2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-029 (12 pages)

Prediction of Entrained Oxide Inclusions and Oxide Induced Defects During Directional Flow in Aluminum

Colin D. Ridgeway The Ohio State University, Columbus, Ohio

Keith Ripplinger Honda Engineering North America, Anna, Ohio

Duane Detwiler Honda R&D Americas, Raymond, Ohio

Alan. A. Luo The Ohio State University, Columbus, Ohio

Copyright 2020 American Society little as 5 seconds at 1292F (700C) the aluminum oxide, 6 ABSTRACT or Al2O3, layer can be as thick as 24nm. The newly formed oxide film often becomes entrained into the melt It is well known that the entrainment of double oxide during ladle transfer, adding charge material, or during bifilms formed during flow and solidification has the pouring process. Alternatively, in alloys that contain detrimental effects on the mechanical properties of cast greater than 2% Mg, MgO oxides films can form on the aluminum products. Degassing, filtration and metal flow surface and are equally stable.5 Once entrained, the oxides design are generally employed to reduce the presence of are extremely difficult to remove, as the they are entrained defects, yet, oxide bifilms are still present in a thermodynamically stable and completely insoluble majority of and their locations are often unknown within the melt.4 Fluxing or degassing can be used to often leading to severe quality issues. In this work, a new remove a majority of the oxides that form on the surface, model, termed Oxide Entrainment Number (OEN), is however, during pouring, fresh oxides will reform on the proposed to predict the location and severity of bifilms surface of the melt pool as it is exposed to the and oxide induced defects in final cast structures. The atmosphere. These new oxides, often called ‘young model is developed by examining bifilms formation oxides,’ are then entrained as the molten material fills the events and tracks newly formed bifilms via free surface mold, making entrained oxide inclusions an unavoidable tracking. Simple geometry patterns promoting directional defect in aluminum castings.8 flow were designed to compare the defect location and severity in optimal vs sub-optimal flow conditions. The OXIDE FILM FORMATION & ENTRAINMENT proposed OEN model has been coupled with ProCAST As previously mentioned, molten aluminum will form a (defined as Software 1 from this point on in this paper) thermodynamically stable oxide film instantaneously and experimentally validated with aluminum A356 when it is exposed to an atmosphere. This oxide layer casting. does not harm the melt, and actually protects molten aluminum below the oxide film. However, when the Keywords: oxide inclusion, bifilm entrainment, aluminum surface of the melt is disturbed, the oxide film will fold casting, modeling, defect prediction over itself and the solid film will be entrained below the surface. This mechanism and other common entrained defects are described in Figure 1. The structure of the INTRODUCTION entrained inclusions consists of two solid layers of film separated by a void of air or gas trapped between them. Entrained oxide inclusions are some of the most common Due to the inability of the solids to re-bond, the inner and most detrimental defects that reside in castings with surfaces of the bifilms remain permanently unbonded and numerous studies citing a severe reduction in the tensile the void between them is permanently entrained.9 This strength, fatigue strength and ductility.1-5 The prevalence structure is most notably seen in Figure 1B & 1C. of oxide inclusions or bifilms in cast structures can be Conversely the outer portion of the bifilm is wetted and attributed to two factors: the rapid formation of oxides, acts as a substrate for nucleation and solidification. and the stability of the oxides within the melt. Whenever the melt is exposed to a gaseous environment such as air, The oxide entrainment mechanisms are simple in nature the molten aluminum will instantaneously react with the but are difficult to avoid. One of the most common ways air and form an oxide layer.6,7 Thiele has shown that in as for oxide films to become entrained below the surface is

Page 1 of 12 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-029 (12 pages) during movement or ladle transfer of the molten material. to pouring and are no longer present in the final casting. As the molten material is transferred for pouring, the Removal of old oxides is often done by drossing the surface is perturbed, and small waves form as seen in surface prior pouring, pouring through a filter, and using Figure 1A. As the waves crash back into the surface, the fluxing agents. If old oxides do enter the cast structure, it surface film folds over itself and the double sided film is often difficult to detect them non-destructively. structure of the bifilm is created while the momentum of Extremely large oxides will occasionally appear on x-ray the wave pulls the bifilm beneath the surface, effectively or micro computed tomography (micro-CT), however, entraining the structure. Bifilms can also be entrained by online inclusion detection techniques such as LiMCA are the addition of charge material to the melt. As additional rarely used in mass production of aluminum castings.10,11 material is added, the solid charge material will break the Thus, old oxides are often only identified after a surface oxide film and drag the film below the surface. component has failure and the fracture surface presents During this process the solid film from the surface will the entrained oxide. tend to wrap itself around the charge material and fold into a network like structure, increasing the surface area Young oxides are much more prevalent in final cast of the defect. structures and are much more difficult to detect. The current maximum resolution for micro-CT is ~400 nm The last major mechanism in which oxides are entrained making it nearly impossible to locate all entrained young occurs during the pouring stage when the molten metal oxides. Furthermore, the same issue as with old oxides enters the mold. As the molten metal exits the crucible the occurs, micro-CT cannot be used for online oxide previously non-oxidized material below the surface is detection in mass production and oxides are often only exposed to the environment. The newly exposed material detected in a destructive manner. forms fresh young oxides which are then entrained due to turbulent flow as the molten material folds over itself Both young and old oxides also have large affinity to inside the mold. Campbell has shown that turbulent flow induce other forms of defects including gas porosity and occurs in aluminum alloys when the gate velocity exceeds shrinkage porosity. Campbell argues that the pressure 0.5m/s highlighting the need for proper molding design.9 required for gas pores to form either homogeneously or heterogeneously is so great that the practicality of such nucleation rarely occurs.4,5 Instead the non-wetted inner of the bifilm offers a perfect initiation site for the excess gas to reside with limited formation energy. Excess gas is able to permeate the solid and inflate or unfurl the bifilm as shown in Figure 1 (Area C). If sufficient gas is within the melt, the bifilm can be completely unfurled into a spherical morphology and give the appearance of a gaseous pore, though the surface area of the pore will be entirely oxidized. Similarly shrinkage porosity requires a critical drop in the metallostatic feeding pressure, and the easiest initiation site for shrinkage is in the presence of a bifilm. As the melt solidifies the dendrites will begin to pull away from one another. Thus the presence of the crack-like structure of the bifilm offers a site for shrinkage initiation and bifilm is pulled apart and creates an irregular shaped void. If this process were to occur without the presence of a bifilm, there would be a significant requirement of formation energy as the solid would need to physically pull apart the solidifying Figure 1: Various oxide entrainment mechanisms where black lines indicate oxidized surfaces (A) Wave structure. entrainment (B) A folded over bifilm (C) Gas precipitation within the bifilm structure. (D) An internal In the case that excess gas within the melt or casting is pore with an oxidized surface and (E) A bubble trail. able to heterogeneously nucleate near the mold wall, it is still likely that this pore will contain some level of oxygen OXIDE DETECTION AND PREVENTION inside. The oxygen trapped in the pore with will then Entrained oxide inclusions are often characterized by their instantaneously oxidize the surface area of the pore as size and are termed either Young or Old Oxides. Old described by Thiele and shown in Figure 1 (Area D). oxides range from 1-5mm in thickness and form when the After oxidation, the gas pore will then float towards the molten surface is exposed to elevated temperatures for an top of the melt or casting due to its low density and in its extended period of time. Young oxides range from 10 - wake leave a bubble train as shown in Figure 1E. The 500nm and form during the pouring process.5 In general, bubble trail left behind will also be made up of small old oxides can be completely removed from the melt prior defects with included surface areas.

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into the fluid and are tracked through solidification. The The culmination of the entrained oxides as well as each of created particles range from 25-31μm which is more these oxide induced defects can then lead to a greater realistic, though it will not capture or accurately depict all volume of the defects within the casting. The greater entrained oxides. The location accuracy of the OFEM was volume of defects will then lead to a further degradation experimentally validated, however, the quantity is still in of mechanical properties as Caceres has shown in question for the reasons mentioned above. Further, the previous publications.12 computation time is greatly increased with increasing resolution for industrial castings, limiting the usage of this For these reasons, a great deal of research has been model. conducted to create standards for mold design to prevent/minimize oxide entrainment.4,9,13–15 Nonetheless, Overall, determining whether a casting will contain oxides are entrained in almost all castings with the final entrained defects is relatively easy; however, predicting location of the entrained oxides and oxide induced defects the locations of the entrained oxide defects is extremely within the casting often unknown. Therefore, a new difficult. Moreover, no model exists that is able to method for predicting oxide entrainment is needed for the accurately predict both the quantity and locations of design and manufacturing high-quality aluminum entrained oxide defects while remaining computationally castings. efficient.

EXISTING MODELING TECHNIQUES MODEL METHODOLOGY Several models have been developed to solve the issue of locating entrained oxides during pouring and mold-filling, In order to create a computationally efficient model that most of which are based on the basic principles of can be used to predict the location and severity of entrainment established by Campbell.9 These principles entrained defects and eventual oxide induced defects, the include the liquid gate velocity, Weber Number, Froude fundamentals set forth by Campbell and others in Number and the free surface of the fluid. Each of these literature must be re-examined. principles or numbers have often been used as a criterion or as a “go-no-go” gauge to whether oxide entrainment The first parameter to examine is the Weber number, We, occurs or not, with no prediction on entrainment quantity which is defined in Equation 1 where is the material or locations. density, is the liquid velocity, is the hydraulic radius of the wall thickness, is the liquid surface𝜌𝜌 tension, is The first breakthrough in determining the locations of the acceleration𝑣𝑣 due to gravity and𝑙𝑙 h is the characteristic oxide entrainment came from tracking the free surface of wave height. The We is𝜎𝜎 a dimensionless number and 𝑔𝑔is the liquid during pouring and mold-filling. Since, it is used to describe when a fluid transfers from laminar to known that only the free surface can oxidize, the free turbulent flow. The We is dominated by the gate velocity surface can be followed through solidification, and the and the wall thickness of the casting. An increase in final location of the defects can be determined. Smoothed velocity results in an increase in the amount or turbulence Particle Hydrodynamics (SPH) was one of the first and a larger channel width provides more room for numerical methods to employ this theory for oxide defect sloshing and increases the number of entraining events, prediction. SPH functions by discretizing the liquid into thus a larger velocity and wall thickness result in greater spherical particles that are able to simulate the pouring turbulence. process. During pouring, only the particles that are on the free surface and exposed to the environment are oxidized = Eqn. 1 and grow at a constant prescribed rate. The oxide growth 2 𝜌𝜌𝑣𝑣 𝑙𝑙 rate is based on a constant oxidation rate and is then 𝑊𝑊𝑊𝑊 𝜎𝜎 followed through solidifcation.16 Unfortunately, this This dependence on velocity falls in line with Campbell’s method has a major drawback due to the resolution or size assertion that there is a critical velocity above which of the particles which ranges from 5-12 mm.17–19 This entrainment will occur. However, unlike the critical resolution is several orders of magnitude larger than the velocity criterion, the We accounts for both the casting young oxides and this method will not capture the true geometry and velocity and better describes the mold- size of the entrained oxides; likely over predicting the filling conditions. The We is thus more insightful than the amount of oxide present. velocity criterion, but it only can be used to determine when entrainment will occur, not where the entrainment A more rigorous model for determining the final locations will occur. of entrained oxides was developed by Reilly et al. and is called the Oxide Film Entrainment Model (OFEM).20 This To determine where the entrainment events occur, the free approach uses a volume of fluid technique in combination surface must be considered. As described previously, with Boolean logic to determine when the fluid flow is oxidation of the melt only occurs on the free surface and turbulent enough for entrainment to occur. When if it can be tracked, it can provide information on the entrainment does occur, particles are created and inserted location of the entrained defects. Thus, by combining the

Page 3 of 12 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-029 (12 pages) free surface with the We, the location at which entrainment events occur can be determined. However, as = Eqn. 4 mentioned previously, oxides vary in size and thickness, 𝑒𝑒 𝑥𝑥 ̇ 𝑊𝑊 𝑂𝑂 𝑇𝑇 with the thickness increasing with exposure time. 𝛷𝛷 𝐺𝐺 3 Consequently, an oxide severity term was developed and The units of Eqn. 4 are equivalent to [m ] and correlate to is shown in Eqn. 2 where is the free surface, and t is the the total volume of entrained oxides present. This is reasonable considering each term has a specific amount of time the free surface has been exposed to the 𝑠𝑠 environment. Now, the location𝐹𝐹 of the entraining event contribution to the determination of the total volume of and the severity, or relative size, of the oxide entrained entrained defects. The oxide severity term in Eqn. 2 tracks are known, but unfortunately the units of the combined the free surface which is only entrained when the We term from Eqn.1 and Eqn. 2 is [m2s] which does not indicates turbulent flow and provides the location of the provide a physical meaning and cannot be validated. entrainment. The final term is the solidification rate which correlates to the number of pores in the system. When

these values are combined as in Eqn. 4, the total volume = Eqn. 2 of entrained defects is calculated. Equation 4 provides a

𝑥𝑥 𝑠𝑠 computationally efficient way to follow oxide entraining Aside𝑂𝑂 𝐹𝐹from∗ 𝑡𝑡 oxides, the most common defect in cast events and the location of entrained defects. This can be structures is the presence of shrinkage and gas porosity. run with common commercial codes, such as Software 1, Shrinkage porosity results from a lack of feeding of MAGMA, or FLOW-3D. molten material that occurs during solidification. Limited feeding will result in insufficient pressure and small voids or pores will form. The most common criterion used for predicting these pores was developed by Niyama et al. DESIGN OF EXPERIMENTS and the critical pressure drop at which this occurs is In order to validate the model developed in this study, two shown in Eqn. 3 where the is the pressure, is the different castings were designed for simulation and instantaneous cooling rate and G is the thermal gradient.21 experimental validation and are shown in Figures 2A and 𝑝𝑝 𝑇𝑇̇ 2B. Both castings contain cylindrical test coupons that are Eqn. 3 100mm in length and 8mm in diameter. The first design 𝑇𝑇 2 shown in Figure 2A was termed the preferred filling mold 𝐺𝐺 ̇ AsΔ𝑝𝑝 the∝ magnitude of the pressure drop increases from and was designed to follow Campbell’s 10 rules of elevated cooling rates or low thermal gradients, pores will casting with bottom feeding, along with appropriate begin to form, often nucleating on pre-existing bifilms.4,22 and gating ratios. These design elements of the preferred Excess gas within the melt is also able to preferentially filling mold will give it a quiescent, laminar flow with diffuse to these low energy voids. Once near the bifilm, minimal chance for entraining events. Conversely, the the gas permeates the solid film and fills the void causing design in Figure 2B possesses less than optimal filling the bifilm to inflate and take on a spherical morphology. conditions and was termed the “poor filling” mold. The The entrapped air within the void then acts to further poor filling mold has a non-tapered sprue, hard corners increase the thickness of the oxide, and the surface area of throughout the cavity which will induce splashing and all pores within the structure will have some level of folding of the melt front or free surface leading to oxidation as shown in Figure 1D. Hence, it can be increased entrainment. Further, the poor filling mold has a ascertained that shrinkage plays a role in the number and water-fall gate that will lead to large turbulence at the gate shape of the entrained bifilms. after which the fluid front will divide before re- converging which is often considered a worst case To determine the correlation between porosity and bifilms scenario for melt flow. In each casting design, the the ratio in Eqn. 3 must be examined. Gu et al. has shown cylinders or test coupons will fill unidirectional (from the that the number of pores and morphology is dependent on bottom up) allowing the effect of directional flow on Eqn. the thermal surroundings of the melt. Specifically, Gu et 4 to be understood. The two designs provide an al. was able to show that as the cooling and solidification opportunity to validate the model and Eqn. 4 for the rate increased, the total number of pores increases, but the best/worst case designs. average size of individual pores decreases.23 When applying this to bifilms, it can be seen that elevated To simulate the model and Eqn. 4, Software 1 was used. solidification rates will increase the overall volume of the Software 1 was setup using a 1mm tetrahedral mesh entrained defects and can be considered in the within the gate, and the cylinders while a global 2mm determination of the total volume of inclusions in the mesh was used in the remaining portion of the model. The casting. If the ratio in Eqn. 3 is altered to / , it is now flow rate used for filling was 0.45kg/sec and the mold equivalent to the solidification rate and it can be material was set as silica with a heat transfer coefficient 2 combined with Eqns. 1 and 2 to yield Eqn.𝑇𝑇̇ 4𝐺𝐺 where Φ is of 500 W/m K. defined as the Oxide Entrainment Number, OEN.

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(A) (B)

Figure 2: (A) Preferred filling mold designed for optimal filling based on Campbell’s 10 rules of casting. (B) Poor filling mold, designed to create increased levels of entraining events.

Once the simulation completed, a custom equation was RESULTS AND DISCUSSION created for the introduction of Eqn. 4. The results for Eqn. 4 only produce volume vs. time plots of oxide generation SIMULATION RESULTS AND ANALYSIS which were then transferred to MATLAB for integration Both the preferred filling and poor filling molds were and analysis at 1 cm increments along the length of simulated using Software 1 and examined using the model cylindrical coupons. established in Eqn. 4. As mentioned, the fluid front velocity is a key variable in determining if turbulent Both the preferred mold and poor filling mold were cast activity will occur and is has a strong influence on Eqn. 4. with aluminum A356 using sand molds. Primary A356 The fluid velocity acts as an initial assessment to ingots were melted in an induction furnace using graphite determine the amount of turbulent flow that will occur crucibles coated with boron nitride. Prior to casting, during filling and can be seen for both mold designs in was skimmed from the surface of the melt and prepped Figure 3. for casting. The sand molds were made using a sodium silicate binder and were allowed 24 hours to dry prior to Ideally the fluid velocity remains below 0.5m/s during casting to eliminate any chance of gas vaporizing and filling to prevent entrainment events from occurring. entering the molten material in the mold cavity during Figure 3A & 3C show that both molds have velocity casting. exceeding the 0.5m/s criterion in the sprue and the runner region. This is somewhat expected as the aluminum Once each of the designs was cast, the test coupons were falling just several inches will exceed the maximum sectioned along the longitudinal and transverse axis for velocity for entrainment as calculated by Campbell. What metallographic examination. When sectioning along the is more important is the gate velocity or the velocity of transverse direction, samples were taken every 10mm and the molten material entering the cast structure. Once the were mounted for polishing with the cross section face fluid front reaches the gate, a slow quiescent flow is down. The first cross section was taken at the bottom of desired to prevent any chance for entrainment. Figures 3B the cylinder and was termed the 0cm cross section. These and 3D display the gate velocity of the fluid as it is cross sections were then cross compared with the data entering the casting and fills the cast structure. Figure 3B obtained from the Software 1 simulation. After polishing, shows large velocities in the entirety of the cylinder the samples were viewed under an optical microscope and coupons indicating entrainment of defects is likely, ImageJ software was used for analyzing the resulting whereas Figure 3D shows a slow quiescent filling velocity micrographs. in the cylinder sections with a laminar flow. This initial check confirms that the design intent of both molds is accurate, and the fluid flow will be mostly turbulent for the Poor Filling Mold and laminar for the Preferred Filling Mold.

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Figure 3: Fluid velocity for the Poor Filling Mold (A)-(B) and the Preferred Filling Mold (C)-(D) at various times. The color scale varies from 0m/s (purple) to 0.6m/s (red)

The second term considered is the oxide severity. The Figures 3C & 3D, the gate velocity is quite low, and the oxide severity term is a cumulative function found within free surface is unlikely to entrain in the preferred filling Software 1 and tracks the location of the free surface upon mold. This would indicate that the free surface shown in solidification. The final time cumulative step for the oxide Figure 4B for the preferred filling mold is likely located at severity term for both molds can be seen in Figure 4. As the surface not in the interior of the casting. The opposite expected, Figure 4 shows little to no free surface in the is true for the poor filling mold where the velocity is sprue and most of the runner for both mold designs. This elevated during filling and when combined with excess is because the free surface, by definition, remains at the free surface, it is likely many oxide entraining events will fluid front and the material solidifying in the sprue and occur and the free surface oxides have become trapped in runners has had little to no exposure to the environment the interior of the casting. prior to entering the mold. Figure 4 also illustrates that the regions with a greater magnitude of the oxide severity The final term to consider in the newly proposed model is term are located in the last-to-fill regions. In the poor the thermal term as it relates to the formation of pores in filling mold this is the majority of the cylinder or test the casting. Theoretically, the Niyama criterion states that coupon which shows elevated levels throughout the pores will form in regions of low thermal gradients and cylinder with the highest magnitude region being located high cooling rates which lead to a large drop in the at the top of the cylinder. This would follow the feeding pressure. The individual terms that make up the conventional wisdom that the last-to-fill or last-to-solidify thermal term are shown in Figure 5. Figure 5A shows that regions would contain the defected material and is one of in all cylinders, the thermal gradient is the lowest in the the reasons why risers are used in casting design to locate center of the cylinders and largest at the ends of the the defects in the unused part of the casting. The preferred cylinder coupons. This is an intuitive result as the edges filling mold shows a good example or locating the excess of the cylinders are able to dissipate heat faster as they free surface in the nonessential regions of the casting have more surface area contacting the mold and results in including the runner and excess gate regions. There are an increased thermal gradient. The next term to consider some elevated levels at the top of the cylinder coupons in is the cooling rate which is shown in Figure 5B. Figure Figure 4B, however, a majority of the cylinder coupons 5B shows that the cooling rate follows the thermal are clean, with limited free surface. gradient with elevated cooling rates at the ends of the As previously described, the free surface is only harmful cylinders and lower levels near the centers. to the casting when it is entrained and when viewing

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Figure 4: Oxide severity term for the (A) Poor Filling Mold and (B) The Preferred Filling Mold. The scale bar shown spans from 0 (purple) to 3.0 (red) and applies to both (A) and (B).

term in Figure 4. Thus, the thermal term is able to provide Individually the thermal gradient and cooling rate do not a strong indicator as to the location of pore formation provide an obvious indicator of the drop in pressure which can be used to calculate the number of oxides required for pore formation and require the examination present in the final casting. of the thermal term shown in Figure 5C. The thermal term shows that the thermal gradient is more dominant than the Each of the terms above provides a good indicator of cooling rate and the majority of the pores will exist in the when and where defects might be within a casting, centers of the cylinders of both mold designs when filled however, as shown in Figures 3–5 there is not a clear using directional flow. This occurs due to the relatively conclusion on where and how severe the entrained defect consistent cooling rate throughout the casting, and the content will be. This illustrates the need for the Oxide greater variance in the thermal gradient across the length Entrainment Number to predict the volume and location of the cylinders. This outcome does not follow of entrained defects. Without the OEN, individual conventional wisdom which would state most of the entrainment terms can be misleading and can lead to poor porosity would float to the top of the casting or would design of patterns/molds. The results of the numerical occur at the last to fill stages similar to the oxide severity simulation using Eqn. 4 can be seen in Figure 6.

Figure 5: Individual terms included in the thermal term for OEN. (A) Thermal Gradient [C/cm] (B) Cooling Rate [C/s] (C) Thermal Term [Cm/s].

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Figure 6: The instantaneous Oxide Entrainment Number generation for both the (A) Poor Filling Mold and (B) Preferred Filling Mold. The scale bar spans from 0 (purple) to 1000 (red) and applies to both (A) and (B)

The contour plot in Figure 6 displays the instantaneous Figure 7 confirms that the total volume generation of the volume generation of oxide content for both mold types. poor filling mold is greater than the preferred filling mold. Figure 6 shows a larger generation in the poor filling However, the ends of the cylinders (0-1cm & 7-9.5cm) mold compared to the preferred filling mold and has a show relatively low and similar volumes for both molds. strong resemblance to the velocity contour plots in Figure These low volumes are likely due to larger thermal 3 highlighting the importance of velocity term. In order gradients at the ends of the bars, while the tip of the to convert these instantaneous values in Figure 6 to cylinder (7-9.5cm) will have a relatively lower fluid cumulative values, the instantaneous data was integrated velocity compared to the rest of the filling. over time to obtain the final OEN or volume of oxide and other induced defects at each element within the simulation. After obtaining the total volume generation, EXPERIMENTAL VALIDATION transverse slices, or cross sections, were taken every centimeter along the cylinder and the average OEN Upon completing the examination of the OEN, the same throughout the cross sections was calculated. The mold designs in Figure 2 were prepared using sodium resulting values are shown in Figure 7 and depict how the silicate bound sand and arranged for casting with total volume of entrained material changes over the length aluminum A356. Following casting the transverse cross of the cylinder. sections were examined at every centimeter along the cylinder for both molds to compare the oxide content and 200

OEN Preferred location to the OEN prediction. The transverse sections 180 OEN Poor are shown in Figure 8. The defects seen in Figure 8 are 160 )

3 two-dimensional making it impossible to calculate the

140 true volume of the oxides present without making

120 assumptions. For this reason, a constant oxidation thickness of 500nm was assumed for all defects present in 100 the cross section. This assumption was employed as it is 80 often not possible to view oxide inclusions using standard

60 microscopy methods. However, it is known that oxide

Total Volume Generated (mm 40 inclusions are often the precursor to or directly cause most defects within the system, it is reasonable to 20 conclude that each of the defects shown in Figure 8 have 0 0 1 2 3 4 5 6 7 8 9 10 some level of oxidation similar to Figure 1. The size of Vertical Position (cm) 500nm thickness was chosen as it is a common size of

Figure 7: The total volume of entrained oxide at each young oxides present in castings, and it is likely that a transverse slice along the vertical axis of the cylinders majority of the large, old oxides are eliminated during in the Poor Filling Mold and the Preferred Filling Mold melt preparation. The thickness assumption allows the obtained from Software 1. total volume of the included oxide volume in combination

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(A) (B) (C) (D)

(E) (F) (G) (H)

Figure 8: Micrographs of the transverse cross sections of the (A-D) Poor Filling Mold and the (E-H) Preferred Filling Mold. The positions of the transverse sections are (A,E) 2cm, (B,F) 4cm (C,G) 6cm (D,H) 9cm. with the oxide induced defects to be calculated by defects. Thus, the OEN requires an additional term to multiplying the surface area of each defect by the account for the amount of gas present in the casting. thickness so it can be compared to the OEN. The quantity of gaseous product within a cast structure is Returning to Figure 8 it can be seen that in general there often determined using an Archimedes test. For aluminum are lower quantities of defects in the cross sections taken melted in an a standard foundry environment the gaseous near the ends (Figures A, D, E and H) compared to the product is often found to range from 0.3-0.4ml/100g Al or centers (Figures B, C, F and G). These results 0.8% volume fraction.23 As previously stated, this gas will qualitatively match the predictions by the OEN shown in not be equally distributed evenly throughout the structure; Figure 7. Additionally, Figure 8 shows that the model instead, more, smaller pores will be present near elevated correctly predicts that there is not a large disparity in the solidification rates while fewer, larger pores will be number of defects for the two different types of molds. present at regions of lower solidification rates.23 Thus, Though these qualitative results are in good agreement, when examining Figure 5 it is now clear why a majority the quantitative volume of defects in the cylinders have of the defects appear in the central regions of the bar as some differences which can be seen when comparing the shown in Figure 8. However, as mentioned, the extent to OEN calculation of defect volume in Figure 7 to the which these unfurled bifilms inflate is limited to the gas experimental observed defect volume in Figure 9. Figure content. 9 shows a low value of defects observed at the two ends

Experimental Inclusion Volume Generated at each Location of the bar (positions 0 and 9.5cm) with the centers of the 45 bars having a relatively constant defect volume in the Preferred Filling Poor Filling centers of the bars. Figure 7, however, predicts a 40 ) maximum defect volume between the 4-5cm locations of 3 the bar. 35

In order to improve the OEN model, an additional 30 parameter should be considered. It is well understood that liquid aluminum has a solubility of gaseous product that 25 upon solidification is trapped within the casting. As Total Volume Generated (mm previously described, these gases are able to permeate the 20 solid oxide film and cause the bifilm to unfurl and inflate. 15 The extent to which the process can occur is obviously 0 1 2 3 4 5 6 7 8 9 10 determined by the amount of gas trapped within the Position (cm) casting. When more gas is present, more bifilms will be Figure 9: Experimental defect volume for both mold unfurled and cause an increase in the overall volume of types.

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Further, if by chance gas pores are able to nucleate entrained oxides and oxide induced defects in complex heterogeneously without the presence of a bifilm, it is geometries by examining the unique flow front after the likely that the pore will possess a low density and float melt has passed the gate region. For simple geometries, towards the top of the casting. Thus, the volume of gas however, the entrapped gas volume term provides trapped within the melt must be accounted for in a manner accurate results for oxide entrainment and oxide induced that accounts for oxide unfurling as well as heterogeneous defects while maintaining a physical reason for its pore floatation. In other words, the OEN should have an inclusion. increased prediction of defect volume at the top portions of the cylinder while limiting limits the defect volume at After applying the entrapped gas volume term to the OEN the centers of the bars for a directionally filled casting. A to both the poor and preferred filling molds the OEN is function that best fits this description is a simple parabolic shown to have good agreement to experimental data for function as shown in Equation 5. both cases as shown in Figure 11. Both mold types show that the cylinders have a relatively constant volume of = + Eqn. 5 defects through the majority of the cylinders excluding 2 very bottom and top of the cylinders. This result is Where𝑔𝑔 𝑣𝑣𝑥𝑥 g is the𝑏𝑏 entrapped gas volume term or oxide consistent with the OFEM predictions of Reilly et al. and induced defect term, v is the volume fraction of gas the experimental results of Green et al. and.20,24Both divided by 10, x is the vertical position of the cylinder and results show even distributions of defects throughout the b is a fitting term. The process of applying the entrapped castings that were filled directionally. gas volume term is displayed in Figure 10. The entrapped gas volume term was centered at a vertical position of The two major differences between Reilly’s OFEM and 4cm as this was the maximum observed in Figure 7 and it the OEN presented in this study are the increase in was only applied between vertical positions 3 thru 9.5cm computational efficiency of the OEN and the ability to as there is no increase in the defect volume at the bottom predict the volume of the defects present in a cast of the bar that will occur from the flotation of any structure. Figure 11 shows some general variance in the heterogeneously nucleated pores. After applying the amount of oxide present, which is to be expected in this entrapped gas volume term the OEN, it can be seen in type of experiments. The complex dynamic situation of Figure 10C that the OEN now provides an extremely the filling process in casting will always have a level of accurate prediction of the volume of defects. It should be variability, which Reilly et al. highlights in their noted that the entrapped gas volume term applied here has development of OFEM. When using the OFEM Reilly et only been validated on casting that are filled directionally al. was not able to correlation between location and as the two molds in Figure 2 show. If a complex casting severity of the oxide content nor were they able to couple with multi-directional flow were to be examined, it is it with other oxide induced defects such as unfurled likely that the entrapped gas volume term or oxide bifilms or shrinkage porosity. Thus, the OEN provides a induced defect term would need to be altered, though the strong predictor of the numeric volume and location of general principles of oxide entrainment and oxide induced entrained oxide inclusions as well as oxide induced defect formation remain the same. It is possible that the defects present in the final cast structure. directional prediction can be used the prediction of

(A) (B) (C)

Figure 10: Comparison of the OEN to the experimentally measured with the inclusion of the entrapped gas volume term for the preferred filling mold. (A) The OEN original prediction. (B) The Entrapped Gas Volume (g) Function. (C) Experimental vs Final OEN Prediction.

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Figure 11: OEN comparison to the experimentally observed defect volume. (A) Poor Filling Mold (B) Preferred Filling Mold.

Manufacturing Research Lab for discussions and design CONCLUSIONS assistance. Finally, the authors would like to thank the Central Ohio Chapter of the American Foundry Society A new Oxide Entrainment Number (OEN) model has for donated materials, especially Keener Sand & Clay Co. been developed to predict location-specific volumes of and Fisher Cast Steel Products Inc. entrained defects as well as oxide induced defects in aluminum castings. The OEN is based on four key components, the Weber Number, the liquid free surface REFERENCES the local solidification rate and the total gas volume in the melt. When these terms are combined, the OEN model 1. Liu, L., and Samuel, F. H., “Effect of inclusions provides an intuitive description of the oxide entrainment on the tensile properties of alloy,” J. Mater. Sci., process and is able to accurately predict the volume and 33, pp. 2269–2281 (1998). location of various entrained defects. It was found, based 2. Nyahumwa, C., Green, N. R., Campbell, J., on the OEN model, that a majority of oxide induced “Influence of Casting Technique and Hot defects are located within the center region of the cast Isostatic Pressing on the Fatigue of an Al-7Si- cylinder coupons while the ends (top and bottom) Mg Alloy,” Metall. Mater. Trans. A, 32, pp. possessed a smaller volume of defects during directional 349–358 (2001). metal flow. This phenomenon was attributed to an 3. Cao, X., and Campbell, J., “Oxide inclusion increased velocity as well as a decreased thermal gradient defects in Al-Si-Mg cast alloys,” Can. Metall. in the centers of the cylinders. Using the entrapped gas Q., 44, pp. 435–448 (2005). volume term based on the volume Gopalan, R. & Prabhu, 4. Campbell, J., “Entrainment defects,” Mater. Sci. N. K. Oxide bifilms in aluminium alloy castings – a Technol., 22, pp. 127–145 (2006). review Oxide bifilms in aluminium alloy castings – a 5. Gopalan, R., and Prabhu, N. K., “Oxide bifilms review. Mater. Sci. Technol. 27, 1757–1769 (2011).of gas in aluminium alloy castings – a review,” Mater. present within the melt, the OEN was also able to Sci. Technol., 27, pp. 1757–1769 (2011). accurately predict that the center portion of the cylinders 6. Thiele, W. G., “Die oxidation von Aluminium would contain more gaseous pores than at the edges or und Aluminiumlegierungs Schmelzen,” ends of the cylinders and was experimentally validated. Aluminium, 38, pp. 707–715 (1962). Overall, the OEN is a computationally efficient model 7. Griffiths, W. D., Caden, A. J., and Chen, Q., that can be implemented into any commercial casting “Effects of transition metal additions on double- simulation software and shows promise for aiding oxide film defects in an Al-Si-Mg alloy,” Mater. casting/ designs for minimized oxide inclusions Sci. Technol., 33, pp. 2212–2222 (2017). and oxide induced defects. 8. Yang, X., Huang, X., Dai, X., Campbell, J., and Tatler, J., “Numerical Modelling of the Entrainment of Oxide Film Defects in Filling of ACKNOWLEDGMENTS Aluminium Alloy Castings,” Int. J. Cast Met. Res., 17, pp. 321–331 (2004). The authors would like to thank Honda Engineering of 9. Campbell, J., “Castings” (Butterworth- America and Honda R&D Americas for continued Heinemann, 2003). financial support and technical contributions. The authors would also like to the members of OSU Light Metals and

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