Aluminum Soldering: Techniques

Soldering is joining two or more different metals by melting them then passing a filler metal. You use a filler metal with a lower melting temperature than your workpiece. Aluminum soldering gives you strong joints which can withstand stress. However, before you solder, you must understand the techniques that will ensure you get the best result of joint.

Joint design

Aluminum soldering joint design assembly is like the ones used in other metals. Mostly, the design forms a T-type shape or a simple lap joint. Joint clearance depends on your specific soldering method, flux composition, solder composition or the base alloy composition. Joint clearance range from 0.13 - 0.51mm when using chemical fluxes or 0.05 - 0.25mm when using a reaction type flux.

Preparation

Before you start soldering aluminum ensure the metal surface is clean. The metal surface should be free from grease, dust and other debris. Use a stainless steel wool or brush to clean your surface. Do solvent degreasing to eliminate grease. If the surface is oxidized then clean off the aluminum surface using a chemical cleaner. 

Materials needed

• Stainless steel brush or wool
• Heat source
• Grease remover
• Clean rag work gloves
• Goggles
• Flux
• Solder metal or filler

Caution:

Always wear specialized welding equipment before soldering. Buy a specialized brazing alloy or solder meant for use on aluminum. Perform the procedure quickly before aluminum oxide film forms

Aluminum Soldering Techniques 

There are four known aluminum soldering techniques you can use. They are;

01. Liquid flux and solder

The liquid flux consists of a mixture of inorganic fluoroborate salts and organic amines. You can add chemicals like alcohol to lower the liquid flux viscosity. Liquid flux is perfect for induction soldering as it rapidly deteriorates after a particular temperature. 

02. Paste flux and solder

To create paste flux add chemical binders to your liquid flux. The paste flux give you more precision placement because you use a needle to disperse it on your metal. 

03. Flux-cored soldering filler metal

The flux-cored filler has a higher viscosity and has more solid contents. When you heat the flux, it turns into liquid which you can disperse on your metal surface using an injector. The filler material cools and hardens holding the two metal pieces firmly together. 

04. Soldering filler metal paste

You can get the soldering filler by converting your inorganic flux. To do this, add the binders and the solder powder. Use the paste to solder the aluminum sheet used in high temperature areas. The filler material can handle extreme temperatures of up to 420 degrees Celsius. However, you need to generate more heat to melt this filler material. 

What Are The Types Of Solders?

Solders are mainly categorized based on their melting points. The types of solders are;

Low temperature solders

These solders have a melting point ranging from 300-500 degrees Fahrenheit and are mostly made of Zinc, Lead, Cadmium and Tin. The solder produce joints with low corrosion resistance. 

Intermediate temperature solders

The melting point of an intermediate temperature solder is roughly 500-700 degrees Fahrenheit. The base material is Cadmium or Tin with Zinc. Other materials present in small amounts are lead, aluminum, copper, nickel or silver.

High-temperature solders

These solders have a high melting point of 700-800 degrees Fahrenheit. The base materials is mostly zinc with the solder containing 3-10 percent aluminum. Other materials present in small amounts include silver, nickel and iron. The composition of the materials gives the solder different wetting and melting characteristics. 

Solders with high zinc content exhibit the most strength and are more resistant to corrosion. 

Rules Of Aluminum Soldering 

The following rules will help you solder aluminum effectively and get the best results. Follow the rules to get quality soldering outcome. 

• Clean the aluminum surface to remove dust and other debris before soldering.
• Clamp the parts firmly to ensure they don’t move while soldering
• Use the right type of flux to solder
• Use the correct temperature to melt your filler material

Important Facts To Understand

It’s difficult to solder aluminum because of the formation of aluminum oxide layer. You must first scrap off the layer before starting the soldering process. Solder quickly before more aluminum oxide film forms. Since aluminum has a low melting point of 660 degrees, you need a soldering product that has a lower melting point. 

Safety Tips and Preparation Before Soldering

The following safety tips and procedures before soldering are important:

Identify the alloy

As much as you can solder pure aluminum, it’s a hard metal that is hard to deal with and you will be mostly dealing with aluminum alloy products. You can solder most aluminum alloys using the same method. If the aluminum alloy is marked with a letter or number, check the specific requirements before you solder.

Choose a low-temperature solder

The melting point of aluminum is 600 degrees, which is lower than most solders can handle. Because of the high heat capacity, don’t use a multi-purpose solder. However, an alloy or aluminum with zinc or silicon can handle general-purpose solder. The main priority here is soldering iron because it has a significantly lower melting point. First, confirm that you have a soldering iron specifically meant for aluminum-aluminum joint before buying. 

Pick the aluminum solder flux

Choose a solder flux specifically meant for aluminum. You can get your solder and flux from the same store. The solder flux temperature should be the same as that of the soldering iron. 

Set up your workspace

Always wear a respiratory mask when working for protection against toxic fumes from the soldering process. Work in a well-ventilated space and wear heavy leather gloves and non-synthetic clothing. 

Soldering Aluminum: Step-By-Step Guide

Here is an comprehensive step by step guideline for soldering are given below:

Step-01: Clean your aluminum pieces

Use a stainless steel wire brush to clean your aluminum carefully to remove aluminum oxide film. The film prevents joining of aluminum. Thoroughly scrub the aluminum piece using and clean the solder flux and solder iron to eliminate all oxide layers. If you are using an old aluminum metal with heavy oxidization, sand or grind it or simply wipe it with isopropyl alcohol or acetone. 

Step-02: Clamp the base metals together

It’s an important step if you are joining two aluminum pieces. Clamp them in the specific joining positions and ensure to leave a slight gap for your iron solder to flow. If the pieces don’t fit smoothly, try sanding or bending them. 

Sep-03: Apply aluminum solder flux

After cleaning the aluminum metal pieces, apply the solder flux along the joining places. The solder flux eliminates formation of oxide film and draws the iron solder along the aluminum joint. If you are using soldering wires, dip them in the liquid flux and if the flux is in powder from then consult your label to know the mixing instructions.

Step-04: Heat your metal pieces 

Use the soldering iron when heating your aluminum pieces. Heat the metal piece next to the join section. Start from the lowest position of your workpiece. Direct flame can overheat the solder and flux. Ensure you move the heat constantly in small motions to evenly heat the workpiece. It can take 10 minutes approximately of heating before you apply your soldering iron.

Step-05: Apply solder

Drag the soldering rod or wire over the join as you continue to heat the area indirectly or heating the aluminum piece from the opposite side. Apply in constant slow movements to get an even outcome.    

Step-06: Confirm your results  

Check if the solder bonded perfectly with the aluminum piece. It not, it means an aluminum oxide film formed which interfered with the joining. Clean the surface and solder again to achieve the best results.

Conclusion

The aluminum soldering techniques eliminate the need to plate your aluminum separately before you solder. Apply the techniques and tips that are most appropriate to get great results. Choose the most appropriate technique based on your aluminum alloy composition that will give you the strongest joint. Always clean your aluminum surface before soldering.

FAQ

Does silver solder work on aluminum?

Regular old silver doesn’t work on aluminum. As much as the silver flux is strong enough to handle stainless steel doesn’t mean it will work on your aluminum alloy. You can use a combination of tin-silver to create a special bond.

Which is used as solder for soldering Aluminum?

Use a solder that is a blend of different metals like tin or a combination of silver, copper, tin,. zinc, silicon and bismuth.

Can you use a soldering iron to weld aluminum?

Yes. You can weld aluminum using a soldering iron. This is doable because you are bonding a blending of two metals which is something soldering does. However, there are limits because the soldering iron can’t generate the high temperatures needed to weld some aluminum alloys. Before soldering, thoroughly clean the aluminum surfaces as contaminants can make the solder to fail to stick because aluminum is highly reactive with air. 

Can I solder an aluminum radiator?

Yes. You need a soldering iron and a thick soldering wire. Heat the gun intensely to ensure the solder melts and bonds well with the aluminum radiator surface. Heat the damaged radiator section using the soldering iron then slowly introduce the wire. Don’t drip solder into your radiator.

What is the best solder for aluminum?

Indium solder is the best as it has 100 percent thermal conductivity than any solder containing tin. Besides, indium solder welds easily. However, it has a lower shear and tensile strength than a solder containing tin. The second best is 48 percent tin solder which has a greater shear and tensile strength but lower electrical and thermal conductivity.