Manufacturers prefer zinc alloys for a variety of applications that range from small electrical connectors to automotive parts and decorative components. According to a research study, the global consumption of zinc casting alloys is more than one million metric tons.Zinc alloys are used in casting applications because of their mechanical characteristics, electrical conductivity, dimensional accuracy and machinability. Functional components are manufactured keeping in mind the stable mechanical properties over the design life of part, along with good dimensional precision. Metals for decorative components are chosen depending on their ability to maintain good corrosion resistance over a variety of exposure conditions. Many castings are designed with both functional as well as decorative attributes, however these characteristics may be vitiated if the zinc alloy is contaminated. For instance, high level of lead may cause intergrannular corrosion during plating works.
The presence of tin, lead, cadmium and antimony at a level higher than the permissible limits can adversely affect the mechanical properties and corrosion resistance of zinc castings.
Both die casting manufacturers and alloy producers are agreeing on the need to improve purity levels by adopting advance processes and sophisticated technology. In recent years, however, several instances of out-of-specification imported castings have been reported. According to experts, these instances are related to the hand sorting of scrap used to produce secondary alloy. Presence of little tin and lead can have significant impact on the purity of zinc alloys.
Presence of tin and lead in zinc alloys can be attributed to several factors, including - solder leftovers and bronze bushings, which are not separated from the secondary feed. Latest technology separation, alloying and alloy analysis methods can be used to rule out the risk of contamination of ingots, however, these must be carefully followed to ensure the supply of in-specification product.
Affect of Impurities
Though tin is considered as the most harmful impurity for zinc alloys, the presence of lead, cadmium and antimony at levels higher than permissible limits can also badly affect the mechanical properties and corrosion resistance of zinc alloys. In addition, when these impurities coexist in the contaminated alloy, their effects can get compounded. These impurities act as microscopic-size cathodes, finely diffused throughout the alloy along its grain boundaries and reduce the properties of zinc alloys. The zinc alloys generally corrodes at these areas.
Quality Control Procedures
To avoid zinc contaminants and to ensure the production of successful die castings, the zinc alloy producers needs to take several steps, including -
- Use only certified special high grade zinc in their manufacturing. This eliminates the risk of lead, cadmium and tin contamination.
- Each lot of zinc and die casting alloy must be produced as per the ISO 9001:2002 documented norms and procedures, which include different assays of the molten material during pouring off of the heats.
- Track and identify the likely harmful sources of contaminant, including customer returned scrap for reclaim.
Die casters can also ensure the quality of alloys that they use by procuring their raw materials from reliable suppliers and receiving certification for the purity of alloy purchased. It is also important for the buyers, designers and specifiers of zinc die castings to look up to a recognized standard, such as ASTM B86 or EN 12844, on all drawings and purchase orders.
