at temperatures of 300 °C and more. It usually forms larger particles with a larger spacing than the other phases and thus does not lead to any increase in strength or even to a drop if its formation takes place at the expense of the other phases. The -phase also occurs at temperatures between 150 °C and 250 °C as precipitation at grain boundaries, as this reduces the interfacial energy. The -phase leads to a partial intergranular fracture; however, the fracture behavior remains ductile overall. The change in fracture behavior is caused by precipitation-free zones at the grain boundaries. The -phase has a greater potential difference compared to the mixed crystal, so that layer corrosion and intergranular corrosion can occur. With longer annealing times, the inside of the grains also separate -phases and the potential difference is lower. Grades, alloying elements and contents As with almost all aluminum alloys, a distinction is made between for rolling and forging and for casting. The copper content is usually between 3 and 6%. Between 0.3% and 6% they are regarded as not weldable or very difficult to weld (by fusion welding), with higher Cu contents they are weldable. Most types also contain additives of magnesium, manganese and silicon to increase strength. Lead and bismuth form small inclusions that melt at low temperatures, resulting in better chip formation, similar to free-cutting steel. The heat resistance is increased by adding nickel and iron. Iron, found as an impurity in engineering alloys, prevents strain hardening. Adding magnesium makes it possible again. Larger amounts of magnesium up to 1.5% increase strength and elongation at break (see Aluminium-magnesium alloy). Manganese is also used to increase strength (see AlMn). Larger amounts, however, have negative side effects, so the content is limited to around 1% Mn. Smaller additions of silicon are added to bind iron, since it prefers to form the AlFeSi phase, while the formation of Al7Cu2 Fe would remove larger amounts of copper from the material, which then no longer leads to the formation of phases that are actually desired (especially Al2Cu, copper aluminide) are present. Larger amounts of silicon are alloyed to form with magnesium Mg 2 Si (magnesium silicide) which, like , improves strength and hardenability. Lithium is added to some alloys with contents between 1.5% and 2.5%. Due to the very low density of Li (0.53 g/cm³ compared to 2.7 g/cm³ of aluminium), this leads