TEM assessment of HVOLF thermally sprayed Al–12 wt.% Sn–1 wt.% Cu alloy

The high velocity oxy-liquid fuel (HVOLF) thermal spray technique has the potential to fabricate thick layers based on the Al–12 wt.% Sn–1 wt.% Cu alloy. The microstructure of the as-sprayed coatings exhibits several levels of complexity due to the effects of rapid quenching and metastable phase formation. The use of tripod polishing and low angle milling procedures have allowed the sectioning of large thin areas of material for plan view and cross-sectional transmission electron microscope (TEM) observation, thereby clearly delineating regions of material that have undergone differing patterns of melting and freezing during the HVOLF process. Comparison has also been made with the pre-sprayed gas-atomised powder. The microstructure of most of the larger gas-atomised powder particles comprises Al dendrites surrounded by Sn interdendrite phase. Conversely, <10 μm sized powders prior to spraying adopt a nanoscale Sn particulate structure embedded in an Al matrix. This emphasises the dominant effect of cooling rate on the resultant microstructure. The HVOLF thermal spraying process causes the powders to adopt a mixture of molten, semi-molten or indeed unmelted states prior to layer on layer impact onto steel substrates. Nanoscale Sn particles embedded in the Al matrix are considered characteristic of fully molten material that has undergone rapid cooling, whereas large scale Sn particles are exhibited by material that was semi-molten at the time of impact.