Formation of a TiB2-reinforced copper-based composite by mechanical alloying and hot pressing

The microstructural changes during mechanical alloysing and subsequent hot pressing of Cu, Ti, and B powder mixtures were studied using scanning electron microscopy, X-ray diffraction, differential thermal analysis, and electron microprobe analysis. In particular, the changes in the Cu grain size, lattice parameter, and lattice strain of the powder mixtures and the formation of new phases (TiCu₄ during mechanical alloying and TiB₂ during hot pressing) were investigated. The mechanism of the in-situ formation of TiB₂ particles in the resultant copper composite was also studied.     

Method for the production of TiAl-based layered composite billets containing a ductile constituent

Various schemes are studied to fabricate layered composite billets consisting of a Ti-48 at % Al alloy (phase composition γ-TiAl + α₂-Ti₃Al) and plates of a ductile metallic constituent made of a titanium alloy or niobium. Hot isostatic pressing followed by hot isothermal pressing are used to fabricate high-quality defectless multilayered billets of layered composite materials intended for high-reduction hot rolling to produce sheets.     

Effect of composition and amount of nickel-molybdenum binder on the properties of a titanium carbide hard metal

Conclusions In titanium carbide hard metals of high metallic binder content, during hot pressing in graphite die sets part of the binder is forced out of their surface layers into gaps in the sets. Under these conditions, the molybdenum content of the alloys in the early stages of hot pressing appreciably falls. The optimum amount of molybdenum in the Ni-Mo binder of hot-pressed titanium carbide hard metals is 30 vol. %. In a large-sized part strength properties vary over its cross section: Maximum strength is exhibited by the central portion, and with increasing distance from it _tr decreases.Translated from Poroshkovaya Metallurgiya, No. 4(268), pp. 18–21, April, 1985.     

The effect of density anisotropy on the yielding and flow behavior of partially consolidated powder compacts

The effect of density anisotropy on the flow behavior of partially consolidated powder compacts of an alpha-two titanium aluminide alloy (Ti-24A1-11Nb) was established by conducting hot compression tests on samples made by hot die pressing or hot isostatic pressing (“hipping”). For the former consolidation method, in which anisotropic density distributions were developed, the upset tests were conducted both parallel and perpendicular to the die pressing direction. The flow stress results from these experiments, as well as those from the tests on the “hipped” material, were interpreted in terms of calculations of the stress intensification factor (φ). When the stress intensification factors were plotted as a function of areal relative density, rather than volumetric relative density, a unique dependence of φ on density was obtained. This dependence showed a smooth transition from a behavior measured by others for the early stages of densification to a behavior theoretically predicted for the latter stages of densification, in which isolated pores are closed.     

Deformation of porous materials during hot forging combined with extrusion

Conclusions A study has been made of the character of the redistribution of volumes of a porous blank during hot forging with elements of extrusion; the feasibility is shown of controlling the process with the aim of ensuring that certain volumes of material occupy the required positions. It has been confirmed by experiment that the process of hot forging of porous materials with elements of extrusion involves more than one stage. The character of pressing pressure variation in various stages has been determined. In pressing in conical dies densification of central volumes of material lags slightly behind that of peripheral regions. For specimens of the geometric parameters investigated, the width of the peripheral blank zone, which in the steady-state extrusion stages moves parallel to the compressing part of the die, is (0.15–0.20)D_c. The degree of deformation and the redistribution of volumes of material in peripheral layers are affected mainly by the temperature conditions of the pressing process and to a much smaller extent by the starting blank porosity.Translated from Poroshkovaya Metallurgiya, No. 6(282), pp. 15–19, June, 1986.     

Effect of the nickel-chromium binder on the densification kinetics of chromium carbide-titanium nitride cermets in hot pressing

The paper studies the densification kinetics in hot pressing of mixtures with equimass content (50 ∶ 50) of Cr₃C₂ and TiN without a binder at 1350, 1470, and 1800°C under 16.2, 21.6, and 35.7 MPa, a mixture of Cr₃C₂-50 wt.% TiN with 5 wt.% of the Ni-Cr binder at 1300, 1350, and 1370°C under 5 and 10 MPa, and a mixture of Cr₃C₂-50 wt.% TiN with 20 wt.% of the Ni-Cr binder at 1180, 1220, and 1270 °C under 5 and 10 MPa with isothermal holding for up to 25 min. The behavior of materials in hot pressing is analyzed using the viscous flow theory for porous body. It is shown that the compaction of an alloy without a binder and with the Ni-Cr binder in hot pressing is described by a generalized equation of volume viscous flow of a porous body. It is controlled by non-linear creep with an effective activation energy of 3.2 eV in materials without a binder and by a viscous flow in alloys with a metal binder. The structure and mechanical properties of the materials obtained are studied. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 29–37, 2007.     

Structure and Properties of B4C - SiC Composites

We have investigated how the composition, grain morphology, and method of preparing the starting mixture affect the processes that form the structure and phase composition of B₄C - SiC composites during hot pressing. We found that, depending on the composition of the initial powder mixtures, which is responsible for different mechanisms of consolidation of ceramic materials during hot pressing, the grain size of the main B₄C phase and its defect content as well as the nature of the SiC phase distribution within the material differ significantly. When B₄C - SiC composites with a low SiC content are made from initial B₄C - B₄Si - B - C powder mixtures those composites have a high cracking resistance because of their fine grain structure.__________Translated from Poroshkovaya Metallurgiya, Nos. 3–4(442), pp. 112–119, March–April, 2005.     

NiAl powder alloys: II. Compacting of NiAl powders produced by various methods

The technological properties of granulated NiAl powders produced by gas spraying of melts and NiAl powders produced by calcium hydride reduction (CHR) of mixtures of nickel and aluminum oxides are compared. The possibilities of production of compact workpieces from these powders using hydrostatic pressing, hot pressing, hot isostatic pressing, and hot extrusion are estimated. To improve compressibility, preliminary milling and/or mechanical activation of the powders are proposed. The strength properties of NiAl rods with a diameter of 20 mm extruded from a temperature of 1100°C and made from the granulated powders are slightly higher than those made from the CHR powders. At temperatures higher than 800°C the properties becomes similar. Transition point t _d.b from the ductile to brittle state of samples made from powders sprayed in nitrogen and argon is 100?C150°C higher than those made from the CHR powders. The difference in the mechanical properties is caused by the structural and chemical microheterogeneity of granules (microingots), which is inherited in the rods after hot deformation and annealing at 1200?C1400°C and is (0.67?C0.88)T _m NiAl (T _m is the melting point, K).     

Thermophysics characteristics and densification of powder metallurgy composites

Abstract

An analytical densification model describing the final stages of hot pressing and sintering has been developed and found to be consistent with empirical findings. The behaviour of composite powders for the matrices of diamond tools has been studied under hot pressing conditions. Differential scanning calorimetry was used to determine the heat capacity at constant pressure C _p of pure Co, 663Cu, and composite iron- and cobalt based powders (also containing WC, Ni and 663Cu). The relationship between C _p and composite densification has been analysed, and it has been found that optimised rare earth additions to the iron based composite powders can produce C _p characteristics close or equivalent to that of pure Co powders. This modified composite powder has been used to hot press diamond drill and saw bits that show good properties. Employing a densification regime guided by the dynamic model has been found radically to improve stability in service (bend strength, hardness, impact, ductility and porosity).     

Recrystallization of silicon nitride powders during hot pressing

Conclusions The hot pressing of comminuted silicon nitride powders enables a uniform fine-grained structure of one and the same mean grain size to be obtained irrespective of the type of starting Si₃N₄, powder. At milling times of more than 100 h no significant decrease in particle size is observed. The recrystallization of milled silicon nitride powders during hot pressing takes place chiefly in the fine fractions appearing during milling. The degree of recrystallization attained is higher with ultrafine active silicon nitride powders (PCS) than with comminuted powders.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 43–47, October, 1982.     

Nanostructure and Mechanical Properties of Bulk Al86Ni6Y6Ce2 Alloy Produced by Hot Consolidation of Amorphous Melt-Spun Flakes

Amorphous Al₈₆Ni₆Y₆Ce₂ (at. pct) flakes produced by melt spinning were consolidated using hot pressing at different conditions. The influence of pressing conditions on the crystallization behavior, thermal stability, and mechanical properties of the alloy has been studied through differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, microhardness, and compression test. The results show rapid solidification combined with the hot consolidation produce a highly dense sample (3.41 ± 0.2 g cm^?3) with the ideal interflake bonding, good thermal stability, good microhardness (381 ± 12 HV), and remarkably high strength (910 ± 7 MPa) combined with 20 pct fracture strain were obtained at T = 748 K (475 °C) and P = 1.2 GPa in a lightweight Al-based material. The high mechanical properties mainly result from structural refinement during the controlled consolidation method.     

Layered composites made from bimetallic strips produced by plasma spraying of TiAl on niobium

The production and structure of a multilayer TiAl/Nb composite material made from bimetallic TiAl/Nb strips fabricated by plasma spraying of TiAl granules onto niobium plates are studied. Here, 3-mm-and 2-mm-thick plates of a layered composite material (LCM) are produced by hot isostatic pressing of a stack of 35 bimetallic plates followed by hot rolling (the total degree of reduction is 78.6 and 85.7%, respectively). The LCM consists of discontinuous TiAl layers separated by niobium layers, and the adhesion between the layers is good. Diffusional intermediate layers form at the TiAl/Nb interfaces in the 3-mm-thick LCM and consist of the following two solid solutions: an α₂-Ti₃Al-based solid solution contains up to 28 at % Nb, and a niobiumbased solid solution contains up to 27 at % Ti and 32 at % Al. The diffusional intermediate layers in the 2-mmthick LCM plates consist of an α₂-Ti₃Al-based solid solution with up to 16.0 at % Nb; a τ-Ti₃Al₂Nb-or Ti₄Al₃Nb-based solid solution with 51.5 at % Ti, 32 at % Al, and 16.5 at % Nb; and a niobium-based solid solution with up to 22 at % Ti and 30.5 at % Al. When a bimetallic TiAl/Nb strip is fabricated by plasma spraying of granules of the Ti-48 at % Al alloy, this alloy is depleted of aluminum to 42–45 at %, and the fraction of the α₂-Ti₃Al phase in the sprayed layer increases. When the LCM is produced by hot isostatic pressing followed by hot rolling, the layer of plain niobium (Nb1) dissolves up to 5 at % Ti and 7 at % Al.     

An X-ray structural investigation of the reactions of yttria with chromium and tungsten

Conclusions During the hot pressing of Y₂O₃-Cr cermets in an argon atmosphere the chromium experience partial oxidation, as a result of which yttrium chromite is formed at the yttrium/chromium interfaces. In Y₂O₃-Cr cermets being being pressed hot in a carbon-containing atmosphere (CO + CO₂) the chromium completely oxidizes, with the formation of yttrium chromite. In Y₂O₃-W cermets produced by hot pressing in argon and carbon-containing atmosphere at temperatures right up to 2100°C no new phases were detected.Translated from Poroshkovaya Metallurgiya, No. 11(203), pp. 75–77, November, 1979.     

Deformation behavior of Zr3Al-Nb alloys I: Room-temperature and high-temperature deformation study

The deformation behavior of the Zr₃Al-Nb alloys was studied by measuring hardness at different temperatures, by hot rolling, and by hot pressing. Using the hardness data, elasticity and plasticity parameters were estimated and were used to determine the suitable temperature range of deformation for these alloys. Hot rolling and hot pressing were applied to determine the optimum temperatures and annealing time for carrying out deformation successfully in these alloys. Microstrural investigation of the hotdeformed samples revealed that the matrix β phase has undergone substantial deformation and the second intermetallic phase, Zr₂(Al,Nb), underwent dissolution and re-precipitation. The hardness of the fully annealed Zr₃Al-Nb alloys showed two types of temperature dependence. Transition temperatures for the change in behavior, intrinsic hardness, and softening coefficient were determined. The hardness of the fully annealed alloys rolled to different extent at various temperatures and subsequently heat treated was used to study the recovery process. The microstructural study of room-temperature deformed binary Zr₃Al alloy showed the splitting of the superlattice dislocations into partials containing superlattice intrinsic stacking faults.     

The structure and properties of aluminum alloys produced by mechanical alloying: Powder processing and resultant powder structures

Mechanical alloying of two aluminum alloy powders to form composite A1-A1₂O₃ powders has been studied. Changes in powder microstructure with processing are reported and interpreted. Mechanical alloying proceeds by the continual cold welding and fracturing of the constituent powder particles when subjected to the large compressive forces of a high speed mill. A suitable organic surfactant must be added so that a balance between cold welding and fracturing is obtained. The organic surfactant is embedded and finely distributed in the powder particles during mechanical alloying and is converted to discrete A1₄C₃ particles after hot pressing. The establishment of steady state processing conditions, characterized by equiaxed powder particles, a constant particle size distribution and a saturation hardness, is found to depend on the size distribution of the initial powders. The oxide particles formed and distributed during mechanical alloying are equiaxed, small (30 nm) and homogeneously distributed with a volumetric center to center distance of about 60 nm. Formerly Graduate Research Assistant in the Department of Materials Science and Engineering at Stanford.     

Manufacture of NiAl-based rods for plasma centrifugal spraying using mechanochemical synthesis

An alternative technology is proposed for the production of NiAl–Co–Cr–Hf–Al₂O₃ alloy rods. It includes the fabrication of a powder by mechanochemical synthesis (MCS) followed by hot isostatic pressing in forming tool. The processes of MCS of the intermetallic alloy in a planetary mill and an attritor are studied. The products of synthesis in various mixers are compared. The microstructure and the properties of compacted samples are studied: their ultimate compressive strength is 1390–1480 MPa at a plasticity of 8.5–8.8%. Spherical granules with a target size of 20–200 μm are fabricated by plasma centrifugal spraying of the rod workpiece formed by the proposed technology.     

THE DISPLACEMENT OF GAS FROM POWDERS DURING COMPACTION

Abstract

A soap-bubble method was used to observe in detail the flow of gas out of powder masses during and immediately after compaction. The effects of powder material and size distribution, die size, pressing speed, and degree of compaction were investigated. The amount of gas trapped in the compact at completion of pressing varied from 12 to 83% of the initial amount of gas present in the spaces between the powder particles, over the range of compaction conditions studied. As expected, the amount trapped increased with increase in die size, with increase in pressing speed, and with decrease in particle size. Very little gas escaped during the later stages of compaction, even at slow pressing speeds. The effect of a small punch/die clearance was examined in typical cases, and shown to be minor, except with a coarse powder pressed at a high speed.

That trapped gas can produce cracks was demonstrated by making compacts containing varying amounts of trapped gas, other conditions remaining constant. With the particular powder used, once the amount of trapped gas had passed a certain level, the compacts tended to be fairly badly cracked. It appears, however, that cracks due to gas pressure alone tend to occur only when powders are rapidly compacted to very high densities.     

Structural and Magnetic Characterization of High Performance Die-Upset Forged Nd-Fe-B Magnets

The processing method of anisotropic Nd-Fe-B magnets, based on rapid solidification of a molten alloy, followed by hot pressing and die-upset forging is currently well established and commonly used for the processing of high performance magnets. In this method uniaxial stress, created in an isotropic, polycrystalline alloy, leads to texture formation with the crystallographic c axis, of the tetragonal structure in each grain, being parallel, to the deformation direction (DD). However, the mechanisms leading to the anisotropic structure and accompanying processes are still not fully understood. In this study the Fe_73.7Nd_13.6Co_6.6Ga_0.6B_5.5 alloy, after rapid solidification by melt spinning and hot pressing was subjected to die-upset forging with strains 30% and 65%, respectively. Systematic studies of the structure, microstructure and magnetic domain structure versus processing parameters were performed for this alloy, using magnetic measurements, transmission electron microscopy and atomic force microscopy.     

HOT PRESSING HIGH-TEMPERATURE COMPOUNDS

Abstract

The technique of hot pressing and the factors to be considered in designing suitable equipment are discussed. Examples are quoted of three main classes of materials: (a) unbonded hard materials, (b) materials containing a small amount of a cementing phase or impurity affecting densification, and (c) materials containing an appreciable amount of a cementing phase. Current theories concerning the mechanism of densification during hot pressing are discussed.     

THE INFLUENCE OF TOOLING METHODS ON THE DENSITY DISTRIBUTION IN COMPLEX METAL-POWDER PARTS

Abstract

The theoretical movements required for the various moving tool elements during the pressing cycle are examined for several typical component sections. These theoretical movements are then compared with the actual movements of the moving tool elements in various types of tooling. The conclusion is reached that the conventional tool designs vary greatly from the ideal in many ways, permitting uncontrolled lateral powder movement during pressing and causing excessive density variation in the pressed compact.

Three major problems are apparent:

(1) The use of a solid die for pressing compacts with varying section thicknesses results in an “average” die movement that is always too great for the thicker section and too small for the thinner section.

(2) The double function of the floating punch, which takes vertical pressure on its horizontal face while its vertical face acts as a die, presents a very difficult problem. The floating punch movement, controlled by its horizontal face, is always greater than the correct movement required to give minimum density variation in the portion of the compact formed by the vertical surface of the punch.

(3) The various types of support used for floating punches rarely provide the correct timing, sufficient maximum load, or progressively increasing resistance essential to ensure equal density distribution within the powder during the pressing operation.

Suggestions for tooling, designed to solve some of these problems, are discussed.