Microstructural evolution of 3000 series multi-port extruded tubes during brazing cycles
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The AA3xxx series alloys are known for the good corrosion resistance and widely used as heat exchanging material. However, 3xxx series alloys shows intergranular corrosion problem after brazing. In this project, a systematic characterization on the microstructure evolution of 3xxx-alloy multi-port tubes during brazing processes has been done. A special focus has been put on the evolution of dispersoids and constituent particles in the bulk of tube and the microstructure characteristics of the bonding layer between tube and fin when different brazing processes, in terms of brazing materials and brazing temperature, are applied. Such an investigation will help to reach a deeper insight about the mechanisms of brazing and the subsequent IGC behavior of brazed tubes. Three set of samples were received from Sapa. The first set has seven different tube samples without coating and fin. They have been heat treated by different processes and the simulated brazing cycles was interrupted at defined temperatures. These samples represent the status of materials before, during and after brazing. The second set of samples are tubes coated with two different types of braze coating layer, alloy coating and ZnSilflux coating. They have been heat treated with three different heat treatments to simulate the performances of the tubes during different brazing cycles and the interaction between the braze coating and the core material. The third set of samples are the real brazed tube-fin components. These samples represent the performer of the materials during the industrial brazing cycles. All brazing sample exposure was done in a laboratory brazing cycle in Karmøy. An investigation about their conductivity, microstructure, chemical composition, particles size have been done and will be given in present work. Different characterization techniques, including Light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) , and conductivity measurement were used. For uncoated tubes, during simulated brazing cycles, with increasing temperature, the grain size of the tube and particle size increases while the number density of dispersoids decreases. The conductivity and mean size of dispersoids increase to a maximum and then decreases again with increasing temperature. For coated tubes, the Silicon in the coating layer will diffuse into bulk while Manganese in the bulk of tube will diffuse into surface layer. This will cause the formation of particle free zone (PFZ) and fine dispersoid zone (FDZ) beneath the surface. The thickness of these zone are dependent on different brazing cycle. Extreme cycle has the thickest PFZ and FDZ, full brazed cycle coming after and the fast heated Karmøy cycle has the thinnest PFZ and FDZ. It is also dependent on coating materials. ZnSilflux coating samples have thicker PFZ than alloy coating samples while alloy coating samples have a thicker FDZ than ZnSilflux samples. Al-Si eutectic structure was also founded in ZnSilflux coating samples but not in alloy coatings. Further discussion will be given in present work. The Alloy coating has less Si in coating layer and it has better corrosion resistance than Znsilflux coating. For the real brazing case, a good binding between fin and tube was reached with the help of capillary force. Large Mn-containing particles and Al-Si eutectic structure still precipitate at the boundary layer between fin and tube. The Mn concentration in the large particles are higher than those samples with coating but without fin. The PFZ and FDZ thickness are also higher in real case.