#1758
Author: Charles A Bishop, Bishop Consulting
Email: CABishopConsulting@cabuk1.co.uk
Subject: Spitting from the aluminium molten pool relates to impurities, the oxide layer on the aluminium wire and the stability of the process.
Spitting from the aluminium molten pool relates to impurities, the oxide layer on the aluminium wire and the stability of the process. If there are impurities these can be a source of spitting and so higher purity wire can give a better quality product. However there are other factors that can be a worse source of spitting and may be worth addressing first.
All aluminium oxidises, it is the dense oxide layer on the surface of aluminium that protects the aluminium from further oxidation/corrosion.
When the aluminium wire hits the hot boat the aluminium melts, wets the boat & forms a molten pool. The aluminium oxide from the surface of the wire does not melt but forms a skin or slag on the surface of the molten pool.
There is a tendency for this slag to accumulate at the edges of the pool.
If the pool size is unstable and so the pool size increases and then decreases and increases again this accumulated slag at the edge of the pool will be pushed onto new and possible hotter parts of the boat and this can be a significant source of spitting causing pinholes/pin-windows. This changing size of the molten pool can be caused by many factors such as the wire feed hunting which can be related to the coating thickness monitoring & feedback control. Hence it is good policy to look to maintain stable operating conditions. If the pool is stable then if the number of spits is high it is then worth looking at paying for higher purity wire.
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