Background and Reviews
Aluminium AlloysAluminium alloys can be separated into two major categories: Non heat-treatable and heattreatable. The initial strength of non heat-treatable alloys depends primarily upon the hardening effect of alloying elements such as silicon, iron, manganese and magnesium. The non heat-treatable alloys are mainly found in 1xxx, 3xxx, 4xxx and 5xxx series. Additional strength is usually achieved by solid-solution strengthening or strain hardening. The initial strength of heat-treatable alloys depends upon the alloy composition, just like the non heat-treatable alloys. In order to improve their mechanical properties they need to undergo solution heat treating and quenching followed by either natural or artificial aging (precipitation hardening). This treatment involves maintaining the work piece at an elevated temperature, followed by controlled cooling in order to achieve maximum hardening. The heat-treatable alloys are found primarily in the 2xxx, 6xxx and 7xxx alloy series (ibid). The 7xxx series alloys contain zinc in amounts between 4 and 8 % and magnesium in amounts between 1 and 3 %. Both have high solid solubility in aluminium. The addition of www.intechopen.com Low speed laser welding of aluminium alloys using single-mode iber lasers 49 magnesium produces a marked increase in precipitation hardening characteristics. Copper additions between 1 and 2 % increase the strength by solid solution hardening, and form the basis of high strength aircraft alloys. The addition of chromium, typically up to 0.3 %, improves stress corrosion cracking resistance. The 7xxx series alloys are predominantly used in aerospace applications, 7075-T6 being the principal high strength aircraft alloy (Ion, 2000). This chapter will focus on fiber laser welding of 7075-T6 because of its predominant use in aircraft components.