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ARTICLE IN PRESSG ModelASCER 67 1–9

Journal of Asian Ceramic Societies xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of Asian Ceramic Societies

jo ur nal ho me page: www.elsev ier .com/ locate / jascer

onitoring of temperature profiles and surface morphologies during laserintering of alumina ceramics

in Qiana, Lauri Taimistob, Antti Lehtib, Heidi Piili b, Olli Nyrhiläc, Antti Salminenb,d, Zhijian Shena,∗

University of Stockholm, Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm, SwedenLappeenranta university of Technology, Laser Processing Research Group, Lappeenranta, FinlandEOS Finland, Turku, FinlandMachine Technology Centre Turku Ltd, Turku, Finland

r t i c l e i n f o

rticle history:eceived 30 November 2013eceived in revised form 21 January 2014ccepted 22 January 2014vailable online xxx

a b s t r a c t

Additive manufacturing of alumina by laser is a delicate process and small changes of processing parame-ters might cause less controlled and understood consequences. The real-time monitoring of temperatureprofiles, spectrum profiles and surface morphologies were evaluated in off-axial set-up for controllingthe laser sintering of alumina ceramics. The real-time spectrometer and pyrometer were used for rapid

eywords:onitoring

aser sinteringeramicslumina

monitoring of the thermal stability during the laser sintering process. An active illumination imagingsystem successfully recorded the high temperature melt pool and surrounding area simultaneously. Thecaptured images also showed how the defects form and progress during the laser sintering process. Allof these real-time monitoring methods have shown a great potential for on-line quality control duringlaser sintering of ceramics.

© 2014 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by

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. Introduction

The term “laser sintering”, when used in this article, is an empiri-al term, which includes solid state sintering, liquid phase sinteringr partial melting and melt-solidification processes. Laser sinterings belonging to the family of Laser Additive Manufacturing (LAM).his is a well-known technology for the fabrication of complexunctional parts [1]. By using a combination of computer aidedesign (CAD) and computer aided manufacture (CAM) bodies cane “printed” directly into their final shape. A controlled high-energy

aser beam is used to fuse particle granules directly into complexet-shaped 3D-components in a layer-by-layer manner. When the

aser beam spot scans over the particle granules, they consoli-ate either through sintering or melting/solidification reactions [2].ften the components include designed functional features that

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

∗ Corresponding author. Tel.: +46 8 162388; mobile: +46 70 8796488.E-mail address: shen@mmk.su.se (Z. Shen).

eer review under responsibility of The Ceramic Society of Japan and the Koreaneramic Society.

187-0764 © 2014 The Ceramic Society of Japan and the Korean Ceramic Society.roduction and hosting by Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jascer.2014.01.009

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Elsevier B.V. All rights reserved.

therwise are difficult or even impossible to fabricate by otherethods. In addition, this bottom-up approach allows the use of

ifferent laser scanning parameters in different parts of the con-olidated body to fit with different quality parameters [3]. Thus,verhanging structure parts are compensated for over-heating thatight induce deformation or curving [4]. Real-time monitoring of

he process has become important for detection of any unpredictedault and for continuous control of the interior quality.

Laser sintered plastic and metal parts, but no ceramic parts, haveeen produced in industrial scale. To prepare ceramic bodies effi-ient lasers are needed with carefully controlled parameters. Theigh melting point, low thermal conductivity and poor thermalhock resistance are examples of the intrinsic difficulties com-on for laser sintering of ceramics [5]. Alumina (Al2O3) is one

f the most widely used ceramics because of its good mechani-al strength, thermal and wear resistance properties, and the usef laser sintering for alumina is reported [6,7]. Laser-materialsnteractions, however, for different ceramics are much more com-

licated than for metals and are yet to be clearly understood inetail. Therefore, real-time monitoring of the laser process would

e important to control the more sensitive sintering of ceramics.easuring of temperatures and/or surface morphologies is used inetal laser sintering, but is less common for ceramics [8,9]. Real-

ime monitoring techniques are expected to become important

ature profiles and surface morphologies during laser sintering of16/j.jascer.2014.01.009

ools for ceramic laser sintering and the different monitoring tech-iques need to be evaluated with respect to their performances.

control technique and general and better understandings of theaser–ceramic interactions will be important for further progress.

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B. Qian et al. / Journal of Asian C

In this study, three different monitoring methods were exam-ned and compared during laser sintering of alumina ceramics. The

ain target is to evaluate real-time measurements by pyrometer,pectrometer and a surface imaging system. Their ability for con-rolling the laser process in production will be observed and tohat extent they might be used for a better understanding of the

undamental phenomena taking place.

.1. Laser sintering principles

.1.1. Standard parametersMost important parameters in laser sintering of ceramics are

sually laser power, focal spot size, scanning speed, hatch line pat-ern and scan distances. The total energy input Q (unit J/m) fromhe laser beam can be easily calculated as Eq. (1) shows.

= P

v(1)

here P is the laser power and v is the scanning speed [10].All these parameters can be set in the laser sintering program

nd kept constant or varied during the process. The complexity ofaser sintering of ceramics, however, demands a detailed controlbtained by real-time monitoring to set proper parameters and tovoid processing faults.

.1.2. The detecting elementIn principle, the detecting element can be installed either in a

o-axial or an off-axial manner [9,11,12]. An appropriate exampleor explaining the difference of these two modes is the tempera-ure monitoring. The co-axial set-up refers to the same scanningnit for material processing and monitoring, while off-axial set-upnly uses the laser beam without an additional scanning system.he advantage of a co-axial set-up is that the detector element islways focusing on the current process zone, but must considerhe restriction of the optical components. The semi-reflective mir-or could reflect almost all light from the laser wavelength and canransmit the melt pool radiation in other spectral ranges. The off-xial set-up does not have restrictions in system design, but thehermal radiation is captured from the entire heat-affected zone.hus, the obtained temperature would be lower than the maximumemperature within the focal spot, but the average temperature stilleflects the overall thermal state.

.1.3. Optical imagingThe co-axial imaging system records the real-time information

rom the melt pool and when coupled with a high-speed CMOSamera a monitor system will be achieved [13–15]. The melt pooleometry and evolution during the laser sintering process cane extracted from the 2D images. Simultaneous observation ofhe high and low temperature areas is difficult. It is possible toeduce the laser radiation intensity by a filter, but the capturedmages will then reflect the brightness contrasts and to enhancehe signals of cooler areas an additional diode illumination (LED) issed. The off-axial imaging system can be used for flaw detectiony identifying any deviations in the powder bed before the lasercanning and inspect the same consolidated surface morphologyfter laser sintering.

. Experiments

The powder material used was high purity �-alumina (99.99%

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

l2O3 from TAIMICRON, ultrafine grade TM-DAR) with a meanarticle size of 0.10 �m. An X-ray diffraction (XRD) analysis ofhe powder (XRD equipment from Seifert, Ahrensburg, Germany)roved a single-phase corundum structure with narrow and strong

alpt

s 2 mm. (b) An illumined surface where a 4 mm square of alumina powder is laserintered with a heat input of 400 J/m. The image was captured by a CCD-cameramaging system and the illumination came from a strong LED laser source.

eaks and no detected crystalline contaminants. Alumina powderas pressed into tablets by uniaxial dry-pressing at 50 MPa. Theensity of pressed pellet is 2.26 g/m3.

To evaluate the different real-time monitoring techniques, theop layer of these alumina tablets is consolidated. Consolidated alu-

ina pieces were cleaned in an ultrasonic bath with a mixturef alcohol and water and characterized with optical microscopyOM, Olympus SZX12) and scanning electron microscopy (SEM,EOL JSM-7000F).

All studies described in this paper were made by using anOSINT M 270 Selective Laser Sintering system from EOS GmbH.he laser was a 200 W continuous wave Nd:YAG fiber laser with

Gaussian intensity profile operating at a wavelength of 1070 nmnd the typical focal spot diameter is 70 �m. All the tests wereased on laser sintering of one alumina layer at a time of the pressedreen bodies. The scanning hatch line pattern used in all tests ischematically illustrated in Fig. 1. The hatch line distance was fixedt 0.05 mm and the stripes can width was 2 mm, where this scan-ing pattern was set to a 4 mm square surface. The XRD analysis of aat surface of laser sintered alumina after cooling showed that thenly crystalline phase was �-alumina, with an obvious texture oflong [1 1 3]. The sintered alumina had a brittle fracture and SEMnspection of fracture sections revealed that very few pores wasresent, indicating a fully dense alumina.

The active illumination imaging system (manufactured by Cavi-ar) consists of a control unit, lighting system, control software and

CCD-camera. The imaging system mounted to the laser sinteringquipment is shown in Fig. 2a–d. The optics and the camera arexed outside the laser sintering machine. To prevent the camerasCD-array to solarize/over exposure, an additional lighting is doney a strong 810 nm light source (diode laser, Cavilux HF) attachedo the laser unit via optical fiber. This laser has a light with muchigher intensity compared to the thermal radiation [16,17]. Theamera has an optical filter with transmission mainly to this wave-ength and will decrease the light emission of the laser spot. Thus,he image contrasts or relates to the absorption and reflection prop-

rties in this wavelength. The camera can expose up to 30 frameser second and from time-expended videos it is possible to detectapid phenomena.

A pyrometer (Temperature-Control-System, TCS) is calibrated

ature profiles and surface morphologies during laser sintering of16/j.jascer.2014.01.009

nd used for on-line temperature measurement and control of theaser process. The optical head of the pyrometer, attached to sup-ort, was mounted inside the chamber and it was adjustable fromhe outside. Optical fibers transmit data into the pyrometer and the

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ig. 2. The monitoring equipment, where (a and b) are the active illumination imocated inside the chamber. In (d) an image from the inside of the sintering chambe

adiation was divided into two measuring ranges 1200–400 nm and400–1700 nm. The chosen radiation ranges are optimal for study-

ng alumina (melting point 2072 ◦C) and relates to temperatureanges about 1400–1800/1800–2130 ◦C. For each recorded range ahoto-detector turns the radiation intensity into a photo-current,here the photo-current ratio is independent of the spectral emis-

ion coefficient at the work piece surface. This allows simpleemperature detection at different surfaces or of different mate-ials. The pyrometer uses a round-shaped detecting area that cane focused and the spectrometer (Ocean Optics HR2000+) uses aCD-array detector. The spectrometer can be used to record themission spectrum as intensity for each specific wavelength inter-al from 200 to 600 nm. It centers on the ultraviolet end of theisible light range (400–700 nm). Note the cut-off toward loweremperatures (600 nm) and that the measuring range is differentrom the pyrometer. The optical head of this spectrometer, attachedo a support, was mounted inside the chamber, see Fig. 2d.

. Results and discussion of monitoring methods

.1. Temperature by a pyrometer

An off-axial pyrometer set-up was used in the laser sinteringystem. The recordings cover the whole heat-affected zone andegister the highest average temperature when alumina powders consolidated. The observed variations of average temperature asunctions of the laser energy input (J/m), laser power (W) and scan-ing speeds (mm/s) are summarized in Fig. 3a–c, respectively. The

aser energy input, after adjusting for scanning speed, is expectedo give similar results. However, when scanning speed was keptonstant and laser power was varied, measured temperature isower than when laser power was kept constant and scanningpeed varied, see Fig. 3a. The temperature of the sintering processncreases rapidly, when energy input increases and reaches a level

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

t 1900–2000 ◦C at heat input values of 400 J/m or laser powersf 20 W at 50 mm/s scanning speed. Thereafter, the average tem-eratures are at the same level, even when energy input increases.he heat diffuses outward as the formed alumina melt equilibrates

rtnm

system installed outside the chamber; (c) is the measuring unit of the pyrometern the spectrometer was in use.

hermally with the surroundings. When the input energy increasehe melt will also penetrate deeper into the body. Scanning speeds a parameter of importance as it influences the added energy perime unit. Increase in scanning speed from 50 mm/s to 150 mm/sesulted in a decrease of 100 ◦C in average process temperature, aseen in Fig. 3c. The faster the laser beam moves, the less time is

vailable to interact with the alumina material.The average temperature is not increasing linearly with the laser

nergy input. The laser beam provides a large amount of concen-rated energy in a blink of time into a limited area (70 �m) of theowder bed, inducing melting of the starting powder material. Theelting of alumina powder consumes energy, but stabilizes on the

ther hand at ∼2070 ◦C in the melted volume. The melt pool alwaysiffuses heat into the surrounding area and this process is influ-nced by many factors. Important is the very short time available

nd the ceramics low thermal conductivity, but also grain contactsnd other physical factors might affect the heat flow. The measuredverage will become a merged result of the different thermal states.uring these short interaction times the heat-affected zone will beomplicated by the laser beam scanning in a zigzag way, where thehort laser and materials interaction time is partly compensatedy the following scanning track only 0.05 mm away, cf. Fig. 2a.he 2 mm jump between separated stripes and any changes incan direction might give the opposite effect with a local temper-ture fall. The interaction efficiencies between the laser beam andhe diffusion heated alumina material might also change by tem-erature as well as surface ability to reflect or adsorb the laser.he balance between all these variables results in strongly fluc-uating temperature at locally heated areas and an instability ofverage temperature. Temperature monitoring has been reportedlso to be affected by uncontrolled surface geometries, such asurving of materials that might result in local high temperatures4,14].

The measurements reported above by pyrometer give only a

ature profiles and surface morphologies during laser sintering of16/j.jascer.2014.01.009

ough estimation of the ongoing process for the materials interac-ion with the laser beam. It might be used for a first estimate whenew ceramics are introduced, but has less value for research or foronitoring a production process.

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ig. 3. The evolution of the average temperature of the hot area detected by an ofpeed in mm/s.

.2. Real-time thermal radiation profiles recorded by apectrometer and a pyrometer

The real-time off-axial spectra of the thermal radiation wereaptured at the same time by a spectrometer and a pyrometerhen a Nd:YAG fiber laser beam (1070 nm) interacts with alu-ina powder. The time-resolved spectra are captured from the

ntire heat-affected zone, but the strong surface laser-ceramicnteractions might dominate in the spectrometer records. The rela-ionships between the increase in heat input and the measuredmitted intensities and wavelengths by elapsed spectrometer timere summarized in Fig. 4a–d. The captured spectra are includingll the emitted radiation from the laser and material interactions.o narrow peak caused by photo- or thermoluminescence was

esolved in the broad emission. The emitted very short wavelengthseflect the laser-material interactions and cannot directly be con-eyed as alumina temperatures (∼6000–7000 ◦C). The figure alsoompares the results with a real-time pyrometer recording, sett longer recorded emission wavelengths, measuring the aluminaurface temperature.

Using the lowest heat input of 100 J/m, the spectrometer recordsielded few low and fluctuating emission intensities whereas theyrometer temperatures were below measuring range, reflectingn unsatisfying and unstable sintering situation. By increasing theeat inputs to 200 J/m, the measured electromagnetic emissionseen by the spectrometer and pyrometer give more consistentecords shown in Fig. 4a and b, respectively. The laser radia-ion interacts preferable in several parallel bands across the lasercanned alumina area, where the local temperature recordings byyrometry consequently show rapid local temperature intensifica-ions. Neither at 100 nor at 200 J/m, are the heat inputs enough tochieve a stable and acceptable sintering condition by the laser-aterial interaction at 1070 nm wavelength.At a heat input of 400 J/m the recording is more stable and the

hole scanned 4 mm squared area is heated to above 2000 ◦C, withne notable exception as a cooler band is running across the area,ee Fig. 4c and d. The pyrometer shows that the time width ofhis band is around 0.3 s. At 700 J/m the band with lower tem-eratures has become much narrower and at the highest input1000 J/m) thermal flow from surrounding heated areas has equal-zed temperatures. In fact, at 1000 J/m pyrometer curve stabilizest a temperature around the melting point of alumina (2072 ◦C).owever, note that a heat input of 1000 J/m saturates the recordedmission intensities and the heat is seen to diffuse well outside ofhe intended laser scanned area. This energy input is too high to be

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

ontrolled.From the results achieved, the changes of the emission spectrum

hape reflect the stability of the alumina laser sintering. The real-ime temperature variation captured by pyrometer is essentially

gtts

pyrometer versus (a) the heat input in J/m, (b) laser power in W and (c) scanning

onsistent with spectrometer observations. The spectrum andemperature profiles are also useful for detailed observation of thenstant local heating behavior. The very short temperature droptill needs an explanation. This is too large to be caused by theaser beam zigzagging within the hatch line block with 0.05 mmeparation, but the laser stop at the end and restart in a differentirection at another distant hatch line block. Thus, a hypotheticalource of the apparent 0.3 s descent in transferred energy mighte a result of the complicated hatch pattern, cf. Fig. 2a. With the

ncrease of the total energy input, the accumulated heat diffusesnd eliminates temperature differences. The laser hatch trackhould be modified to allow a uniform temperature within the

mm × 4 mm area.The real-time monitoring by both spectrometry and pyrometry

as shown to be efficient tools to track even short-time events dur-ng the fast laser scan. At higher energy inputs the recordings showess fluctuation and reflect a more stable sintering process. In theeat input interval 400–700 J/m the average process temperature

ndicates that an acceptable sintering condition will be found andther energy inputs within this region should be tried to find theptimal level.

.3. Real-time surface morphology by an active imaging system

An off-axial imaging system is used to record the real-time laserintering process. It has a laser lighting and the captured imagesill reflect the brightness and emissions from the heated area that

ould be affected by several factors. The appearing contrasts inhite/black images result from difference of light reflectivity from

he surface. The reflectivity might be influenced by illumination andetecting angle, the surface roughness (powder vs. consolidatedlumina) and the melt pool. In addition, the contrast is influencedy the difference in materials absorption ability in the illuminat-

ng laser wavelength (810 nm). The high absorption by aluminanduces an overall dark-gray contrast in the recorded images. Theigh temperature melt pool radiates a thermal signature and athe same time have strong absorption, similar to a “black body”,ecoming black. In this way, the optical imaging system grasps theeal-time recording of the high and low temperature zone at theame time, but the interpretation needs training. Typical imageshat are captured during laser sintering of alumina are shown inig. 5.

The imaging system illustrates the quality of consolidated lay-rs when the heat input increase by the same energies used in theection above, i.e. 200, 400, 700 and 1000 J/m, see Fig. 5a–c, d–f,

ature profiles and surface morphologies during laser sintering of16/j.jascer.2014.01.009

–i and j–l, respectively. Three images at each energy level followhe process after 1/3, 2/3 and full sintering time (∼6 s). Only par-ial melting of alumina powder along parallel bands crossing thecanned area can be seen at 200 J/m. This should be compared with

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Please cite this article in press as: B. Qian, et al., Monitoring of temperature profiles and surface morphologies during laser sintering ofalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.1016/j.jascer.2014.01.009

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Fig. 4. Comparison of spectrometer (left column) and pyrometer (right column) recordings of the laser-material interaction process when alumina was laser sintered.Different heat input levels were used and the real-time spectrometer and pyrometer profiles were measured, comprising the heat inputs of 200 J/m (a and b), 400 J/m(c and d), 700 J/m (e and f) and 1000 J/m (g and h). See details in text.

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ig. 5. Top view images captured by the active illumination imaging system when00 J/m, in (d–f) 400 J/m, in (g–i) 700 J/m and in (j–l) 1000 J/m. For each heat input l

he band structure of emitted light shown in Fig. 4a. There are moreeated bands at the first half of sintering then after the laser beam

ump to the next scan block, cf., Fig. 5a and c. The opposite ordereems to occur at 400 J/m and the circumstances ruling preferredaterial-laser interaction is unclear. Total melting of the alumina

rain layer was achieved at 400 J/m and resulted in a consolidatedoating kept within the scanned area, see Fig. 5f. The heat accu-ulating effect by time is indicated at 700 J/mw here the final melt

one seem to exceed the scanned area, see Fig. 5i. This is even moreronounced at the highest heat input 1000 J/m, see Fig. 5l. Too largenergy inputs in a restricted area might cause the melt pool pene-rating deeper into the alumina body. A lack of underneath supportan cause a slight concave look of the melt pool surface. In sum-ary, the level of energy input has a strong effect upon the apparent

uality of consolidated alumina.

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

The time-evolution of a melt pool is followed with a new imagevery 1/30 s in Fig. 6a–f. The image is exposed at the edge of theompacted alumina powder disc where the reflective edge is seens a curved white band. At the top dark-gray surface the laser

emlA

ina was consolidated with different heat input values. In (a–c) the heat input was sequence of three images was exposed after 1/3, 2/3 and full time.

rack has reached the edge leaving a trail of melted alumina that isooling and the laser beam is turning direction. Once the laser irra-iates upon the alumina powder body again a small dark melt poolorms, see yellow arrow in Fig. 6b. Following the time and the

ovement into the alumina disc, the melt pool can be followed,ee Fig. 6c–f. The diameter of the melt pool increases and it causeslso a deeper penetration into the material. By observing theseetails of the laser sintering process the best laser parameters cane set.

The imaging system is good for on-line control during laser sin-ering and it is also possible to follow how a critical defect mayorm. The formation of a defect, being an un-melted body, withinhe consolidated alumina can be followed, see Fig. 7a–d. Duringintering of alumina a tiny crack forms at the front of the movingaser spot, see yellow arrow in Fig. 7a. This crack at the powder

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dge of the melt pool creates a small fragment that moves into theelted area, see Fig. 7b. This fragment is only partially melted and

eaves a porous defect in the consolidated part, see Fig. 7c and d.nother example is the evolution of a thermally induced crack into

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Fig. 6. The melt pool is followed by the imaging system when alumina was laser sintered. The images are captured at the edge of a tablet of compacted alumina powder andt ed at

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he white contrast band is the edge. Both the side view and top view can be observime in (b)–(f). The laser spot follow a direction parallel a previous scan line, but in

ight give a concave surface.

he alumina green body from a low-angle contact point and therack continue to growth as the sintering proceeds, see Fig. 7d–f.hese examples illustrate that the imaging system can be used tonderstand the formation of defects.

Defects can also be found by optical microscopy (OM) andcanning electron microscopy (SEM) examinations of consolidatedlumina bodies, as exemplified in Fig. 8a–f. Too low heat inputs like00 J/m will leave volumes of poorly consolidated alumina powder

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

ithin the bodies, see Fig. 8a and d. A defect of this size woulde detrimental for the mechanical properties. It is well knownhat dense alumina ceramics are sensitive for thermal shock muchigher than about 400 ◦C and intense micro-cracking will occur

pcod

ig. 7. The time evolutions of two types of defects during laser sintering of alumina are frogress of cracks in (e–f), cf. the text.

the same time. Yellow arrows indicate the position of a melt pool that enlarges byite direction. Heat accumulation enlarges the melt pool also into the material that

aused by the internal stresses of a high linear thermal exponent. Anpparently well-consolidated body at 400 J/m will receive a greathermal shock at the rapid cooling and the stresses will find anyarge enough defects in the consolidated body, as illustrated byhe release of large cracked fragment, see Fig. 8b and e. In thenlarged SEM image the defect is easily found in the middle of arack mirror plane. Too high energy input (1000 J/m) will certainlyntroduce deformation, curved solidification lines, crack initiation

ature profiles and surface morphologies during laser sintering of16/j.jascer.2014.01.009

oints and internal stresses; see Fig. 8c and f. Curved surfaces afterooling of large melt areas are found and crack propagation isften perpendicular to these as it follows the temperature gradientirection.

ollowed by the imaging system. A poorly consolidated powder defect in (a–d) and

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Fig. 8. OM micrographs (a–c) and SEM images (d–

We observed frequently sever thermal cracking in the sinteredlumina ceramics formed in this article to demonstrate the use ofonitoring equipments. It was not surprising that the great tem-

erature shock during cooling of alumina caused problems. It isell-known that alumina is prone to crack by thermal shock and

t has been shown that a pre-heating/controlled cooling of con-olidated alumina is necessary by Hagedorn et al. [7] The use ofonitoring will be a useful tool to control temperature rates in

uture fabrication of oxide ceramic bodies.

.4. Comparison of monitoring techniques

All the monitoring devices used in this study are off-axialnstalled, as the installation is easier compared with a co-axialet-up. The off-axial set-up measures the average value from theeasuring zone, but the results presented show that it is sensi-

ive enough in real-time mode to record even small activities. Onexception is the measurement of the average temperature by ayrometer that has limited practice.

All the studied real-time techniques are useful in monitoring theaser sintering process. The temperature variation is the most cru-ial parameter when controlling laser sintering of alumina. Thus,he real nature of optimization laser scanning parameters is toontrol the temperature variation and heat distribution behavioror each ceramic. The relationship between the heat input and theemperature profile captured by the spectrometer and pyrometers important for controlling the laser sintering process. The stabilityf the laser sintering process can be judged. It will help to set thearameters for an optimal laser sintering process. These techniquesill be of value for other ceramic materials as well.

The active imaging system is more useful in defect detection byirect observation of the image. The resolution is limited by theCD camera, however, and the contrast is affected by the differ-nce in materials absorption ability in the illuminating laser. Thehort response times of the pyrometer or spectrometer are goodomplements to the imaging system.

For ceramic materials, rapid temperature fluctuations and theiragnitude can be detrimental for the subsequent engineering

Please cite this article in press as: B. Qian, et al., Monitoring of temperalumina ceramics, J. Asian Ceram. Soc. (2014), http://dx.doi.org/10.10

roperties in application. Occurrence of residual stresses or cracknitiation points must be avoided. The real-time spectrometer andyrometer may be used for optimizing both the stability duringhe laser sintering process and for obtaining an alumina ceramic

ifferent defects in consolidated alumina, cf. text.

ith a minimum of thermally induced defects. However, the alu-ina should not be allowed to be thermally shocked and a thermal

ontrol is recommended both for pre-heating and, especially, forooling.

. Conclusions

This study examines possibility of real-time monitoring tech-ique attached to a laser sintering system. During laser sinteringf alumina, different monitoring equipments have been installedff-axial and tested. The pyrometer installed inside the laser sin-ering chamber has measured the average temperature at differentaser parameters and can be used for a first setting. The real-timepectrometer and pyrometer could be used for rapid monitor-ng of the thermal stability during the laser sintering process. Anctive illumination imaging system has successfully recorded theigh temperature melt pool and surrounding area simultaneously.he captured images also have shown how the defects form androgress during the laser sintering process. All these real-timeonitoring methods have shown a great potential for on-line qual-

ty control during laser sintering of ceramics.

cknowledgements

The work was supported by the Swedish Research Coun-il through grant 621-2008-5730 and InnoBusiness-ArvoBusinessroject consortium (TEKES). Support from the Knut and Alice Wal-

enberg foundation for the purchase of the laser sintering facilitysed in this study is gratefully acknowledged. Authors would alsoindly thank EOS Finland and Esmaeilzadeh Saeid for all help duringhis study. We thank professor Thommy Ekström for his valuableomments and language revision.

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