Processing Methods for Reaching Full Density Powder Metallurgical Materials

Licentiate thesis, 2017

Powder metallurgy (PM) is one of the most cost effective methods for manufacturing structural components with complex shapes. Utilisation of the metal powder to shape the components allows to minimise material waste and increase energy efficiency. However, with the increased usage of PM parts in high performance applications, there is a demand for components with properties that can withstand extreme loading conditions and are equivalent or better than the properties of their wrought counterparts. The PM steels fabricated with press and sinter, even with all its advantages have limitations due to the presence of residual porosity. Hence, reaching full density is necessary to meet the high performance demands. The scope of this study covers different approaches of powder consolidation with the aim of reaching near full density. Liquid phase sintering and double pressing-double sintering approach for water atomised powders, high velocity compaction approach for agglomerated gas atomised powders, and finally additive manufacturing approach to produce samples with almost full density were utilised. These approaches were complimented by capsule free hot isostatic pressing (HIP) to reach full density and improve component performance.

The liquid phase sintering approach utilised is based on addition of Ni-Mn-B master alloy for enhanced sintering and densification. The results obtained clearly indicated that the master alloy addition effectively enhances the densification process through the liquid phase generation. The mechanical properties were improved when boron content was optimised from 0.2 to 0.12 wt. %. Density levels up to 98% were realised after sintering with addition of this master alloy.

In the double pressing-double sintering process, cylindrical and gear samples, fabricated from water atomised powder, prealloyed with Mo were studied. The studies included the comparison of using both standard and fine (<63µm) powder for the compacts investigated. The process route provides pore free surface and full density is then realised by means of subsequent HIP after second sintering. Fine powder shows better densification but the density variation after the first pressing persists as a low density region in the middle of the compact, i.e. at the neutral zone. Hence, optimisation during first pressing is necessary in order to avoid this phenomenon.

High velocity compaction (HVC) is a means of tailored densification. There is an industrial processing route that takes advantage of this effect, involving conventional pressing of agglomerated gas-atomised powder and sintering followed by HVC re-strike and hot isostatic pressing for reaching full density. In this study, the said approach was evaluated for agglomerated alloyed tool steel powder type 100Cr6 when processed to reach almost full density of about 99.5% relative density.

The additive manufacturing approach includes the assessment of material fabricated by means of electron beam melting (EBM), involving both solid and shell samples from Ti-6Al-4V powder. The studies showed that either samples were printed solid or as shells filled with powder, the subsequent HIP led to full densification. Furthermore, good bonding between the shell and the interior material was obtained after HIP.

The above presented approaches all represent ways of achieving full density. Based on the analysis of the different methods it can be concluded that the combination of the tailored alloy concepts and consolidation techniques allows manufacturing complex-shaped full-density components for high-performance applications.