A strict process control is critical when brazing aluminium. Tenacious aluminium oxide makes most brazing processes with conventional means difficult. In addition, care must be taken when choosing the metal mixture. Aluminium alloys, lengths and temperature often show very varied brazing results. Before commencing production, a careful study of how aluminium accepts or rejects heat during brazing is required.
Joining can be done by soldering (Sn-based, low temperature) or brazing (Zn-based, high temperature) and with appropriate fluxes that suit the processing temperatures. Soldering is by definition a low-temperature joining process. Therefore, less deformation of the component is expected with aluminium soldering than with brazing, welding or other melt bonding processes. Soldering temperatures of 225 to 490°C are way below the aluminium melting temperature of 661°C, although 490°C is above the glow point.
Different aluminium alloys have different solderability: 1xxx, 2xxx, 3xxx, 4xxx and 7xxx are easier to solder than alloys from the 6xxx-series. Due to their magnesium content, and alloys from the 5xxx-series are the most difficult to solder.
Due to the high thermal conductivity and reflectivity of aluminium, the heat source requires to be tailored to the specific process and product and its geometry.
Use of Flux
The rapid formation of an aluminum oxide layer and the difficulty in removing it so that the solder can wet aluminium are the reasons for using flux. In “normal” copper soldering, removing copper oxide is relatively easy with mild organic and inorganic fluxes. Aluminium oxide, however, is not easily removed and may require stronger fluxes such as an organic amine-based flux (up to 285°C), inorganic fluxes (chloride or fluoride up to 400°C), and complex fluoroaluminate salts (above 550°C). The use of mechanical rubbing, ultrasonic or thermal spraying depends on the use of molten zinc to scrub or remove the aluminum oxide layer and enable wetting of aluminium. No flux is used. For soldering with tin or zinc the first two fluxes are used, as their melting point is lower than 330°C and the zinc portion helps prevent galvanic corrosion. For solder-based zinc products, Mat-Tech uses fluxes that offer higher melting temperatures.
The residue of some soldering fluids may still be active after soldering and need to be removed. Solders used for aluminium usually contain zinc with lead, cadmium, tin, copper, or aluminium. However, solder containing tin can cause an electrochemical corrosion problem. Solders containing cadmium have been banned due to health concerns they could pose. Soldering with lead is also restricted.
Lead-free and cadmium-free alloys commonly used for soldering aluminium are 91Sn9Zn, 70Sn30Zn, and 98Zn2Al. Other alloys in the Zn/Al family are 85Zn/15Al, 90Zn/10Al, and 97Zn/3Al. Other variations include 60Sn/40Zn and 80Sn/20Zn, which are part of the Sn/Zn family.
Aluminium often has other elements added to improve strength, stiffness, corrosion resistance, workability, and formability. Some additions pose no problem for soldering, but magnesium is the exception. Magnesium-containing aluminum alloys (e.g., 5xxx and 6xxx series) are used to expand the strength-to-weight ratio and for better corrosion resistance in some applications.
Magnesium oxide reforms very quickly and does not allow solder wetting to occur. Titanium and some exotic additives such as vanadium and chromium can also cause problems. The series 1xxx (99% Al or higher), 2xxx (added copper), 3xxx (added manganese), 4xxx (added silicon), and 7xxx (added zinc) are generally suitable for soldering. The 5xxx (added magnesium) and 6xxx (added silicon and magnesium) are not suitable for soldering.