Thermal Modelling and Experimental Validation in the Perspective of Tool Steel Laser Polishing

Abstract

Laser polishing (LP) is a process that allows a significant reduction of the surface roughness of a metal workpiece via re-melting a shallow layer of material. However, the practical use of LP is limited due to the difficulty of satisfying the high surface polishing quality for industries such as jewelry. Hence, this study presents a thermal model based on the Chernoff formula for fixed and moving LP processes. The model was validated via a sensitivity analysis of the coefficient of absorption of the solid and liquid phase and through comparing the results with the corresponding experimental observations of the melt pools produced. In this investigation, a continuous wave, high power diode laser (980 nm wavelength, Ø 0.9 mm spot size with a flat top distribution) was coupled to a fixed focusing head. The material was a hardened tool steel (X38CrMVo5-1 steel-1.2343) with an initial surface state obtained by electrical discharge machining (EDM) of CH30 on the Charmilles scale for EDM, which corresponds to a roughness Ra of 3.15 µm. The results show that the model is able to represent the thermal behavior of the tool steel during LP. The best results are obtained with two constant coefficients, one for the solid based on the measurement at room temperature (as = 35%) and one for the liquid fixed so that the model fits the experiment data (al = 25%).

Optimization of Process Parameters for the Laser Polishing of Hardened Tool Steel

“…As mentioned in the introduction, laser power and velocity are the main parameters taken into account in most studies on LP. In contrast, in this study and based on previous experimental works on laser processing [ 16 , 22 , 23 , 24 ], we selected four different factors, and they are: the spot size ( Ø ), the velocity ( ν ), the maximum temperature ( T ) and the overlap ( O ) between two successive parallel lines as illustrated in Figure 1 . The limits of the values were set after some preliminary tests to insure a complete line of fusion on the sample [ 16 ].…”

“…The major advantage of using this parameter is that it allows direct comparison of the different experiments realized at different speeds or with different spot sizes. In this work, the maximum temperature has been determined via numerical simulations developed in a prior work by Meylan et al [ 22 ]. Hence, only a summary of the model/methods is presented in this work.…”

“…To identify the melt pool created by a laser, finite element simulation software was used by Meylan et al [ 22 ] to model the heat transfer in a 3D part that has a shape in the form of a parallelepiped. In this contribution, the main problem was to retrieve the two space- and time-dependent fields.…”

“…In addition, on the six faces of the parallelepipedon, we also imposed some boundary conditions for the temperature even though only the boundary conditions on the surface processed by the laser ( z = H ) are pertinent if the domain is sufficiently large when compared to the polished region. Finally, Meylan et al [ 22 ] used a space discretization that was gotten by applying a standard finite element method (FEM) (Galerkin formulation) in agreement with Kolossov et al [ 25 ]. The material properties of non-linearity are implemented via a semi-implicit method.…”

“…An example of such simulation for Optim 5 is shown in Figure 2 . All details on the thermal model can be found in Meylan et al [ 22 ].…”