As the aluminium based materials being extensively used in industries, especially in automobile manufacturing, the demand for aluminum brazing alloy was increased over time.
Aluminum brazing alloys of the Al-Si series have the most outstanding overall performance. Their wettability, strength, colour and anti-corrosion properties are all sophisticated. This series of brazing products are based on Al-Si system with other selected alloying elements such as copper, magnesium, nickel and rare-earth elements added for various applications. Also, brazing requirements for different types of base metals and joints can be satisfied by choosing the right brazing method, and the options include flame brazing, dip brazing, induction brazing, furnace brazing and vacuum brazing.
Typical products of the Al-Si series include BAiSi-2, BAlSi-5, BAlSi-4, etc. Flux-cored BAlSi-4 brazing wires and rings are made of an Al-Si alloy shell and non-corrosive brazing flux core, assuring quality and stable even in moisture. This type of products are usually applied to braze aluminum parts and aluminum-copper dissimilar material components for HVACR, automobile and electric machinery industries. Flux-cored BAlSi-4 products smartly combine brazing alloy with its compatible flux, thus eliminating the need to apply flux separately. It is well-suited for automatic brazing processes, and are helpful to improve production efficiency.
Pre-braze cleaning of the base metals is essential to ensure quality of the joints. The steps for pre-braze cleaning include, a. degreasing the surface by detergent; b. soaking the braze parts in NaOH solutions, followed by water rinsing; c. Surface neutralisation by soaking the braze parts in diluted HNO3 solution, followed by water rinsing; d. air-drying the base metals. Finally, the air-dried base metals should be free from any acidic or alkaline residue and watermark. Furthermore, the cleaned braze parts ought to be processed at the same day, so as to prevent any new contamination or oxidation.
Gap width of the braze joints is critical to determine compactness and strength of the braze joints. In general, gap width designed for Al-Si brazing alloys usually is in a range of 0.15-0.61mm, and width needs to be even over all the gap. When brazing lap joints, one shall also pay attention to the length of the lap between the base metals. Lap in a range of 4 – 5 t is suggested where t is the thickness of the thinnest part of base metal concerned. When brazing socket joints, blind holes ought to be avoided in the joint design.
Preheating on the base metals may reduce the risk to overheat a local spot during brazing. The brazing process involves the follows, firstly brazing flux is melted by heat; then after BAlSi-4 alloy melts and wets the base metals, driven by siphon force. Finally, BAlSi-4 alloy fills the braze joint, dissolves and diffuses into the base metals. When brazing is completed, slag and residues shall be cleaned away at once.
Relevant tests ought to be performed under the quality specification of the brazing materials applied, which may concern appearance inspection, bonding strength evaluation and micro-structural analysis. A qualified joint should be defect-free from slag, blow hole, crack, incomplete penetration and so on. Metallurgical bonding was formed and evaluated at the brazing interface to optimize the joint strength. Penetration inspection is really needed for socket joints.In addition, tests for evaluating air-tightness and corrosion resistance sometimes are required.