Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

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SESSION APPLICATION-ORIENTED TECHNOLOGIES Fig. 2: FTIR-Spectra while heating from room temperature to 605°C (left), summary <strong>of</strong> <strong>the</strong> peak area depending on heating temperature for <strong>the</strong> three phases existing at <strong>the</strong> material surface (right) mospheres. In <strong>the</strong> subsequently described analyses <strong>the</strong> following materials and atmospheric conditions were used: 1. Material: Solder plated (both sides) Alsheets a. Base material: EN AW-Al Mn1Cu, thickness: 0.32 mm b. Solder: EN AW-Al Si10, thickness: 0.04 mm (on both sides) 2. Atmospheric conditions for <strong>the</strong> analysis <strong>of</strong> <strong>the</strong> influence <strong>of</strong> <strong>the</strong> storage and transport conditions a. Normal climate (NC) – 23°C/ 50% rel. humidity b. Humid climate (HC) – 40°C/ 92% rel. humidity c. Condensation (C) – 23°C/ > 100% rel. humidity 3. Heating tests for <strong>the</strong> analysis <strong>of</strong> <strong>the</strong> influence <strong>of</strong> increased temperatures a. Heating temperature: 605°C b. Ambient atmosphere: Nitrogen The challenge for <strong>the</strong> analysis <strong>of</strong> <strong>the</strong> influence <strong>of</strong> real conditions on brazing materials is <strong>the</strong> choice <strong>of</strong> suitable methods which are able to analyse nm-thick <strong>layer</strong>s and to distinguish between <strong>aluminium</strong> <strong>oxide</strong> and hydr<strong>oxide</strong> phases. With XPS-measurements, <strong>the</strong> composition <strong>of</strong> <strong>the</strong> surface can be measured. The spectra shows <strong>the</strong> intensity depending on <strong>the</strong> binding energy, see Fig. 1. For <strong>natural</strong> <strong>aluminium</strong> surfaces oxygen, carbon, magnesium as well as <strong>aluminium</strong> can be measured. The important phases and <strong>the</strong>ir binding energies for <strong>the</strong> present analyses are [18]: Metallic Al 72,5 – 73 eV γ-Al 2O 3 73,7 – 74,5 eV Al(OH) 3 73,4 eV. Obviously it is not possible to distinguish between oxidic and hydroxidic phases on <strong>the</strong> base <strong>of</strong> <strong>the</strong> binding energy. Ano<strong>the</strong>r possibility to distinguish between <strong>the</strong>se two phases is described by Wittenberg et. al. [19]. In this study, <strong>the</strong> oxidic and hydroxidic phases are classified by <strong>the</strong> formation <strong>of</strong> <strong>the</strong> relation <strong>of</strong> <strong>the</strong> atomic concentration <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> peak to <strong>the</strong> oxygen peak (Al/O). However, with <strong>the</strong> industrial material used in <strong>the</strong> present analysis, <strong>the</strong> measured <strong>aluminium</strong> intensity consists always <strong>of</strong> <strong>the</strong> oxidic bound <strong>aluminium</strong> and <strong>the</strong> metallic <strong>aluminium</strong>. The signal <strong>of</strong> <strong>the</strong> metallic bound <strong>aluminium</strong> originates from <strong>the</strong> metallic <strong>aluminium</strong> <strong>of</strong> <strong>the</strong> base material directly under <strong>the</strong> <strong>aluminium</strong> <strong>oxide</strong> <strong>layer</strong>. Therefore, this metallic <strong>aluminium</strong> signal cannot be taken into account for <strong>the</strong> determination <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> surface phases. Hence, only <strong>the</strong> oxidic bound <strong>aluminium</strong> is set into relation to <strong>the</strong> oxygen intensity in <strong>the</strong> present analysis. For Al 2O 3 <strong>the</strong> ratio <strong>of</strong> Al <strong>oxide</strong> to O is about 2/3, for <strong>the</strong> hydroxidic bound <strong>aluminium</strong> Al(OH) 3 this ratio is about 1/3. Additionally, <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> thickness can be estimated using <strong>the</strong> ratio <strong>of</strong> <strong>the</strong> intensity <strong>of</strong> <strong>the</strong> oxidic to metallic bound <strong>aluminium</strong> [20]. The XPS-measurement <strong>of</strong> <strong>natural</strong> <strong>aluminium</strong> surfaces as well as <strong>of</strong> materials stored under normal climate, humid climate and condensation show, that <strong>the</strong> hydroxidic <strong>layer</strong> is existent on all surfaces, even on <strong>the</strong> surface <strong>of</strong> <strong>natural</strong> (not stored) <strong>aluminium</strong> materials. In contrast to this, <strong>the</strong> storage conditions have a measurable influence on <strong>the</strong> <strong>oxide</strong> thickness. Especially, <strong>the</strong> influence <strong>of</strong> condensed water on <strong>the</strong> surface causes a significant increase <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong>, see Fig. 4. The temperature increase during <strong>the</strong> brazing process has ano<strong>the</strong>r significant influence on <strong>the</strong> thickness and composition <strong>of</strong> <strong>the</strong> <strong>layer</strong>s at <strong>the</strong> surface <strong>of</strong> <strong>aluminium</strong> materials. The behaviour <strong>of</strong> <strong>the</strong>se <strong>layer</strong>s during heating from room temperature till 605°C (= brazing temperature) can be analyzed with <strong>the</strong> help <strong>of</strong> FTIR measurements under a N 2-atmosphere (= brazing atmosphere). All tests were done at Innoval Technology Ltd. Banbury, where a FTIR-spectrometer with an additional heating device is available.. A spectrum is taken <strong>of</strong> <strong>the</strong> surface every minute during <strong>the</strong> heating process. All spectra for one heating cycle are displayed in Fig. 2. It is obvious, that <strong>the</strong>re are three phases existing at <strong>the</strong> surface during <strong>the</strong> heating process. At room temperature <strong>the</strong> amorphous Al 2O 3-phase dominates <strong>the</strong> surface. With rising temperatures, especially at temperatures above 400°C, <strong>the</strong> Al/O-signal moves to lower wave numbers. Unfortunately, <strong>the</strong> phase could not be defined until now. This phase can consist <strong>of</strong> ei<strong>the</strong>r crystalline Al 2O 3 or MgAl 2O 4. For a precise determination <strong>of</strong> this phase, TEM analyses are planned. At temperatures above 577°C, <strong>the</strong> solidus temperature <strong>of</strong> <strong>the</strong> solder, MgO is growing at <strong>the</strong> surface. With <strong>the</strong> help <strong>of</strong> this analyzing method, <strong>the</strong> behaviour <strong>of</strong> materials, which were stored under condensed water, was also analyzed. These materials have a major growth <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> during <strong>the</strong> heating process. Correlation between surface condition and brazeability Brazing method for analyzing influence <strong>of</strong> <strong>oxide</strong> <strong>layer</strong> on brazeability: The brazing tests were done in a laboratory shielding gas furnace. To see only <strong>the</strong> influence <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> on <strong>the</strong> brazeability, a mechanical surface activation method, see Fig. 3, was used instead <strong>of</strong> a chemical surface activation. To realize <strong>the</strong> cracking <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong>, <strong>the</strong> upper joining partner is pressed inside <strong>the</strong> liquid solder by applying a weight. Therefore, tensions are induced inside <strong>the</strong> <strong>oxide</strong> <strong>layer</strong>, which cause <strong>the</strong> cracking <strong>of</strong> this <strong>layer</strong>. Afterwards <strong>the</strong> liquid solder can flow to <strong>the</strong> upper joining partner and wet it. Brazing results for different surface conditions: A storage under humid climate as well as under condensation for a short period <strong>of</strong> time (3 days) causes a growth <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong>, but does not influence <strong>the</strong> brazing Fig. 3: Mechanism <strong>of</strong> <strong>the</strong> mechanical surface activation 58 ALUMINIUM · EAC CONGRESS 2011
level significantly, see Fig. 4. A longer storage under condensation suppresses <strong>the</strong> brazing process completely. However, storage in a normal climate after <strong>the</strong> condensation leads to a significant improvement <strong>of</strong> <strong>the</strong> brazeability <strong>of</strong> <strong>the</strong> material. It has to be followed, that <strong>the</strong> water, which is stored inside <strong>the</strong> pores <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> due to <strong>the</strong> condensation, influences <strong>the</strong> brazing level. This water leads to an increased growth <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> during <strong>the</strong> heating. This thickened <strong>oxide</strong> <strong>layer</strong> cannot be cracked by <strong>the</strong> weight <strong>of</strong> <strong>the</strong> upper joining partner. Hence, <strong>the</strong> flow <strong>of</strong> <strong>the</strong> solder is suppressed completely. In contrast to this, <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> thickness has a minor influence on <strong>the</strong> brazeability. Conclusions The present analysis characterizes <strong>the</strong> influence <strong>of</strong> <strong>the</strong> humidity <strong>of</strong> <strong>the</strong> atmosphere as well as <strong>of</strong> condensed water at <strong>the</strong> surface <strong>of</strong> <strong>aluminium</strong> materials on <strong>the</strong> <strong>natural</strong> <strong>aluminium</strong> <strong>oxide</strong> <strong>layer</strong>. With <strong>the</strong> help <strong>of</strong> XPS-measurements, it has been shown, that storage under normal or humid climate does not influence <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> thickness significantly. In contrast to this, <strong>the</strong> influence <strong>of</strong> condensed water on <strong>the</strong> surface causes an intense growth <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong>. FTIR measurements with an heating device have also shown that materials, which were stored under <strong>the</strong> influence <strong>of</strong> condensed water, have a significant thicker <strong>oxide</strong> <strong>layer</strong> at brazing temperature. Subsequent brazing tests have been done in a shielding gas furnace with a mechanical surface activation to see <strong>the</strong> influence <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> on <strong>the</strong> brazeability <strong>of</strong> <strong>the</strong> material. These tests have shown that <strong>the</strong> <strong>oxide</strong> thickness before brazing is not <strong>the</strong> critical parameter for <strong>the</strong> brazing result. In fact, <strong>the</strong> water inside <strong>the</strong> pores <strong>of</strong> <strong>the</strong> hydroxidic <strong>layer</strong> causes an increased growth <strong>of</strong> <strong>the</strong> <strong>oxide</strong> <strong>layer</strong> during <strong>the</strong> heating process. This effect <strong>of</strong> <strong>the</strong> condensed water can be avoided, if <strong>the</strong> materials are stored under normal climate after having water present at <strong>the</strong> surface <strong>of</strong> <strong>aluminium</strong> materials. The analyses showed that not only <strong>the</strong> solder or <strong>the</strong> brazing conditions are important for <strong>the</strong> brazing result <strong>of</strong> <strong>aluminium</strong> materials, but also <strong>the</strong> <strong>natural</strong> <strong>aluminium</strong> <strong>oxide</strong> <strong>layer</strong>. The <strong>natural</strong> <strong>oxide</strong> <strong>layer</strong> can be influenced significantly by <strong>the</strong> atmospheric conditions during <strong>the</strong> transport and <strong>the</strong> storage <strong>of</strong> <strong>the</strong> materials. Fur<strong>the</strong>rmore, it has to be assumed that also <strong>the</strong> surface <strong>of</strong> different brazing partners, e. g. <strong>aluminium</strong> <strong>oxide</strong> or stainless steels, can influence <strong>the</strong> brazeability. References APPLICATION-ORIENTED TECHNOLOGIES [1] Ostermann, F.: Anwendungstechnologie Aluminium – ein Werkst<strong>of</strong>fhandbuch. Springer Berlin Heidelberg; Auflage: 2., 2007. [2] Kohlweiler, A.: Hartlöten von Aluminium – so geht es auch. In: Der Praktiker.(1996) Heft 2, S. 40-45. [3] Meurer, C.; Belt, H.-J.; König, H.: Das Nocolok- Flux-Hartlötverfahren. In: Die Kälte und Klima- technik. (1997) Heft 10, S. 802-808. [4] Neitz, G.; Wielage, B.; Trommer; F.: Schutzgas- löten von Al-Wärmetauschern. In: Hart- und Hochtemperaturlöten und Diffusionsschweißen DVS-Berichte Band 212. Düsseldorf: DVS-Verlag, 2001, S. 240-244. [5] Senaneuch, J.; Nylén, M.; Hutchinson, B.: Metallurgy <strong>of</strong> brazed <strong>aluminium</strong> heat-exchangers, Part I. In: Aluminium. 77 (2001) Heft 11, S. 896-899. [6] Müller, W.; Müller, J.-U.: Löttechnik – Leitfaden für die Praxis. Düsseldorf: Deutscher Verlag für Schweißtechnik DVS-Verlag GmbH, 1995. [7] Terrill, J. R.; Cochran, C. N.; Stokes, J. J.; Haupin; W. E.: Understanding <strong>the</strong> Mechanisms <strong>of</strong> Aluminium Brazing. In: Welding Journal. (1971) S. 833-839. [8] Ashburn, L. L.: Fluxless Vacuum Furnace Brazing <strong>of</strong> Aluminum particularly Advantageous for more critical Applications: I. In: Industrial Heating. (1994) S. 47-51. [9] Byrnes, E. R.: Vacuum Fluxless Brazing <strong>of</strong> Aluminium. In: Welding Journal. (1971) S. 712-716. [10] Füssel, U.; Six, S.: Entwicklung eines NIR- Lötverfahrens für die Fertigung von Solarabsorbern aus Aluminium. LÖT, 9. International Conference Brazing, High Temperature Brazing and Diffusion Welding. In: DVS-Berichte: Band 263 (2010) S. 358-360 [11] Möller, F.; Thomy, C.; Vollertsen, F.: Flussmittelfreies Hartlöten von Aluminiumlegierungen mit einem koaxialen Laser-Plasma-Hybridlötprozess. In: Schweißen und Schneiden 62/5 (2010) 294. [12] Bach, F.W.; Möhwald, K.; Holländer, U.; Langohr, A.: Niedrig schmelzende Aluminiumhartlote aus dem System Al-Si-Zn. 9. International Conference Brazing, High Temperature Brazing and Diffusion Welding. In: DVS-Berichte. Düsseldorf: DVS-Verlag. Band 263 (2010) Seite 117-121. [13] Tillmann, W.; LIU, C.; Wojarski, L.: Dotierung von Aluminiumbasisloten zum flussmittelfreien Hartlöten von Aluminiumlegierungen. 9. International Conference Brazing, High Temperature Brazing and Diffusion Welding. In: DVS-Berichte. Düsseldorf: DVS-Verlag.: Band 263 (2010) Seite 106-112. [14] Melander, M.; Woods, R.: A corrosion study <strong>of</strong> brazed <strong>aluminium</strong> heat exchangers after field service. In: International Aluminium Journal. Band 86 (2010) Heft 7/8, Seite 56-61. [15] Gray, A.; Flemming, A. J. E.; Evans, J. M.: Optimising <strong>the</strong> Properties <strong>of</strong> Long-life Brazing Sheet Alloys for Vacuum and Nocolok Brazed Components. VTMS 4 Conference Proceedings, London, May 1999. [16] Swidersky, H.-W.: Aluminium brazing with non-corrosive fluxes – state <strong>of</strong> <strong>the</strong> art and trends in Nocolok flux technology. In: Hart- und Hochtem- peraturlöten und Diffusionsschweißen DVS-Berichte Band 212. Düsseldorf: DVS-Verlag, DVS-Berichte Band 212 (2001), S. 164-169. [17] Altenpohl, D.: Aluminium und Aluminium- legierungen. Berlin: Springer Verlag, 1965. [18] Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D.: Handbook <strong>of</strong> X-Ray Photoelectron Spectroscopy: A Reference Book <strong>of</strong> Standard Spectra for Identification and Interpretation <strong>of</strong> XPS-Data. Physical Electronics, 1995. – ISBN-10: 096481241X [19] Wittenberg, T. N.; Douglas, W. J.; Wang, P. S.: Aluminium hydr<strong>oxide</strong> growth on <strong>aluminium</strong> surfaces exposed to an air/1% NO 2 mixture. In: Journal <strong>of</strong> Materials Science, 23 (1988), S. 1745-1747. [20] Kozlowska, M.; Reiche, R.; Oswald, S.; Vinzelberg, H.; Hübner, R.; Wetzig, K.: Quantitative ARXPS investigation <strong>of</strong> systems at ultrathin <strong>aluminium</strong> <strong>oxide</strong> <strong>layer</strong>s. Surface Interface Anal. 36 (2004) 1600. � Fig. 4: <strong>Influence</strong> <strong>of</strong> storage conditions and duration on <strong>oxide</strong> thickness and brazing level ALUMINIUM · EAC CONGRESS 2011 59
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