WWW Copp-Weld Deox Copper
The following describes the processes and filler metals used to weld coppers and high copper alloys. Applicable processes include gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) and, to a lesser degree, oxyfuel welding (OFW). Shielded metal arc welding (SMAW) and resistance welding are generally not recommended or copper. Less conventional processes such as friction welding and electron beam (EB) welding can produce high quality joints in copper and high copper alloys, but since they are mainly used in special situations, they are not discussed here.
For Welding Copper And High-Copper Alloys
DIFFERENT TYPES OF COPPERThere are actually several types of copper. Differences among them, which result from the way the metal was produced, are based mainly on the metals' oxygen content, the presence of alloying elements (if any), and whether or not deoxidizers are added. These factors have a strong influence on weldability. Most coppers are readily weldable, but some can be welded only if certain precautions are observed.
A list of copper and high copper alloys, along with their welding characteristics, is given in Table 1. Entries are classified according to the Unified Numbering System (UNS), in which copper metals are designated by five-digit numbers preceded by the letter "C". At the top of the list are grades simply called "coppers". These include wrought coppers (i.e., rolled, extruded or drawn), numbered UNS C10100 through C15760, and three cast coppers, UNS C80100, C81100 and C81200. Here's how they differ:
Oxygen-Free (OF) Coppers, UNS numbers C10100-C10800 and C80100, are the purest commercial forms of the metal, containing at least 99.95% copper and less than 0.05% of other elements. Very low oxygen content (less than 10 parts per million, or 0.001%) gives these coppers good weldability. The lack of impurities other than oxygen gives these coppers their excellent electrical conductivity, and they are used mainly for electrical or electronic products.
But all coppers, including OF grades, also have high thermal conductivity. That affects weldability because it tends to draw heat away from the weld zone, causing reduced penetration and, in the worst cases, incomplete fusion. There are two ways to counteract this effect:
|Apply sufficient preheat and maintain good inter-pass temperature control. A high workpiece temperature reduces the tendency for heat to spread to cold areas. The thicker the workpiece, the more preheat is required. Tables 2 and 3 list recommended preheat temperatures.
|Use a higher current setting (more heat input), especially when welding heavy sections. See Tables 2,3 and 4 for recommended settings.
High thermal conductivity also widens the heat-affected zone (HAZ). If the metal has been cold-worked, any strengthening gained will be annealed out during welding. Some grades of OF copper contain a little silver, which helps copper retain its strength at high temperatures (increases annealing resistance) without reducing electrical conductivity significantly. Silver-bearing coppers nevertheless require preheat for good penetration.
Cold-worked coppers are prone to cracking in the HAZ due to a combination of abrupt softening, high thermal expansion and thermal and/or residual stresses, if any. Again, thicker sections are more sensitive than thin ones. The application of preheat and interpass temperature control reduce the level of risk.
Oxygen-Bearing Coppers, UNS C11000-C11900 and C12500-C13000, may contain up to several hundred parts per million (i.e., more than 0.0100%) of oxygen in the form of copper oxide particles distributed throughout the metal. The oxides have little negative effect on properties, and they actually improve workability in some cases.
Weldability is another matter:
Oxygen-bearing coppers, including both electrolytic (ETP) and fire-refined "tough pitch" coppers, are not recommended for gas shielded arc welding.
They have a tendency to crack, either during welding or in service. If welding is necessary, arc welding using strongly deoxidized filler metal such as WWW COPP-WELD is preferable to OFW.
Problems arise because hydrogen is easily introduced into the weld puddle from any of a number of possible sources, such as water left on the metal surface, moist electrodes or even high relative humidity. High arc temperatures break the water down into hydrogen and oxygen. The hydrogen rapidly diffuses into the metal, where it reacts with the cop-per oxide particles, reducing them to copper metal and forming water vapor, which appears as porosity at grain boundaries. These defects seriously reduce the strength of the weld joint.
Oxygen-bearing coppers are best joined by brazing and soldering, where the temperatures involved arenŐt high enough to permit the harmful reactions to occur. Friction welding, a solid-state process, is also immune from this form of cracking.
Deoxidized Coppers (UNS C12000-C12300) contain phosphorus, which combines with, and locks up oxygen in a harmless form, enabling the metal to resist hydrogen damage. Deoxidized coppers have good weldability, and along with OF coppers, these are the grades usually specified for welded assemblies. The trade- off is that phosphorus reduces electrical conductivity: the more phosphorus, the lower the metal's IACS1 conductivity rating.
The deoxidized grade with the best electrical properties is Phosphorus-Deoxidized, Low Residual Phosphorus (DLP) Copper, C12000. It contains between 0.004 and 0.012% phosphorus and has nearly 100% IACS conductivity. The most common grade is Deoxidized High Residual Phosphorus (DHP) Copper, C12200. It can contain between 0.015 and 0.040% phosphorus and has a conductivity of 75% IACS conductivity. WWW COPP-WELD deox copper (UNS C18900 and ANSI/AWS 5.7 ERCu), also falls into this category.
Free-Machining Coppers owe their high machinability to additions of lead and/or tellurium or selenium. Those elements also make the alloys hot short and susceptible to cracking, and the free-machining coppers are not considered to be weldable. They can, however, be joined by brazing or soldering.
1 IACS refers to the International Annealed Copper Standard, a measure of electrical conductivity agreed upon in the early 20th Century. OF and other the very pure coppers that became widely available after the standard was accepted have conductivities somewhat higher than 100% IACS. Copper has a higher IACS conductivity than all other metals except silver, which exceeds it only slightly. By comparison, pure aluminum (the next most conductive metal) has an IACS conductivity of only 66% IACS. Steels, stainless steels, and even complex copper alloys like copper-nickels and aluminum bronzes have conductivities less than 10% IACS.
HIGH-COPPER ALLOYSThe high-copper alloys are a family of metals that contain less than 99.3%, but more than 96% copper (in wrought grades) and more than 94% copper in cast grades. The family includes cadmium coppers (C16200 and C16500), beryllium coppers (C17000-C17500), chromium coppers (C18100-C18400), zirconium copper (C15000) and chromium-zirconium copper (C14500). Alloy C18000, another member of the group, contains nickel, silicon and chromium. These alloys can be thought of as trade-offs, offering high strength and hardness as well as reasonably high electrical and thermal conductivity.
With the exception of cadmium copper, the alloys achieve their properties through heat treatment, which involves heating the alloy to a high temperature, cooling rapidly by water quenching, then reheating (or "age hardening") at an intermediate temperature. This process creates a structure containing microscopic particles containing the alloying elements) dispersed throughout the copper matrix. Like the aggregate in concrete, it is these particles that give the alloys their strength. Even higher strength can be gained by combining heat treatment with cold work. Cadmium copper, which is not heat-treatable, is primarily strengthened by cold working.
The high-copper alloys are weldable, but there are reservations. The main factor to bear in mind is that high temperatures in the HAZ can wipe out the effect of heat treatment by over-aging the alloy. Heating also reduces the effects of cold work. That brings us to a few more rules:
|Age-hardenable alloys can be welded in the hardened condition or in the annealed (soft) condition. Hardening can be conducted after welding.
|For repairs, or if maximum as-heat-treated proper-ties are not required, alloys can be welded with WWW-A2 BRONZE WELD (aluminum bronze, AWS 5.7 ECuAI-A2) or WWW SIL-WELD (silicon bronze, AWS 5.7 ECuSi).
|The alloys can also be welded using WWW COPP-WELD deox copper, but the weld metal will have lower strength than when aluminum bronze or silicon bronze are used.
|Cold-worked (work hardened) and heat treated alloys lose strength as a result of welding (and brazing, as well) due to annealing and over-aging, and welding procedures may have to be developed to take this effect into account.
Because high-copper alloys have somewhat lower thermal conductivities than pure copper, they could require lower welding currents (less heat) and less preheat than pure coppers.
SHIELDING GASESArgon, helium, or mixtures of the two are commonly used as shielding gases for GTAW and GMAW welding of copper and high copper alloys. The choice of gas depends on the degree of heat input needed. Helium produces a higher heat input than argon and is normally used with thick sections. Helium-argon mixtures give intermediate heat input. As rule of thumb, use argon with GTAW when manually welding sections less than 0.09 in (2.3 mm) thick, although a mix ture of 75% helium-25% argon can also be used for the automatic welding of thin sections. See Table 2 for details. With GMAW, straight argon requires approximately 100°F (55°C) higher preheat temperatures than when welding with 100% helium. With GTAW, preheat temperatures should be raised by 200° F (110°C) for pure argon. See Tables 3 and 4 for details.
WELDING PROCESSESHigh-quality joints are best made using GTAW or GMAW processes. Plasma arc welding (PAW) is also used successfully, and comments made below with regard to GTAW also generally apply to this process. SMAW can be used for less-critical joints, but the process will not be discussed here.
GTAW. Manual GTAW is generally preferred for thin sections. Some fabricators use automatic GTAW for relatively thick sections, claiming that the process affords better control than GMAW and is capable of producing x-ray quality welds for especially critical applications. That preference may reflect special circumstances, such as work that is routinely fixtured for flat position welding, and it is not meant to imply that GMAW is an inferior process. GMAW is widely and routinely used to weld copper at economically favorable speeds.
Preheat should be considered for all thicknesses, but it is absolutely necessary for work thicker than about 0.1 inch (0.25 mm). Preheat temperatures with GTAW are about 100°F (55°C) higher than those used tor GMAW. See Table 2 for details.
GTAW can be used in all positions and is best for out-of-position welding. Pulsed current is recommended for vertical and overhead work. Use direct current, with electrode negative.
GMAW. GMAW is normally used when welding heavy sections in copper. Preheat is important, and straight helium or a 75% helium-25% argon mixture is normally used as a shielding gas. When welding with 100% argon, recommended preheat temperatures should be about 100°F (55°C) higher than what they would be with helium. GMAW can be used for vertical and overhead work but pulsed cur-rent and small-diameter wire is recommended in such cases. See Tables 3 and 4 for typical welding conditions.
JOINT PREPARATIONJoint designs for welding copper and copper alloys are based on the metals' high thermal conductivity and the need to gain good penetration. Joint angles are generally 10°-20° wider than those used with steels and low-conductivity alloys. A separation of 3/32 inch (2.4 mm) should be used for square-groove butt joints. For double V-groove joints, use a total included angle between 80° and 90° and a 3/32-to-1/8 inch (2.4-3.2 mm) root opening. An 80° to 90° included angle is also recommended for single V-groove joints, although no joint separation is necessary in this case.
All welded surfaces should be thoroughly cleaned, dry and degreased. Wire brushing with a stainless steel brush normally suffices for pure coppers, but beryllium coppers and other copper alloys that form tightly adhering oxide films may require grinding or chemical cleaning to provide completely bare surfaces.
DISTORTION CONTROLThe thermal expansion of copper metals is about 50% higher than that of carbon steels. Thermal conductivity can be as much as eight times higher. The potential for distortion during welding is therefore considerably greater when welding copper than with steel. Small or light-gage components should be firmly clamped or fixtured to minimize warping. Multiple tack welds are also helpful, especially with large items or thick sections. Preheating, which is necessary in any case, is likewise beneficial since it tends to reduce temperature differences across large areas.
The thermal stresses that cause distortion can also lead to cracking - and that's another reason for using the right amount of preheat, which minimizes these stresses. Root passes should be large. High heat input during the initial pass, along with copper's high conductivity, creates more uniform temperatures around the weld zone and therefore avoids the sharp thermal gradients that can lead to cracking. Copper or ceramic backing, when used in conjunction with GTAW, help control root-pass penetration.