The CIP-sinter-HIP (CHIP) process is a multi-step PM process in which a green body/preform is produced via cold isostatic pressing (CIP), conventionally sintered under vacuum, and then hot isostatically pressed to close the remaining porosity [158]. A schematic of this process is given in Figure 19. The CHIP process has enjoyed significant commercial attention. In fact, this process is currently reported to be the sole qualified PM production route by the Boeing Material Specification for PM Ti–6Al–4V [168]. Additionally, CHIP has been identified as a possible ‘green’ manufacturing technology for sustainably producing Ti alloys [158,169].
As mentioned, HIP is utilised solely to close the residual porosity after vacuum sintering during the CHIP process. Therefore, the microstructure after HIP is similar to that achieved in conventional pressureless (vacuum) sintered compacts of BE powder. An optical micrograph of Ti–6Al–4V alloy produced via the CHIP process is shown in Figure 19 [158]. As seen, the microstructure is comparable in size and morphology to the lamellar structure produced via conventional pressureless sintering. As also shown in Figure 19, CHIP may also be used with subsequent TMP, such as extrusion and forging [158]. In addition to serving as a forming process, TMP can break up the coarse lamellar grains and facilitate the formation of a refined equiaxed or bi-modal microstructure through recrystallisation of the α grains. However, as pointed out earlier, TMP processes are not near-net-shape compatible, are inherently energy-intensive and, therefore, would increase the cost of the final product.
CHIP, as with other pressure-assisted consolidation, is shown to effectively produce fully dense PM Ti alloys. However, when BE powders are used, sintering temperatures well above the β-transus are required to homogenise the alloying elements. Therefore, the microstructures of PM Ti–6Al–4V produced via CHIP are similar to those available with conventional pressureless sintering. If more expensive PA powders are used, lower sintering temperatures may be used, enabling the formation of different microstructures.
https://www.tandfonline.com/doi/full/10.1080/09506608.2017.1366003