is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. a b s t r a c tThe direct metal deposition (DMD) with laser is a free-form metal deposition process for manufacturing dense pieces, which allows generating a prototype or small series of near net-shape structures. One of the most critical issues is that produced pieces have a deleterious surface finish which systematically requires post machining steps. This problem has never been fully addressed before. The present work describes investigations on the DMD process, using an Yb-YAG disk laser, and a widely used titanium alloy (Ti-6Al-4V) to understand the influence of the main process parameters on the surface finish quality. The focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish.In order to understand the physical mechanisms responsible for deleterious surface finishes, we have carried out: (1) a precise characterization of the laser beam and the powder stream; (2) a large number of multi-layered walls using different process parameters (P(W), V(m/min), D m (g/min), Gaussian or uniform beam distribution); (3) a real time fast camera analysis of melt pool dynamics and melt-pool -powder stream coupling; (4) a characterization of wall morphologies versus process parameters using 2D and 3D profilometry.The results confirm that surface degradation depends on two distinct aspects: the sticking of nonmelted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters. Last, a simple analytical model was proposed to correlate melt-pool geometries to resulting surface finishes.

Derived from laser cladding, the direct laser metal deposition (DLMD) process is based upon a laser beam–powder–melt pool interaction and enables the manufacturing of complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DLMD parts usually necessitate postmachining steps. Within this context, the focus of our work is to improve the understanding of the phenomena responsible for deleterious surface finish by using numerical simulation. Mass, momentum, and energy conservation equations are solved using comsol multiphysics® in a 2D transient model including filler material with surface tension and thermocapillary effects at the free surface. The dynamic shape of the molten zone is explicitly described by a moving mesh based on an arbitrary Lagrangian–Eulerian method (ALE). This model is used to analyze the influence of the process parameters, such as laser power, scanning speed, and powder feed rate on the melt pool behavior. The simulations of a single layer and multilayer claddings are presented. The numerical results are compared with experimental data, in terms of layer height, melt pool length, and depth of penetration, obtained from high speed camera. The experiments are carried out on a widely used aeronautical alloy (Ti–6Al–4 V) using a Nd:YAG laser. The results show that the dilution ratio increases with increasing the laser power and the scanning velocity or with decreasing the powder feed rate. The final surface finish is then improved.

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Derived from laser cladding, the Direct Metal Deposition (DMD) laser process, is based upon a laser beam – projected powder interaction, and allows manufacturing complex 3D shapes much faster than conventional processes. However, the surface finish remains critical, and DMD parts usually necessitate post-machining steps. In this context, the focus of our work was: (1) to understand the physical mechanisms responsible for deleterious surface finishes, (2) to propose different experimental solutions for improving surface finish. Our experimental approach is based upon: (1) adequate modifications of the DMD conditions (gas shielding, laser conditions, coaxial or off-axis nozzles), (2) a characterization of laser-powder-melt-pool interactions using fast camera analysis, (3) a precise check of surface aspects using 3D profilometry, SEM, (4) preliminary thermo-convective simulations to understand melt-pool hydrodynamics. Most of the experimental tests were carried out on a Ti6Al4V titanium alloy, widely investigated already. Results confirm that surface degradation depends on two aspects: the sticking of non-melted or partially melted particles on the free surfaces, and the formation of menisci with more or less pronounced curvature radii. Among other aspects, a reduction of layer thickness and an increase of melt-pool volumes to favor re-melting processes are shown to have a beneficial effect on roughness parameters.

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