Research On Brazing And Interfacial Characteristic Of Aluminum Alloys To Precoating Stainless Steel

In recent years, the hybrid structures of aluminum and stainless steel, which are widely valued in the field of air conditioning pipe, attract the great attention for its properties of excellent corrosion resistance and low temperature performance. Large amounts of brittle intermetallic compounds(IMCS)were formed during welding of aluminum/steel dissimilar alloys due to the great difference in their thermal physical properties. Therefore, the key to obtain excellent joint of aluminum/steel is controlling IMCS. Liquid solder is used to wet parent metal in brazing. Because the soldering temperature is lower than the parent metal melting point, this method can control the formation of IMCS effectively. However, the major problem is the poor wettability of filler metal on steel substrates, retards its research and application on joining of aluminum/steel hybrid structures. Thus brazing of precocating-steel and present to get good quality joints.Aluminum coating on stainless steel was applied to enhance the wetability and spreadability of the filler and control the growth of IMCS.This paper has developed systemic research on joining of Al/Steel dissimilar alloys with brazing method. Firstly,a layer of aluminum or Al Si12 was plated on the surface of stainless steel by hot-dipping. Interface Structures of the stainless and dip coating with different coating materials have been researched and analyzed. The wetting process of liquid Al on solid steel during hot-dip aluminize has been analyzed. The interfacial characteristic and the interface compounds forming rules has been analyzed through thermodynamic calculation. Based on the conditions, research of precoating stainless steel/Al brazing techniques, using high frequency induction brazing and flame brazing two methods to obtain a good brazed joints by changing the dipped material, welding current, the heating time and other conditions. Then, we compare and analyze the microstructure, interfacial reaction layer morphology and phase characteristics of the joints with different brazing parameters. We build the relationship of technology, organization and performance. Finally, we combin with the thermodynamics, kinetics theoretical knowledge and test results to analysis growth behavior of the interface reaction layer; Mechanical properties of brazed have been evaluated and analyzed by micro-hardness, tensile test.The results showed that uniform aluminized coating was obtained through hot-dip aluminum method with the assistance of flux under atmospheric conditions. Stainless steel is immersed in liquid aluminum plating process has four stages including: non-wetting, incipient wetness, wetting extension and complete wetting. The interfacial layer was composed of Fe2Al5 and Fe Al3. Hot-dip Al Si12 tight reaction layer was composed of Fe Al3 and Fe2Al8 Si. In brazing procedure, coating improves the solder wettability on stainless steel. Brazed joint is mainly composed of steel, brazing seam, reaction layer and aluminum. brazing seam is mainly composed of α-Al and Al-Si eutectic. It has been identified that the proper frequency brazing current is 270 A ~ 330 A, brazing time is 25 s ~ 35 s. To maintain a good solder flux conditions, the heating time is constant, with the induced current increases, the reaction layer thickness and aluminum-rich phase increase, the mechanical properties decrease; When the current is constant, with the increase of heating time, kinds of reaction layer are same, the reaction layer thickness increases, the mechanical properties decrease.Not-dipped interface reaction layer is mainly composed of 5~7μ m Fe(Al,Si)3 and 1~2μ m serrated Fe2Al8Si; Hot-dipped aluminum reaction layer is mainly composed of 3~5μ m edge shaped Fe(Al,Si)3 and 1~2μ m flat Fe(Al,Si)2; When hot-dipped Al Si12, interfacial reaction layer can be divided into three layers distinctly. With the inhibition of hot dip reaction layer, growth of the interfacial layer was suppressed. As a result, thickness of the interfacial layer decrease. The shear strength of hot-dipped aluminum brazed joint is 164.2MPa which is higher than not hot-dipped aluminum brazed joint whose shear strength is 102 MPa and the fracture location is in base metal.