Technical Field

The inventive method relates to the art of metal wire forming. Typically, wire is made by casting or extrusion of large diameter rod, followed by many suc¬ cessive drawing operations to reduce the diameter and to shape and work harden the wire.

To reduce the number of drawing steps, several processes have been proposed to produce near-net-shape or net-shape wire or rod. U.S. Patent 405,914 to F. W. Schultz discloses a method for casting wire solder in grooves. However, in the Schultz system, nozzles over or inside the grooves deliver molten metal into the grooves. Experimentation with nozzles has shown that they are very difficult to control. The rate at which the metal flows out of the nozzle (the speed of movement) must be approxi¬ mately equal to the speed of the groove, otherwise tur¬ bulence is created which interferes with the uniformity of the casting. Further, the amount of metal flowing through the orifice per unit time must be exactly right to fill the groove. It is relatively easy to reach these two object¬ ives at very slow speeds, but difficult for higher speeds. Since one of the control means is the speed at which the groove moves, it would be extremely difficult to use the Shultz-type nozzle on a multiple-strand production oper¬ ation.

U.S. Patent 3,939,900 to Polk and Bedell dis¬ closes a process for casting metal in a V-shaped groove lined with thermal insulating material which is not wetted by the molten metal. Once again, control over the volume delivery of molten metal is a problem.

Summary

Despite some work in the area, a practical method for near-net-shape casting of metal wire has not

yet been proved. The present invention has, as its objective, the provision of a practical method for com¬ mercial production of such near-net-shape elongated metal products, particularly wire or rod, by rapid solidifi- cation.

The method generally comprises metering molten metal from a source thereof to an elongated, moving cast¬ ing channel in a chill block. The molten metal is metered to the casting channel under positive pressure from the source through a flow-restricting orifice comprised of the casting channel and a complementary volume-control chan¬ nel. The orifice is designed such that it is not sub¬ stantially larger than the cross-section of the desired elongated metal product and, preferably, that it is equal to or slightly smaller in cross section.

Preferably, wire or rod is cast using a semi-cylindrical casting channel. In that case, the complementary volume-control channel is also semi-cylin¬ drical with a diameter slightly less than the casting channel. The casting channel preferably has sharp corners at the upper surface of the chill block which, combined with surface tension, prevents the molten metal from escaping the channel and causes the unconfined molten metal to form a rounded upper surface.

Brief Description of the Drawings

Fig. 1 is a view of a metal wire cast in a chill block.

Fig. 2 is a plan view of apparatus according to the invention for casting metal wire in a casting channel. Fig. 3 is a front, sectional elevation of the apparatus of Fig. 2.

Fig. 4 is a side, sectional elevation of the apparatus of Fig. 2.

Fig. 5 is a front, sectional elevation view of apparatus according to the invention showing an alterna¬ tive complementary volume-control channel.

Fig. 6 is a front, sectional elevation view of apparatus having a second alternative complementary volume-control channel.

Fig. 7 is a front, sectional elevation view of apparatus according to the invention for forming angle iron.

Description of the Invention

The invention relates to an apparatus and met¬ hod for forming elongated metal products, particularly near-net-shape and net-shape products, by rapid solidifi-, cation. Typical continuous casting methods to produce net-shape precursors such as billets, ingots, rods, etc., typically operate at speeds on the order of 1 meter per minute. Subsequent drawing operations further slow the overall production process and add to energy costs. The present process is useful to cast near-net-shape and net-shape products at rates of at least about 1 meter per second.

The method and apparatus are exemplified in the Figures. Fig. 1 shows a chill block 1 having a casting channel 2 in its upper surface. The disclosed casting channel has the preferred semi-circular cross-section (semi-cylindrical elongation). If the edges of the cast¬ ing channel are "sharp", molten metal delivered to the channel resists pouring out of the channel. Then, if the channel is over filled with melt, surface tension and the sharp edges tend to cause the melt to stay within the channel and to form a rounded upper surface. In the smaller diameters, the cross-sectional shape of the melt may be substantially circular. Chilling the cast melt in this shape results in elongated rods or wire.

By the term "sharp" we mean that the channel wall and the upper surface of the chill block should meet at an edge with a very small radius of curvature, pre¬ ferably on the order of less than a quarter of a rnilli- meter. It is also preferred that they meet at an angle of about 90°. The "sharpness" is clearly a matter of degree and those practicing the invention can easily find the necessary conditions to meet the desired melt shape in the channel. Longitudinal score lines resulting from ma¬ chining the casting channel appear not to adversely affect casting. Transverse score lines appear to interfere with melt flow by causing turbulence. Burrs, laps or other high relief marks definitely adversely affect castings and hin- der release of the solidified products.

Critical to casting net-shape products at high rates according to the invention is the apparatus and method for controlled delivery of molten metal to the casting channel. Prior methods of delivering a stream of melt through a nozzle lack the control for high rate production of uniform product.

Figs. 2-4 exemplify apparatus useful in meter¬ ing molten metal to the casting channel. The chill block 1 again has a semi-cylindrical casting channel 2 in the upper surface. Molten metal 7 is contained in tundish 4. Appropriate superstructure (not shown) is applied to the tundish or the casting channel (generally in a chill block) to provide relative motion therebetween. Pre¬ ferably, the chill block/casting channel is a continuous caterpillar type track providing the relative movement to the fixed tundish.

The tundish 4 is made of a refractory ma¬ terial and has a drain 6 in the bottom leading to a flow-restricting orifice which acts to meter the volume and pressure of molten metal to the casting channel. The

orifice cross section at its smallest point is preferably equal to or less than the cross section of the metal product. The larger diameter or shorter length the ori¬ fice, the larger the flow rate of melt; the smaller or longer the orifice, the smaller the flow rate. An orifice of slightly larger size than the product might temporarily be tolerated by increasing the speed of the casting chan¬ nel, but the speed also causes more drag of the melt and eventually an excess of melt would upset the balance of the system making control difficult.

The flow-restricting orifice is not so much a pressure-reducer as it is a volume and velocity reducing element. Since the casting channel forms part of the orifice, the orifice adjusts (lowers) the velocity of melt flowing through the drain to the relative velocity of the casting channel, thereby reducing turbulence for a more uniform product. It also limits the volume of melt to that necessary to complete the product and may also contribute roughly to shape of the product (the surface tension ultimately determines the final shape).

In theory, the size of the drain and not the orifice could be used to control the volume of melt delivered to the casting channel. However, this ap¬ proaches the problem of former nozzles, in that the drain would have to be exactly the right size. Turbulence would also still be a problem.

In the Figures, the orifice is comprised of the casting channel, itself, and a complementary volume-con¬ trol channel 5 in the underside of the tundish. The volume-control channel 5 is shown in its preferred shape (semi-cylindrical) and size (slightly smaller in diameter than the casting channel). As shown best in Fig. 4, the volume-control channel extends the thickness of the front wall of the tundish 4 and communicates with the drain 6.

A barrier 8, also of semi-cylindrical shape, is preferably formed on the underside of the tundish. The barrier is desirable in the process to guide the tundish relative to the casting channel and to prevent molten metal from moving the wrong direction in the casting channel when etered from the tundish.

The drain 6 in the tundish should be large enough to avoid plugging. It preferably has a slight angle and taper toward the flow-restricting orifice to give a forward momentum to the metal flow, but this does not appear critical.

When casting rod or wire, the volume-control channel is preferably a mirror image of the casting chan¬ nel. Other shapes may also be used for the volume-control channel, such as the square or triangular channels 15 and 17, respectively, of Figs. 5 and 6, but the cross-sec¬ tional shape of the cast rod tends to be somewhat affected such as shown by rod 13 in Fig. 5. But surface tension tends to round the melt after it leaves the orifice. Other elongated products may also be formed using the invention. Fig. 7 shows the formation of angle iron 18. A variety of tees, squares, ovals, triangles, bars or other shapes may also be formed. The rod or wire tend to be the most practical due to the rounded upper surface. The wire size depends somewhat on the density and surface tension of the particular melt. In general, wire of between about 1 and 10 mm diameter is formable. Larger sizes begin to have the noticeable effects of gravity whereas smaller sizes begin to cause problems relating to small orifice and drain delivery holes.

In the inventive process the tundish is fairly small to maintain a short residence time for the molten metal. A relatively smooth and continuous flow of molten metal is desirable. To this end, a constant head height should be maintained in the tundish. The head pres-

sure forces the molten metal out the drain and into the flow-restricting orifice. The barrier prevents the molten metal from moving in an upstream direction relative to the motion of the casting channel. At higher speeds, the barrier is not as important because the casting channel tends to drag the molten metal from the orifice and drain. Flow rate is controlled by a combination of the head height, the orifice diameter and length, and the casting speed. As molten metal is metered to the casting chan¬ nel it forms a metallurgical bond therewith and a thin shell is solidified. The metering system provides suffi¬ cient additional molten metal to complete the cross-sec¬ tion of the product. Thereafter, the product shrinks as it cools and then releases from the channels.

The casting channel does not need to be formed in a chill block, though, for purposes of effective cool¬ ing it may be desirable. Successful castings have been made, for example, in a piece of cooled, thin-walled angle iron.

Examples

A 13 mm-thick copper plate, 76 cm in diameter was used as a chill block to cast round wire. Casting channels having semi-circular cross-sections and 3.2 mm, 4.8 mm and 6.35 mm diameters were machined around the face of the plate.

Small tundishes, such as shown in Figs. 2-4, were made out of a fibrous refractory material (Kaowool) . The drain hole was about 2 mm in diameter. Type 304 stainless steel was cast in each of the casting channels. In each case the volume-control channel and the barrier were semi-circular in cross section and slightly smaller in diameter than the corresponding casting channel.

Most of the effort concentrated on the 6.35 mm casting channel. A steady pour of molten steel into the tundish produced a steady flow to the casting channel. The chill block was rotated to produce a casting speed of between about 15 and 70 cm/sec. Wire with a substantially circular cross section was cast, however, vibration ap¬ peared to cause some irregularities. The steel wire was successfully reduced in diameter from 6.35 to 3 mm by drawing. Other castings were made in the same fashion with Kanthal (Fe-20Cr-5Al) , tin, copper and cartridge brass. All were reasonably round and could be swaged or drawn to half their original 6.35 mm diameter.