Comment on processing

Powder compact forging of HDH titanium and HDH Ti-6Al-4V with an initial relative density of 0.75 was carried out. The density of a powder compact increased during forging and the final relative density increased with increases in the initial forging temperature. High density HDH titanium powder compacts could be produced by a single action cold powder compaction, whereas HDH Ti-6Al-4V powder compacts required the maximum capacity of the 100 ton press to achieve the required density for the experiments. During the experimental work surface cracks were visible on

HDH Ti-6Al-4V powder compacts forged at a temperature of 1250 o _{C. The cracks}

further widened when forged at a lower initial temperature of 1000 o _{C. In practice, as} the powder forging operation progresses a powder compact will lose heat to the die

and cracks might occur at regions where the temperature is below 1000 o _{C; thus}

powder forging HDH Ti-6Al-4V powders at the investigated relative density is not recommended. On the other hand, no cracks were seen in HDH titanium powder compacts even when forged at a lower temperature. The measured densities of HDH titanium were much higher than those for HDH Ti-6Al-4V parts.

8.1.2 Cylindrical component

Hot-repressing and extrusion of HDH titanium powder compacts were carried out to produce a cylindrical titanium component. A hot-repressing method of powder

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consolidation is of interest in the production of high performance titanium fastener products. The experimental results showed that cylindrical components with a high and uniform density can be produced by this method. The higher density is achieved because of the hydrostatic pressure generated in the powder compact when forged in a confined die. This can be explained by the fact that the yielding behaviour of any porous material is sensitive to the applied hydrostatic stresses. With the application of compressive hydrostatic stresses, the pores within the material will collapse resulting in higher densification. Due to the design of the process which includes combined hot-repressing and extrusion, the density increases at the extrusion die radii during the initial stages of the process. After the forward leg region is filled, the hydrostatic pressure increases in the rest of the component. A uniform density was seen throughout the component, except at the bottom leg, where the lowest measured relative density was 0.98.

Despite success in producing cylindrical titanium fastener grade components in the laboratory scale experiments, manufacturing to industrial trials did not move forward for the following reasons (1) The high hydrostatic pressure and temperature involved in the process exerts high stresses beyond the yield stress of the die material. Also, the original design for this product had a hole at the top end as shown in figure 7.1. The tooling material failed on several occasions during backward extrusion of the product. (2) Critical structural components such as fasteners rely on standardisation and material traceability for safety and reliability. At the time the experiments were being carried out there was no standardisation for non-melt P/M titanium structural materials available for a specialised process such as the hot-pressing. This situation has since changed with the introduction of a new ASTM B988-13 specification for P/M titanium for structural components, published in July 2013 [130]. The release of these new standards is expected to facilitate wider industrial application of titanium P/M products with reduced cost and lead time. However, only a few HDH titanium powder manufacturers are currently able to meet these new standards.

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8.1.3 Ring shaped titanium component

A closed die powder forging route was used to produce an HDH titanium ring shaped part. The material flowed outwards during the powder forging operation. The highest relative density was seen at the centre of the component and this reduced towards the edge. A lack of uniformity in density in the HDH titanium powder compact was a major concern. The non-uniform density distribution was due to the varying horizontal cross section of the powder compact and problems in uniform die filling in the experimental compaction die setup. The die filling problem can be solved by re- designing the die such that the filling depth is equal to the depth of the compaction die cavity. During the experimental trials, radial cracks were seen in the final powder compact forged part. The reason for the occurrence of these cracks was due to improper bottom die design which led to high radial stresses applied to the powder compact (revealed by the simulation) during the early stages of powder forging.

Powder compact forging trials were not successful in producing sound ring shaped titanium forgings. This was because of cracking during forging and a requirement for a specialised argon or vacuum chamber for industrial production trials. As an alternative approach a sinter-powder forging route was used for this the HDH titanium powder compacts were sintered in a vacuum furnace and powder forged in air. No cracks were seen in sinter-forged titanium powder compacts; this might have been because of higher densification of the powder compacts after vacuum sintering. It is interesting to point out that an increase in densification through vacuum sintering increases the fracture strain limits of the powder compacts during forging [131].