METHOD OF MAKING METAL COMPOSITE MATERIALS

Abstract: According to the method of the present invention one or more metal salts of at least one iron group metal containing organic groups are dissolved and complex bound in at least one polar solvent with at least one complex former comprising functional groups in the form of OH or NR3, (R=H or alkyl). Hard constituent powder and, optionally, a soluble carbon source are added to the solution. The solvent is evaporated and remaining powder is heat treated in inert and/or reducing atmosphere. As a result coated hard constituent powder is obtained which after addition of pressing agent can be compacted and sintered according to standard practice to a body containing hard constituents in a binder phase.

Description: Method of making metal composite materials The present invention relates to a method of produc¬ ing metal composite materials such as cemented carbide. Cemented carbide and titaniumbased carbonitride al¬ loys often referred to as cermets consist of hard con¬ stituents based on carbides, nitrides and/or carbonit- rides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder phase essentially based on Co and/or Ni. They are made by powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering. The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is on the order of several hours up to days. Such processing is believed to be ne¬ cessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further be¬ lieved that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure. GB 346,473 discloses a method of making cemented carbide bodies. Instead of milling the hard constituent grains are coated with binder phase with an electrolytic method, pressed and sintered to a dense structure. This and other similar methods are, however, not suited for cemented carbide production in a large industrial scale and milling is almost exclusively used within the ce¬ mented carbide industry today. However, milling has its disadvantages. During the long milling time the milling bodies are worn and contaminate the milled mixture which has to be compensated for. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore even after an extended milling a random rather than an ideal homogeneous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure sintering has to be performed at a higher tem¬ perature than necessary. Thus, the properties of the sintered metal composite materials containing two or more components depend to a great extent on how well the starting materials are mixed. An ideal mixture of particles of two or more kinds especially if one of the components occurs as a minor constituent (which is the case for the binder phase in ordinary metal composite materials) is diffi¬ cult to obtain. In practice, after extended mixing a random rather than an ideal homogeneous mixture is ob¬ tained. In order to obtain an ordered mixing of the com- ponents in the latter case, the minor component can be introduced as a coating. The coating can be achieved by the use of various chemical techniques. In general it is required that some type of interaction between the coated component and the coating is present, i. e. ad- sorption, chemisorption, surface tension or any type of adhesion. It has now surprisingly been found that using a technique related to the SOL-GEL technique the hard con¬ stituent grains, cubic as well as hexagonal, can be coated with binder phase layers. The coating process seems not to pass a gel state and therefore is not a strict SOL-GEL process but should rather be regarded as a "solution-chemical method". Figs 1 - 3 show in 1000X the microstructure of ce- mented carbide compositions made with the method of the present invention. According to the method of the present invention one or. more metal salts of at least one iron group metal containing organic groups are dissolved and complex bound in at least one polar solvent with at least one complex former comprising functional groups in the form of OH or NR3, (R=H or alkyl). Hard constituent powder and optionally a soluble carbon source are added to the solution. The solvent is evaporated and remaining powder is heat treated in inert and/or reducing atmosphere. As a result, coated hard constituent powder is obtained which after addition of pressing agent can be compacted and sintered according to standard practice. The process according to the invention comprises the following steps where Me= Co, Ni and/or Fe, preferably Co: 1. At least one Me-salt containing organic groups such as carbooxylates, acetylacetonates, nitrogen con¬ taining organic groups such as schiff bases, preferably Me-acetates, is dissolved in at least one polar solvent such as ethanol, acetonitrile, dimetylformamide or di- etylsulfoxide and combinations of solvent such as methanol-ethanol and water-glycol, preferably methanol. Triethanolamine or other complex former especially mole- cules containing more than two functional groups, i. e. OH or R3 with R = H or alkyl (0.1-2.0 mole complex for¬ mer/mole metal, preferably about 0.5 mole complex for¬ mer/mole metal) is added under stirring. 2. Optionally, sugar(Ci2 I '*22 ( - ' 1 ll) or other soluble carbon source such as other types of carbohydrates and/or organic compounds which decompose under formation of carbon in the temperature interval 100-500°C in non- oxidizing atmosphere can be added (<2.0 mole C/mole metal, preferably about 0.5 mole C/mole metal), and the solution heated to 40°C in order to improve the solubi¬ lity of the carbon source. The carbon is used to reduce the MeO formed in connection with heat treatment and to regulate the C-content in the coating layer. 3. Hard constituent powder such as WC, (Ti,W)C, (Ta,Nb)C, (Ti,Ta,Nb)C, (Ti,W) (C,N) , TiC, TaC, NbC, VC and Cr 3(--2' preferably well-deagglomerated e.g. by jet mill¬ ing, is added under moderate stirring and the tempera¬ ture is increased to accelerate the evaporation of the solvent. When the mixture has become rather viscous, the dough-like mixture is kneaded and when almost dry smoothly crushed in order to facilitate the evaporation (avoiding inclusions of solvent) . 4. The loosened powder lump obtained in the preced¬ ing step is heat treated in nitrogen and/or hydrogen at about 400-1100°C, preferably 500-900°C. To achieve a fully reduced powder a holding temperature might be needed. The time of heat treatment is influenced by process factors such as powder bed thickness, batch size, gas composition and heat treatment temperature and has to be determined by experiments. A holding time for reduction of a 5 kg powder batch in pure hydrogen atmos¬ phere at 700°C of 120-180 minutes has been found suit¬ able. Nitrogen and/or hydrogen is normally used but Ar, NH3, CO and CO2 (or mixtures thereof) can be used whereby the composition and microstructure of the coat¬ ing can be modulated. 5. After the heat treatment the coated powder is mixed with pressing agent in ethanol to a slurry either alone or with other coated hard constituent powders and/or uncoated hard constituent powders and/or binder- phase metals and/or carbon to obtain the desired compo¬ sition. The slurry then is dried, compacted and sintered in the usual way to obtain a sintered body of hard con¬ stituents in a binder phase. Most of the solvent can be recovered which is of great importance when scaling up to industrial produc¬ tion. Alternatively the pressing agent can be added to¬ gether with the hard constituent powder according to step 3, directly dried, pressed and sintered considering the conditions according to step 4. Example 1 A WC-6 % Co cemented carbide was made in the follow¬ ing way according to the invention: 134.89 g cobaltace- tatetetrahydrate (Co(C2H3O2)2 ' 4H 2°) as dissolved in 800 ml methanol (CH3OH) . 36.1 ml triethanolamine ((C2H5θ)3N (0.5 mole TEA/mole Co) was added during stirring and af- ter that 7.724 sugar (0.5 mole C/mole Co) was added. The solution was heated to about 40°C in order to dissolve all the sugar added. After that 500 g jet-milled WC pow¬ der was added and the temperature was increased to about 70°C. Careful stirring took place continuously during the time the methanol was evaporating until the mixture had become viscous. The dough-like mixture was worked and crushed with a light pressure when it had become al¬ most dry. The powder obtained was fired in a furnace in a po- rous bed about 1 cm thick in nitrogen atmosphere in a closed vessel, heating rate 10°C/min to 700°C, no hold¬ ing temperature, cooling 10°C/min and finally completed with reduction in hydrogen, holding temperature 800°C for 90 minutes. The powder obtained was mixed with pressing agent in ethanol with no adjustment of carbon content, dried, compacted and sintered according standard practice for WC-Co alloys. A dense cemented carbide structure was ob¬ tained with porosity A00. Fig 1 shows the microstructure of a compacted body before sintering and Fig 2 after sintering. Example 2 A (Ti,W)C-ll % Co powder mixture was made in the following way according to the invention: 104.49 g co- baltacetatetetrahydrate (Co (C2H3O2)2 ' 4H 2°) s dissolved in 630 ml methanol (CH3OH) . 28 ml triethanolamine ((C2H5θ)3N (0.5 mole TEA/mole Co) was added during stir¬ ring and after that 5.983 g sugar (0.5 mole C/mole Co) was added. The solution was heated to about 40°C in or¬ der to dissolve all the sugar added. Subsequently 200 g jet-milled (Ti,W)C powder was added and the temperature was increased to about 70°C. Careful stirring took place continuously during the time the methanol was evaporat- ing until the mixture had become viscous. The dough-like mixture was worked and crushed with a light pressure when it had become almost dry. The powder obtained was fired in a furnace in a porous bed about 1 cm thick in nitrogen atmosphere in a closed vessel, heating rate 10°C/min to 700°C, no holding temperature, cooling 10°C/min and finally completed with reduction in hydro¬ gen, holding temperature 800°C for 90 minutes. The powder obtained was mixed with the WC-Co powder from example 1 and pressing agent in ethanol with no ad- justment of carbon content, dried, compacted and sinter¬ ed according standard practice. A dense WC- (Ti, )C-7 % Co-cemented carbide structure was obtained with porosity A02, Fig 3. Example 3 A WC-6 % Co cemented carbide was made according to Example 1 but with a modified combined heat treatment cycle set forth below: The powder was fired in nitrogen atmosphere in a closed vessel, heating rate 10°C/min to 500°C completed with reduction in hydrogen for 180 minutes, finally fol¬ lowed by cooling in nitrogen atmosphere at 10°C/min. In contrast to Example 1, no cooling step between burning off and reduction step was used. The powder obtained was mixed with pressing agent in ethanol with no adjustment of carbon content, dried, compacted and sintered according to standard practice for WC-Co alloys . A dense cemented carbide structure was obtained with porosity A00. Example 4 A WC-6 % Co cemented carbide was made according to Example 1 but with no sugar added to the solution and a modified combined heat treatment cycle set forth below: The powder was fired in nitrogen atmosphere in a closed vessel, heating rate 10°C/min to 600°C completed with reduction in hydrogen for 180 minutes, finally fol¬ lowed by cooling in nitrogen atmosphere 10°C/min. In contrast to Example 1, no cooling step between burning off and reduction step was used. The powder obtained was mixed with pressing agent in ethanol with adjustment of carbon content according to standard practice, dried, compacted and sintered accor- ding to standard practice for WC-Co alloys. A dense ce¬ mented carbide structure was obtained with porosity A00. Example 5 A WC-6 % Co cemented carbide was made according to Example 1 but with a modified combined heat treatment cycle set forth below: The powder was fired in nitrogen/hydrogen atmosphere (75% N2/ 25%H2 ) in a closed vessel, heating rate 10°C/min to 700°C completed with reduction in the same nitrogen/hydrogen atmosphere (75% 2/ 25%H2 ) for 180 minutes, finally followed by cooling in nitrogen/hydro¬ gen (75% 2/ 25%H2 ) at 10°C/min. In contrast to Example 1, no cooling step between burning off and reduction step was used. The powder obtained was mixed with pressing agent in ethanol with no adjustment of carbon content, dried, compacted and sintered according to standard practice for WC-Co alloys. A dense cemented carbide structure was obtained with porosity A00. Example 6 A WC-6 % Co cemented carbide was made according to Example 1 but with no sugar added to the solution and a modified combined heat treatment cycle set forth below: The powder was fired in nitrogen atmosphere in a closed vessel, heating rate 10°C/min to 700°C completed with reduction in hydrogen for 180 minutes, finally fol¬ lowed by cooling in nitrogen atmosphere at 10°C/min. In contrast to Example 1, no cooling step between burning off and reduction step was used. The powder obtained was mixed with pressing agent in ethanol with adjustment of carbon content according to standard practice, dried, compacted and sintered accord- ing to standard practice for WC-Co alloys. A dense ce¬ mented carbide structure was obtained with porosity A00.

Claims: Claims 1. Method of making a hard constituent powder coated with at least on iron group metal c h a r a c t e r i s e d in comprising the following steps - dissolving and complex binding at least one salt of at least one iron group metal containing organic groups in at least one polar solvent with at least one complex former comprising functional groups in the form of OH or NR 3 , (R=H or alkyl) - adding hard constituent powder and, optionally, a soluble carbon source to the solution - evaporating the solvent - heat treating the remaining powder in inert and/or reducing atmosphere to obtain said hard constituent pow¬ der coated with said at least one iron group metal 2. Method according to the preceding claim c h a r a c t e r i s e d in that pressing agent is added together with said hard constituent powder and said optional soluble carbon source.

Inventors: EDERYD STEFAN, EKSTRAND AASA, FISCHER UDO, NYGREN MATS, WALDENSTROEM MATS, WESTIN GUNNAR, EDERYD, STEFAN, EKSTRAND, AASA, FISCHER, UDO, NYGREN, MATS, WALDENSTROEM, MATS, WESTIN, GUNNAR
Applicants: SANDVIK AB
Application year: 1995
Main classification: B22F1
IPC codes: B22F1, B22F9, C22C1, C22C29