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Application of 3D stereoscopic visualization technology in casting aspect

*Kang JinwuBorn in 1970, Ph.D., Associate Professor: His research and teaching interests mainly focus on numerical simulation of casting processes.

E-mail: kangjw@tsinghua.edu.cn

Received: 2014-03-12 Accepted: 2014-04-25

*Kang Jinwu, Zhang Xiaopeng, Zhang Chi and Liu BaichengKey Laboratory for Advanced Materials Processing Technology, Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Abstract: 3D stereoscopic visualization technology is coming into more and more common use in the field of entertainment, and this technology is also beginning to cut a striking figure in casting industry and scientific research. The history, fundamental principle, and devices of 3D stereoscopic visualization technology are reviewed in this paper. The authors’ research achievements on the 3D stereoscopic visualization technology in the modeling and simulation of the casting process are presented. This technology can be used for the observation of complex 3D solid models of castings and the simulated results of solidification processes such as temperature, fluid flow, displacement, stress strain and microstructure, as well as the predicted defects such as shrinkage/porosity, cracks, and deformation. It can also be used for other areas relating to 3D models, such as assembling of dies, cores, etc. Several cases are given to compare the illustration of simulated results by traditional images and red-blue 3D stereoscopic images. The spatial shape is observed better by the new method. The prospect of 3D stereoscopic visualization in the casting aspect is discussed as well. The need for aided-viewing devices is still the most prominent problem of 3D stereoscopic visualization technology. However, 3D stereoscopic visualization represents the tendency of visualization technology in the future; and as the problem is solved in the years ahead, great breakthroughs will certainly be made for its application in casting design and modeling and simulation of the casting processes.

Key words: 3D stereoscopic visualization technology; casting;

modeling and simulation; observation and analysisCLC numbers: TP391.92 Document code: A Article ID: 1672-6421(2014)04-308-06

The 3D stereoscopic visualization technology based on red-blue color for photos emerged as early as the 1920s.

Also, 3D grating technology dates back decades [1,2]. However, these visualization technologies progressed slowly due to their limitations and the whole technology status during the last century. In recent years, 3D stereoscopic visualization technology has been growing rapidly with the development of electronic and computer graphics technologies. Especially, 3D films and games have aroused people’s enthusiasm about 3D stereoscopic visualization in the entertainment area. Three-dimensional television sets, almost the same price as conventional ones, are becoming widespread in the family

home. Meanwhile, 3D stereo TV channels are mushrooming [3]. Rapid development has also been achieved in 3D stereoscopic displays and projectors, which are manufactured and sold in large quantities. Some digital cameras and camcorders have 3D functions as well; and even naked-eye 3D visualization has been realized in some cell phones. Several companies, such as the German International Electronic Company, SAMSUNG, SANYO, and a domestic company, Super Dimension Technology Corporation, have invested and achieved breakthroughs in the research and development of new 3D stereoscopic visualization technology and displays [4]. The above mentioned 3D stereoscopic visualization is mainly applied to

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two kinds of information: photos and the images related to 3D models constructed by computer software. The latter is more important in the areas of scientific research, engineering and the games industry. A series of computer aided technologies in product design and manufacturing, such as CAD, CAE, CAPP and CAM have been deeply rooted in modern manufacturing industries. However, until now, these kinds of 3D data formats are still mainly displayed by monitors or projectors with a 2D image; the depth characteristics must be shown by occlusion, perspective view, light shading, rotation, and slicing. Therefore, the 3D stereoscopic visualization technology brings great potential in the area of 3D modeling and simulation. Meanwhile, it has also been incorporated into virtual reality, serving the fields of military, medical care, architectural design, advertising media, education, training and culture [5-11]. For example, the 3D molecule research software Chimera developed by the University of California at San Francisco is used in the observation of molecular structure [12]. American 3D stereoscopic visualization software EnSight is used in post-processing and visualization of all kinds of engineering data [13]. The IC.IDO visual decision-making platform for design and manufacture developed by the ESI group from France can bring about a stereo virtual environment [14]. There is a similar immersion virtual reality system DVS3D in China[15]. PortalTM [16], a virtual reality system developed by the University of Iowa can be used for simulations and analysis of designs where the user is immersed in 3-D geometry of a CAD design. In the casting area, 3D stereoscopic visualization has a great potential to illustrate the complicated shapes and various numerically simulated results. We exhibited the 3D stereoscopic visualization system at the 2012 China International Foundry Expo. Meanwhile, Magma Corporation, a company in the modeling and simulation of casting process, also demonstrated a 3D video related to castings.

1 Principle of 3D stereoscopic visualization technology

Three-D stereoscopic visualization technology is mainly based on the principle of binocular parallax. First, two images with parallax information are acquired, and then technical means are utilized to make sure that the two images are seen by the corresponding eyes; and the stereo effect is perceived by the observer when the information received by their two eyes is transmitted to the brain. Two images displayed on the monitor or projector screen simultaneously or in a very short interval are distinguished by the device. One method of distinguishing two images is to separate them on the monitor, the other one is to separate them in front of the eyes, as shown in Fig. 1. The former method can be realized by parallax raster, which lets the two eyes receive two different corresponding images. This kind of monitor is relatively small because of its strict requirement of the observer’s position and distance. However, the advantage

of this method is that it can bring about stereo effect for the observer without wearing 3D glasses, that is, naked-eye 3D effect is realized. In the latter method, two images are produced in front of the observer using 3D glasses, which have varieties of forms, such as electronic shutter type, polarizing type, red-blue type and so on [17].

The two eyes see images from two directions through the fast switching of images controlled by an electronic shutter in the glasses. Meanwhile, synchronous switching of the monitor or projector is required. The monitor or projector displays the left and right images in turn and remains synchronized with the glasses, as a result, when the device is displaying the left image, the left glass is switched on while the right one is switched off. When the device is displaying the right image, the right glass is switched on while the left one is off. The switching frequency is generally higher than 120 Hz, which guarantees that the two eyes receive two different images.

Due to the alternating display of the left and right images, the electronic shutter glasses need the support of 3D graphics cards, such as NVIDIA.

Polarizing glasses work with a polarizing monitor, which displays two images polarized in different directions simultaneously. The polarizing angles of the pair of glasses are different, in this way, only the image corresponding to the specific glasses can be received; as a result, the two eyes see two different images. Three-D stereoscopic visualization can also be realized through the co-work of polarizing glasses and polarizing projectors. In this case, two projectors are required to project two parallax images overlapping on the screen, whose polarization properties must be guaranteed.

It is not necessary for the red-blue glasses to function with the help of any special performance hardware. Conventional media such as monitors, projectors, even color printings can exhibit the 3D stereo effect with the help of such glasses. The technology commonly called red-blue display is actually a form of red-cyan display. The color cyan is a combination of blue and green. The principle of red-blue glasses is that the left and right parallax images are displayed complementarily in red and cyan. In the RGB mode, any kind of color consists of a mixture of

Fig.1: Principle of 3D stereo imaging

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Fig. 2: Generation process of a 3D red-cyan image

(a) Left image

(b) Right image

(c) Left image after red filtering (d) Right image after cyan filtering (e) Synthetic 3D red-cyan image of temperature field of a steel casting

red, green and blue. The observer’s left eye can only receive red images through the red-light filter on the left side, as only the red component is passed; while on the right side, only cyan images can be seen by the observer’s right eye as a result of the filtering of cyan light. In this way, different parallax images can be received by the two eyes respectively and stereo visualization is realized. Based on the same color filtering theory, the generation of a 3D stereo red-cyan image is the combination of the red-filtered left image and the cyan-filtered right image, as shown in Fig. 2. Anamorphic color often occurs in 3D stereo images seen through the red-blue glasses. There are also 3D glasses based on other kinds of color combination, such as red-green, brown-green, pink-green, and green-pink glasses.

There are other ways to obtain the 3D effect without the help of glasses or headgear, called auto-stereoscopic display [18]. One way is lenticular; an array of cylindrical lenslets is placed in front of the pixel raster, directing the light from adjacent pixel columns to different viewing slots at the ideal viewing distance so that each of the viewer’s eyes sees light from only every second pixel column. Another way is by parallax barrier; a barrier mask is placed in front of the pixel raster so that each eye sees light from only every second pixel column. There are other ways by using a combination of projectors and mirrors. However, the auto-stereoscopic display technology is still under development. Holographic technique can also be applied in 3D stereoscopic visualization.

2 Application of 3D stereoscopic visualization technology in the casting area

It is necessary for complex graphics to be displayed in the software window for every step of modeling and simulation of the casting process, such as building 3D casting models, generating finite-element or finite-difference grids, analysis of simulation results and so on. The display of various simulation results, including temperature, flow, stress and strain, displacement, solvent and microstructure fields, and casting defects such as shrinkage/porosity, deformation and cracks, is also in need of special graphic techniques. Conventional 2D observation methods are difficult for castings with complicated shapes. It is very difficult especially for people who are not familiar with the casting configuration to understand the images. In order to figure out the details of local areas, the

observers have to resort to the help of functions such as rotation, perspective view, slicing, X-ray like view, and magnification. The 3D stereoscopic visualization technology can assist observers to understand the casting structure and analyze the simulated results in pre-processing and post-processing easily and efficiently. This technology is especially suitable for the observation of objects such as casting, chills, risers, gating system, and insulation sleeves; and makes it convenient for observers to check out the positions of casting defects such as shrinkage/porosity, deformation and cracks.

A 3D stereoscopic visualization system has been developed based on EnSight. It can accept the data format from FT-STAR, developed by Tsinghua University for modeling and simulation of castings; and other commercial software for castings, such as Magma, ProCast and ADSTEFAN; and finite element method based analysis software, such as Ansys, Abaqus, I-Deas and Deform. Most importantly, it can accept STL files which are

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Fig. 3: Application of 3D stereoscopic visualization

widely used in modeling and simulation. The 3D stereoscopic visualization can be shown in EnSight by images and movies; all kinds of operations such as rotation, slicing, zoom and transparency are also provided. Meanwhile, the basic 3D format, side by side format, can be saved; and then can be displayed in 3D visualization software by typical ways such as anaglyph, polarization and shutter. The last two ways need the help of a 3D graphic card, 3D display or 3D projector and related glasses. For the red-blue method, there is no special requirement for the display: an ordinary monitor, display, TV set, or projector can display it. The only unique requirement is for the observers to wear red-blue glasses. This visualization system can be used for engineers to do research and for discussion or presentation in conferences, as shown in Fig. 3. The red-blue images can be printed out as paper, book or poster and the 3D stereoscopic effect still exists, which provides a convenient tool for the communication of complicated shapes and related things.

The application cases of 3D stereoscopic technology by red-

blue format in comparison with traditional images are shown in Figs. 4-7. Those 3D images must be observed with the help of red-blue glasses; otherwise, it may seem unclear with shadows. In Fig. 4, it is hard to judge the position of the shrinkage because of overlapping in the traditional image; while it is easy to see the shrinkage defects in the four risers and in the casting even though there is overlapping in the 3D stereo image. In Fig. 5 the mold filling of an aluminum cylinder block by low pressure casting is shown in both traditional and 3D stereoscopic imaging. In Fig. 5(a), it is hard to judge the relative height of the melt level; while, in Fig. 5(b), the melt level is easily discerned, the edge is lower than the middle, and the flow is downward from the middle to the lower edge. In Fig. 5 (c) and (d), similar results can be seen. Figure 6 shows the comparison of the two methods of illustration of the deformation of the hydro turbine blade for the Three Gorges Project. In Fig. 6(a), the shape of the blade can’t be figured out, while in Fig. 6(b), a spatially twisted shape can be seen. The original shape is the yellow contour, the

(a) Personal use (b) Discussion

(a) (b)

Fig. 4: Comparison of traditional illustration (a) and 3D stereoscopic visualization (b) of the shrinkage/porosity of the steel casting in Fig. 2

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Fig. 5: Comparison of traditional illustration (a) (c) and 3D stereoscopic visualization (b) (d) of the filling of a cylinder block casting at different moments

other one is the deformed shape, their relationship is also clearer than in the traditional method. A core assembly for an aluminum casting is shown in Fig. 7. There are eight pieces of sand blocks,

(a) (b)

i.e., a pair for the box, and three pairs for sand cores. Therefore, The 3D stereoscopic visualization can be used to show the assembly process of cores as well.

(d)(c)

Fig. 6: Comparison of traditional illustration (a) and 3D stereoscopic visualization (b) of the deformation of the hydro-turbine blade casting for the Three Gorges Project (The yellow contour is the original shape.)

(a) (b)

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Fig. 7: Comparison of traditional illustration (a) and 3D stereoscopic visualization (b) of core assembly for an aluminum casting

3 Prospect of 3D stereoscopic visualization technology

The problem with 3D stereoscopic visualization technology at present is that the images must be observed through glasses. Currently, many corporations and institutes are making efforts to develop naked-eye 3D visualization technology, so a great breakthrough should be achieved in the near future. Once naked-eye 3D visualization technology becomes reality, all of the visualization equipment may be upgraded, and a revolution in visualization technology will be brought about. In the current situation, 3D digital technology is gradually permeating into all fields, many of which are relevant to the casting industry, such as 3D scanning system, varieties of 3D design and modeling software such as AutoCAD, UG, Pro/e, Solidworks, Maya, and Sketch; as well as the tomography technique used in perspective scanning [19,20]. All of these fields provide a huge potential market for 3D stereoscopic visualization technology.

4 Conclusions3D stereoscopic visualization technology, which is currently developing rapidly, is a revolutionary technology in the visualization platform and has great potential in the field of modeling and simulation. It is applied in the modeling and simulation of casting processes, including the visualization and observation of temperature, flow, stress and strain, displacement and microstructure fields, and casting defects such as shrinkage/porosity and cracks. It could greatly improve the observation and analysis efficiency and the investigation of detailed results of complicated casting structures. Revolutionary change will take place in the post-processing of modeling and simulation of castings once breakthroughs in naked-eye visualization technology are achieved in the future.

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