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Author: search.filters.author.Mishra, Radha RamanSubject: search.filters.subject.Microstructure

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    Investigation on Microwave Joining of Mild Steel Plates at 2.45 GHz and Joint Characterization
    (Springer, 2021-02) Mishra, Radha Raman
    Microwave joining of materials is a recently developed advanced joining process in which electromagnetic energy at 2.45 GHz is used to develop butt joint between different metallic plates. Rapid selective hybrid heating of the targeted area depends upon the location of the metallic samples inside the applicator cavity during microwave exposure. A high electric field intensity location for rapid hybrid heating of samples was identified inside the resonating cavity with the help of the COMSOL Multiphysics 5.2. Accordingly, experimentation was done to develop joints of mild steel (MS) samples using microwave energy at 2.45 GHz and input power of 900 W. The nickel powder was used as an interface material between metallic plates. The fabricated mild steel butt joints were characterized to analyse the microstructures and the micro indentation hardness of the joints. The microstructural characterization of the joints revealed complete melting of nickel powder and its fusion with base mild steel plates. The presence of iron in the joint zone indicated a metallurgical fusion of the interface layer with the base metal; however, oxides and carbides presence in the joint indicated interaction with atmospheric oxygen and carbon in the susceptor. The hardness of the developed joint zone was 405 ± 12 Hv which is higher as compared to the base metals (211 ± 12 Hv).
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    Effect of input microwave power and insulation on microstructure and tensile properties of cast Al 7039 alloy produced at 2.45 GHz
    (Taylor & Francis, 2020-10) Mishra, Radha Raman
    In the present work, microwave energy was used for casting Al 7039 alloy at 2.45 GHz in the ambient cavity environment. Effects of input power and insulation of the mould assembly during irradiation on charge melting and mould preheating were studied. Five different casts were produced at 1000 W, 1200 W, 1400 W, 1400 W with an insulated pouring basin and 1400 W with insulated mould assembly. Melting time of the charge was the least while using 1400 W with insulated mould assembly, whereas preheating of the mould was observed minimum during casting at 1400 W inside an insulated pouring basin. Cast microstructures revealed that less preheating of the mould resulted in finer grains and intermetallics, which improve tensile properties of the cast. Fractographic analyses showed the presence of coarse intermetallics in the casts produced with insulated mould assembly, which resulted in significant reduction of tensile properties.
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    Multi-physics simulation of in situ microwave casting of 7039 Al alloy inside different applicators and cast microstructure
    (Sage, 2018-06) Mishra, Radha Raman
    In the present study, finite element models of three different applicators (A1, A2, and A3) having different power densities were developed to study melting of the charge and solidification of the melt during in situ microwave casting. Multi-physics simulations were carried out to understand the effect of applicator specific processing conditions on the distribution of electric field inside the cavities at 2.45 GHz for Al 7039 alloy as charge. The alloy was cast inside the selected applicators and the mold temperature was monitored. The experimental results showed reasonable agreement with the simulation data. Simulation results revealed that the distribution of electromagnetic field inside A3 offers the lowest melting time of the charge (141% less than A1); however, it also caused the highest preheating of the graphite mold with respect to A1 (30% higher) and A2 (25% higher). It was found that the applicator-specific solidification conditions affect grain structure, intermetallic precipitation, and their distribution inside the casts. Coarser intermetallic phases (57 µm) and grains (97 ± 54 µm) were present in the Cast 3 developed using A3 due to higher preheating of the mold and slower cooling rate of the melt as compared to that in A1 and A2.
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    Effect of Solidification Environment on Microstructure and Indentation Hardness of Al–Zn–Mg Alloy Casts Developed Using Microwave Heating
    (Springer, 2017-09) Mishra, Radha Raman
    In the present work, microwave casting of Al 7039 was carried out using microwave energy at 2.45 GHz and 1400 W. A set of casts were developed through the in situ microwave casting process inside the applicator cavity applying three different solidification conditions, i.e., closed cavity cooling (cast C1), open cavity cooling (cast C2) and water-cooled cavity cooling (cast C3), whereas another cast was developed through conventional microwave casting (cast C4). Microstructure and microindentation hardness studies of the developed casts revealed that dense cast with smaller equiaxed grains could be obtained in the cast C3. Presence of the intermetallic phases, MgZn2, Mg2Si, Al3Fe and Al8Fe2Si, was observed in the in situ casts, whereas the cast C4 contains intermetallic phases: MgZn2, Mg2Si, Al2Cu, Al2CuMg and Al7Cu2Fe. It was found that the grain structure and major attributes of the intermetallic precipitates (size, shape and distribution) in the casts significantly depend on the solidification conditions. Average microindentation hardness of the cast C4 was found to be 191 ± 32 HV which is higher than other casts. The study showed that microindentation characteristics of the casts depend more on attributes of the precipitated intermetallic phases during solidification than the grain size
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    Effect of susceptor and mold material on microstructure of in-situ microwave casts of Al-Zn-Mg alloy
    (Elsevier, 2017-10) Mishra, Radha Raman
    In-situ microwave casting is a novel technique; it is based on the principles of microwave hybrid heating. The dynamics of the process and the cast quality are significantly influenced by the materials used in the microwave irradiation. In the present work, role of susceptor and mold on exposure time, melting time, mold preheating and cast properties is studied. Physics of the process in the context of exposure time and mold materials is discussed. The aluminum alloy 7039 casts were developed in ambient atmosphere inside an applicator using microwaves at 2.45 GHz and 1400 W. Charge was hybrid heated using susceptors – SiC and ceramic crucible to melt and cast in-situ in the preplaced alumina and graphite molds. Characterization reveals that grain structures of the casts were influenced by mold preheating and mold material. Finer grains with higher micro-pores were observed in the casts developed in alumina mold with SiC susceptor. The casts contain MgZn2, Mg2Si, Al3Fe and Al8Fe2Si as intermetallics; however, their distribution and size depend upon the cooling pattern of the melt. Microindentation hardness of the casts developed in alumina mold with SiC susceptor was observed to be the highest (146 ± 10 HV) among the developed casts.
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    Structure-property correlation in Al–Zn–Mg alloy cast developed through in-situ microwave casting
    (Elsevier, 2017) Mishra, Radha Raman
    Casting is one of the preferred routes to manufacture net shape parts. Major concern in this process is to obtain suitable microstructure in the cast parts and thereby achieving desired mechanical properties. Recently, a new approach in metal casting called ‘in-situ microwave casting’ (MWC) has been reported in which microwave energy is used during melting, pouring and solidification of the cast material. The complete process is carried out inside the applicator cavity which adds more flexibility due to control over the mold temperature and solidification inside the applicator. In the present work, some studies/analysis on microstructure and mechanical properties of the developed in-situ microwave casts of aluminum 7039 alloy has been presented. The alloy was cast in-situ in an ambient atmosphere inside a microwave applicator using microwave energy at 2.45 GHz and 1400 W. Characterization of the in-situ casts was carried out to study the grain structure, phases and their distribution, porosity and mechanical properties. A dense cast was obtained with porosity less than 2%. The in-situ cast consists of MgZn2, Mg2Si, Al3Fe and Al8Fe2Si precipitates and intermetallic phases. Ultimate tensile strength (UTS) of the in-situ microwave cast was observed to be 148.46±10 MPa and the average micro indentation hardness was observed higher near the gain boundaries (maximum 132 HV) than inside the α-Al grains (maximum 120 HV).
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    On mechanism of in-situ microwave casting of aluminium alloy 7039 and cast microstructure
    (Elsevier, 2016-12) Mishra, Radha Raman
    The demand for processing of aluminum alloys is increasing owing to their extensive use in high specific strength products. Consequently, need for development of a faster and economic route to process such materials has been felt. In the present work, the mechanism of a new process, called ‘in-situ microwave casting’, is discussed vis-à-vis the cast microstructure. The AA 7039 alloy was cast in-situ using microwave energy at 2.45 GHz and 1400 W in ambient environment inside an industrial microwave applicator. Principles of microwave hybrid heating (MHH) were used to heat the charge. The mechanisms of heating and melting of the charge are discussed; significant heating phases of the alloy during irradiation are explained through time–temperature characteristic. The role of oxide layer at elevated temperature during irradiation is presented in the context of microwave absorption by the alloy. The developed casts were characterized in terms of identification of microstructures and metallurgical phases, pattern of microstructures across the cast section, porosity and micro indentation hardness. The results reveal a dense cast with porosity < 2%; phase analysis indicates presence of MgZn2, Al3Fe and Al8Fe2Si intermetallic phases. The micro indentation hardness of the in-situ microwave cast was 85.5 ± 26.5 HV.