SELECTION OF FLUXES FOR ALUMINUM BRAZING

The flux for aluminum brazing is constituted by three agents, e.g. the cov-ering, the oxide-film removal and the interfacial activating agent. Fifteen systemsof various covering agents were examined based on the spreading area of filler onparent metal. The systems of (Li+Sr)Cl, (Li+Na+K)Cl and (Ca+Na+K)Cl werefound to be suitable, those containing iodides and bromides, however, were worstbecause of deposition during melting. The oxide-film removal agents tested werefluorides or fluorine-containing complex compounds, in which LiF was the best.Many chlorides adaptable as interfacial activating agents were scanned. TlCl wasfound to be suitable for all the three metallic base plates tested, e.g. the pure al-uminum, the Al-Mg2.8 and the Al-Cu4.9-Mg1.8. ZnCl_2 and CdCl_2 were good forpure aluminum. BiCl_3 and PbCl_2 were rejected because of causing "dirt" on theparent metal. The most effective content of interfacial activating agent was foundto be about 3 mol%. Owing to the absence of a liquid-metal activated layer, theaction of the flux would not be apparent during brazing. In such a case, ZnCl_2acted the worst, and TlCl the best. The non-fusible loose-cotton-like clusters form-ed in the pool of molten fluxes may be resulted from the LiCl dehydration pro-cess. For convenience, suitable constituents of fluxes for aluminum brazing arelisted in the text.