Aluminum soldering can be simple but has a number of critical areas that need tight process control. Tenacious aluminum oxide makes most attempts to solder using conventional means difficult. In addition, care must be taken regarding alloy choice due to potential galvanic corrosion consequences because of aluminum’s dissimilarity with many conventional solders. The varieties of aluminum alloys, gauges, and tempers often display widely varying soldering results, and how aluminum accepts or rejects heat during soldering must be carefully studied for each individual job.
Soldering can be done with either soft solders (Sn-based, lower temperature) or hard solders (Zn-based, higher temperature) and with appropriate fluxes to fit processing temperature ranges. By definition, soldering is a low-temperature joining process. Therefore, less distortion of the aluminum component is expected by soldering than by brazing, welding, or other fusion joining processes. Soldering temperatures of 225 to 490°C are well below the 661°C aluminum melting temperature, although 490°C is above the annealing point. Stresses in the aluminum from shearing, drawing, and heat treating are changed by the localized heating encountered during soldering, and distortion may result. Preheating, noncontinuous joints, and careful selection of joint geometry becomes critical.
Various aluminum alloys have different solderability: 1xxx, 2xxx, 3xxx, 4xxx, and 7xxx are easier to solder than the 6xxx series alloys. Due to their magnesium content, 5xxx series alloys are the most difficult to solder.
Left. Coupons of Alloy 6111, 2 ¥ 4 ¥ 0.036 in. with 2-in. overlap. The top coupon has a 0.125-in. hole centered in the overlap area to facilitate introduction of Zn/15Al hard solder wire. Solder flows to every edge providing complete wetting of the joint.
Methods or processes in aluminum soldering involve mechanical rubbing of aluminum with solder, ultrasonic bath soldering, thermal spray (these three do not use fluxes), heating the assembly by induction, flame, infrared, hot plate, furnace, soldering iron, laser, and arc lamp (all of which usually involve the use of fluxes). Soldering aluminum requires an adequate volume of heat on the component, not the solder. Because of the high thermal conductivity and reflectivity of aluminum, the heat source must be tailored to the job.
Use of Flux
The rapid formation of an aluminum oxide layer and the difficulty in removing that oxide layer so the solder can wet the aluminum are the reasons for the use of flux. In “normal” soldering of copper, removal of the copper oxide is relatively easy with mild organic and inorganic fluxes. Aluminum oxide is not so 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, ultrasonics, or thermal spray depends upon using the molten zinc to abrade or blast away the aluminum oxide layer and allowing subsurface wetting of the aluminum. No flux is used. Tin/zinc soft solders are typically used with the first two fluxes since their melting point is under 330°C and the zinc portion helps in preventing galvanic corrosion. Zinc-based hard solders use fluxes that offer higher melting temperatures to activate. The residues of some soft soldering fluxes may be still active after soldering and must be removed. Solders used for aluminum generally contain zinc with some lead, cadmium, tin, copper, or aluminum. However, any solder that contains tin may cause an electrochemical corrosion problem due to its galvanic potential. With the anticipated worldwide ban on lead in solder, most industries have already or are switching to lead-free solders. This removes some of the more ductile and/or higher-temperature soft solders available. Cadmium-bearing solders have been effectively banned due to worker health issues.
Right. Close-up confirms total wetting. Appearance changes where reaction occurs between the flux and surface oxidation, yet residues are considered noncorrosive. Joints of this type are generally stronger than the base material.
Lead-free and cadmium-free alloys that are commonly used to solder aluminum include 91Sn9Zn, 70Sn30Zn, and 98Zn2Al. Other alloys in the Zn/Al family include 85Zn/15Al, 90Zn/10Al, and 97Zn/3Al. Other variations are 60Sn/40Zn and 80Sn/20Zn, which are in the Sn/Zn family.
Aluminum often has other elements added to improve strength, rigidity, corrosion resistance, machinability, and formability. Some additives cause no problem for soldering, but magnesium is the exception. Magnesium-containing aluminum alloys (e.g., 5xxx and 6xxx series) are used for extending the strength-to-weight ratio and to provide better corrosion resistance in some applications. However, the authors are not aware of any solder or flux that is very effective with magnesium-containing aluminum alloys. The magnesium oxide reforms very quickly and does not allow solder wetting to take place. Titanium and some exotic additives such as vanadium and chromium may also cause problems. The 1xxx (99% Al or higher), 2xxx (copper added), 3xxx (manganese added), 4xxx (silicon added), and 7xxx (zinc added) series are generally solderable. The 5xxx (magnesium added) series is probably not solderable and the 6xxx (silicon and magnesium added) series may or may not be solderable depending upon the individual alloy. The 6061 alloy is definitely solderable and the 2xxx series in sheet form may have a 6xxx cladding that could change its solderability.
Left. Coupons of Alloy 6111 soldered using Zn/15Al and a flux based on complex fluoroaluminate salts. For the purpose of this test, one length of 0.093-in.-diameter solder was placed on one side of the joint, then pulled through to the opposite side with heat.
Cladding or Coatings
In some cases the aluminum can be clad with a more solderable alloy, plated with nickel, or coated with zinc by thermal spray or other methods. This surface is then more solderable and eases the above problem since they are both easier to solder than just aluminum. Soldering aluminum to other metals (steel, galvanized steel, copper, brass, stainless, etc.) is also done, but with some difficulty since the joint design must allow for differential thermal expansion and many fluxes do not work for both metals. The simple job of heating the assembly at the joint area becomes difficult since the aluminum conducts heat away from the joint very rapidly vs. other metals’ tendency to conduct heat away much more slowly (stainless steel comes to mind). A general rule of thumb in soldering is “heat the component, not the solder.” This allows the substrate to transfer heat to the solder and melt the solder once it is up to the melting temperature. Fluxes can insulate the solder from the substrate and cause the reactivity of the flux to expire before the solder melts or, perhaps, leave a hard residue that the solder cannot penetrate in order to wet the substrate. Cored soft solders may be used to eliminate this problem since the flux is not released until the solder melts; however, not all aluminum solders are available with flux cores.
Dangers of Overheating
Due to its low melting temperature, aluminum may be annealed or tempered at temperatures as low as 325–350°C in a relatively short time. This suggests that any joining process approaching these temperatures for more than a brief interval may begin to alter the properties of the base metals being joined. Overheating may result in stress relieving, sagging or warping panels, altering hardness, temper, surface condition, re-alloying of the base metal in the immediate joint area, hot cracking, or even a dreaded meltdown.
Left. Close-up confirms good fillets on both sides. Zn-based hard solders may not be as pretty as soft solders, yet they are not susceptible to galvanic corrosion when soldering aluminum, as are Sn-based alloys.
Generally speaking, soft solders do not pose much of a risk to the base materials from heating, provided the parts are not held at soldering temperatures for an extended period of time. However, in some cases, exposure of aluminum to a molten zinc alloy for even a short period of time may result in re-alloying of the base metal within the heat-affected zone (HAZ). This may change its properties and cause what appear to be heat cracks that emanate beyond the HAZ.
One final tip: Working in the laboratory can aid in process, alloy, and flux selection. A mock-up might be helpful to determine the type, location, and volume of heat required to accomplish the desired result. As in other processes, preheating or hybrid heating may be helpful and may change the original process selection. Cooldown times and delay before handling may vary substantially from the laboratory to the production floor. Aluminum soldering is not difficult, but neither is it very forgiving. Control the process tightly.
Metals Handbook, 10th Ed.,Vol. 2. 1990. Properties of wrought aluminum and aluminum alloys. Materials Park, Ohio: ASM International, pp. 102–103.
ASM Desk Editions Online — Metals Handbook Desk Edition. Brazing and soldering. Retrieved August 1, 2002, from http://www.asminternational.org/asm/servlet/Navigate. Materials Park, Ohio: ASM International.
Fenton, E. A. 1963. Brazing Manual. New York, N.Y.: American Welding Society. p. 9.
Rivard, John D., Sabau, Adrian S., and Schwartz, M. M. 1987. Brazing. Metals Park, Ohio: ASM International.
A. E. Gickler and F. H. LePrevost, Jr., are with Johnson Manufacturing Company, Princeton, Iowa.