Powder Metallurgy

The main asset of the powder metallurgy route is its ability to produce structural components that meet functional requirements in a cost efficient manner. Many PM parts are produced at densities lower than 7·1 g cm⁻³ because achieving higher densities often requires additional processing steps, which result in increased part cost. Although warm powder warm die processing is not new, new methods that employ die heating only have been introduced that enable high green density via single press/single sinter. This simplified processing will potentially lead to greater market acceptance of high density PM parts. However, drawbacks of die heat only are part size restrictions and higher compaction loads. Both processes are useful in the production of high density PM components. This paper will detail the advantages and disadvantages of both processes. Practical part production will be discussed with mechanical properties achievable via high density processing.

Increasing density is one approach to improve the performance of PM components. Lower lubricant contents are often required to increase the compressibility of a mix, and heating of the tool die can help to achieve sufficient lubrication with less lubricant. Chromium prealloyed powders are also a cost effective means of achieving high mechanical strength. However, solution hardening effects mean that prealloyed mixes have lower compressibilities than those based on pure iron powders, making Cr alloyed powder more demanding to compact and requiring more efficient lubricants. The performance of an efficient lubricant system developed for demanding applications is compared with that of amide wax in mixes based on chromium alloyed powders, including for compaction of multi-level components under production conditions.

This paper describes the viscoelastic behaviour of sintered steels with porosities of 12, 20 and 33%, using a dynamic mechanical analyser. Test specimens were prepared from premix powders of 100–150 μm size by a process of die compaction, delubrication and sintering. The influences of test temperature and vibrational frequency on storage and loss modulus and tangent delta have been investigated. The investigated operating temperature and frequency was varied from 25 to 280°C and from 10 to 50 Hz respectively.

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.

A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed.

Ni, Cu and in some cases Mo are the alloying elements which have traditionally been used in sintered steels. High performance of powder metallurgy (PM) structural parts from Fe powders is reached mainly by alloying of Ni. The use of Mn in Fe base PM structural parts has been avoided because of its high affinity to oxygen. It is difficult to sinter Mn steel, without oxidation, in industrial atmospheres. However, the PM industry follows also possibilities in order to develop Ni free sintered steels which render as high mechanical properties as diffusion alloyed Ni containing sintered steels and further fulfil the requirements of health protection. In recent years Mn have been introduced as alloying element in Fe based structural parts, on laboratory scale and also for pilot scale production. In this paper the factors that contribute to the structure and mechanical properties of sintered Mn steels are summarised.

In an earlier study, the authors presented a characterisation of the FC-0205 Ancorsteel powders containing 0·6 and 1·0% Acrawax to define the evolution of the failure line and cap surface of the modified Drucker/Prager cap model during compaction. Using the results of that study (i.e. FC-0205 material parameters), this paper presents sensitivity and uncertainty analysis of the microstructure–property relationships for powder metallurgy compaction. It is found for all of the responses of interest (the compressibility curve, the interparticle friction, the material cohesion, the cap eccentricity and the elastic modulus) that the most dominant parameter is the initial (or tap) density. It is also observed that the uncertainty in output parameters for the case of 1% wax is much larger than those for the case of 0·6% wax, due to the large uncertainty in the failure stress (in particular, the compressive failure stress).

The development of novel extractive metallurgy techniques for titanium offers the prospect of lower cost Ti powder and therefore wider application of Ti. This review is largely confined to coverage of the low cost press and sinter methods of powder metallurgy, consisting of cold pressing of mixed elemental powders followed by sintering without the application of external pressure. Cold die compaction, sintering behaviour and densification are reviewed in detail. Some information on powders and cold isostatic pressing is included. Microstructure, mechanical properties and applications are considered in less detail. The review deals mostly with the sintering of alloys, but there is some reference to synthesis of intermetallic compounds, such as the shape memory alloy NiTi and titanium aluminides for high temperature applications. Densification is discussed in terms of the four fundamental processing variables: compaction pressure, particle size, sintering temperature and sintering time. Other factors such as alloy composition, the form of alloying addition, type and impurity content of powders and heating rate are also considered.

This paper presents the preliminary results obtained in developing a powder metallurgy process involving an approach based on the use of press (compaction) and sintering together with subsequent extrusion on one hand and press and extrusion on the other hand. Two systems have been compared: an unreinforced AA6061 alloy (matrix) and AA6061-2 wt.% fly ash particulate composite. Mechanical mixing of the matrix powder, obtained from elemental powders, and particulate reinforcement was followed by compaction. Pressurisation was carried out at 345 MPa with zinc stearate as die wall lubricant. Some green compacts were extruded subsequently at 5–10 mm min⁻¹ rate with a reduction ratio of 16:1 at 500°C. And some were sintered at 620°C for 4 h before extrusion. From XRD, SEM EDAX and mechanical testing studies it was observed that the press extruded samples were showing improvement in density, hardness, 0·2% proof stress and tensile strength over press sinter extruded samples.

In the present work, a powder mixture of pure WO₃, graphite and Mg with a definite atomic ratio was milled at room temperature using a high energy ball mill method, and ball milled powders were analysed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results indicated that after ball milling for a period of time, an oxidation–reduction reaction was successfully achieved among the Mg, graphite and WO₃ powders to obtain MgO and WC. The extension of the ball milling led to the refinement of the powders. After ball milling 50 h, nanocrystalline WC grains (25 nm) were embedded into the fine matrix of MgO and formed fine nanocomposite MgO/WC powders (∼100 nm in diameter). The experimental results and thermodynamic analysis showed that the formation of nanocomposite MgO/WC was a mechanically induced self-propagating reaction, and very short milling time was needed to complete the reaction.

In this research, synthesis of Co₃W–Cu composite nanopowders based on Co₃W intermetallic compound by mechanical milling and hydrogen reduction process was investigated. Powder mixture of Co₃O₄, WC and CuO with Co₅₀W₄₀Cu₁₀ stoichiometry was first milled by high energy planetary ball mill and then reduced in a hydrogen reduction system. Crystallite structure of milled mixture and reduced powders was determined by X-ray diffraction. Particles size, morphology and cross-section of reduced samples were studied by SEM, TEM and SEM back scattered electron microscopy. Optimum condition of reduction under hydrogen atmosphere was found at 900°C. Particles have Cu coring structure surrounded with Co₃W intermetallic compound. Mean particle size was observed less than 50 nm with six fold hexagon morphology.