Casting and Solidification of Light Alloys

1. Introduction and Scope

At present, light alloys based on aluminum, magnesium, titanium, etc. have been intensively studied. Improving the properties of these alloys by modifying the structure and reinforcement makes it possible to solve the technological problem of increasing the working properties of mechanical engineering products while simultaneously reducing their weight. Concurrent to the study and development of methods for strengthening light alloys, casting technologies also require improvement since traditional casting technologies do not allow for the improvement of alloy properties when hardening particles are introduced into the liquid phase of the base metal. Modern methods of processing light alloys during casting make it possible to increase the physical and, ultimately, the operational properties of the obtained alloys by degassing, reducing the average grain size, increasing the uniformity of the alloy composition, reducing the amount of agglomerates and impurities at grain boundaries, increasing wettability, etc. The development of unconventional casting methods and the study of properties of light alloys are reported in the articles of this special issue.

2. Contributions

This special issue consists of one review and 11 original research articles. Of these 12 articles, 10 articles are devoted to aluminum alloys, one article is devoted to titanium alloys and the review article is devoted to aluminum, titanium, copper and magnesium alloys.

The review article [1] provides an overview of recent advances in light alloys (aluminum, copper, titanium and magnesium alloys) modified using friction stir processing (FSP). The general mechanisms of the formation of subsurface gradient structures in metal alloys processed by FSP under various conditions are described. It is shown that FSP can be used to produce light alloys with subsurface gradient structure, composite subsurface gradient structure and “in-situ” composites.

In the first research article [2], aluminum alloys of the Al-Mg system with titanium diboride particles of different dispersion were obtained. The introduction of titanium diboride particles using ultrasonic treatment of the melt made it possible to significantly reduce the average grain size of the alloy AA5056. The greatest effect of structure refinement was obtained using a master alloy containing titanium diboride particles with a size of 1 µm. It was also found that the introduction of titanium diboride particles led to an increase in the yield strength, tensile strength and plasticity from 57 to 71 MPa, from 155 to 201 MPa, and from 11.5 to 18.8%, respectively.

The effect of aluminum oxide nanoparticles introduced into the melt using ultrasonic treatment of the melt on the structure, and the properties of pure aluminum were studied in [3]. It was found that the introduction of nanoparticles of aluminum oxide using ultrasonic treatment of the melt into commercially pure aluminum allowed microstructure refining and reduced the average grain size from 200 to 69 µm. The introduction of nanoparticles made it possible to increase the hardness from 19 to 22 HB, the yield strength from 12 to 27 MPa, and the tensile strength from 48 to 79 MPa in commercially pure aluminum.

In [4], the effect of porosity on the properties of aluminum composites reinforced with SiC particles was studied using 2D models and finite elements analysis. The authors found that when the particle content did not exceed 11%, and the theoretical density of the material was reached, the shape of the particles (circular or square) did not affect the mechanical properties of the composite. With SiC particle content of more than 11%, the angular particles were more effective in improving the mechanical properties. Furthermore, it was shown that the particles transferred the stress to the soft matrix even in the presence of pores in the composite.

The analysis of the effect of processing the Al-Mg-Si solution on the transformation of β-AlFeSi particles into α-(FeMn) Si and the aging of the Al 6063 alloy were presented in [5]. It was found that the maximum hardness of 104 HV was reached following a solution treatment at a temperature of 600 °C for 2 h and aging at a temperature of 160 °C for 12 h.

In work [6], the Al-Si-Cu-Mg alloy was cast at casting speeds of 1, 2, 3 and 4 mm/s by the Ohno continuous casting (OCC) process. It was shown that the spacing between secondary dendrite arms of α-Al dendrites in the samples decreased significantly with an increase in the casting speed. Moreover, an increase in the casting speed to 4 mm/s allowed the authors to increase the tensile strength of the alloy, which was not possible with the alloy obtained at the casting speed of 1 mm/s.

In work [7], the authors obtained the Al₈Zn₇Ni₃Mg hypereutectic alloy, combining an Al-8% Zn-3% Mg matrix reinforced by an Al₃Ni intermetallic compound. The specified structure of the alloy was obtained by changing the cooling rate in the range from 0.1 K/s to 2.3 × 10⁵ K/s. It was shown that an increase in the cooling rate made it possible to grind intermetallic compounds in the alloy, the size of which was 50 nm, at a cooling rate of 105 K/s. At the same time, the hardness of the alloy increased to 220 HV.

In work [8], the aluminum alloy containing 0.6 wt.% Zr, 0.4% Fe and 0.4% Si was obtained by electromagnetic casting at a high cooling rate (140 K/s). The subsequent procedure of direct cold drawing was used to synthesize Al₃Zr nanoparticles in the alloy with a uniform distribution in the matrix and an average size not exceeding 10 nm. The composite wire sample obtained by the authors had a tensile strength and electrical conductivity of 234 MPa and 55.6 IACS, respectively.

A study of the process of treating commercially pure titanium by friction with stirring using a tool made of a nickel-based superalloy ZhS6U was presented in [9]. It was found that the transfer layer in titanium contained chemical elements of the ZhS6U alloy. After such treatment, the tensile strength of commercially pure titanium increased by 25%.

In work [10], an experiment was carried out on friction stir welding of a sheet made of a fine-grain Al-Mg-Sc-Zr alloy to study the peculiarities of the plasticized metal flow and microstructural evolution. It was shown that the stir zone macrostructure might contain either a single or many nugget zones, depending on sheet thickness and the seam length. Despite the finer-grained structure, the hardness of the welded seam was lower than the hardness of the base alloy, and the tensile strength of the welded seam was comparable to the tensile strength of the base alloy.

The structural phase characteristics of the friction-treated Al-Cu alloy zone were studied in [11]. The presence of Al₂Cu, Al₂Cu₃, AlCu₃, Al₂MgCu, etc. intermetallic phases with a nonuniform distribution over volume and size was revealed.

In work [12], the structural and phase states of the TiAl system alloyed with rare earth metals were studied. Tantalum, yttrium and dysprosium were used as additives. It was found that the studied systems Ti(49 at.%)–Al(49 at.%) with additions of Ta, Y and Dy contained intermetallic compounds consisting of AlTi₃, TiAl in hexagonal, tetragonal and triclinic forms.

3. Conclusions and Outlook

Future development of new casting methods and techniques for local control of crystallization of light alloys will significantly change the approach to industrial production of high-tech and efficient products. This will make it possible to create light alloys for specific structural elements of automobiles, airplanes, etc., where a set of specific operational properties is required. There is also great interest in joining new light alloys, including those with a composite structure, which also indicates the widespread use of new light alloys.

Implementation of this concept will create a strong demand for innovations in the field of new light alloys and their casting and joining technologies, which are one of the leading topics in this special issue. As a guest editor of this special issue, I am very pleased with the final result and hope that these articles will be useful to both researchers and technologists working in the production and processing of light alloys. I would like to express my gratitude to the authors for their contributions and the reviewers who helped with the review process. I would also like to extend a special thanks to the Metals Editor Office staff for working on the special issue.