**1. Introduction**

Sliding bearings are key components that are commonly used in machinery; antifriction alloys are the main materials employed in their manufacture. Bearing antifriction alloy materials generally have two metallographic structures. The first type of structure is based on a soft-phase matrix, where the hard phase is evenly distributed in the form of particles, such as in tin- and lead-based alloys. The second type of structure is based on a hard-phase matrix, where the soft phase is uniformly distributed in the form of particles, such as in aluminum–tin alloys and copper–lead alloys [1,2]. The Babbitt alloy belongs to the first type of antifriction alloy. It exhibits good compliance, compatibility, and embedment with other materials, although it has poor bearing capacity and heat resistance. It is prone to sticking and corrosion because of its lead content. The Babbitt alloy is therefore suitable for use under stable load working conditions, but not for use under heavy load conditions, and has been gradually phased out of industrial production. The second type of antifriction alloy for preparing self-lubricant bearings presents significant advantages. During working, the hard matrix structure of the bearing ensures that the bearing bush is not deformed, whereas the soft phase is easily worn out, forming a gap between the bearing bush and the bearing

**Citation:** Huang, S.; Zhu, B.; Zhang, Y.; Liu, H.; Wu, S.; Xie, H. Microstructure Comparison for AlSn20Cu Antifriction Alloys Prepared by Semi-Continuous Casting, Semi-Solid Die Casting, and Spray Forming. *Metals* **2022**, *12*, 1552.

https://doi.org/10.3390/ met12101552

Academic Editors: Marcello Cabibbo and Paolo Ferro

Received: 10 August 2022 Accepted: 10 September 2022 Published: 20 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that contains the lubricant. As a representative example of the second type of antifriction alloy, aluminum–tin and copper–lead alloys are widely used in high-load mechanisms. However, preparation of this second type of antifriction alloy is more difficult than that of the first, where the main difficulty is related to controlling the distribution morphology of the soft phase in the matrix. For aluminum–tin alloys, the tin-phase morphology in the alloy is one of the most critical technical indicators for potential applications, and the fine, uniform, and nearly equiaxed Sn phase is the ideal application state for alloys [3–5].

Semi-continuous casting of aluminum–tin alloy ingots, followed by processing the aluminum–tin alloy bearing blanks through multiple deformations and annealing, is presently the most commonly used process. The morphology of the tin phase of alloys prepared in this manner is affected by the deformation, and the phase forms a flat strip along the deformation direction. Annealing treatments above the melting point of tin are used to obtain a granular tin phase, which causes tin liquefaction and overflow from the matrix [6–10]. Aluminum–tin alloys can be prepared by powder metallurgy [11–15]. However, this process is extremely complicated, involving multiple processes such as powder milling, powder mixing, wrapping, degassing, and sintering. In addition, if the sintering temperature is higher than the melting point of the tin phase (232 ◦C), the tin phase becomes reticular and segregates. However, if the sintering temperature is lower than the melting point of the tin phase, the aluminum matrix is not able to metallurgically bond. Although there are many reports on the preparation of aluminum alloys by spray forming, few studies describe aluminum–tin alloy preparation by spray forming, although these studies [16–19] confirmed the feasibility of preparing Al-Sn alloys using this approach. However, because spray forming involves the use of high-speed airflow as the driving force for deposition and forming, defects are inevitably introduced; few studies have considered the analysis and elimination of these defects. In contrast, the preparation of aluminum alloys by semi-solid die casting is a mature technology [20–22], although there have been no studies describing the preparation of aluminum–tin alloys by semi-solid die casting.

In this study, three methods: semi-continuous casting, semi-solid die casting, and spray forming are used to prepare aluminum–tin alloy billets for analyzing the variation of microstructure with fabrication method. Different processes are used for the subsequent processing (except semi-solid die casting) to obtain self-lubricating and antifriction bearing bush blanks. The microstructures of the materials obtained by these three processes are compared, providing valuable data that may be used in the improvement of aluminum– tin alloy processing technologies. This study also provides a meaningful experimental reference for the preparation of alloys such as copper–lead alloys, in which a low-meltingpoint phase is uniformly distributed in a high-melting-point matrix.
