Advanced Manufacturing Processes

1. Introduction

The advancement of engineering and the emergence of increasingly individualized and specific applications have heightened the need to develop advanced manufacturing processes capable of delivering all of the required characteristics. These processes, which have very particular features or have been developed more recently, deserve special attention in current manufacturing engineering research in order for their challenges to be overcome. This progress will spread the use of these processes, resulting in better-quality components with lower manufacturing costs and lower environmental impact [1].

Processes such as welding and joining are constantly improving and they remain a crucial technology for modern engineering [2,3]. In turn, subtractive manufacturing is one of the most important steps in the manufacturing of a component and represents an essential part of manufacturing, even with the development of additive processes [4,5]. Material forming and assembly are also topics of interest that still have their areas of relevance in the current manufacturing industry [6,7]. In fact, component manufacturing is so varied and diverse that each manufacturing process has its own space and application.

The aim of this Special Issue is to illustrate the state of the art of advanced manufacturing processes, giving a unique relevance to recent developments achieved in this field, which may provide innovative engineering solutions to the industry.

2. Overview of Published Articles

The present Special Issue encompasses 11 original research papers, each addressing different areas of manufacturing technology. Specifically, four-papers address material welding and joining, five papers are focused on subtractive manufacturing (machining/cutting processes), one paper addresses material forming, and one paper focuses on dimensional control in assembly. Different research topics and different methodological approaches have been studied/followed among the works published in this Special Issue, highlighting its multidisciplinary nature.

The works addressing welding and joining processes encompass studies on solid-state welding techniques, like friction welding, friction stir welding and magnetic pulse welding, and fusion welding techniques, specifically laser welding. It is important to stress that all of these works address the joining of dissimilar materials with very different chemical, physical, and mechanical properties. Regarding the study on friction welding, Winiczenko et al. (Contribution 1) used this technique to produce dissimilar welds of tungsten-heavy alloy and 5XXX aluminum alloy. The research approach followed by these authors was based on experimental and finite-element results. A good correlation between the experimental and the numerical results was found in terms of the weld geometric characteristics and temperature. The maximum weld temperature was found to be 581 °C, which was slightly higher than the temperature measured during the experimentation. The flash width, flash height, and axial weld shortening were predicted with small deviation values, which ranged between 2.34% and 4.45%.

Also using a friction-based welding technique, Torabi et al. (Contribution 2) addressed the dissimilar friction stir welding of aluminum and magnesium. These authors explored the influence of tool rotation speed on the formation of eutectic structures during the friction stir welding of aluminum to magnesium in order to avoid liquation and solidification problems. Through many characterization techniques, they found that changing the rotation speed directly affected the eutectic formation, whereas the welding speed did not present any influence. A lower rotation speed during the process resulted in a thin, continuous intermetallic layer, whereas a higher speed led to the formation of a massive Mg-Al₁₂Mg₁₇ eutectic microstructure. The formation of a eutectic mixture, as indicative of liquation, may affect the material flow during the process due to the decreased friction coefficient between the tool and material.

Similar to the previous two works, a solid-state welding technique was also studied by Shim and Kim (Contribution 3). However, in order to produce Al/steel joints, these authors used an impact-based technique, i.e., magnetic pulse welding. In fact, according to these authors, the use of evolutionary algorithms to reach optimal process parameters in magnetic pulse welding has been poorly explored in the literature. This way, their work was dedicated to maximizing the quality of Al/steel magnetic-pulsed (MP) welds by using evolutionary algorithms, i.e., an Imperialist Competitive Algorithm (ICA) and a Genetic Algorithm (GA). Prediction models were developed; the peak current, gap between workpieces, and frequency were selected as the input parameters; and the maximum load and weld length were selected as the output parameters. Their results showed a good agreement between the predicted values and experimental results for the maximum load and weld length, mainly when the ICA-based model was used.

Unlike the welding works described above, Wang et al. (Contribution 4) worked on fusion welding—in particular, on laser direct joining. Specifically, they studied the influence of temperature distribution on the interfacial bonding process between carbon-fiber-reinforced thermoplastic (CFRTP) composites and 6061 aluminum alloy during laser direct joining. To better characterize the interfacial bonding phenomena, these authors developed a novel online observation device that allowed them to observe the polymer melting, flowing, and bonding with the metal throughout the process. The temperature distribution/gradient in the weld zone was characterized using numerical simulation. Ultimately, the experimental and simulated results allowed the authors to correlate the temperature distribution with the melting and flow of the polymer. The melting of CFRTP composites and the formation of bubbles in the polymer matrix, which are associated with thermal degradation of the polymer and hinder its melting, could be determined by the temperature distribution in the bonding zone. This study also led the authors to conclude that the flow of the polymer during welding is responsible for thermal gradients. Moreover, the flow of the polymer was found to have a close relationship with the greater or lesser formation of bonding defects.

A significant number of works addressing subtractive manufacturing are published in the present Special Issue. These works encompass different processes, such as turning, milling, sawing, and grinding, with varying working scales (micro vs. macro). For example, the objective of the research developed by González-Sierra et al. (Contribution 5) was to evaluate the wear of ceramic and coated carbide inserts in the finishing turning of age-strengthened gray cast iron. Several experimental characterization techniques were used to analyze the wear of the inserts and the microstructure of the workpiece materials (i.e., 5- and 12-day age-strengthened gray cast iron). The authors observed a slight increase in the tensile strength of each workpiece material by increasing the degree of age strengthening from 5 to 12 days. They also report a 50% reduction in flank wear for inserts that machined 5-day age-strengthened cast iron when compared with inserts that machined the cast iron with a greater degree of age strengthening. The main wear mechanisms in the inserts were found to vary according to the insert type—while abrasion and adhesion were the main wear mechanisms of ceramic inserts, adhesion and oxidation were the main wear mechanisms for coated carbide tools.

Jia et al. (Contribution 6) contribute to this Special Issue with a paper on diamond wire sawing, which is the most widely used method of cutting Al₂O₃ ceramic due to its high accuracy and the minimal surface damage it causes. Due to the need to improve the material removal rate, park discharges were generated around the diamond wire based on the electrochemical discharge machining (ECDM) process. An oil-film-assisted electrochemical discharge machining process was applied to solve the difficulty of generating spark discharges. In this work, the authors found that the combination of oil-film-assisted electrochemical discharge machining and diamond wire sawing improved the material removal rate of Al₂O₃ ceramic, improved the surface quality of the machined parts, and reduced the wear on the diamond wire. The results of their study bring valuable information for the application of diamond wire sawing combined with oil-film-assisted ECDM.

An abrasive material removal technique was addressed by Wang et al. (Contribution 7), who studied the surface quality of large-shaft multi-pass grinding. Two of the most important surface quality indices (surface roughness and glossiness) are affected by the process parameters and the surface quality of the previous grinding pass, which leads to difficulty in modeling. Moreover, an uneven distribution of the actual grinding depth usually leads to inconsistent surface quality throughout the shaft, which results in the need for multiple spark-out grinding passes to ensure consistency. Thus, these authors developed surface quality evolution models for assessing surface roughness and glossiness based on an Elman neural network, which builds regressions between process parameters, surface quality indices of the previous grinding pass, and surface quality indices of the current grinding pass. These authors also propose a consistency control method for the surface quality that involves adjusting the actual grinding depth within the dimensional accuracy tolerance range at the rough grinding stage. Their study shows that this proposed consistency control method can guarantee a consistent surface quality, reduce the grinding passes, and increase the grinding efficiency.

At a lower scale than in the previous works on subtractive manufacturing, the paper published by Wang et al. (Contribution 8) addresses micro-milling, another particular process within engineering that presents many applications such as terahertz slow-wave structures, microfluidic chips, and micro-molds. These authors indicate that even with this process consuming more energy than traditional milling, research on micro-milling is usually focused on parameter optimization, with limited attention given to energy efficiency. Citing the importance of developing or improving such processes to be more sustainable, the authors propose a micro-milling parameter-based power consumption model for use with evolutionary algorithms to optimize micro-milling parameters. This model allowed them to achieve comprehensive enhancements in both machinability and sustainability, improving surface quality, dimensional accuracy, the material removal rate, and specific energy consumption during the micro-milling process for thin-walled micro-structures.

Wang et al. (Contribution 9) also worked on machining, but with a different focus, specifically the use of double-sided collaborative machining. In fact, these authors investigated the tool path in double-sided collaborative machining of a propeller blade. The concomitant vibration and deformation produced by propeller blades in single-sided machining significantly affect the surface machining precision. These authors indicate that double-sided symmetrical machining can improve system rigidity through mutual shoring on both sides, but it cannot be applied to blade machining due to the complexity of a blade’s shape. Thus, they investigated a tool-path planning algorithm, showing that the algorithm can achieve smooth fitting and correspondence of bilateral cutter position points through double-curve interpolation and position data alignment. The authors also verified the feasibility and superiority of double-sided collaborative machining through machining experiments.

The multidisciplinary nature of the present Special Issue also allowed the inclusion of the work conducted by Zhang et al. (Contribution 10). These authors investigated the variations in surface shape during gas jet forming for optical aspherical mirror blanks. The inference was verified using numerical simulation and compared to the experimental results. The authors found that the average prediction deviation for the diameter was 1.07 mm, while the average prediction deviation for the principal curvature was 0.03665 mm⁻¹, which is challenging to correct in simulation. Therefore, they developed a dimensionless prediction model for forecasting the surface curvature and surface diameter of mirror blanks by considering the jet parameters using experimental data. The model’s average prediction error for the surface diameter of the formed surface was found to be 0.3192 mm, and the average prediction error of the principal curvature for the formed surface was 0.00269 mm⁻¹.

The assembly stage of component manufacturing is also addressed in detail in this Special Issue. Zhang et al. (Contribution 11), taking the core machine of a micro gas turbine as the research object, conducted a study with the aim of constructing a simulation model for assembly-deviation prediction based on the design tolerance and actual measurement data. The authors analyzed the assemblability of the design model and the main factors promoting assembly deviation. The current assembly position was found to be non-optimized, which led the authors to propose an optimal solution, resolving the interference issue resulting from the design error. The authors also report an assembly deviation analysis showing that when using an assembly-deviation prediction and analysis model that is constructed based on actual measurement data, one can achieve results that exhibit high consistency with the actual outcome of the assembly.

3. Conclusions

This set of articles positively demonstrates the exact objective of this Special Issue: to address arguments regarding advanced and unconventional manufacturing processes in order to take a step forward in the development and improvement of knowledge surrounding these processes.

In a world that is increasingly valuing and urgently needs more sustainable and efficient processes, manufacturing processes require constant improvement and development—and components must be manufactured with better quality. To keep pace with this evolution, it is important to study not only the main manufacturing processes, but also processes that are typically less discussed. This Special Issue presents research addressing topics such as welding, material removal processes, numerical simulation, tool wear, and sustainability, among others. These are all extremely important processes that are not commonly addressed in the existing literature, despite the fact that the knowledge surrounding these topics requires improvement in both its amount and its quality—thus, this corroborates the relevance of this Special Issue.

Finally, it is important to highlight the fact that the publications in this Special Issue come from various research groups spread widely across the world. The level of expertise among the researchers who have contributed to this Special Issue is outstanding, constituting the excellence of this compilation of scientific articles for the manufacturing sector.

As the Editors of the present Special Issue, we are pleased with the success of this project. We are profoundly grateful to all of the authors, the reviewers, and the Editorial Office that contributed to the making of this Special Issue.