TECHNICAL ARTICLE:
| Alloy Conductor Developments in Automotive and Aerospace Wire | ||||||||||||
| By: Eric S. Fisk - President, Fisk Alloy Wire Inc. - USA | ||||||||||||
Introduction Alloy conductors have long been used in wire and cable - but the cadmium alloyed with copper used to make these conductors is hazardous to the environment. Developments in copper alloys can now satisfy both the engineering and environmental aspects in environmental concerns. A material of choice as conductor Copper's main attribute is its high electrical conductivity. The electrical conductivity of pure copper, which is in excess of 101% IACS (International Annealed Copper Standard), sets the standard and makes it the material of choice as a conductor. However, copper has low strength and softens readily at relatively low temperatures. Many applications require higher strength and greater softening resistance while still maintaining high electrical conductivity. Adding various elements to copper will modify its properties and increase its strength. The tradeoff is that additions to copper, whether an impurity or an intentional alloying element, reduce its electrical conductivity. Of the various elements commonly alloyed with copper, silver, cadmium, and zinc have the least effect in reducing conductivity; phosphorus, silicon, and iron are among the most potent. Figure 1 illustrates the effect of various elements on the electrical conductivity of copper.
Commercially pure copper is the most widely used conductor material. It has excellent conductivity, attains limited strength, is easily processed, and is a commodity item. But pure copper falls short in other properties and fails in many demanding high-performance applications. The basic requirements for a high-performance conductor alloy are:
Copper alloys are not new to the wire and cable industry. The specifications ASTM B 105 and ASTM B 624 deal with alloy conductors. As with all metals, each alloy system currently used for conductors has its own attributes and deficiencies as compared to pure copper:
Cadmium copper (C16200) and cadmium chromium copper (PD135) are the two most commercially established alloy systems employed by the wire and cable industry. These are established alloy systems and are well tested in wire and cable applications. However, both of these copper alloy systems contain cadmium, an element of increasing environmental concern and product liability. Copper alloy metallurgy Cadmium copper and tin copper are examples of solid-solution alloys in which the alloying addition is completely dissolved in the copper, forming a single phase. These alloys have higher strength than pure copper, but they also have lower conductivity. Solid-solution alloys can be further strengthened by work such as rolling or drawing. However, increasing amounts of cold work result in further reductions in conductivity. Precipitation strengthened alloys, such as PD135, contain elements with linked solubility at lower temperatures and increasing solubility at higher temperatures. This solubility-temperature relationship forms the basis for precipitation hardening. These alloys are initially heated to a high temperature, where most or all of the alloying elements are dissolved in the copper, then rapidly cooled (quenched). Quenching essentially freezes the high-temperature structure to form a supersated solid solution. In this “solutionised” condition, the alloy is soft and has the lowest electrical conductivity for the specific composition. The alloy is then heated (aged) at an appropriate temperature to allow controlled precipitation of the second-phase particles out of the copper matrix. Precipitation of the second phase simultaneously increases the strength and electrical conductivity of the alloy. Similar to solid-solution alloys, precipitation hardened alloys can be additionally strengthened by cold work. Dispersion strengthening is another method of increasing copper alloy strength. Here, stable particles that do not easily dissolve in the copper matrix are introduced by various techniques. The particles increase in strength by a mechanism similar to the second-phase particles in precipitation hardening. As the dispersion particles do not dissolve into the copper matrix, there is minimal effect on the electrical conductivity. The alloy may not require solutionising or aging heat treatments. As is the case with most metals, the mechanical properties of the dispersion hardened alloys can be controlled through cold work treatment processes. The metallurgy of an alloy determines its properties; however, casting limitations may restrict the alloy’s utilisation to specific product categories, such as tube or strip. Copper alloys may be cast into special shapes, billets, and bars, or it can be continuously cast. Recent technologies have developed alloys utilising powder or spray metallurgy techniques. An alloy can posses the desired engineering attributes for a good alloy conductor; however, the alloy is only commercially cast in a form that cannot be processed into a rod or wire. Further, commonly deployed metal casting technologies that are used for producing copper alloy rod often have difficulties maintaining the melt-chemistry when the alloy contains the volatile elements often used in high-performance alloys. Keeping the alloys homogeneous during cooling, and free of impurities that impede wire drawing and affect final properties, can also prove to be problematic when using these technologies.
A technique developed by Fisk Alloy Wire, Inc., USA, to convert copper alloy bars into rod has enabled many new high-performance alloys to be produced in wire form. In addition, this method bypasses a major problem where alloy chemistry renders it incapable of being cast into iron form. This technique has proven to be a very effective method in increasing the availability of many of the new high-performance copper alloys for wire. Copper alloy development The increasing demand placed on integrated circuits, connectors, and terminations in the operation of electronic components has caused the development of a number of high-performance copper alloys. These alloys have been designed to meet specific performance requirements. Part miniaturisation and higher contract forces require alloys with higher strength. The need to carry more current and dissipate a larger amount of heat demand alloys with higher conductivity. These alloys must also be highly ductile to be formed into parts. The alloys must operate at higher temperatures, requiring improved stress relaxation resistance. Lastly, and of great importance, all these new alloys must employ environmentally responsible chemistries and processes.
Regardless of the metallurgy used to develop these alloys, the conductivity-versus-strength tradeoff is the principle challenge facing the alloy designer. Plotting the combination of conductivity and strength in different conductor alloys gives a comparison of this performance tradeoff. Figure 2 illustrates the essential conductivity versus tensile strength for conductor alloys in their commercially employed soft temper. Figure 3 illustrates the electrical conductivity versus tensile strength for hard temper conductor alloys. These graphs also present the developments in alloy conductors. The progression of conductor alloys offering higher strength with the same or minimum sacrifice to conductivity is evidenced by the continued shifting of the performance curves to the right
The Percon® alloys are new and offer similar or superior properties without the use of cadmium, beryllium, or other environmentally hazardous substances. Environmental issues A legal reality facing many existing and new products is the ever-increasing regulation of hazardous materials being released to the environment. The list of regulated materials covers heavy metals, solvents, and other compounds that affect the quality of human life when excessive levels are introduced into the environment or food chain. Many materials, once considered environmentally safe and acceptable, are now being regulated or are listed for future regulation. Cadmium appears on the Environmental Protection Agency (EPA) list of persistent bio-accumulative and toxic (PBT) chemicals and chemical categories that may be found in hazardous wastes regulated under the Resource Conservation and Recovery Act (RCRA). The list was created to help implement the EPA’s national RCRA waste minimisation policy to reduce the generation of PBT chemicals. When cadmium enters the food chain and is ingested, it accumulates in the liver and kidneys, causing damage. High concentrations of cadmium in kidneys can lead to proteinuria (protein in the urine) and the excretion of cadmium from bones. Cadmium affects the body at levels lower than that specified for lead. The EPA considers cadmium a “probable humane carcinogen” and has classified it as a Group B1 carcinogen. It therefore falls under the Waste Minimisation National Plan. Elemental cadmium and cadmium compounds have had a wide variety of uses in the wire and cable industry as pigments for insulation, stabilisers for PVC, plating for electrical connector shells, and in alloy conductors. Cadmium occurs naturally in various ores. But large industrial releases have been attributed to manufacturing operations involving cadmium and zinc. Release of cadmium can occur during the many stages of a product’s life. For cadmium-containing alloys, the manufacture of raw materials, casting, waste products during manufacture, disposal in landfills, and remelting or improper recycling are all potential sources of release to the environment.
Cadmium restrictions differ in various parts of the world, and more severe restrictions are passed each year. As previously mentioned, in the USA cadmium appears on the EPA hazardous materials list. Cadmium’s use is not prohibited in the USA, but minimisation of its use and control of its release into the environment have been a priority. For example, USA automobile manufacturers have effectively banned the use of cadmium-containing materials in all their products. In Europe, a January 1, 2007 deadline has been proposed by the European Commission to reduce Waste from Electrical and Electronic Equipment (WEEE) as part of the Restriction of Hazardous Substances (RoHS) directive. Hazardous materials such as cadmium, lead, and mercury, among others, will be restricted for use in a wide variety of equipment.
Environmental regulations are now addressing the entire life cycle of a product. Legislation is pending to require electrical and electronic equipment manufacturers to be responsible for their products throughout their lifetime - up to and including their disposal. This “take-back” policy will require manufacturers to bear the costs of proper recycling and disposal if their products contain listed hazardous materials. As previously noted, the two predominant copper alloy systems in wire and cable are cadmium copper (C16200) and cadmium chromium copper (PD135). Because of environmental issues, the number of cadmium copper alloy producers in the world has been decreasing. There has been no indication that additional capacity will be added in the near future.
Until recently, there was little research and development to provide alternatives to these cadmium-containing alloys for wire and cable. Other industries have responded to the increasing cadmium restrictions with new, environmentally safer products. The pigment industry has responded with new colourants based on organic materials and other elements to replace cadmium-based colours. PVC stabilisers based on zinc and calcium have been effective against the effects of light and heat. Alternatives to cadmium plating such as Ion Vapor Deposition (IVD), nickel-boron plating, unplated connector systems, and sophisticated plastics are being implemented. The copper alloy strip industry has also developed cadmium-free alloys but, as previously mentioned, not in rod form for wire. Before the introduction of Percon alloys, material development in the high-performance conductor industry has not followed the lead of other industries. Cadmium-free alloy conductors The environmental regulations on cadmium, already in force in the electronics and connector industries, will affect the wire and cable industry. Percon 11, Percon 17 and MicroShield are examples of cadmium-free alloys, originally developed for and widely accepted in electronics. These alloys have excellent mechanical and electrical properties, but were neither readily available in rod form nor easily drawable to fine sizes for use as stranded conductors. Percon 19 and Percon 24 are also cadmium-free alloys. They were developed specifically for applications in alloy conductors. Percon 11 is similar to copper alloy C150, the traditional zirconium copper alloy. It contains less zirconium, about 0.1% by weight. The lower zirconium concentration, combined with the special casting techniques employed for this alloy, controls the size and distribution of zirconium particles. This results in higher elongation and typical use in hard drawn temper in conductor applications. But it may also be heat treated to increase elongation. The alloy has a hard drawn tensile strength in excess of 80,000psi (522MPa) with electrical conductivity of 90% IACS minimum. Additionally, Percon 11 does not contain any hazardous elements and is graded as Class 1 scrap, permitted to be mixed in with copper scrap. Percon 17 was engineered specifically to replace cadmium copper C162 in applications where alloy conductivity is the first order of importance. The alloy is a dispersion-strengthened alloy with iron and magnesium phosphates in the copper matrix. Percon 17 can be used in the hard drawn condition or it can be heat-treated to soft temper at finish for higher elongation. In hard temper the tensile strength of Percon 17 can exceed 95,000psi (655Mpa) with electrical conductivity of 80% IACS minimum; in soft temper, 58,000psi (400Mpa) and 85% IACS. Percon 19 is an alloy of copper, magnesium, phosphorus and tin, designed to replace cadmium copper C162. The alloy and its processing have been engineered for applications where the design criteria are for conductor strength and high flex life. In these applications, its flex life exceeds the performance of C162. These applications require that the alloy be used in the hard drawn condition where its tensile strength is commonly 120,000psi (827Mpa) and electrical conductivity is 73% IACS. Percon 24 is an alloy designed to exceed the specification of ASTM B624. It is a precipitation-hardened alloy system of copper, chromium, and zirconium. This alloy is cast to obtain a highly clean melt with an oxygen content of less than 10ppm. The casting method also insures uniform distribution of the alloy constituents, which assures drawing to ultra-fine wire diameters. Percon 24 is offered in the heat-treated condition. Typical tensile strengths are greater than the ASTM B624 minimum of 60,000psi (414Mpa). Minimum electrical conductivity is 90% IACS, also exceeding the requirement. Conductor flex life and softening resistance are excellent. MicroShield is a solid-solution alloy of copper, nickel, and tin and has been long used in telecommunication electronics for its solderability and corrosion resistance. A beneficiary of the improvements in casting and processing, the alloy can now be reliably drawn to very high strengths and to ultra-fine gauge sizes. Tensile strengths of over 150,000psi (1,034Mpa) and 10% electrical conductivity make the alloy an ideal candidate for high-strength coaxial shielding applications, or to replace stainless steel shielding in other applications where the low conductivity of stainless steel is of concern. In addition to its strength (and unlike stainless steel), MicroShield is highly solderable when bare or can be plated with nickel, silver, or tin. Performance testing Military, aerospace, automotive, and medical applications are the primary uses for alloy conductors. With few exceptions, military wire and cable specifications require qualifications of the manufacturer. Many commercial specifications are based on comparable military specifications, and users often accept a manufacturer’s military qualification. Before approval as a qualified supplier, the manufacturer must show that its product consistently meets specification regardless of whether pure copper or copper alloy is used as the conductor. The qualification tests have historically focused on the properties of the insulation. There are many requirements for the insulation material, tested before and after long-term environmental exposure, but few requirements for the conductor. The conductor must meet physical, mechanical, and electrical requirements. However, most of these parameters are measured as-received and not after long-term environmental exposure. There are no standard qualification tests for conductor materials in military and commercial wire specifications, as these generally specify only three basic attributes: minimum tensile strength, maximum electrical resistance, and minimum elongation in the finished wire. In addition to developing new conductor alloys it was necessary to develop a programme to validate long-term reliability of these alloys when used in wire and cable. The objective was to not only test Percon 24 for military and commercial application, but also establish a test programme for future alloy development and qualification. Testing was designed and conducted to compare the long-term performance of Percan 24 with PD135 and copper wire. Electrical and mechanical tests were performed on terminated and un-terminated specimens of MIL-W-22759 (M22759) insulated wire in the as-received condition, after thermal aging, after thermal shock, and after exposure to controlled-vibration environments. Additional tests were performed on wire specimens terminated after thermal aging to simulate a repair operation on wire already in service. Thermal aging and thermal shock conditioning were performed at the maximum rated operating temperatures of the specific conductor/insulation as specified in the mil-spec, and standard mil-spec termination contacts (M39029/56-348 and /58-360) were used
Samples of 24AWG (19 x 36AWG or 0.127mm Unilay) silver-plated Percon 24 and PD 135 and 22AWG (19 x 34AWG or 0.16mm Unilay) silver-plated copper conductors were insulated with a 6 mil (0.1524mm) extruded wall of cross-linked ethylene-tetraflouroethylene copolymer (XL-ETFE) per M22759/33 (for alloys) and M22759/44 (for copper). Also, stranded nickel-plated Percon 24, PD135 and copper samples were insulated with 6 mil (0.1524mm) thick composite tape-wrap insulation according to M22759/82 and M22759/92. Wires containing both alloy conductors were simultaneously exposed to environmental conditions. Comparative testing would then quantitatively determine the relative performance of each alloy. Copper wire samples were prepared in four different conditions:
Conditioned wire samples were subjected to these tests: conductor dimensions and properties, insulated wire dimensions and properties, solderability, voltage drop, crimp tensile strength, centre of wire tensile strength, flex life of crimp terminations, centre of wire flex life, and SAE AS4373 flex test. Test performance summary The battery of tests performed showed that the alloy properties and long-term performance of Percon 24 prove it to be comparable to PD135 in all respects. The test results were forwarded to Naval Air Systems Command (NAVAIR) for review. NAVAIR will accept wire containing Percon 24 conductor for qualification to military high-strength wire and cable specifications. Conclusions The new casting technologies used to make Percon alloys produce alloys that are processed with more control and greater uniformity, and offer the user a selection of new conductor alloys for wire and cable. Waste stream and environmental responsibilities are driving development of new materials and processes in many industries. The wire/cable industry must be proactive in removing hazardous materials from products and processes. The testing programme developed to qualify Percon 24 for military and aerospace applications is thorough and rigorous. It is an excellent model for the testing and qualification of new conductor alloys. Company profile Founding in 1973, Fisk Alloy Wire, Inc. manufactures high-quality copper alloy wires, engineered to meet customers’ manufacturing challenges, product performance requirements, and marketplace needs. The growth of Fisk Alloy Wire, Inc. has led to the founding of two additional companies, Electroplated Wire Corp. and Fisk Alloy Conductors, Inc., each with distinct but synergistic products and manufacturing technologies. The companies are organised and staffed to assure a focused effort on their respective products and adherence to the highest quality standards. Fisk Alloy Wire, Inc. manufactures copper alloy wire to meet specific alloy, shape, and quality requirements. The company manufactures wires in flat, round, square, and special shapes to customer specification. Fisk Alloy has unique technical abilities and commercial capacities for producing high-performance copper alloys for the most demanding applications. Components and assemblies produced from the company’s wires can be found in computers, automobiles, telecommunications, industrial controls, appliances, and many other electronic and electrical applications. Electroplated Wire Corp. (EPW) is organised and equipped specifically to electroplate wire. The company plates to exacting thicknesses across a variety of alloys, sizes, shapes, and plating systems. The plating capabilities comprise tin, tin-lead, nickel, copper, gold, silver, and palladium. EPW’s technology permits the plating of single layers, such as silver and nickel, or the continuous production of multiple layers such as nickel/tin-lead or copper/nickel/gold. References [1] Annual Handbook of ASTM Standards, Section 2 Non-Ferrous Metal Products, Vol. 02.03 Electrical Conductors, ASTM, Philadelphia, PA. |
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