Parametric review of microwave-based materials processing and its applications

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Available online at www.sciencedirect.com

www.jmrt.com.br

Review Article

Parametric review of microwave-based materials processing and its applications Praveen Kumar Loharkar a,∗ , Asha Ingle b , Suyog Jhavar c a

Department of Mechanical Engineering, MPSTME, SVKM’s Narsee Monjee Institute of Management Studies (NMIMS), Shirpur Campus, Maharashtra 425405, India b Department of Mechanical Engineering, MPSTME, SVKM’s Narsee Monjee Institute of Management Studies (NMIMS), Mumbai 400056, Maharashtra, India c Department of Mechanical Engineering, Atria Institute of Technology, Bangalore 560024, India

a r t i c l e

i n f o

a b s t r a c t

Article history:

The realization of microwave’s heating ability is a significant discovery in the history of sci-

Received 12 November 2018

entific research. The thermal ability of microwave has led to the invention of devices and

Accepted 4 April 2019

processes that could replace conventional heating methods and reduce humankind’s depen-

Available online 28 May 2019

dency on fossil fuels. In the early days, microwave heating was limited to food processing

Keywords:

processing and showed encouraging outcomes. Currently, microwave-based materials pro-

and drying. Researchers over the years have examined the utility of microwave in materials Cladding

cessing research is aligned toward exploring the processing of novel materials and achieving

Heating

greater reliability and control in the existing processes. This, in turn, requires extensive

Joining

knowledge of the parameters of the microwave heating processes. This review provides

Melting

insights into microwave-based heating parameters and their effect on the response param-

Microwave

eters of major microwave processing applications. Four significant parameters viz., heating mechanisms, dielectric and magnetic properties, load and applicators have been identified and factors influencing these parameters have been critically examined to understand the requirements of efficient microwave-based materials processing. This review analyzes the existing state of research and identifies the future scope of research in this field. It reveals optimization studies, commercialization of the process at large scale, simulation studies for better understanding of heating mechanisms and innovation in processing set-ups as few of the potential areas with a wide scope for investigation. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

∗

Corresponding author. E-mails: [email protected] (P.K. Loharkar), [email protected] (A. Ingle), [email protected] (S. Jhavar). https://doi.org/10.1016/j.jmrt.2019.04.004 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Praveen Kumar Loharkar obtained his Bachelor of Engineering (BE) degree in Mechanical Engineering from University of Pune, India and M.Tech. in Industrial Design from Maulana Azad National Institute of Technology, Bhopal, India. He has three years of industrial and more than five years of academic experience. He is currently working as an Assistant Professor in the Department of Mechanical Engineering, SVKM’s NMIMS, Mukesh Patel School of Technology Management and Engineering, Shirpur Campus, India and pursuing Ph.D. from SVKM’s NMIMS (Deemed-to-be University), Mumbai, India. He has around 14 research publications in various international and national journals and conferences. His research interests include mechanical engineering design and advanced manufacturing processes.

Abbreviations

Dr. Asha Ingle obtained her Bachelor of Engineering (BE) degree from Visvesvaraya Regional College of Engineering, Nagpur, India in Metallurgical Engineering and M.Tech. and Ph.D. from Indian Institute of Technology, Mumbai, India. She has worked in the corporate R&D division of Crompton Greaves Ltd, Mumbai, India for twelve years before moving to Academics seven years ago. She is currently a Professor in the Department of Mechanical Engineering at SVKM’s NMIMS (Deemed-to-be University), Mukesh Patel School of Technology Management and Engineering, Mumbai, India. She has published extensively and also has 10 filed patent applications and 5 design registrations to her credit. Her areas of interest include characterization of materials, process optimization, silicon steels, advanced manufacturing processes and failure analysis of materials.

Dr. Suyog Jhavar is currently working as an Associate Professor in Mechanical Engineering at Atria Institute of Technology, Bangalore. He is also professor in-charge for Centre of Excellence in Additive Manufacturing at the institute. He received Ph.D. in Metal Additive Manufacturing from Indian Institute of Technology Indore, India in 2014 and Master of Technology in Welding engineering from Indian Institute of Technology Roorkee, India in 2010. He has served in various national and international organizations including University of Texas, USA as Assistant Research Scientist. He is specialized in metal additive manufacturing, welding engineering, advanced and hybrid materials processing techniques. He has published several book chapters, research papers in SCI-indexed journals of international repute. He has addressed several international technical programs and has visited Singapore, USA, China, and Israel for his academic interests. He is also a recognized reviewer for many international journals in his field of specialization and a member of expert committee at Shanghai Additive Manufacturing Association, China.

3307

MOR YS UTS

modulus of rupture, psi yield strength, MPa ultimate tensile strength, MPa

Symbols ε* ε ε tan ı tan ıε r r εr εr c εeff

complex permittivity, Fm−1 dielectric constant, Fm−1 dielectric loss factor, Fm−1 magnetic loss tangent dielectric loss tangent relative magnetic permeability relative magnetic loss factor relative dielectric constant relative dielectric loss factor velocity of light, ms−1 effective dielectric loss factor

ε0 Erms f ω eff 0 Hrms L L0 B0 t D R T Q

1.

permittivity of free space, 8.854 × 10−12 Fm−1 local value of the electric field, Vm−1 frequency, cps angular frequency, rad/s effective magnetic loss factor magnetic permeability of vacuum, 4␲ × 10−7 Hm−1 local value of the magnetic field, Am−1 change in length of the compact, mm initial length of the compact, mm collection of material parameters sintering time, s particle diameter, ␮m gas constant, J kg−1 K−1 absolute temperature, K activation energy of diffusion, J

Introduction

Microwaves are electromagnetic radiations having a wavelength between 1 mm and 1 m and are an important constituent of many modern technologies [1]. These radiations are used extensively in the field of network and communications, medical treatments, remote sensing, and food processing applications [2]. Microwaves exhibit unique uniform heating characteristics that have led to the development of applications beyond food processing, which involve thermal energy [1–6]. For example, various materials processing applications, such as sintering, joining, cladding/coating, melting/casting, etc. require high temperature for processing of materials [7]. The unique heating ability of microwaves has inspired researchers to explore avenues for utilizing this source of energy in materials processing and industrial applications [8]. The use of microwaves in these applications have produced encouraging results in terms of shorter processing times, better mechanical properties and superior control over the process [2,8–16]. Another aspect of using

3308

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

2. Parameters affecting microwave-based heating processes

Fig. 1 – Microwave processing of metals: chronological development [16].

microwaves is its economic viability and emission-less characteristics. Subsequently, development of microwave-based processing technology is significant as it minimizes dependency on fossil fuels as well as other high-energy based heating systems. As there is an increased awareness of reduced emissions and attempts toward the use of greener technologies in the field of manufacturing, microwave heating presents itself as an effective alternative to conventional heating based processes. In the context of processes involving metals, microwavebased heating methodologies are still at the preliminary stages of development. It is mostly limited to laboratory experiments. Fig. 1 shows the chronological order of development of microwave-based metal processing [16]. It is evident that microwave-based heating has the potential to replace their conventional counterparts at the commercial level in metal processing applications, such as sintering, surface engineering, joining, melting and drilling. Large-scale commercialization of microwave-based processing applications is a potential area for research. The major challenge, however, lies in the unique source of energy in the case of microwaves, which differentiates it from the conventional processes and involves an altogether different mode of heating. It requires knowledge of physics involved in the propagation of electromagnetic radiations and its behavior. The lack of synchronization between researchers from physics/electronics domain and materials processing domain is one of the major cause of the slow pace of development in this domain of research. Heating efficiencies can be improved with the design and development of dedicated applicators through collaboration. There have been several studies to explore theories, various interaction mechanisms, applications and numerical simulations in different microwave processing domains but there is a need for consolidated information on parametric evaluation for microwave-based metal processing [1,2,8–20]. The objective of this review is to summarize and discuss parameters and corresponding factors affecting microwave processing including discussion on input variables involved in microwave processing and their effect on response variables. The following sections will discuss the process and material parameters related to major applications of microwave heating developed in the past two decades. In addition, the paper summarizes this novel processing concept and methodologies in developing microwave-assisted materials processing.

It is imperative to identify and understand the parameters and the factors that govern these parameters to optimize and control microwave-based materials processing. Since microwaves are electromagnetic radiations, the heating process is primarily influenced by dielectric and magnetic properties of the materials being processed. The authors have identified four significant parameters affecting microwave heating as (i) dielectric and magnetic properties, (ii) heating mechanisms, (iii) load subjected to microwave heating and (iv) applicators used for heating the materials and underlying factors that govern these parameters. Fig. 2 depicts parameters and corresponding factors affecting microwave-based heating processes.

2.1.

Dielectric and magnetic properties

Electromagnetic nature of microwaves results in the interaction of inherently present electric and magnetic fields with the electrons, ions, and molecules at micro-levels. In order to understand the microwave-heating phenomenon, a numerical study has also been used [21]. The results have indicated that dielectric properties have a strong effect on heating. The dielectric parameter that characterizes heating ability is known as complex permittivity, ε∗ [2]. It is expressed by Eq. (1): ε∗ = ε − jε

(1)

where the real term, ε , quantifies the ability to store energy while the imaginary term and ε , is the measure of the ability to convert the stored energy into useful heat. The ratio of the imaginary term and the real term is known as loss tangent, tan ıε , which assesses if the material is suitable for microwave-based heating. Several methods for determining dielectric constant and dielectric loss factor have been presented in the literature [22,23]. Table 1 presents the principle of few methods used for characterization of electromagnetic properties. The magnetic component of microwaves also contributes to heating. Yoshikawa et al. [24] studied the effect of microwave interaction on iron particles and ferromagnetic materials. It has been found that the source of heating in these materials was their magnetic effect. In the case of magnetic materials, the property characterizing loss and storage of magnetic energy is a complex permeability [25]. Eq. (2) expresses permeability in real and imaginary terms. ∗ =  − j

(2)

The real term of complex permeability, ∗ is known as magnetic permeability,  while the imaginary term is called magnetic loss factor,  . The magnetic properties result in heating due to losses depending on the resonant frequencies corresponding to a given material. For example, the resonant frequency of a water molecule is 2.45 GHz [26]. Both complex permittivity and magnetic permeability are variable quantities and are affected by frequency, temperature and material

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

3309

Fig. 2 – Parameters affecting microwave-based heating processes.

composition. Complex permittivity is a function of dielectric constant (refer to Eq. (1)). With the increase in temperature, dielectric constant and imaginary part of complex permittivity increases for a particular frequency and drops with the increase in frequency [27]. Fuzerova et al. [28] discussed variation in magnetic permeability in soft magnetic composites. The variation in the real and imaginary part of magnetic permeability using a mixture of Somaloy 700 powder (insulated iron particles) and amorphous Vitroperm (Fe73 Cu1 Nb3 Si16 B7 ) with respect to frequency and sample composition. Thus, by controlling the material composition and frequency of radiations, the magnetic properties of the specimen can be altered. The chemical nature of a specimen is another factor that affects the rate of microwave heating. Evaluation of the effect of composition, structure, and impurities in the substance was carried out by Yixin and Liu [29]. The experimental investigation revealed that microwave absorption is a function of the chemical state of the material subjected to microwaves and thus, it plays an important role in heating efficiency. In conclusion, the factors such as frequency of radiations are governed by the standards laid down by governing bodies [2,25] and temperature of processing is limited by the application and control available. Therefore, the only factor that can be controlled by the researcher is the material being used and its composition. In cases where the material to be processed do not have required dielectric or magnetic properties, the use of hybrid heating is devised [15]. Refer Section 2.2.3 for detail discussion on this mechanism.

2.2.

Mechanism of heating

Prediction and control over the response of the material subjected to the microwave environment are significantly affected by mechanisms involved in heating. Researchers, such as Hao et al. [30], Sun et al. [20], Mishra and Sharma [19], have studied microwave-heating mechanisms. Hao et al. [30] specifically developed a model termed as “flat-ball” to express the relationship between sintering time and density. It is clear that microwave heating is primarily due to the electric field and magnetic field heating. Subsequent sections discuss the various aspects of the two heating modes and the advent of a hybrid-heating concept.

2.2.1.

Electric field heating

Heating due to the electric field is attributed to various mechanisms, namely polarization of dipolar molecules, ionic conduction and Maxwell–Wagner effect [20]. The mechanisms involved in the electric field and magnetic field heating are presented in Fig. 3. The polarization of dipolar molecules occurs under the effect of the applied electric field, which is an inseparable part of any electromagnetic radiation. The heat generated through this phenomenon is because of frictional interaction due to “to and fro” movement of these molecules under the effect of a time-varying electric field [2]. In the mechanism involving ionic conduction, the charge carriers, such as electrons and ions, undergo forward-reverse movement when the electric field is applied [20]. This leads

Table 1 – Methods for characterization of electromagnetic properties [22,23]. Method Transmission/reflection line method Open-end co-axial line method Free space method Resonance cavity method

Principle The sample is placed within a wave-guide, a device such as vector network analyzer (VNA) is used for the measurement of transmitted/reflected signal parameters. These parameters help in identifying the electromagnetic characteristics of the sample. A co-axial probe is made to get in contact with the sample and the reflection coefficient is used to identify material properties using VNA. In this method, vector network analyzer is connected to two antennas, the sample is placed between these two antennas and the signals are analyzed using VNA for determining material properties. Presence of a sample within a cavity leads to variation in the resonant frequency, which is detected by the vector network analyzer. This aids in the determination of properties.

3310

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Table 2 – Comparison between electric field heating and magnetic field heating. Particulars

Heating-electric field

Heating-magnetic field

Mechanisms

Polarization due to dipolar effect Conduction of ions Maxwell–Wagner polarization

Materials

Aqueous electrolytic solutions Carbon materials

Microwave power loss density (per unit volume) at any location (x,y,z)

P= ω · εeff · ε0 · E2r.m.s where ω = 2f

Losses due to eddy current Losses attributed to Hysteresis Magnetic resonance losses Residual losses Magneticdielectric materials (ferrite) Powder of conductive materials Ferromagnetic materials (steel, nickel etc.) P= 2 ω · eff · 0 · Hr.m.s where ω = 2f

Table 3 – Susceptor material properties at different temperatures [32]. Fig. 3 – Mechanisms of electric and magnetic field heating.

to the induction of currents which releases heat. In materials that consist of excess electrons like carbon, a cause of electric field heating is attributed to the Maxwell–Wagner effect [20]. Here, the heat is generated due to interfacial polarization. The excess electrons also termed as  electrons available in carbon, are unable to couple with the changing phase of the electric field component of the microwave, thereby, resulting in dissipation of energy [31]. In the context of microwavebased materials processing applications, there are quite a few materials that do not interact with microwaves or reflect the microwaves altogether. This is quite prominent in the case of metals. Therefore, susceptors such as charcoal, graphite, and silicon carbide are used due to their heating capability as a result of the Maxwell–Wagner effect. These susceptors are used as per the requirement of the set-up as described in detail in Section 2.2.3.

2.2.2.

Magnetic field heating

The magnetic field component of microwaves also causes heating under the effect of microwave irradiation. Several loss mechanisms have an effect on temperature rises, like in the case of magnetic materials and conductors. These are losses due to eddy current, Hysteresis, magnetic resonance and residual losses. Sun et al. [20] and Mishra and Sharma [19] discussed this in detail. Table 2 is a comparison between the two modes of heating under the influence of microwaves. Mathematical modeling of heat transfer and generation mechanisms in microwave-based materials processing is an area that needs further investigation to achieve greater reliability and reproducibility of experimental results.

Material

tan ıε (25 ◦ C)

tan ıε (500 ◦ C)

tan ıε (900 ◦ C)

Al2 O3 ZrO2 AlN

0.0005 0.0002 0.004

0.002 0.04 0.004

0.01 0.25 >0.01

2.2.3.

Microwave hybrid heating

Energy generated in electric and magnetic field heating is inadequate to achieve desired heating effects. This is primarily due to the inability of materials to interact with microwaves effectively. In microwave hybrid heating, high microwave absorbent material, known as susceptors are used, such as carbon, silicon carbide and alumina for rapid heating. This absorption property is a function of temperature [32]. The loss tangent increases with increase in temperature thus enhancing the microwave heating characteristics. Table 3 shows the value of loss tangents at different temperatures for selected materials that are used as susceptors. The loss tangent (tan ıε ) value >0.01 suggests good absorption capacity. Owing to the in microwave hybrid heating, applications, such as sintering, coating and joining have attracted the attention of several researchers. Table 4 summarizes research on powder metals and their use in microwave-hybrid heating.

2.3.

Load

Load in the context of microwave hybrid heating refers to the material being heated inside the chamber. The factors that affect this parameter selection are size, penetration depth – a factor dependent on the electrical conductivity of the material – and load positioning in the applicator [2].

3311

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Table 4 – Microwave direct/hybrid heating applications. Reference

Material

Gupta and Sharma [33] Gupta and Sharma [34] Gupta et al. [35]

Sharma and Gupta [36] Pathania et al. [37]

Powdered/bulk

Austenitic stainless steel (SS-316)/EWAC powder as clad material (40 ␮m) Austenitic stainless steel (SS-316)/WC10Co2Ni Clad material (40 ␮m) Austenitic stainless steel (SS-316)/(EWAC (Ni-based) + 20% Cr23 C6 ) Clad material (40 ␮m) Austenitic stainless steel (SS-316)/EWAC + 20% WC10Co2Ni Clad material (40 ␮m) Mild steel/EWAC + 20% WC10Co2Ni Clad material (40 ␮m)

Zafar and Sharma [38–40]

SS304/WC-12Co

Roy et al. [41]

Al, Cu, Co, W, Ni Fe and steel Mo, WC and Sn and their alloys Ni (99.8%, 45 ␮m), SUS316L (45 ␮m), Co (99.5%, 5 ␮m) Cu (99.5%, 75 ␮m) and Fe (99.5%, 45 ␮m) Copper-steel W (99.95%, 5–7 ␮m) Al (purity: 96%, average diameter: 120 ␮m) Ferrite

Saitou [42]

Agrawal [7] Prabhu et al. [43] Xu et al. [44] Shannigrahi et al. [45] Bykov et al. [46] Thuault et al. [47] Yang et al. [48] Li et al. [49] Ghobadi et al. [50]

2.3.1.

Al2 O3 –Ni (2–2.5 ␮m) Kaolin bowls SiC SiC (125 ␮m) Al2 O3 nanocomposites

Graphite

Cladding

Graphite

Cladding for wear resistance

Graphite

Cladding for wear resistance

Graphite

Cladding for wear resistance

Charcoal

Cladding for wear resistance

Charcoal

Cladding for wear resistance

SiC/MoS2

Sintering

Powder

SiC powder

Sintering

Powder Powder Powder

Direct Direct Direct

Sintering Sintering Sintering

Powder

SiC

Sintering

Powder Bulk Fiber bundles Powder Powder

Direct SiC Direct Direct Direct

Sintering Sintering Sintering Sintering Sintering/composites synthesis

Penetration depth

√ 2c

ω



εr r



(1 + tan2 ıε · tan2 ı + tan2 ıε + tan2 ı )

1/2

tan ı tan ıε

Application

Bulk substrate and powder as clad material Bulk substrate and powder as clad material Bulk substrate and powder as clad material Bulk substrate and powder asclad material Bulk substrate and powder asclad material Bulk substrate and powder as clad material Metal powders

Selection of a load for microwave interaction is based on an understanding of the parameter known as penetration depth. A material with all other parameters constant would behave differently under the microwave environment when the dimensions change. Penetration depth is defined as: “The depth at which electric field component drops to 1/e times the magnitude of electric field intensity at the surface” [20]. Mathematically, it is expressed by Eq. (3): d=

Susceptor/direct

−1/2 (3)

where tan ı = r /r and tan ıε = εr /εr . The magnitude of penetration depth is used to categorized materials as transparent, reflectors and absorbing. Due to inherent properties, conventionally processed materials such as plain carbon steels, alloy steels, aluminum and copper have low penetration depths and therefore, microwaves do not effectively heat these materials. These are termed as transparent materials. This might be a reason for the slow rate of progress in microwave-based processing technologies. Eq. (3) shows that frequency has an inverse relationship with penetration depth. Since the frequency band for industrial heating is limited to specific values such as 915 MHz, 2.45 GHz, etc.

[25], the penetration depth is fixed for a given operational frequency of the applicator. A typical value of penetration depth (at 2.45 GHz) for conductors (opaque/reflecting materials), absorbing materials and transparent materials is shown in Table 5adapted from [51]. This categorization helps in understanding load attributes and facilitates selection of crucible materials.

2.3.2.

Size

Size of the load under treatment affects the uniformity in heating. Maximum efficiency is attained only when load dimensions are close to the order of penetration depth. In the case of materials with unfavorable di-electric and magnetic properties, powdered materials have exhibited superior ability to interact with microwaves. Studies have attempted to understand the behavior and utility of powdered metals in microwave-based applications. Whittaker and Mingos [52] found that powder metals show good coupling characteristics with microwaves. Experiments were carried out in a power controlled 2.45 GHz microwave oven. The study revealed that there was no morphological change compared with conventional syntheses after microwave syntheses. To understand the nature of heat absorption in a powder sample, Mondal et al. [53] examined the influence of microwave field on the

3312

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Table 5 – Penetration depth for different category of materials [51]. Sr. No.

Reflectors Material

Penetration depth

Absorbers Material

Penetration depth

◦

Transparent Material

1 2

Silver Aluminum (100% purity)

0.33 ␮m 0.86 ␮m

Water + 20 C Nitrile rubber, natural

16 mm 650 mm

Quartz, pure Aluminum oxide

3 4 5

Brass (70% Cu) Stainless steel (304) Titanium, pure

1.4 ␮m 4.3 ␮m 3.3 ␮m

Cooked meat + 40 ◦ C Sand soil, dry Spruce 10% moist.

11 mm 2000 mm 77 mm

PTFE (Teflon ) Ice, −12 ◦ C Polythene, 25 ◦ C

thermal profile of copper powder with different particle sizes and porosity under the condition of constant power. A thin layer of graphite was used as a susceptor. It was observed that particle size inversely affects the heating rate that is, the heating rate increases with reduced particle size while higher porosity provides higher heating rates. Ghosh et al. [54] combined powdered aluminum, alumina, to synthesize composites. The results were quite effective owing to the exposure of each powdered particle to radiation. All these studies revealed that the size of the sample is an important factor in the processing of materials and therefore, a key component of process effectiveness.

2.3.3.

Load positioning in the applicator

Several studies have attempted to identify the optimum location for the positioning of the load. Microwaves form a standing wave pattern inside the chamber due to the interference phenomenon. The location with maximum amplitude has the maximum possible energy for conversion into thermal energy. Therefore, it is important to know these locations for load positioning.

2.4.

Applicators

Applicators are chambers in which microwave processing is carried out. The applicator type i.e., single mode or multimode and cavity design and size have a direct effect on microwave heating efficiency. It influences the availability of power for processing [55]. Researchers have for long been finding ways to customize applicators for improving power concentration and in turn heating efficiencies. It has led to the development of various industrial microwave heating systems and customized set-ups. Based on the modes of microwave frequencies generated by the source, the applicators are classified as single mode and multi-mode. Rana and Rana [18] discussed in detail microwave reactors. The following sections discuss the applicator types and their impact on applications.

2.4.1.

Single mode applicators

These applicators operate at single mode microwave frequency. The radiations propagate through waveguides. These waveguides are transmission media, similar to that of an electrical cable used for passing electricity and can be circular or a rectangular cross-section. In rectangular waveguides, either a transverse electric mode or a transverse magnetic mode of wave propagation can exist depending on the boundary conditions. These applicators provide better control over the microwave process parameters due to the predictability of

®

Penetration depth 200 m 30 m 90 m 12 m 40 m

the field across the load. The challenge with the single mode applicators is the proper selection of system components and precise impedance matching to avoid loss of power. A typical single mode applicator configuration is shown in Fig. 4. The microwave is generated by the magnetron and passed through a series of components placed inside a transmission line known as waveguides, analogous to electrical transmission lines. Theisolator protects the generator from cutting off the reflected microwave transmission back to the source [8]. Sometimes circulators are also used to perform the task of directing microwaves to desired space using multiple ports. In order to enable thetransfer of waves without obstruction and reversal, which will damage the source, it is important to have a matching of impedance. Stub tuners are used for impedance matching and minimizing reversal of waves [56]. Maximum heating in the cavity or the heating zone is achieved when the standing waves developed after interference with the reflected wave are constructive in nature. Constructive interference is achieved by having a movable shorting plunger. The load or the material to be processed has to be placed in this heating zone [57]. Researchers have designed single mode applicators for various heating applications to achieve closer process control. For instance, Gower [58] developed a beam applicator for melting of ceramicsand Uhm et al. [59] fabricated a unique microwave torch to synthesize carbon nanotubes. A single mode applicator prototype developed by Pavuluri et al. [60] was 10 times faster in heating the integrated circuit packages required for electronic components. The single-mode applicator with a cylindrical cavity developed by Prapuchanay et al. [61] exhibited superior heating performance compared with multi-mode microwave ovens. Customized single-mode applicators have been developed for microwave drilling applications [62–68]. These applicators have provided novel means for drilling concrete, silicon, glass [65], bones [66] and Litz wire [68]. Saitou [42] carried out series of experiments in a single mode applicator using selected metal powders, such as iron, cobalt, nickel, copper and stainless steel to investigate the mechanism of microwave sintering. Sintered SiC tube was used as a susceptor. The applicator had the provision for establishing a controlled atmosphere (nitrogen gas for nickel and cobalt and hydrogen gas for other materials). It was achieved by providing two ports to the applicator chamber: one for creating a vacuum which in turn, created space for the gas to be introduced while the other port acted as an inlet to the gas; H2 /N2 mixture in this case. Nitrogen was used as it is one of the best low-cost alternative to argon for creating inert atmosphere to avoid the formation of undesired compounds during

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

3313

Fig. 4 – Typical single-mode microwave processing system.

applicators while Srinath et al. [73] used simulations to predict microwave-heating behavior in a multi-mode setup. There was a good agreement between the experimental and simulation results. These simulation results revealed the distribution of electric and magnetic fields along with thermal losses and the temperature distribution in the applicator.Thus, favorable results obtained using experiments and simulation of microwave-based heating can be used for development of large-scale furnaces for industrial applications.

Fig. 5 – Multi-mode microwave processing system.

the sintering process [69]. However, there is a tendency for nitrogen uptake by the material and formation of nitrides [70]. Therefore, to ensure sinter quality, hydrogen is used for iron, copper and stainless steel. It develops a reducing atmosphere in the furnace and absorbs the oxygen from the oxide layers between the particles formed under the high temperature of the furnace. This improves bonding and reduces porosity [71]. Yoshikawa et al. [24] carried out a similar experiment using a 1.5 kW single mode applicator. This enabled easier prediction of the locations with higher strength of magnetic fields and electric fields. The area with higher strength magnetic field showed greater efficiency in heating ferromagnetic materials while nickel-oxide, another material used here, showed better heating results at the location with higher strength of the electric field. The factor that limits the utilization of single-mode applicators is its inability to support larger size specimen.

2.4.2.

Multi-mode applicators

Multi-mode applicators, shown in Fig. 5 are designed to accommodate multiple modes of microwave radiations emitted from a microwave source such as a magnetron. This leads to the distribution of energy in a larger cavity. Eventually, the energy efficiencies obtained in these applicators are lower compared with that achieved in single-mode applicators. For the material processed in multi-mode applicators, the position of the load is insignificant as the uniformity in heating is obtained by rotating the load or by using a mode stirrer, i.e. a component that reflects the microwaves to desired locations. Multi-mode applicators are the most commonly used test chambers for microwave treatment. The domestic ovens are a prime example of a multi-mode set-up. Vollmer [26] described the physics of how these ovens operate while the use of simulations has been reported in applications involving multi-mode applicators as well. For instance, Ju and Zhao [72] used a simulation software (HFSS) in the designing of

2.4.3.

Design modifications and accessories in applicators

In order to understand the effects of process variables, several researchers have done modifications in multi-mode applicators, such as provision for temperature measurement, gas supply and vacuum creation. The set-up used by Roy et al. [41] was an industrial one with a maximum power rating up to 6 kW, working at frequency 2.45 GHz. Facility for mounting susceptors was also available to enable hybrid heating. The temperature at 1400 ◦ C was suitable for achieving sintering of all the combinations of metal and alloy samples. Various metal powder components were successfully sintered. On evaluation, these sintered components exhibited superior metallurgical properties. Agrawal [74] also used an industrial batch type microwave furnace with a high power rating (up to 6 kW) at 2.45 GHz to sinter copper-steel powdered components. Temperature, to the order of 1260 ◦ C, was achieved in this experiment. These studies suggested that judicious usage of multi-mode applicators could lead to a significant magnitude of temperature. A significant investigation in this context has been carried out by Prabhu et al. [43]. Sintering of high melting point material, tungsten was performed in a multimode microwave furnace with variable power input facility at 2.45 GHz. The furnace had the facility of creating a vacuum. Even using domestic ovens at different power ratings yielded desired results in cladding and coating processes [14]. Although there was no clear mention of the temperature levels achieved in these experiments the quality of clads and their properties reveal that complete melting of the clad materials, such as EWAC (Ni-based) + 20% Cr23 C6 , WC10Co2Ni and EWAC, etc. have taken place. Table 6 shows the temperature achieved in different applicators. An important component of the microwave applicator design is the selection of microwave source. Generally, commercial ovens use a magnetron. Apart from this, other generators, such as power grid, gyrotron, traveling wave tube and klystron are available [25]. The selection of the generator is factorized by power. Gyrotron and klystron are high power devices with a frequency range greater than the one used for domestic heating (2.45 GHz) while power grids exhibit very

3314

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Table 6 – Applicator type and temperature attained. Temperature ◦ C

Reference Das et al. [75] Gupta and Sharma [33] Gupta et al. [35] Gupta and Sharma [34,76] Pathania et al. [37] Roy et al. [41] Saitou[42]

Agrawal[7] Prabhu et al. [43] Shannigrahi et al. [45] Bykov et al. [46] Thuault et al. [77] Yang et al. [48] Li et al. [49] Hassan et al. [78] Ghobadi et al. [50]

1310 Melting point of WC10Co2Ni Melting point of EWAC (Ni-based) + 20% Cr23 C6 Melting point of EWAC + 20% WC10Co2Ni Melting point of EWAC + 20% WC10Co2Ni 1200 Fe: 775–1250 Co: 900–1200 Ni: 900–1300 SS316L: 900–1200 Cu: 600–1000 1260 1600, 1700, 1800 900–950 (1400-capacity of furnace) 1250 1200 1200 1800 1300 1520

high-power characteristics at lower frequencies [25]. Thus, one can choose these microwave sources as per the requirement of the application. The parameters discussed above are the basis for the successful implementation of microwave heating in materials processing. This field of research has much to offer in terms of exploring materials for processing, the design of applicators, controlling factors, such as frequency, the atmosphere inside the furnace, analyzing mechanisms of heat generation among others for getting desired experimental outcomes.

3.

Applications of microwave processing

The extent of control over the parameters discussed above determines the effectiveness of the microwave processing applications. The following sections discuss the development in microwave-based heating applications and the relationship between corresponding response parameters and input parameters discussed in previous sections.

3.1.

Power/frequency

Microwave sintering applications

The conventional methods of sintering use resistance heating or heating through conventional modes of heat transfer that is, conduction, convection and radiation. These methods have limitations such as slow heat transfer rate and uneven heating. The development of microwave heating attracted attention when it was observed that microwave sintering offered several advantages over conventional sintering. The energy transfer from microwaves to materials is achieved by absorption. This heating is uniform in nature and therefore yields superior characteristics of the processed material quantified as response parameters [79].

Applicator

800 W/2.45 GHz 900 W/2.45 GHz 900 W/2.45 GHz

Multi-mode Multi-mode Multi-mode

900 W/2.45 GHz 900 W/2.45 GHz 1.4 kW/2.45 GHz 1.5 kW/2.45 GHz

Multi-mode Multi-mode Multi-mode Single Mode

6 kW/2.45 GHz2 kW/2.45 GHz 3 kW/2.45 GHz 0.5–3 kW/2.45 GHz

Multi-mode-Batch furnace Multi-mode Multi-mode

5 kW/24 GHz 6 kW/2.45 GHz 4 kW/2.45 GHz 3 kW/2.45 GHz 5 kW/30 GHz 900 W/2.45 GHz

Multi-mode Multi-mode Multi-mode Multi-mode Multi-mode Multi-mode

Gradually, it was understood that apart from drying microwave energy could be used in efficient sintering of ceramics [74]. For instance, sintering of WC/Co composites through microwave led to a drastic reduction in processing time and offered superior mechanical properties [7,10,74]. Similar advantages have also been reported in the case of metal powders [41,42]. The outcome of the sintering process is measured in terms of three distinct parameters: mechanical properties, sintered density, and porosity. In order to understand the effect of microwave heating on mechanical properties, Roy et al. [41] used components of varied alloy compositions, such as iron, nickel, molybdenum, aluminum, tungsten, tungsten carbide and tin, manufactured through powder metallurgy route on a commercial basis. The applicator had controlled environment and provision for single mode as well as multi-mode operations while susceptors (SiC and MoS2 ) were used to enhance heating efficiency. The results indicated the superiority of microwave sintering process, which boosted toughness and ductility of the materials. For materials with high melting points (such as tungsten – about 3683 K), these characteristics are relevant as it is processed primarily through powder metallurgy route. Prabhu et al. [43] carried out sintering of tungsten powder and activated tungsten powder. Activation of the tungsten powder was achieved using the ball milling process. The experiment was carried out with three different experimental conditions based on variable sintering temperatures (1873 K, 1973 and 2073 K) and varying power input in a 3 kW, 2.45 GHz-controlled atmosphere (5% H2 and 95% N2 ) multi-mode microwave furnace. It was observed that activated tungsten showed better properties in terms of sintered density as shown in Fig. 6. This enhancement is due to the increased surface area, smaller particle sizes and increased strain energies developed during activation using the ball milling process.

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

3315

Fig. 6 – Sintered density versus temperature for activated tungsten and as received tungsten [43].

A unique output parameter identified by Saitou [42] for evaluating the sintering process effectiveness is “Shrinkage Measurement” [80]. The shrinkage parameter can be measured using Eq. (4).

 L n/2 L0

= B0 t(2n Dm RT)

−1

exp

 −Q  RT

(4)

Values of numerical constants, n and m, depending on the sintering mechanism: n = 2, m = 1 for viscous or plastic flow; n = 5, m = 3 for volume diffusion; n = 6, m = 4 for grain boundary diffusion; and n = 7, m = 4 for surface diffusion. The samples used in the study consisted of several metal powders, viz. copper, cobalt, iron, nickel and stainless steel subjected to sintering in single mode applicator within a controlled environment. It was observed that microwave-heating process has an impact on shrinkage, which is dependent on material parameters as expressed in Eq. (4). It was also pointed out by the author that the reasons for shrinkage are still unknown and further studies are required to understand it [42]. Ferrite is an important component in many engineering applications, especially in electrical and electronic devices due to its magnetic properties. It is processed through powder metallurgy route and requires sintering. Conventional sintering causes a loss in fineness of the particles as a result of high temperatures [45]. Microwave sintering of the ferrite powders led to superior properties [45]. Microwaves, exhibit selective heating characteristics due to different interaction abilities of materials as discussed in Section 2.3.1. This ability of microwaves was used in the sintering of Ni–Al2 O3 functionally graded material [46]. The samples obtained were free from defects, such as cracks and delamination. Fig. 7a–c compares a few response parameters of microwave sintering with conventional sintering of selected samples [41,74]. Microwave sintering resulted in a greater degree of

hardness, modulus of rupture (MOR) and reduced processing time. In short, the response parameters in sintering are sintered density, hardness, flexural strength and toughness in general and magnetic properties and shrinkage in particular. The superior properties of microwave-sintered components are attributed to the following reasons [41]: • Bulk heating characteristics of microwaves resulting in heat transfer in dual directions (from penetration depth to both the sides of the sample. • Fast processing speed leading to microstructure-dependent property enhancement such as finer grain size and development of round edge porosities. It also minimizes the formation of undesired compounds and at the same time, provides energy advantage due to low consumption of power. Sintering is a well-known application in microwave processing wherein a lot of development in design and development of dedicated microwave furnaces has taken place. There is very little understanding of the mechanisms involved in sintering. Large-scale commercialization is also yet to be realized. At the same time, the optimization of the process parameters and predictable control over the process is still a challenge and demands extensive research.

3.2.

Cladding and surface coating applications

Many engineering applications require materials that exhibit strong resistance to chemical attacks and wear with an ability to perform intended function even in hostile environments and operating conditions [81]. Marine applications, aeronautical applications, oil and gas refineries and drilling applications, agricultural applications, and defense applications are few of those applications that involve extreme as well as adverse environments [82,83]. The materials used in these applications also need to have sufficient strength, hardness and toughness to withstand the various structural and thermal loads experienced by them [84–88]. The effects of adverse

3316

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Fig. 7 – (a) Comparison of hardness achieved in the microwave and conventional sintering. (b) Comparison of MOR achieved in the microwave and conventional sintering. (c) Comparison of time required to achieve the desired temperature [41,74].

environmental and severe operating conditions are primarily limited to surfaces. There are different ways to modify or enhance surface properties, such as modification of chemical composition by surface nitriding, surface cyaniding, carburizing, vanadizing and modification in the surface characteristics by mechanical and thermal processes and development of layers or overlays on the surfaces using coating and cladding processes. Conventional surface modification methods are thermal spraying [82], high-velocity oxyfuel spraying [89], tungsten inert gas (TIG) cladding, laser cladding, and submerged metal arc welding (SMAW) based cladding, etc [84] . Laser cladding is the most effective process in terms of its superior properties resulting in reduced cracks, porosity and errors in bonding along with the controlled cooling rate. The major issue with laser-based processes is the cost and thermal stresses developed due to the concentration of heat [33]. With a greater understanding of microwave heating phenomenon, several applications of coating and cladding using microwave thermal energy have been reported. Gupta and Sharma [33] demonstrated successful cladding of EWAC (97% nickel) over austenitic stainless-steel substrate. The process was carried out in the domestic microwave oven (1 kW, 2.45 GHz). The clad exhibited metallurgical bonding with the substrate and developed intermetallic compounds, such as iron-nickel and nickel-silicide. The average microhardness achieved was about 304 + 48 HV, which is twice than that of the substrate material. Additionally, the clads were observed to be of a superior quality which was evident through its pores-free characteristics as can be seen in Fig. 8.

Fig. 8 – Microstructure of EWAC microwave cladded cross-section (Reprinted from Surface and Coatings Technology, 205 /21-22, Dheeraj Gupta, A.K. Sharma, Development and microstructural characterization of microwave cladding on austenitic stainless steel, 5147-5155, 2011, with permission from Elsevier.) [33].

In extension to this work, Gupta and Sharma [29] carried out cladding experiments to gauge the effectiveness of microwave cladding of a composite (EWAC-Ni-based and 20% Cr23 C6 powder) over austenitic stainless steel. The results were encouraging as the obtained clads exhibited desired properties. Using similar methodologies, the cladding of antiwear powders to improve wear resistance have been achieved [37,38,40].

3317

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Table 7 – Parametric summary of microwave-based coating and cladding research. Reference

Material

Power

Remarks 1. Preheating: 200 h Muffle furnace heating in air (600 ◦ C) 2. Coating thickness:17 ± 3.2 ␮m (porous coating) 3. Preheating: 200 h Muffle furnace heating in air (600 ◦ C) + 60 min: microwave heating (1050 ± 25 ◦ C) 4. Coating thickness: 42 ± 0.1 ␮m (porous coating) 5. Preheating: 200 h Muffle furnace heating in air (600 ◦ C) + 90 min: microwave heating (1310 ± 25 ◦ C) 6. Coating thickness: 661 ± 58.9 ␮m (thick and dense coating) 1. Vicker’s microhardness •Clad: 304 ± 48 HV •SS-316 substrate: 175 HV 2. Processing time: 360 s 3. Preheating: the EWAC powder was pre-heated at 100 ◦ C for 24 h in a conventional Muffle furnace 1. Vicker’s microhardness: 1064 ± 99 HV 2. Porosity: 0.89% 3. Wear resistance. 4. Processing time: 360 s

Das et al. [75]

Commercial aluminum (purity ∼99%). Cylindrical samples/aluminum oxide coating solid (length ≈ 10 mm, ϕ ≈ 18 mm)

800 W, 2.45 Hz

Gupta and Sharma [33]

Austenitic stainless steel (SS316)/EWAC powder as Clad material (40 ␮m)

900 W, 2.45 Hz

Gupta and Sharma [34]

Austenitic stainless steel (SS-316)/WC10Co2Ni Clad material (40 ␮m) Solid substrate (35 mm × 12 mm × 6 mm) and powder as Clad material Austenitic stainless steel (SS-316)/(EWAC (Ni-based) + 20% Cr23C6) Clad material (40 ␮m) Solid substrate (35 mm × 12 mm × 6 mm) and powder as Clad material Austenitic stainless steel (SS-316)/WC10Co2Ni Clad material (40 ␮m) Solid substrate (35 mm × 12 mm × 6 mm) and powder as Clad material Austenitic stainless steel (SS-316)/EWAC + 20% WC10Co2Ni Clad material (40 ␮m) Solid substrate (35 mm × 12 mm × 1 mm) and powder as Clad material Mild steel/EWAC + 20% WC10Co2Ni Clad material (40 ␮m)

900 W, 2.45 GHz

SS304/WC-12Co as Clad material Samples of size 20 mm × 10 mm × 6 mm

1.4 and 1.1 kW, 2.45 GHz

Gupta et al. [35]

Sharma and Gupta [36]

Gupta and Sharma [76]

Pathania et al. [37]

Zafar and Sharma [38]

900 W, 2.45 GHz

1. Vicker’s microhardness: 2. Clad: 425 ± 140 HV 3. Porosity: 0.9% 4. Processing time: 360 s 5. Preheating: EWAC and Cr23 C6 powder particles were preheated at 100 ◦ C for 24 h

900 W, 2.45 GHz

1. Flexural strength (3-point bend test): 629 ± 8 N Deformation: 0.76 mm. 2. Vicker’s microhardness: •Clad: 416 ± 20 HV •SS-316: ∼200 HV 3. Processing time: 360 s 4. Preheating: EWAC and WC10Co2Ni particles were preheated at 100 ◦ C for 24 h. 1. Vicker’s microhardness: •Clad: 1042 ± 124 S HV •SS-316: ∼250 HV •SS-316, microhardness near the clad surface: 377 ± 68 HV 2. Processing time: 120 s 3. Preheating: WC10Co2Ni particles were preheated at 100 ◦ C for 24 h.

900 W, 2.45 GHz

900 W, 2.45 GHz

The quality of the coating and cladding is determined by characterizing and measuring response parameters, such as micro-hardness, wear resistance, deformation index, and thickness. In order to understand the conditions under which these parameters have attained the desired

1. Wear test parameters •Sliding distances (meters) 500, 1000, 1500 •Sliding speed (m/s) 0.5, 1, 1.5 •Normal load (kg) 0.5, 1, 1.5 2. Wear resistance is measured to be 128 times mild steel 3. Vickers micro-hardness: 980 ± 52 HV 1. Vicker’s microhardness •NM Clad: 1564 ± 53 HV •MM Clad: 1138 ± 90 HV

levels, the results of various researchers are presented in Table 7. Microwave-based cladding or coating has the potential to be developed as an alternative to conventional surface modification techniques. Microwaves ability to achieve uniform

3318

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

heating makes a case for strong thermal stress-free surface modification. Another important observation in the area of surface modification application is the lack of studies on the effect of process and response parameters. This could be a result of the lack of suitable experimental set-ups and the early days of development of such an application. This offers tremendous scope for future research on the reliability of the process control, the advent of clads of novel materials and optimization of processes.

3.3.

Microwave joining of materials

A wide variety of materials such as plastics, mild steel, stainless steel, copper have been tested for suitability of joining through microwave energy. The process is based on utilizing the heat developed by microwave energy for melting interfacing powder material at the joint either directly or through hybrid heating and fusion of the melted powder with the rectangular plates or cylindrical specimen to be joined [90–100]. Siores and Diego [101] carried out pioneering work in microwave joining. Set-ups for carrying out joining of plastics and metals were developed and bonding agents having low loss factors were used for the joining purpose to enhance microwave absorption. The set-ups were specially devised to utilize microwave energy with minimum losses. Diagnostic tools were incorporated for measuring process parameters, such as the strength of the electric field, the temperature of the specimen, incident and reflected power and joining interface pressure. This provided an opportunity to detect and control joining processes with the help of microcomputer. Investigations showed that microwaves can be successfully used for joining not only high loss materials such as mullite, alumina, but also low loss materials viz. plastic polymers and metals. Rods of the composite of alumina (48%), zirconium (32%), SiO2 (20%) were butt welded in a single mode applicator using microwaves generated by the variable power source with pulsating input [90]. It was observed that at higher power ratings, 1 and 1.25 kW, and loading pressure of about 1 MPa, the joint strength was found to be reduced. It was due to the formation of blowholes and flash. On the other hand, very low power and loading pressure of magnitude 0.5 MPa resulted in incomplete joints. Ahmed and Siores [90] have also reported that joint quality is affected by the rate of temperature rise which can be controlled with the use of incident power pulsing. Therefore, an efficient microwave joining process requires close control over the rate of temperature rise. A significant development in microwave joining was the advent of methodologies for joining metals. Experiments were carried out to weld bulk SS316L using microwave hybrid heating [102]. The joint was formed by fusing powdered sandwich layer heated through microwave interaction in 900 W multi-mode applicator at 2.45 GHz frequency. The joints were characterized for morphology and mechanical properties and were found to have good strength and microhardness owing to the formation of carbides in the intermetallic phases due to the use of charcoal as a susceptor. Using a similar methodology, copper plates were joined using microwave hybrid heating with the help of charcoal as a susceptor material [98]. Hybrid

heating provided the initial rise in temperature to a critical one of the sandwich layers comprising a powdered slurry of copper. Beyond this temperature, the powder started coupling with microwaves. It was found that the joint had a uniform microstructure, an outcome of uniform heating. This resulted in superior microhardness, denser microstructure, significant tensile strength, and high elongation. This development is significant due to poor weldability of copper through conventional methods. Another important field in microwave-based joining is the preparation of high-quality dissimilar joints. Upon successful processing of high-quality similar metal joints of copper–copper and SS316L–SS316L, experiments were carried out for joining two different metals SS316L and MS with Ni as an interfacing material [103]. Due to strong intermetallic bonding, high ultimate tensile strength (up to 346.6 MPa) and hardness of 133 HV were obtained. This method was found to be time-efficient. The use of clean energy in the form of microwave radiations made the process eco-friendly too. Bansal et al. [92] examined MS-MS joints formed with nickel as an interfacing material in a multi-mode applicator of 900 W rating at an exposure time of 600 s and a frequency of 2.45 GHz. Complete melting of the interfacing material at the interface led to diffusion bonding which resulted in superior joint quality. The SEM micrograph showed that the joint had a predominant ductile fracture behavior. Badiger et al. [97] showed the joining of bulk Inconel-625 alloy using a novel technique based on microwave heating. The researchers followed a similar procedure with that of earlier experimental studies. Butt joining of the square specimens was carried out using nickel powder as an interfacing material. The joint strength obtained was observed to be greater than 35% of the base metal strength. Most of these experiments were conducted in a multi-mode domestic microwave oven. Hoyt et al. [104] carried out soldering of copper with lead telluride using a customized microwave-based welding system. It was cost and time effective. Numerical simulation of the joining process has also been done on COMSOL, a commercial package, to understand the temperature distribution and the results were compared with experimental values [73]. It can be concluded that main response parameters in the microwave-based joining of materials are: joint hardness, joint strength, porosity, percentage elongation of the joint and time required for processing as shown in Fig. 9. The magnitude of response parameters of the joint obtained using microwave-based heating for various combinations of metals, interfacing powders and susceptors used is shown in Table 8. The magnitude of joint hardness, porosity, joint ultimate tensile strength proves the efficacy of microwave-based joining methodology. All these studies focused on investigating the feasibility of joining different materials using microwaves and quantified the magnitudes of response parameters using experiments. The parametric effects of variables, such as powder size, time of materials’ ‘microwave exposure have seldom been reported. Gamit et al. [96] reported the results of time dependency on the strength of the bond formed between the two metal pieces. Although there was not much improvement in the yield strength of the joint with time, it was revealed that the exposure time has a synergistic relationship with the

3319

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Fig. 9 – Response parameters in microwave-based joining.

Table 8 – Comparison of microwave joining parameters. Reference

Material combination

Srinath et al. [102]

SS 316L–SS 316L Plate sizes: (25 × 12 × 6) mm3 Copper–copper Plate sizes: 15 × 12 × 4) mm3 Coin size: 18 mm × 12 mm) SS 316L and MS Plate sizes: (25 × 12 × 6) mm3 MS-MS Plate sizes: (15 × 10 × 4) mm3 and (10 × 10 × 4) mm3 SS 316–SS 316 Plate sizes: (25 × 15 × 4) mm3 Inconel-625–Inconel 625 Plate sizes: (12 × 102 × 6) mm3

Srinath et al. [98]

Srinath et al. [103] Srinath et al. [92]

Bansal et al. [91] Badiger et al. [97]

Susceptor

Charcoal

Charcoal

Charcoal

Charcoal

Silicon Carbide Charcoal

Interfacing material

Joint UTS (MPa)

Joint hardness (HV)

Porosity (%)

% Elongation Processing time (s)

Ni-based powder (40 ␮m) Copper powder (50 ␮m)

309

290 ± 14

0.78

11.5

900

164.4

78 ± 7

1.92

29.21

900

Ni-based powder (40 ␮m) Ni-based powder (40 ␮m)

346.6

133

0.58

13.58

450

240

420 ± 30

–

6

600

SS 316 powder (50 ␮m) Ni-based powder (40 ␮m)

425

275 ± 20

0.94

9.44

720

326

350 ± 10

–

9.05

1260

metallurgical bonding formed between the interface material and the bulk pipes of mild steels. Investigation on the effect of powder size on joint hardness, ultimate tensile strength (UTS) and percentage elongation in the joining of SS304 specimen was carried out by Bagha et al. [94]. Four joints were prepared with interfacing powder Nickel with a size of 50 ␮m, 40 ␮m, 30 ␮m, and 20 ␮m. The effect of variation of powder size on the response variables is as shown in Fig. 10. It was found that the size of powders has an inverse relationship with joint homogeneity, strength, and hardness. That is, there is an increase in the joint hardness and ultimate tensile strength with the reduction in powder size; while the ductility tensile strength reduces with decreasing powder size. This is due to increased microwave interaction with reducing powder size. This is

attributed to the close proximity of the powder size to the penetration depth of the material. The materials joined by microwave heating have shown good characteristics and shorter processing times in different investigations. Apart from conventional materials such as steel and copper, this technique of joining can be used in different applications involving difficult-to-join materials, such as aluminum and its alloys, magnesium and its alloys and various dissimilar joints. The major challenges posed by the microwave-based joining process is selection of suitable interfacing powder or filler material compatible to the materials to be joined, preparation of set-up that includes selection and positioning of susceptor and masking materials to avoid bulk specimen from getting exposed to microwaves while ensuring maximum heat

3320

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Fig. 10 – Effect of particle size.

transfer from the susceptor to the joint area. In addition, the most significant challenge is to estimate the processing time, which is still based on trial and error [96].

3.4.

Microwave casting/melting applications

Researchers have also explored possibilities in utilizing microwave-based heating technology in melting or casting processes. One of the earlier usages of microwave energy for casting process is the de-waxing operation in the investment casting process. Brum et al. [105] evaluated the feasibility of using microwave technology in dewaxing while Yahaya et al. [106] implemented the concept of microwave hybrid heating for in microwave dewaxing. It offered advantages, such as nondegradation of wax quality and faster processing time. Although microwave-based melting of copper, aluminum and steel specimen have been reported earlier by Agrawal [7], however, not many details are available on the experimental parameters. Chandrasekaran et al. [107] carried out a similar investigation to understand the heating characteristics of metals, such as lead, tin, copper, and aluminum in a multimode oven under hybrid heating environment. The process parameters selected were microwave power and the quantity of material being processed (load). The results of the experiment were compared with conventional melting. Microwave heating resulted in rapid temperature rise attributed to the increased interaction of microwaves with the material. Three different power levels 100% (1300 W), 70% (910 W) and 40% (520 W) and at various sample loads (25–150 g) were used for investigation. An important outcome was the fact that melting through microwave power was found to be twice faster than conventional heating-based melting and it is also a cleaner process with reasonable control. The success achieved in the melting of metals using microwave energy led to the research on casting. An in situ

casting set-up was developed in an industrial multi-mode microwave oven (1.4 kW at 2.45 GHz) by Mishra and Sharma [108]. This was used to melt a cylindrical piece of 100 g aluminum 7079 alloy. Major parts of the set-up were made up of materials that exhibit good interaction with microwaves. This included a pouring basin with SiC susceptor; alumina sprue and a graphite mold in two pieces (cope and drag). The casting process got completed with microwave exposure for only 8 min and 45 s. COMSOL Multiphysics, a commercial package was also used to indicate the thermal history of the load under the influence of microwave irradiation. The simulation results demonstrated that the location of charge to be melted had a significant effect on heating characteristics. An important parameter in microwave processing is the rate of temperature rise. Xu et al. [44] performed the melting of copper powder of three different sizes of 25 ␮m, 38 ␮m, and 74 ␮m. An additional experimental factor was quantity. These were used in three different quantities, viz. 50 g, 100 g, and 150 g. The samples were exposed to 1.3–1.8 kW power at 2.45 GHz in a multi-mode industrial applicator. The applicator had a provision for temperature measurement and setting up of a controlled atmosphere of Ar gas. The research resulted in the development of a model for the rate of temperature rise in copper of powder size 74 ␮m. The rise in temperature has been expressed as a function of ‘t’ using Eq. (5).

 T=

8.8t + 443.7, R2 = 0.9993

(0 < t ≤ 26)

−294.5 + 49.9t − 0.46t2 , R2 = 0.9793

(26 ≤ t ≤ 55)

(5)

Xu et al. [44] also showed that an increase in temperature leads to densification from 2.13 g/cm3 at room temperature to 3.31 g/cm3 , 4.29 g/cm3 , 4.85 g/cm3 and 8.07 g/cm3 at 900 ◦ C, 1060 ◦ C, 1083 ◦ C and 1090 ◦ C respectively for the powder size of

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

3321

Fig. 11 – Response parameters in microwave-based casting/melting.

74 ␮m. Thus, the desired level of cast quality can be obtained by controlling the temperature rise. In general, the response parameters in any casting process are the strength of the cast, porosity, microhardness of the cast, the rate of temperature rise, different phases obtained after the process, microstructure, and elemental analysis as shown in Fig. 11. For developing the understanding of the structure and property of the cast, Mishra and Sharma [109] determined the characteristics of the in situ Al–Zn–Mg alloy cast. Characterization was done to obtain cast microstructure, elemental analysis, phase analysis, porosity, micro-indentation hardness and tensile strength. The porosity of the cast was found to be less than 2%. Fig. 12 shows the comparison between properties of received aluminum alloy material and cast alloy. The cast alloy exhibited superior characteristics with increased yield strength, 86 MPa against 70 MPa; ultimate tensile strength, 149 MPa against 112 MPa and larger percentage elongation, 1.92% versus 1.59%. Melting application or casting through microwave energy is a recent development in the context of microwave-based heating applications and therefore, further research is anticipated to establish its reliability at the commercial level for different castable materials. The characteristics that make it a lucrative research domain is the amount of energy savings

that can be achieved through this process and superior cast product quality in terms of mechanical properties that can be obtained with better control over the process.

4. Conclusion and potential areas for research Efficient microwave-based processing techniques save cost, time and energy. There is wide scope for introduction of novel materials in microwave-based coating and cladding such as the use of conducting polymers for anti-corrosion and stealth applications, microwave-based joining and microwave-based casting. Development of experimental set-ups is required for dedicated application areas. It can enhance the predictability of newly developed processes. Further research and analysis are required to describe the relationships between the input and response parameters for accurately achieving reliable process control. Summary of the present research in the field of microwave processing is described as follows: • Four significant input parameters have been identified that control the microwave-based heating process. These input parameters are dielectric and magnetic properties, heating mechanisms, load and applicators.

3322

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

Fig. 12 – Comparison between as received alloy and cast alloy properties.

• Each of these parameters is influenced by several factors such as morphology and size of the material being processed, frequency, positioning inside the applicator, the type of applicator and the set-ups used, the dominant heating mechanisms, etc. • The type of applicator i.e. single mode or multi-mode, size of the material, heating mechanism and the temperature of the process need finer attention and control. • The material, especially metals when used in the powder form with a size of the order of penetration depth (around 10 ␮m) increases interaction, which results in uniform heating and superior properties along with a reduction in processing time. This reduction is quite drastic that is, from hours required in conventional processes to minutes in microwave-based applications. • Microwave-based processing scope can be widened using a microwave hybrid heating approach at a large scale to process the materials with poor microwave interaction. • Selection of susceptors such as silicon carbide, charcoal, graphite, alumina, mullite, zirconia, etc. requires a thorough understanding of the applications. In addition, the selective heating capability of the microwave can be utilized in composite materials synthesis as these have distinct phases with variable microwave interaction capability. • Future studies can focus on overcoming the challenge of an uncontrolled rise of loss tangent at elevated temperatures, which leads to excessive undesired heating. • The common response parameter of all the microwavebased heating applications is the rate of temperature rise that indirectly affects product characteristics. Control over the heating rate offers advantages in the form of improved morphology and properties of the processed material. • In the case of sintering, the response parameters are sinter density, modulus of rupture, hardness, the rate of

temperature rise. Sintering has been the most widely practiced microwave-based materials processing application and has shown to be effective in microwave-based processing. • Cladding and coating applications have exhibited enhanced wear properties, improved micro-hardness and sufficient thickness. Still, these are in their infancy stage and demands further development for large scale processing. • Microwave-based joining is characterized by joint strength, joint hardness, porosity, percentage elongation. Joint quality is a function of the rate of temperature rise. Hence, hybrid heating and careful selection of applicator with precise control is an area that needs to be intensively investigated. Loss of weld strength (up to 50%) during microwave joining. Therefore, the development of higher strength welds and microwave joining of novel materials are impending challenges. • Melting or casting processes have exhibited advantages and desired output parameters such as mechanical and physical properties have been obtained in the cast product. However, quantitative analysis of energy conserved using this clean production methodology needs further extensive research.

Conflicts of interest The authors declare no conflicts of interest.

Acknowledgment The work is a part of the research project titled, “Study on the effect of microwave heating on metals(Grant ID NMIMS/MPSTME/PROJECT/41)” funded by SVKM’s NMIMS (Deemed-to-be University), Mumbai, India. The authors would

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

like to thank the university for providing funding and research facilities.

references

[1] Thostenson ET, Chou T-W. Microwave processing: fundamentals and applications. Compos Part A Appl Sci Manuf 1999;30:1055–71, http://dx.doi.org/10.1016/S1359-835X(99)00020-2. [2] Chandrasekaran S, Ramanathan S, Basak T. Microwave material processing – a review. AIChE J 2012;58:330–63, http://dx.doi.org/10.1002/aic.12766. [3] Pal R, Jha AK, Akhtar MJ, Kar KK, Kumar R, Nayak D. Enhanced microwave processing of epoxy nanocomposites using carbon black powders. Adv Powder Technol 2017;28:1281–90, http://dx.doi.org/10.1016/j.apt.2017.02.016. [4] Aguilar-Reynosa A, Roman A, Ma. Rodrguez-Jasso R, Aguilar CN, Garrote G, Ruiz HA. Microwave heating processing as alternative of pretreatment in second-generation biorefinery: an overview. Energy Convers Manag 2017;136:50–65, http://dx.doi.org/10.1016/j.enconman.2017.01.004. [5] Beneroso D, Monti T, Kostas ET, Robinson J. Microwave pyrolysis of biomass for bio-oil production: scalable processing concepts. Chem Eng J 2017;316:481–98, http://dx.doi.org/10.1016/j.cej.2017.01.130. [6] Hazarika A, Deka BK, Kim D, Kong K, Bin Park Y, Park HW. Microwave-synthesized freestanding iron-carbon nanotubes on polyester composites of woven Kevlar fibre and silver nanoparticle-decorated graphene. Sci Rep 2017;7:1–11, http://dx.doi.org/10.1038/srep40386. [7] Agrawal D, Material RI. Microwave sintering, brazing and melting of metallic materials. Non-Ferrous Mater Extr Process 2006;4:183–92, http://dx.doi.org/10.1080/0371750X.2006.11012292. [8] Kim J, Mun SC, Ko HU, Kim KB, Khondoker MAH, Zhai L. Review of microwave assisted manufacturing technologies. Int J Precis Eng Manuf 2012;13:2263–72, http://dx.doi.org/10.1007/s12541-012-0301-2. [9] Das S, Mukhopadhyay AK, Datta S, Basu D. Prospects of microwave processing: an overview. Bull Mater Sci 2009;32:1–13, http://dx.doi.org/10.1007/s12034-009-0001-4. [10] Agrawal D. Latest global developments in microwave materials processing. Mater Res Innov 2010;14:3–8, http://dx.doi.org/10.1179/143307510X12599329342926. [11] Oghbaei M, Mirzaee O. Microwave versus conventional sintering: a review of fundamentals, advantages and applications. J Alloys Compd 2010;494:175–89, http://dx.doi.org/10.1016/j.jallcom.2010.01.068. [12] Yahaya B, Izman S, Konneh M, Redzuan N. Microwave hybrid heating of materials using susceptors – a brief review. Adv Mater Res 2013;845:426–30, http://dx.doi.org/10.4028/www.scientific.net/AMR.845.426. [13] Leonelli C, Mason TJ. Microwave and ultrasonic processing: now a realistic option for industry. Chem Eng Process Process Intensif 2010;49:885–900, http://dx.doi.org/10.1016/j.cep.2010.05.006. [14] Singh S, Gupta D, Jain V. Recent applications of microwaves in materials joining and surface coatings. Proc Inst Mech Eng Part B J Eng Manuf 2016;230:603–17, http://dx.doi.org/10.1177/0954405414560778. [15] Singh S, Gupta D, Jain V, Sharma AK. Microwave processing of materials and applications in manufacturing industries: a review. Mater Manuf Process 2015;30:1–29, http://dx.doi.org/10.1080/10426914.2014.952028.

3323

[16] Mishra RR, Sharma AK. A review of research trends in microwave processing of metal-based materials and opportunities in microwave metal casting. Crit Rev Solid State Mater Sci 2016;41:217–55, http://dx.doi.org/10.1080/10408436.2016.1142421. [17] Zhao X, Huang K, Yan L. Review of numerical simulation of microwave heating process. INTECH Open Access Publ; 2011. p. 27–8, http://dx.doi.org/10.5772/13387. [18] Rana KK, Rana S. Microwave reactors: a brief review on its fundamental aspects and applications. OALib 2014;01:1–20, http://dx.doi.org/10.4236/oalib.1100686. [19] Mishra RR, Sharma AK. Microwave–material interaction phenomena: heating mechanisms, challenges and opportunities in material processing. Compos Part A Appl Sci Manuf 2016;81:78–97, http://dx.doi.org/10.1016/j.compositesa.2015.10.035. [20] Sun J, Wang W, Yue Q. Review on microwave–matter interaction fundamentals and efficient microwave-associated heating strategies. Materials (Basel) 2016:9, http://dx.doi.org/10.3390/ma9040231. [21] Reeja-Jayan B, Harrison KL, Yang K, Wang CL, Yilmaz AE, Manthiram A. Microwave-assisted low-temperature growth of thin films in solution. Sci Rep 2012;2:1–8, http://dx.doi.org/10.1038/srep01003. [22] Chen LF, Chen LF, Ong CK, Ong CK, Neo CP, Neo CP, et al. Microwave electronics measurement and materials characterisation; 2004, http://dx.doi.org/10.1002/0470020466. [23] Angela A, DAmore M. Relevance of dielectric properties in microwave assisted processes. In: Microw. mater. charact. InTech; 2012, http://dx.doi.org/10.5772/51098. [24] Yoshikawa N, Ishizuka E, Taniguchi S. Heating of metal particles in a single-mode microwave applicator. Mater Trans 2006;47:898–902, http://dx.doi.org/10.2320/matertrans.47.898. [25] Council NR. Microwave processing of materials; 1994, http://dx.doi.org/10.17226/2266. [26] Vollmer M. Physics of the microwave oven. Phys Educ 2003;39:74–81, http://dx.doi.org/10.1088/0031-9120/39/1/006. [27] Solanki GK, Patel DB, Patel KD, Gosai NN, Gafur Mansur Y. Growth and dielectric properties of DVT grown GeS0.25Se0.75 single crystals. Phys Chem 2012;2:67–72, http://dx.doi.org/10.5923/j.pc.20120205.02. [28] Fuzerova J, Fuzer J, Kollar P, Hegedus L, Bures R, Faberova M. Analysis of the complex permeability versus frequency of soft magnetic composites consisting of iron and Fe73Cu1Nb3Si16B7. IEEE Trans Magn 2012;48:1545–8, http://dx.doi.org/10.1109/TMAG.2011.2173173. [29] Hua Y, Liu C. Heating rate of minerals and compounds in microwave field. Trans Nonferrous Met Soc China (English Ed) 1996;6:35–40. [30] Hao H, Xu L, Huang Y, Zhang X, Xie Z. Kinetics mechanism of microwave sintering in ceramic materials. Sci China, Ser E Technol Sci 2009;52:2727–31, http://dx.doi.org/10.1007/s11431-008-0217-3. [31] Menéndez JA, Arenillas A, Fidalgo B, Fernández Y, Zubizarreta L, Calvo EG, et al. Microwave heating processes involving carbon materials. Fuel Process Technol 2010;91:1–8, http://dx.doi.org/10.1016/j.fuproc.2009.08.021. [32] Kirby BW. Alternative crucibles for U–Mo microwave melting; 2017. [33] Gupta D, Sharma AK. Development and microstructural characterization of microwave cladding on austenitic stainless steel. Surf Coatings Technol 2011;205:5147–55, http://dx.doi.org/10.1016/j.surfcoat.2011.05.018. [34] Gupta D, Sharma AK. Investigation on sliding wear performance of WC10Co2Ni cladding developed through

3324

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

microwave irradiation. Wear 2011;271:1642–50, http://dx.doi.org/10.1016/j.wear.2010.12.037. Gupta D, Bhovi PM, Sharma AK, Dutta S. Development and characterization of microwave composite cladding. J Manuf Process 2012;14:243–9, http://dx.doi.org/10.1016/j.jmapro.2012.05.007. Sharma AK, Gupta D. On microstructure and flexural strength of metal–ceramic composite cladding developed through microwave heating. Appl Surf Sci 2012;258:5583–92, http://dx.doi.org/10.1016/j.apsusc.2012.02.019. Pathania A, Singh S, Gupta D, Jain V. Development and analysis of tribological behavior of microwave processed EWAC + 20% WC10Co2Ni composite cladding on mild steel substrate. J Manuf Process 2015;20:79–87, http://dx.doi.org/10.1016/j.jmapro.2015.09.007. Zafar S, Sharma AK. Dry sliding wear performance of nanostructured WC-12Co deposited through microwave cladding. Tribol Int 2015;91:14–22, http://dx.doi.org/10.1016/j.triboint.2015.06.023. Zafar S, Sharma AK. Structure–property correlations in nanostructured WC-12Co microwave clad. Appl Surf Sci 2016;370:92–101, http://dx.doi.org/10.1016/j.apsusc.2016.02.114. Zafar S, Sharma AK. Abrasive and erosive wear behaviour of nanometric WC-12Co microwave clads. Wear 2016;346–347:29–45, http://dx.doi.org/10.1016/j.wear.2015.11.003. Roy R, Agrawal D, Cheng J, Gedevanlshvili S. Full sintering of powdered-metal bodies in a microwave field. Nature 1999;399:668–70, http://dx.doi.org/10.1038/21390. Saitou K. Microwave sintering of iron, cobalt, nickel, copper and stainless steel powders. Scr Mater 2006;54:875–9, http://dx.doi.org/10.1016/j.scriptamat.2005.11.006. Prabhu G, Chakraborty A, Sarma B. Microwave sintering of tungsten. Int J Refract Met Hard Mater 2009;27:545–8, http://dx.doi.org/10.1016/j.ijrmhm.2008.07.001. Xu L, Srinivasakannan C, Peng J, Guo S, Xia H. Study on characteristics of microwave melting of copper powder. J Alloys Compd 2017;701:236–43, http://dx.doi.org/10.1016/j.jallcom.2017.01.097. Shannigrahi SR, Pramoda KP, Nugroho FAA. Synthesis and characterizations of microwave sintered ferrite powders and their composite films for practical applications. J Magn Magn Mater 2012;324:140–5, http://dx.doi.org/10.1016/j.jmmm.2011.07.050. Bykov YV, Egorov SV, Eremeev AG, Holoptsev VV, Plotnikov IV, Rybakov KI, et al. Temperature profile optimization for microwave sintering of bulk Ni–Al2 O3 functionally graded materials. J Mater Process Technol 2014;214:210–6, http://dx.doi.org/10.1016/j.jmatprotec.2013.09.001. Thuault A, Marinel S, Savary E, Heuguet R, Saunier S, Goeuriot D, et al. Processing of reaction-bonded B4C–SiC composites in a single-mode microwave cavity. Ceram Int 2013;39:1215–9, http://dx.doi.org/10.1016/j.ceramint.2012.07.047. Yang H, Zhou X, Yu J, Wang H, Huang Z. Microwave and conventional sintering of SiC/SiC composites: flexural properties and microstructures. Ceram Int 2015;41:11651–4, http://dx.doi.org/10.1016/j.ceramint.2015.05.126. Li Y, Xu F, Hu X, Luan Y, Han Z, Wang Z. Micro-focusing effect of electromagnetic fields and its influence on sintering during the microwave processing of ceramic particles of SiC. Ceram Int 2015;41:14554–60, http://dx.doi.org/10.1016/j.ceramint.2015.07.172. Ghobadi H, Ebadzadeh T, Sadeghian Z, Barzegar-Bafrooei H, Nemati A. Microwave-assisted sintering of Al2 O3 –MWCNT nanocomposites. Ceram Int 2017;43:6105–9, http://dx.doi.org/10.1016/j.ceramint.2017.02.003.

[51] No Title n.d. http://www.por.se/references/dieldata.html [accessed 04.10.18]. [52] Whittaker AG, Mingos DMP. J Chem Soc Dalton Trans 1995:2073–9. [53] Mondal A, Agrawal D, Upadhyaya A, Engineering M. Microwave heating of pure copper powder with different n.d.:517–20. [54] Ghosh S, Pal KS, Dandapat N, Mukhopadhyay AK, Datta S, Basu D. Characterization of microwave processed aluminium powder. Ceram Int 2011;37:1115–9, http://dx.doi.org/10.1016/j.ceramint.2010.10.012. [55] Houˇsová J, Hoke K. Microwave heating – the influence of oven and load parameters on the power absorbed in the heated load. J Food Sci Czech 2002;20:117–24. [56] Chaichumporn C, Ngamsirijit P, Boonklin N, Eaiprasetsak K, Fuangfoong M. Design and construction of 2.45 GHz microwave plasma source at atmospheric pressure. Proc Eng 2011;8:94–100, http://dx.doi.org/10.1016/j.proeng.2011.03.018. [57] Jamróz P, Kordylewski W, Wnukowski M. Microwave plasma application in decomposition and steam reforming of model tar compounds. Fuel Process Technol 2018;169:1–14, http://dx.doi.org/10.1016/j.fuproc.2017.09.009. [58] Gower SA. Development of a high power microwave plasma beam applicator. Rev Sci Instrum 2001;72:4273, http://dx.doi.org/10.1063/1.1406920. [59] Uhm HS, Hong YC, Shin DH. A microwave plasma torch and its applications. Plasma Sources Sci Technol 2006:15, http://dx.doi.org/10.1088/0963-0252/15/2/S04. [60] Pavuluri SK, Goussetis G, Desmulliez MPY, Adamietz R, Tilford T, Ferenets M, et al. Open-ended single mode resonant applicator for microelectronics packaging applications n.d.:1–3. [61] Prapuchanay E, Puangngernmak N, Chalermwisutkul S. A single mode cylindrical microwave cavity for heating applications of liquid material. In: 2012 9th int conf electr eng comput telecommun inf technol ECTI-CON 2012. 2012., http://dx.doi.org/10.1109/ECTICon.2012.6254353, 0–3. [62] Jerby E, Dikhtyar V, Aktushev O, Grosglick U. The microwave drill. Science 2002;298:587–9, http://dx.doi.org/10.1126/science.1077062. [63] Jerby E, Aktushev O, Dikhtyar V. Theoretical analysis of the microwave-drill near-field localized heating effect. J Appl Phys 2005:97, http://dx.doi.org/10.1063/1.1836011. [64] Jerby E, Dikhtyar V. Drilling into hard non-conductive materials by localized microwave radiation. In: 8th ampere proc. 2001. p. 1–9. [65] Jerby E, Aktushev O, Dikhtyar V, Livshits P, Anaton A, Yacoby T, et al. Microwave drill applications for concrete, glass and silicon. In: Proc 4th world congr microw radio-frequency appl. 2004. p. 156–65. [66] Eshet Y, Mann RR, Anaton A, Yacoby T, Gefen A, Jerby E. Microwave drilling of bones. IEEE Trans Biomed Eng 2006;53:1174–82, http://dx.doi.org/10.1109/TBME.2006.873562. [67] Lautre NK, Sharma AK, Kumar P, Das S. Performance of monopole concentrator during microwave drilling of perspex; 2014. p. 1–6. [68] Lautre NK, Sharma AK, Kumar P, Das S. Microwave drilling with litz wire using a domestic applicator, vol 4; 2014. p. 9756, http://dx.doi.org/10.9756/BIJIEMS.6053. [69] Kurgan N. Effects of sintering atmosphere on microstructure and mechanical property of sintered powder metallurgy 316L stainless steel. Mater Des 2013;52:995–8, http://dx.doi.org/10.1016/j.matdes.2013.06.035. [70] Shvab R, Bergman O, Bengtsson S. Effect of sintering atmosphere on the microstructure of high Cr-alloyed sintered stainless steel, vol 13; 2013. p. 103–8.

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

[71] Sintering Fundamentals n.d. https://www.abbottfurnaceco.com/sintering-fundamentals/ [accessed 23.02.19]. [72] Ju H, Zhao Q. Simulation and experimental method for microwave oven, vol 7; 2009. p. 188–91. [73] Srinath MS, Murthy PS, Sharma AK, Kumar P, Kartikeyan MV. Simulation and analysis of microwave heating while joining bulk copper. Int J Eng Sci Technol 2012;4:152–8. [74] Agrawal D. Microwave sintering of ceramics,composites, metals and transparent materials. J Mater Educ 1997;19:49–57. [75] Das S, Mukhopadhyay AK, Datta S, Basu D. Novel method of developing oxide coating on aluminum using microwave heating; 2003. p. 1635–7. [76] Gupta D, Sharma AK. Microwave cladding: a new approach in surface engineering. J Manuf Process 2014;16:176–82, http://dx.doi.org/10.1016/j.jmapro.2014.01.001. [77] Thuault A, Savary E, Bazin J, Marinel S. Microwave sintering of large size pieces with complex shape. J Mater Process Technol 2014;214:470–6, http://dx.doi.org/10.1016/j.jmatprotec.2013.09.030. [78] Hassan MN, Mahmoud MM, Link G, El-Fattah AA, Kandil S. Sintering of naturally derived hydroxyapatite using high frequency microwave processing. J Alloys Compd 2016;682:107–14, http://dx.doi.org/10.1016/j.jallcom.2016.04.266. [79] Chen Y, Wu Q, Wang X, Xie Q, Tang Y, Lan Y, et al. Microwave-assisted synthesis of arene Ru(II) complexes induce tumor cell apoptosis through selectively binding and stabilizing bcl-2 G-quadruplex DNA. Materials (Basel) 2016:9, http://dx.doi.org/10.3390/ma9050386. [80] German RM. Powder metallurgy science. 2nd edn Princeton, NJ: MPIF; 1994, n.d. [81] Girisha KG, Rao KVS. Improvement of corrosion resistance of AISI 410 martensitic steel using plasma coating, vol 5; 2018. p. 7622–7. [82] Fauchais P, Vardelle A. Thermal sprayed coatings used against corrosion and corrosive wear. Adv Plasma Spray Appl 2012:3–39, http://dx.doi.org/10.5772/34448. [83] Ott EA, Groh JR, Mannan SK. Environmental behavior of low thermal expansion Inconel alloy 783. Superalloys 2004;2004:643–52, http://dx.doi.org/10.7449/2004/Superalloys 2004 643 652. [84] Sandhu SS, Shahi AS. Metallurgical, wear and fatigue performance of Inconel 625 weld claddings. J Mater Process Technol 2016;233:1–8, http://dx.doi.org/10.1016/j.jmatprotec.2016.02.010. [85] Verdi D, Garrido MA, Múnez CJ, Poza P. Mechanical properties of Inconel 625 laser cladded coatings: depth sensing indentation analysis. Mater Sci Eng A 2014;598:15–21, http://dx.doi.org/10.1016/j.msea.2014.01.026. [86] Verdi D, Garrido MA, Múnez CJ, Poza P. Microscale evaluation of laser cladded Inconel 625 exposed at high temperature in air. Mater Des 2017;114:326–38, http://dx.doi.org/10.1016/j.matdes.2016.11.014. [87] Liu W, Xu F, Li Y, Hu X, Dong B, Xiao Y. Discussion on microwave-matter interaction mechanisms by in situ observation of “core–shell” microstructure during microwave sintering. Materials (Basel) 2016:9, http://dx.doi.org/10.3390/ma9030120. [88] Chang C, Verdi D, Angel M, Ruiz-hervias J. Micro-scale mechanical characterization of Inconel cermet coatings ˜ deposited by laser cladding. Boletín La Soc Espanola Cerámica y Vidr 2016;55:136–42, http://dx.doi.org/10.1016/j.bsecv.2016.01.001. [89] Chinnakurli Suryanarayana R, Rupanagudi SK, Gudi HR, Udupa VV. Corrosive wear behaviour of plasma sprayed

[90] [91]

[92]

[93] [94]

[95] [96]

[97]

[98]

[99]

[100]

[101]

[102]

[103]

[104]

[105]

[106]

3325

inconel 718 and titania coatings on low carbon steel in marine environment. Soc Tribol Lubr Eng Annu Meet Exhib 2016;2016:5–7. Ahmed A, Siores E. Microwave joining of 48% alumina ±32% zirconia ±20% silica ceramics, vol 118; 2001. p. 88–95. Bansal A, Sharma AK, Kumar P, Das S. Characterization of bulk stainless steel joints developed through microwave hybrid heating. Mater Charact 2014;91:34–41, http://dx.doi.org/10.1016/j.matchar.2014.02.005. Bansal A, Sharma AK, Das S. Metallurgical and mechanical characterization of mild steel–mild steel joint formed by microwave hybrid heating process. Sadhana Acad Proc Eng Sci 2013;38:679–86, http://dx.doi.org/10.1007/s12046-013-0142-4. Kumar A. Microwave joining of stainless steel and their charactrization; 2015. Bagha L, Sehgal S, Thakur A, Kumar H. Effects of powder size of interface material on selective hybrid carbon microwave joining of SS304–SS304. J Manuf Process 2017;25:290–5, http://dx.doi.org/10.1016/j.jmapro.2016.12.013. Aravindan S, Krishnamurthy R. Joining of ceramic composites by microwave heating; 1999. p. 245–9. Gamit D, Mishra RR, Sharma AK. Joining of mild steel pipes using microwave hybrid heating at 2.45 GHz and joint characterization. J Manuf Process 2017;27:158–68, http://dx.doi.org/10.1016/j.jmapro.2017.04.028. Badiger RI, Narendranath S, Srinath MS. Joining of Inconel-625 alloy through microwave hybrid heating and its characterization. J Manuf Process 2015;18:117–23, http://dx.doi.org/10.1016/j.jmapro.2015.02.002. Srinath MS, Sharma AK, Kumar P. A new approach to joining of bulk copper using microwave energy. Mater Des 2011;32:2685–94, http://dx.doi.org/10.1016/j.matdes.2011.01.023. Tamang S, Aravindan S. An investigation on joining of Al6061-T6 to AZ31B by microwave hybrid heating using active braze alloy as an interlayer. J Manuf Process 2017;28:94–100, http://dx.doi.org/10.1016/j.jmapro.2017.05.027. Bajpai PK, Singh I, Madaan J. Joining of natural fiber reinforced composites using microwave energy: experimental and finite element study. Mater Des 2012;35:596–602, http://dx.doi.org/10.1016/j.matdes.2011.10.007. Siores E, Do Rego D. Microwave applications in materials joining. J Mater Process Technol 1995;48:619–25, http://dx.doi.org/10.1016/0924-0136(94)01701-2. Srinath MS, Sharma AK, Kumar P. A novel route for joining of austenitic stainless steel (SS-316) using microwave energy. Proc Inst Mech Eng Part B J Eng Manuf 2011;225:1083–91, http://dx.doi.org/10.1177/2041297510393451. Srinath MS, Sharma AK, Kumar P. Investigation on microstructural and mechanical properties of microwave processed dissimilar joints. J Manuf Process 2011;13:141–6, http://dx.doi.org/10.1016/j.jmapro.2011.03.001. Hoyt D, Y. Adonyi SK. Microwave joining – Part 1: Closed-loop controlled microwave soldering of lead telluride to copper. Weld J 2016;95:141–5. Brum FJB, Amico SC, Vedana I, Spim JA. Microwave dewaxing applied to the investment casting process. J Mater Process Technol 2009;209:3166–71, http://dx.doi.org/10.1016/j.jmatprotec.2008.07.024. Yahaya B, Izman S, Idris MH, Dambatta MS. Effects of activated charcoal on dewaxing time in microwave hybrid heating. Procedia CIRP 2015;26:467–72, http://dx.doi.org/10.1016/j.procir.2014.07.039.

3326

j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):3306–3326

[107] Chandrasekaran S, Basak T, Ramanathan S. Experimental and theoretical investigation on microwave melting of metals. J Mater Process Technol 2011;211:482–7, http://dx.doi.org/10.1016/j.jmatprotec.2010.11.001. [108] Mishra RR, Sharma AK. On melting characteristics of bulk Al-7039 alloy during in-situ microwave casting. Appl Therm Eng 2017;111:660–75, http://dx.doi.org/10.1016/j.applthermaleng.2016.09.122.

[109] Mishra RR, Sharma AK. Structure-property correlation in Al–Zn–Mg alloy cast developed through in-situ microwave casting. Mater Sci Eng A 2017;688:532–44, http://dx.doi.org/10.1016/j.msea.2017.02.021.