Overview: Copper-to-aluminum welding is performed primarily to reduce costs and weight. Since aluminum has approximately one-third the density and one-fifth the price of copper, it is frequently used as a substitute—particularly in the power industry—necessitating copper-aluminum welding. However, both copper and aluminum surfaces oxidize easily.
k：167 W/m·K（25°C）
Cp：900 J/kg·K
ρ：2700 kg/m³
α：23.6×10⁻⁶ /K
Aluminum does not have a single, fixed HV hardness value; figures vary significantly depending on whether it is pure aluminum or a specific alloy, as well as its heat-treatment condition. Pure aluminum is very soft, measuring only around 20–30 HV, whereas heat-treated, high-strength 7075-T6 aluminum alloy can reach approximately 175 HV. Therefore, specifying the exact alloy grade and temper (condition) is essential for hardness values to be meaningful.
Reference Hardness Values for Common Aluminum Materials
Pure aluminum (1xxx series): Annealed (O-temper) is approx. 25–35 HV; fully work-hardened (H18 temper) is approx. 45–55 HV.
3xxx series (Al-Mn alloys): Approx. 60–90 HV.
5xxx series (Al-Mg alloys): Approx. 70–120 HV.
6061-T6: Approx. 95–115 HV.
6063-T5/T6: Approx. 80–105 HV.
7075-T6: Approx. 160–190 HV.
Hard-coat anodized layer: Approx. 300–500 HV (hardness of the surface treatment layer).
(2).C1100
k：398 W/m·K（25°C）
Cp：385 J/kg·K
ρ：8940 kg/m³
α：16.5×10⁻⁶ /K
Hardness: HV 343~369MPa
2.Copper-to-aluminum welding
However, both copper and aluminum surfaces oxidize easily. The aluminum oxide film, in particular, is extremely stable, heat-resistant (with a melting point of 2050°C), and highly resistive. Mechanical connections often result in unstable contact during operation, leading to issues such as smoking, explosive failure, and potentially serious consequences like fires. In general, copper-aluminum welding can be achieved through fusion welding, pressure welding, or brazing.
A. Challenges in Copper-Aluminum Welding:
(1). Welding copper to aluminum involves joining dissimilar (non-ferrous) metals, which is significantly more difficult than copper-to-copper or aluminum-to-aluminum welding.
(2). Both metals oxidize readily; the formation of high-melting-point oxides during welding hinders complete fusion of the weld metal, complicating the welding process.
(3). Welded joints between copper and aluminum are brittle and prone to cracking. During fusion welding, eutectics such as CuAl2 tend to form in the weld zone near the copper side; their distribution along grain boundaries makes the joint susceptible to intergranular cracking.
(4). There is a significant difference in melting points between copper and aluminum. During fusion welding, aluminum melts while copper remains solid; by the time the copper melts, a large amount of aluminum has already liquefied, increasing the difficulty of the process.
(5). Porosity is common in the weld seam. Because both copper and aluminum have high thermal conductivity, the molten pool solidifies rapidly, trapping gases generated by high-temperature metallurgical reactions before they can escape, thereby causing porosity.
B. Introduction to Copper-Aluminum Welding Methods
(1). Argon Arc Welding ① Pre-welding preparation:
a. Clean the surfaces of the copper and aluminum workpieces.
b. Prepare a groove on the copper workpiece; determine the groove geometry based on the principles of maximizing the weld area on the copper side while minimizing the copper content within the weld itself. Typically, a single-sided U-groove is used on the copper side, while no groove is prepared on the aluminum side (see Figure 1).
c. Selection of welding materials: Ag65Cu20Zn15, Q203, ER4043.
d. Equipment: WES315 welding machine and an oxy-acetylene setup.
② Welding operation:
a. Assemble and securely fixture the copper and aluminum workpieces to prevent deformation or displacement during heating and welding.
b. First, use an oxy-acetylene flame to braze a layer of high-silver filler metal (approximately 1 mm thick) onto the copper side.
c. While the workpiece is still hot, perform argon arc welding using aluminum-silicon filler wire; during welding, direct the center of the tungsten arc toward the aluminum workpiece (note: this differs from direct tungsten argon arc welding of copper to aluminum). The guiding principle for the welding process is twofold: minimizing copper content in the weld and achieving brazing on the copper side while performing fusion welding on the aluminum side.
d. The inclination angle between the welding torch and the workpiece is typically 75–85°; start the arc at 90°, then maintain the standard working angle (tilted toward the aluminum side).
e. The tungsten arc length is generally around 5 mm; the arc length should be shorter for thinner workpieces.
f. Position the aluminum filler wire at the edge of the weld pool, maintaining an angle of less than 15° with the workpiece; the wire tip must not touch the weld pool but must remain within the argon gas shielding zone at all times—avoid lifting the wire too high or using an excessive inclination angle.
g. Pre-heat the aluminum filler wire near the arc before feeding it into the center of the arc to fuse with the base metal, avoiding the deposition of large segments of wire at once.
h. If the tungsten electrode sticks to the workpiece, do not hastily pull the welding torch away; instead, release the control switch first, then gently wiggle the torch to free the electrode, as forced removal can easily break the tungsten electrode.
i. To prevent defects such as craters, cracks, or collapse at the end of the argon arc weld, increase the filler wire feed rate appropriately during the termination of the weld.
③ Grind the weld after completion to meet technical specifications.
2. Submerged Arc Welding (SAW)
① Generally, prepare a bevel (usually U-shaped) on the copper side, but not on the aluminum side.
② During SAW, direct the arc toward the aluminum side, with an optimal offset distance of 5–7 mm.
③ Appropriately introduce elements such as Si, Zn, Ag, and Sn into the weld seam.
④ Example of welding parameters: For 10 mm thick copper and aluminum plates using ø2.5 mm SAl2 aluminum filler wire, the welding current should be 400–420 A, welding voltage 38–39 V, and welding speed 0.58 cm/s.
3. Electron beam welding:
Direct electron beam welding tends to produce a weld metal that is hard and brittle due to the formation of a CuAl2 eutectic structure (θ phase) alongside significant amounts of η and β phases. To avoid this, silver (Ag) is typically used as an intermediate alloy layer; since Ag, Cu, and Al can form mutually soluble solid solutions, using an Ag interlayer approximately 1 mm thick yields a high-quality joint.
4. Flash butt welding of copper and aluminum:
① Pre-weld preparation: a. Precisely machine the copper and aluminum workpieces to ensure flat, straight mating surfaces. b. Clean the surfaces to remove contaminants and oxides. c. Anneal the workpieces (copper: 600–650°C, held for 40–60 min, water-cooled; aluminum: 400–450°C, held for 40–60 min, air-cooled) to reduce hardness, increase plasticity, and improve weld joint quality.
② Flash butt welding allows brittle intermetallic compounds to be squeezed out of the joint and induces significant plastic deformation at the contact interface, resulting in a superior joint; it is a key method for joining copper and aluminum.
③ Example of flash butt welding machine parameters (using Model LQ-200): Butt-welding interface area of 6 × 60 mm²; electrode overhang of 29 mm for Cu and 17 mm for Al; clamping pressure of 0.44 MPa; upsetting pressure of 0.29 MPa; flashing time of 4.1 s; live-upset time of 1/50 s; cam angle of 270°.
5. Friction Welding
① Low-temperature friction welding (currently the mainstream friction welding method):
a. The temperature of the friction interface is controlled below the copper-aluminum eutectic point (548°C); specifically, the welding is performed within the 460–480°C range. This is the optimal range for obtaining satisfactory copper-aluminum welded joints. It is a form of semi-heated pressure welding; the required upsetting pressure per unit area is lower than that for cold pressure welding but higher than that for flash butt welding. It is primarily suitable for friction welding copper-aluminum joints with medium cross-sections (300–800 mm²).
b. Example of technical parameters for low-temperature friction welding: Copper and aluminum workpiece diameter: ø10 mm; rotational speed: 450 r/min; friction time: 6 s; upsetting pressure: 392 MPa; holding time: 2 s; copper protrusion length: 20 mm; aluminum protrusion length: 20 mm; upsetting speed: 12.6 cm/min; pre-weld pressure: 490–588 N; friction pressure: 166–196 MPa. ② High-temperature friction welding: a. During high-temperature friction welding, the rotational linear velocity can reach 0.58 m/s, and the interface temperature can reach the melting point of aluminum (660°C), well above the copper-aluminum eutectic temperature (548°C). At this temperature, copper and aluminum atoms undergo mutual diffusion, enabling the formation of a high-quality welded joint. b. Pre-weld preparation: Machine the copper end face to a 90° cone angle; anneal the copper and aluminum workpieces; clean the parts after annealing.
c. Example of technical parameters for high-temperature friction welding: Copper-aluminum workpiece diameter: ø10 mm; rotational speed: r/min; outer-rim linear velocity: 0.71 m/s; friction pressure: 147 MPa; copper component axial angle: 60°. 6. Cold Pressure Welding: The essence of cold pressure welding lies in utilizing plastic deformation at the weld zones of workpieces at room temperature. This process extrudes oxide films from the contact interface, bringing metal atoms close enough for interatomic forces to act; accompanied by atomic diffusion, this creates a strong joint through interatomic bonding. It is primarily suitable for welding copper and aluminum components with small to medium cross-sections.
① Butt cold pressure welding of copper and aluminum: a. Butt cold pressure welding is performed at room temperature without external heat sources; the metal microstructure undergoes neither recrystallization nor softening (annealing), and joint strength is not lower than that of the base metal. Suitable for cross-sectional areas ranging from 1 to 1000 mm², this method is applied in electric motors, transformers, and overhead power lines. b. Example of technical parameters for copper-aluminum butt cold pressure welding: pre-weld annealing; workpiece diameter ø10 mm; workpiece protrusion length: 10 mm for copper, 10 mm for aluminum; upsetting performed in 3 stages; upsetting pressure: 333 MPa.
② Lap cold pressure welding of copper and aluminum: a. For cold pressure welding of ductile copper and aluminum materials—including plate-to-plate, wire-to-wire, wire-to-plate, foil-to-wire, and foil-to-plate configurations—the lap joint is the preferred form. Spot cold welding can be performed using a method similar to resistance spot welding. The shape of the welding indenter can be circular (d = 1–2δ) or rectangular (a = 1–2δ, b = 5–6a). For multi-spot welding, the spots should be arranged in a staggered pattern, with the center-to-center distance between spots exceeding 2d. b. Example of parameter selection for copper-to-aluminum lap cold pressure welding: workpiece dimensions 40 × 4 mm²; lap length 70 mm; number of weld points: 6; indentation diameter: Al ø7 mm, Cu ø8 mm; indenter total length: Al 30 mm, Cu 55 mm; center-to-center distance between indentations: 10 mm; pressure: 235 MPa.
7. Capacitor-discharge welding
① Capacitor-discharge welding is a specialized form of resistance welding and is classified as a solid-state welding process. The fundamental principle involves storing energy beforehand and releasing it through the workpieces within an extremely short timeframe; this instantaneously generates a large amount of thermal energy at the joint interface, while a welded joint is formed under rapid forging pressure. Since both copper and aluminum possess excellent electrical and thermal conductivity, the welding process requires parameters characterized by high current and short duration. Capacitor-discharge welding is particularly well-suited for joining small-cross-section copper and aluminum conductors and is currently considered an ideal method for this application; however, the adjustable range for process parameters is narrow, necessitating strict adherence to established parameters to ensure the production of high-quality copper-aluminum joints.
② Example of parameter selection for capacitor-discharge welding of fine copper and aluminum conductors: conductor diameter: Al ø1.81 mm, Cu ø1.56 mm; capacitance: 8000 µF; welding voltage: 190–210 V; extension length: Al 2.5 mm, Cu 2.0 mm; upsetting pressure: 608 MPa; clamping force: 2650 N; transformer ratio: 60:1. 8. Vacuum diffusion welding
① Vacuum diffusion welding is primarily used to manufacture copper-aluminum joints for refrigeration and power equipment. The process parameters are strictly controlled, resulting in more consistent joint quality compared to fusion welding, brazing, or flash butt welding.
② Before welding, the surfaces of the copper and aluminum workpieces must undergo rigorous precision machining, grinding, re-polishing, and cleaning to ensure they are as smooth as possible and free of any impurities. Stack the copper and aluminum plates and place them into the vacuum chamber; factors influencing the vacuum diffusion welding of copper and aluminum include heating temperature, welding pressure, holding time, vacuum level, and workpiece surface quality.
③ Example of technical parameters for copper-aluminum vacuum diffusion welding: For vacuum diffusion welding of copper and ZAl2 hard aluminum alloy (thickness 0.2–0.5 mm), the parameters are: heating temperature 480–500°C; holding time 10 min; pressure 4.9–9.8 MPa; vacuum level 1.33×10⁻² to 1×10⁻³ Pa.
9. Brazing ① Low-to-medium temperature brazing
a. Friction brazing (scratch welding): Use HL501 (Zn58Sn40Cu2); commercially available specifications are typically cast rods measuring 5×20×350 mm³. The welding method is very simple: heat the copper and aluminum workpieces to approximately 300°C. Without using any flux, attempt to scratch the workpiece surface with the rod; if the filler metal does not melt, continue heating until the filler metal fills the joint upon scratching.
b. M51 + M51-F: This method uses a combination of filler metal and flux. The brazing temperature is very low (179°C). Do not heat the filler metal directly during welding; instead, heat the copper and aluminum workpieces and use the heat from the workpieces to melt the filler metal. Suitable heating tools include electric soldering irons, heat guns, LPG torches, kerosene blowtorches, gasoline blowtorches, and oxy-acetylene torches.
c. Dip brazing: The filler metal is Zn (96–98%) and Pb (2–4%); the flux consists of rosin (C20H30O2) and anhydrous alcohol (ratio > 1). Welding method: Quickly dip the copper-aluminum joint—coated with the rosin-alcohol solution—into the molten filler metal at approximately 440°C to perform the brazing.
d. WE CuAl-Q303: This is a flux-cored copper-aluminum welding wire with a brazing temperature of approximately 400°C. It features an integrated, non-corrosive organic flux. As the metal sheath melts, the flux rapidly breaks down surface oxide films and overcomes surface tension, while also preventing re-oxidation. By wetting the metal surfaces, the flux enables the filler metal to fill the entire joint via capillary action; the process is simple, generally requires no post-weld cleaning, and is highly popular among welders.
e. Introduction to other welding materials: WE53 + WE53-F consists of a solid wire used with a separate flux (brazing temperature approx. 400°C); XR-FC22, XR-FC0215, ER15, E9802, and FR301 are flux-cored wires, which offer greater convenience in use. ② High-temperature vacuum brazing
a. Filler metal: AlSi12
b. Pre-weld preparation: Press the wire-form filler metal into rectangular sheets 0.5–1 mm thick; thoroughly clean the copper and aluminum workpieces and the filler metal.
c. Assembly: Use stainless steel plates to clamp the components in an aluminum–filler metal–copper stack, layering them in that specific order. d. Furnace brazing: Begin evacuation after loading the furnace; activate the heating system once the vacuum level reaches 10⁻² Pa. Heat at a rate of 10–20°C/min, holding at 150°C for 5 minutes, and at 350°C and 540°C for 5 minutes each; then continue heating to 624°C, hold for 6 minutes, and begin cooling. Once cooled below 600°C, turn off the heating system; upon cooling below 450°C, introduce nitrogen gas to accelerate the cooling rate; the furnace door may be opened once the temperature drops below 100°C.
10. Other welding methods: In addition to the copper-to-aluminum welding methods mentioned above, techniques such as explosion welding and ultrasonic welding can also be used.
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