alloying, the sample can be flipped and remelted without opening the chamber. It also has provisions for piston suction casting and cap casting for small specimens. For a wide range of alloy compositions and sample sizes, the complete process from pre-alloying to high-quality net-shape casting can be carried out in a continuous sequence using this apparatus. Furthermore, the critical feedthroughs in this apparatus feature ultra-high vacuum (UHV) construction methods, using flexible metal bellows for all moveable parts, and an all-metal gas line connects the chamber to a supply of high-purity inert gas. Thus, a high-purity atmosphere can be maintained throughout the entire processing sequence.

(6) The invention will be described more specifically in the following with reference to the attached drawing the only FIG. 1 of which shows schematically an exemplary embodiment of the present invention. Only those features which are needed for understanding the invention have been shown in the FIG. 1 and it should be understood that the apparatus includes several other features necessary for its operation but they are obvious for one skilled in the art and, therefore, they are not considered necessary to be described here.

(7) The apparatus 1 as shown in FIG. 1 includes a casing 13 having a high-vacuum chamber 12 therein. The arc-melting electrode 4 having, e.g. a tungsten tip 4, passes through the top side of the casing. A water-cooled hearth having a melting trough 7 and pouring means 7 is placed in the vacuum chamber. A mold 2 having a melt receiving orifice 8 is also inside the vacuum chamber 12. The apparatus includes also vacuum generating means (not shown), sealing means (not shown) for maintaining the vacuum in the chamber and tilting means (not shown) for tilting the apparatus to cause the melt 6 to flow from the melting trough 7 via the pouring means in the mold 2 through the mold orifice 8. An arrow A shows tilting of the apparatus. The improved feature of the present invention is that the hearth and mold are connected together and moveable as a single unit 9 in the vacuum chamber and out from the chamber. In the embodiment shown in FIG. 1 the unit 9 is raised from the bottom side of the apparatus through an opening 14.

(8) As seen in FIG. 1, the unit 9 may form the hearth, with its melting trough 7 and pouring nozzle 7 on its upper surface. The unit 9 also defines a slot that opens to the upper surface of the unit 9 and extends into the unit 9. The slot holds the mold 2 such that the mold orifice 8 is oriented with respect to the melting trough 7 and nozzle 7 so as to cause a melt 5 to flow from the melting. trough 7 via the nozzle 7 and through the mold orifice 8, when the apparatus 1 is tilted.

(9) Connection 5 for suction casting and means 3 for cap casting are also provided in the embodiment shown. These means are preferably connected to the unit 9 by connecting means (not shown) to be moved in the vacuum chamber and out from the chamber together with the unit 9.

(10) The water-cooled copper hearth inside the chamber features a single large melting trough 7 with a pouring nozzle 7 leading to the mould orifice 8, and a smaller trough (not shown) for titanium gettering. The hearth is attached from below, to avoid any internal leaks from gas pockets that might otherwise he trapped between the hearth and the chamber. A standard ISO-K 200 O-ring seal (not shown) with centering ring separates the vacuum from the cooling water circulating underneath the copper hearth. Belleville spring washers (not shown) ensure that differential thermal expansion when the furnace is operated does not cause excessive decreases or increases in the clamp force maintained on the O-ring seal. To load a new charge into the furnace, the unit 9 is released from the rest of the chamber and lowered on a pneumatic lift (not shown) provided for that purpose.

(11) An arc-melter necessarily includes a feedthrough for the arc melting electrode. The electrode is a water-cooled conductor that can carry an electrical current up to 500 A and can handle the 30 kV high-voltage arc-ignition spark. This current needs to be electrically isolated from the chamber potential at operating pressures and atmospheres to avoid damage to the chamber. Furthermore, the electrode should be moveable; with a freely moveable electrode tip, the operator can deliver the energy of the plasma arc precisely where it is needed to melt the sample. The feedthrough should allow a range of motions covering every possible position of the sample in the melting trough as well as the titanium getter. This freedom of movement is realized with a flexible edge-welded metal bellows (not shown), between the top side of the chamber and the electrical feedthrough (not shown) for the electrode. The electrical feedthrough is constructed of two fluoropolymer (PTFE) insulators clamped onto either side of a copper flange which is brazed onto the electrode rod. Standard ISO-K 100 O-ring seals and centering rings seal the vacuum side. The tungsten electrode tip 4 is secured with two screws to the brazed electrode tip assembly which seals the end of the water-cooled electrode rod.

(12) To assist the operator with delicate movements of the electrode tip 4 and to prevent movements that would damage the hearth or the bellows, a mechanism for supporting the electrode is also necessary. The weight of the electrode rod and the atmospheric pressure when the chamber is evacuated amount to a force in excess of 800 N drawing the electrode towards the copper hearth. A mechanism (not shown) with pneumatically actuated servo control in the vertical direction carries this load.

(13) Tilt casting requires a mechanism for pouring the melt from the crucible into the mold. Often this is done with a sliding O-ring seal, in which a rigid connector carrying cooling water for the metal crucible also allows to tilt the crucible towards the mold. In the present apparatus, the whole chamber is tilted. This eliminates a potentially troublesome sliding O-ring seal.