Al brazing under severe alterations of the background atmosphere: A new vs. traditional brazing sheet

This paper summarizes a comparative in situ and in real time study of the onset of melting, clad flow, and joint formation involving a novel composite clad aluminum brazing sheet vs. traditional clad brazing sheet material. The adverse impacts considered involve changes of (i) the oxygen level, and (ii) the humidity of the nitrogen background atmosphere. A series of hot stage microscopy studies of the brazing process evolution have been performed within the range of ∼2 ppm to 2×10 ⁵ ppm of the oxygen content (i.e., ∼2, 2×10 ², 5×10 ², 2×10 ³, and 2×10 ⁵ ppm), and the humidity in range between - 48°C and + 6°C. Traditional brazing sheet in both series of tests was covered with potassium fluoroaluminate flux, while the brazing with the composite material was executed without any flux addition. Standard set of the temperature profile brazing parameters was selected with the peak temperature at 600°C, and kept as constant so that only an adverse impact of the background atmosphere was considered. A consistency of the behavior vs. adverse effects was identified with respect to the clad flow and joint formation, with a few outliers. Newly developed material demonstrated a remarkable resilience against an adverse effects impact. New material performed well without flux added for up to 2000 ppm Oxygen and even within the domain of small negative (in degree Celsius) dew point temperature values. Joint formation without the added flux has been repeatedly registered even in air atmosphere. Real time video clips of the sequence of event during brazing (melting, reactive flow, joint formation and solidification) document fully these findings.