Method of hardening titanium and titanium alloys

Abstract

A method of hardening the outer surface of a titanium or titanium alloy substrate under standard atmospheric conditions. The method comprises focusing an electromagnetic beam from a laser generating apparatus, absent the disposition of a chemical compound, onto at least a portion of the substrate to heat it to a point below the melting point of the substrate. The treated substrate has a substantial increased harness and durability compared to an untreated surface of titanium or titanium alloy.

Description

FIELD OF THE INVENTION

The invention generally relates to a method for hardening the surface of metals or alloys; particularly, a method for hardening titanium and titanium alloys using a laser for reducing wear and improving corrosion resistance of items made from the hardened titanium or titanium alloy.

BACKGROUND OF THE INVENTION

Titanium is an excellent lightweight material, whose strength, chemical resistance, and biocompatibility make it particularly suitable for various applications in the aerospace, medical and chemical industries. However, titanium's low resistance to wear caused by sliding friction and its low surface hardness often precludes its use in the above noted applications. Numerous techniques have been developed to increase the surface hardness of titanium so that it will not gall or chip off when used in abrasive mechanical and environmental conditions (saltwater, acid, autoclave ovens, etc.)

The term “hardness” refers to the resistance of a material to localized deformation. Deformation includes, albeit not limited to, indentation, scratching, cutting or bending. In metals, the deformation occurs mainly at the surface of the workpiece. There are a large variety of testing methods used for determining the hardness of a substance, some of which include the Brinell hardness test, Rockwell Hardness test, Vickers hardness test and Knoop hardness test. The methodology for each of these aforementioned tests is well known and will not be described herein for the sake of brevity. Metallic parts exposed to abrasive and/or corrosive conditions should be resistant to corrosion and have a surface Rockwell Hardness of at least 40 HRC to 55 HRC (Rockwell Hardness on the “C” scale) to prevent galling, seizing, and wear when the substrate is in rubbing and sliding contact with other materials (e.g., metal). The approximate surface hardness values for common materials are given below in both Rockwell Hardness C (HRC) numbers and Knoop hardness numbers (KHN).

Material                   Surface Hardness
303 stainless steel        19 HRC; 180 KHN
Commercially Pure titanium 16 HRC; 175 KHN
Titanium alloy (Ti—6Al—4V) 34 HRC; 363 KHN

Standard 303 stainless steel is prone to corrosion, whereas, the commercially pure titanium (about 98% to about 99.5% Ti) and titanium alloy are both resistant to corrosion. However, the low surface hardness makes titanium containing workpieces generally unsuitable for use in conditions where the unhardened titanium workpiece is in physical contact with other materials.

Surface hardening of metals is a process that includes a wide variety of techniques designed to improve the wear resistance of parts without affecting the tough interior of the part. Presently available techniques of hardening the surface of titanium and it alloys include nitriding, anodizing, surface alloying, metallic and ceramic coatings. However, these techniques are elaborate and often require expensive equipment, such as a furnace, vacuum chamber, heat source, or other means for supplying a specific atmosphere environment (nitrogen, argon, etc.).

Heat treating using a laser to focus onto and harden a metallic substrate has been utilized. Prior to using the laser, these substrates are coated or preconditioned in a manner to form a uniform layer of oxides and/or phosphates, often referred to as “black oxidizing” to make it more absorbent to the light of the laser. It is critical that this coating is uniform and substantially thick. Otherwise, a substantial portion of the laser's light energy may be reflected away from the surface of the object. As a result of laser treating, the coating creates a textured layer on the workpiece with a high coefficient of friction so that the surface must be smoothed or polished, thereby adding an additional step to the process.

What has been heretofore lacking in the art is a simple and inexpensive method of hardening titanium and titanium alloy materials. Desirably, a hard coating is formed on the titanium which does not gall or chip off when in moving contact with other metal parts. It has been discovered by the present inventor that substantial hardening of titanium and titanium alloys is achieved with a combination of parameters on a laser. It is theorized that these specific parameters can be harmonized to create the hard surface of varying depth. The present invention could be used in various applications, such as hardening cutting tools, hardening working areas on hand tools, strengthening stressed areas of existing titanium tools, etc.

DESCRIPTION OF THE PRIOR ART

Numerous patents have been directed to hardening substrates composed of titanium and titanium alloys, however, none of the known prior art discloses a method of focusing a beam from a laser generating apparatus onto at least a portion of the substrate surface to harden the substrate in the open, uncontrolled atmosphere.

For example, U.S. Pat. No. 5,145,530, to Cassady discloses a method of hardening the surface of titanium and its alloys to form hard carbides, by treating the surface thereof with a moving and continuously energized carbon arc. A carbon arc is created by an electrical lead connected to both an electrode, formed of carbon in any of its allotropic forms, and the workpiece and passing an electrical current between them. The carbon arc liquefies the workpiece surface and creates craters on the workpiece surface. The regions of the creators on workpiece are hardened.

U.S. Pat. No. 4,304,978 to Saunders is drawn to a method and apparatus utilizing a laser for heat treating a transformation hardenable workpiece. The workpiece is initially coated with oxides or phosphates to absorb the wavelength of the laser. Sufficiently high laser power densities are provided at the workpiece surface to cause an incandescent reaction with the workpiece. The incandescent reaction only occurs at temperatures above the melting point of the workpiece. In the areas where work-hardening has occurred a textured surface of oxide results. This must be removed by wire brushing. In the areas where work-hardening has not occurred hydrochloric acid must be employed to remove the oxide layer. This in direct contrast with the present invention where the laser heats the workpiece to point lower than the melting points of the titanium or titanium alloy so as to avoid possible deformation of the workpiece.

U.S. Pat. No. 4,434,189, to Zaplatynsky, is directed to coating metal substrates, preferably titanium and titanium alloys, by forming TiN on the substrate surface. A laser beam strikes the surface of a moving substrate. Unlike the present invention, this process is performed in a purified nitrogen gas atmosphere. This heated area reacts with the nitrogen gas to form a solid solution. The alloying or formation of TiN occurs by diffusion of nitrogen into the titanium.

U.S. Pat. No. 6,231,956 to Brenner et al., discloses a process for creating a wear-resistant edge layer for titanium and its alloys which can be subjected to high loads and has a low coefficient of friction. Unlike the present invention, this process involves melting the surface of the substrate in a controlled atmosphere.

Therefore, there remains a need in the art for a simple and cost-effective process by which a titanium or titanium alloy workpiece may be substantially hardened, without the need for special environments or pretreatments, so that the workpiece may be used in abrasive mechanical and environmental conditions.

SUMMARY OF THE INVENTION

Accordingly, the instant invention is related to a method of hardening an outer surface of a metallic substrate under standard atmospheric conditions and without the use of inert gases or a vacuum. The method comprises the steps of providing a substrate of titanium or titanium alloys and focusing an electromagnetic radiation beam formed by a laser generating apparatus onto at least a portion of the substrate surface to heat the substrate surface to a point below the melting point of the substrate and then cooling the substrate surface. The laser intensity and duration is limited such that a disposition of a chemical compound of the surface of the substrate does not occur. The laser treated surface of the metallic substrate has increased hardness and durability compared to the untreated surface of the substrate.

It is an objective of the instant invention to provide method of treating a metallic substrate with a laser having a high power density and a short exposure time such that selected areas of the substrate may be hardened as desired under normal atmospheric conditions.

It is a further objective of the instant invention to provide a method for hardening a metallic substrate which does not require preconditioning, pretreatment, or coating prior to treatment.

Yet another objective of the instant invention to provide a method of hardening a metallic substrate where minimum distortion and/or selective hardening of the workpiece are achieved.

Still another objective of the invention to teach a method of hardening a metallic substrate where the contact time of the laser on the substrate is sufficiently short so that no significant melting of the substrate occurs.

Other objects and advantages of this invention will become apparent from the following description taken in conjunction with any accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. Any drawings contained herein constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 is a scanning electron micrograph (SEM) image of a cross section of a titanium alloy (Ti-6Al-4V) substrate magnified 200×, treated by the method of the present invention;

FIG. 2 is a scanning electron micrograph image, at low magnification, of the titanium substrate of FIG. 1 mounted on a specimen stub with wire (scale bar is 1000 microns);

FIG. 3 is a detail of the surface of the titanium substrate seen in FIG. 2 at a higher magnification (scale bar is 200 microns);

FIG. 4 is another SEM micrograph of the center of the titanium substrate of FIG. 1 in cross-section (scale bar is 100 microns);

FIG. 5 is an Energy Dispersive Spectrum (EDS) collected from the central region of the titanium substrate's cross-section illustrated in FIG. 4;

FIG. 6 is another EDS spectrum representative of fractured surfaces of the outer layer of the titanium substrate;

FIG. 7 is an EDS spectrum representative of surface elemental composition of the outer layer of the titanium substrate;

FIG. 8 is a SEM image of the area from which the spectrum presented in FIG. 6 was collected (scale bar is 5 microns);

FIG. 9 is a SEM image representative of the scan area from which the FIG. 7 surface spectrum was collected (scale bar is 20 microns)

FIG. 10 illustrates the test results for titanium alloy (Ti-6Al-4V) treated by the method of the present invention at various laser parameters, such as frequency and current;

FIG. 11 is a photograph of a bar of 303 stainless steel with a helical groove cut into it by a titanium cutter hardened by the present inventive method;

FIG. 12 is a photograph of a bar of 303 stainless steel which was attempted to be cut by a titanium cutter which was not hardened by the present inventive method; and

FIG. 13 is a photograph of the untreated titanium cutter used in the FIG. 12 photograph.

DETAILED DESCRIPTION OF THE INVENTION

Detailed embodiments of the instant invention are disclosed herein, however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific functional and structural details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representation basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.

It has been discovered by the present inventor that hardening of titanium and titanium alloys is achieved using a laser generating apparatus operating under a specific set of parameters. These parameters result in a surface hardness that does not gall or chip off when in moving contact with other metal parts. This makes the treated substrate particularly desirable when used in implant or medical applications. The hardening process is performed under open atmospheric conditions, e.g. normal air and at room temperature. Thus, no specialized equipment (vacuum chamber, gas chamber, supply of inert gas, etc.) is required. This makes the present process easier and inexpensive to perform than the methods taught by the prior art. Moreover, the use of a laser generating apparatus for hardening titanium and titanium alloys allows for selective treatment of the workpiece wherein only certain areas on the surface of a workpiece can be hardened without affecting the other surface areas thereby reducing the cost of the process.

According to a preferred, albeit non-limiting, embodiment of the invention, surface hardening was accomplished with a Nd—YAG laser used under the following range of parameters. Other lasers can be used such as a carbon dioxide laser without departing from the scope of the invention. Utilizing a Q-switch on the laser the frequency (KHz) of the pulsed laser was from about 25 to about 50 KHz; the power or current (Amps) was from about 8 to about 25 amperes; and the speed at which the laser beam traveled across the treated surface was from about 0.01 to about 5.0 inches/sec (IPS). Also, the number of laser cycles, repeats of beam travel across the treated surface, was variable. The focal length of the laser was variable and dependent on the type of lens used in the laser. Further, the use of a continuous wave (CW) or non-pulsed laser beam also resulted in hardening of the substrate surface.

EXAMPLE 1

This is an illustrative example in which an alloy of titanium has been treated. It is within the purview of the present invention that pure titanium or any titanium alloy could be similarly treated.

A substrate made of the Alpha-Beta alloy of titanium comprising about 6 wt. % of aluminum and about 4 wt. % of vanadium (also referred to as Ti-6Al-4V) with the dimensions of 10×40×60 mm3 which was cleaned by a solvent to removal all residues. The substrate was positioned on the work table under standard atmospheric conditions (temperature and pressure). A Q-switched Nd:YAG laser having a power density of 60.6664×10⁴ watts/mm² at the surface of the substrate was employed. The laser was pulsed in the frequency range of 10-50 KHz. It was also operated in a continuous wave mode. The current applied was 10-20 apms. The laser beam was in constant motion so there was no “dwell time” of the beam on the substrate. The electromagnetic radiation beam formed by the laser generating apparatus was focused onto a portion of the surface of the Ti-6Al-4V substrate. The surface was heated to a point below the melting point of the substrate. The substrate was then cooled. The cooling could be performed by any means of cooling deemed suitable, e.g., water, air, etc. The laser-treated surface of the cooled substrate exhibited increased hardness and durability than the untreated surface of the substrate as evidenced by the results illustrated in the table of FIG. 11.

In addition a sample of the titanium alloy hardened by the method described above was sent to Matco Associates, Inc. (Pittsburgh, Pa.) to perform a Knoop microhardness test to determine its surface hardness. The Knoop microhardness test was conducted at room temperature (RT). A transverse cross-section through the coating and substrate was prepared for subsequent metallographic inspection. In the polished condition a Knoop microhardness inspection, using a 200 gram load, was performed in the outer 0.0015 inches of the coating as is known in the art.

Table 1 below provides the results of this Knoop microhardness test and includes one approximate value for Rockwell Hardness C scale (HRC). The ten individual tests results shown in TABLE 1 for Rockwell Hardness C scale (HRC) are approximate values. The resultant average Knoop hardness number (KHN) of the ten tests is about 1080 and Rockwell hardness is above about 69.7, which, as discussed above, is well above that needed to prevent galling, seizing, and wear (about 40-C to about 55-C Rockwell hardness) in the substrate when in rubbing and sliding contact with other materials.

TABLE 1
Microhardness Test
KHN  Approx. HRC
965  69.7
980  —
980  —
1160 —
1040 —
1040 —
1190 —
1230 —
1020 —
1200 —

Analysis of Elemental Composition of Outer Coating on the Treated Titanium of Example I.

A sample rod of the titanium alloy hardened by the aforementioned inventive procedure was submitted to Impact Analytical (Midland, Mich.) for identification of the composition of the hard surface layer. Energy dispersive spectroscopy (EDS) of the central and outer areas of the rod was performed in the scanning electron microscope (SEM), to compare the elemental composition of the surface layer (FIGS. 6 & 7) with the inner rod (FIG. 5). It was discovered that the surface layer contains significant oxygen which is not present in the bulk rod.

The treated sample of titanium was first rinsed with acetone and methanol, blown dry with filtered nitrogen, and tied to a SEM sample stub with wire to avoid contamination, as illustrated in FIG. 2. The resulting specimen was inserted in the SEM at the accelerating voltage of approximately 20 keV. The EDS spectra and digital images were collected from the outer layer and the center of the sample. Additional spectra and images were collected from fractured surfaces of the outer layer produced by the Knoop microhardness test. Spectra were deconvoluted to determine elemental composition. The surface layer and bulk spectra were compared and the results are presented in TABLE 2.

FIG. 1 presents an overview of the titanium sample as mounted in the scanning electron micrograph (SEM). The photograph of the sample is magnified at 200×, showing the coating on the titanium substrate sample and one of the Knoop hardness indentations, after etching with Kroll's reagent.

FIG. 2 provides further detail of the sample surface morphology. This figure is a low magnification scanning electron micrograph (SEM) of the sample of titanium Ti-6Al-4V treated by the present inventive hardening process. As described above, the sample is mounted on a specimen stub (not shown) with wire. Note the distinctive surface morphology of the surface, characterized by parallel band domains with overlapping orientations.

FIG. 3 is an image detail of the surface seen in FIG. 2, at a higher magnification.

FIG. 4 is a SEM micrograph of the center of the sample bar in cross-section. This is the surface area scanned for x-ray collection comprising the spectrum seen in FIG. 5.

FIG. 5 is the spectrum derived from an EDS collected from the central region of the rod cross-section shown in FIG. 4, and is representative of the bulk rod material. SEM is used in conjunction with EDS to perform elemental analysis on the microscopic section of the material being test or contaminants that may be present as is well known in the art. The EDS spectrum of FIG. 5 illustrates the x-ray energy (keV) seen along the abscissa versus the relative of counts of the detected x-rays (y-axis). The energy of the x-ray is characteristic of the element from which the x-ray was emitted. This spectrum provides both the qualitative and quantitative values for the elements present in the sample.

As seen in FIG. 5, the dominant titanium peak has been truncated, such that the other peaks can be scaled for visibility. Note that an overlap with a secondary feature of Ti (K beta peak or second peak) exaggerates the apparent signal from vanadium. The presence of small V beta peak supports the conclusion that vanadium is present at greater than trace levels. The asterisk indicated a peak artifact, associated with the large Ti signal.

FIG. 6 is another EDS spectrum representative of fracture surfaces of the outer layer, providing evidence of the composition of the outer layer without surface contamination or other variations associated with the extreme outer surface of the coating. Comparison with the bulk spectrum in FIG. 5 reveals that oxygen is now significantly detected. This element is not present in the bulk material. The vanadium signal is again exaggerated by overlap with Ti as noted in the FIG. 5 caption. The Ti peak artifact is again noted by an asterisk.

FIG. 7 is EDS spectrum representative of the surface elemental composition of the outer layer. Although the sample was cleaned with solvents as noted above (acetone, methanol), due to the rough surface microstructure some difference with the FIG. 6 spectrum may be due to trapped contamination. Aluminum (Al), Carbon (C), and Oxygen (O) are significantly more prevalent than in previous regions, as are several other elements as summarized in TABLE 2 above. The vanadium signal is again exaggerated by overlap with Ti as noted in the FIG. 5 caption. The Ti peak artifact is again noted by an asterisk.

TABLE 2
Elements detected by Energy Dispersive Spectroscopy (EDS).
Sample Position	Major elements	Minor elements	Trace elements
Center (bulk)	Ti	Al, C, V	Si
Surface layer	Ti	Al, C, O, V	Ca, Fe, Si
fracture site
Surface	Ti, Al, C, O	Ca, Fe, Si, V	Cl, K, Na, S

FIG. 8 is another SEM image of the area of the sample from which the EDS spectrum presented in FIG. 6 was collected. These regions of micro-fracture in the surface coating enabled the generation of the x-rays from the internal structure of the surface layer of interest.

FIG. 9 is a SEM image which is representative of the area of the sample from which the FIG. 7 spectrum was collected.

Observation of the sample in the stereomicroscope revealed chipped, fractured areas in the surface coating on one cross-cut end of the rod sample. These regions afforded the opportunity to gain an approximate measure of the layer thickness of about 60 to about 100 microns. Additionally, these fractured surfaces enables elemental analysis of the internal composition of the outer layer. Again, significant differences between the internal and surface composition of the outer layer of interest are noted in Table 2. Although some of the differences may be due to contamination, trapped by the rough morphology of the external surface (see FIG. 2), it is unlikely that the significant increase in the signal for aluminum, carbon and oxygen, relative to titanium content, is attributable solely to contamination. The different elemental composition at the surface of the outer layer is more likely to originate from the layer forming process.

EXAMPLE 2

In this example, the ability of a cutting tool made from the untreated Ti-6Al-4V substrate and a cutting tool made from the same Ti-6Al-4V substrate but treated by the inventive process to cut into 303 stainless steel were compared. FIG. 11 is a photograph of a bar of 303 stainless steel with a helical groove cut into it by a titanium cutter treated by the method of the present invention. The resultant groove is about 1/16 of an inch deep. FIG. 12 is a photograph of a bar of 303 stainless steel which was attempted to be cut by a titanium cutter which was not treated by the method of the present invention. It can be seen that there are only minimal abrasions on the surface of the bar. There is no penetration into the bar as shown in FIG. 11. FIG. 13 is a photograph illustrating the damage done to the untreated titanium cutter which was used to attempt to cut the bar of 303 stainless steel shown in FIG. 12.

All patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. All patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference.

It is to be understood that while a certain form of the invention is illustrated, it is not to be limited to the specific form or arrangement herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification and any drawings/figures included herein.

One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objectives and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments, methods, procedures and techniques described herein are presently representative of the preferred embodiments, are intended to be exemplary and are not intended as limitations on the scope. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art are intended to be within the scope of the following claims.

Claims

1. A method of hardening an outer surface of a metallic substrate in open atmospheric conditions comprising the steps of:

providing a substrate of titanium or titanium alloys;

focusing an electromagnetic radiation beam formed by a laser generating apparatus onto at least a portion of said substrate surface at sufficiently high power densities to cause an incandescent reaction above the substrate melting temperature with the substrate; and

limiting the incandescent reaction at any given area of the substrate to a sufficiently short period of time to prevent any substantial melting of the substrate, whereby said laser treated surface of said substrate has a substantial increased hardness and durability compared to an untreated surface of said substrate.

2. The method as set forth in claim 1, wherein said laser generating apparatus is operated at a frequency between about 25 to about 50 kHz.

3. The method as set forth in claim 1, wherein said laser generating apparatus operates at a power density of about 606,664 watts/mm2.

4. The method as set forth in claim 1, wherein said laser generating apparatus operates at a scanning speed between about 0.01 to about 5.0 inches/sec.

5. A method of hardening an outer surface of a metallic substrate in open atmospheric conditions consisting of the steps of:

providing a substrate of titanium or titanium alloys;

focusing an electromagnetic radiation beam formed by a laser generating apparatus onto at least a portion of said substrate surface at sufficiently high power densities to cause an incandescent reaction above the substrate melting temperature with the substrate; and

limiting the incandescent reaction at any given area of the substrate to a sufficiently short period of time to prevent any substantial melting of the substrate, whereby said laser treated surface of said substrate has a substantial increased hardness and durability compared to an untreated surface of said substrate.

6. The method as set forth in claim 5, wherein said laser generating apparatus is operated at a frequency between about 25 to about 50 kHz.

7. The method as set forth in claim 5, wherein said laser generating apparatus operates at a power density of about 606,664 watts/mm2.

8. The method as set forth in claim 5, wherein said laser generating apparatus operates at a scanning speed between about 0.01 to about 5.0 inches/sec.

9. A titanium containing composition having a hardness in the range of 965 to 1200 Koops hardness number produced by the process of claim 1.

10. A titanium containing composition having a hardness in the range of 965 to 1200 Koops hardness number produced by the process of claim 5.