Selective Laser Sintering (SLS) of Metal Powder
SLS is an emerging technology of solid freeform fabrication (SFF) that allows three-dimensional parts to be built from CAD data (Beaman et al., 1997). A schematic of the SLS process is illustrated in the figure on the right. SLS involves fabrication of near-full-density objects from powdered material via layer-by-layer sintering or via melting induced by a directed laser beam (generally CO2 or YAG). Thin (100–250 um) powder layers are laser-scanned to fuse a densified two-dimensional slice to an underlying solid piece that consists of a series of stacked and fused two-dimensional slices. After laser scanning, the part bin is lowered by one layer-thickness, a fresh powder layer is spread, and the scanning process is repeated. Loose powder is removed after the part is extracted from its bin. Extremely complex part shapes can be formed from a variety of materials, which may be amorphous (e.g., polycarbonate), semi-crystalline (e.g., nylon), or crystalline (e.g., metal). The advantage of the SLS process is that complex parts can be made in a single step without any part-specific tooling or human intervention. However, this technology is still in its infancy, and many parts produced by the SLS process do not meet the requirements for functional strength. Further efforts at modeling, and experimental investigation of this technique, are still urgently needed in order to expand its application.
For sintering of a metal powder, the latent heat of fusion usually is very large. Therefore, melting and resolidification phenomena have a significant effect on the temperature distribution in the parts and powder, the residual stress in the part, local sintering rates, and the final quality of the parts. A significant change of density accompanies the melting process, because the volume fraction of gas(es) in the powder decreases from a value as large as 0.6 to nearly zero after melting. In addition, the liquid metal infiltrates the unsintered region due to capillary and gravitational forces. The modeling of SLS of metal powder is a very challenging task because the melting and resolidification processes are highly nonlinear, and the process is further complicated by the shrinkage phenomena. Successful modeling of SLS of metal powder can provide the transient temperature distribution in sintered and unsintered regions, the local sintering rate, and the distribution of concentration for cases involving more than one component powder. A thorough survey of the existing literature indicates that scant attention has been paid to thermal modeling of the sintering of metal powders, and it remains an ongoing effort (Zhang and Faghri, 1998; 1999a; Zhang et al., 2000; Chen and Zhang, 2006).
Laser machining includes laser drilling and laser cutting, which are processes important to the automotive, aerospace, electronics, and materials-processing industries. Laser machining involves removing material by vaporizing the portion of the workpiece that interacts with the laser beam. The mechanism of vaporization is different for metal and ceramic workpiece materials.
During laser drilling on a metal substrate, a laser beam is directed toward a solid target material at an initial temperature of Ti, which is below the melting point of the metal. The laser-material interaction can be divided into three stages (Zhang and Faghri, 1999b). During the first stage, the temperature of the solid remains below the melting point so that no melting or vaporization occurs. The solid absorbs thermal energy and its temperature increases with time. When the highest temperature of the solid – located at the center of the laser beam – reaches the target material’s melting point, continued laser beam irradiation results in melting of the target material; at this point, the process enters its second stage. In the second stage, the surface temperature of the liquid is below the saturation temperature, and the vaporization required by thermodynamic equilibrium is negligible. When the liquid surface’s highest temperature reaches the vaporization temperature of the material, vaporization occurs at the liquid surface and the third stage starts. During the third stage, the locations of both the solid-liquid and liquid-vapor interfaces are unknown and must be determined. Vaporization creates a backpressure on the free surface of the liquid, which pushes the melt away in the radial direction. Thus the material is removed through a combination of vaporization and liquid expulsion. Due to strong evaporation occurring in the laser drilling process, the gas near the liquid-vapor interface is not in translational equilibrium; the translational equilibrium is achieved within a few mean free paths by collisions between particles in a thin region referred to as the Knudsen layer. Above this layer, lying stacked in the vertical direction, are the layers of vapor, disturbed air, and undisturbed quiescent air.
The phase changes occurring in laser cutting are similar to those in the laser drilling process, but the problem can no longer be modeled as axisymmetric because the laser beam is moving. Although the workpiece material is removed via melting and vaporization, many researchers have assumed that phase change occurs in a single step, directly from solid to vapor (Kim and Majumdar, 1995; Modest 1996). This assumption is acceptable for a number of ceramics and other nonmetals, such as graphite and silicon nitride. However, this assumption may be inappropriate for metal, because in this case melting always occurs before vaporization takes place.
Selective Area Laser Deposition (SALD) and SALD Vapor Infiltration (SALDVI)
SALD and SALDVI are methods of building functional structures by using a laser beam to deposit solid materials from gas precursors in an environmentally-controlled chamber (Jakubenas et al., 1997). Both techniques utilize laser chemical vapor deposition (LCVD; Mazumder and Kar, 1995), which can be based on reactions initiated pyrolytically, photolytically, or a combination of both (Marcus et al., 1993), to deposit film to a desired location or to join powder particles together. While the SALD technique uses precursors to directly create free-standing parts, or to join together simple shapes to create parts with higher complexity, the SALDVI uses gas precursors and powder particles to build three-dimensional parts. This is similar to other SFF techniques, such as SLS. The advantages of SALDVI over SALD include (a) uninfiltrated powder provides necessary support for producing overhangs, (b) confining the deposition to thin powder layers provides dimensional control in the direction of growth, and (c) it allows for tailoring of local chemistry and micro structures. The manufacturing process using SALDVI is very similar to that using SLS because both of them fabricate objects from powdered material via a layer-by-layer process induced by a directed laser beam. The only difference is that the powder particles in SALDVI are bonded together by LCVD, a process that occurs on the surface of the powder particles, while binding of powder particles in SLS is accomplished through sintering or melting.
Since the temperature gradient in the precursor near the laser spot is very high, and the consumption of the reactants near the laser spot creates a concentration gradient, a natural convection driven by both temperature and concentration gradients occurs. In addition, the precursors are usually transported to the reaction zone by convection. Therefore, convection plays a significant role in the SALD/SALDVI processes. The phase change process involved in SALD/SALDVI is completed by a chemical reaction that takes place on the substrate/powder particle surface.
Beaman, J.J., Barlow, J.W., Bourell, D.L., Crawford, R.H., Marcus, H.L., and McAlea, K.P., 1997, Solid Freeform Fabrication: A New Direction in Manufacturing, Kluwer Academic Publishers, Bordrecht.
Chen, T., and Zhang, Y., 2006, “Three-Dimensional Simulation of Selective Laser Sintering of a Two-Component Metal Powder Layer with Finite Thickness,” ASME Journal of Manufacturing Science and Engineering, Vol. 128, pp. 299-306.
Faghri, A., and Zhang, Y., 2006, Transport Phenomena in Multiphase Systems, Elsevier, Burlington, MA.
Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.
Ganesh, R. K., Faghri, A., and Hahn, Y., 1997, “A Generalized Thermal Modeling for Laser Drilling Process, Part I – Mathematical Modeling and Numerical Methodology,” International Journal of Heat and Mass Transfer, Vol. 40, No. 14, pp. 3351-3360.
Jakubenas, K.J., Birmingham, B., Harrison, S., Crocker, J., Shaarawi, M.S., Tompkins, J.V., Sanchez, J., and Marcus, H.L., 1997, “Recent Development in SALD and SALDVI,” Proceedings of 7th International Conference on Rapid Prototyping, San Francisco, CA, pp. 60-69.
Kim, M.J., and Majumdar, P., 1995, “Computational Model for High-Energy Laser-Cutting Process,” Numerical Heat Transfer, Part A., Vol. 27, pp. 717-733.
Marcus, H.L., Zong, G., and Subramanian, P.K., 1993, “Residual Stresses in Laser Processed Solid Freeform Fabrication, Residual Stresses in Composites,” Measurement, Modeling and Effect on Thermomechanical Properties, eds. Barrera, E.V., and Dutta, I., TMS, pp. 5257-5271.
Mazumder, J., and Kar, A., 1995, Theory and Application of Laser Chemical Vapor Deposition, Plenum Publishing Co., New York, NY.
Modest, M.F., 1996, “Transient Model for CW and Pulsed Laser Machining of Ablation/Decomposing Materials-Approximate Analysis,” ASME Journal of Heat Transfer, Vol. 118, pp. 774-780.
Zhang, Y., Faghri, A., Buckley, C.W., and Bergman, T.L, 2000, “Three-Dimensional Sintering of Two-Component Metal Powders with Stationary and Moving Laser Beams,” ASME Journal Heat Transfer, Vol. 122, pp. 150-158.
Zhang, Y., and Faghri, A., 1998, “Melting and Resolidification of a Subcooled Mixed Powder Bed with Moving Gaussian Heat Source,” ASME Journal of Heat Transfer, Vol. 120, No. 4, pp. 883-891.
Zhang, Y., and Faghri, A., 1999a, “Melting of a Subcooled Mixed Powder Bed with Constant Heat Flux Heating,” International Journal of Heat and Mass Transfer, Vol. 42, pp. 775-788.
Zhang, Y., and Faghri, A., 1999b, “Vaporization, Melting and Heat Conduction in the Laser Drilling Process,” International Journal of Heat and Mass Transfer, Vol. 42, pp. 1775-1790.