DISPERSION-STRENGTHENED ALUMINIUM PRODUCTS MANUFACTURED BY POWDER BLENDING

Abstract

The microstructure of dispersion-strengthened aluminium products manufactured by powder blending has been examined by optical and transmission electron microscopy, and the mechanical properties have been determined at room temperature and at elevated temperatures by tensile- and creep-testing.

Powder variables, such as the size of the aluminium-powder particles and the size, volume concentration, and type of oxide (Al₂O₃, SiO₂, ZrO₂) used as the dispersed phase, have been investigated, together with manufacturing variables, such as temperature of the extrusion billet, reduction ratio in extrusion, and heat-treatment after extrusion. Major variables are the size of the aluminium particles and the oxide concentration, and generally it has been found that the strength increases and the elongation decreases for decreasing size of aluminium particles and increasing oxide concentration. The elongation measured after extended creep-testing is, however, practically the same for all products, of the order of 1-3%.

A subgrain structure is formed in the aluminium matrix during manufacturing. Subgrain-boundary-strengthening, which is effective at room temperature, is superimposed on oxide-strengthening; at elevated temperatures, oxide-strengthening only is of importance.

A model has been proposed that relates the flow stress (0.2% offset) to the size of aluminium particles and the oxide concentration, and good agreement with the experimental data has been found.     

STRUCTURAL AND MECHANICAL PROPERTIES OF SINTERED MAGNESIUM PRODUCTS AT ELEVATED TEMPERATURES

Abstract

The use of products sintered from pure magnesium or a magnesium alloy is envisaged for certain structural components in the field of nuclear technology. The mechanical properties at elevated temperatures (up to 525°C) of sintered compacts of oxidized Mg-Zr or Mg-MgO of various particle-size ranges (～20–800 μm) are discussed. Comparison of the tensile properties and of the values of the creep parameters in tension and compression emphasizes the direct dependence of the increase in mechanical strength at elevated temperatures on the content and distribution of the oxide and on the particle-size distribution of the oxidized powders. Metallographic examination of the various materials confirms these observations. The ductility varies with the size of the original particles.

Finally, data are presented for comparison on a material obtained by sintering magnesium-zirconium and magnesium-aluminium cuttings. In this case hardening occurs as a result of the formation of intermetallic compounds between the zirconium and the aluminium.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

THE CONTROL OF GRAIN SIZE IN THE MANUFACTURE OF SINTERED HARD-METAL

Abstract

In sintered hard-metals grain-size control is of exceptional importance because the basic properties of wear-resistance and strength are critically dependent upon it. In some applications a change in average grain size from 5·0 to 1·0 μm can increase wear-resistance by as much as twenty times, while the same change can halve the transverse rupture strength.

Each step in manufacture, from the chemical processing of the ore to the final sintering operation, can influence the final grain size. The factors operating at each stage are not yet completely understood. However, the most important processes of tungsten reduction and carburizing have been closely studied and these are chosen to control and monitor grain size during manufacture. The factors influencing grain size during these operations are discussed in some detail. The difficulty of relating powder-particle size and sintered-product grain size is mentioned. This is largely due to the weaknesses of conventional methods of measurement. Size changes in reduction, carburizing, and milling have been examined by metallographic means, and this has shown clearly the effect of aggregation and of polycrystalline particles in confusing the relationship between powder-particle size and sintered grain size.

The metallographic method of mounting products in copper has been especially informative with products with a grain size of > 1·5 μm, but optical limitations make the results less clear with a finer size. However, inferences drawn from this work have been largely confirmed by the scanning micrograph.     

Comparison of mechanical properties of Fe-1.75Ni-0.5Mo-1.5Cu-0.4C steels made from PIM and press and sinter processes

Abstract

Powder injection moulding (PIM) is a relatively new process and only a few alloy standards have been recognised. To further promote the application of this technology, new alloys with competitive mechanical properties need to be developed. Fe-1.75Ni-0.5Mo1.5Cu-xC is one of the compositions widely used in the conventional powder metallurgy industry in making press and sinter parts. To benefit from the excellent mechanical properties and the accumulated knowledge of this alloy system, the same composition was employed in this study to make powder injection moulded compacts with the expectation that evenbetter mechanical properties would be attained. The results obtained on the compacts sintered at 1200°C for 1h showed a tensile strength, hardness, and elongation of 685MPa, 91 HRB, and 7 5% respectively. With heat treatment, the tensile strength and hardness increased to 1530MPa and 52 HRC, respectively. However, the elongation decreased to less than 1 0%. These properties are better than those of the press and sinter counterparts owing to higher sintered density, finer grain size, and more homogeneous microstructure.     

The Effect of Prior Beta Grain Size on the Ductility of Zirconium-2.5 Weight Per Cent Niobium Alloy

Abstract

Data was collected establishing a time-temperature-beta grain size relationship for the alloy zirconium-2.5 weight per cent niobium. The effect of multiple heat treatments on the beta grain size was also investigated to some extent. Tests were done to determine the effect of the prior beta grain size on the room temperature tensile properties of the alloy. It was found that the ultimate tensile strength and the offset yield strength were not significantly changed with increasing prior beta grain size. There was, however, a change in ductility; over the mean grain diameter range of 0.002 inch to 0.10 inch, the elongation decreased from 20 per cent to 8 per cent and the reduction in area from 63 per cent to 12 per cent.     

Influence of die wall lubrication on tensile properties of high temperature sintered and sinterhardened low alloy steel

ABSTRACT

The influence of die wall lubrication on the green strength and the tensile properties of a 1.5%Mo and 0.5%C steel, both as sintered and sinterhardened at 1250°C was investigated.

Samples compacted using die wall lubrication had green strengths more than 100% higher than samples compacted at the same pressure using bulk lubrication, and a sintered density up to 98% of the pore-free ones may be achieved, compared to a maximum of 95% with bulk lubrication.

Pore morphology and the matrix microstructure and microhardness were not affected.

The tensile properties of both sintered and sinterhardened materials were much better for die wall lubrication than for bulk lubrication. Tensile strength increased up to 20%, tensile elongation up to 60%. The effectiveness of the system that delivers the lubricant on the die wall surface every stroke was verified successfully in the production of tensile specimens that do not have an axisymmetric geometry.     

TENSILE EVALUATION OF UNWORKED TUNGSTEN-25% RHENIUM ALLOYS CONSOLIDATED BY SINTERING ELEMENTAL POWDERS

Abstract

The use of refractory metals in the as-sintered condition has been precluded by the belief that useful engineering properties cannot be obtained in the unwrought alloys.

A comparison of tensile data for commercial wrought tungsten-25% rhenium rod in the recrystallized condition and unwrought as-sinteredrod directly converted from elemental powders is presented, to show that at comparable grain size there need be no marked difference in properties. After exposure to very high temperatures the sintered material is markedly superior to the wrought recrystallized alloy as a result of the differences in grain-growth characteristics.     

THE MECHANICAL PROPERTIES OF HOT-RECOMPACTED IRON-NICKEL SINTERED ALLOYS

Abstract

Powder-metallurgy components which are to withstand high dynamic stress are frequently required to possess both high strength and great toughness. This combination of properties can best be achieved by increasing the density of the sintered component and one method of doing so is bot pressing.

This paper deals with the mechanical properties of sintered iron–nickel alloys produced by hot compacting in six stages, as follows:

(1) Preparation of the powder mix.

(2) Production of compacts under a pressure of 8 Mp/cm²

(3) Heating the compacts to 1000°C (1275 K).

(4) Re-pressing the hot compacts in a die heated to 300°C (575 K).

(5) Cooling in air.

(6) Sintering at optimum temperature and time under optimum furnace conditions.

The investigation covered the dependence of tensile strength, elongation at fracture, and Brinell hardness of alloys with Ni contents of 1–10% on the sintering temperature and time, on the furnace conditions, and on raw-material variables.

It was found that Fe–Ni powder-metallurgy parts with a maximum tensile strength of ～60 kp/cm² could be produced. The Brinell hardness reached 190 kp/mm² with 10% Ni content. Elongation at fracture was in the region of 45% with 1% Ni and remained comparatively satisfactory even with high Ni contents if very pure raw materials were used. Powder-metallurgy materials with a tensile strength of 60 kp/cm² and an elongation at fracture of 17% can be obtained by the process.     

Sintering parameters and mechanical properties of injection moulded aluminium powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded aluminium powder. Gas atomised aluminium powder was injection moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for feedstock. Binder debinding was performed in solvent and thermal method. After debinding, the samples were sintered at different temperatures and times in high purity N₂. Metallographic studies were conducted to determine the extent of densification and the corresponding microstructural changes. The results show that gas atomised aluminium powder could be sintered to a maximum 96·2% of theoretical density. Maximum density, tensile strength and hardness were obtained when sintered at 650°C for 60 min.     

On development of press and sinter Al–Ni–Mg powder metallurgy alloys

Abstract

The objective of this research was to initiate the development of powder metallurgy alloys based on the Al–Ni–Mg system. In doing so, binary (Al–Mg) and ternary (Al–Ni–Mg) blends were prepared, compacted and sintered using elemental and master alloy feedstock powders. Research began with fundamental studies on the sintering response of the base aluminium powder with additions of magnesium. This element proved essential to the development of a well sintered microstructure while promoting the formation of a small nodular phase that appeared to be AlN. In Al–Ni–Mg systems a well sintered structure comprised of α aluminium plus NiAl₃ was produced at the higher sintering temperatures investigated. Of these ternary alloys studied, Al–15Ni–1Mg exhibited mechanical properties that were comparable with existing commercial 'press and sinter' alloys. The processing, reaction sintering and tensile properties of this alloy were also found to be reproducible in an industrial production environment.     

OPTIMIZATION OF PROPERTIES OF POWDERED IRON AND STAINLESS-STEEL PARTS BY STATISTICAL DESIGN AND COMPUTER ANALYSIS

Abstract

A statistically designed experiment was formulated to study the effect of several major powder variables on the strength properties of porous iron and stainless-steel parts. The resulting data were analysed by means of a suitable computer programme to develop individual response equations relating the chosen dependent variables with selected independent variables. Computer analysis of the data and the optimization techniques adopted led to an improvement of ～50% in the strength of sintered parts by comparison with those made by conventional processes. A certain set of powder properties and process variables resulted in a tensile strength of 170 MN/m², 11.5% elongation, and very low dimensional change in a sintered iron sample with 25% porosity. In a 316L stainless-steel part with 25% porosity, a tensile strength of 435 MN/m², 0.2% yield strength of 269 MN/m², and 12.6% elongation were reached–values far above those that can be obtained without the benefit of statistical design.     

Effect of Mg on sintering phenomenon of aluminium alloy powder particle

Abstract

The effect of Mg on the sintering phenomenon of aluminium alloy powder particles has been examined using XPS analysis of the chemical reaction at the top most surface of the particle during heating. The relative density of the sintered material increases by 9% according to the increase of Mg content. The mechanical properties of the sintered material also increase remarkably as the Mg content in the particle increases. The ratio of the dimple patterns observed at the fractured surface after the tensile test also increases. It is considered that Mg acts to deoxidise the Al₂O₃ film that covers the particle surface as a barrier and helps sintering between the particles.     

THE PROPERTIES OF POROUS NICKEL PRODUCED BY PRESSING AND SINTERING

Abstract

The effects of compacting pressure and of sintering temperature and time on the properties of porous sintered nickel compacts have been studied, using three carbonyl and two reduced nickel powders. For all five powders, the density of the green compacts and the porosity of the sintered compacts were linearly related to the log compacting pressure. Similar relationships with pressure were observed for strength and electrical conductivity.

Photomicrographs of sections through the sintered compacts made from the reduced nickel powders show that there are pores in two different size ranges, originating from the porosity between the original powder particles and the pores within the particles. It is concluded that sintered compacts from all five powders containing 40–50% porosity have adequate strength and conductivity for use in fuel-cell electrodes.     

Statistical experimental design of Al–Cu–Mg–Si P/M alloys

The principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties required of them in service has not been widely applied to pressed and sintered P/M aluminium alloys. Most commercial alloys made from mixed elemental blends are identical to standard wrought alloys. Here we use a statistical design of experiment approach to re-evaluate the composition spectrum of the 2xxx alloys. Copper has the major influence on the tensile properties. Magnesium and silicon do effect some properties, but to a lesser extent. The major value of magnesium is breaking up the surface oxide layer. Conversely, an extensive secondary pore network develops from excess magnesium and this severely limits the ductility. Hard intermetallic particles, which are a residue of the sintering liquid, also reduce the tensile ductility. The tensile strength is limited by the low ductility, which is related to the equilibrium liquid volume at the sintering temperature, as well as the sintered density.     

TITANIUM—ALUMINIUM—MANGANESE ALLOYS PREPARED BY POWDER-METALLURGICAL TECHNIQUES

Abstract

The preparation of titanium hydride powder and the method of adding aluminium and manganese are described. Sintered materials had poor ductility and this is attributed to the presence of an acicular structure. The tensile strength and ductility of sintered and forged alloys, in which the acicular structure is destroyed, were as good as, and in some cases superior to, those of the equivalent conventional cast and wrought alloy. Both the sintered and the sintered and forged material exhibited good creep properties at 200 and 400°C. Attempts to destroy the acicular structure of the sintered alloy by heat-treatment were unsuccessful.     

Influence of Lubricants on Dimensional Changes and Mechanical Properties of Sintered Ferrous Compacts

Abstract

The influence of admixed zinc stéarate on the shrinkage of uniaxially pressed iron powder compacts has been studied. For pressing conditions which caused inhibition of compaction the removal of the stéarate during sintering produced an increase in shrinkage parallel to the pressing axis and in direct proportion to lubricant content. Additions of stearic acid (varying particle size), zinc stearate, lithium stearate, stearamide, and Cosmic 64 wax were used to investigate the influence of lubricant on mechanical properties of green and sintered iron powder compacts. Green strength was reduced relative to unlubricated material only by lubricants whose physical and chemical properties enabled them to produce and maintain extensive interparticle films during pressing. Vapour from the rapid initial decomposition of lubricants which reduced green strength could have a deleterious physical influence on the tensile strength of dewaxed or sintered Fe compacts. Decomposing lubricants also produced undesirable chemical effects. These arose from reactions between lubricant decomposition products and the matrix or by these products interfering with reactions between matrix and sintering atmosphere.     

Effect of beryllia inclusions on the anisotropy of the physicomechanical properties of sintered beryllium

Conclusions A study was made of the anisotropy of the physicochemical properties of sintered beryllium. It was established that the anisotropy of and is attributable chiefly to the crystallographic texture of semifinished products (hot-extruded rods), and does not exceed 15–20%, whereas the anisotropy of the tensile strength (up to 50%) and elongation is mainly due to grain boundary texture and a banded distribution of beryllia particles along grain boundaries.Translated from Poroshkovaya Metallurgiya, No. 4(196), pp. 36–41, April, 1979.     

A STUDY OF THE FACTORS CONTROLLING GRAIN SIZE IN SINTERED HARD-METAL

Abstract

Previous experimental work concerning the grain growth observed during the sintering of tungsten carbide–cobalt alloys is reviewed. Particle-sizing methods suitable for the examination of hard-metal powders are described, and techniques for the evaluation of the carbide grain size in the sintered compacts are discussed.

By using a Model A Coulter Counter to examine the size distribution of the carbide grains (obtained from the milled hard-metal powders by dissolution of the cobalt with hydrochloric acid), and by counting techniques on electron photomicrographs of carbon replicas of the sintered compacts, it has been established that the increase in grain size during sintering is quantitatively related to the carbon content of the material after pre-heating. The results presented indicate that the cobalt content exerts little influence on the average grain size of the sintered structures A cobalt content >10% by weight is shown to exert a strong damping effect on the rate of comminution during milling.

The linear relationships between the specific surface area of the carbide grains in milled powders (obtained using a Perkin–Elmer Sorptometer) and the specific surface of the carbide phase in sintered compacts are given. The influence of sintering temperature and time on average grain size and contiguity in a commercial alloy is shown. Some preliminary work indicates that the morphologies of the initial carbide powders may be important factors with respect to the grain size of sintered hard-metal.     

Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract

Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densities of up to 7·44 g cm^-3 with tensile strength of 482 MPa, hardness value of 153 HV10 and 15% elongation were obtained with ball milled plus 3 wt-%Si addition. Low levels of interconnected porosities (~4%) were recorded within the temperature range 1250-1300°C suggesting the possibility of good corrosion resistance.

The sintered microstructures consisted of ferrite and austenite (duplex structure), complex silicide and eutectic phases within grains and at grain boundaries, pools of liquid (rich in Si) and some medium and small pores preventing full density to be achieved despite the liquid phase formation.