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Aluminum's mechanical and physical properties are enhanced with the use of alloying elements. These alloying elements are commonly referred to as hardeners.

Aluminum based master alloys which contain the hardener elements in high concentrations provide a convenient and economical way to add them to aluminum to achieve desired properties. These master alloys readily go into solution at lower liquid aluminum temperatures, thus minimizing dross formation and solubility of hydrogen. Lower furnace temperatures also mean reduced energy consumption and longer furnace life. High concentration aluminum hardeners in the form of compacts remain a popular method for hardener additions. These products are a mixture of hardener elements such as Mn, Cr, Fe, Ti and Cu compacted with high grade aluminum powder. Generally compacts have a slower recovery time and lower recovery rate than that of master alloys as the elements in question are not in solution with aluminum.

Many cast-house operations have adopted an economical practice that includes the use of both high concentration compacts and lower concentration hardener waffle to achieve the desired recovery and productivity results.