High-Performance Full Density Structural and Functional Components - Processing-Microstructure-Property Relationship in PM and AM

Doctoral thesis, 2024

Powder Metallurgy (PM) and Additive Manufacturing (AM) offer resource- and cost-efficient manufacturing for the fabrication of near-net shape components with tailored properties and high precision. These fabrication technologies are regarded as a source of consistent innovation, particularly in the development of new materials and novel processing routes.

This thesis study encompasses both PM and AM technologies, with the objective of investigating their utilization for structural and functional applications. Water-atomized and gas-atomized steel powder grades were used to fabricate the structural components using PM process. Cold isostatic pressing (CIP) process achieves uniform density distribution and enables geometrical complexity through lubricant-free isostatic compaction. The materials included have been the steel powder grade pre-alloyed with 1.8%Cr and admixed with 0.3% graphite and 2% Ni, the Vanadis 4E tool steel, and the FeCrAl powder. To apply CIP, the latter two gas-atomized powders were granulated prior to compaction using an appropriate binder and freeze-drying process.

Sintering of these materials has been achieved to remove the surface oxides. In addition, sintering at high enough temperatures has facilitated the complete elimination of interconnected pores into the components. Hereby, hot isostatic pressing (HIP) has offered a final stage for reaching full density, without the need for capsule. Hence, the combination of CIP, sintering and capsule-free HIP enables the creation of novel components with full density.

The AM technologies enable fabricating complex designs compared to conventional manufacturing methods. Pure copper and 316L austenitic steel are used for the functional applications demonstrated, following the use of powder bed fusion - electron beam (PBF-EB) and powder bed fusion - laser beam (PBF-LB), respectively. Optimized process parameters using melt strategies such as hatch and point melting with PBF-EB resulted in full density with high thermal and electrical conductivity for pure copper. The EBSD analysis revealed the influence of low angle- and high angle grain boundaries on the conductivity of the components. Precision parts of 316L were manufactured as prototype components for waveguide applications using PBF-LB and the geometrical accuracy in the range of tens to micrometers was demonstrated. The feature control capability was depicted by RF resonance frequency measurements and small spread of RF resonance frequency between the samples was shown.