Disclosure: This post contains affiliate links and I will be compensated if you make a purchase after clicking through my links. Learn More
Here’s how to braze aluminum: clean the joint, coat it with aluminum brazing flux, heat the base metal to about 1,080°F to 1,120°F, and feed in a 4047 (BAlSi-4) aluminum-silicon filler rod. The filler melts and flows into the joint before the aluminum part melts.
The catch is the narrow heat window. Pure aluminum melts at about 1,221°F, so you have roughly 100°F between a good braze and a slumped part.
Time: about 30 minutes for a small part | Difficulty: moderate | Tools: torch, 4047 filler, aluminum brazing flux, stainless brush
What Is Aluminum Brazing?
Brazing joins metal with a filler that melts above 840°F but below the base metal. The base aluminum never melts, so the joint relies on the filler wetting and flowing between close-fitting parts.
The common filler is AWS A5.8 BAlSi-4, also sold as alloy 4047. It’s about 12% silicon and the rest aluminum. Lucas-Milhaupt lists its solidus at 1,070°F (577°C) and liquidus at 1,080°F (582°C).
For the bigger picture on brazing and soldering, the main differences come down to filler melting point and joint strength.
What You’ll Need
- Filler rod: 4047 (BAlSi-4) aluminum brazing wire, such as Harris Al-Braze 1070 or Lucas-Milhaupt Al 718.
- Flux: an aluminum brazing flux matched to the filler, such as Al-Braze flux. Use a cesium-based flux for magnesium-bearing alloys.
- Torch: one with enough heat to bring the whole joint area up to brazing temperature.
- Cleaning tools: a stainless steel wire brush, acetone or a degreaser, and clean rags.
- Clamps or fixtures to hold a tight, even gap.
- PPE: brazing goggles or a face shield, leather gloves, and fume extraction.
| Spec | Value (4047 / BAlSi-4) | Source |
|---|---|---|
| Silicon content | About 12% | Lucas-Milhaupt Al 718 TDS |
| Solidus | 1,070°F (577°C) | Lucas-Milhaupt, Harris |
| Liquidus | 1,080°F (582°C) | Lucas-Milhaupt, Harris |
| Brazing range | 1,080°F to 1,120°F (582°C to 604°C) | Lucas-Milhaupt, Harris |
| Joint clearance | 0.003 to 0.005 in. per side | Lucas-Milhaupt Al 718 TDS |
| Al-Braze 1070 flux working range | 915°F to 1,115°F | Harris spec sheet |
How to Braze Aluminum: Step by Step
Step 1: Check the Base Alloy
Confirm the part is a brazeable aluminum alloy. Its melting point must sit well above the filler’s 1,080°F liquidus.
Magnesium-bearing alloys such as 6063 need a cesium fluoroaluminate flux, per Lincoln Electric’s Aluxcor data sheet. Harris also says its Al-Braze kit is for aluminum to aluminum, not aluminum to other metals.
Step 2: Clean the Joint
Degrease both parts with acetone. Then brush the joint with a stainless steel brush used only on aluminum.
Brush right before you braze. Aluminum oxide reforms fast, and flux works best on a fresh surface.
Step 3: Fit Up the Gap
Clamp the parts with a gap of 0.003 to 0.005 in. per side. Lucas-Milhaupt rates that range as optimum for the highest joint strength with Al 718.
Too tight and the filler can’t flow in. Too loose and the joint fills with weak filler instead of drawing in by capillary action.
Step 4: Apply the Flux
Harris says to mix the powdered flux with water into a paste and brush it on the joint. You can also heat the rod tip and dip it in the flux powder.
Keep the flux jar closed between uses. Harris notes the flux is hygroscopic and absorbs moisture from the air.
Credit: www.youtube.com
Step 5: Heat the Base Metal
Heat the parts, not the rod. Move the flame across the whole joint area so it warms evenly, and heat the thicker part more.
Watch the flux. When it melts and turns clear and liquid, the joint is close to brazing temperature.
Step 6: Feed the Filler
Touch the 4047 rod to the joint at the edge away from the flame. The hot base metal should melt the rod and pull the filler through the gap.
If the rod balls up and sits on top, the part isn’t hot enough. Keep heating the base metal, not the rod.
Don’t linger. Lucas-Milhaupt warns that long heating lets the filler erode into the base metal.
Step 7: Cool and Clean
Let the part air cool. Then scrub off flux residue with hot water and a brush, since chloride flux left on the joint absorbs moisture and can attack the aluminum.
Flux-cored rods with a non-hygroscopic fluoroaluminate core are different. Lincoln Electric says that residue does not need to be removed.
Common Mistakes to Avoid
- Heating the rod instead of the part. The filler balls up and never wets the joint.
- Skipping the flux. Harris says the flux is required except in vacuum brazing.
- Overheating. Past the narrow window, the base aluminum sags or melts.
- Dirty metal. Oil and thick oxide keep the filler from wetting.
- Wrong flux for the alloy. Magnesium-bearing alloys need a cesium-based flux.
- Brazing aluminum to steel or copper with Al-Braze. Harris warns the joint may turn brittle.
Credit: www.youtube.com
Brazing vs Welding Aluminum
Brazing keeps the base metal solid, so thin parts warp less and you don’t need a TIG machine or a spool gun. It suits thin sections, tubing, and repairs.
Welding melts the base metal and fuses it. It gives full-strength structural joints on thicker parts. If you don’t own a welder, brazing is one way to weld aluminum without a welder in the everyday sense.
Safety Tips
- Keep your head out of the fumes and use exhaust at the flame, as Harris and Lincoln Electric direct.
- Read the flux SDS. Aluminum brazing fluxes contain chloride and fluoride salts.
- Wear eye protection, leather gloves, and flame-resistant clothing, and follow ANSI Z49.1.
- Clear flammables from the area and keep an extinguisher close.
Frequently Asked Questions
Can You Braze Aluminum With a Propane Torch?
Yes, on small, thin parts. A propane torch can reach the 1,080°F to 1,120°F brazing range on small joints, but it struggles on thick or large parts that pull heat away. A hotter MAPP or oxy-fuel torch heats bigger parts faster and more evenly.
What Temperature Do You Braze Aluminum At?
Most aluminum brazing uses 4047 (BAlSi-4) filler. Its brazing range is 1,080°F to 1,120°F (582°C to 604°C), per the Lucas-Milhaupt and Harris data sheets. That’s only about 100°F below the melting point of pure aluminum, so watch the flux and control the heat closely.
Do You Need Flux to Braze Aluminum?
Yes, for torch brazing. Harris says its Al-Braze flux is required except in vacuum brazing. Flux removes the oxide layer and lets the filler wet the metal. Flux-cored rods carry the flux inside, so you don’t need a separate jar of flux.
Is Brazed Aluminum as Strong as Welded Aluminum?
Usually not for heavy structural loads. Brazing is strong on a well-fit lap or sleeve joint with a 0.003 to 0.005 in. gap. Harris lists tensile strength up to 35,000 psi for its 1070 joints, but welding suits thick, load-bearing parts better.
Can You Braze Aluminum to Copper or Steel?
Not with a standard aluminum-silicon kit. Harris warns that Al-Braze joints to non-aluminum metals may become brittle. Aluminum to copper calls for special fillers, and aluminum to steel usually needs a transition insert, a coating, or a bolted mechanical joint instead.
Final Words
Brazing aluminum comes down to clean metal, the right flux, a tight gap, and careful heat. Keep the part between 1,080°F and 1,120°F and let the hot base metal melt the rod.
Start with scrap pieces of the same alloy. A few practice joints teach you how the flux looks when the part is ready.
