Prior to the 1960s, Nippes [59] characterised three regions of a fusion weld, which were roughly composed of the fusion zone, the heat affected zone and the base metal. These regions are commonly categorised by the temperature experienced during the welding process. Each zone can be characterised by the following details and as shown in Figure 2.7:
• Fusion Zone (Weld Material): During welding, this zone is completely molten and has a maximum temperature above the liquidus temperature of the alloy. Grain structure development occurs by epitaxial growth, competitive growth (columnar grains), followed by the formation of equiaxed grains. Moreover, solidification cracking or hot tearing (mostly along the centre of the weld) are common phenomena.
• Heat Affected Zone: This zone is not melted during welding and exhibits a maximum temperature below the eutectic temperature of the alloy.
• Base Material: The metal to be welded but not affected by the heat input.
Figure 2.7: The three regions of a fusion weld, which is composed of the fusion zone, the heat affected zone and the base material [59]
Savage et al. [60] identified two additional regions. They proposed that the fusion zone consisted of a composite region where the filler metal and base metal are uniformly mixed and an unmixed zone where base metal melts and resolidifies without mixing with the filler metal. The heat affected zone was further subdivided into a partially melted zone and a true heat affected zone. The partially melted zone represents the transition from 100 per cent liquid at the fusion boundary to 100 per cent solid.
2.1.6.1 Epitaxial Solidification of Fusion Welds
For heterogeneous nucleation, the free energy required is a function of the wetting angle (Φ) between the substrate and the embryo formed. This is typically the case in a weld where the partially melted base metal at the fusion boundary acts as a substrate to the fusion weld.
Figure 2.8 illustrates the relationship between wetting angle and interfacial energy for heterogeneous nucleation. Assuming that the interfacial energy between the embryo and the liquid is isotropic, it can be shown, for given volume of the embryo, that the interfacial energy of the whole system is minimised if the embryo has the shape of a spherical cap.
Under such conditions, the following relationship exists between the interfacial energies (equations 2.4 to 2.6): with the embryo formation, γ is the interfacial energies, r is the radius of the stable nucleus, and f(Φ) is the shape factor. During welding, the liquid weld metal completely wets the base metal (substrate), rendering the wetting angle (Φ) is zero and so the free energy, ∆Ghet*, is also zero. In autogenous welding, the composition of the fusion zone is identical to the base metal and the grains at the fusion boundary of partially melted base metal generally act as seed crystals for the growing grains [60]. If the solidifying weld has a crystallographic orientation identical to that of the adjoining solid grain at the fusion boundary, epitaxial crystalline growth can occur to create an additional grain boundary parallel to the fusion boundary. This is evident from the continuity of grains across the fusion boundary observed in melt runs on some autogenous welds (as shown in Figure 2.8).
Figure 2.8: Schematic representation of (a) heterogeneous nucleation, and (b) epitaxial nucleation and competitive growth in the weld fusion zone
2.1.6.2 Solidification Parameters and Weld Microstructures
The parameters that determine the final welding microstructures are growth rate (R), also referred to as the alloy composition solidification rate, temperature gradient (G), cooling rate (T) and supercooling (undercooling) (T) [56].
The solidification rate (growth rate, R), also referred to as the solidification front, is the rate at which the solid/liquid interface advances across the weld pool. During steady state welding, the weld pool moves along the welding direction and the shape of the pool remains constant. Thus, the solidification rate is directly related to the welding speed. Figures 2.9 and 2.10 illustrate a schematic geometry of a 3-D weld pool where R denotes the solidification rate, v denotes the welding speed, and n is the unit vector normal to the solidification front. Since the solidification rate R is along the maximum temperature
gradient, which is normal to the liquid/ solid interface, it is related to the welding speed v by equation 2.7:
θ cos n v
R = •
Equation 2.7Assuming that the solidification front is normal to the surface, the growth rate would vary from R= 0 when θ = 90o along the fusion line to a maximum of R = v when θ = 0o long the centerline of the weld. Consequently, the growth direction of the columnar grains will change continuously from the fusion line towards the centre of the weld due to a corresponding shift in the direction of the maximum temperature gradient.
Figure 2.9: Schematic diagram showing relationships between welding speed and solidification rate, (a) weld pool in 3-D cross-section with a unit vector normal to the solidification front, (b) top view of the weld
Figure 2.10: Effects of temperature gradient and growth rate for different shape of weld pools. (a) elliptical shape, low and moderate welding speed (b) tear-drop shape, high welding speed
2.1.6.3 Supercooling
The supercooling, sometimes referred to as undercooling, ∆T, is generally defined as the temperature difference between the equilibrium temperature of a system and its actual temperature [59]. The latter is lower than the equilibrium temperature when the melt is supercooled. David and Vitek [56] have reviewed that the total supercooling for growth of solidification during welding which is composed of four terms, as shown in equation 2.8:
k r c
th T T T
T
T =∆ +∆ +∆ +∆
∆ Equation 2.8
Where ∆Tth isthe thermal supercooling at the interface as well as within the weldpool, ∆Tc
is the constitutional supercooling present at the interface as well as in the liquid, ∆Tr is the
supercooling due to curvature at the solid/liquid interface present at the interface and ∆Tk is the kinetic supercooling present at the interface.
Process Cooling Rate (K/s)
Directional Solidification 10-1 to 101
Casting 100 to 102
Arc welding 101 to 103
Electron and Laser welding 102 to 106 Rapid Solidification Processing 103 to 107 EB or LB surface modification 105 to 107
Single laser pulse 107 to 108
Table 2.4: Estimated cooling-rate ranges for various solidification processes [61]
Among these, the ∆Tk is so small that its contribution can be ignored. In contrast, much higher solidification rates are found in welding than in casting. Therefore, the ∆Tr term is believed to be significant. Thermal supercooling is not commonly found under welding conditions since nucleation of the solid is not difficult and therefore it is not possible to cool the liquid far below the transformation temperature. A summary of cooling rates for selected solidification and welding processes is given in Table 2.4.
Since the solidification is accompanied by the evolution of heat, in a pure metal the solidification rate is determined by the rate of heat extraction from the solid-liquid interface.
This is of purely academic interest only in welding, since the welding of pure metal is impractical. Most engineering metals are alloyed and their melting points are composition dependent during solidification. The change in transformation temperature due to compositional effects is known as constitutional supercooling (see Figure 2.11) which described by Savage et al. [62]. The extent of constitutional supercooling in a given alloy depends on its solidification rate and temperature gradient. Thus, by plotting the value of G/(R)½ as the abscissa and the solute content as the ordinate, the solidification microstructures can be summarised as shown in Figure 2.11. In the broadest sense, low values of G/(R)½ indicate an increased tendency for constitutional supercooling, thus favouring the dendritic growth during solidification, whereas steep G in the liquid and slow R favour planar and cellular growth.
Figure 2.11: Influence of cooling rate and direction of base metal crystallisation on crystal configuration, crystal direction and constitutional supercooling. Modified after Savage[62]