Studio della bagnabilità del sistema metallo-ceramico AgCuTi/YAG (Y3Al5O12) per la realizzazione di giunti brasati YAG-trasparente/Ti6Al4V -- Evaluation of wettability of the transparent YAG/AgCuTi metal-ceramic system for the production of brazed joints (YAG/AgCuTi/Ti6Al4V)

This paper is mainly focused on the wettability and the reactivity evaluation of the YAG/AgCuTi systems due to define the performances of a new kind of joints (YAG/AgCuTi/; Ti6AI4V). The transparent YAG (Y3Al5O12) ceramic oxides, coupled to a corrosion resistant alloy (Ti6AI4V) using the AgCuTi brazing materials, is being considered as an optical window to be utilized in the marine sector. The YAG/AgCuTi systems are evaluated at different temperatures (820, 850, 950°C) in terms of wettability by the "sessile drop technique" and in terms of interfacial reactivity by microstructural and EDS analysis. The role of the active element (Ti) in determining the interactions between the brazing alloys and the YAG materials has been quantified and discussed using CALPHAD method with the final aim of interpreting the wetting behaviour of the samples. The YAG/AgCuTi contact angle values obtained after the tests have shown an excellent wettability at 950°C (?eq=10°) and fair wettability at 850°C and 820°C (?eq=73°and 71°). The interfacial analyses have shown a significant reactivity at the solid-liquid interfaces at the different temperatures and the formation of new phases. The same metal-ceramic phase was found in the 1st layer in contact with the YAG substrate for the all AgCuTi/YAG samples at all temperatures, while a 2nd layer, identified as "M4O", was found only at the interface of the samples tested at 950°C. Its presence was associated to an important improvement in the wettability due to its metallic behaviour. The different phases formed at 820, 850, 950°C, were also identified analysing the calculated Ag-Cu-Ti phase diagram. The YAG/AgCuTi/Ti6AI4V specimens were tested under the same conditions and temperatures chosen for the wetting experiments. From these preliminary results the best joints in terms of adhesion were obtained at the lower temperatures (820, 850°C); in fact for these samples no pores or defects were found at the interface. The microstructures of the interfacial zone have been reported analysing the different compositions of the layers grown between the two parts joined. The good wettability reached and the maintained transparency of the ceramic substrate allow these systems to be considered as potential candidates for the production of brazed optical windows to be used under extreme conditions.

In questo lavoro vengono riportati i risultati dei test di bagnabilità di YAG (Y3Al5O12) - substrato ceramico trasparente - da parte di leghe AgCuTi, condotti in atmosfera controllata e a diverse temperature e tempi di misura al fine di definire e ottimizzare le condizioni di brasatura di questi ceramici con la lega Ti6Al4V. Inoltre, vengono riportati studi preliminari di giunzione YAG/Ti6Al4V. La lega di Ti è stata scelta in virtù della sua resistenza alla corrosione ed alle sue ottime proprietà meccaniche, che la rendono ampiamente utilizzata nelle applicazioni marine. La valutazione della bagnabilità e della reattività interfacciale è stata eseguita mediante la tecnica della sessile drop (14). In particolare, sono state eseguite misure di angolo di contatto di gocce metalliche liquide sul substrato ceramico in funzione della temperatura e della composizione delle leghe. La morfologia e la composizione delle fasi formate all'interfaccia lega/YAG sono state valutate mediante la microscopia ottica (LOM) ed elettronica (SEM) accoppiata con la spettroscopia a dispersione di energia (EDS).