GB2328221A - Surface treatment of titanium alloys - Google Patents

Abstract

A method of modifying the surface of a titanium alloy comprises the steps of using a tungsten metal electrode to provide surface melting in an atmosphere of pure nitrogen or argon and nitrogen mixture. The electrode may be a non-consumable electrode. The electrode may be held stationary and at an angle of about 45‹ to the alloy surface, the power supply being gradually increased.

Description

TITANIUM ALLOY The present invention relates to the surface modification of a titanium alloy.

Titanium alloys such as Ti-6AI4V are extensively used in the aerospace industry where high strength to weight ratio is of prime consideration. These alloys also possess excellent corrosion resistance and high temperature mechanical properties, however, they suffer from poor surface wear which limits further applications in tribological systems, as explained in "Titanium" by A.D.

McquiUan and A.K. Mcquillan, Academic Press, New York, U.S.A. 1956.

A number of surface modification techniques have been used to improve the surface wear properties and these include conventional nitriding and chemical/ physical vapour phase coating processes, as described in B. Edenhofer: Metallurgical and Materials Technology, 1976, pp 8 & 421; and in H.E.

Hintermnn: Wear, 1984, pp 100 & 381. However, nitriding is a high temperature technique requiring a long processing time, and the success of vapour deposition to produce titanium nitride coatings is limited by the adhesive strength of the coat to the titanium alloy.

More recently, laser processing methods have been used for surface modification and offer a shorter processing time, but are an expensive alternative. These have been described by S. Yerramareddy and S. Bahadur in Wear, 1992, pp 157 & 245.

The present invention seeks to provide an improved titanium alloy.

According to an aspect of the present invention, there is provided a method of modifying the surface of a titanium alloy comprising the steps of using a tungsten metal electrode to provide surface melting in an atmosphere of pure nitrogen or argon and nitrogen mixture. The electrode is preferably a non-consumable electrode.

The advantages of the preferred embodiment over those currently available include: 1. much shorter processing time (a few minutes); 2. a portable process allowing for onsite repair; 3. the process is independent of any stringent requirements such as a vacuum system; 4. an overall cheaper process.

The improvements in the surface wear properties are shown in this document by the use of standard metallographic techniques, micro-hardness and dry sliding wear methods.

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an embodiment of surface modification apparatus; Figure 2 is a micrograph of a treated alloy; Figure 3 is an X-ray diffraction spectrum taken from an untreated surface; Figure 4 is an X-ray diffraction spectrum taken from a surface melted under a nitrogen gas shield; Figure 5 is an X-ray diffraction spectrum taken from a surface melted under argon/nitrogen gas mixture; Figure 6 is a graph showing a comparison of micro-hardness depth profiles for surface treated alloy; Figure 7 is a dry sliding wear graph showing variation in the scar width with load for surface in the untreated condition; and Figure 8 is a dry sliding wear graph showing variation in the coefficient of friction with load for surfaces before and after treatment.

EXPERIMENTAL PROCEDURE The equipment used for this process was a Murex TransTIG (a tungsten inert gas welding apparatus) conventionally used for the fusion welding of metals and their alloys. During welding processes argon gas is passed through the electrode gun so that it shields the joint region from the atmosphere thereby preventing oxidation of the weld. In the process discussed in this document, nitrogen gas is used to induce the formation of hard phases at the surface with the aim of increasing the wear resistance of the surface. The hardness of the surface can be controlled by using a mixture of nitrogen and argon. A 3mm diameter solid tungsten electrode was used to create an arc between the tip of the electrode and the titanium alloy surface. This was achieved by holding the electrode stationary and at an angle of about 45 " to the titanium alloy surface, and then by gradually increasing the power supply to the electrode by careful control of parameters such as the current and voltage. Shielding gases were channelled through the electrode gun, and flow regulators used to control the flow rate to give either a pure nitrogen shield gas or a mixture of argon (80% by volume) and nitrogen (20 % by volume).

Larger diameters of up to 6mm can be used if a greater surface melted width is required, and also gas could be passed through a hollowed electrode.

The conditions necessary to produce an arc at the surface for a pure nitrogen shield are as follows: Nitrogen flow rate = 92 cm3/s Current to electrode = 112 A Electrode to metal distance = 1-2 mm Conditions necessary to produce an arc in the argon/nitrogen shield gas mixture was: Nitrogen flow rate = 25 cm3/s Argon flow rate = 83 cm3/s Current to electrode = 120 A Electrode to metal distance = 2-3 mm Changes at the surface after modification were investigated using light microscopy and X-ray diffraction (in order to identify the formation of hard phases). The surface wear properties were investigated using a Leitz microhardness tester with the indentation load set at 500g. The wear resistance was assessed further using a reciprocating diamond pin on plate test, performed in air and changes in surface properties compared with the untreated (reference) titanium alloy. The wear tests were carried out using a constant sliding distance of 72 m with a range of applied loads (2.0 kg, 0.8 kg, 0.4 kg and 0.2 kg). The effect of surface treatment of the wear behaviour was monitored by measuring changes in the wear scar and the coefficient of friction.

RESULTS Figure 2. The micrograph shows a dendritic microstructure produced under a nitrogen gas shield and is characteristic of a surface which has melted and then resolidified.

Figure 3. X-ray diffraction spectrum taken from the untreated (reference) Ti6A14V surface.

Figure 4. X-ray diffraction spectrum taken from surface melted under a nitrogen gas shield. There are prominent peaks showing the formation of hard nitride phases such as TiN and Ti2N.

Figure 5. X-ray diffraction spectrum taken from surface melted under argon nitrogen gas mixture. Peaks showing the formation of titanium nitrides are again evident but have a lower intensity (hence concentration) due to the lower available concentration of nitrogen in the gaseous mixture.

Figure 6. A graph showing a comparison of micro-hardness depth profiles for surface treated Ti-6Al-4V alloy. The highest Vickers hardness number (VHN) was achieved when surface melting under a nitrogen gas shield (between 800 to 1012). In addition the tungsten metal arc produces a wider melted zone which has a more uniform hardness to a greater depth than that possible with laser nitrided surfaces.

Figure 7. A dry sliding wear graph showing variation in the scar width with load for surface in the untreated condition; shielded by nitrogen gas and under a gaseous mixture of argon/nitrogen. The least change in scar width was recorded for surfaces shielded by nitrogen, showing that the wear resistance of the surface is much improved over that of the untreated surface.

Figure 8. A dry sliding wear graph showing variation in the coefficient of friction with load for surfaces before and after treatment. The results show clearly a reduction in the coefficient of friction for the treated surfaces. This further confirms the increase in wear resistance mainly attributed to the increase in surface hardness of the titanium alloy due to the formation of the nitride phases.

It has been shown that a tungsten metal arc heat source is capable of accurately surface melting to a depth of 2mm and more, and when performed in a controlled atmosphere of pure nitrogen or a mixture of nitrogen and argon can produce a wear resistant surface alloy. The use of standard micro-hardness techniques together with wear assessment methods show superior wear behaviour over the untreated titanium alloy. Metallographic and X-ray diffraction techniques attribute this to the formation of TiN in the resolidified region.

EXAMPLE Commercial alloy Ti-6Al-4V was cut into rectangular plates (50mm x 20mm x 10mm) and the surface prepared to give a flat, polished finish followed by a degreasing treatment in acetone before surface melting. A 3mm diameter tungsten electrode was used to create a metal arc between the tip of the electrode and the titanium alloy surface. This was achieved by holding the electrode stationary and at an angle of about 45" to the titanium alloy surface, a metal arc was produced by adjusting the distance between the tip and alloy surface and careful control of parameters such as current and voltage supply to the electrode.

Shielding gases were channelled through the electrode gun, and flow regulators used to control the flow rate to give either a pure nitrogen shielding gas or a mixture of argon (80 % by volume) and nitrogen (20 % by volume).

Characterization of the resolidified surface microstructure was undertaken using light microscopy and X-ray diffraction performed to identify the formation of my nitride phases. Changes in the surface wear properties were investigated using a Leitz micro-hardness tester with the indentation load set at 500g. The wear resistance of modified surfaces was assessed using a reciprocating diamond pin on plate test carried out in air and changes in properties compared with the untreated (reference) alloy. The wear test was performed using a constant sliding distance of 72m with a range of applied loads (2.0 kg, 0.8 kg, 0.4 kg, 0.2 kg).

The effect of surface treatments on wear behaviour were monitored by measuring changes in the wear scar size, the frictional force and the coefficient of friction.

Surface melting to a depth of 2mm in an atmosphere of pure nitrogen or an argon/nitrogen mixture was possible and resulted in the formation of dendrites in the resolidified microstructures as shown by Fig. 2. The X-ray diffraction analysis taken from the surfaces show the prominent peaks for TiN present for surfaces treated with pure nitrogen and weaker intensities for surfaces treated by the argonlnitrogen mixture. It is reasonable then to say that the lower concentration of nitrogen (only 20% by vol.) in the argon/nitrogen mixture corresponds to a lower level of nitride formation in the resolidified surface. The shielding gases were also successful in preventing the formation of titanium oxides in the surface melted zone, the presence of which could have been detrimental to surface wear properties.

The formation of hard nitride phases on surface wear was further assessed using micro-hardness measurements. The surface melted in the presence of nitrogen gave the highest Vickers hardness number (VHN) between 800 to 1012, and a decrease in hardness was recorded outside the heat affected zone at a depth of 1.2mm from the surface. As expected, the surface treated by a mixture of argon/ nitrogen gave lower VHN values of 400-600 which would correspond to a lower concentration of nitride phases in the surface. Some increase in hardness, particularly in the heat affected zone, has been attributed to the interstitial solidsolution strengthening of the a-phase of the titanium alloy. For comparison, the results showing variation in micro-hardness with depth for a laser nitrided surface have been superimposed in Fig. 6. For laser nitriding, although high VHN values can be achieved at the surface, the hardness begins to drop rapidly with increasing distance from the surface. In comparison, the tungsten metal arc produces a much wider melted zone (which is determined by the diameter of the electrode used), but also a deeper, more uniform hardened surface region.

The resistance to surface wear was assessed by monitoring variations in the wear scar width with increasing applied load, and making a direct comparison with the untreated (reference) titanium alloy.

Surfaces treated by melting in either pure nitrogen or argon/nitrogen mixture show better wear resistance than the untreated titanium surface. However, the least change in scar width was recorded for surfaces shielded by nitrogen. These results are consistent with the micro-hardness values, showing that the greater concentration of nitrides formed in the surface improve the surface wear properties of the titanium. In both cases, in which the surface was shielded by nitrogen or argon/nitrogen mixture, severe adhesive wear was absent, and smooth regions were visible within the wear scar. These regions appear to have worn far less than other surface areas. The dry sliding wear test was carried out in air, and these regions could be "oxidised zones" however, because titanium oxides were not detected in the wear debris, these hardened regions could be then attributed to the formation of nitride phases.

It is thus possible to modify the surface of the titanium alloy Ti-6A14V by using a tungsten metal arc to provide surface melting under the influence of a shielding gas such as pure nitrogen or a mixture of argon/nitrogen. The use of nitrogen gas for shielding produces a wear resistant surface with surface hardness values of 1000 VHN. This increase in wear resistant properties is attributed to the formation of titanium nitride phases in the resolidified surface microstructure.

It will be apparent from the above that the preferred embodiment proposes a much cheaper and faster method for surface modification by surface melting using a tungsten arc heat source under the shield of a nitrogen gas or an argon nitrogen mixture. Metallurgical analysis together with surface hardness measurements and dry sliding wear assessment techniques show that this modification process is successful in producing wear resistant surfaces.

Claims (6)

1. A method of modifying the surface of a titanium alloy, comprising the steps of using a tungsten metal electrode to provide surface melting in an atmosphere of pure nitrogen or argon and nitrogen mixture.

2. A method according to claim 1, wherein the electrode is a nonconsumable electrode.

3. A method according to claim 1 or 2, wherein nitrogen gas is used to induce the formation of hard phases at the surface of the alloy.

4. A method according to claim 1, 2 or 3, wherein a mixture of nitrogen and argon is used to control the hardness of the surface of the alloy.

5. A method according to any preceding claim, wherein a solid tungsten electrode is used to create an arc between the tip of the electrode and the alloy surface.

6. A method according to claim 5 wherein,the electrode is held stationary and at an angle of about 45" to the alloy surface, the power supply being gradually increased.