Magnesium-based Alloys

1
(No Transcript)
2
Magnesium-based Alloys
Magnesium is HCP at all temperatures up to its
melting point of 649 oC It has relatively high
strength but limited ductility at room
temperature It can be easily worked at high
temperatures i.e., at 400 oC Mg is a highly
reactive metal It reacts with air and moisture
so must be covered with a flux during melting For
covered crucibles the flux is 20 KCl, 50 Mg2Cl,
and 15 CaF2. For open pots the flux is 55
KCl, 34 Mg2Cl, 9 BaCl2 and 2 CaF2. -
strong reducing agents.
3
Magnesium-based Alloys
Magnesium reacts with the SiO2 in clays to form
Mg2Si but it can be safely melted in iron or
graphite crucibles. To obtain a bright, clean
casting the mold is covered with sulphur boric
acid or KBF4. To dissolve magnesium alloy
precipitates, it is solution treated at 390 410
oC If the solution temperature is too high 1)
It will burn where low melting grain boundary
phases are exuded at the surface. 2) A
grey-black powder appears on the surface 3)
Internal voids form due to evolution of gaseous
phases.
4
General Properties of Mg-Alloys

• The corrosion resistance of Mg alloys is
  improved by using high purity starting materials
  and modifying practices with respect to caustic
  fluxes.
• Mg alloys are still susceptible to corrosion in
  salt atmospheres a problem for mag wheels in
  snow belt regions and for marine applications.
• Aircraft are not so critical but low flying
  over the ocean or the use of reactive de-icing
  fluids can create problems

5
General Properties of Mg-Alloys

• Mg is also notch sensitive so care has to be
  taken in design to remove sharp corners and
  abrupt changes in section
• Mg has excellent machining properties - but
  poor machining practice can introduce severe
  notch brittle effects
• Mg has a modulus of elasticity of 45 GPa
  compared to 71 GPa for Al and 200 GPa for steel

6
General Properties of Mg-Alloys

• Mg density is 1.8 g/cm compared to 2.8 g/cm
  for Al and 7.9 g/cm for steel on a mass basis,
  Mg has the greatest stiffness/weight and steel
  the least
• Mg is relatively difficult to weld as it must
  be protected from the atmosphere by an inert gas
  using a tungsten arc or consumable Mg
• It can be welded - like Al using a gas torch
  with suitable flux for temporary repairs in the
  field.

7
Mg-Al Alloy System
Al is soluble up in Mg up to 12.6 wt Alloys
containing up to 3 wt Al are solution
strengthened Alloys with 6-9 wt Al can be
precipitation hardened
8
Mg-Al Alloy System
At 437 oC aMg forms a eutectic with an
intermediate phase d which has a mean
composition of 32 wtAl or 33 atAl Its
chemical formula is actually Mg17Al12 As d is
brittle - the eutectic is also brittle as d is
the major constituent the eutectic contains
71.4 d and 28.6 aMg
9
Mg-Zn Alloy System
Zn is soluble in Mg up to 8.4 wt At 341 oC aMg
forms a eutectic with an intermediate phase MgZn
which has a mean composition of 54 wtZn As
MgZn is brittle the eutectic is also brittle
as MgZn is the major constituent
10
Mg-Zn Alloy System
The eutectic contains 71d and 29 aMg
(d)
11
Mg-Mn Alloy System
Mn is soluble in Mg up to 3.4 wt The aMg phase
forms by a peritectic reaction at 652 oC. As
there are no intermediate phases for
precipitation hardening Mg-Mn alloys are
strengthened by solid solution hardening alone.
12
ASTM Designation for Mg Alloys

• Two capital letters indicate the two principal
  alloying elements.
• A Aluminium M Manganese
• B Bismuth N Nickel
• C Copper P Lead
• D Cadium Q Silver
• E Rare Earth R Chromium
• F Iron S Silicon
• H Thorium T Tin
• K Zirconium Z Zinc
• L Beryllium

13
ASTM Designation for Mg Alloys

• Two digits indicate the rounded off percentages
  of the alloying elements, e.g., AZ63 Mg 6Al
  3Zn
• A following capital letter indicates the
  chronological order of an alloy with the same
  major constituents but with different minor
  elements.
• A letter and number indicate condition and
  properties
• F As fabricated
• O Annealed
• H10, H11 Slightly strain hardened
• H23, F24, H26 Strain harden and partially
  annealed
• T4 Solution treated
• T5 Artificially aged
• T6 Heat treated and artificially aged

Similar to Al alloys
14
Compositions of Mg-Alloys
Mg Mn (1.2 1.5) solution hardening Mg Al
(3-6) Zn (0.4 1.5) solution hardening Mg
Al (6 10) Zn (2 -3) precipitation
hardening Mg Zn (3.5 6.5) Zr (0.55 1.0)
precipitation hardening Mg Rare Earths (0.75
1.75) Zn (3.5 5.0) Zr (0.4 1.0)
precipitation hardening Mg Ce (6)
precipitation hardening These alloys are
solution treated at 390 410 oC and then air
cooled. Due to the low melting temperature this
allows ageing at room temperature, i.e., natural
ageing after solution treatment they do not
have to be tempered.
- Mo, Nb, Ta, W
15

• - as fabricated
• - artificially aged

We will discuss these alloys in turn
16
Wrought Mg-Alloys

• All solid solution Mg alloys can be hot forged
  at 300 400 oC in hydraulic presses rather
  like hammers.
• Extrusions can also be made from all alloys to
  obtain a fine grain size extrusions are made from
  very fine pellets
• M 1A, AZ31B and AZ61A can be rolled into sheet
  at temperatures 200 oC
• These alloys are not heat treatable.

17
Wrought Mg-Alloys

• AZ80A and ZK60A are effectively solution treated
  after forging because of the hot working
  temperature is close to 400 oC so precipitation
  hardening during subsequent aging at room
  temperature occurs.
• AZ80A and ZK60A are used for high temperature
  150 oC applications
• ZK60A T5 contains no Al so is more
  expensive but has greater strength and
  ductility than AZ80A.

18
Microstructures of Mg-Alloys - 1
AZ31 Alloy Annealed after hot working
AZ31 Alloy cold rolled into sheet work
hardened
19
Microstructures of Mg-Alloys - 2
M 1A Alloy Annealed Particles in grain
boundaries are impurities
20
Microstructures of Mg-Alloys - 2
ZK60 Alloy Extruded from pellets To obtain fine
grain size (0.001 mm)
21
Sand Cast Mg-Alloys

• Mg reacts with SiO2 causing the skin of the
  casting to be blackened (oxidized) to an
  appreciable depth below the surface.
• To obtain a bright surface inhibitors such
  as sulphur, boric acid or KBF4 are mixed with
  the molding sand.
• The reactive nature of Mg also means that sand
  cast alloys are subject to microporosity caused
  by evolution of hydrogen with a consequent
  deterioration of its mechanical properties
• Insoluble gases such as He and Cl are
  bubbled through the melt before casting to remove
  reactive gases such as H.
• - similar to Al alloys

22
Sand Cast Mg-Alloys

• It is also evident from the phase diagrams that
  sand cast alloys will contain brittle networks of
  eutectic constituents
• To improve the ductility of these castings they
  can be solution treated to dissolve the eutectic
  constituents and this treatment also increases
  the tensile strength
• Aging a solution-treated alloy strongly
  increases the yield point and slightly lowers
  the ductility but has relatively little effect
  on the ultimate strength
• Increasing the amount of Al increases the
  strength compared AZ63 with AZ92 but lowers
  the casting quality and increases the amount of
  microporosity
• The stronger Mg-Zn-Zr alloys are also more
  difficult to cast.

23
Microstructures of Sand Cast Mg-Alloys - 1
Grain boundary constituent is Mg17Al12
Grain boundary constituent is local Mg17Al12
24
Microstructures of Sand Cast Mg-Alloys - 2
EM62 Alloy As Cast The eutectic constituent is
Mg9Ce
AZ91B Die Casting Alloy As Cast The Mg17Al12
eutectic is very fine because of chill casting
25
Die Cast Mg-Alloys

• Die cast alloys have excellent dimensional
  tolerances and can be formed in complicated
  shapes as the liquid is forced into a steel mold
  under pressure.
• Alloy AM60A is used for auto wheels.
• Alloy AS41A is used for crankcases for air
  cooled engines like VWs
• AZ91B is a general purpose alloy recently used
  for dash boards in GM trucks
• Die cast alloys are significantly stronger than
  sand cast alloys as they are not susceptible to
  microporosity.

26
Mechanical Properties of Mg-Alloys
(intermediate step)
27
Effect of Grain Size on Mechanical Properties

• Superheating Mg-Al alloys to about 250 oC above
  the melting point just before casting refines the
  grain size and improves the strength.
• Note This is the only metal that can be grain
  refined by superheating usually it has the
  opposite effect!

28
Effect of Grain Size on Mechanical Properties
The grain size can also be refined by applying
one of the following treatments at 760 oC i.e.,
just before casting 1) Vigorous stirring 2)
Bubbling acetylene, methane, propane or carbon
tetrachloride 3) Stirring in 0.003 carbon as
graphite or lamp black or Al4C3.
29
The EndAny questions or comments?