Microstructure and Properties of Semi-solid ...

metals Article

Microstructure and Properties of Semi-solid ZCuSn10P1 Alloy Processed with an Enclosed Cooling Slope Channel Yongkun Li 1 , Rongfeng Zhou 1,2, *, Lu Li 1,2 , Han Xiao 1 and Yehua Jiang 1 1

2

*

Faculty of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China; [email protected] (Y.L.); [email protected] (L.L.); [email protected] (H.X.); [email protected] (Y.J.) Research Center for Analysis and Measurement, Kunming University of Science and Technology, Kunming 650093, China Correspondence: [email protected]; Tel.: +86-137-0886-8341

Received: 14 March 2018; Accepted: 16 April 2018; Published: 17 April 2018







Abstract: Semi-solid ZCuSn10P1 alloy slurry was fabricated by a novel enclosed cooling slope channel (ECSC). The influence of pouring length of ECSC on the microstructures of ZCuSn10P1 alloy semi-solid slurry was studied with an optical microscope an optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). Liquid squeeze casting and semi-solid squeeze casting were performed under the same forming conditions, and the microstructure and properties were compared. The results show that primary α-Cu phase gradually evolved from dendrites to worm-like or equiaxed grains under the chilling action of the inner wall of the ECSC. The mass fraction of tin in the primary α-Cu phase increased from 5.85 to 6.46 after the ECSC process, and intergranular segregation was effectively suppressed. The finest microstructure can be obtained at 300 mm pouring length of ECSC; the equivalent diameter is 46.6 µm and its shape factor is 0.73. The average ultimate tensile strength and average elongation of semi-solid squeeze casting ZCuSn10P1 alloy reached 417 MPa and 12.6%, which were improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting. Keywords: enclosed cooling slope channel; ZCuSn10P1; semi-solid; microstructure refinement; properties

1. Introduction Tin bronze has excellent flexibility, wear resistance, corrosion resistance, and high strength, and is widely used in the ship-building industry and is important in gears, valves, worm gears, etc. [1–3]. However, the primary α-Cu phase is a coarse dendritic structure and the segregation of tin in traditional tin-bronze ZCuSn10P1 is significant [4], which leads to microstructural heterogeneity and poor properties, limiting its uses in industry. Therefore, it is very necessary to find a technology that can improve the uniformity of microstructures and properties. Semi-solid process is widely studied due to its many technical and economic advantages; it includes rheo-casting and thixo-casting [5]. In recent years, rheo-casting has attracted the interest of many researchers, and there are several ways to prepare non-dendrite microstructures [6–8]. The cooling slope plate method is a simple way to prepare semi-solid slurry. Abdelsalam et al. [9] fabricated A356/Al2 O3 metal matrix nano-composites by a combination of stir casting and cooling slope casting typically. The results show that the average grain size of samples fabricated is fine, the finer α-Al grains were obtained at the bottom of the ingots. However, the addition of Al2 O3 nanoparticles did not significantly influence the shape factor of the primary α-Al grains. Das et al. [10] studied the microstructural evolution Metals 2018, 8, 275; doi:10.3390/met8040275

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of semi-solid slurry generation of Al-Si-Cu-Fe alloy and the effect of microstructural morphology [10]slurry studied evolution of semi-solid slurry generation of Al-Si-Cu-Fe alloyAZ31B and of the onthe its microstructural rheological behavior. Yoshida et al. [11] studied the formation of hollow the effect of microstructural morphology of the slurry on its rheological behavior. Yoshida et al. [11] magnesium alloy pipe made by vertical continuous casting using semi-solid slurry prepared on an studied the formation of results hollowshow AZ31B magnesium alloy pipe made bypipe vertical continuous casting inclined cooling plate. The that it was possible to make the using both a taper mold ◦ C of molten semi-solid slurry prepared cooling alloy plate.has Theno results showof that was possible andusing a core rod. However, holdingon at an 650inclined tapering theit mold, and the make the pipe bothon a taper andand a core rod.surface However, holding at 650 °C of molten alloy coretorod 1 mm crackusing appears both mold outside inside of the hollow material. Cit et al. [12] has no tapering of the mold, and the core rod 1 mm crack appears on both outside and inside surface prepared the semi-solid slurry of tin bronze alloy by the slope plate method, and the effect of tin of the hollow material. Cit et al. [12] prepared the semi-solid slurry of tin bronze alloy by the slope mass fraction on the microstructure of semi-solid slurry was studied. The results show that the size of plate method, and the effect of tin mass fraction on the microstructure of semi-solid slurry was primary α-Cu phase decreases with the increase of tin mass fraction. The size of the primary α-Cu studied. The results show that the size of primary α-Cu phase decreases with the increase of tin mass phase is about 60 µm when the tin mass fraction reaches 3%. Kose et al. [13] prepared the semi-solid fraction. The size of the primary α-Cu phase is about 60 μm when the tin mass fraction reaches 3%. slurry of ZCuSn10P0.1 alloy by Shearing Cooling Roll (SCR). The effect of mold temperature on the Kose et al. [13] prepared the semi-solid slurry of ZCuSn10P0.1 alloy by Shearing Cooling Roll (SCR). tensile properties was studied. The results show that ZCuSn10P0.1 alloy was suitable for semi-solid The effect of mold temperature on the tensile properties was studied. The results show that process and an excellent semi-solid slurry can be prepared. The of direct-squeeze casting ZCuSn10P0.1 alloy was suitable for semi-solid process and anelongation excellent semi-solid slurry can be parts was increased by 60% dueoftodirect-squeeze the improvement of tinparts segregation in the microstructure, prepared. The elongation casting was increased by 60% due the to mold the ◦ C. According to the above study, it is known that there are few temperature at this time was 700 in improvement of tin segregation the microstructure, the mold temperature at this time was 700 °C. studies on semi-solid rheo-casting copper According to the above study, itof is ZCuSn10P1 known that there arealloys. few studies on semi-solid rheo-casting of In this investigation, we attempted to make semi-solid slurry of ZCuSn10P1 with fine solid grains ZCuSn10P1 copper alloys. for rheo-casting, the influence the pouring lengthsemi-solid of ECSC on the microstructures of ZCuSn10P1 In this investigation, weofattempted to make slurry of ZCuSn10P1 with fine solid grains for rheo-casting, influence of the pouring length of was ECSC on the from microstructures of alloy semi-solid slurry wasthe studied. Subsequently, a shaft sleeve obtained the semi-solid ZCuSn10P1 alloy semi-solid slurry was studied. Subsequently, a shaftofsleeve was obtained theon slurry with a favorable solid morphology by squeeze casting. Effect melt processing byfrom ECSC semi-solid slurry with a favorable solid morphology by squeeze casting. of melt processing by microstructure and properties of squeeze casting ZCuSn10P1 alloy were Effect investigated. ECSC on microstructure and properties of squeeze casting ZCuSn10P1 alloy were investigated. 2. Materials and Methods 2. Materials and Methods 2.1. Materials 2.1. Materials The chemical composition of the ZCuSn10P1 alloy used in this study is listed in Table 1. The alloy The chemical composition of the ZCuSn10P1 study is listed in Table 1. Theby had liquidus and solidus temperatures of 1020.7 ◦ Calloy andused 830.4in◦ this C, respectively. Measurements alloy had liquidus and solidus temperatures of 1020.7 °C and 830.4 °C, respectively. differential scanning calorimetry (STA 449F3, NETZSCH, Bavaria, Germany) wereMeasurements carried out on by differential scanning calorimetry (STA 449F3, NETZSCH, Bavaria, Germany) were on−1 , about 10 mg samples obtained from the as-cast alloys under an argon at a heating ratecarried of 10 Kout ·min −1 about 10 samples as shown inmg Figure 1. obtained from the as-cast alloys under an argon at a heating rate of 10 K·min , as shown in Figure 1. Table 1. Chemical composition of ZCuSn10P1 alloy (mass %). Table 1. Chemical composition of ZCuSn10P1 alloy (mass %).

Cu Cu 88.9088.90

Sn Sn 10.22 10.22

P P Other Other 0.71 0.71 0.17 0.17

Figure DSCheating heatingcurve curveof of as-cast as-cast ZCuSn10P1 ZCuSn10P1 alloy. Figure 1.1.DSC alloy.

2.2. Fabrication of the ZCuSn10P1 Alloy Semi-Solid Slurry 2.2. Fabrication of the ZCuSn10P1 Alloy Semi-Solid Slurry A schematic illustration of the ECSC equipment and forming process curve is shown in Figure A schematic illustration of the ECSC equipment and forming process curve is shown in Figure 2. 2. The slurry-making equipment consists of two parts: an enclosed cooling slope channel and an angle Theadjuster slurry-making equipment consists of two parts: an enclosed cooling slope channel and an angle (Figure 2a). The channel thickness of metal flow was 5 mm [14]. The enclosed cooling slope

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adjuster (Figure 2a). The channel thickness of metal flow was 5 mm [14]. The enclosed cooling ◦ with respect to the horizontal plane slope channel wasfrom made from stainless steel and fixed at 45 channel was made stainless steel and fixed at 45° with respect to the horizontal plane and was and was water circulation in the gap. The pouring to different in cooled bycooled water by circulation in the gap. The pouring length length was setwas to set different valuesvalues in this this investigation. investigation.

Figure illustration of the: (a) ECSC equipment; (b) forming process curve; (c) Figure 2.2.Schematic Schematic illustration of the: (a) ECSC equipment; (b) forming processand curve; temperature recording curve during slurry transfer ① ECSC process, ② slurry transfer, ③ mold 1 ECSC process, 2 slurry transfer, 3 mold and (c) temperature recording curve during slurry transfer filling,④ ⑤ partsair aircooling. cooling. 4 holding 5 parts filling, holding pressure, pressure,

Fabrication of the ZCuSn10P1 alloy semi-solid slurry was carried out according to the following Fabrication of the ZCuSn10P1 alloy semi-solid slurry was carried out according to the following procedures (Figure 2b): about 6.5 kg of ZCuSn10P1 alloy in a graphite crucible was melted at 1200 ± procedures (Figure 2b): about 6.5 kg of ZCuSn10P1 alloy in a graphite crucible was melted at 10 °C in a medium frequency induction furnace. After complete melting, the temperature dropped to 1200 ± 10 ◦ C in a medium frequency induction furnace. After complete melting, the temperature 1080 °C and it was poured immediately into the ECSC. The ECSC casting of the ZCuSn10P1 involved dropped to 1080 ◦ C and it was poured immediately into the ECSC. The ECSC casting of the pouring the melt over an enclosed cooling slope channel and its width is 100 mm. The prepared slurry ZCuSn10P1 involved pouring the melt over an enclosed cooling slope channel and its width is was collected by a high-purity graphite crucible preheated to 950 °C to ensure that the temperature 100 mm. The prepared slurry was collected by a high-purity graphite crucible preheated to 950 ◦ C to of the slurry was almost constant during the transfer process (Figure 2b). ensure that the temperature of the slurry was almost constant during the transfer process (Figure 2b). By changing the pouring length of ECSC, the semi-solid slurry of ZCuSn10P1 alloy under By changing the pouring length of ECSC, the semi-solid slurry of ZCuSn10P1 alloy under different different pouring length conditions was obtained. A 10 mm × 10 mm × 10 mm cube was removed pouring length conditions was obtained. A 10 mm × 10 mm × 10 mm cube was removed from the from the slurry and water quenching immediately. The microstructure of these specimens was slurry and water quenching immediately. The microstructure of these specimens was observed using observed using an optical microscope. The number of primary α-Cu phases in the metallographic an optical microscope. The number of primary α-Cu phases in the metallographic images under images under five different fields of view was counted to calculate the average value under each field five different fields of view was counted to calculate the average value under each field of view. Then, of view. Then, the total number of per unit area based on the area of the field of view and the average the total number of per unit area based on the area of the field of view and the average number of number of primary α-Cu phases was calculated. Measurements of the equivalent diameter (D) and primary α-Cu phases was calculated. Measurements of the equivalent diameter (D) and shape factor shape factor (F) of the primary α-Cu phase were carried out using image-analysis techniques [15]. (F) of the primary α-Cu phase were carried out using image-analysis techniques [15]. The equivalent The equivalent diameter (D) and shape factor (F) were determined from the following equations [15]: diameter (D) and shape factor (F) were determined from the following equations [15]: r 𝐴A D= = 22√ 𝐷 𝜋π F=

4πA p2 4π𝐴

(1) (1) (2)

𝐹 = (2) where A is the area of primary α-Cu phase and p𝑝2is the perimeter. We obtained them using the Image-pro plus (Media Cybernetics, Rockvile, MD, USA) software suite. At F = 1, the shape is a where A is the area of primary α-Cu phase and p is the perimeter. We obtained them using the Imageperfect circle. pro plus (Media Cybernetics, Rockvile, MD, USA) software suite. At F = 1, the shape is a perfect circle. 2.3. Squeeze Casting Mold map and the product of ZCuSn10P1 alloy are shown in Figure 3. Squeeze casting of the semi-solid was at pressure 100 MPa and speed 21 mm/s to obtain a four-cavity part, as shown in

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2.3. Squeeze Casting MetalsMold 2018, 8,map x FORand PEER REVIEW the product

4 of 11 of ZCuSn10P1 alloy are shown in Figure 3. Squeeze casting of the semi-solid was at pressure 100 MPa and speed 21 mm/s to obtain a four-cavity part, as shown in Figure 3a. The semi-solid slurry was prepared at a pouring length of 300 mm. Liquid squeeze casting Figure 3a. The semi-solid slurry was prepared at a pouring length of 300 mm. Liquid squeeze casting at casting temperature 1050 °C was also carried out for comparison with semi-solid squeeze casting. at casting temperature 1050 ◦ C was also carried out for comparison with semi-solid squeeze casting. The forming process parameters of liquid squeeze casting and semi-solid squeeze casting were the The forming process parameters of liquid squeeze casting and semi-solid squeeze casting were the same. The mold temperature was 450 °C. As shown in Figure 3c, samples were taken from different same. The mold temperature was 450 ◦ C. As shown in Figure 3c, samples were taken from different positions A, B and C for microstructure observation and phase analysis. To accurately reflect the positions A, B and C for microstructure observation and phase analysis. To accurately reflect the properties of different position in the part, we took four tensile samples symmetrically on both sides properties of different position in the part, we took four tensile samples symmetrically on both sides of of the parting surface (Figure 3b). The average value of four tensile samples as the final property the parting surface (Figure 3b). The average value of four tensile samples as the final property index index was calculated. Mechanical properties of ZCuSn10P1 alloy parts were measured using a was calculated. Mechanical properties of ZCuSn10P1 alloy parts were measured using a CMT300 CMT300 tensile machine. tensile machine.

Figure 3. Mold map and the product of ZCuSn10P1 alloy: (a) mold map; and (b,c) squeeze part Figure 3. Mold map and the product of ZCuSn10P1 alloy: (a) mold map; and (b,c) squeeze part (A, B (A, B and C are sampling positions for microstructure observation, while D is sampling position and and C are sampling positions for microstructure observation, while D is sampling position and shape shape for tensile test, 1 to 4 are sampling positions for tensile test from above the parting surface to for tensile test, 1 to 4 are sampling positions for tensile test from above the parting surface to below below the parting surface). the parting surface).

3. Results and Discussion 3. Results and Discussion 3.1. Influence of ECSC Process on the Microstructures of ZCuSn10P1 Alloy 3.1. Influence of ECSC Process on the Microstructures of ZCuSn10P1 Alloy Figure 4 shows the as-cast microstructures and semi-solid slurry microstructures of ZCuSn10P1 4 showssample the as-cast and semi-solid slurry microstructures of ZCuSn10P1 alloy.Figure The as-cast wasmicrostructures obtained by direct water quenching of the molten at 1050 ◦ C. alloy. The as-cast sample was obtained by direct water quenching of the molten at 1050 The ◦ The processing parameters of ECSC were a pouring temperature of 1080 C, and the pouring°C. length processing parameters of ECSC were a pouring temperature of 1080 °C, and the pouring length and ◦ and angle of 300 mm and 45 , respectively. As can be seen, the two types of samples are mainly angle of 300ofmm and 45°, respectively. can bephases. seen, the two types ofα-Cu samples areismainly composed composed primary α-Cu and (α+δ As +Cu3P) The primary phase the coarse mesh of primarystructure α-Cu and phases.phase The primary α-Cu in phase is the coarse mesh in dendritic dendritic and(α+δ the +Cu3P) (α+δ +Cu3P) is distributed the dendrite clearance as-cast structure(Figure and the4a). (α+δ +Cu3P) phase is distributed in the dendrite clearance ingrains as-castby samples (Figure samples The primary α-Cu become worm-like grains or equiaxed ECSC process 4a). The4b). primary α-Cu become worm-like grainsby oraequiaxed grains bysolid ECSC process (Figure 4b).with The (Figure The primary α-Cu was surrounded liquid phase, the grain filling together primary α-Cu was surrounded by a liquid phase, the solid grain filling together with the liquid phase the liquid phase in the process of forming improved the synergistic effect of solid-liquid flows. Note in the of forming improved the synergistic effect of solid-liquid flows. Note at that the liquid that theprocess liquid content shown in the micrographs is not suggested: the number present temperature content shown in therate micrographs is enough not suggested: number present at temperature because the because the quench is not rapid for the the content of liquid to be entirely “frozen in” [16]. quench rate is not rapid enough for the content of liquid to be entirely “frozen in” [16]. During the During the quench, some liquid deposits onto the solid, appearing to be “solid” in the quenched quench, some liquid deposits onto the solid, appearing to be “solid” in the quenched microstructure microstructure [16,17]. [16,17]. Phase identification was accomplished by comparison of the observed XRD peaks with published Phase identification was accomplished by comparison of the observed XRD peaks with crystallographic data (JCPDS database, International Centre for Diffraction Data, Powder Diffraction published crystallographic data (JCPDS database, International Centre for Diffraction Data, Powder Standards Joint Committee, Newtown Square, PA, USA). Figure 5 shows the XRD traces from as-cast Diffraction Standards Joint Square, PA, USA). Figure were 5 shows the XRD traces and semi-solid slurry of the Committee, ZCuSn10P1 Newtown alloy. Primary α-Cu and δ-Cu31Sn8 detected in as-cast from as-cast and semi-solid slurry of the ZCuSn10P1 alloy. Primary α-Cu and δ-Cu31Sn8 were sample and semi-solid sample; the Cu3P phase was not detected, but the Cu3P phase was nevertheless detected[18]. in as-cast samplebecome and semi-solid sample; phase was not detected, butindicating the Cu3P present Some peaks weaker and widerthe (asCu3P shown in Figure 5, marked “2”), phase was nevertheless present [18]. Some peaks become weaker and wider (as shown in Figure 5, that the grains were more refined (see also Figure 4). marked “2”), indicating that the grains were more refined (see also Figure 4).

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Figure 4. MicrostructuresofofZCuSn10P1 ZCuSn10P1 alloy: and (b)(b) semi-solid. Figure 4. alloy: (a)as-cast; as-cast; and semi-solid. 4. Microstructures of ZCuSn10P1 alloy:(a) as-cast; and (b) semi-solid.

Figure 5. XRD spectrum of ZCuSn10P1 alloy.

Figure 6 shows the as-cast and5.semi-solid slurryof SEM micrograph of ZCuSn10P1 alloy. They are Figure ZCuSn10P1 alloy. Figure 5. XRD XRD spectrum spectrum of ZCuSn10P1 alloy. composed of three regions due to the different tin mass fraction (dark grey areas, light grey areas, and bright whitethe areas). The and points showing slurry EDS areSEM marked in Figureof 6 ZCuSn10P1 and the EDS alloy. resultsThey are are Figure 6 shows as-cast semi-solid micrograph Figure 6 shows the as-cast and semi-solid slurry SEM micrograph of ZCuSn10P1 alloy. They are listed in The tin mass fraction graduallytin increases as the color becomes lighter, light as indicated composed ofTable three2.regions due to the different mass fraction (dark grey areas, grey areas, composed of three regions due to the different tin mass fraction (dark grey areas, light grey areas, and by the arrows in Figure 6. The EDS results contrast the as-cast and semi-solid slurry: the tin mass and bright white areas). The points showing EDS are marked in Figure 6 and the EDS results are the α-Cu of semi-solid is higher than as-cast, and the mass fraction brightfraction whitein areas). Thephase points showing slurry EDS are marked inthe Figure 6 and the tin EDS results areinlisted listedthe in liquid Table 2. Theoftin mass fraction gradually increases as the color becomes lighter, as indicated semi-solid is lowerincreases than the as-cast, indicating that the distribution of tin is in Table 2. Thephase tin massthe fraction gradually as the color becomes lighter, as indicated by the by the arrowsuniform in Figurethe 6. The EDS results contrast the as-cast andpeak semi-solid slurry:inthe tin mass relatively semi-solid slurry and segregated. Some shift (as arrows in Figure 6. TheinEDS results contrast the less as-cast and semi-solid slurry: theshown tin massFigure fraction in fraction in the α-Cu phase ofthat semi-solid slurry is higher than the as-cast, and the tinTable mass fraction in 5, marked indicated the solid solubility has changed (consistent The the α-Cu phase“1”) of semi-solid slurry is higher than of thetinas-cast, and the tin masswith fraction in2).the liquid the liquid phase of the semi-solid is lower than the as-cast, indicating that the distribution of tin is Cu3P phase is lamellar structure and distributed on the edge of the primary α-Cu phase [18]. It may phase of the semi-solid is lower than the as-cast, indicating that the distribution of tin is relatively be affected by the phase when measuring two-segregated. or three-point energy spectrum, showsinthe relatively uniform in Cu3P the semi-solid slurry and less Some peak shift (asand shown Figure uniform in the semi-solid slurry and less segregated. Some peak shift (as shown in Figure 5, marked accumulation of P near that primary 5, marked “1”) indicated the α-Cu solidphase. solubility of tin has changed (consistent with Table 2). The

“1”) indicated that the solid solubility of tin has changed (consistent with Table 2). The Cu3P phase is Cu3P phase is lamellar structure and distributed on the edge of the primary α-Cu phase [18]. It may lamellar structure distributed onmicrostructure the edge ofofthe α-Cu phase [18]. It may Tableand 2. EDS results for the theprimary as-cast and semi-solid ZCuSn10P1 alloy.be affected by be affected by the Cu3P phase when measuring two- or three-point energy spectrum, and shows the the Cu3P phase when measuringAs-Cast two- or three-point energy spectrum, Semi-Solid and shows the accumulation of accumulation of P near primary α-Cu phase. P near primary Element α-Cu phase. 1 2 3 1 2 3 Wt. %

At. %

5.85 0.36

3.21 0.75

Wt. %

At. %

Wt. %

At. %

Wt. %

At. %

13.73 7.28 As-Cast 7.02 14.26 As-Cast 2

27.94 0.53

17.08 1.24

6.46 0.32

3.56 0.66

Wt. %

At. %

Wt. %

At. %

14.85 8.48 20.59 Semi-Solid 0.56 1.22 8.71 Semi-Solid 2

11.83 17.7

Table 2. EDS results for the microstructure of the as-cast and semi-solid ZCuSn10P1 alloy. Cu 96.04 for 79.25 78.46 71.53 of 81.68 93.22 and 95.78 84.6 ZCuSn10P1 90.30 70.69 alloy. 70.47 Table 2. 93.79 EDS results the microstructure the as-cast semi-solid Sn P

Element Element

Cu CuSn Sn P P

1 3 Wt. % At. % Wt. %2 At. % Wt. %3 At. % 1 Wt.93.79 % At.96.04 % Wt.79.25 % At. % Wt. % At. % 78.46 71.53 81.68 93.79 96.04 79.25 78.46 71.53 81.68 5.85 3.21 13.73 7.28 27.94 17.08 5.85 7.28 17.08 0.36 3.21 0.75 13.73 7.02 14.26 27.94 0.53 1.24 0.36

0.75

7.02

14.26

0.53

1.24

1 Wt. % 1 Wt. % 93.22 93.22 6.46 6.46 0.32 0.32

At. % At. % 95.78 95.78 3.56 3.56 0.66 0.66

Wt. % 2 At. % Wt. At. % 84.6% 90.30 84.6 90.30 14.85 8.48 14.85 8.48 0.56 1.22 0.56

1.22

3 Wt. % 3At. % Wt. % 70.47 At. % 70.69 70.69 70.47 20.59 11.83 20.59 11.83 8.71 17.7 8.71

17.7

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Figure 6. SEM micrograph of ZCuSn10P1 alloy: (a) as-cast; and (b) semi-solid. Figure 6. SEM micrograph of ZCuSn10P1 alloy: (a) as-cast; and (b) semi-solid. Figure 6. SEM micrograph of ZCuSn10P1 alloy: (a) as-cast; and (b) semi-solid.

3.2. Influence of Pouring Length on the Microstructure of the ZCuSn10P1 Semi-solid Slurry. 3.2. Influence of Pouring Length on the Microstructure of the ZCuSn10P1 Semi-solid Slurry. 3.2.The Influence of Pouring Length onaffects the Microstructure of the ZCuSn10P1 Semi-solid Slurry. pouring length directly the cooling length of the alloy on the cooling plate, so it is an important factor affecting the microstructure. Whenlength the pouring temperature is constant 1080 The pouring length directly affects the cooling of the alloy on the cooling plate,atso it is°C an The pouring length directly affects the cooling length of the alloy on the cooling plate, so it is◦an and the pouring length is 200 mm, the alloy melt flows through the cooling channel rapidly, thus the important factor affecting the microstructure. When the pouring temperature is constant at 1080 C important factor affecting the microstructure. When the pouring temperature is constant at 1080 °C cooling is shorter. number of nucleation andcooling the melt temperature is thus higher. and thetime pouring lengthThe is 200 mm, the alloy melt sites flowsdecreases through the channel rapidly, the and the pouring length is 200 mm, the alloy melt flows through the cooling channel rapidly, thus the The microstructure was The mainly worm-like and dendrite-like, as shown Figure 7a. With pouring cooling time is shorter. number of nucleation sites decreases and theinmelt temperature is higher. cooling time is shorter. The number of nucleation sites decreases and the melt temperature is higher. length increased to 300 the flow time ofand thedendrite-like, melt in the cooling channel increases. The number The microstructure wasmm, mainly worm-like as shown in Figure 7a. With pouring The microstructure was mainly worm-like and dendrite-like, as shown in Figure 7a. With pouring of nucleation sitestoincreases, the temperature is more uniform. microstructure is basically length increased 300 mm, and the flow time of the melt in the coolingThe channel increases. The number length increased to 300 mm, the flow time of the melt in the cooling channel increases. The number composed of worm-like grainsand or the equiaxed grains due to suppression of microstructure the formation ofis dendrite of nucleation sites increases, temperature is more uniform. The basically of nucleation sites increases, and the temperature is more uniform. The microstructure is basically structures, in Figure with the increase of the pouring to 400 the composed as of shown worm-like grains7b. orHowever, equiaxed grains due to suppression of thelength formation of mm, dendrite composed of worm-like grains or equiaxed grains due to suppression of the formation of dendrite flow time inasthe cooling channel further increased. The primary α-Cu phase is to finer structures, shown in Figure 7b. is However, with the increase of the pouring length 400and mm,rounder the flow structures, as shown in Figure 7b. However, with the increase of the pouring length to 400 mm, the due pouring coolingincreased. length, as The shown in Figure The temperature of the melt timetointhe theincrease cooling in channel is further primary α-Cu7c. phase is finer and rounder dueat to flow time in the cooling channel is further increased. The primary α-Cu phase is finer and rounder the of thein cooling channel is very low to the of The the pouring cooling length. easier theexit increase pouring cooling length, asdue shown in increase Figure 7c. temperature of the meltItatisthe exit due to the increase in pouring cooling length, as shown in Figure 7c. The temperature of the melt at to crusts channel inside the cooling which is not the preparation ofeasier a semi-solid ofform the cooling is very lowchannel, due to the increase of conducive the pouringtocooling length. It is to form the exit of the cooling channel is very low due to the increase of the pouring cooling length. It is easier slurry. crusts inside the cooling channel, which is not conducive to the preparation of a semi-solid slurry. to form crusts inside the cooling channel, which is not conducive to the preparation of a semi-solid slurry.

Figure alloy at at different differentpouring pouringlengths lengthsatataapouring pouringtemperature temperatureof Figure7.7.Microstructures Microstructures of of ZCuSn10P1 ZCuSn10P1 alloy ◦ of 1080 °C: (a) 200 mm; (b) 300 mm; and (c) 400 mm. 1080 C:7.(a)Microstructures 200 mm; (b) 300ofmm; and (c) 400 mm. Figure ZCuSn10P1 alloy at different pouring lengths at a pouring temperature of 1080 °C: (a) 200 mm; (b) 300 mm; and (c) 400 mm.

Figure 8 shows the relationship between equivalent diameter and shape factor of the primary αFigure 8 shows the relationship between equivalent diameter and shape factor of the primary Cu phase with pouring length (Figure 8a), and distribution of primary α-Cu phase equivalent α-Cu Figure phase 8with pouring length (Figure 8a),equivalent and distribution of and primary shows the relationship between diameter shapeα-Cu factorphase of theequivalent primary αdiameter (Figure 8b) at different pouring lengths of semi-solid ZCuSn10P1 alloy. Figure 9 shows the diameter 8b) at different of distribution semi-solid ZCuSn10P1 the Cu phase(Figure with pouring lengthpouring (Figure lengths 8a), and of primaryalloy. α-CuFigure phase9 shows equivalent relationship between the amount of primary α-Cu phase per unit area and pouring length in semirelationship between amount ofpouring primarylengths α-Cu phase per unit ZCuSn10P1 area and pouring diameter (Figure 8b)the at different of semi-solid alloy.length Figurein9semi-solid shows the solid ZCuSn10P1 alloy. In Figures 8 and 9, it can be found that the equivalent diameter of the primary ZCuSn10P1 In Figures 8 and 9, can be found theper equivalent of thelength primary relationshipalloy. between the amount ofitprimary α-Cu that phase unit areadiameter and pouring in α-Cu semiα-Cu grains was 50 μm and the shape factor was 0.72 when the casting temperature was constant ◦at grains was 50 µm and theInshape factor was when the casting temperature constant at 1080 C solid ZCuSn10P1 alloy. Figures 8 and 9, 0.72 it can be found that the equivalent was diameter of the primary 1080 °C and the pouring length was 200 mm. The equivalent diameters are mainly in the range of 30 α-Cu waslength 50 μmwas and200 the mm. shapeThe factor was 0.72diameters when the are casting temperature wasofconstant and thegrains pouring equivalent mainly in the range 30 µm toat μm to 60 μm, but there are also some larger grain diameters exceeding 100 μm. At this point, the 1080 and the pouring lengthlarger was 200 mm. The equivalent diameters areAtmainly in thethe range of 30 60 µm,°Cbut there are also some grain diameters exceeding 100 µm. this point, number number of grains per square centimeter of primary α-Cu phase is about 33,939 and the solid fraction of grains square centimeter of primary α-Cu phase is about exceeding 33,939 and 100 the solid fraction is about μm to 60per μm, but there are also some larger grain diameters μm. At this point, the number of grains per square centimeter of primary α-Cu phase is about 33,939 and the solid fraction

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isisabout after The diameter ofofprimary α-Cu isis46.6 μm and the about63.58% 63.58% afterquenching. quenching. Theequivalent equivalent diameter primary α-Cugrains grains 46.6 the 63.58% after quenching. The equivalent diameter of primary α-Cu grains is 46.6 µm andμm theand shape shape factor isis0.73 when the pouring length isis300 mm. The number ofofgrains with ananequivalent shape factor 0.73 when the pouring length 300 mm. The number grains with equivalent factor is 0.73 when the pouring length is 300 mm. The number of grains with an equivalent diameter diameter 20 μm and 6060μm, for number of grains. At this point, diameterbetween between μm,accounting accounting for83% 83%ofnumber ofthe thetotal total number grains. this point, between 20 µm and20 60μm µm,and accounting for 83% of the total of grains. Atofthis point,Atthe number the number per square centimeter ofofprimary α-Cu phase grains isismaximum about 34,909 and the the number per square centimeter primary α-Cu phase grains maximum about 34,909 and the per square centimeter of primary α-Cu phase grains is maximum about 34,909 and the solid fraction solid fraction was 66.34% after water-quenching. The equivalent diameter ofofprimary α-Cu phase solid fraction was 66.34% after water-quenching. The equivalent diameter primary α-Cu phase was 66.34% after water-quenching. The equivalent diameter of primary α-Cu phase grains increased grains increased toto53.2 and decreased toto0.68 the pouring length was 400 grains 53.2μm μm andthe theshape shapefactor factor 0.68when when thewas pouring length to 53.2 increased µm and the shape factor decreased to 0.68decreased when the pouring length 400 mm. Thewas size400 of mm. The size ofofprimary α-Cu phases isismainly distributed within 4040μm and 7070μm, accounting for mm. The size primary α-Cu phases mainly distributed within μm and μm, accounting for primary α-Cu phases is mainly distributed within 40 µm and 70 µm, accounting for 78.8% of the total 78.8% number grains. The per centimeter ofofprimary α-Cu phase grains 78.8%ofofthe thetotal total number Thenumber number persquare square centimeter primary α-Cu phaseand grains number grains. The numbergrains. per square centimeter of primary α-Cu phase grains was 31,757 the was 31,757 and the solid fraction was 75.3% after water-quenching. The actual solid fraction was 31,757 and the solid fraction was 75.3% after water-quenching. The actual solid fraction solid fraction was 75.3% after water-quenching. The actual solid fraction calculated is much higher calculated isismuch than theoretical calculated muchhigher higher thanthe the theoreticalsolid solidfraction fraction[16,17]. [16,17]. than the theoretical solid fraction [16,17].

Figure Figure8.8. 8.Relationship Relationshipbetween betweenequivalent ofprimary primaryα-Cu α-Cuphase phasewith with Figure Relationship between equivalentdiameter diameterand andshape shapefactor factorofof primary α-Cu phase with pouring length (a); and distribution of primary α-Cu phase equivalent diameter (b) at different pouring length (a); and distribution of primary α-Cu phase equivalent diameter (b) at different pouring length (a); and distribution of primary α-Cu phase equivalent diameter (b) at different pouring pouring ofofsemi-solid ZCuSn10P1 pouringlengths semi-solid ZCuSn10P1 alloy. lengths oflengths semi-solid ZCuSn10P1 alloy. alloy.

Figure Figure9.9. 9.Relationship Relationshipbetween betweennumber numberofof ofprimary primaryα-Cu α-Cuphase phaseper perunit unitarea areaand andpouring pouringlength lengthinin in Figure Relationship between number primary α-Cu phase per unit area and pouring length semi-solid ZCuSn10P1 alloy. semi-solid ZCuSn10P1 alloy. semi-solid ZCuSn10P1 alloy.

Figure Figure10a 10ashows showsa aschematic schematicdiagram diagramofofthe theCrystal CrystalSeparation SeparationTheory Theory[19] [19]and andFigure Figure10b 10b Figure 10a shows a schematic diagram of the Crystal Separation Theory [19] and Figure 10b shows showsthe theshear shearstress stressand andvelocity velocitydistribution distributionofofthe themelt meltduring duringflow flowthrough throughthe theECSC ECSCprocess. process. shows the shear stress and velocity distribution of the melt during flow through the ECSC process. Many Manyprimary primaryα-Cu α-Cuphases phasesare arenucleated nucleatedon onthe thesurface surfaceofofthe thecooling coolingplate platedue duetotothe themelt meltchilling chilling Many primary α-Cu phases are nucleated on the surface of the cooling plate due to the melt chilling effect when the liquid metal is poured onto a cooling slope plate [19]. The crystal nucleus and effect when the liquid metal is poured onto a cooling slope plate [19]. The crystal nucleus andthe the effect when the liquid metal is poured onto a cooling slope plate [19]. The crystal nucleus and the cooling coolingplate plateare aredetached detachedinto intothe theinterior interiorofofthe thesolution solutionunder underthe theaction actionofofthe themelt meltflow. flow.The The cooling plate are detached into the interior of the solution under the action of the melt flow. The surface surface surfaceofofthe thecooling coolingslope slopeplate plateforms formsnew newnuclei nucleiagain againsosothat thatthe thedownstream downstreamliquid liquidmelt meltbecomes becomes of the cooling slope plate forms new nuclei again so that the downstream liquid melt becomes the the thesemi-solid semi-solidslurry. slurry.Therefore, Therefore,the theformation formationofofslurry slurryisisrelated relatedtotothe thetemperature temperaturefield fieldand andthe the semi-solid slurry. Therefore, the formation of slurry is related to the temperature field and the velocity velocity velocityfield. field.The Thecooling coolingchannel channelthickness thicknessofofECSC ECSCisisonly only5 5mm, mm,which whichmeans meansthe thetemperature temperature field. The cooling channel thickness of ECSC is only 5 mm, which means the temperature field and the field fieldand andthe thevelocity velocityfield fieldare arerelatively relativelyuniform. uniform.The Thesemi-solid semi-solidslurry slurrywith withequiaxed equiaxedororworm-like worm-like grains grainsisisfinally finallyformed formedunder underthe thecombined combinedaction actionofofheterogeneous heterogeneousnucleation nucleationand andhomogeneous homogeneous nucleation. nucleation.

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velocity field are relatively uniform. The semi-solid slurry with equiaxed or worm-like grains is finally Metals 2018, 8, x FOR PEER REVIEW 8 of 11 formed under the combined action of heterogeneous nucleation and homogeneous nucleation.

Figure 10. Schematic diagram of: (a) the mechanism of neck formation of α-Cu grains; and (b) principle Figure 10. Schematic diagram of: (a) the mechanism of neck formation of α-Cu grains; and (b) of generation of new semi-solid slurry using the ECSC. principle of generation of new semi-solid slurry using the ECSC.

In Insummary, summary,the thepouring pouringlength lengthdetermines determinesthe theflow flowtime timethrough throughthe thecooling coolingchannel, channel,too tooshort short or prepare excellent semi-solid slurry. There is notisenough time for chilling when ortoo toolong longtime timecannot cannot prepare excellent semi-solid slurry. There not enough time for chilling the pouring length islength too short (200short mm).(200 Themm). melt chilling time is too long it islong not conducive to when the pouring is too The melt chilling time and is too and it is not prepare of semi-solid when theslurry pouring length too longlength (400 mm). ensure theTo melt has conducive to prepareslurry of semi-solid when the is pouring is tooTo long (400that mm). ensure athat certain chilling and the fine semi-solid slurry be prepared continuously, the recommended the melt hastime a certain chilling time and the finecan semi-solid slurry can be prepared continuously, pouring length in this paperlength is 300 mm. the recommended pouring in this paper is 300 mm. 3.3. 3.3. Rheo-Casting Rheo-Casting Figure Figure11 11shows showsthe thecomparison comparisonof ofmicrostructures microstructuresof ofZCuSn10P1 ZCuSn10P1alloy alloyparts partsobtained obtainedby byliquid liquid squeeze casting and semi-solid squeeze casting. It can be found that liquid squeeze casting is made up squeeze casting and semi-solid squeeze casting. It can be found that liquid squeeze casting is made of dendrites or mesh and large area eutectoid phase segregation. upcoarse of coarse dendrites or mesh and large area eutectoid phase segregation.This Thissort sortofofmicrostructure microstructure would be detrimental to mechanical properties. During liquid squeeze casting, would be detrimental to mechanical properties. During liquid squeeze casting, liquid liquid metal metal was was adopted. adopted. The The squeeze squeeze casting casting mold mold and and liquid liquid metal metal had had aa huge huge temperature temperature differential, differential, which which led led to a large temperature gradient; therefore, coarse dendrites are easily formed in the alloy. At once, to a large temperature gradient; therefore, coarse dendrites are easily formed in the alloy. At once, eutectoid eutectoidphase phasecould couldsolidify solidifyrapidly rapidlydue dueto to high high cooling cooling rate. rate. Moreover, Moreover,the themicro-segregation micro-segregationalso also occurred seen that dendritic at position A isArelatively less less thanthan at position B, and occurredininthe theparts. parts.ItItcan canbebe seen that dendritic at position is relatively at position B, minimum at position C. The different filling sequence of these positions leads to different solidification and minimum at position C. The different filling sequence of these positions leads to different speed and cooling rateand (Figure 3a). Filling mouth away position the temperature of alloy solidification speed cooling rate (Figure 3a). from Filling mouthA,away from position A,was the higher and the mold temperature was lower at position A. Thus, the solidification speed is fastest at temperature of alloy was higher and the mold temperature was lower at position A. Thus, the position A, and slowest at position C. solidification speed is fastest at position A, and slowest at position C. However, it is discovered that the microstructures of ZCuSn10P1 alloy parts prepared by semi-solid squeeze casting is made up of worm-like or equiaxed grains and eutectoid structure. The average size of grains of semi-solid squeeze casting is finer than that of liquid squeeze casting. Microstructures at position A and B have almost no difference, therefore, the average size of grains and grain shape are almost the same. In addition, eutectoid phase segregation almost disappears. However, grain size at position C is bigger than at positions A and B, and solid fraction is higher than at positions A and B. The reason is that the filling sequence of these positions are different (Figure 3a), resulting in different solidification speed and cooling rate. The uniformity of semi-solid squeeze casting has been greatly improved compared to liquid squeeze casting, which reveals that melt processing by ECSC process has a significant effect. Semi-solid squeeze casting process and liquid squeeze casting have different solidification modes. Semi-solid squeeze casting process consists of two solidification stages, namely in the enclosed cooling slope channel and in the mold. However, liquid squeeze casting only has solidification in the mold. Semi-solid alloy had a relatively higher solid fraction than liquid alloy, so the cooling rate in the mold was slower, and the solidification was much closer to equilibrium solidification in the mold, thus suppressing microstructure segregation.

eutectoid phase could solidify rapidly due to high cooling rate. Moreover, the micro-segregation also occurred in the parts. It can be seen that dendritic at position A is relatively less than at position B, and minimum at position C. The different filling sequence of these positions leads to different solidification speed and cooling rate (Figure 3a). Filling mouth away from position A, the temperature of alloy was higher and the mold temperature was lower at position A. Thus, the Metals 2018, 8, 275 9 of 11 solidification speed is fastest at position A, and slowest at position C.

Figure 11. Comparison of microstructures of ZCuSn10P1 alloy parts obtained by liquid squeeze casting and semi-solid squeeze casting.

Table 3 shows ultimate tensile strengths and elongations of ZCuSn10P1 alloy parts produced by liquid squeeze casting and semi-solid squeeze casting for different positions. The average ultimate tensile strengths of ZCuSn10P1 alloy parts prepared by semi-solid squeeze casting reach 417 MPa and the average elongation reach 12.56%. The average ultimate tensile strengths of ZCuSn10P1 alloy parts prepared by liquid squeeze casting reach 342 MPa and average the elongation reach 6.5%. We can see that the average ultimate tensile strength and average elongation of ZCuSn10P1 alloy by semi-solid squeeze casting are higher than liquid squeeze casting. The average ultimate tensile strength and average elongation are improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting. This is attributed to the grain refinement and homogeneity of microstructure. Therefore, the squeeze casting combined with the ECSC process is an effective technique to prepare tin bronze alloy sleeve with good mechanical properties. Table 3. Ultimate tensile strengths and elongations of ZCuSn10P1 alloy parts produced by semi-solid squeeze casting and liquid squeeze casting for different positions. Process

Positions 1

2

3

4

Average Value

Mechanical Properties

Semi-solid squeeze casting

Ultimate tensile strength/MPa Elongation/%

411 12.03

420 12.96

422 13.07

415 12.18

417 12.56

Liquid squeeze casting

Ultimate tensile strength/MPa Elongation/%

325 5.5

352 7.43

358 7.87

333 5.2

342 6.5

4. Conclusions The conclusions obtained from the present study are described below:

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2.

3.

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The processing by ECSC can refine grain size and suppress tin segregation as well as regional segregation which usually appears in liquid squeeze casting. The mass fraction of tin in the primary α-Cu phase increased from 5.85 to 6.46 after the ECSC process. The pouring length of ECSC process is an important factor affecting the microstructure and successful preparation of semi-solid slurry. The optimal pouring length is 300 mm; at this time, the equivalent diameter of primary α-Cu is 46.6 µm and its shape factor is 0.73. The average ultimate tensile strength and average elongation of semi-solid squeeze casting ZCuSn10P1 alloy reached 417 MPa and 12.6%, which were improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting. This is attributed to the grain refinement and homogeneity of microstructure.

Acknowledgments: 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. Author Contributions: Rongfeng Zhou and Yongkun Li designed most of the experiments, analyzed the results and wrote this manuscript. Yongkun Li and Lu Li performed most experiments. Han Xiao and Yehua Jiang helped analyze the experiment data and gave some constructive suggestions about how to write this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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