Showing papers on "Powder metallurgy published in 1991"

Processing of metals and alloys

Abstract: Solidification Processing Rapid Solidification Surface Modification by Lasers Powder Metallurgy Mechanical Milling and Alloying Ion Implantation and Ion-Beam Mixing Thin Films: Epitaxial Deposition Metallic Multiloayers Recrystallization and Recovery Measurement and Control of Texture Electrodeposition of Metals and Alloys Solidification Processing Under Microgravity Cluster Assembly of Nanophase Materials

Melt processing of bulk high T/sub c/ superconductors and their application

Abstract: The authors report a melt-powder-melt-growth (MPMG) process which results in high J/sub c/ for bulk Y-Ba-Cu-O superconductors. The Y-Ba-Cu-O pellets or powders are melt quenched. The quenched plates are crushed into powder and mixed well. The powder is then compacted into desired shapes, remelted, and slowly cooled in a thermal gradient. When the starting composition is changed from the 1:2:3 stoichiometry toward the Y/sub 2/BaCuO/sub 5/

The processing and properties of discontinuously reinforced aluminum composites

Abstract: Discontinuously-reinforced aluminum (DRA) SiC whisker or particle-reinforced Al-alloy matrix composites produced by P/M methods have progressed toward commercial applications, supported by growing data bases and large-scale production facilities. Attention is presently given to the elastic modulus, plastic, ductile, and toughness characteristics of representative DRA formulations, as well as to the DRAs commercially available in the forms of sheets, extrusions, and optical and instrument grade structures able to supplant beryllium. 36 refs.

Synthesis of iron aluminides from elemental powders: Reaction mechanisms and densification behavior

Abstract: Pressureless sintering and hot pressing experiments were conducted on elemental powder compacts of Fe-15.8 wt pct Al and Fe-32 wt pct Al, corresponding approximately to the compositions of stoichiometric Fe3Al and FeAl, respectively. Upon heating near the melting point of aluminum, an exothermic reaction was initiated in the compacts, resulting in synthesis of the desired compounds with reaction times on the order of seconds. Thermal analysis and microstructural observations indicate the formation of a transient liquid phase during rapid exothermic compact heating. The mechanisms shown to be responsible for microstructural development include initial compound formation in the solid state, appearance of an aluminum-rich liquid at the aluminum particle sites, iron dissolution accompanied by outward spreading of the liquid, and subsequent precipitation of the iron-rich compounds. Apparent enthalpies of formation,ΔH f °(298), estimated from reaction temperature measurements were −18 and −31.8 kJ/mol for Fe3Al and FeAl, respectively. The influences of heating rate, green density, and aluminum particle size on sintered density were studied for pressureless reaction sintering in vacuum. The effects of processing variables on densification were explained as the net result of swelling during heating and subsequent shrinkage due to the transient liquid phase. Near full density Fe3Al and FeAl compounds were obtained through the application of external pressures near 70 MPa during reaction in a hot press. These alloys were partially ordered, chemically homogeneous, and exhibited an equiaxed grain structure with an average grain size below 10μm. The Fe3Al material exhibited significantly higher fracture strength and somewhat lower ductility than coarse-grained wrought material of the same composition.

Structure and properties of rapidly solidified ultrahigh strength AlZnMgCu alloys produced by spray deposition

Abstract: A high solute, ultrahigh strength 7XXX series aluminium alloy (EURA1 alloy) with solute contents close to the equilibrium solid solubility limits of the AlZnMgCu system has been produced by rapid solidification using a new process of spray deposition (Osprey process) which yields massive preforms directly from the liquid state. The same alloy was also produced following the traditional powder metallurgy (PM) route. The microstructures and resulting mechanical and corrosion properties of the extruded products in the T6 and T7X conditions were evaluated and compared. Under peak-aged conditions only the spray-deposited (SD) products exhibited an acceptable strength-ductility combination, with strengths in excess of 800 MPa and fracture elongations of 4.9%. The EURA1 SD products also exhibited the largest increase in fatigue strength compared to the commercial IM 7075-T6 alloy, while maintaining a comparable crack propagation resistance as well as similar corrosion behaviour. The maximum strength was mainly associated with a very high volume fraction of plate-like η′ precipitates (ranging up to 50 A in diameter), which were identified as having a hexagonal structure with lattice parameters a = 0.489 nm and c = 1.374 nm. Nevertheless, within the T6 microstructure, small spherical, possibly ordered, zones (5–10 A in diameter) were also observed. The EURA1 extrusions exhibited a loss in fracture toughness associated with predominantly intergranular dimpled ruptures. However, the structural and fractographic observations suggested that the properties of the EURA materials could be considerably improved and some preliminary results from the ongoing optimization work are presented.

Gear wheels rolled from powder metal blanks

Abstract: A gear wheel is formed from a pressed and sintered powder metal blank (2) by surface hardening the tooth, root and flank regions to establish densification in the range of 90 to 100 per cent to a depth of at least 380 microns

Strengthening in deformation-processed Cu-20% Fe composites

Abstract: Three Cu-20% Fe composites with different iron powder sizes were fabricated using powder metallurgy processes. The strengths of these composites after extensive deformation processing by rod swaging and wire drawing were shown to be anomalously higher than those predicted by rule of mixtures equations. However, the strengths obey a Hall-Petch type relationship with the iron filament spacings. The strengths of the Cu-20% Fe composites after equivalent deformation processing increased with decreasing initial iron powder size. Comparison of a Cu-20% Fe composite with a similar Cu-20% Nb composite showed that Cu-20% Fe was stronger after an identical degree of deformation processing. This increase in strength of a Cu-20% Fe composite over that of a Cu-20% Nb composite correlated with the greater shear modulus of iron compared to niobium using a barrier model for hardening.

Structural development during the extrusion of rapidly solidified Al-20Si-5Fe-3Cu-1Mg alloy

Abstract: An investigation concerning the changes of powder structure and microstructure during the extrusion of an important Al-Si-Fe-Cu-Mg alloy prepared from rapidly solidified powder has been carried out. The fragmentation of needle-shaped intermetallics in the alloy has been regarded as one of the main features of the process, which happens concurrently with the interparticle bonding and the shaping of the porous billets. The as-extruded microstructure is found to be mainly composed of the dynamically recovered α-Al matrix with numerous microcells, which are retained because of the inhibiting effect exerted by massive, fine second-phase particles on cell wall motion. Some recrystallized grains are also observed but their growth is effectively prevented. The refined intermetallics together with massive silicon particles and precipitates dispersed in the matrix can be expected to improve the thermal stability and high-temperature strength of the alloy to a great extent.

Rapid solidification processing of titanium alloys

Abstract: This review surveys rapid solidification (RS) processing of titanium alloys. The attributes of and requirements for RS are discussed along with techniques of RS including powder, flake, fibre, and wire production. Compaction and processing techniques are presented followed by a description of the metastable constitutional effects which can result from RS – solid solubility extension and formation of metastable crystalline, quasicrystalline, and amorphous phases. The microstructures that can develop through RS, and their potential subsequent decomposition, are discussed. Effects of additions of dispersoid forming elements, metalloids, and eutectoid formers to enhance mechanical property behaviour are described. The final section deals with the impact of RS on titanium base intermetallics. Throughout the review, the emphasis is on the correlation of microstructures with properties and it is shown where advantages are to be gained from the RS approach compared with conventional ingot metallurgy techn...

Formation of amorphous fe-b alloys by mechanical alloying

Abstract: Using a novel ball mill with controlled ball movement we have produced for the first time amorphous Fe50B50 and Fe40B60 alloys and nanocrystalline Fe80B20 and Fe66B34 alloys by mechanical alloying. The structural evolution of elemental powder mixtures is studied following milling and subsequent thermal treatment. Upon heating both amorphous and nanostructural mechanically alloyed Fe‐B alloys transform into a mixture of equilibrium phases.

Metal matrix composites made by spray forming

Abstract: Spray-formed metals have a finer structure, near zero segregation and improved mechanical properties resulting from rapid solidification, when compared with conventional materials. A further benefit of spray forming is that it gives greater freedom to add any second or third phase, liquid of solid, by entraining a stream of the added phase into the atomized spray before deposition. This paper shows that metals matrix composites made by spray forming can incorporate both of these benefits in one operation. A comparison with the equivalent materials made by powder metallurgy or by the melt mixing route is made.

Dynamic compaction of titanium aluminides by explosively generated shock waves: Experimental and materials systems

Abstract: Different approaches to compact cylinders of titanium aluminide powders by explosively generated shock waves were explored. Two basic compositions of the titanium aluminide powders produced by the rapid solidification rate (RSR) technique were used: Ti-21 wt pct Nb-14 wt pct Al and Ti-30.9 wt pct Al-14.2 wt pct Nb. A double-tube design utilizing a flyer tube was used in all experiments. Experimental parameters that were varied were initial temperature, explosive quantity, and explosive detonation velocity. The major problem encountered with shock consolidation of titanium aluminides was cracking. Titanium aluminide powders were also mechanically blended with niobium powders in one case and elemental mixtures of aluminum and titanium powders in the other case. Enhanced bonding and decreased cracking were observed in both cases. In the former case, the addition of niobium powder provided a ductile binder medium which assisted in consolidation. In the latter case, due to the additional heat generated and melting produced by the shock-induced reactions between Ti and Al, significant improvements in bonding of the titanium aluminide powders were observed.

Powder processing of intermetallic alloys and intermetallic matrix composites

Abstract: This presentation covers the use of powder metallurgy for the formation of monolithic intermetallics and intermetallic matrix composites. A notable development has been the fabrication of homogeneous high density compacts from elemental powders by reactive sintering. A variant process involving simulataneous pressurization in a hot isostatic press, termed reactive hot isostatic pressing, is applicable to those compounds that prove difficult to consolidate by pressureless reactive sintering. This paper describes the effects of various processing factors on fabrication of compounds including Ni 3 Al, NiAl, TaAl 3 , MoSi 2 and their composites. A key concern is with processing effects on microstructure, selection of compatible ceramic reinforcing phases, and whisker alignment through injection molding.

Composite strengthening in 6061 and Al-4 Mg alloys

Abstract: Metal matrix composites using prealloyed 6061 Al (containing 1% Mg) and elemental blend Al-4 Mg alloys with 10 vol% SiC particulate reinforcements were fabricated using powder metallurgy techniques. The consolidation of the powders was effected by the section rolling process recently developed at the Defence Metallurgical Research Laboratory. This process involves the successive steps of cold isostatic pressing, vacuum sintering and special canning followed by section rolling. This resulted in a high-integrity composite product. An interfacial layer containing magnesium-rich precipitates observed in both the composites is suggested to be the major reason for the low (compared to the value predicted by the rule of mixtures) modulus and strength values in these composites. This layer also appeared to promote interfacial failure at the alloy/SiC interface. The Al-4 Mg alloy, which is known to be non-heat treatable, was found to respond to precipitation hardening heat treatment in the composite. The enhanced generation of dislocations due to the presence of SiC, promoting a more homogeneous precipitation of the second phase and the possibility of an inhomogeneous distribution of magnesium (as a result of elemental blending) are suggested to be the major factors responsible for rendering the Al-4 Mg alloy amenable to the precipitation hardening heat treatment.

Process for direct shaping and optimization of the mechanical characteristics of penetrating projectiles of high-density tungsten alloy

Abstract: A process for shaping penetrating projectiles useful in the manufacture of military ammunition, comprising: preparing an alloy of tungsten, nickel, iron and copper by powder metallurgy, compacting the alloy mass into a rough shaped blank having an axis of revolution, sintering the rough shaped blanks thereby producing a blank having a density of at least 17,000 kg/m 3 , and work-hardening the sintered blank at a temperature ranging from ambient temperature to 500° C., thereby producing a blank having a variable degree of reduction in section in a direction parallel to the axis of the blank.

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

Abstract: A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al13Cr2 intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al13Cr2 primary intermetallic particles. The solidification of these powders is discussed in terms of so...

Effect of microstructure on the mechanical properties of an Al alloy 6061-SiC particle composite

Abstract: The mechanical properties, microstructures and fractures of a powder metallurgy Al alloy 6061-15vol.% SiC particle composite produced by extrusion were investigated. In order to build a basis for comparison, the composite was rolled at a warm temperature. The rolled composite after extrusion presents a much lower dislocation density than the extruded composite after T6 heat treatment. This cannot be explained by the difference between the coefficients of thermal expansion of aluminium alloy and SiC. In addition, the composite shows a non-uniform dislocation density and precipitate distribution so that matrix strength is non-uniform. A comparison of the properties between the rolled and unrolled composites indicates that the proper distribution of the matrix strength may be important to increase the ductility and toughness of the composite.

The effect of reaction condition on composition and properties of ultrafine amorphous powders in (Fe, Co, Ni)-b systems prepared by chemical reduction

Abstract: Amorphous ultrafine powders in (Fe, Co, Ni)-B binary systems were prepared in different reduction conditions of metal ions in an aqueous solution by use of KBH4, with the aim of clarifying the effect of reaction conditions on the composition, thermal stability, and magnetic properties of the resultant amorphous powders. As the mol ratio of KBH4 to metal ions decreases, the structure of the ultrafine powders changes from amorphous to crystalline phase. The morphology of these powders is in a nearly spherical shape with a particle size of about 20 nm for the amorphous phase and changes to the chain-like or net-like shape for the crystalline phase. The B content in the Fe-B amorphous powder decreases with a decrease of the ratio of KBH4 to metal ions, and the powder size decreases with an increase of the reduction temperature.

Optimized double press-double sinter powder metallurgy method

Abstract: Methods for preparing sintered components from iron-containing and alloy steel powder are provided. The methods includes compacting a powder mixture in a die set at a pressure of at least about 25 tsi to produce a green compact which is then presintered at a temperature of about 1100°-1600° F. (593°-870° C.) for at least about 5 minutes to produce a presintered preform. The presintered preform is then compacted at a pressure of at least about 25 tsi to produce a double-pressed presintered preform, which is, in turn, sintered at a temperature of at least about 1000° C. for at least about 5 minutes to produce a sintered component having improved transverse rupture strength and a higher density.

Powder metal process for producing multiphase NI-AL-TI intermetallic alloys

Abstract: A powder metallurgy process for producing near-net shape, near-theoretical density structures of multiphase nickel, aluminum and/or titanium intermetallic alloys is provided by employing pressureless sintering techniques. The process consists of blending a brittle aluminide master alloy powder with ductile nickel powder, so as to achieve the desired composition. Then, after cold compaction of the powdered mixture, the compact is liquid phase sintered. The four step liquid phase sintering process is intended to ensure maximum degassing, eliminate surface nickel oxide, homogenize the alloy, and complete densification of the alloy by liquid phase sintering.

Metastable phases in mechanically alloyed AlMn powder mixtures

Abstract: Elemental aluminum and manganese powder mixtures have been mechanically alloyed in a Spex mill. It has been shown that supersaturated solid solutions of aluminum containing as much as 18.5 at.% Mn can be produced in the as-milled condition. Room temperature aging produced a metastable f.c.c. phase with a = 0.4772 nm . The equilibrium Al 6 Mn phase formed by decomposition of the supersaturated solid solution on elevated temperature exposure of the powder at temperatures higher than 623 K. The activation energy for decomposition has been calculated to be 2.04 eV, the same as the activation energy for the diffusion of manganese atoms in an aluminum matrix.

Sintering Behavior of

Abstract: The pressureless sintering of ultra fine SiC powder (average particle size: 0·01-0·12!l1n) obtained by vapor phase reaction method was investigated with an emphasis on the effects of powder properties and sintering aids. The ultra fine SiC powder showed a remarkable densification in the presence of both carbon and boron as sintering aids, but the sintering aids were indispensable to densification. The homoge­ neous addition of sintering aids was important to enhance the effects thereof. The deposition of an adequate amount of free carbon and boron from the vapor phase was available on the preparation of SiC powder, and the resulting SiC powder showed very high sinterability due to the widespread dispersion of sintering aids on the surface of SiC particles. When the atomic ratio (C-O)/ Si was near 1·0, SiC powder had high sinterability because free carbon accelerated SiC sintering by removal of the surface oxide layer of SiC particles while excess carbon retarded sintering .• The effect of the particle size of the starting powder was significant: a particle size below 0'05!l1n was required for densification above 95% of theoretical by 2050°C sintering. The binary mixing of SiC powders with different particle sizes improved sinterability, being especially effective in promoting the densification of coarse powders. In conclusion, the conditions for SiC powder with good sinterability are the reduction of particle size, the control of chemical composition, and the homogeneous addition of sintering aids.

Correlation of shock initiated and thermally initiated chemical reactions in a 1:1 atomic ratio nickel‐silicon mixture

Abstract: Shock initiated chemical reaction experiments have been performed on a 1:1 atomic ratio mixture of 20- to 45-µm nickel and –325 mesh crystalline silicon powders. It has been observed that no detectable or only minor surface reactions occur between the constituents until a thermal energy threshold is reached, above which the reaction goes to completion. The experiments show the energy difference between virtually no and full reaction is on the order of 5 percent. Differential scanning calorimetery (DSC) of statically pressed powders shows an exothermic reaction beginning at a temperature which decreases with decreasing porosity. Powder, shock compressed to just below the threshold energy, starts to react in the DSC at 621 °C while powder statically pressed to 23% porosity starts to react at about 30 °C higher. Tap density powder starts to react at 891 °C. The DSC reaction initiation temperature of the shock compressed but unreacted powder corresponds to a thermal energy in the powder of 382 J/g which agrees well with the thermal energy produced by a shock wave with the threshold energy (between 384 and 396 J/g). (Thermal energies referenced to 20 °C.) A sharp energy threshold and a direct correlation with DSC results indicates that the mean thermal energy determines whether or not the reaction will propagate in the elemental Ni+Si powder mixture rather than local, particle level conditions. From this it may be concluded that the reaction occurs on a time scale greater than the time constant for thermal diffusion into the particle interiors.

Magnetic materials and process for producing the same

Abstract: This invention relates to a process for producing a rare earth-containing powder comprising crushing a rare earth-containing alloy in water, drying the crushed alloy material at a temperature below the phase transformation temperature of the material, and treating the crushed alloy material with a passivating gas at a temperature from the ambient temperature to a temperature below the phase transformation temperature of the material. Rare earth-containing alloys suitable for use in producing magnets utilizing the powder metallurgy technique, such as Nd-Fe-B and Sm-Co alloys, can be used. The passivating gas can be nitrogen, carbon dioxide or a combination of nitrogen and carbon dioxide. If nitrogen is used as the passivating gas, the resultant powder has a nitrogen surface concentration of from about 0.4 to about 26.8 atomic percent. Moreover, if carbon dioxide is used as the passivating gas, the resultant powder has a carbon surface concentration of from about 0.02 to about 15 atomic percent. The present invention further relates to the production of a permanent magnet comprising the above steps for producing the rare earth-containing powder, and then compacting the powder, sintering the compacted material at a temperature of from about 900° C. to about 1200° C., and heat treating the sintered material at a temperature of from about 200° C. to about 1050° C. An improved permanent magnet in accordance with the present invention includes the type of permanent magnet comprised of, in atomic percent of the overall composition, from about 12% to about 24% of at least one rare earth element selected from the group consisting of neodymium, praseodymium, lanthanum, cerium, terbium, dysprosium, holmium, erbium, europium, samarium, gadolinium, promethium, thulium, ytterbium, lutetium, yttrium, and scandium, from about 2% to about 28% boron and at least 52% iron, wherein the improvement comprises a nitrogen surface concentration of from about 0.4 to about 26.8 atomic percent. The improved permanent magnet can also have a carbon surface concentration of from about 0.02 to about 15 atomic percent if carbon dioxide is used as a passivating gas.