ULTRASONIC PROCESSING OF MAGNESIUM ALLOY MELTS

Abstract

Magnesium alloy developments have traditionally been driven by requirements of aerospace and automobile industries. Interest in Mg based alloys stems from their low density, high strength-to-weight ratio and low inertia which results from its low density. The primary problem in all alloy systems is that dendrites are produced in normal castings because of their large freezing ranges, whereby a constitutionally undercooled region can be readily produced ahead of the growing front. Therefore, suppressing the formation of these coarse dendrites is a key issue in improving the alloy properties. During solidification of alloys, there are generally two ways to achieve a refined microstructure. One is to break up the growing dendritic structure so that each secondary dendritic arm becomes a grain. Another way is to provide a large quantity of nuclei so that there is no excess room for each grain to grow into a dendrite. Several techniques such as inoculation, electromagnetic vibrations, and mechanical stirring are employed for the modification of grains and the grain size distribution in AZ series magnesium alloys. Use of physical methods such as mechanical vibrations/ stirring and electromagnetic vibration techniques during solidification are known to result in finer grain size in cast products. A new effective but less practiced method of grain refinement is the ultrasonic treatment (UST). Ultrasonication is environmentally cleaner and one of the simple and effective physical methods used to refine the grain size of alloys during solidification process. Application of high intensity ultrasonic vibrations during solidification produces two effects-ultrasonic streaming of the liquid (increased convection) and ultrasound induced cavitations. These lead to various physical and indirect chemical effects in the liquid. Scientific literature documents the vast capability and potential of high power ultrasonics in materials processing. Much work has been carried out to study the solidification behavior during ultrasonic treatment, in particular the microstructure refinement and properties of various alloys. However, it mostly concerns with the studies in aluminum alloys. A major problem with Mg-Al alloys is their large and variable grain size with coarse dendritic morphology which results in their poor mechanical properties. Unlike the Zr containing magnesium alloys, Mg-Al alloys are Zr free and no potent inoculants are available in order to obtain uniform and finer grain size. iv Therefore, physical method of microstructure modification such as ultrasonication during solidification may prove useful. Further, Al containing Mg alloy systems suffer from lack of a potent nucleating agent. In this regard, grain refining techniques such as superheating of the melt, ferric chloride inoculation (Elfinal process), and carbon inoculation have been tried with moderate success. Among these methods, carbon inoculation using reagents like C2Cl6, CCl4, and graphite offer many practical advantages because of lower operating temperatures and less fading effects with long holding times, at a lower cost. Carbon black is an easily available and inexpensive form of carbon that has nano size morphology. Present work investigates the inoculation potency of these nano particles in Mg-Al alloy melts. This approach will also help replace the conventionally used toxic waste generating hexachloroethane, C2Cl6, grain refiner. When ultrasonic vibrations are coupled to the melts, it is expected to accelerate the wetting, de-agglomeration, and dispersion of inoculant particles and also facilitate degassing of the melts thereby reducing the microporosity. Similarly, use of ultrasonic vibrations is also being explored in a new solidification processing technique in order to fabricate fine-grained, wear resistant MMCs reinforced with nano and microsized ceramic particles, in order to overcome the drawbacks of stir casting. In view of the potential of in situ synthesis approach and the UST, both these techniques can be coupled for the fabrication of AZ91 alloy matrix composites by the addition of suitable reactant to the melt. Such approach is expected to result in refined micro/nano composites because of the physical and concomitant chemical phenomena induced by UST. In view of the above, there is a need to study the grain refinement, distribution of intermetallic phases and microstructure modification of magnesium alloys by high intensity ultrasonic vibration and different mechanisms operating under varying ultrasonic processing conditions. In the present work, individual and combined effects of physical (ultrasonic vibrations) and chemical (inoculation) techniques on the microstructural development and mechanical properties in AZ series magnesium alloy melts is studied. Reactively synthesized and ultrasonically processed MMCs are fabricated and their microstructural evolution, mechanical and sliding wear properties is investigated. Formation of hard oxide reinforcement particles by in situ reactions is studied. The microstructure of the composites is characterized for the uniformity in the distribution of reinforcement particles and is correlated with their mechanical properties. By evaluating the dry sliding wear behavior over a range of loads, the operating wear mechanisms are analysed. v The thesis consists of eight chapters. The first chapter consists of introduction to the topic and gives a brief idea about the overview and benefits of magnesium alloys used for different applications. The second chapter gives a comprehensive literature review on the recent developments in the field of magnesium alloys; recent processing techniques available for magnesium alloys, history of ultrasonic processing techniques and different theories related to ultrasonic processing. It also defines objectives of the present work based on literature review, scope of the work and the methodology adopted in the present investigation. The experimental study carried out in line with the scope of the work is explained in detail in the third chapter. Results obtained are discussed in next three chapters. In the fourth chapter results of various UST parameters on the microstructural evolution and mechanical properties of magnesium alloys is discussed. Application of ultrasonic processing to the molten magnesium alloys are carried out by two different processing approached: first one is continuous cooling during the application of UST and other one is isothermal UST processing followed by cooling. In the Fifth chapter, synergy of nano carbon black inoculation and high intensity ultrasonic processing in cast magnesium alloys is discussed. Commercial AZ series magnesium alloy containing three, six, and nine weight pct of aluminum (AZ91, AZ61 and AZ31) are investigated for their response to carbon black nano-particle inoculation and high intensity ultrasonic processing. Potency of inoculants is assessed under varying experimental conditions of untreated, carbon black treated and combined carbon black inoculation treatment followed by ultrasonic treatment conditions. Resulting grain refinement is studied and explained based on existing theories of nucleation and ultrasound induced phenomena in molten metal. In chapter six, AZ91 and AZ31 alloys matrix composites are synthesized by in situ reactive formation of hard MgO and Al2O3 particles by the addition of 1, 5, and 10 wt pct magnesium nitrate (Mg(NO3)2) as a reactant to the molten alloy. Application of ultrasonic vibrations to the melt increased the uniformity of particle distribution, avoided agglomeration, helped to reduce porosity in the castings. In situ chemical reactions were promoted and accelerated because of the ultrasound induced physical phenomena such as cavitation and liquid streaming. Presence of well dispersed, reactively formed, hard oxide and intermetallic particles increased the hardness, ultimate strength, and strain-hardening exponent of the AZ91 and AZ31 alloys matrix composites. Their improved sliding wear resistance is attributed to vi presence of hard oxide particles and further ultrasonic treatment produced distribution and cavitation enhanced wetting of reactively formed particles in the AZ91 and AZ31 alloys matrix. The chapters seven and eight contain the conclusions and scope for future work, respectively.