Light Alloy and Its Application (2nd Edition)

1. Introduction and Scope

Light alloys, primarily with a aluminum, magnesium, or titanium base, are fundamental to modern engineering due to their advantages of low density and high strength-to-weight ratio. Light alloys offer a suite of advantages beyond their well-known weight-saving properties. They exhibit excellent thermal conductivity, which is crucial for managing heat in automotive and electronics applications, and provide outstanding electromagnetic shielding, protecting sensitive devices from interference. Current research is intensely focused on overcoming key challenges to further their adoption. A primary research emphasis lies in enhancing their absolute strength, creep resistance, and manufacturability through novel alloy design and advanced processing techniques such as additive manufacturing. These include the inherent low ductility and poor formability of some alloys at room temperature, susceptibility to corrosion in specific environments, and the high cost of raw materials and processing.

Given these dynamic research fronts and existing hurdles, this Special Issue dedicated to light alloys is both timely and critical. It aims to gather the latest breakthroughs in basic understanding, processing science, and performance characterization, providing a platform for disseminating knowledge and accelerating the development of the next generation of light alloys. It must be pointed out that the present Special Issue is a continuation of the previously completed Special Issue “Light Alloy and Its Application”. All researchers interested in light alloys are welcome to read and disseminate it.

2. Contributions

Ten papers of high scientific quality have been published in the present Special Issue of Metals titled “Light Alloy and Its Application (2nd Edition), covering different research areas featuring titanium alloys (1), aluminum alloys (8) and magnesium alloys (1). The contents of the published manuscripts are briefly described below.

Liquid films are an important part of liquid metal granulation in the process of centrifugal spray forming. Their use in this research has enhanced our understanding of the flow characteristics of liquid films and how they can provide guidance for forming blanks. Taking an A390 aluminum alloy as the research object, force analysis of a liquid film on the surface of a high-speed rotating centrifugal disk used in centrifugal spray-forming technology was carried out using D’Alembert’s principle and Newton’s law of viscosity. The objective was to theoretically elucidate the effects of process parameters on liquid film thickness, flow velocity, and trajectory, and to clarify the relationship between process parameters, trajectory length, and liquid film thickness. The results show that (contribution 1) the trajectory of the liquid film formed by the liquid metal on the surface of the rotating centrifugal disk is a spiral. The liquid film thickness increases with increases in the volume flow rate and kinematic viscosity, and decreases with increases in the centrifugal disk rotation speed. The average radial velocity and circumferential slip increase with increases in the volume flow rate, and decrease with increases in the kinematic viscosity and rotation speed of the centrifugal disk. The trajectory length increases with increases in the kinematic viscosity and centrifugal disk speed, and decreases with increases in the volume flow rate.

The automotive industry is undergoing rapid evolution, demanding accelerated processes for producing new component prototypes and conducting validation testing. In this context, rapid sand casting (RSC), based on additive manufacturing technology, offers a promising solution for the quick production of sand molds. Experimental analysis was conducted on automotive prototypes with different geometries made from the aluminum alloy EN AC 42100-T6. The findings demonstrate the considerable potential of RSC, giving, in general, high mechanical properties (contribution 2). A comparative analysis with prototypes produced through traditional sand casting revealed similar results, with RSC exhibiting superior yield strength and stress at brake. However, both technologies revealed a reduced elongation percentage, as expected. Future efforts will focus on standardizing the RSC process to enhance ductility levels.

Although the aluminum alloy AlSi5Cu2Mg is suitable for manufacturing high-stress cylinder head castings, it exhibits a high tendency for hot cracking. An effective method to reduce this hot cracking tendency is to submit the alloy to grain refinement treatment. The author investigated the effects of graded titanium addition on the solidification process and hot cracking susceptibility of the AlSi5Cu2Mg alloy (contribution 3). The results show that the solidification time of the experimental alloy is significantly longer than that of the reference alloy. The extension of the solidification interval may increase susceptibility to hot tearing. Surprisingly, the addition of Ti reduced the thermal tearing susceptibility of the AlSi5Cu2Mg alloy. The addition of Ti resulted in an effective refinement of the primary α-(Al) phase and a transformation of the columnar grains into equiaxial grains, resulting in better melt-filling ability in the interdendritic spaces. The improved melt-filling ability in the interdendritic spaces resulted in higher tear resistance.

The gas nitriding and quenching process (GNQP) on the Ti–18 mass% Nb alloy was applied to obtain a high damping capacity and wear resistance. The results indicate that the surface and interior hardness of the Ti–18 mass% Nb alloy at 1223 K is higher than that at 1023 K (contribution 4). The coefficient of friction of the GNQP specimen obtained at 1023 K was lower than that obtained at 1223 K. It can thus be considered that GNQP at 1023 K is suitable for improving the damping capacity and wear resistance of the Ti–18 mass% Nb alloy.

This work investigated the effect of adding La–Ce mixed rare earths and Sr to enhance the microstructure and mechanical properties of the AlSi10MnMg alloy (contribution 5). Under this combined modification, the addition of 0.02 wt.% Sr and 0.1 wt.% RE (La–Ce mixed rare earths) exhibited the most pronounced refining effect. The secondary dendrite arm spacing (SDAS) was reduced by 59.18%. The eutectic silicon phase transformed from coarse needle-like shapes to fine fibrous or granular forms, with an aspect ratio reduction of 69.39%. The tensile strength increased to 240 MPa, achieving an increase of 23.08%; the yield strength increased to 111 MPa, achieving an increase of 18.09%; and the elongation reached 7.3%, achieving an increase of 73.81%. This indicates that the proper addition of Sr and mixed rare earths can significantly optimize the microstructure and enhance the mechanical properties of the AlSi10MnMg alloy, providing an effective method for the preparation of high-performance heat-treatment-free aluminum alloys.

Enhancing the strength and toughness of aluminum alloys using microstructure optimization remains a key challenge. In contribution 6, an AA2024 aluminum alloy with a double-layer multi-gradient structure was fabricated using 50% constrained deformation and single-stage peak aging at 150 °C. The results revealed a heterogeneous microstructure with variations in grain size, dislocation gradient, and precipitation phases between the constrained and deformation layers. Mechanical testing demonstrated a 30.9% increase in yield strength, a 16.4% increase in tensile strength, and a 13.9% improvement in uniform elongation compared to the T6 temper. Corrosion tests showed enhanced resistance, with a shallower intergranular corrosion depth and higher self-corrosion potential.

Mg–Sc alloys with a dual-phase structure (HCP + BCC) exhibit superior plasticity compared to single-phase HCP magnesium alloys. In contribution 7, the deformation behavior of dual-phase Mg-19.2 at.% Sc alloy was investigated. Experimental findings indicate that with the rise in annealing temperature, the volume fraction of the α phase progressively declines, while that of the β phase expands. Moreover, the grain size of the α phase first grows and then reduces, whereas the grain size during the β phase consistently enlarges. When the annealing temperature reaches 600 °C, the alloy exhibits an optimal strength–ductility combination, with an ultimate tensile strength of 329 MPa and an elongation of 20.5%. Microstructural analysis indicates that the plastic incompatibility between the α and β phases induces significant heterogeneous deformation-induced (HDI) strengthening.

In contribution 8, various surface preparation approaches prior to the measurement of the hydrogen content in the Al-2Cu aluminum alloy were investigated. Substantial errors in the hydrogen content quantification in aluminum alloys using the TDA technique can be introduced by grinding in water. Chemical pickling in concentrated nitric acid is a safe and simple surface preparation method for samples without adherent corrosion products. For the proper preparation of corroded surfaces with adherent corrosion products, successive steps of grinding, electrochemical polishing, and chemical pickling are recommended.

AlSi10Mg has been one of the most studied and employed aluminum alloys for additive manufacturing via laser powder-bed fusion (L-PBF). In contribution 9, in addition to the optimization of the manufacturing parameters of the AlSi10Mg alloy via powder bed fusion, the effects of the T6 heat treatment and direct aging on the microstructure and mechanical properties of this alloy were investigated and compared. The optimized manufacturing conditions were 300 W power, 800 mm/s scan speed, 30 µm layer thickness, and an argon atmosphere, which led to lower porosity and better finishing. The initial microstructure of the as-built samples had a fine cellular structure, composed of a network of α-Al cells surrounded by a eutectic silicon network and silicon nanoparticles dispersed within the α-Al cells. The direct aging treatment at 170 °C for 6 h, promoted the highest hardness, reaching a peak of approximately 195 HV, which represents an increase of about 14.7% over the as-built state. The T6 treatments resulted in lower hardness values than those observed in direct aging, due to the loss of fine cell microstructure and the growth of silicon particles.

The contribution 10 reported the development of a novel Al-Si-Mg-based composite reinforced by micron-sized Cu-Mn binary solid solution phases and submicron-sized α-Al(Mn,Fe)Si dispersoids. The microstructure analysis confirmed the presence of micron-sized Cu-Mn binary, eutectic Mg2Si, and Al15(FeMn)3Si2 intermetallic phases, submicron-sized α-Al(Mn,Fe)Si dispersoids, and nano-sized precipitates in the Al-based composite. At room temperature, tensile results represented a yield strength of 287 MPa and a tensile strength of 306 MPa, with an elongation of 17%. Moreover, the Al-based composite maintained a yield strength of 277 MPa up to 250 °C, with a slight increase in elongation. The composite also exhibited excellent high-temperature high-cycle fatigue properties and showed a high-cycle fatigue limit of 140 MPa at 130 °C, which is ~2.3-fold higher than that of the commercial A319 alloy. Additionally, Cu-Mn binary solid solutions and Al15(FeMn)3Si2 phases were found to be effective in reducing the crack propagation rate by hindering the movement of the propagated crack.

3. Conclusions and Outlook

Seeking to bridge the gap between foundational knowledge and industrial practice, the contributions in this Special Issue explore diverse aspects of light alloys. It is our intention that this curated work will not only inform a wide audience, but also spark further innovation and development in these critical materials.

As Guest Editor, I am very pleased to report the success of this Special Issue and hope that the papers will be useful to researchers researching areas related to light alloys. I am sincerely grateful to the authors for their contributions and the reviewers for their significant efforts in providing high-quality publications. I give sincere thanks to the editors and editorial assistants of Metals for their continuous support during the preparation of this volume. In particular, I would like to warmly acknowledge Ms. Reese Kong for her valuable assistance.