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Technical Column | Copper And Its Welding With Dissimilar Materials

2024-05-01 10:00:00

Copper has excellent electrical conductivity, thermal conductivity, corrosion resistance, ductility and strength. In pure copper (copper) add other alloying elements to form a solid solution of various types of copper alloys, such as adding zinc for brass, adding nickel for white copper, adding silicon for silicon bronze, adding aluminum for aluminum bronze.

Copper and copper alloys can be connected by brazing, resistance welding and other processes. In today's industrial development, fusion welding has occupied a dominant position, with welding electrode arc welding, TIG welding, MIG welding and other processes can achieve the welding of copper and copper alloys. There are four main technical difficulties affecting the weldability of copper and copper alloys: first, the influence of high thermal conductivity. The thermal conductivity of copper is 7-11 times greater than that of carbon steel, and when the process parameters used are similar to the welding of carbon steel with the same thickness, the copper is difficult to melt, and the filler metal and the base metal can not be well fused. Second, the hot cracking tendency of welded joints is large. During welding, the copper and the impurities in the molten pool form a low melting point eutectic, which makes the copper and copper alloy have obvious thermal brittleness and easy to produce hot cracks. Third, the defects of pores are much more serious than carbon steel, mainly hydrogen pores. The fourth is the change of the performance of the welded joint. Grain coarsening, plasticity decline, corrosion resistance decline and so on.

1. Welding of copper

The welding methods of red copper include gas welding, manual carbon arc welding, electrode arc welding and manual argon arc welding, and automatic welding can also be used for large structures.

(1) Gas welding of copper is most commonly used in butt joints, and lap joints and T-joints are used as little as possible. Two kinds of wire can be used for gas welding, one is the wire containing deoxygenated elements, such as wire 201 and wire 202; The other is a general cutting of copper wire and base material, using gas agent 301 as a flux. Neutral flame should be used in gas welding of copper.

(2) arc welding of copper electrode. When the thickness of the welded part is greater than 4 mm, it must be preheated before welding, and the preheating temperature is generally 400~500℃. Welding with copper 107 electrode, the power supply should be DC reverse connection. Short arc should be used when welding, and the electrode should not be swayed horizontally. The shape of the weld can be improved by the reciprocating linear motion of the electrode. Long welds should adopt the step back welding method. The welding speed should be as fast as possible. In multi-layer welding, the slag between layers must be thoroughly removed. Welding should be carried out in a well-ventilated place to prevent copper poisoning. After welding, use a flat head hammer to knock the weld to eliminate stress and improve the quality of the weld.

(3) Manual argon arc welding of red copper. In the manual argon arc welding of copper, the welding wire used has wire 201 and wire 202, and the copper wire can also be used, such as T2. Before welding, clean the oxide film, oil and other dirt on the workpiece welding edge and the welding wire surface to avoid porosity, slag inclusion and other defects. Cleaning methods include mechanical cleaning method and chemical cleaning method. When the thickness of the butt joint plate is less than 3 mm, do not open the groove; When the plate thickness is 3 ~ 10 mm, the V-shaped groove is opened, and the Angle of the groove is 60o ~ 70o; When the plate thickness is greater than 10 mm, the X-shaped groove is opened, and the Angle of the groove is 60o ~ 70o; In order to avoid incomplete welding, generally do not leave blunt edges. According to the plate thickness and groove size, the assembly clearance of the butt joint is selected in the range of 0.5 ~ 1.5 mm.

Manual argon arc welding of red copper usually uses DC positive connection, that is, tungsten electrode to negative electrode. In order to eliminate porosity and ensure reliable fusion and penetration of weld root, it is necessary to increase welding speed, reduce argon consumption, and preheat the weldment. When the plate thickness is less than 3 mm, the preheating temperature is 150 ~ 300℃; When the plate thickness is greater than 3 mm, the preheating temperature is 350 ~ 500℃. The preheating temperature should not be too high, otherwise the mechanical properties of the welded joint will be reduced.

Carbon arc welding of copper, carbon arc welding electrodes are carbon electrode and graphite electrode. The wire used in copper carbon arc welding is the same as that used in gas welding, and the base metal cutting strip can be used, and the flux of gas welding copper can be used, such as gas agent 301.

2, brass welding

Brass welding methods are: gas welding, carbon arc welding, electrode arc welding and argon arc welding.

(1) Gas welding of brass. Due to the low temperature of the gas welding flame, the evaporation of zinc in the brass during welding is less than that when using electric welding, so in brass welding, gas welding is the most commonly used method.

The welding wires used in brass gas welding are: wire 221, wire 222 and wire 224, etc. These wires contain silicon, tin, iron and other elements, which can prevent and reduce the evaporation and burning loss of zinc in the weld pool, which is conducive to ensuring the performance of the weld and preventing the formation of pores. There are two kinds of fluxes commonly used in gas welding brass: solid powder and gas fluxes. Gas fluxes consist of methyl borate and methanol. Flux such as gas agent 301.

(2) Brass electrode arc welding. In addition to copper 227 and copper 237 for welding brass, homemade welding rods can also be used.

When welding brass electrode, DC power supply should be used for positive connection, and the electrode should be connected to the negative electrode. The surface of the weldment should be carefully cleaned before welding. The Angle of the groove should generally not be less than 60o ~ 70o, in order to improve the weld formation, the weldment should be preheated by 150 ~ 250℃. During operation, short arc welding should be used, without lateral and forward swing, only linear movement, and the welding speed should be high. Brass welds in contact with corrosive media such as seawater and ammonia must be annealed after welding to eliminate welding stress.

(3) Manual argon arc welding of brass. Brass manual argon arc welding can use standard brass wire: wire 221, wire 222 and wire 224, can also use the same composition of the base material as the filler material. Welding can be done with direct or alternating current connections. When welding with AC, zinc evaporates less than when welding with direct current. Usually, there is no need to preheat before welding, only when the plate thickness difference is relatively large. Welding speed should be as fast as possible. After welding, the welds should be heated at 300 ~ 400℃ for annealing treatment to eliminate welding stress to prevent cracks in the use of welds.

(4Brass carbon arc welding. Wire is selected according to the composition of the base material when brass carbon arc welding is performed221silk222silk224Welding wire, also can be made of brass wire welding. Welding can use gas agent301Isouse as flux. Welding should be short arc operation to reduce zinc evaporation and burning loss.

Dc TIG welding process is widely used in the welding of copper and copper alloys, with good welding air forming and excellent internal and external quality. Under the protection of argon, the molten pool is pure, the porosity is less, the influence of hot cracking is less, and the operation is easy to master. When the thickness is less than 4mm, it is not necessary to preheat before welding, but directly preheat with argon gas. When the temperature of the melting pool is close to 600℃, the filler wire can be added to melt the base metal to achieve welding. For copper with a thickness greater than 4mm, pure copper should be preheated 400 ~ 600℃. Copper alloy welding preheat 200 ~ 300℃. Dc TIG welding machine can weld pure copper, silicon bronze, phosphorus bronze, brass, white copper and other copper alloys. The AC/DC TIG welding machine can be used to weld aluminum bronze with AC TIG (using AC square wave to clear the surface oxide film) and the above copper with DC TIG welding.

In recent years, the construction of copper and copper alloys using MIG method is more and more, especially for aluminum bronze, silicon bronze and white copper with thickness ≥3mm, MIG welding method is the best choice. Copper and copper alloys with a thickness of 3 ~ 14mm or >14mm are almost always selected for MIG welding, because of high deposition efficiency, deep penetration and fast welding speed (generally 3 ~ 4 times that of TIG welding), to achieve efficient, high-quality and low-cost economic benefits. Before welding, the copper should meet the preheating temperature requirements (400 ~ 600℃ for pure copper, 200 ~ 300℃ for copper alloy), the welding wire and the base metal are fully similar in chemistry, and the purity of argon is ≥99.98%

3, copper and stainless steel welding

The main problem of welding between copper and stainless steel is that cracks and heat affected zone penetration cracks are easy to occur in weld zone and fusion zone, especially the improper selection of welding filler metal.

      If the stainless steel is used as a filler metal material, because copper is also an element of the formation of the Austrian structure, the weld is still the Austrian structure. There is a layer of liquid copper with a low melting point between the grains during crystallization, which is easy to cause cracks.

When Monel alloy is used as filler metal material (such as Ni70%+Cu30%), due to the high nickel content in the weld, the melting degree of copper in the Austenite structure can be increased, and the harmful effect of copper can be reduced, so that the thermal crack tendency is reduced.

      For example, when some copper alloys (such as aluminum bronze) and copper are used as filler metal materials, due to the high copper content in the weld, there is more copper liquid at low melting point between the crystals during crystallization, and there is a certain healing effect, so there is less tendency to hot crack.

However, the Fe, M, Cr fused into the weld makes the weld brittle, reduces the impact toughness, and may still produce penetration cracks in the heat affected zone on the stainless steel side, so this type of filler metal material can be used only when the mechanical properties of the joint are not high.

Since nickel can be immiscible with copper in either liquid or solid state, the use of pure nickel (Ni≥90%,Fe≤8%) to fill metal materials when welding can greatly exclude the harmful effects of copper, but also effectively prevent penetration cracks, is the best filled metal materials.

Welding process:

① Pure nickel is used as the filling wire, or cast iron electrode Z308 can be removed and wiped clean as the filling wire.

② Weld a transition layer on stainless steel or copper (contact welding surface), and then weld.

③ The welding edge of the two workpieces needs to remove the oxide layer and polish it with sandpaper until it is bright metal.

④ Because the copper heat dissipation is fast, the nickel electrode arc should be slightly more on the copper side, which is only conducive to good weld fusion.

The use of AC power welding can reduce the evaporation of metal melting, especially the welding of stainless steel and aluminum bronze.

The welding current can be adjusted accurately according to the thickness and size of the workpiece.

Argon flow rate, 12 ~ 15 liters/min, the best choice of high purity argon.

4, copper and steel welding

(1) Gas welding, plus copper wire.

(2) Argon tungsten arc welding, plus copper wire.

(3) With J507 electrode, wrapped with Φ1.25 pure copper wire (if it is enameled wire must be cleaned), baked for 2 hours at 350℃ before welding, and used as needed when it drops to 100℃. Before welding, clean the workpiece and preheat it with oxyacetylene flame. The preheating temperature is 650 ~ 700℃, and this temperature should be maintained during welding. Electrode selection Φ3.2mm, current selection of the upper limit, DC reverse welding.

      In order to ensure welding penetration, the arc stays on one side of the copper plate for a little longer, and the copper and steel are fully and evenly mixed by the arc blowing force. According to the thickness of the plate to determine the degree of winding density, the general spacing is2~3mmThe copper wire should not be in contact with the welding core and the jaws.

In principle, copper and most metals are immiscible. Copper wire or brass wire can be used for welding. Taking copper welding as an example, because copper conducts heat faster, if the copper plate is thicker, the copper plate should be preheated to 200~300 degrees first, and gas welding or argon arc welding can be used

Since the melting point of the steel plate is nearly 500 degrees higher than that of copper, when welding, the arc is tilted to the side of the steel plate first, so that the arc heat is heavier than the steel plate, and then the arc is tilted slightly to the side of the copper after the molten pool is formed on the side of the steel plate, and the welding wire can be sent to the molten pool.

5, copper and aluminum welding

      Aluminum and copper can be fused, pressed and brazed, of which the pressure welding application is the most.

The main difficulty of fusion welding is that the melting point of aluminum and copper is very different (up to 423°C), and it is difficult to melt at the same time when welding. Aluminum is strongly oxidized at high temperatures, and measures should be taken to prevent oxidation and remove oxides in the weld pool during welding. Aluminum and copper are immeasurably miscible in the liquid state, and finite in the solid state. Aluminum and copper can form a variety of solid solution phases dominated by intermetallic compounds, including AlCu2, Al2Cu3, AlCu, Al2Cu and so on. When the copper content in the aluminum-copper alloy is below 12% to 13% (mass fraction), the comprehensive performance is the best, so the fusion welding should try to control the copper content in the aluminum copper alloy of the weld metal does not exceed this range, or the aluminum base alloy is used.

      Aluminum and copper are both very plastic metals, so they are suitable for pressure welding, especially cold pressure welding, friction welding, diffusion welding and so on.

When using argon arc welding, the arc center should be biased to one side of the copper plate, and the offset is equivalent to 1/2 of the thickness to achieve simultaneous melting on both sides. Pure aluminum or aluminum-silicon can be used as filler wire. Adding alloying elements to weld metal can improve the quality of aluminum-copper fusion welding joint, and adding zinc and magnesium can limit the transition from copper to aluminum. The addition of calcium and magnesium can activate the surface and easily fill the gap of dendritic crystals. Adding titanium, zirconium, molybdenum and other refractory metals helps to refine the structure; Intermetallic compounds can be reduced by adding silicon and zinc. The addition method can be applied to the surface of copper to be welded before welding.

When submerged arc welding is used, the deviation value between arc and upper edge of copper groove l=(0.5 ~ 0.6)δ, δ is the thickness of the solder. The copper side has a J-shaped groove, and the aluminum side has a straight edge. A φ3mm aluminum welding wire is preset in the J-groove. When the thickness of the workpiece is 10mm, the welding wire diameter is φ2.5mm, the welding current is 400 ~ 420A, the arc voltage is 38 ~ 39V, and the w(Cu) in the weld metal after welding is in the range of 8% ~ 10%, satisfactory mechanical properties of the joint can be obtained.

When friction welding is used for bar material butt joint, the workpiece should be annealed before welding, file the joint surface, and weld as soon as possible to avoid contamination or re-formation of oxide film. Friction welding heating temperature should be lower than the aluminum copper eutectic temperature (548°C), generally controlled at 460 ~ 480°C. This not only prevents the formation of brittle intermetallic compounds, but also ensures sufficient plastic deformation.

Friction stir welding can be used for plate butt welding. As long as the welding parameters are suitable, good welded joints can also be obtained. Taking aluminum alloy 5A06(LF6)+ pure copper T1 butt welding with plate thickness of 2mm as an example, the mixing head speed is 375 ~ 1180r/min. Welding speed 30 ~ 150mm/min in a wide range of welding can obtain good weld formation. Among them, the joint tensile strength at the speed of 1180r/min and the welding speed of 30mm/min is the highest, reaching 297 ~ 62MPa, which is 95% of the aluminum alloy 5A05(314MPa).

Another characteristic example is 5A05+T2 butt friction stir welding with a plate thickness of 4mm. During welding, the pure copper T2 is placed on the forward side, and the stirring needle of the stirring head is tilted to the aluminum alloy 5A05 side by 0.9mm. Before welding, 200°C is preheated, and the welding is performed with the stirring head rotating speed of 1250r/min and the welding speed of 40mm/min. Good weld formation is also obtained.

Both aluminum-copper flash butt welding and capacitive energy storage butt welding rely on increasing the top forging speed and sufficient top forging force, while strictly controlling the electric top forging time, and extruding the formed intermetallic compounds out of the interface with the liquid metal to make the brittle layer as thin and discontinuous as possible to ensure that the contact surface has a large plastic deformation. For example, in flash butt welding, high current (twice as large as steel welding), high feed speed (4 times higher than steel welding), high voltage fast forging (100 ~ 300mm/s) and very short galvanizing forging time (0.02 ~ 0.04s). Sometimes, in order to prevent the formation of brittle compounds, zinc, aluminum, or silver filler metal is coated on the surface of copper in advance.

Aluminum-copper vacuum diffusion welding can obtain strong joints with good electrical and thermal conductivity. There is no need to add an intermediate transition layer during welding

Aluminum - copper brazing is often used in the manufacture of electrical components.

Fusion welding-brazing is a kind of welding technique which combines fusion welding and brazing for aluminium-copper joint. It is usually brazed for copper, that is, a zinc base filler metal or a zinc layer of 50 ~ 60μm is plated on the surface of copper to be welded. It is then fused with aluminum, melting only one side of the aluminum. If gas welding is used, CJ401 flux and pure aluminum wire are used for welding; When using argon tungsten arc welding, only the aluminum welding wire is filled.

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