THE STRUCTURE AND PROPERTIES OF SOME INTERNALLY OXIDIZED ALLOY POWDER COMPACTS

Abstract

The advantages of internally oxidized structures produced by powder-metallurgical techniques are briefly reviewed and discussed. A metallographic survey of the structures of some internally oxidized copper alloy and nickel alloy powder compacts is presented, and the effect of pressing and sintering variables upon the density and hardness of the product is established. Hot-hardness data up to 800°C, and also some tensile data up to 620°C, are presented for certain alloys.

It is concluded that although dispersion-hardened structures can be prepared by the pressing, sintering, and internal oxidation of appropriate alloy powders, and although the methods described offer a valid comparison of the properties of the various alloys studied, optimum mechanical properties are undoubtedly developed only after the powders are hot worked. The latter treatment densifies the product, removes any porosity in the structure, and increases the stored energy in the material.     

Hot rolling of blanks from copper and nickel powders

Summary The authors investigated the hot rolling of preformed blanks from electrolytic copper and carbonyl nickel powders. The blanks were prepared by cold pressing, cold rolling, wedge pressing, and sintering loosely poured powder. The hot rolling was carried out in a special mill, in a hydrogen atmosphere. The investigation showed that at high rates of deformation, a rolling temperature of 0.8–0.9 T_melt, and a reduction of 50% the hot rolling of preformed copper and nickel blanks in one pass can yield strip with a relative density of 100%, a fine-grained structure, and mechanical properties which are not inferior to those of materials obtained by sintering powder compacts or cold-rolled strip.Translated from Poroshkovaya Metallurgiya, No. 6 (66), pp. 25–28, June, 1968.     

Sintering and tribological properties of lead-free bronze alloy for friction materials

Abstract

A lead-free bronze alloy powder with a dispersed sulphide solid lubricant phase has been produced by atomisation as a substitute for lead bronze friction materials. The powder has been processed to produce a bronze/steel bimetal strip by a sinter/roll/sinter process route, via both solid state and liquid phase sintering. Friction properties of the developed materials and a sintered bronze were compared in pin-on-disc tests; the Cu–Sn–S material shows excellent properties, equivalent to those of a lead bronze, and is being assessed for use in high pressure bushes.     

Inorganic interfacial engineering: processing of hard materials

Abstract

Inorganic interfacial engineering may be regarded as the core of powder metallurgical processing of hard materials. The present paper reviews recent results from an interdisciplinary research effort, BRIIE (the Brinell Centre for Inorganic Interfacial Engineering), a joint effort between five industrial companies, three universities, two research institutes and VINNOVA (the Swedish Agency for Innovation Systems). The research involves experimental work on the aqueous processing of powders and the use of surface actants is reviewed as well as the colloidal processing of ceramics. Pressing and sintering of agglomerated powders have been studied both theoretically and experimentally. Models for the simulation of pressing and sintering of hard metal powders are developed. Results on ceramic materials obtained by spark plasma sintering and their resistance to thermal shock are reported.     

THE ISOSTATIC PRESSING,VACUUM SINTERING,AND SWAGING OF THORIUM POWDER

Abstract

The isostatic pressing of calcium-reduced thorium powder into round bars and tubes, prior to vacuum sintering and swaging, is described. The simple equipment allows powder filling to be carried out in an enclosed system, thus avoiding fire and health hazards at this stage. A number of bars or tubes, of varying length and diameter, can be pressed simultaneously. The technique has certain advantages over conventional die compacting.

The effect of pressure on density and hardness through the bar section before and after sintering at various temperatures has been studied, and the optimum conditions determined for the manufacture of dense bars with high metal efficiencies (98%). Details are given of the microstructure and mechanical properties of thorium bars in the sintered, swaged, and annealed conditions.     

SINTERED IRON-COBALT-MOLYBDENUM ALLOYS

Abstract

The magnetic properties and certain physical properties of sintered iron-cobalt-molybdenum alloys have been investigated, and a study made of the effect of processing variables, i.e. particle size, compacting pressure, sintering temperature and time. The compacting and sintering conditions which result in optimum properties have been determined. Consideration is also given to the influence on magnetic properties of variations in heat-treatment procedure, involving such factors as quenching temperature and media, ageing time and temperature, &c.

The substitution of ferro-molybdenum powder for elemental molybdenum powder reduces the cost of raw materials substantially without greatly diminishing the magnetic properties. Commercial powders with a normal particle-size distribution give good properties.

It was found that the alloys had somewhat better qualities than those made by casting.     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

Netshaping concepts for tungsten alloys and composites

Abstract

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.     

Iron based partially pre-alloyed powders as matrix materials for diamond tools

Abstract

The hardness and bend strength of diamond tool matrix materials prepared from partially pre-alloyed Fe–Cu powders and from elemental powder mixes of equivalent composition has been compared. After sintering at 890^oC, the hardness of the partially pre-alloyed powder matrix was 81.72?HRB, 9.3% higher than that of the mixed powders matrix; sintering at 870^oC gave the highest bend strength (634.85 MPa), 11.6% higher than that of the mixed powders matrix. The improved homogeneity of alloy elements distribution achieved in the matrix by pre-alloying was shown generally to increase the hardness and strength. The cutting performance of diamond tool segments prepared with the pre-alloyed matrix material met standard requirements; overall, the pre-alloyed material is felt to offer significant manufacturing and performance benefits.     

Fatigue crack growth in some PM low alloy steels consolidated by rotary forging and sintering

Abstract

Fatigue crack propagation rates under plane strain conditions have been investigated for three PM low alloy steels consolidated to high densities by rotary compaction followed by sintering and heat treatment. It is shown that the densities and properties are intermediate between those of pressed and sintered materials and of powder forged materials. Threshold stress intensities compare satisfactorily with those for wrought counterparts, but resistances to crack growth are inferior to those of wrought steels. Possible reasons for the properties of the rotary compacted materials are considered in the light of their microstructures and the behaviour of other PM materials.     

Sintering transformations in mixtures of austenitic and ferritic stainless steel powders

Abstract

The microstructural transformations and the dimensional evolution of green specimens obtained by pressing mixtures of austenitic and ferritic stainless steel powders have been investigated by sintering at 1120 and 1240°C. Dilatometry experiments show that the linear shrinkage is influenced by the amount of ferritic powder. Moreover, during sintering Ni diffuses into the ferritic grains causing austenite destabilisation and the formation of a mixed constituent, whose constitution has been investigated by means of EDXS and interpreted on the basis of the Schaeffler diagram. Sigma phase also forms during sintering of the duplex mixtures.     

OBSERVATIONS ON THE SINTERING OF COMPACTSFROM A MIXTURE OF IRON AND COPPER POWDERS

Abstract

Three grades of iron powder-an atomized steel powder, a sponge iron powder reduced from magnetite with carbon, and a powder reduced from mill scale with hydrogen were mixed with 3% of copper powder and pressed into compacts. The diametral dimensional changes of the compacts during sintering below and above the melting point of copper were measured, their microstructures examined, and both related to the characteristics of the powders, particularly their specific surface. During sintering below the melting point of copper, compacts of all three powders shrank. Micrographic examination showed that the copper is transported by solid-state diffusion along the surfacesand grain boundaries of the iron powder particles. During sintering above the melting point of copper, compacts of the atomized and the MH-100 sponge iron powders grew while those of the hydrogen reduced mill-scale powder shrank. This phenomenon is related to the different mode of penetration of liquid copper in the compacts from the three powders, observed in the microstructures of the compacts.     

PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids

Abstract

A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640^°C for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ≥340 MPa. Near full sintered densities of , ～98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys.     

THE FABRICATION OF HIGH-SPEED TOOL STEEL BY ULTRAFINE POWDER METALLURGY

Abstract

Various powder-metallurgy techniques have been developed during recent years to avoid segregation effects associated with the conventional methods of casting and forming high-speed steels. These techniques have generally involved the consolidation of hot working or hot pressing of 50–500μm prealloyed powders into dense billets or rods.

The work described has demonstrated that much finer, 0·5–5μm, powders of M2 and M50 steels may be cold pressed and sintered to produce bodies with densities of 99% theoretical containing uniformly distributed 1–2μm particles of carbides. It is anticipated that the method will have application for the manufacture of complex-shaped parts with very small material losses and little machining.

An account is given of the preparation of the fine powders by ball-milling and their subsequent compaction, sintering, and microstructure. The control of carbon and oxygen levels by carbon addition to the powders is described.     

Wear of Sintered Metals

Abstract

A pin-on-disc technique is described which produces a graphical record of the whole duration of a wear experiment. Small changes to the experimental conditions produce large differences in wear rates which are related to oxidative and metallic wear mechanisms. This technique has been used to evaluate materials pressed and sintered under various conditions and to investigate the effect of alloying and oil impregnation. Increases in pressing pressure and sintering temperature improved the wear resistance. The addition of phosphorus in the form of a hard phosphide eutectic brought about an appreciable reduction in wear rate. The presence of lubricant impregnated into the pores of the sintered compact reduced the wear rate to a very low value until all the lubricant had been used up. A correlation is presented between wear of piston rings and cylinder liners in a diesel engine and wear of the same materials investigated on the pin-on-disc machine, on which it was possible to reproduce the wear mechanism observed in service and to study the mechanism of failure. It was also possible to advise on the optimum combination of materials for this application.     

POROUS BRONZE: THE EFFECTS OF SOME POWDER CHARACTERISTICS ON THE PRESSING AND SINTERING PROPERTIES

Abstract

The various tests usually applied to powders for the manufacture of porous bronze are listed. Stress is laid on the importance of consistency of materials. Variations in the important properties of each of the metallic and non-metallic ingredients, and their effect on pressing and sintering behaviour, are discussed. The conclusion reached is that a compromise on powder properties must be accepted in order to achieve economic production.     

Optimised sintering route for cold work tool steels

Abstract

Two different powder metallurgy cold work tool steels, D7 and a nickel alloyed D7 modification, were compacted to full density by utilisation of super solidus liquid phase sintering in vacuum and nitrogen atmospheres. The experiments were carried out with respect to the fabrication of wear resistant metal matrix composites. These materials exhibit a higher durability than cast materials and are increasingly used in the mineral and mining industry. In the present paper, sintering exhibits a more cost effective processing of these materials compared with hot isostatic pressing. The experiments were supported by thermodynamic calculations with Thermo-Calc. Significant differences of the sintering behaviour in vacuum and nitrogen atmospheres were detected. Full density could be obtained at a sintering temperature as low as 1210°C.     

Comparison of thermal and microwaveassisted plasma sintering of nickel–diamond composites

Abstract

R&;D efforts at CSIRO, Australia, into the production of ‘lower cost’ titanium powders are complemented by a strong, downstream PM programme. One of these efforts has focused on the direct powder rolling of commercially pure (cp) titanium powder with a view to the continuous production of fully dense strip. Considerable research is also being undertaken to produce titanium alloy strip, initially from the Ti–6Al–4V alloy, using this process. An experimental design approach has been employed to establish key parameters, maximise the process window and meet property specifications. Demonstration of a proof-of-system at pilot scale is well advanced and the focus is now shifting to seeking industrial engagement with a view to collaboration, technology transfer and commercialisation of the technology. The current status of the technology is surveyed including aspects of the associated market trends and commercial feasibility.     

STUDIES OF THE SINTERING AND HOMOGENIZATION OF NICKEL-COPPER COMPACTS

Abstract

Studies are described of the progress of sintering and alloying in compacts of similar compositions made from nickel-coated copper, copper-coated nickel, and mixed nickel and copper powders. Density losses observed in the early stages of sintering were lower in magnitude and were more quickly recovered in the case of the composite powder compacts. Alloying by diffusion at both 1900 and 2200°F (1040 and 1205°C) progressed most rapidly in compacts prepared from nickel-coated copper powders, and the probable reasons for this observation are discussed in detail. Electrical resistivity was used to follow homogenization of the compacts, and samples were rendered nearly 100% dense by cold working and annealing before making resistivity measurements. Resistivity / sintering-time curves for dense specimens showed no maxima of the type reported by earlier investigators for porous compacts, which were attributed to alloying effects.