Method for producing titanium aluminide weld rod

A process for producing titanium aluminide weld rod comprising: attaching one end of a metal tube to a vacuum line; placing a means between said vacuum line and a junction of the metal tube to prevent powder from entering the vacuum line; inducing a vacuum within the tube; placing a mixture of titanium and aluminum powder in the tube and employing means to impact the powder in the tube to a filled tube; heating the tube in the vacuum at a temperature sufficient to initiate a high-temperature synthesis (SHS) reaction between the titanium and aluminum; and lowering the temperature to ambient temperature to obtain a intermetallic titanium aluminide alloy weld rod.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
One object of the invention is to provide a process for producing 
intermetallic titanium aluminide alloy weld rod using powder metallurgy. 
A further object of the invention is to produce intermetallic titanium 
aluminide alloy weld rod utilizing a sustained high-temperature synthesis 
(SHS) reaction to manufacture the weld rod. 
A yet further object of the invention is to provide a process for producing 
intermetallic titanium aluminide alloy weld rod in which thin-wall 
metallic tubes are used to contain and confine the sustained 
high-temperature synthesis (SHS) reaction, and in which the thin-wall tube 
may or may not become part of the weld rod alloy. 
In general, the invention process is accomplished by attaching a niobium 
tube to a vacuum line at one end; placing means over the vacuum line at a 
junction between the vacuum line and niobium tube to prevent powder from 
entering the vacuum system, pulling a vacuum in the tube; placing a 
mixture of titanium and aluminum powder in the tube, and packing the 
mixture; heat treating the tube in vacuum at a temperature of about 
700.degree. C. for a sufficient period of time to initiate 
high-temperature synthesis (SHS) reaction between the titanium and 
aluminum; and permitting the temperature to fall to room temperature to 
obtain a titanium aluminide alloy weld rod.

DETAILED DESCRIPTION OF THE INVENTION 
Example 1 
One end of a 0.088 inch ID niobium tube of 0.0025 inches wall thickness was 
attached to a vacuum line. 
A piece of rag was drawn over the vacuum line at the junction of the vacuum 
to the niobium tube to prevent powder from entering the vacuum system. A 
vacuum of about 25 inches was pumped on the tube. A 50/50 mixture of 
titanium and aluminum powder was funneled into the open end of the tube 
while a small vibrator was moved up and down the tube length. At several 
intervals during the process, a rod was inserted into the tube for the 
purpose of tamping and packing the powder. 
The tube was weighed before and after the powder addition to determine the 
packing density and the percentage of the niobium. The niobium constituted 
13.9% of the total weight of the filled tube. 
After filling, the tube was rolled in one pass in a small rolling mill that 
was machined with circumferential grooves measuring 0.075" diameter. The 
length of the filled tube increased from about 36 inches to 38 inches 
after rolling. The rolling produced a slight flange on the tube but no 
breaks. 
The tube was heat treated in vacuum @700.degree. C. A sustained 
high-temperature synthesis (SHS) reaction occurred between the titanium 
and aluminum, but the niobium was not affected, and remained metallic 
looking. 
After cooling to ambient temperature, the finished weld rod was used to 
simulate a repair weld on a section of a titanium aluminide casting. 
Preheating and postheating of the casting to 800.degree. C., followed by 
furnace cooling were utilized to prevent cracking from thermal shock. 
Welding was no different than tungsten inert gas (TIG) and metal inert gas 
(MIG) welding of other metals. The welds showed no cracks, and the weld 
grain size was somewhat larger than the grain size of the casting. 
It was subsequently found that the step of rolling (to further pack the 
powders within the tube), was not absolutely essential and that welds of 
comparable quality are produced when the rolling step was eliminated from 
the manufacturing process. 
ALTERNATE EMBODIMENTS OF THE INVENTION 
Several variations of the process were tested as alternative processing 
means, or as an effort to eliminate processing steps and reduce the 
niobium content of the alloy. 
The method of loading and consolidating the powder in the tubes was the 
same as described in the Example 1 embodiment. The variations were as 
follows: 
A) Cold pressing the powders in the niobium tube without heating. No 
titanium-aluminum SHS reaction occurred. 
B) Cold pressing the powders in the niobium tube, with heating to 
700.degree. C. for 15 minutes to initiate the SHS titanium-aluminum 
reaction, followed by air cooling, and flaking off of NbO. 
C) Cold pressing the powders in the niobium tube, with heating to 
700.degree. C. for 15 minutes in vacuum to initiate the SHS 
titanium-aluminum reaction, and furnace cooling in a vacuum. 
Tubes which were heated in vacuum did not oxidize, and the niobium tube 
remained intact and shiny. Test welds were made with tubes A, B, and C on 
a 1.5" diameter piece of titanium aluminide heated to 850.degree. C. The 
welds were made in shallow 3/8" diameter holes that were drilled into the 
titanium aluminide to simulate casting defects that had been gouged out. 
After welding, the piece was immediately put in a furnace and furnace 
cooled to room temperature. The welds and heat-affected zones (HAZ) showed 
no external cracking. 
The following welding characteristics were noted: 
Sample A--Splattering and sparkles. Tube flamed after weld completed. 
Sample B--Best, performed well, no splatter. 
Sample C--Second best, very little splatter. 
Metallography of weld cross sections showed the following: 
Sample A--(cold pressed tube). The weld contained 14+ pits, one of which 
was fairly large; however, overall the structure was fine grained. 
Sample B--(cold pressed and furnaced--tube removed). No pits. Fine grained 
in general. Larger elongated grains toward heat affected zone (HAZ). More 
dendritic than A. One crack to HAZ. 
Sample C--(cold pressed and vacuum furnaced) 7+ pits, one large, two 
cracks. Very large grained and dendritic. 
Cracking is believed due to inexperience with pre and post heat treatment. 
Weld C which contained the most niobium also had very large grains. 
Although weld A was produced from a rod containing the maximum niobium, it 
appears that a large portion of the niobium spattered off. The best weld 
in terms of microstructure and ease of welding was weld B. Less niobium 
appears to produce finer grains and less pitting. However, the niobium 
tube adds rigidity to the rod and prevents it from breaking, and in this 
respect, niobium is necessary. 
Attempts were also made to produce weld rods by consolidating titanium and 
aluminum powders in plastic and glass tubes prior to initiating the SHS 
reaction. Powders were loaded and consolidated in the plastic and glass 
tubes as described in the method of Example 1. 
Plastic tubes were cold pressed at 60,000 psi to densify the powders. The 
plastic was cut off to expose the densified powder. Numerous radial cracks 
had developed during the cold pressing, and the largest length of rod 
measured not more than 1/4 in. long. Additional plastic tubes containing 
the mixed powders were put in a furnace at 650.degree. C. after cold 
pressing to initiate the SHS reaction and burn off the plastic tube. The 
plastic burned off almost immediately, and the unconfined SHS reaction 
spewed powder particulates throughout the furnace. 
Experiments with the glass rods also failed. Powders were packed in quartz 
glass tubes using the method described in Example 1. Cold pressing was not 
performed in order to keep the glass intact. The tubes containing the 
titanium and aluminum powders were transferred directly to a furnace at 
800.degree. C. The SHS reaction occurred without breaking the glass. The 
tube remained intact as it was furnace cooled to room temperature. The 
only apparent means to separate the densified powder from the glass was to 
break the glass. In breaking the glass, the rod, if it was not fractured 
beforehand, also broke. 
Tests employing titanium and aluminum powders in titanium tubes were not 
tried, due to an inability to locate thin-wall titanium tubing. However, 
this combination may be ideal. The ratio of titanium to aluminum in the 
powder mixture could be adjusted to reflect the powder packing density and 
tubing thickness to achieve an optimum weld rod composition. 
The foregoing description of the specific embodiments will so fully reveal 
the general nature of the invention that others can, by applying current 
knowledge, readily modify and/or adapt for various applications such 
specific embodiments without departing from the generic concept, and, 
therefore, such adaptations and modifications should and are intended to 
be comprehended within the meaning and range of equivalence of the 
disclosed embodiments. It is to be understood that the phraseology or 
terminology employed herein is for the purpose of description and not of 
limitation.