The Brazing Behavior of Aluminum-Steel Cladding Strip with Pre-Cladded Brazing Layer Used in Air Cooling System of Power Plants

The brazing behavior of 3003 fin assemble with 4343(filler layer)/4A60(transition layer)/08Al(steel layer) brazing sheet (the ratio of filler layer thickness to transition thickness were constant 1:2) were investigated. This study elucidates the brazing results vary with the layer thickness and brazing temperatures. Results showed that keeping the holding time at brazing temperature constant (the holding time of this study was 10 minutes in all brazing progress), under the condition of brazing temperature less than or equal to 580 °C, thinner filler layer led to well formed joint, while thicker filler layer led to defects at brazing joints because of poor mobility. When brazing temperature was up to 600 °C, a thinner transition layer trended to lead to the formation a thick intermetallic compounds layer on the Aluminum-Steel surface. It was inferred that Si diffused from molten filler metal to the interface of transition layer and steel layer and promoted the formation of intermetallic compounds. Furthermore, it was found that in the surface away from brazing joints, with lower Si content, the intermetallic compounds were mainly η (Fe2Al5), while higher Si content region nearby brazing joints trended to form τ5(Al8Fe2Si) and τ6(Al4.5FeSi). Meanwhile, a transition layer with thickness above 134μm could avoid the formation of intermetallic compound. The optimum brazing process was that temperature range was 590 °C to 595 °C with 10-minutes holding time and the transition layer thickness was suggested to be equal or more than 134μm