Analytical investigations of the modification shift in laser-sintered solids by furnace treatment

Transcription

1 Proceedings of the Fifth International WLT-Conference on Lasers in Manufacturing 2009 Munich, June 2009 Analytical investigations of the modification shift in laser-sintered solids by furnace treatment T. Süß* 1, P. Regenfuß 1, A. Streek 1, F.Ullmann 1, M. Horn 1, R. Ebert 1, H. Exner 1 1 Hochschule Mittweida, Germany Abstract Structures obtained from ceramic powders by laser micro-sintering are characterized by a high porosity and an inhomogeneous material composition, in particular if powder blends are processed. During laser micro sintering of metal powder blends, only incomplete fusion of the materials to the alloy corresponding to the material ratio of the powder is obtained. By an additional furnace treatment homogenization of compound and equilibration of its modification is achieved, yielding a material more suitable for technical applications. Examples of metals and a ceramic material, subjected to an additional furnace process after laser micro sintering, are presented. For example, in the case of ceramic sintering, it can be shown that the as laser sintered solid from a mixture of alumina and silica needs additional furnace sintering to achieve its final modification of the corresponding alumina silicate. Keywords: laser micro sintering, heat treatment, compound equilibration, brass, mullite Laser micro sintering was developed at Laserinsitut Mittelsachsen (Mittweida/ Germany) in 2002 [1,2] as a modification of selective laser sintering, which in turn dates back to 1995 [3]. It can be applied for generative free-form fabrication of metal and ceramic. The maximum resolution is 30 µm. The advantages of free-form fabrication compared to other technologies are the possibility of mouldless generation of solid bodies and the lack of mechanical wear-out. Operating-time dependent attenuation of laser power or deterioration of alignment can be controlled and compensated usually much easier and more systematically. The current mechanistic process hypothesis of laser sintering and especially laser micro sintering reads as follows [4]: Over a very short time (with Q-switched pulses) a high amount of energy is irradiated in to a comparably small powder volume. The simultaneous reactions of the material are melting as well as partial evaporation and formation of plasma. Also, the result of rapid boiling events can be observed. From the transition and condensing of the powder material by these thermal and mechanical effects, a solid body results. The dynamic course of the temperature, specific for the applied laser regime, becomes even less predictable or controllable due to the poor reproducibility of the powder layer. Heating and cooling phases usually are very rapid. Therefore, the resulting compounds consist of modifications that do not have a high order on the atomic scale and differ noticeably from the product of a reaction under equilibrium conditions. There may be exceptional cases, in which such modifications are intended; as a rule, however, industrial production and technical application require compound structures of the product that are comparable to the equilibrium crystalline modifications. Furthermore a certain porosity cannot be avoided due to the mentioned boiling events and plasma effects. This feature may also be desired for a few special cases; but in the most cases of functional parts the properties of a massive solid compound are aspired. A third important undesired consequence of the laser process are the remaining centers of thermal stress respectively the fissures by which the material relieves itself from the tensions during or after solidification. In order to approach the equilibrium crystalline properties as close as possible with the laser generated material, efforts are made to achieve the transition of laser sintered compound structures into the equilibrium modification through a subsequent heat treatment. In this article, only examples for subsequent furnace sintering or tempering of laser micro sintered probes will be reported. Results of infiltration processes, a technology that also belongs to the group of heat treatments, will be discussed in later publications. *Corresponding author: suess@htwm.de The presented results have been obtained in the course of the projects INNOPROFILE Rapid Micro tooling mit laserbasierten Verfahren (pr. no. 03IP506) and KONAMI Kontrollierter Einsatz von Pulvern mit nanoskaligen Charakteristiken zur Erzeugung von Mikrokörpern (pr. no. 03X0033B), funded by German Bundesministerium für Bildung und Forschung. 1

2 Fig. 1: Probes from Cu/Zn powder blends as laser sintered. The components are yet very incompletely alloyed. Phases of elemental Cu (red) and Zn (blue/grey) are clearly visible. 1 Materials and Methods 1.1 Materials With the following laser generated compounds subsequent equilibration by heat treatment was attempted: 1. Bicomponent metal compounds from four different copper/zinc powder blends. 2. A ceramic compound from an alumina/silica powder blend. The two components of the metal blend were a copper and a zinc powder, both from Alpha Aesar GmbH & Co KG (Karlsruhe/ Germany). For the generation of an oxide-ceramic compound a mixture of alumina and silica was used; both powders were acquired from Sigma-Aldrich Chemie GmbH (München/ Germany). 1.2 Laser micro sintering The equipment and setup for the laser micro sinter technology has been described in earlier publications [5]. Contrary to furnace sintering laser sintering entails, at least partially, locally restricted transient liquefication of the processed material. The bicomponent metal compound was sintered with a solid-state laser from FOBA GmbH (Lüdenscheid/ Germany). The average laser power was varied between 6 and 8 Watt (λ=1064 nm); the power had to be increased with the copper content. The oxide-ceramic compound was produced with a solid state laser (p av =20W; λ=350 nm) from Coherent GmbH (Dieburg/ Germany). 1.3 Sintering and tempering in a furnace Each one of the described probes was, subsequent to laser sintering, tempered in a furnace, acquired from GERO GmbH (Neuhausen/ Germany). The three-hour heat treatment of the metal probes was conducted under a flow of argon. Increasing furnace temperatures were used in accordance with the rising copper content of the compounds: Probe A: 700 C; Probe B und C: 650 C; Probe C: 450 C (Fig. 2). The oxide-ceramic laser sintered probe was tempered for six hours under ambient gas conditions at a temperature of 1450 C. 2 Results 2.1 Cu/Zn-compounds The initial compositions of the powder blends A-D are listed in the second column of Tab. 1. Laser sintered probes Fig. 1 shows the opto-microscopic views of crosssection preparations through the as laser sintered probes; considerable amounts of un-alloyed copper and zinc are still present. During furnace tempering the nonoxidized metal fractions of the probe are completely turned into the corresponding brass alloys. Some of the cross-sections shown in Fig. 3 allow the discrimination of brass alloys with different copper/zinc ratios. XRD analyses give evidence that the sintered probes have 2

3 Tab. 1: Respective composition of the powder blend and the phases detected by XRD in the as laser sintered compound. Tab. 2: Metal composition of the compound phases detected by XRD after furnace tempering. considerable lower zinc content than the initial powder mixture, presumably due to material-specific loss during sintering (Tab. 1, 2). Except for probe D the relative zinc fraction of the metal-phase is reduced stepwise during laser sintering as well as during the furnaces process. Fig. 2 shows the shift in composition of the metal-phases and their respective positions in the phase diagram. The amount of zinc in the concomitant zinc oxide is purposely excluded from this consideration as it is supposed that this phase is not accountable for the crystal modification of the brass alloy. Because of the rapid transitions of the aggregational states during laser sintering the as laser sintered compound cannot be expected to feature the equilibrium material modifications; consequently in none of the as laser sintered probes the equilibrium crystal-structure of the corresponding brass composition could be observed. Fig. 3 gives evidence that the metal mixtures are alloyed during the tempering process. As already mentioned, also during this process shifts of the copper/zinc ratios occur (Tab. 2). In three of the probes (A, B, C) reduction of the zinc is observed. Remarkably, the composition of probe D is shifted towards a higher relative zinc fraction. Furthermore, the XRD-results (Tab. 2) corroborate that the crystal structures after tempering correspond to the equilibrium brass modifications of the respective Cu/Zn-ratios. The copper/zinc ratios of the tempered alloy are symbolized by III in the phase diagram (Fig. 2). Furnace tempered probes Fig. 2: Cu/Zn-ratios of the probes A, B, C, D (Tab.1) as unprocessed powder blends (I), as laser sintered (II), and after tempering (III). 3

4 2.2 Oxide ceramics The probes were generated from an Al 2 O 3 /SiO 2 powder mixture with a respective mass ratio of 80:20. Laser sintered probe The crushing strength of the as laser sintered probe was 350 MPa. The XDR diagram (Fig. 4 a.) shows the peaks of Al 2 O 3, SiO 2 and of the compound Al 5 Si 2 O 13 (mullite). The quantitative evaluation of the signals yields a mullite content of 8 mass-percent. From the mass-ratio of the components (Al 2 O 3 ):(SiO 2 ):(Al 5 Si 2 O 13 ) ~ 66:26:8 a mass-ratio of (Al):(Si) ~ 1:0,37 results. This deviates noticeably from the respective Al:Si value of 1:0,23 in the initial powder blend. It must be mentioned that the percentages refer to the crystalline fraction of the material, as no amorphous material can be analyzed from the XDR spectrum. It can be assumed that the amorphous fraction of the as laser sintered ceramic compound is Fig. 4: XRD diagrams of a laser sintered (a) and an additionally furnace tempered (b) sample from an Al 2 O 3 /SiO 2 blend. Fig. 3: Probes from Fig 1 after additional tempering under a stream of Argon. The metal phase is completely alloyed. Several modifications of brass can be differentiated (refer to text). considerable and, therefore, that the Al/Si ratio obtained from the XRD signals does not reflect the true ratio of the elements in the compound. SEM-views of the surf ace and of the cross section of an as laser sintered probe is shown in Figs. 6 a and c. Furnace sintered probe Through furnace tempering the initially grey and semitransparent laser sintered probe had gained a bright and opaque appearance (Fig. 5). The shrinking during the furnace process amounted to less than 1%. Figs. 6 b and d show surface and cross sectional SEM-views of a subsequently tempered laser sintered oxide ceramic probe. Upon closer examination it becomes evident that the fissures, apparent in the cross section of the as laser sintered probe (Fig. 6 a and c), have been healed out during tempering. The crushing strength was enhanced by this step to 1400 MPa. Larger pores can be detected in the cross sections of the tempered probe compared to the untempered samples, although, as already mentioned, only insignificant shrinking of the bodies occurs in the furnace process. For this phenomenon too, an 4

5 explanation could be that the as laser sintered body consists to a significant amount of amorphous material that loses volume by transition into a crystalline state during tempering. The observation that, instead of shrinking, the pores are enlarged could indicate that in the pores of the as laser sintered probe a minor amount of gas is enclosed or develops during tempering, which, at the furnace temperature of 1600 C, exerts sufficient counter pressure against the contraction forces. Fig.5: Al 2 O 3 /SiO 2 probe: laser sintered (a.); tempered (b.). The XRD-signals show that the relative mullite fraction of the crystalline material is enlarged significantly during the tempering process (Fig. 4 b). The composition (Al 2 O 3 ):(SiO 2 ):(Al 5 Si 2 O 13 )~27:15:58 corresponds to a (Al):(Si)-mass ration of around 1:0,33, which is closer to the initial powder-blend ratio of 1:0,23 than the ration detected in the as laser sintered compound. Under the assumption that material loss during tempering is negligible the conclusion is also that the crystalline fraction of the compound has increased during furnace tempering and the amorphous part of the compound in the as laser sintered probe consists over-proportionally of alumina or its decomposition products. Fig. 6: Surface and cross section of an as laser sintered probe (a, c) and of the sample after additional tempering in a furnace (b, d). 3 Summary & Conclusion Changes of the crystalline structures within five different laser sintered bodies (four Cu/Zn-alloys and an Al 2 O 3 /SiO 2 -ceramic blend) have been achieved by subsequent heating processes in a furnace. With both of the material classes the compounds of the laser generated bodies result from rapid solidification, of a liquid whereas merely solid body reactions take place during the subsequent furnace process. Material specific loss could be observed from microscopic views as well as from XRD analyses during laser sintering as well as furnace tempering of the metal probes. Due to the rapid solidification, the resulting material modifications differ from their equilibrium formations. Upon tempering in a furnace the crystal structures of the metal phase complied with the equilibrium structures of the respective alloy compositions. The analyses of the ceramic probe were made under the assumption that with this material significant materialspecific loss occurs neither during laser sintering nor during the furnace process. The results allow the interpretation that the as laser sintered compound contains a significant fraction of non-crystalline material. From the change in XRD signals upon subsequent heating in a furnace an increase in crystallinity is deduced. The crushing strength of the compound is enhanced by the process from 350 MPa to 1400 MPa. Subsequent tempering in a furnace of the investigated laser sintered bodies has proved an efficient method to homogenize and equilibrate the material. Equilibration often leads to enhanced firmness and inertness of the compound. Bibliography [1] Regenfuss, P.; Hartwig, L.; Klötzer, S.; Ebert, R.; Exner, H.: Microparts by a Novel Modification of Selective Laser Sintering. SME Technical paper TP04PUB185 (2004) [2] Regenfuss, P.; Streek, A.; Hartwig, L.; Klötzer, S.; Maaz, A.; Ebert, R.; Exner, H.: Advancements in Laser Micro Sintering. In: E. Beyer et al. (Edtrs.): Proceedings of the Third International WLT-Conference on Lasers in Manufacturing. Munich, Germany June 13-16, 2005, ATV-Verlag GmbH, ISBN , pp (2005) [3] Deckard, C.: Method and apparatus for producing parts by selective sintering. US patent 4,863,538. filed: October 17th, 1986, published; September 5th, [4] Regenfuss, P.; Streek, A.; Hartwig, L.; Klötzer, S.; Brabant, T.; Horn, M.; Ebert, R; Exner, H.: Principles of laser micro sintering. In: Rapid Prototyping Journal vol.13 (2007) no.4, [5] Streek, A.; Regenfuß, P.; Süß, T.; Ebert, R.; Exner, H.: Laser micro sintering of Silica with an Nd:YAG Laser. In: Proceedings FLAMN-07 "Fundamentals of Laser Assisted Micro- & Nanotechnologies" St. Petersburg (Russia), ISBN: , Vadim P. Veiko, Editors, 69850Q (Jan. 15, 2008) 5