Ultrasonic assisted squeeze casting of a wrought aluminum alloy

Accepted Manuscript Title: Ultrasonic assisted squeeze casting of a wrought aluminum alloy Authors: Gang Chen, Ming Yang, Yu Jin, Hongming Zhang, Fei Han, Qiang Chen, Zude Zhao PII: DOI: Reference:

S0924-0136(18)30474-6 https://doi.org/10.1016/j.jmatprotec.2018.10.032 PROTEC 15985

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

15-8-2018 17-10-2018 26-10-2018

Please cite this article as: Chen G, Yang M, Jin Y, Zhang H, Han F, Chen Q, Zhao Z, Ultrasonic assisted squeeze casting of a wrought aluminum alloy, Journal of Materials Processing Tech. (2018), https://doi.org/10.1016/j.jmatprotec.2018.10.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ultrasonic assisted squeeze casting of a wrought aluminum alloy

Gang Chen a, #, Ming Yang b, #, Yu Jin a, Hongming Zhang c, Fei Han a, Qiang Chen , Zude Zhao d

School of Materials Science and Engineering, Harbin Institute of Technology, Weihai 264209, China

Micro-Joining Center, Korea Institute of Industrial Technology (KITECH), Incheon

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b

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a

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d,*

Department of Civil Engineering, Harbin Institute of Technology, Weihai 264209,

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c

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21999, Republic of Korea

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Southwest Technology and Engineering Research Institute, Chongqing 400039, China

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d

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China

These authors contributed equally to this work

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*Corresponding author. E-mail address: [email protected]; [email protected] (Qiang Chen)

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Abstract:

In the present work, an ultrasonic assisted squeeze casting method is proposed for processing wrought aluminum alloys. A frame-shaped part was fabricated to verify the feasibility of ultrasonic assisted squeeze casting technology. The results show that a 2024 alloy part with a complex shape and good surface quality can be produced by

the proposed ultrasonic assisted squeeze casting method. As the ultrasonic power increasing, the microstructures of the squeeze cast parts were clearly refined, and the coarse polygonal or dendritic structures evolved to fine and equiaxed grains. Mechanical properties, such as strength and plasticity, were also improved

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significantly as the ultrasonic power increasing. When the ultrasonic power was 1.8 kW, the UTS, YS and elongation to fracture were 372 MPa, 246 MPa and 8.5%,

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which were improved by 20.8%, 21.2% and 84.8%, respectively, compared to a

conventional squeeze cast part. Finally, the effect of ultrasonic vibration on mold-

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filling and solidification was analyzed.

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1. Introduction

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Keywords: Squeeze casting; ultrasonic vibration; wrought aluminum alloy

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Squeeze casting is a near net shaping technology where a molten alloy is squeezed

by a punch to fill dies, solidified under a high pressure and finally deformed slightly

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(the amount of deformation is equal to the alloy shrinkage during solidification) (Ghomashchi and Vikhrov, 2000). Therefore, squeeze casting is suitable for producing high performance parts with a complex shape. Moreover, wrought aluminum alloys with low fluidity and a high hot-crack tendency can also be processed by squeeze casting.

In recent years, squeeze casting of aluminum alloys has received extensive attentions. Li et al. (2017) studied the microstructures and mechanical properties of squeeze cast hypereutectic Al-xSi alloys, and found that both Si phases and α-Al can be refined by squeeze casting. Jahangiri et al. (2017) found that the dendrite arm

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spacing and porosity of 2024 aluminum alloys can be decreased by increasing the squeeze casting pressure and reducing the pouring temperature. Youn et al. (2004)

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designed a squeeze casting process for an automobile part by computer-aided

engineering and experiments, and found that the mechanical properties depended on

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the injection velocity and holding time after complete filling.

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To widen the application of squeeze casting technology for important components,

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further improvement to the formability and mechanical properties of a squeeze cast part has become a great challenge for researchers. Much work has been carried out to

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optimize the squeeze casting method. Lü et al. (2012) proposed a rheo-squeeze

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casting method in which an A356 aluminum alloy was pretreated by indirect ultrasonic vibration in the semi-solid state followed by direct squeeze casting. Guan et

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al. (2016) also proposed a similar method that the molten A356 alloy was first treated by a vibrating slope for preparing fine and equiaxed primary grains, and then

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processed by squeeze casting. The mechanical properties of squeeze cast parts were improved in comparison to conventional squeeze casting. Jiang et al. (2012) proposed a combined casting-forging forming method (termed as double control forming), which involves the molten alloy first being processed by die casting followed by forging on the partially solidified alloy. Components with high mechanical properties

can be produced by removing the casting defects, e.g. porosities, and refining the microstructures. The method has been verified on AZ91D (Jiang et al. 2012) and AM60B (Jiang et al. 2013). In the metallurgical industry, ultrasonic treatment has been widely used for grain

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refinement and degassing, which are attributed to the cavitation and acoustic

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streaming effects generated by ultrasonic vibration in a melt (Eskin and Eskin, 2015). The current ultrasonic casting technology mainly refers to applying an ultrasonic vibration in the melt treatment stage (Haghayeghi et al., 2015). To the best of our

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knowledge, few studies have been reported on combining ultrasonic treatment and

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squeeze casting. If ultrasonic vibration can be applied in the squeeze casting process,

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whether it can play positive roles in both mold-filling (fill complex-shape dies) and solidification (refine the microstructures)?

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In this work, a novel ultrasonic assisted squeeze casting method is proposed. A

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frame-shaped part was fabricated to verify the feasibility of ultrasonic assisted squeeze casting technology. The microstructures and mechanical properties of

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squeeze casting parts were investigated. The effect of ultrasonic vibration on moldfilling and solidification was analyzed. The study will be of significance for widening

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the application of squeeze casting technology in important components that require high mechanical properties. 2. Experimental 2.1 Technical principle

The technical principle of ultrasonic assisted squeeze casting is shown in Fig. 1. The squeeze casting system contains cavity dies, a punch, an ultrasonic device, springs and a limitation block. Before casting, the ultrasonic probe is contacted with the punch along the axis, and a pre-tightening force is applied between the punch and

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ultrasonic probe by means of springs (Fig. 1(a)). The ultrasonic vibration can be transmitted through the punch and applied into the melt. Therefore, during the

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squeeze casting process, the molten alloy can be squeezed by the punch to fill dies under the effect of ultrasonic vibration. As the cavity die is filled, the springs are

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further compressed if the hydraulic cylinder continues moving upward. As the

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limitation block contacts with the punch (Fig. 1(b)), all pressure of the hydraulic press

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can be applied to the molten alloy. Finally, the melt is solidified and deformed slightly

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(the deformation degree is equal to the amount of shrinkage) under the coupled effects

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of high pressure and ultrasonic vibration.

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(a)

(b)

Part

Cavity dies Molten alloy Punch Ultrasonic probe Limitation block Springs

Fig. 1. Technical principle of ultrasonic assisted squeeze casting: (a) before and (b) after casting.

2.2 Materials A commercial wrought aluminum alloy 2024 was used as the raw material. The chemical composition of the 2024 alloy is listed in Table 1. According to the DSC result in previous work (Chen et al., 2016), the solidus and liquidus temperatures are

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505 °C and 647 °C, respectively. Such a wide solidification interval causes a mushy

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freezing characteristic with low fluidity and a high hot-crack tendency, leading to a challenge in processing the 2024 alloy by the casting method.

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2.3 Method

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Dimensional drawings of the target frame-shaped part and squeeze cast part are

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shown in Fig. 2. As indicated in Fig. 2(a), the length and width of the frame-shaped

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part are 170 mm and 109 mm, respectively, and the minimum thickness is 4 mm. As

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shown in Fig. 2(b), in order to obtain a good mold filling condition, one primary runner and three sub-runners were set on the squeeze cast part. Moreover, three

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overflows were also set on every side for pushing the solidification front (which

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contains some oxides) out of the matrix.

(a)

Overflow

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(b)

Runner

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Fig. 2. Schematic diagrams of the target frame-shaped part (a) and squeeze cast part

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(b).

Fig. 3 shows the schematic diagram of the ultrasonic assisted squeeze casting dies

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in this work. As presented in Fig. 3, the pre-tightening force between the punch and

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ultrasonic probe is realized via a series of disk springs. An inner support tube is used to transmit the forming force of the lower hydraulic cylinder, and an outer frame is

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used to support the clamping force applied by the upper hydraulic cylinder. To transmit the ultrasonic vibration from the punch to the molten alloy effectively,

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the shape and dimension of the punch should be designed to ensure the first-order natural frequency matches the frequency of the ultrasonic vibration. In this work, the punch is cylindrical with a diameter of 60 mm. The height of the punch was determined by modal analyses conducted on ANSYS Workbench 14.0. Fig. 4 shows

the modal analysis results of punches with varying heights. As shown in Fig. 4, punches with heights of 76 mm to 82 mm have first-order natural frequency orientations along their axes. The first-order and second-order natural frequencies of punches with varying

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heights are listed in Table 2. As the height of the punch increases from 76 mm to 82

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mm, the first-order natural frequency decreases from 20595 Hz to 19088 Hz, which matches the frequency of ultrasonic vibration (20 kHz). Therefore, the height of the

Disk springs

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punch was determined to be 80 mm in this work.

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Fig. 3. Schematic diagram of the ultrasonic assisted squeeze casting dies. 1-Lower pattern plate; 2-Hydraulic cylinder; 3-Inner support tube; 4-Outer support frame; 5- Ultrasonic device; 6-Fixing plate; 7-Punch; 8-Upper pattern plate; 9-Cavity die; 10-Runner; 11-Cavity die sleeve; 12-Backing ring; 13-Guide sleeve

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(a)

Fig. 4. Modal analysis results of punches with heights of (a) 76mm, (b) 78mm, (c)

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80mm and (d) 82mm.

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The ultrasonic assisted squeeze casting dies are shown in Fig. 5. The squeeze casting was carried out on a double action hydraulic press that contains upper and

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lower hydraulic cylinders with the maximum force of 2000kN and 500kN, respectively. For squeeze casting, the 2024 alloy was melted in an electric resistance

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furnace at 730 °C and then poured into the cavity dies, which were pre-sprayed by a graphite lubricant and preheated to 300 °C. Subsequently, the upper hydraulic cylinder moved downward to clamp the cavity dies, and the ultrasonic device was started simultaneously. Finally, the lower hydraulic cylinder moved upward to squeeze the molten alloy filling dies, and make it solidified under the coupling effects

of a high pressure (60 MPa) and ultrasonic vibration (1.8 kW) for 30 s. During the squeeze casting process, the ultrasonic transducer was air cooled to ensure its temperature was in a safe range (less than 80 °C). The squeeze cast parts were then subjected to T6 heat treatment according to

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previous work (Chen et al., 2016). Tensile samples with a reduced section of 6 mm

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and a gauge length of 45 mm were cut from the squeeze cast parts. Optical microscopy and tensile tests were used to examine the microstructures and

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mechanical properties.

Fig. 5. Ultrasonic assisted squeeze casting dies.

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3. Results and discussion Fig. 6 shows the 2024 alloy frame-shaped parts prepared by conventional and

ultrasonic assisted squeeze casting methods. As shown in Fig. 6, the contours of all squeeze cast parts are clear without obvious defects, indicating that the mold filling processes of both conventional and ultrasonic assisted squeeze casting are

satisfactory. It should be attributed to the high forming pressure (60 MPa) during squeeze casting. The ultrasonic assisted squeeze casting parts before and after machining are shown in Fig. 7. After cutting the primary runner, sub-runners and overflows and cleaning the graphite lubricant, a component with good surface quality

Ultrasonic assisted squeeze casting

100 mm

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Conventional squeeze casting

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produced by the proposed ultrasonic assisted squeeze casting method.

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is exhibited in Fig. 7(b). A 2024 alloy component with a complex shape can be

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Fig. 6. 2024 alloy frame-shaped parts prepared by conventional and ultrasonic

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assisted squeeze casting methods

Fig. 7. Ultrasonic assisted squeeze casting parts before (a) and after (b) machining

Fig. 8 shows the tensile mechanical properties of 2024 aluminum frame-shaped parts prepared by ultrasonic assisted squeeze casting. When the ultrasonic power was 0 kW (i.e., conventional squeeze casting), the ultimate tensile strength (UTS), yield strength (YS) and elongation to fracture were 308MPa, 203MPa and 4.6%,

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respectively. As the ultrasonic power increased (0 to 1.2 kW), the mechanical properties, especially the elongation to fracture, were improved significantly. As the

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ultrasonic power further increased (1.2 to 1.8 kW), the mechanical properties were improved slightly. When the ultrasonic power was 1.8 kW, the UTS, YS and

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elongation to fracture were 372 MPa, 246 MPa and 8.5%, which were improved by

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20.8%, 21.2% and 84.8% respectively compared to the conventional squeeze cast

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part.

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Fig. 8. Tensile mechanical properties of 2024 aluminum frame-shaped parts prepared by ultrasonic assisted squeeze casting. The optical micrographs of 2024 aluminum frame-shaped parts prepared by squeeze casting with and without ultrasonic vibration are shown in Fig. 9. The

microstructure of a conventional squeeze cast part consists of coarse polygonal grains and some dendrites (as indicated by the wright circles), and the average grain size is about 100μm (Fig. 9(a)). When the ultrasonic power is 0.6 kW, the microstructure is refined significantly, although some positions still remain dendritic (as indicated by

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the wright circles in Fig. 9(b)). As the ultrasonic power further increased, the microstructures consist of fine and equiaxed grains leaving a few small dendrites (as

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indicated by the wright arrows in Fig. 9(c) and (d)). When the ultrasonic power is 1.8

kW, the average grain size is about 50μm. Fig. 9 indicates that the ultrasonic vibration

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has a significant effect on the solidification behavior during the squeeze casting

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process. As illustrated in Fig. 10, the microstructure was refined evidently as the

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ultrasonic power increasing. Besides grain refinement, modification of grain shape

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(from coarse dendrite to fine equiaxed grains) and removing of micro-defects (micro-

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porosities and gas cavities) are all beneficial for improving the mechanical properties. Therefore, the mechanical properties, including strength and plasticity, are improved

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significantly as the ultrasonic power increasing (Fig. 8).

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(b)

100μm

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100μm

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100μm

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100μm

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Fig. 9. Optical micrographs of 2024 aluminum frame-shaped parts prepared by

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squeeze casting with no ultrasonic vibration (a), and under ultrasonic power of 0.6kW

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(b), 1.2kW (c) and 1.8kW (d).

Fig. 10. Average grain size of 2024 aluminum frame-shaped parts prepared by ultrasonic assisted squeeze casting. During the ultrasonic assisted squeeze casting process, the effects of ultrasonic vibration on mold-filling and solidification are shown in Fig. 11. When the molten

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alloy is poured into the primary runner, a thin chilled layer is formed on the inner wall

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of the runner, although the cavity dies and runners have already been pre-heated to 300 °C (still much lower than the liquidus temperatures of the 2024 alloy), as

illustrated in Fig. 11(a). The chilled layer could not only have a negative effect on

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mold filling, but also degrade the mechanical properties if the chilled layer is

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squeezed into the cavity dies.

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When the ultrasonic vibration is applied in the melt, nonlinear effects including cavitation and acoustic streaming will be generated if the ultrasonic intensity or sound

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pressure intensity exceed the corresponding threshold values (100 W/cm2 for

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ultrasonic intensity, and 1.0×106 Pa for sound pressure intensity). According to previous work (Qin et al. 2015), the ultrasonic intensity was calculated to be 344

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W/cm2, and the sound pressure intensities were calculated to be 4.3×106 Pa, 6.1×106 Pa and 7.45×106 Pa when the ultrasonic power is 0.6 kW, 1.2 kW and 1.8 kW,

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respectively. In this work, both the ultrasonic intensity and sound pressure intensity are much higher than the threshold values. Therefore, instantaneous pressure, highintensity shock waves, and local high temperature will be caused in the melt under the effects of cavitation and acoustic streaming. According to previous work (Qin et al. 2015), the instantaneous pressure caused by acoustic streaming was calculated to be

4.44 MPa, and the local high temperatures caused by instant collapse of cavitation cavities were calculated to be 14422 K, 20321 K and 24745 K when the ultrasonic power was 0.6 kW, 1.2 kW and 1.8 kW, respectively. Therefore, the chilled layer can be ruptured to small solid particles under the effects of instantaneous pressure, high-

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intensity shock waves, and local high temperature (Fig. 11(b)). Moreover, the ruptured particles can be dispersed or even remelted due to instantaneous pressure and

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local high temperature (Fig. 11(c)).

During the solidification process of squeeze casting, the nucleation rate can also be

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improved by ultrasonic vibration (Fig. 11(d)), which is attributed to two reasons. (Ⅰ)

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The melting point of some local regions may be increased due to a local high pressure

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caused by the cavitation effect, so the degree of super-cooling can be improved leading to an increased nucleation rate. (Ⅱ) The insoluble particles (e.g., Al2O3) can

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be activated by removing the gas absorbed on the particle surface. Therefore, the

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wettability between Al melt and insoluble particles can be improved, and the particles can appear as heterogeneous nuclei. Subsequently, the initial dendrites can be

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ruptured to fine particles under the effects of cavitation and acoustic streaming, which corresponds to an increase in the amount of nuclei (Fig. 11(e)). Finally, the parts with

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fine and equiaxed microstructures, and therefore high mechanical properties, are produced by the ultrasonic assisted squeeze casting method.

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(c)

(e)

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Fig. 11. Schematic diagram of the effect of ultrasonic vibration on squeeze casting: (a) A chilled layer is formed after the molten alloy is poured into the dies; (b) The

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chilled layer is ruptured under the effects of cavitation and acoustic streaming; (c) The

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ruptured particles are dispersed and remelted; (d) The nucleation rate is improved; (e) The initial dendrites are ruptured; (f) The final part with fine and equiaxed structures.

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4. Conclusion

An ultrasonic assisted squeeze casting method was proposed for processing

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wrought aluminum alloys. A frame-shaped part was fabricated to verify the feasibility of ultrasonic assisted squeeze casting technology. Some important results are summarized as follows. 1.

For ultrasonic assisted squeeze casting, the shape and dimension of the punch

should be designed to ensure the first-order natural frequency matches the frequency of ultrasonic vibration. As the height of the punch increases from 76 mm to 82 mm, the first-order natural frequency decreases from 20595 Hz to 19088 Hz, and it matches the frequency of ultrasonic vibration (20 kHz). Mechanical properties, including strength and plasticity, were also improved

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2.

significantly as the ultrasonic power increased. When the ultrasonic power was

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1.8 kW, the UTS, YS and elongation to fracture are 372 MPa, 246 MPa and 8.5%, which were improved by 20.8%, 21.2% and 84.8% respectively

As the ultrasonic power increased from 0 to 1.8 kW, the microstructures of the

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3.

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compared to a conventional squeeze cast part.

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squeeze cast part were refined from 100 μm to 50 μm, and the coarse polygonal

During the ultrasonic assisted squeeze casting process, instantaneous pressure,

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4.

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or dendritic structures evolved to fine and equiaxed grains.

high-intensity shock waves, and local high temperatures were caused in the

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melt due to cavitation and acoustic streaming, which can play positive roles in

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both mold-filling and solidification.

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Acknowledgments The authors express their appreciation for the financial support of National Natural

Science Foundation of China under Grant Nos. 51875121and 51405100, Postdoctoral Science Foundation of China under Grant Nos.2014M551233 and 2017T100237, Plan of Key Research and Development in Shandong Province under Grant No.

2017GGX202006, and Plan of Co-Development of University in Weihai under Grant No. 2016DXGJMS05.

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References

Chen, G., Zhou, T., Wang, B., Liu, H.W., Han, F., 2016. Microstructure evolution and

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segregation behavior of thixoformed Al−Cu−Mg−Mn alloy. Trans. Nonferr. Met. Soc. China 26, 39–50.

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Eskin, G.I., Eskin, D.G., 2015. Ultrasonic Treatment of Light Alloy Melts, second ed.

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CRC Press, New York.

Process. Technol. 101, 1–9.

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A

Ghomashchi, M.R., Vikhrov, A., 2000. Squeeze casting: an overview. J. Mater.

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Guan, R.G., Zhao, Z.Y., Li, Y.D., Chen, T.J., Xu, S.X., Qi, P.X., 2016. Microstructure

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and properties of squeeze cast A356 alloy processed with a vibrating slope. J.

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Mater. Process. Technol. 229, 514–519. Haghayeghi, R., Heydari, A., Kapranos, P., 2015. The effect of ultrasonic vibrations

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prior to high pressure die-casting of AA7075. Mater. Lett. 153, 175–178.

Jahangiri, A., Marashi, S.P.H., Mohammadaliha, M., Ashofte, V., 2017. The effect of pressure and pouring temperature on the porosity, microstructure, hardness and yield stress of AA2024 aluminum alloy during the squeeze casting process. J. Mater. Process. Technol. 245, 1–6.

Jiang, J., Wang, Y., Chen, G., Liu, J., Li, Y., Luo, S., 2012. Comparison of mechanical properties and microstructure of AZ91D alloy motorcycle wheels formed by die casting and double control forming. Mater. Des. 40, 541–549. Jiang, J., Chen, G., Wang, Y., Du, Z., Shan, W., Li, Y., 2013. Microstructure and

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mechanical properties of thin-wall and high-rib parts of AM60B Mg alloy

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formed by double control forming and die casting under the optimal conditions. J. Alloys Compd. 552, 44–54.

Li, R., Liu, L., Zhang, L., Sun, J., Shi, Y., Yu, B., 2017. Effect of Squeeze Casting on

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Microstructure and Mechanical Properties of Hypereutectic Al-xSi Alloys. J.

A

Mater. Sci. Technol. 33, 404–410.

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Lü, S., Wu, S., Dai, W., Lin, C., An, P., 2012. The indirect ultrasonic vibration

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process for rheo-squeeze casting of A356 aluminum alloy. J. Mater. Process. Technol. 212, 1281–1287.

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Qin, J., Chen, G., Wang, B., Hu, N., Han, F., Du, Z., 2015. Formation of in-situ Al 3

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Ti particles from globular Ti powders and Al alloy melt under ultrasonic vibration. J. Alloys Compd. 653, 32–38.

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Youn, S.W, Kang, C.G, Seo, P.K, 2004. Thermal fluid/solidification analysis of automobile part by horizontal squeeze casting process and experimental evaluation. J. Mater. Process. Technol. 146, 294–302.

Table 1 Chemical composition of the 2024 aluminum alloy used in this study (wt. %). Mg

Mn

Fe

Si

Ni

Zn

Ti

Al

3.8~4.9

1.2~1.8

0.3~0.9

0.5

0.5

0.10

0.30

0.15

Balance

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Cu

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Table 2. Natural frequency of punches with varying heights 76

78

80

82

First-order frequency (Hz)

20595

20066

19565

19088

Second-order frequency (Hz)

24586

23751

22058

22204

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A

N

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Height (mm)