Characterization on Fracture Surfaces of 304 Stainless Steels Joined by Brazing Using Silicon Nanoparticles
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Characterization on Fracture Surfaces of 304 Stainless Steels Joined by Brazing Using Silicon Nanoparticles L. Santiago-Bautista, H. M. Hernández-García, R. Muñoz-Arroyo, M. Garza-Castañón, F. García-Vázquez, J. Acevedo-Dávila Corporación Mexicana de Investigación en Materiales S.A. de C.V., Ciencia y Tecnología No. 790 Fracc. Saltillo Coah., C.P. 25290, México. E-mail: [email protected].
ABSTRACT Silicon nanoparticles of 100 nm obtained by high-energy ball milling were characterized by X-ray diffraction (XRD) and transmission electronic microscopy (TEM). Results show dark areas due to a staking of defects. On the other hand, brighter areas exhibit a combination of small crystalline and amorphous zones. To fulfill and cover the micro-cracking and micro-pores generated during the welding process of 304 stainless steels joined by brazing, these nanoparticles were deposited directly in the fracture. The amorphous silicon drove the Transient Liquid Phase (TLP) at 1000°C for 20 min. This amorphous silicon decreases the energies of reaction between the substrate and melting filler. TLP increases the wettability and capillary forces between micro-cracking and micro-pores; due to that, the eutectic phase contained by the melting filler forms a liquid. Moreover, the weld beads were characterized by Scanning Electron Microscopy (SEM) to analyze the effect of silicon nanoparticles on the weld beads. These results showed that the interaction of the Si nanoparticles with metallic filler in the melting zone decreases the size and change the morphology of the present phases as well as the zone of isothermic growth. Keywords: welding, steel, nanostructure, brazing, fracture. INTRODUCTION Stainless steels are essential for medical, chemical, food, and biotechnological applications because of their excellent properties such as corrosion resistance, weldability and hardness at room temperature. However, under certain conditions, are susceptible to corrosion, which reduces their mechanical properties giving place to fracture initiation. These adverse effects are mainly present near the areas joined by welding. Micro cracking without soldering is mainly because the flux used during the brazing process does not adequately penetrates cracks or spaces given their complex morphologies. Therefore, binding has to be used after the formation of a transient liquid phase (TLP). This is a new process that welds base material with an intermediate layer. The intermediate layer is melted and the inserted element diffuses into the substrate material, causing isothermal solidification [1-3]. If the isothermal solidification is not completed, the liquid in the intermediate layer is solidified through eutectic phases during cooling, causing a decrease of the mechanical properties of the weld bead. Xiaowei Wu et al [4] reported that TLP bonding process is carried out in four steps: 1) melting of the intermediate layer, 2) dissolution of base metals (in thermodynamic equilibrium solid-liquid interface) 3) isothermal solidification
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