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International Journal of Cast Metals Research

ISSN: 1364-0461 (Print) 1743-1336 (Online) Journal homepage: http://www.tandfonline.com/loi/ycmr20

Numerical simulation and experimentalinvestigation of a thin-wall magnesium alloycasting based on a rapid prototyping core makingmethod

Yu Fu, Han Wang, Chen Zhang & Hai Hao

To cite this article: Yu Fu, Han Wang, Chen Zhang & Hai Hao (2017): Numerical simulationand experimental investigation of a thin-wall magnesium alloy casting based on a rapidprototyping core making method, International Journal of Cast Metals Research, DOI:10.1080/13640461.2017.1365477

To link to this article: http://dx.doi.org/10.1080/13640461.2017.1365477

Published online: 30 Aug 2017.

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InternatIonal Journal of Cast Metals researCh, 2017https://doi.org/10.1080/13640461.2017.1365477

Numerical simulation and experimental investigation of a thin-wall magnesium alloy casting based on a rapid prototyping core making method

Yu Fu, Han Wang, Chen Zhang and Hai Hao

Key laboratory of solidification Control and Digital Preparation technology (liaoning Province), Department of Materials science and engineering, Dalian university of technology, Dalian City, China

ABSTRACTThe preparation process of complex thin-wall magnesium alloy castings based on numerical simulation and rapid prototyping core making method is presented in this paper. The filling and solidification processes were simulated using ProCAST to optimise the gating system. The simulation results showed that compared with bottom gating system, vertical slitting gating system equipped with a slot ingate and slag tank contributed to provide a stable filling pattern and eliminate casting porosity. Additionally, Profile Failure-based Rapid Prototyping (PFRP) technology was adopted to obtain sand cores due to its complexity. Mg-4.2Zn-1.7Ce-0.8Zr-0.2Ca-0.2Sr (wt.%) alloy was selected as the alloy material because of the excellent fluidity and flame retardancy. JMatPro was utilized to calculate thermo-physical properties of the alloy to ensure the simulation validity. Casting experiments were conducted, indicating that numerical predictions agreed well with experimental results. Sound thin-wall accessory gearbox casting was obtained based on the optimised gating system and PFRP-fabricated sand cores.

1. Introduction

The current adaptation of energy and fuel conserva-tion is leading to a compelling demand for magne-sium alloys due to their high strength to weight ratio, excellent castability and recyclability [1]. The ability to obtain high quality magnesium alloy castings has a fundamental influence on the further development of the transportation and aerospace industries [2]. Among manufacturing methods for high-performance aero-space components, gravity casting provides the most direct production route to get near net shape from liquid metal and offers manufacturing flexibility in terms of size and shape complexity [3]. The gravity castings make use of the gravitational force in feeding molten metals into a mould cavity through the gating and running system.

Casting defects, such as porosity, entrapment of gas or oxide film, cold shut and misrun, which are caused by improper mould filling, are closely related to surface tur-bulence [4]. Recently, there was an increasing demand to produce thin-wall and curved-shape casting products such as transmission housings and accessory gearboxes. Because of the pronounced oxidation tendency and the low density of molten magnesium alloy, the gating sys-tem need to be designed to minimize the entrance of oxides on the surface of the molten metal into the cast-ing, prevent turbulence in the mould filling and ensure sequential solidification of magnesium alloy castings [5]. Therefore, a well-designed gating system is crucial to secure the casting quality. The conventional trial and error approach to designing casting process based on

© 2017 Informa uK limited, trading as taylor & francis Group

KEYWORDSnumerical simulation; thin-wall magnesium alloy casting; gating system; profile failure-based rapid prototyping (PfrP) technology; gravity casting; Mg-Zn-Ce-Zr-Ca-sr alloy

ARTICLE HISTORYreceived 3 May 2017 accepted 4 august 2017

CONTACT hai hao haohai@dlut.edu.cn

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validated via temperature measurements. Although the example is given in an accessory gearbox casting, the fundamental methodology is applicable to the manu-facture of magnesium alloy casting products.

2. Model description

2.1. Mathematical models

The FEM simulation software ProCAST™ is applied to simulate the mould filling and solidification behaviours of the accessory gearbox casting and identify the opti-mal gating system before conducting experiments. The governing equations describing fluid dynamics, heat and mass transfer phenomena in gravity casting include mass, momentum and energy conservation equations [19]. The molten metal is assumed to be incompressible and Newtonian. The continuity equation, the Naiver-Stokes equation and the energy conservation equation are written as [20]:

The volume of fluid (VOF) method, which was developed by Hirt and Nichols, is commonly used to track free sur-faces [21]. In this technique, a function F is defined to represent the fractional volume of the cells occupied by fluid. The fractional volume of fluid in each cell can be determined by solving the following equation:

Since the melt liquid and gas are immiscible, F corre-sponds to values of 1 in liquid and 0 in gas. When the two fields are separated by the interface, F has a value between 0 and 1 (0 < F < 1).

Based on the ProCast User Manual, the ‘POROS = 1’ model can couple micro- and macro-porosity and pile shrinkage, which is applied to gravity casting and die

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experimental investigations is costly and time consum-ing [6]. Numerical simulation technology has become a powerful tool for predicting and controlling the cast-ing process, thereby reducing the extend of required experimental studies. A series of studies has reported on the simulation of mould filling and solidification stages, based on which defect regions can be further predicted [7–9]. Numerical optimisation of mould filling patterns through improvement of gating system param-eters becomes significant for producing thin-wall mag-nesium alloy castings [10,11], and the volume of fluid (VOF) method are commonly utilized to analyse free sur-face problems in the modelling of mould filling [12,13]. Furthermore, casting solidification simulation provides various physical phenomena such as time-temperature contours and evolution of latent heat, which contributes to identify shrinkage porosity effectively [14,15].

In recent years, there has been an increasing inter-est in the Rapid Prototyping (RP) Technology as it pro-vides an opportunity to fabricate complex-shaped and customized parts [16]. RP-fabricated parts can be obtained using alloying powders in a timely manner [17]. Nevertheless, the application of RP-fabricated parts is still very limited, mainly due to their low surface roughness, the anisotropic microstructure and porosity defects [18]. Recently, Profile Failure-based Rapid Prototyping (PFRP) method for precoated sand was widely used in quick casting owing to its unique ability to create complicated sand cores. However, studies on using PFRP method to achieve quick casting of thin-wall magnesium alloy cast-ings are quite limited.

The present study is carried out to demonstrate how the combined application of numerical optimisation techniques and rapid prototyping core making method can be used to prepare the magnesium alloy castings with complex-shaped thin-wall structures. Figure 1 describes an experimental procedure detailing the meth-odological approach used in this work for accessory gear-box castings manufacture. It begins with the design of the model of accessory gearbox. Then, mould filling and solidification processes of the accessory gearbox casting are simulated with the ProCAST, which attempts to opti-mise the gating and runner systems to eliminate the cast-ing defects. On the basis of the optimised gating system, casting experiments combined with the PFRP-fabricated core pattern method are carried out to prepare the thin-wall magnesium casting. Finally, the numerical model is

Figure 1. an experimental procedure detailing the methodological approach used in this work.

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INTERNATIONAL JOURNAL OF CAST METALS RESEARCH 3

casting. Shrinkage porosity field is employed to ana-lyse the shrinkage porosity in a casting. When POROS is activated, it can visualize the contour called ‘Shrinkage porosity’. Values corresponding to a level below 0.01 are considered as micro-porosity and those above 0.01 are considered as macro-porosity [19].

2.2. Geometric and finite element models

The entity model of the accessory gearbox component is designed by means of UG software. Figure 2(a) depicts the three-dimensional (3D) model of the casting with the contour size of 350 mm × 190 mm × 100 mm and varying wall thickness of 2~20 mm. In this experiment, bottom filling of the mould is employed on the accessory gear-box casting. The preliminary design with bottom gating system is shown in Figure 2(a). The detailed parts of the gating and runner systems are labelled and the top riser is essential to ensure to vent gas. The complex geometry of the casting requires special attention for mesh gener-ation. Three-dimensional 4-node linear tetrahedral ele-ments (C3D4) are adopted in the finite element model. The enmeshed solid model of the preliminary design is presented in Figure 2(b), consisting of 104080 nodes and 446027 tetrahedral elements. The geometry is meshed by using UG automatically and convert the .unv file into the .ideas. Then, the preprocessor module of ProCAST read the .ideas file as the finite element model.

2.3. Thermo-physical properties of materials

An alloy with tailored properties plays an important role in producing defect free and sound thin-wall castings. Based on the authors’ previous study, Mg-4.2Zn-1.7Ce-0.8Zr-0.2Ca-0.2Sr magnesium alloy (all the alloy compo-sitions in wt.% except otherwise stated) is used in this study for the preparation of accessory gearbox casting due to its excellent fluidity and flame retardancy. In addi-tion, to ensure the validity of numerical simulation, the thermophysical parameters of the alloy are calculated by the multi-platform software program JMatPro [22], and

then imported into the material database of ProCAST. The mould material is assigned to Resin Bonded Sand accord-ing to the material database of ProCAST software. Table 1 presents the thermophysical parameters of the casting alloy and the mould materials used in the simulation.

2.4. Initial and boundary conditions

The initial and boundary conditions used in the pres-ent simulation are listed in Table 2, which were defined according to the actual experiment condition.

3. Numerical simulation results and analysis

3.1. The preliminary design

Figure 3 presents the representative results during the mould filling and solidification processes for the preliminary design. The highest temperature in the temperature scale is set as 730 °C which is the pouring temperature. Figure 3(a) indicates that in the early stage of mould filling, the molten metal flows into the mould cavity through its runner under the action of gravity, accompanied by the turbulence and splash behaviour as marked in circles. As the filling process advances, as shown in Figure 3(b), it is observed that the occurrences of the fluctuation and recirculation will result in porosi-ties and the entrapment of gas and oxides. Furthermore, the liquid level rises gradually and reaches up to the top riser until the end of filling (Figure 3(c) and (d)). The sim-ulated filling time is about 1.60 s. The temperature dis-tribution during the solidification process is presented in Figure 3(e) and (f ). The melt solidification begins from the thin wall portions at the bottom of the casting and exhibits gradual solidification. The final solidification region is located in the top riser and the runner.

3.2. The modified design

In consideration of the above results, besides, mag-nesium alloy is susceptible to react with oxygen and

Figure 2. (a) the solid model of the casting with bottom gating system (units: mm) and (b) the finite element mesh of the casting.

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Tabl

e 1.

 the

rmo-

phys

ical

pro

pert

ies o

f the

cas

ting

allo

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ould

mat

eria

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Varia

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Mou

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ater

ial

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tem

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ture

(°C)

635

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535

–D

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ty (g

 cm

−3 )

1.59

ther

mal

con

duct

ivity

(W m

−1  K

−1 )

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and the final solidification regions occur in the riser, the runner and the slag tank. As discussed above, it can be demonstrated that the optimised gating system makes the mould filling more stable than that in the bottom gating system. Consequently, employing the slot ingate is an effective method to eliminate the turbulent flow, reducing gas and oxides entrapment. Additionally, the vertical slitting gate system equipped with a slag tank is conducive to enhance the inclusions’ floating and decrease the inclusions mixed in the casting.

3.3. Shrinkage porosity analysis

Figure 6(a) gives the simulated result for the prediction of shrinkage porosity in the casting with bottom gating

nitrogen existing the atmosphere [23]. To alleviate the problem of liquid splash and reduce the entrapment of gas and oxides, the gating system is modified to the vertical slitting gate system. Figure 4 describes the detailed geometry of the slot gate system. Simulation on the improved design is performed under the identical numerical setup conditions of the preliminary design.

Figure 5 shows the mould filling and solidification processes of the casting with the slot gate system. During the initial stage of the mould filling, the molten metal flows into the cavity along the slot ingate and fills the mould cavity smoothly, accompanied by a mild fluctu-ation, as shown in Figure 5(a) and (b). As the filling pro-cess proceeds, the phenomena of the recirculation and fluctuation slowly fade away with rising the liquid level, as depicted in Figure 5(c). Figure 5(d) indicates that the filling process is completed, and the total filling time is approximately 2.01  s which is longer than that of the preliminary design. It can be seen from Figure 5(e) and (f ) that the casting exhibits the sequential solidification

Table 2. Initial and boundary conditions used in the simulation.

Parameter (unit) ValueInitial temperature (°C) Casting 730

Mould 200Boundary definitions (heat transfer coeffi-

cient) (W m−2 K−1)Casting-mould 500

Mould-air 10ambient temperature (°C) 20Casting velocity (m s−1) 0.5

Figure 3. simulated filling and solidification processes in the bottom gating system at different time: (a) t = 0.29 s; (b) t = 0.51 s; (c) t = 0.97 s; (d) t = 1.60 s; (e) t = 56.8 s; (f ) t = 81.8 s.

Figure 4.  the solid model of the casting with the slot gate system (units: mm).

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mould wall will bridge by the cooling effect from the mould and then the feeding channels of the liquid metal flowing into the portion will be blocked. In addition, because of the obvious occurrences of the turbulence and gas entrapment induced by the bottom gating sys-tem, the solubility of gas is reduced significantly close to

system. The intensity of porosity is graded in colour. It can be seen that shrinkage porosity accumulates in the top riser and runner, besides it can be found of the casting porosity defect near the ingate, as arrowed in Figure 6(a). The reason can be attributed to the fact that, as the solidification proceeds, the dendrites close to the

Figure 5. simulated filling and solidification processes in the vertical slitting gate system at different time: (a) t = 0.65 s; (b) t = 0.95 s; (c) t = 1.25 s; (d) t = 2.01 s; (e) t = 84.5 s; (f ) t = 182 s.

Figure 6. Comparison of the shrinkage porosity simulations in different gating systems: (a) the bottom gating system; (b) the slot gate system.

Figure 7. schematic illustration of the PfrP manufactured the complex-shaped core of the gearbox casting.

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The complex-shaped core is obtained by stripping waste materials (see Figure 7(d)). The effective laser power and scanning velocity are key factors which greatly influence the final core quality. Moreover, the layer thickness can be adapted depending upon the desired geometry to achieve both high fabrication quality and short build time. The process parameters are listed in Table 3. It is worth noting that the alcohol-based flame retardant coating is used to prevent the casting-mould/core reac-tions and oxidative combustion of the magnesium alloy melt. Figure 8 presents the desired complicated core and sand moulds of the accessory gearbox casting. Figure 8(c) and (d) present the sand moulds of vertical slitting gate system and bottom gating system, respectively. Take the production efficiency and large-size moulds into account, numerical control engraving technology is adopted to manufacture the sand moulds.

4.2. Casting experiments

It is a common way to validate the simulation results by experiments. The alloy used in this study has a nominal composition of Mg-4.2Zn-1.7Ce-0.8Zr-0.2Ca-0.2Sr (wt.%). Casting parameters used in the experiments are the same as those in the simulation. The alloys were melted in an electrical resistance furnace using a mild steel crucible and protected by 2 wt.% RJ-2 flux additions. The mould cavity was made of resin bonded sand and preheated to 200 °C. The pouring temperature was designed to be 730 °C. In order to measure the temperature history, a K-type thermocouple was placed in the sprue base where the coordinate of the temperature measuring point was (–202, 90, –105), and the temperature-time curve was

the mould wall, forming the casting porosity. Figure 6(b) shows the simulated result for the prediction of shrink-age porosity in the casting with the slot gate system. It indicates that shrinkage porosity aggregates in the top riser, the runner and slag tank. Thus, the slitting gate sys-tem successfully avoids the existence of the shrinkage porosity in the casting.

4. Experimental

4.1. Complicated core fabrication

Taking the complexity of the core configuration, Profile Failure-based Rapid Prototyping (PFRP) is utilized to fab-ricate the sand core. In this study, PFRP machine (Zhu Jie CAD/CAM RP Tester) is used for fabrication of the complex structure cores. Figure 7 illustrates the process schematic of the PFRP fabricating the complex-shaped core of the gearbox casting. The coated sand begins in powder form with the particle size of approximately 250 μm. PFRP uses a mobile laser head to form the 2D cross section of the desired 3D geometry. The laser beam is capable of emitting photons to a focal point located on a substrate and the collision of these photons with the surface produce enough heat to form a failure profile in each layer. The parting surfaces are built up in this layer-by-layer process (see Figure 7(b)). Then the coated sand solidified under heating conditions (see Figure 7(c)).

Table 3. Process parameters used in the PfrP machine.

Effective laser power (W)

Scanning velocity (mm s−1)

Layer thickness (mm)

100 300 0.30

Figure 8. the desired complicated core and sand moulds of the gearbox casting: (a) the overall size of the complex-shaped core; (b) the core coated by the alcohol-based flame retardant coating; (c) the mould of vertical slitting gate system without painting the alcohol-based coating; (d) the mould of bottom gating system with painting the alcohol-based coating.

Figure 9. Photographs of the casting with vertical slitting gate system.

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in liberated latent heat results in the rapid decrease of temperature. However, it can be found that the meas-ured result is smaller than that calculated from ProCAST simulation. It may be caused by the assumption and boundary conditions in this work.

5. Conclusions

(1) ProCAST was used to optimise the gating sys-tem of a thin-wall magnesium alloy casting. Bottom gating system and vertical slitting gate system were modelled and simulated numeri-cally. Mg-4.2Zn-1.7Ce-0.8Zr-0.2Ca-0.2Sr (wt.%) alloy was selected as the cast alloy. Thermo-physical properties of the magnesium alloy were calculated by JMatPro.

(2) Compared with the bottom gating system, the optimised gating system made the filling process more stable, which was beneficial to eliminate the entrapment of gas and oxides. The simulations of shrinkage porosity indi-cated that the vertical slitting gate system con-tributed to reduce shrinkage porosity in the

acquired by using a paperless recorder. Figure 9 presents the photographs of the accessory gearbox casting with the optimised gating system. It indicates that the cast-ing with vertical slitting gate system is structural inte-grated and no porosity defect is observed in the casting. However, for the casting with bottom gating system, the porosity defect regions are found in the casting, as circled in Figure 10. It is found that the casting defect of porosity lies on the edge of the casting, which corresponds to the simulation results of shrinkage porosity. The casting porosities are formed due to the escape of dissolved and wrapped gases during the mould filling, following the significant drop of solubility in the solidification process of casting procedure. It is concluded that the porosity defects are eliminated effectively by the optimised gat-ing system under the same casting conditions.

Figure 11 shows the comparison of temperature-time curves between the experimental measurement and the simulation result. From Figure 11, it can be concluded that the numerical prediction agrees well with the exper-iment. The temperature keeps approximately 618 °C at the early stage of solidification as a result of the libera-tion of latent heat of freezing during the phase trans-formation. As the solidification proceeds, the reduction

Figure 11. (a) the schematic diagram of the temperature measuring point and (b) the comparison of the predicted and measured cooling curves.

Figure 10. Photographs of the casting porosity defect of the casting with bottom gating system.

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the National High-tech Research and Development Program of China [grant number 2015AA042502].

References

[1] Liu Y, Ren H, Hu WC, et al. First-principles calculations of strengthening compounds in magnesium alloy: a general review. J Mater Sci Technol. 2016;32:1222–1231.

[2] Pan FS, Yang MB, Chen XH. A review on casting magnesium alloys: modification of commercial alloys and development of new alloys. J Mater Sci Technol. 2016;32:1211–1221.

[3] Ravi B. Metal casting: computer-aided design and analysis. New Delhi: PHI Learning Private Limited; 2005.

[4] Hong CH. Computer modelling of heat and fluid flow in materials processing. London: IOP Publishing Ltd; 2004.

[5] Luo AA. Magnesium casting technology for structural applications. J Magn Alloys. 2013;1:2–22.

[6] Fu PX, Kang XH, Ma YC, et al. Centrifugal casting of TiAl exhaust valves. Intermetallics. 2008;16:130–138.

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[11] Sun ZZ, Hu H, Chen X. Numerical optimization of gating system parameters for a magnesium alloy casting with multiple performance characteristics. J Mater Process Technol. 2008;199:256–264.

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[15] Jeong SI, Jin CK, Seo HY, et al. Mold structure design and casting simulation of the high-pressure die casting for aluminum automotive clutch housing manufacturing. Int J Adv Manuf Technol. 2016;84:1561–1572.

[16] Murr LE, Gaytan SM, Ramirez DA, et al. Metal fabrication by additive manufacturing using laser and electron beam melting technologies. J Mater Sci Technol. 2012;28:1–14.

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casting effectively and was considered as the optimised design.

(3) Complicate sand cores were fabricated by PFRP method and sand moulds were produced by the numerical control engraving technology. The time-temperature characteristic of the casting experiment illustrated that the simula-tion results matched the experimental results very well. Sound accessory gearbox casting was obtained based on the optimised gating system.

Highlights

(1) The complex-shaped sand core is fabricated by the Profile Failure-based Rapid Prototyping (PFRP) technology in this paper.

(2) The vertical slitting gating system of the acces-sory gearbox casting is proposed based on the mould filling and solidification simulation.

(3) Integrated thin-wall accessory gearbox casting is obtained based on the optimised gating sys-tem and the PFRP-manufactured core.

(4) Mg-4.2Zn-1.7Ce-0.8Zr-0.2Ca-0.2Sr (wt.%) alloy is selected for the casting alloy due to its excel-lent fluidity and flame retardancy.

Nomenclature

V velocity vector

Fg gravity force vectorP pressurec specific heatQ an internal power sourceT temperaturet timeu, v, w velocity components∇ divergence∇2 laplace operator

Greek symbols

λ thermal conductivity

ν kinematic viscosityρ density

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by the National Key Research and Development Program of China [grant number 2016YFB0701204];

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[21] Hirt CW, Nichols BD. Volume of fluid (VOF) method for the dynamics of free boundaries. J Comput Phys. 1981;39:201–225.

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