Joel W. Barlow

Selective laser sintering of alumina with polymer binders

The selective laser sintering (SLS) process is used to prepare test bars from Al2O3/polymer binder powders. Finds that binder‐coated A12O3 particles formed bars that were approximately twice as strong as could be formed from mixtures of alumina and polymer binder at the same binder level and processing conditions. In mixed systems, bar strengths increased nearly in proportion to increases in polymer binder content over the 20‐40 per cent volume binder range. Parts made in any particular laser scanning mode showed optimum values for strength and density as the laser energy density was systematically increased from 2‐8cal/cm2. Suggests that optima result from the counteracting influences of energy density on binder fusion and thermal degradation. The optimum energy density is mode or geometry sensitive and shifts to lower values as the laser scanning vector is reduced. Concludes that this behaviour is probably the result of the lower heat losses. Equivalently better utilization of laser energy is associated with the shorter scan vectors. Some of the SLS fabricated bars were infiltrated with colloidal alumina, fired to remove the binder, and sintered at 1,600°C to achieve alumina bars with 50 per cent relative densities, interconnected porosity, and strengths between 2 and 8MPa.

Post‐processing of selective laser sintered metal parts

Gives a brief overview of post‐processing of selective laser sintered (SLS) metal parts to improve structural integrity and/or to induce a material transformation. Presents results which show the effect of post‐processing liquid phase sintering temperature and time on material properties. Describes the hot isostatic pressing process, and discusses its application to SLS metal parts. Results gained from using this process show that it is suitable for achieving almost full‐density parts.

Selective Laser Sintering of Bioceramic Materials for Implants

Selective Laser Sintering (SLS) process is employed for fabrication of biocerarnics for orthopedic implants. Hydroxyapatite and Calcium Phosphate ceramics are coated with polymer as a intermediate binder by using a spray drier. Polymer coated materials are SLS processed to make green parts, which are infiltrated and fired to remove the polymer. SLS processed green parts of hydroxyapatite have low density due to the small particle size with large specific surface area. This paper discusses the possibilities and problems in free-form fabrication of bioceramic. INTRODUCTION Many attempts have been made to find material that will assist in the regeneration of bone defects and injuries. Calcium phosphate ceramics, particularly hydroxyapatite(HA), Cas(OH)(P04h, has received special attention as potential bone implant material because of its biocompatibility with the tissue and its compositional similarities to human bone and tooth. Many studies and methods, from powder compaction sintering to hot isostactic pressing, have been reported for the fabrication of HA. However, sintered HA materials by conventional techniques are as weak as sea coral even at high compacting pressure, because HA decomposes at temperatures lower than the required temperature for sintering. Selective Laser Sintering (SLS) processes for preparing ceramic green parts with polymer as intermediate binder and post processing with the aid of ceramic cement have been discussed in detail in literature [1]. One advantage of SLS process for fabrication of bioceramic is the accurate construction of a complete facsimile bone structure from the geometric information obtained from either patient computed tomographic (CT) data or a computer Aided Design (CAD) software package[2]. Another advantage is the ability to controlling pore structure for biogenesis through control of polymer content. MATERIALS and METHODS HA, obtained from Monsanto Inc. as Tricalcium Phosphate, TCP, was used as starting material. HA powders are very cohesive and consist of very porous agglomerates with mean particle size of 1 to 2 /lm and bulk density of less than 0.4 g/cm3 [3]. The surface area determined by Mercury intrusion analysis is about 60 m2/g, suggesting very small particles. Stoichiometric HA contains constitutional water in the form of OHions. This water can be driven off at 1200 oc. Figure 1 shows the microstructure of finely divided HA powders.

A thermal model of polymer degradation during selective laser sintering of polymer coated ceramic powders

Reports that measurable amounts of polymer degradation occur during the fabrication of objects from polymer coated ceramic powders by selective laser sintering (SLS). Argues that because the binder is important in achieving strong green parts that can be handled with minimal breakage during post‐processing operations, it is essential to minimize the extent of binder losses. As the first step towards understanding the mechanisms of binder degradation, this paper presents a thermal model of the physical system, noting that the agreement between theory and experiment are good. The model is used to help determine the most influential parameters affecting binder losses during fabrication from polymer coated powders. Predicts that adjustments to laser beam diameter, laser scanning distance and gaseous environment will strongly affect polymer binder degradation during processing. Further predicts correctly that polymer degradation during SLS processing is not sensitive to the inherent degradation kinetics of the polymer.

A rapid mould‐making system: material properties and design considerations

Presents the mechanical properties of a new mould‐making material, proposed for producing rapidly prototyped injection mould inserts for plastics by selective laser sintering. Explains that although the strength of this material is far below that of the tool steel usually used to fabricate moulds, design calculations indicate that it can still be used for mould insert production. Points out that the thermal conductivity of this material is lower than that for steel but higher than that for plastic melts. Indicates from the calculations that proper choices of conduction length and cycle time can minimize differences, relative to steel moulds, in the operational behaviour of moulds made of the new material. Discusses the longevity of example moulds.

Direct SLS processing for production of cermet composite turbine sealing components

Abstract This paper presents the development to date of Selective Laser Sintering (SLS) technologies for production of cermet composite turbine sealing components, the particular application being an abrasive blade tip. The component chosen for the application is an integral part of the low pressure turbine in a. IHPTET (Integrated High Performance Turbine Engine Technology) demonstrator engine. Both indirect and direct SLS techniques are being developed. Initial trials and process development involved the use of fugitive polymeric binders. Sequential refinements were performed to develop a binderless direct SLS process. Results from mechanical testing indicate that acceptable microstructure and properties are attainable by SLS with substantial cost savings as compared to the currently employed production method. This is the first instance of direct Solid Freeform Fabrication (SIT) method applied to a functional component.

Parametric Analysis of the Selective Laser Sintering Process

Qualitative and quantitative analyses are required to develop Selective Laser Sintering into a viable Manufacturing process. A simplified mathematical model for sintering incorporating the heat tJ;ansfer equation. and the sintering rate equation, but using temperature independent thermal properties, is presented in this paper. A practical result is the calculation of sintering depthdeftned as the depth of powder where the void fraction is less than 0.1 as a function of control parameters, such as the laser power intensity, the laser scanning velocity, and the initial bedtemperature. We derive the general behavior of laser sintering. A comparison of model predictions with laser sinterlng tests is provided. Introduction The previous works on the Selective Laser Sintering have been concentrated on exploring suitable sintering materials and experimentally determining the control parameters. Little theoretical research has been done beyond Frenkels sintering model [1]. However, a better model that predict the sintering behavior correctly could save a lot of experimental work. We here propose a simplified integrated model for Selective Laser Sintering which, through the verification of true sintering experiments, proved to be a valuable basis of the more sophisticated models to be developed in the future. It can also be used in the instructive parametric analysis of the sintering behavior. Treating the Selective Laser Sintering process as a system, the input variables are laser power and scanning rate. Output variables of this system should practically describe the quality of the sintered part.One important indication of part quality is its void fraction. Temperature acts as a intermediate state variable in this model. There are some other factors affecting the sintering process, such as the initial powder bed temperature, air flow condition, the optical, thermal and rheological properties of the powder, and the morphology of the powder. A conceptual model for selective laser sintering is thus derived as shown in Fig. 1. power ........._ .....o;;;r,... ... ....._-., degree of absorbed qAT sintering OPTICAL r- THERMAL SINTERING SUBMODEL L-....( SUBMODEL SUBMODEL laser jwer> laser scanning velocity environmental control parameters (temperature, air ..------...., Figure 1. The Integrated Model of Selective Laser Sintering

Solid Freeform Fabrication Using Gas Phase Precursors

In addition to Selective Laser Sintering two other Solid Freeform Fabrication approaches that involve precursor gas phases are being explored at The University of Texas. The first is a variation of the Selective Laser Sintering process, where a layer of powder is spread and the laser scans under the identical conditions described earlier, but the action of the scanning laser is to heat the powder surface to a temperature where it will react with the gas phase to create a compound. This is another version of Selective Laser Reactive Sintering (SLRS) defined earlier (Birmingham, 1992; Birmingham, 1993; Birmingham, 1995a; Bourell, 1992; Subramanium, 1992) for powder/powder reactive sintering. Typical precursor gases include O2, CH3, C2H2, tetramethylsilane, N2, and NH4. An example is Si powder in the presence of CH4 forming SiC. In this case, Si was locally melted by the scanning laser beam and reacted with the CH4 gas. In a similar case, Si powder under the laser beam reacts with a precursor of ammonia to produce shapes of Si3N4. Finally, a mixture of Si3N4 and Si powders, where Si reacts in the presence of the CH4 gas, results in a composite of SiC and Si3N4.

Direct SLS Fabrication of Metals and Ceramics

In the following, direct fabrication of high-density metal and ceramic structures is discussed. This discussion is primarily centered on the SLS (Selective Laser Sintering) process, but a brief discussion of other direct techniques is given at the end of this chapter. All of these techniques (including SLS) are still in the research stage.

Current and future trends in solid free-form fabrication

Solid freeform fabrication (SFF) technologies have exploded exponentially since the early 1980s. One manifestation of this growth is a proliferation of new SFF processes and variations. A review of currently developed SFF processes is presented, and future trends are discussed.