Soldering copper and aluminum in the refrigerator: technology, materials and life hacks

Refrigerator repairs are often faced with the need to connect copper and aluminum tubes - for example, when replacing the evaporator, repairing a circuit or eliminating freon leaks. The main problem: these metals have different melting points, coefficients of thermal expansion and are prone to melting when in direct contact. Conventional soldering with tin does not work here - the connection quickly collapses, and freon begins to etch through microcracks. galvanic corrosion with direct contact. Conventional soldering with tin does not work here - the connection quickly collapses, and freon begins to etch through microcracks.

In this article we will examine three reliable methods of soldering (including low-temperature for beginners), we will dwell in detail on the choice of fluxes and solders, as well as let's reveal the secret of joint durability - how to avoid oxidation at the junction of metals without expensive equipment. We will separately discuss typical mistakes due to which craftsmen damage refrigerator parts and why some “folk” methods (for example, soldering with rosin) are categorically not suitable for repairing refrigeration systems.

Why copper and aluminum cannot be soldered in the usual way

The main difficulty lies in the physical properties of metals:

• 🔥 Melting point difference: copper melts at 1083°C, aluminum - at 660°C. When heated, aluminum can deform before copper reaches the desired temperature.
• ⚡ Galvanic couple: upon contact in the presence of moisture (and there is plenty of it in the refrigerator), an electrochemical reaction is formed that destroys the connection in 1-2 years.
• 🛡️ Oxide film: aluminum is instantly coated with oxide (Al₂O₃), which blocks solder adhesion. It must be removed mechanically or chemically.

In addition, refrigeration systems use thin-walled tubes (wall thickness 0.6–1.2 mm), which are easy to burn when overheated. And if you are working with a circuit filled with freon, then improper soldering can lead to explosion of refrigerant vapors when heated.

⚠️ Attention: Soldering tubes with freon residues (even after evacuation) requires mandatory ventilation of the room. Refrigerant vapors R134a or R600a, when heated above 120°C, decompose to release phosgene, a poisonous gas used in chemical weapons.

3 working methods for soldering copper with aluminum

The choice of method depends on the available equipment, the thickness of the metal and strength requirements. Let's consider the options from the most reliable (but complex) to the simplest.

1. Argon arc welding (TIG) with filler wire

An ideal method for professionals, guaranteeing 100% tightness and strength. Used in service centers for repairing industrial refrigerators. Requires:

• 🔧 TIG welding machine with AC/DC function (alternating current for aluminum).
• 💨 High purity argon (not lower than 99.99%).
• 🧵 Filler wire ER4043 (AlSi5) or ER5356 (AlMg5).

Technology:

1. Sand the edges of the tubes to a shine (sandpaper P120–P240).
2. Connect the ground to copper, the burner to aluminum (polarity depends on the mode).
3. Heat the joint to 600–650°C, add an additive and weld the seam "overlapped" or "butt" with tacks.
4. Cool the joint in an argon environment (without access of air).

⚠️ Attention: When welding aluminum with a thickness of less than 1 mm, use the pulse mode to avoid burn-through. The arc power should not exceed 30–40 A.

2. Brazing with zinc-based flux

Suitable for home repairs, if not available. welding machine. refractory solders (melting temperature 400–500°C) and active fluxes are used. The best options:

• 🔥 Solder: Castolin 192 FB (Zn-Al-Cu) or Lugand ALU.
• 🧪 Flux: Castolin Aluflux 2000 or F-SW21 (dissolves aluminum oxide).

Step-by-step instructions:

Key point: heating should be uniformIf you overheat the copper and the aluminum remains cold, the solder will not adhere. Use gas burner with flame adjustment (For example, Rothenberger Super Fire).

3. Low-temperature soldering with pre-tinning

The most accessible method for beginners, but less reliable. Suitable for tubes with a diameter of up to 10 mm. You will need:

• 🔥 Solder: POS-61 (tin-lead) or Sn97Cu3 (lead-free).
• 🧪 Flux: F-64 (for aluminum) + LTI-120 (for copper).
• 🛠️ Soldering iron with a power of 100–150 W or mini-burner.

Algorithm:

1. Tin the copper in the usual way (solder + rosin).
2. Apply flux to the aluminum F-64, rub with a hard brush to remove oxide.
3. Heat the aluminum to 300–350°C and apply a thin layer of solder (tinning).
4. Connect the tubes with an overlap (the gap is no more than 0.2 mm), heat the joint and add solder.

Which flux to choose: comparison table

Flux is 50% of success when soldering aluminum. It removes oxides, reduces the surface tension of the solder and protects the seam from oxidation. Below is a comparison of the most effective compositions:

For refrigeration systems, it is better to avoid fluxes based on chlorides (for example, F-34A) - they cause corrosion when in contact with moisture. The optimal choice: F-SW21 or Castolin Aluflux (despite the high price, they do not require washing and do not destroy the seam over time).

📊 What soldering method have you tried?

Argon arc welding

Hard solder with flux

Low-temperature soldering

Haven't soldered yet

Typical mistakes and how to avoid them

Even experienced craftsmen sometimes make mistakes that lead to freon leaks or destruction of the seam. Here are the most. common:

• 🔥 Overheating of aluminum: at temperatures above 400°C, the metal loses strength. Use thermal paint or an infrared thermometer for control.
• 🧴 Insufficient amount of flux: the oxide film on aluminum is restored in seconds. Apply flux immediately before soldering.
• 🔧 Soldering without gap: the ideal gap between the tubes is 0.1–0.3 mm With “dry” contact, the solder will not penetrate. joint.
• 💧 No flushing after flux: residues F-64 or Castolin cause corrosion. Rinse the seam with hot water and soda (50 g/l).

Another critical error is the use of acid fluxes is permissible (for example, soldering acid). They corrode aluminum within a few months, even if the seam looks strong. For refrigerators, only neutral or slightly alkaline fluxes.

What will happen if you do not wash the flux?

Residues of active flux (especially chloride) react with moisture from the air, forming hydrochloric acid. It corrodes aluminum and after 3–6 months microcracks appear. In the refrigerator, this process is accelerated due to condensation on the tubes.

How to check the quality of a seam

It is not enough to visually inspect the connection - even a beautiful seam can have microdefects. Here 3 teststhat need to be carried out:

1. Visual inspection:
   • The seam must be uniform, without pores and cracks.
   • Color - silver (for Zn-solders) or yellowish (for Sn-solders). Dark spots indicate oxidation.
2. Leak test:
   • Put into the tube nitrogen at a pressure of 15–20 bar (do not use oxygen or air!).
   • Immerse the seam in a soap solution - bubbles will indicate a leak.

Mechanical test:

• Carefully bend the joint (if the design allows). A high-quality seam should not crack.

For refrigerators with freon, a vacuum test is required vacuum test:

1. Connect the vacuum pump to the system.
2. Pump out the air to pressure –0.9 bar (hold for 15–20 minutes).
3. Close the valve and watch the pressure gauge: if the pressure rises by more than 0.1 bar/hour, there is a leak.

⚠️ Attention: Never check the tightness of the freon system with pressurized air. A mixture of air with oil and freon vapor is explosive when heated (risk of detonation during soldering!).

Alternative connection methods (if soldering is not possible)

In some cases, soldering is impractical - for example, if the tubes are heavily oxidized or their diameters do not match. Then use:

• 🔧 Flange connections:
  • Suitable for tubes with a diameter of 12 mm.
  • Requires copper-aluminum adapters (for example, KF-16) and gaskets from nitrile rubber.
• 🧲 Compression fittings:
  • Quick installation without heating (for example, fittings Alcoa or Parker).
  • Required correct tightening - a pinched fitting deforms the aluminum.
• 🔗 Threaded adapters:
  • Use brass nipples s Teflon tape.
  • Not suitable for high pressure (more than 10 bar).

The advantage of alternative methods is no risk of overheating however, they take up more space and can create hydraulic resistance in the system. For major repairs. (for example, replacing a compressor), it is better to combine soldering with fittings.

FAQ: Frequent. questions about soldering copper with aluminum

Is it possible to solder copper with aluminum using ordinary solder (POS-60)?

No. Tin-lead solders do not adhere to aluminum due to the oxide film. At most, it is a temporary connection that will collapse in a few months. solders based on zinc (for example, Castolin 192 FB) or silicon (for example, Alusi).

How to solder if the tubes are different diameter?

Use adapter sleeve (for example, copper-aluminum adapter with flaring). An alternative is overlapping soldering with a copper backing ring (0.5 mm thick). Heat the thicker tube first, then add solder to it. thin.

How to clean the tubes before soldering?

Last sequence:

1. Degrease acetone or isopropyl alcohol.
2. Clean sandpaper (P180–P240) or metal brush.
3. Apply phosphoric acid (10% solution) for 30 seconds, then rinse with water.

Do not use emery from previous work - particles of steel or cast iron will accelerate corrosion.

How long does a soldered joint last in the refrigerator?

With the right technology:

• 🔥 Argon welding: 15–20 years (comparable to factory seam).
• 🛠️ Hard solder: 8–12 years (with flux flushing).
• ❄️ Low-temperature soldering: 3–5 years (higher risk of corrosion).

The period depends on the humidity in the refrigerator and the quality of the vacuum system.

Is it possible to solder tubes without draining the freon?

Absolutely not, even if there is little freon left. vapors:

• Explosive when heated (risk of formation phosgene).
• They will turn the solder into a porous mass (due to reaction with the refrigerant).

Before soldering required pump out the freon with a vacuum pump and flush the system with nitrogen.