Thermal Experiments in Direct Metal Laser Sintering

Transcription

1 Thermal Exeriments in Direct Metal Laser Sintering Mr. C. Martin Taylor, rof. Thomas H.C. Childs School of Mechanical Engineering, University of Leeds, Leeds LS 9JT, UK Tel. : +44 () ABSTRACT This aer discusses recent work undertaken to quantify energy absortance and material conductivity during the laser rocessing of a owder bed. A carbon dioxide laser was used in exeriments, directly sintering single lines and layers on the surface of a stainless steel owder bed. Thermocoules ositioned under the bed s surface measured temerature changes with time. Using adated equations, exerimental quantities were combined to calculate the roortion of incident laser energy absorbed and held by the bed. Resonse to different situations was tested, such as use of an inert atmoshere and change of scanning conditions. When scanning layers at various values of scan sacing, it was discovered that this change in scan sacing led to a change in energy absortance. Keywords: Direct laser rocessing, heat transfer, absortance, metal owders, carbon dioxide laser, inert atmoshere 1. INTRODUCTION 1.1 Heat transfer in selective laser sintering In the case of the raid rototying rocess selective laser sintering (SLS), a laser beam is directed by otical means to scan the surface of a bed comrising material in owder form. Where the beam makes contact with the owder material, the owder absorbs laser energy and the local temerature increases. This temerature increase causes melting and sintering of the surface owder. Adjacent owder articles bond together to form a solid structure, which layer by layer is built u to create solid three-dimensional arts. Energy efficiency is of interest in such a rocess. Heat is transferred to the surroundings at various oints in the laser system: for instance, a small ercentage of energy is lost in the beam exanding and focussing otics. At the surface of the owder bed a roortion of the incident laser energy is absorbed by the owder, and the remainder is reflected away or lost through other modes. Kizaki (1) and Tolochko () used otical (electromagnetic wave) sensors to measure the laser radiation reflected by the owder bed: the remaining radiation would be that absorbed. The roortion of absorbed to incident radiation is known as the absortance. The tye of laser being used affects absortance (examles are Nd:Yag and CO), as does the tye of material rocessed (whether olymer, metal or ceramic). After radiation absortion has occurred, a ercentage of absorbed energy is then lost from the owder bed surface to its surroundings over time. The relative significance of this loss is not known, but it has been suggested () that if rocessing ower is too high, surface material can vaorize, thus removing heat by mass transfer. Aside from radiation methods to measure absortance there are calorimetric methods, i.e. measuring the increase in a body s temerature and relating this to the energy the body has absorbed. Sih and Barlow (3) used temerature measurements to estimate the absortance of a constant, low energy density laser source. 1. Scoe of reorted exeriments Calorimetric exeriments were carried out in a situation reresentative of that occurring in selective laser sintering. Heat absortance and conduction were measured during the sintering of lines and layers of stainless steel owder. As an extension to the study, the roerties of a commercial olymer-binder steel owder were also tested.. THEORY.1 A method for measuring the effective conductivity of a material owder bed The method used takes as its starting oint Carslaw and Jaeger s equation for an instantaneous thin line heat source of infinite length inside an infinite isotroic body (4). The body is at constant temerature before heat alication.

2 In SLS the heat source is alied at the owder bed surface, so Carslaw and Jaeger s equation has been adated. See Figure.1 for a visualisation of the geometry described. A single line scan is being considered initially. The infinite body can be slit in half, to exist where x. No heat can leave the body, so the heat emitted is equivalent to that absorbed in SLS. If the infinite line runs along the z-axis on the surface of what is now a semi-infinite body, twice the amount of heat will be sulied to the remaining half of the body. Under such conditions, a temerature rise dt will be seen at ositions directly below the line source (y): dt x t Q 4. κ. e. π. κ. t (1) Infinitely long thin line on z axis Surface of semiinfinite body: x where x is deth and Q is the magnitude of the heat emission, which occurs at time t. κ is the body s diffusivity, equal to k/ρ.c, or thermal conductivity divided by density and secific heat. Differentiating the above, then equating the differential to zero, the time to reach maximum temerature at a secific deth, t T MAX, is introduced: t T MAX x 1 4.κ () Figure.1 Visualisation of system reresenting the SLS rocess If a lot is made of t T MAX versus x at various values of x, the gradient of the best fit line will be equal to given owder bed density and secific heat, the effective conductivity of the bed can be calculated.. A method for measuring the absortion of heat into a owder bed from a scanning laser In SLS, the absorbed ower is the roduct of inut laser ower and owder absortance, α: A O. α (3) z x y Body exists where x ρ. c 4k. For a The source strength Q in equation (1) is the temerature rise er unit volume of material caused by the heat transferred. Energy inut er unit length Q. ρ.c. For a laser source, moving with seed U:. α U (4) Q. ρ. c Substituting t for t T MAX, equation (), and Q, equation (4), into (1), T MAX.34. α. U. ρ. c. x If a lot is made of T MAX versus α. e. π ρ.c U. x at various values of, U and x, the gradient of the best fit line will be equal to. Knowing the owder bed density and secific heat, the absortance of the bed can be calculated..3 Use of develoed theory in the following work Some examles of the two tyes of lot discussed in Sections.1 and. are generated in Section 4. These form the basis for calculating values of conductivity and absortance, reorted in Section 4. Similar rocedures are followed for the case of raidly scanning single layers. 3. EXERIMENTS 3.1 Equiment and materials owder bed temeratures were measured with the aaratus in Figure 3.1. Five tye K thermocoule wires were used, functional u to 13 C. To electrically insulate the wires, ceramic tubes of mm diameter were used. Exeriments were carried out in a sealed chamber. A window in the chamber ceiling allowed the laser beam to enter. A removable viewing window allowed observation of the rocess, and access between exeriments. A um and argon bottle were attached to the chamber so that the air inside could be relaced with an inert atmoshere for sintering. No owder reheating was rovided. The rocessing laser was of CW carbon dioxide tye, wavelength 1.6µm, beam diameter 1.1mm. The laser sintered the (5)

3 material directly. In all but one case reorted, the material used was stainless steel 314S owder, size distribution 75-15µm and comosition: Element Fe Ni Cr C Si Mn S ercent Bal The excetion is where DTM Raid Steel owder was tested, redominantly stainless steel 316 with aroximately 3 ercent olymer binder content. Section A-A 5 1 Removable utty 3. rocedure The tray shown in Figure 3.1 was filled with owder, and the surface levelled with a ruler. The laser was guided to scan single lines or layers on the surface of the bed, directly over the thermocoule ositions. Using Labview software and an instrument board, temerature data was logged from the thermocoules at.6s intervals. Curves of heating and cooling at a osition were collected over several minutes after laser alication. Between exeriments, the owder bed was emtied and fresh owder was added. Readings were taken over a range of bed deth (1-11mm), ower (1-16W), scan seed and atmosheres. To View 14 A 17 Unused NC holes Insulated thermocoule wires A In the first situation reorted, single 6mm long lines were scanned in an air atmoshere. For the second situation, the atmoshere was relaced by argon and again a 6mm line was scanned. After the owder bed had cooled, the same osition was scanned again, over the reviously rocessed material. This rescanning was done 4 times. Figure 3.1 Temerature measurement aaratus. All dimensions in mm Aluminium owder tray For the third situation, single layers 4xmm in area were created at constant ower, but varying scan sacing inversely to scan seed so that the total time taken to scan the layer was constant. This occurred in argon. In the fourth situation, single 6mm long lines were scanned in an argon atmoshere. A different material was tested: Raid Steel. 4. RESULTS 4.1 Data rocessing For each exerimental situation tested, over 15 readings were taken. Quantities were lotted on grahs, and gradients used to evaluate effective conductivity (as Section.1) and absortance (as Section.). Figure 4.1 shows an examle of the lot described in Section.1. Figure 4. shows an examle of the lot described in Section.. According to the theory develoed, both lots should yield straight lines Linear (Exerimental oints) (Measurement Deth)^ (mm^) Exerimental oints Figure 4.1 Finding conductivity: Raid Steel in argon Linear (Exerimental oints) (/U)/(Deth^) Exerimental oints Figure 4. Finding absortance: stainless steel in air

4 4. resentation of results These are best summarised in tabular form. α absortance, k conductivity. Situation No. Sintered Geometry Atmoshere Notes α (.-1.) k (W/m.K) 1 6mm single line Air α examle, Fig mm re-scanned line Argon See Figure xmm single layer Argon See Figure mm single line Argon DTM Raid Steel. k examle, Fig Table 1 Exerimental results 7 6 Re-scan 3 Re-scan Re-scan 1 4 Original line Original line 3 Re-scan 1 Re-scan Re-scan 3 Re-scan 4 1 Re-scan (/U)/(Deth^) Figure 4.3 The change in absortance with number of re-scans: stainless steel in argon Absortance Sread of oints Ratio, Scan Sacing/ Beam Diameter Figure 4.4 Absortance versus scan sacing: single layer scans, changing scan sacing and seed- st. steel, argon 5. DISCUSSION 5.1 Absortance results Examining the absortance results resented (Table 1) it can be seen that all values lie below.5. In the closest comarable situation found, Tolochko () reorted an absortance of.45 for iron owder rocessed with a C laser in inert gas. However, data reference books demonstrate how sensitive absortance (often quoted as emissivity) can be to such factors as material constituents, geometry and surface finish. In the exeriments resented here, the quantity measured is heat retained by the owder bed; further losses are accounted for comared to radiation-tye measurements, i.e. heat convection and radiation from the bed surface. These may be significant in the seconds after laser contact. Figure 4.3 dislays the result of re-scanning a line (situation in Sections 3 and 4). The gradient of each best fit line is roortional to absortance. With each successive scan, the measured absortance reduces from an initial.13 down to.5 finally. This is believed to be due to the material surface condition: each ass of the laser causes further bonding and melting. The surface becomes more smooth and shiny, i.e. reflective. The same theory can be alied to what occurs on the left-hand side of Figure 4.4 for single layer scanning (situation 3). The larger the scan sacing, the higher the absortance. This is because a higher roortion of unrocessed owder is being scanned, which has a higher absortance than the reviously rocessed material. Towards the right-hand side of Figure 4.4, it is believed this rule breaks down, because the scanning seed is being reduced in roortion to the increase in scan sacing. Below a certain scanning seed, the high energy density causes fast melting of the steel owder, making it more reflective. In situation 4, the resulting absortance of the Raid Steel is lower than for the stainless steel owder. A ossible exlanation is the small ercentage of olymer binder contained. The energy density used in rocessing the RS owder was reduced to a minimum ossible, because at higher owers degradation of the olymer occurred. Wiss of olymer could be observed floating in the chamber atmoshere. In exeriments, the olymer could still be resonsible for removing heat by mass transfer.

5 5. Conductivity results The effective conductivity of a owder bed deends on the roerties not only of the solid owder articles, but also of the surrounding gas hase. Yagui and Kunni (5) roosed an equation redicting that the conductivity of a steel/argon mixture will be.68 times that of a steel/air mixture. This agrees reasonably with the comarison between k in situation 1 versus situations 3 and 4 (Table 1). The conductivity of the atmosheric gas makes a larger difference to the effective owder conductivity than the change in solid constituents (comaring situations and 3 against 4). 5.3 Discussion of method The method reorted has been devised to reresent the conditions of the selective laser sintering rocess. In articular: Laser ower and scanning seed values used are roven for roduction of good quality arts, so full sintering takes lace. This leads to changing surface orosity and therefore absortance over the laser exosure time (). The absortance values measured reresent an average over time; The calorimetric method chosen takes into account heat lost through convection and thermal radiation after laser alication, not accounted for by radiation measurements; Realistic scanning rocedures are followed whilst measurements are taken; An inert (argon) atmoshere is used. 6. CONCLUSIONS A method has been develoed which can measure energy absortance and owder effective conductivity during the laser rocessing of a owder bed. Using this method, the absortance of 314S stainless steel owder, size distribution 75-15µm, rocessed in air was found to be.1. For the same conditions the effective conductivity of the owder was found,.5w/m.k. When a different steel-based owder was tested it became clear that a change in material constituents leads to different absortance values. It was found, testing a steel, that rerocessed material had a reduced absortance, whether comletely (re-scanning) or artially (scanning single layers) rerocessing an area. Stainless steel owder in an argon atmoshere was found to have aroximately.7 times the effective conductivity of the same steel owder in air. 7. REFERENCES 1. Kizaki, Y. et al, henomenological Studies in Laser Cladding. art I. Time-Resolved Measurements of the Absortivity of Metal owder, Jaanese Journal of Alied hysics, Vol.3, No.1A, Jan. 1993,.5-1. Tolochko, N.K. et al, Absortance of owder Materials Suitable for Laser Sintering, Raid rototying Journal, MCB University ress, Bradford, UK, Vol.6, No.3,, Sih, S.S., Barlow, J.W., The Measurement of the Thermal roerties and Absortances of owders Near Their Melting Temeratures, roceedings of the Solid Freeform Fabrication Symosium 199, University of Texas, Austin, Texas, USA, 199, Carslaw, H.S., Jaeger, J.C., Conduction of Heat in Solids, Clarendon ress, Oxford, UK, Yagi, S., Kunni, D., Studies on Effective Thermal Conductivities in acked Beds, A.I.Ch.E. Journal, Setember 1957,