Development of a low-melting-point filler metal for brazing aluminum alloys

The study is concerned with developing low melting-point filler metals for brazing aluminum alloys. For this purpose, thermal analyses of a series of Al-Si-Cu-Sn filler metals have been conducted and corresponding microstruct ures observed. The results showed that the Liquidus temperature of Al-Si-Cu filler metals dropped from 593 degrees C to 534 degrees C, when the amount of copper was increased from 0 to 30 pet. As the copper content reached fu rther to 40 pet, the liquidus temperature would rise to 572 degrees C. By a dding 2 pet tin into the Al-Si-20Cu alloys, the liquidus and solidus temper ature would fall from 543 degrees C to 526 degrees C and from 524 degrees C to 504 degrees C, respectively. The main microstructures of Al-Si-Cu alloy s consist of the alpha-Al solid solution, silicon particles, the CuAl2 (the ta) intermetallic, and the eutectic structures of Al-Si, Al-Cu, and Al-Si-C u. For further improvement of the brazability of this filler metal, magnesi um was added as a wetting agent, which would remove the residual oxygen and moisture from the brazed aluminum surface and reduce the oxide film. Based on results gleaned from the thermal analyses, a new filler metal with the composition Al-7Si-20Cu-2Sn-1Mg is proposed, which possesses a melting temp erature range of 501 degrees C to 522 degrees C and a microstructure that i ncludes an Al-Si solid solution, silicon particles, a tin-rich phase, and C uAl2, CuMgAl2, and Mg2Si intermetallic compounds. When this filler metal wa s used to braze the 6061-T6 aluminum alloy, an optimized bonding strength o f 196 +/- 19 MPa was achieved.