MICROSTRUCTURE AND MECHANICAL PROPERTIES OF RHEOCAST ALUMINIUM-COPPER ALLOYS

Abstract

Rheoeasting is an emerging casting technique in which a liquid alloy is vigorously agitated during its partial solidification to yield a slurry comprising of non-dendritic solid particles uniformly suspended in the remaining liquid. Such a slurry is subsequently cast into desired shapes by conventional methods. Most of the investigations relating to this process deal with the structure and theological behaviour of alloy slurries and are primarily aimed at process development rather than product characterisation. A ciritical survey of the published literature reveals that the interrelationship of processing , microstructure and mechanical properties of rheocast products has not yet been clearly understood and therefore, the need for a detailed study of the microstructural features of rheocast products and their role in modifying the mechanical properties cannot be overemphasized. In view of the above, the present investigation has been undertaken to study the microstructure and mechanical properties of rheocast kl-Cu alloys in the composition range of 4.5 to 10 Wt./. copper. The ail- Cu system has been chosen for the study because of large amount of available information regarding its solidification characteristics and also because this system forms base for a large number of industrially important alloys in this composition range. Three alloys of nominal composition Al-4.5 Wt. 's. Cu ;i-6 Wt.'/. Cu and Al-10 Wt.'/. Cu have been used for making rheocastings. The microstructural features of rheocastings such as, morphology, size and distribution of proeutectic a-phase have been studied by optical microscopy and the effects of rheocasting parameters, pouring temperature and stirring speed, have been examined. Tensile tests have been performed to evaluate the strength of castings and tensile fractured surfaces have been examined under Scanning Electron Microscope (SEM) to study the fracture behaviour. For a better understanding of the physical metallurgy of the mechanical properties, the rheocastings have been modeled as particulate composites containing ductile a-particles and brittle %+CuAil- eutectic. Two theoretical models for the strength of rheocast alloys- one for the case when eutectic is continuous and the other, for the case when x-particles join to form a continuous phase- have been proposed. The experimental data have been compared with theoretical predictions. The entire work reported in this thesis has been spread over six chapters. In Chapter 1, a critical review of the published literature has been presented. Various hypotheses concerning morphology, size and distribution of primary particles have been critically examined in the light of classical theories of nucleation and growth. Theories relating to the mechanical properties of composite materials have been presented. Lastly, in the perspective of present understanding, the problem under investigation has been posed. Chapter II deals with the theoretical analysis of the mechanical properties of model rheocast alloys as conceived in the present investigation. After defining the physical state of constituent phases , two theoretical models on the basis of the concept of particulate composites have been developed - one, for the case when eutectic is continuous and the other, when a--phase is continuous. A parametric model has been developed to examine the effect of porosity on the strength of real life castings. In Chapter III, the details of experimental set-up, alloy selection, choice of process parameters and experimental procedure have been presented., The results pertaining to microstructural features of the rheocast alloys have been presented and discussed in Chapter IV. The effects of process parameters on morphology$ size and distribution of proeutectic a--phase have been examined on the basis of prevailing solidification condition. In Chapter V, mechanical properties of rheocastings have been presented and analysed on the basis of theoretical models developed in Chapter II. From the least square fit of experimental data the effects of average size of a-particles and porosity on the tensile strength have been evaluated. Strength values of rheocastings have been compared with those of conventional casting at equivalent porosity level. In the end of the chapter, results of SEM fractographic observations have been presented to throw light on the mechanism by which strengths of rheocast Ey1-Cu alloys is governed. I1stly, conclusions based on the present investigation have been presented in Chapter VI.