part ii: partially melted zone - eng.sut.ac.theng.sut.ac.th/metal/images/stories/pdf/04_weld...
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Part II: Partially melted zone
Suranaree University of Technology Sep-Dec 2007
• 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
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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
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• 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
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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