WO2007039870A2

WO2007039870A2 - Composite metal matrix materials based on titanium and their use for the production of cutting tools

Info

Publication number: WO2007039870A2
Application number: PCT/IB2006/053606
Authority: WO
Inventors: Silvia Spriano; Sante Bugliosi; Ildiko Matekovits
Original assignee: Politecnico Di Torino
Priority date: 2005-10-04
Filing date: 2006-10-03
Publication date: 2007-04-12
Also published as: ITTO20050692A1; WO2007039870A3

Abstract

Composite material, useful especially for manufacturing cutting tools, including a metal matrix containing strengthening particles, where the matrix is a titanium alloy including alloying elements chosen among copper, nickel, aluminium and their mixtures. It is preferred that the material of the matrix is cobalt free. The materials are useful for manufacturing cutting tools, as replacement of the conventional cobalt matrix composites and they go over the environmental and health risks related to the production and use of conventional cobalt based composites.

Description

Composite metal matrix materials based on titanium and their use for the production of cutting tools

The present invention deals with composite materials comprising a metal matrix, including strengthening particles and their use for manufacturing cutting tools.

The most part of cutting tools are currently made by using composite materials based on a cobalt matrix, and they are reinforced by tungsten carbide (cemented carbides or hard metal, cermets) or diamond (diamond tools) . They are mainly manufactured by sintering fine powders (about 1 μm) at high temperature. These materials show an excellent wear resistance: the trade mark WIDIA, which is their commercial name, means a material similar to diamond. The diamond tools we will speak about are those for the machining of hard stones, as for instance granite or natural stone of Luserna (Italy) .

The exposure to powders during manufacturing or use of all these tools can generate a lung pathology called "Hard metal Lung Disease". The labelling of the mixtures of Co and WC powders must reproduce the word "toxic" and a symbol representing the cancerous nature of the mixture when it is breathed, according to the Directive 2001/58/CE.

Some recent epidemiological researches and some in vitro and in vivo tests show that hard metals can be involved as causes of bronchial cancer. They were performed studying workers employed as manufacturers of these products. This suspect of potential cancerous nature was considered highly probable in a recent report of the International Agency for Research on Cancer (October 2003) . >The high professional hazard due to the exposure to cobalt powders was also revealed by a recent research performed by ARPA by studying some manufacturers of diamond tools in the Turin area (M.Fontana et al . - Proceedings of the 19^th National Congress AIDII 2001) .

The cellular and molecular mechanism is based on a reaction at the Co-WC interface and it was accepted by the international scientific community. It causes the production of oxygen reactive species (as for instance the superoxide radicals) and they are the probable cause of the observed cytotoxic and genotoxic effects .

Moreover cobalt can be progressively solubilized as ion in a higher amount than what is due to its low water solubility, because of the presence of endogenous -CeIIs with a chelating action.

The WC/Co system has cytotoxic, genotoxic and clastogenic properties, which are not found when cobalt alone is present. On the other hand the carbide is inert in cellular systems and animal models.

Moreover it must be mentioned that cobalt is in any case a toxic and hazardous material, when it is spread into the environment. This is the case of wear debris produced during cutting of stones.

The present invention was grown out during a research performed by the inventors with the aim of finding some innovative matrices, alternative to cobalt, for the production of cutting tools, presenting a lower risk than the commercial ones .

So the aim of this invention is finding new composite materi- als of the above mentioned type useful, particularly, for the production of cutting tools, which, therefore, show mechanical properties, for instance hardness and wear, erosion and abrasion resistances, comparable to the commercial materials now currently used. Their manufacture and use must be free from the described environmental and professional risks.

In view of said aim, the subject of the present invention is a composite material presenting the characteristics described in the following claims .

Moreover a subject of the present invention is the use of said composite material for making cutting tools and the tools produced by using such materials.

The composite materials of the present invention show a matrix based on titanium, containing nickel, copper and/or aluminium as alloying elements .

The use of titanium as base element of the matrix is an innovative characteristic in the scenario, of the materials now currently used for making cutting tools. We mean that titanium is now employed in the commercial materials for cutting tools not in the metallic state, as base for the matrix, but it is present into carbides or nitrides as strengthening particles . Titanium is advantageous to be used as base for the matrix, because it is an atoxic element, presenting a low elastic modulus, it is strong carbide former and it has a low expansion thermal coefficient.

Respect to the low elastic modulus it is of relevance because the occurring of plastic deformation around strengthening particles is the cause of early pull-out. So it is suitable that, considering the same energy coming into the system, the metal matrix does not go over its yielding strength.

Respect to the strong ability of titanium as carbide former, it is of interest because a surface reaction between the matrix and diamond and/or tungsten carbide can be useful for having a strong interface and an adequate retention ability.

Moreover the use of titanium is useful for obtaining a lowering of the production costs of the tools. In fact it is possible to employ a pressure-less sintering process by using titanium, because of its low thermal expansion coefficient.

The titanium amount into the metal alloy is usually equal or higher than 50 weight%, it is preferred from 80% to 90 weight %. The remaining part of the alloy includes one or more metal elements. They are chosen among copper, nickel and aluminium or their binary or ternary mixtures .

These metal elements are usually included into the alloy in the following weight percentages :

Copper from 0% to 40 weight%, it is preferred from 5% to 15 weight%;

Nickel from 0% to 40 weight%, it is preferred from 5% to 15 weight %;

Aluminium from 0% to 10 weight %, it is preferred from 1% to 5 weight %, being understood that, according to the invention, the presence in the alloy of at least one among these metal elements, is contemplated.

Other elements such as, by way of example C, N, O, Fe, Cr, Sn, Zn, Co, V can be added in a small amount, usually in a quantity not higher than 10 weight % respect to the alloy- composition. Organic binders can be also included into the mixture of the starting powders, as for instance ethylenic glycol or paraffin wax.

According to the preferred embodiment, the amount of cobalt is zero in the matrix. This characteristic is an innovative issue respect to the matrices now currently used for making cutting tools. They have a minimum amount of cobalt of 25 weight % .

The addition of copper or nickel allows a strengthening of the alloy by the presence of a second phase, which is an in- termetallic compound such as Ti₂Ni, TiNi, Ni₃Ti, Ti₂Cu, TiCu, and Ti₃Cu₄. On the other hand the addition of aluminium allows the strengthening by the formation of a solid solution with titanium. According to the present invention the hardness of the used alloy, because of these strengthening mechanisms, is comparable to that of cobalt, now currently employed for these purpose. So the alloys used as matrices show Vickers hardness (Hv) in the range from 290 to 380.

The addition of Cu, Ni into the alloy,, owing to the presence of a eutectic point in the Ti- (Cu, Ni) phase diagram, allows sintering at lower temperature respect to those usually employed for the commercial titanium alloys, which means a temperature lower than 1000⁰C. This issue is of relevance for reducing diamond graphitization, in the case of diamond containing composites.

The amount of titanium alloy into the composite material can be chosen in the range 2-40 weight % in the case of cemented carbides (preferred values are about 10 weight %) and they can be chosen in the range 70-90 weight % in the case of diamond composites (preferred values are 90-95 weight %, 93 weight %, for instance) .

All the currently used strengthening particles can be used as strengthening of the composite material. The strengthening particles can be tungsten carbide and/or other carbides (TiC, TaC, NbC for instance) , nitrides (TiN, BN for instance) , carbo nitrides ( (Ti, Mo, W, Hf, Ta, Nb) (C, N) for instance) , borides (ZrB₂, TiB₂ for instance) , with the optional presence of doping elements as V, Cr. When using diamond, it can be both natural and synthetic. It is preferred without coating.

The sintering process involves, concerning the metal matrix, the use of powders of pure metal elements or of their compounds, such as hydrides or metallorganic or pre-alloyed powders (that means the Ti- (Cu, Ni) alloy,_ with the optional addition of aluminium or other eventual elements) . It can also involve both fine powders and granulates. Fine powders have a dimension usually comprised between 0.5 μm and 300 μm. It is preferred a dimension of about 40 μm.

Respect to the strengthening phase (WC, carbides, nitrides, carbo nitrides, borides) , it is considered the use of the powders of the ceramic compound (s) of interest. The process involves a first step of mixing and compaction, followed optionally by a thermal process of debinding (200-500⁰C) and as last the sintering performed in vacuum or in inert atmosphere (vacuum is preferred) . The sintering process can be performed by using or not pressure, at a temperature between 800⁰C and 1700⁰C, it is preferred lower than 1000⁰C (it is preferred about 930-980⁰C) in the case of diamond composites. The manufacturing process by pressure-less sintering is largely useful and preferred. It is a low cost process, in particular respect to the other ones now currently used for the production, particularly, of diamond tools.

When elemental powders are used, the reaction among metals occurs ^• during sintering . As consequence the alloy is formed during sintering and it includes the mentioned Ti-Cu, Ti-Ni intermetallic compounds or solid solutions. Moreover the surface reactions between the metal alloy and the strengthening particles occur.

The sintered samples show a relative density equal to 95-100% respect to the theoretical one, which is comparable with the commercial materials (about 98%) . Titanium carbide can be present on the interface between the metal alloy and strengthening particles after sintering.

Composite materials object of the present invention are of interest for application as cutting tools, in fact they show a high abrasion/erosion resistance and good diamond/WC retention ability. In fact it was observed on fracture surfaces of specimens broken in tensile tests that the most part of strengthening particles (diamond or WC) are still embedded into the metal matrix, while a quite small part of particles is pulled out during fracture. This eyidences a good retention ability. This property of the metal matrix is due to surface reactions with the strengthening particles, with a strong chemical interface as consequence .

Respect to the resistance to erosion and abrasion due to free particles, it is a relevant property. In fact the cutting elements are the protruding diamond grits, during the cutting process by using diamond tools. They protrude out of the surface of the tool, while the metal matrix is not in direct contact with the surface to be cut, but with the debris produced during cutting. It is unexpected that titanium alloys show good performances to erosion and abrasion by free particles. In fact they show low trybological properties respect to abrasion wear with direct contact between the two surfaces with relative motion. In the case of composites with a metal matrix and WC, the abrasion action is due mostly to the ceramic, while the metal matrix, which is a small amount, has essentially a binding function.

Example

Diamond composites were made with a Ti-Cu metal matrix (Ti 87%, Cu 13 weight %) . This alloy was sintered starting from elemental fine powders (Ti < 40 μm, Cu < 4 μm) . Synthetic diamonds were employed into the composite, with a ratio of 0.35 g/cm³, which is 1.75 Kt/cm³. The mean dimension of diamonds grits was 0.7-0.8 mm. The powders were cold pressed at 500 MPa and sintered under high vacuum by pressure-less sintering at 93O⁰C for 1 hour.

The metal matrices and diamond composites were characterized by density measurements, hardness test, XRD diffraction, SEM and optical microscope observations.

In the attached drawings :

- in fig. 1 it is reported an XRD diffraction pattern of the Ti-Cu alloy sintered at 93O⁰C for 1 hour, according to the example ; in fig. 2 it is reported an SEM micrograph showing the biphasic microstructure of the cobalt-free metal matrix (the dark, phase is Ti, the light one is a Ti-Cu intermetallic compound) ; in fig. 3 it is reported an SEM micrograph of the fracture surface of a Ti-Cu-diamond composite sintered at 930⁰C. The micrograph shows a diamond embedded into a cobalt-free metal matrix. The interphase between the two components does not show cracks and the morphology of the diamond is not damaged; in fig. 4 it is reported an XPS pattern showing the presence on the diamond grits (C_D) taken out of the composite of a reaction layer made of TiC; in fig. 5 it is reported an SEM micrograph showing the two complementary fracture surfaces of a composite bead. It was broken during a radial strength test . Numbers refer to the correspondence between a diamond grit and its crater on the complementary surface.

The obtained relative density (98%) and hardness (H_v40 = 318) of the Ti-Cu metal matrix are satisfactory and they are comparable with the cobalt now currently used.

The microstructure of the alloy is formed by a titanium rich solid solution and the intermetallic compound Ti₂Cu. At the same a continuous metal-diamond interface was observed in the composites. It is crack free and it shows that the thermal expansion coefficients are consistent with a pressure-less sintering process.

The diamond surface does not show evident corrosion or graph- itization. At the same time, some fragments rich in Ti are observable on some parts the diamond surface. This evidences that a chemical reaction between matrix and diamond occurred and this inaction makes the interface strong. Moreover the fracture surfaces show that the most part of diamonds does not pull-out during fracture and that the retention ability by the matrix is good.

Abrasion tests performed by using watery slurry of stone powders (for 1 hour) showed an abrasion resistance of these materials higher than that of hot-pressed cobalt (the abraded volume is 30% lower) .

The flexural strength of the metal matrix is 810 MPa, which is comparable with the commercial ones .'

Claims

1. Composite material, especially useful for manufacturing cutting tools, including a metal matrix, with strengthening particles inside, characterized in that said metal matrix is a titanium alloy, which includes an alloying element chosen among copper, nickel, aluminium and mixtures thereof.

2. Composite material according to claim 1, characterised in that the titanium amount into the alloy is equal or higher than 50 weight %.

3. Composite material according to claims 1 or 2, characterized in that the titanium amount into the alloy is 80-90 weight % .

4. Composite material according to any of claims 1 to 3 , characterised in that the alloy of the matrix includes 0-40 weight % of copper, preferably from 5 to 15 weight %.

5. Composite material according to any of claims 1 to 4, characterised in that the metal matrix includes 0-40 weight % of nickel, preferably from 5 to 15 weight %.

6. Composite material according to any of claims 1 to 5, characterised in that the metal matrix includes 0-10 weight % of aluminium, preferably from 1 to 5 weight %.

7. Composite material according to any of claims 1 to 6, characterised in that the metal matrix includes also alloying elements selected from the group consisting of C, N, O, Pe, Cr, Sn, Zn, Co, V in an amount not higher than 10 weight %.

8. Composite material according to any of the previous claims, characterised in that the mixture of starting powders includes also organic binders chosen among ethylene glycol or polyethylene and paraffin waxes .

9. Composite material according to any of the previous claims, characterised in that the metal matrix is cobalt free .

10. Composite material according to any of the previous claims, characterised in that it includes 2-40 weight % of metal matrix.

11. Composite material according to any of claims 1 to 9, characterised in that it includes 70- 99 weight% of the metal matrix.

12. Composite material according to any of the previous claims, characterised in that said metal matrix shows a Vick- ers hardness of 290-380.

13. Composite material according to any of the previous claims, characterised in that it includes strengthening particles chosen among carbides, nitrides, carbon nitrides, bor- ides and/or diamond.

14. Composite material according to claim 13, characterized in that it includes carbides chosen among Tungsten, titanium, tantalum, niobium carbides and their mixtures

15. Composite material according to any of the previous claims, characterised in that it includes titanium and/or boron nitrides .

16. Composite material according to any of the previous claims, characterised in that it includes carbon nitrides chosen among titanium, molybdenum, tungsten, hafnium, tantalum, niobium carbon nitrides and their mixtures.

17. Composite material according to any of the previous claims, characterised in that it includes zirconium and/or titanium borides.

18. The use of a composite material according to any of the previous claims, for manufacturing cutting tools.

19. Cutting tools including a cutting element, including a composite material according to any of claims 1 to 17.