Investigation and characterization of fluidity and microstructure of recycled AL-B4C materials /

Abstract

In recent years, AI-B4C Metal Matrix Composites (MMCs) have been increasingly used as neutron absorber components in the nuclear industry. During manufacturing processes, the process scrap from the casting and transformation processes (extrusion and rolling) can reach 50 to 60% of the total materials produced. The need to recycle AI-B4C composites becomes more urgent to meet environmental goals and to reduce the production costs. Remelting is a promising method to recycle process scrap materials because of its simplicity. For the liquid metal casting process, good fluidity is a basic requirement for the materials to be recycled with the remelting process. The reinforcement characteristics such as size, shape and volume fraction of ceramic particles have an effect on the fluidity of Al-based composites. Moreover, the particle agglomerates, particle settling and pushing, presence of oxide films, and the appearance of reaction-induced particles influence the flow behavior of the composite melt.The study presented in this thesis was carried out to investigate the fluidity evolution of AI-B4C recycled scrap. In this project, the fluidity evolution of scrap from AA6063-10 vol.% B 4 C cast billets and extruded plates, as well as AA1100-16 vol.% B 4 C cast ingots and rolled sheets is investigated by means of vacuum fluidity tests. In order to understand the different materials, the microstructure evolution of original materials, crucible samples, as well as fluidity samples as different holding times is examined with optical and electron microscopes. The morphology and distribution of the B4C particles and its reaction products in these samples are quantitatively characterized by an image analyzer. Furthermore, the influence of microstructure on fluidity behavior of these materials is discussed.The methods for characterization of AI-B4C composites microstructure are developed in this research. These methods have been successfully applied to describe and quantitatively analyze the particle volume fraction, distribution, agglomeration and particle effective volume fraction. General image analysis techniques are introduced to distinct particles for subsequent quantitative measurements. An appropriate homogeneity parameter P for charactering the distribution homogeneity is proposed to be the ratio of variance of distribution of cell areas to the corresponding variance obtained from a random particle distribution with the same amount of particles. The particle agglomerates in the form of clusters and networks induced by oxide films are identified. And the effective volume fraction of particles is introduced to reflect the flow resistance of particle segregation and agglomeration in the AI-B4C MMC microstructure.The results show that the fluidity of either cast or extruded AA6063-10 vol.% B4C metal matrix composites decreases with the increase of the holding time. The fluidity decline of the cast billets is much faster than that of the extruded plates during the melt holding period (8.5 h). The microstructures of orig...