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[Hiroshi Honda](https://orcid.org/0000-0003-0011-9063), [Makoto Watanabe](https://orcid.org/0000-0002-5064-9583)

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[Quantifying Laser Absorptivity of Ti–6Al–4V Powder through Additive Manufacturing Systems](https://mdr.nims.go.jp/datasets/0491b4a4-5626-4cf2-89d7-47d9ac41dc84)

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Quantifying Laser Absorptivity of Ti–6Al–4V Powder through Additive Manufacturing SystemsQuantifying Laser Absorptivity of Ti–6Al–4V Powder through AdditiveManufacturing SystemsHiroshi Honda+ and Makoto WatanabeResearch Center for Structural Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, JapanIn laser metal-based powder-bed fusion additive manufacturing, it is important to know the laser absorptivity of metal powder to elucidateor optimize the manufacturing process and numerical simulation. Laser absorptivity depends on the manufacturing process conditions and thecircumstances of the additive manufacturing machine. Therefore, we tried to develop a simple method of measuring laser absorptivity under thesame circumstances present in the manufacturing process by using a commercially available additive manufacturing machine.[doi:10.2320/matertrans.MT-M2023156](Received September 28, 2023; Accepted October 31, 2023; Published January 25, 2024)Keywords: additive manufacturing, power-bed fusion, Ti–6Al–4V powder, laser absorptivity1. IntroductionIn laser powder-bed fusion additive manufacturing,powder’s absorption of laser energy represents an initialphenomenon with far-reaching effects on subsequent process-es and the properties of the resulting product. Therefore, theability to measure laser absorptivity is necessary for theelucidation or optimization of the manufacturing processes.This ability is also indispensable for numerical simulations ofadditive manufacturing.However, the number of materials with known absorptivityis not high, and the light wavelengths of absorptivity arelimited. Furthermore, absorptivity is generally measured forideal flat surfaces. The materials used in laser power-bedfusion additive manufacturing are spherical, and the surfaceconditions are usually not ideal. Therefore, laser absorptivityis thought to change depending on the surface conditions ofthe powder used. The temperature dependence of laserabsorptivity is also important in order to understand themanufacturing process. However, there are only a fewmaterials for which the temperature dependence of laserabsorptivity is known.One method of measuring light absorptivity involves theuse of an integrating sphere. Brandau et al.1) measuredabsorptivity across a wide range of light wavelengths forvarious metal powders. However, the measurement circum-stances differed from those of additive manufacturing, andthe temperature dependence of absorptivity was notmeasured.Calorimetric methods have also been used to measure laserabsorptivity. Wieting et al.2) measured the laser absorptivityof stainless steel sheets for a CO2 laser by analyzing theheating rate during laser irradiation, followed by the coolingrate post-irradiation, within a vacuum furnace. They alsoinvestigated the temperature dependence of laser absorptivityby varying the furnace temperature. Haag et al.3) measuredthe laser absorptivity of powder when exposed to a CO2 laserby considering factors such as laser irradiation time,temperature elevation of the irradiated sample, and sub-sequent cooling rate. These measurements were conductedin different atmospheres—air, argon, and helium—usingspecialized equipment designed for this purpose. However,they did not measure the temperature dependence of the laserabsorptivity. Rubenchik et al.4,5) investigated the temper-ature-dependent laser absorptivity of both plate and powderforms of various materials. They employed a dedicatedmeasurement system that utilized a laser diode, eliminatingthe need for a furnace setup. However, the laser wavelengthand the measurement circumstances in the measurementsystem may differ from those in the additive manufacturingmachine to be used. In such cases, it is necessary to considersuch differences in order to understand the manufacturingprocess.Trapp et al.6) made a dedicated device that imitated laserpowder-bed fusion additive manufacturing, and used it tomeasure laser absorptivity based on the temperatureevolution of the specimen. The conditions closely mimickedadditive manufacturing conditions, encompassing scenariosthat lead to the formation of keyholes. However, themeasured laser absorptivity encompasses the entire spectrumof phenomena occurring during the additive manufacturingprocess, and the precise temperature dependence of laserabsorptivity remains uncertain. Moreover, the laser and themeasurement circumstances of the dedicated equipmentmay differ from those of the additive manufacturing machineto be used.In real situations, the laser, the powder, and themanufacturing circumstance change depending on theadditive manufacturing machine and the manufacturingprocess. The laser absorptivity will also change, and it isnecessary to know the laser absorptivity in each situation inorder to realize the desired product. Therefore, we tried tomeasure the laser absorptivity of metal powder by usinga commercially available additive manufacturing machineunder the same circumstances present in the manufacturingprocess.2. Experimental ProcedureLaser absorptivity in this experiment was measured by acalorimetric method similar to that employed by Rubenchiket al.4,5) In the method we used, the temperature of a+Corresponding author, E-mail: HONDA.Hiroshi@nims.go.jpMaterials Transactions, Vol. 65, No. 2 (2024) pp. 194 to 198©2024 The Japan Institute of Metals and Materialshttps://doi.org/10.2320/matertrans.MT-M2023156specimen during laser irradiation was measured and the laserabsorptivity was calculated from energy conservation duringthe temperature variation. The laser irradiation conditionswere chosen such that they could be used by a commerciallyavailable additive manufacturing machine.In the method used by Rubenchik et al.,5) the powder ona tray was irradiated uniformly and continuously by a laserlight. In our method, the laser light scans the powder arearepeatedly as shown in Fig. 1, under the same circumstancesof the manufacturing process that would be used for anadditive manufacturing machine. The laser absorptivity wascalculated by using the energy conservation in each laserlight scan of the powder area.The laser absorptivity can be calculated by eq. (1), whichrepresents the power conservation:AðT ÞP ¼ ðm1c1ðT Þ þm2c2ðT ÞÞdTdtþ LðT Þ: ð1ÞHere, P is laser power, T is temperature, and A(T ) is laserabsorptivity at temperature T. m1 and m2 are the masses of thepowder and tray, respectively. c1(T ) and c2(T ) are the specificheats of the powder and tray, respectively. t is time and L(T )is the thermal loss to the surroundings.The thermal loss depends on the measurement environ-ment of the additive manufacturing machine. When the laserpower is zero, namely P = 0, eq. (1) becomes as follows:LðT Þ ¼ �ðm1c1ðT Þ þm2c2ðT ÞÞdTdt: ð2ÞThe thermal loss in the measurement environment can beevaluated from the specific heats and cooling rate attemperature T. In this experiment, the thermal loss wasestimated from the temperature history of the cooling processafter the laser heating under the same circumstances as in theheating process.Equation (1) can be differentiated by using the timeinterval of laser scanning of the powder area ¦t and solvedfor A(T ) as follows:AðT Þ ¼ ðm1c1ðT Þ þm2c2ðT ÞÞ�T þ LðT Þ�tE: ð3ÞHere, E and ¦T are the irradiated energy and the temperaturerise during the time interval ¦t, respectively. The laserabsorptivity in this experiment was calculated by eq. (3).The laser powder-bed fusion additive manufacturingmachine used in this experiment was the SLM SolutionsSLM280. The laser wavelength was 1070 nm. The processchamber was filled with argon gas. The specimen as shownin Fig. 1 was set in the process chamber, whose environmentwas the same as that of the manufacturing process of themachine.The material of the metal tray and powder was titaniumalloy Ti–6Al–4V. The tray size was 10mm © 10mm ©1mm. The top surface of the tray had a machined recessedflat portion with 0.10mm depth. The metal powder wasevenly placed on the recessed portion to the height of the rim.A commercially available powder, CONCEPT LASER CL41TI ELI, was used. The mass of the tray was 0.4083 g,and the mass of the tray with the powder was 0.4242 g. Thetemperature was measured by using a thermocouple attachedto the center of the bottom surface of the tray.The laser beam was Gaussian and scanned the powder areain a meander hatch pattern as shown in Fig. 1. The scanningprocess was replicated 10 times, with 1 s intervals betweenscans. The incident angle of the laser beam on the specimenwas almost normal, as shown in Fig. 1. To avoid the meltingof the metal powder, the spot size of the laser beam on thespecimen was expanded and the scan speed was set to high.The spot size and scan speed were estimated to be about2.2mm and 16m/s, respectively. There were 161 hatch linesin the powder area. The laser power was changed in theorder of 50W, 60W, and 70W without exchanging the metalpowder in this experiment.Moreover, the laser absorptivity of a machined flat surfaceof the titanium alloy Ti–6Al–4V was measured by irradiatingthe metal tray without the metal powder under the sameexperimental conditions.For the calculation of laser absorptivity in eq. (3), thetemperature-dependent specific heat was incorporated,utilizing a fitting formula derived from experimental datafor the titanium alloy Ti–6Al–4V.7)3. Results and DiscussionFigure 2 illustrates the recorded temperature history of thespecimen. It was measured for the tray without powder at70W laser power. The temperature history shows a steplikepattern aligned with the 1 s time interval between scans.For the calculation of laser absorptivity using the valuesof ¦T and ¦t in eq. (3), the differences between the localmaximums of adjacent steps were employed, as depicted inthe inset of Fig. 2. The differences between the localminimums of adjacent steps were also utilized. As seenfrom the temperature history magnified in the inset in Fig. 2,the cooling curve gradient changed largely around 10.5 sfollowing the peak temperature in this experiment.To investigate the change in the cooling curve gradient, asimplified numerical simulation was performed by using anLaser beamThermocoupleTrayPowderBeam scanningFig. 1 Schematic of the experimental setup.Quantifying Laser Absorptivity of Ti–6Al–4V Powder through Additive Manufacturing Systems 195FEM calculation through commercially available software,Wolfram Mathematica. A 2D model was adopted byassuming uniformity in the hatch line direction due to thevery high scan speed. For simplification, the simulationregion was set to be a rectangle 10mm wide and 1mm high(Fig. 3). The initial temperature of the simulation was 40°C,and the boundary condition of the calculation region wasno thermal loss. On the top surface of the region, the lineheat source, whose intensity profile corresponded to the laserirradiation intensity, moved with speed u from left to right.The intensity profile of the line heat source S(x, t) was set tobe a Gaussian shape as follows:Sðx; tÞ ¼ ¡PDN¸1¸ffiffiffiffiffiffiffiffi2³r2re�2ðx�x0�uðt�t0 Þr Þ2 : ð4ÞHere, α is laser absorptivity, P is laser power, and D is thedepth of the specimen in the direction of the hatch line. N isthe number of hatch lines, τ1 is the time to scan one hatchline, τ is the time to finish scanning all of the hatch lines, andr is the laser spot radius. u is the mean moving speed in theperpendicular direction x to the hatch line during scanning ofthe hatch lines. x0 and t0 are the start position and the starttime of scanning, respectively. In the numerical simulation,the laser power and the laser spot diameter were set to 70Wand 2.2mm, respectively. The material for the numericalsimulation was titanium alloy Ti–6Al–4V. The physicalproperty values were held as constants, adopting the valuesspecified in the fitting formula of the experimental data7) at40°C. The laser absorptivity was assumed to be 0.4.Figure 4 shows the temperature history at the center of thebottom surface of the calculation region as the result of thenumerical simulation for one complete scan of all of thehatch lines. The start time of scanning was set to be 0. Thetemperature rose as the heat source went through the center ofthe top surface. After that, the temperature fell, approachinga constant value over time the temperature distribution ofthe calculation region became uniform. The time to reach analmost uniform temperature distribution was about 3 s.Considering the temperature history of the simulation, thechange in the cooling curve gradient is thought to be causedby the uniformization of the temperature distribution in thespecimen. Therefore, the initial part of the cooling process,which is influenced by the uniformization of the temperaturedistribution, was eliminated in the estimation of thermal lossfrom the cooling process. The thermal loss L(T ) wasestimated from the temperature history from 3 s after thetemperature was maximized.The measured temperature history after 3 s in the coolingprocess was smoothed and differentiated to estimate thecooling rate in eq. (2). The estimated cooling rate dT/dt isplotted against temperature Twith a fitting function in Fig. 5.The fitting function of the estimated cooling rate was used toestimate L(T ) while accounting for the temperature-depend-ent specific heat.In addition, it is thought that the temperature measuredearly in the heating process was also influenced by theuniformization of temperature distribution. To estimate theinfluence of this uniformization on the measured temperaturehistory, a 2D numerical simulation was performed. Thesimulation conditions remained consistent with those describ-ed above, except for the number of heat source movements:there were 10 movements at 1 s intervals. The simulationresult is shown in Fig. 6. In spite the absence of thermal lossin the simulation, the step heights of the temperature historyearly in the process are higher than they are later. In thenumerical simulation with one movement of the heat source,as shown in Fig. 4, it takes about 3 s for the temperature tobe almost constant by uniformization of the temperaturedistribution. In the numerical simulation with 10 movements0 20 40 60 80 10030405060708090100110120130Temperature, T/°CTime, t/s6 8 10 12 148090100110120130Temperature, T/°CTime, t/stTFig. 2 Temperature variation of only the tray irradiated at 70W laserpower.10 mm1 mmHeat sourceuFig. 3 Schematic of the numerical simulation model.0 1 2 3 4 5 640424446485052Temperature, T/°CTime, t/sFig. 4 Temperature variation at the center of the bottom surface in thenumerical simulation with one movement of the heat source.H. Honda and M. Watanabe196of the heat source, it also seems that the higher temperaturesteps early in the process, i.e., about 3 s, are caused by thenonuniformity of temperature distribution. Of course, thetemperatures measured after about 3 s are also influenced bythe nonuniformity of temperature distribution. Nevertheless,the temperatures recorded approximately 3 s into the heatingprocess encompass the impact of temperature distributionnonuniformity at the same level. Consequently, it is plausiblethat the effect on the temperature difference ¦T could bemitigated through cancellation.To confirm the influence of the nonuniformity of thetemperature distribution on the temperature difference ¦T,the temperature difference between adjacent maximums orminimums from in the numerical simulation shown in Fig. 6are plotted in Fig. 7. The time of the plot is the intermediatevalue of times of the adjacent maximums or minimums. Asseen from Fig. 7, the early temperature differences are largerthan the latter differences, even with the same input energy.The temperature differences become almost constant afterabout 3 s. Therefore, laser absorptivity was calculated for themeasured temperature history from about 3 s after the heatingwas started.The measured laser absorptivity data are shown in Fig. 8.The laser absorptivity measured for the machined surfaceof the tray is labeled Plate. The laser absorptivity measuredfor the powder bedded in the tray is labeled Powder. Thetemperatures are intermediate values of temperatures of theadjacent maximums or minimums for ¦T calculation, andthe range of error bars corresponds to the maximums orminimums. The temperatures used to calculate laserabsorptivity are intermediate values of temperatures of theadjacent maximums or minimums for ¦T calculation, and theranges of error bars of laser absorptivity are calculated fromthe temperatures of the adjacent maximums or minimums.The laser absorptivity of the powder was about 0.6, and thatof the machined flat surface was about 0.4 for a normalincidence of laser light. The increase in laser absorptivityfrom the flat surface to the powder is likely attributable to thephenomenon of laser light undergoing multiple reflectionswithin the powder. Although the measurement temperaturerange was narrow, no significant temperature dependence oflaser absorptivity was seen.40 60 80 100−3−2−10 Experiment Fitting curveCooling rate, dT/dt/°Cs-1Temperature, T/°CFig. 5 Temperature dependence of cooling rate in the cooling process andits fitting curve.0 5 10 15 2030405060708090100110120130Temperature, T/°CTime, t/sFig. 6 Temperature variation at the center of the bottom surface in thenumerical simulation with 10 movements of the heat source.0 2 4 6 8 100246810 Local maximum Local minimumTemperature difference, T/°CTime, t/sFig. 7 Temperature difference of the adjacent temperature maximums andminimums in the heating process in Fig. 5.0 50 100 1500.00.20.40.60.81.0 Plate, 50 W Plate, 60 W Plate, 70 W Powder, 50 W Powder, 60 W Powder, 70 WAbsorptivityTemperature, T/°CFig. 8 Measured laser absorptivities of the tray and the powder.Quantifying Laser Absorptivity of Ti–6Al–4V Powder through Additive Manufacturing Systems 197In the experiments by Rubenchik et al., the laserabsorptivity of titanium alloy Ti–6Al–4V powder was about0.7 for the laser wavelength of 970 nm,5) and that for theflat surface of the same alloy was about 0.5 for the laserwavelength of 780 nm.4) In the experiments using anintegrating sphere by Brandau et al., the absorptivity oftitanium alloy Ti–6Al–4V powder was 0.7633 for the lightwavelength of 1070 nm.1) The laser absorptivity valuesmeasured in our experiments were slightly lower than those.However, the increase in laser absorptivity from the flatsurface to powder was close to the ray-tracing simulationresult by Boley et al.8)Due to the use of a thermocouple, which couldn’t bepositioned precisely at the laser irradiation site, there existeda disparity between the measured temperature and the actualtemperature at the laser irradiation position. The measuredtemperature is thought to be less than the temperature at thelaser irradiation position.To check the influence of the measurement position, thetemperature distribution was investigated by using thenumerical simulation described above. The simulationconditions were the same as described above. Figure 9shows the temperature distributions of the top and bottomsurfaces and the laser intensity profile on the first movementof the laser from left to right. The temperature distributionon the top surface follows the laser intensity profile withalmost the same shape immediately. On the other hand, thetemperature distribution on the bottom surface experiences adelay due to the time taken for heat propagation from thetop surface. The temperature on the top surface in the laserirradiation region is higher than that on the bottom surfaceand varies in the irradiation region, typically ranging a fewtens of degrees Celsius higher. Therefore, the temperature oflaser absorptivity shown in Fig. 8 is supposed to be a fewtens of degrees Celsius lower than the laser irradiationarea. To obtain more accurate results of the temperaturedependence of laser absorptivity, it is necessary to reduce theuncertainty of temperature measurement.In the actual additive manufacturing process, the temper-ature becomes above the melting point. However, themeasured laser absorptivities were at the temperatures below150°C. Expansion of the measurement temperature rangeto higher temperatures is needed in order to study laserabsorption behavior at high temperatures as a future work.4. ConclusionTo assess laser absorptivity in the context of laser powder-bed fusion additive manufacturing, we endeavored tomeasure laser absorptivity under conditions mirroring thoseof the actual manufacturing process. We achieved this byutilizing a commercially available additive manufacturingmachine.(1) We have succeeded in measuring the laser absorptivityof titanium alloy Ti–6Al–4V powder and the tem-perature dependence below 150°C by using a com-mercially available additive manufacturing machineunder the same circumstances as in the manufacturingprocess.(2) For titanium alloy Ti–6Al–4V powder used in additivemanufacturing, the measured laser absorptivity wasabout 0.6 at a laser wavelength 1070 nm.(3) For a machined surface of titanium alloy Ti–6Al–4V,the measured laser absorptivity was about 0.4 whenexposed to laser light under normal incidence.AcknowledgmentsWe would like to thank Mr. Masaru Suzuki for theexperiments and Mr. Mitsugu Sato for preparation of theexperimental devices.REFERENCES1) B. Brandau, A. Da Silva, C. Wilsnack, F. Brueckner and A.F.H. Kaplan:Mater. Des. 216 (2022) 110591.2) T.J. Wieting and J.L. DeRosa: J. Appl. Phys. 50 (1979) 1071–1078.3) M. Haag, H. Hügel, C.E. Albright and S. Ramasamy: J. Appl. Phys. 79(1996) 3835–3841.4) A.M. Rubenchik, S.S.Q. Wu, V.K. Kanz, M.M. LeBlanc, W.H.Lowdermilk, M.D. Rotter and J.R. Stanley: Opt. Eng. 53 (2014) 122506.5) A. Rubenchik, S. Wu, S. Mitchell, I. Golosker, M. LeBlanc and N.Peterson: Appl. Opt. 54 (2015) 7230–7233.6) J. Trapp, A.M. Rubenchik, G. Guss and M.J. Matthews: Appl. Mater.Today. 9 (2017) 341–349.7) S. Sinha: J. Laser Appl. 31 (2019) 032008.8) C.D. Boley, S.C. Mitchell, A.M. Rubenchik and S.S.Q. Wu: Appl. Opt.55 (2016) 6496–6500.−5 −4 −3 −2 −1 0 1 2 3 4 520406080100Temperature, T/°Cx/mm Top surface Bottom surface0510 Laser intensityLaser intensity, I/arb. unitFig. 9 Temperature distributions on the top and bottom surfaces and thelaser intensity profile on the first movement of the laser from left to rightin the numerical simulation.H. Honda and M. 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