Microstructure characteristics and strengthening mechanism of semisolid CuSn10P1 alloys

Highlights

• High cooling rate and the semisolid slurry isothermal treatment induce the formation of metastable phase β′-Cu_13.7Sn.

• The liquid-solid separation in rheo-squeeze casting can be significantly improved by holding the semisolid slurry for 20 s.

• The strength and ductility of CuSn10P1 alloy can be improved simultaneously.

• The fracture mechanisms has transformed from the brittle fracture to a combination of cleavage fracture and ductile fracture.

Abstract

Studies on the rheoforming of high melting point alloys are scarce due to the difficulties in preparing semisolid slurries. This work applied a novel enclosed cooling slope channel (ECSC) to prepare a semisolid slurry and squeeze cast a CuSn10P1 (weight percent) alloy. The microstructure and phase formation of the semisolid slurry, properties of liquid squeeze casting, rheo-squeeze casting with or without isothermal treatment of the semisolid slurry were investigated through X-ray diffraction, scanning electron microscopy, electron probe microanalysis, nanoindentation, and transmission electron microscopy. The results show that the microstructure is transformed from coarse dendrites of as-cast to equiaxed grains of semisolid. The high cooling rate of the ECSC process and the semisolid slurry isothermal treatment suppressed tin diffusion to induce the formation of the metastable phase β′-Cu_13.7Sn. The average values of the modulus and micro-hardness of the rheo-squeeze part after soaking for 20 s were 121.99 ± 14.03 GPa and 2.01 ± 0.94 GPa, which was lower than that without isothermal and liquid squeeze casting, indicating that it has good deformability. The tensile strength and elongation of the rheo-squeeze part after 20 s of soaking treatment are 417 MPa and 12.6%, which increase by 26.3% and three times, respectively, compared to those without isothermal and liquid squeezing, which is likely attributed to solution strengthening, fine grain strengthening, and microstructure homogenization. In addition, the fracture mechanism was transformed from the brittle fracture of liquid squeeze and rheo-squeeze casting without isothermal treatment to a combination of cleavage fracture and ductile fracture of rheo-squeeze casting with 20 s of soaking treatment.

Introduction

Tin bronze is widely utilized in aerospace applications, high-speed railways, automobiles, and other fields because of its excellent elasticity, wear resistance, and corrosion resistance [[1], [2], [3]]. There are various microstructure features, including primary α phase grain size, element distribution, peritectic phase content, and grain-boundary morphology that can influence the mechanical properties of tin bronze [4]. Although cast and wrought tin bronze are commonly used in many fields, some defects are still generated by conventional forming processes [5]. Traditional squeeze casting shows improved performances of parts and process yield because of forming under pressure and in a mold, while the service life of the mold is substantially reduced due to the high pouring temperature, which leads to higher production costs and limited application space. Therefore, a series of methods to improve the mechanical properties of tin bronze have been developed to broaden its industrial applicability. These methods primarily include selective laser melting (SLM) [6,7], advanced casting technologies (such as centrifugal casting [8]), second phase particle strengthening [9,10], liquid phase sintering (LPS) [11], and their combination [12,13].

Semisolid processing (SSP) has been widely studied as an advanced new metal forming technology that combines the characteristics of liquid forming and solid forming since it was discovered by Flemings [14]. The microstructure of traditional liquid casting parts is dendrites, and the microstructure of traditional forging methods is always greatly deformed. However, the microstructure of SSP consists of fine and equiaxed or spherical grains and a liquid phase [15,16]. Compared with the traditional casting method, the solidification shrinkage rate is smaller and the forming temperature is lower for SSP. Therefore, the parts prepared by SSP have advantages of high dimensional accuracy and excellent surface quality, which reduces subsequent machining loss and processing time. In addition, the low forming temperature can reduce the thermal shock to the mold and increase the mold service life [17]; in particular, the advantages of casting and forming high-temperature alloy parts are more prominent. As a consequence, the near-net-shaped complex parts can be prepared by SSP [18].

Semisolid formation includes thixoforming and rheoforming [18]. At present, the main research methods of SSP on high melting point alloys are concentrated on thixoforming methods, such as strain-induced melting activation (SIMA) [19] and recrystallization and partial remelting (RAP) [20]. However, the preparation process of thixoforming is long, the energy consumption is high, and the production cost increases due to the large plastic deformation and isothermal treatment process [21]. The rheoforming process has a short process and high efficiency because it is directly formed after preparing the semisolid slurry. It has been extensively studied in low melting point aluminum [22,23] and magnesium alloys [24]. However, the rheoforming of high melting point alloys has not been widely studied due to the difficulty in preparing semisolid slurries [25]. Some researchers have studied the rheoforming of steel by cooling slope [26]. The cooling slope is a promising method to fabricate semisolid slurries of high melting point alloys. For copper alloys, there is less research on the rheoforming process. Motegi et al. [27] showed that the microstructure of CuSn alloy in a semisolid state consisted of equiaxed solid grains through the cooling slope, and the microstructure was gradually refined with increasing tin element content. Kose et al. [28] studied the effect of the rotational speed on the microstructure of the CuSn10P0.2 alloy semisolid and mold temperature on the properties of high-pressure casting parts.

This paper adopts a novel enclosed cooling slope channel (ECSC) for fabricating a semisolid slurry of a CuSn10P1 alloy. Based on the ECSC method, the microstructure and phase formation of the semisolid slurry were studied. In addition, the microstructure and properties of liquid squeeze and rheo-squeeze casting with or without isothermal treatment of the semisolid slurry were investigated.