Binding Mechanisms in Selective Laser Sintering and Selective Laser Melting

Layer Manufacturing (LM) technologies like Selective Laser Sintering (SLS) were developed in the late 80’s as techniques for Rapid Prototyping (RP). Today, SLS - as well as its derived technology Selective Laser Melting (SLM) - is used as well for prototyping, tooling and manufacturing purposes. This widening of applications is caused mainly by the possibility to process a large variety of materials, resulting in a broad range of physical and mechanical properties. This paper presents a survey of the various binding mechanisms in SLS and SLM, which are responsible for the broad range of materials and applications. Basic binding mechanisms involve solid state sintering, chemically induced binding, liquid phase sintering, partial melting and full melting. Many subcategories can be distinguished based on the type of structural or binder powder composition: single component powder grains (single material or alloy), composite powder grains, mixtures of different powder grains, distinct binder material (sacrificial or permanent), etc. The paper will explain how these binding mechanisms apply for sintering various types of materials: plastics, metal, ceramics and composites (e.g. glass reinforced polymers, cermets, hardmetals, etc.). It gives a survey of research done at the University of Leuven, Belgium, as well as at other European and non-European organizations.Mechanical Engineerin

Functional Connectivity fMRI of the Rodent Brain: Comparison of Functional Connectivity Networks in Rat and Mouse

At present, resting state functional MRI (rsfMRI) is increasingly used in human neuropathological research. The present study aims at implementing rsfMRI in mice, a species that holds the widest variety of neurological disease models. Moreover, by acquiring rsfMRI data with a comparable protocol for anesthesia, scanning and analysis, in both rats and mice we were able to compare findings obtained in both species. The outcome of rsfMRI is different for rats and mice and depends strongly on the applied number of components in the Independent Component Analysis (ICA). The most important difference was the appearance of unilateral cortical components for the mouse resting state data compared to bilateral rat cortical networks. Furthermore, a higher number of components was needed for the ICA analysis to separate different cortical regions in mice as compared to rats

Selective Laser Sintering/Melting of Iron-Based Powders (Selectief laser sinteren/smelten van ijzergebaseerde poeders)

Door selectief laser sinteren / smelten (SLS/SLM) van Fe Cu Fe3+xP Ni poeder werd de performantie van de beschikbare machines voor de prod uctie van ijzergebaseerde objecten met hoge dichtheid geëvalueerd. Een t weede deel van deze studie handelt over het effect van procesparameters, legeringelementen (C, Si, Ti en Cu) en zuurstof op het gedrag van Fe ti jdens selectief laser smelten (SLM). In gepulste laser mode en bij een hoge laserintensiteit veroorzaakt verd amping van het materiaal een druk op het oppervlak van het smeltbad. Dez e druk verbetert bij gematigde waarden de verdichting tijdens SLM. Voorw erpen met een dichtheid tot 95% werden geproduceerd uitgaande van Fe C u Fe3+xP Ni, Fe en Fe C (0.2 0.8 gew%) poeders. De stabiliteit va n een ijzergebaseerd smeltbad tijdens SLM op een poederbed wordt in cont inue laser mode bepaald door de scansnelheid, het zuurstofgehalte in de atmosfeer en de aanwezige legeringelementen. Deze afhankelijkheid wordt hoofdzakelijk verklaard door het effect van deze factoren op de lengte t ot diameter verhouding van het smeltbad en op de kinetica van Rayleigh i nstabiliteit. Een correlatie tussen enerzijds de oppervlaktemorfologie e n de porositeit van driedimensionale stukken en anderzijds het effect va n proces- en materiaalparameters op de fysische fenomenen die optreden t ijdens SLM wordt voorgesteld. Er werd vastgesteld dat de warmtevrijgave als gevolg van oxidatie van Fe of van legeringelementen zoals C, Si of T i in vele gevallen het gedrag van ijzergebaseerde poeders tijdens SLM be paalt.status: publishe

Fundamentals of selective laser melting of alloyed steel powders

The successful fabrication of dense iron-based parts by selective laser melting (SLM) is still limited to a narrow range of materials. This study aims at gaining an understanding of the effect of elements such as oxygen, carbon, silicon, titanium and copper on the quality of two-dimensional and three-dimensional iron-based objects. The results are related to the effect of the elements on physical phenomena such as laser absorption, heat transfer, wetting and spreading of the melt, oxidation, Rayleigh instability and Marangoni convection.status: publishe

Alkaline corrosion properties of laser-clad aluminum/titanium coatings

Purpose– The purpose of this paper is to study the use of titanium as a protecting element for aluminum in alkaline conditions.Design/methodology/approach– Aluminum coatings containing up to 20 weight per cent Ti6Al4V were produced using laser cladding and were investigated using light optical microscope, scanning electron microscope – energy-dispersive X-ray spectroscopy and X-Ray Diffraction, together with alkaline exposure tests and potentiodynamic measurements at pH 13.5.Findings– Cladding resulted in a heterogeneous solidification microstructure containing an aluminum matrix with supersaturated titanium (&lt;1 weight per cent), Al3Ti intermetallics and large partially undissolved Ti6Al4V particles. Heat treatment lowered the titanium concentration in the aluminum matrix, changed the shape of the Al3Ti precipitates and increased the degree of dissolution of the Ti6Al4V particles. Corrosion testing showed significant localized dissolution of the aluminum matrix.Research limitations/implications– Increased titanium concentration and heat treatment gave improved alkaline corrosion properties. At pH 13.5, the Al3Ti phases were protected, while the aluminum matrix corroded.Practical implications– For alkaline corrosion-protection of aluminum in the automobile industry, titanium might be useful at pH values below 13.5 or by using other coating techniques.Originality/value– This is the first study testing the use of titanium as a protective element of aluminum in stringent alkaline conditions.</jats:sec

Investigation of sectoral scanning in Selective Laser Melting

Selective laser melting (SLM), a powder metallurgical (PM) additive manufacturing (AM) technology, is able to produce fully functional parts directly from standard metal powders without using any intermediate binders or any additional post-processing steps. During the process, a laser beam selectively scans a powder bed according to the CAD data of the part to be produced and completely melts the powder particles together. Stacking and bonding two-dimensional powder layers in this way, allows production of fully dense parts with any geometrical complexity. The scanning of the powder bed by the laser beam can be achieved in several different ways, one of which is island or sectoral scanning. In this way, the area to be scanned is divided in small square areas (‘sectors’) which are scanned in a random order. This study is carried out to explore the influence of sectoral scanning on density, surface quality, mechanical properties and residual stresses formed during SLM. The experiments are carried out on a machine with an Nd:YAG laser source using AISI 316L stainless steel powder. As a result of this experimental study, it is concluded that sectoral scanning has some advantages such as lower residual stresses and better surface quality. However, the selection of parameters related to sectoral scanning is a critical task since it may cause aligned porosity at the edges between sectors or scanned tracks, which is very undesired in terms of mechanical properties.status: publishe

Photopyroelectric measurement of thermal conductivity of metallic powders

The effective thermal diffusivity of metal powders in air at room temperature is measured by the photopyroelectric technique. The thermal conductivity is calculated from the diffusivity, the relative density, and the specific heat obtained from literature. Maxwell's model is a good prediction but underestimates the measured effective thermal conductivity, especially for irregular particles. Due to the large difference between the thermal conductivity of metals and air, the effective conductivity is mainly determined by the relative density of the powder bed but not by the properties of the powder material. A theoretical model showing the influence of grain size and gas pressure is presented. The dependence on the particles' size and pressure is explained by the gradual transition from the free molecular to conductive mechanism of heat transfer in gaps between particles. The theory gives a precise estimation of effective thermal conductivity for metallic powders with a narrow size distribution of spherical particles and it underestimates this value in the case of a wide size distribution or in the case of irregular particles. (C) 2005 American Institute of Physics.status: publishe

Charpy impact testing of metallic selective laser melting parts

Selective laser melting (SLM) is a layer-additive manufacturing technology which makes it possible to create fully functional parts directly from standard metals without any intermediate binders or any additional post-processing steps. During SLM, a high intensity laser beam selectively scans a powder bed according to the CAD data of the part to be produced and powder particles are completely melted. SLM is capable of producing near full density parts with an almost infinite geometric freedom. However, the mechanical properties obtained with SLM may differ from the ones of bulk material. In this study, Charpy impact tests are applied on the samples produced by SLM from different materials; Ti-6Al-4V, AISI 316L (X2CrNiMo18-14-3) and maraging steel 300 (X3CoMoTi18-9-5). The influence of the building axis as well as of various heat treatments applied on the samples after SLM is investigated. The evolution of the microstructures of the sample parts is also studied.status: publishe

Wax based binder for indirect selective laser sintering of alumina

In this study the production of alumina parts by indirect selective laser sintering is investigated. A production process for homogeneous, free-flowing composite powder composed of fine alumina powder and a wax based binder has been developed. Two binder formulations have been studied: 100% carnauba wax and carnauba wax mixed with 18 wt% low density polyethylene (LDPE). Three dimensional parts could successfully be produced by SLS using both binder systems. However, the geometrical stability during debinding and sintering of the parts composed of the 100% carnauba wax binder was insufficient. The presence of low density polyethylene in the binder resulted in good geometrical stability during post-processing but at the ex-pense of the green density after SLS. After thermal debinding and furnace sintering relative densities o 75% could be obtained at optimal processing parameters using the carnauba wax based binder containing LDPE.status: publishe