SESSION 12: SEGREGATION AND CRACKING PHENOMENA
Fusion-Boundary Macrosegregation in Welds Made with Dissimilar Filler Metals
Y. K. Yang and S. Kou
Introduction
Dissimilar filler metals, that is, filler metals different in composition from the workpiece,
are more often used than not in arc welding. It has been long recognized that at the fusion
boundary the melted base metal tends not to mix well with the weld pool and forms the socalled “unmixed” zone, which can be susceptible to cracking. Two mechanisms have been
proposed to explain fusion-boundary macrosegregation based on basic solidification concepts,
instead of just attributing it to weak convection near the pool boundary as in the past, and
verified with aluminum welds made with dissimilar filler metals.
Technical Approach
Aluminum GMA welds were made with dissimilar filler metals. Pure Al and eutectic AlCu were selected for welding because of their easily recognizable microstructures (featureless
and lamellar, respectively). Commercially pure Al (alloy 1100) was welded with Al-Cu filler
metals. Eutectic Al-33Cu alloy was cast and welded with commercially pure Al filler metal
(alloy 1100). The eutectic was welded either in the as-cast condition or after heat treating to
coarsen the lamellar structure. The resultant microstructure at the fusion boundary was
examined. The composition profiles across the fusion boundary were measured to determine
macrosegregation at the fusion boundary.
Results/Discussion
The trailing portion of the weld pool boundary, that is the solidification front, is at the
liquidus temperature of the weld metal (TLW). However, if some melted base metal near the
fusion boundary is unmixed, it solidifies at the liquidus temperature of the base metal (TLB).
The mechanism for the formation of the filler-deficient zone (FDZ) is shown in Fig. 1 for
the case where the filler metal makes TLW < TLB. The region of the liquid weld metal
immediately ahead of the solidification front is below TLB. The liquid base metal, if it is swept
by convection nearly parallel to the pool boundary, can enter this cooler region and freeze
quickly without much mixing. Depending on convection, “peninsulas” or “islands” roughly
parallel to the fusion boundary can form as well as “beaches” along the fusion boundary.
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SESSION 12: SEGREGATION AND CRACKING PHENOMENA
TLW < TLB: Non-isothermal pool boundary;
quick freezing of liquid base metal into
peninsulas/islands roughly parallel to
fusion boundary
TLW > TLB: Non-isothermal pool boundary;
quick freezing of liquid weld metal into
intrusions in beach; randomly oriented
peninsulas/islands
θ+L
weld pool
base
metal
weld
metal
TE
TLB
cooler liquid base
metal; T < TLW
fillerdeficient
peninsula
filler
metal
weld
metal
fusion boundary
flow
FDZ
weld-metal
intrusion
CW
CF
Solute content, wt% B
(c)
liquid base metal
mixes with warmer
liquid weld metal
before freezing
bulk weld pool
m
zo ush
ne y
bu
m lk
et w
al el
d
liquid weld metal TLW
freezes quickly in
cooler liquid
base metal
CB
weld pool
base
metal
fusion boundary
(c)
bulk weld
pool
flow
TLW
eutectic
(b)
welding
direction
filler
metal
θ+L
eld
hy
us lk w
m u
b
FDZ
TLB
fillerdeficient
island
filler-deficient
stagnant or
beach
laminar-flow filler-deficient
filler-deficient
layer of liquid
peninsula
island
base metal
filler-deficient beach
Fig. 1 Mechanism for formation of
filler-deficient zone (FDZ) when filler
metal makes TLW < TLB: (a) phase
diagram; (b) longitudinal weld pool
cross-section; (c) filler-deficient beach,
peninsula and island.
liquid base metal
freezes quickly in
cooler liquid weld
metal T
TLW TE
LB
LB
(b)
welding
direction
CW
CB
Solute content, wt% B
filler
metal
zo
ne
me
tal
CF
α
A
eutectic
A
TLB
TE
α+L
L
T
α+L
base
metal
TLB
TLW
<
TLW
TLB
TE α
L
base
metal weld
metal
T
filler
metal weld
metal
Temperature, T
Temperature, T
(a)
(a)
Fig. 2 Mechanism for formation of fillerdeficient zone (FDZ) when filler metal
makes TLW > TLB: (a) phase diagram; (b)
longitudinal weld pool cross-section; (c)
filler-deficient beach, peninsula and
island.
The mechanism for the formation of the filler-deficient zone (FDZ) is shown in Fig. 2 for
the case where the filler metal makes TLW > TLB. The liquid base metal near the fusion
boundary is below TLW. The liquid weld metal, if it is pushed by convection toward the pool
boundary, can enter this cooler region and freeze quickly as intrusions without mixing. The
unmixed liquid base metal remaining along the pool boundary can solidify as a “beach” while
that between the intrusions as randomly oriented “peninsulas” or “islands.”
With similar convection, the chance of a wider macrosegregation zone is higher with a
larger difference between TLW and TLB.
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SESSION 12: SEGREGATION AND CRACKING PHENOMENA
Figure 3 shows an “unmixed” island roughly parallel to the fusion boundary in an
aluminum weld made with TLW < TLB. Figure 4 shows beaches, peninsulas and an island in an
aluminum weld made with TLW > TLB. The latter two form in the space between weld-metal
intrusions and are randomly oriented. These results confirm the proposed mechanisms.
TLW < TLB
island: unmixed base metal, roughly parallel
to fusion boundary
(a)
bulk weld metal
TLW > TLB
beach: continuous, intruded
peninsula/island: randomly
oriented
A
welding direction
B
bulk
weld
metal
C
filler-deficient
island
D
base metal
filler-deficient beach
base metal
Cu Content, wt %
filler-deficient island
40
30
20
10
(b)
B
A
weld
metal
fillerdeficient
island
C
weld
metal
0
(b)
bulk weld
metal
D
base
metal
E
0
200 μm
50 μm
E
(a)
filler-deficient weld-metal
peninsula
intrusions
filler-deficient beach
50 100 150 200 250 300 350
Distance, μm
base metal
Fig. 3 Macrosegregation across fusion
boundary when TLW < TLB: (a) transverse
micrograph; (b) composition profile.
Workpiece: 1100 Al; filler: Al-52.5Cu.
200 μm
Fig. 4 Longitudinal micrographs along
fusion boundary when TLW > TLB: (a)
beach and peninsulas; (b) beach and
island. Workpiece: heat-treated Al33Cu eutectic; filler: 1100 Al.
CONCLUSIONS
Mechanisms have been proposed to explain macrosegregation near the fusion
boundary, including “beaches”, “peninsulas”, or “islands” similar to the base metal in
microstructure and composition. With TLW < TLB, the melted base metal near the fusion
boundary swept into the cooler liquid weld metal immediately ahead the solidification front can
quickly freeze without much mixing. With TLW > TLB, the liquid weld metal pushed into the
cooler layer of liquid base metal near the fusion boundary can quickly freeze without much
mixing. These mechanisms have been confirmed with aluminum welds made with dissimilar
filler metals.
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SESSION 12: SEGREGATION AND CRACKING PHENOMENA
274