DESIGN OPTIMIZATION OF GATING SYSTEM BY FLUID FLOW AND SOLIDIFICATION SIMULATION FOR WHEEL HUB BY SAND CASTING

Transcription

1 DESIGN OPTIMIZATION OF GATING SYSTEM BY FLUID FLOW AND SOLIDIFICATION SIMULATION FOR WHEEL HUB BY SAND CASTING Keertikumar 1, Bharat.S.Kodli 2 1 Post graduate student, Department of Mechanical Engineering, PDA College of Engineering, Kalaburagi, VTU, Karnataka, India. 2 Professor, Department of Mechanical Engineering, PDA College of Engineering, Kalaburagi, VTU, Karnataka, India. Abstract: Sand casting is a manufacturing process most widely used in manufacturing industries, especially in automotive products. Many researchers reported that about 90% of the defects in castings are due to wrong design of gating and risering system and only 10% due to manufacturing problems. In this p aper optimization of gating and risering system by replacing existing trial and error method with the help of CAD modeling (CATIA V5) and casting simulation software ADSTEFAN was carried out. The simulation results are used to optimize the gating system to improve Directional Solidification and reduce shrinkage porosity. Through several simulation iterations, it was concluded that defect free casting could be obtained by modifying the sprue location and providing the risers and exothermic sleeves at location porne to formation of shrinkage porosity lead to the decreasing size of shrinkage porosity and shifting the shrinkage porosity from component to the risers. Keywords Casting Simulation, Gating Design Optimization, Shrinkage, Fluid flow and Solidification and Wheel Hub. I. INTRO DUCTIO N Casting is a manufacturing process for making complex shapes in which a molten material is poured into a mould cavity, which contains a mold cavity of the desired shape and then allowed it to solidify. The solidified part is also known as a casting, which is removed or broken out of mould to complete the process [1]. Inspite of conventional knowledge of gating and riser system design and suggestions by experienced foundry engineer s wheel hub showed the presence of shrinkage cavity. Producing defect free casting is a challenge in manufacturing environment. The formation of various casting defects is directly related to fluid flow phenomena during the mould filling stage and in the cast metal. The rate of solidification greatly affects the mechanical properties such as strength, hardness, machinability etc [2]. One of the critical elements that has to be considered for producing a high quality sand casting product is the gating system design and risering system design [3-4]. Any improper designing of gating system and risering system results in cold shut and shrinkage porosities. Therefore adequate care is necessary in designing gating and risering system to obtain defect free casting. Casting simulation minimizes shop floor trials, time, cost and work force to achieve th e desired internal quality at the highest possible yield. Hence with conventional approach, finding an acceptable gating system design proves to be an expensive process so a number of casting simulation software s are available today, such as ADSTEFAN, AutoCAST, CAPCAST, AnyCasting, CastCAE, MAGMA, MAGMASOFT, Flow-3D, Novacast, NCADOMS-2016 Special Issue 1 Page 233

2 NovaFlow, SoftCAST, SUTCAST, Virtual Casting, WINCAST, ProCAST and SolidCAST [5]. Most of them use Finite Element Method to discretize the component to solve the solidification and fluid flow equations. Presently use of casting simulation software is increasing, as it essentially replace or minimizes the shop floor trails to achieve sound casting. With the availability of modern numerical software and good hardware capabilities, simulation has become an important tool for design, analysis and optimization of casting processes. Use of casting process simulation software can significantly reduce the casting cost, lead time and enhance the quality of casting [6]. ADSTEFAN(Advanced Solidification Technology for Foundry Aided by Numerical Simulation) is three dimensional solidification and fluid flow package developed to perform numerical simulation of molten metal flow and solidification phenomena in various casting processes, primarily sand casting and die casting (gravity, low pressure and high pressure die casting). It is particularly helpful for foundry application to visualize and predict the casting results so as to provide guidelines for improving product as well as mold design in ord er to achieve the desired casting qualities. Prior to applying the ADSTEFAN extensively to create sand casting and die casting models for the simulation of molten metal flow(mould filling) and solidification(crystallization in the process of cooling).the cast and mold design of the experiment is transformed into a 3D model and imported into ADSTEFAN to conduct the sand casting process simulation. Many software use finite element method (FEM) to simulate casting process, which needs manual meshing and are prone to human errors. The casting simulation software used in the present work uses Finite Difference Method (FDM) using cubes as the basic elements and has a major advantage over FEM. It meshes automatically eliminates the need to recheck the meshing connectivity there by speeding up analysis. In the present riser system has been designed and optimized by iterative process through fluid flow and solidification simulation for a wheel hub to produce defect free casting [6]. The main inputs include the mould cavity geometry (includes the shape, size and location of cores, bosses, ribs, mold cavity, risers, runners, ingates and sprue.), thermo-physical properties (density, specific heat, latent heat, volumetric contraction during solidification, viscosity, surface tension and Thermal conductivity of the cast metal as well as the mold material, as a function of temperature), boundary conditions (such as the casting -mold, castingchill, casting-exothermic sleeve, casing-die, die-cooling channels heat transfer coefficient, for normal mould as well as feed-aids including chills, insulation and exothermic materials) and process parameters (such as pouring time, pouring rate and temperature). The results of solidification simulation include color-coded freezing contours at different instants of time starting from beginning to end of solidification. This provides a much better insight into the phenomenon compared to shop-floor trials (real molds being opaque). The user can verify if the location and size of feeders are adequate, and carry out iterations of design modification and simulation until satisfactory results are obtained. Sometimes, it is not possible to achieve the desired quality by changes to method (mainly feeding and gating) alone. In such an event, it may become necessary to redesign the part design. The size and location of the runner, ingates, riser and sprue is an important input parameter for solidification simulation. Considerable re-designing and experience of the user will help in taking the right decision. Further, by NCADOMS-2016 Special Issue 1 Page 234

3 using the CAD software (CATIA V5) the solid model of the component with runner, ingates, riser and sprue is to be designed by the engineer and imported STL ( Stereo Lithography) file into the casting simulation program (ADSTEFAN) for each iteration. These all tasks requires computer skills and designing knowledge. The accuracy of the results (such as solidification time, fluid flow and shrinkage defects) are influenced by geometry of t he component and availability of temperature dependent material property database. The simulation of complex intricated casting may consume more time and cost than shop-floor trials and further delay and expenses occur due to the wrong feeding of the input parameters in the casting simulation program [5]. The sand casting (green sand) casting process utilizes a cope (top half) and drag (bottom half) flask of sand (usually silica), clay and water. When the water is added it develops the bonding characterist ics of the clay, which binds the sand grains together. When applying pressure to the mold material it can be compacted around a pattern, which is either made of metal or wood or wax or plastic to produce a mold cavity having sufficient rigidity to enable metal to be poured in it to produce a casting. The process also uses cores to create cavities inside the casting. After the molten metal is poured into mold cavity and allowed it to cool, then the core is removed from the casting. In this process material cost is low and the sand casting process is exceptionally flexible. In this process simulation is carried out for manufacturing of Wheel Hub and the results were obtained [7]. II. CASTING SIMULATIO N Computer simulation of casting process has emerged as powerful tools for achieving quality assurance without time consuming trials. This includes mold filling, fluid flow, solidification, stresses and distortion. It requires part model of component and tooling (parting line, mould layout, cores, feeders, chills, exo thermic sleeves and gates), temperature dependent properties of component and mold materials, input process parameters (pouring time, pouring rate, direction of fluid flow, etc.). The simulation results are interpreted to predict casting defects such as shrinkage porosity, hot spots, blow holes, cold shut, cracks and distortion. For a product design engineer inputs are not easily available which required considerable experience and expertise in the simulation software. In the simulation process the tooling and product design process will run simultaneous in parallel manner to evolve the quality product. This approach towards improve the quality of product simultaneously is referred as concurrent engineering [5]. III. MATERIAL AND METHO DO LO GY Wheel Hub is usually made of cast iron, it is a bridge between Shaft and wheel. Wheel hub limited to a revolution rate of few thousand RPM. Chemical analysis of cast iron material is as shown below table. Table 1: Chemical composition of Cast Iron Alloyant C Si Mn S P Mg Fe NCADOMS-2016 Special Issue 1 Page 235

4 Wt% bal Figure 1 shows the CAD model of Wheel Hub. The wheel hub casting model with the essential elements of gating system are sprue, runner, ingates and riser system were generated in CATIA V5 CAD software. In the first iteration (fig 1) the sand riser is used for the casting of wheel hub, after the simulation of first iteration the shrinkage porosity defect is occurred. In order to obtain sound casting the model has to be re-designed in such way that in the second iteration the exothermic sleeves are used to keep riser metal in the molten condition so that it is used to compensate the shrinkage porosity to achieve the directional solidification (fig 2). The dimensions used in iteration 1 and 2 are tabulated in the below table (2). Table 2: Iteration design dimensions No Sprue(mm) Runner(mm) Ingates(mm) Riser(mm) Sleeve(mm) Yield Øb Øt H W L H W L H Ø H Øi Øo H (%) Not Used Fig: 1 Top and bottom view of Wheel Hub (Iteration 1) Fig: 2 Top and bottom view of Wheel Hub (Iteration 2) NCADOMS-2016 Special Issue 1 Page 236

5 Fig: 3 Methodology used in simulation process IV. SIMULATIO N PRO CESS ADSTEFAN is casting simulation software developed by Hitachi Corporation Ltd Japan. This was used to simulate fluid flow and solidification of sand casting of wheel hub. Casting simulation and result analysis was done to predict the molten metal solidification and fluid flow behavior inside the mould. The casting component with gating system was imported in STL (Stereo Lithography) format to the ADSTEFAN software and meshing of the model was done in the pre-processor mesh generator module. The mesh size of casting is taken as 5mm. The structural boundary conditions are automatically taken care by the software. As signment of material properties, fluid flow and solidification parameters: The meshed model was taken into the precast environment of the software, where the material, type of mold used, density of cast material, liquidus and solidus temperatures of cast Iron and other input parameters of fluid flow and solidification conditions like pouring time, pouring type, direction of gravity etc. were assigned. Table 3&4 show the material properties, fluid flow & solidification parameters. After the assignment of material properties and simulation conditions, predication of air volume, filling temperature, filling velocity, solidification pattern, temperature distribution and soundness of degree are carried out. Casting simulation program provides output files in the form of graphical images and video files which are analyzed to predict defects after the successful execution [6]. NCADOMS-2016 Special Issue 1 Page 237

6 Table 3: Input material properties and conditions Parameters Type of Mold Conditions Sleeves Material Green sand SG 500/7 (FCD500) - Density 1.5 gm/cm gm/cm gm/cm 3 Initial Temperature Liquidus Temperature Solidus Temperature Reaction Heat (cal/gm) Reaction Time sec Ignition Temperature C Table 4: Input and output data of fluid flow and solidification parameters Parameters Input Conditions Filling time 56 Seconds Pouring type Gravity pouring Gating Ratio 1:2: Air Entrapment 2. Filling Temperature Output files 3. Filling Velocity 4. Solidification pattern 5. Temperature Distribution 6. Shrinkage porosity V. RESULTS AND DISCUSSIO N 1. Air Entrapment Figures 4 (a) & (b) shows the molten metal (grey color) at the bottom portion and air sweeping (blue color) from the top portion of mould cavity. From the simulation results it is clear that from the nine ingates mold cavity is filled with molten metal, air escapes through the top of the housing i.e. from the mold cavity to the atmosphere through risers. Fig (a) and (b) shows pattern of air escape from the mold cavity. Hence this simulation results helps to identify air entrapment defect in the casting. By this simulation result it is clear that there is no air entrapment defect in the casting hence no need of modification in the design of gating system. NCADOMS-2016 Special Issue 1 Page 238

7 a. Slide no 51 (50%) b. Slide no 101 (100%) Fig: 4 Air Entrapment The ingates and runner are placed in a proper location due to which even flow of molten metal makes the air gently to rise above, as the metal starts filling from the bottom of the cavity. This allows all the air and gases to escape from the mould cavity. There is no air entrapped zone in the casting component and gating system in both iterations. 2. Filling Temperature a. Slide no 51 (50%) b. Slide no 101 (100%) Fig: 5 Filling Temperature Figure 5 (a) and (b) represent the temperature distribution of the casting at different regions at specific time. Figure (a) shows the temperature distribution of the casting at 27 seconds, figure (b) shows the temperature distribution of the casting at 55 seconds. The red color represent the molten state of the casting material and dark blue color represent the solidified casting. From the figure it is clear that, there is no sudden temperature drop occurred during the fluid flow process, the fluid flow is laminar or uniform flow such that there is no fluid flow associated defects are present in casting of both simulations. NCADOMS-2016 Special Issue 1 Page 239

8 3. Filling Velocity a. Slide no 51 (50%) b. Slide no 101 (100%) Fig: 6 Filling Velocity The fig 6 (a) and (b) represent Filling velocity at which the particular part of the component is filled by the molten metal. The figure (a) represent the 50% portion of mold is filled within seconds and figure (b) represent the 100% portion of mold is filled by molten metal within 55 seconds, it clearly depicts that the part that last to be filled is the riser. This is again a positive result of the casting simulation as riser solidify at the last, can compensate material for casting. So there is no filling defects occurred this results are favorable to obtain sound casting in both cases. 4. Solidification Pattern In order to achieve sound casting it is necessary to provide the directional solidification. The directional solidification starts from thinnest section to thickest section and which ends at riser. The actual solidification of metal begins at liquidus temperature of 1410 C. The solidification of metal ends at solidus temperature 222 C. NCADOMS-2016 Special Issue 1 Page 240

9 a. Slide no 100 (100% ) (Iteration 1) b. Slide no 100 (100% ) (Iteration 2) Fig: 7 Solidification pattern In figure 7 (a) first iteration the sand risers are used for the wheel hub casting process where the isolated regions or hot spots are observed at the neck of wheel hub component so isolation area prone to defective area. So in second iteration the figure (b) shows the outer surface of the component which is in direct contact with atmosphere are solidified faster as heat transfer take place earlier. In order to solidify riser at last, in second iteration exothermic sleeves are used which prevent the transfer of heat from the riser and restrict the solidification of metal in the riser. In this simulation result we come to know that the riser solidifies at the last which provide the directional solidification of wheel hub casting in the second iteration. Hence second iteration results the sound casting of wheel hub. 5. Temperature Distribution a. Slide no 101 (100% ) (Iteration 1) b. Slide no 101 (100% ) (Iteration 2) Fig: 8 Temperature distribution The actual solidification of metal begins at liquidus temperature of 1410 C (reddish yellow color). The solidification of metal ends at solidus temperature 222 C (blue color). Figure 8 (a) shows the temperature distribution of the molten metal in the first iteration of the gating system. There is no sudden temperature drop below the liquidus temperature. In second iterations as shown in figure 8 (b) the temperature distribution is also uniform. In all the iterations it can be seen that runner bars and in-gates have temperature distribution within the limit i.e. above liquidus temperature. Any sudden drop in temperature within the gating elements would have resulted in formation of cold shuts and blockage of further entry of molten metal which has not been observed in the both simulations results. NCADOMS-2016 Special Issue 1 Page 241

10 6. Soundness of Degree Figure 9 (a) shows shrinkage defect is present at the neck of wheel hub component in the first iteration simulation. The redesign of the gating system is necessary to eliminate the shrinkage defect. But in the second iteration simulation fig 9 (b) represents these shrinkage defect present in the component are reduced by the increasing the height of riser and providing exothermic sleeves at the proper location. Thus volume of shrinkage defect decreased significantly. The shrinkage defect is completely shifted to the riser this leads to the defect free wheel hub casting by simulation process using ADSTEFAN casting simulation software. These studies helps to optimize gating system. There is further no requirement of redesign the gating system. Hence the second iteration yields sound wheel hub casting. Fig: 9 a. Slide no 100 (100% ) (Iteration 1) b. Slide no 100 (100% ) (Iteration 2) Soundness of Degree VI. CO NCLUSIO NS In the present work a three dimensional (3-D) component model was developed by CATIA V5 and using casting simulation software ADSTEFAN to evaluate possible casting defects for sand casting of Wheel Hub. Notable conclusions from this study are: To overcome the problems of current gating or riser system, a method based on CAD and simulation technology is implemented. By adopting the pressurized gating system, the fluid flow was smooth and air was expelled without any entrapment inside the mould cavity. Simulation showed that the molten metal was able to fill the mould within the desired time. Therefore heat distribution was good and no cold shut was observed. In first iteration improper location of riser and ingates led to formation of shrinkage porosities wh ere in the second iteration the height of riser is increased and exothermic sleeve are used for the wheel hub component casting to achieve directional solidification, which leads to sound casting. NCADOMS-2016 Special Issue 1 Page 242

11 The second iteration resulted in reducing the shrinkages and the defect associated with the casting is eliminated and the sound cast is achieved by the using the exothermic sleeves. By analyzing simulation results, the optimized riser system is determined. From the above study it can be concluded that the defect analysis done by simulation help a practice foundry man to take decision and corrective actions can be taken to eliminate defects with lesser efforts. By modifying the design of gating system which includes sprue, runner, gates and rises by trial and error method using the ADSTEFAN simulation tool, one can able to determine the amount of material to be used, time required to fill mold cavity and can determine the cost of different manufacturing products. ACKNO WLEDGEMENT The author s wishes to thank research paper review committee, department of mechanical engineering. HOD and Principal of P.D.A.college of Engineering, Kalaburagi for their suggestions, encouragement and support in undertaking the present work. I also express special gratitude to my guide Professor Bharat S Kodli for his inspiration, guidance, constant supervision, direction and discussions in the present work. REFERENCES [1]. MohdRizuan Mohammed Shafiee, "Effects of gating design on the mechanical strength of thin section castings", ELSEVIER: Journal of Materials Processing Technology, Vol-105, Pg , [2]. T.Nandi, "Optimization of Riser size of Aluminium alloy (LM6) castings by using conventional method and computer simulation technique", International Journal of Scientific & Engineering Research, Vol-2, 2011, ISSN [3]. Lee, P.D, Chirazi, A and see, D (2001). Modelling micro porosity in aluminium -silicon alloys: a review. Journal of light metals. Vol.1 Pg [4]. Naveen Hebsur, Sunil Mangshetty, Casting simulation for sand casting of fly wheel Vol 11, Issue 4 Ver. VII (Jul-Aug. 2014), PP [5]. Naveen Kumar, Bharat s kodli, Design optimization of gating system by fluid flow and solidification simulation for pump casing Vol 2, Issue 4, Aug-Sept, [6]. B. Ravi, R.C. Creese and D. Ramesh, Design for Casting A New Paradigm to Prevent Potential Problems, Transactions of the AFS, 107, [7]. Mazhar Iqbal, Sushil Patel, Ganesh Vidyarthee, Simulation of casting and its validation by experiments Iqbal, 3(8): August, [8]. Dr.B.Ravi, Casting simulation and optimization: Benefits, Bottlenecks, and Best Practices Technical paper for indian foundry journal January 2008 special issue. [9]. Vivek S. Gondkar, K.H.Inamdar, Optimization of Casting Process Parameters Through Simulation Vol 3, Issue6, June [10]. K.Srinivassulu Reddy, Casting Simulation of Iron Rotor Disc using ProCAST, Vol.4, No.6 (Dec 2014). [11]. Wang, D., Li, Y.H., Guo, G.S.: The feeding mechanism and mathematical model during solidification of casting, Foundry, 45 (1996), [12]. F. Bradley, S. Heinemann, A hydraulics based optimization methodology for gating design, Applied Mathematical Modeling, 17 (1993) pp [13]. Masoumi A., Effect of Gating Design on Mould Filling, American Foundry Society, USA, NCADOMS-2016 Special Issue 1 Page 243

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