Characterization of AlN Inclusion Particles Formed in Commercial Purity Aluminum
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RODUCTION
THE formation and entrainment of inclusions in Al alloy melts is inevitable during melting and casting processes due to the interactions with environment and atmosphere. Under normal conditions, these inclusions exist in the form of particles, films, and clusters/ agglomerates and are considered as defects affecting the production of Al alloys and detrimental to the performance of final products. It is found that inclusions are often associated with the casting defects such as porosity, hot tearing, and cracking process and lead to poor strength, ductility, machinability, and corrosion resistance of the castings.[1] Therefore, enormous effort has been devoted to the maximum reduction of inclusions from the melt prior to casting. By far, a number of methods including flotation, filtration, sedimentation, and centrifugal and electromagnetic separation have been proposed to remove inclusions from the Al melt.[2–4] Recently, however, it has been demonstrated that the inclusions can be uniformly dispersed throughout the volume of melt and be positively used as nucleation
FENG WANG and ZHONGYUN FAN are with the EPSRC Centre for LiME, BCAST, Brunel University London, Uxbridge, UB8 3PH, UK. Contact e-mail: [email protected] Manuscript submitted October 25, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
substrates for the matrix phase or other primary solid phases via special melt treatment such as intensive melt shearing[5–9] and high-intensity ultrasonication[10,11] in Al and Mg alloys. Such dispersed inclusion particles enhance the heterogeneous nucleation of primary solid phase during solidification and promotes the refinement of the primary solid phase which eventually improves the properties of the castings. In theory, this method of harnessing inclusions as nucleation substrates for the primary solid phase provides a sustainable and economic solution to achieve grain refinement in Al alloys and offers an opportunity for the development of closed-loop recyclable Al alloys since no extra inoculation particles are required. From either removal or utilization point of view, it is important to obtain the comprehensive details of all the inclusions formed and entrained in the Al melt. It is now generally recognized that the major constituent of the inclusions in Al melt are the oxides.[12–14] There are mainly amorphous Al2O3, c-Al2O3, a-Al2O3, MgAl2O4, and MgO that present in the form of particles, agglomerates, and/or films depending on the specific conditions such as melt temperature, alloy composition, pouring atmosphere, and holding/handling time. The formation mechanisms, behavior in the Al melt after entrainment, morphology, and crystallography features of the oxides have been extensively investigated and are readily available in the literature.[15–22] Other common inclusions include carbides, borides, nitrides, chlorides, and fluorides.[12] Among these, nitrides are one type of the
inclusions that are considered as potential candidates of nucleation substrates for a-Al grains[23] and yet few details are avai
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