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Effects of Solidification Cooling Rate on the Microstructure and Mechanical Properties of a Cast Al-Si-Cu-Mg-Ni Piston Alloy Lusha Tian, Yongchun Guo, Jianping Li *, Feng Xia, Minxian Liang and Yaping Bai Materials and Chemical Engineering School, Xi’an Technological University, Xi’an 710021, China; [email protected] (L.T.); [email protected] (Y.G.); [email protected] (F.X.); [email protected] (M.L.); [email protected] (Y.B.) * Correspondence: [email protected] Received: 25 May 2018; Accepted: 6 July 2018; Published: 18 July 2018

 

Abstract: The effects of cooling rate 0.15, 1.5, 15, 150, and 1.5 × 105 ◦ C/s on the microstructures and mechanical properties of Al-13Si-4Cu-1Mg-2Ni cast piston alloy were investigated. The results show that with an increase of solidification cooling rate, the secondary dendrite arm spacing (SDAS) of this model alloy can be calculated using the formula D = 47.126v − 1/3. The phases formed during the solidification with lower cooling rates primarily consist of eutectic silicon, M-Mg2 Si phase, γ-Al7 Cu4 Ni phase, δ-Al3 CuNi phase, ε-Al3 Ni phase, and Q-Al5 Cu2 Mg8 Si6 phase. With the increase in the solidification cooling rate from 0.15 to 15 ◦ C/s, the hardness increased from 80.9 to 125.7 HB, the room temperature tensile strength enhanced from 189.3 to 282.5 MPa, and the elongation at break increased from 1.6% to 2.8%. The ε -Al3 Ni phase disappears in the alloy and the Q phase emerges. The δ phase and the γ phase change from large-sized meshes and clusters to smaller meshes and Chinese script patterns. Further increase in the cooling rate leads to the micro hardness increasing gradually from 131.2 to 195.6 HV and the alloy solidifying into a uniform structure and forming nanocrystals. Keywords: Al-13Si-4Cu-1Mg-2Ni alloy; cooling rate effect; solidification microstructure; second phase; mechanical property

1. Introduction Piston, known as the “heart” of the engine, is a key component of internal combustion engine. Operating in a harsh environment, piston endure high temperature, high pressure, complex friction, and coupled thermo-mechanical load. With the improvement of engine power density (kW/L), demanding requirements imposed on the piston material increased greatly [1–4]. Among various piston alloys, near-eutectic Al-Si-Cu-Mg-Ni alloys, involving -13Si-4Cu-1Mg-2Ni alloy, were widely used in high performance diesel engine piston since the near-eutectic alloys have a better combination of light weight, high mechanical properties and a better castability combination than other alloys. Up to date, intensive studies have been made on the heat treatment process and the microscopic thermal characteristics of the Al-13Si-4Cu-1Mg-2Ni alloy [5]. Yongchun Guo, Tao Xu et al. studied the precipitation phase in the Al-Si-Cu-Mg-Ni piston alloy and analyzes the five kinds of Al-13Si-1Mg-2Ni-xCu (x = 0%, 2%, 3%, 4%, 5% (mass fraction)) of common liquid precipitation phase composition and element Cu on its liquid phase precipitation formation regularity [6]. Manasijevic et al. examined the thermal and microscopic features of the piston alloys and found that changes in the concentrations of various alloying elements changed the composition and shape of the phases due to segregation and redistribution [7]. The microstructure of Al-13Si-4Cu-1Mg-2Ni alloy is composed

Materials 2018, 11, 1230; doi:10.3390/ma11071230

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1Mg-2Ni is composed Al3Ni, 2Cu, Mg2Si, Al3CuNi, Al7Cu4Ni, Al5Cu2Mg8Si6 and Al9FeNi. of Al3 Ni, alloy Al2 Cu, Mg2 Si, Al3of CuNi, AlAl 7 Cu4 Ni, Al5 Cu2 Mg8 Si6 and Al9 FeNi. The size and shape of The size and shape of these phase changes closely related to theThus, solidification process. Thus, these phase changes are closely related to theare solidification process. solidification control is solidification control is very important to alter the microstructure of the alloy. However, the research very important to alter the microstructure of the alloy. However, the research on the influence of on the influence of solidification rate on the microstructure of relatively the alloy islittle. still solidification cooling rate on thecooling microstructure and properties ofand the properties alloy is still relatively little. This paper discusses the properties of the alloy under different solidification cooling This paper discusses the properties of the alloy under different solidification cooling rates, in pursuit rates, in pursuit of providing formanufacture the design and manufacture of the piston alloy. of providing guidance for the guidance design and of the piston alloy. 2. Experimental Experimental Methods Methods and and Processes Processes 2. In the the experimental experimental study, study, an an alloy alloy model model of of Al-13Si-4Cu-1Mg-2Ni Al-13Si-4Cu-1Mg-2Ni (wt.%) (wt.%) was was prepared prepared in in an an In intermediate frequency induction melting furnace. Pure aluminum ingot blocks were added into the intermediate frequency induction melting furnace. Pure aluminum ingot blocks were added into the graphite crucible crucibleand and were melted firstly, then the ofpieces of pure silicon, Al-Niintermediate and Al-Cu graphite were melted firstly, then the pieces pure silicon, Al-Ni and Al-Cu intermediate alloys were added into the melt, respectively. Furthermore, mischmetal, rich in alloys were added into the melt, respectively. Furthermore, mischmetal, rich in elements Laelements and Ce, La and Ce, with Al-P alloy agents as the were modifier agents were added into the molten alloy. together withtogether Al-P alloy as the modifier added into the molten alloy. Hexachloroethane as Hexachloroethane theadded degassing agent ◦was added andMg held for was 15 min. Then Mg the degassing agentaswas at 720–740 C and heldat for720–740 15 min.°C Then block added under block was added under the melt completely to oxygen keep it from contacting oxygen and burning. the melt completely to keep it from contacting and burning. After skimming off the After slags skimming covered on the melt surface, melt wasmolds poured into the different covered onoff thethe meltslags surface, the clean melt was pouredthe intoclean the different with various cooling moldsand with variousto cooling rates and solidified to the ingots. rates solidified the ingots. The cooling rate is calculated recording the temperature andpoint time ofpoint of solidification The cooling rate is calculated by by recording the temperature and time solidification process process the temperature paperless in different moldsmold, (sand metal mold, mold, metal mold, with thewith temperature paperless recorderrecorder in different molds (sand coppercopper mold ◦ mold with water cooling). The solidification temperature range is from 650 to 450 °C as demonstrated with water cooling). The solidification temperature range is from 650 to 450 C as demonstrated in in Figure Three setsofofdates dateswere weretested testedfor foreach eachsample sampleand and the the approximations approximations were were obtained. Figure 1. 1. Three sets obtained. The preparation preparation methods methods of of the the test test alloys alloys with with different differentcooling coolingrates ratesare areshown shownin inTable Table1.1. The

Figure 1. 1. The (differential scanning scanningcalorimetry) calorimetry)curve curveofofthe thealloy alloywith withthe thecooling coolingrate rateofof1515◦ C/s. °C/s. Figure The DSC DSC (differential Table 1. The The preparation preparation methods methods of of different different cooling cooling rate rate alloys. alloys. Table 1. Cooling Rate °C/s

Cooling Rate ◦ C/s

0.15 0.15

1.5 1.5

15 15

150

150

150,000

150,000

Casting Mold

Mold Size MoldCylinder Size Ø50 mm Sand mold Ø50 mm Cylinder length:150 mm Sand mold length: 150 mm Ø55 mm Cylinder Metal mold Ø55 mm Cylinder length:150 mm Metal mold length: 150 mm copper mold with Ø50 mm Cylinder copper with Ø50 mm Cylinder water mold cooling length:150 mm water cooling length: 150 mm copper mold with Ø4 mm Cylinder copper with Ø4 mm Cylinder water mold cooling length:10 mm Casting Mold

water cooling

melt-spinning melt-spinning

length: 10 mm

strip

strip

Type of Furnace Cooling Rate Measurement Type of Furnace Rate Measurement induction melting Cooling paperless recorder induction melting paperless recorder furnace thermocouple GZJ-800G furnace thermocouple induction melting paperlessGZJ-800G recorder induction melting paperless recorder furnace thermocouple GZJ-800G furnace thermocouple GZJ-800G induction melting paperless recorder induction melting paperless recorder furnace thermocouple GZJ-800G furnace thermocouple GZJ-800G vacuum induction paperless recorder vacuum paperless recorder meltinginduction furnace thermocouple GZJ-800G melting furnace thermocouple GZJ-800G vacuum induction analog computation vacuum meltinginduction furnace analog computation melting furnace

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The composition of the alloy was measured by a direct-reading spectrometer (model Bruker Q4 The composition of the alloyinwas measured by a direct-reading spectrometer (model Bruker Q4 Tasman). The results are shown Table 2. Tasman). The results are shown in Table 2. Table 2. Element content in the test alloy (wt.%). Table 2. Element content in the test alloy (wt.%). Material Si Cu Mg Ni

Material Si Al-13Si-4Cu-1Mg-2Ni 12.76 Al-13Si-4Cu-1Mg-2Ni 12.76

Cu 4.03 4.03

Mg 1.06 1.06

Al

Ni Al 2.17 balance 2.17 balance

The as as cast microstructures microstructures of the Al-13Si-4Cu-1Mg-2Ni Al-13Si-4Cu-1Mg-2Ni alloy alloy solidified solidified with with different different cooling cooling The rates were were analyzed analyzed using using aa Nikon Nikon EPIPHOT300 EPIPHOT300 optical optical microscope microscope (Nikon, (Nikon, Tokyo, Tokyo, Japan) Japan) and and aa rates Quanta 400F (FEI, Hillsboro, OR, USA) thermal field emission scanning electron microscope equipped Quanta Oregon, USA) thermal field emission scanning electron microscope equipped with with an INCA energy dispersive spectrometer. phase compositions contained in the alloy prepared an INCA energy dispersive spectrometer. The The phase compositions contained in the alloy prepared at at different cooling rates were examined usingananXRD-6000 XRD-6000X-ray X-raydiffract diffractmeter meter(Shimadzu, (Shimadzu,Kyoto, Kyoto, different cooling rates were examined using Japan). Brinell Brinellhardness hardnesstested testedwith witha HB3000 a HB3000 hardness tester (Shanghai shangcai testing machine Japan). hardness tester (Shanghai shangcai testing machine co. Co. Ltd., Shanghai, China) a load of 250 Vickers hardnessload loadisis200 200NNwith withaaWilsonV32D265 WilsonV32D265 LTD, Shanghai, China) hashas a load of 250 N.N. Vickers hardness Duisburg, Germany). Germany). ItIt was was utilized utilized to to measure measure the the hardness hardness of of the the three three ingot ingot samples samples (Wilson, Duisburg, prepared at different cooling rates. The tensile strengths of the cast Al-13Si-4Cu-1Mg-2Ni alloy was prepared The tensile strengths Al-13Si-4Cu-1Mg-2Ni alloy was tested with with the Instron 1195 mechanical testing machine (Instron, Shanghai, China). tested 3. Results Results and and Discussion Discussion 3. 3.1. Microstructural Analysis of the Samples at Different Cooling Rates 3.1. Microstructural Analysis of the Samples at Different Cooling Rates Figure 2 shows the microstructures of the model alloy solidified with different cooling rates. Figure 2 shows the microstructures of the model alloy solidified with different cooling rates. The The solidification phase composition of alloy varies with cooling rate. The figure indicates that for solidification phase composition of alloy varies with cooling rate. The figure indicates that for cooling cooling rates in the range of 0.15–1.5 × 105 ◦ C/s, the microstructure of the alloy turns to be noticeably 5 rates in the range of 0.15–1.5 × 10 °C/s, the microstructure of the alloy turns to be noticeably refined refined as the cooling rate increases, and dark gray eutectic silicon changes from continuous small strips as the cooling rate increases, and dark gray eutectic silicon changes from continuous small strips to to small granules. The size of white Ni- and Cu-rich alloy phase becomes smaller, and the distribution small granules. The size of white Ni- and Cu-rich alloy phase becomes smaller, and5 the distribution of the phases is more uniform in the structure. When the cooling rate exceeds 1.5 × 10 5 ◦ C/s, a uniform of the phases is more uniform in the structure. When the cooling rate exceeds 1.5 × 10 °C/s, a uniform granular α-Al grain formed, and the second phase which is small in size is distributed around the α-Al granular α-Al grain formed, and the second phase which is small in size is distributed around the αgrain, because the second phase produced during solidification is too fast to grow. Al grain, because the second phase produced during solidification is too fast to grow.

Figure Al-13Si-4Cu-1Mg-2Ni alloy solidified with different cooling rates: (a) Figure 2. 2. SEM SEManalysis analysisininthe the Al-13Si-4Cu-1Mg-2Ni alloy solidified with different cooling rates: 5 0.15 °C/s; (b) 1.5 (c) 15(c) °C/s; 150(d) °C/s; 1.5 (e) × 101.5°C/s. ◦ C/s; ◦ C/s; (a) 0.15 (b)°C/s; 1.5 ◦ C/s; 15 ◦(d) C/s; 150 (e) × 105 ◦ C/s.

It can be seen from Figure 2, as the cooling rate increases, the secondary dendrite arm spacing It can be seen from Figure 2, as the cooling rate increases, the secondary dendrite arm spacing (SDAS) obviously decreases. This is due to the fact that with the increased cooling rate, both the (SDAS) obviously decreases. This is due to the fact that with the increased cooling rate, both the critical nucleation radius and the nucleation activation energy decrease and the nucleation rate increases [8–10]. The high cooling rate results in higher temperature gradient at the leading edge of

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critical nucleation radius and the nucleation activation energy decrease and the nucleation rate increases [8–10]. The high cooling rate results in higher temperature gradient at the leading edge of the solid–liquid interface, thereby leading to more heterogeneous nucleation and inhibiting the grain Materials The 2018, results 11, x FOR REVIEW 4 of 9 growth. ofPEER SDAS at different cooling rates are shown in Table 3.

the solid–liquid interface, thereby leading more heterogeneous nucleation Table 3. SDAS of the to alloy ingot at different cooling rates. and inhibiting the grain growth. The results of SDAS at different cooling rates are shown in Table 3. Cooling Rates/◦ C/s 0.15 1.5 15 150 1.5 × 105 Table 3. SDAS of84.1 the alloy43.9 ingot at different cooling SDAS/µm 24.8 13.5 rates. 0.82

Cooling Rates / °C/s 0.15 1.5 15 150 1.5 × 105 SDASrelationship / μm 84.1 the43.9 24.8 rate 13.5 0.82 The following classical between metal cooling and SDAS is derived from the theory of heat transfer and mass transfer: The following classical relationship between the metal cooling rate and SDAS is derived from D = βv−1/3 (1) the theory of heat transfer and mass transfer: (1) law = βv−1/3 where β is a constant related to the alloy D composition and cooling conditions. Using power regression analysis, a quantitative between the cooling rateconditions. (v) and theUsing SDAS power (D) of the where β is a constant related to relationship the alloy composition and cooling law model alloy analysis, can be obtained by fitting the data: between the cooling rate (v) and the SDAS (D) of the regression a quantitative relationship model alloy can be obtained by fitting the data: D = 47.126v−1/3 (2) (2) D = 47.126v−1/3 Figure 3 shows the XRD patterns of Al-13Si-4Cu-1Mg-2Ni alloy at different cooling rates. Figure 3 shows the XRD patterns of Al-13Si-4Cu-1Mg-2Ni alloy at different cooling rates. The The figure reveals that the composition of the alloy is similar to that of solidification under different figure reveals that the composition of the alloy is similar to that of solidification under different cooling rates, but the content of each phase is different. There is ε-Al3 Ni in the XRD analysis of a cooling rates, but the content of each phase is different. There is ε-Al3Ni in the XRD analysis of a sample at cooling rate of 0.15 ◦ C/s, and there is almost no ε-Al3 Ni phase in the alloy when the cooling sample at cooling rate of 0.15 °C/s, and there is almost no ε-Al3Ni phase in the alloy when the cooling rate is increased greatly, as demonstrated in the SEM analysis in Figure 4. It also indicates that there rate is increased greatly, as demonstrated in the SEM analysis in Figure 4. It also indicates that there are almost no other alloying phases forms in the Al alloy during solidification with the cooling rate of are almost no other alloying phases forms in the Al alloy during solidification with the cooling rate 1.5 × 105 ◦ C/s except for α-Al phase. Because α-Al precipitated from under-cooling melt, the residual of 1.5 × 105 °C/s except for α-Al phase. Because α-Al precipitated from under-cooling melt, the melt did not have enough time to form other big second alloying phases, and therefore nanocrystal residual melt did not have enough time to form other big second alloying phases, and therefore exists in the alloy. nanocrystal exists in the alloy.

Figure 3. XRD analysis in the Al-13Si-4Cu-1Mg-2Ni alloy solidified with different cooling rates. Figure 3. XRD analysis in the Al-13Si-4Cu-1Mg-2Ni alloy solidified with different cooling rates. (a) (a) 0.15 ◦ C/s; (b) 1.5 ◦ C/s; (c) 15 ◦ C/s; (d) 150 ◦ C/s; (e) 1.55 × 105 ◦ C/s. 0.15 °C/s; (b) 1.5 °C/s; (c) 15 °C/s; (d) 150 °C/s; (e) 1.5 × 10 °C/s.

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Al7Cu4Ni

20μm

(d) ɛ-Al3Ni

(f) γ-Al7Cu4Ni

20μm

20μm

(e) δ-Al3CuNi

(g) Q-Al5Cu2Mg8Si

Figure 4. SEM analysis in the Al-13Si-4Cu-1Mg-2Ni alloy solidified with different cooling rates:(a) Figure 4. SEM analysis in the Al-13Si-4Cu-1Mg-2Ni alloy solidified with different cooling rates: 0.15 °C/s; (b) 1.5 °C/s; (c) 15°C/s; (d) EDS of ε-Al3Ni; (e) EDS of δ-Al3CuNi; (f) EDS of γ-Al7Cu4Ni; (g) (a) 0.15 ◦ C/s; (b) 1.5 ◦ C/s; (c) 15◦ C/s; (d) EDS of ε-Al3 Ni; (e) EDS of δ-Al3 CuNi; (f) EDS of γ-Al7 Cu4 Ni; EDS of Q-Al5Cu2Mg8Si6. (g) EDS of Q-Al5 Cu2 Mg8 Si6.

Figure 4 presents an enlarged view of the SEM micrograph of the model alloy at: (a) 0.15 °C/s (b) ◦ Figure presents an enlarged view of the SEManalysis micrograph the model alloy at: (a) 0.15 1.5 °C/s (c) 154 °C/s. Energy dispersive spectroscopy of theof white phase has confirmed it toC/s be ◦ C/s (c) 15 ◦ C/s. Energy dispersive spectroscopy analysis of the white phase has confirmed it to (b) 1.5 δ-Al 3CuNi phase and γ-Al7Cu4Ni phase. It has been reported that the major strengthening formed be δ-Al3the CuNi phase and process γ-Al7 Cuin has been reported that the major strengthening formed 4 Ni during solidification thephase. pistonItalloy are δ-Al 3CuNi phase and γ-Al7Cu4Ni phase [7]. during the solidification process in the piston alloy are δ-Al CuNi phaseingot and γ-Al [7]. 3 7 Cu 4 Ni phase rate, Figure 4a suggests that there are ε-Al3Ni phases in the solidified at the subcooling Figure 4a suggests that there are ε-Al Ni phases in the solidified ingot at the subcooling rate, 3 and there are few black M-Mg2Si phases. With the increase in the cooling rate, the black M-Mg 2Si and there few black M-Mg With thefrom increase the cooling rate, the black M-Mg 2 Siγphases. 2 Si phase risesare slightly, and the δ and phases changed largein meshes and clusters to smaller meshes phase rises slightly, the δ and phases changed from large meshes and 5clusters and Chinese script and features. At aγcooling rate of 15 °C/s, there are Q-Al Cu2Mg8to Si6smaller phasesmeshes in the ◦ C/s, there are Q-Al Cu Mg Si phases in the and Chinese script features. At a cooling rate of 15 5 2 8 6 microstructure of the ingot, and most of them are around the δ phase. However, no Q phase is found microstructure of the ingot,at and most ofrate them the δ phase. no Q phase is found in the ingot microstructure a cooling ofare 0.15around °C/s because Mg2SiHowever, is preferentially precipitated. ◦ C/s because Mg Si is preferentially precipitated. in the ingot microstructure at a cooling rate of 0.15 The δ and Q phases both increase with rising cooling rate. Belov et al.2 studied the phase diagrams of TheAl-Cu–Fe–Mg–Ni–Si δ and Q phases both system increaseofwith rising cooling al.Al-Si-Cu-Mg-Ni studied the phase diagrams of the aluminum piston rate. alloysBelov [11], et the alloy reported the Al-Cu–Fe–Mg–Ni–Si system of aluminum piston alloys [11], the Al-Si-Cu-Mg-Ni alloy reported in the literature is the same as the alloy system studied in this paper, so the DSC curve is similar, in the literature is the as the system studied in thisLpaper, which is shown in the same literature: thealloy Q phase formed through → (Al)so+ the (Si) DSC + θ +curve Q + γisatsimilar, about which shown the literature: the Q phase formed through L →so(Al) (Si)the + θhigh + Q temperature + γ at about 503 °C.isWhen theincooling rate increases, the temperature decreases fast+that 503 ◦ C. When thenot cooling increases, theprecipitate. temperature so alloy, fast that high temperature precipitates did have rate enough time to Indecreases this model thethe morphology of the precipitates did not have enough time to precipitate. In this model alloy, the morphology of the second second phase is characterized by irregularity, which is generally attributed to the alterations in the phase is characterized irregularity, whichofis generally attributed to the alterations in the temperature temperature gradientbyand the degree overcooling and composition changes during the gradient and the degree of overcooling and composition changes during the solidification of the solidification of the alloy. It is the cooling rate of the alloy in the solidification stage that influences alloy. It is the cooling rate of the of alloy the solidification that influences the microstructural the microstructural morphology theinalloy; it allows thestage secondary phase of the alloy to have a morphology of the alloy; it allows the secondary phase of the alloy to have a significant change in size significant change in size and morphology [11–14]. When the cooling rate is accelerated, the energy and morphology [11–14]. When the cooling rate is accelerated, the energy fluctuation is larger and the fluctuation is larger and the nucleation rate increases. For the crystal grain, the liquid-solid phase nucleation rate Forbefore the crystal liquid-solid phase transitions takefine place even and before transitions takeincreases. place even the grain, crystalthe grain grows up, thus producing grains a the crystal grain grows up, thus producing fine grains and a deviation from the equilibrium state. deviation from the equilibrium state. Figure 5 shows a high-magnification scanning microscopy image for the cooling rate of 150 °C/s and a line scanning elemental profile. In the figure, the gray-colored region is the aluminum matrix, and the fine white phase may be the γ-Al7Cu4Ni through EDS and XRD in Figure 6. From the line-

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Figure 5 shows a high-magnification scanning microscopy image for the cooling rate of 150 ◦ C/s and a line scanning elemental profile. In the figure, the gray-colored region is the aluminum matrix, Materials 2018, 11, x FOR PEER REVIEW 6 of 9 and the 2018, fine 11, white phase be the γ-Al7 Cu4 Ni through EDS and XRD in Figure 6. From the line-scan Materials x FOR PEERmay REVIEW 6 of 9 distribution pattern, the complex compounds produce in a uniform symmetrical growth manner scan distribution pattern, the complex compounds produce in a uniform symmetrical growth manner scan distribution pattern, the complex compounds produce incooling a uniform symmetrical growth when manner during rate. Inthe themeantime, meantime, the duringthe theα-Al α-Alsolidification solidification process process due due to to the the excessive excessive cooling rate. In when the 5 ◦ during the α-Al solidification process due to the excessive cooling rate. In the meantime, when the cooling C/s(Figure (Figure5), 5),the theSDAS SDASisistoo toosmall smallto todistinguish distinguish coolingrate ratecontinues continues to toincrease increase to to1.5 1.5×× 10 105 °C/s cooling rate continues to increase to 1.5 × 105 °C/s (Figure 5),one thecan SDAS issee toothat small to distinguish gray-colored regions from bright white-colored regions, and only the elements the gray-colored regions from bright white-colored regions, and one can only see that the elements ofofthe gray-colored regions from bright white-colored regions, and one can only see that the elements of the alloy alloyare areevenly evenlydistributed distributedto to form form aa solid solid solution. solution. alloy are evenly distributed to form a solid solution.

10 μm 10 μm

◦ C/s. Figure5.5.SEM SEManalysis analysis with with the the cooling rate of 150 °C/s. Figure Figure 5. SEM analysis with the cooling rate of 150 °C/s.

Figure 6. SEM analysis in the Al-13Si-4Cu-1Mg-2Ni alloy solidified with cooling rate of 1.5 × 1055 °C/s. ◦ C/s. Figure 6. SEM SEM analysis analysis in the Al-13Si-4Cu-1Mg-2Ni alloy solidified with cooling rate of 1.5 ××10 105 °C/s.

Figure 7 shows the TEM image and EDS analysis of the sample at the cooling rate of 1.5 × 105 °C/s. Figure 7 shows the TEM image and EDS analysis of the sample at the cooling rate of 1.5 × 105 °C/s. It is clearly demonstrated that something like a polycrystalline ring or an amorphous ring is formed It is clearly demonstrated that something like a polycrystalline ring or an amorphous ring is formed around the [110] zone axis of Al matrix. EDS analysis of the matrix shows a small amount of Si and around the [110] zone axis of Al matrix. EDS analysis of the matrix shows a small amount of Si and Cu in the Al matrix, whereas the so-called grain boundaries are alloying elements in the states. Such Cu in the Al matrix, whereas the so-called grain boundaries are alloying elements in the states. Such

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Figure 7 shows the TEM image and EDS analysis of the sample at the cooling rate of 1.5 × 105 ◦ C/s. It is clearly demonstrated that something like a polycrystalline ring or an amorphous ring is formed Materials 2018, 11, x FOR PEER REVIEW 7 of 9 around the [110] zone axis of Al matrix. EDS analysis of the matrix shows a small amount of Si and Cu in the Al matrix, whereas the so-called grain boundaries in theto states. Such crystals crystals are called nanocrystals because the matrixare Alalloying contentelements is too high completely form are called nanocrystals because the matrix Al content is too high to completely form amorphous. amorphous.

1

(a)

(b)

2

(c)

(d)

Figure 7. TEM analysis with the cooling rate 1.5 × 105 °C/s. Figure 7. TEM analysis with the cooling rate 1.5 × 105 ◦ C/s.

3.2. Analysis of the Mechanical Properties of the Samples at Different Cooling Rates 3.2. Analysis of the Mechanical Properties of the Samples at Different Cooling Rates As seen from Table 3, with the increase in cooling rate, the SDAS changes rather significantly. As seen from Table 3, with the increase in cooling rate, the SDAS changes rather significantly. Table 4 lists the test results of the cast model alloy solidified with different cooling rates. The results Table 4 lists the test results of the cast model alloy solidified with different cooling rates. The results show that as the solidification cooling rate accelerated, hardness changes from 80.9 to 125.7 HB and show that as the solidification cooling rate accelerated, hardness changes from 80.9 to 125.7 HB and the the micro hardness changes from 130 to 195 HV. Figure 8 shows the stress strain curve of different micro hardness changes from 130 to 195 HV. Figure 8 shows the stress strain curve of different cooling cooling rate of the alloy. rate of the alloy. Table 4. Mechanical properties of the cast model alloy solidified with different cooling rates. Table 4. Mechanical properties of the cast model alloy solidified with different cooling rates.

Colling Rate °C/s Colling0.15 Rate ◦ C/s 0.15 1.5 1.5

15 15

150 150 15,000 15,000

Hardness Hardness 80.9 HB 80.9 HB 118.6 HB 118.6 HB 108.6 HV matrix 125.7 HB 108.6 HV matrix 125.7 HB 131.2 HV phases 131.2 HV phases 150.4 HV 150.4 HV 195.6 HV 195.6 HV

Tensile Strength/MPa Elongation/% Tensile Strength/MPa Elongation/% 189.3 1.6 189.3 1.6 213.3 2.2 213.3

282.5

2.2

2.8

282.5

2.8

---

---

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Figure 8. The stress strain curve of different cooling rate of the alloy.

Tensile strength at room temperature has also increased significantly by 49.2% and the elongation increased from 1.6% to 2.8% since as the cooling rate increases, the grains become considerably finer, and the various second phases dispersed more evenly, thereby giving rise to the dual effect of fine grain strengthening and second phase strengthening. It is evident from Figure 8 that the tensile strength is the point at which the premature failure occurs. This means that the improved tensile strength at high cooling rates is related to the refinement of secondary phases that delays the premature failure. Both the strengthening by fine phases and the delay of premature failure contribute to the improved mechanical properties at high cooling rates. The fine alloy phase size is the main reason for the reinforcement. With the strengthening in these manners, there are improvements in the strength of the alloy material, the processing performance of the material and the toughness of the alloy material. 4. Conclusions By comparing the different cooling rate, the microstructure, hardness and tensile strength of the alloy all changed obviously. The main findings are summarized as follows: 1.

2.

With the increase in the solidification cooling rate from 0.15 to 1.5 × 105 ◦ C/s, the SDAS of Al-13Si-4Cu-1Mg-2Ni alloy is refined from 84.1 µm to 0.82 µm. With the increase in the solidification cooling rate from 0.15 to 1.5 × 105 ◦ C/s, the main strengthening phases, δ-Al3 CuNi phase and γ-Al7 Cu4 Ni phase, increased and the shape changed from the larger continuous mesh and long chains into smaller clusters and granules. The formation of Q-Al5 Cu2 Mg8 Si6 phase gradually increases while the ε-Al3 Ni phase decreases. There is a rapid change in alloy structure, from different polymetallic phases to a single nanocrystal. The mechanical properties of the as cast Al-13Si-4Cu-1Mg-2Ni alloy at room temperature including hardness, tensile strength and elongation were noticeably improved due to the effect of fine grain strengthening, fine second phase strengthening, and dispersion strengthening. The hardness increased from 80.9 to 125.7 HB, the room temperature tensile strength enhanced from 189.3 to 282.5 MPa and the elongation at break increased from 1.6% to 2.8%. Further increase in the cooling rate leads to the micro hardness increases gradually from 131.2 to 195.6 HV.

Author Contributions: L.T. wrote the main part of the manuscript and took part in the planning and execution of the experiments. Y.G. conceived and designed the study. J.L. participated in the coordination of the study and reviewed the manuscript. F.X. carried provided experimental alloys and analytical samples. M.L. analyzed part of experimental results. Y.B. performed performance tests and made a final examination of the article.

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Funding: The authors would like to thank the National Basic Science Development Foundation of China (613224), National Natural Science Foundation of China (Grant No. 51705391) and Creative Talents Promotion Plan—Technological Innovation Team (2017KCT-05) for financial support. Conflicts of Interest: The authors declare no conflicts of interest.

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