Soldering is an economical and practical means of joining aluminum on a production basis. With careful attention to such details as surface preparation, solder composition, temperature, and application of heat, a variety of joints can be soldered. Although less heat is required to raise the temperature of a piece of aluminum sheet of a given thickness than is  required for a sheet of copper or steel of the same thickness, aluminum must be heated from 55 to 110°C (100 to 200°F) higher than either of these metals when it is to be soldered. The higher  temperature is specified to produce joints with good resistance to corrosion, and is one of the key factors in producing successful soldered joints in aluminum.

Preparing the Surface

As a first step, it is necessary to remove the oxide film on aluminum so that the filler metal can contact and bond with the parent metal. This is accomplished by one of the following methods:

(1) Mechanical abrasion

(2) Application of ultrasonic energy

(3) Electroplating

(4) Use of either chemical or reaction-type fluxes

Mechanical Abrasion

Scraping is the simplest way to remove oxide. Due to the rapid rate at which the film re-forms on aluminum, scraping is impractical unless it is accomplished in the  presence of molten solder. The solder then wets and bonds with the parent metal and results in a precoated or “tinned” surface.

Although there are many variations of the process, one example is as follows: Two sheets of aluminum are heated to the melting temperature of the solder. A small amount of solder is then melted on the sheets and rubbed with an abrasion tool until the solder wets the surface. The two precoated sheets are then placed together and held in contact until the solder solidifies. A strong joint results.

A fibrous glass brush is one of the most satisfactory abrasion tools, since no corrosion hazard is created and the close-packed strands remove the oxide without damage to the parent metal.

Some solder rods, called “abrasion solders,” have melting characteristics which permit them to perform the dual role of solder source and abrasion tool. However, only a precoated or “tinned” surface is produced, and a second operation is generally required to complete the joining.

Ultrasonic Cleaning and Soldering

Cleaning- Ultrasonic energy can be used to remove oxide film on aluminum. An electronic power oscillator is used to generate electrical impulses (currents) at frequencies from 15 to 50 kHz; these electrical impulses are converted to mechanical motion by a device known as a magnetostrictive transducerCommercial transducers used in soldering tools consist of a nickel core and a coil around the core that is connected to the oscillator. When the nickel core (a laminated nickel core is generally used to reduce eddy currents) is subjected to an electromagnetic impulse resulting from electric current flowing through the coil, it constricts a maximum of 30/1 000 000 (30 x of its length. If the end of the vibrating core is brought into contact with molten solder, the vibrating core will produce numerous holes, or voids, within the liquid.When aluminum is immersed in the liquid solder, the collapse of the voids  reates an abrasive effect known as cavitation erosion on the surface of the metal. This erosive action removes the oxide film.

Soldering- In ultrasonic aluminum soldering, the area to be precoated, or “tinned,” is cleaned, heated to soldering temperature, about 190°C (375”F), and the solder, usually a 90-10 tin-zinc combination, is applied. A quantity of solder is melted on the surface to form a molten puddle, and the end of the transducer is swept over this surface. The ultrasonic energy removes the oxide from the aluminum, allowing a firm solder bond.

The ultrasonic method can also be applied in dip soldering, or, with modifications, in brazing and welding. The primary advantages of the ultrasonic process are that no flux is required, and joint quality is equal to that of joints soldered by any other process using the same solder and parent metal. The disadvantages are high cost of equipment, small capacity of the units, and the limitation that direct soldering of lap or crimp joints is not practical.

Plated Surfaces for Soldering

It is possible to prepare the aluminum surface to be soldered by electrolytically plating it with a metal, such as copper. Before deposition of the copper, the aluminum surface is treated by immersing the aluminum in a solution of alkaline sodium zincate. The zincated surface is then electrolytically plated with copper to produce a surface that can be easily soldered with the conventional solders and fluxes used to solder copper.

Fluxes for Soldering Aluminum

Chemical and reaction fluxes are the types generally used for soldering aluminum. Chemical fluxes are usually recommended when the joint temperature is less than 275°C (525°F). However, in some applications, the maximum temperature limit can be successfully raised to 325°C (620°F). At temperatures exceeding 275°C (525″F), the chemical fluxes decompose; at temperatures above 325°C (620″F), this decomposition becomes so rapid that it is impractical to use this type of flux.

In general, chemical fluxes are used with the tinlead-cadmium-zinc solders. For best results, the magnesium content  if the aluminum alloy being soldered should not exceed 1%, and the silicon content  should not exceed 5%.

All of the common commercial reaction fluxes deposit zinc or tin, or both, on the aluminum surfaces. These metals alloy with the aluminum, and a thin alloy layer is formed in the area near the original surface of the material.

Solders for Aluminum

There are four groups of commercial solders for aluminum: zinc base, zinc- admium base, tin-zinc base, and the tin-lead base. All these may contain appreciable quantities of other metals. Table A-6 shows the composition of typical solders for aluminum.

The zinc-base solders produce joints with shear strengths of 103 MPa (15 000 psi) and higher, with good corrosion resistance. These solders require soldering temperatures ranging from 370 to 435°C (700 to 820°F).

The zinc-cadmium base solders develop joints with shear strengths in excess of 70 MPa (10 000 psi), with intermediate corrosion resistance. They require soldering temperatures of 265 to 400°C (5 10 to 750°F).

The tin-zinc base solders develop joints with shear strengths in excess of 48 MPa (7000 psi), with intermediate corrosion resistance. They require soldering temperatures of 290°C (550°F) or higher. The tin-lead solders containing cadmium or zinc produce joints with shear strength in excess of 34 MPa (5000 psi), with corrosion resistance adequate for interior applications only. These solders are applied at soldering temperatures of 230°C (450°F) or higher.

Solders high in zinc content are applied to aluminum for a soldered system that is very resistant to corrosive attack. Hot dip tinned surfaces are used in special applications to produce readily solderable surfaces, since tin quickly wets an aluminum surface from which the oxide has been removed. Thus, pretinned aluminum soldering materials and techniques cannot be used. However, molten tin penetrates aluminum- magnesium alloys along the grain boundaries, and alloys containing more than 0.5% magnesium can be seriously damaged by this penetration. Cadmium is only slightly soluble in solid  aluminum and forms a very limited diffusion zone in aluminum soldered joints. Cadmium is not usually used as a solder by itself, but is used effectively to improve the  properties of zinc- and tin-base solders. Lead is practically insoluble in solid aluminum and is not normally used as a solder by itself. In combination with tin, zinc and cadmium, lead forms an important class of solders for aluminum.

Joint Design

The joint designs used for soldering aluminum are similar to those used with other metals. The most common designs are lap, crimped, and T joints. Capillary spacing varies with method, alloy, solder, joint, and flux. Generally, joint spacings from 0.25 to  0.60 mm (0.010 to 0.025 in.) are maintained when a chemical flux is used, and from 0.05 to 0.25 mm (0.002 to 0.010 in.) with reaction fluxes.

Torch Soldering

Air-fuel gas or oxyfuel gas torches are used effectively to solder aluminum assemblies. The flame temperature (gas mixtures) and heat output (torch size) can be independently adjusted to provide optimum conditions for specific applications. The flux is usually painted on the joint, and the solder is either pre-placed or manually fed into the joint using solder wire. The best torch soldering technique involves heating the assembly initially on both sides of the joint area until solder flow can be initiated in the joint area. The flame can then be moved to a position directly over the joint and slightly behind the front of the solder flow. In this way the flame does not come into direct contact with the flux before it has performed its function, and the speed and ease of soldering is at a maximum.

Furnace Soldering

Furnace soldering is a highly productive, efficient method for fabricating aluminum assemblies. In this process, the entire assembly is raised to temperature, thus minimizing distortion. The solder is usually preplaced in the joint, using wires, shims, or washers of filler material. Flux is applied by spraying, painting, or immersing the part in the flux by flowing a liquid flux over the assembly. The assembly is then placed in a furnace and brought to temperature. The flux must be carefully protected against charring or volatilization before it has performed its function. Joint design and furnace characteristics should be such that all sections of the joint are brought to temperature at the same time in order to prevent excessive alloying and penetration by liquid solder.

Dip Soldering

Dip soldering is an efficient process for joining assemblies at a high production rate. It is a versatile process because the same techniques used for other metals can often be utilized for soldering aluminum by merely changing solder and flux. Any of the solders listed in Table A-6 can be used for dip soldering. Solder selection should be based on service and operating characteristics required, and cost of the solder.

In dip soldering, the flux tends to insulate the part to be soldered from the solder, thus a heavy coat of flux will reduce the rate at which the part is brought to soldering temperature. Since the rate of heating will be greatest if a small amount of flux is used, and because solder will prevent the surface from being reoxidized, a dilute liquid flux is recommended for dip soldering. Also, the flux should be selected to operate at the  optimum temperature of the solder to minimize drossing, dissolution, and liquid metal penetration, and to provide the best operating characteristics possible.

Soldering Aluminum Alloys

While aluminum and all the aluminum alloys can be satisfactorily joined by soldering, the alloying elements influence the ease with which they are soldered. Alloys commonly used in commercial applications are 1100, 1145,3003,5005, and 6061.

Commercially pure aluminum (1100), aluminum of higher purity (1145), and aluminum-manganese (3003) alloys can be readily joined using all soldering techniques. Aside from ensuring that the surface is reasonably free of extraneous dirt or corrosive produced, no special surface preparation is needed for soldering these alloys. They are also resistant to intergranular penetration by liquid solder.

Use of molten tin solders results in intergranular penetration in alloys containing 0.5% or more magnesium. Zinc solders will also cause intergranular penetration of aluminum-magnesium alloys, but the extent of penetration is usually not significant until the magnesium content of the parent alloy exceeds 0.7%.

Aluminum alloys containing more than 5% silicon are not usually soldered by procedures requiring the use of a flux.

The addition of zinc or copper to aluminum does not materially reduce the solderability. However, these metals are used in combination with other elements to form high-strength, heat-treatable alloys. Films formed on the surface during heat treatment reduce the solderability, so a chemical surface pre-treatment is usually recommended. In some instances, alloys such as 2024 and 7075 have been satisfactorily soldered using reaction fluxes without using chemical pretreatment. If chemical fluxes are used, a chemical pretreatment is usually required.

Additions of small amounts of magnesium and silicon to aluminum produce an alloy system commonly referred to as the aluminum-magnesium-silicate alloys. These alloys, 6061 and 6063, are easily soldered and are not as susceptible to intergranular penetration by liquid solder as the binary aluminum-magnesium alloys of a similar magnesium content.

Excellent Solderability

Binary aluminum-magnesium alloys, in sheet and other forms, provide excellent solderability, and include:

1030, 1050, 1060, 1070, 1075, 1080, 1085, 1090, 1095, 1099, 1100, 1130, 1145, 1160, 1171, 1180, 1187,1197, and 3003.
Chemical or reaction fluxes may be used.
Good Solderability

Alloys considered “good” for soldering are:

3004, 5005, 5357, 6053, 6061, 6062, 6063, 6151, 6253, 6951, 7072, and 8112.
With the exception of the first two, reaction type flux is recommended.
Fair Solderability

Fair solderability is accorded alloys:

2011, 2014, 2017, 2018, 2024, 2025, 2117, 2214, 2218, 2225, and 5050.
Poor Solderability

The alloys rated as poor for soldering are:

5052, 5652, 7075, 7178, 7277, 4032, 4043, 4045, 4343, 5055, 5056, 5083, 5086, 5154, 5254, and 5356.

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