IN-SITU MAGNESIUM BASED COMPOSITES - DEVELOPMENT AND TRIBOLOGICAL BEHAVIOUR

Abstract

Particle reinforced cast metal matrix composites (PMMCs) result from solidification of a metal or alloy in the restricted spaces between the reinforcing phases. By carefully controlling the relative amounts and distributions of the ingredients constituting a composite and by controlling the solidification conditions, one may synergize the constituents to impart a tailored set of useful engineering properties that cannot be realized in conventional monolithic materials. In addition, the microstructure of the of the matrix is often refined and modified because of solidification in presence of the reinforcing particles, indicating the possibility of controlling micro-segregation, macro-segregation and grain size in the matrix. PMMCs combine metallic properties (ductility and toughness) of the matrix with the characteristics of particle reinforcements, often leading to greater strength, higher wear resistance and higher elevated temperature properties. Stable properties at elevated temperatures require thermally stable reinforcements. In PMMC's where reinforcement is added externally, the resulting combination in ex-situ composite may not be thermodynamically stable at elevated temperatures. Over the last two decades, considerable research efforts have been directed towards the production of in-situ PMMCs, in which the reinforcements are formed in-situ in a metallic matrix by chemical reaction between the constituents during the fabrication of composites. Compared to conventional ex-situ PMMCs, the in-situ PMMCs exhibits the following advantages: (a) the in-situ formed reinforcements are thermodynamically stable and do not dissolve at higher temperature or do not generally have a reaction layer, thereby leading to less degradation during elevated-temperature services; (b) the in-situ generated reinforcements are finer in size and their distribution in the matrix is relatively more uniform, which may lead to better mechanical properties; and (c) the reinforcement-matrix interfaces are clean and uncontaminated which may result in good interfacial bonding, leading to higher ductility and toughness and also, improved wettability during melt processing. The tribological behaviour of particulate composites depends mainly on the type, size and volume fraction of the reinforcing particles. Friction and wear in a material observed during its dry sliding against the counterface of another material depends on the nature, the extent and the strength of contact between the asperities of the mating surfaces. The hardness of the materials in dry sliding contact determines its extent. Higher hardness at the contact reduces the wear of the material significantly. Therefore, it is desirable to design the metal matrix composites which contain hard in-situ formed reinforcing particles like alumina. Solidification processing offers a low cost route for synthesizing such in-situ composites but the resulting cast in-situ composite may often have significant porosity content. The impact of porosities on the tribological properties is of practical concern. However, these porosities may not necessarily be a handicap as they will provide space for storage of lubricants and improve the performance of these cast in-situ composites under lubricated conditions. The rising oil prices compelled the automobile industries to look for better fuel economy and lighter vehicles which has opened the door to expanded usage of lightweight metal matrix composites. Weight saving in a vehicle reduces the requirement of the engine capacity for the same performance and thereby, results in better fuel economy. Therefore, the agenda of weight saving in a car is alive in the automobiles sector which is a high volume material consumer. Development of cast metal matrix composites, the cheapest material in its category, is primarily motivated by the interest of automobile industries which can afford its price range. It is perceived that composite materials are going to make a big difference in automobiles, particularly in view of escalating oil prices. Engines including those used in automobiles have a number of bearings. In addition, there are components like valve seat/valve insert and gudgeon pin where tribological properties are important factors deciding their service life. By replacing the currently used denser alloy based components made of lead, tin, copper or iron by those made of lightweight materials, it is possible to achieve reduction in the weight of engines resulting in considerable fuel economy. Magnesium and its alloys being soft materials, may provide a suitable matrix of a composite for developing bearing by incorporation hard load bearing constituents in it. Recent efforts have led to the development of PMMCs with magnesium as matrix material, and borides, carbides, nitrides, oxides, silicides and their mixtures as second-phase particles. Magnesium and its alloys, owing to their uniquely low density, good damping characteristics, dimensional stability and excellent machinability have become potential candidates for light-weight structural materials. It is easy to fabricate this metal pnd its alloys by a number of conventional techniques. The demand for magnesium alloys has increased rapidly owing to increased efforts by the automotive industry to adopt light-weight materials in order to achieve improved fuel economy and lower emission levels. The use of magnesium in automobile parts is predicted to increase globally at an average rate of :1°/c, per year. As the usage of magnesium alloy components in automobiles and other energy saving applications increase, the demand also increased for further improvement of some the properties of these materials, such as high elastic modulus, hardness, wear resistance and ii lower thermal coefficient of expansion etc. Chapter 1 presents the general context of the present investigation as explained above. Literature review of the last two decades given in chapter 2 indicates that extensive research has been conducted on the development of in-situ composites. These studies show that the objectives for the synthesis of in-situ composites are to generate finer particles with cleaner interfaces, which has been achieved by many novel processing techniques developed. Synthesis of in-situ magnesium based composites has just been initiated and literature is sparse. Therefore, the present study has been undertaken to explore solidification processing of in-situ magnesium based composites with a new approach to inherit constituents of another composite or to initiate the reaction between the matrix alloy of magnesium and minor phase of another alloy. The aim of developing these composites is to employ them in components used in automobiles, particularly components having relative motion, and readily acceptable by the industry people from economical as well as product development and fabrication point of view. In view of the gaps in the present state of knowledge of cast in-situ composites as outlined in literature review on cast in-situ magnesium based composites, the present study has been formulated with the following objectives: 1. To explore the feasibility of solidification synthesis of low cost magnesium alloy based in-situ composite via stir casting in such a way as to generate reinforcing intermetallics (A13Ti, Mg2Si) and/or oxides particles (such as A1203) in-situ from a different composite developed by addition of low cost metal oxide, such as titanium dioxide (Ti02) or the minor phase of silicon existing in an alloy. 2. To determine the influences of different key processing parameters like processing temperature and the amount of titanium dioxide (Ti02)/silicon content in the constituent composite/alloy added to magnesium melt, on the evolution of microstructure and their impact on the mechanical properties of the resulting magnesium based cast in-situ composites. 3. To evaluate the tribological behaviour of the resulting cast in-situ composites including interaction between particle/intermetallic content and porosity in the context of dry sliding friction and wear properties. The experimental procedure followed in the present investigation is given in chapter 3. Special purpose furnace has been designed and fabricated for the solidification processing of intended cast in-situ magnesium or magnesium alloy composites in inert atmosphere. Comprehensive experimental studies have been carried out to achieve the objectives outlined above, in order to enable and to identify the most promising combination of parameters in the context of mechanical and tribological properties. iii maximum size up to 2 um. Formation of intermetallic, Mg2Si, in another type of composite, cast in-situ Mg-Al/Mg2Si composite, has been observed from the XRD pattern of the composite powder and further confirmed in the microstructures of in-situ composite through EDS analysis. In the cast in-situ Mg-Al/Mg2Si composites, the intermetallic, Mg2Si, occurs in blocky shapes with an average size of about 20 lam. In both the types of cast in-situ composites, relatively higher porosities are observed at the top than at the bottom of cast ingot and relatively higher volume fraction of intermetallics are observed at the bottom than at the top of cast ingots. It may be due to gravity segregation due to settling of intermetallics, being heavier than the matrix alloy. In the cast in-situ Mg-Al/Mg2Si composites, the volume fraction of Mg2Si and the porosity increases with increasing addition of silicon. Also, the porosity increases with increasing volume fraction of Mg2Si. In both the cast in-situ composites - Mg-Al/A13Ti-A1203 and Mg-Al/Mg2Si, Brinell hardness decreases gradually along the height of cast ingot from the bottom to top, which may be due to combined effect of decreasing reinforcing phases and increasing porosity along the height of cast ingot from the bottom to the top. Brinell hardness values for both the types of cast in-situ composites are significantly higher than those observed in cast Mg-9 wt% Al alloy and cast commercial magnesium. Brinell hardness decreases with increasing processing temperature in the cast in-situ Mg-Al/A13Ti-A1203 composites, while, in case of the cast in-situ Mg-Al/Mg2Si composites, Brinell hardness increases slightly with increasing processing temperature, however, at higher processing temperatures it decreases, which may be attributed to higher porosities. Brinell hardness increases with increasing reinforcing phases, for both the types of cast in-situ composites. In both the types of cast in-situ composites, comparatively better tensile properties (UTS and percentage elongation) are observed at the bottom than at the top of cast ingots where particle content is relatively more and porosity is less. In cast in-situ Mg-Al/Mg2Si composites, both of the tensile properties, UTS and percent elongation increase initially but decreases at higher processing temperatures. While, in the case of cast in-situ Mg-Al/A13Ti-A1203 composites, UTS increases with increasing processing temperature, but the percentage elongation show slightly decreasing trend with increasing processing temperature. In both the types of cast in-situ composites, both of the tensile properties, UTS and percent elongation, decrease with increasing porosity. In both the types of cast in-situ composites, lower values of tensile properties, UTS and percentage elongation, are observed over Mg-9 wt% Al alloy. vi The results on tribological behaviour of both the cast in-situ composites as well as of cast commercial magnesium and Mg-9 wt% Al alloy are given in chapter 5 and it is observed that the cumulative volume loss increases linearly with increasing normal load for all these materials. In cast commercial magnesium, the wear rate initially increases gradually with increasing normal load but at higher loads it increases rapidly. In Mg-9 wt% Al alloy, the wear rate increases with increasing normal load. In both the types of cast in-situ composites, the wear rate increases with increasing normal load and at higher normal loads. In both the types of cast in-situ composites, the' cumulative volume loss and wear rate decreases considerably, particularly at higher loads, compared to those observed in cast commercial magnesium and Mg-9 wt% Al alloy. In both the types of cast in-situ composites, the wear coefficient decreases gradually with increasing porosity. In both the cast in-situ composites containing similar porosity content, the wear rate decreases gradually and continuously with increasing reinforcing phase where as the wear coefficient increases with increasing reinforcing phase. In cast in-situ Mg-Al/A13Ti-A1203 composites, the average value of coefficient of friction is slightly higher than those observed in cast Mg-9 wt% Al base alloy where as its value for cast in-situ Mg-Al/Mg2Si composites is similar to those observed in cast Mg-9 wt% Al alloy. The of coefficient of friction decreases with increasing normal load in both type of in-situ composites and Mg-9 wt% Al alloy and it decreases at a higher rate for lower loads than that at higher normal loads. The decreasing rate of coefficient of friction is relatively more for cast commercial magnesium and the coefficient of friction decreases from about 0.53 at 10 N to 0.27 at 40 N. In both type of in-situ composites and Mg-9 wt% 4d alloy, there are large fluctuations in the coefficient of friction during sliding only at the lowest load, 10 N where as large fluctuations in the coefficient of friction are observed at all the normal loads for cast commercial magnesium. From the examination of the worn surfaces of wear specimens and the debris, oxidative and abrasive wear mechanisms are observed in both the types of cast in-situ composites. Finally, chapter 6 summarizes the conclusion of the present investigation. vii