The relationship between the sawing force caused by force and friction and the thickness of the workpiece and the thickness of the workpiece, the feed rate of the workpiece, the sawing speed of the wire, and the nature of the workpiece material provide a theoretical basis for the development of the multi-wire cutting machine.
1 Introduction Multi-wire cutting machine is a kind of hard and brittle material cutting equipment that has been rapidly developed in the past ten years, including the use of free abrasives and fixed abrasives. According to the movement mode of the wire and the structure of the machine, it is divided into reciprocating and unidirectional. Currently the most widely used in the optoelectronics industry is the reciprocating multi-wire cutting machine. Multi-wire cutting machines use high-hardness SiC as the abrasive. The most typical is the abrasive particles used in solar cell wafer cutting. The size is ten micrometers, which can be used for precision and narrow kerf cutting of hard and brittle materials. Achieve forming processing. With the application and development in the cutting of large-size semiconductors and solar cell sheets, multi-line cutting machines show a series of unparalleled advantages: small surface damage, small deflection, thin slices, and good sheet thickness consistency. It can cut large-sized silicon ingots, save materials, produce large quantities, and have high efficiency.
2 Multi-wire cutting machine key technology The process of cutting brittle materials with multi-wire cutting machine was first proposed in the 1970s. In the 1980s, a multi-wire cutting machine for silicon wafer cutting was realized. In the 1990s, especially in recent years, foreign multi-line cutting machines have developed rapidly, and their research is deeper. However, China's development of multi-wire cutting machine has just started. Although a lot of research has been carried out, there is no domestic multi-wire cutting machine. Many key technologies have not yet been put into practical use, so it is necessary to study key technologies of multi-wire cutting machines. However, the design and processing of multi-wire cutting machines must study the relationship between sawing force and SIC abrasive grains, and study the influence of sawing process parameters on sawing ability. The following are several aspects to be studied: In the movement, in addition to the composition ratio of the refining, the movement and action of the refining in the cutting zone, the influence on the cutting characteristics of the wire saw. By cutting single crystal silicon.
In the figure: the speed of the wire is Vs; the feed speed of the workpiece is Vw; the cutting thickness of the workpiece Lw is the radius of the wire R (diameter d); the density of the abrasive grain C. Since the sawing force of the multi-wire cutting machine is all Participate in the sum of the sawing forces of the cut SiC particles, so first analyze and calculate the sawing force, and establish the following coordinate system: the center of the over-saw, the direction of the workpiece feed motion is the x-axis, the over-wire axis and the wire The direction of the same direction of motion is the z-axis, and the line perpendicular to the x, the axis and passing through the center of the wire is the y-axis.
For example, in x, plane, take a micro-segment ds=R*d屮 at a small angle, then the micro-area of ​​the axial direction of the saw wire is dAn=Lw*ds=:Lw*R*d屮 in the micro-area dAn The number of inner abrasive grains is dNc=CdA. According to the theory of SmalKin and NHCOOK, the grinding force consists of two parts: the cutting deformation component and the friction force. Since the sawing process is essentially the grinding process, the sawing force can also be used. Divided into cutting deformation component and friction force, so the sawing force in the single-line micro-area is: force; dFt is the tangential sawing force in the micro-area along the direction of the wire movement; dFm is the normal direction of the cutting deformation in the micro-area Sawing force; dFns is the normal sawing force generated by friction in the micro area; dFtc is the tangential sawing force generated by the cutting deformation in the micro area; dFte is the tangential sawing force generated by the friction in the micro area.
For the cutting force of a single abrasive cutting edge (see), the conclusions using S.MaLKin are: the sawing force; Fet is the tangential sawing force of a single abrasive grain; Fen. is the single caused by the cutting deformation The normal force on the abrasive grain perpendicular to the surface of the wire; Fetc is the tangential force on a single abrasive grain caused by the cutting deformation; the Fens is the normal of the individual abrasive grains generated by the friction perpendicular to the surface of the wire Friction; Fets is the tangential friction on a single abrasive grain produced by friction.
2.1.1 Single SiC abrasive grains sawing force caused by chip deformation During the sawing process, under the action of high-speed feed pressure of the workpiece, SiC particles are pressed into the crystal at a certain depth, so the single SiC particles are The normal sawing force caused by the cutting deformation is: S is the average sectional area of ​​the chip.
The average sectional area S of the chips should be the product of the average thickness h of the undeformed chips and the average width b: according to the principle of volume invariance: Uw = U. * Nc: Uw is the volume of the workpiece material removed per unit time.
Nc is the number of SiC particles that participate in cutting per unit time.
Then, the volume of the material removed in the micro-area of ​​the SiC abrasive grains per unit time is: therefore, the average cutting thickness of the individual particles in the micro-area is: the average cutting area of ​​the single abrasive grains is: the tangential direction due to the cutting deformation Force, such as only considering the normal force acting on the front of the abrasive particles and ignoring the friction of the cutting and abrasive grains, and assuming that the abrasive grains are conical, the axis of the cone is perpendicular to and intersects the axis of the wire, according to the geometric relationship: The tangential sawing force caused by the cutting deformation on a single abrasive grain in a micro-area is: 2.1.2 The sawing force generated by friction is the normal sawing force and the tangential sawing force generated by the frictional force of SiC and silicon. It can be calculated by the following method.
The sawing process for silicon by a single SiC particle is similar to grinding, while normal grinding is the average contact pressure between the abrasive grain and the workpiece material. It is proportional to the hardness of the workpiece material under grinding conditions, so that: 4 is the average contact pressure of the SiC particles and the material to be cut; and hc is the indentation depth of the SiC particles, according to the geometric relationship: r=h, tg6 The normal sawing force of a single SIC particle caused by friction is: (Also the same, similar to the friction, the tangential sawing force is also linearly added with the contact area between the sic particle and the workpiece, and the friction between the SIC particle and the workpiece The coefficient is constant. If the friction coefficient is M, the tangential force generated by the friction between the single SIC particle and the workpiece is: the normal sawing force and the tangential sawing force of the single abrasive grain perpendicular to the surface of the wire are: Decomposing the normal sawing force perpendicular to the surface of the wire into the feed resistance Fenx along the x-axis and the force FF along the y-axis will cancel each other out. According to the geometric relationship, the feed resistance Fenx can be calculated as follows: The sawing force and the tangential sawing force are respectively: the above formula is integrated, and the total feed resistance and the total tangential sawing force caused by the cutting deformation and friction during the sawing process are obtained.
Simplification of 0 is a parameter related to the properties of the abrasive material. The shape of the abrasive grain is not discussed here, so it is considered to be a relatively fixed value. Then: r is the radius of the vertical projection of the SiC particles; 4 is the average contact pressure between the SiC particles and the silicon material; "is the friction coefficient; the relevant process parameter of the 601 type multi-wire cutting machine is R = 0.07 mm (the wire diameter L = 156mm (cutting solar energy; square piece 0.3~0.8mm/min (when cutting solar panels, the work can be seen from the above formula: the sawing force of the multi-wire cutting machine is proportional to the thickness of the workpiece and the feed rate of the workpiece, and The sawing speed of the wire is inversely proportional to the material properties of the workpiece.
Station standing speed range); Vs=2001000mm/min (the speed range is often set when cutting solar panels); 3.1 Analysis of the influence of the sawing speed on the sawing process When Vw-time, the wire movement Vs takes 200,400, 600,800,1000mm/min, the relationship between Vs and single saw wire normal sawing force Fn, tangential sawing force Ft, can be seen from the formula, normal sawing force and tangential sawing force The speed of the wire is inversely proportional and decreases as the speed of the wire moves. The effect of the speed of the sawing on the sawing process was calculated using a particle size of SiC abrasive of 1 000. It can be seen from the calculation that the sawing force decreases with the increase of the wire speed when the feed pressure is constant. For example, the analysis reason: as the sawing speed increases, the cutting is taken in unit time. The number of SiC abrasive grains will be large, so the sawing efficiency increases as the sawing speed increases. On the other hand, the increase in the sawing speed reduces the plunging depth of the single SiC particles, thereby lowering the sawing force.
3.2 Analysis of the influence of workpiece feed rate on sawing process When Vs-time, the table feed speed Vw is 0.2, 0.4, 0.6, 0.8mm/min, single saw wire normal sawing force Fn, tangential saw The relationship between the shear force Ft and Vw. It can be seen from the calculation that the normal sawing force and the tangential sawing force are proportional to the workpiece feeding speed, and the feeding speed of the workpiece is large and large.
When the particle size of the SiC abrasive is 1000, the wire speed is calculated. It can be seen that as the feed rate of the workpiece is large, the feed pressure is significantly large, and the workpiece feed speed is basically proportional to the feed pressure; as the feed pressure is large, the normal sawing force, the tangential saw The cutting force is also big. The analysis result: the feed rate of the workpiece is large, and the feed pressure is large, which increases the indentation depth of the SiC particles, thereby improving the sawing force and thereby improving the sawing efficiency. On the other hand, the feed pressure is large, and the cutting area of ​​a single SiC particle is added, so that the sawing force is also large. That is, the relationship between the feed rate of the table and the sawing force is that the normal sawing force and the tangential sawing force are proportional to the feed speed of the workpiece, and the sawing force will be large as the workpiece feed speed is high. However, the sawing force is too large and it is easy to cause wire breakage, so the workpiece feed speed should be in the range of 0.20.8 mm/min.
The relationship between the feed rate and the sawing force of the table 4 Conclusion In this chapter, under the condition of analyzing the force of SIC particles and saw wire, according to the sawing mechanism of multi-wire cutting machine, it is considered that the sawing force of multi-line cutting is mainly composed of two parts, cutting Deformation component and friction. From the analysis of the force of SIC particles, the influence of sawing process parameters on the sawing ability is studied. The sawing force is proportional to the cutting thickness of the workpiece and the workpiece feeding speed. With the feeding speed of the workpiece, The sawing force is also large, but the feed speed is not as high as possible; it is inversely proportional to the sawing speed of the wire, and the sawing force decreases as the wire speed increases. The sawing force is also related to the nature of the workpiece material.
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