Machining surface roughness and tool design of the

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Surface roughness and tool design of particle reinforced composites (II)

1 Test conditions

in order to improve the machining surface quality of particle reinforced metal matrix composites and design reasonable cutting tools, firstly, the effects of cutting conditions and composite structure on the machined surface roughness of particle reinforced metal matrix composites are studied, and the following cutting tests are carried out

three SiC particle reinforced aluminum matrix composites with different particle sizes and volume fractions were selected for the test. The particle size parameters are: material 1: 14 m, 10%; No.2 material: 40 m, 20%; Material 3: 63 m, 20%. The base material is cast aluminum alloy ZL109. The test piece is shaped by 160MPa, extruded and cooled to form a hollow cylinder, and then heat treated by T

use polycrystalline diamond tools (PCD) for cutting. The geometric parameters of the tool are: front angle 0=0, rear angle 0=11, main deflection angle r=75, auxiliary deflection angle R =15, and tool tip arc radius r=0.20mm. Considering that there are large SiC particles in No. 3 material and there is a large impact during cutting, the tool adopts 0 rake angle

the range of cutting parameters selected in the test is: cutting speed vc=80 ~ 250m/min, feed rate f is 015mm/r, 0.20mm/r, 0.24mm/r, 0.28mm/r, 0.30mm/r respectively, and cutting depth (back feed) AP is 0.2mm, 0.4mm, 0.6mm, 0.8mm, 1.0mm

2. Test results and analysis

precision technology is a set of specially set tolerance values

from the curve obtained from the test, with the increase of cutting speed VC, the machined surface roughness decreases slightly. This is because with the increase of cutting speed, the cutting deformation decreases, and the cracks and scale thorns also decrease. At the same time, the increase of temperature reduces the elastic recovery of the reinforced phase particles pressed during cutting. Due to the increase of cutting deformation, with the increase of cutting depth AP, the change of machined surface roughness of the material is not obvious, but increases slightly

according to the test results, the roughness of the machined surface will increase sharply with the increase of the feed rate F. Under different cutting speeds of vc=80 ~ 200m/min, changing the feed rate for cutting, the test results have the same regularity. RA values vary from 3.15 to 11.0 M

the comparative test results also found that the surface roughness values obtained when changing the cutting speed and cutting depth were lower than those when changing the feed rate, because the former adopted a smaller feed rate (f=0.10mm/r). Because the feed rate is less than the arc radius r of the tool tip, the cutting surface is repeatedly extruded in the cutting process, so the surface roughness is improved. When the feed rate f is equal to or greater than the arc radius r of the tool tip, the cutting surface is cut out by the tool at one time, and there is no ironing and finishing effect. Therefore, the machined surface has the cutting surface characteristics of particle reinforced composites

from the test results, we can also see the influence of the material structure on the machined surface roughness. Since the reinforced particle size (14 m) of material 1 is smaller than that of material 2 (40 m), the machined surface roughness value obtained by cutting is also small. This is because in particle reinforced composites, the synergistic effect between the matrix and the reinforcement has a great impact on its cutting behavior. The reinforcement in the material is an obstacle to the plastic deformation of the matrix, so the cutting deformation increases and the machined surface roughness increases. This conclusion is consistent with the research results of many scholars. However, the test results show that the cutting surface roughness of material 3 with large particle size is almost smaller than that of the other two materials, which is contrary to the conventional conclusion. According to the analysis, this is due to the non-uniformity of the composite structure containing coarse reinforced particles, which leads to serious plastic deformation of the local matrix during processing, and the sharp wear and damage of the tool caused by cutting coarse hard particles. Therefore, the cutting temperature is very high, and the matrix is softened by heating and extruded by the worn flank, so that the machining surface is relatively flat

IV. tool design to improve the cutting performance of particle reinforced metal matrix composites

1 Cutting tool wear when cutting particle reinforced metal matrix composites

in sicp/al materials, the hardness of silicon carbide particles is as high as hv2700 ~ 3200, while the hardness of conventional YG cemented carbide tools is only about hv1800. Therefore, during the processing process, silicon carbide particles have severe friction with the front and rear tool surfaces, thus accelerating the life cycle of tool wear products. The test results show that the abrasive wear of the main and auxiliary flanks of the tool is the main failure form of the tool when turning cylindrical workpiece, especially when the particle size of the material is small. When the SiC particle size is large, the tool often breaks at the main cutting edge due to the impact on the tool due to the local inhomogeneity and discontinuity of the material in the cutting process. With the increase of cutting time, the broken edge is blunt and grooved, resulting in the deterioration of the machined surface quality

2. Design of cutting tools

the difficulty of cutting particle reinforced metal matrix composites is that the tool wear is very fast. After cutting with ordinary cemented carbide tools for 30s, the tool wear VB has been greater than 0.4mm, and the machining surface has great randomness and poor quality. This is mainly because this material has a discontinuous multiphase structure with very different hardness. The cutting performance of the material shows plastic, brittle and amphoteric (the overall hardness is low, but the chips are in the form of units or crumbs). The blunt tool often forms a rough surface by squeezing a soft substrate. From the above analysis of sicp/al machining surface forming mechanism and surface roughness cutting test results, it can be seen that the size, morphology, volume fraction of reinforced SiC particles in particle reinforced composites and the feed rate in cutting parameters are the main factors affecting the cutting surface roughness. Therefore, in the design of cutting tools, it is necessary to effectively reduce the extrusion and sliding between cutting tools and workpieces according to the characteristics of elastic and plastic deformation during material cutting. By reasonably designing the cutting tool, changing the stress condition of the material, inducing the propagation of shear bands and cracks in the material along the direction of the ideal cutting line, forming a potential separation surface in the material in the front edge area, and then pressing and ironing it by the cutting edge, so as to actively control the cutting process, obtain good cutting surface quality and prolong the service life of the tool

after repeated tests, the geometric parameters of the designed cutting tool are: the rake angle of the tool is 5 ~ 10, with a large rake angle and a large polished arc radius; The front knife face has a large negative chamfer front angle. The width and front corner of the chamfer are related to the particle size of the material reinforcement phase, followed by the ironing belt with arc transition, so as to make the protruding hard points press down on the machining surface and reduce the scratch and wear of the cutting edge and ironing belt. The large rake angle enables the chip to flow smoothly along the rake face with a certain pressure. Due to the friction effect of SiC particles in the chip on the rake face, the self sharpening of the tool can be realized to a certain extent

according to this idea, a carbide cylindrical turning tool with ironing and smoothing function is designed. The tool material is YG8, and the geometric parameters of the tool are: front angle 0=6, main rear angle 0=12. About the metrological characteristics 0, the main deflection angle r=90, auxiliary deflection angle R =15, and the arc radius of the tool tip r=0.20mm. In addition, at the joint between the auxiliary cutting edge and the tool tip, the edge grinds a large arc transition, and a negative chamfer is grinded on the auxiliary cutting edge

3. The test results of ironing and finishing tool

the test results show that the tool with 90 main deflection angle can effectively reduce the radial component of the tool on the workpiece during cutting and reduce the rebound of hard particles in the material. Then, the roughness of the machined surface is significantly reduced through the ironing and finishing of the polished edge surface of the tool. In addition, the ironing effect of the tool can make the machined surface extend and stretch, and produce the plastic flow of the matrix material, which can bridge the surface micro cracks and reduce the phenomenon of stress concentration

it can be seen from the test that in order to obtain the best cutting surface effect, the height of the guiding smooth edge surface of the tool should be less than the average height of the micro roughness of the contour, so as to reduce the compression elastic deformation of the parent material; Its width should be greater than the feed rate f to achieve continuous ironing. For sicp/al materials with different particle sizes and volume fractions, the size of the guiding finishing surface will change slightly, which can be optimized through experiments

in order to verify the effect of the new tool, cutting tests were carried out on materials 1 and 2. The test results showed that after turning material 1 for 15 minutes, the tool did not produce obvious wear bands. After turning material 2 for about 40 minutes, the width of the tool's smooth arris increased by only 0.1mm. In the whole cutting test, the roughness value of the machined surface is basically stable, and the maximum Ra value does not exceed 0.8 M. After cutting and finishing, the test results of the machined surface roughness of the test material are as follows: the average roughness Ra of the machined surface of material 1 is 1.08 m; The average roughness Ra of material 2 is 0.52 m, which is smaller than the machined surface roughness test results cut by PCD tools. Compared with conventional tools, ironing tools have the effect of ironing and smoothing, and the roughness of the machined surface is significantly reduced

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