Welding of Copper Alloys

Copper is an alloy of choice for critical applications due to its high thermal and electrical conductivity, as well as its corrosion resistance in saltwater environments and against many chemical substances.

Copper, which has very low mechanical strength in its pure form, can be alloyed to enhance its mechanical properties and be formed into various structural components.

When alloyed, copper takes the name of the primary alloying element added to it. Copper alloys are primarily classified into five types:

1. Brass (Zinc-Copper Alloy)
2. Phosphor Bronze (Phosphorus-Tin-Copper Alloy)
3. Aluminum Bronze (Aluminum-Copper Alloy)
4. Silicon Bronze (Silicon-Copper Alloy)
5. Nickel Bronze (Nickel-Copper Alloy)

Before understanding the properties of different copper alloys, it is helpful to know the properties of pure copper.

Pure Copper

Pure copper is obtained by reducing copper found in nature to its purest form. Pure copper alloys can be produced using three basic methods: electrolytic copper, phosphor bronze, and oxygen-free copper.

Electrolytic copper is a material containing a maximum of 0.03% oxygen and possessing high electrical conductivity. As the oxygen content in copper decreases, its weldability increases.

The properties of pure copper alloys are listed in Table 1.1.

1) Brass (Zinc-Copper Alloy)

Brass is a copper alloy with zinc as the primary alloying element. Brass has been used for many years and is preferred for its good mechanical properties and corrosion resistance. The zinc added to copper lowers the alloy’s melting point, density, electrical and thermal conductivity, and modulus of elasticity. However, zinc increases the alloy’s strength, toughness, and hardness. As the zinc content in brass increases, the alloy’s color changes from red to gold, light gold, and yellow. The general applications of brass alloys are listed below.

While brass alloys containing less than 20% zinc are generally weldable, the weldability of alloys with a higher zinc content decreases significantly. Lead, added to improve the machinability of brass materials, positively affects mechanical properties but negatively impacts weldability. Lead that vaporizes during welding causes porosity and leads to defects such as hot tearing.

Cast brass materials contain between 2% and 41% zinc and are generally not homogeneous. This situation makes welding cast brass materials quite difficult due to the concentration of alloying elements in different regions or the presence of lead in the material. Welding lead-free cast brass materials is also difficult. Brass materials containing lead are generally considered unweldable.

Brass casting alloys containing iron, aluminum, and manganese are used in ship propellers due to their resistance to cavitation and corrosion. Propellers manufactured from this alloy can be repaired if damaged by cavitation or mechanical stress.

Table 1.3 Chemical composition and general applications of brass (copper-zinc) alloys

2) Phosphor Bronze (Phosphorus-Tin-Copper Alloy)

This alloy, known as phosphor bronze due to the phosphorus added to copper-tin alloys for deoxidation purposes, has high wear resistance compared to other copper alloys. In its processed state (rolling, etc.), phosphor bronze is a very tough, relatively hard material that is resistant to cracking; has very poor weldability due to the high affinity of the tin it contains for oxygen and its tendency to become trapped in the solidification zone during welding, causing hot tears.

To prevent a decline in machinability, the tin content in cast phosphor bronze is generally kept below 10%. In most commercial cast alloys, the tin content is also below 10%. However, as the tin content decreases, zinc, nickel, and lead may be added to the alloy to mechanically improve the material properties. In rare cases, cast phosphor bronze materials containing more than 20% tin are also available.

3) Aluminum Bronze

Copper alloys containing 3–15% aluminum by weight are called aluminum bronze. Unlike other copper alloys, aluminum bronze alloys can exist in two different morphologies. Alloys containing up to 7% aluminum typically have a single-phase structure after solidification, whereas materials containing 9.5–11.5% aluminum exhibit a two-phase (alpha and beta) solidification morphology. These alloys with a two-phase structure can be heat treated. Hardness can be increased in these alloys by subjecting them to heat treatment to obtain a martensitic structure. The desired mechanical properties can be achieved in the material after it is tempered following the hardening process. Depending on the aluminum alloy content, the material—heated to a temperature between 1010 °C and 843 °C—can be quenched in water or oil and then tempered in the range of 427–649 °C, again depending on the alloy content.

Two-phase aluminum bronzes can be produced by casting or plastic forming. The mechanical properties of the materials are nearly identical in both production methods. Like brass materials, aluminum bronze materials can also be used in the manufacture of ship propellers. However, compared to brass materials, they have higher mechanical strength and greater resistance to seawater corrosion and cavitation. Propellers made from this alloy can be repaired if damaged by cavitation or mechanical stress.

Table 1.8 Mechanical Properties of Aluminum Bronze Alloys

4) Silicon Bronze

Silicon bronzes, which possess high mechanical and corrosion resistance as well as excellent weldability (the best), have a wide range of applications in various fields such as bearings, bells, pump housings, valves, and sculpture. Silicon added to the copper alloy increases the fluidity of the molten metal during casting and welding processes. Additionally, silicon acts as an effective degasser during welding by binding free molecular oxygen in the atmosphere and allowing it to pass into the slag, thereby reducing porosity.

5) Nickel Bronze

Copper-nickel alloys are widely used, particularly in offshore applications and in facilities that desalinate seawater, due to their high resistance to seawater corrosion. Typically, the nickel content in these alloys ranges from 10% to 30%. Additionally, elements such as iron, manganese, and zinc may be present in the alloy in small amounts. In Turkey, these alloys—which commonly contain 10% nickel—are known as CuNiFe (commonly referred to as “künefe”). Since there is a high risk of cracking in the ITAB zone during welding, care must be taken with the welding method, and welding procedures must be followed meticulously.

PHYSICAL PROPERTIES OF COPPER ALLOYS USED IN WELDING

The physical properties of copper alloys used in welded joints are of great importance in terms of parameters such as the coefficient of thermal expansion, electrical resistance, thermal conductivity, and melting point. These properties are critical not only during welding but also for understanding the performance of the weld metal after welding. The table below presents the physical properties of copper alloys used for welding in comparison with iron, nickel, and Monel materials.

As can be seen from the table, as alloying elements are added to pure copper, the electrical conductivity decreases (resistance increases), and thermal conductivity decreases in parallel. It is particularly important to note that the melting point and coefficient of thermal expansion of the weld metal are of critical importance in many applications.

Table 1.13 Physical properties of copper alloys and some other alloys for comparison purposes

2) WELDING OF COPPER AND ITS ALLOYS

2.1 General Information on Consumables Used in the Welding of Copper Alloys

In the welding of copper alloys, consumables such as electrodes, MIG (Gas Shielded) wire, and TIG (Argon) wire are widely used commercially. The first table below lists the chemical and physical properties of covered electrodes according to AWS A5.6 standards; the second table lists the chemical and physical properties of MIG (Gas-Shielded) and TIG (Argon) wires, also according to AWS A5.6 standards. The final table lists the general properties and application areas of these materials used in the welding of copper alloys.

2.2 Weldability of Copper Alloys and General Welding Methods

Welding copper alloys can be performed just as easily as welding steel alloys when appropriate welding methods are followed. However, since copper alloys have more distinctive properties than steel alloys, they require special attention both before and during the welding process.

The five main issues that may be encountered when welding copper alloys are as follows:

1. Insufficient energy applied during welding due to copper’s high thermal conductivity, leading to penetration issues.

2. Copper’s high coefficient of thermal expansion leads to dimensional changes, increasing the risk of distortion and crack formation.

3. Copper’s low melting point causes slag to become trapped within the weld metal during electrode welding before it has a chance to escape into the molten metal.

4. Rapid changes in grain structure due to heat input can significantly reduce mechanical properties.

5. Elements such as lead, tin, bismuth, and sulfur cause embrittlement and cracking; zinc, on the other hand, vaporizes during welding, creating porosity.

2.2.1 Welding of Pure Copper Alloys

Pure copper rapidly transfers the heat from the weld pool created by the welding arc to the base material due to its high electrical and thermal conductivity (8 times that of steel). This leads to insufficient penetration problems caused by inadequate melting.

In deoxidized copper or oxygen-free pure copper, hot tearing during welding of thick sections is a common welding defect. Additionally, porosity formation due to the dissolution of hydrogen in the weld metal is also a frequently encountered problem in pure copper.

Electrolytically refined pure copper makes welding quite difficult due to its high oxygen content. It is more prone to hot tearing and porosity than deoxidized copper.

Electrode welding, TIG (argon) welding, and MIG (gas-shielded) welding methods can be used for welding pure copper. If electrical conductivity is the primary selection criterion after welding, ErCu series pure copper welding wires or ECu pure copper electrodes may be preferred.

In cases where electrical and thermal conductivity are not the primary selection criteria after welding, ERCuSi-A and ERCuSn-A TIG (argon) or MIG (gas-shielded) welding wires, or ECuSi, ECuSn-A, and ECuSn-C electrodes can be used. Unlike pure copper alloys, these alloys melt more easily due to their low thermal conductivity and thus provide better penetration.

Since TIG (argon) and MIG (gas-shielded) welding generate more heat input than electrode welding, they should be preferred, especially when welding thick sections. These methods are performed in a shielding gas atmosphere; therefore, argon and helium gases can be used effectively in the welding of pure copper. Helium requires less preheating than argon. Generally, TIG (argon) welding can be used for welding pure copper alloys up to 6 mm thick, while MIG (gas-shielded) welding is recommended for parts thicker than 6 mm.

2.2.2 Welding of Brass (Copper-Zinc) Alloys

The vaporization of zinc during welding is a significant problem for welding brass alloys. Therefore, while alloys with a low zinc content are relatively weldable, those with a high zinc content are generally considered unweldable.

While most brass alloys are not weldable, for those that are, TIG (argon) or MIG (gas-shielded) welding methods are recommended over electrode welding. TIG (argon) welding reduces the risk of zinc vaporization because it provides a narrower and more controlled arc, though it cannot eliminate it entirely.

Although there is no welding filler metal that provides a perfect color match due to zinc vaporization, color matching after welding is good when using ERCuSn series alloys. ERCuSi series alloys, on the other hand, require a lower preheating temperature due to their low thermal conductivity, and their silicon content makes the weld bead more fluid. ERCuAl series alloys may be preferred due to their high mechanical strength. Electrode welding is generally not recommended for brass alloys because it requires slag entrapment and wide weld openings.

The preheating temperature for brass alloys varies depending on the part thickness and the zinc content in the alloy. Generally, a preheating temperature between 100 and 350 °C is sufficient. During the welding process, it is important to keep the welding arc within the weld pool. If the welding arc moves in front of or behind the weld pool, it will increase zinc evaporation and negatively affect the weld.

2.2.3 Welding of Phosphor Bronze (Copper-Tin-Phosphorus Alloys)

Phosphor bronze carries a risk of cracking because it forms a weak dendritic structure during solidification. In multi-pass welding applications, hammering the weld metal (using manual or pneumatic methods) after each pass will help reduce the risk of cracking.

For thick materials and heavy-duty welding applications, MIG (Gas Shielded) welding should be preferred. For MIG welding, ERCuSn-type welding wires and argon gas should be used. For TIG (Argon) welding, ERCuSn series welding wires and helium gas may also be preferred. In electrode welding, ECuSn-C or ECuSn-A type electrodes are used. For all welding methods, it is recommended to use a linear pass to achieve the best mechanical properties.

To improve the fluidity of the phosphor bronze weld metal, preheating between 150–250 °C is required. However, if the temperature exceeds 250 °C during welding, the risk of hot cracking increases.

Due to copper’s high thermal conductivity, preheating is absolutely necessary before the welding process. The preheating temperature is determined based on the thickness of the workpiece and the type of gas used.

2.1.1 Welding of Brass (Copper-Zinc) Alloys

The vaporization of zinc during welding poses a significant problem for the welding of brass alloys. Therefore, while alloys with a low zinc content are relatively weldable, those with a high zinc content are generally not weldable.

While most brass alloys cannot be welded, for those that are weldable, TIG (argon) or MIG (gas-shielded) welding methods are preferred over electrode welding. TIG (argon) welding reduces the risk of zinc vaporization because it creates a narrower and more controlled arc, though it does not eliminate it entirely.

Although there is no welding filler metal that provides an exact color match due to zinc vaporization, color matching after welding is good when ERCuSn series alloys are used. ERCuSi series alloys require a lower preheating temperature due to their low thermal conductivity, and the silicon content in these alloys contributes to a more fluid weld bead. ERCuAl series alloys, on the other hand, may be preferred due to their high mechanical strength. Electrode welding of brass alloys is generally not recommended due to slag entrapment and the need for a wide weld groove.

The preheating temperature for brass alloys varies depending on the part thickness and the zinc content in the alloy. A preheating temperature between 100 and 350 °C is generally sufficient. During welding, care must be taken to ensure that the welding arc is always maintained within the weld pool; because if the arc moves in front of or behind the pool, it can increase zinc vaporization and adversely affect the weld.

2.1.2 Welding of Phosphor Bronze (Copper-Tin-Phosphorus Alloys)

Phosphor bronze forms a weak dendritic structure during solidification, which increases the risk of cracking. In multi-pass welding operations, tapping the weld metal manually or pneumatically after each pass will reduce the likelihood of cracking.

For thick materials and heavy-duty welding applications, MIG (Gas Shielded) welding should be preferred. For MIG welding, ERCuSn-type welding wires and argon gas should be used. For TIG (argon) welding, ERCuSn series welding wires and helium gas may also be used. For electrode welding, ECuSn-C or ECuSn-A electrodes can be used. For all welding methods, applying linear passes is recommended to achieve the best mechanical properties.

To improve the fluidity of the phosphor bronze weld metal, a preheating temperature of 150–250 °C must be applied. However, if the temperature exceeds 250 °C during the welding process, the risk of hot cracking increases.

2.1.1 Welding of Aluminum Bronze (Copper-Aluminum Alloys)

Copper alloys that exhibit single-phase solidification and contain up to 7% aluminum carry a risk of hot tearing in the ITAB during the welding process. However, when the aluminum content exceeds 7% (alloys containing 8% Al), aluminum bronzes exhibit two-phase solidification. This leads to a reduction in grain size, significantly reducing the risk of cracking.

During the welding of aluminum bronze alloys, the aluminum in the alloy tends to form Al₂O₃ (aluminum oxide). This can cause insufficient fusion during welding (Al₂O₃ melts at approximately 2300 °C) and may appear as residue during the solidification of the molten weld metal.

In MIG (Gas-Shielded) welding, the oxide layer can be removed by using a positive polarity and argon gas. In TIG (Argon) welding, the use of alternating current (AC) enhances oxide removal. Electrode welding should be used for aluminum bronze alloys only when MIG (Gas-Shielded) and TIG (Argon) welding cannot be used.

While ERCuAl-A1, ERCuAl-A2, and ERCuAl-A3 welding consumables can be used in MIG (Gas Shielded) and TIG (Argon) welding processes, ECuAl-A2 or ECuAl-B electrodes are recommended for electrode welding.

2.1.2 Welding of Silicon Bronze (Copper-Silicon) Alloys

Silicon bronze alloys can be welded easily because their low thermal conductivity results in minimal heat transfer from the weld metal to the base material. The silicon in the alloy not only acts as an effective deoxidizer but also improves the fluidity of the weld metal, resulting in better weld quality compared to other alloys.

For welding silicon bronze alloys, ERCuSi welding wires and ECuSi electrodes can be used in electrode, MIG (Gas Shielded), and TIG (Argon) welding methods. In special cases, ERCuAl-A2 welding wires and ECuAl-A electrodes may be preferred for welding these alloys.

Preheating is generally not required for thin parts; however, welding in linear passes is recommended to reduce heat input. Additionally, the temperature between passes must not exceed 700 °C. In multi-pass welding, it is important to reduce grain size by hammering the weld with a welding hammer or pneumatic hammer after each weld pass.

2.1.3 Welding of Nickel Bronze (Copper-Nickel) Alloys

Nickel bronze alloys are relatively easier to weld because their low thermal conductivity results in less heat transfer from the weld metal to the base material during welding. However, since nickel bronzes carry a risk of hot tearing during welding, it is important to keep the heat input low.

These alloys are prone to porosity due to moisture absorbed from the air or their surfaces. For this reason, titanium is included as an additional element in the consumables used for welding nickel bronze alloys. Titanium binds oxygen in the weld metal, preventing it from escaping as a gas and allowing it to be released as slag in the form of oxide compounds onto the surface of the weld metal.

When welding alloys containing 90% Cu and 10% Ni (90/10 Copper-Nickel) and alloys containing 70% Cu and 30% Ni (70/30 Copper-Nickel), it is recommended to use welding consumables with a similar chemical composition to ensure color matching after welding.

When welding nickel bronze alloys, ERCuNi welding wires and ECuNi electrodes can be used in electrode, MIG (Gas Shielded), and TIG (Argon) welding methods.

Preheating is generally not required; however, it is recommended to weld in straight, short passes to minimize heat input. Additionally, it should be noted that the interpass temperature must not exceed 1000 °C.

2.1.4 Welding Copper to Different Materials

Copper and copper alloys are used in combination with different materials—particularly in petrochemical plants, chemical plants, and power plants—to enhance corrosion resistance, ensure mechanical strength, and reduce material costs. Copper alloys play a significant role in the cladding of plain carbon steels, the joining of stainless steels, and the integration of copper-nickel alloys.

Since the risk of cracks and welding defects is high when welding different materials, these joints must be thoroughly examined from a metallurgical perspective, and the predefined methods must be meticulously applied during welding.

The following section provides recommendations regarding welding methods for three different types of joints:

a) Joining copper and copper alloys to plain carbon steels

b) Joining copper and copper alloys to stainless steels

c) Joining copper and copper alloys to coated steels

a) Joining Copper and Copper Alloys to Plain Carbon Steels

Copper and steel are alloys that can pose metallurgical problems when dissolved in one another. However, due to their different melting points, joining is possible provided that controlled welding is performed. While copper melts in the 900–1200°C range, the melting temperature of steel and steel alloys—approaching approximately 1500°C—allows for welding with low mixing ratios.

At temperatures near copper’s melting point, steel transitions to the austenitic phase. In this case, molten copper seeps into the grain boundaries of the austenitic steel, leading to a phenomenon known as “copper creep.” Copper creep causes microcracks in copper-steel joints due to the intermetallic phases formed by copper and iron. This condition, which increases the risk of failure—particularly in bending tests—requires the steel surface to be coated with nickel weld metal when microcracks are observed in the weld.

To coat the steel surface with nickel, ENi-1 welding rods for electrode welding or ERNi-1 welding wires for MIG/TIG welding, as well as ENiCu-7 electrodes and ERNiCu-7 MIG/TIG wires, can be used. However, these alloys may contain small amounts of titanium, and titanium can cause microcracks in copper.