A New Al-Si-Li Filler Metal that Enhances Brazeability of High-Strength Alloys in CAB and Vacuum

A new family of Al-Si filler metals that contain low levels of Li has been developed for use in both vacuum and NOCOLOK™ brazing processes. The use of Li in the filler metal makes it possible to use the same core-clad combinations for both brazing processes; specifically, higher strength core alloys that contain up to 0.5% Mg. When a Mg-containing core alloy has been clad with the new Li filler metal and is controlled atmosphere brazed, it develops fillets that are equal to or larger than those obtained with conventional Mg-free CAB alloys. No flux penalty was observed when NOCOLOK brazing core alloys that contained about 0.5% Mg. Vacuum brazing the Li-containing filler metals clad to a 0.5% Mg-containing core alloy resulted in fillets that were 200% to 400% larger than typically observed in conventional vacuum brazing sheet that contained 0.25% Mg in the liner. Lithium is much more surface active than Mg; at very low Li levels, near 0.1% for example, the filler metal (liquid-vapor) surface tension is reduced about 25% when compared with conventional Al-10%Si-0.2%Mg alloys. This decrease in surface tension enhances wettability and allows fillet formation to occur before significant Mg poisoning of the flux occurs. The optimum Li range for CAB was found to be 0.01% to 0.05% wt. The optimum Li level for the vacuum brazing process ranges from 0.01% to 0.3% wt.