The Effect of Degassing on Grain Refinement in Commercial Purity Aluminum

摘要

Degassing of molten aluminum is used to remove dissolved hydrogen and impurity particles prior to casting. The most common method, rotary degassing, gives a small bubble size and distributes the bubbles throughout the melt by means of vigorous stirring. Although this is an efficient method for hydrogen removal, the purge gas may also inadvertently remove grain refining particles and thus reduce refinement efficiency. During degassing, particle removal occurs by physical attachment to the degassing bubbles and flotation, by turbulent transport due to the flow field generated by stirring, or by sedimentation. The experiments reported here were undertaken using a static graphite degassing lance (i.e., nonrotating) in order to prevent the formation of strong flow fields and to quantify the rate of grain refiner loss due to bubble attachment and floatation. By varying either (a) gas flow rate, (b) grain refiner addition level, or (c) type of grain refiner, it was found that, although grain size increased with time, the increase was predominantly due to particle sedimentation and not caused by attachment.