MIXING CHARACTERISTICS AND MECHANICAL PROPERTIES OF CAST Al (Mg)-A1203 PARTICULATE COMPOSITE

Abstract

Aluminium alloy particulate composites have received wide spread attention from the technologists due to its improved specific strength (strength/unit weight) and stiffness especially at elevated temperatures. A number of processes have been tried so far to fabricate these particulate composites but the foundry technique has been considered as a more direct and simpler one as compared to other techniques used. However, the primary L2 difficulties in making cast composites by vortex method area-- to attain a significant retention of particles, their homogeneous distribution in the matrix and reproducible characteristics. The maintenance of suitable mixing conditions during agitation of a partially solid aluminium.alloy is necessary to surmount the present difficulties. However, no systematic investigation of the parameters influencing the mixing had been undertaken. The mechanical properties of the compocast particulate composites are very much sensitive to the volume fraction of porosity, the particle content, their shape, size and distribution in the matrix. The shape, size and the distribution of porosity can not be fully controlled during the casting process. It is common practice to correlate the mechanical properties of the cast materials directly to the volume fraction of porosity. However, -the role of particle porosity interaction in cast particulate composites influencing its mechanical properties at ambient and elevated temperatures have not been clearly understood. iii In this study an effort has been made to optimise the mixing parameters for the incorporation of alumina particles in partially solid aluminium alloy melt so as to achieve the maximum incorporation of the particles •and a homogeneous distribution. The role of porosity in reducing the ambient and elevated temperature strength of the cast particulate composites has been investigated with the help of a phenomenological model and the experimental results. The adverse contribution of porosity on the strength has been estimated in the cast composites having different volume fraction of alumina particles with narrow and broad spectrum of sizes. The projected ultimate tensile strength at zero porosity level has been determined with the help of the model. The effect of porosity content on the engineering fracture strain of the composites containing different levels of alumina content has been found out at ambient and elevated temperatures. The detailed perspective of the problem under investigation has been introduced in chapter 1. The critical review of the literature on the variations of vortex method used for making cast particulate composite e.g., liquid metal process, partially solid melt process (compocasting) have been explained in chapter 2. The chapter 2 also contains a review of the mechanical properties of the composites. In chapter 3 the experimental method used to study the relevant process parameters influencing the fabrication of particulate composite in the vortex method and the procedure for characterisation of the mechanical properties have been iv described. The particulate composites have been prepared by adding pre-heated (1072K) alumina particles of narrow and wide spectrum of sizes from the top to the vortex of the vigorously agitated freshly alloyed aluminium-4wt.%Mg alloy at a given holding temperature. The slurry has been cast through bottom pouring and the ingot has been quenched by spraying water on it. The chemical composition of the composites has been determined by spectrographic analysis. The volume fraction of alumina particles and the porosity in each casting has been estimated by determining density by weight loss method and point counting under optical microscope. The impact of process parameters like the holding temperature of the melt, the speed of stirring the size and the position of the impeller inside t the melt on the retention of alumina particles in the cast composites and its porosity content has been investigated. Flat four blade impellers of different sizes have been used. The microstructures of the cast composites have been studied under optical microscope with a view to determine the roles of various process parameters. The particle-matrix interface has been of particular interest for observations under scanning electron microscope (SEM). The composites have been analysed by X-ray diffraction to detect products of interfacial reaction between the particles and the alloy. Mechanical properties of the composites have been investigated under tension at ambient and elevated temperatures by taking samples from different positions of the ingots. The volume fraction of alumina and the porosity content of each u specimen have been determined. The effects of true strain and porosity on the nucleation and growth of voids during testing under tension at ambient temperature have been estimated under SEM. The fractured, surface of the specimens have been investigated under SEM. Chapter 4 deals with the influence of holding temperature, stirring speed, the diameter and the position of the impeller on the particle retension, porosity content and the particle-matrix interface in the cast composites. The retention of alumina particles in the cast composites has been found to increase with the decrease in melt temperature between the liquidus (915K) and the solidus (875K) temperature of Al-4wt.%Mg alloy for a given stirring speed, diameter and position of the impeller. However at the holding temperature below 883K the casting of the slurry has been found extremely difficult. The retention of alumina particle in the partially, solid alloy at 900K has been found to increase initially with the increase in the stirring speed, the diameter and the position of the impeller from the bottom of the crucible 'upto a certain value followed by a decrease with further increase in the values of these parameters. The optimum values of the stirring speed, the diameter of the impeller and the position of the impeller for the maximum retention of alumina particles ( ^ 12.5 wt.%) in the partially solid aluminium alloy at a temperature of 900K have been found as — 16 revolution.' , 0.63D and 0.81H respectively where D is the diameter of the melt surface at rest and H is the height of the melt at rest from the bottom of the crucible. A deviation in mixing parameters from the vi optimum value has been found to result in a significant clustering of alumina particles in the matrix. The various, features of the retention of alumina particles in the matrix resulting from different mixing conditions are evident from the micrographs of different castings. The analysis of the mixing process carried out with the help of a model expression correlating the various parameters of"a concentrically agitated liquid system influencing the liquid profile in the vessel shows that in all the choices of process parameters in this work the impeller surface has been exposed except during mixing at the holding temperature of 883K. The exposing of the impeller has been found to play ,a helpful role in retaining the particles in the agitated slurry. However, the retention of particles has been found to be controlled by the radius of the cylindrically rotating zone (c.r.z.), rc, which increases with the increase in the diameter of the impeller at a given holding temperature. The retention of particles in the slurry increases so long as rc remains below r3, the radius of the central region of the impeller excluding the blades. In case of low holding temperature of 883K a large ✓ retention of alumina inside the melt without any exposure of the impeller may have been possible due to mechanical entrapping of the particles. The influence of the holding temperature, the stirring speed, the diameter and the position of the impeller inside the melt on the microstructures observed at the bottom and the vii top of the castings show that under all the combinations of mixing parameters a tendency to form a dendritic microstructure is always there especially at the top of the ingot. The extent of the dendritic growth at the top of the ingot has increased with a reduced holding temperature, stirring speed and size of the impeller. During pouring the stirring was on and the slurry from the central regions of the crucible has come out first through the bottom hole of the crucible followed, by the slurry from the side of the crucible wall which may have undergone a limited dendritic growth in the boundary layer. In the composites fabricated at a lower holding temperature of 883K the zones of coarse dendrite and large pri" solid particles have been observed having no alumina particles entrapped in this region. An examination of microstructure also reveals a reacted layer at the boundry of the alumina particles. The thickness of this reacted layer reduces with an increase in the stirring speed and the holding temperature. It has been attributed to detachment of the reacted layer from particle (caused by stirring) due to its viscous liquid nature. This effect is reduced with a reduction of the stirring speed. Lower fluidity at a lower holding temperature also counters the tendency of detachment. The reacted layers are of irregular shape in the composite and often cracks during solidification as it has been observed in the microstructures under scanning electron microscope. viii The cold model experiments have been conducted with water and poorly wetting plastic beads to observe visually the role of stirring speed and the size of the impeller on the particle retention. It has been noticed that the deviation of these parameters from the optimum level cause either non-immersion or rejection of the particles and sometimes, even accumulation of the particles at the bottom of the impeller. The observations of the cold model experiments have been found to be in qualitative agreement with the results obtained in the cast composites. The porosity in the composites has been found to increase initially with the increase in stirring speed, the diameter of the impeller and its distance from the bottom of the crucible upto a certain value followed by a decrease with •a further increase in these process variables. However, the porosity of the composites has always been found to increase with the decrease in holding temperature of the melt. It is interesting to note that the porosity in the composites increases linearly with the level of retention of alumina. The increase in porosity in a composite is accompanied with a larger inhomogeneity in the distribution of particles. The scanning electron microscopic observation on the microstructures of the composites shows the presence of pores primarily at the boundry of the p-AmAyy solid particles. Some pores have also been observed inside the primary particles. ix The influence of porosity and alumina content on the mechanical properties of compocast Al-4wt.%Mg + alumina particulate composites at ambient and. elevated temperatures are presented in chapter 5.- The contribution of porosity on the reduction of the strength of the composite at various levels of alumina content has been expressed as a linear function of porosity with two experimentally determined parameters, o°o the ultimate tensile strength at zero porosity and oC , a weakening factor, as p /do = 1 - oC P where, o'p is the tensile strength of the composite at the volume percent porosity P, in the composite. At ambient temperature the weakening effect of porosity expressed as aC has been found to decrease significantly with the increase in alumina content (with wide spectrum of sizes) of the composite upto - 7 vol.%. But the rate of reduction slows down at higher alumina levels in the composite. With d , the corresponding values of o also follow a similar trend of reduction with an increase in the alumina content. For a given alumina content the increase in average particle size from 22 um to 115 um has been found to push the value of oC a little up followed by a significant decrease with a further increase in the particle size upto 195 Um. However, the increase in average particle size from 22 pm to 195 μm has always been found to reduce the value of d o . The behaviour of the curve for cc versus alumina content of the composite X obtained with the help of model equation has been found in agreement to the trend of variation in ultimate tensile strength of a composite with its alumina content for a given level of porosity. The role of porosity on the engineering fracture strain, ef, of the composite has been found to increase almost linearly with the inverse of porosity beyond a critical level of porosity. The fracture of the composite within this linear regime of high porosity content has been caused primarily by the growth and coalescence of the existing pores along with some nucleated voids in the matrix as evident from scanning electron micrographs. In the region of lower porosity beyond the critical level the fracture of the composite is possibly dominated by the nucleation of voids by particle debonding and their coalescence. The wide spread debonding has been reflected in the observed serrations in tensile stress-strain curve of the composite. The critical level of the inverse of porosity below which the linear behaviour of of versus P-1 is observed increases with the decrease in alumina content of the composite and the composite having higher alumina content shows a linear behaviour upto a lower ef value. This may have been due to lower strain for void nucleation, ETN, caused by a decrease in particle content has resulted into new voids assuming a primary role in the fracture of composites with high alumina content. At elevated temperatures of 473 K and 573 K the weakening factor, cC , has been found to decrease with an increase in the xi volume fraction of alumina in the composite similar to that observed at ambient temperature. But, here, or is more sensitive to the alumina content of the composite. The weakening factor in a composite increases significantly with the increase of temperature but this effect gradually reduces with an increase in the alumina content of the composite. Finally, in a composite having '•• 10.3 Vol.% alumina the weakening factor, c , reduces with an increase in the temperature. The tensile strength at zero porosity level, do, of the composite estimated at elevated temperatures has been found to reduce with the increase of alumina, content upto a certain level ( — 9 vol.%) similarly as it has been observed at ambient temperature. In contrast to the observations at the ambient temperature a sharp fall of do has been observed at higher alumina contents at the elevated temperature of 473K and 573K. The o of a composite of a given alumina content does not vary significantly with the increase in temperature upto about 473K but drops sharply with a further increase in temperature. This reduction in tensile strength at zero porosity, c, is attributed to dynamic recrystalisation of the matrix under tensile deformation. The tendency to recrystallisation has been found comparatively more in the matrix around the alumina particles which may have caused a sharp fall in do at a higher alumina level of about 10 Vol.%. An increase in the porosity level has been found to reduce the engineering fracture strain, ef, of a composite at elevated temperature and this effect is more pronounced xii at higher particle content of the composite. At a given porosity level the engineering fracture strain of a composite decreases with an increase in the temperature. At elevated temperatures the fracture of the composite has been marked by the growth and coalescence of voids at the boundary of primary sol-LcUparticles and the recrystalised zones.