Soldering aluminum joints
Types of Soldered Joints
To ensure effective penetration of solder into the seam, there must be a suitable gap between the metals to be joined. It means, that in almost all cases it is necessary to guarantee tight geometric tolerances as for individual products of two elements, so for complex multi-element designs.
When brazing aluminum, two main types of joints are used (Figure 1):
- butt joints and
- lap joints.
All other types of compounds are modifications of these two basic types..
Figure 1 – Solder joint design 
The lap joint has increased strength due to the additional soldered area and thickness of the connected elements. They are easy to carry out and they require minimal fixation before soldering or do without it at all..
Butt joints are not so strong, like lap joints. Therefore always take into account, that the soldered butt joint will always be weaker, than the base metal.
In practice, combinations of these two types of solder joints are usually used (see. below picture 2).
In the butt joint, the ends of both metal elements are joined opposite each other. Solder between the ends of the elements or applied in advance, or placed on top of the joint after, how both elements are connected together. In the process of soldering, the solder melts and flows into the gap between the elements due to the phenomenon of capillarity.
Butt joints are usually used there., where strength requirements are not critical or when the use of lap joints is not possible, for example, due to thickness limitations.
The main weakness of the butt joint is the small soldering area. It is therefore important, so that the ends of the joints are rectangular and parallel. Rounded ends can significantly reduce the effective brazing area (see. Figure 1).
In lap joints two metal elements, which are soldered, just stacked on top of one another. The strength of the brazed lap joint depends on the size of the lap and the thickness of the solder joint itself. An acceptable overlap size is three thicknesses of the thinnest of the components to be joined. A wider overlap does not increase the strength of the soldered joint, and too small overlap can lead to damage to the soldered seam, not the base metal. For good strength of the overlap brazed joint, the surfaces to be joined must be sufficiently close to each other and parallel (see. Figure 2).
General requirements for solder joints
Soldered joints of aluminum and aluminum alloys - solid and soft - must meet certain requirements :
- First of all, solder joint design - weld construction - should allow free application of flux on the joined surfaces immediately before soldering.
- Moreover, must be foreseen, so that the solder flux is replaced by solder, since the remaining flux at the junction is a potential source of corrosion.
- The design of the solder joint should also allow gas removal and complete penetration of the solder throughout the seam.
Typical Solder Joint Designs
Typical solder joint designs for aluminum and aluminum alloys are shown in the figure. 2.
Figure 2 - Typical solder joint designs (hard and soft)
for aluminum and aluminum alloys 
When designing solder joints using fluxes, special attention is paid to the ways of supplying flux to the connection and ways to remove it from it. for instance, lap seams, for which a long distance solder flow is required, construct like this, so that this flow occurs in only one direction. Otherwise, if solder flows from both ends, flux is trapped inside the seam.
Solder Seam Design Requirements
A good solder joint design should take into account the following :
- The contact area of the solder should be sufficient to provide the necessary strength, tightness and good thermal and electrical conductivity.
- Sufficient clearance must be provided between the surfaces to be joined for the flow of flux and solder.
- The method for applying solder should be like this, so that it is clearly visible and accessible for manual adjustment.
- Possibility of removing flux reaction products and its residues from the surface of the compound.
- Minimum access for corrosion.
- If it's necessary, then the joint shape should be suitable for subsequent protective coating.
Solder joint clearances
When soldering aluminum, two types of joints are mainly used - lap joints and T-joints. Purely butt joints are rarely used..
When soldering with overlapping soft solders, the size of the gaps from 0,13 to 0,40 mm, and for solders - from 0,05 to 01,25 mm. These data apply to overlaps no more 6-7 mm. For wider laps, wider gaps are required - up to 0,65 mm - so, to allow solder to flow into internal areas of the joint.
- EEA Aluminium Automotive Manual – Joining – 6. Brazing, EEE, 2015
- Aluminum and Aluminum Alloys, ASM International, 1996