Microstructural features, properties

and application of magnesium alloy

Vinayak kumar

NIT-TRICHY

Industrial metallurgy

• Found 2.8% in sea water and other forms, i.e., dolomite (CaMg(CO3)2),

magnesite (MgCO3) and Carnallite (KMgCl3.6H2O).

• Magnesium with 99.8% purity are readily available but rarely used in

this stage for engineering applications.

Applications

phone bodies. Magnesium side panels

Alloyed wheel

Gearbox housing in the VW-Passat

Aerospace applications

Cam and mobile

Physical properties of magnesium

• Low strength and toughness and corrosion

resistance.

• Easily flammable with oxygen.

• Alloyed with Al, Zn, Th, rare earth metals to produce alloys with high-

strength-to weight ratios.

• Tends to form compounds with negative valence ion (due to strong

electropositive) rather than solid solution.

• Not readily plastically deformed at room temperature due to HCP structure.

• Cast magnesium alloys dominate 85-90% of all magnesium alloy products,

with Mg-Al-Zn system being the most widely used.

Crystal structure HCP a = 0.3202, c = 0.5199, c/a = 1.624

Atomic diameter 0.320 easily alloyed with Al, Zn, Ag, Zr

Density (g.cm-3) 1.74

Melting point (oC) 650

Classification of magnesium alloys

• ASTM system designated by two capital letter

followed by two or three numbers.

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

A – Aluminium B – Bismuth C – Copper

D – Cadmium

E – Rare earths

• The letters stand for the two major alloying elements

- first letter - the highest amount

- second letter - the second highest amount

• The numbers stand for the amount of the two major

alloying elements

- first number following the letters stands for

the wt% of the first letter element.

- second number stands for the wt% of the second

letter element.

Example

The magnesium alloy contains

2 wt% silver, 2% rare earths

and in the A modification.

T6 – solution heat-treated, quenched

and artificially aged.

QE22A-T6 -

AZ92A - The magnesium alloy contains

9 wt% aluminium, 2 wt% zinc.

Commercial magnesium alloys

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Magnesium- Aluminum based alloys

Magnesium-Zinc based alloys

Magnesium-Rare Earth alloys

Magnesium-Thorium based alloy

High temperature Mg casting alloys

Mg-Al based alloy

• Al is alloyed to increase strength, castability and corrosion resistance

• Maximum solid solubility is ~ 12.7% at 818F • Decreases to 3.2% at 400 F.

• This group includes Magnesium-Aluminum- Manganese and Magnesium- Aluminum- Zinc.

• AZ63A is used where maximum toughness along with moderately high yield strength.

• AZ92A is used where maximum yield strength plus good pressure-tightness are required.

• The microstructure of AZ92A alloy is shown

• The structure in sand-cast condition shows a network of massive δ (Mg17Al12)

However most of the eutectic has separated, the ε phase has joined the

primary ε grains, leaving behind the compound as a white network.

By rapid cooling and subsequently artificial aging can cause precipitation of

the compound as fine particle within the grain.

The wrought alloy M1A (1.2% Mn) is low cost, low strength has excellent

weldability, corrosion resistance and hot formability.

AZ31 with low calcium content(PE alloy) is widely used for magnesium

photoengraving sheet.

AZ61A and AZ80A has good strength and ductility

Chemical composition of Mg-Al based

alloy and applications

Mechanical properties of Mg-Al based

alloys

Mg-Zn based alloys Essentially magnesium-zinc-zirconium (ZK) and magnesium-zinc-thorium

(ZH) casting alloys

Zirconium is added for grain refinement so increases the mechanical

properties

Zn controlling is important as it causes microporosity and hot cracks so

Thorium and rare-earth metal is added with effect on mechanical properties

ZK51A, ZK61A,the highest tensile strength and ductility of any Mg casting

alloy

ZH62A-T5 Lamellar form of Eutectic Mg-Th-Zn

Mg rich solid

solution

Compound

of Mn-Th-

Zn

Chemical composition, application and

mechanical properties of Mg-Zn based alloy

Magnesium-Rare Earth-based alloys Includes of magnesium-rare earth-zirconium(EK), magnesium- rare earth-

zinc

By addition of rare earth elements we get improvement in elevated-

temperature properties

So these alloys are suitable for use up to 480˚C

Alloy EZ33A-T5 sand casing

Mg –E compound

Mg rich solid solution

Magnesium-thorium-based Alloys

Includes magnesium-thorium-zirconium(HK) and magnesium-thorium-

zinc(HZ)

Improved elevated temperature properties

HK31A- short-time elevated temperature under high stress

HZ32A –long-time lower stress applications

High temperature Magnesium casting alloys Primarily used for aerospace applications due to light weight (major consideration)

Application range 200-250oC with tensile strength ~240MPa

Mg-Ag-RE alloys

QE22-has been used for aerospace applications, i.e., landing wheels, gear box

housings.

Mg-Y-RE alloys

WE43-has been developed for improved elevated temperature tensile properties.

Mg-Ag-Th-RE-Zr alloys

Forging of Mg alloys

Advantages of forged magnesium components compared to commonly

used die cast magnesium parts:

Excellent strength, especially with the fibres lying parallel to the main

load direction.

Very good properties for pressure-sealed components because of a

forging process in preventing a porous microstructure

Joining of magnesium alloys

Arc welding ( TIG, MIG )

Laser beam welding

Electron beam welding

Diffusion welding

Friction welding

Spot welding

Engineering design with Magnesium alloys Advantage

Ability to die cast at high productivity rate

Good creep resistance

High damping capacity due to ability to absorb energy elastically

High thermal conductivity permitting rapid heat dissipation

Good machinability

Easily gas-shield arc-welded

Disadvantage High tendency to galvanic corrosion with dissimilar metals or electrolyte. Difficult to deform by cold working

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