METHOD OF MAKING METAL COMPOSITE MATERIALS

Abstract: According to the method of the present invention one or more metal salts of at least one of Co, Ni and/or Fe is dissolved/suspended in water. To the solution/suspension at least one hard constituent powder is added to form a slurry. The solvent is evaporated, preferably by spray drying, and the resultant powder is heat treated in 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. In addition to or instead of said salt of Co, Ni and/or Fe, a soluble salt of Mo and/or W can be used.

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 (the latter often referred to as cermets) are formed of hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder phase essentially based on Co and/or Ni and/or Fe. These alloys are made by powder metallurgical methods comprising 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 of the order of several hours up to days. The lengthy milling time is believed to be necessary in order to obtain a uniform distribu¬ tion of the binder phase in the milled mixture. It is further believed 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 for which contamination, compensation has to be made. 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 temperature 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 compo¬ nents in the latter case, the minor component can be in¬ troduced 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. Swedish patent application 9401078-2 discloses a method of applying a coating to hard constituent grains. According to the method of said 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 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. Swedish patent application 9401150-9 discloses an¬ other method of applying a coating to hard constituent grains. According to the method of that application hard constituent powders are coated with cobalt and/or nickel metal in solution by reducing the metals from a suitable salt with a polyol while keeping the powder in suspen¬ sion. The polyol functions both as a solvent and as a reducing agent at the same time and is present in an amount of at least >5 times more moles polyol than moles metal. There is obtained an even distribution of the co¬ balt and/or nickel over the surface of the hard con¬ stituent powder without the formation of islands of pure metal . It has now surprisingly been found that by spray drying a slurry of hard constituent grains in a solu¬ tion/suspension of Co, Ni, Fe, W and/or Mo-salts the hard constituent grains can be coated with layers of Co, Ni, Fe, W and/or Mo. Fig 1 shows in 5000X C-powder coated with Co ac- cording to the method of the present invention. Figs 2 - 4 show in 1200X 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 Co, Ni and/or Fe, preferably ace¬ tates, hydroxides, tungstates and olybdates, are dis¬ solved or suspended in water. At least one hard consti¬ tuent powder such as WC, (Ti, )C, (Ta,Nb)C, (Ti,Ta,Nb)C, (Ti,W) (C,N), (Ti.Mo) (C,N) , TiC, TaC, NbC, VC and Cr 3 C2 is added under vigorous stirring to a slurry. The amounts of said salts and said hard constituent powders are chosen such that the desired final composition is obtained. The slurry is dried in a spray drier and resultant powder is heat treated in reducing atmosphere, preferably H2 • In an alternative embodiment a water soluble tung¬ sten and/or molybdenum salt, preferably ammonium meta- tungstate (AMT) and/or ammoniummolybdate, is dissolved together with said salts of Co, Ni and/or Fe. In the first alternative, the slurry is dried in a spray drier followed by heat treatment at 450°C to 700°C, preferably 500°C to 550°C, in reducing atmos¬ phere, preferably H2. for a time sufficient to reduce the salts to the metals, generally for about V2 to 2, preferably for about 1, hours. In the second alternative, the heat treatment is performed at 700°C to 1000°C, preferably 750°C to 900°C, in a reducing atmosphere, preferably H2, for a time suf¬ ficient to reduce the salts to the metals, generally for about V to 2, preferably for about 1, hours followed by an additional carburization step. That carburization can be either the classical carburization by carbon black or by gas phase carburization at 800°C to 1250°C, preferab¬ ly 900°C to 1000°C, forming an extremely fine grained C and/or M02C-C0, Ni and/or Fe coating of the hard consti¬ tuent grains. In another alternative embodiment a solution con¬ taining only tungsten and/or molybdenum salts is used resulting in tungsten and/or molybdenum coated hard con- stituents. The coated powder is mixed with conventional 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 composition. The slurry then is dried, compacted and sintered in the usual way to obtain a sintered body of hard constituents in a binder phase. Example 1 A WC-6 % Co cemented carbide was made in the follow¬ ing way according to the invention: 1000 g of WC (d FSSS = 4.1 μm) was introduced in 896.2 ml of an aqueous solu- tion of cobalt acetate (71.3 g Co/1) . The suspension was vigorously stirred in order to keep a homogeneous dis¬ tribution of the WC particles in the suspension. The suspension was spray dried in a classical way and par¬ ticles of WC coated by cobalt acetate were obtained. The reduction step was then performed in a push fur¬ nace at 520°C for 1 h. A WC-Co powder was obtained which was made of partially coated WC particles plus some free cobalt particles (Fig 1) . EDX spot analysis on appar¬ ently non coated WC surfaces showed the presence of co- bait indicating that all WC particles were coated by a thin layer of cobalt. The powder obtained was then mixed with pressing agent (2 weight-% PEG) in ethanol, dried, compacted and sintered according to standard practice for WC-Co al- loys. A dense cemented carbide structure was obtained with porosity A00. Fig 2 shows the microstructure of a compacted body after sintering at 1450°C. Example 2 A WC- (Ti,W)C(80/20) -7 % Co cemented carbide was made in the following way according to the invention: 800 g of WC (d FSSS = 4.1 μm) and 200 g (Ti,W)C (d FSSS = 4.9 μm) was introduced in 1056 ml of an aqueous solution of cobalt acetate (71.3 g Co/1) . The suspension was vigorously stirred in order to keep a homogeneous distribution of the carbide particles in the suspension. The suspension was spray dried in a classical way and particles of car¬ bide coated by cobalt acetate were obtained. The reduction step was then performed in a push fur- nace at 520°C during 1 h. A WC- (Ti,W)C-Co powder was ob- tained which was made of partially coated carbide par¬ ticles plus some free cobalt particles. EDX spot analy¬ sis on apparently non coated WC and (Ti,W)C surfaces showed the presence of cobalt indicating that all car- bide particles were coated by a thin layer of cobalt. The powder obtained was then mixed with pressing agent (2 weight-% PEG) in ethanol, dried, compacted and sintered according to standard practice for WC-(Ti,W)C- Co alloys. A dense cemented carbide structure with good cobalt-gamma phase (cubic phase) distribution was ob¬ tained. Fig 3 shows the microstructure of a compacted body after sintering at 1450°C. Example 3 A WC 85.9- TiC 2.6- TaC 3.6- NbC 2.4- Co 5.5 ce¬ mented carbide was made in the following way according to the invention. A solution containing 2395 g of ammonium metatung- state (86 % of WO3) previously dissolved in 1.2 1 of water was first mixed with 350 ml of an aqueous suspen¬ sion of cobalt hydroxide containing 110 g of cobalt. Then 52 g of TiC, 72 g of TaC and 48 g of NbC were added to the suspension. The suspension was vigorously stirred in order to keep a homogeneous distribution of the car- bide particles added. The suspension was spray dried in a classical way and particles of the suitable composi¬ tions were obtained. The reduction step was then performed in a push fur¬ nace at 900°C for two hours. The W-TiC-TaC-NbC-Co powder were then mixed with the suitable amount of carbon black and carburized at 1000°C during 4 hours in a continuous push furnace. A WC-TiC- TaC-NbC-Co-powder was obtained. The powder obtained was then mixed with pressing agen (2 weight-% PEG) in ethanol, dried, compacted and sintered according to standard practice for WC-(Ti,W)C- Co alloys. A dense cemented carbide structure with good cobalt+gamma phase (cubic phase) distribution was ob¬ tained. Fig 4 shows the microstructure of a compacted body after sintering at 1520°C.

Claims: Claims 1. Method of making a hard constituent powder coated with Co, Ni and/or Fe c h a r a c t e r i s e d in com¬ prising the following steps - forming a water solution/suspension of at least one Co, Ni and/or Fe-salt - adding to said solution at least one hard con¬ stituent powder to form a slurry - drying said slurry, preferably by spray drying, to obtain a powder - heat treating said powder in reducing atmosphere to obtain said at least one hard constituent powder coated with at least one of Co, Ni and/or Fe. 2. Method according to the preceding claim c h a r a c t e r i s e d in that in addition a soluble tungsten- and/or molybdenum-salt is added to said solu¬ tion. 3. Method according to the preceding claim c h a r a c t e r i s e d in that said reduced powder is further carburized to form fine grain WC- and/or M02C- Co, Ni and/or Fe-coating on the at least one hard constituent. 4. Method according to claim 1 c h a r a c t e r i s e d in that instead of said at least one Co, Ni and/or Fe-salt at least one soluble W- or Mo-salt is used.

Inventors: CHARDON NICOLAS, EDERYD STEFAN, PASTOR HENRI, WALDENSTROEM MATS, CHARDON, NICOLAS, EDERYD, STEFAN, PASTOR, HENRI, WALDENSTROEM, MATS
Applicants: SANDVIK AB
Application year: 1996
Main classification: B22F1
IPC codes: B22F1, C22C1