Light Alloys- Magnesium 2

Question 1

Classification system for Mg alloys

Answer 1

E.g AZ31B
First letter is the alloying element of the highest amount
Second letter is the alloying element of the second highest amount
First number is wt% of first element
Second number is wt% o second element
Last letter indicates the stage of development of the alloy (A, B, C…)

Question 2

Code letters for each of Mg’s alloying elements

Answer 2

See table page 3 of magnesium 2 lecture
Often is the first letter of the element like A is aluminium

Question 3

Temperature designations for Mg alloys

Answer 3

F- as fabricated/casted
O- annealed recrystallised (wrought products only)
H- strain hardened
T- thermally treated to produce stable tempers
W- solution heat treated (unstable temper)
H and T have subdivisions like T6 or H1

Question 4

Effect of adding Ag, Al, Be, Ca on melting and casting behaviour, mechanical properties, corrosion behaviour

Answer 4

Ag: improves high T tensile and creep properties in presence of REs, bad for corrosion.
Al: improves castability and tendency to microporosity, solid solution hardener and precipitation hardening under 120C.
Be: significantly reduces oxidation of melt surface under 30ppm leading to coarse grains.
Ca: grain refining effect and slight suppression of oxidation of molten metal, improves creep properties and ideal for biomaterials, bad for corrosion

Question 5

Effect of adding Cu, Fe, Li, Mn, Ni on melting and casting behaviour, mechanical properties, corrosion behaviour

Answer 5

Cu: improves castability as system with easily forming metallic glasses, bad for corrosion.
Fe: Mg hardly reacts with mild steel crucibles, bad for corrosion.
Li: increases evaporation and burning behaviour and melting only in protected and sealed furnaces, solid solution hardener at ambient T, reduces density, enhances ductility, bad for corrosion and need coating to protect from humidity.
Mn: control of Fe content by precipitating Fe-Mn and refinement of precipitates, increase creep resistance, good for corrosion as control Fe.
Ni: system with easily forming metallic glasses, bad for corrosion

Question 6

Effect of adding REs, Si, Sn, Sr on melting and casting behaviour, mechanical properties, corrosion behaviour

Answer 6

RE: improves castability and reduce microporosity, solid sol and precipitation hardening at high T, improve high T tensile and creep properties, good for corrosion
Si: decrease castability forms stable silicide compounds with other alloying elements, compatible with Al, Zn, Ag, weak grain refiner, bad for corrosion.
Sn: increases ductility m solid sol hardener, less effective precipitation hardening, bad for corrosion.
Sr: increased castability and reduce microporosity, improve creep resistance, good for biomaterials, bad for corrosion.

Question 7

Effect of adding Th, Y, Zn, Zr on melting and casting behaviour, mechanical properties, corrosion behaviour

Answer 7

Th: suppresses microporosity, improves high T tensile and creep, improves ductility and most efficient alloying element.
Y: grain refining and increased ignition T of molten metal, improves high T tensile and creep, good for corrosion.
Zn: increases fluidity of melt and weak grain refiner, tendency to microporosity, precipitation hardening, improve strength at ambient, tendency to brittleness and hot shortness unless Zr refined, sufficient Zn compensates for bad corrosion of Cu.
Zr: most effective grain refiner but incompatible with Si, Al, Mn, removes Fe, Al, Si from melt, slightly improves ambient tensile properties.

Question 8

What can form in Mg-Al based alloys?

Answer 8

Mg17Al12 is a brittle intermetallic (β phase) that forms above the solubility limit.

Question 9

Two uses of Mg alloys in cars

Answer 9

Gearbox housing
Seat frame

Question 10

Mg-Zn based alloys

Answer 10

Zn important alloying element but rarely the major one. Response to age hardening (MgZn2 forms at GP zones (Guinier-Preston)). Lowers melting point and Zn is much denser than Mg

Question 11

Mg-Zr based alloys

Answer 11

Zr very effective grain refiner in Mg. Same crystal structure and virtually identical lattice parameters. Zr could form heterogeneous nuclei for Mg alloy to solidify on. Can’t be used with Al as forms intermetallic compound

Question 12

Mg-Zn-Zr alloys

Answer 12

ZK51 and ZK61 are sand cast
5-6% Zn addition for solid solution strengthening
1% Zr addition for grain refinement
Alloys have limited use due to their susceptibility to microporosity during casting and not weldable due to high Zn content

Question 13

Mg-RE-Zn-Zn alloys

Answer 13

RE (Ce, Nd) is added to produce EZ33 and ZE41 (sand cast) giving good castability due to low melting point eutectics formed as networks in GBs during solidification.
Ageing used to improve mechanical properties particularly creep

Question 14

High temperature Mg casting alloys

Answer 14

Primarily for aerospace due to light weight. Application 200-250C with tensile strength about 240MPa.
Mg-Ag-RE alloys
Mg-Y-RE alloys
Mg-Ag-Th-RE-Zr alloys
Thorium best known to improve high temperature properties due to age hardening and refined grain but is slightly radioactive so not commercially available

Question 15

Mg-Ag-RE alloys

Answer 15

QE22. Outstanding age-hardening response. Good tensile properties up to 200C. Improved elevated temperature strength and creep resistance

Question 16

Mg-Y-RE

Answer 16

Mg-Y alloys capable of age-hardening with solid solubility of Y up to 12.5wt%. Good strength and creep resistance up to 300C. Y expensive. Difficult to alloy due to high Tm about 1500C.
WE43 improves high T properties, maintained RT tensile strength of 250MPa after long term exposure at 200C. For advanced aerospace applications

Question 17

Mg-Li alloys

Answer 17

Super light weight alloys
Li density 0.53g/cm3
Only 11% Li needed to form a new β phase
MA18 and MA21 are examples with good strength and stiffness properties on page 16 magnesium lecture 2