COPPER – From the 1st metal used by humans to the essential element in technology development_PART I – What is Copper and why is useful?

I love to talk about Copper. I must say that before to become materials science engineer I was very much in touch with electronics and initialy I wanted to become electronics engineer but it was Copper the metal that fascinated me since the beginning when I created my first PCBs for my electronic circuits. So indeed Copper is a remarcable metal. During my years at the techincal university I had the opportunity study Copper even more, the amout to details here is huge. I will cover few of them in this post. I consider to write more posts about Copper becasue this metal has a lot to offer. But let me start with its most important characteriscis. So WHAT is COPPER?

THE RAW METAL

The Beginnings of Bronze

The story of copper and its principal alloys, bronze (Cu-Sn) and brass (Cu-Zn), is virtually a chronicle of human endeavor since man emerged from the Stone Age. The ubiquity of the copper metals and their contribution to every civilization since Sumeria and Pre-Dynastic Egypt gives them a unique position in the history of technology.

Copper is one of those 6 metals that man started using very early. As a matter of fact, copper was the first metal that man discovered in 10.000 years ago. The other 5 metals used in pre-historic times were gold (Au), silver (Ag), tin (Sn), lead (Pb), and iron (Fe).

The earliest definite date usually assigned to true bronze casting is about 2500 B.C., i.e. 700 years or more after copper is known to have been in use; nevertheless numerous analyses show that copper artifacts of around 3000 B.C. sometimes contain small and variable percentages of tin (Sn). These may be regarded as “accidental bronzes.” One of the first things that the early coppersmith must have learned was that when he hammered copper he hardened it and, conversely, by heating the object he could soften or anneal it again. Thus the unalloyed metal could be fabricated and cut in a number of different ways. But when some unknown inventor conceived the idea of deliberately adding fixed proportions of tin (Sn) ore to the melt, he produced true bronze and thereby started the Bronze Age. As bronze was harder, almost equally durable and decidedly easier to cast than copper, although much more liable to fracture if not properly made, its use spread rapidly. In the Mediterranean countries bronze was not supplanted for over 2000 years and it lasted a good many centuries longer in north-western Europe, where methods of extracting and working iron were slower to follow those of Hallstadt and Rome. Meanwhile, both bronze and copper ran side by side. Museum labels on exhibits are not to be trusted unless analyses have been made and it is only in recent years that this has been systematically undertaken.

The majority of the surviving relics of early copper work are in cast form, an art which the Egyptians quickly brought to a high state of perfection. It is less easy to cast copper than bronze; but once they had learned to alloy the metal deliberately with tin, and frequently also with a little lead, the operation became much easier. When a little tin (Sn) or lead (Pb) is added, even accidental amounts like 1%, the production of sound castings becomes much easier; and this must have hastened the development of bronze as a definite alloy.

The melt flow was improved, and thereafter there was no limit to their fertility of invention. In this connexion, it must be remembered that the abundant remains, which the world possesses today, are but a fraction of what once existed in Egypt, the rest having been stolen or melted down and recast into other forms.

Small, decorative pendants and other items discovered in the Middle East have been dated about 8700 B.C. These objects were hammered to shape from nuggets of “native copper,” pure copper found in conjunction with copper-bearing ores. The earliest artifacts known to be made from smelted metal were also copper. These were excavated in Anatolia (now Turkey) and have been dated as early as 7000 B.C. The discovery of a copper-tin (Cu-Sn) alloy and its uses led to the Bronze Age, which began in the Middle East before 3000 B.C. More recent discoveries in Thailand, however, indicate that bronze technology was known in the Far East as early as 4500 B.C. The Bronze Age ended about 1200 B.C., after which iron technology (the Iron Age) became common.

The Beginnings of Brass

The Romans were the first to use brass on any significant scale, although the Greeks were well acquainted with it in Aristotle’s time (c. 330 B.C.). They knew it as ‘oreichalcos’, a brilliant-and-white copper, which was made by mixing tin (Sn) and copper (Cu) with a special earth called ‘calmia’ that came originally from the shores of the Black Sea. Pure zinc was not known until quite modern times, the ore employed being calamine which is an impure zinc carbonate rich in silica. The earliest brass was made by mixing ground calamine ore with copper and heating the mixture in a crucible. The heat applied was sufficient to reduce the zinc to the metallic state but not to melt the copper. The vapour from the zinc, however, permeated the copper and formed brass which was then melted.

Some Roman armour, particularly the helmets worn on ceremonial occasions, was made of brass. A large number of fine specimens of these helmets still survive. Spears and swords, daggers and palstaves, were originally of bronze, but later for weapons the Romans turned entirely to iron. The Romans also used brass for brooches (fibulae), personal ornaments and for decorative metalwork. The alloys employed contained from 11 to 28% Zinc, and the Romans clearly knew the value of different grades of brass for different purposes. The quality specified for delicate decorative work, for instance, had to be very ductile and of a good colour; and the Roman mixture contained about 18% Zinc and 80% Copper, i.e. it was about the same as the modern ‘gilding metal’ so widely used today for imitation gold jewellery.

Copper during the Industrial Revolution and beyond

Copper boomed during the Industrial Revolution as a material for objects, fixtures, and buildings. Copper is still widely used in power networks—the copper market is, in fact, growing because the material is such an effective conductor. But the material has been pushed out of many building applications by a wave of new materials from the 20th century. Plastics, tempered glass, aluminum, and stainless steel are the materials of modernity—used for everything from architecture to Apple products.  Brass door knobs and handrails went out of style as architects and designers opted for sleeker-looking (and often cheaper) materials.

Today, copper and copper alloys remain one of the major groups of commercial metals, ranking 3rd behind only iron/steel and aluminum in production and consumption. They are widely used because of their excellent electrical and thermal conductivities, outstanding resistance to corrosion, ease of fabrication, and good strength and fatigue resistance. They are generally nonmagnetic. They can be readily soldered and brazed, and many coppers and copper alloys can be welded by various gas, arc, and resistance
methods. For decorative parts, standard Cu alloys having specific colors are readily available.

Copper alloys can be polished and buffed to almost any desired texture and luster. They can be plated, coated with organic substances, or chemically colored to further extend the variety of available finishes. Pure copper is used extensively for cables and wires, electrical contacts, and a wide variety of other parts that are required to pass electrical current. Coppers and certain brasses, bronzes, and cupronickels are used extensively for automobile radiators, heat exchangers, home heating systems, panels for absorbing solar energy, and various other applications requiring rapid conduction of heat across or along a metal section. Because of their outstanding ability to resist corrosion, coppers, brasses, some bronzes, and cupronickels are used for pipes, valves, and fittings in systems carrying potable water, process water, or other aqueous fluids.

Typical applications of cold-worked wrought Cu alloys (cold-worked tempers) include springs, fasteners, hardware, small gears, cams, electrical contacts, and components. Certain types of parts, most notably plumbing fittings and valves, are produced by hot forging simply because no other fabrication process can produce the required shapes and properties as economically. Copper alloys containing 1 to 6% Pb are free-machining grades. These alloys are widely used for machined parts, especially those produced in screw machines.

Coppers are metals that have a designated minimum copper content of 99.3% or higher. Dilute or high-copper alloys (~94% Cu min) contain small amounts of various alloying elements, such as beryllium (Be), cadmium (Cd), or chromium (Cr), each having less than 8 at.% solubility. Because high-copper alloys retain the face-centered cubic structure of copper, their physical properties are similar to those of the pure metal. Alloying generally serves to impart higher strength, thermal stability (resistance to softening), or other mechanical attributes while retaining sufficient electrical conductivity for the intended use. The term “modified copper” has also been used to describe metal for which the specified minimum copper content is less than 99.88% but not less than 99.3%, silver (Si) being counted as copper (Cu). However this term is no longer recommended for usage. So let’s dive a little more and see what exacty Copper is cappable of.

The Major Groups of Copper and Copper Alloys

The elements most commonly alloyed with copper are aluminum (Al), nickel (Ni), silicon (Si), tin (Sn), and zinc (Zn). Other elements and metals are alloyed in small quantities to improve certain material characteristics, such as corrosion resistance or machinability. Copper and its alloys are divided into 9 major groups. These major groups are:

  1. CoppersCu, which contain a minimum of 99.3% Cu
  2. High-copper alloysHigh Cu, which contain up to 5% alloying elements
  3. Copper-zinc alloys Cu – Zn (brasses), which contain up to 40% Zn
  4. Copper-tin alloys Cu- Sn (phosphor bronzes), which contain up to 10% Sn and 0.2% P
  5. Copper-aluminum alloys Cu-Al (aluminum bronzes), which contain up to 10% Al
  6. Copper-silicon alloys Cu-Si (silicon bronzes), which contain up to 3% Si
  7. Copper-nickel alloys Cu-Ni, which contain up to 30% Ni
  8. Copper-zinc-nickel alloys Cu-Zn-Ni (nickel silvers), which contain up to 27% Zn and 18% Ni
  9. Special alloys, which contain alloying elements to enhance a specific property or characteristic, for example, machinability
Fig1. The Copper Alloy Tree

Properties of Importance

Along with strength, fatigue resistance, and ability to take a good finish, the primary selection criteria for copper and copper alloys are:

  • Electrical conductivity
  • Thermal conductivity
  • Corrosion resistance
  • Color
  • Ease of fabrication

Electrical Conductivity

A little more than 60% of all copper and copper alloys consumed in the United States and Europe each year are used because of electrical conductivity.The bulk of these applications are wire and cable, for example,telecommunications, wire and cable, electronic wire and cable, building wire, magnet wire, power cable, and automotive wire and cable.

The electrical conductivity scale established in 1913 was based on a copper standard defined as 100%, and the electrical conductivity of any material is still expressed as percent IACS (International Annealed Copper Standard), equal to 100 times the ratio of the volume resistivity of the annealed copper standard (0.017241 μΩ m) at 20 °C (68 °F) to the value measured for the material concerned.

The highest purity copper produced today (99.999% Cu) has been found to be 103% IACS. Only silver has a higher electrical conductivity than copper.

Table 1. – Electrical Conductivity values of various metals

The electical conductivity of metals in general is influenced by 4 main pameters:

  • the temperature;
  • the grain size and cold working;
  • the composition
  • the alloying & condition

In case of Copper we have them as follows:

The Effect of Temperature => Electrical conductivity is sensitive to temperature: for copper it drops from 800% IACS at –240 °C (–400 °F) to 38% IACS at 425 °C (800 °F).

The Effect of Grain Size and Cold Working => The conductivity of copper is independent of its crystal orientation and does not vary significantly with grain size. Cold working an annealed copper to about 90% reduction can cause a drop of 2 to 3% IACS

Effect of Composition => All additives to pure copper reduce its electrical conductivity, depending upon the element and amount in solid solution. Only small decreases are caused by elements added in excess of solubility.

Table 2 – Solubility limits and electical resistivity of few of the main copper alloying elements

There is a cumulative effect when more than one element is added. The drop in electrical conductivity caused by additions of commonly used alloying elements is illustrated by Fig. 2, which shows the strongly detrimental effects of phosphorus (P) and iron (Fe) and the relatively mild decreases caused by silver (Ag) and zinc (Zn) additions.

Fig.2 – Effect of alloying elements on the conductivity of oxigen-free high-conductivity copper.

Oxygen (O) in standard grade copper reacts with many impurities, yielding insoluble oxides and thereby greatly reducing the harmful effects. Where oxygen free or deoxidized copper is used, impurity levels must be reduced below those in cathode copper to achieve 100% IACS.

Effect of Alloying and Condition => As with other metal systems, copper is intentionally alloyed to improve strength without unduly degrading ductility or workability. However, it should be recognized that additions of alloying elements also degrade electrical and thermal conductivity by various amounts, depending on the alloying element and the concentration and location in the microstructure (solid solution, precipitate, dispersoid). The choice of alloy and condition is most often based on the trade-off between strength and conductivity. When additional demands are placed on the material—corrosion or oxidation resistance, for example—the combinations become more complex. Hence, understanding the properties demanded by a given application is of paramount importance.

Thermal Conductivity

Copper and its alloys are also good conductors of heat, making them ideal for heat-transfer applications, for example, radiators and heat exchangers. Changes in thermal conductivity generally follow those in electrical conductivity in accordance with the Wiedemann-Franz relationship, which states that thermal conductivity is proportional to the product of electrical conductivity and temperature. Table 3 compares the thermal conductivities of various metals and alloys.

Table 3.- Thermal conductivity values of various metals

Corrosion Resistance

Copper is a noble metal, but unlike gold and other precious metals, it can be attacked by common reagents and environments. Pure copper resists attack quite well under most corrosive conditions. Some copper alloys, however, have limited usefulness in certain environments because of hydrogen embrittlement or Stress Corrosion Cracking (SCC). Hydrogen embrittlement is observed when tough pitch coppers, which are alloys containing cuprous oxide, are exposed to a reducing atmosphere. Most copper alloys are deoxidized and thus are not subject to hydrogen embrittlement.

Stress-Corrosion Cracking most commonly occurs in brass that is exposed to ammonia or amines. Brasses containing more than 15% Zn are the most susceptible. Copper and most copper alloys that either do not contain zinc or are low in zinc content generally are not susceptible to SCC. Because SCC requires both tensile stress and a specific chemical species to be present at the same time, removal of either the stress or the chemical species can prevent cracking. Annealing or stress relieving after forming alleviates SCC by relieving residual stresses. Stress relieving is effective only if the parts are not subsequently bent or strained in service; such operations reintroduce stresses and resensitize the parts to SCC.

Dealloying is another form of corrosion that affects zinc-containing copper alloys. In dealloying, the more active metal is selectively removed from an alloy, leaving behind a weak deposit of the more noble metal. Copper-Zinc alloys containing more than 15% Zn are susceptible to a dealloying process called dezincification. In the dezincification of brass, selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper. Unless arrested, dealloying eventually penetrates the metal, weakening it structurally and allowing liquids or gases to leak through the porous mass in the remaining structure.

Color

Copper and certain copper alloys are used for decorative purposes alone, or when a particular color and finish is combined with a desirable mechanical or physical property of the alloy. Table 4 lists the range of colors that can be obtained with standard copper alloys.

Table 4. Standard color-controlled wrought copper alloys

Ease of Fabrication/ Fabrication Characteristics

As stated previously, ease of fabrication is one of the properties of importance for copper and copper alloys. These materials are generally capable of being shaped to the required form and dimensions by any of the common forming or forging processes, and they are readily assembled by any of the various joining processes. For a brief review of the fabrication characteristics of copper and its alloys, their ease of fabrication goes as follows:

Workability => Copper and copper alloys are readily cast into cake (slabs of pure copper, generally 200 mm thick and up to 8.5 m long), billet, rod, or plate—suitable for subsequent hot or cold processing into plate, sheet, rod, wire, or tube—via all the standard rolling, drawing, extrusion, forging, machining, and joining methods. Copper and copper alloy tubing can be made by the standard methods of piercing and tube drawing as well as by the continuous induction welding of strip. Copper is:

  • hot worked over the temperature range 750 to 875°C,
  • annealed between cold working steps over the temperature range 375 to 650 °C, and
  • thermally stress relieved usually between 200 and 350 °C.

Copper and copper alloys owe their excellent fabricability to the face-centered cubic crystal structure and the 12 available dislocation slip systems. Many of the applications of copper and copper alloys take advantage of the work-hardening capability of the material, with the cold processing deformation of the final forming steps providing the required strength/ductility for direct use or for subsequent forming of stamped components. Copper is easily processible to more than 95% reduction in area. The amount of cold deformation between softening anneals is usually restricted to 90% maximum to avoid excessive crystallographic texturing, especially in rolling of sheet and strip.

Fig. 3 – The Hall-Petch Equation

Although copper obeys the Hall-Petch relationship (Fig.3) and grain size can be readily controlled by processing parameters, work hardening is the only strengthening mechanism used with pure copper. Whether applied by processing to shape and thickness, as a rolled strip or drawn wire, or by forming into the finish component, as an electrical connector, the amount of work hardening applied is limited by the amount of ductility required by the application. Worked copper can be recrystallized by annealing at temperatures as low as 250 °C, depending on the prior degree or cold work and the time at temperature. While this facilitates processing, it also means that softening resistance during long-time exposures at moderately elevated temperatures can be a concern, especially in electrical and electronic applications where resistance (I2R) heating is a factor.

Weldability => Copper and copper alloys are most frequently welded using gas tungsten arc welding, especially for thin sections, because high localized heat input is important in materials with high thermal conductivity. In thicker sections, gas metal arc welding is preferred. The weldability varies among the different alloys for a variety of reasons, including the occurrence of hot cracking in the leaded (free-machining) alloys and unsound welds in alloys containing copper oxide. Tin (Sn) and zinc (Zn) both reduce the weldability of copper alloys. The presence in the alloy of residual phosphorus (P) is beneficial to weldability because it combines with absorbed oxygen (O), thereby preventing the formation of copper oxide in the weld. Resistance welding is also widely used, particularly in alloys with low thermal conductivity. Oxygen-bearing coppers can be subject to gassing and embrittlement, particularly in oxyacetylene welding.

Solderability => Copper is among the easiest of all engineering metals to solder. Oxides or tarnish films are easily removed by mild fluxing or precleaning in a dilute acid bath. A superior metallurgical bond is obtained with the use of a general-purpose solder composed of tin (Sn) in the range of 35 to 60% and the balance lead (Pb). Alloys of copper exhibit a range of solderability, dependent upon the type and level of alloying addition and method of soldering.

The immersion test is one common method to evaluate solderability. It involves immersion of a substrate alloy in a molten solder bath. The sample after removal is graded on a scale of I to V, based on the surface characteristics of the solder coat. Variations in solderability are the result of the effect of alloying additions on formation of the metallurgical bond at the substrate-solder interface. Under these conditions, most copper alloys are easily solderable using mildly activated rosin fluxes. Table 5 ranks various representative alloy groups in order of decreasing solderability, showing the adverse effects of zinc (Zn) and nickel (Ni).

Table 5. Solder immersion test ranking of copper alloys using a mildy activated rosin flux.

For most conditions, the use of a more aggressive flux achieves the desired class I or II solderability, even for the alloys more difficult to solder. However, aggressive fluxes are not used for electronic applications. Soldering involving slower heating than in the immersion test amplifies the alloy effects noted in Table 5 or requires more severe fluxes to remove oxides.

Brazeability =>The effects of alloying on brazing are similar to those for soldering, but because brazing is carried out at a higher temperature than soldering, the presence of reactive alloying elements intensifies the problem of detrimental oxide formation. Again, more aggressive fluxes and faster heating reduce the adverse effects caused by such alloy additions. Braze materials that melt at higher temperatures may also cause base-metal erosion or, in the case of the zinc brasses, give rise to zinc fuming, which degrades the structural integrity of the braze joint.

Machinability => All copper alloys are machinable in the sense that they can be cut with standard machine tooling. High-speed steel suffices for all but the hardest alloys. Carbide tooling can be used but is rarely necessary, and while grinding may be required for a few alloys in very hard tempers, these are not conditions to be expected in high-speed production. For mass-produced screw machine parts made from free-cutting brass or one of the other leaded copper alloys, high-speed steel is the standard tool material.

Surface Finishes => For decorative parts, standard alloys in specific colors are readily available. Copper alloys can be polished and buffed to almost any desired texture and luster. They can be plated, coated with organic substances, or chemically colored to further extend the variety of available finishes.

Production of Copper

Primary copper is produced from sulfide copper minerals and oxidized copper minerals. These materials are processed pyrometallurgically and/or hydrometallurgically to produce a high-purity electrorefined or electrowon copper containing less than 40 parts per million (ppm) impurities, which is suitable for all electrical, electronic, and mechanical uses. Secondary copper is produced from recycled scrap. Recycling of scrap accounts for approximately 40% of copper production worldwide.

The Industrial Revolution brought about a tremendous change in the production of copper and its alloys. In the first place, an insistent demand arose for more and better raw material. Therefore from mid of 18th century the copper has literally put humanity again in another copper Age: The Electrical Age and one more time today in the Digital Age. The technology we have today would have not been possible without Copper.

As the biggest technological milestones ever made by humans because of Copper, I would mention:

  • The Invention of the Stamping Press (which inevitably was followed by coining presses)
  • The Navigational Instruments
  • The Brass Clocks and Watches
  • The Architecture and the Fine Arts
  • The Shipbuilding
  • The Lightning Conductor
  • The Voltaic Pile and its Consequences
  • The electrical transformer
  • The electromagnets and the electric motor
  • The Dynamo
  • The Electric Telegraph
  • The Submarine Cables & The Atlantic Cable
  • The Cables & The Electricity Generation and Supply
  • The Telephone
  • The Electric Lighting
  • The Radio and The Radar
  • The Modern Architecture, Building and Plumbing
  • The Railways and Other Traction on Land
  • Money
  • The Paper Manufacture
  • Printing (of books, magazines, photographs)
  • The Printed Circuit Boards
  • The large brewing vats
  • The compounds and fertilizers for Agriculture and Horticulture

Just to name the few of them, all these applications were only possible because of Copper. Thus, in the 20th & 21st Century we have turned the complete circle. Industry began with man, the agriculturist, picking up shining pieces of copper and wondering what they were; and we conclude with man, the horticulturist, putting back the same element in solution out of a watering-can. The modern Agriculture depends very much on Copper.

We need COPPER more than ever. This metal is just super useful.

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