Powder Metallurgy

Duplex stainless steels prepared from mixes of elemental powders with martensitic 410L steel have been shown to possess stable dual phase microstructures. Following heat treatment, the steels possess good mechanical properties. The addition of boron as a sintering aid reduces compressibility but promotes densification through transient liquid phase sintering; its effect on corrosion resistance is particularly favourable.

The EPMA's triennial meeting on hard materials and diamond tooling took place this year in Lausanne on 7–9 October. A broad mix of industry and research papers provided an overview of developments in these important sectors, while the plenary sessions, summarised below, reviewed industry trends and challenges.

The oxygen content in the process of the production of oxide dispersion strengthened copper by mechanical alloying was traced by carrier gas-hot extraction. First, the usual determination of the total oxygen content was carried out. Second, the analytical method was checked and modified with respect to the selection and detection of different oxygen species in the sample. Applying this analytical approach the relevance of various technological treatments (milling, heat treatment) on the oxygen concentration was evaluated.

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.

Low porosity powder metallurgy compacts have been manufactured from treated elemental iron and cobalt powders sintered at 1150°C under an H_2(g) atmosphere. Their microstructures consist of an interconnected mixed oxide network which encapsulates both the iron and cobalt phases. The production technique employed is an innovative process termed reacto-thermitic sintering (RTS), which leads to near full density and near net shape parts utilising conventional uniaxial compaction and mesh belt furnace practices. The RTS technique relies on microscale exothermic reaction between small quantities of added elemental Al and oxides present on the surface of the bulk powder, together with the bulk powder itself. This results in the production of a transient liquid phase which freezes rapidly and consolidates the compact without slumping. In order to generate an interconnected mixed oxide network, experiments were designed such that the Al powder reacts with the cobalt and the surface of the iron powder which is artificially doped with Fe and Cr oxides.

Differential thermal analysis (DTA) and energy balance calculations revealed that the Al and the oxide coating reaction does not proceed directly. Instead the main contribution to the exothermic process is the reaction between Al and Co/Fe. The system does not exhibit true RTS behaviour and the interconnected network of mixed Al, Cr, and Fe oxides is created by subsequent reaction of Co-Al and Fe-Al intermetallics with the artificial Fe-Cr oxide coating on the Fe. The microstructure obtained exhibits negligible porosity with the metallic particles on the whole fully encapsulated by the oxide.

A short overview of works on spark plasma sintering (SPS) is given in the present paper. SPS is a newly developed rapid sintering technique with a great potential for achieving fast densification results with minimal grain growth in a short sintering time. It is proven by obtained experimental data that enhanced sinterability of powders subjected to SPS mainly associated with particle surface activation and increased diffusion rates on the contact zones caused by applied pulse current. Application of rapid heating results in bypassing of low temperature regions where surface transport controlled sintering is dominant. This preserves the powder surface area to temperature levels where bulk transport is significant. However, the nature of activation effects, especially in its regards to acceleration of diffusion processes, is not clearly established. A lot of research work reports about the occurrence of plasma during the application of pulse current. However, the appearance of thermal plasma during SPS is a controversial issue and plasma-particle interaction is a complex phenomenon. Presently the industrial application of SPS processing is considered mainly in the areas where it is difficult or extremely difficult to obtain high performance materials and where normally HP or hipping processes are applied. Apparently significantly more work is required in this field for better understanding of the entire process and for making a stable ground for more extended industrial application of SPS.

Most of the numerical simulation software on the hot isostatic pressing (hipping) process is based on viscoplastic modelling of densification, such as the well known Abouaf's model. These constitutive equations are generally obtained from a viscoplastic potential depending on Green's equivalent stress. An implicit coupling between isotropic and deviator parts is therefore only defined by an equivalent stress. A new viscoplastic formulation proposed by Stutz introduces an explicit coupling between isotropic and deviator parts of the stress state, allowing then more flexibility to take into account the experimental results achieved from isotropic, die compaction, and creep tests. This paper deals with the presentation of this new formulation which has been implemented in the finite elements software PreCAD, and the subsequent changes observed in numerical simulations. These simulations achieved with PreCAD software, are compared with an experiment on a complex part manufactured by CEA Grenoble.

The paper presents a summary of two case studies that were carried out by the scientific team in the Thematic Network PM Modnet. During the life of this project, the compaction of complex multilevel ferrous components was investigated. These formed a vehicle to explore methods to characterise the yield and friction properties of the powder, perform simulation of the compression stage of the forming process, complete experimental trials, and compare experimental and simulated results. Density comparisons were made with results from Archimedes, quantitative metallography, and computerised tomography and force levels were compared with recordings from the pressing trials. The results highlight differences between equipment and experimental techniques used in characterising powders. They also show that hardness, metallographic analysis, and computerised tomography may be used to measure density variations throughout the compact. The prediction of density variation was reasonably consistent when using different simulations, whereas punch force prediction showed good consistency. It was found that predicted and measured density distributions agree within 0·05 to 0·5 g cm^-3 and that punch force levels may be predicted within 10 to 30%. The study effectively establishes a benchmark with which to compare and improve future simulations.

The paper explores the use of a shear plate technique to measure the frictional characteristics between a compacted powder and a target surface. The study confirms that the shear plate technique is valid to measure these frictional characteristics and that any variability in the data is a physical characteristic of the powder being tested. Surface roughness and hardness were explored fully for both iron and an alumina powder. This confirmed the major impacts of surface hardness, roughness, and roughness orientation on the friction coefficient. The static and dynamic values were found to be a minimum for the hardest surface (tungsten carbide). With regard to static friction, benefit may be obtained by using a very smooth surface finish, however, the minimum level of dynamic friction coefficient is not always associated with the smoothest surface.

The final mechanical properties of PM parts are strongly affected by the processing parameters: processing controls porosity, composition, and microstructure. Variations within the processing sequence lead to significant property variations. Variations in strength values are often accounted for by the publication of typical values and minimum values. Other mechanical properties, important for critical applications, such as toughness, are even more sensitive to processing. This paper discusses the use of Weibull statistics to analyse the properties of PM parts and suggest new ways to determine property variability for design application. Examples of different properties and materials are used to illustrate the variation. The effects of processing parameters in controlling the variability are discussed. Weibull parameters are suggested as being more suitable for designers than the current system.

In this paper, the properties of Ag-Sn, Ag-Fe, Ag-Mo, and Ag-W samples are presented. The samples under consideration were produced using the powder metallurgy technology and the silver, a main component of metal composites, was manufactured in the cathodic reduction process, during which a sparingly soluble AgCl compound was reduced. That silver is characterised by a fine grained structure and a very good compressibility, and its properties effectively improved the density and the homogeneity of samples investigated.

Blending metal powders and compacting them at 500 MPa produced silver based metal composites. The green compacts were sintered for 60 min in a hydrogen atmosphere, at a temperature of 1123 K. The following properties of metal composites were examined: density, hardness, electrical conductivity, and magnetic susceptibility. Structural investigations of composites manufactured were also carried out.

In this paper, it is stated and proved that the character of specimens in Ag-Sn, Ag-Fe, Ag-Mo, and Ag-W systems depends on the components’ attributes, and also on their density. The impact of silver on the samples’ density was calculated on the basis of the experimental data obtained, and it was expressed in the form of equations. In addition, the dependence between the electrical conductivity of tested metal composites and the silver content was established.