Microstructural changes in Mo steels during sintering and effect on electrical conductivity

Abstract

An electrical conductivity measurement method was used for studying the sintering mechanism and microstructural changes of low alloyed PM Mo steels in a temperature range between 600-1300°C. The influences of alloying method (elemental or prealloyed), Mo content (1·5 and 3·5 wt-%), and sintering temperature were investigated. The results show that the effects of, for example, formation of Mo carbide(s), ferrite-austenite phase transformation, as well as liquid phase formation during heating of the steel compacts can be detected by the technique cited. Mo dissolution during sintering of compacts from mixed powders results in a decrease of the conductivity with increasing sintering temperature while compacts from Fe-Mo prealloyed powders exhibit the standard behaviour of higher conductivity after sintering at higher temperature. Moreover, the relationship between Mo dissolution, formation of sintered contacts, and mechanical properties was demonstrated to assess the viability of the conductivity measurement method for studying the sintering behaviour of PM materials and its influence on physical and mechanical properties. An approach was also demonstrated for relating the conductivity to the microstructural parameters, e.g. total porosity and contiguity between solid phase, that would be useful for predicting relative changes in mechanical properties dependent on porosity and pore morphology.     

THE CORRELATION BETWEEN RAW MATERIALS,PREPARATION CONDITIONS,AND PROPERTIES OF SINTERED IRON

Abstract

Three plain iron powders of different types (sponge-iron, atomized and electrolytic iron powder) were studied with respect to their sintering behaviour and to the influence of manufacturing parameters—i.e., compacting pressure, sintering temperature, and sintering atmosphere—on the microstructure and the properties of sintered compacts. The changes of length, electric conductivity, and strength during sintering are explained in physical and chemical terms. Technical sintering diagrams are presented. The influence of sintering atmospheres on the mechanical properties of sintered compacts is shown for the three types of powder. The correlation between pore structure and strength is discussed; analytical relationships are developed which are in agreement with the experimental results.     

Electrical conductivity and microstructure of sintered ferrous materials: sintered iron

Abstract

The viability of electrical conductivity as a tool for describing the microstructure of sintered iron compacts was investigated, the sintering temperature being varied from dewaxing to high temperature sintering. The relationships between formation of sintered contacts, presence of lubricants, and mechanical properties were evaluated through determination of conductivity and effective load bearing cross-section A_c . The latter parameter was measured via quantitative fractography of specimens impact fractured at 77 K. The role of porosity and sintering temperature on grain growth in iron was also evaluated using quantitative metallography. It was found that the conductivity of pressed compacts increases during the dewaxing stage, while the effect of the sintering parameters at higher temperatures is less conspicuous. In any case, the conductivity can be related to the load bearing cross-section by a logarithmic equation. Using the already established relationships between A_c and the mechanical properties, the latter can be predicted by using the conductivity, which might be helpful in quality control of PM components.     

Effect of carbon content,sintering temperature,density, and cooling rate upon properties of prealloyed Fe–1·5Mo powder

Abstract

Mixtures of prealloyed Fe–1·5Mo (Astaloy Mo) with and without additions of 0·5–1·2 wt-%C were prepared and their sintering, as well as their mechanical, properties investigated under different process conditions. It was found that carbon content, sintering temperature, and cooling rate had marked effect on physical and mechanical properties. Sintered density decreased with increase in carbon content and sintering temperature. On the other hand, UTS, TRS, and hardness values improved with up to 0·8 wt-%C addition, sintering temperature, and cooling rate. Percentage elongation decreased with increase in carbon content and cooling rate, but was higher for specimens sintered at higher temperatures. The as sintered microstructures consisted of either fine or coarse pearlite, upper or lower bainite, and their mixture depending on the carbon content and cooling rate. The heat treated mechanical properties showed some improvement for the specimens containing 0·5 and 0·8 wt-%C. It became evident that a variety of ternary low alloy steels consisting of Fe + 1·5Mo + 0·5–0·8 wt-%C can be produced and used in the as sintered or heat treated conditions for PM structural parts having good physical and mechanical properties as well as high dimensional accuracy with acceptable microstructures.     

Strain hardening behaviour of powder metallurgy alloys

Abstract

A model to describe the strain hardening behaviour of porous alloys, produced by powder metallurgy, is presented. It accounts for the influence of the matrix strain hardening ability, the initial porosity content and the pore geometry, and its predictive ability is verified with reference to sintered iron and an Fe–0·3%C sintered alloy. The necessity to introduce in the model a parameter able to account for the internal notch strengthening effect exerted by pores is emphasised. This parameter is found to be dependent on the sintering degree of the materials.     

Sintering Behaviour and Mechanical Properties of Al–Cu–Mg–Si–Sn Aluminum Alloy

The present study investigates the effect of compaction pressure and sintering temperature on densification response and mechanical properties of the Al–3.8Cu–1Mg–0.8Si–0.3Sn (2712) alloy. The compacts were pressed at 200 and 400 MPa and sintered at temperatures ranging from 570–630°C in vacuum (10^?6 Torr). The objective of the present work is to obtain an optimum sintering conditions for achieving higher sintered densities and mechanical properties. The effect of sintering temperature is evaluated by measuring the sintered density, densification parameter, microstructure, phase changes and mechanical properties. While a higher sintering temperature results in densification enhancement, it also leads to microstructural coarsening. Significant improvement in mechanical properties is obtained through age-hardening of sintered alloy under various ageing conditions (T4, T6 and T8).     

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.     

PM processing and characterisation of Ti–7Fe low cost titanium alloys

Abstract

This work studies a set of low cost beta alloys with the composition Ti–7Fe, processed by conventional powder metallurgy (PM). The materials were prepared by conventional blending of elemental Ti hydride–dehydride powder with three different Fe powder additions: water atomised Fe, Fe carbonyl and master alloy Fe–25Ti. The optimal sintering behaviour and the best mechanical properties were attained with the use of Fe carbonyl powder, which reached a sintered density of up to 93% of the theoretical density, with UTS values of 800 MPa in the ‘as sintered’ condition. Coarse water atomised powder particles promoted reactive sintering, and coarse porosity was found due to the coalescence of Kirkendall porosity and by the pores generated during the exothermic reaction between Ti and Fe. The addition of Fe–25Ti produced brittle materials, as its low purity (91·5%) was found to be unsuitable for formulating Ti alloys.     

Effect of Compacting Pressure and 0·5%Mo on Properties of Sintered Iron Powder and Manganese Steels

Abstract

An investigation has been carried out on the effect of compacting pressure, in the range 150–600 MPa, and of the addition of 0·5%Mo on the properties of sintered Hametag iron powder and manganese steels. Higher sintering activity compared with standard iron resulted in higher density and higher mechanical properties in the presence of manganese vapour. The addition of 0·5%Mo in the form of ferromolyb-denum caused an increase in density and strength properties in the Fe–C and Fe–Mn–C steels. PM/0157     

IRON-COPPER-TIN SINTERED COMPACTS

Abstract

A wide range of copper and tin powder additions to iron powder sintered compacts hasbeen studied. From mechanical-property tests it has been shown that when using sinteririg temperatures of 900–1100°C in nitrogen/10% hydrogen atmospheres there is an optimum copper: tin ratio of 15:2. The mechanical properties obtained from compacts pressed from iron mixed with 4% copper+tin in this ratio and sintered at 900°C were similar to those obtained from iron ?l0% copper powder compacts sintered at 1100°C. Moreover, the iron-copper-tin components showed improved dimensional accuracy.

In a further series of experiments, it was shown that tin additions to iron-copper alloy compacts increased the solubility of iron in the liquid phase at the sintering temperature and simultaneously decreased the rate of diffusion of copper into the iron particles. At the same time, tin improved the wettability of the liquid, reducing its surface tension and allowing it to disperse more completely throughout the matrix. The mechanical properties of compacts containing larger amounts of tin were decreased by the presence of brittle compounds, although the sintering rate was increased. It is concluded that the optimum properties of iron-copper-tin compacts are obtained by making correct additions of copper and tin to the iron powder and giving careful consideration to the sintering atmosphere.     

Effects ofcontaminationonproperties of W-15Cu preparedfrom mechanically alloyed powders

Abstract

To eliminate the contamination of activator elements, such as Fe and Ni, W-15Cu compacts were prepared from mechanically alloyed powders using an attritor with a zirconia tank, balls and agitator arms. Coarse tungsten and copper powders, 9·9 μm and 13·3 μm, respectively, were milled to 1·26 μm composite powders after 145h of milling. The milled powder contained little free copper and was highly combustible in air. After sintering, the 50 vol.-% dense green compacts attained a density of 15·8g cm^-3 or 96·2%. The microstructure consists of uniformly interdispersed tungsten and copper. When stainless steel grinding balls were used, the powder was heavily contaminated with Fe and Ni. The contamination improved the density slightly, but the grain size and the electrical resistivity increased significantly as well. The sintering behaviours of the two composite powders were similar. Most densification occurred during heating before reaching the melting point of copper.     

Determination of the physical and mechanical properties of iron-based powder materials produced by microwave sintering

Microwave energy is highly efficient for heating and processing different materials. In recent years, this type of heat transfer has been used in sintering process. Rapid and highly efficient heating, time and energy saving, and improved properties of sintered materials are advantages of microwave sintering. In this paper, Fe and Fe-Cu powder compact samples (cylindrical and bone shapes) are sintered both in microwave and electrical tube furnaces. The microwave generator has 2.45 GHz frequency and 1 KW power. Times are selected in the range of 5–25 min for microwave sintering and 5–40 min for electrical heating. The sintering temperature is set at 1120°C. Samples are sintered in the reducing atmosphere of 95% N₂ + 5% H₂ mixture. The density, hardness, and tensile strength of the samples are measured. The results are compared. The results show that the microwave-sintered materials have a finer microstructure. The microwave-sintered materials have 6–8% higher density, 5–10 HV5 higher hardness, and about 10% higher tensile strength than conventionally sintered materials.     

Sintered copper–graphite powder compacts forindustrial applications

Abstract

Studies were made on copper/graphite based powders and sintered compacts for industrial applications. The dependence of particle shape on friction in the powder mass, compression ratio, and electrical receptivity of powder metallurgy components was studied using near spherical precipitated copper powders and angular or flakelike powders generated by mechanical comminution. Results reveal that powders with particles that are nearly spherical in shape have lower friction, lower compression ratios, and higher electrical resistivities in sintered compacts than powders with acicular or flakelike particles. Also, the effects produced by the small additions of lead and zinc (up to 2·5 wt-%) on the electrical resistivity and hardness of sintered copper–graphite compacts are also presented, and the influence of variation of briguetting pressure is discussed.     

Pressing vacuum sintering of multipowders for manufacturing novel engine valve seat on Gleeble 1500 simulator

Abstract

Patented Fe based multipowders for manufacturing novel engine valve seat were compacted and sintered using a Gleeble 1500 thermal simulator system. Microstructures and properties of the sintered alloy have been studied by hardness test, optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) analysis. Results indicate that a sintered alloy with a hardness of 38 HRC and a density of 7·23 g cm^?3 can be obtained when the green compacts are sintered at 1240°C for 30 min. It is found that lattice parameters of the alloy matrix increase with increasing sintering temperature. Additionally, the investigation of green compacts' shrinkage during sintering shows that there is a threshold value for the density of the green compacts. When the density of the green compacts is lower than that value, the density of the sintered alloy remains almost constant with increasing density of green compacts. An equation to calculate the threshold density has been suggested as νρ² = C ₁ ρ² + C ₂ m² + C ₃ m ρ.     

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.     

Effect of compacting pressure on microstructure and mechanical properties of carbide cutting tools

Abstract

The effects of compaction pressure on properties of carbide cutting tools containing 80·5 wt-%WC, 5 wt-%TiC, 5 wt-%TaC–NbC and 9·5 wt-%Co were studied. These tools were formed by powder metallurgy with different compacting pressures ranging from 77 to 231 MPa (5–15 tons in^?2) and sintered in a vacuum furnace at a constant sintering temperature (1450°C) and at a constant heating and cooling rate of 5°C min^?1. Green and bulk densities, shrinkage and hardness of the produced compacts were measured. Tool cutting performance has been assessed based on machining a medium alloyed steel workpiece under different cutting conditions and measuring the tool flank wear and the workpiece surface roughness. The microstructure of the compacts was metallographically examined using scanning electron microscopy. The results have revealed that both densities and hardness figures increase with increasing compaction pressures, while shrinkage decreases. Cutting performance has not demonstrated a substantial improvement of the tool's performance and life due to the increasing compacting pressure of these tools.     

THE POWDER METALLURGY OF RUTHENIUM

Abstract

An investigation of the powder metallurgy of ruthenium is described, from the reduction of ammonium ruthenium chloride to the working of sintered compacts. The powder properties measured were specific surface area, by a simplified BET method, and tap density. The dependence of these properties on the conditions of reduction has been determined. The surface area of powders varies from 1 to 10 m²/g in the temperature-of-reduction range 700-350°C. The tap density is also variable (1–3 g/c.c.) and is generally related to the surface area. The effects of compacting pressure and temperature on sintering are described, the progress of sintering being observed by measurements of the “open” and “closed” porosity present in samples. Compact densities up to 95% of theoretical can be obtained by sintering at 1500°C. The selection of powder properties and compacting pressures to be used in the production, by vacuum sintering at 1500°C, of high-density compacts for working, is governed by the necessity to maintain open porosity during the heating cycle up to at least 1200°C, as considerable gas evolution occurs at this temperature; at the same time it is essential that good densification shall have occurred even at this stage. These conditions can be met by using powder with a surface area of 2–5 m²/g and compacting pressures in the range 0·5–25 tons/in ².

Observations on the hot working of sintered compacts indicate that ease of working is related to the surface area of the powder.     

Effect of conventional and microwave sintering on the properties of yttria alumina garnet-dispersed austenitic stainless steel

The current study examines the effect of heating mode, temperature, and varying yttria alumina garnet (YAG) addition (5 and 10 wt pct) on the densification and properties of austenitic (316L) stainless steel. The straight 316L stainless steel and 316L-YAG composites were heated in a radiatively heated (conventional) and 2.45 GHz microwave sintering furnace. The compacts were consolidated through solid state as well as supersolidus sintering at 1200 °C and 1400 °C, respectively. Both 316L and 316L-YAG compacts couple with microwaves and heat to the sintering temperature rapidly (∼45 °C/min). The overall processing time was reduced by about 90 pct through microwave sintering. As compared to conventional sintering, compacts sintered in microwaves exhibit higher densification and finer microstructure but no corresponding improvement in mechanical properties and wear resistance. This has been correlated to elongated, irregular pore structure in microwave-sintered compacts.     

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.