Factors Affecting the Nucleation Kinetics of Microporosity Formation in Aluminum Alloy A356

Abstract

Metal cleanliness is one of the most critical parameters affecting microporosity formation in aluminum alloy castings. It is generally acknowledged that oxide inclusions in the melt promote microporosity formation by facilitating pore nucleation. In this study, microporosity formation under different casting conditions, which aimed to manipulate the tendency to form and entrain oxide films in small directionally cast A356 samples was investigated. Castings were prepared with and without the aid of argon gas shielding and with a varying pour surface area. Two alloy variants of A356 were tested in which the main difference was Sr content. Porous disc filtration analysis was used to assess the melt cleanliness and identify the inclusions in the castings. The porosity volume fraction and size distribution were measured using X-ray micro-tomography analysis. The measurements show a clear increment in the volume fraction, number density, and pore size in a manner consistent with an increasing tendency to form and entrain oxide films during casting. By fitting the experimental results with a comprehensive pore formation model, an estimate of the pore nucleation population has been made. The model predicts that increasing the tendency to form oxide films increases both the number of nucleation sites and reduces the supersaturation necessary for pore nucleation in A356 castings. Based on the model predictions, Sr modification impacts both the nucleation kinetics and the pore growth kinetics via grain structure.

Characterization of the Effect of Melt Treatments on Melt Quality in Al-7wt %Si-Mg Alloys

“…This mechanism is in agreement with the results of Liu et al [54] and provides an explanation on how the number of pores increased with Sr additions [55]. Yao et al [56] had shown that in the presence of Sr, oxidation increased and nucleation of pores decreased.…”

Effect of electromagnetic transport process on the improvement of hydrogen porosity defect in A380 aluminum alloy

“…When the casting temperatures are 670 C, 700 C and 730 C, the hydrogen porosity defect number (N) of the A380 ingots decreases from No EMT casting's 274, 473, and 572 to EMT casting's 193, 204 and 297, while the hydrogen porosity defect distribution density (Nd) decreases from No EMT casting's 7.6, 13.1 and 16.1 cm À2 to EMT casting's 5.5, 5.8 and 8.2 cm À2 . The diameter of the hydrogen porosity defect in No EMT and EMT casting A380 ingots ranges from 50 mm to 500 mm, and the average diameter of the hydrogen porosity defect (d m ) is about 250 mm, which is consistent with references [3,29,37,42]. Fig.…”

“…For the heterogeneous nucleation mechanisms of hydrogen pores in aluminum alloys during the solidification process, the conventional theory suggests that oxide inclusions in the melt serve as heterogeneous nucleation sites [29], while new nucleation mechanism based on the entrainment of surface oxide bi-films during the pouring process was proposed by Campbell [30,31], and there is still no final conclusion for the two argumentative mechanisms [42]. As shown in Fig.…”

“…For the control of hydrogen porosity defect in aluminum alloy castings, Caceres et al [39] investigated the effect of Cu content on the level of porosity in AleSieCueMg casting alloy, Zhang et al [40] studied the effect of strontium addition on the hydrogen content and porosity shape in AleSi casting alloys, Anyalebechi [41] investigated the effects of hydrogen, solidification conditions and grain refining on the porosity formation in as-cast AleCueMg alloy. However, the formation mechanism of hydrogen porosity is still argumentative [42], and seldom did researchers consider the improvement of hydrogen porosity defect from the view point of the transport process of liquid aluminum alloys during casting.…”

Cellular Automaton Modeling of Microporosity Formation during Solidification of Aluminum Alloys

“…41) It is known that the porosity with larger size will have important impact on the fatigue strength and tensile strength. 16,39,40) Figure 7 presents the maximum porosity radius varying with cooling rate at different initial hydrogen concentrations. Note that with the decrease of cooling rate, the final maximum porosity radius increases.…”