The Effect of Degassing on Grain Refinement in Commercial Purity Aluminum

Abstract

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.

Novel Degasification Design for Aluminum Using an Impeller Degasification Water Physical Model

“…In a previous work [6], we have included an extensive list of references of studies that have been done in this regard. In the case of rotating impellers this technique has been employed widely for indirect studies on the degasification operation [7,8] and particle removal kinetics [9]. For example, Saternus and Botor [7] used the oxygen desorption technique in water to simulate the hydrogen desorption process in a continuous aluminum refining reactor, obtaining the range of flow rate producing uniform bubble dispersion and optimal degassing conditions.…”

The Separation Behavior of TiB2 during Cl2-Free Degassing Treatment of 5083 Aluminum Melt

“…Schaffer et al [16] Lance CPAl 1 Ar 5 kg Limited Sedimentation Khorasani [17] Impeller A356-Sr N 2 /Ar 450 kg Significant Floatation Gu et al [18] N.A. CPAl-5% TiB 2 C 2 Cl 6 N.A.…”

“…The discrepancy between the model predictions and experimental data may even be greater should the real bubble size in our degassing unit be substituted in the expression of the model, as the real bubble size in our degassing unit is supposed to be bigger than the bubble size given in Table 4. Another contrasting piece of evidence concerning the applicability of the conventional deterministic flotation model in predicting the TiB 2 removal rate was found by Khorasani and Schaffer et al [16,17]. These authors reported that increasing the gas flow rate has no significant impact on the TiB 2 removal rate.…”

“…Table 1. Compilation of results from the literature concerning TiB 2 removal during degassing treatment [15][16][17][18][19].…”

Modeling of Microporosity Size Distribution in Aluminum Alloy A356

“…[25] Previous work showed that the degassing process is capable of removing (floating) oxide inclusions. [26][27][28] Fully understanding the dependence of nucleation site distribution on oxide content is beyond the scope of the current article and is the subject of a forthcoming publication. In contrast to the fitting parameters used in the expression for the nucleation kinetics, a single cooling rate dependent function for m (Figure 7) was suitable for both castings.…”