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Controlled atmosphere brazing of aluminum air cooled oil cooler without flux was investigated.The materials of plates and mounting plate used in the researchare fluxless brazing alloy with five layers.The materials of the other parts are convemntiidodnlael 3003 single layer aluminum alloy.After assembling, the oil cooler is put into the brazing furnace, without appended any flux on it.After brazing, the surface of the products is dark, and the brazing seams between middle plates can hardlybe seen from the outside of the oil cooler by naked eyes.The metallographic analysis of the sections of the oil cooler shows that the fillets of the brazing seams near the outside of the oil coolerare very small, but the fillets of the brazing seamsof the interior are very big.This phenomenon shows that the molten brazing filler flows from outside of the oil cooler to the interior during brazing.The SEM and EDS analysis results shows that a flux layer is covered on the surface of the middle plates of the outside of the oil cooler.The existence of the flux layer effects the oxide removing ability of the fluxless brazing materials.Consequently, the wetting and spreading abilities of the brazing filler on the surface of the aluminum alloy after melting are influenced.