STUDY OF JOINING MECHANISM(S) IN BRASS/STEEL/BRASS SANDWICH MATERIALS PREPARED BY SCRATCH BRUSHING METHOD

In this research,the mechanism of joining three sheets of metals, i.e. brass-steel-brass, by cold roll welding process has been studied.For this purpose, the two surfaces of steel sheets were roughened with stainless steel wire brush by different amounts , then the brass sheets were put on both sides of the steel sheets, before they were subjected to cold roll process. During rolling, peaks of the asperities on the surfaces of the steel sheet were pressurized,i.e. deformed, much more than that of trough .Hence ,more hardening due to formation of higher dislocation denscity in the peaks regions were detected in comparison to the trough regions. Therefore, due to the differences in the amounts of work hardening occurred during cold rolling in the peaks & trough of the scratches and also due to the nature of the rough surfaces of the steel sheets , which causes the smooth surface of soft brass sheets laied over the rough surface of the steel sheet to be shappend according to the profile of the steel sheet scratches during cold rolling , mechanical locking occurred at the interface of brass & steel sheets. In addition, while the extrusion of brass took place through cracks within the surface of hardend peaks and metal bonding occurred on the contact points of the brass sheet & the vergin steel. Therefore , it seems two mechanisms were in operation is making a suitable joining between the sheets . One was a locking mechanism due to the roughness of the steel sheets & the other was bonding mechanism due to the bonding between the peak points of the scratches & soft brass surface. The strength of the bonded points in the interface were later increased by annealing the composite, so that by annealing the samples within the 500-900°C range for a period of 1 ½ hr the interface strength increase substantially. The results of peeling test indicated that the interface strength of the samples annealed at 700°C or more increased so much that the brass sheet torns during peeling & the fracture did not pass through the interface.