How to Braze Aluminum: A Guide

Aluminum brazing has become a widely used method for repairing cracks, fixing leaks in aluminum pipes or tanks, repairing castings, or fabricating aluminum parts. Not only is brazing often preferred over welding due to its cost-effectiveness, but it also requires minimal metalworking and welding experience.

Unlike welding, brazing does not necessitate the use of heavy and expensive welding machines, making it an ideal solution for those seeking a more accessible and efficient alternative.

In this article, we will go over how to braze aluminum, the different techniques used, and the common applications of aluminum brazing.

What Is Aluminum Brazing?

Aluminum brazing is a joining process that connects two aluminum parts using a filler metal with a lower melting point than the base material. The filler metal is heated until it melts and flows into the joint, where it solidifies and forms a bond between the two parts. The process requires precise temperature control and cleanliness to ensure a successful bond. Aluminum brazing is commonly used in the automotive and aerospace industries for applications such as heat exchangers, air conditioning systems, and engine components.

The Different Types of Aluminum Brazing

There are several different types of aluminum brazing. Let’s briefly discuss each kind to help you choose the appropriate brazing method.

Manual and Automatic Brazing

As with many other related joining processes, there are both manual and automatic brazing processes.

In industrial and manufacturing settings, there are advanced automated brazing systems that are both large in size and expensive. These systems utilize a temperature-controlled enclosure where components are loaded into a chamber, the parts are brazed in a temperature-controlled enclosure, and the filler metal is fed into the joint automatically at a carefully controlled rate.

Manual brazing involves a hand-held torch that uses either propane or oxy-acetylene, where the operator heats up the entire part as uniformly as possible and then adds the filler metal in a hand-held brazing rod.

Aluminum Dip Brazing

Aluminum dip brazing uses a salt bath containing flux. The braze filler metal is formed around the part and then lowered into the bath at a high temperature (liquidus temperature of over 840 degrees Fahrenheit). This method is often used for brazing small parts, resulting in a relatively neat and uniform connection when properly performed.

Furnace (Controlled Atmosphere) Brazing

Furnace brazing is a semi-automatic process in which aluminum is brazed by using low-melting temperature filler metal in a furnace. This is done using flux or in a vacuum-enclosed chamber where no flux would be necessary for the absence of atmospheric contaminants. The temperature is controlled so the part is brought up to the correct temperature, and the brazing rod is added to complete the desired connection. This technique is used mainly for complex designs, which must be welded at different stages or all at once.

Induction Brazing

Another way to braze aluminum is with the use of induction coils. First, the part is inserted into an electromagnetic field created by the use of induction coils. Next, the electromagnetic field generates the necessary heat, and the filler rod is added to the part. This technique is frequently employed for joining small sections of pipe and is known to yield uniform and high-quality brazed joints.

What You Need To Braze Aluminum

To braze aluminum, you will need the following:

A torch, either a small propane torch or a small oxy-acetylene setup
A pack of brazing rods, with the right rod specified by the manufacturer's instructions on the package for the base metal being joined
Flux paste to coat brazing rods if they are bare
A wire brush, scuff pad, or fine-grit sandpaper to clean the parts to be joined.

How To Braze Aluminum

Now let's get down to the practical discussion regarding how to braze aluminum with a hand-held MAP gas torch or a propane torch and aluminum brazing filler rod.

Clean Your Workpieces

As with any other welding or brazing process, cleaning the part is essential to quality workmanship. You can use many different cleaning methods depending on the size and shape of the pieces involved in the brazing operation. For example, fine-grit sandpaper or a scuff pad can work well on newer materials.

A wire brush on a grinder may be necessary for materials with odd size and shape configurations or those that have come into contact with grease or paint. Aluminum is a nonferrous metal that can be oxidized and form a surface oxide layer of aluminum oxide (AL2O3) under normal room temperature conditions. Therefore, proper cleaning of the surface using wire brushes and fluxes is necessary to achieve a successful brazed joint.

Acetone is a widely used solvent that effectively removes oxidation and surface contaminants from the workpiece. Although muriatic acid can also be useful in cleaning aluminum, it should be handled with care.

Prepare Gas for Torch

It is crucial to use a clean tip to ensure a uniform flame, regardless of the gas used (MAP gas, propane, or oxy-acetylene). Additionally, it is essential to reduce the flame or use a neutral flame when brazing aluminum, as aluminum is prone to oxide formation when subjected to an oxidizing flame.

Be sure you have enough gas or propane to complete the joint. Running out part way through will only mean you have to start the process all over again. Adequate heat distribution on all the intended joints is necessary to get a sound joint without any voids due to the lack of filler metal distribution.

Joint Fit Up

In brazing, the molten filler metal flows in a horizontal or vertical direction within the joint due to capillary action, which is caused by the surface energy of the two lapped surfaces. Therefore, to get a valid sound joint, the joint gap should be narrow as per the standard to get the capillary action during brazing.

Start Heating the Joint Gradually

The hardest part in brazing aluminum, and the most critical factor in achieving a proper joint, is keeping the heat uniform throughout the part being joined. Excessive heat will only melt the aluminum, and insufficient heat will result in a lack of uniform bonding.

1200 degrees Fahrenheit in the base metal is a good general rule of thumb. When the base metal has been heated to the desired temperature, you are ready to add the brazing rod. You do not need to apply heat directly to the brazing rod. The brazing rod will melt at a lower temperature than the base metal and will melt when brought into close contact with the base metal.

The now liquified brazing rod will melt into the joint by capillary action. Check your work for any places where there may be gaps or voids, and repeat the process as necessary. Be sure to clean off any excess flux with a rag or cleaning cloth upon completion. The amount of heat required depends mainly on the thickness of the parts to be brazed. This amount is controlled mainly by selecting the proper torch tip size, where the heat increases as the tip size increases.

Benefits of Aluminum Brazing

There are several benefits to brazing aluminum. The first is the comparatively low cost of brazing instead of welding aluminum. Welding requires expensive machines and a source of electricity sufficient to supply the required amp and volts to the machines. All you need to braze aluminum is a handheld torch which makes brazing inexpensive, highly portable, and versatile.

In some types of aluminum applications, the lower heat input from the brazing process might result in lower residual stress in the base metal than you would get in a welded joint.

It is also possible to join dissimilar metals by brazing that would not be possible to do so by welding. Aluminum to copper is one such example.

Risks of Aluminum Brazing

However, there are a few risks associated with brazing that you should be aware of before choosing the joining process for your particular application.

The joints made by brazing should not be used in joining structural or pipe assemblies in high-temperature applications. Many brazing rods melt at temperatures around 700 to 800 degrees Fahrenheit, so temperatures exceeding that range could result in failure in the joint.

It’s important to note that many fluxes used in brazing aluminum may contain toxins. Always read the manufacturer’s recommendations and use the required personal protective equipment.

Common Applications of Aluminum Brazing

Brazing is a versatile technique that can be employed in various repair and fabrication applications, including repairing aluminum boats, fabricating aluminum furniture or decorative items, repairing radiators, small sections of pipe, or aluminum fuel tanks.

Aluminum Brazing FAQ

How strong is aluminum brazing?

When performed correctly, a brazed joint can be just as strong as a welded joint. Additionally, since brazing is done at much lower temperatures than welding, the metallurgical properties of the base metal are undisturbed or unaltered, which can mean less residual stress in the finished product.

What is the best kind of aluminum brazing rod?

The 4043 alloy rod is a common type of aluminum brazing rod. It has a lower melting point compared to other aluminum alloys, making them easier to work with, and they provide good corrosion resistance.

5356 rods are another common type of aluminum brazing rods with a higher melting point and provide better strength and crack resistance compared to the 4043 rods.

It is important to note, however, that the choice of the best kind of aluminum brazing rod will always depend on the specific application and the type of aluminum being brazed.

Is brazing hard to learn?

Brazing aluminum does not take a great deal of skill and experience to learn. A beginner should consider a couple of practice tries on some scrap metal of comparable size and thickness before working on the finished product, but with a little trial and error, you will have no trouble finding the optimal temperature to achieve a proper brazed joint.

How much cleaning is required to achieve a good brazed connection?

For new aluminum, minimal cleaning to remove any minor oxidation may be sufficient. However, if you are trying to braze an aluminum fuel tank, more extensive cleaning may be required due to aluminum’s porous nature.

In cases where grease or paint is present, heat up the material a bit with a torch and use a wire brush attachment on a grinder. Muriatic acid or acetone can also be used to clean grease and paint.

Sources: Image courtesy Mastars.com via Unsplash.