198
S. S. Nayak et al.
provides the ideal nucleation site. Few solidification modes that exist at the S–
L interface are (a) planar growth (Fig. 6.2a), (b) cellular growth (Fig. 6.2c), (c)
cellular dendritic growth (Fig. 6.2e), (d) columnar dendritic growth (Fig. 6.2g) and
(e) equiaxed dendritic growth (Fig. 6.2i)[5]. The grain structure which can be seen in
a typical weld is planar grains, cellular grains, dendritic grains and equiaxed grains.
Factors that govern different types of grain structure are: (a) composition of weld
metal which is solidifying (it basically affects the constitutional supercooling) and (b)
heat transfer rate away from weld metal (cooling rate experienced by the weld metal
and it actually affects the temperature gradient in weld metal). Cooling rate determines the rate of growth of S–L interface movement. In Fig. 6.2a, c, e, g, solidified
regions as well as liquid region which is to be solidified can be seen. Heat is rejected to
the BM from the fusion boundary. Due to heat extraction, the grains grow in opposite
direction of the heat flow and perpendicular to the fusion boundary. First, it forms the
planar grain structure next to the fusion boundary. Thereafter, the cellular grains form.
Width of these zones is governed by the composition as well as the cooling condition
experienced by the weld metal during the solidification. Thereafter, the dendritic
structure forms and in the centre of the weld, equiaxed grain structure forms. Similar
kind of things is repeated from both the sides and gets accommodated in centre of the
weld, where solidification occurs at the end. In the alloy, solidification starts with the
formation of pure metal or the metal with a minimum concentration of the alloying
element and one having the higher liquidus temperature. The solid region is formed
by consuming the liquid metals and rejecting the excess alloying elements which are
beyond the solubility limit in the solid state. The region next to the S–L interface is
enriched with alloying elements. Therefore, in this region, equilibrium temperature
gradient is different from the remaining portion due to compositional variation. In
S–L interface, if the equilibrium temperature gradient is below the actual temperature gradient as per the cooling condition, then planar grains are formed (Fig. 6.2b,
dark line). If this condition is reversed, it results in the formation of zone having
an effect of constitutional supercooling (Fig. 6.2d, f, h, j). When the zone is very
limited, cellular grain results and plane interface are broken and cells start to grow.
Further increase in constitutional supercooling leads to the formation of dendritic
structure, which causes equiaxed grain structure.
In fusion welding, the molten weld pool is surrounded by the solid metal. The
heat flow is always directed towards the cold metal. Therefore, the weld metal attains
a column-like shape with long grains that are parallel to the heat flow direction. If
solidification is extremely rapid, a cell-like microstructure [6] is obtained where
dendrites are not fully developed.
The dendrites grow when the molten metal gets solidified. The tree-like shape
is produced because of the faster growth of grains in favourable crystallographic
direction. In the beginning, one solid nucleus grows in the melt. In metals, this solid
tries to minimize the area of the surface having high surface energy. The dendrites,
which are initially formed, solidify first. The solid rejects the solutes which are
most soluble in liquid, and these, in turn, get diffused into the remaining liquid
and become concentrated near the S–L interface. The growth of crystals is locked
in that direction. Dendritic arm characteristics of as-solidified metals are produced
S. S. Nayak et al.
provides the ideal nucleation site. Few solidification modes that exist at the S–
L interface are (a) planar growth (Fig. 6.2a), (b) cellular growth (Fig. 6.2c), (c)
cellular dendritic growth (Fig. 6.2e), (d) columnar dendritic growth (Fig. 6.2g) and
(e) equiaxed dendritic growth (Fig. 6.2i)[5]. The grain structure which can be seen in
a typical weld is planar grains, cellular grains, dendritic grains and equiaxed grains.
Factors that govern different types of grain structure are: (a) composition of weld
metal which is solidifying (it basically affects the constitutional supercooling) and (b)
heat transfer rate away from weld metal (cooling rate experienced by the weld metal
and it actually affects the temperature gradient in weld metal). Cooling rate determines the rate of growth of S–L interface movement. In Fig. 6.2a, c, e, g, solidified
regions as well as liquid region which is to be solidified can be seen. Heat is rejected to
the BM from the fusion boundary. Due to heat extraction, the grains grow in opposite
direction of the heat flow and perpendicular to the fusion boundary. First, it forms the
planar grain structure next to the fusion boundary. Thereafter, the cellular grains form.
Width of these zones is governed by the composition as well as the cooling condition
experienced by the weld metal during the solidification. Thereafter, the dendritic
structure forms and in the centre of the weld, equiaxed grain structure forms. Similar
kind of things is repeated from both the sides and gets accommodated in centre of the
weld, where solidification occurs at the end. In the alloy, solidification starts with the
formation of pure metal or the metal with a minimum concentration of the alloying
element and one having the higher liquidus temperature. The solid region is formed
by consuming the liquid metals and rejecting the excess alloying elements which are
beyond the solubility limit in the solid state. The region next to the S–L interface is
enriched with alloying elements. Therefore, in this region, equilibrium temperature
gradient is different from the remaining portion due to compositional variation. In
S–L interface, if the equilibrium temperature gradient is below the actual temperature gradient as per the cooling condition, then planar grains are formed (Fig. 6.2b,
dark line). If this condition is reversed, it results in the formation of zone having
an effect of constitutional supercooling (Fig. 6.2d, f, h, j). When the zone is very
limited, cellular grain results and plane interface are broken and cells start to grow.
Further increase in constitutional supercooling leads to the formation of dendritic
structure, which causes equiaxed grain structure.
In fusion welding, the molten weld pool is surrounded by the solid metal. The
heat flow is always directed towards the cold metal. Therefore, the weld metal attains
a column-like shape with long grains that are parallel to the heat flow direction. If
solidification is extremely rapid, a cell-like microstructure [6] is obtained where
dendrites are not fully developed.
The dendrites grow when the molten metal gets solidified. The tree-like shape
is produced because of the faster growth of grains in favourable crystallographic
direction. In the beginning, one solid nucleus grows in the melt. In metals, this solid
tries to minimize the area of the surface having high surface energy. The dendrites,
which are initially formed, solidify first. The solid rejects the solutes which are
most soluble in liquid, and these, in turn, get diffused into the remaining liquid
and become concentrated near the S–L interface. The growth of crystals is locked
in that direction. Dendritic arm characteristics of as-solidified metals are produced