DE10133498A1 - Improving the wear resistance of titanium and titanium alloys used in the production of a vehicle valve by using a welding additive made from solid composite material, and welding as a hard metallic layer on the surface - Google Patents

Abstract

Process for improving the wear resistance of titanium and titanium alloys comprises using a welding additive made from a solid composite material consisting of a titanium alloy matrix with dispersed fine hard material reinforcing particles; and welding as a hard metallic layer on the surface of a component made from titanium or titanium alloy. Preferred Features: The welding additive consists of TiAl6V4 or TiAl6V6Sn2 as base material. The fine hard material reinforcing particles are made from titanium carbide or titanium boride having an average particle size of not more than 40 mu m.

Description

The invention relates to a method for improving the Wear resistance of titanium and titanium alloys through Surface hardening.

The development of new high-performance materials is closely linked the development of new coating and manufacturing processes connected. New materials require new ones Production technologies or new aftertreatment processes. In addition to the Development of new technologies and procedures is the Main task of the technical practice, by further development and modification of known methods of material and Component manufacturing as well as improved after-treatment, especially with composite materials, recognizable Property improvements and thus significant product improvements too to reach.

The low density, the high specific strength, the excellent corrosion resistance and good thermal Durability characterizes titanium and its alloys as innovative light metal material. These advantageous Properties explain the increasing scope of use of Titanium alloys for a wide variety of components. On the other hand, it is also known that titanium and titanium alloys have a low wear resistance, so it difficult is titanium alloys for those areas of use mechanical components, which with others Workpieces are in sliding contact. The low Resistance to all tribological Stress is generally considered a limitation for the technical Application of titanium and titanium alloys for different Components, for example for automotive components, and especially for engine intake valves, which is a good Wear resistance must have. Therefore, it is necessary to Such components perform a surface hardening.

In case of wear, the possible applications of Titanium components without a suitable method for Surface hardening often restricted, and premature failure of components due to high wear is high cost connected. Among the already known methods, the Improvement of the wear resistance of titanium and Titanium alloys have been developed, include u. a. Laser alloying, CVD and PVD methods, thermal spraying, ion implantation and Electroplate. Each of these methods has specific Disadvantages. An application of these methods is therefore accurate to check. Very often, such as B. in spray coatings and PVD Layers, are due to poor adhesion detachments of the Surface layer observed from the coated component. at the production of hard, crack-free surface layers (Armor layers) by laser alloying or plasma powder Build-up welding occur unwanted material changes, the adversely affect operation or later use can. This is in the publication: Takahashi, W. et al. Development of New Wear Resistant Carbide Dispersed Titanium Based Composite and Its Application to Automobile Parts ", in SAE Technical Paper Series No. 900535, (1990); International Congress and Exposition Detroit, Michigan: February 26 - March 2, (1990). Crack-free, wear-resistant Surface layers were only with the PPA process can be produced if all added during the melting process Hard substances were completely dissolved and thereby at the same time a transformation of the original as a powder supplied (α + β) alloy TiAl6V4 in a β-alloy took place.

A method for surface hardening by build-up welding of titanium and titanium alloy components is known from JP 63-282435 or JP 63-318783 or US 5 068 003. The process described in these documents describes a titanium alloy which has a good wear resistance without a specific surface treatment and which can be used to form wear-protection layers applied by build-up welding. As a disadvantage of this solution, already cited (Takahashi, W. et al., In SAE 1990), it should be emphasized that with this method it was not possible to produce a wear-resistant composite material consisting of a TiAl6V4 matrix alloy and dispersed TiC particles. In this process, wear-resistant crack-free wear protection layers (armor layers) are only observed for composite materials made of TiAl6V4, reinforced with W ₂ C, or TiAl6V4, reinforced with Cr ₃ C ₂ . In the composite TiAl6V4, however, reinforced with TiC particles, but microstructural changes of the welded material compared to its initial state by the Aufschweißvorgang occur. Thus, there are changes in the titanium matrix phase and the composition of the titanium component of the applied material range. Furthermore, there is a risk of remaining unresolved in the melt (α'-Ti martensitic phase matrix) grains (TiC particles) of the added refractory hard materials and fine particles that are crystallized during the build-up welding process. This leads to a significant reduction in the wear resistance of the composite material. Furthermore, fine cracks occur, indicating a brittle matrix. On the other hand, miniaturization of the method is poorly possible. Smallest and precisely positioned welds are difficult to implement in this method, which, for example, adversely affects the application for automotive engine valves listed in US Pat. No. 5,068,003.

The object of the present invention is therefore to a Specify method that succeeds in the To improve wear resistance of titanium and titanium alloys and while also avoiding the cracking and phase changes.

The invention is characterized by the features of claim 1 played. The tribologically stressed surface of a Titanium or titanium alloy component is provided with a Layer of a selected composite material provided, wherein through build-up welding their wear resistance against Stress is improved by sliding and friction. The further claims contain advantageous Aus and Further developments of the invention.

According to the invention used as welding filler material already known metal matrix composite material whose basic or matrix material of a titanium alloy TiAl6V4 (typical (α + β) alloy) or TiAl6V6Sn2 and its amplification phase preferably consists of homogeneously dispersed TiC particles. you However, TiB can also be used as finely divided reinforcing particles use, which has a TiC similar HV hardness of over 1000 having. This material was called structural and Functional material developed with the aim of wear resistance increase, the thermal expansion value to decrease, the modulus of elasticity and to increase the strength.

The invention is explained in more detail below with reference to the accompanying drawing Fig. 1, which shows a schematic representation of the application of a wear protection layer according to the application of the welding filler for build-up welding, in particular for the so-called. TIG welding (tungsten inert gas welding).

In this way, in the solution according to the invention a hard metallic layer on the surface of a component Titan applied. As filler material is a Welding wire or rod inserted, which consists of 80 volume percent a titanium alloy such as. TiAl6V4 or TiAl6V6Sn2 and 20 Volume percent of finely divided particles of hard material Titanium carbide TiC exists.

According to this invention as o. G. a new field of application this metal matrix composite (MMC) proposed. Of the Welding filler material is TIG-controlled by remote maintenance of oxygen and nitrogen on the surface of a Titanium component melted. This is how the original ones dissolve TiC particles completely in the melt, which on the component of molten component material (low proportion) and melted filler material (overwhelming proportion) arises in the electric arc. at the re-solidification of this melt on the component The TiC crystallizes in the newly formed surface Material ("weld bead") completely out again, but not in the form of the original, relatively coarse particles, but with considerably smaller dimensions and in completely altered Shape.

The hard material TiC and the base material titanium alloy (TiAl6V6Sn2) have the same density of 4.52 cN / cm ³ . Therefore, 20 volume percent of TiC in the composite material corresponds to 20 mass percent. From this, the mass percentage of 4% carbon in the composite material (MMC) is calculated. Neglecting the alloy components Al, V, and Sn, this value corresponds to an atomic percentage of about 12.5 atomic percent carbon in the MMC. This material begins to melt in the electric arc of the TIG welding at around 1650 ° C and, after complete dissolution of the TiC particles, completely melts at around 2400 ° C. The resulting melt has the following composition in mass percent: 84.8% titanium (68.8% + 16%), 4.8% Al, 4.8% vanadium, 1.6% Sn, 4.0% C.

When this melt is cooled, the process proceeds in the reverse order: first, the primary carbides are crystallized out at around 2400 ° C. Their amount is constantly increasing with decreasing temperature, whereas the amount of remaining melt constantly decreases. At 1650 ° C, the remaining melt, which contains only 0.45% by mass of dissolved carbon, crystallizes to a microstructure of pure titanium and the lower-carbon titanium carbide Ti ₂ C from. Thus, the same amount of carbide re-crystallizes, which was previously dissolved. As a rule, even in this case, the primary solidified phase (primary carbides) crystallizes in the form of dendrites (fir tree crystals). The approximately 20 volume percent TiC are in the form of fine dendrites in the solidified melt of the newly created surface material. They are evenly distributed in this and have a significantly smaller particle size than the particles present in the original welding filler material (MMC). In this process, according to the invention, the metallic base substance of the welding consumable material (titanium) has not changed, which is of considerable importance in corrosive stress of the weld.

This wear resistant applied according to the invention Surface layer can now be used to produce a desired Surface quality or, in order to reach certain Component dimensions, by means of a machining aftertreatment to be edited.

The inventive method provides a useful supplement the powder metallurgical possibilities for the production wear-resistant titanium components and also allows additional / other possibilities of surface armor. On particular advantage of the invention is the good adhesion between the surface layer and the titanium or Titanium alloy base material. This procedure is the one above enumerated methods for improving the Wear resistance of titanium and titanium alloys also by the Similarity or equality of the chemical composition of coated material and superior to the coating material, since no disturbing differences of metallurgical, physical and mechanical properties of component and Surface layer exist. As more benefits of this Process can be the formation of a crack-free Wear protection layer, with no detectable phase changes are to be determined relative to the initial state and the preferred suitability of the method for the coating of smallest and very precise welds, especially on Automotive parts, such as car valves, called. Of special The advantage is the freely selectable layer thickness of the welded hard material.

Further advantages and explanations of the invention will become apparent from the following description of embodiments and from the accompanying drawing and the illustrations to which Reference is made. It shows:

Fig. 1 is a schematic diagram for explaining the formation of a wear-resistant layer according to the invention by hardfacing

Fig. 2 shows the microstructure of the armor layer on the alloy at a scale build-up welding on the cylindrical peripheral surface of a bar made of pure titanium (25 × magnification)

Fig. 3 shows the formation of the armor layer with dendritic growth in a flat surfacing on the cylinder surface of a rod of pure titanium (magnification 200 ×).

In the following first embodiment is shown schematically in Fig. 1, the apparatus design for the production of wear protection layers according to the invention and to see in Figs. 2 and 3, the structural formation of the armor layer. Heat is generated by the electric arc in FIG. 1 between the non-consumable tungsten electrode 3 and the base material 1 , in particular a titanium and titanium alloys, and thereby a hard metallic filler material 2 , preferably a metal matrix composite material reinforced with TiC hard material particles. melted in a layer 4 on the surface of a component of the softer titanium alloy. The molten bath, the electrode and the filler material are surrounded by inert gas 5 argon, which is supplied via an inert gas 6 . A preferred application of the build-up welding operation in this example is based on the surface welding by the TIG method of FIG. 1 of several overlapping, parallel beads or strands (or layers) of the welding filler material 2 on the titanium alloy TiAl6V4. The consumable filler 2 used consists of 80% by volume of metallic base alloy-TiAl6V6Sn2 and 20% by volume of TiC particles, used in the form of rods for the TIG process on build-up welding. The microstructure of the welded-on armor layer is shown in different magnification in FIGS. 2 and 3. The metallographic findings in the field of build-up welding at the cross-section laid to the welding direction a well-performed build-up welding, with good bond to the base material 1 , since the base material only a relatively thin layer was melted and no pores / voids and oxides are observed. From the metallographic micrographs of Fig. 2 and 3, the structure of the armor layer is shown. Due to the set process parameters, the TiC crystallizes in dendrites. The metallographic findings show that the overlaying of the armor layer according to the invention leads to a hardness increase of 24% and thus to a significant increase in wear resistance.

In a second embodiment, a point-like build-up welding is performed on the flat end face of a rod. The welding filler is shown in Example 1. The build-up welding was performed by TIG method of FIG . It was applied only as much welding filler material, as was necessary for the formation of a flat dome on the face. The composition in percentage by mass of the titanium alloy of the rod is as follows: 2.5% Al, 11% tin, 5% zircon, 1% molybdenum, 0.2% silicon. The metallographic investigations of punctiform weld overlay on a longitudinal grinding provide the results corresponding to Example 1. The structure of the armor layer is identical to Example 1 and leads to a significant increase in hardness of approximately 30%.

As a third example, a surface version of this method is listed below on the cylinder jacket surface of a rod made of pure titanium. The composite material used in Examples 1 and 2 was also used here as a filler metal. In this example, the build-up welding according to FIG. 1 is carried out on the cylinder jacket surface of a rod with a diameter of 15 mm. The rod material consists of pure titanium with 0.25 Ma% iron and 0.2 Ma% oxygen as alloying elements. The welding is carried out according to Example 1 and 2 with the TIG method of FIG. 1 using a rod-shaped filler metal. Compared to the preceding examples, a plurality of overlapping, parallel caterpillars or strands (or layers) of the welding filler material are welded on the cylinder jacket surface in the axial direction. These parallel beads allow complete coverage of the surface of the rod. The microscopic examinations resulted in an error-free build-up welding, pores / voids and oxides within the weld beads are not observed. The metallographic micrographs Fig. 2 and 3 show the armor layer structure. In FIG. 2, at a magnification of 25 times, a dendritic growth of the microstructure in the wear-resistant layer is clearly recognizable. According to FIG. 3, no formation of pores / voids and oxides is detectable at a magnification of 200 times. The armor has a hardness increased by a factor of 3.

After all the examples listed, the inventive Welding to a very effective hardness increase in the Area of armor.

Claims (9)

A method for improving the wear resistance of titanium and titanium alloys by surface hardening, characterized in that a filler metal ( 2 ) is composed as a solid composite of a titanium alloy matrix with dispersed fine reinforcement particles and as a hard metallic layer ( 4 ) on the surface of a component in particular made of titanium or titanium alloys is welded. 2. The method according to claim 1, characterized in that using the welding consumable material from one Titanium alloy TiAl6V4 or TiAl6V6Sn2 as base material, with a content between 70 and 95, preferably 80 Volume percent of the composite, and from between 5 and preferably 20% by volume Hard material reinforcing particles is composed. 3. The method according to claim 1 or 2, characterized that as hard material reinforcing particles titanium carbide or Titanium boride can be used. 4. The method according to claim 1 to 3, characterized that the hard material reinforcing particles of the Filler metal has a mean particle diameter preferably not exceed 40 microns and a homogeneous Have distribution. 5. The method of claim 1 to 4, wherein the hard metallic layer with a selected layer thickness the surface of a titanium alloy Component is welded. 6. Method according to one or more of the preceding Claims, characterized in that the Welding filler on the surface, especially on one small area is applied as wear protection. 7. Method according to one or more of the preceding Claims, characterized in that the Filler metal in the form of thin rods or thin ones Wires is applied. 8. Components, in particular compressor blades made of titanium or a titanium alloy having a sliding surface in which at least the sliding surface of the component of a Composite material made of a wear-resistant titanium alloy according to one of claims 1 to 7. 9. automobile engine valve, made of a Wear-resistant titanium alloy according to one of claims 1 to 7.