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Research progress on drilling technology of CFRP

Time: 2019-08-22 17:11  View:63

In the past few decades, the demand for high-performance, lightweight structures in various industries (such as aircraft, spacecraft, automobiles, ships, chemical processing equipment, and sporting goods) has increased, stimulating carbon fiber reinforced resin-based composites < The strong development of Carbon Fiber Reinforced Plastics (CFRP)>. At present, large aircraft body materials are replacing aluminum alloys with composite materials as the main material, and 50wt.% of composite materials is the starting point for aircraft in future. Typically, mechanical drilling is the final process of composite laminate parts. However, composite laminates are recognized as difficult to machine materials due to low drilling efficiency and undesired drilling delamination. Therefore, it is urgent to conduct in-depth research on the basic theories of CFRP cutting mechanism, tool material and structure, and processing technology.
 
1. CFRP cutting mechanism and numerical simulation technology
The anisotropy and non-uniformity of CFRP composites make significant differences in many aspects from the drilling of metals and their alloys, so CFRP drilling is a new challenge in the field of machining. Wern CW et al. conducted a photoelastic study of the stress field during the cutting process of fiber reinforced plastics (FRPs), measured the cutting force during processing, and used it for stress field analysis. The surface morphology of the machined surface indicates that The fibers are machined by shear and tensile fracture; when the fibers are tilted toward the tool, the fibers break by shearing and bending. Furthermore, it was observed that the maximum value of the fiber matrix debonding occurred when the fiber orientation angle was 45°, so the fiber orientation greatly affected the cutting force and stress. In order to explore the formation of CFRP cutting chips and its influence on surface roughness, Robert Voss et al. used a rapid knife-cutting device to orthogonally cut unidirectional CFRP composites to study chip formation and morphology and fiber orientation angles in different fiber directions. The relationship between the relationship and the variation law shows that for the CFRP laminate, the fiber laying direction directly affects the chip shape and surface roughness. Niu Bin et al. based on the micromechanical model to study the material removal mechanism of the CFRP unidirectional plate, and established the Microscopic to macroscopic cutting force prediction model; Chen Ming et al. cut T800 unidirectional laminates using right angle free cutting test, studied the cutting force and cutting heat of different fiber direction angles, and then drilled CFRP layer with three-point and eight-face drilling The slab is used to study the variation of cutting force, torque and cutting temperature during the drilling process of the two drills and its influence on the formation of drilling defects, and the surface of the hole wall is analyzed.
 
Due to the multi-scale characteristics of the CFRP composite itself, its structural response (such as stress, strain and damage failure process) under mechanical load also exhibits strong multi-scale characteristics, compared with conventional metal cutting. The process is complicated. Therefore, the research on the drilling mechanism of CFRP unidirectional laminates and the formation mechanism of pore defects can not simply apply the classical metal cutting theory, nor can it be studied unilaterally at a certain scale.
 
 
In recent years, it is difficult to accurately realize the traditional cutting simulation in the simulation of anisotropic composite fiber breakage defects. The new CFRP cutting numerical simulation technology has begun to develop, and Calzada KA has established a microstructure-based CFRP. The finite element model of composite processing introduces a new interface modeling approach that uses continuous elements to model the material interface and allow it to fail during stretching or compression. The model can describe the fiber fracture mode occurring during the whole chip formation process, and study the fiber fracture characteristics and cutting force in the chip at 0°, 45°, 90° and 135°. At the same time, the 3D numerical simulation technology of CFRP drilling is slowly developing. Phadnis V.A. studied the influence of cutting parameters on drilling axial force and torque through experiments and numerical calculations. A unique three-dimensional finite element model is established for the complex kinematics of composite laminates at the bit-workpiece interface. The cohesive zone unit was used to simulate the interlayer delamination in the composite material, and the drilling damage was detected by X-ray micro-computer tomography. The results show that the surface numerical model is in good agreement with the experiment. Therefore, this model can be used to predict the optimal drilling parameters for CFRP composites. Isbilir O. and Ghassemieh E. established a three-dimensional drilling finite element model of CFRP composites based on Hashin failure theory, using a layered debonding model of bonded contact zones to compare various step drills and twist drills. The drilling performance proves the feasibility of the model for the optimization of the drill geometry.
 
There are three main methods for numerical simulation modeling of composite two-dimensional orthogonal cutting: meso-mechanical analysis, macro-equivalent homogenization (EHM) and a combination of these two methods. Micromechanical methods and macroscopic equivalent mean methods (EHM) have their own advantages and disadvantages. G.V.G.Rao et al. combined the meso-mechanical method and the EHM method to simulate two-dimensional orthogonal cutting. The effects of fiber direction on cutting force and chip formation, as well as the degree of fiber damage, matrix damage and peeling were studied. With the development of micro-nano manufacturing technology, multi-scale analysis methods have been applied in numerical simulation of cutting process. Multi-scale analysis is a new method that considers the cross-scale and cross-level features of space and time and couples related scales. It is an effective method and technique for solving various complex computational materials science and engineering problems. Composite material refers to a multi-phase material system formed by complex space combination of two or more materials with different physical and chemical properties, which are microscopic, mesoscopic or macroscopic. The multi-scale analysis method is quite effective for the simulation of mechanical properties of composite structures.
 
2. Technology of CFRP cutting tools for hole
Hole making is an important part of CFRP secondary processing. However, the high hardness, anisotropy and low interlayer stress of CFRP unidirectional laminates hinder the development of CFRP hole making technology. In the CFRP laminate drilling process, delamination damage is considered to be the main hole defect. It has been reported that in the final assembly of the aircraft manufacturing field, the damage caused by the delamination damage of the CFRP laminate is as high as 60%, and the loss is also enormous. The research shows that the axial force during CFRP laminate drilling is the main factor causing delamination damage. There are many factors affecting the axial force of drilling, such as tool material and geometry, drilling parameters and coolant, but the tool The geometry has the greatest impact on it. Therefore, in order to achieve high-efficiency precision hole making of CFRP composite materials, it is necessary to improve the material properties of the tool, improve the tool geometry, and obtain a high-performance hole-making tool with high reliability and long service life. Liu Yang et al. optimized the structure of the double-point drill to improve the processing quality. Jain S. found that the width of the chisel edge is the most important factor affecting the axial force, and the increase in the number of cutting edges can reduce the maximum axial force. Therefore, the multi-blade processing method of “wearing and grinding” is introduced into the hole making process of CFRP. For example, Bao Yongjie and others use electroplated diamond nesting to drill CFRP composite materials. Similarly, Xu Jiuhua et al. use brazed diamond nesting drills. CFRP is made in the hole. Studies have shown that diamond nesting can achieve smaller axial force and better drilling quality when drilling CFRP, which is more suitable for CFRP processing. Jia Zhenyuan et al. used four different drill bits to drill T800 composite materials in order to influence the geometry of the drill bit on the axial and outlet delamination. The experimental results show that the greater the axial force reduction rate during the drilling phase, the larger the stratification factor. Tsao CC et al. studied the effects of drilling processes such as twist drills, saw drills, core drills, and step drills on the delamination defects of fiber-reinforced materials and axial force delamination. The impact of defects. Feito N. et al. compared the drilling characteristics of step drill and twist drill through simulation and experimental analysis. The results show that the step drill can obtain lower thrust and delamination coefficient at low feed rate. Xinyi Qiu et al. optimized the diameter ratio of the first and second drilling stages of the step drill based on the critical stratification force and the sidewall quality. The ratio is preferably 0.3-0.75 under the experimental conditions. Qiu Xinyi et al. analyzed the advantages and disadvantages of double-point drill, three-point two-edged drill and dagger, and designed a composite drill that can effectively reduce cutting force and outlet damage. Jia Zhenyuan et al. proposed a new type of drill bit structure to change the cutting environment of the exit, and the result can effectively reduce the export damage.
 
Drilling delamination and tool wear have become the two most challenging challenges in CFRP drilling. Improving tool materials and optimizing geometry are one of the effective ways to reduce drilling damage and achieve low-caliber hole making. It is also an effective way to improve tool life. The emergence and application of a new generation of high modulus CFRP places higher demands on the hole making tool, especially when the CFRP/metal laminate structure with more complicated hole making environment is more prominent.
 
3. CFRP hole quality control method
CFRP laminates are prone to burrs, delamination, ablation and other processing defects during the hole making process, which seriously affects the strength and fatigue life of the joint structure. S.R. Karnik et al. studied the relationship between drilling process parameters and stratification behavior at the entrance of CFRP flat drilling. CCTsao et al. comprehensively analyzed the relationship between bit wear and axial force and drilling delamination of twist drill carbon fiber composites. The wear rate was higher under different spindle speeds and feed rates. The greater the force, the more likely the stratification is to occur. Based on the Taguchi method, Paulo Davim et al. established a drilling experiment plan to study the relationship between cutting speed, feed rate and material delamination of CFRP sheets under special cutting pressure. Bonnet C. et al. studied the relationship between drilling force, delamination damage and fiber direction, gave the fiber direction of the maximum drilling force, and established the relationship model between delamination damage and fiber direction. Girot F. et al. studied the distribution law of drilling force along the main cutting edge and chisel edge, and established a CFRP layering model. Zhang Houjiang et al. studied the effects of cutting parameters on delamination defects, tearing at the exit of the drill hole, and fuzzing defects. Sun Luhua et al. studied the influence of drilling parameters such as tool, rotation speed, feed rate, number of drilled holes and material thickness on axial force under high-speed drilling conditions, and studied material thickness, rotation speed, feed, axial force and The relationship between tool wear. In CFRP, the thermal expansion coefficients of carbon fiber and epoxy resin are very different, and the temperature field gradient formed during the drilling process is large, resulting in the generation of thermal stress, deteriorating the processing quality, and causing ablation defects in severe cases. Bao Yongjie et al. established a drilling temperature field model for diamond abrasive tools. Li N.Y. et al. studied the effects of fiber grating temperature measurement technology and cutting temperature on drilling defects. These efforts enrich the research on the thermal effects of the CFRP drilling process. In addition, in recent years, the online detection technology for CFRP hole delamination has also been carried out, which shows that the online detection method of CFRP delamination damage is indispensable for maintaining the reliability of CFRP structure.
 
Although CFRP has been widely used in high-end fields such as aerospace and automotive, its quality of hole making is still problematic. Machining defects in composite drilling have been the underlying cause of the development of composite drilling processes. CFRP laminate hole defects are high-quality, low-cost manufacturing stumbling blocks, so it is an eternal theme to clarify the causes of hole defects and explore the technology of defect-free hole making.
 
Summary
The above analysis shows that the research of CFRP drilling has not yet formed a mature theory and method, and its pore making behavior, tool wear mechanism, pore defect and damage generation mechanism need to be further studied.
 
(1) At present, the research on CFRP cutting mechanism is limited to the appearance, such as cutting force, cutting heat, matrix damage and fiber breakage. It is not enough for deeper damage research and subsurface damage of the workpiece, especially lacking based on The drilling mechanism of CFRP laminates and the formation mechanism of pore defects are studied in depth.
 
(2) CFRP physical simulation modeling and finite element simulation technology are effective means to study the characteristics of cutting process. Due to the complexity of the simulation tools and the required computation time, it is still a challenge to implement the machining model in the industry. Compared with experimental research, the numerical simulation technology of drilling processing is not mature. It is worth noting that the finite element simulation analysis for anisotropic CFRP is just beginning. The construction of material constitutive model, multi-phase interface contact model and failure model It is an important prerequisite for accurate simulation of composite materials.
 
(3) Drilling is a complex machining process. The geometric parameters of the cutting edge of the twist drill change during the machining process. The rake angle of each point on the cutting edge varies greatly, making the conditions for chip formation very complicated. A large amount of literature has focused on how to change the geometric parameters of the drill tip to reduce the axial force of the drilling, thereby reducing delamination damage. There are few studies on the influence of the cutting edge edge structure on the drilling quality and the bit wear.