1. Field of the Invention
The present invention relates to a sustained high-temperature synthesis (SHS) reaction process for making titanium aluminide welding rod or wire. In particular, the invention pertains to a process for producing a titanium aluminide weld rod by utilizing thin-wall metallic tubes to contain and confine the SHS reaction between a powdered mixture of titanium and aluminum, and in which the thin-wall tube may or may not become part of the weld rod alloy. After the thin-wall tube is packed with the titanium and aluminum powder, the tube may be rolled on a rolling mill, subjected to a vacuum, and heat treated in vacuum at about 700.degree. C. to initiate the sustained high temperature synthesis (SHS) reaction between the titanium and aluminum. Upon cooling of the thin-wall metallic tube, a finished weld rod is obtained.
2. Background of the Invention
It is known that intermetallic materials can promote higher fuel efficiencies in internal combustion engines and reduce environmental pollutants in off gases by allowing increased engine operating temperatures. One intermetallic material, titanium aluminide, has outstanding hot strength properties, thereby making it desirable as a material for jet and turbine engine hot section components. However, the cost of titanium aluminide is higher than most conventional materials, not only because the elemental costs are higher but because titanium aluminide has poor toughness at room temperature and extra precautions must be taken during processing to prevent fracture and breakage.
Manufacturers are testing titanium aluminide alloys for the next generation of aircraft engines, and most components are expected to be made by investment casting, since titanium aluminide cannot be easily forged or otherwise worked. Also, casting temperatures for titanium aluminide alloys are critical, and caution must be used to relieve stresses before cooling through about 700.degree. C. Nevertheless, even when proper cooling is done, cracking and defects in a significant number of parts occurs. To and repair slightly damaged parts that may contain surface cracks, inclusions, or other imperfections, the defects are removed by grinding away the defective material and replacing it with new material via welding. Welding rod of the same material is required to accomplish the weld repair.
Titanium aluminides have low room temperature ductility, and for this reason, it is difficult to fabricate small diameter (0.078") weld rod. Presently, titanium aluminide weld rod is made by explosive forming, and the cost is about $1,000.00/lb. At this high price, there is incentive to find less expensive means for producing titanium aluminide weld rod, since a less expensive rod will lower the cost of weld repairing, increase the number of parts worth repairing, and lower the overall cost of producing titanium aluminide components. As the cost goes down, more applications for titanium-aluminide will be found.
Titanium aluminide is extremely brittle at room temperature, and small diameter welding wire of this alloy is prone to breakage under normal handling. Also, because of this brittleness, titanium aluminide is not easily produced from bars or ingots to obtain small rod and wire diameters. A need exists to provide processes for reducing the cost of titanium aluminide welding rods, reducing the cost of repairing titanium aluminide castings, and reducing the cost of producing fabrications.
Titanium aluminide is a relatively new alloy and has not been produced in large quantities; however, it is expected that within about five years, most jet engines will be utilizing this alloy to a significant extent. To achieve high production efficiencies, a means of joining fabricated sections together is necessary. However, welding is made difficult because titanium aluminide is an intermetallic alloy characterized by little room temperature ductility and toughness. This lack of ductility prevents use of this alloy in many conventional fabrication processes, such as bar forging or swaging, and wire drawing. It has been found that, when small diameter bars of this alloy are heated to increase ductility prior to size reduction, rapid surface cooling usually occurs and cracking follows.
Although small diameter (0.078") titanium aluminide welding wire is made by explosive forming in a series of several steps, the process is time consuming and expensive. Therefore, a need exists to provide a less time consuming and less expensive process for providing titanium aluminide weld rod.