Microstructure and Properties of Semi-solid CuSn10P1 Alloy under Different Filling Velocity by Squeeze Casting

Yongkun Li 1,a, Rongfeng Zhou 1,2,b*, Lu Li 1,2,c, Han Xiao 1,d and Yehua Jiang 1,e

1 Faculty of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China

2 Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China

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Keywords: CuSn10P1 alloy, semisolid, microstructure and properties, squeeze casting

Abstract.Semi-solid CuSn10P1 alloy slurry was fabricated by a novel enclosed cooling slope channel (for short ECSC). The effect of filling velocity on microstructure and properties by squeeze casting was studied. The results showed that primary α-Cu phase gradually formed from dendrites evolved into worm-like or equiaxed crystals by ECSC. As the filling velocity increases, the ultimate tensile strengths and elongations of the shaft sleeve increase first and then decrease. The ultimate tensile strength and elongation of semi-solid squeeze casting CuSn10P1 alloy reached a maximum of 417MPa and 12.6% when the forming pressure is 50t and filling velocity is 21mm/s, which were improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting.

1. Introduction

With the improvement of the industry’s performance requirements for products, Cu-Sn alloys attract more and more attention because of excellent flexibility, wear resistance, corrosion resistance and high strength [1-5]. CuSn10P1 alloy have the most widely applications among Cu-Sn alloys and is widely used in the shaft sleeve, bearing, gears, valves, etc. [6-8]. However, the primary α-Cu phase is a coarse mesh dendritic structure, intergranular segregation and negative segregation are serious in traditional casting, which leads to poor properties, limiting its uses in industry [9-10]. It is necessary to do multi-pass heat treatment to eliminate segregation and improve performance [11]. Continuous casting can produce low-segregation and high-performance bars or tubes [12], Centrifugal casting can make all kinds of pipes and simple parts [13], but none of them can prepare complex parts. Liquid squeeze casting is an advanced near net forming process for producing complex parts [14], but it is easy to produce intergranular segregation and negative segregation [15], which can’t meet the requirements of high-performance parts.

Semi-solid metal processing technology is known as one of the most promising near-net forming technologies in the 21st century and has received increasing attention [16]. Semi-solid processing is widely used due to its many technical and economic advantages, it includes rheo-casting and thixo-casting [17]. In recent years, rheo-casting has been widely research, and there are several ways to prepare non-dendrite structures [18-19]. Squeeze casting can refine grain and reduce part defects, improve the overall properties of the part. The technology of semi-solid slurry combined with squeeze casting has been widely studied. Guan Renguo et al. [20] adopt novel sloping plate process prepared AlSi3Mg2 alloy slurry and a small car hub wheel was thixoformed. The slurry with good microstructures and small car hub wheel pattern and inner microstructure are fine. Mao Weimin et al. [21] adopt electromagnetic stirring prepared A356 alloy semi-solid slurry with spherical grains and then rheo-squeezed. The parts are completely filled when the pressure reaches 34MPa. The ultimate strength, yield strength and elongation have been improved after T6 heat treatment. Vanluu Dao et al. [22] preparation of AlSi9Mg semi-solid slurry by electromagnetic stirring and a connecting-rods fabricated by the semi-solid squeeze casting. The ultimate strength and elongation of semi-solid squeeze casting are improved by 16.35% and 23.40%, respectively, compared with liquid squeeze casting.

In the present work, the effect of filling velocity on microstructure and properties of semi-solid squeeze casting was investigated.

2. Experimental procedures

Commercial CuSn10P1 copper alloy blanks were used in this work, its chemical composition (mass fraction) is Sn 10.22%, P 0.71%, others 0.17% and Cu balance. The alloy with liquidus and solidus temperatures of 1020.7 °C and 830.4 °C, respectively, were determined by Differential Scanning Calorimetry.

A schematic illustration of the ECSC rheo-casting equipment, used in the present work to prepare semi-solid slurry, is shown in Fig.1. The cooling slope channel was made from stainless steel and fixed at 45° with respect to the horizontal plane, and its three-dimensional is 300×100×5mm. After preparing the melt at 1200 °C within an electrical resistance furnace and poured on the ECSC at a desired pouring temperature of 1080 °C to obtain a semi-solid slurry. Take a 10x10x10mm square immediate water quench from semi-solid slurry to observe the microstructure. In contrast, take the same size block from liquid metal of 1080°C and immediately water quench.

Fig.1 Schematic illustration of the ECSC equipment.

Mold map and the product of CuSn10P1alloy is shown in Fig.2. Squeeze casting of the above semi-solid slurry at forming pressure is 50t and different filling velocity to obtain a four-cavity shaft sleeve parts, as shown in Fig.2a. The single shaft sleeve part as shown in Fig.2b. Liquid squeeze casting at casting temperature 1050°C was also carried out at forming pressure is 50t and filling velocity is 21mm/s for comparison with semi-solid squeeze casting. The squeeze casting mold temperature is 450°C. As shown in Fig.2c, samples were taken from positions A for microstructure observation. Mechanical properties of ZCuSn10P1 alloy parts were measured by using a CMT300 tensile machine, the sampling positions are shown in Fig.2c.After preparing metallographic samples, the specimens were polished and etched by a solution of 5% FeCl3. The microstructure observation by a Nikon ECLIPSE MA200 metallographic microscope.

Fig.2Mold map and the product of CuSn10P1 alloy: (a) mold map, (b) and (C) squeezed part (A is sampling position for microstructure observation, B is sampling position for tensile test).

3. Results and discussion

3.1. Influence of ECSC on the microstructures of CuSn10P1 alloy.

The as-cast microstructure and semi-solid slurry microstructure of CuSn10P1 alloy as shown in Fig.3. It can be found that the microstructures of as-cast and semi-solid slurry are composed of primary α-Cu phase and eutectoid (α+δ +Cu3P) phases. The primary α-Cu phase in as-cast is a coarse mesh dendritic structures and eutectoid (α+δ +Cu3P) phases distributed in the dendrite space (Fig.3a). The primary α-Cu phase in the semi-solid slurry is rose grains or equiaxed grains, and the eutectoid (α+δ +Cu3P) phase was surrounded by the primary α-Cu phase. The melt is strongly chilled by the enclosed cooling tank when the superheated metal is poured onto the ECSC. A large number of nuclei are formed on the upper and lower surfaces of the cooling tank. The formed nuclei enter the collected crucible with the flowing liquid and resulting in a semi-solid slurry consisting of fine grains.

Fig.3 Microstructures of CuSn10P1 alloy: (a) as-cast (b) semisolid.

3.2. Effect of melt processing on microstructure and properties.

Fig.4 shows comparison of microstructures of CuSn10P1 alloy shaft sleeve parts obtained by liquid squeeze casting and semi-solid squeeze casting. The forming process parameters are the forming

Fig.4 Comparison of microstructures of CuSn10P1 alloy shaft sleeve parts obtained by liquid squeeze casting(a) and semisolid squeeze casting(b).

pressure is 50t, the filling velocity is 21mm/s and the mold temperature is 450°C. It can be found that the microstructure of liquid squeeze casting is coarse dendrite and the growth direction is disorder(Fig.4a). The primary α-Cu phase of liquid squeeze casting is obvious refinement compared with the direct water quenching of liquid melt. There is a large temperature difference between the overheated metal melt and the mold, the melt is undercooled and a large amount of nucleation occurs during melt filling. The number of nuclei increase leads to refinement of microstructure. The microstructure of semisolid squeeze casting also is rose grains or fine equiaxed grains. This microstructure inherits the microstructure of the semi-solid slurry. However, the primary α-Cu phase size of the semi-solid squeeze casting is larger than the size of the primary α-Cu phase in the semi-solid slurry. The reason is that the slurry cooling rate in semi-solid squeeze casting is slower than that of direct water quenching, and the primary α-Cu phase has a certain period of time to grow.

The ultimate tensile strengths and elongations of CuSn10P1 alloy shaft sleeve parts prepared by liquid squeeze casting and semi-solid squeeze casting as shown in Fig.5. It can be found that the ultimate tensile strength and elongation of CuSn10P1 alloy shaft sleeve by semi-solid squeeze casting are higher than liquid squeeze casting. The ultimate tensile strength and elongation of semi-solid squeeze casting reach 417MPa and 12.6%, which were improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting.

Fig.5 Ultimate tensile strengths and elongations of CuSn10P1 alloy shaft sleeve parts produced by liquid squeeze casting and semisolid squeeze casting.

3.3. Effect of different filling velocity on microstructure and properties of CuSn10P1 alloy by semi-solid squeeze casting.

Fig.6 shows microstructures of CuSn10P1 alloy shaft sleeve parts under different filling velocity by semi-solid squeeze casting. It can be found that the microstructure consists of rose grains or equiaxed grains under different filling velocity. The filling velocity is slow when the filling velocity

Fig.6 Microstructures of CuSn10P1 alloy shaft sleeve parts under different filling velocity by semi-solid squeeze casting:(a) 17mm/s (b) 21mm/s (c) 25mm/s.

is 17mm/s,the solid-liquid cooperating fluidity is poor, and the primary α-Cu phase easily forms agglomerates. Defects such as shrinkage porosity and shrinkage tend to form during solidification in the liquid-rich areas (Fig.6a). The liquid flow rate can drive the primary α-Cu phase to flow together when the filling velocity is 21mm/s and 25mm/s. The solid-liquid cooperating fluidity is good and the shaft sleeve parts have a uniform microstructure(Fig.6b,6c). The increase in the filling velocity leads to a greater filling capacity of the alloy melt. The probability of collisions between grains and grains is also greatly increased and the microstructure is relatively fine (Fig.6b). The solid phase grains that first solidified in contact with the mold cavity are easily entered into the interior of the alloy under the action of high shear rate when the filling velocity continues to increase. Defects such as shrinkage porosity and shrinkage are formed inside the shaft sleeve part after solidification is complete.

The ultimate tensile strengths and elongations of CuSn10P1 alloy shaft sleeve parts prepared under different filling velocity by semi-solid squeeze casting as shown in Fig.7. It can be found that as the filling velocity increases, the ultimate tensile strengths and elongations of the shaft sleeve increase first and then decrease. The corresponding ultimate tensile strengths and elongations at filling velocity of 17 mm/s, 21 mm/s and 25 mm/s were 378.3 Mpa and 8.1%, 417.6 Mpa and 12.6%, 407.6 Mpa and 11%, respectively. The property is affected by the microstructure. Defects such as agglomeration, shrinkage porosity and shrinkage in the microstructure lead to the lowest performance when the filling velocity is 17mm/s. When the filling velocity is 25 mm/s, a small amount of shrinkage porosity and shrinkage defects in the microstructure lead to higher property than the filling velocity of 17 mm/s. The microstructure is the most uniform and the defects are less when the filling velocity is 21 mm/s, so the property is also the best. Therefore, the filling velocity is too small or too large to affect the microstructure and property. The filling velocity was selected to be 21mm/s in the present work.

Fig.7 Ultimate tensile strengths and elongations of CuSn10P1 alloy parts under different filling velocity by semi-solid squeeze casting:(a) 17mm/s (b) 21mm/s (c) 25mm/s.

4. Conclusions

1. The processing by ECSC can refine the primary α-Cu phase into worm-like or equiaxed grains.

2. The ultimate tensile strength and elongation of semi-solid squeeze casting CuSn10P1 alloy reached 417MPa and 12.6%, which were improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting.

3. The ultimate tensile strengths and elongations of the shaft sleeve increase first and then decrease with the filling velocity increases. The filling velocity was selected to be 21mm/s in the present work.

Acknowledgment

The authors acknowledge funding for the research from National Science Foundation of China (51765026) and (51665024). This work is supported by National-local Joint Engineering Laboratory of Metal Advanced Solidification Forming and Equipment Technology of China.

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