Brazing of the A7003 with Low Melting Temperature Aluminum Fillers

Abstract

Brazing of A7003 AI-Zn-Mg alloy is considered to be difficult because of its low melting temperature and high content of oxidation susceptible elements such as Mg and Zn. Obtaining high joint strength by vacuum brazing with conventional fillers, where no flux in used, is practically impossible.
In this study, Al-Ge-Si-Mg-Cu and Al-Ge-Si-Mg alloys were examined as brazing fillers for A7003 alloy. These fillers were formed into 100 μm thick foils by PFC (Planer Flow Casting) method and used in the brazing experiment to investigate relation between brazing temperature and joint strength. SEM observation of the brazed interfaces and surface analysis of the specimens were performed.
In Al-Ge-Si-Cu-Mg fillers, the joint strength values measured by tensile test dispersed widely, whereas average strength was 310 MPa, which is 78% of A7003 alloy's tensile strength. In Al-Ge-Si-Mg fillers, the joints brazed with the filler of 753 K melting temperature showed 340 MPa of joint strength in average with narrower dispersion.
Auger electron spectroscopy (AES) carried out on the surfaces of specimens revealed the following facts ;
(1) Before brazing, surface of the alloy is covered with aluminum oxide.
(2) After the brazing at 773 K, aluminum oxide converts to magnesium oxide partially.
(3) Surface oxide changes to magnesium oxide perfectly after the heating at 823 K.
Mg in the fillers therefore acts as wetting agent only in the conditions where the brazing temperature is lower than 773 K, being ineffective at 823 k. This is responsible for the higher strength with narrower dispersion in the joint brazed with the Al-Ge-Si-Mg filler of 753 K of melting temperature.