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Magnesium Alloys

Magnesium is a silvery white metal and has the lowestdensity of the common structural materials.

Very Light: 1738 kgm-3. Magnesium has a melting point of 650C.

Modulus low: E = 44.7 GPa

Crystal structure is hcp,

Alloying is for precipitation hardening and (wroughtalloys) grain refinement.

Excellent machining properties

Alloys are weldable (inert gas)

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Two main classes of alloys:

Impact strengths low

High damping capacity (useful in machine casings) Corrosion resistance very poor. Can coat chromate,

anodise, epoxy resin.

alloyed with elements such as Al, Zn, Mn, Zr, etc.

Alloying increases strength and corrosion resistance

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Alloying additions

Al, Zn, Th Produce precipitation hardening very complexseries of metastable precipitates, depending on alloy

composition Th very stable precipitates, good for creep resistance

Mn corrosion resistance (ties up Fe and otherimpurities)

Zr strong grain refinement, reacts with Al and Mn, Mg-Zralloys must be Al, Mn free.

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Magnesium alloys possess

High strength to weight ratio

Good fatigue strength.

Good dimensional stability in service. Good damping capacity.

High thermal conductivity.

Relatively high electrical conductivity.

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Welding Characteristics of Magnesium

Most magnesium alloys are readily weldable.

Weld strengths above 90% are possible when the fillermetal used

T

he weldability of magnesium alloys may be affectedby :

1. Oxidation.2. Thermal expansion.3. Susceptibility to hot cracking (hot shortness).

4. Grain growth and aging effects

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Base Metal Surface Preparation

usually supplied with an oil coating, an acid pickledsurface or a chromate coated surface

Mechanical cleaning Chemical cleaning

This cleaning solution is kept in a tank made up ofceramic or stainless steel

The job is dipped in the bath, which is kept at 22 to32C, for about 3 minutes

then is rinsed thoroughly in hot water and dried in air.

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Welding Processes

The various processes employed for welding magnesiumalloys are

1. Gas Tungsten Arc Welding (TIG or GTAW)2. Gas Metal Arc Welding (MIG or GMAW).3. Resistance (Spot) Welding.4. Gas Welding.5. forge welding.6. Other processes (Pr Welding, Brazing, EB welding,TIG spot welding, Stud Welding).

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GTAW for Mg Alloys

most popular process for welding Mg alloys

DC or reverse polarity and AC with superimposed are

commonly used. Both manual and automatic methods are suitable.

On materials over 4.5 mm thick, A.C. is preferred,because it provides deeper penetration.

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GMAW for Mg Alloys

welded using DCRP. Argon shielding is mostsatisfactory.

yields welding speeds 2 to 4 times faster than TIGwelding.

Speeds range from 60 to 150 cm per minute and evenhigher.

Increased welding speeds result in reduced distortion.

Involves high metal deposition rates limited to flat, horizontal and vertical down welding

positions.

MIG welding are equal to TIG welds in strength and

soundness.

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Resistance Welding for Mg Alloys

either AC or DC used to join Mg alloys.

can be welded as spot, seam and flash welding.

In Spot welding thicknesses up to about 4.5 mm Spot weld penetration in Mg should not be less than

20%, nor more than 80%, into each of the parts beingjoined

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Gas Welding for Mg Alloys

Gas welding is generally restricted to pure Mg and toMn-Mg alloys

oxy-acetylene, oxy-hydrogen and oxy-carbon hydrogengases may be employed.

Oxy-acetylene is preferred for welding heavier gauges.

The filler rod should be either of the same compositionas the base metal or have a lower melting point than

BM.

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Forge Welding for Mg Alloys

upset of 50 mm are used to weld 19 mm diameter rod.

Heating and welding times of less than 30 seconds at

temperatures between 280 to 316C are used for Mg-3% Al-l % Zn alloy.

developed to join high strength Mg alloys.

Joint efficiencies as high as 95-100%

Upset pressures of 35 tonnes with a total

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PROBLEMS ASSOCIATEDWITHWELDINGOF Mg AND ITS ALLOYS

SOLIDIFICATION CRACKING:Solidification cracking or hot cracking, consists offractures at the weld metal boundaries in the solidificationprocess, during which the liquid phase of the mushy meltbecomes rich in impurities, mainly S and P

This tendency is augmented by addition of Zn and Ca to Mgand its alloys

Al, Mn and Zr have little or no effect on this tendency whileThorium inhibits solidification cracking

The most high strength high alloying types of Mg alloys arethe most prone to such craking

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STRESS CORROSION CRACKING:

Al bearing Mg alloys are susceptible to such type of cracking. To

aviod this, the weld must be heat treated to 250C to relieve thestresses.

Zr and Th inhibits this type of cracking tendency

During joining of Mg foils, through arc welding or resistancewelding, there is a risk of catching fire. So precautions must be

taken to avoid any fatalities