Plasma descaling of titanium and titanium alloys
a technology of titanium alloys and descaling, which is applied in the field of surface treatment of metals and metallic alloys, can solve the problems of large amount of metal waste, and large amount of metal waste, and achieve the effect of cost-effectiveness and low waste of materials
Inactive Publication Date: 2000-06-27
THE BOEING CO
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Benefits of technology
The invention provides a method of removing the oxide scale produced by the heat treatment of crystalline titanium and titanium alloys. The method is particularly well suited for the removal of such scales from large titanium or titanium alloys substrates, such as aircraft components. Moreover, the method generates very little hazardous waste, in comparison with the chemical bath immersion technique. The method is cost-effective and removes scale at a rate of at least about 0.0001, and preferably at least about 0.0005 to about 0.002, inches per hour.
The method includes heating the surface scale-covered titanium or titanium alloy substrate to a temperature that is sufficiently high to promote reaction of chemical components of the scale at commercially useful rates with a plasma generated from a gas that produces fluoride ions, such as CF.sub.4 and SF.sub.6, and the like. Usually, the substrate is heated to a temperature in the range from about 100.degree. C. to about 600.degree. C. The plasma reacts with the scale, removing the oxide scale and any alpha case. Importantly, this is achieved without intergranular attack on the underlying crystalline titanium or titanium alloy substrate. The plasma reaction self-terminates when the plasma has reacted with the scale and the plasma encounters the underlying metallic substrate. Consequently, the invention provides a method that is not only capable of removing the surface scale, but is also capable of doing so uniformly, on all surfaces of the substrate.
Problems solved by technology
However, heat treatment results in the formation of a dense, tightly adherent oxide on outer surfaces of the metal.
Also, alpha case scale can cause cracking of the titanium that may result in catastrophic failure.
Some of these chemical baths contain concentrated alkaline solutions while others contain highly toxic and corrosive acids, including nitric acid and hydrofluoric acid.
As a consequence, the baths and ancillary equipment that come into contact with these corrosive chemicals must be fabricated from expensive exotic materials that are resistant to attack.
Some of the hydrogen generated during acid-etching may also migrate into the alloy structure causing "hydrogen-embrittlement"--a serious problem that reduces fatigue strength significantly.
Such disposal is becoming increasingly costly.
Method used
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example 1
Two samples of heat-treated titanium alloy were descaled, one in SF.sub.6 and the other in CF.sub.4 plasmas. Each sample measured 0.5.times.1.5 inches and was 0.125 inches thick. Since the 6.3 liter volume plasma chamber used for descaling was only able to accept 5-inch wide wafers, each sample 10 was adhered to an upper surface of a 5-inch silicon wafer 12 with photoresist material 14 in order to lead the sample into the chamber, as illustrated in the FIGURE. Moreover, in order to provide a comparison between the descaled and original surfaces, one side of the upper surface 10a of each sample was covered with a strip of silicon 16 adhered to the face of the sample with photoresist 14 to provide a mask, while the other side 10b was exposed to the plasma.
The descaling with CF.sub.4 was carried out with a flow rate of 45 ccs per minute of CF.sub.4 through the chamber, along with 2 ccs per minute of oxygen. The plasma descaling was carried out for in periods of 30 minutes, that include...
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Abstract
The invention provides a method of removing surface scale from a titanium or titanium alloy substrate. The method includes the steps of heating the substrate to a temperature in the range from about 100 DEG C. to about 600 DEG C., and thereafter subjecting the heated surface to a plasma formed from a gas selected from the group of consisting of CF4 and SF6. The plasma reacts with the surface scale, removing the scale, without attacking the underlying crystalline titanium or titanium alloy. Properly controlled, the plasma reaction terminates when the plasma has penetrated the scale, and encounters the underlying crystalline metal. As a result, the method of the invention is capable of uniform removal of the entire surface scale of a crystalline titanium-containing substrate, without intergranular attack of the substrate.
Description
FIELD OF THE INVENTIONThe invention relates to the surface treatment of metals and metallic alloys to remove surface scales that arise naturally or from heat treatment processes. More particularly, the invention relates to a method of subjecting the surfaces of titanium and titanium alloy aircraft components to a reactive plasma to remove these surface scales.BACKGROUND OF THE INVENTIONTitanium and its alloys are used in the fabrication of aircraft. These metals are used to form not only the outer skin of the aircraft, but also internal support structures because of their light weight and high strength. In order to achieve desired physical properties, the titanium alloys are first heat treated. However, heat treatment results in the formation of a dense, tightly adherent oxide on outer surfaces of the metal. This oxide ranges in thickness from about 0.001 to about 0.010 inches and must be removed before subsequent machining, forming or joining operations. Scale covered parts cannot ...
Claims
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CPCC23G5/00
InventorGOODE, JR., HERBERT S.NIELSEN, JEAN A.NITZSCHE, LARRY E.
OwnerTHE BOEING CO