Protect Al-Cu joints
Coat finished aluminium-to-copper joints with a suitable sealant, heat-shrink sleeve or paint, especially on outdoor units and in coastal areas.
Brazing filler guide
78/22 Zinc-Aluminium Flux-Cored Brazing Rod
A 78 % zinc, 22 % aluminium brazing rod with a non-corrosive flux core joins aluminium to aluminium and aluminium to copper at about 430 – 540 °C, far below the melting point of the base metals. It is the standard low-temperature choice for HVAC/R coil and line repairs and other light aluminium joints made with a torch. This guide covers its composition, melting range, joint design, technique, corrosion behaviour and how it compares with Al-Si fillers.
- Alloy
- 78 % Zn / 22 % Al
- Melting range
- ≈ 427 – 482 °C
- Brazing range
- ≈ 482 – 538 °C
- Joins
- Al–Al and Al–Cu
What a 78/22 zinc-aluminium brazing rod is
Brazing joins metals with a filler that melts above 450 °C but below the melting point of the parts. Classic aluminium brazing uses aluminium-silicon fillers such as 4047, which flow at about 580 – 600 °C, only a few tens of degrees below the solidus of many aluminium alloys. A zinc-aluminium filler with about 22 % aluminium melts roughly 100 °C lower, giving the operator a much wider margin before the base metal sags or melts.
The rod is usually a thin aluminium-zinc sheath wrapped around a core of caesium-based, non-corrosive flux. As the rod is heated, the flux melts first, breaks down the tough aluminium oxide film and lets the molten filler wet both surfaces. Because the flux residue is non-corrosive and non-hygroscopic, joints in most applications do not need washing after brazing, which is a major advantage for field repairs on air conditioning and refrigeration equipment.
The low working temperature also makes aluminium-to-copper joints practical. Copper and aluminium form brittle intermetallic compounds when they are held hot together, and their large difference in melting point makes high-temperature brazing difficult. A zinc-aluminium filler wets copper well at temperatures where intermetallic growth is limited, provided the joint is heated quickly and not overheated.
Composition and physical properties
| Property | Typical value | Comment |
|---|---|---|
| Zinc (Zn) | Balance, about 78 % | Lowers melting range and improves wetting |
| Aluminium (Al) | About 22 % (± 1 %) | Raises strength and corrosion resistance compared with pure zinc |
| Cadmium, lead | Kept to very low levels | Cadmium-free fillers avoid toxic fumes |
| Solidus | About 427 °C (800 °F) | Filler begins to melt |
| Liquidus | About 482 °C (900 °F) | Filler is fully molten |
| Recommended brazing range | About 482 – 538 °C (900 – 1,000 °F) | Heat the joint into this range |
| Density | About 5.2 g/cm³ (0.19 lb/in³) | Calculated from composition |
| Flux | Caesium fluoroaluminate type, non-corrosive | Active at lower temperature than potassium fluoroaluminate flux |
| Typical rod sizes | 1.6 – 3.2 mm diameter; 0.090 in × 20 in is common | Wire and preform rings also used |
Melting figures vary slightly between producers; use the datasheet supplied with your batch. Values above are typical of 78/22 Zn-Al flux-cored products.
Zinc-aluminium alloys of this composition lie on the aluminium-rich side of the zinc-aluminium eutectic, so they melt over a range rather than at a single temperature. That mushy range is useful for filling slightly uneven gaps and building small fillets, but it also means the joint must be heated evenly so that the filler does not freeze part-way through the gap.
Zinc-aluminium versus aluminium-silicon fillers
| Feature | 78/22 Zn-Al flux-cored | 4047 / 4045 Al-Si fillers | Silver / phosphorus-copper alloys |
|---|---|---|---|
| Working temperature | ≈ 480 – 540 °C | ≈ 580 – 620 °C | ≈ 650 – 800 °C |
| Margin below base metal melting | Large | Small: needs good temperature control | Not suitable for aluminium |
| Aluminium to copper | Yes | Difficult | Copper to copper only |
| Flux | Built-in, non-corrosive | Separate or cored; usually non-corrosive | Self-fluxing on copper (BCuP) |
| Joint corrosion resistance | Good when dry; zinc-rich fillet is anodic in wet service | Very good; close to base metal | — |
| Colour match after anodising | Poor | Grey; moderate | — |
| Typical use | Field repair, HVAC/R, low-heat assemblies | Production and furnace brazing, heat exchangers | Copper refrigerant lines |
Choose the zinc-aluminium rod when heat input must be kept low, when joining aluminium to copper, or when an operator with a hand torch needs a forgiving process. Choose an Al-Si filler, preferably in a controlled furnace or with careful torch work, for production joints, thick sections, parts that will be anodised, and heat exchangers that will see long-term wet service.
Joint design and preparation
- Clearance: aim for about 0.08 – 0.13 mm (0.003 – 0.005 in) per side for best capillary flow and strength; wider gaps can still be filled but produce weaker, larger fillets
- Overlap: lap and socket joints with an overlap of about three times the thinner wall, or at least 6 mm on small tubes, give joints stronger than annealed tube
- Cleaning: remove oil, paint and corrosion; abrade aluminium with a stainless brush immediately before brazing to thin the oxide film
- Fit: support parts so they do not move while the filler solidifies, and avoid relying on the filler to bridge poor fit-up
- Copper side: clean copper to bright metal; tin-plated or heavily oxidised copper wets poorly
How to braze with a zinc-aluminium flux-cored rod
- Heat the base metal, not the rodUse a soft, neutral to slightly reducing flame with propane, MAPP-type gas or oxy-fuel set low. Heat the thicker part and the copper side first; aluminium shows little colour change, so watch for the flux to melt as a temperature guide.
- Test the temperatureTouch the rod to the joint away from the flame. When the base metal is hot enough, the flux flows and the filler melts on contact.
- Feed the fillerApply the rod at the joint edge and let capillary action draw it through the gap. Move the flame to pull the filler towards the heat.
- Avoid overheatingExcess heat burns the flux, boils zinc (white fumes), and on Al-Cu joints grows brittle intermetallic layers. Stop heating as soon as a continuous fillet forms.
- Cool and inspectLet the joint cool naturally without quenching. Inspect for a smooth, continuous fillet and leak-test refrigerant joints with nitrogen pressure.
On thin aluminium tubes the biggest risk is melting the tube itself, which happens quickly because aluminium does not glow before it melts. Practise on scrap of the same wall thickness, and use heat sinks or wet rags to protect nearby brazed joints and components.
Corrosion behaviour and long-term durability
A zinc-rich fillet is electrochemically more active than aluminium and copper. In dry or intermittently damp conditions this is not a problem; the joint performs well for many years. In continuously wet, salty or condensate-laden conditions, the zinc can corrode preferentially, and on aluminium-to-copper joints the galvanic couple between copper and aluminium accelerates attack.
Keep condensate away
Avoid locating joints where condensate drips or pools, such as the lowest point of an evaporator.
Flux residue
Non-corrosive caesium flux residue can usually stay on the joint, but remove heavy deposits if the part will be painted or sealed.
Safety when brazing with zinc-aluminium filler
- Work with local exhaust ventilation; zinc oxide fumes form if the filler is overheated and can cause metal fume fever
- Use cadmium-free fillers and check the safety data sheet for flux components
- Follow ANSI Z49.1 or local equivalent rules for gas cylinders, hot work permits and fire watch
- Wear shaded eye protection, gloves and flame-resistant clothing; flux fumes irritate eyes and lungs
- Purge refrigerant systems and recover refrigerant before applying heat to any line
References and standards
- ANSI Z49.1 – Safety in Welding, Cutting, and Allied Processes (American Welding Society)
- AWS Brazing Handbook (American Welding Society)
- ISO 17672 – Brazing: filler metals (Al-Si fillers for comparison)
- ASM Handbook, Vol. 6A – Welding Fundamentals and Processes (brazing and soldering of aluminium)
Zinc-aluminium brazing rod: frequently asked questions
What temperature does a 78/22 zinc-aluminium brazing rod melt at?
It starts melting at about 427 °C (800 °F) and is fully liquid at about 482 °C (900 °F). Joints are made by heating the base metal to roughly 482 – 538 °C (900 – 1,000 °F).
Can it join aluminium to copper?
Yes. This is one of its main uses, for example connecting aluminium coil tubes to copper lines. Heat quickly, avoid overheating, and protect the finished joint from moisture.
Do I need extra flux with a flux-cored rod?
Normally not. The core contains non-corrosive caesium-based flux. Extra flux can help on heavily oxidised parts or wide joints, but clean surfaces are more important.
Is zinc-aluminum brazing as strong as welding?
A well-designed lap or socket joint with adequate overlap is usually stronger than annealed thin-wall tube, so the tube fails first. Butt joints rely on the filler alone and are much weaker, so avoid them.
Why does my rod ball up instead of flowing?
The base metal is not hot enough or the surface is dirty. Heat the parts, not the rod, clean to bright metal, and let the hot joint melt the filler.
Is the joint suitable for outdoor HVAC units?
Yes, if it is protected. Zinc-rich fillets corrode in persistently wet or salty conditions, so seal or coat aluminium-to-copper joints on outdoor units.
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