Soldering and Brazing of Aluminum Alloys
This article will discuss the operations of soldering and brazing aluminum alloys. There will be a definition of each techique. Comparisons and differences will be discussed as well. Brazing and soldering aluminum alloys is a very useful method to join this kind of metal. This is especially true where crimp joints, rivets, nuts and bolts, or any other cold connection are impractical or impossible. With skill and practice, the metalsmith can do impressive and effective joints rather inexpensively with tools already owned. This information will be presented in a manner that caters to metal workers but is usefull to any resourceful DIY'er. Below is a breakdown of the contents of this article.
- Misinformation of Aluminum welding
- What is soldering and brazing and how do they compare?
- What aluminum alloys are suitable for brazing and soldering?
- How to heat the base metal and apply the flux.
- Cleaning joint and removing flux
- Soldering aluminum to nonaluminum alloys
Misinformation of Aluminum Welding
There is a lot of incorrect information about soldering and brazing both online and offline. Many salesmen pitch they have a "magic" alloy that can "weld" aluminum without flux of any kind. This is blatently wrong and misleading. Welding means to actually melt the edges of the two pieces of the base metal together. You will quickly realize these hacks are incorrect when you see their "magic" alloy flow into the joint without melting the base metal. This will be discussed in further detail in the soldering section.
Aluminum Brazing Alloy and Flux
What is Soldering And Brazing And How Do They Compare
Both soldering and brazing use a filler metal that is melted by heat and used to join a base metal(aluminum alloy) that is not to be melted. Soldering refers to the process of using a low melting alloy and either flux or no flux. Soldering occurs when the filler has a melting point below 800 Fahrenheit(426 Celsius). Because brazing uses temperatures over 800 Fahrenheit(426 Celsius) the use of flux is an absolute must. In the case of aluminum, the alloys used are usually 4043 or 4047 with 4043 being most common by far. These are silicon/aluminum alloys and they melt at just 100 or 200 less than the base metal being joined. The silicon adds fluidity to the alloy. This assist in capillary action which leads to a stronger joint. It shoul also be mentioned that casting alloys are also silicon alloys so intricate details can be filled out in castings.
What Aluminum Alloys are Suitable for Brazing and Soldering
Most of the common aluminum alloys can be brazed with a couple of notable exceptions. The alloys not brazeable are the alloys from the 2000 and 7000 series. The 2000 series are commonly known as aviation alloys. The active alloying element is copper. These alloys ,when brazed or welded, form stress cracks. The 7000 are also high strength aerospace alloys and will suffer the same fate as the 2000 series.
The most common alloys commercially available are 3003 and 6061. The 3003 alloy is a manganese alloy that takes to brazing and welding very well. It can be bent without cracking or tearing like other aluminum alloys. The 6061 alloy is an alloy that contains magnesium. It is far stronger and tougher than 3003 aluminum. This alloy is heat aged to add to its strength. While this alloy is weldable and brazaeable, the heat affected zone(HAZ) Is significantly weaker that the parent metal due to loss of age tempering properties. Also, this alloy is more prone to crack while bent unless careful attention to the recommended bend radius is heeded.
Aluminum brazing rod
How to Heat the Base Metal and Apply Flux
A very important point to remember with aluminum is that its melting point is considerably lower than incandescent heat. In other words, you are not going to heat it to glowing red like steel to braze it. Heat management can be tricky with aluminum since there is no noticeable color change.
For metal less than 1/8" and not too large, a propane or MAPP torch shall suffice. While slowly moving the flame back and forth, dip filler rod in powdered flux a deposit on joint. The flux will undergo some changes with heat. First, it will start as a white gooey sticky mass. In time it gets fluid but is still thick. Finally, when about 900 fahrenheit(500 Celsius) it gets clear and watery. This is when you can start applying filler rod with additional heat. The flux removes oxides and provides a protective coating over the filler rod which will soften and eventually flow penetrating the joint.
For heavier gauge metals, you will need oxyacetylene torch as the heat source. It is more challenging to get thicker aluminum up to 900 degrees Fahrenheit to help the flux flow. One trick is to go over the joint with a sooty acetylene only flame to deposit black carbon on the base metal. Then apply heat until the soot burns off which is around 900 degrees. You apply flux and then filler run as usual. For thicker metals, it is advised to champher the joint to allow more penetration.
Soot as a heat indicator
Aluminum Brazing with Flux
Cleaning Joint and Removing Flux
After satisfactory penetration of the filler metal into the base metal, it is time to lets it cool. Once cool the flux needs to be removed completely since it is corrosive. The best way is to have a stainless steel wire brush dedicated to just aluminum work to avoid contamination from other metals. WIth hot running water and wire brush, scratch at the joint to ensure all flux is removed.
As mentioned above, soldering encompases joining metals together with a filler metal that melts below 800 fahrenheit(426 Celsius). There are two major kinds of solder for joining aluminum alloys to each other and to other metals and alloys. One is the fluxless zinc based alloy that melts at 730 Fahrenheit(388 Celsius). The fluxless zinc alloys are marketed by several companies as miracle welding rods and are usually stageringly overpriced. You can obtain a tube of these at your nearest Harbor Freight and are called Alumiweld. Because there is no flux used, this alloy can be difficult to apply. The area around the joint must be thoroughly cleaned to remove as much the oxide coating as possible.
Another alloy that is used for soldering is a tin and zinc alloy that melts at under 500 fahrenheit. A flux is used that changes brown when suitable temperature is reached then alloy can be applied.
Al Solder 500
Soldering Aluminum to Non Aluminum Alloys
Solder, because of its low melting point and wetting capability with many metals, is ideal for joining aluminum to dissimilar metals such as brass, copper, and even ferrous alloys. The heating methods are the same and flux is applied when needed as in the case with tin based solder filler metal.