Direct laser sintering of metal powders: Mechanism, kinetics and ...

A. Simchi / Materials Science and Engineering A 428 (2006) 148–158 153Fe–Cu, Fe–C–Cu–P, M2 HSS, i.e. full melting of the powderparticles occurred (Fig. 7).4.2. Densification rateMany factors influence the densification of metal powdersin DMLS process. In this paper, we classified these parametersin two categories as “process parameters” and “material properties”.Process parameters are the defined variables that influenceand control the laser sintering process. These parameters aregenerally considered as those determine the amount of energydelivered to the surface. Material properties include chemicalconstitutions and the purity (oxygen, carbon, and nitrogen) ofthe material, method of alloying (elemental or prealloy) andparticle characteristics (size, distribution and shape). Assumingfixed material properties, the amount of delivered energy duringthe laser irradiation-material interaction period is dependent onthe intensity of the laser beam, the period of a single exposure,the number of total exposures and the time between each exposure.The parameters that are most influential in governing theintensity and method of the energy delivered to a single layer ofpowder with thickness (w) are the laser power (P), laser beamspot size (d), scan rate (v), scan line spacing (h) and geometricalscanning strategy including the length of scan vector and themethod of irradiation between each successive layers. The effectof scanning method will be addressed in the following section.Removing any geometrical constraint, the energy density can beexpressed as:Fig. 6. SEM micrographs show the surface morphology of laser sintered iron.P = 215 W, v =50mms −1 , w = 0.1 mm, and h = 0.3 mm (a) and 0.1 mm (b).graph illustrates the surface temperature distribution at differentdimensionless scan rates defined according to the followingequation:v ∗ = vd(1)4αIt can be noted that the temperature exceeds the melting pointat high scan rates whereas at low scan rates, the temperature canbe beyond the boiling point. Based on the experimental resultsreported previously [18], sound iron parts cannot obtained atv ∗ > 0.58, determining that for particle bonding full melting mustbe occurred. The SEM micrographs taken from the surface oflaser sintered specimen are shown in Fig. 6. The surface picturesclearly show that the powder particles were melted and formeda track of molten cylinder. The molten cylinder was then solidifiedto form continuous rows of metal agglomerates. The surfacetension effect [19] and solidification shrinkage also resulted inthe formation of large inter-agglomerates pores. Meanwhile,formation of metal balls due to instability of the molten cylinderaccording to Marangoni effect is likely to occur. The highthermal gradients present in the materials are accompanied bythermal stresses, which in fact may cause delamination of sinteredlayers (Fig. 3). Similar observations were noticed for Fe–C,Q = πηP(2)4dvSince DMLS process exposes a region to the laser beamirradiation several times by following an overlapping scanningpattern, the fraction of overlap is simply defined as:O = d (3)hTherefore, the delivered energy per volume of each laser trackis given by:Q = πηP(4)4hvwThis energy rapidly heats up the particles above the meltingpoint and cause particle bonding to occur. Therefore, the apparentsintering temperature (T s ) in very simple form is related tothe relevant process parameters as:T s = T o + 1 C[( πη4ρ)( Phvw)− HNote that if material evaporation occurs, the latent heat ofevaporation must also be considered in this equation. In thisarticle, we define the specific energy input (ψ) as the total energyinput per volume of each sintered track according to Eq. (6).ψ =Phvw](5)(6)