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be strengthened by precipitation hardening. However, the resulting precipitate is rather coarse and causes only moderate hardening. When zinc is added to the composition, it refines the precipitate and increases the strength by a combination of solid solution strengthening and precipitation hardening, as shown in Fig. 3.4. Even then, the degree of strengthening is minimal compared to that achievable in the heat treatable aluminum alloys. With tensile strengths in the range of

Fig. 3.4. Aging Curves for Mg-9%Al Alloy with Various Zn Additions

31-35 ksi and elongations of 1-8%, the Mg-Al-Zn alloys are not particularly strong or ductile but have low densities and are easy to cast.

More corrosion resistant Mg-Al-Zn alloys were developed in the mid-1980s by using higher purity starting materials and by limiting the amounts of iron (<0.005%), nickel (<0.001%), and copper (<0.015%). The low levels of nickel and copper are controlled by the purity of the starting materials, while the low iron levels are controlled with MnCl2 additions. For example, the high purity alloy AZ91E, due to its lower iron content, has improved corrosion resistance compared to the earlier alloy AZ91C. Magnesium alloys are limited to a total aluminum and zinc level of less than 10%; at higher levels, the ductility is drastically reduced due to the formation of intermetallic compounds. Thus, if the zinc content of an Mg-Al-Zn alloy is raised to 3%, the aluminum content must be reduced to about 6%, as in AZ63. However, as the zinc contents increase in the Mg-Al-Zn alloys, there is an increase in microporosity and shrinkage. The Mg-Al and Mg-Al-Zn alloys are limited to usage temperatures of about 200° F.

Mg-Zn-Zr and Mg-Zn-Rare Earth-Zr casting alloys. Alloys, such as ZK51 and ZK61, were developed as sand casting alloys by combining 5-6% zinc for increased strength with about 0.7% zirconium for grain refinement. Although these are relatively high strength alloys, they are not widely used because of their susceptibility to microporosity during casting, and they cannot be weld repaired due to their high zinc contents. RE additions to the Mg-Zn-Zr casting alloys improved their castability due to the formation of low melting point eutectics that form at the grain boundaries during solidification, which tends to suppress microporosity and hot cracking, while improving strength and creep resistance. However, the room temperature tensile strengths of EZ33-T5 of 20ksi and ZE41-T5 of 29ksi are low due to the removal of zinc from solid solution to form the Mg-Zn-RE phases in the grain boundaries. At low stress levels, these alloys do have respectable creep resistance up to 320° F.

Mg-Ag-Rare Earth casting alloys. The addition of 2.5% silver and 2.5% REs produces better precipitation hardening with good tensile properties up to 400° F in the alloy QE22, which has tensile strength of 35ksi in the T6 condition. Casting alloys with about 4-5% yttrium have also been developed which have better elevated temperature properties. For example, alloy WE43 has a room temperature tensile strength of 36 ksi when heat treated to the T6 condition. This alloy maintains a tensile strength of 36ksi after long-term aging (5000 h) at 400° F. The effect of 400° F exposure on the room temperature strength of WE43 is shown in Fig. 3.5. A relatively new alloy, Elektron 21 as specified in

Aging Time (h)

Fig. 3.5. Effect of 400° F Aging on Tensile Properties of WE43A-T6

Source: Magnesium Electron, Ltd

Aging Time (h)

Fig. 3.5. Effect of 400° F Aging on Tensile Properties of WE43A-T6

Source: Magnesium Electron, Ltd

AMS 4429, with design data available in Metallic Materials Properties Development and Standardization (MMPDS), offers many of the advantages of the WE43; however, the cost is lower and the castability is better. Instead of using yttrium, neodymium and gadolinium are used along with zinc and zirconium.

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