Powder Metallurgy

The hardness and bend strength of diamond tool matrix materials prepared from partially pre-alloyed Fe–Cu powders and from elemental powder mixes of equivalent composition has been compared. After sintering at 890^oC, the hardness of the partially pre-alloyed powder matrix was 81.72 HRB, 9.3% higher than that of the mixed powders matrix; sintering at 870^oC gave the highest bend strength (634.85 MPa), 11.6% higher than that of the mixed powders matrix. The improved homogeneity of alloy elements distribution achieved in the matrix by pre-alloying was shown generally to increase the hardness and strength. The cutting performance of diamond tool segments prepared with the pre-alloyed matrix material met standard requirements; overall, the pre-alloyed material is felt to offer significant manufacturing and performance benefits.

The development of multilayered polymeric/metallic materials by a PM route employing spark plasma sintering (SPS) has been explored. The aim is to produce composites that join a polymer with a metal for structural applications. In a first approach, the consolidation of polyimide-based composites by SPS was studied. For temperatures as low as 350°C, homogeneous mechanical properties were achieved in compression tests. In a second set of experiments, multilayered polyimide/aluminium composites were consolidated by SPS. These polymer-based composites could be used as an interlayer material to join effectively polyimide and aluminium.

The formation of a liquid phase during sintering of prealloyed Cu–28Zn powder is potentially attractive for densification, but the effect of gravity results in graded densification which tends to result in heterogeneous cross-sections. The formation of a necklace structure along grain boundaries due to the accumulation of liquid phase with high Zn content, especially at higher temperatures, is also of interest. Evaporation of zinc is another essential feature to consider, since loss of Zn during sintering can influence strongly the mechanical properties of brass products. A macroscopic visualisation of Zn evaporation has been achieved using a copper substrate placed within the gas stream near the sample. FEGSEM observation and XRD analysis of the deposited white mass revealed the formation of nanocrystalline ZnO as a consequence of Zn evaporation. It is proposed that this method could usefully show the evaporation of other alloying elements during sintering of similar alloys.

There have been few systematic studies of the fabrication and properties of porous MAX phase microstructures despite the potential applications of these materials. A simple, low cost, eco-friendly PM method has been developed to prepare MAX phase foams from commercial Ti₂AlC powder, using crystalline carbohydrate as space holder. This method involves: mixing Ti₂AlC powder with crystalline carbohydrate, pressing to form a green body, removal of the space holder and sintering. Compaction was achieved by uniaxial pressing (UP) and cold isostatic pressing (CIP). Control of porosity was achieved by varying the particle size (three ranges between 250 and 1000 μm), and volume fractions (20, 40 and 60%) of space holder. The foams were characterised and the porosity compared with the expected values. Optimal conditions for this novel processing technique were established with the aim of controlling the final microstructures and properties of the Ti₂AlC foams.

An orthogonal experiment scheme was employed to study the influences of forming pressure, sintering temperature and holding time and Cu content on microstructure, hardness and electrical resistivity of the Cu/Invar composites prepared by the powder metallurgy (PM) technique. The interdiffusion of the Fe, Ni and Cu atoms of the composites during sintering was also investigated. The results show that the Invar alloy is distributed continuously in the composites, when the Cu content is 30 wt-% and below; when the Cu content is 40 wt-% and above, a continuous net structure of Cu forms. Properties, especially the electrical and thermal conductivities, depend on the relative density and atom interdiffusion of the Cu/Invar composites. Taking the electrical resistivity of the composites as index, the optimum processing parameters are: forming pressure of 600 MPa, sintering temperature of 1000°C, holding time of 60 min and Cu content of 50 wt-%.

The present paper introduces a new concept of a sintered soft magnetic composite (SSC): a combination of a sintered soft magnet and soft magnetic composite. The approach is to create a Fe–Si based bulk material with a microstructure of Si rich, highly insulating ferromagnetic particle boundary phases by a high temperature heat treatment (sintering). The concept is realised via a powder metallurgical route, starting from an initial mixture of iron, silicon and copper. Silicon forms a graded boundary phase around the iron particles. Copper is added to reduce the melting point of the silicon and to ensure a uniform wetting of the silicon-rich layer around the iron particles. A systematic composition range of such sintered materials are evaluated as to magnetic and mechanical properties. The best material composition found has 96 wt-% iron+3 wt-% silicon+1 wt-% copper with the optimum sintering parameters of 1050 to 1100°C for 30 min in a nitrogen atmosphere.

The effects of gas flow pattern on the distortion in Al–7Zn–2.5Mg–1Cu alloy were experimentally examined by sintering three rectangular samples, equally spaced 2–40 mm, in each batch at 620°C for 45 min under flowing nitrogen. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed to investigate the gas flow behaviour surrounding three samples during isothermal sintering. Streamlines, gas flowrate into each cavity between two adjacent samples and wall shear stress distribution along the surfaces of each sample were found dependent on sample separation distance. The flow patterns could affect the distortion levels of the three samples by changing the oxygen content in the local sintering atmosphere and the evaporation rates of Mg and Zn from the sintering surfaces. The 3D CFD model developed was also applied to the experimental design of the sintering of six samples to ensure minimum distortion as well as maximum densification.

The present study focuses on modelling, optimisation and experimental investigation into properties and microstructure of supersolidus liquid phase sintered Cu–28Zn brass from prealloyed powder. The experiments are designed using response surface methodology based on central composite rotatable design to evaluate the effect of process variables on liquid phase sintering of prealloyed brass powder. Three variables namely temperature, time and atmosphere were changed during sintering. The mathematical equations were derived to predict densification and impact energy using second order regression analysis. The optimum condition was predicted when the sintering variables were set at about 876°C, 43 min and N₂ atmosphere. Selecting optimum sintering parameters is an important factor for achieving improved properties and relatively homogeneous microstructure. Gravitational force has a detrimental influence on homogeneity which is reflected by a graded structure that is formed especially at higher sintering temperature and extended time. Also structural coarsening occurs at higher sintering temperatures and longer times. It is concluded that both gravitational effect and structural coarsening should be considered in manufacturing of Cu–28Zn alloy parts. Furthermore, a combination of modelling and experimental investigation provide a new concept for better understanding and analysing the sintering process of brass and related structures.

The addition of alloying elements in low alloyed PM steels in the form of a master alloy gives the advantage of introducing oxidation sensitive but less expensive elements and also allows manipulation in composition adjustment to achieve desired properties. In this work, interrupted sintering trials of the Fe–2MA–0.5C (%) (MA = Cu based master alloy) are performed. The behaviour of the liquid forming master alloy, for instance in terms of liquid phase formation, alloying element redistribution and effect on the dimensional changes, is investigated. The results show that master alloy particles melt over a range of temperature, which is also supported by the thermodynamic calculations. The low swelling in the master alloy system, compared to a reference system of Fe–2Cu–0.5C, is attributed to the progressive melting of the master alloy. The mean diffusion distance of Cu in Fe at the interparticle boundaries is 5.8 μm after 34 min of isothermal holding.

Stainless tool steels highly alloyed in niobium can be produced by powder metallurgy using diffusion alloying. Steel powder atomised without carbon is subsequently mixed with graphite and hot isostatically pressed. The atomised powder contains the intermetallic Laves phase NbFe₂ that transforms into MC-type carbides during HIP when graphite has been added. The obtained structure features a fine distribution of carbides to increase wear resistance and chromium fully dissolved in the matrix to provide corrosion resistance. X-ray diffraction (XRD) measurements and reflection position analysis with additional scanning electron microscopy (SEM) have been conducted to study the phase transition of NbFe₂ Laves phase into NbC carbides in two high Nb alloyed stainless tool steels. The results show that carburisation starts at 1000–1050°C and also confirm the correlation between oxide reduction and carburisation. The formed carbides are distinctly understoichiometric, which leads to an overestimation of the suitable quantitiy of added carbon in the thermodynamic calculations.

Cell shape plays a crucial role on mechanical properties of titanium foam as scaffold in bone tissue engineering. In the present research, titanium foam was prepared using space holder technique. Sodium chloride and ammonium bicarbonate were utilised as spacer agent separately. The effect of cold compaction pressure and spacer agent type on the cell morphology was investigated by scanning electron microscopy (SEM) and optical stereo microscopy. Image analysing technique was performed to evaluate the microscopic images quantitatively. Exact salt leaching time was introduced by a new approach using electrical conductivity measurement. True and apparent porosities and compressive mechanical properties of the synthesised foams were evaluated. Finally, the superior spacer agent and appropriate cold compaction pressure were determined. It was shown that sodium chloride, due to maintaining its morphology during cold compaction pressure and absence of chemical side effects on titanium, is the superior spacer agent.