Selective Laser Sintering and Post Processing of Fully Ferrous Components

Commercially available steel for indirect SLS (LaserFormtm A6 tool steel) is normally post-process infiltrated with a copper-based material. While such parts have high thermal conductivity necessary for short- and medium-run injection molding dies, they are weakened by the second phase with limited high temperature stability. This paper deals with a modification to the commercial process whereby a low-melting-point cast iron is substituted for the copper alloy infiltrant. A predictive model is presented that describes the part equilibrium solid fraction at the infiltration temperature as a function of the green density and infiltration temperature. In an experimental study, green parts were fabricated using LaserFormtm A6 tool steel powder. They were then heated in vacuum to drive off the binder and infiltrated with ASTM A532 white cast iron. During infiltration, an equilibrium state is established between the solid SLS steel part and liquid cast iron associated primarily with carbon diffusion from the cast iron into the tool steel. The equilibrium state is governed by the carbon content of the steel and cast iron, the relative density of the steel part prior to infiltration and the infiltration temperature. In some cases guided by Ashby densification maps, pre-sintering of the tool steel green part was performed to increase the initial relative density of the solid metal.Mechanical Engineerin

Fabrication of Electrically Conductive, Fluid Impermeable Direct Methanol Fuel Cell (DMFC) Graphite Bipolar Plates by Indirect Selective Laser Sintering (SLS)

Graphite bipolar plates are highly desirable due to their properties of high electrical conductivity and low weight but are associated with prohibitive machining costs arising from poor mechanical properties. Bipolar plates made by indirect Selective Laser Sintering (SLS) offer the advantages of complex part production in shorter times at lower cost. Due to the nature of the SLS process, the as-produced (green part) plates are porous and possess low electrical conductivity which can be improved by carbonizing the phenolic resin binder at high temperatures (brown part). It has been found that the electrical conductivity increases significantly (> 200 S/cm) with a corresponding increase in pyrolyzing temperature which correlates well with literature on the carbonization of phenol formaldehyde resins. The brown parts are subsequently infiltrated with low viscosity (~5 - 10 cps) cyanoacrylate to seal up the open pores, rendering the plates fluid impermeable.Mechanical Engineerin

Improvement of Electrical Conductivity of SLS PEM Fuel Cell Bipolar Plates

Previous work in this research demonstrated the feasibility of fabrication of proton exchange membrane (PEM) fuel cell bipolar plates by an indirect selective laser sintering (SLS) route. Properties of the SLS bipolar plate, such as flexural strength, corrosion resistance and gas impermeability, etc. are quite promising and satisfactory. However, initial results showed that there was still room for the improvement in electrical conductivity. This paper summaries the strategies investigated in an effort to increase the electrical conductivity, among which are: (1) infiltration of brown parts with conductive polymer (2) addition of a liquid phenolic infiltration/re-curing step prior to final sealing and (3) reduction of glassy carbon resistivity by curing process parameter control. Results show that the electrical conductivity value may be improved from 80 S/cm to around 108 S/cm, which is equivalent to a 35% jump, when the phenolic infiltration/re-curing step is applied before final epoxy sealing.Mechanical Engineerin

An Initial Assessment of Infiltration Material Selection for Selective Laser Sintered Preforms

High-temperature infiltration is an important process that is used to add strength to skeletal microstructures. In this study, particulate metal matrix composites (MMCs) are fabricated. MMCs are applied in a wide variety of industries, including military, aircraft, tooling and automotive. In this paper, various materials for infiltrating selective laser sintered (SLS) silicon carbide and titanium carbide preforms are considered based on fundamental knowledge of SLS and infiltration mechanics. Proposed infiltrant materials systems include an aluminum-silicon alloy infiltrant and a silicon carbide preform, ductile iron infiltrated into a titanium carbide preform, and commercially pure silicon infiltrated into a silicon carbide preform. The first two infiltrants are considered because they add ductility to the brittle silicon carbide or titanium carbide part, thus broadening the range of applications. They also will model a broader field of possible infiltrants, including magnesium and iron-based materials, (e.g., steel). Silicon is investigated because it adds strength to silicon carbide, is robust at high temperatures, and has a comparable coefficient of thermal expansion. Presented is a feasibility assessment of these systems based on infiltration theory.Mechanical Engineerin

Solidification Morphology Analysis of SLM of Cu Powder

The solidification morphology analysis of fine Cu powder melted by a raster scanned energy beam from a focused Nd:YAG laser is presented here. The powder was processed inside of sealed chamber where it was subjected to a high vacuum cycle. The laser fusion process consisted raster scanning a narrow rectangular pattern with a high density of scanning lines, the chamber was purged with inert gas during the process. Up to a 3.3 mm/s laser travel speed and maximum laser power level of 240 W were used to melt a 2 mm thick bed of loose powder. The resulting solidified ingots were separated into categories based on their shape integrity. Metallographic analysis by means of optical microscopy and scanning electron microscopy was performed on the cross section and longitudinal section of the ingots with homogeneous surface and complete shape integrity. Characterization revealed an elongated columnar grain structure with a grain orientation along the direction of the laser travel direction, some degree of porosity was observed too in some of the specimens. It was observed that grains diameter ranged from 10 to 100 µm and contained a two phase eutectic microstructure of copper and it oxides. Oxygen content was accounted from a 5.5 up to 8.1 atomic percent, a small percentage of chlorine was present, too. A 2 to 8 percent variation in the Vickers microhardness values were found between the different specimens when measured along the longitudinal section. These HV values corresponded to approximate 20-25% cold rolled oxygen free copper (80-90 HV). The ingots thus produced suggest that a multilayer structure from Cu powder could be build by the SLM process having sufficiently adequate compositional, microstructure and mechanical properties for functional applications.Mechanical Engineerin

Fabrication of PEM Fuel Cell Bipolar Plates by Indirect SLS

The paper presents a new manufacturing technique involving Selective Laser Sintering (SLS) for proton exchange membrane fuel cell (PEMFC) bipolar plate fabrication. A material system for bipolar plate fabrication was identified to satisfy both the cell performance requirement and SLS operation restriction. Carbonization and liquid epoxy infiltration are subsequently performed following the completion of SLS green bipolar plate. The finished SLS bipolar plate showed impressive surface finish and mechanical strength, and a single fuel cell was assembled with two SLS end plates and membrane electrode assembly (MEA) in between. Various physical property tests were performed with positive results. Fuel cell performance (voltage vs. current density, voltage vs. time, etc.) will be assessed in the near future.Mechanical Engineerin

Sustainability Study in Selective Laser Sintering – An Energy Perspective

This paper presents a sustainability analysis of Selective Laser Sintering (SLS) from an energy standpoint. Data of electrical power consumed by the system over an entire build were acquired using a LabVIEW 8.6 circuit. The power drawn by individual subsystems were also measured, and an energy balance was performed. These data were then used to arrive at a Total Energy Indicator of the process with the help of a specific type of Environmental and Resource Management Data (ERMD) known as Eco-Indicators, which indicates the level of sustainability of the process.Mechanical Engineerin

Design, Fabrication and Testing of Graphite Bipolar Plates for Direct Methanol Fuel Cells

Direct Methanol Fuel Cells (DMFCs) are clean energy devices that convert chemical energy into electrical energy. The bipolar plate component of the DMFC is required to be fluid impermeable to prevent fuel leakage and electrically conductive to collect the electrons produced within the cell. Graphite possesses the properties of high electrical conductivity, low weight and resistance to corrosion that make it an attractive material for bipolar plates. However, the poor mechanical properties of graphite lead to prohibitive machining costs. The indirect Selective Laser Sintering (SLS) process, involving the laser sintering of graphite powders mixed with a phenolic resin binder, offers the advantage of complex part production and testing of prototype bipolar plates in short times. The current research deals with the production of bipolar plates for DMFC's by utilizing indirect SLS followed by post processing steps.Mechanical Engineerin

An Evaluation of the Mechanical Behavior of Bronze-NI Composites Produced by Selective Laser Sintering

Mechanical properties of Bronze-Nickel composites produced by Selective Laser Sintering (SLS) were evaluated by constant displacement tension tests. These were studied as a function of SLS process parameters - laser power density, scan speed, scan spacing, scan direction and layer thickness. The strength data was then correlated to the microstructure and the part bulk density. To further enhance the part densities and the mechanical properties, post-SLS sintering was studied. The relationships between SLS process parameters, post-SLS sintering parameters and the resulting microstructures, part bulk density and the mechanical properties will be described.Mechanical Engineerin

Selective Laser Sintering of Zirconia with Micro-Scale Features

Recent work in Selective Laser Sintering of ceramics at the University of Texas at Austin demonstrates the capability to produce zirconia parts with fine features. Zirconia powder was pre-processed into spherical particles, laser sintered with a sacrificial polymer binder, infiltrated and post-sintered to higher density. Optical micrographs show that hole sizes of 180 /-tm are possible in fully ceramic components.Mechanical Engineerin