Selective Laser Sintering State-of-the-art

Selective Laser Sintering: State-of-the-art J.P Kruth*, T Laoui* * Catholic University of Leuven, Celestijnenlaan 300B, B-3001 Leuven, Belgium Abstract Selective Laser Sintering (SLS) is a rapid prototyping process that allows to progressively produce (build) complex geometrical parts by fusing and solidifying successive layers of fine powder material deposited on top of each other Fusion is induced by heat delivered by a laser beam that melts or sinters the powder grains together and to the previous solidified layer The paper describes the state-of-the-art of SLS one decade after its first appearance (First SLS machine sold in 1990) It deals with progresses in terms of SLS machines (hardware, optics, lasers,…), SLS processible materials (polymers, metals, ceramics, foundry sand, composites, cermets,…) and applications (prototypes, tooling, moulds and dies, functional parts,…) Key words: Selective laser sintering, polymer, metal powders, cermets, simulation INTRODUCTION Selective Laser Sintering (SLS) is a Rapid Prototyping (RP) process that fuses or sinters powder particles together to generate solid parts The fusing or sintering energy is provided by the heat of a laser beam that scans across successive layers of pre-deposited powder (Fig 1) The ‘green’ product obtained after laser sintering might still be porous Since this porosity is normally not desired (e.g limited strength), a post-treatment is often applied that mostly involves infiltration of the pores with a polymer (e.g epoxy) or a metal (e.g Cu or bronze) 1),2) Alternatively, a furnace post-sintering or HIP cycle may be applied to reach full density 3) beam d e f le c t io n s y s t e m fo c u s e d a n d d e fle c te d b e a m la s e r p o w d e r d e p o s it io n s y s te m e n e rg y b e a m s in te r e d p a tte rn lo o s e p o w d e r CAD c o n t a in e r Fig 1: Schematic overview of SLS process SLS might be considered the RP technology with the highest potential, mainly because it can be applied to produce parts in almost any (powder) material: polymers, reinforced polymers, metals, ceramics, composites, foundry sand, etc The process may be used to sinter many off-the-shelf powder materials not specially developed for SLS, even though the two well-known vendors of SLS equipment (DTM and EOS) offer special purpose well flowing SLS powder materials Those powders depict a fast sintering reaction tuned to the short laser/powder interaction time that results from the fast scanning of the laser beam across the powder layers The use of off-the-shelf powders adds a second major advantage over other rapid prototyping (RP) processes, like stereo-lithography (SL), laminated object manufacturing (LOM) or fused deposition modelling (FDM), that respectively require dedicated photo-polymers tuned to a specific light wavelength (SL), pre-shaped material foils generally provided with glue on one side (LOM) and calibrated polymer wire coils or sheet cartridges (FDM) 56),57) Generally, all powder materials may be processed on the same SLS machine, even though one of the two SLS vendors opted for marketing dedicated machines for the various materials: EOSint-P machine for polymers, EOSint-M for metals, EOSint-S for sands Notwithstanding these potentials and the fact that SLS was, after stereo-lithography, one of the first RP processes (originally developed at University of Texas-Austin4)), SLS has till now been characterised by a relatively small market penetration: see Table However, in recent years it depicted one of the biggest growth (Table 1), that may be partly associated with the fact that RP applications are no longer limited to the production of visual polymer prototypes, but extend more and more to the production of functional prototypes, functional parts and rapid tools made from polymer, metal or sand This paper gives a survey of the state-of-the-art of SLS, one decade after the first industrial applications emerged SLS EQUIPMENT 2.1 Principle A basic SLS machine is depicted in Fig The SLS part is produced by depositing successive thin Table : RP systems unit sales Process Stereo-Lithography (SL) Fused Dep Mod (FDM) Ink Jet Printing (IJP) Laminated Object Mfg (LOM) Selective Laser Sintering (SLS) Sales 1999 291 units (24.4%) 293 units (24.5%) 389 units (32.5%) 94 units (9%) Diff ‘97 + 3,5 % + 13 % + 47 % - 42 % 115 units (9.6%) + 53 % 2.2 Powder deposition Depositing the powder layers is a critical issue in the SLS process Several studies 5),6) were devoted to develop suited powder deposition mechanisms to deposit smooth thin layers of powder even if the powder morphology does not yield good powder flowability (e.g in case of irregular or very fine powder particles that tend to coagulate rather than to spread out regularly) Those mechanisms often use a combination of specially shaped feeding slots, freerolling or counter-rolling cylinders, scrapers, vibrators and other devices to deposit and pack or densify powder in ever-thinner layers Ideally the powder should reach the tap density when deposited This is the highest achievable density of the powder obtained by vibrating or tapping the powder without enforcing any plastic deformation of powder grains The best packing density is 74% for monosized spherical powders Achieving this density or even higher (using a careful selection of a particle size distribution) may be favoured with vibrators or pressing devices Some studies tried to levitate and deposit thin powder layers with electrostatic charged plates similar to those used to deposit toner in photocopying devices 7),8) The development of those deposition systems run in parallel with the development of better flowing powders (e.g using fluxing additives) and the trend to work with finer powder and thinner layers (Present-day minimum layer thickness is about 0.05 mm with typically 30µm powder) 2.3 Type of lasers Commercial SLS machines (DTM and EOS) are all equipped with CO2 lasers with maximum power ratings between 50 and 200W The university of Leuven developed two SLS machines equipped respectively with Nd:YAG lasers of 300 and 500W 9) , while the University of Connecticut and University of Manchester used a 60W diode laser of 810nm 10),11) The University of Liverpool explored the use of Q-switched Nd:YAG and short-pulse Cu- C O Laser N d :Y A G Laser A b s o r p t io n c o e ff ic ie n t  layers of powders (typically 0.1 to 0.3mm thickness) in a building container Each layer of powder is first sintered in accordance with the product’s geometry coinciding with that layer or section of the product, before a new layer of powder is deposited and sintered 2 S te e l Iro n 0 C opper 0 0 5 W a v e l e n g t h  (  m ) 10 20 vapour lasers 35) In future, other type of lasers might show up, like diode pumped solid state lasers and others Fig 2: Absorption of light by some metals and polymers for different wavelengths Optimally, the laser wavelength should be adapted to the powder material to be sintered, because the laser absorption coefficient greatly changes with the material and the frequency or wavelength of the laser light (Fig 2) 12), 13) From Fig 2, it could be anticipated that CO2 lasers (wave length 10.6µm) might perform better with polymer powders, while Nd:YAG (wavelength of 1.06µm) will be superior for metallic materials, since metals depict higher absorption at shorter wavelengths This is confirmed by several studies of the University of Leuven in which Fe-Cu metal powder and WC-Co hardmetal or cermet powder were alternatively sintered with both laser types 14) Results proved that for the same amount of energy or at similar settings (laser power and scan speed), a Nd:YAG laser results in higher green part density, a larger sintering depth (allowing thicker layers to be sintered, hence reducing production time) and a higher yield (process efficiency) Few results are presented further in this paper (see Figs and 6) The study also proved that, for those materials, the processing window of a YAG laser is larger than that of a CO laser (Fig 5) This allows a wide variation of the processing parameters (power and scan speed) or of the powder composition (here mixture ratio between Fe and Cu), while still ensuring good sintering results (i.e good liquid phase sintering – see section on SLS of metals) This enforces good process controllability and reliability 2.4 Optics and scanners The optics serve to bring the laser beam from the laser source to the processing area and to focus it onto the powder surface, while the scanner moves the beam across the surface Scanning is normally done with oscillating galvano mirrors that yield higher scan speeds than those achievable with XY tables translating mirrors and focussing devices Nd:YAG lasers allow the use of fibre optics to guide the beam The University of Leuven used such glass fibre to guide a 500W CW Nd:YAG beam to a focussing unit fixed to an XY table 9) The use of such combination ‘fibre optics/table scanner’ eliminates the need for an expensive flat field lens (needed with galvano scanners to convert the spherical focussing plane into a flat plane coinciding with the powder surface) Moreover, this solution does not need any mirror and allows for higher power (here 500W), whereas galvano mirrors can not support over 300W (Above this power, mirrors need powerful internal water cooling and their inertia puts unrealistic requirements to the galvano scanner) Using a fibre guiding a YAG beam has the advantage to act as a beam integrator (i.e conversion of Gaussian or other TEM power profile to a constant power profile across the beam cross section) It has the disadvantage to induce loss in the beam quality, thus lower focability Using high integrated powers with defocused beams, on the other hand, allows to sinter wide tracks (10-15mm wide) at once, thereby reducing the scan time for sintering large areas However, the fibre/table solution was abandoned in a later stage in favour of a faster 300W YAG-beam galvano scanner, since powers of 100-300W seemed sufficient for sintering most materials (including refractory materials like hardmetals, cermets and ceramics) The University of Erlangen developed special optics to split a laser beam in two, defocus one beam and recombine the two beams concentrically into an intensive focused spot surrounded by a larger less intensive (defocused) spot 15) The large spot is used to pre-heat the powder in front of the moving intensive sintering spot 2.5 Processing chamber, atmosphere, powder pre-heating To achieve proper laser sintering, oxidation of the powders should be avoided This is mainly crucial for metals depicting very high affinity to O 2, but is not negligible for polymers and some types of ceramics Therefore, SLS machines are generally equipped with processing chambers working with a protective atmosphere (mostly N2 in case of polymers or metals, Ar for sensitive metals) Some machines have been designed 9),16) or modified 17) to operate under vacuum Those machines are aimed at sintering metals, although sintering in vacuum or low pressure might induce sublimation of the metal powder Today, the vacuum facility of the chamber of the proprietary SLS machine of the University of Leuven is mainly used for O2-degassing of the powder (extracting O2 from the chamber or O2 trapped in the pores within the powder) After degassing, the chamber is filled with low pressure N or Ar, ensuring an O2-concentration below 0.5% Many SLS processing chambers are further equipped with powder pre-heating systems Preheating the powder prior to sintering yields several advantages including drying the powder (removing the adsorbed humidity), reducing the laser power and interaction time needed for sintering, and lowering the thermal gradient generated in this process thus reducing the subsequent thermal stresses and distortions SLS OF POLYMERS Polymer powders were the first and are still the most widely applied materials in SLS Amorphous polymers, like polycarbonate powders, are able to produce parts with very good dimensional accuracy, feature resolution and surface finish (depending on the grain size), but they are only partially consolidated As a consequence, these parts are only useful for applications that not require part strength and durability Typical applications are SLS models for the manufacture of silicone rubber and cast epoxy moulds 18) Semi-crystalline polymers, like nylons (polyamide), on the contrary can be sintered to fully dense parts with mechanical properties which approximate those of injection moulded parts On the other hand, the total SLS process shrinkage of these semi-crystalline polymers is typically - % 19) , which complicates production of accurate parts The good mechanical properties of these nylon based parts makes them particularly suited for high strength functional prototypes New grades of nylon powders (i.e Duraform PA1220)) even yield resolutions and a surface roughness close to those of polycarbonate (PC), making polyamide (PA) also suited for casting silicone rubber and epoxy moulds, even though parts with higher resolution and smoother surfaces can be produced from amorphous powders Those “Duraform” polyamides yield improved accuracy, processibility and recyclability, as well as reduced nitrogen consumption 21) Other polymer-based materials available commercially are acrylic styrene (PMMA/PS) for Table 2: Overview of the mechanical properties of some SLS polymer materials (DTM) PolyFine Nylon Glass filled Elastomer carbonate Nylon Tensile modulus (MPa) 1200 1400/1800* 2800/4400* 20 Tensile strength (MPa) 23 36/44* 49/42* Break elongation (%) 6/22* 1.8* 111 Surface roughness Ra as 12/8.5* 15 SLS processed (m) * value for DuraForm PA investment casting and an elastomer for rubber-like applications Table gives an overview of the mechanical properties of some SLS polymer materials (DTM) SLS OF REINFORCED AND FILLED POLYMERS Polyamide powders can be relatively easily reinforced with other materials in order to further improve their mechanical and thermal properties Several grades of glass fibre reinforced polyamide powders are readily available on the market 22) Polyamide coated copper powder (Cu-PA) is also available for the production of plastic-metal composite injection tools 23) This Cu-PA powder mixture contains 70 wt% Cu (rest is PA) Compared to plain PA parts, Cu-PA SLS parts are 3.5x heavier (density of 3.45 g/cm3), 4x more thermally conductive (1.28 W/m°C) and exhibit a similar tensile strength (34 MPa) but a higher tensile modulus (3.4 GPa) Applications include producing Cu-PA inserts for building moulds Those Cu-PA moulds can be used as laser sintered without need for removal of the PA phase or without any postdensification process To improve the surface finish of the SLS Cu-PA inserts, the latter are first surface coated with a resin (epoxy, acrylate, Imprex Superseal) to fill the porosities and then finished with a flexible cloth Molds containing such inserts can deliver 200-400 parts in common plastics 23) SLS OF METALS AND HARDMETALS/CERMETS The production of functional components from metallic or ceramic powders or a combination of those by SLS process is a very promising area Industrial applications have already emerged particularly in rapid tooling Other potential applications include: one-of-a-kind complex metallic or ceramic parts, prototyping of cutting tools, moulds and inserts, EDM electrodes, etc The following sections summarise the main developments related to powders and laser sintering mechanisms investigated so far in this rapidly developing field 5.1 SLS of metals and cermets with polymer binder or infiltrant DTM Corporation (Austin, USA) has developed a process that applies polymer-coated steel powders (1080 steel, 316 or 420 stainless steel particles coated/mixed with a thermoplastic/thermoset material) for the SLS of metal parts During laser sintering, the polymer melts and acts as a binder for the steel particles A post treatment is necessary in which the polymer is burned out and the porous part is infiltrated with copper or bronze 18),24) Over several years, DTM improved continuously their production process by reducing the number of postprocessing cycles and their total duration For the third generation of RapidSteel called LaserForm ST100, composed of 60% 420 stainless steel and 40% bronze (89Cu-11Sn), DTM reported an infiltration time of 24h performed in one single step under pure nitrogen These developments resulted in improving several material properties of the final SLS parts such as strength, hardness, machinability, weldability, wear rate and thermal conductivity 24) Using this indirect SLS process, the University of Texas at Austin sintered SiC particles coated with a proprietary polymer binder to obtain a SiC preform with a typical density of 40Vol% 25) After polymer debinding (at 400°C) the SiC preform becomes quite fragile preventing further handling To improve the strength of the SiC preform, an additional firing step (1100°C, 2hrs, formation of SiO2 layer) was utilized followed by (a pressureless) infiltration (670°C) with a Mg-based (AZ91D) die casting alloy 25) EOS GmbH (Munich, Germany) avoids the use of a polymer binder by directly sintering metal powders with a low melting point: i.e bronze-nickel based powders (EOS-Cu 3201 containing Cu-Sn, Cu-P and Ni particles) developed by Electrolux Co 2) After SLS, the part is infiltrated with epoxy resin to fill in the porosities Hence the final part is a bronze-epoxy composite, rather than a plain metallic part and its mechanical and thermal properties are limited Infiltration with a metal like Cu or bronze is not possible in this case, since the green part would melt during infiltration Lately, EOS put into market a new powder (EOS-DMLS Steel 50-V1 containing steel, Cu-P and Ni particles) yielding improved mechanical properties 26) The SLS part is about 70% dense and thus can be used as such for inserts and small mold components 5.2 SLS of metals and hardmetals/cermets by liquid phase sintering Many research institutes study the possibility of directly laser sintering metal and ceramic powders without use of any polymer component 16),27)-35) Several approaches for binding the powder particles together using laser beam energy have been investigated at the University of Leuven including: solid state vs liquid phase sintering, loose vs precoated metal binder phase, mixed vs milled powders 30),36) Further research mainly focussed on liquid phase sintering of uncoated powder mixtures The basic material used in liquid phase sintering (LPS) consists of a mixture of two metal powders (or a metal and a ceramic powder) : a high melting point metal/ceramic, called the structural material, and a low melting point metal, called the binder Applying heat to the system causes the binder to melt and to flow into the pores formed by the non-molten particles The classical stages of LPS are schematically shown in Figure The main advantage of liquid phase sintering is Loose pow der B in d in g e le m e n t takes place during laser sintering because of the very short laser-material interaction time (fraction of a second) Early studies to sinter steel powder mixed up with copper grains acting as binder material were performed at the universities of Aachen (Germany) 16) , Leuven (Belgium) 27) and Erlangen (Germany) 28) The universities of Texas at Austin (USA) 38) and Leuven (Belgium) 32)-34) and some German Fraunhofer research centres 37) succeeded to laser sinter hardmetals (i.e cemented carbides) and cermets by SLS Figure 4a shows a typical microstructure of a WC-9wt%Co powder mixture sintered by a CO2 laser showing a good bonding between the WC particles surrounded by a Cobinder layer and the presence of large pores (5060%) After infiltration, Cu filled these open porosities (Fig 4b) Such LPS green part has enough strength to withstand a post-processing cycle to bring the part to full density This post-processing consists of a furnace post-sintering to proceed with the next stages of LPS shown schematically in Fig (solution reprecipitation and solid state sintering) or an infiltration with a low melting point metal R e a rra n g e m e n t S tr u c tu r a l e le m e n t ( s o lid p h a s e ) S o lu t io n r e p r e c ip it a t io n S o lid s ta te s in te r in g P o r o s ity Fig 3: Different stages of Liquid Phase Sintering the very fast initial binding This binding is based on capillary forces, which can be very high: the reaction speed in this stage is determined by the kinetics of the solid-melt transformation This transformation is several orders of magnitudes faster then solid-state diffusion occurring in solid phase sintering Once the binder metal is molten and spread out into the solid lattice, the system cools down (because the moving laser beam no longer feeds energy into the material) and the situation is frozen Only the first stage (rearrangement) of the LPS mechanism A wide variety of powder combinations have been investigated in Leuven using LPS mechanism, including: Fe-Cu, Cu-coated Fe, Fe 3C-Fe, Stainless Steel-Cu, Stainless Steel-CuFeCo-Co, Stainless Steel-CuP-Co, WC-Co, Co-coated WC, WC-Cu, WC-CuFeCo, TiC-Ni/Co/Mo, TiB2-Ni, ZrB2-Cu, It is worth noting that finding the processing window yielding good sintering behaviour using LPS mechanism is not always evident and easy to obtain For instance in case of Stainless Steel-Cu powder mixture, the processing window turned out WC C o P o ro s ity a) b) Fig 4: SEM micrographs of WC-9wt%Co powder processed by SLS revealing (a) the distribution of WC particles within the Co binder matrix and (b) Cu filling the open pores after infiltration to be quite narrow when this powder was sintered with a CO2 laser and relatively larger using a L iq u id Phase S in te r in g 25 20 CO Laser 15 10 M e lt F r a g ile % B in d e r ( V o l% C u ) 30 30 % B i n d e r ( V o l% C u ) For WC-9wt%Co material, it has been shown that the mechanically alloyed powders resulted in higher N d :Y A G L a s e r 25 L iq u id P h a s e S in te r in g 20 M e lt 15 10 M e lt F r a g ile 300 1200 600 900 S p e c ific In d u c e d E n e r g y ( J /c m ) 1500 300 600 900 1200 S p e c ific In d u c e d E n e r g y ( J /c m ) 1500 Fig 5: Processing window for Stainless Steel-Cu sintered with both CO2 and Nd:YAG lasers as a function of specific induced energy and vol% binder (Cu) Nd:YAG laser as shown in Figure The small difference in melting points between the structural element (stainless steel) and the binder element (Cu) and the high reflectivity of Cu particles for laser light, particularly with CO lasers having a longer wavelength (10.6 µm), induced mostly simultaneous melting of both elements Owing to the larger difference in their melting points, such behaviour was not observed in the WC-Co powder mixture and most of the SLS tests resulted in LPS without much difficulty Further details about these results can be found elsewhere 14),34),47) The type of laser (and thus wavelength) was also found to have an effect on the density of the SLS parts Parts sintered with Nd:YAG laser gave a higher density at similar processing parameters for both Stainless Steel-Cu and WC-Co powder mixtures (Fig 6) 14) It has also been shown that by optimising the process parameters related to both laser (power, scan speed, scan spacing) and powder (particle size, powder composition, mixing vs milling of both elements), the properties (such as surface quality, density) of the sintered product were improved 34), 36) green densities with better surface roughness compared to the layers prepared from the mixed powders 36) Furthermore, for a given scan speed, the surface quality of the sintered layers prepared from milled powders (as well as mixed powders) is improved with smaller particle size The main difficulty with the milled powder is to obtain a smooth and uniform layer during powder deposition For that, further research is underway to develop a deposition mechanism capable of depositing fine powders (mixed or milled) with particle sizes below 20 µm 5.3 SLS of metals by through melting The Fraunhofer Institutes ILT and IPT (Aachen, Germany) applied a 300W Nd:YAG laser to completely melt metal powders (bronze, steel, stainless steel such as 316L) deposited in a standard way using a wiper (scraper) and producing directly dense parts (density > 95%) 39) Due to the melting nature of metals (tendency to form droplets to minimise their surface energy), a careful control of the process parameters is needed Moreover, R e la t iv e d e n s it y f o r b o t h la s e r s o u r c e s 55 R e la tiv e d e n s ity ( %  m a x ) 50 45 Nd:YAG P P P P P P 40 CO 35 30 ow ow ow ow ow ow der der der der der der -3 -2 -2 -3 -2 -2 N N N C C C d :Y A G d :Y A G d :Y A G O O O 25 20 375 500 625 125 250 S p e c ific In d u c e d E n e r g y ( J / c m ) 750 Fig 6: Relative density of Stainless Steel-Cu SLS parts processed by CO2 and Nd:YAG lasers overhangs with angles higher than 60° could not be built with this process To maintain dimensional control and improve accuracy and surface roughness (Ra 50-80 µm), a final touch by high speed milling of the lateral sides is utilised When this process was used to sinter Al-30%Si, a maximum density of 9095% was obtained 37) EOS recently came to the market with a plain steel powder that is laser sintered by through melting The average particle size is 50µm, but an enhanced steel powder with 20µm size is announced Osaka University (Japan) utilised a pulsed Nd:YAG laser (50W mean power, 3kW maximum peak power) to melt pure Ti spherical powders (200 µm and 50 µm average particle size) to produce medical parts (dental crowns and bone models) 40) For the coarse Ti powder, the SLS part delivered a maximum relative density of 84% yielding a maximum tensile strength of 70MPa Using fine Ti powder (25 µm), a higher relative density (maximum 93%) was achieved with a tensile strength of 150MPa Due to the presence of remaining porosity, the tensile strength of these SLS parts is still lower than that of bulk pure Ti material (275 - 481 MPa) 40) 5.4 Combination SLS/HIP To produce fully dense metallic parts, the University of Texas at Austin used a combination of SLS and HIP processes 41),42) The objective of that research project was to produce fully dense functional metal parts using a SLS process generating a porous core encapsulated in a fully dense skin (i.e integrated SLS canning) followed by a HIP treatment The can or skin material formed directly by SLS (with density > 95%) around the laser sintered metallic component (60-80% dense) plays the role of an encapsulation material during HIPing and becomes part of the final component This process reduces the production time and costs associated with the encapsulation and can/skin removal after HIP The metals considered in that research project were Inconel 625 Superalloy, Stainless Steel (17-4 PH), Ti-alloys (Ti6Al4V) and Molybdenum The University of Texas at Austin claimed successful results in using the SLS/HIP combination to process metallic parts 42) temperatures leading to a disintegration of part/surface of SiC particles into Si and C The free Si then oxidises and forms SiO2, which plays a role of a binder between the SiC particles 39),43) After laser sintering, the SiC parts could be infiltrated with Si and reaction bonded to full density Zirconium silicates were also laser sintered by almost fully melting the powder particles forming large agglomerates 43),44) Similar to DTM’s polymer coated powder process, graphite coated with phenolic resin was also processed by SLS by melting only the polymer binder, which is burned out afterwards However, the resulting graphite part becomes very fragile 43) SLS OF FOUNDRY SAND Both commercial SLS machine vendors (DTM and EOS) offer sand powders that can be laser sintered in order to produce foundry sand moulds DTM, for instance, offers both Zr and Si sand: SandForm ZrII and Si released in 1997 45) Key characteristics include Shell Foundry Sand of given AFS grain fineness number (GFN# = 97 for Si and 99 for ZrII) and dimensional tolerances of 0.5mm SandForm Si, used predominantly for Al castings, is based on silica, which is prevalent in the market and has a low density SandForm ZrII can be used for both Al and Fe castings and its binder system matches silica Demonstrated applications are castings of power-train components, manifolds, automotive and heavy machinery parts APPLICATIONS OF SLS SLS OF CERAMICS The ILT and IPT Fraunhofer Institutes used also the SLS process in an attempt to produce directly ceramic parts without polymer binder material The absence of any binder element makes the ceramic laser sintered part very fragile and viable to breakage Due to the short reaction time involved in SLS, solid state sintering is not feasible To sinter SiC powder material, a sufficient amount of laser energy was supplied to induce high local Fig 7: Polyamide SLS prototype (DTM) The largest application of SLS is still the production of rapid prototypes from plastic material (Fig 7) One of the advantages is the good strength of SLS polymer prototypes as compared to prototypes made by e.g stereolithography or ink jet printing SLS allows to make functional prototypes from traditional engineering thermoplastics (e.g nylon) with properties nearly equal to injection moulded parts parameters and cycle times used with such mould for various plastic materials are reported in Table Today, tool life times of 100,000 shots are reported with Rapidsteel moulds Several WC-Co hardmetal injection moulds were produced at the University of Leuven 32), 47) Figure shows a number of 3-D green parts produced in Leuven, some of which representing mould inserts The parts are made from WC-9wt %Co powder mixtures After laser sintering, the green part exhibits a density of about 40-47vol% A post-treatment is thus necessary to achieve full densification This is done by an infiltration process in a furnace with a low melting point metal (e.g Cu or bronze) under a controlled atmosphere (a mixture of nitrogen and hydrogen) Fig 8: A plastic injection mould whose cavity and core were made with the Rapidsteel powder Table : Injection parameters for Rapid steel SLS mould 46) Material Closing pressure Injection temp Injection pressure Cycle time ABS 35 ton (350kN) 240° C 600 bar 40 sec - 160°C Polyamide 6.3% glass Fig 9: Various WC-Co green parts made by SLS (60MPa) 20 bar 14 sec (2MPa) Today, SLS of metals (with or without polymer binders) mainly finds industrial applications for ‘rapid tooling’, i.e for fast production of moulds and dies Fig shows a plastic injection mould whose cavity and core were made with the Rapidsteel SLS process of DTM In comparison with conventional tool making, the lead time for this mould was reduced by 45% (22 days down to 12 days), while the cost dropped by 38% (inclusive time and cost for CAD mould design, moulds assembly, use and machining of standard mould plates and accessories, etc.) 46) Typical injection Further improvements in the SLS parts including a minimum layer delamination (due to thermal stresses) and a reduction in the production costs of parts made of WC-Co powder mixture (due to the high price of Co), were achieved using alternative binders to replace partially or completely the Co phase A number of multi-component binder substitutes (to Co) considered in this study are alloys based on Fe-Cu/Ni-Co 48) SLS may also be used to produce functional metal/cermet parts or prototypes An example is given in Fig 10 It represents a plain WC/Co drill bit head for drilling in stone material as it appears directly after SLS (i.e before Cu infiltration) The geometrical features look quite reasonable, however the final surface roughness and strength (even after a b Fig 10: Photographs showing (a) Hilti TE-CX drill bit with full hardmetal head welded to a tool steel and (b) a WC-9Co drill bit head made by SLS process MODELLING AND SIMULATING THE SLS PROCESS The final properties of parts fabricated by SLS are very much dependent on the process parameters as well as on the powder characteristics As a result, each powder/machine combination may require extensive testing in order to identify the processing window and to optimise the processing parameters suitable to achieve the desired properties for a specific application This can be expensive and time consuming To gain a better understanding of the physical phenomena that are taking place in this relatively new process, more fundamental and modelling work is needed Moreover, this will reduce the number of experiments required to optimise the processing parameters for a given powder system Several research institutes have taken initiatives to tackle this task using different approaches The University of Leuven applied a simple ray-tracing model to evaluate the laser radiation penetration into a metal powder bed and the resulting powder absorptance 49),50) At the University of Texas and University of Leeds, thermally-based finite element models have been used to simulate SLS of amorphous polymers 51)-52) Recently, the two-dimensional model developed at the University of Leeds has been extended to the study of crystalline polymers and metals in two- and three-dimensions 53),54) The ray tracing model of the university of Leuven has been used to simulate laser sintering of Fe-Cu and WC-Co powders using Nd:YAG and CO lasers The results include the evaluation of the total energy incoupling (also called absorptance), the optical penetration of the laser beam, and the estimation of the sintering zone dimensions (thickness and width of a single laser sintered track) As an example of the results obtained so far, Fig.11 shows the calculated and measured sintering zone dimensions when sintering Fe-Cu powder with a Nd:YAG laser 55) 10 CONCLUSION On the long term, selective laser sintering could turn out to be one of the most successful rapid prototyping processes, mainly because of its unique ability to process nearly any material During just one decade of existence, it has been demonstrated that SLS may successfully be applied to produce parts in a wide range of polymers (elastomers, amorphous and semi-crystalline technical polymers), metals, hardmetals/cermets, ceramics and sand Among others, it has been demonstrated that SLS is well suited to produce a variety of composite materials: glass reinforced polymers, metal/polymer composite (e.g Cu/PA), metal/metal composites (e.g Fe/Cu), cermets (e.g WC-Co) and others In many cases, standard off-the-shelf powder materials can be used, without the need to develop dedicated powders It is expected that further development of the SLS process, equipment and of appropriate powder materials will further boost this rapid prototyping process in the years or 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(1998) 43)F Klocke, H Wirtz, Selective laser sintering of Ceramics, Proc LANE’97, Laser Assisted Net shape Eng 2, 589-596, (1997) 44)F Klocke, H Wirtz, Selective laser sintering of zirconium silicate,... Bonse, Selective Laser Sintering of WC-Co ‘hard metal’ parts, Proc 8th Int Conf on Production Engineering (ICPE), Japan, pp 149-156, (1997) 33)T Laoui, L Froyen, J.-P Kruth, ? ?Selective Laser Sintering. .. shows the calculated and measured sintering zone dimensions when sintering Fe-Cu powder with a Nd:YAG laser 55) 10 CONCLUSION On the long term, selective laser sintering could turn out to be one