Influence of CFRP properties on drilling

Transcription

1 Influence of CFRP properties on drilling R. Royer a, E. Merson a a. Sandvik Coromant, CTSD, Advanced Manufacturing Park, Unit 8, Morse Way, Waverley, Sheffield, S60 5BJ, United Kingdom Résumé : Alors que l usinage des matériaux classiques est relativement bien appréhendé, ils restent de nombreux point d interrogations autour de l usinage des matériaux composites. Pourtant, le perçage des matériaux composites est un enjeu essentiel pour l industrie, en particulier pour l industrie aéronautique. Les matériaux composites, de par leur définition, sont un assemblage de différents matériaux aux propriétés différentes. Les conditions d usinage devraient être différentes pour ces différents matériaux impliquant des durées de vie d outils différentes. Cette étude a pour objectif de présenter l influence de différentes propriétés des matériaux telles que le type et le taux de fibres, constituant les composites. Cette étude a été menée sur une variété d outils ayant chacun des avantages et inconvńients en ce qui concerne la qualit du trou obtenu et la durée de vie de l outil. Un accent particulier est mis de même sur les propriétés de la résine telle que son type, ses propriétés mécaniques et la température de transition vitreuse. Enfin des indications d usure chimique sur les outils sont mises en évidence. Abstract : While the machining of classic materials is relatively well understood, there remain many questions about the machining of composite materials. Yet, the drilling of composite materials is a key issue for industry, especially for the aerospace industry. Composite materials, by their definition, are a blend of different materials with different properties. The machining conditions should be different for each different materials therefore implying different tool life. This study aims to present the influence of different material properties such as the type and proportion of fibres constituting the composites. This study was conducted on a variety of tools, each with advantages and disadvantages with regard to the quality of the resulting hole and tool life. A particular emphasis is put on the resin properties such as its type, its mechanical properties and the glass transition temperature. Finally some indications of chemical wear on the tools will be highlighted. Mots clefs : Machining ; composite materials ; drilling 1 Introduction Nowadays, the use of high performance composites has become very common in fields such as the aerospace industry. Although there is an extensive body of literature covering the damage mechanisms in drilling of composites and optimum geometries for composite drilling, there is little information on the machinability of different composite materials. Here, we are particularly interested in drilling of Carbon Fibre Reinforced Plastics (CFRP), although it is likely that the conclusions reached are as relevant to other machining operations such as trimming and milling. Although several papers have reported on tool wear of carbide tools in drilling of these materials [2, 3], none have yet extensively examined the effects that specific variables relating to the composite have on wear rate. Authors have concluded that the primary wear mechanism in drilling of CFRP is abrasive wear by the hard carbon fibres. Rawat et al. break down the abrasive wear further into two modes : Hard abrasion mode, where the fibres initiate cracks in the carbide grains, leading to their subsequent brittle fracture in a fatigue-like process. These grains then themselves act to abrade the surface of the tool. 1

2 Soft abrasion mode, where the cobalt binder is worn away by the fibres, thus increasing the surface area that undergoes fatigue cracking. In addition, anisotropy of the carbide grains is thought to lead to shearing of grains in certain orientations. In one case, the possibility of chemical wear or corrosion of the tool was considered [5]. However, this was rejected on the basis that the low cutting temperatures involved in composite machining (compared with metal cutting) would rule out chemical wear. Although the majority of the work in this field has focussed on machining of CFRP composites, extensive abrasive wear has also been reported in the machining of glass fibre reinforced composites [4]. If the key wear mechanism in CFRP drilling is abrasion due to the fibres, we would expect that the properties of the resin would have very little effect on tool wear, while the variables relating to the fibre properties would be highly significant. This work aims to examine these theories. 2 Geometry analysis In this section, the effect of different drill geometries will be assessed on six different materials. Three drill geometries have been studied : a classical drill with a 120 point angle, a spur point drill, and a double angle drills. Figures 1, 2 and 3 are representations of the geometries. All the drills are tugsten carbide drills. The six materials were chosen for their type, their toughness and their glass transition temperature. Table 1 shows the deciding properties of the resins. All the materials have the same woven fibre fabric, were produced using the same method and have the same fibre volume fraction. Figure point drill Figure 2 Spur point drill Figure 3 Double angle drill LTM 12 MTM44-1 VTM 264 MTM 28B MTM110 HTM515-1 Type Epoxy Epoxy Epoxy Epoxy Cyanate Ester BMI Toughness Low High Low High Low Low Tg ( C) Table 1 Type, Glass transition temperatures and toughness of the resins 2.1 Testing methodology An experiment was carried out with 5 different feed rates and 3 different drilling speeds. For each condition, hole quality and thrust force was examined. For the hole quality, both splintering and delamination were measured. A splintering ratio was calculated using equation 1. The delamination was measured and the values will are given in mm. The delamination was measured as the surface of composite where a debonding of a composite layer was visible using an optical microscope. The delamination value corresponds to the maximum distance from the hole periphery where debonding was observed. S f = A A c (1) 2

3 2.2 Results and analysis Table 2 shows the results for hole quality for the six materials. This table is the result of 3240 measurements. For each material, the optimal drill with the best cutting condition is given. The combination of the tool and cutting conditions were chosen in order to achieve minimum splintering and delamination. If an optimal solution could not be chosen, priority was given on delamination as it is an important measurement for the aerospace industry. When looking at all the tests, it is possible to see that the Tg is clearly important in influencing how a material behaves when drilled. High Tg materials will generally have quite poor hole quality compared with the low Tg materials. Another generalisation is that the spur drill appears to perform particularly well in the high Tg materials, while the double angle geometry is better in the low Tg resin composites. Tg ( C) Drill Speed (m/min) Feed (mm/rev) Splintering ratio Delamination (mm) LTM Spur MTM Spur VTM Double MTM28B 75 Double MTM Double HTM Double Table 2 Splintering and delamination for optimum drill and cutting conditions The average maximum thrust force (measured over 10 holes), in each material with each drill geometry for a speed of 40 m/min and a feed of 0.02 mm/rev, is shown in Table 3. These results suggest that for the low Tg materials, the double angle drill creates the lowest average thrust force, while for the higher Tg epoxies, the 120 point drill produces the least thrust. Neither the glass transition temperature or the toughness appear to have a direct effect on the thrust force. However, the MTM44-1, with it s high toughness and glass transition temperature, does give the highest thrust forces. Spur point 120 point Double angle MTM28B VTM LTM MTM MTM HTM Resin effect on tool wear Table 3 Average measured thrust forces in N In order to look into more detail the effect of the different material variables, it was decided to limit the study to one drill geometry, the double angle drill. This section will take a look the tool life for different resins and postcuring conditions. 3.1 Testing methodology A tool life test was carried out, using a single uncoated carbide tool with double angle geometry, using a speed of 4504 RPM and 0.08 mm/rev. Images of the tool were taken after drilling every 15 holes, and from these, flank wear measurements were taken on both cutting edges. These were then used to make a measure of how many holes the drill could produce before an unacceptable level of flank wear was produced. The limit taken for the flank wear was 3.5% of the tool diameter. 3

4 3.2 Composites with differing resins The tool life for the uncoated double angle drill in each of the different resin composites is shown in Table 4. From the data, we see that there is a huge range in tool life with different resins, with tool life a factor of 8 higher in some composites than in others. An important point to keep in mind is that the fibre reinforcement is the same in all the materials. Taking this data along with the glass transition temperature, we see that the main factor in determining tool life, particularly for the epoxy based resins, is the glass transition temperature. Another thing to note from the results is the difference in tool life in M21E and resins. Those two resins are similar in their hardness and Tg. This would suggest further that the wear mechanism is partly chemical as the exact chemical composition of the two resins probably differ. Resin Tg ( C) Tool life (holes drilled) MTM28B VTM LTM MTM LTM MTM M21E MTM HTM Table 4 Tool life and Tg for different resins 3.3 Composites with different postcure conditions The results shown in Table 5 show how tool life varies with postcure temperature. At low postcure temperatures (and therefore low glass transition temperatures), the wear rate is extremely low. However, it increases rapidly at intermediate levels. A useful conclusion of this is that should a drill be developed specifically for drilling composites before they have been postcured, there is less need for high value coatings than with tools for general composite use. The reason for the sudden drop in wear resistance as postcure temperature increases is currently unclear, and has not previously been reported in the literature. A possible explanation is that it is linked to the temperature reached when drilling. Where drilling temperature becomes higher than the glass transition temperature, the polymer will soften locally around the hole. This could then allow movement of fibres away from the drill, meaning that fewer fibres are cut through, and so the wear rate drops rapidly. Postcure Temperature ( C) Tool life (holes drilled) none Table 5 Tool life for different postcure temperature The results in Table 5 confirm that the condition of the resin is highly significant in determining tool life. This is not expected ; in papers discussing wear of tools cutting CFRP, the extreme abrasiveness 4

5 of the fibres is always cited as the reason for poor tool life, and mechanical wear is considered to be the predominant wear mechanism. Because the fibres are so much harder than the resin, it is unlikely that the resin would contribute significantly to mechanical wear. Therefore these results suggest that the wear is at least in part, chemical. Although tungsten carbides are generally considered to be relatively inert, it is well known that carbide cutting tools used in wood undergo a combination of chemical and mechanical wear [1]. The chemically active group in wood is thought to be tannins. In polymers, it is possible that the reaction is with an OH group, but further research on this is required. In the case of wood, the chemical wear is thought to primarily affect the cobalt, which then is more easily abraded away, to leave the surfaces of the carbide grains exposed. This mechanism is very similar to that suggested in work by previous authors, which may explain why it has not been identified previously. 4 Fiber effect Carbon fibre reinforcements are commonly divided into four categories based on stiffness ; Ultra high Modulus (Young s Modulus <450 GPa) High modulus (HM) fibres (Young s Modulus GPa) Intermediate Modulus (Young s Modulus GPa) High strength (Young s Modulus >200 GPa) The stiffness of the fibres is increased by increasing their crystallinity ; however, this has a detrimental effect on tensile strength. It also means that the fibres become more brittle, and will shear easily. Given brittle fracture is the key mode of cutting of the carbon fibres in the drilling process, we would expect that a change to the fracture properties of the fibre would result in differences in hole quality, thrust force, hole size and tool wear. Only tool wear was considered in thus study. 4.1 Testing methodology To examine the effect of fibre properties on tool life, three types of fibre were combined with two different resin systems, as shown in Table 6. All composites were produced from prepreg in an autoclave, to produce plates of thickness 10mm. The prepregs all had the same fabric weight, tow size, weave design and volume fraction reinforcement, and all were postcured as recommended by the manufacturer based on the resin used as before. 4.2 Results and analysis Table 6 show the results from the tool life test for different fibre types. It is clear for both the sets of material tested that tool life is higher in the high modulus carbon reinforced composite than in the intermediate modulus and high strength fibres. There are a few possible explanations for this : 1. The high level of crystallinity in the HM fibres will lead to lower toughness than in the high strength and intermediate modulus fibres. Where fibres are in effect single crystals (as with HM fibres) with only very low rotation angle grain boundaries between grains, there will be few barriers to crack initiation, which in effect will mean that the fibres fracture when cut with less force. 2. The fibres have a lower strain to failure, they will not brush the cutting edge to the same extent as other fibres. Because of this, the contact time for abrasion will be lower, and the tool life extended. This could mean that the relatively soft cobalt is not removed from between the grains to the same extent as in fibres that bend more during machining. 3. The temperature that the drill reaches is much lower for the high modulus fibre composite, because the fibres are so much easier to cut. This means the chemical wear due to the resin will be significantly reduced. 5 Additional proof of chemical wear mechanism As well as the evidence from the testing of different composite materials, there is also evidence to support the theory of a chemical wear mechanism in the results of testing of different substrates and coatings for machining composites. If the wear mechanism was purely mechanical, then we would 5

6 Resin Fibres Tool life (holes drilled) MTM44-1 High strength 210 MTM44-1 Intermediate Modulus 225 MTM44-1 High Modulus 360 VTM 264 High strength 823 VTM 264 Intermediate Modulus 788 VTM 264 High Modulus 1014 Table 6 Tool life for different fibre types in two different resins expect an almost direct correlation between the hardness of a tool material, and its wear rate. In fact, we do not see this happening. When an uncoated 4.2% cobalt grade (hardness 2200 Hv30) was tested alongside an uncoated 10% cobalt grade (hardness 1590 Hv30), no significant difference in tool life was observed. When a silicon nitride grade was tested (hardness 1530 Hv30), despite being only slightly softer than the 10% cobalt carbide grade, the tool life was reduced from 180 holes to 45 holes in the same material. This suggests that either there is a highly complex relationship between substrate hardness and wear resistance, or that the SiN is simply more susceptible to chemical attack from the resin than the carbide. The second explanation is considered to be the more likely. A chemical attack mechanism would also explain why non-carbon based coatings have poor wear resistance in CFRP composites, despite the fact that the hardness is in some cases superior to diamond like coating. 6 Conclusions The experimental work carried out for this study has allowed us to show that the resin used in composite materials has a significant effect in drilling on the extent to which damage occurs, and the thrust force measured. It was also shown that resin properties have a significant effect on tool life. Generally, composite materials with high glass transition temperature resins are more abrasive than the same composite with a lower glass transition resin. The effect of resin glass transition temperature on tool life was confirmed by testing the same composite, postcured to different temperatures. Material postcured to higher temperature was more abrasive than those postcured at lower temperatures. Based on these findings, a wear mechanism partially dependent on chemical wear was proposed. Références [1] Faraz, A., Biermann, D., Weinert, K Cutting edge rounding ; an innovative tool wear criterion in drilling CFRP composite laminates. International Journal of Machine Tools and Manufacture [2] Lin, S.C., Chen, I.K Drilling carbon fibre-reinforced composite material at high speed. Wear [3] Rawat, S., Attia, H Wear mechanisms and tool life management of WC-Co drills during dry high speed drilling of woven carbon fibre composites. Wear [4] Sakuma, K., Seto, M Tool wear in cutting glass fiber reinforced plastics. Bulletin of Japan Society of Mechanical Engineers [5] Sheik-Ahmad, J.Y Machining of Polymer Composites. pp. 125, Springer 6