Preparation and Properties of Special Vehicle Cover Via a Novel Squeeze Casting Quantitative Feeding System of Molten Metal

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Article Preparation and Properties of Special Vehicle Cover via a Novel Squeeze Casting Quantitative Feeding System of Molten Metal

Yushi Qi 1 , Lili Chen 1, Gang Chen 2,*, Kefeng Li 3, Chao Li 4 and Zhiming Du 1,*

1 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (Y.Q.); [email protected] (L.C.) 2 School of Materials Science and Engineering, Harbin Institute of Technology, Weihai 264209, China 3 Harbin First Machinery Group Co., Ltd., Harbin 150056, China; [email protected] 4 Beijing Benz Automobile Co., Ltd., Beijing 100176, China; [email protected] * Correspondence: [email protected] (G.C.); [email protected] (Z.D.); Tel.: +86-155-8843-0177 (G.C.); +86-135-0450-6990 (Z.D.) Received: 7 January 2020; Accepted: 15 February 2020; Published: 18 February 2020

Abstract: A quantitative feeding system of molten metal was designed based on a special vehicle cover, which achieved the quantitative transport of molten metal in a fully enclosed environment. There are lots of advantages of this novel device, such as improving the accuracy of the molten metal pouring, reducing the oxide inclusions defects, improving the quality of forming parts, etc. The numerical simulation of the casting process of filling mold was carried out based on the software ProCAST, which optimized the processing parameters of the quantitative feeding device. The result shows that the optimal movement speed of the punch piston is 0.08 m/s. The forming process of the aluminum alloy cover was designed and the process was conducted on the novel quantitative feeding device of molten metal. The structure and process performance of the forming parts were summarized and the defects were analyzed. The influence of technological parameters such as pouring temperature, filling speed, initial mold temperature, and feeding distance on the quality of forming parts was studied, which resulted in the optimum process parameters for the dosing device.

Keywords: 7075 aluminum alloy; squeeze casting; quantitative feeding system of molten metal; numerical simulation

1. Introduction Aluminum (Al) alloy materials are widely used in mechanical industries due to their excellent characteristics such as low density and high strength [1–3], and excellent physical properties and mechanical properties can be obtained through heat treatments. The traditional casting process of Al alloy restricts the production efficiency due to the difficulty of controlling the process precision and the poor quality of produced parts [4]. Currently, the squeeze casting process is widely used in the automobile, aerospace, and military fields due to its high quality, low cost, and easy management [5–7]. Squeeze casting, also known as liquid die forging, is a process for obtaining a part or a blank, which directly injects molten metal or semi-solid alloy into an open mold. Then, the mold closes to apply a clamping force on the molten metal, and the unsolidified metal is subjected to isostatic pressing to produce a forced feeding. The solidified metal (outer shell) is plastically deformed, and high-pressure solidification occurs at the same time [4,8–11]. The squeeze casting process usually consists of three procedures: (1) mold preparation and liquid metal pouring, (2) mold clamping and pressing, and (3) the mold opening and demolding [12–14]. As a near net forming process for light metal, squeeze

Metals 2020, 10, 266; doi:10.3390/met10020266 www.mdpi.com/journal/metals Metals 2020, 10, 266 2 of 16 casting combines the advantages of casting and forging, indicating low production cost, fewer casting defects, and reliable quality [15–17]. The pouring of molten liquid metal is the vital process in squeeze casting, as well as the key point which limits the development of squeeze casting. The traditional pouring process is hand-cast by workers, resulting in high labor intensity and poor working environment [18,19]. It is difficult to accurately control the flow rate due to manual pouring, causing poor dimensional accuracy of the casting product. On the other hand, due to the long period exposure of molten metal in air, defects such as oxide inclusions are caused, which have a negative effect on product quality [20,21]. Therefore, improving the flow accuracy and automation of the molten metal casting process is of vital importance to the development of squeeze casting equipment [22]. The enclosed die-casting system “LEOMACS” designed by Toshiba machines, combining magnetic pump injection for molten metal with indirect metal forging, improved not only the production efficiency but also the product quality [23,24]. A metal transfer tube delivering molten magnesium to a die casting machine (Courtesy of Metamag, Inc., Strathroy, ON, Canada) was employed in magnesium casting technology, as reported by Luo, which effectively prevented the molten magnesium from oxidation [25]. In Yu et al.’s work, a molten metal transfer system was designed to investigate the interfacial heat transfer behavior at the metal/shot sleeve interface in the high-pressure die casting process of AZ91D alloy [26]. The shot sleeve was implemented with three blocks of thermocouples in the bottom side along the direction of movement of plunger to collect temperature readings at the metal–sleeve interface. In this work, a metal liquid quantitative feeding system was designed and further improved, which achieved the quantitative transport of molten metal in a fully enclosed environment, improving the accuracy of the molten metal pouring, reducing the oxide inclusion defects, and improving the quality of forming parts. The forming process of the 7075 Al alloy cover was designed, and the process was conducted on a novel quantitative feeding device. The optimum process parameters were obtained via a numerical simulation based on the software ProCAST and the analysis of sample properties.

2. Experiment The composition of 7075 Al alloy (provided by NORTHEAST LIGHT ALLOY Co., Ltd., Harbin, China) used in this work is shown in Table1. Figure1 shows the Al alloy liquid metal quantitative feeding device, which connected the molten liquid metal and the forming hydraulic pressure machine, and which achieved the automatic quantitative transportation of the liquid metal. The feeding process of liquid metal was carried out in a fully enclosed device, avoiding contact with air and reducing defects such as oxidation and spatter of liquid flow. At the same time, the liquid metal was pressed by the main punch at a constant speed, and subsequently injected smoothly into the mold cavity, avoiding air entrapment and liquid flow splashing. Later, through the programmable logic controller (PLC), the entire squeeze casting process was automated, and the production efficiency was effectively improved. The simulation of the liquid metal filling process was carried out using the software ProCAST (version 2009.1, UNIVERSAL ENERGY SYSTEM).

Table 1. Composition of 7075 Al alloy (wt.%).

Al Zn Mg Cu Si Cr Mn Fe 89.89 6.16 1.29 1.53 0.25 0.17 0.29 0.36

In the forming process, the 7075 Al alloy blank was placed in a well-type melting furnace for melting, and a certain amount of Al alloy reﬁning agent (hexachloroethane, purchased from Aladdin, Shanghai, China) was added during the melting to remove impurities contained in the blank. The initial temperature of the liquid metal pouring was controlled at 700–740 ◦C. The ceramic heating ring was used to heat and maintain the temperature of the feeding sleeve, and the local temperature of the feeding sleeve was heated to 450 ◦C to reduce the heat loss of the liquid metal during the transport Metals 2020, 10, 266 3 of 16 process. The forming mold shown in Figure2 was heated and insulated by a heating rod with a power of 2 kW, and the heating temperature was set to 100–300 ◦C. When the mold temperature reached 100 ◦C, the grease graphite lubricant (purchased from QingDao KunYu Graphite products Co., Ltd., Qingdao, China) was sprayed on the surface of the upper and lower molds in order to facilitate demolding. The distance between the upper mold and the limit plate, known as the feeding distance, was controlled by adjusting the disc spring rod. The feeding distance was 0 mm when the upper mold Metals 2020, 10, x FOR PEER REVIEW 3 of 16 contacted the limit plate, and the feeding distance was set to 0–4 mm in the forming experiment.

1—main hydraulic cylinder; 2—threaded connection plate; 3—main punch; 4—heating ring; 5— feeding sleeve; 6—mold; 7—sub punch; 8—sub hydraulic cylinder; 9—upper feeding sleeve; 10— melting furnace.

Figure 1. Quantitative feeding system device for squeeze casting forming.

AfterIn the beingforming poured process, into the feeding 7075 sleeve,Al alloy the blank liquid was metal placed was in pusheda well-type by the melting main furnace punch and for injectedmelting, intoand thea certain mold amount cavity at of a Al certain alloy movementrefining agent rate. (hexachloroethane, The movement rate purchased of the main from punch Aladdin, was simulatedShanghai, andChina) determined was added by during the software the melting ProCAST. to remove The simplified impurities model, contained surface in mesh, the blank. and solid The meshinitial aretemperature shown in Figureof the3 .liquid The adaptive metal pouring meshing was method controlled was adopted at 700–740 to mesh °C. The the geometryceramic heating model. Afterring was meshing, used to the heat total and number maintain of elements the temperatur was 621,054.e of the The feeding mesh size sleeve, was and set asthe 5 mm.local Thetemperature interface typeof the was feeding set as sleeve the “COINC was heated (coincident)” to 450 °C mode to redu and thece the interfacial heat loss heat-transfer of the liquid coe metalfficient during was set the as 3000transport W/(m process.2 K). The The thermophysical forming mold parameters shown in of Figure the 7075 2 was Al alloy heated are givenand insulated in Table2 .by The a heating hydropress rod · waswith controlled a power toof make2 kW, the and upper the mold heating move temperature downward atwas a speed set to of 30100–300 mm/s. °C. The When upper moldthe mold first contactedtemperature the reached liquid metal, 100 °C, pushing the grease the liquid graphite metal tolubricant the flange. (purchased As the upper from moldQingDao continued KunYu to descend,Graphite theproducts upper Co., mold Ltd., sleeve Qingdao, acted on China) the liquid was metal,sprayed and on the the upper surface mold of the moved upper up and to achieve lower themolds partial in order filling to at facilitate the end ofdemolding. forming. UntilThe distance the upper between mold contacted the upper the mold limit and plate, the thelimit forming plate, pressureknown as was the completelyfeeding distance, applied was on thecontrolled sample by with adju a dwellsting timethe disc of 20 spring s. Then, rod. the The remaining feeding distance metal in thewas feeding0 mm when sleeve the fell upper from themold discharge contacted opening the limit as plate, the main and punch the feeding advanced distance and thewas sub set punchto 0–4 drewmm in back. the forming At last, experiment. the punch returned stroke, and the ejector rods ejected the forming sample out of theAfter concave being mold poured to complete into feeding the squeezesleeve, the casting liquid of metal the 7075 was Al pushed alloy cover.by the The main experimental punch and schematicinjected into is shownthe mold in Tablecavity3 .at a certain movement rate. The movement rate of the main punch was simulated and determined by the software ProCAST. The simplified model, surface mesh, and solid mesh are shown in Figure 3. The adaptive meshing method was adopted to mesh the geometry model. After meshing, the total number of elements was 621,054. The mesh size was set as 5 mm. The interface type was set as the “COINC (coincident)” mode and the interfacial heat-transfer coefficient was set as 3000 W/(m2∙K). The thermophysical parameters of the 7075 Al alloy are given in Table 2. The hydropress was controlled to make the upper mold move downward at a speed of 30 mm/s. The upper mold first contacted the liquid metal, pushing the liquid metal to the flange. As the upper mold continued to descend, the upper mold sleeve acted on the liquid metal, and the upper mold moved up to achieve the partial filling at the end of forming. Until the upper mold contacted the limit plate, the forming pressure was completely applied on the sample with a dwell time of 20 s. Then, the

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remainingremaining metal metal in in the the feeding feeding sleeve sleeve fell fell from from the the discharge discharge opening opening as as the the main main punch punch advanced advanced andand the the sub sub punch punch drew drew back. back. At At last, last, the the punch punch re returnedturned stroke, stroke, and and the the ejector ejector rods rods ejected ejected the the Metalsforming2020forming, 10, 266sample sample out out of of the the concave concave mold mold to to complete complete the the squeeze squeeze casting casting of of the the 7075 7075 Al Al alloy alloy cover. cover.4 of 16 TheThe experimental experimental schematic schematic is is shown shown in in Table Table 3. 3.

1—upper1—upper plate; plate; 2—upper 2—upper padding padding plate; plate; 3—bolts; 3—bolts; 4—upper 4—upper mold mold sleeve; sleeve; 5—lower 5—lower mold mold sleeve; sleeve; 6— 6— moldmold insert; insert; 7—lower 7—lower mold; mold; 8—ejector 8—ejector rod; rod; 9—mol 9—mold dlocating locating plate; plate; 10—bolts; 10—bolts; 11—lower 11—lower plate; plate; 12— 12— spruesprue bush; bush; 13—upper 13—upper mold; mold; 14—limit 14—limit plat plate;e; 15—disc 15—disc spring; spring; 16—disc 16—disc spring spring rod. rod.

FigureFigureFigure 2. 2. Schematic2. Schematic Schematic of of of forming forming forming mold. mold.

Figure 3. Simplified model (a), surface mesh (b), and solid mesh (c). FigureFigure 3. Simpliﬁed 3. Simplified model model (a (),a), surface surface mesh mesh ((bb),), and solid solid mesh mesh (c ().c ).

TableTableTable 2. 2.Thermophysical 2. Thermophysical Thermophysical parameters parameters of ofof the thethe 7075 7075 Al Al Alalloy. alloy. alloy. Thermal Specific Heat DensityDensityThermal ConductivityThermal SpeciﬁcSpecific Heat Capacity Heat SolidusSolidus LiquidusLiquidus Density Conductivity Capacity Solidus Liquidus (Kg/m3)(Kg/m3) (W mConductivity1 K 1) (kJ kgCapacity1 K 1) (°C)( C) (°C) ( C) (Kg/m3) − −−1 −1 − −1 −−1 (°C)◦ (°C) ◦ · (W·m· −·K1 −)1 (kJ·kg· · ·K−1 )− 1 3 (W·m ·K ) (kJ·kg ·K ) 2.75 10 3 97.68 1.28 600.9 663.2 × 2.752.75 × × 10 10 3 97.6897.68 1.28 1.28 600.9600.9 663.2663.2

TableTableTable 3. 3. Experimental3. Experimental Experimental schematic. schematic. schematic.

No. Pouring Temperature ( C) MoldMold TemperatureMold Temperature Temperature ( C) FeedingFeedingFeeding Distance Distance Distance (mm) No. Pouring Temperature◦ (°C) ◦ No. Pouring Temperature (°C) (°C) (mm) 1 700 250(°C) (mm) 2 211 710700700 250 250 250 2 22 32 720710 250 250 2 2 42 730710 250 250 22 3 720 250 2 53 740720 250 250 22 6 730 100 2

7 730 150 2 8 730 200 2 9 730 250 2 10 730 300 2 11 730 250 0 12 730 250 1 13 730 250 3 14 730 250 4 Metals 2020, 10, 266 5 of 16

The characteristic points were distributed in the bottom, side wall, and flange of the prepared cover. Therefore, the metallographic samples were chosen from the three characteristic positions, and the sample size was 15 mm 15 mm 4 mm. The metallographic samples were ground using × × 100#, 500#, 1000#, 1500#, and 2000# sandpaper. After grinding, the polishing treatment and etching treatment were conducted. The etching agent was self-mixed 2.5% HNO3-1.5% HCl-1% HF aqueous solution, and the etching time was set as 20 s. The microstructure was analyzed using a metallographic microscope (Olympus-PEM-3, Tokyo, Japan). Tensile test specimens with an effective stretch length of 13 mm were cut from the flange and bottom of the cover. Then, the heating treatments were conducted on the tensile specimen. For the solution treatment, the specimens were heated at 490 ◦C 5 C for 2 h followed by water quenching; For the aging treatment, the specimens after solution ± ◦ treatment were heated at 190 5 C for 9 h, and then air-cooled. Tensile tests at room temperature ± ◦ were carried out using an electronic universal testing machine (AG-Xplus250kN, SHIMADZU, Kyoto, Japan) at the tensile rate of 1.5 mm/min. The tensile fracture of the sample was observed by a scanning electron microscope (SEM, Merlin Compact, ZEISS, Oberkochen, Germany) to further explore the fracture mechanism.

3. Results and Discussion

3.1. Determination of the Rate of Punch Movement The horizontal movement rate of the punch is the vital process parameter for designing a metal liquid quantitative feeding device. If the movement rate is too slow, the liquid metal cooling time is long in the feeding sleeve, the fluidity becomes lower, and the solidification occurs even before the mold cavity is filled, resulting in blocking in the feeding sleeve. Conversely, if the movement rate of the punch is too fast, the liquid metal splashes easily when the liquid metal enters the mold cavity, which increases the inclination of defects such as oxidation of the gas. Therefore, the punch should push the liquid metal at a uniform rate, thereby smoothly filling the mold cavity and reducing the occurrence of defects. Due to the limited power of the hydraulic system, the maximum advancement speed of the main punch was 0.1 m/s. Therefore, the speeds were set as 0.02 m/s, 0.04 m/s, 0.06 m/s, 0.08 m/s, and 0.10 m/s during the simulation. Under the guiding principle of reducing the cooling time as much as possible, the fastest movement rate of 0.1 m/s was selected. When the punch movement rate was set as 0.1 m/s, the metal liquid pouring temperature was 700 ◦C, the initial mold temperature was 200 ◦C, and the screenshot step was a 20% filling rate. According to the analysis of the cloud image (Figure4), when the main punch movement rate was 0.1 m/s, the metal liquid uniformly filled the mold cavity. When the filling rate reached about 50%, the metal liquid began to enter the cover flange from the side wall; at the same time, a slight entrapment of air occurred at the front of liquid flow, which lead to defects in the samples. Therefore, the punch movement rate needs to be less than 0.1 m/s. The main punch movement rate was set as 0.08 m/s, the metal liquid pouring temperature was 700 ◦C, the initial mold temperature was 200 ◦C, and the screenshot step was a 20% filling rate. The cloud images (Figure5) shows that the metal liquid entered the mold cavity smoothly throughout the filling process, and there was no obvious entrapment of air or liquid-flow splashing. Therefore, the punch movement rate of 0.08 m/s was determined as the optimum process parameter. Metals 2020, 10, x FOR PEER REVIEW 6 of 16

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Figure 4. Temperature cloud diagrams for different filling rates when the punch movement rate was 0.1 m/s: (a) 0%; (b) 20%; (c) 40%; (d) 60%; (e) 80%; (f) 100%.

The main punch movement rate was set as 0.08 m/s, the metal liquid pouring temperature was 700 °C, the initial mold temperature was 200 °C, and the screenshot step was a 20% filling rate. The cloud images (Figure 5) shows that the metal liquid entered the mold cavity smoothly throughout the filling process, and there was no obvious entrapment of air or liquid-flow splashing. Therefore, the punchFigure movement 4. TemperatureTemperature rate cloud of 0.08 diagrams m/s was for de different diterminedﬀerent fil ﬁllingling as the rates optimum when the process punch movement parameter. rate was 0.1 m/s: m/s: ( (aa)) 0%; 0%; ( (bb)) 20%; 20%; ( (cc)) 40%; 40%; ( (d)) 60%; 60%; ( (e)) 80%; 80%; ( (f)) 100%. 100%.

FigureFigure 5. 5. TemperatureTemperature cloud cloud diagrams diagrams for for different diﬀerent fil ﬁllingling rates rates when when the the punch punch movement movement rate rate was was 0.080.08 m/s: m/s: ( (aa)) 0%; 0%; ( (bb)) 20%; 20%; ( (cc)) 40%; 40%; ( (dd)) 60%; 60%; ( (ee)) 80%; 80%; ( (ff)) 100%. 100%.

3.2.3.2. Macroscopic Macroscopic Morphology Morphology Analysis Analysis of of Formed Formed Parts Parts FigureFigure 66 showsshows the the macroscopic macroscopic morphology morphology of of the th formede formed parts. parts. As shownAs shown in Figure in Figure6a, the 6a, cold the shut and misrun defects were observed, which could be ascribed to the fluidity of liquid metal being cold shut and misrun defects were observed, which could be ascribed to the fluidity of liquid metal beingtoo poor too poor to fill to the fill moldthe mold cavity cavity suffi sufficientlyciently during during the the filling filling process, process, resulting resulting inin the incomplete incomplete Figure 5. Temperature cloud diagrams for different filling rates when the punch movement rate was formationformation of partsparts andand the the occurrence occurrence of defects.of defects. In orderIn order to fill to the fill mold the mold cavity cavity sufficiently, sufficiently, the process the 0.08 m/s: (a) 0%; (b) 20%; (c) 40%; (d) 60%; (e) 80%; (f) 100%. processparameters parameters should be should adjusted be accordingly.adjusted accordingly. The thermal The insulation thermal of insulation metal liquid of was metal ensured liquid during was the delivery process, the rate of the main punch was appropriately adjusted to between 0.08 m/s and 3.2. Macroscopic Morphology Analysis of Formed Parts 0.1 m/s, and the dwell time was elongated. Figure 6 shows the macroscopic morphology of the formed parts. As shown in Figure 6a, the cold shut and misrun defects were observed, which could be ascribed to the fluidity of liquid metal being too poor to fill the mold cavity sufficiently during the filling process, resulting in the incomplete formation of parts and the occurrence of defects. In order to fill the mold cavity sufficiently, the process parameters should be adjusted accordingly. The thermal insulation of metal liquid was

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Metalsensured2020 ,during10, 266 the delivery process, the rate of the main punch was appropriately adjusted7 of to 16 between 0.08 m/s and 0.1 m/s, and the dwell time was elongated.

Figure 6.6. MacroscopicMacroscopic morphology morphology of of formed formed parts: parts: (a) cold(a) cold shut shut and misrun;and misrun; (b) micropores; (b) micropores; (c) surface (c) cracks;surface (cracks;d) surface (d) withsurface no with defects. no defects. During the forming process, pores and cracks could be observed on the surface of formed parts, During the forming process, pores and cracks could be observed on the surface of formed parts, as shown in Figure6b. The semi-spherical pores exhibited di fferent sizes and smooth walls. When the as shown in Figure 6b. The semi-spherical pores exhibited different sizes and smooth walls. When upper mold moved downward and contacted with metal liquid, excessive impact force caused the gas the upper mold moved downward and contacted with metal liquid, excessive impact force caused to be trapped in the liquid flow, and the gas could not come out due to the short dwell time, which the gas to be trapped in the liquid flow, and the gas could not come out due to the short dwell time, resulted in porosity surface defects. In order to avoid the occurrence of such defects, the downward which resulted in porosity surface defects. In order to avoid the occurrence of such defects, the speed of the upper mold was controlled during the process, and the dwell time was adjusted to downward speed of the upper mold was controlled during the process, and the dwell time was guarantee that the gas could completely discharge from the mold cavity. adjusted to guarantee that the gas could completely discharge from the mold cavity. The surface crack defects are depicted in Figure6c. A tear-like crack surface and metal oxidation The surface crack defects are depicted in Figure 6c. A tear-like crack surface and metal oxidation could be observed, indicating that the crack was a thermal crack. When the metal liquid entered could be observed, indicating that the crack was a thermal crack. When the metal liquid entered the the mold cavity, the heat was lost mainly through the molds and, thus, the metal liquid adjacent to mold cavity, the heat was lost mainly through the molds and, thus, the metal liquid adjacent to the the surface of the part firstly solidified. As the metal liquid solidified, a large number of dendrites surface of the part firstly solidified. As the metal liquid solidified, a large number of dendrites continuously overlapped to form a skeleton, and the metal liquid began to shrink, while there was also continuously overlapped to form a skeleton, and the metal liquid began to shrink, while there was unsolidified metal liquid between the dendrite skeletons. Since the metal mold lacked deformability, also unsolidified metal liquid between the dendrite skeletons. Since the metal mold lacked the inner stress in the formed part increased due to the large cooling rate. Tensile stress was generated deformability, the inner stress in the formed part increased due to the large cooling rate. Tensile stress since the shrinkage of dendritic skeleton was hindered by inner stress. When the tensile stress exceeded was generated since the shrinkage of dendritic skeleton was hindered by inner stress. When the the material strength limitation, cracks formed between the dendrites could not be replenished by the tensile stress exceeded the material strength limitation, cracks formed between the dendrites could metal liquid in a timely manner. In view of the thermal crack, the dwell time was adjusted to make not be replenished by the metal liquid in a timely manner. In view of the thermal crack, the dwell sure that the metal liquid could continuously be pushed to fill the porosity caused by the shrinkage time was adjusted to make sure that the metal liquid could continuously be pushed to fill the porosity during the solidification process. The overlapped dendrites were broken and the inner stress caused caused by the shrinkage during the solidification process. The overlapped dendrites were broken and by the shrinkage of the dendritic skeleton could be released. Figure6d shows the formed parts after the inner stress caused by the shrinkage of the dendritic skeleton could be released. Figure 6d shows adjusting the process parameters, which demonstrated that surfaces of the cover parts were relatively the formed parts after adjusting the process parameters, which demonstrated that surfaces of the smooth, and no defects such as cracks, hook grooves, and wrinkles were observed. cover parts were relatively smooth, and no defects such as cracks, hook grooves, and wrinkles were observed.

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3.3. Effects Effects of Pouring Temperature onon MicrostructureMicrostructure andand PropertiesProperties ofof FormedFormed PartsParts Figure7 7shows shows thethe metallographicmetallographic photographsphotographs ofof thethe covercover flangesflanges atat didifferentfferent pouringpouring temperatures. The crystal grains with a cell-like crystal morphology were coarse, showing an uneven size distribution, when the pouring temperature waswas 700 °C.◦C. As the pouring temperature increased, the nucleation rate of the metalmetal liquidliquid increased,increased, and the grain size was refinedrefined at the pouringpouring temperature ofof 710710 ◦°C.C. Small grain agglomerations around thethe large grainsgrains couldcould bebe observed.observed. When the metalmetal liquidliquid pouring pouring temperature temperature reached reached 720 720◦C, °C, the the grain grain morphology morphology and sizeand distributionsize distribution were relativelywere relatively uniform, uniform, and the and metal the metal liquid liquid nucleation nucleati rateon reached rate reached the highest, the highest, resulting resulting in crystal in crystal grains withgrains small with and small dense and morphology. dense morphology. When the When pouring the pouring temperature temperature continued continued to be elevated, to be elevated, the grain sizethe grain increased, size increased, and the morphology and the morphology distribution distribution became uneven. became uneven.

Figure 7.7. Metallographic photographsphotographs of of the the cover cover ﬂange flange at at di ﬀdifferenterent pouring pouring temperatures: temperatures: (a) 700(a) 700◦C; (°C;b) 710(b) 710◦C; (°C;c) 720(c) 720◦C; (°C;d) 730(d) 730◦C; °C; (e) 740(e) 740◦C. °C.

The yield strength and tensile strength of the sa samplesmples taken from the flange ﬂange at different diﬀerent pouring temperatures are shown in FigureFigure8 8a.a. TheThe yieldyield strengthstrength andand tensiletensile strengthstrength werewere thethe lowest,lowest, i.e.,i.e., 305.4 MPa and 346.5 MPa, respectively, whenwhen thethe pouringpouring temperature of the metal liquid was 700 °C. As the pouring temperature increased, the yield strength tended to increase and became stable.

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Metals 2020, 10, x FOR PEER REVIEW 9 of 16 ◦C. As the pouring temperature increased, the yield strength tended to increase and became stable. TheThe initial initial tensile tensile strength strength rose rose rapidly rapidly and and decreased decreased slightly slightly in in the the later later stage. stage. The The maximum maximum yield yield strengthstrength was was 340.4 340.4 MPa MPa at at 730 730◦C, °C, and and the the maximum maximum tensile tensile strength strength was was 367.4 367.4 MPa MPa at 720at 720◦C. °C.

Figure 8. Strength and elongation of the samples at different casting temperatures: (a) strength; Figure 8. Strength and elongation of the samples at different casting temperatures: (a) strength; (b) (b) elongation. elongation. The elongations of the samples taken from the flange at different pouring temperatures are shown The elongations of the samples taken from the flange at different pouring temperatures are in Figure8b. When the pouring temperature was in the range of 700–740 C, the plasticity of the sample shown in Figure 8b. When the pouring temperature was in the range◦ of 700–740 °C, the plasticity of fluctuated. With the increase in pouring temperature, the elongation varied slightly. At the pouring the sample fluctuated. With the increase in pouring temperature, the elongation varied slightly. At temperature of 700 C, the sample had the highest elongation of 5.3%, and the lowest elongation was the pouring temperature◦ of 700 °C, the sample had the highest elongation of 5.3%, and the lowest 2.9% at the pouring temperature of 730 C. The elongation fluctuated within the range of 0.8% in the elongation was 2.9% at the pouring temperature◦ of 730 °C. The elongation fluctuated within the range pouring temperature range. of 0.8% in the pouring temperature range. 3.4. Effect of Mold Pre-Heating Temperature on Microstructure and Properties of Formed Parts 3.4. Effect of Mold Pre-Heating Temperature on Microstructure and Properties of Formed Parts Figure9 shows the metallographic photographs of the cover flanges at di fferent pre-heating temperaturesFigure of9 shows the mold. the metallographic In the process, thephotographs movement of rate the ofcover the mainflanges punch at different was 0.08 pre-heating m/s, the temperatures of the mold. In the process, the movement rate of the main punch was 0.08 m/s, the pouring temperature was 730 ◦C, and the feeding distance was 2 mm. When the mold pre-heating pouring temperature was 730 °C, and the feeding distance was 2 mm. When the mold pre-heating temperature was 100 ◦C, there were a large number of coarse grains with uneven distribution of grain sizes.temperature As the pre-heating was 100 °C, temperature there were ofa large the mold number increased, of coarse the grains grain with became uneven finer distribution and denser. of Finer grain grainssizes. with As the uniform pre-heating distribution temperature were observedof the mold as increased, the mold pre-heatingthe grain became temperature finer and increased denser. Finer to grains with uniform distribution were observed as the mold pre-heating temperature increased to 200–250 ◦C. When the pre-heating temperature of the mold was 300 ◦C, the grain demonstrated the tendency200–250 of °C. growth. When the In addition, pre-heating if the temperature mold temperature of the mold was was too high,300 °C, the the slow grain cover demonstrated cooling rate the resultedtendency in grainof growth. coarsening. In addition, Thus, if thethe optimummold temp pre-heatingerature was temperature too high, the of slow the moldcover wascooling set asrate resulted in grain coarsening. Thus, the optimum pre-heating temperature of the mold was set as 200– 200–250 ◦C under the conditions of the test forming equipment. 250The °C yieldunder strength the conditions and tensile of the strength test forming of the samples equipment. taken from the flange at different pre-heating temperatures of mold are shown in Figure 10a. When the mold pre-heating temperature was 100 ◦C, the yield strength and tensile strength were the lowest, i.e., 290.4 MPa and 325.5 MPa, respectively. The strength of the formed part firstly increased and then decreased as the pre-heating temperature of the mold increased. The yield strength and tensile strength reached the maximum of 340.4 MPa and 361.7 MPa, respectively, at the mold pre-heating temperature of 250 ◦C. As the mold pre-heating temperature was elevated continually, the strength of the formed part tended to decrease.

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The initial tensile strength rose rapidly and decreased slightly in the later stage. The maximum yield strength was 340.4 MPa at 730 °C, and the maximum tensile strength was 367.4 MPa at 720 °C.

Figure 8. Strength and elongation of the samples at different casting temperatures: (a) strength; (b) elongation.

The elongations of the samples taken from the flange at different pouring temperatures are shown in Figure 8b. When the pouring temperature was in the range of 700–740 °C, the plasticity of the sample fluctuated. With the increase in pouring temperature, the elongation varied slightly. At the pouring temperature of 700 °C, the sample had the highest elongation of 5.3%, and the lowest elongation was 2.9% at the pouring temperature of 730 °C. The elongation fluctuated within the range of 0.8% in the pouring temperature range.

3.4. Effect of Mold Pre-Heating Temperature on Microstructure and Properties of Formed Parts Figure 9 shows the metallographic photographs of the cover flanges at different pre-heating temperatures of the mold. In the process, the movement rate of the main punch was 0.08 m/s, the pouring temperature was 730 °C, and the feeding distance was 2 mm. When the mold pre-heating temperature was 100 °C, there were a large number of coarse grains with uneven distribution of grain sizes. As the pre-heating temperature of the mold increased, the grain became finer and denser. Finer grains with uniform distribution were observed as the mold pre-heating temperature increased to 200–250 °C. When the pre-heating temperature of the mold was 300 °C, the grain demonstrated the tendency of growth. In addition, if the mold temperature was too high, the slow cover cooling rate Metalsresulted2020 in, 10 grain, 266 coarsening. Thus, the optimum pre-heating temperature of the mold was set as10 200– of 16 250 °C under the conditions of the test forming equipment.

Metals 2020, 10, x FOR PEER REVIEW 10 of 16

Figure 9. Metallographic photographs of the samples at different mold pre-heating temperatures: (a) 100 °C; (b) 150 °C; (c) 200 °C; (d) 250 °C; (e) 300 °C.

The yield strength and tensile strength of the samples taken from the flange at different pre- heating temperatures of mold are shown in Figure 10a. When the mold pre-heating temperature was 100 °C, the yield strength and tensile strength were the lowest, i.e., 290.4 MPa and 325.5 MPa, respectively. The strength of the formed part firstly increased and then decreased as the pre-heating temperature of the mold increased. The yield strength and tensile strength reached the maximum of 340.4Figure MPa 9. andMetallographicMetallographic 361.7 MPa, photographs photographsrespectively, of of theat the thesamples samples mold at pre-heating atdifferent diﬀerent mold moldtemperature pre-heating pre-heating oftemperatures: 250 temperatures: °C. As (thea) mold pre-heating temperature was elevated continually, the strength of the formed part tended to decrease. (100a) 100 °C; ◦(C;b) (150b) 150 °C; ◦(C;c) 200 (c) 200°C; ◦(C;d) (250d) 250°C; ◦(eC;) 300 (e) 300°C. ◦C.

FigureFigure 10. 10.Strength Strength andand elongationelongation ofof thethe samples at different diﬀerent mold mold pre-heating pre-heating temperatures: temperatures: (a) a b ( )strength; strength; ( (b)) elongation. elongation.

Figure 10b shows the elongations of the tensile specimen at different pre-heating temperatures of the mold. The elongation of the sample was 5.5% at most when the mold temperature was 100 °C. As the pre-heating temperature of the mold increased, the elongation certainly fluctuated. The elongation of the samples exhibited an obvious downward trend when the mold pre-heating temperature was elevated to 200 °C, which could be ascribed to the grain growth at a high mold Figure 10. Strength and elongation of the samples at different mold pre-heating temperatures: (a) strength; (b) elongation.

Metals 2020, 10, 266 11 of 16

Figure 10b shows the elongations of the tensile specimen at different pre-heating temperatures of the mold. The elongation of the sample was 5.5% at most when the mold temperature was 100 ◦C. As the pre-heating temperature of the mold increased, the elongation certainly fluctuated. The elongation of the samples exhibited an obvious downward trend when the mold pre-heating temperature was elevated to 200 C, which could be ascribed to the grain growth at a high mold temperature resulting Metals 2020, 10,◦ x FOR PEER REVIEW 11 of 16 in a decrease in plasticity. When the mold temperature was 250 ◦C, the elongation was reduced to the lowesttemperature value. resulting in a decrease in plasticity. When the mold temperature was 250 °C, the elongation was reduced to the lowest value. 3.5. Effect of Feeding Distance on Microstructure and Properties of Formed Parts 3.5. Effect of Feeding Distance on Microstructure and Properties of Formed Parts The metallographic photographs of the cover flanges with different feeding distances are shown in Figure The11. Themetallographic process conditions photographs were of as the follows: cover flan theges movement with different rate offeeding the main distances punch are was shown 0.08 m/s, in Figure 11. The process conditions were as follows: the movement rate of the main punch was 0.08 the pouring temperature was 730 ◦C, and the initial mold temperature was 250 ◦C. The microstructure was loosem/s, the without pouring feeding, temperature and coarse was grains730 °C, with and uneven the initial distribution mold temperature were observed. was 250 As °C. the The feeding microstructure was loose without feeding, and coarse grains with uneven distribution were observed. distance increased, the grains became dense and refined. When the feeding distance was 3 mm or As the feeding distance increased, the grains became dense and refined. When the feeding distance 4 mm, the grain size was relatively small, and the distribution was uniform. was 3 mm or 4 mm, the grain size was relatively small, and the distribution was uniform.

FigureFigure 11. 11.Metallographic Metallographic photographs photographs of of the the samples samples with with different diﬀerent feeding feeding distances: distances: (a) 0 mm; (a) (b 0) mm; 1 mm; (c) 2 mm; (d) 3 mm; (e) 4 mm. (b) 1 mm; (c) 2 mm; (d) 3 mm; (e) 4 mm. Figure 12 shows the yield strength and elongation of the samples taken from the flange with different feeding distances. As the feeding distance increased, the yield strength and tensile strength of the samples increased. When the feeding distance exceeded 2 mm, the strength tended to be stable. The elongation of the samples varied with the feeding distance. The elongation was 5.2% at most when the feeding distance was 0 mm, and the elongation was at least 0.9% with the feeding distance

Metals 2020, 10, 266 12 of 16

Figure 12 shows the yield strength and elongation of the samples taken from the ﬂange with diﬀerent feeding distances. As the feeding distance increased, the yield strength and tensile strength of the samples increased. When the feeding distance exceeded 2 mm, the strength tended to be stable.

TheMetals elongation 2020, 10, x ofFOR the PEER samples REVIEW varied with the feeding distance. The elongation was 5.2% at most12 of 16 when the feeding distance was 0 mm, and the elongation was at least 0.9% with the feeding distance ofof 4 mm.4 mm. As As the the feedingfeeding distancedistance increased, the the el elongationongation of of the the sample sample decreased. decreased. Combined Combined with withmetallographic metallographic analysis, analysis, the the mechanical mechanical properties properties were were closely relatedrelated toto the the microstructure microstructure characteristics.characteristics. The The samples samples with with a densera denser and and ﬁner finer grain grain demonstrated demonstrated considerable considerable strength. strength.

Figure 12. Strength and elongation of the samples with different feeding distances: (a) strength; Figure 12. Strength and elongation of the samples with different feeding distances: (a) strength; (b) (b) elongation. elongation. 3.6. Analysis of Microstructure and Properties at Different Position of Formed Parts 3.6. Analysis of Microstructure and Properties at Different Position of Formed Parts The process conditions were as follows: the pouring temperature of the metal liquid was 730 ◦C, the movementThe process rate conditions of the main were punch as follows: was 0.08 the m pour/s, theing pre-heating temperature temperature of the metal of liquid the mold was 730 was °C, the movement rate of the main punch was 0.08 m/s, the pre-heating temperature of the mold was 250 250 ◦C, and the holding time was 20 s. The metallographic photographs of the samples taken from the°C, flange, and the side holding wall, and time bottom was 20 of s. the The formed metallogra partsphic are shown photographs in Figure of 13the. Theresamples were taken significant from the diffflange,erences side among wall, the and microstructure bottom of the at formed these three part locations.s are shown The in grains Figure at 13. the There flange were were significant flat and spherical,differences and among they had the an microstructure uneven size distribution, at these three while lo shrinkagecations. The micro-holes grains at couldthe flange also be were observed. flat and Thespherical, grains at and the side they wall had were an relativelyuneven size coarser, distribu demonstratingtion, while a shrinkage more uneven micro-holes distribution, could and also there be wasobserved. nucleation The of microcracks,grains at the thereby side reducingwall were the re servicelatively life coarser, of the formingdemonstrating parts. The a grainsmore atuneven the bottomdistribution, of the formed and there parts was had nucleation a small size of microcracks, and fuzzy boundary. thereby reducing A dense the structure service was life of observed the forming in theparts. samples The subjectedgrains at tothe plastic bottom deformation, of the formed but parts there werehad a also small numerous size and shrinkage fuzzy boundary. micro-holes. A dense structureThe yield was strength observed and in tensilethe samples strength subjected of the to samples plastic takendeformation, from the but flange there and were the also bottom numerous are shownshrinkage in Figure micro-holes. 14. The average yield strength and the average tensile strength at the flange were 342.8 MPa and 367.3 MPa, respectively. The average yield strength and the average tensile strength at the bottom were 317.3 MPa and 345.2 MPa, respectively. The elongations of different samples at different positions are shown in Figure 15. The average elongation at the flange was 1.6%, and the average elongation at the bottom was 2.1%. The comparison indicated that the mechanical properties at the bottom were poorer than those at the flange. Specifically, the yield strength was about 10.5% lower, the tensile strength was about 8.3% lower, and the elongation was about 2.3% lower. The molten metal filled the mold through the bottom of the cover, and it was easy to form shrinkage holes due to lacking of feeding. When stretching, these shrinkage holes would evolve into a crack nucleation site, which would reduce the strength of the forming parts.

Metals 2020, 10, x FOR PEER REVIEW 12 of 16 of 4 mm. As the feeding distance increased, the elongation of the sample decreased. Combined with metallographic analysis, the mechanical properties were closely related to the microstructure characteristics. The samples with a denser and finer grain demonstrated considerable strength.

Figure 12. Strength and elongation of the samples with different feeding distances: (a) strength; (b) elongation.

3.6. Analysis of Microstructure and Properties at Different Position of Formed Parts The process conditions were as follows: the pouring temperature of the metal liquid was 730 °C, the movement rate of the main punch was 0.08 m/s, the pre-heating temperature of the mold was 250 °C, and the holding time was 20 s. The metallographic photographs of the samples taken from the flange, side wall, and bottom of the formed parts are shown in Figure 13. There were significant differences among the microstructure at these three locations. The grains at the flange were flat and spherical, and they had an uneven size distribution, while shrinkage micro-holes could also be observed. The grains at the side wall were relatively coarser, demonstrating a more uneven distribution, and there was nucleation of microcracks, thereby reducing the service life of the forming parts.Metals The 2020 grains, 10, x FOR at PEERthe bottom REVIEW of the formed parts had a small size and fuzzy boundary. A dense13 of 16 MetalsstructureMetals2020 2020, was10,, 10 266 observed, x FOR PEER in REVIEW the samples subjected to plastic deformation, but there were also numerous1313 of 16of 16 shrinkage micro-holes.

Figure 13. Metallographic photographs of the samples at different position of forming parts: (a) Metals 2020flange;, 10, x (FORb) side PEER wall; REVIEW (c) bottom. 13 of 16 Figure 13. Metallographic photographs of the samples at different position of forming parts: (a) flange; (b) side wall; (c) bottom. The yield strength and tensile strength of the samples taken from the flange and the bottom are shown in Figure 14. The average yield strength and the average tensile strength at the flange were The yield strength and tensile strength of the samples taken from the flange and the bottom are 342.8 MPa and 367.3 MPa, respectively. The average yield strength and the average tensile strength shown in Figure 14. The average yield strength and the average tensile strength at the flange were at the bottom were 317.3 MPa and 345.2 MPa, respectively. The elongations of different samples at 342.8 MPa and 367.3 MPa, respectively. The average yield strength and the average tensile strength different positions are shown in Figure 15. The average elongation at the flange was 1.6%, and the at the bottom were 317.3 MPa and 345.2 MPa, respectively. The elongations of different samples at average elongation at the bottom was 2.1%. The comparison indicated that the mechanical properties different positions are shown in Figure 15. The average elongation at the flange was 1.6%, and the at the bottom were poorer than those at the flange. Specifically, the yield strength was about 10.5% average elongation at the bottom was 2.1%. The comparison indicated that the mechanical properties lower, the tensile strength was about 8.3% lower, and the elongation was about 2.3% lower. The at the bottom were poorer than those at the flange. Specifically, the yield strength was about 10.5% molten metal filled the mold through the bottom of the cover, and it was easy to form shrinkage holes lower, the tensile strength was about 8.3% lower, and the elongation was about 2.3% lower. The due to lacking of feeding. When stretching, these shrinkage holes would evolve into a crack molten metal filled the mold through the bottom of the cover, and it was easy to form shrinkage holes nucleation site, which would reduce the strength of the forming parts. due to lacking of feeding. When stretching, these shrinkage holes would evolve into a crack Figure 13.13. MetallographicMetallographic photographs photographs of of the the samples samples at di atﬀ erentdifferent position position of forming of forming parts: (parts:a) ﬂange; (a) nucleation(flange;b) side ( wall;b )site, side ( cwhich )wall; bottom. ( cwould) bottom. reduce the strength of the forming parts.

The yield strength and tensile strength of the samples taken from the flange and the bottom are shown in Figure 14. The average yield strength and the average tensile strength at the flange were 342.8 MPa and 367.3 MPa, respectively. The average yield strength and the average tensile strength at the bottom were 317.3 MPa and 345.2 MPa, respectively. The elongations of different samples at different positions are shown in Figure 15. The average elongation at the flange was 1.6%, and the average elongation at the bottom was 2.1%. The comparison indicated that the mechanical properties at the bottom were poorer than those at the flange. Specifically, the yield strength was about 10.5% lower, the tensile strength was about 8.3% lower, and the elongation was about 2.3% lower. The molten metal filled the mold through the bottom of the cover, and it was easy to form shrinkage holes due to lacking of feeding. When stretching, these shrinkage holes would evolve into a crack nucleation site, which would reduce the strength of the forming parts. FigureFigure 14. 14.Strength Strength of of the the samples samples in in di ﬀdifferenterent positions: positions: (a) ( ﬂange;a) flange; (b )(b bottom.) bottom. Figure 14. Strength of the samples in different positions: (a) flange; (b) bottom.

Figure 15. Elongation of the samples in diﬀerent positions: (a) ﬂange; (b) bottom. Figure 14. Strength of the samples in different positions: (a) flange; (b) bottom. Figure 15. Elongation of the samples in different positions: (a) flange; (b) bottom. Figure 15. Elongation of the samples in different positions: (a) flange; (b) bottom.

Figure 15. Elongation of the samples in different positions: (a) flange; (b) bottom.

Metals 2020, 10, 266 14 of 16 Metals 2020, 10, x FOR PEER REVIEW 14 of 16 3.7. Analysis of Tensile Fracture Behavior 3.7. Analysis of Tensile Fracture Behavior Figure 16 shows the fracture morphology of the samples formed under the following process Figure 16 shows the fracture morphology of the samples formed under the following process conditions: the movement rate of the main punch was 0.08 m/s, the pouring temperature was 730 C, conditions: the movement rate of the main punch was 0.08 m/s, the pouring temperature was 730 °C,◦ the initial mold temperature was 250 C, and the feeding distance was 2 mm. As shown in Figure 16a, the initial mold temperature was 250 °C,◦ and the feeding distance was 2 mm. As shown in Figure 16a, a small number of cleavage steps and numerous pores could be found in the fractured morphology due a small number of cleavage steps and numerous pores could be found in the fractured morphology to incomplete feeding, which caused the molten metal to shrink during solidiﬁcation. Considerably due to incomplete feeding, which caused the molten metal to shrink during solidification. equiaxed dimples and microscopic pores could be observed at position A as shown in Figure 16b. Considerably equiaxed dimples and microscopic pores could be observed at position A as shown in At position B, there were a large number of tearing dimples caused by the fracture across the grain. Figure 16b. At position B, there were a large number of tearing dimples caused by the fracture across When the sample was stretched and deformed, some holes formed. As the deformation progressed, the grain. When the sample was stretched and deformed, some holes formed. As the deformation the micro-holes expanded at the minimum cross-sectional area of the necking, and cracks formed progressed, the micro-holes expanded at the minimum cross-sectional area of the necking, and cracks gradually. With the cracks expanding, more holes were congested, and a ﬁbrous region formed in formed gradually. With the cracks expanding, more holes were congested, and a fibrous region the fracture morphology. When the cracks approached the free surface of the sample, the sample formed in the fracture morphology. When the cracks approached the free surface of the sample, the underwent plastic deformation, which resulted in instability and rapid propagation of the cracks. sample underwent plastic deformation, which resulted in instability and rapid propagation of the Above all, the cover samples demonstrated a ductile intergranular fracture. cracks. Above all, the cover samples demonstrated a ductile intergranular fracture.

Figure 16. SEMSEM micrographs ofof tensiletensile fracturefracture specimen:specimen: (a(a)) whole whole area; area; ( b(b) position) position A; A; (c ()c position) position B; B;(d )(d position) position C. C.

4. Conclusions Conclusions A metal liquid quantitative feeding system wa wass designed and improved, which could achieve thethe quantitative transport transport of of molten molten metal metal in in a fully enclosed environment, improving improving the the accuracy accuracy of the molten metal pouring, reducing the oxide inclusions defects, and improving the quality of formingforming parts.parts. TheThe forming forming process process of theof Althe alloy Al coveralloy wascover designed, was designed, and the processand the was process conducted was conductedon the novel on quantitative the novel quantitative feeding device feeding of moltendevice of metal. molten The metal. optimum The optimum punch movement punch movement rate was rate0.08 was m/s obtained0.08 m/s viaobtained the numerical via the numerical simulation simulation carried out carried using software out using ProCAST. software When ProCAST. the pouring When thetemperature pouring wastemperature 730 ◦C, the was initial 730 mold°C, the temperature initial mold was temperature 250 ◦C, and thewas feeding 250 °C, distance and the was feeding 2 mm, distancewhereas was the average2 mm, whereas yield strength the average and the yield average strength tensile and strength the average at the tensile ﬂange strength were 342.8 at the MPa flange and were 342.8 MPa and 367.3 MPa, respectively. The average yield strength and the average tensile strength at the bottom were 317.3 MPa and 345.2 MPa, respectively. The average elongation at the

Metals 2020, 10, 266 15 of 16

367.3 MPa, respectively. The average yield strength and the average tensile strength at the bottom were 317.3 MPa and 345.2 MPa, respectively. The average elongation at the ﬂange was 1.6%, and the average elongation at the bottom was 2.1%. When the sample was stretched and deformed, the main fracture mode was ductile intergranular fracture.

Author Contributions: Conceptualization, G.C. and Z.D.; methodology, Y.Q.; software, L.C.; validation, Y.Q., G.C. and Z.D.; formal analysis, K.L.; investigation, C.L.; resources, Z.D.; data curation, G.C.; writing—Original draft preparation, Y.Q.; writing—Review and editing, Y.Q.; visualization, C.L.; supervision, L.C.; project administration, L.C.; funding acquisition, G.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China, grant number 51875121 and Natural Science Foundation of Shandong Province, grant number ZR2019MEE039. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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