Surface Over-Melt During Laser Polishing of Indirect-SLS Metal Parts
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Surface Over-Melt During Laser Polishing of Indirect-SLS Metal Parts J. A. Ramos†, D. L. Bourell, J. J. Beaman Texas Materials Institute, Department of Mechanical Engineering, The University of Texas at Austin † Department of Mechanical and Metallurgical Engineering, Pontificia Universidad Católica de Chile ABSTRACT Laser polishing of indirect-SLS parts made from 420 stainless powder infiltrated with bronze has been achieved using CO2 and Nd:YAG lasers. Two mechanisms have been previously proposed for the reduction in surface roughness, namely: shallow surface melting (SMM) and surface over-melt (SOM). In SMM reflow of the molten surface minimizes the peak-valley height driven by capillary pressure and liquid curvature. On the other hand, during SOM the melting depth is such that the entire surface becomes liquid and formation of surface periodical structures dominates driven by a surface tension gradient. This surface morphology was identified by means of optical and scanning electron microscopy (SEM). The onset of this regime is dictated by the energy density (i.e., ratio of laser power to scan speed and beam diameter) as well as the initial roughness Ra value prior to laser surface polishing. In contrast with SMM, onset of the latter mechanism increases the roughness Ra with speed reduction. A thermo-physical model is presented, signaling good agreement with roughness Ra and characteristic surface wavelength results obtained for varying laser beam scan speeds. Understanding the surface over-melt mechanism is critical for determining the optimum polishing conditions that minimize roughness. INTRODUCTION Rapid Prototyping has proven to be a successful technique in shortening the manufacturing cycle for parts with complex geometry [1,2]. Selective Laser Sintering (SLS) is one such RP technology that benefits from the advantage of building up solid object from polymer, ceramic and metal-alloyed materials in powder form [1-3]. To build objects out of metal alloys, two variations of the SLS are currently available, namely, indirect and direct SLS. The former requires postinfiltration of a SLS formed green part (high melting point metal) by metal of low fusion point. The latter technique, as its name indicates, involves consolidation of metal powder by direct laser induced local fusion. However, the surface roughness achieved during the manufacture of rapid prototyping articles by these SLS techniques is still in the range of several microns [3]. At the same time, a sub-micron surface roughness Ra value is determinant when functional mechanical parts are to be built [3,4]. This is an issue yet to be solved along the path of rapid manufacturing evolution. An intermediate step in solving this problem is to employ the hardware available to the SLS process, specifically a high power laser unit and galvanometer scanner mirrors, to provide for laser polishing of the rapid prototyped object. Pursuing this objective, Ramos et al [5] were able to obtain decrements in Ra values up to 3 times the as-received values on ind
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