The coating of steel strip with aluminium powder by roll compaction

This thesis examines a process in which mild steel strip is coated with aluminium to produce a cheap, strong, corrosion-resistant material. The steel strip is initially wetted and sprinkled with a thin layer of aluminium powder. After subsequent drying the strip is passed through a set of two high rolls to compact the coating and provisionally bond it to the steel. The product can be coiled and finally sintered. A survey of the existing literature reveals that one of the main problems with the process is that air is expelled from the powder as it compacts. At higher roll speeds this tends to fluidise the powder entering the roll nip and cause uneven coating. Several methods of preventing this have been suggested but previous workers have tended to concentrate on adding binders to the powder. This work therefore confines itself to the actual mechanical behaviour of powder and substrate in the roll nip. A technique is presented in which the results from an unusual item of powder compaction equipment can be used to predict pressure profiles and final coating densities for this type of two-component system. Experimental evidence indicates that the analysis is not valid at higher roll speeds even if allowance is made for the additional pore pressure caused by entrained air. However, if more information could be obtained on the behaviour of powders at high strain rates then the technique could be a valuable design tool at all roll speeds for any powder–solid system.