Patent Application
20120082801
1. Field of the Invention
The present invention relates generally to surface hardening of metals, and particularly to a method of laser treating Ti-6Al-4V to form barrier surface compounds using gas-assisted laser nitriding.
2. Description of the Related Art
Titanium alloys are metallic materials which contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness (even at extreme temperatures), are light in weight, exhibit extraordinary corrosion resistance, and have the ability to withstand extreme temperatures. Although “commercially pure” titanium has acceptable mechanical properties and has been used for orthopedic and dental implants, for most applications titanium is alloyed with small amounts of aluminum and vanadium, typically 6% and 4%, respectively, by weight. This mixture has a solid solubility which varies dramatically with temperature, allowing it to undergo precipitation strengthening. This heat treatment process is carried out after the alloy has been worked into its final shape but before it is put to use, allowing much easier fabrication of a high-strength product.
The American Society for Testing and Materials (ASTM) classifies titanium alloys by numerical grades. “Grade 5”, also known as Ti-6Al-4V, is the most commonly used alloy. It has a chemical composition of 6% aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen, and the remainder titanium. Grade 5 is used extensively in the aerospace, medical, marine, and chemical processing industries. Ti-6Al-4V is significantly stronger than commercially pure titanium while having the same stiffness and thermal properties. Among its many advantages, it is heat treatable.
This grade also exhibits an excellent combination of strength, corrosion resistance, weld and fabricability. Generally, it is used in applications up to 400° C., and its properties are very similar to those of the 300 stainless steel series, particularly stainless steel 316.
Titanium dioxide dissolves in titanium alloys at high temperatures, and its formation is very energetic. These two factors mean that all titanium, except the most carefully purified, has a significant amount of dissolved oxygen, and so may be considered a Ti—O alloy. Oxide precipitates offer some strength, but are not very responsive to heat treatment and can substantially decrease the alloy's toughness. In order to protect a titanium alloy, the formation of surface barrier compounds is desirable. Thus, a method of laser treating Ti-6Al-4V to form surface compounds solving the aforementioned problems is desired.
The method of laser treating Ti-6Al-4V to form surface compounds is a method of forming barrier layers on surfaces of Ti-6Al-4V plates or workpieces. The Ti-6Al-4V workpiece is first cleaned, both with a chemical bath and then through an ultrasonic cleaning process. Any suitable type of chemical bath for cleaning titanium alloys may be used, as is conventionally known. Similarly, any suitable type of ultrasonic cleaning process may be used.
Following cleaning of the workpiece, a water-soluble phenolic resin is applied to at least one surface of the Ti-6Al-4V workpiece. The Ti-6Al-4V workpiece and the layer(s) of water soluble phenolic resin are then heated to carbonize the water soluble phenolic resin, thus forming a carbon film on the at least one surface. TiC particles are then inserted into the carbon film.
Following the insertion of the TiC particles, a laser beam is scanned over the Ti-6Al-4V workpiece. Preferably, the laser beam is produced by a carbon dioxide laser with a power intensity output of approximately 110 W/m2. Scanning preferably occurs at a rate of approximately 10 cm/sec. A stream of nitrogen gas, which may be atomic or diatomic nitrogen formed by any suitable method (such as dissociation from ammonia at high temperature), is sprayed on the surface of the Ti-6Al-4V workpiece coaxially and simultaneously with the laser beam at a relatively high pressure, such as approximately 600 kPa, thus forming a barrier layer of TiCxN1-x, TiNx, Ti—C, and Ti2N compounds in the surface region, typically at a depth of 15 μm in the laser-irradiated region.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The method of laser treating Ti-6Al-4V to form surface compounds is a method of forming barrier layers on surfaces of Ti-6Al-4V plates or workpieces. Such barrier nitride or carbonitride layers harden the surface, protect the available oxidizing metallic species of the titanium alloy, and further impede egress of surface dislocations, which tend to cause increases in fatigue and creep strengths.
The Ti-6Al-4V workpiece or plate P is first cleaned, both with a chemical bath and then through an ultrasonic cleaning process (step 10 in
Similarly, any suitable type of ultrasonic cleaning process may be used. Ultrasonic cleaners are well known in the art. One example of such a cleaner is shown in U.S. Pat. No. 6,630,768, which is hereby incorporated by reference.
As diagrammatically illustrated in
As illustrated in
As illustrated in
A stream of nitrogen gas, which may be atomic or diatomic nitrogen formed by any suitable method (such as dissociation from ammonia at high temperature) is sprayed on the surface of the Ti-6Al-4V workpiece P coaxially and simultaneously with the laser beam B at a relatively high pressure, such as a pressure of approximately 600 kPa (step 20 in
It should be understood that sprayer S in
During the laser-irradiated heating of the surface of the plate P, the nitrogen diffuses into the material, starting at the surface and working inwardly, particularly via the grain and subgrain boundary regions and the dislocation lines. The nitrogen then combines with the constituents of the alloy to form complex nitrides. The nitride buildup (extending from the surface inwardly to a depth of approximately 15 μm) restricts the high diffusion paths and slows down the initial rate of oxidation diffusion of titanium or of any other material in the alloy that would normally be oxidized. The nitriding further increases resistance against both creep and fatigue.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
