El proceso del registro ampliado: origen, resultado, balance y desafíos.

The physical and chemical properties of powders have a strong influence on the characteristics of the material during

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pressing and sintering. Zapf has shown that even small

percentages of alloying constituents and non-metallic admixtures present in commercial iron powders have a deleterious influence

on compressibility. Thus reduced iron and atomised powders

were found to be less compressible than high purity electrolytic *

iron powder and consequently possessed lower strength after identical sintering treatments. However, when the above three powders were compared at identical densities, the strengths of the compacts made from electrolytic iron powder were lower than those made from the other two powders.

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Zapf has also discussed the effects of hydrogen loss, screen analysis and particle .shape and size on the pressing and sintering behaviour of commercial iron powders. He pointed out that oxidation of green compacts increased sinterability in contrast to the decreased sinterability of pre-oxidised powders. This was ascribed to the fact that oxidation of

green compacts did not alter the existing metallic bridges and, only tended to activate the remainder of the powder surface.

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Zapf , in a separate paper, also evaluated the influence of powder type on the mechanical properties of hot re comp acted iron-0-10% nickel alloys. He found that the best mechanical properties were achieved using electrolytic iron powders.

Reduced iron powder (MH300) had comparable strength but slightly lower ductilities. Although water atomised iron powder was not included in this investigation, the author believed that it would give properties similar to electrolytic iron powder.

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Krishnamoorthy studied the influence of the type of iron powder on the properties of sintered iron within the

p

density range of 6.8 - 7»87 kg/dm . He concluded that the use of carbonyl iron powder generally gave the best mechanical properties. The strength values of fully dense compacts were similar for all types of powder except the carbonyl iron. Although, the latter had the highest strength at lower

densities, it had the poorest tensile strength in the fully dense condition. It was suggested that this was due to the porous compacts ability to pick up carbon from the admixed

lubricant and thus have greater strength. The fully dense" compacts were unable to absorb carbon from admixed lubricant and therefore gave relatively poor tensile strengths. Amongst the other powders, electrolytic and high purity atomised

powders gave the best compressibilities, tensile ductilities and impact strengths. However, the high purity of these powders produced relatively poor tensile strengths at any

given density. The highest strengths were obtained with

normal purity atomised powder, although this was coupled with (93)

poor ductility. However, Krishnamoorthy also pointed out that the behaviour of the powders investigated could alter when alloyed with carbon or other alloying elements.

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Squire ’ reported that both reduced and electrolytic iron powder of similar mesh sizes exhibited similar strengths and ductilities and that there was an improvement in strength levels when finer powder was used. Cundill et al^96^

confirmed that the tensile strength of samples made from various types of powders, including atomised powder, were

essentially similar but that the ductility and impact

Electrolytic powder gave the best ductility and water

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atomised iron powder the best impact resistance. Hulthen has pointed out that the evaluation of different iron powders on the basis of compressibility can be very misleading. He examined four types of iron powder that possessed different specific surface areas and found that the powder with the lowest specific surface area had the highest compressibility although the green strengths of compacts made from this

powder were very low. The strength of sintered low alloy steel compacts were shown to increase as the specific surface area of the base iron powder increased. However, very high specific surface area was also found to be detrimental on account of large dimensional changes and low apparent density. Hulthen then went on to evaluate the different types of iron powder on the basis of relative material requirements and raw material costs and concluded that reduced sponge iron powder was the most economical choice for sintered density levels of up to 7-2 kg/dm^.

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Esper et al studied three different types of plain iron powders e.g. sponge iron, - atomised and electrolytic iron powder, in relation to their pressing and sintering behaviour. Their results showed that, in spite of some pronounced differences in the sintering behaviour of the three types of iron powders investigated, the associated

variation in the mechanical properties was of little signifi­ cance for all practical purposes.

The sintering behaviour of three types of iron powder (carbonyl electrolytic and sponge iron powder) was examined

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by Poster and Hausner , who reported that carbonyl iron powder compacts produced the highest density when sintered

at 870°C. Furthermore the density decreased as the sintering temperature was raised. The strength values of the compacts made from carbonyl iron powder were higher than those made

from the other two powders, after sintering at any temperature, although a maximum was again observed after sintering at

870°C. A major reason for the very high strengths of the compacts made from this material was the small particle size

(-325 mesh) which led to small individual closed pores even at relatively low densities. This fine powder also contributed to the small grain size, which also increased the strength

of the sintered compact.

A similar study, that involved a comparison of sintering behaviour of atomised and carbonyl powder was carried out by lenel et a l ^ 00l They also reported that the rate of sintering was much faster in the case of compacts made

from carbonyl iron powder. The relatively poor sinterability of atomised powder was the result of the particle size and the contaminated surface of the atomised powders used.

However, the cost of carbonyl and electrolytic iron powders is prohibitive for most applications despite their superior properties. Thus, for economic reasons, sponge iron and

atomised iron powders are the most widely used material in the powder metallurgy industry.

2.7*2. Mixing of Powders (101-103)

Several authors have expressed the view that improper mixing can offset the benefits of deliberate use of fine alloy powders for the purpose of rapid homogenisation

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during sintering. Dixon and Clayton pointed out that consistent results from pressing and sintering operations could only be expected when consistent uniform mixtures of

powders were used. Although a great deal of importance has

been attached to the mixing process, Marshall (1968) and

Hausner^ 05 ^ (1970) pointed out that the mixing process was not fully understood and that the present state of knowledge was insufficient to allow the prediction of the optimum

mixing conditions for any given powder.

Goetzel^106^ pointed out that mixing of metallic powders was influenced by several factors, viz., specific gravity, particle shape and size, structure and surface condition of the particles and the type of mixing equipment. Hausner ^'L05 ^ in a review of the mixing process, expressed the view that mixing was predominantly governed by friction within the

powder mass which in turn was dependent on powder characteristics such as particle shape, size and surface condition. His

preliminary investigations confirmed this view point and showed that the degree of mixing of a powder mass increased as the friction within the mass was reduced, although a

reduction in the amount of friction also facilitated segregation. These conflicting processes allowed a low-friction powder

mixture to reach a state of maximum mixing very rapidly, after which the process of segregation became dominant. This led to poorer homogeneity of mixing after the use of prolonged mixing times.

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Marshall , in a review of the production of powder metallurgy parts, referred to the various types of industrial mixers designed to give optimum mixing in as short a time as possible. He pointed out that long mixing times were not only uneconomical but that they could alter the powder

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characteristics. Dixon and Clayton recommended the use of air mixers which reduce the mixing time to less than ten minutes.

2.7-3* Compaction of Powders (108 109)

Kunin and Yurchenko * were of the opinion that the applied pressure in compaction of metal powders was the sum of two pressures, viz:-

(i) pressure needed to overcome external friction (ii) deformation and compacting pressure

The effectiveness of the applied pressure was reduced on account of the external friction; here the major loss came from the friction between powder particles and the die-wall. In normal commercial practice lubricants are used, either in admixed form or applied to the die-wall, in order to

reduce losses in applied pressures resulting from the effect

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of frictional forces. However, Donachie and Burr

have reported that the effects of lubricants on pressure- density relationship were negligible except at veaylow pressures. A similar view was expressed by Hausner and

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Sheinhartz , who reported that lubricants only contributed to a reduction in ejection pressures and had little effect on pressure-density relationships. The Hoganas iron powder

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handbook also confirmed this viewpoint and pointed out that there was an initial increase in the density followed by a sharp decrease as the lubricant content was increased. The maximum density was associated with a lubricant content

(*1*18 )

of between 1-2%. Yarnton and Davies reported that the application of a lubricant in the form of monomolecular layer over each particle brought about a 20% increase in packing density relative to unlubricated mixes. However,

any lubricant in excess of the monomolecular layer reduced the packing density. The decrease in the packing density

in the case of high lubricant contents, was attributed to

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an increase in particle size. Afanas'ev et al observed

that additions of zinc stearate decreased the compaction

pressure required to produce any given density level, although

additions of more than 0.25 weight percent of zinc stearate

did not provide any further reduction in the compaction (118 )

pressure. Beid studied the influence of various com­ mercially used lubricants on pressure density relationships

and concluded that the metal stearates were the most effective A compaction pressure of 462 N/mm was required to achieve a density of 6.6 gm/cm^ for sponge iron compacts using any of the metal stearates, whereas the use of stearic acid as lubricant required a pressure in excess of 617 N/mm , if the same density was to be achieved. Ejection pressures were very similar irrespective of the type of lubricant employed with the exception of stearic acid, which gave marginally

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lower ejection pressures. Geiger and Jamison reported that additions of zinc stearate and stearic acid reduced the sintered strength slightly but increased the compressibility relative to unlubricated compacts.

Several workers have reported that die-wall lubrication is more effective than admixed lubrication and does not produce harmful reactions during sintering nor does it

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