Part II: Partially melted zone

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• Grain boundary segregation is

one of the serious problems

observed in partially melted zone.

• This unfortunately lowers the

mechanical properties of the

welds.

• The liquated and re-solidified

material along the grain

boundary is shown in fig (a dark-

etched area is eutectic GB and a

lighter etched area is αααα (Al-rich)

band along GB). � making grain

boundary weaker.

(a) PMZ microstructure of GMAW in

6061 aluminium made with 4145

aluminium filler wire, (b) enlarged.

Partially

melted zone

Liquated metal along GB

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Formation of partially melted zone

Suranaree University of Technology Sep-Dec 2007

Formation of PMZ in 2219 aluminium weld (a) Al-rich side of Al-Cu phase

diagram, (b) thermal cycles, (c ) transverse cross section.

WM

PMZ

BM

Consider welding of 6.3% Cu 2219 alloy

• Metal at position a is

completely melted.

• Metal at position b is

heated upto between the

eutectic temperature TEand the liquidus

temperature TL. The

metal becomes a solid +

liquid mixture � partially

melted.

• Metal at position c is not

melted at all.

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Microstructure evolution in PMZ

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Microstructure evolution in PMZ of

2219 Al alloy.

• At T < TE, microstructure of the base metal

consists of αααα and θθθθ (Al2Cu) within grains

and along GB, see fig (a).

• At T = TE, eutectic reaction occurs

α + θα + θα + θα + θ ���� LE (liquid of eutectic composition,

CE). Cooling at this point gives

microstructure of eutectic particles and

some GB eutectic, fig (b) and (c).

• At T > TE, liquation inside PMZ intensifies,

more αααα dissolve into liquid phase changing

composition to hypoeutectic liquid. Cooling

at this point gives Cu-depleted αααα and Cu

rich eutectic.

Consider welding of 6.3% Cu 2219 alloy

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Solidification of grain boundary liquid in PMZ

Solidification of grain boundary

liquid in PMZ

• The grain boundary liquid has a tendency to

solidify essentially upward and toward the

weld due to high temperature gradient toward

the weld pool.

Solidified

GB liquid

Solidified

GB liquid

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Problems associated with the Problems associated with the partially melted zonepartially melted zone

Suranaree University of Technology Sep-Dec 2007

• Liquation cracking

(PMZ cracking or hot cracking)

• Loss of strength and ductility

• Hydrogen-induced cracking

The partially melted zone can suffer from problems as described

below, which are induced by grain boundary liquation in the PMZ

during welding.

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Liquation crackingLiquation cracking

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‘Liquation cracking’ is also called ‘PMZ cracking’ and in the

more commonly used name ‘hot cracking’.

PMZ cracking in 2219 Al welded with

filler metal 1100.

• Liquation in the PMZ is an

intergranular type and can also

occur along the fusion

boundary.

• The presence of a liquid phase

at the intergranular fracture

surface can be either evident or

unclear.Partially

melted zone

Weld

Cracks

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Liquation cracking Liquation cracking –– susceptibility testssusceptibility tests

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Circular patch testing

Schematic

of a circular

patch test

• Specimen is sandwiched between two copper

plates while welding is employed on the top.

• Liquation cracking occurs at the outer edge

of the weld because the outer edge is

restrained in tension while the inner edge is

in compression due to contraction during

welding.

Cracking in circular patch welds

(a) 6061 Al and (b) 2219 Al both

made with a 1100 filler wire.

Liquation

cracking

Solidification

cracking

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Liquation cracking Liquation cracking -- mechanismsmechanisms

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• Since PMZ is weakened by grain

boundary liquation, it cracks when

the solidifying weld metal contract

and pulls it.

• Metals having a wide freezing

temperature range especially most

aluminium alloys are susceptible for

liquation cracking (hot cracking).

• It is also due to large

solidification shrinkage and large

thermal expansion.

Formation of PMZ cracking

in a full-penetration

aluminium weld.

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Weld metal pulling and tearingWeld metal pulling and tearing

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Weld metal pooling and tearing PMZ in 7075 aluminium welded with filler 1100.

• The papillary (nipple) type penetration is common in GMAW of Al alloys with Ar

shielding with spray transfer mode.

• Small amount of weld metal in the nipple solidifies much more quicker than the

surrounding area. Contraction occurs in the nipple and pulls the PMZ that is

weakened by grain boundary liquation.

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Loss of strength and Loss of strength and ductilityductility

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• Microhardness of the Cu-depleted αααα is

much lower than that of Cu-rich eutectic.

• This segregation results in a weak PMZ

microstructure with a soft ductile αααα and a

hard brittle eutectic next to each other.

• During tensile loading, the αααα yield without

much resistance while the eutectic fractures

badly.

• Fracture is along the GB and large eutectic

particles.

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HydrogenHydrogen--induced crackinginduced cracking

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• Since hydrogen dissolves in liquid iron ~ 3-4 times better

than in solid, GB liquated films in the PMZ therefore act as

‘pipelines’ for hydrogen to diffuse from the weld metal through

the fusion boundary.

Hydrogen-induced cracking in the

PMZ of HY-80 steel.

Hydrogen induced crackingHydrogen

induced cracking

Supersaturated hydrogen

GB films with higher

hardenability.

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Remedies for liquation crackingRemedies for liquation cracking

Suranaree University of Technology Sep-Dec 2007

1) Use proper filler metals

Ex: for Al weld, Al-Mg filler is not preferred, but Al-Si filler is more

favourable for the reduction of liquation cracking

2) Reducing heat input

Heat input PMZ

3) Reducing degree of restraint

• The presence of tensile stress can be the cause of liquation cracking.

• Reducing the degree of restraint and hence the level of tensile stresses.

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Remedies for liquation crackingRemedies for liquation cracking

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4) Controlling the base metal

Impurities Reducing S and P is known to reduce

the freezing temperature range,

reducing chances for liquation

cracking, i.e. nickel base alloy.

Grain size and orientation

Microsegregation

Grain size Liquation cracking

Liquation cracking

Small grains

Large grainsTapany Udomphol

Part III:Part III: The heatThe heat--affected zoneaffected zone

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• Temperature distribution outside the fusion zone is not sufficiently high

to melt the base metal in the adjacent area of the weld.

• Grain growth, recrystallisation, phase transformation or tempering/annealing

can occur, depending on temperature gradient and the nature of the alloy.

Temperature gradient and grain

structure transformation

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RecrystallisationRecrystallisation and grain growth and grain growth in weldingin welding

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If the base material has been cold worked prior

to welding, the effect of work hardening is

completely gone in the fusion zone due to

remelting and is partially lost in HAZ due to

recrystallisation and grain growth.

Grain growth in electron beam

weld of molybdenum.

Note: Strength loss should be taken into

account in structural designs. (even

toughness)

The harder the base metal, the greater the

strength loss is.

Softening of work-

hardened material

caused by welding

(a) thermal cycles

(b) strength or

hardness profile.Fusion

zone

Grain

growthBase

metal

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Effect of welding parameters Effect of welding parameters and process and process

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Heat input per unit length

HAZ width

Loss of strength/hardness

Effect of heat input per unit length of weld on (a)

width of HAZ, (b) thermal cycles near fusion

boundary (c ) strength or hardness profile.

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Effect of heat input on HAZEffect of heat input on HAZ

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Small HAZ in single pass

electron beam welding of beta

titanium alloy. (Low heat input)

0.5 mm

HAZ HAZFusion Zone

2 mm

HAZ HAZFusion ZoneLarge HAZ in multipass

gas tungsten arc welding

of beta titanium alloy.

(High heat input)

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ReferencesReferences

• Kou, S., Welding metallurgy, 2nd edition, 2003, John Willey and

Sons, Inc., USA, ISBN 0-471-43491-4.

• Evans, G.M., Bailey, N., Metallurgy of basic weld metal, 1997,

Abington publishing and William Andrew Inc., ISBN 1 884207 57 X

• David, S.A., Babu, S.S., Vitek, J.M., Welding : Solidification and

microstructure, JOM, June 2003.

• Koteswara Rao, S.R.,Madhusudhana Reddy, G., Kamaraj, M.

andPrasad Rao, K., Grain refinement through arc manipulation

techniques in Al–Cu alloy GTA welds, Material Science and Engineering A, Vol. 404, 2005, p. 227-234.

Suranaree University of Technology Sep-Dec 2007Tapany Udomphol