Development and Characterization of Metal-Ceramic Composite Castings through Microwave Hybrid Heating

Abstract

The manufacturing sector of any country plays an important role in its economic development, but it consumes more than 30% of the energy resources. Within the manufacturing domain, the melting and casting of steel and iron consumes majority of the primary energy resources and responsible for 38% of CO2 emissions worldwide. To handle such situations, technologists, researchers and academicians are continuously working towards efficient processing of materials such that production cost can be reduced and energy can be saved. The conventional manufacturing practices are getting obsolete due to higher energy consumptions; higher pollution levels, low productivity and formation of higher defects in developed component. The alternative or new processing methods are required to reduce production or manufacturing costs, processing times, and to enhance product quality. Further, the developed technology should be widely acceptable for all types of materials such as metal matrix composites, ceramics, alloys, and fiber reinforced plastics. In current scenario, the focus is on the development of energy efficient and green processing methods, which are sustainable. Thus, it is of prime importance for the researchers to investigate alternative processing methods/techniques, which have the potential to overcome/lower the deficiencies of conventional one. The developed techniques are expected to be highly energy efficient, reduces CO2 or other unwanted toxic gas emission, and produces better quality products at an effective processing cost. In mid of year 90’s, microwave heating effects were discovered and microwaves were successfully utilized in the cooking sector with the discovery of the microwave oven. During 1980-1990 the utilization of microwave ovens spread throughout the world and become the essential part of home appliances due to the better efficiency and lower processing time characteristics. However, at the same time some commercial applications of microwave heating were also discovered such as drying of wood, processing of rubbers, chemical processing, etc. The continuous efforts of researchers led to the development of microwave sintering process, which has emerged as one of the most promising technology in the materials processing field. However, microwave sintering was confined to the ceramics and ceramic based composites in different forms. Processing of metallic powders/metals through microwaves was unsuccessful due to lower skin depth associated with such materials. However, researchers led to the development of microwave hybrid heating techniques to process such materials like metals in the form of powders. But, still processing of metallic based materials is a challenging task at lower frequencies. Lately, a lot of work in the field of microwave joining of bulk metals and cladding of metallic based powders on metallic substrates was reported. The joining and cladding was achieved with partial melting of metallic powders which motivated the researchers to explore the potential of microwave heating in powdered metal casting and in situ casting processes. Still, the majority of the work is reported on sintering of metallic powders and provided an opportunity to investigate the potential of microwaves to melt metallic based powders. In the present work, microwave energy is utilized for melting and casting of powdered metal-ceramic composites. The domestic microwave oven working at 2.45 GHz and 900 W maximum power is used as microwave applicator. Microwave hybrid heating was used with charcoal as a susceptor material; for melting the nickel based metal powder particles. For the development of various MMC’s, commercially available EWAC 1004EN powder was selected as matrix material, which is approximately 97% nickel and ceramic reinforcements of silicon carbide (SiC), alumina (Al2O3) and tungsten carbide (WC-8Co) were selected. The matrix and reinforcements powders were premixed in a mechanical mixer to obtain homogeneously mixed powders; with reinforcement volume fraction of 5% and 10%. The premixed powder was preheated to 200° C and placed in the graphite cavity; which is exposed to microwave radiations for optimum processing times. The cavity was allowed to cool under the atmospheric conditions for solidification of the castings. The developed castings were characterized using various relevant techniques to study the X-ray diffraction patterns (phase analysis), microstructural characterizations (using optical microscope and scanning electron microscope equipped with EDS), mechanical properties (microhardness, tensile strength and percent elongation) and functional characteristics (dry sliding wear behavior). The XRD study of castings revealed the formation of various intermetallics which was due to higher processing time and intense heating by microwaves. On adding SiC reinforcement, intermetallic of Ni2Si was observed, whereas pure EWAC casting revealed peak of intermetallic FeNi3. Similarly, on adding alumina, nickel aluminide intermetallic peaks were observed, whereas Ni4W major peak was observed in WC-8Co casting. All these intermetallics were favored due to the thermal degradation of reinforcements at higher temperatures. The EDS analysis revealed that the hard phases of chromium carbides, nickel carbide and cementite were precipitated along the grain boundaries. The microwave processed castings revealed the formation of equiaxed grains throughout the microstructures and this was due to the volumetric heating characteristics of microwaves. There was no evidence of dendritic transition from the cellular/equiaxed structures. The porosity content of developed castings was low, which is again due to the uniform heating profile obtained in MHH process. The formation of various hard intermetallic compounds within the castings led to the higher average microhardness. The EWAC casting revealed average Vickers microhardness of 410±55 HV, whereas, the addition of 10% SiC led to microhardness of 980±208 HV, addition of 10% alumina led to microhardness of 985±80 HV and 10%(WC-10Co) reinforced casting revealed the microhardness of 1012±108 HV. Tensile tests revealed that EWAC casting possesses an average tensile strength of 330±15 MPa with average percent elongation of 33±2%. By adding the ceramic reinforcements the load carrying capacity increased, but hard phases of reinforcement restricted the plastic deformations thus producing lower elongations. By increasing the content of SiC to 10% volume fraction, the strength further increased by 26.67% (450±21 MPa) in comparison to EWAC casting with an average elongation of 10.5±2%. The average tensile strength of 10% alumina reinforced composite was 355±10 MPa, with percent elongation of 18±3%. The tensile strength of EWAC+10%(WC-8Co) composite increases to 508±6 MPa, which is 53.93% higher than EWAC casting and average percent elongation of 18±4%. The functional characterization (in terms of dry sliding wear tests) results revealed lower wear rates in microwave processed MMC’s in comparison to pure EWAC casting and this was due to higher microhardness and uniform dispersion of reinforcements. In comparison to the EWAC casting wear rate (10.82x10-3 mg/s), the EWAC+10%(WC-8CO) casting revealed 12.8 times lower wear rate (0.84x10-3 mg/s) under the 20 N load, 1.0 m/s sliding velocity and after 2000 m of sliding. The EWAC+10%SiC casting revealed 5.15 times lower wear rate (2.1x10-3 mg/s) and EWAC+10% Al2O3 casting revealed 3.53 times less wear rate (3.06x10-3 mg/s) in comparison to EWAC casting. The overall results led to the conclusion that microwave energy was successfully employed in melting and casting of various MMC’s with less energy consumption. The developed metal-ceramic composites can be used in many industrial applications where high strength and anti wear characteristics are required.