Transcript Slide 1

Mech 473 Lectures
Professor Rodney Herring
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.
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.
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
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
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.
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
Mg-Al Alloy System
At 437 oC aMg forms a eutectic with an intermediate phase d –
which has a mean composition of 32 wt%Al – or 33 at%Al –
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
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 wt%Zn
As MgZn is brittle – the eutectic is also brittle – as MgZn is
the major constituent
Mg-Zn Alloy System
The eutectic contains 71%d and 29% aMg
(d)
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.
ASTM Designation for Mg Alloys
1.
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
ASTM Designation for Mg Alloys
2.
3.
4.
Two digits indicate the rounded off percentages of the
alloying elements, e.g., AZ63 = Mg + 6%Al + 3%Zn
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
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
* - Mo, Nb, Ta, W
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.
*
**
We will discuss these alloys in turn
•- as fabricated
** - artificially aged
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.
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.
Microstructures of Mg-Alloys - 1
AZ31 Alloy – Annealed after hot working
AZ31 Alloy – cold rolled into sheet – work hardened
Microstructures of Mg-Alloys - 2
M 1A Alloy – Annealed
Particles in grain boundaries are impurities
Microstructures of Mg-Alloys - 2
ZK60 Alloy – Extruded from pellets
To obtain fine grain size (0.001 mm)
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
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.
Microstructures of Sand Cast Mg-Alloys - 1
Grain boundary constituent is Mg17Al12
Grain boundary constituent is local Mg17Al12
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
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.
Mechanical Properties of Mg-Alloys
(intermediate step)
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!
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.
The End
Any questions or comments?