Selective Laser Sintering Parts Research Articles

Powder Densification Maps in Selective Laser Sintering

Selective Laser Sintering (SLS) is a manufacturing process in which a part is produced without the need for part-specific tooling. It competes effectively with other manufacturing processes when part geometry is complex and the production run is not large. Traditionally, this was limited to prototype production, although tooling applications are now appearing. This paper describes several applications of powder densification maps to advance solutions in direct SLS of metallic and ceramic powders. Time-dependent plasticity issues arise in pre-processing of powder to make it suitable for SLS and in post-processing of SLS parts to obtain desired density.

Characterization of SLS parts for drug delivery devices

There are many applications for rapid prototyping systems and application in the biomedical field is an important domain. Uses selective laser sintering (SLS) in this study to build porous cylindrical disc matrices for use as drug delivery devices (DDD). Studies the part‐bed temperature to ascertain its influence over the porosity of the disc matrices. They are found to have an inverse linear relationship. Also investigates the dense walls, the inherent consequences of building porous structures with the SLS, in the disc matrix as they have a direct impact on the performance of the DDD. Discusses the size constraint of the disc matrix due to the limitations of the SLS process. Also investigates the possibility of creating disc matrices of varying porosity. Experimental results demonstrate that SLS is viable in producing DDDs that have variable porosity and micro‐features.

Infiltration of alumina sol into SLS processed porous Al2O3-Al4B2O9 ceramic composites

Selective laser sintering (SLS) is a form of the rapid prototyping technology which produces geometrical objects directly from a three-dimensional computer image without part-specific tooling or human intervention [1]. These rapid prototyping techniques have recently been developed to overcome some of the barriers of conventional manufacturing techniques, such as difficulties in tooling complex-shaped ceramic parts and the long production time in fabricating prototypes. The computer image is generated using CAD/CAE software or a computer imaging process. The three-dimensional computer image is numerically sliced into a series of two-dimensional cross-sections. The SLS machine regenerates these two-dimensional patterns on the powder bed by selectively scanning the powder bed with a focused laser beam and binding the loose powder. The thickness of the sintered powder layer is generally 125–250 μm. The sintered layer is lowered from the sintering plane and a new layer of the powder is spread again. The laser scans again, resulting in sintering of the powder particles, and bonding of the present layer to the underlying previous layer. The desired object is generated by laying down a number of such layers and successively sintering them [2]. The primary advantage of the SLS process is the flexibility of selection of material systems compared to other SFF techniques [3]. The two-phase powder approach to SLS, which involves binding high temperature ceramics such as alumina and silicon carbide with a low melting inorganic binder, is a promising technology to fabricate ceramic composite parts [4–6]. The selection of an optimum materials system for this approach depends on materials properties such as the melting point of the binder material and interparticle wetting between the components in the composite powder blend [7]. In a suitably chosen system, the low melting phase melts completely or partially under the laser beam and binds the high melting phase particles. Furthermore, it can react in some cases with the high melting phase or with the atmosphere. Further reaction can also takes place in a subsequent heating step. As a result, either another compound may incorporate into the matrix or a single phase compound may result. The samples made by SLS and simple post-thermal processing of an alumina-boron oxide composite system had only about 35–40% of the theoretical density due to a lower powder bed density arising from a lower apparent density (34% of theoretical density) of 15 μm alumina powder. This resulted in poor bend strengths (around 6 MPa) of the test bars. Therefore, it was necessary to increase the density for functional applications of SLS parts. Infiltration with colloids and solutions into porous SLS parts is an attractive option to increase the density [8]. The infiltration technique densifies a porous material by filling or partially filling the interconnected porosity of a particular compact with a liquid, melt, or vapor [9]. Infiltration can incorporate additional phases into a body that was formed by well-established techniques such as pressing, extrusion, tape casting, slip casting, and injection molding. Furthermore, infiltration may provide a uniform mixing of various phases. During subsequent heat treatment of a porous powder compact infiltrated with a suitable liquid infiltrant, processes such as decomposition of infiltrant, solid-state reaction, liquid phase formation, and densification can take place depending on the system [10–12]. For the infiltration study, test bars with dimension of 0.076 m× 0.025 m× 0.00625 m (3′′ × 1′′ × 0.25′′) made from alumina-25 wt.% boron oxide (melting point; 450 ◦C) powder blend with a laser energy density between 15.1 and 27.2 joules/cm2 fired at 900 ◦C for 6 h were selected. During that post-thermal processing step, boron oxide powder reacted with alumina powder, resulting in aluminum borate (2Al2O3 · B2O3) whiskers at the surface of the alumina particles [4]. The energy density is defined as laser power/(scan spacing× scan speed) [13]. The SLS operational parameters employed to fabricate these test bars are given in Table I. An infiltrant such as alumina sol was selected for densification of alumina-aluminum borate (2Al2O3 · B2O3) composites. Dispal alumina sol provided by the Vista chemical company was employed. The solid content was 25 wt.%, which was confirmed by a direct drying experiment and the particle size was 150 nm. X-ray diffraction (Fig. 1) of the dried gel reveals it to be boehmite (γ -AlOOH). At about 500 ◦C, boehmite transforms to γ -Al2O3 which has a defect spinel structure with 1/3 of the cation sites

Titanium alloy powder preparation for selective laser sintering

Selective laser sintering (SLS) is a solid freeform fabrication process whereby a part is built layerwise by scanning a powder bed. The processability of metal powder varies depending on the state of the powder prior to SLS. A powder thermal pre‐treatment was developed which involved degassing the powder at an elevated temperature in a vacuum. Without powder thermal pre‐treatment, the powder may flow poorly and may “ball” or form molten clumps during the laser exposure rather than wetting into the present and previous layer. These effects result in SLS parts with poor surface finish, mechanical properties and density. The purpose of this study was to identify for titanium alloy powder the mechanisms responsible for the improvements obtained after powder thermal pre‐treatment and to optimize the thermal excursion.

Improving surface quality of selective laser sintered rapid prototype parts using robotic finishing

Because of ‘stair-step’ and ‘shrinkage’ effects, most rapid prototyping (RP) parts have need of surface finishing in post-processing to obtain good surface quality. Instead of manual operations or homogeneous polishing techniques, a robot, with its high flexibility and sophistication, can effectively finish the surfaces of selective laser sintering (SLS) parts. After the sintering process, an SLS part that requires finishing is placed on a fixture with the help of a special base fitting. By calibrating the base fitting, the position and orientation of the part relative to the robotic system are determined. The robot holds a finishing tool, which can be changed according to different materials and different surfaces, as its end-effector. Once the calibration data, finishing tool, surfaces to be finished and finishing parameters are determined, the robotic finishing path can be automatically programmed from the computer aided design/computer aided modelling (CAD/CAM) model. The robot then moves a high-speed finishing tool over the desired surfaces according to the finishing path. Some experiments have been carried out on different SLS parts using different materials. It is shown that surface quality, which includes surface roughness, dimensional accuracy as well as geometrical accuracy, can be improved using robotic finishing.

Characterization of surface fault patterns with application to a layered manufacturing process

Abstract Detection of faults is a continuing need in all manufacturing processes. This need is more pronounced in new manufacturing processes where continual product improvement is desired. In this paper, a systematic approach is developed for addressing this need. Contemporary experimental and surface analysis techniques provide a basis for incrementally diagnosing the fault mechanisms leading to surface deviations. Based on this general approach, a maturing layered manufacturing process is investigated, notably, Selective Laser Sintering (SLS). First through experimental analysis it is shown that layer thickness and part orientation affect surface quality. Using these experimental results, random process analysis is carried out to determine the specific fault patterns. The quantification of these fault patterns leads to the identification of fault sources, providing a foundation for monitoring and control. The paper concludes with a summary of fault mechanisms on SLS parts, implementation issues for improving part quality, and insights for applying the approach to other manufacturing processes.

Full densed alumina-glass composites by SLS and ceracon forging process

Selective Laser Sintering (SLS) process has been employed to fabricate alumina-glass composites using monoclinic HBO2 as an inorganic binder. Subsequent post-thermal processing of green SLS parts at various temperatures yielded glass-ceramic composites. Composites with higher glass contents showed higher density and bend strength for all firing temperatures than those with lower glass contents. For a firing temperature of 900°C, the glass redistribution arising from a better viscous flow of glass gave all composites maximum density and bend strength. In addition, fully dense SLS glass-ceramic parts were obtained by employing the Ceracon forging process. Bend strength of these Ceracon forged composite parts reached as high as 70 to 110 MPa.