Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials using powder metallurgy: A review

The densification response of aluminum powder reinforced with 5 vol.% nanometric alumina particles (35 nm) during uniaxial compaction in a rigid die was studied. The composite powder was prepared by blending and mechanical milling procedures. To determine the effect of the reinforcement nanoparticles on the compressibility of aluminum powder, monolithic Al powder, i.e. without the addition of alumina, was also examined. It was shown that at the early stage of compaction when the rearrangement of particles is the dominant mechanism of the densification, disintegration of the nanoparticle clusters and agglomerates under the applied load contributes in the densification of the composite powder prepared by blending method. As the compaction pressure increases, however, the load partitioning effect of the nanoparticles decreases the densification rate of the powder mixture, resulting in a lower density compared to the monolithic aluminum. It was also shown that mechanical milling significantly impacts the compressibility of the unreinforced and reinforced aluminum powders. Morphological changes of the particles upon milling increase the contribution of particle rearrangement in densification whilst the plastic deformation mechanism is significantly retarded due to the work-hardening effect of the milling process. Meanwhile, the distribution of alumina nanoparticles is improved by mechanical milling, which in fact, affects the compressibility of the composite powder. This paper addresses the effect of mechanical milling and reinforcement nanoparticles on the compressibility of aluminum powder.

An investigation on the compressibility of aluminum/nano-alumina composite powder prepared by blending and mechanical milling

The present study investigates the microstructural evolution and densification behavior of water- and gas-atomized 316L stainless steel powder. Dilatometry and quenching studies were conducted to determine the extent of densification and corresponding microstructural changes. Results indicate that water-atomized powder could be sintered to 97% of theoretical density, while gas-atomized powders could be sintered to near-full density. The difference in the densification behavior is examined in terms of the particle morphology, initial green density and the particle chemistry.

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Microstructural evolution of injection molded gas- and water-atomized 316L stainless steel powder during sintering

Powder injection molding (PIM) is a technology for manufacturing complex, precision, netshape components from either metal or ceramic powder. The potential of PIM lies in its ability to combine the design flexibility of plastic injection molding and the nearly unlimited choice of material offered by powder metallurgy, making it possible to combine multiple parts into a single one (Hausnerova, 2011). Furthermore, PIM overcomes the dimensional and productivity limits of isostatic pressing and slip casting, the defects and tolerance limitations of investment casting, the mechanical strength of die-cast parts, and the shape limitation of traditional powder compacts (Tandon, 2008).

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Powder Injection Molding of Metal and Ceramic Parts

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.

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

Metal injection molding (MIM) uses powders which differ considerably from the ceramics powders used for ceramic injection molding. Metal powders are hardly available in the sub-micron ranges that are almost standard in ceramics. The reason for this lies in the ductility and reactivity of the metals which make it difficult and very expensive to produce fine powders. In this paper the specifications for MIM powders are discussed with reference to the different materials processed by MIM and the resulting properties of the materials. Subjects will include stainless steel, tungsten alloys, intermetallics and titanium. The paper will deal with the activation and characterisation of the powders as well as with their processing by MIM.

Powders for metal injection molding

An instrumented die was used to investigate the behaviour of metal powders during cold (ambienttemperature) and warm (up to 140°C) compaction. This instrument enables simultaneousmeasurement of density, die wall friction coefficient, the triaxial stresses acting on the powderduring the course of compaction and ejection pressure. Commercial iron, titanium, aluminium,316L stainless steel (SS) and aluminium–silicon powders were employed for investigation. Theresults demonstrated the advantages of powder preheating on the compaction behaviour of metalpowders concerning green density, dimensional changes, frictional behaviour, ejectioncharacteristics and compactibility. However, the outlines also determined that the response ofthe non-ferrous powders to powder preheating is somehow different from those of the ferrouspowders. In this context, the behaviour of prealloy aluminium–silicon powders during compactionwas found of particular interest, as their compactibility is strongly affected by powder prehe...

Behaviour of metal powders during cold and warm compaction

The effects of warm compaction on the green density and sintering behaviour of aluminium alloys were investigated. Particular attention is paid to prealloyed powders, i.e. eutectic and hypereutectic Al-Si alloys, regarding their potential applications in the automotive industry. The effects of chemical composition, alloying method, compacting temperature and the amount of powder lubricant were studied. The compaction behaviour was examined by an instrumented die enabling simultaneous measurement of density, die wall friction coefficient, the triaxial stresses acting on the powder during the course of compaction and ejection pressure. The sintering behaviour was studied via dilatometeric analysis as well as normal batch sintering. The results show that warm compaction could be a promising way to increase the green density of aluminium alloys, especially prealloyed powders, and to decreased imensional instability during sintering. Moreover, it reduces the sliding friction coefficient and the ejectio...

Investigation of warm compaction and sintering behaviour of aluminium alloys

Metal Injection Molding (MIM) was performed with water atomized and gas atomized 316L stainless steel powders and powder blends thereof. Feedstocks were prepared using a thermoplastic binder system and subsequently molded into tensile test specimens. Different debinding procedures and sintering treatments were applied and their influence on carbon content in the product was compared. Chemical decomposition processes of the binder and the influence of powder morphology on debinding and sintering behaviour are discussed. Shrinkage of the MIM-fabricated parts was examined and correlated to the powder characteristics. As a result a procedure is suggested to achieve mechanical properties expected for 316L stainless steel.

G. Veltl

Papers

Powders for metal injection molding

Behaviour of metal powders during cold and warm compaction

Investigation of warm compaction and sintering behaviour of aluminium alloys

Investigations on Metal Injection Molding of 316L Stainless Steel

Uneven shrinkage causes problems for cheaper hardmetal tooling