201209_PSpaw.pdf 27Przegląd sPawalnictwa 9/2012 mariusz Frankiewicz Edward Chlebus Karol Kobiela APS sprayed coatings onto the selective laser melted substrates Powłoki aPs natryskiwane na podłoża przetapiane  wiązką laserową  Dr inż. Mariusz Frankiewicz, prof dr hab. inż. Edward Chlebus, mgr inż. Karol Kobiela – Politechnika Wrocławska. abstract Additive layer-based selective laser melting techno- logies (SLm), provide a new range of possibilities for ob- jects manufacturing. In the present study the alumina/ titania Al2O3/TiO2 coatings were applied onto SLm-ma- nufactured substrates by the atmospheric plasma spray- ing method. The microstructure and porosity of coatings were examined by scanning electron microscopy (SEm), and industrial computer tomography. Bonding strength of the coatings was analysed. Streszczenie Generatywne technologie selektywnego przetapiania laserowego (SLm) zapewniają nowe możliwości wytwa- rzania części maszyn. W pracy badaniom poddano po- włoki tlenku glinu i tytanu Al2O3/TiO2 nanoszone za pomo- cą metody natryskiwania plazmowego APS na podłoża wykonane w procesie SLm. Przy zastosowaniu skanin- gowej mikroskopii elektronowej i przemysłowej tomogra- fii komputerowej przeprowadzono ocenę mikrostruktury i porowatości powłok, jak również poddano analizie przy- czepność powłok. Introduction Selective laser melting SLm is an additive manufac- turing process based on the layered forming of physical objects by fusing of materials in the form of powder. This method allows processing for wide range of me- tal powders including stainless steels, tool steels, as well as Cr-Co and Ti alloys. The SLm technology is the process of total metal powders melting using the con- trolled beam of 100 W (up to 400 W), 1064 nm wave length, nd:YAG fibre laser [1, 2]. The process of melting runs in the protective atmosphere of argon preventing against oxidation of the fused metal powders. Due to the layered incremental process of manufacturing and the wide range of available materials the technology could be applied for creating physical objects of com- plex geometrical structure both, internal and external. The SLm process finds application in manufacturing prototypes, final products [2], as well as tools, involving inserts of injection moulds [3] or stamping die parts. Operating properties of objects manufactured using the selective laser melting method may be improved by applying additional surface engineering treatment such as the functional coating deposition. One of the methods of improving properties of com- ponents manufactured in the SLm process is plasma spraying. In the plasma spraying processes the ma- terial in a form of powder is supplied to plasma gun (plasmatron), fused and then deposited onto a sub- strate surface with a velocity of approximately 150 m/s. Functional coatings deposited with this method may be, among others, characterised with: resistance to dif- fusion, low friction coefficient, resistance to low- and high-temperature corrosion in aggressive chemical environment, as well as high surface hardness and high wear resistance [4, 5]. The plasma-sprayed Al2O3/TiO2 coatings provide a high wear resistance properties, are applied among others, in the textile, machinebuilding and paper indu- stries [6÷8]. Alumina/titania coatings are known to be dense, with smooth as-sprayed surface. Alumina/tita- nia with 3% share by weight of TiO2 (97/3) has been applied for preventing wear, as well as chemical and cavitational erosion in pump components [4]. Also, the variation of Al2O3/TiO2 composition with increased 28 Przegląd sPawalnictwa 9/2012 to 13% share by weight of TiO2 is used (Al2O3 – 13%, TiO2). Increase in share of titanium dioxide improves the crack resistance [4]. For tests the material supplied by BayState Surface Tech designated as PP-37 (Al2O3 – 3%, TiO2) was used. The research were aimed at initial analysis of co- atings deposited by plasma spraying at substrates ma- nufactured by the selective laser melting process. For that purpose the tests of surface profile, using confocal microscopy, industrial computer tomography and SEm were applied. The tests were performed with the pur- pose of evaluating the influence of substrate surface characteristics at the coating adhesion degree. Sample preparation The substrates were made of the Ti6Al7nb titanium alloy [1] by the SLm Realizer II system. Orientation of the analysed substrate surface during SLm manu- facturing process has been presented in Fig. 1. The research was carried out on four sets of five samples each. Three sets were grit-blasted with three different energies of abrasive material stream each (Al2O3 with grain size of 120 µm), related to the three values of alu- minium oxide feeding pressure, presented in the Table I. One set of the samples was coated without additional machining of the substrate surface. The first stage of the research was aimed on identi- fication of the influence of the grit-blasting parameters on the geometrical structure of the substrate. Rough- ness analysis of the surfaces was performed using the Rank Taylor Hobson Form Talysurf 1201 profilographo- meter. Example results of the measurements for non- machined surfaces have been presented in Fig. 2, and for the grit-blasted ones in Fig. 3. In addition, an analysis of surface roughness was performed using the LEXT OLS4000 confocal micro- scope. Results of the analyses of substrate surface not subjected to grit-blasting processing have shown that they are characterised with a structure containing non-melted powder grains. They adhere to the mel- ted surfaces of the samples during the selective laser melting process. Character of the non-machined SLm surface may be observed in Fig. 4. Diameters valu- es of the non-melted grains, ranging from several to about 70 µm, lead to high surface roughness of the SLm manufactured objects. Results of the roughness measurements carried out on the grit-blasted surfa- ces shows decreasing values of the Ra parameter, on average from above 15,25 µm up to about 8,58 µm, as shown in Fig. 3. Character of the Ra changes can be explained as a result of removal of the non-melted powder grains partly agglomerated with the surface. Increasing of roughness for the third set of samples, Fig. 1. View of the analysed surface of the SLm manufactured sample Rys. 1. Analizowana powierzania próbki wytwarzanej metodą SLm Fig. 2. Profilogram of the unmachined sample surface Rys. 2. Profil nieobrobionej powierzchni próbki Fig. 3. Profilogram of the grit-blasted sample surface. Feeding pres- sure of 0,6 mPa Rys. 3. Profil obrobionej strumieniowo powierzchni próbki. Ciśnienie wejściowe 0,6 mPa table I. Results of the substrate average roughness analyses before coatings depositing tablica I. Wyniki analizy średnich wartości chropowatości przed na- łożeniem powłoki Type of sample surface Grit-blasting parameter Al2O3 feeding preassure, mPa Average values of roughness parameters Ra µm Unmachined surfaces - 15,2509 Grit blasted surface, set no. 1 0,4 10,5504 Grit blasted surface, set no. 2 0,5 8,58285 Grit blasted surface, set no. 3 0,6 14,2604 29Przegląd sPawalnictwa 9/2012 which was processed with 0,6 mPa feeding pressure, can be linked to the higher energy of the abrasive ma- terial stream and should be investigated in the further research. microscopic view of the grit-blasted surface has been presented in Fig. 5. Results of the surface roughness analyses have been collected in Table I. Coating deposition Coatings were sprayed by the APS 60 kW roboti- sed plasma spraying system equipped with the Bay- State Surface Tech SG100 plasma burner. Sample coatings were deposited with parameters presented in Table II. The second stage of investigations was analyses of the internal structure with the using of the indu- strial tomography method (CT). It was carried out by the Zeiss metrotom CT equipment with a detector of 1024x1024 pixels resolution and the 225 kV lamp ena- bling achievement of 10 µm measurements resolution. The CT tests enabled determining porosity of the de- posited coatings. Exemplary result of porosity analysis for coating deposited on the grit-blasted surface, has been presented in Fig. 6. It was characterised with ban- ding location of the pores resulting from the coating de- position process. Subsequent layers of the deposited Fig. 5. View of the substrate surface after grit-blasting with 0,5 mPa feeding pressure Rys. 5. Powierzchnia podłoża obrobiona strumieniowo przy ciśnieniu wejściowym 0,5 mPa table II. Parameters of plasma burner operation during test coatings deposition tablica II. Parametry procesu podczas natryskiwania powłok prób- nych Current Gases Voltage, V Intensity, A Ar flow, l/min He flow, l/min 40 450 66 14 Fig. 6. Example of CT image of the coating (top) and substrate (bot- tom) porosity. Coating deposited on the grit-blasted substrate Rys. 6. Przykład obrazu CT porowatości powłoki (u góry) i podłoża (u dołu). Powłoka naniesiona na obrobione strumieniowo podłoże Fig. 7. View of spatial qualita- tive analysis of arrangement and morphology of the coating discontinuity Rys. 7. Jakościowa analiza przestrzenna rozmieszczenia i morfologii nieciągłości powłoki Fig. 8. Example of SEm images of the coating (top) and substrate (bottom) cross-section: a) overall view with coating thickness mar- ked, b) detailed view of the coating pores. Coating deposited at the non-machined substrate Rys. 8. Przykładowe obrazy SEm przekroju powłoki (u góry) i pod- łoża (u dołu): a) widok całej grubości powłoki, b) powiększenie ob- szaru z porowatością powłoki. Powłoka nanoszona na nieobrobione podłoże a) b) Fig. 4. View of the substrate sur- face as observed in the confocal microscope Rys. 4. Powierzchnia podłoża ob- serwowana na mikroskopie współ- ogniskowym 30 Przegląd sPawalnictwa 9/2012 of coating thicknesses were performed at them, which have shown similar values amounting to about 450 µm. The coating structure discontinuities observed du- ring CT analysis, corresponded to its defects found during analysis using SEm, see Fig. 8 and Fig. 9. The- ir presence could be related to the complex geome- trical surfaces structure of substrates manufactured with the SLm. The non-melted Ti6Al7nb powder gra- ins agglomerated to its surface could prevent correct deposition of coating at the whole substrate surface, which is visible in Fig. 9. In the SEm images of the cross-sections of coating samples deposited at grit- blasted substrates much lower number of coating de- fects was observed at the junction with the substrate surface, see Fig. 10. The observed regularity results from removal of the Ti6Al7nb powder grains weakly bound with the substrate by means of grit-blasting, which resulted in better adhesion of the coating, as seen on Fig. 10. Fig. 9. SEm image of defects in the Al2O3/TiO2 coating resulting from complex structure of the substrate surface. Coating deposited at the non-machined substrate Fig. 9. Obraz SEm wad w powłoce Al2O3/TiO2 wynikających ze złożo- nej struktury podłoża. Powłoka nanoszona na nieobrobione podłoże Fig. 10. Example SEm image of coating (top) deposited at grit-bla- sted substrate (bottom) Rys. 10. Przykładowy obraz SEm powłoki (u góry) nanoszonej na obrobione podłoże (u dołu) Literature [1] Chlebus E., Kuźnicka B., Kurzynowski T., Dybała B.: micro- structure and mechanical behaviour of Ti-6Al-7nb alloy produ- ced by selective laser melting, materials Characterization 62, 2011, s. 488-495. [2] Chlebus E.: Rapid prototyping and advanced manufacturing. Springer handbook of mechanical engineering / [Karl-Hein- rich] Grote, [Erik K.] Antonsson (eds.). Berlin: Springer, 2009, s. 733-768. [3] Frankiewicz m., Kurzynowski T., Dybała B., Chlebus E.: Rapid Tooling application in manufacturing of injection moulds, Virtu- al design and automation/ed. by Z. Weiss. Poznań : Publishing House of Poznan University of Technology, 2008. s. 155-162. [4] Heinmann R.: Plasma Spray Coating, John Wiley & Sons, Ltd, Chichester, UK, 2008. [5] Pawlowski L.: The Science and Engineering of Thermal Spray Coatings, John Wiley & Sons, Ltd, Chichester, UK, 2008. [6] Rico A., Poza P., Rodríguez J.: High temperature tribological behavior of nanostructured and conventional plasma sprayed alumina-titania coatings, Vacuum, Available online 2 march 2012, ISSn 0042-207X. [7] Venkataraman R., Gautam Das a, Venkataraman B., narashi- ma Rao G.V., Krishnamurthy R.: Image processing and stati- stical analysis of microstructures of as plasma sprayed Alumi- na-13 wt.% Titania coatings, Surface & Coatings Technology 201, 2006, s. 3691-3700. [8] Yugeswaran S., Selvarajan V., Vijay m., Ananthapadmana- bhan P.V., SreekumarK.P.: Influence of critical plasma spray- ing parameter (CPSP) on plasma sprayed Alumina–Titania composite coatings, Ceramics International 36, 2010, s. 141- 149. coating were separated by bands of pores. Fig. 6 presents the pore bands located in 4 layers between 5 deposited layers of the coating. It was not observed pores on the layer between the substrate and the coating, which allo- wed assumption on its high adherence to the substrate. The results of porosity analysis for coatings deposited on the non-machined substrates, showed discontinuity of the coating on the layer between coating and sub- strate. Also, porosity of the coated substrate has been observed, as seen in Fig. 6, that is characteristic for substrates manufactured with the SLm method. Size and arrangement of the pores could be parameters, the values of which are controlled by selecting the proper parameters for the selective laser melting process, in- volving, among others, the laser power and scanning rate of the layer fused with a laser beam [1]. CT analysis results have been verified by SEm tests, performed at transverse microsections of the samples with deposited coatings. measurements