Lidija Ćurković, Rea Veseli, Ivana Gabelica, Irena Žmak, Ivana Ropuš, Milan Vukšić: A Review of Microwave-Assisted Sintering Technique

The present study examines the potential of microwave heating as an emerging and innovative energy-efficient alternative to conventional heating techniques used for different materials, with a focus on the processing of ceramic materials. Modern ceramics are studied extensively, and their use and different applications are wide due to many advantages of these materials. The most important factor in microwave sintering which differentiates it from conventional heating techniques is a unique heat transfer mechanism. Microwave energy is absorbed by the material, hence the transfer of energy takes place at the molecular level. This way, the heat is generated throughout the material, i.e. on the inside as well on the outside. This allows a very low temperature gradient throughout the material cross section. When conventional sintering is used, typically at high heating rates, high temperature gradients pose a problem. The accelerated microwave heating occurs through the whole volume, so the heating is uniform, which limits the grain growth and coarsening, and leads to a uniform and fine microstructure. The densification is accelerated as well during the unique heat transfer mechanism of microwave sintering, which enhances the mechanical properties of the sintered materials.
This paper discusses the use of microwave sintering in the manufacturing of different modern technical materials, namely ceramics, composites, metals and alloys, and glasses. The improvement of different properties is described using the available literature.

Key words: microwave sintering; conventional sintering; ceramics; materials

Original scientific paper

Yun Ho Kim, Dong Woo Jung, Hyung Do Song, Young Uk Won, Dong Ho Jung
Original scientific paper: 
Experimental Study on Coordinated Heading Control of Four Vessels Moored Side by Side

A floating type liquefied natural gas (LNG) bunkering terminal has been under development in Korea since 2014; the terminal is designed to receive LNG from an LNG carrier (LNGC) and transfer it to two other LNG bunkering shuttles (LNGBS) simultaneously. The operational feasibility of the LNG loading and unloading processes has been confirmed. When four vessels are moored side by side with mooring ropes and fenders, their positions must be maintained within the designed allowable criteria. In addition, the floating bunkering terminal (FLBT) has its own mooring system, an internal turret with catenary mooring lines and stern tunnel thrusters to maintain its own position and control the vessel heading. In this study, we investigated the operational feasibility of the FLBT during the LNG loading and unloading operations with four vessel mooring configurations and heading controls. A series of model tests was done in the ocean engineering basin of the Korea Research Institute of Ships and Ocean engineering. The motion responses of the four vessels were determined using an optical measurement system, and the tensile loads on ship-to-ship mooring ropes and the compressive loads on ship-to-ship fenders were measured using one-axis load cells. A white noise test was done and the results were compared with the numerical results for the purpose of validation. Then, four combined environmental conditions were presented both without heading control and with several heading control cases. Finally, we determined the available safe bunkering heading ranges taking into account the tensile loads on the mooring ropes.

Key words: Floating LNG bunkering terminal (FLBT); side-by-side mooring; heading control; ocean engineering basin test; bunkering operability

Original scientific paper

Gabriel-Radu Frumusanu, Cezarina Afteni, Alexandru EpureanuData-Driven Causal Modeling of the Manufacturing System

In manufacturing system management, the decisions are currently made on the base of ‘what if’ analysis. Here, the suitability of the model structure based on which a model of the activity will be built is crucial and it refers to multiple conditionality imposed in practice. Starting from this, finding the most suitable model structure is critical and represents a notable challenge. The paper deals with the building of suitable structures for a manufacturing system model by data-driven causal modelling. For this purpose, the manufacturing system is described by nominal jobs that it could involve and is identified by an original algorithm for processing the dataset of previous instances. The proposed causal modelling is applied in two case studies, whereby the first case study uses a dataset of artificial instances and the second case study uses a dataset of industrial instances. The causal modelling results prove its good potential for implementation in the industrial environment, with a very wide range of possible applications, while the obtained performance has been found to be good.

Key words: manufacturing system; causal modelling; ‘what-if’ analysis; instance-based learning

Original scientific paper

Stipe Perišić, Jani Barle, Predrag Đukić, Hinko WolfA Bayesian Conjugate Model for the Estimation of Friction Intensity

This paper addresses the Coulomb dry friction force as a technical indicator for fast and efficient condition-based maintenance. To estimate the value of friction force, the Bayesian analysis is used. Instead of the complex Markov Chain Monte Carlo numerical method, a closed-form analytical solution is applied. Thus, a simple and efficient procedure for friction estimation is described. Such a solution in the Bayesian context is known as the conjugate prior. The procedure presented here is verified numerically and experimentally by directly comparing the estimated value with the measured one. Two families of conjugate priors, the gamma-exponential and the normal-gamma, are compared. It is shown that the latter is suitable for friction estimation. An additional parameter, the precision parameter, was proposed as a criterion for the acceptance of estimation.

Key words: Bayesian inference; Conjugate priors; Coulomb dry friction; Experimental friction estimation

Original scientific paper

Dengqiu Ma, Yongping Liu, Zhenhuan Ye, Yongqiao Wei, Jing LiuAnalysis of the Tooth Surface Contact Area of a Circular-Arc-Tooth-Trace Cylindrical Gear under Load

To reconstruct the tooth surface of a circular-arc-tooth-trace cylindrical gear (CATT cylindrical gear), a 3D model has been developed and the contact characteristics have been investigated. Based on the development principle and meshing theory, the tooth surface equation, tooth surface curvature equation and tooth surface contact ellipse equation of the CATT cylindrical gear were deduced, and it was proved that the contact was a point contact. Then, the tooth surface was reconstructed and a 3D model was developed. Next, by performing the finite element analysis and meshing impression experiment, it was proved again that the contact is the point contact, and the contact area became an ellipse under loading. Finally, the influences of the design parameters on the contact ellipse were investigated. The general tendency is that the elliptical contact area increases from the tooth root to the gear top; the elliptical contact area decreases when the modulus and the gear tooth number near the tooth root increase and it increases when the modulus and the gear tooth number near the tooth top increase; the elliptical contact area increases when the tooth line radius increases. The elliptical contact area decreases in a cliff-like manner near the tooth top. The research results provide a reference for the design, profile modification and lubrication of the CATT cylindrical gear.

Key words: circular-arc-tooth-trace cylindrical gear; tooth surface equation; contact ellipse; finite element method; tooth line radius; module and gear tooth number.

Original scientific paper

Mohammed A. Noman, Adel AL-Shayea, Emad Abouel Nasr, Husam Kaid, Abdulrahman Al-Ahmari, Ali K. Kamrani, Haitham A. MahmoudA Model for Maintenance Planning and Process Quality Control Optimization Based on EWMA and CUSUM Control Charts

The performance of a production system is highly dependent on the smooth operation of various equipment and processes. Thus, reducing failures of the equipment and processes in a cost-effective manner improves overall performance; this is often achieved by carrying out maintenance and quality control policies. In this study, an integrated optimization method that addresses both maintenance strategies and quality control practices is proposed using an exponentially weighted moving average (EWMA) chart, in which both corrective and preventive maintenance policies are considered. The integrated model has been proposed to find optimal decision variables of both the process quality decision parameters and the optimal interval of preventive maintenance (i.e., Ns, Hs, L, λ, and t_PM) to result in overall optimal expected hourly total system costs. A case study is then utilized to investigate the impact of cost criteria on the proposed integrated model and to compare the proposed model with a model using the cumulative sum (CUSUM) control chart. The improved model outputs indicate that there is a reduction of 34.6% in the total expected costs compared with those of the other model using the CUSUM chart. Finally, an analysis of sensitivity to present the effectiveness of the model parameters and the main variables in the overall costs of the system is provided.

Key words: optimization model; maintenance planning; process quality control; EWMA chart; CUSUM chart

Original scientific paper

C. Devanathan, A. SureshBabuMulti Objective Optimization of Process Parameters by Firefly Algorithm during the Friction Stir Welding of Metal Matrix Composites

The use of composite materials in practical applications can be further enhanced by joining them effectively. In the current research, composites of Al 5083 reinforced with 10% B4C were prepared using the stir casting technique and were joined by friction stir welding. The process parameters, i.e. tool revolving speed, welding feed, and vertical plunge force were considered and taken in three levels to analyse their influence on mechanical properties, such as ultimate tensile strength and micro hardness. To make a joint, two tools with a square tool pin profile were machined, one coated with titanium nitride (TiN) and the other with diamond like carbon (DLC).Experiments were planned as per Box – Behenken’s design which suggested 15 experiments to analyse the output response. A mathematical model was developed for each output response for both the tools and the reliability of the model was confirmed by conducting the ANOVA test. The effect of varying each parameter on output parameters was studied and compared for both tools. Multi objective optimization was done by the firefly algorithm using the MATLAB 2018a software. For the TiN coated tool, maximum tensile strength of 133.92 MPa and Vickers micro hardness value of 102.54 were obtained with the following process parameters: 1540 rpm, 20 mm/min, 6 KN. As far as the DLC tool is concerned, the maximum values obtained are 129.67 MPa and 102.47 with the following parameters: 1360 rpm, 55 mm/min, and 8 KN.

Key words: Al 5083 + B4C composites; Titanium nitride; Diamond like carbon; Box- Behenken’s design; Multi objective optimization

Original scientific paper

Shengyuan Li, Longxi ZhengVibration Attenuation Mechanism of the Rotor System with Anisotropic Support Stiffness

Using the one-dimensional finite element method, the dynamic behaviour of a double-disk rotor system with anisotropic supports is studied in this paper. First, the natural frequencies, whirl state and unbalance response of the rotor system are analysed. Then, the vibration attenuation mechanism of the rotor system under the effect of bearing damping and accelerating rotor is discussed in detail. The research results show that stiffness anisotropy makes natural frequency lines of the rotor system tend to diverge from each other. The damping in bearings not only decreases the amplitude of forward and backward whirls but also reduces the spin speed range of backward whirl. The whirl orbit of the rotor system approximates a straight line at the spin speeds near the critical point of the backward and forward whirls. For the unbalance response of the rotor system with anisotropic supports, the forward natural frequency dominates in the direction of the strong stiffness axis, while the unbalance response in the direction of the weak stiffness axis is mainly affected by the backward natural frequency. Increasing the run-up acceleration can decrease the amplitude of backward and forward whirls, but it cannot reduce the spin speed range of backward whirl.

Key words: rotor system; anisotropic supports; vibration attenuation mechanism; bearing damping; accelerating rotor

Original scientific paper

Weixuan Jiao, Li Cheng, Di Zhang, Bowen Zhang, Yeping SuInvestigation of Key Parameters for Hydraulic Optimization of an Inlet Duct Based on a Whole Waterjet Propulsion Pump System

The hydraulic performance of an inlet duct directly affects the overall performance of a waterjet propulsion system. Key parameters for the hydraulic optimization of the inlet duct are explored using the computational fluid dynamics (CFD) technology to improve the hydraulic performance of the waterjet propulsion system. In the CFD simulation and experiment, an inlet duct with different flow and geometric parameters is simulated. By comparing grid sensitivity and different turbulence models, a suitable grid size and a turbulence model are determined. The comparison between the numerical simulation and the experiment shows that the numerical results are reliable. The results of the calculation and analysis of different speed cases show that the ship speed affects the efficiency of the waterjet propulsion system. In particular, the system efficiency increases first and then decreases with an increase in the ship speed. Under the conditions of constant ship speed and rotational speed, the influence of the length and dip angle of the inlet duct on the waterjet propulsion system is investigated using a single factor method. The results show that the dip angle has an obvious effect on the hydraulic performance of the inlet duct, and an extremely small angle of inclination will lead to poor flow patterns in the inlet passage. When the length is approximately six times the inlet duct outlet diameter, and the dip angle is 30°–35°, the hydraulic performance of the waterjet propulsion pump system is satisfactory.

Original scientific paper