Microstructure formation

Besides chemical composition, it is often microstructure characterized by phase and growth morphology, which controls the functional properties and the quality of the final coating obtained. The microstructure formed is largely dependent on solidification process and chemical composition of the coating alloy. In laser cladding, microstructure formation frequently falls in the category of rapid solidification since growth rates of the solid-liquid interface (Vs) are often >600 mm/min [299]. Typically, three kinds of microstructures are detected in laser coatings; planar, cellular and dendritic. The latter two are usually columnar in shape. They grow perpendicular to temperature isotherms, i.e. along the steepest temperature gradient. In cubic metals, crystallographic effects influence the grain growth by favoring the growth along preferred crystallographic directions. These preferred directions are

<100> directions, which is usually seen in XRD pattern taken from the surface of fcc-ordered alloy parallel to coating/base material interface as the strongest peak arises from the plane (002) instead of plane (111), which would give the strongest peak if randomly oriented. In practice, this means that growth direction remains essentially perpendicular to coating/base material interface (direction [001]) except for the very top surface where the growth is parallel to coating/base material interface (direction [100]) due to a minimum velocity criterion given by equation 2 (Figure 10) [300]:

These different types of microstructural growths depend strongly on the local shape of the solid-liquid interface, which in turn depends on solidification conditions, which are determined by thermal gradient (G, °C/m) and solidification speed (Vs, m/s) at the solid-liquid interface. The latter is the same as the growth rate of the solid-liquid interface mentioned earlier. Solidification conditions (G and Vs) are dependent on processing conditions and they vary as a function of depth of the formed bead as shown in Figure 10 and explained in caption. Vs is dependent on the traverse speed of the cladding process (Vb) and angle θ. Angle θ depends on the shape of the melt pool, which is determined by heat flux. Heat flux depends on the traverse speed (Vb), dimension of the laser beam spot along cladding direction (d1) and temperature dependent thermophysical properties of the alloy, namely the diffusion coefficient for heat (α(T), mm²/s), i.e. thermal diffusivity (α(T) = k(T) / (ρ • c(T))). The higher the traverse speed (Vb) in relation to factor α(T)/d1, the more elongated the melt pool [301].

This also means that Vs cannot be increased indefinitely since the angle θ stays large with high Vb even on the surface. Consequently, Vs would never reach the Vb at high traverse speeds [302]. Thermal gradient (G), in turn, depends on the traverse speed (Vb), temperature difference between melt pool and base material as well as thermal conductivity [303]. The

following general trends regarding the variation in G were identified; when Vb increases, G increases; when the base material is preheated, G decreases and when the thermal conductivity increases, G decreases. Examples of magnitudes of Vs and G was given by Frenk and Kurz [81], who calculated values for Vs and G for the different traverse speeds (Vb) at the solid-liquid interface of Stellite 6 throughout the coating thickness. With the traverse speed of 100 mm/min average values of Vs and G were 84 mm/min and 2 • 10⁵ K/m. Increasing traverse speed to 10 000 mm/min, increased average values of Vs and G to 8400 mm/min and 1.5 • 10⁶ K/m. These values gave cooling rates of 280 and 2.1 • 10⁵ K/s since cooling rate (Tc) is defined by: Tc = G • Vs.

Talking about the planar front growth, according to solidification theory, it becomes possible when G/Vs ratio is high. These conditions are encountered at the bottom of the melt pool.

When G/Vs ratio starts to decrease, planar solid-liquid interface destabilizes leading to cellular, dendritic and again cellular growth in this sequence. The critical growth velocity (or thermal gradient) after which the planar growth destabilizes and cellular and dendritic growth starts is given by:

ΔT0 = equilibrium liquidus-solidus interval (= width of the mushy zone) (°C)

It can be seen that the wider the mushy zone of the alloy with given G, the lower the Vc. This means that the wider the mushy zone, the narrower the planar front zone near the coating/base material interface. Planar microstructure would be highly desirable throughout the coating since it is free of microsegregation and grain boundaries. Planar growth is also possible with very high growth rates since ΔT0 approaches zero (= partition coefficient (k) approaches unity), but these velocities are not usually achieved in laser cladding. For typical laser coating alloys and used cladding parameters planar growth zone is just couple of microns in thickness and majority of the coating microstructure consist of cellular or dendritic columns.

Another form of above equation, which gives the conditions for planar growth, is:

Ds

T V

G > Δ _{0 (5)}

It can be seen that conditions are fulfilled the best at the coating/base material interface where the G/V ratio is at maximum.

This destabilization of solid-liquid interface and the initiation of cellular and dendritic growth are closely related to the constitutional supercooling, which takes place in liquid near the solid-liquid interface. With growth velocities higher than Vc solid-liquid interface rejects

solute atoms ahead of the solid-liquid interface. Simultaneously, solute profile forms in liquid near the solid-liquid interface so that the solute content is the highest near the solid-liquid interface and decreases going further into the liquid. Width of this composition profile is defined by Ds/Vs. As the composition of the liquid changes, also liquidus temperature changes. In the case of k < 1, (k is partition coefficient = Csolid/Cliquid) liquidus temperature decreases when the solute content increases. Consequently, liquidus temperature becomes lower near the solid-liquid interface compared to the liquid further away. Depending on the actual thermal gradient across the solid-liquid interface, certain portion of the liquid with increased solute content may experience a temperature, which is below its liquidus temperature. In this case of thermal gradient G, that portion of liquid is said to be constitutionally supercooled. In consequence of this, nucleation ahead of the solid-liquid interface takes place, i.e. planar front growth destabilizes and cellular and dendritic growth starts. This nucleation ahead of the interface may also lead to change from columnar to equiaxed growth [304]. These are often called stray grains. In transverse cross-section perpendicular to cladding direction, they are easily mixed with the dendrites growing parallel to coating/base material interface. Convection movements in melt pool may also fragment dendrites and initiate the formation of stray grains or mix the orientations of columnar trunks.

Figure 10. Transverse cross-section parallel to cladding direction cut through the centerline of the single bead. Solidification speed (Vs) depends on traverse speed (Vb) and angle θ between vectors Vs and Vb; Vs = Vb • cos θ. Since the angle θ is 90° at the bottom and approaches 0° at top of the melt pool, solidification speed is at minimum at the bottom and maximum at the top of the melt pool, i.e. Vs increases from the bottom to the free surface of the coating. In contrast, thermal gradient (G) is at maximum at coating/base material interface and minimum at the top of the coating, i.e. G decreases from the bottom to the free surface of the coating. G and Vs define also the local cooling rate (Tc) since Tc = G • Vs (°C/s).

It increases from the bottom to the free surface of the coating.

In addition to the type of microstructural growth (= shape of the grains), Vs and G affects the scale of the microstructure via cooling rate Tc. Frenk and Kurz [81] noted that secondary dendrite arm spacing λ2 followed λ2 • Tc1/3 = constant relationship when Stellite 6 was laser clad with traverse speeds of 100–10 000 mm/min. This relationship does not differ much from the frequently used general λ² • Vs = constant relationship given in Ref. [299], where λ is the

S

V_s = V_bcosθ V_hkl

ψ₁

primary or secondary dendrite arm spacing. Scale of the microstructure, in turn, affects the hardness of the alloy. For instance, Frenk and Kurz [184] obtained 30% higher hardness values for laser clad hypoeutectic Stellite 6 alloy (λ2 = 0.5–0.8 μm) compared with corresponding cast microstructure (λ2 = 15–20 μm) despite lower carbide (M7C3) fraction (carbide fraction decreased when traverse speed was increased). Traverse speed in this laser cladding experiment was exceptionally high; 10 000 mm/min. Positive effect of the finer scale of the microstructure on hardness and yield strength can be explained via Hall-Petch grain boundary strengthening equations, which are based on the observation that grain boundaries impede efficiently the dislocation movements. If compared with other overlay welding techniques, Monson and Steen [305] reported on the following cooling rates deduced from SDAS of Stellite 6; 9 for oxyacetylene, 46 for TIG, 112 for PTA and 3045 K/s for laser cladding.

1.5.1.1 Microsegregation

Microsegregation, which appears as compositional differences between dendrite cores and interdendritic regions, takes place during solidification. Principle is that the solute atoms, which decrease the temperature of the alloy segregates to interdendritic regions, i.e. solute is rejected by the solid/liquid interface and builds up ahead of the advancing front. Equilibrium partition coefficient (k) for these alloying elements is less than 1 (k = Csolid/Cliquid). According to equilibrium binary phase diagrams, such solute atoms as Nb (<23.2 wt.%), Mo (<47.7 wt.%) and Cr segregates to interdendritic regions when alloyed with pure Ni. Similarly, Cr (<42.4 wt.%) and Mo (<38.0 wt.%) segregate into interdendritic regions when alloyed with pure Co. Presence of other alloying elements may, however, change the segregation behaviour. If the solute increases the temperature of the alloy, it segregates to dendrite core and its k > 1. The more the k deviates from 1, the more severe the segregation. In other words, the larger the temperature interval between solidus and liquidus, the larger the segregation. In rapid solidification, partition coefficient is, however, dependent on growth rate of the solid-liquid interface (Vs). When the growth rate increases, k approaches unity (and width of the solute profile becomes smaller), i.e. temperature interval between liquidus and solidus approaches zero. The most immediate consequence of this dependence of k on growth rate is that at high solidification rates, less solute redistribution occurs with the result that the solidified structure is more uniform in composition [306]. They can be further reduced by solid-state diffusion during subsequent post-heat treatment or immeadiately during slow cooling [103]. In bcc ordered structures, this homogenization is easier than in fcc structures [103].

1.5.1.2 Macrosegregation

Macrosegregation stands for compositional differences in macro-level in final coating layer. It depends on mass transport in melt pool, which is in turn dependent on stirring motions known as Marangoni convection originating from the steep thermal gradients, which cause surface tension gradients [307]. The magnitude of this convection depends primarily upon the surface temperature gradients. Thus, processing with beam where the power density is non-homogeneously distributed increases temperature gradients and the magnitude of fluid flow producing more homogeneous compositional distribution [308]. Besides Marangoni convection, gravitational force and impact of the powder jet [90] affect the fluid flow. In laser cladding fluid flow velocities are typically several times greater than the traverse speeds as was discussed in sections 1.3.1.1 and 1.3.3 resulting in homogeneous compositional distributions. With increased traverse speeds non-homogeneous distributions have been reported [309]. This was encountered in situation where the coating was prepared from pure

Fe, Cr and Ni powders. Influence of convection on liquid homogenization is often described with surface tension number S [310]:

)

When S is low, convection is negligible, and mass transport in the melt pool is predominantly diffusive resulting in non-homogeneous compositional distribution. When S is high, convection plays a dominant role and homogeneous compositional distribution can be expected.