Cadmium in Alloys
Although cadmium has only limited use as a pure metal it forms many binary and more complex alloys which have useful properties for many commercial applications.
Most commercial alloys containing cadmium fall into three major groups, where
(a) the presence of cadmium improves some feature of the alloy. Small amounts of cadmium can improve the hardness and wear resistance, mechanical strength, fatigue strength, castability and electrochemical properties of a number of alloys. Cadmium is added principally to alloys based on copper, tin, lead and zinc although several others benefit from its presence.
(b) lower melting points are obtained. These alloys range from the low melting point eutectic ('fusible') alloys to high melting point non-eutectic alloys used in metal joining. In the EU, the use of these alloys is restricted to defence and aerospace applications and to brazing fillers used for safety reasons.
(c) easy to cast alloys for decorative application. The low melting point of cadmium makes this an interesting choice for casting. In such alloys, cadmium is often combined with zinc. Use of such alloy applications is banned in Europe. Children’s jewellery containing cadmium are also facing a ban in a growing number of countries.
These alloy groups will be considered in more detail:
(a) Alloys with special features to enhance properties
A series of alloys with small additions of cadmium were developed to achieve a combination of interesting properties:
- Copper-cadmium alloys which have almost double the mechanical strength and wear resistance of pure copper, yet still retain 90 per cent of its conductivity, contain between 0.8 to 1.2 per cent cadmium. In addition, copper-cadmium alloys can be severely cold rolled without making them susceptible to softening during soldering. Major uses for such alloys include telephone drop wires, contact wire and catenary strand for railway overhead electrification, tinsel conductor for flexible telephone cords, special cables for military and aerospace uses and electrical components such as contact strips. In many copper conductors used in diverse applications not only the conductivity isof importance but also the tensile strength. Examples of applications are free hanging conductors in railway catenary systems and power lines, but also for small power and signalling wires in airplanes. Almost all elements that can be added to copper to improve its strength will have devastating effect on the metal conductivity. On the whole it can be stated that in order to achieve the required overall tensile strength of the conductor, the conductivity of it will at best be 50% of what could be achieved with pure copper. In some applications it is even not possible to introduce a separate strength element. To improve the strength performance of these conductors without jeopardizing the conductivity and therefore also the efficiency, copper alloys were developed that combine conductivity with strength. In this family of solution strengthened alloys, the combined strength and conductivity characteristics of copper cadmium alloys are unsurpassed. Strengths of copper can easily be increased by more than 50% while still reaching conductivities of 90% or more. Only the newest of alloys that are precipitation hardening can match these values, but do so at a significantly higher energy consumption than do the copper cadmium alloys. Copper cadmium alloys used in electrical applications can be manufactured with the lowest carbon footprint of all. At the same time the characteristics of the conductors allow to save on energy losses during the total lifetime of the conductor representing huge amounts of CO2 that do not have to be produced. Although cadmium has a poor reputation linked to its toxicity, this argument is not valid for copper cadmium alloys for electrical applications. The risks during production of these alloys are well under control and once the copper cadmium alloy is made, it remains inert until after many decades when it ends its life cycle, after which it can be easily recycled.
- Copper-cadmium alloys can withstand service temperatures as high as 150°C. As a result they are used in both domestic and automotive radiators and fittings.
- Zinc alloys containing cadmium in the range 0.025 to 0. 1 5 per cent are used as sacrificial anodes in the corrosion protection of structural steelwork immersed in seawater.
- Lead alloys with up to 0.075 per cent cadmium are sometimes used as sheaths for cables subject to cyclic stress.
- Tin-based white metal bearing alloys with up to 1 per cent cadmium have improved tensile and fatigue strength for use in marine engines and gearboxes.
- Precious metal alloys for jewellery incorporate cadmium for improved hardness and strength. Levels of up to 5 per cent cadmium in gold-silver-copper alloys make Greek gold, a greenish-tinged gold.
- Silver electric contacts incorporating 10 to 15 per cent cadmium or cadmium oxide are useful in many heavy duty electrical applications such as relays, switches and thermostats. The presence of cadmium gives some degree of arc-quenching and improves resistance to material transfer and electric erosion.
(b) Joining and fusible alloys
Intermediate temperature soldering alloys Cadmium alloyed with silver, zinc and/or tin makes excellent solders with a tensile strength two to three times greater than most common solders in this temperature range. They are also often used in applications where temperature sensitivity prohibits the use of silver solders with higher tensile strength. Zinc-cadmium alloys are useful for soldering aluminium whilst cadmium-zinc-tin alloys are used for soldering magnesium.
Silver-brazing alloys Cadmium is an important component in quaternary alloys with silver, copper and zinc in the lower temperature range of brazing alloys. They are distinguished by their ability to produce high strength joints at working temperatures more than 100°C below the melting points of the ternary silver-copper-zinc brazing alloys.
An eutectic alloy melts to become a free running liquid at a single temperature which is lower than the melting point of any of its component elements. The lowest melting point eutectic cadmium alloy is at 46.8°C and also contains bismuth, lead, indium and tin. The rapid fusing characteristics of these and the near-eutectic alloys are useful in many temperature sensitive applications. A non-eutectic alloys melt over a range of temperatures and have a 'pasty' range in which they can be easily worked and shaped as solders, fillers and brazes.
- Fusible alloys Low temperature fusible alloys containing cadmium employ their low melting points and rapid fusing or solidifying characteristics in a variety of uses. Heat sensitive fusible links in fire safety devices or kilns and ovens can activate control mechanisms when they melt at specific temperatures.
- Other cadmium containing solders are useful in light electrical and electronic assemblies and when it is necessary to make a lower temperature second joint adjacent to a conventional tin-lead solder joint.
- The tin-lead-bismuth-cadmium alloy which melts at 70° C is more commonly known as Woods Metal and is used in the bonding of metallized ceramic and glass components to metal frames and chassis where higher soldering temperatures are not possible. The alloys are used to firmly mount glass lenses during grinding operations. Similarly, jet engine turbine blades and vanes are machined whilst held by such low melting point alloys (shown in Fig. 1). Metal fabrication operations on complex or delicate assemblies are also easy when they are encased in a supporting jacket of metal. This technique also produces wrinkle-free bends in pipes and tubes. Woods metal in water sprinkler valves automatically activates the water supply as it melts when the local temperature exceeds 70° C. Proof castings in these alloys can be made from wood, plastic or plaster foundry patterns for short run dies for sheet metal and thermoplastics.
Fig.1 Atypical work holding application of a low melting point alloy - an encapsulated turbine blade.
(c) Casting alloys for decorative articles
Cadmium has a melting point of 321°C but by adding zinc, the eutectic melting temperature can be decrease to 266°C at 17.4wt%Zn. The CdZn alloys have a modest melting temperature and therefore easy to cast by slush casting or gravity casting. The mechanical strength is modest which make the alloys less suitable for technical parts. Cadmium zinc alloys are therefore mainly used for casting of decorative parts and cheap jewellery. Further, in India there is a significant amount of silver alloyed with cadmium to make cheaper decorative articles with a silver look. Use of such alloy applications is banned in Europe. Children’s jewellery containing cadmium are also facing a ban in a growing number of countries.
Fig.1 Atypical work holding application of a low melting point alloy - an encapsulated turbine blade
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The International Cadmium Association (ICdA) is a non-profit association representing the interests of the world’s cadmium industry. Its Members include producers, processors, recyclers and consumers of cadmium metal, cadmium compounds, and products to which cadmium or its compounds have intentionally been added.
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