**5. LST applications**

*Engineering Steels and High Entropy-Alloys*

hence vaporization of surface materials is restricted. LG is used to level the surface and develop a dense re-melted layer. Due to such a system microhardness, erosion

Since LST processes are highly nonlinear processes, hence for clear understanding of the process, numerical modeling is essential [45]. Besides laser beam interaction with the substrate, LC and LSA impose additional constraints of clad and alloying materials that may be supplied in the form of preplaced powder or through powder feeder. In case powder is supplied through powder feeder, then the study of powder flow dynamics becomes important. Hence, LC and LSA processes can be broken in three stages: powder stream dynamics, melt pool region, and quality variables. Experimental study of laser surface treatments is not sufficient to understand complex phenomena such as powder stream pattern, laser and substrate interaction, heat transfer mode, and melt pool behavior. Hence, analytical models and numerical simulations have been developed. Some of the brief advancements

Powder stream dynamics is a significant area in the blown powder technique because we can predict powder stream characteristics such as motion, flow profile, powder with laser system interaction, etc. These parameters may be further used in optimization of parameters and calculate powder efficiency. The behavior of powder flow is governed by the type of nozzle employed. The turbulent flow of carrier gas can be assumed to be a continuum; hence Navier-Stokes equations can be used. A dispersed powder in carrier gas is generally solved using the Lagrangian equation [46]. During powder–laser interaction, attenuation of laser energy takes place due to absorption, reflection, radiation, beam scattering, and ricocheting of powder particles. Models based on particle attenuation [47], ray tracing [48], resolution [49], and light propulsion force model [50] have been reported to predict the behavior of attenuated laser energy fairly. Finally, powder substrate interaction depends on the standoff distance. For high convergence the substrate should lie at

The substrate melts due to absorption of remaining incident laser energy. The focuses of interest in this region are heat transfer, geometry of melt pool, fluid flow velocity, cooling rate, and solidification rate. These variables have been calculated using kinetic approach [52], volume method [53], and finite element method [54, 55]. Usually commercial multi-physics platforms such as ANSYS, COMSOL,

The simulation of desirable properties is usually done in combination with developed thermal model. Phase transformation models along with thermal models provide base for the measurement of quality variables. Diffusion and diffusion-less phase transformations may occur in different material systems. Diffusion phase transformations have been modeled using the Johnson-Mehl-Avrami equation,

resistance and thermal shock behavior of the substrate may be improved.

**4. Numerical modeling of laser surface treatment processes**

and understandings in these areas are presented.

FLUENT, and ABAQUS are employed for the problem.

**4.1 Powder stream dynamics**

the focus of the nozzle [51].

**4.3 Quality variables**

**4.2 Melt pool**

**246**

LST has a wide application in aerospace, automobile, medical, nuclear, oil recovery, and refinery industries [62–66]. Aluminum and its alloys are widely used in aerospace industry; they have been efficiently clad with other novel metals to improve their surface properties [67]. Stainless steel is used in automobile and household applications [68, 69]. Titanium and its alloys are used in the medical sector [70]. LC on Ti6Al4V has been studied frequently to improve its surface properties [71].


**Table 2.** *Some applications of LST.*

LST applications can be classified in two categories. First is remanufacturing or refurbishing products to restore their properties and dimensions [72]. Second is development of new materials with improved properties. **Table 2** presents some critical applications of LST.
