WO1991000373A1

WO1991000373A1 - Manufacture of metallic alloys

Info

Publication number: WO1991000373A1
Application number: PCT/GB1990/000998
Authority: WO
Inventors: Alfred Richard Eric Singer
Original assignee: Alcan International Limited
Priority date: 1989-06-29
Filing date: 1990-06-28
Publication date: 1991-01-10
Also published as: GB8914996D0; AU5843690A

Abstract

A metal alloy is produced by arc melting and atomizing a composite wire consisting of a group of strands of metal wire which is such that at every cross-section the group has the required constituents of the alloy in their correct proportions. The resulting atomized metal alloy is sprayed on a substrate which is maintained at a temperature below that of the spray to promote rapid solidification of the alloy. The substrate may be stationary or moving. The method may be used to produce a metal alloy in forms including strip, sheet, tubes and billets and may be used to form metal alloy coatings on metal or ceramic articles.

Description

MANUFACTURE OF METALLIC ALLOYS

Many metal alloys are known to have attractive mechanical, physical or chemical properties but are not available and never used for engineering purposes either because they cannot easily be melted and cast, or because they cannot be fabricated into useful forms, or both. The term "alloy" is used herein to include both simple mixtures of metals and intermetallic compounds .

High temperature alloys, particularly those containing large quantities of titanium or other highly reactive metals are often difficult to melt because in some cases they attack the crucible and nozzle refractories leading to contamination of the melt and erosion of the nozzles. Such material can often be melted using an arc process and a water cooled copper hearth or levitation melting which gives freedom from contamination of the melt. However, it is difficult and expensive to devise accurate methods of pouring from such melt sources and very difficult to carry out spray forming operations which require a continuous and uniform stream of molten metal.

Difficulties of fabrication are a further cause of concern. Even though it may be possible to cast small slabs or billets of these alloys it has often been found impossible, as a commercial operation, to process the alloys into the form of strip, sheet, tube, rod or wire. Extrusion is a favoured procedure but not all such alloys extrude successfully either because of very limited ductility and/or because of the very high extrusion temperatures that might be required. Powder metallurgy offers a possible route to certain products except that either elemental powders have to be used or some means has to be found of melting and atomizing the alloys to form powder. The first of these alternatives suffers from the difficulty that alloying during sintering is a lengthy and expensive process that has to be carried out at high temperatures. The second alternative has all the drawbacks of the melting and casting procedure mentioned before.

Another method of fabrication is to use the new technology of spray forming because it is known that spray formed products have a much finer structure than conventional ones, can be made with near zero segregation and respond well to fabrication. However the main difficulty of melting and atomizing remains. According to the present invention there is provided a method of making a metal alloy comprising the steps of forming into a group strands of wire made from constituents of the said alloy so that at every cross-section along the grouped wires the group has in correct proportions the required constituents of said alloy, feeding the group progressively to a station at which the wires are melted and atomized to form a spray of molten metal particles and directing the spray on to a cool substrate which is cooler than the spray thereby to form the alloy in a solid state.

The invention also provides an alloy when made by the method described in the preceding paragraph.

The strands of wire in the group may be twisted or braided together. Individual strands may themselves be alloys, the only requirement being that such alloys can readily be drawn into the form of wire and that the total cross-section of the group of strands comprises the final desired composition. The arc melting procedure may be made by metallic arc, tungsten arc or plasma arc. In one example two similar groups of strands of wire are used in a conventional wire arc spray gun. In a second example a heavy gauge group of strands of wire is used as the consumable electrode in an arc melting unit for generating a spray of alloy particles in which the second electrode is a rapidly rotating water cooled copper disc causing centrifugal atomizing. In a third example the group of strands of wire is used as a consumable electrode in a melting and gas atomizing unit in which the second electrode is of tungsten.

The atomization may be carried out using an inert gas such as argon, or using air in certain cases, or by centrifugal means.

Spray deposition, using the group of strands of wire as a source, can be used for making the same wide range of shapes as can be made using a conventional melt source. These shapes include strip a d sheet, tubes and billets. They also include coatings and laminates having the alloy compositions of the combined components of the group of strands of wire on other metal or ceramic bases.

Two examples of the manufacture of alloy products that are extremely difficult to make by conventional methods are the making of a titanium- aluminium alloy strip having the composition Ti-Al, and coating a steel tube with Ti,Al. TAAl is a brittle alloy that is very difficult to melt and fabricate using conventional processing. However its constituent metals titanium and aluminium are both ductile and freely available in the form of wire and tape of different cross-sectional form.

In the application of the present invention to the first example, seven titanium wires of circular cross-section 0.752 mm diameter were twisted around a central aluminium wire of circular cross-section 1.1 mm diameter, as shown in Figure 1 , in a wire rope making machine. Two lengths of the resulting group of wires were fed into a conventional two-wire arc spray gun in which the atomizing gas was argon. When the arc spray gun was operated the wires were melted in the arc to form molten Ti-Al which was atomized by a pressurized jet of argon gas and directed on to a flat cool copper substrate that has been lightly grit blasted. The substrate was moved at constant speed in a plane normal to the axis of the spray to produce a spray formed deposit on the substrate. When the deposit was removed from the substrate it constituted a strip having a very fine grain size caused by the high rate of solidification and near-to-zero segregation. Because of the fine structure and very low segregation such strip can successfully be given limited amounts of hot or cold rolling.

In the application of the invention to the second example two groups of strands of wire of the same composition as in the first example were used in a two-wire arc spray gun to produce a coating on a tube. The tube was rotated by a manipulator at 180 rpm and simultaneously the arc spray gun traversed the full length of the product in a number of passes. Adhesion was improved by preheating the tube slightly ahead of the spray deposit by means of a plasma torch although this is not essential.

In all two wire spray guns it is important to have good electrical contact between the guide tubes, usually made of copper, and the wire that runs through them. In the case of the use of a group of strands of wire this is an important factor because the group has a smaller area of contact with the smooth bores of the guide tubes which conduct the current and the wire strands than monolithic wire. In most cases the use of a group of strands of wire presents no difficulty but if contact is poor it is possible to draw the outer wires to the shape of part of a sector having regard to the need for the cross-sectional areas to be such that the correct composition is laid down. A preferred alternative is to carry out a light drawing or rolling operation of the group. This fixes the strands firmly in position and gives the group a smoother external surface. Figure 2 shows a typical way of achieving this objective by presenting to the guide tube a group of strands of wire comprising 6 strands of Ti wire disposed on a single strand of Al wire and providing the group with a smooth external surface. In this case the central aluminium wire is of 1.2 mm diameter and each part sector of titanium wire is 0.63 mm thick making a group with a diameter of 2.46 mm.

In the case of twin wire arc spraying it is necessary to ensure that the group of wires does not bend on leaving the guide tubes. If bending takes place the two groups may not meet at the required position to form the arc. To avoid such bending it may be necessary to straighten the group before entering the guide tubes and it may also be beneficial to use wire that is in the half hard or hard condition so as to increase stiffness.

In the illustrated examples the titanium wires were placed on the outside of the composite stranded wire because of the small total cross sectional area of the aluminium and in the interests of avoiding protrusion of a central wire during the operation of the arc. It is usually desirable to place the stiffer wire at the centre and strand the smaller softer wires around it. This gives the composite wire the maximum stiffness so avoiding bending after leaving the guide tubes.

It is of course, possible to avoid having a central straight wire by twisting all the strands together. However, this practice is not recommended because flexibility is increased and the composite wire is less easy to produce. In a further example a stranded wire was made by winding 6 aluminium wires around a central half hard low carbon steel wire giving a composition 32% Al by weight with 68% Fe. It was noticeable that the iron and aluminium wires melted off at the same rate when used with nitrogen as an atomizing gas, shown by the fact that on switching off the arc the iron wire was found to be level with the aluminium.

The method described above completely overcomes the problems of melting and contamination and allows spray formed products of virtually any composition to be made.

The gases used for atomizing should be those appropriate for the metals being used. In the case of spraying Ti-Al argon is the preferred gas because it is inert to both Ti and Al, helps to produce a stable arc and is relatively cheap.

Claims

1. A method of making a metal alloy comprising the steps of forming into a group strands of wire made from constituents of the said alloy so that at every cross-section along the grouped wires the group has in correct proportions the required constituents of said alloy, feeding the group progressively to a station at which the wires are melted and atomized to form a spray of molten metal particles and directing the spray on to a substrate which is cooler than the spray thereby to form the alloy in a solid state.

2. A method as claimed in Claim 1, wherein the strands of wire in the group are twisted together.

3. A method as claimed in Claim 1, wherein the strands of wire in the group are braided together.

4. A method as claimed in any one of the preceding claims, wherein a straightening operation is carried out on said group of wires during feeding thereof to said station.

5. A method as claimed in any one of the preceding claims, wherein said metal alloy is Ti,Al.

6. A method as claimed in any one of the preceding claims, wherein the alloy forms a bonded coating on said substrate.

7. A method as claimed in any one of Claims 1 to 6, wherein the grouped strands comprise a central strand about which the other strands are disposed.

8. A method of coating a tube with a metal alloy comprising the steps of forming into a group strands of wire made from constituents of the said alloy so that at every cross-section along the grouped wires the group has in correct proportions the required constituents of said alloy, feeding the group progressively to a station at which the wires are melted and atomized to form a spray of molten metal particles and directing the spray on to the tube while maintaining the tube at a lower temperature than the spray and rotating the tube about its lengthwise axis and simultaneously reciprocating the tube and the spray relative to each other in a direction lengthwise of the tube.

9. A metal alloy when produced by a method as claimed in any one of Claims 1 to 7.