Magnesium Alloys and Its Heat Treatment | Metallurgy

Magnesium alloys have characteristics, which place them in a separate class of alloys, like great chemical stability, ease of machinability and fabrication, light weight (density – 1.8 gm/cc) as a consequence of which, alloys have high strength-to-weight ratios.

Magnesium alloys are two-third the weight of aluminium and one-fourth the weight of iron and steel. Thus, half of the magnesium produced is used as alloys for structural applications, primarily in the air-craft and missile industries. Magnesium alloys are well adopted for use in structures which can be easily fabricated from extrusions, plates, sheets, castings and forgings.

Their ultra-light weight permits the design of structures, which have equal stiffness and strength with lower weight than is obtainable from other metals. While replacing steel by magnesium alloy, a change in design is required. To equal the strength of the steel, greater cross section has to be made out of magnesium alloy.

Strength increases in direct ratio to any increase in thickness (assuming constant width), but stiffness increases as the cube of the increase in thickness. Thus, weight may be reduced, strength maintained, and stiffness increased with greater resistance to bending, buckling, elastic deformation and stresses.

Cast magnesium alloys have predominated over wrought products such as forgings and extrusions, and rolled sheet forms. Though, magnesium alloys have HCP structure to possess low ductility, but warm working (above 260°C) results in a few more slip planes operating to improve plasticity of magnesium alloys.

Wrought magnesium alloys provide a very light structural metal for use in such applications as oil and fuel tanks, air-craft wings, bus and trailer bodies. In the form of sheets and plates, these alloys are used for parts, such as cover plates, engine cowling, bus trailer and truck roofs, side panels, aircraft wings and ailerons, floor plates, seats, conveyers and other uses. Magnesium extrusions are used for applications such as floor beams, mouldings, stiffener elements in aircraft structures and frame sections, etc.

Since wrought magnesium alloys have excellent strength to weight ratios, they are used in many highly stressed structures. In castings, UTS of 242 MN/m² to 310 MN/m² are attained, and in the wrought condition the UTS may be 310 MN/m² to 380 MN/m². In many cases, magnesium alloy causes weight savings up to one-third of replacing aluminium and one half for ferrous alloys.

Some magnesium alloys show response to age-hardening but the degree is less than in aluminium alloys. The precipitation processes are quite complex, but most alloys show the formation of an ordered, hexagonal precipitate with DO₁₉ (Mg₃Cd) crystal structure which is coherent with magnesium lattice and is present at peak hardness over a wide temperature range, and contributes to age hardening in the alloys and creep resistance. This is analogous to θ” (GP Zone 2) phase in Al-Cu aged alloys.

Alloying Elements:

The commonly added alloying elements are aluminium, manganese, zinc. A new series of cast and wrought zirconium containing alloys with improved mechanical properties at room and elevated temperatures are being used in aerospace industries.

Aluminium is an important alloying element. With the increase in aluminium content, hardness, strength (tensile, compressive and yield) increase with grain refinement but ductility decreases. Alloys with more than, 13 percent aluminium are too brittle. Those having 6 to 10 percent of aluminium can be age hardened.

Manganese has little effect on the mechanical properties but increases the corrosion resistance (by removing iron and other impurities into relatively harmless intermetallic compounds). Zinc is usually added 1-3 percent to improve corrosion resistance under saline conditions. It is an important element for products, which are to have high strength.

Alloys Nomenclature (B 275-61 & B 296-61):

The most common designation of magnesium alloys is based on ASTM (The American Society of Testing Materials) publication. The temper designations are similar to aluminium alloys.

The first two letters of ASTM nomenclature indicate principal alloying elements, such as. A-aluminium: B-bismuth; C-copper; D-cadmium; E-rare earths; F-iron; G-magnesium; H-thorium; K-zirconium; L-lithium; M-manganese; N-nickel; P-lead; Q-silver; R-chromium; S-silicon; T-tin; Y-antimony; Z-zinc. The two alloying elements are arranged in order of decreasing percentages, or in alphabetical order if of equal percentages followed by respective percentages rounded off to whole numbers.

Classification of Magnesium Alloys:

The wrought magnesium alloys which are given precipitation hardening treatment can be broadly classified based on the composition as:

(a) Magnesium, aluminium, and zinc

(b) Magnesium, thorium and zirconium

(c) Magnesium, thorium and manganese

(d) Magnesium, zinc and zirconium

Table 14.9 illustrates the chemical composition, heat treatment and properties of some of the wrought magnesium alloys, whereas Table 14.10 illustrates the chemical composition, heat treatment and properties of some casting magnesium alloys.

Heat Treatment Processes for Magnesium Alloys:

Most wrought magnesium alloys obtain maximum mechanical properties by cold working and, in general, either are used without subsequent heat treatment, or are merely aged to T 5 temper. Many times, solution treatment, or a combination of solution treatment with strain hardening and artificial ageing, will substantially improve mechanical properties.

The common heat treatments given to the magnesium alloys are:

(a) Annealing

(b) Stress relieving

(c) Solution and precipitation hardening

(a) Annealing:

Wrought magnesium alloys, which have undergone various degrees of cold work, i.e., different tempers, if required, can be given annealing treatment to increase maximum possible softness and ductility. The alloys are heated to range of temperature 290 to 455°C depending on the composition of the alloy for one, or more hours.

Table 14.11 gives annealing temperatures of some wrought magnesium alloys. As warm working (above about 225°C) is commonly done of magnesium alloys as the alloys are then more ductile, as these alloys can slip on (1011) type planes as well as on the usual (0001) planes. Thus, annealing may not commonly be required.

(b) Stress-Relieving:

(i) Wrought Alloys:

Stress relieving treatment is given to wrought magnesium alloys to remove, or reduce the residual stresses in them due to earlier cold and hot working, shaping and forming, straightening, and welding. Table 14.12 gives temperatures and times for stress- relieving annealing of some wrought magnesium alloys. As far as possible lower temperature and longer times at it are used commonly.

(ii) Castings:

Castings may have residual stresses as a result of solidification due to mould restraints, due to non-uniform cooling after heat treatment, or from quenching. Machining operations may also induce some residual stresses. Weld-repair may also cause residual stresses.

Although magnesium castings do not normally contain large residual stresses, but as these alloys have low modulus of elasticity, even low residual stresses could cause large elastic strains. Thus, castings of Mg-Al-Mn alloys are stress relieved by heating at 260°C for 1 hour. So, are the castings of Mg-Al-Zn alloys. Castings of ZK 61 A (T 5) are heated at 330°C for 2 hours and at 130°C for 48 hours.

(c) Solution and Precipitation Hardening:

Solution Heat Treatment:

Magnesium alloys are solutionised in temperature range of 350° to 500°C, depending on the composition of the alloy. The aim is to dissolve all the alloying elements to form a single phase homogeneous solid solution at that temperature. The time of soaking varies from 8 to 24 hours, which too depends on the thickness, section size, thermal history of the alloy, i.e., the nature of microstructure. Mg-Al-Zn alloys are charged in a furnace at 260°C and slowly heated to its solutionising temperature so that eutectic does not melt.

Alloy HK 31 A is heated quickly to its solutionising temperature and kept for least time (≈ 2 hours) at the temperature to avoid possible grain coarsening. Normally the magnesium alloys are air cooled after solutionising treatment. Table 14.10 gives the solutionising temperature of some of magnesium alloys.

Precipitation Hardening:

Precipitation hardening is done of solutionised magnesium alloys in the range of 130-230°C for period of 5-16 hours, though some alloys may be aged for even 48 hours. Ageing is done to get a critical dispersion of fine sized precipitates to obtain a high peak hardness. Table 14.10 gives ageing time of some magnesium alloys.

Magnesium-Lithium:

These alloys have now been used as very light weight sheets and plates. Lithium (relative density = 0.53) is the lightest of all metals. Alloy LA 141 (Mg-14 Li-I Al) is weldable and has been used for armour plates and for aerospace components.