Light Alloys- Magnesium 2 Flashcards
Classification system for Mg alloys
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…)
Code letters for each of Mg’s alloying elements
See table page 3 of magnesium 2 lecture
Often is the first letter of the element like A is aluminium
Temperature designations for Mg alloys
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
Effect of adding Ag, Al, Be, Ca on melting and casting behaviour, mechanical properties, corrosion behaviour
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
Effect of adding Cu, Fe, Li, Mn, Ni on melting and casting behaviour, mechanical properties, corrosion behaviour
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
Effect of adding REs, Si, Sn, Sr on melting and casting behaviour, mechanical properties, corrosion behaviour
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.
Effect of adding Th, Y, Zn, Zr on melting and casting behaviour, mechanical properties, corrosion behaviour
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.
What can form in Mg-Al based alloys?
Mg17Al12 is a brittle intermetallic (β phase) that forms above the solubility limit.
Two uses of Mg alloys in cars
Gearbox housing
Seat frame
Mg-Zn based alloys
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
Mg-Zr based alloys
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
Mg-Zn-Zr alloys
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
Mg-RE-Zn-Zn alloys
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
High temperature Mg casting alloys
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
Mg-Ag-RE alloys
QE22. Outstanding age-hardening response. Good tensile properties up to 200C. Improved elevated temperature strength and creep resistance