Effect of Thermomechanical Treatment on Erosion Behaviors of 4343/3xxx Aluminum Clad Sheets during Brazing Heat Treatment

Lightweight aluminum clad sheets are widely used for brazing materials in the automotive heat exchangers due to their high thermal conductivity and specific strength. As a tube material for an automotive heater core, aluminum clad sheets are commonly used, where the clad sheets are composed of 3000-series core alloy cladded each side with 4000-series filler alloy and 7000-series sacrificial alloy. Conventionally, aluminum clad sheets are produced using ingot-cast core and filler alloys. The core alloys contains relatively high Mn concentration (about 1.5wt% Mn), which can be readily produced by the strip casting and the strip-cast core alloy showed enhanced tensile strength with comparable erosion resistance. In this study, AA4343/3xxx clad sheets are fabricated by the roll bonding process using strip-cast bare alloys. The clad sheets are further cold-rolled down to the thickness of 0.2 mm with different heat treatment. Annealing treatment is optionally conducted after roll bonding and final heat treatment is done at the final thickness for preferred tube formability and improved erosion resistance during brazing process. The effect of thermomechanical treatment on the tensile and erosion properties of aluminum clad sheets is elucidated by means of microstructural analysis. Tensile strength of the clad sheets at heat-treated states is inversely proportional to the annealing temperature. The clad sheet annealed after roll bonding shows poor erosion resistance (more than 60% of erosion depth ratio) while the clad sheets without intermediate annealing show less than 10% of erosion depth ratio. The clad sheets without intermediate annealing have larger reductions in thickness at the final thickness and heat-treated microstructures are mixture of cold-rolled and grain growth. However, the intermediately annealed clad sheet has only grain growth microstructure, which may lead severe erosion behavior by easy penetration of Si along the grain boundaries.