Effect of Laser Power and Scan Speed on the Microstructure and Texture Evolution in Cr Claddings Developed over V Substrate Using Laser-Induced Directed Energy Deposition

Abstract

In the present study, microstructures and texture of Cr clads developed on V plate using laser-assisted Directed Energy Deposition (DED) at different laser power and scan speeds have been investigated using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM) techniques. This investigation has shown that at low laser power grains at the interface region of clad and substrate show equiaxed morphology with random orientation and at high laser power grains have columnar morphology with strong rotated cube texture <001> {110}, while middle region of all clads showed columnar morphology. In the top region of the clad, at low laser power the grain morphology remained columnar and at high laser power, columnar grains transformed to equiaxed morphology. Theoretically estimated temperature profiles and energy calculations have shown that the equiaxed grains developed at low laser power is due to incomplete melting of powder particles and transformation of columnar to equiaxed grains at the top region of clad at high laser power was due to constitutional super cooling resulted by compositional dilution. A process parameter map between laser energy density and number of powder particles has been developed to predict the microstructure of clads at a given laser power and scan speed. The experimental observation of all the clads showed that the clads developed at high laser power and low scan speed exhibit superior physical integrity and higher micro-hardness which is favorable for the development of crack-free transition joints of V and Cr. The optimized laser parameters can be used to develop the SS-Ti joint by DED process using V and Cr interlayers.

Appendix A

The analytical solution for a concentrated energy source having a Gaussian geometry developed by Eagar and Tsai^[57] is provided in Eq. [A–1].

$$ \theta = \frac{n}{{\sqrt {2\pi } }}\mathop \smallint \limits_{0}^{{\frac{{v^{2} t}}{2a}}} d\tau \frac{{\tau^{{\frac{ - 1}{2}}} }}{{\tau + u^{2} }}e^{{ - \frac{{\xi^{2} + \varphi^{2} + 2\xi \varphi + \tau^{2} }}{{2\tau + 2u^{2} }} - \frac{{\Gamma^{2} }}{2\tau }}}, $$

(A-1)

where $a, c, v, {T}_{\text{c}},{T}_{0}, T, \sigma , \theta , P, u, n, \xi ,\varphi ,\Gamma$ represents thermal diffusivity (cm² s⁻¹), specific heat (J g⁻¹ K⁻¹), scan speed (cm s⁻¹), melting temperature (K), initial temperature (K), instantaneous temperature (K), distribution parameter, dimensionless temperature ($(T- {T}_{0})/({T}_{\text{c}} - {T}_{0})$), laser power (W), dimensionless distribution parameter ($v\sigma /2a$), operating parameter ($qv/4\pi {a}^{2}\rho c[{T}_{\text{c}}-{T}_{0}]$), dimensionless distance along x axis ($v(x-vt)/2a$), dimensionless distance along y axis ($vy/2a$), dimensionless distance along z axis ($vz/2a$), respectively (Table AI).

Table AI The Values of Specific Heat, Thermal Diffusivity, and Thermal Conductivity of Cr and V Obtained from Database^[70]

The concentration coefficient ($\sigma$) was estimated by considering radius of laser beam. The laser interaction time t was estimated by dividing laser beam diameter with scan speed. The above analytical solution does not consider effect of convection and radiation. Therefore, values of convection coefficient and Stefan–Boltzmann constant are not considered. The attenuation of laser power by powder stream was included by considering an average 15 pct decrease in laser power. The absorptance of V was estimated using Eq. [A–2], where value of R was obtained using Fresnel equation.

$$ A + R = 1;\;{\text{where}}\;R = \left| {\frac{{\left( {n - 1} \right)^{2} + k^{2} }}{{\left( {n + 1} \right)^{2} + k^{2} }}} \right|. $$

(A-2)