The 4xxx series of aluminum alloys contain silicon as the main alloying element. There are two types of silicon-bearing alloys in this series: those with silicon alone, which are not very strong but have excellent flow and finishing properties, and those that also include copper and/or magnesium along with silicon. The latter type gains strength through solution heat treatment and aging.
4xxx alloys are not highly resistant to atmospheric corrosion and tend to turn “gray” over time in humid environments. Interestingly, this grayish characteristic is used advantageously in finishing techniques like anodizing to achieve various shades of gray.
Due to the added silicon, some 4xxx alloys, such as 4032, are used for intricate forgings like pistons because they have good “flow” characteristics during shaping. These alloys are also easily weldable and, in fact, include some of the most widely used welding filler alloys, largely because of their high fluidity.