March_2022_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A R C H 2 0 2 2 4 0 M agnesium alloys are produced in both wrought and cast product forms. Cast alloys are much more widely used, and die castings constitute the larg- est segment. Well over 90% of magnesium alloys are used for high-pressure die casting, but heat treatment plays a small role in die casting. The emphasis here is on gravity- cast alloys. WROUGHT MAGNESIUM ALLOYS A relatively limited number of magnesium alloys are available in wrought form, because magnesium is some- what more expensive than aluminum, and aluminum is also much more easily cold formed. The lower ductility of magnesium alloys is due to the hexagonal close-packed structure of magnesium, which offers only three slip sys- tems. This is also true of zinc, but the c / a ratio of zinc al- lows mechanical twinning in tension and new slip systems, which leads to great ductility (e.g., 50%) in zinc alloys. In contrast, the c / a ratio of magnesium allows only mechan- ical twinning in compression. Thus, magnesium alloys in- variably have low (e.g., less than 10%) ductility in tension, although magnesium alloys can be fabricated by forming operations with a compression component (e.g., rolling, extrusion). THE BASICS OF HEAT TREATING MAGNESIUM ALLOYS Three types of thermal treating processes are commonly applied to magnesium alloys, including annealing, solution heat treatment, and precipitation, or aging. 9 TABLE 1 — MAGNESIUM ALLOY TEMPER CODES Code Description F As-fabricated O Annealing H10, H11 Slightly strain hardened H23, H24, H26 Strain hardened and partially annealed T Heat treated to produce stable tempers other than F, O, or H T2 Annealed (cast products only) T3 Solution heat treated and cold-worked T4 Solution heat treated T5 Artificially aged only T6 Heat treated and artificially aged T7 Solution heat treated and stabilized T8 Solution heat treated, cold-worked, and artificially aged T9 Solution heat treated, artificially aged, and cold-worked Fig. 1 — Phase diagram (a) and age hardening response (b) of magnesium-yttrium alloy. Source: Ref. 1. (a) (b) 8 The basic temper designations used to designate the various types of heat treatment for magnesium alloys are shown in Table 1.