DEVELOPMENT OF Cu-Cr ALLOY BASED COMPOSITE AND THEIR PHYSICAL AND TRIBOLOGICAL PROPERTIES

Abstract

The particle-reinforced MMCs are of particular interest due to their ease of fabrication, lower costs, and isotropic properties. Traditionally, discontinuously reinforced MMCs have been produced by several processing routes such as powder metallurgy, spray deposition, mechanical alloying (MA) and various casting techniques. All these techniques are based on addition of ceramic reinforcements to the matrix materials which may be in molten or powder form. For the conventional MMCs, the reinforcing phases are prepared separately prior to the composite fabrication. Thus, the scale of the reinforcing phase is limited by the starting powder size, which is typically of the order of microns to tens of microns and rarely below 1 μm. It is widely recognized that the properties of MMCs are controlled by the size and volume fraction of the reinforcements as well as the nature of bonding at the matrix-reinforcement interfaces. The external addition of particles often leads to poor bonding due to interfacial reactions between the reinforcements and the matrix, and surface contamination of the reinforcements. During the past decade, another approach to the synthesis of metal matrix composites (MMCs) has been undertaken, where a reaction takes place during processing to result in reinforcing particles in-situ. A large number of composites have been synthesized through this approach, with a wide range of matrix materials based on aluminum, titanium, copper, nickel and iron generating reinforcing particles of borides, carbides, nitrides, oxides and their mixtures. Because of the formation of ultra fine and stable ceramic reinforcements, these MMCs are found to exhibit excellent mechanical properties. As compared to the conventional MMCs produced by external addition of particles, the MMCs having internally generated reinforcements exhibit the following advantage: (a) the in situ generated reinforcements are thermodynamically stable at the matrix leading to less degradation during elevated-temperature service, (b) the reinforcement matrix interfaces are clean, resulting in a strong interfacial bonding and (c) the in -situ formed reinforcing particles are finer in size and their distribution in the matrix is more uniform, yielding better mechanical properties. Cast and wrought copper alloys find applications in resistance welding electrodes, electrical contacts, shipping hulls and electrode holders where physical and tribological properties are important. Often the required shape of the component is complex and H uneconomical for machining and so, cast alloy is preferred. Copper alloys developed for these applications are chromium-copper, chromium- zirconium-copper, beryllium-copper and tungsten-copper alloys. Copper-chromium alloys are commonly used in the wrought forms as electrode rod for spot welding and bar for high strength conductors, and as forged wheels for seam welding and discs for aircraft brake. In the present investigation, the aim is to develop Cu-Cr alloy based composite taking advantage of strong carbide forming tendency of chromium in the alloy. The reinforcements particle are to be generated internally during processing by reaction between either SiC or graphite particles added during solidification processing of Cu-Cr-SiC and Cu-Cr-G (graphite) composites. The effects of reinforcement particle on the physical property like thermal, electrical and mechanical properties have been investigated apart from dry sliding friction and wear behaviour. The thesis has the following six chapters. Chapter-1: The perspective of the problem under investigation has been outlined in chapter1. Chapter-2: The literature review as reported in this chapter of the thesis describes the detailed knowledge regarding the development of cast composites with particular emphasis on solidification processing where the reinforcements are generated internally within the matrix alloy by reaction with added particles. The present understanding of physical properties of composites like thermal, electrical and mechanical properties, have been reviewed but there is lack of studies involving copper or copper alloy based composites reinforced by ceramic particles. Chapter-3: This chapter of the thesis contains the details of experimental procedures followed in the current investigation. In the present study, commercial copper has been selected as the base material and chromium for alloying it and the alloy is to become the matrix of cast composites synthesized. The chemical composition of the as received copper ingots was determined by atomic absorption spectroscopic. Powders of hard silicon carbide (SiC) and soft graphite particles have been selected for reacting with the matrix alloy, which is the well known Cu-Cr alloy containing strong carbide former chromium. The carbide particles may impart strength, wear resistance and the composites may still have acceptable electrical and thermal properties. 11 About 4 kg of commercially pure copper was melted in a clay-graphite crucible placed in an induction furnace and 4 wt% of chromium metal powder was added using graphite blade stirrer. At temperature of around 1400-1500 °C, the surface of the melt was cleaned by skimming and weighed quantity of SiC or graphite was introduced at a controlled rate through a vibratory feeder, into the Cu-Cr alloy melt while stirring by graphite blade stirrer driven by a 1 HP motor, having ,a maximum rated speed of 4000 RPM. The position of stirrer inside the crucible was always kept constant at a given level. A digital temperature indicator connected to a platinum-rhodium thermocouple was placed at a depth of 10-20 mm inside the melt to record the temperature. The tip of the thermocouple was covered by alumina paste and sintered so as to prevent any reaction between the thermocouple and the melt. During stirring, the temperature of the slurry was maintained within ±50 °C around the processing temperature. After 10 minutes, the stirring was stopped. No degassing practice of the melt or the slurry was carried out at any stage of processing. The melt-particle slurry in the crucible was poured into a sand mould and allowed to cool in air. For comparison, of microstructure, mechanical and tribological properties of the cast composites, as received copper and Cu - 4 wt% Cr alloys were also cast by following the same procedure used for the synthesis of cast composites, as described above. There are broadly two types of composites depending on the addition of SiC and graphite into the molten melt of Cu-Cr alloy. These composites developed by addition of SiC into Cum 44CCrraallloy, have been designated on the basis of nominal composition as..Cu-4Cr-2SiC, Cu-4Cr-3SiC and Cu-4Cr-4SiC. Similarly, composites developed by addition of graphite have been designated based on the nominal composition as Cu-4Cr-2G, Cu-4Cr-3G and Cu-4Cr-4G where G stand raphite. The composites have been examined for characterization i ution of phases by X-ray diffraction, optical and scanning electron microscopy. The hardness and the tensile properties have been determined in samples whose particle contents have been measured. The dry sliding friction and wear behaviour has been determined on a pin-on-disc set-up at different loads but a fixed sliding speed. The thermal and the electrical properties have also been measured respectively by using Searle's bar apparatus and by four-probe measurement. Chapter-4: The results on characterization and physical properties of composites are reported in this chapter. The chemical composition and the particle content of the composites 111 are followed by the results of structural characterization by XRD, microstructural examination through optical and scanning electron microscope. After the results on characterization of the composite, the Mechanical behaviour as determined through hardness and tensile properties has been reported and it is followedb'y`the results on thermal and electrical properties. The chapter ends with a section on discussion of all the results. On the basis of the results presented in this chapter it has been observed that there is substantial reaction between the SiC and graphite particles with molten Cu-Cr alloy, resulting in the formation of chromium carbides. It is possible that presence of SiC or graphite particles are providing heterogeneous sites for the nucleation of chromium precipitates, thereby refining their size. It is interesting to note that the composites contain a host of particles of varying sizes from nanometers to micrometers. The tensile properties indicate that the formation of carbides contributes to strengthening the composites and in Cu-Cr-SiC composites, there is also strengthening of the matrix by alloying with silicon released through the reaction. The tensile strength increases linearly with increasing addition of particles overcoming the adverse effect of increasing porosity. However, a slightly higher strength is observed consistently for the composites developed by addition of SiC particles compared to those observed for similar addition of graphite. The percent elongation decreases almost linearly with increasing addition of particles. Similarly, the famed and both forged and annealed Cu-Cr-SiC and Cu-Cr-G composites have been characterized following the same procedures along with as received copper, cast copper and Cu-4Cr alloy after forging as well as after forging and annealing. The hardness and tensile strength of composites increase on forging but the hardness of the forged composite decreases after annealing to a value lower than the cast composites. The increase in tensile strength and hardness due to forging could be attributed to reduced porosity and work hardening of the matrix. Forged composites show lower hardness after annealing due to modification of cast structure and elimination of strain hardening. Thermal conductivity of as received copper, cast .copper, Cu-Cr alloy and cast Cu-4Cr-4SiC and Cu-4Cr-4G composites have been determined by Searle's bar apparatus. Thermal conductivity of as received, cast copper and Cu-4 wt% Cr alloy are 349.72, 316.14 and 257.57 W/m°K respectively while cast Cu-4Cr-4SiC and Cu-4Cr-4G have thermal ry conductivities of 130.16 and 145.32 W/m°K respectively. The electrical conductivity of as received copper, cast copper, Cu-Cr alloy, and cast Cu-4Cr-4SiC and Cu-4Cr-4G composites have been measured by using the Keithley nano-voltmeter. The results reveal that the electrical conductivity of as received copper, cast copper and Cu-4Cr alloy are 47.76, 30.49 and 27.97 %IACS (International Annealed Copper Standard) respectively, while those observed in cast Cu-4Cr-4SiC and Cu-4Cr-4G are 20.64 and 21.97 %IACS. The thermal and electrical conductivity of as received copper decrease on casting and alloying but dispersion of SiC /graphite particles has decreased both the conductivities significantly as observed in Cu-4Cr-4SiC and Cu-4Cr-4G composites. For the application of resistance spot welding highly pure about 99 % copper along with 1 % of chromium have the 78 %IACS while for the cast alloy of Cu-Cr this value is decreases to 70 %IACS. While Cu-Cr alloy containing other materials like 0.5 % beryllium, 1 % nickel, I % cobalt incorporating for imparting the high hardness and good wear resistance, electrical conductivity reduced to 45-50 % IACS niake it a good material for electrodes used in spot welding resistance application where high pressure and workpiece resistance are high. In the present investigation the electrical conductivities of Cu-4Cr-4SiC and Cu=4Cr-4G are 20.64% and 21.97% IACS respectively. It may be remembered that the present study has been conducted with commercial copper having conductivity of 47.76% IACS and it has been reduced by alloying, particle dispersion and casting by less than half. For electrical conductivity also, it is observed that with increasing particle content, the electrical conductivity in %IACS decreases linearly irrespective of the nature of particles dispersed. Chapter-5: Dry sliding wear tests have been carried out by sliding cylindrical pin with a flat polished end against counterface of hardened SAE 4615 steel disc under ambient conditions using a pin-on-disc machine. The sliding wear tests have been conducted for the specimens of cast Cu-Cr-SiC and Cu-Cr-G composites at four different normal of loads of 10, 20, 30 and 40 N and a constant sliding speed of 0.786 m/s. The cumulative weight loss with increasing sliding distance has been measured and converted into cumulative volume loss on the basis of density of a test specimen. The wear rate, in units of mm3/m, for a given specimen under a given condition of sliding, has been determined from the slope of the linear variation of cumulative volume loss with sliding distance as estimated by the linear least square fit. When the variation of wear rate with normal load is linear, the wear coefficient has been v calculated by multiplying the slope of the variation of wear rate with normal load by the hardness of the specimen. The cumulative volume loss with increasing sliding distance at different normal loads of 10,20,30 and ON and sliding speed of 0.786 m/s for different cast Cu-Cr-SiC and Cu-Cr-G composites containing the different compositions of Cu-Cr-SiC and Cu-Cr-G. For a given normal load, the cumulative volume loss increases linearly with increasing sliding distance. It is observed that when the amount of reinforcing particles increases in the cast Cu-Cr-SiC and Cu-Cr-G composites, the cumulative volume loss decreases significantly with increasing particle content. Amongst different cast Cu-Cr-SiC and Cu-Cr-G composites, the cast Cu-4Cr-4G composite after sliding through a distance of 1,398 m shows the lowest cumulative volume loss at different loads. The minimum cumulative volume loss in Cu-4Cr-4G composite may be attributed to the presence of soft layer of graphite on the sliding surface of the composite. The variation of cumulative volume loss with sliding distance for as received copper, cast copper and cast Cu-4Cr alloy under similar sliding test conditions as those of cast composites, shows that the cumulative volume loss increases linearly with increasing sliding distance and increasing normal load similarly as those observed in cast composites, indicating that Archard's adhesive wear equation for single phase material is being followed. However, cast copper has shown a higher volume loss as compared to as received copper and cast Cu-4Cr alloy at all the normal loads and these monolithic materials show higher volume loss compared to those observed for cast composites. Similarly, in Cu-Cr-SiC and Cu-Cr-G composites after hot forging as well as after hot forging and annealing, the cumulative volume loss increases linearly with increasing sliding distance at a given normal load. The cumulative volume loss also increases with decreasing of particle content. The minimum cumulative volume loss is found in forged Cu-4Cr-4G composite. The cumulative volume loss of both forged and annealed composite shows higher cumulative volume loss compared to those in forged as well as cast composites. For similar conditions of dry sliding forged composites show a higher volume loss compared to cast composites with similar particle content. The variation of cumulated volume loss in wear with sliding distance for hot forged as well as hot forged and annealed as received copper, cast copper and cast Cu-4Cr alloy shows that forged and annealed cast copper has higher cumulative volume loss compared to all the monolithic materials investigated, either forged, cast or forged and annealed. The observed increase in hardness with higher addition of particles is expected to decrease the real area of contact. Also, at higher particle content in the composite, there is higher amount of reinforcing particles in the debris to form relatively larger cover of transfer layer. Thus, higher particles addition results in decreased wear rate under the same condition of dry sliding wear. The composites developed by addition of graphite show slightly lower wear rate consistently compared to that in the composites developed by the addition of the same amount of SiC particles. The wear rates in Cu-4Cr-4G composite at different loads are relatively small, in the range between 0.93X104 mm3/m and 2.18X10-4 mm3/m, under different test conditions used in the study and it is lower than all the composites containing either SiC particles or with lower graphite addition. For the hot forged as well as annealed after hot forging composites, the wear rate at a given load decreases with increasing the particles contents similarly as observed in cast composite. Amongst all the forged or forged and annealed Cu-Cr-SiC composites investigated, the wear rate has been found to be lowest. for forged Cu-4Cr-4SiC composite, lying in the range of 0.87 X 10-4 to 10.97 X 10 for Ci-Cr-G composites, the forged and annealed Cu-4Cr-4G composite shows the lowest wear rate in range of 5.24 X 10-4 to 12.28 X 10-4. The wear coefficients of composites containing graphite particles are relatively lower in the range between 0.38 X 10-6 and 2.18 X 106 compared to those observed in Cu-Cr-SiC composites. The wear coefficients of the forged as well as the hot forged and annealdd'after hot forging Cu-4Cr-3SiC composite are relatively lower at 1.96 X 10-6 and 1.30 X 10-6 respectively, in spite of lower wear rates for Cu-4Cr-4SiC and Cu-4Cr-4G composites. The friction behaviour of as received copper, cast copper, Cu-4 wt% Cr alloy and the Cu-Cr-SiC and Cu-Cr-G composites have been examined in terms of the variation of coefficient of friction during dry sliding wear tests under different loads. The friction force rises in the initial run-in period and then fluctuates around a mean during dry sliding. he mean has been determined from the individual values of coefficient of friction excluding the initial rising part and it has been observed that the mean coefficient of friction increases with increasing load for the as received, cast copper, Cu-4 wt% Cr alloy and the composites. In vii the composites, the coefficient of friction decreases with increasing addition SiC/graphite particles. The hot forged as well as annealed after hot forging as received copper, cast copper, Cu-4 wt% Cr alloy and Cu-Cr-SiC and Cu-Cr-G composites show slightly higher coefficient of friction than those observed in cast composites at lower load of 10 N. At a higher load, the frictional force increases leading to larger dissipation of energy resulting higher temperature at contact, which may help better compaction and spread of transfer layer leading to lowering of coefficient of friction. In the hot forged as well as annealed after hot forging composites, the coefficient of friction decreases with increasing addition SiC/graphite particles similarly as in cast composite. Sliding generates frictional heating which cause a rise in the specimen pin temperature and it is observed that for all the loads, there is fluctuation of temperature around mean. For the all materials, at lower loads of 10 and 20 N, the fluctuation in temperature is relatively small but at higher loads of 30 and 40 N, there is increased fluctuations. The mean temperature increases with increasing load but decreases with increasing, particle content. The. composites containing graphite has shown relatively lower temperature compared to that observed in Cu-Cr-SiC composite. Surfaces of test specimens of dry sliding wear have been examined before and after test to find the changing surface features with sliding. The basic . statistics of surface roughness (in terms of Ra, Rq, Rz and Rt) and the bearing ratio of the surfaces (in terms of Mn, Mr2, RK, Rpk and Rk) of as received copper, cast copper and in-situ composite of Cu-4Cr-4SiC specimens show that the average roughness, Ra, as well as the rms roughness, Rq, both varies over a limited range with increasing load for as received copper, cast copper, Cu-4Cr alloy and Cu-Cr-SiC and Cu-Cr-G composites under the loads investigated. In all the materials, Ra is consistently lower than Rq. The sensitive parameter like Rt, which is the difference in height between the highest peak and the lowest valley, decreases at intermediate loads but increases at higher loads for cast materials. The same trend is followed by Rz, which is the average of difference in height between the ten highest and the ten lowest points in the data set. The bearing or material ratio, tp in %, is defined as the ratio of the length of the surface, at a specified depth of the profile to the length over which evaluation is carried out and it has been determined as Mnl and Mr2 respectively for the heights where Rpk and viii RK meets and also, where RK and Rk meets in the depth profile of the surface. The surface should be smooth enough to provide sufficient bearing area to improve the wear properties and hence, the resulting tribological performance of the component. At the same time it should also allow the lubricant to be retained in small pockets so to provide lubricant to reduce friction and improve the efficiency. In comparison to surface before dry sliding wear, the oil retaining property of surface of cast composite after dry sliding is better particularly at higher load. At higher load, the composite surfaces after dry sliding also show better bearing capacity. Both hot forged as well as annealed after hot forging as received copper, cast copper, Cu-4Cr alloy and cast Cu-Cr-SiC and Cu-Cr-G composites show similar roughness and bearing ratio after dry sliding. Thus, the present study indicates Cu-4Cr-4G shows good wear resistance as well as low friction in comparison to all materials investigated. The dry sliding friction and wear behaviour of the low cost composites Cu-Cr-SiC and Cu-Cr-Graphite shows their potential as candidate tribological materials for engineering applications. Chapter-6: This chapter lists the salient conclusions of the present study on the solidification processing, microstructure, mechanical, electrical, thermal properties and tribological behaviour of the copper based cast Cu-Cr-SiC and Cu-Cr-G composites, either cast, hot forged or annealed after hot forging.