Thermophysical Phenomena in Metal Additive Manufacturing by Selective Laser Melting: Fundamentals, Modeling, Simulation, and Experimentation

Abstract

Among the many additive manufacturing (AM) processes for metallic materials, selective laser melting (SLM) is arguably the most versatile in terms of its potential to realize complex geometries along with tailored microstructure. However, the complexity of the SLM process, and the need for predictive relation of powder and process parameters to the part properties, demands further development of computational and experimental methods. This review addresses the fundamental physical phenomena of SLM, with a special emphasis on the associated thermal behavior. Simulation and experimental methods are discussed according to three primary categories. First, macroscopic approaches aim to answer questions at the component level and consider for example the determination of residual stresses or dimensional distortion effects prevalent in SLM. Second, mesoscopic approaches focus on the detection of defects such as excessive surface roughness, residual porosity or inclusions that occur at the mesoscopic length scale of individual powder particles. Third, microscopic approaches investigate the metallurgical microstructure evolution resulting from the high temperature gradients and extreme heating and cooling rates induced by the SLM process. Consideration of physical phenomena on all of these three length scales is mandatory to establish the understanding needed to realize high part quality in many applications, and to fully exploit the potential of SLM and related metal AM processes. arXiv:1709.09510v1 [physics.app-ph] 4 Sep 2017 1.4. Differentiation of related additive manufacturing processes for metals References and comparisons to other powder-bed metal AM processes such as electron beam melting (EBM) and selective laser sintering (SLS) will be useful from time to time in the foregoing discussion [97,98,50,52,68]. Similar to SLM, the EBM process represents a powder bed-based additive manufacturing process where pre-defined contours are selectively melted in successively deposited powder layers. While SLM applies a laser beam as energy source, EBM is based on an electron beam. EBM is only applicable to electrically conductive materials and has to be

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“…As the laser beam is shone over the powders, solidification structures such as cells or dendrites are formed in fine and anisotropic microstructures due to the high scanning speeds and small melt pools [51]. Altogether, the particle morphology, particle size distribution, and voids between the powders affect the absorption, reflection, and penetration of the laser radiation [52][53][54]. Consequently, the characteristics of the powder are related to the shape of melt pools and lattice structure.…”

An Overview of Additive Manufacturing Technologies—A Review to Technical Synthesis in Numerical Study of Selective Laser Melting

“…As the laser or electron beams are shone over the powders, they may be absorbed or reflected due to the spaces and voids between the powders. There are multiple reflections of light off the powder particle surfaces as each particle acts as a spherical mirror that is not perfectly shiny, which provides some absorptivity; in addition the laser penetration is under the influence of powder packing [ 32 ]. After accomplishing the scanning of the current layer, then the recoater feeds a thin layer of powders on the top of the solidified layer and the process is repeated until the desired 3D part is achieved [ 33 ].…”

“…SLM-made part’s quality is significantly under the influence of three key parameters–laser/material interaction, heat transfer and fluid dynamics of the melt pool as well as flow of vapor and gas over the build area. Melt pool geometry and stability affects the grain growth and microstructure [ 32 ]. As the laser beam moves over the powder bed, the melt pool is elongated behind it and extended a bit in front of the laser, and due to the dependence of surface tension over the melt pool on temperature, causes the melt pool to be non uniform.…”

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“…At this point, as manufacturing constraints are geometry-dependent [43], components with different geometries have different active AM constraints in their function model (FM) trees.…”

“…Additive manufacturing powder bed processes are based on complex physical phenomena, and AM manufacturing constraints are largely geometry dependent [43]. For this reason, each manufacturing constraint obtained from the literature must be evaluated for the product geometry of interest.…”