Influence of feedstock pre-treatment on dynamic flowabilty of HVOF powders

Statistical investigation of spray parameters is the background for the understanding and optimization of thermal spray processes. For the HVOF processes, the mainly investigated spray process parameters are oxygen flow and kerosene amount, lambda value, as well as the feeder settings. The influence of the spray powder characteristics (size and morphology) and flowability of the powders are also subject of research. Nevertheless, conventional flow tests (like Hall flow test) are not always suitable to analyse a powder in flow like during feeding from powder container to the spray gun.

Different powder pre-treatment methods have become established industrially to improve the spray powder flowability and hence, to improve the feeding behaviour. Classical methods are powder mixing to remove agglomerates or heating respectively drying to reduce the content of moisture. Often the powder pre-treatment process is based on individual experience.

In the present work, the dynamic flowability of four different HVOF spray powders is measured. In the study, WC-Co, WC-Co with 100 nm carbides, WC-FeCrAl and Cr₃C₂-NiCr are used as feedstock. The influence of pre-heating with different temperatures and different exposure times on the dynamic flowability is analysed. Furthermore, the pre-mixing and homogenization of the powders is included in the examination.