The preparation of mechanicomposites tungsten-metal and sintering materials
T. Grigoreva1, L. Dyachkova2, A. Barinova1, S. Tsibulya3, N. Lyakhov1
1 Institute of solid state chemistry and mechanochemistry SB RAS, 18 Kutateladze str., 630004 Novosibirsk, Russia
2 Institute of powder metallurgy NAS B, Minsk, Belarus
3 Boreskov Institute of catalysis SB RAS, Novosibirsk, Russia
Tungsten-based materials are used for manufacture of electro-technical items, spot welding electrodes, spraying cathodes etc.
The preparation of the high-melting materials is power consumptive as two-stage high-temperature sintering is used: tungsten pre-sintering temperature is 1150 – 1300 C, final tungsten sintering temperature is 2900 - 3000 C [1].
Metal additives with a lower melting temperature are introduced into the high-melting material for sintering temperature reduction, and since the tungsten powder has a bad moldability level, more plastic metals, such as copper, nickel, iron are introduced for the moldability improvement.
Tungsten – copper mixture has been studied the best so far.
The mixture W-Cu sintering process research has shown [2] that the product density depends on the initial powders dispersion degree and the mixture composition. So, at the tungsten particles size 10-15 mm the maximum densification is observed at the copper weight ration 50 %. The blend density sharply decreases with the copper content decrease (less than 35 – 40 wt.%). At the same time mixtures with the copper content not higher than 10% are needed. Special methods have to be used for the preparation of the tungsten alloys.
The active densification (from 44 till 12 %) is known to take place at 1100 - 1200 C at sintering of mixtures W-20 vol. % Cu with tungsten particles size lower than 1 mm [3]. Even higher densification speed is observed in a blend, attained with copper tungsten reduction, when components mixing practically achieves a molecular level [4], i.e. the second element concentration reduction is possible at tungsten particles size decrease and homogeneous distribution of the both components. The original blends mechanical activation process [5–7] is very perspective in this trend, since grinding and formation of larger contact surface between the original components take place during mechanical activation. This process is especially effective at mechanical activation of solid and liquid metals and plastic – non-plastic metals pair. The composite nucleus (non-plastic component) – cover (plastic metal) can be created in this case. The possibility of chemical interaction on between tungsten and plastic metal the contact surface during mechanical activation should be considered here.
The work aim is to study structure and morphology of the composites, formed at mechanochemical activation of the tungsten with a small content (till 10 %) of plastic metals both interacting (nickel, iron) with it and not interacting (copper) with it. The influence of the structure and morphology of the mechanocomposites on the processes of forming and sintering was studied.
Powders of tungsten, nickel, iron, copper, were used for preparation of mechanocomposites. Mechanical activation of the mixtures was carried out in a high energy planetary ball mill with water cooling in argon atmosphere (drum volume – 250 cm3, balls diameter – 5 mm, the load – 200 g, the sample - 10 g, the velocity of rotation of the drums around a common axis ~ 1000 rpm).
X-ray analysis was carried out with diffractometer D8 Advance Bruker (Germany) at the CuKa radiation. Research of the structure and morphology of the mechanocomposites was carried out with the scanning electronic microscope (SEM) “Mira LMH” with the add-on device for micro-x-ray analysis. The electronic probe comprised 5, 2 nm, the actuation area comprised 100 nm. The research was carried out in modes of registration of absorbed (AE) and backscattered (BSE) electrons and also of characteristic radiation of tungsten, copper, nickel and iron. The sintered materials research is carried out with the metallographic microscope MEF-3 (Austria) at zoom ×200 and ×950.
The compressibility was determined via density in compliance with the ISO 3927-1985 of cylindrical samples with diameter 10 mm, height 12 mm, pressed in a steel die-mold at pressure 200, 400, 600 and 800 MPa. The pressed samples were sintered in vacuum at temperature of 1100 – 1450 C.
Compression strength of mechanically activated blends was determined via the samples of diameter 10 mm, height 12 mm, transverse strength – via prismatic samples with height 5 mm, width 10 mm, length 55 mm. The tests were preformed on the testing machine “Instron” with the loading speed 2 mm/min.
Sintered samples microstructure was studied on metallographic sections, etched with solution (10 g K3Fe(CN)6, 10 g KOH, 100 ml H2O), via metallographic microscope MEF-3 of the company “Reihert” (Austria).
Mechanical activation was carried out in two stages for attaining mechanical composites tungsten – metal (Cu, Ni, Fe). The first stage saw grinding only tungsten for 4 min. At the second stage 7 – 10 % copper (nickel, iron) was added and joint mechanical activation was carried out for 1 – 2 min.
In compliance with the x-ray data, the initial tungsten sample is a well-crystallised powder (Fig 1a). The intensity of the diffraction peaks shows the texture (of the preferred orientation) presence in trend 110. The X-ray pattern of the tungsten samples, activated during 4 min (Fig. 1b) has widened peaks. The X- ray analysis shows that widening is mostly caused because of micro-defects in the tungsten structure (at the large particles sizes retaining). It should be also noted that the distribution intensity of the peaks shows the texture absence (the equal particles distribution in powder from the point of view of their crystallographic orientation).
a / bFig. 1. X-Ray patterns for initial W (a) and activated for 4 min (b)
During the mechanical activation in a high energy planetary ball mills plastic metals tend to stick to balls and the drums walls even at short-time activation, because of that they were introduced to the blends into the already activated for 4 minutes tungsten and the mixture was treated for 2 minutes more.
The different X-Ray patterns were received for the samples with Cu, Ni, Fe additives (Fig. 2). The second metal phase is seen to be present in a well-crystallised form besides the phase W in all cases; the copper picks relative intensity is however considerably higher than the nickel picks intensity that in turn exceeds the iron reflection intensity. Formation of intermetallic compounds in the X-ray-amorphous state on contact surface W/Ni, W/Fe can be supposed to be possible for chemically interacting metal pairs (tungsten – nickel, tungsten – iron). X-Ray research data are indirect confirmation of this supposition. These data have shown that mechanochemical efforts don’t allow to receive homogeneous distribution of copper in the tungsten matrix. Mechanocomposites W + 10 % Cu is arranged in compliance with the “sandwich” principle where copper phase of micrometric size is located in the tungsten die (Fig. 3).
The second metal phase is seen to be present in a well-crystallised form besides the phase W in all cases; the copper picks relative intensity is however considerably higher than the nickel picks intensity that in turn exceeds the iron reflection intensity. Formation of intermetallic compounds in the X-ray-amorphous state on contact surface W/Ni, W/Fe can be supposed to be possible for chemically interacting metal pairs (tungsten – nickel, tungsten – iron). X-Ray research data are indirect confirmation of this supposition. These data have shown that mechanochemical efforts don’t allow to receive homogeneous distribution of copper in the tungsten matrix. Mechanocomposites W + 10 % Cu is arranged in compliance with the “sandwich” principle where copper phase of micrometric size is located in the tungsten die (Fig. 3). Electron microscopy and X-Ray research of mechanocomposites for interacting metals (W + 10 % Ni) has shown homogenous nickel distribution.
а /
b
c
Fig. 2. X-Ray patterns for mechanocomposites W (4 min) + additives Cu (a), Ni (b), Fe (c) (2 min)
The received result allows to suggest that metals distribution homogeneity depends on the thermodynamical parameters of their mixture (DНmix(W-Ni) = - 2 kJ/mol, DНmix(W-Cu) = + 10 kJ/mol [8]) and on a possibility of the chemical interaction between them. The thin layers of intermetallic compounds form on the continuously renewing contact surface in the systems W-Ni and W-Fe for this time period (1-2 min) and because of distance these thin layers do not manage to form a crystalline phase that could be fixed in X-Ray way.
а / bFig. 3. Micrographs of the mechanocomposites W-Cu (a), W-Ni (b) in characteristic radiation Cu and Ni
The research of compressibility of various mechanocomposites has shown that non-interaction metals (W-Cu) couldn’t compressed and the compressibility of the interaction metals (W-Ni, W-Fe) depends of the contents of additives. Research of compressibility of mechanically activated powders of various composition has shown that tungsten – 10% iron mixture powder has the best compressibility level, and tungsten – 7% nickel mixture powder has the least compressibility level (Fig 4).
But it should be noted that mechanically activated powders compressibility level is not high; moreover, some mechanocomposites do not have compressibility at specific pressure 200 – 300 MPa, and the samples layering is observed at pressure higher than 600 MPa. The relative density of the pressed samples is 50 – 78%. It indicates at the necessity of the additional lubricants introduction into the mechanically activated powders for their compressibility increase.
Fig. 4. Tungsten-based mechanocomposites compressibility curve
For the powders compressibility improvement the lubricants are introduced directly into initial mixture or plated to the press-mould surface for decrease of friction between the powder and the press-mould wall and also between the powder particles. The lubricant removal temperature depends on the lubricant melting or dissociation temperature. The melting and boiling temperature or the lubricants dissociation temperature, generally used in powder metallurgy, are given in table 1 [9].
Stearates, especially zink stearates, have the leading place. The rest lubricants have not got such a wide use, since residual remains after their removal [10].
Nowadays nylon-binding-based lubricant has been developed abroad. This nylon binder is introduced during the charge mixing process and needs warm pressing [11-14]. Such a lubricant allows attaining high (θ is no less than 95 %) density of iron-based materials.
The lubricant addition as a rule retains ~1 wt. % as higher content leads to the pressing growth, if the lubricant is present in the sintering process till the sintering temperature.
The lubricant burning-out process is carried out in the protective-reducing atmosphere in separate furnaces or in a sintering furnace (in the area, separated from the sintering area). The lubricant burning-out temperature is as a rule not high and comprises 600 – 800 C.
Table 1. Temperature of melting and dissociation of solid lubricants
Lubricant / Lubricant formula / Melting point, С / Boiling or dissociation point, СZink stearate / Zn(C18H35O2)2 / 140 / 335
Calcium stearate / Ca(C18H35O2)2 / 180 / 350
Aluminium stearate / Al(C18H35O2)2 / 120 / 360
Magnesium stearate / Mg(C18H35O2)2 / 132 / 360
Plumbum stearate / Pb(C18H35O2)2 / 116 / 360
Lithium stearate / LiC18H35O2 / 221 / 320
Stearinic acid / CH3(CH2)16CООH / 69,4 / 360
Oleinic acid / С8Н17СН=СН-(СН2)7СООН / 13 / 286
Benzol acid / С6Н5СООН / 122 / 249
Hexoic acid / СН3(СН2)4СООNН2 / -4 / 205
Paraffin / From С22Н46 till С27Н56 / 40-60 / 320-390
Molybdenum disulfide / MoS2 / 1185 / -
Tungsten disulfide / WS2 / 1250 / -
Manganous sulphide / MnS / 1655 / -
Graphite / С (crystalline) / 3500 / -
Molybdenum trioxide / MoO3 / 795 / -
During one-component materials heating till 100 – 150 C, the change of the contact character between the particles connected with water evaporation and elastic stress relief tale place. As a result, some contact areas rupture and, as a consequence, general inter-particle contact surface decrease are possible.
The elastic stress relief is ended, the further gases are removed and burning-out of the lubricants and binders, introduced to the powder, take place during heating from 150 C till the temperature, comprising 40 – 50% of the metal melting temperature. The oxide films reduction and non-metal contact replacement with a metal one take place at higher temperatures, although visible pressings density change does not take place.
This work saw lubricants introduction during mechanical composite formation. zink stearate, stearinic acid and lauric acid were used. The lubricants were introduced in amount of 0, 1; 0, 2; 0, 3; 0, 5 wt. %. During mechanical activation metal – organic acid the latter is melted (the melting temperature is lower than 70 C) and thus it wets the metal surface and flows with the formation of a larger contact surface. In case of good wettability and sufficient amount of the low-melting constituent all the solid-phase surface becomes contact, i.e. mixture nucleus (metal) – cover (organic substance) is formed [15]. The compressibility level has to be naturally higher in this case, and mechanochemical approach allows a substantial reduction of plasticizing agents’ concentration.
Research of compressibility of powders with lubricants has shown that Zink stearate has the least influence in comparison to other lubricants used (Fig. 5).
Fig. 5. The compressibility curves of the mechanocomposites W-Fe with the lubricant: 1 – zink stearate, 2 – lauric acid
The lubricant content increase leads to the mechanically activated powders compressibility improvement (Fig. 6), but at the lubricant content more than 0, 3 % the samples destruction takes place at sintering because of intensive gas release. Plasticizing agents introduction has allowed mechanical composites formation also for non-interacting metals (tungsten – copper) (Fig. 6, 7).
Fig. 6. The compressibility curve of the mechanically activated blend W-Cu with stearinic acid: 1 – 0, 1%; 2 – 0, 3%; 3 – 0, 5%.