Reduced Pressure Test Verification of Healing of Double Oxide Film Defects in Al-Mg Alloys
- PDF / 1,305,967 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 14 Downloads / 171 Views
INTRODUCTION
THE possibility of formation of bonding (dubbed healing) between the two layers of a double oxide film defect[1] in Al melt was first suggested by Nyahumwa et al.,[2] who suggested that, after oxygen and nitrogen of the trapped atmosphere between the two layers are consumed by the reaction with the surrounding Al melt, the two layers come into contact with one another and might bond to each other. This hypothesis was investigated and confirmed recently by this research team for different Al alloys. Aryafar et al.[3] put two Al bars on top of each other in a steel tube so that the two oxide layers covering the bases of the Al bars, where the bars were in contact with each other, and the atmosphere between them modeled an oxide film defect. The steel tube was then held in an electric furnace for times ranging from 5 seconds to 50 hours. The Al bars melted inside the steel tube and the two oxide layers were in contact with one another and maintained in the liquid metal during the experiment. The researchers studied any changes in the composition and morphology of the oxide layers by scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS). Aryafar et al.[3] found that the two layers of a newly formed oxide film defect began bonding with each other after being held in Al-0.3 wt pct Mg melt for 13 minutes and the bonding became almost complete in about AMINEH SHAFAEI, Postgraduate Student, and RAMIN RAISZADEH, Associate Professor, are with the Department of Metallurgy and Materials Science, School of Engineering and Mineral Industries Research Center, Shahid Bahonar University of Kerman, Jomhoori Eslami Blvd., Kerman, Iran. Contact e-mail: rraiszadeh@ yahoo.com Manuscript submitted December 4, 2013. Article published online July 19, 2014. 2486—VOLUME 45B, DECEMBER 2014
5 hours. The bonding was suggested to occur as a result of the transformation of Al2O3 to spinel (MgAl2O4) and spinel to MgO in short and long holding times, respectively. These researchers suggested that two criteria had to be met for the layers of an oxide film defect in Al melts to bond to each other (i.e., to heal): (1) almost complete consumption of O and N within the defect through the reaction with the surrounding melt and (2) occurrence of a transformation involving rearrangement of atoms on the internal surfaces of the oxide layers. The bonding between two layers of an oxide film defect was also observed by Najafzadeh-bakhtiarani and Raiszadeh[4] to start forming in commercial purity Al alloy after incubation time of about 5 hours. They suggested that the bonding occurred as a result of the transformation of c- to a-Al2O3. In Al-0.05 wt pct Sr melt,[5] A12O3 layers of a double oxide film defect started to transform to SrO gradually and this transformation caused the two layers to bond with each other over time. Amirinejhad et al.[6] studied the possibility of formation of bonding between the two layers of a double oxide film defect in Al melts containing different Mg concentrations between 0.3 and 4.5 wt pct using the t
Data Loading...
