1. Introduction and Scope

Metal matrix composites, MMCs, are materials that combine the attractive properties of metals and those of a reinforcing phase. Thus, they form a category of metallic materials that transcends the possibilities offered by the classic thermo-mechanical treatments used for monolithic metals to control and improve their properties. By means of increasingly sophisticated manufacturing and processing methods, it is possible to produce MMCs with properties that are inaccessible to monolithic metal alloys. These are materials with very specific properties, tailored to meet certain performance requirements. This approach to material design enables the improved in-service performance of components in many industrial sectors, and is very well suited to moderating increasing energy demand and greenhouse gas emissions. Initially, this philosophy was conceived to improve mechanical properties, but it did not take long to understand that it could also be extended to the improvement of other physical properties, such as electrical properties, thermal properties, etc.

2. Contributions

This Issue summarizes representative studies of this family of composites, with particular emphasis on the correlation processing—microstructure—properties of materials.

In the first paper, entitled “Microstructure and Mechanical Properties of Cu-Coated Carbon-Nanotubes-Reinforced Aluminum Matrix Composites Fabricated by Ultrasonic-Assisted Casting”, carbon nanotubes are used as the reinforcing phase to fabricate an aluminum alloy matrix composite [Contribution 1]. The authors attempt to take advantage of the excellent mechanical properties of carbon nanotubes to improve the performance of the aluminum alloy matrix. The commercial ADC12 alloy (an Al-Si-Cu alloy) is selected as the matrix phase. The problem of reinforcing particle agglomeration is explored here by using high-energy ultrasound-assisted casting for material fabrication.

The next paper, entitled “Effects of Graphite Particle Content and Holding Time on the Microstructure and Mechanical Properties of the Graphite/AZ91D Composite”, focuses on a Mg alloy matrix composite [Contribution 2]. Specifically, the commercial AZ91 alloy is selected for the study, and the interfacial reaction between the Grp reinforcing phase with the Mg alloy matrix during composite fabrication by a casting route is addressed.

The paper entitled “Effect of Carbon Nanotube Content and Mechanical Milling Conditions on the Manufacture of AA7075/MWCNT Composites” also focuses on the use of carbon nanotubes for the fabrication of MMCs [Contribution 3]. Here, the efforts are devoted to identifying a manufacturing method that can be translated to an industrial scale. This study develops an efficient and economical process that can be employed at a large scale to manufacture profiles of AA7075 MMCs. The process involves powder metallurgical techniques, mechanical milling, and a hot extrusion stage for material consolidation.

The fourth paper, entitled “Influence of Different Addition Ratios of Fly Ash on Mechanical Properties of ADC10 Aluminum Matrix Composites”, is aimed at using fly ash, a coal-fired waste resulting from thermal power plants, in different amounts—namely, in the range of 3–15 wt%—as the reinforcement to be added to die-cast alloy ADC [Contribution 4].

The subsequent paper, entitled “An Experimental Investigation on the Material Removal Rate and Surface Roughness of a Hybrid Aluminum Metal Matrix Composite (Al6061/SiC/Gr)”, deals with the machinability of MMCs. The machinability of MMCs remains one of the very complex issues that must be solved if MMCs are to be used on a massive scale [Contribution 5]. Specifically, this paper is aimed at the optimization of the processing parameters in the machinability of a new hybrid MMC. The composite was fabricated using the stir casting method, which is among the very well-known and economical methods for the preparation of these materials.

In the next work, entitled “On the Strength of the CF/Al-Wire Depending on the Fabrication Process Parameters: Melt Temperature, Time, Ultrasonic Power, and Thickness of Carbon Fiber Coating”, the effect of several processing parameters—in particular, the ultrasonic treatment power—on the strength of a CF/Al composite is investigated [Contribution 6]. The fabrication process involves drawing a carbon fiber filament through an aluminum melt exposed to ultrasonic treatment. While most of the parameters involved in the fabrication of these MMCs have been investigated to some extent, the effect of ultrasonic power on the strength is barely known.

The last investigation included in this Issue, and entitled “Laser Beam Welding of a Ti-15Mo/TiB Metal–Matrix Composite”, discusses the welding of a Ti alloy-based MMC. The laser beam welding (LBW) technology is selected for this research, in which several pre-heating temperatures are used [Contribution 7]. It is concluded that the quality of the joints is not markedly dependent on the pre-heating temperature.

3. Conclusions

This Special Issue compiles a representative number of studies on these materials, with special emphasis on MMCs in which the matrix is an aluminum alloy. Generally, aluminum alloys, and, in particular, commercially available alloys, are used in the development of this family of materials. The generalized use of commercial alloys already available on the market to develop these materials is also surprising. The lack of need to develop new alloys specific to the manufacture of MMCs has greatly stimulated their growth and their use in various applications. The effect of the processing route on the final properties of the materials is the subject of research in some of the papers presented.

The purpose of this Special Issue is, in short, to serve as a stimulus and inspiration for technologists and researchers in their daily work on this family of structural materials. Although these materials have been the subject of much interest for several decades, their implementation in various industrial sectors is, to a certain extent, still a pending issue. As has been mentioned, these are materials that still have much to contribute, particularly if we consider the progressive requirements for reducing energy consumption, minimizing environmental impact and, in short, contributing to the achievement of a desirable sustainable society.