Editorial for Special Issue “Casting Alloy Design and Characterization”

1. Introduction

Solidification, the genesis of metallic materials, is a complex phenomenon encompassing fluid flow, heat transfer, phase transformation, liquid–solid interface, solute redistribution, gas trapping, and others. Thermal solidification parameters, such as the cooling rate, thermal gradient, and solidification rate, directly result from variables such as mold material and design, metal composition, metal–mold interface, molten metal temperature, and pouring conditions. Understanding the interaction of these variables is essential for predicting the performance of cast products, as well as the behavior in subsequent processes, e.g., heat treatment, forming, machining, surface treatment, etc. [1,2,3,4,5,6,7,8].

Although the topic has been extensively investigated in the past sixty years [9,10,11,12,13,14], many questions remain to be answered in the development of alloys to meet specific characteristics such as biocompatibility, high entropy, shape memory behavior, or high-temperature corrosion and wear resistances, for example. The pursuit for improving casting processes, reducing costs and time, saving energy, and maximizing quality also remains a constant. The successful development of design casting critically depends on understanding the fundamental knowledge of heat and fluid flow during solidification, mainly for processes producing high-quality products and conducting near-net-shape casting. With the evolution of computers since 1980, several analytical and numerical mathematical models have been proposed. The heat and mass transfer conditions during solidification and the consequent simulation of cooling patterns in castings have improved casting processes. The use of simulators contributes to increasing our understanding of these processes; however, some uncertainties must be eliminated before such simulations can be widely accepted as realistic descriptions. In general, these models can be grouped into two categories: solidification under steady-state heat and unsteady-state heat flow conditions. Reliable prediction in unsteady-state heat flow is of fundamental importance since this flow regime covers most industrial solidification processes [15,16,17,18].

Consequently, the imposition of a wide range of heat and mass transfer conditions in the casting process generates diverse solidification structures. Hence, structural parameters, such as grain size and morphology, dendritic characteristics, the distribution and number of phases, precipitated particles, the porosity distribution, and others, are highly influenced by the metal/mold thermal behavior during solidification, consequently imposing a close correlation between casting design and the resulting microstructure and subsequent processes [19,20,21,22,23]. The properties of alloys depend on the solidification microstructural arrangement. Under these circumstances, the as-cast microstructure will define the physical, chemical, and mechanical properties of the alloy. Expressions correlating these responses with microstructure parameters are useful, providing insight into the preprogramming of solidification in terms of the desired casting properties [24,25,26,27,28]. Despite the existing knowledge of casting design and alloy characterization, the relationship between the solidification conditions, microstructure evolution, and final casting properties has yet to be fully understood. Therefore, this Special Issue of Metals is dedicated to research related to the design and characterization of cast alloys, especially regarding correlations between processing, properties, and microstructures; numerical and analytical heat and mass transfer simulations; and industrial applications.

2. Published Works

In this Special Issue, ten studies were published based on the following topics: special casting alloys; new trends in industrial applications; interrelationships between material composition, casting conditions, and final properties; and unconventional rheocasting processes for Al-Si alloys.

High-entropy alloys, Fe-based, and Co-based multicomponent alloys were investigated in studies (1) and (6), respectively. Different chemical compositions have been proposed for the design of new alloy systems, including high-entropy and multicomponent doped alloys using experimental approaches. Mahmoud et al. (1) developed six new Fe-Mn-Ni-Cr-Al-Si alloys, exploring relationships between chemical compositions and mechanical properties such as hardness and compression strength. Cabrera-Peña et al. (6) studied the multicomponent CoCrFeMoNi alloy in both pure and Zr-doped conditions for its corrosion resistance response in a simulated seawater environment.

Study (2) reviews the evolution of cast bells, covering the historical evolution of casting bell technology in Asia; the main materials used, such as bronze, brass, cupronickel, and iron alloys; different microstructures; and the technologies currently applied in the casting process. The design of a new gating system to reduce liquid metal turbulence during the sand casting of aluminum alloys was proposed in study (7). Experimental and theoretical analyses were carried out by Bruna et al. (7) to achieve the highest-quality casting considering aspects such as gate design; melting velocity; mold filling conditions; impurity and gas entrapment, and their consequences on mechanical properties; and the porosity of the casting. Similarly, Xie, Lv, and Dong (10) conducted an investigation to improve the casting of a high-power engine exhaust elbow. They used numerical simulations to optimize the operational parameters of the precision casting process. Despite the complex casting, their results demonstrate that the castings exhibit high internal and surface quality, in addition to high dimensional accuracy.

Al-based alloys are widely used in engineering applications due to their enhanced physical–chemical properties—lightweight, high mechanical strength, high thermal/electrical conductivity, corrosion resistance, and manufacturing feasibility. Hence, studies (3), (4), (5), and (8) concern this alloy system. In general, the microstructure of Al alloys is characterized by an α-Al matrix. When alloying elements are added, characteristics such as toughness, hardness, castability, formability, and resistance to elevated temperatures can be improved. Noé et al. (3) investigated the influence of small additions of Be on the formation of the eutectic microconstituent of an Al-33Cu alloy and the effects on the microstructural characteristics and hardness. Under transient solidification, the base alloy presented a microstructure composed of eutectic colonies surrounded by a eutectic mixture of coarser cellular morphology, while the addition of Be promoted dendritic morphology. The effect of adding Y or Er on the formation of the microstructure, tensile property, and heat resistance of an Al-Zn-Mg-Cu alloy was studied by Glavatskikh et al. (4). The alloys were prepared and solidified in different metallic molds, subjected to precipitation hardening heat treatment and hot rolling. Both the as-cast heat-treated and hot-rolled conditions were investigated. With the results, the authors proposed a route to obtain heat-resistant alloys with improved casting characteristics and hot deformation behavior.

The influence of Cr additions to hypoeutectic Al–Cu alloys on the solidification condition, microstructure, hardness, and linear reciprocating sliding wear response was investigated by Lantmann et al. (5). Based on the results, interrelationships between microstructure features and mechanical properties were established considering unsteady-state heat transfer conditions. The study concluded that the addition of Cu and Cr affected solidification transformation temperatures and improved wear response. A study on the impact of different Fe/Mn ratios on the microstructure after the homogenization heat treatment of an Al–Mg–Si alloy was reported by Avalos, Torres, and Valdés (8). They aimed to analyze the transformation of harmful Fe-intermetallic compounds with needle- or plate-shaped morphologies (β-Al₆FeSi) into an α-Al₁₅(FeMn)₃Si₂ phase with Chinese-script morphology, which was successfully achieved.

Finally, the unconventional casting process of semisolid rheocasting technology was investigated in study (9) to produce semi-solid slurries from A380 and A356 alloys. Two sets of serpentine channel pouring (SCP) were designed and tested, the first with a vertical configuration and the second with a horizontal design. According to the authors, these designs induce conditions of high heat transfer and nucleation in the channel, obtaining a homogeneous and globular microstructure through an interruption in the nucleus growth by the slurry flow in the channel. The results show that these methods can be applied to modify the primary solid from dendrites to equiaxed grains in the processed alloys. However, the authors advise for further investigation.

3. Conclusions

Since the main properties of casting are directly linked to the microstructure, which is formed depending on the solidification conditions, heat and mass transfer, and fluid flow, among others, it is essential to understand the influence of these characteristics on the final casting properties. This Special Issue overviews state-of-the-art casting designs and alloy characterizations, with different approaches and points of view.

We believe that this subject is extensive and deserves in-depth and constant investigation. Moreover, we hope that these selected contributions are useful for professionals, engineers, and researchers working with casting alloy designs and characterizations.