Gildas Guillemot

Boundary layer correlation for dendrite tip growth with fluid flow

Abstract A correlation is presented for the calculation of the size of a stagnant film located in the liquid ahead of a paraboloidal dendrite, which grows steadily into an undercooled melt in the presence of fluid flow. The coefficients of the correlation are deduced from a fitting exercise of the supersaturation versus dimensionless numbers to the analytical solution developed for the Stokes flow approximation. It is shown that the correlation presented in this contribution offers several advantages compared with the Wang and Beckermann correlation previously published in the literature (Metall. Mater. Trans. 27A (1996) 2754). Not only the fit is improved, but it can also be extended to account for the angle between the dendrite growth direction and the fluid flow direction. Applications are given for an Al–7 wt.% Si alloy.

A new cellular automaton—finite element coupling scheme for alloy solidification

A cellular automaton (CA) method was developed in the 1990s for the prediction of solidification grain structures. It was coupled with a finite element (FE) method to solve the energy conservation equation at the scale of the casting. Such coupling is revisited in this contribution thanks to a recently proposed front tracking method that gives the correct solution to a heat flow problem in which the undercooling of the growing fronts that define the limits of the mushy zone is taken into account. Validation is provided by considering unidirectional solidification. Comparison with a formerly proposed coupling scheme of the CAFE model clearly demonstrates the improvement achieved. The new coupling scheme developed should thus be preferred for further developments of solidification models that include undercooling of the growth fronts.

3D parameter to quantify the anisotropy measurement of periodic structures on rough surfaces

In this paper, a new 3D roughness parameter, Sreg, is proposed to quantify the regularity of a surface, independent of the amplitude and the scanning length units of the surface. The efficiency of this parameter is tested on noisy periodical surfaces with degrees of anisotropy. This parameter lies between zero (perfect noise) and 100% (a perfect periodic surface). This parameter enables the identification of the anisotropy directions of regularity for a given surface. For a periodical surface, the greater the noise, the lower the anisotropy. A direction function is proposed to analyse the direction of regularity of a rough surface, which then permits characterization of the directional regularity of the topography. The regularity parameter can be used for several purposes: to identify the direction of periodical structures formed by laser-pulsed radiations on the surface of solid workpieces; to examine the reproducibility of surface machining methods such as finishing process; and to identify the surface regularity produced by abrasive machining, such as precision surface grain, abrasive slotting, and lapping.

Investigation of the Effect of Residual Stress Gradient on the Wear Behavior of PVD Thin Films

The control of residual stresses has been seldom investigated in multilayer coatings dedicated to improvement of wear behavior. Here, we report the preparation and characterization of superposed structures composed of Cr, CrN and CrAlN layers. Nano-multilayers CrN/CrAlN and Cr/CrN/CrAlN were deposited by Physical Vapor Deposition (PVD) onto Si (100) and AISI4140 steel substrates. The Cr, CrN and CrAlN monolayers were developed with an innovative approach in PVD coatings technologies corresponding to deposition with different residual stresses levels. Composition and wear tracks morphologies of the coatings were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction and 3D-surface analyzer. The mechanical properties (hardness, residual stresses and wear) were investigated by nanoindentation, interferometry and micro-tribometry (fretting-wear tests). Observations suggest that multilayer coatings are composed mostly of nanocrystalline. The residual stresses level in the films has practically affected all the physicochemical and mechanical properties as well as the wear behavior. Consequently, it is demonstrated that the coating containing moderate stresses has a better wear behavior compared to the coating developed with higher residual stresses. The friction contact between coated samples and alumina balls shows also a large variety of wear mechanisms. In particular, the abrasive wear of the coatings was a combination of plastic deformation, fine microcracking and microspallation. The application of these multilayers will be wood machining of green wood.

Finite Element Modeling of Ceramic Deposition by LBM (SLM) Additive Manufacturing

The Marangoni effect caused by surface tension gradient is modeled. The resulting convection flow in the melt pool is demonstrated with different values of the Marangoni coefficient. Its influence on the temperature distribution, the shape of melt pool and thus the shape of solidified track is presented. The role of Marangoni convection on the stability of melt pool and the surface quality of the final track are also discussed.

Macroscopic Finite Element Thermal Modelling of Selective Laser Melting for IN718 Real Part Geometries

A 3D finite element model is developed to study heat exchange during the selective laser melting (SLM) process. The level set functions are used to track the interface between the constructed workpiece and non-melted powder, and interface between the gas domain and the successive powder bed layers In order to reach the simulation in macroscopic scale of real part geometries in a reasonable simulation time, the energy input and the formation of the additive deposit are simplified by considering them at the scale of an entire layer or fraction of each layer. The layer fractions are identified directly from a description (e.g. using G-code) of the global laser scan plan of the part construction. Each fraction is heated during a time interval corresponding to the exposure time to the laser beam, and then cooled down during a time interval equal to the scan time of the laser beam over the considered layer fraction. The global heat transfer through the part under additive construction and the powder material non-exposed to the laser beam is simulated. To reduce the computational cost, mesh-adaptation is adopted during the construction process. The proposed model is able to predict the temperature distribution and evolution in the constructed workpiece and non-melted powder during the SLM process at the macroscale, for parts made of complex geometry. Application is shown for a nickel based material (IN718), but the numerical model can be easily extended to other materials by using their data sets.