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

Selective Laser Sintering of Cu-Pb/Sn Solder Powders

Solid Freeform Fabrication (SFF) as an overall approach in the manufacturing field has been applied to very different areas and on different materials systems including polymers, ceramics, metals and vapor phase. Even though it is clear that polymers have enjoyed a rapid and successful advance in terms of applications, research in the other areas have shown potential applications as well. In this paper, Cu-Sn/Pb solder and Cu-Sn blends have been used as starting material for the Selective Laser Sintering (SLS) manufacture of SFF objects. The general properties of the objects thus produced are presented. In particular, density and microstructural characterizations results are discussed..Mechanical Engineerin

Selective Laser Sintering of Binary Metallic Powder

A selective laser sintering technique has been used to process metal powders and powder blends. Precursor powders include copper, tin, a 70Pb-30Sn solder and their blends. Excessive balling due to surface free energy effects occurred in single layer tests when the laser fluence was sufficient to cause melting of monolithic tin or solder. Improvements in single layer quality were obtained using copper-solder powder blends in a reducing atmosphere. The binary powder layers were characterized metallographically and the effect of processing parameters such as laser fluence and scan speed were assessed. Post-process annealing improved interparticle wetting and part strength. The influence of ZnCl2 flux was investigated when present as a coating in copper-solder blends. Multiple layer tests were performed on the most promising powder blends and the results are presented.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

Laser Polishing of Silica Rods

Lasers have been widely used in surface modification. In this research a CO2 continuous wave laser has been used to polish the slot surface of the silica rods. The strong absorption of the lO.6 um C02 radiation by the silica surface promotes the softening of a very thin layer of material that flows under the action of surface tension. As a result, a mirror smooth glassy surface has been formed which decreases the surface roughness without any substantial change in the surface geometries. The effect of laser to surface inclination angle on the requisite power requirement was assessed experimentally and theoretically. With laser beam scanning controlled by a computer aided design (CAD) database without specific tooling or human intervention, reliability and reproducibility of this process have been greatly improved compared to conventional fire polishing. The potential use of laser polishing as a post-processing step for freeform-fabricated parts is very promising.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

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

Solid Freeform Fabrication An Advanced Manufacturing Approach

Solid freeform fabrication (SFF) is the production of freeform solid objects directly from a computer model without part-specific tooling or human intervention. SFF has been realized in the last ten years through the merging of several previously distinct technologies: computer science, mechanical design, controls, high-energy beam technology and materials science and engineering. Their combination has produced over a relatively short time..frame numerous SFF methods. The value of SFF to the commercial sector is usually articulated in terms of reduced time to market (prototyping), low production "one-of-a-kind" parts and patterns for casting. The purpose of this introductory paper is to describe briefly some of the approaches to SFF as a background for the articles included in this proceedings.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