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Advances in Mechanical Engineering
Volume 2013 (2013), Article ID 628027, 6 pages
Mechanism and Motion of Semifixed Abrasive Grit for Wire-Saw Slicing
Key Laboratory of E&M, Zhejiang University of Technology, Ministry of Education & Zhejiang Province, Hangzhou 310023, China
Received 18 April 2013; Accepted 9 May 2013
Academic Editor: Shengyong Chen
Copyright © 2013 Chunyan Yao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The currently dominant method in the production of wafers is to use slurry wire-saw slicing. This paper reports a new wire-saw slicing technology, namely, semifixed abrasive wire-saw slicing. The traditional smooth wire is replaced by a patterned wire with a textured surface that can help the wire carrying the abrasives. It aims to improve the number of transient fixed abrasives in the machining process. Transient fixed abrasives can produce “scratch-indenting” processes that are similar to fixed-abrasive wire slicing thereby improving the efficiency of the wire-saw cutting. This study focuses on the behavioral mechanics of the abrasive grits in the slurry during the slicing process. The dynamic images of the movement of abrasive grits in the slicing process are obtained by a high-speed camera. At the microlevel, using statistical methods, the behavioral mechanics of the abrasive grits are investigated by changing the slicing parameters. The results contribute toward a more intuitive and profound understanding of the principle of free-abrasive wire sawing.
Wire-saw slicing technology has conventionally been used for cutting monocrystalline and polycrystalline silicon ingots [1–6]. The mainstream technology currently adopted is free-abrasive slurry wire-saw slicing. In the slicing process of the free-abrasive wire-saw, material is continuously removed through the interaction of the abrasive particles below the moving wire and the silicon surface. Different behavioral mechanics of the abrasive grits in the slurry determine the mechanism of material removal. Thickness uniformity of wafers is a critical quality measure in a wire-saw slicing process. Nonuniformity occurs when the material removal rate changes over time during a slicing process, and it poses a significant problem for the downstream processes such as lapping and polishing . A partial differential equation-constrained Gaussian process model is developed by Zhao et al. by global Galerkin discretization with three features incorporated into the statistical model .
So far, studies on the mechanism of free-abrasive wire saw have made some important advances. The earlier work done by Bhagavat and Kao described the removal mechanism during the wire-sawing process as a “rolling-indenting” model with three-body abrasion [3, 4, 9]. In this case, rolling abrasives between the moving wire and the workpiece are randomly rotated and indented into the surface under film pressure to generate cracks and make the material chip away from the substrate. Based on the rolling, indenting, and scratching modes of the free abrasives, slicing models of “rolling-indenting” and “scratch-indenting” were indicated by Yang . Bhagavat et al. studied hydrodynamic pressure in the slicing area, and the “rolling-indenting” model was verified with the observation of the surface morphology of the silicon wafer produced .
Through a comprehensive research of cutting mechanisms and abrasive movement states of multiwire sawing, Möller et al. pointed out the abrasive particles as freely dispersed in the cutting zone , which generate both a “semicontact” and “noncontact” case as illustrated in Figure 1. In the first case, the positive pressure normal to the surface and the shear stress parallel to it on the grains are exerted by the moving wire directly to rotate and indent into the surface, and the “rolling-indenting” mode is mainly the grinding pattern. In the second case, the force on the grits is supplied by the shear stress alone in the moving slurry to create a rollover effect.
Besides, Ge found similar results through the research and its mechanism of large-diameter silicon ingot precise slicing with low damage . Cheng et al. proposed a “rolling-indenting” and “rolling-indenting-chipping” model with multiple movement states . The main results of the experimental investigations suggested that the particles in the semicontact case would remove the material more efficiently, whereas, in the noncontact case, they only rotate without any efficient energy transmission to the surface [11, 12]. Thus, a novel semifixed abrasive wire-saw slicing technology, in which the wire is characterized typically by a surface texture that will expand the quantity of semicontact abrasives to improve the efficiency in the process of machining, is proposed.
Worldwide research on cutting mechanisms and abrasive movement states of multiwire sawing is based on assumptions and experimental verification [15–17]; the motion state cannot be revealed directly during the process, and, thereby, the cutting mechanisms are essentially studied by means of microscopic observations of the moving abrasive state in semifixed abrasive wire-saw slicing. In this paper, all the experiments are observed using high-speed camera equipment, and the dynamic images of the moving-grits state in the cutting process are obtained and the motion properties analyzed by computers. Analysis of the motion state of abrasive particles in the cutting process in terms of different process parameters and different wire patterns helps in the intuitive and profound grasp of the cutting principle involved in free-abrasive wire sawing [18–20].
2. Semifixed Abrasive Wire-Saw Slicing
Studies have discovered that surface material removal mainly depends on the semicontact particles driven by the moving wire in efficient cutting by the “rolling-indenting” mode, whereas the non-contact ones simply involve rolling and crashing. Therefore, a new kind of slicing method termed semifixed abrasive wire-saw slicing has been put forward, which is novel compared to the traditional free-abrasive wire sawing, wherein the wire appearance is reformed or rather the special surface microgroove structures lead to more particles into the semicontact state. By heading the cutting region, the particles are adhered to wire surfaces in the dual action of fluted geometry tessellation and slurry hydrodynamic effect, and the transient fixed abrasives emerge incidentally, referred to as semifixed abrasive state, as illustrated in Figure 2. It is these channels, in favor of turning more abrasive particles into the semifixed state, that make the abrasives embedded in the microgroove and no longer show rolling motion but produce “scratch-indenting” processes that are similar to fixed-abrasive wire slicing, thereby improving the efficiency of wire-saw cutting. In the experiment, a multistrand wire with naturally formed grooved patterns with individual strands of the same diameter is used as a semifixed abrasive sawing wire.
3. System Design
3.1. Experimental Observation Platform
The motion states of abrasive particles of free-abrasive and semifixed abrasive wire sawing were accurately observed and recorded in real time by the use of the Keyence dynamic three-dimensional digital microscope VW-6000/5000 and were analyzed by computer programs. WXD170 reciprocating wire-saw machine was also used in the simulated cutting experiment. The entire observation platform is depicted in Figure 3.
3.2. Experimental Conditions
For observations of the abrasive grits’ movements, the cutting parameters in simulation are different from those in reality, and the high-speed digital camera was set at the rate of 1000 frames per second (fps) with a magnification of 200× in the experiment. In addition, the shooting was carried out from the kerf side to enhance the viewing effect. As in the experimental conditions given in Table 1, mixed abrasive slurries were prepared by mixing black corundum abrasives and PEG300 at a proportion of 1 : 20. All sawing wires had the same diameter of 0.5 mm, including an individual strand called wire A, 1 × 7 multistrand wire called wire B, and 7 × 7 multistrand wire called wire C. Meanwhile, the pregrooved optical glass K9 was adopted as the workpiece.
3.3. Granulometry of Abrasive Particles
Owing to the purchased abrasive particle having a very extensive grain size distribution, sieving was done first for the determination of grain size and then the granularity was measured by a Malvern Mastersizer 2000 laser particle-size analyzer. The experimental abrasives have a mean size of 87 μm. Figure 4 gives the complete distribution.
3.4. Velocity of Moving Abrasives
Although the grits’ movement was triggered by the joint action of the moving wire and abrasive suspension, the high-speed camera equipment does not actually move during photography. The real-time continuous sequence of images was analyzed, and the adjacent marked points’ displacements were measured to get the net slip of a single grit. The average velocity of the particle was computed by the time lag between adjacent frames identified using the camera shooting parameters.
4. Experiments and Analysis
4.1. Abrasive Motion State in Semifixed Abrasive Slicing
During the semifixed abrasive wire-saw slicing, which is analogous to the free-abrasive wire sawing, the cutting area between the sawing wire and the workpiece is filled with mixed slurry driven by the moving taut wire to cut the workpiece. By the film formed with sufficient slurry swarming into the processing region, the wire appears to undergo elastic deformation and floats freely. With the wire running in high-speed feed, the slurry generates some hydrodynamic effect. Thus, the general mechanical behavior depends, to a considerable extent, on the hydrodynamic behavior of the abrasive suspension film in the cutting zone . Given the diverse film thicknesses and abrasive sizes, advancement of the abrasive comes in different forms of state probabilities. The abrasive motion state is plentifully observed in real time during the wire-sawing processes. Based on the abrasive’s own movement and in combination with the moving wire, the motion state can be classified into essentially two categories as the semifixed state and the free-rolling state. Abrasives in the former state are in closer contact with both the wire and the workpiece, playing a scratching role, which is similar to plastic cutting deformation. However, in the latter state, like a roller coaster, crashing with the wire and the workpiece and suspending in the slurry occasionally, abrasives thrive on the hydrodynamic effect to rotate and indent the workpiece surface.
4.2. Abrasive Motion Analysis in the Cutting Process
As mentioned above, the use of a dynamic three-dimensional digital camera during the simulated process allows easy observations of various abrasive movement states. Depending on the film thicknesses between the wire and the workpiece, the particles are either in direct contact with both the wire and the workpiece (semicontact case) or the particles are floating freely (noncontact case). As Figure 5(a) illustrates, three frames are continuously captured from the video image sequences, and it can be seen that an abrasive can readily get in touch with the moving wire and the workpiece simultaneously when the slurry film thickness is close to that of the abrasive particle diameter. The abrasives remain at the bottom of the cutting region, away from being attached to the work surface, and are partially embedded onto the moving wire leading to the removal of the workpiece material through an action similar to “ploughing,” and then the semifixed abrasives (semifixed state I) emerge. Figure 5(b) shows another facet of the semifixed state. The images are captured in every other frame during the process. It is also evident that a bigger grit has fixed itself onto the surface microgroove, rolled along it, and slid into the flank of the cutting area. When the abrasive particle’s diameter is greater than that of the slurry film thickness, it will produce the cutting effect by making contact with the workpiece. Otherwise, it just rolls through the abrasive suspension. It can also be seen in Figure 5(c) that the typical abrasive movement state in both the free- and semifixed abrasive wire-sawing processes is rolling characterized by a contact between the moving wire and the workpiece that is more abrasive. It is evident that, owing to the existence of the microgroove, more abrasives are transformed as transient fixed abrasives, that is, “semifixed abrasive,” to scratch the surface material and improve the efficiency of the wire-saw cutting process. Correspondingly, however, the slicing wire in the conventional free-abrasive wire sawing has no existing microgroove or surface structure, and, therefore, most of the abrasive particles collide back and forth during material removal under the actions of the moving wire and slurry hydrodynamics.
4.3. Abrasive Motion State Statistical Analysis in the Cutting Process
The abrasive movement state of the cutting process is visualized in a portion of the screenshots and then, considering the current practice, classified and analyzed, using statistical methods, in the computer by means of handling a video. Figure 6 gives a statistical analysis graph of different abrasive motion states in the central cutting area, showing the diverse movements of the sawing wires at a feed rate of 0.25 m/s.
The results of general free-abrasive wire-saw studies show that the slurry film is thickest on entry and gradually diminishes in thickness throughout the cutting region. Owing to the different abrasive particle diameters, the slurry film thickness in the intermediate section is usually larger than the grain size  leaving some big abrasive particles aside, which results in a relatively low quantity of transient fixed abrasives. Some abrasive particles have greater thickness than the wire and, hence, are only partially in contact with the wire and the workpiece, which coincides with the results of the granulometry of the abrasives. Under the same test conditions, however, the peak value of the hydrodynamic pressure in the semifixed abrasive state is always lower than in the free-abrasive state, which has relatively small film thickness. Thus, there are more transient fixed abrasives in the former state which can be easily carried into the work region to increase the quantity because of the existing surface microgrooves. It should be noted that the transient fixed chance of wire B mentioned above is slightly higher than wire C. This is due to the fact that there is a correlation between abrasive diameter and microgroove size within the same particle-size range, and, as a result of their similar surface structures, the bigger surface microgroove size gives wire B the ability to carry more abrasive into the work area to be converted to a semifixed state. Different abrasive motion states depending on slurry hydromechanics have distinct cutting mechanisms across the processing region. Hence, the results of our study show that higher cutting efficiency and quality can be achieved by controlling the abrasive motion state in order to regulate the manner of material removal.
To study the effect of feed rate on abrasive motion state, wire B was used to simulate the cutting action on the workpiece at two different feed rates. Figure 7 illustrates the effect of sawing rate on abrasive motion state and average velocity in the mid-cutting area. The overall percentage of number of grains in the semifixed state dropped off along with the feed speed increases. Based on two typical ways in which the abrasive is fixed in the semifixed state, the average moving velocity of the abrasive is further improved along with the increase in feed rate, which is in accordance with Professor Mölle’s results. The study done by Mölle assumed that, treating the abrasive suspension as a Newtonian fluid, the shear stress applied to the abrasive is proportional to the feed rate; that is, the shear stress of the abrasive particles increases with the increase in sawing rate so that the number of semifixed abrasives correspondingly decreases. This can be the important reason for the improvement in the average moving speed with the increase of feed rate.
In summary, we have investigated and proposed a novel semifixed abrasive wire-sawing technology for PV and microelectronic applications based on the above-mentioned theoretical and experimental results. This technology can increase the transient fixed abrasives by the use of the special sawing wire with a surface microgroove, which produces the “scratch” and “ploughing” action, which is akin to the fixed abrasive, to improve the slicing efficiency. In the experiment, a multistrand wire and an individual strand wire were used as the analog experimental wires. Many dynamic images, both in the semifixed and free-abrasive wire sawing processes, were captured through the dynamic three-dimensional digital microscope, which were analyzed in the computer, thereby contributing to the intuitive and profound understanding of the cutting principle of the free-abrasive wire sawing.
The main findings through the statistical analysis of the cutting process are as follows. First, there are distinct abrasive motion states, which, as mentioned above, are the semifixed state and the free-rolling state depending on the slurry hydromechanics across the work region in both the semifixed abrasive wire sawing and in the free-abrasive wire sawing. Second, the semifixed abrasive state can be classified into two categories of movement during the cutting process: the abrasives with diameters greater than the slurry film thickness between the wire and the workpiece are embedded in the microgroove, which roll with the moving wire and slide into the flank of the cutting area. Third, in comparison with the normal free-abrasive wire sawing, the semifixed abrasive wire sawing uses a different type of wire whose surface is characteristic of microgrooves texture that can make more grits turn into the transient fixed state to remove the material with a “ploughing” action. Finally, as the proportion of the semifixed state depends on the abrasive size distribution, higher cutting efficiency and quality can be achieved by controlling the abrasive diameter distribution and motion state to regulate further material removal in the semifixed process.
The work was supported by the National Natural Science Foundation of China (51075367) and Natural Science Foundation of Zhejiang Province (Y1090931).
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