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

Abstract

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 microstructures observed. The results showed that the liquidus temperature of Al-Si-Cu filler metals dropped from 593 °C to 534 °C, when the amount of copper was increased from 0 to 30 pct. As the copper content reached further to 40 pct, the liquidus temperature would rise to 572 °C. By adding 2 pct tin into the Al-Si-20Cu alloys, the liquidus and solidus temperature would fall from 543 °C to 526 °C and from 524 °C to 504 °C, respectively. The main microstructures of Al-Si-Cu alloys consist of the α-Al solid solution, silicon particles, the CuAl2 (ϑ) intermetallic, and the eutectic structures of Al-Si, Al-Cu, and Al-Si-Cu. For further improvement of the brazability of this filler metal, magnesium 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 temperature range of 501 °C to 522 °C and a microstructure that includes an Al-Si solid solution, silicon particles, a tin-rich phase, and CuAl2, CuMgAl2, and Mg2Si intermetallic compounds. When this filler metal was used to braze the 6061-T6 aluminum alloy, an optimized bonding strength of 196 ± 19 MPa was achieved.