Soldering of aluminum
Unlike aluminum brazing, aluminum soft brazing uses solders with a melting point below 450 oC. The melting point of aluminum is 655-660 oC.
(For aluminum brazing, see. here)
Solderability
Wrought Aluminum Alloys
All aluminum alloys can be soldered. However, the ease and quality of soldering is affected by:
- the chemical composition of the alloy
- solder type
- soldering method.
The relative low temperature soldering ability is shown in the figure 1.
Figure 1 – Relative ability of wrought aluminum alloys for low-temperature soldering [1]
Casting aluminum alloys
Most casting alloys contain a significant amount of alloying elements.. This increases the likelihood that, These elements will be dissolved in the solder, which reduces their wettability., what is important for good soldering.
Any roughness or porosity on the cast surface will trap the flux and make it difficult to remove..
Three casting alloys, which are relatively easy to wet with solder, are alloys 443.0, 443.2 and 356.
Other metals and non-metals
Any industrial materials, which can be soldered can be soldered to aluminum using a suitable flux. Table 2 shows the possibility of brazing aluminum with various other materials.
Table 1 – The ability of aluminum alloys to low-temperature soldering with other metals and non-metals [1]
Solders
Solders for aluminium are divided into:
- low temperature
- medium temperature
- high temperature.
Table 2 shows the chemical composition of typical solders for aluminum brazing. Common characteristics of solders include:
- melting range (solidus and liquidus temperatures);
- wetting ability;
- type of flux used: active, organic or one of them;
- corrosion resistance of solder and seam after soldering.
Table 2 – Chemical composition and properties of typical solders for aluminum soft soldering [1]
Low Temperature Solders
Low-temperature solders melt and flow at a temperature from 150 to 260 oC. They consist mainly of low-melting metals such as:
- lead,
- lead,
- zinc,
- cadmium,
- and sometimes bismuth.
These solders may also contain small amounts of high melting metals., such as aluminum, copper, nickel and silver.
Strength low-temperature solders – very soft - shear is only 34 MPa. The melting point of this group start with solders 110 oC. aluminum compounds, those made too soft solders, They have relatively low resistance to corrosion and therefore rarely used for applications exposed to corrosive atmosphere, or.
Tin-lead solders
Low-temperature tin-lead solders are subject to creep. At sufficiently high load may suddenly deteriorate. Typically, the compound of 60Sn-40Pb solder at a tensile load 9600 kPa "bursts" within a few hours. By reducing the load to 690 kPa a brazed joint will stay 165 days. Therefore, these solders are used mainly for sealing, compound twisted wire, as well as for such tasks, as, for example, Hold metal edges in some kind of grooves.
Lead-tin solders
Binary lead-tin solders, which are conventionally used for brazing copper, can in principle also be used for soldering aluminum and. However, they are in the application of certain difficulties. Formed compound obtained weak, and so they are rarely used for aluminum. Lead alone is almost insoluble in aluminum , and tin additives only slightly increase its wettability. Lead-tin solders, designed for aluminum, usually contain small amounts of zinc, cadmium and other metals for improving the wettability and fluidity.
Tin-zinc solders
Tin-zinc eutectic mixture 91Sn-9Zn melts at only 199 ° C, good spreads and wets the aluminum. This solder is the most corrosion-resistant of all the low-temperature solders. Its strength is comparable with medium-strength soft solders.
Medium temperature solders
Medium-soft solders melt temperature of from 260 to 370 oC. They preferably contain tin, or cadmium, in various combinations with zinc. These solders may also contain minor amounts of aluminum, copper, lead, nickel or silver. Since these solders contain 30-70 % zinc, they are well wetted and aluminum form a more durable and more corrosion-resistant compound, than the low-temperature solders.
Medium- tin-zinc solders exhibit high shear strength - to 3800 kPa. These solders are often used for exterior joints, but for protection from exposure to the atmosphere are coated with paint or similar materials. Tin-zinc solders melt at a temperature 290 ° C or higher.
High temperature solders
High-temperature solders melt temperatures between 370 and 450 oC. They consist of 90-100 % zinc and may contain 2-10 % aluminum and small amounts of copper, gland, nickel or silver. These additives reduce the temperature of the soldering, They provide a wide melting range and improve solder wettability aluminum. Aluminum zinc-aluminum solders reduces their ability to intergranular penetration.
High-temperature solders are the most durable and least expensive of those, which are used for soldering aluminum. They also provide a higher corrosion resistance as compared with low-temperature and medium-temperature solders. To ensure high corrosion resistance of these solders must have a minimum of impurities, Hoping to bismuth, cadmium, lead, tin and other low melting point metals.
Zinc-based solders
Zinc-based solders may contain from 90 to 99,999 % zinc. They provide the highest strength of all industrial "soft" solders. Because the shear strength reaches 125 MPa or above. Soldering temperature above their, than all other solders - from 380 to 425 ° C. Zinc solders provide the highest corrosion resistance. The more zinc in the solder, the higher the corrosion resistance.
Zinc solders have a particularly high corrosion resistance, when they do not contain a low-melting metal impurities, such as bismuth, cadmium, lead or tin. Smaller quantities of copper, silver, Titanium and other metals are added to zinc solder to improve their flowing and wetting properties. In small amounts, these components hardly reduces the corrosion resistance of zinc solders.
To reduce the tendency of zinc to penetrate between the grains of some aluminum alloys, it adds aluminum. However, aluminum additions reduce the corrosion resistance of zinc solders, decrease of their melting temperature and plasticity.
Fluxes
There are two types of fluxes for low-temperature aluminum brazing (table 3):
- organic fluxes
- chlorine-containing inorganic salt fluxes (reactive).
Table 3 – Characteristics of typical soldering fluxes [1]
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