"PERFORMANCE OF PVD-COATINGS ON CUTTING TOOLS FOR MACHINING INCONEL"
PERFORMANCE OF PVD-COATINGS ON CUTTING TOOLS FOR MACHINING INCONEL 718, AUSTENITIC STEEL AND QUENCHEND AND TEMPERED STEEL F. Klocke1, D. Lung1, S. E. Cordes1, K. Gerschwiler1 1. Laboratory for Machine Tools and Production Engineering, Chair of Manufacturing Technology, RWTH Aachen University, Aachen ABSTRACT The main objective of this research work is the examination of γ-Al2O3 coatings in dry and wet machining of difficult-to-cut materials, such as austenitic steel and nickel-based alloys and the quenched and tempered steel 42CrMo4+QT. Due to the outstanding properties of coatings based on Al2O3 for cutting operations, they have been deposited on indexable carbide inserts for turning and milling operations for several years now with the aid of CVD processes. The development of pulse tech- nology has made it possible to nowadays use even PVD processes to deposit oxidic coating systems at low temperature on cutting tools in a cost-efficient way. For a better understanding of the wear mechanism, the thermal loads when turning were measured. KEYWORDS: Coatings, Aluminiumoxide, Thermal load, Cutting performance 1. INTRODUCTION Coating systems based on aluminium oxide are predestined for machining tasks due to several properties, e.g. the chemical stability, the high resistance to abrasive wear and oxidation and a low proneness to adhesion. The α-phase of alumina, which is in general deposited by a CVD-process, is a common used coating material. Due to the excellent properties of alumina for cutting operations, every en- deavour has been made to realise as well the deposition of alumina with PVD-processes. The facility to use the PVD instead of the CVD-process for this task is opened since several years, but the coating processes have not proved satisfying because of the low achievable coating rates. Nowadays, with the use of the pulse technology, it is possible to deposit crystalline alu- mina coatings on tools in an efficient way /1/. 2. ALUMINIUM OXIDE AS A COATING MATERIAL The important characteristics of alumina for cutting operations are a high thermal and chemical stability, a high hardness at elevated temperatures and a very low tendency to adhesive effects. In general, aluminiumoxide coatings obtained from chemical vapour deposition are not applied as single layers, but are combined with other hard materials to give multi-layer coatings /2/. To observe if an interlayer is even necessary for coatings systems based on alumina and deposited with PVD- processes, cemented carbide inserts were coated with TiAlN/γ-Al2O3, on one hand with the TiAlN interlayer but also without it. The coatings were made by the Chair of Surface Engineering, RWTH Aachen University. These samples were analysed with calo tests and scratch tests, see figure 1. These tests pointed out the necessity of the TiAlN-interlayer. The coating system without interlayer already failed at a load 50 N, in contrast to the sample coated with TiAlN/γ-Al2O3, here was the critical load 110 N. Proceedings of the 7th International Conference Coatings in Manufacturing Engineering, 1-3 October 2008, Chalkidiki, Greece Edited by: K.-D. Bouzakis, Fr.-W. Bach, B. Denkena, M. Geiger, Published by: Laboratory for Machine Tools and Manufacturing Engineering (ΕΕΔΜ), Aristoteles University of Thessaloniki and of the Fraunhofer Project Center Coatings in Manufacturing (PCCM), a joint initiative by Fraunhofer-Gesellschaft and Centre for Research and Technology Hellas 101 Figure 1: Calo-Test and Scratch-Test. Additionally, these coatings systems were used in machining tests to confirm the necessity of the TiAlN-interlayer. These tests were carried out under dry conditions. Indexable cemented carbide inserts coated with TiAlN/γ-Al2O3 were used when turning X6CrNiMoTi17-12-2. The SEM-photos of the inserts from the machining tests in figure 2 show, that the cohesion of γ- Al2O3 to the cemented carbide substrate is not high enough. The coating is chipped in a large scale when using alumina as coating material without interlayer. In contrast, the TiAlN/γ- Al2O3 coating is nearly intact, there are only some areas with material transfer from the work- piece. Figure 2: Effect of the TiAlN-interlayer on coating cohesion as seen on turning austenite steel. 102 7th Coatings – 2008 3. TEMPERATURE MEASUREMENTS To investigate the thermal loads on cutting tools, on possible method is the measuring of chip upper and under side temperatures Tch. Chip upper side temperatures when turning austenite steel and Inconel 718 were measured, see figure 4. On the other hand, chip under side tem- peratures when turning 42CrMo4+QT were simulated with the FE-method and measured during real cutting operations, figure 5. The chip upper side temperature measurement is a relatively easy method to obtain a temperature band for a specific workpiece material and cutting parame- ters combination and gives a good value for the estimated magnitude of the thermal loads on the tool. All test results in the following are taken with a 2-colour-pyrometer. Figure 3 shows the func- tional principle of the used pyrometer /3/ and figure 3b the two used positions of the quartz fibre /4/. The austenite steel was machined in the continuous and interrupted cut, the nickel-based alloy Inconel 718 in the interrupted cut. The cutting test were realised with different cutting velocities. As the tables in figures 4 show, the cutting velocity has effect on the chip upper side tempera- tures. Chip upperside temperatures increase with higher cutting velocity. In contrast, the cutting forces decreases with higher cutting velocity. In summary, the thermal load increases and the mechanical load decreases by raising up the cutting velocity. a) b) dichroic 2-colour- object surface lens beam-splitter pyrometer quartz L fibre D quarz fibre IR-filter 2-colour- photodiode amplifier pyrometer Figure 3: 2-colour-pyrometer and positons of the quartz fibre for upper and under side tem- perature measurements. Cutting process modelling based on FEM showed that the higher thermal conductivity of nitridic coatings leads to a greater heat flux into the tool than it is the case with the oxidic coating based on γ-Al2O3, see figure 5a. During real machining tasks, the temperature measurements on the under side of the chip con- firm that the temperatures lie at a much higher level when using oxidic coating systems than when using the nitridic nanocomposite coating, nc-TiN/a-AlN. The reason for the higher tem- peratures measured on the under side of the chip when turning using the crystalline γ-Al2O3 may be found in the lower thermal conductivity of γ-Al2O3. This leads to more heat being led away via the chip, and thus less heat flowing into the substrate. This effect is particularly beneficial during dry-cutting processes. The temperature measurements were carried out during turning the quenched and tempered steel, SAE 4140+QT (42CrMo4+QT) using a 2-colour pyrometer. To measure the chip under side temperature, holes were eroded into indexable inserts in the region of the chip breaker in order to accommodate the glass fibre, see figure 5b. The measurement is carried out directly after the chip has left the contact zone. An integral value is calculated via the cross section of Coatings in Manufacturing Processes 103 Figure 4: Chip upper side temperature measurement when turning austenitic steel and nickel- based alloys Figure 5: Simulated and measured chip under side temperatures. the glass fibre (d = 0.5 mm). The measured chip upper side temperatures are good approximate values of the prevailing temperatures in the friction zone on the under side of the chip. 104 7th Coatings – 2008 4. MILLING 42CrMo4+QT In milling operations of the quenched and tempered steel, SAE 4140+QT (42CrMo4+QT) under dry conditions, the cemented carbide inserts coated with a multi-layer coating system, ml-TiAlN/γ-Al2O3, still did not reveal any comb cracks after a milling path of 12 m under a light microscope within the contact zone on the tool face, see figure 6a. Only when using a SEM very fine comb cracks could be detected even in the cemented carbide coated with TiAlN/γ-Al2O3, see figure 6c. By contrast, the inserts with the nitridic coating revealed initial comb-like cracks in the coating when examined under a light microscope after a milling path of only 4 m. These comb cracks then promoted either partial or even complete wear of the coating on the tool face and tool flank of these inserts. In the case of the milling inserts coated with crystalline γ-Al2O3, the contact zones remained completely coated until the end of the test (figure 6c); on the tool flank, a considerably smaller wear zone formed, see figure 6b. a) number of comb cracks b) flank wear VB / µm fc-TiAlN 15 150 TiAlN fc-TiAlN TiAlN 10 100 * fc-AlTiN 5 50 ml-TiAlN/γ-Al2O3 ml-TiAlN/γ-Al2O3 fc-AlTiN 0 0 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 feed travel per tooth lfz / m feed travel per tooth lfz / m * end of test due to c) rake face wear coating system: coating system: ml-TiAlN/γ-Al2O3 fc-AlTiN material: 42CrMo4+QT, process: face milling, ap = 2.50 mm, ae = 27.5 mm, vc = 250 m/min, fz = 0.20 mm tool diameter: d = 65 mm, insert geometry: SEKN1203AF.N-1, substrate: HW-P25, dry cut fc: fine-crystalline, ml: multilayer Figure 6: Milling 42CrMo4+QT: Effect of the coating system on the formation of comb-like cracks and flank wear. In milling operations, comb cracks are generated due to the thermal alternating stress in the hard-material coating and the cemented-carbide substrate occurring as a result of the inter- rupted cut. The observation that no comb cracks formed in the cutting part of the inserts coated Coatings in Manufacturing Processes 105 with TiAlN/γ-Al2O3 may be attributed not only to altered friction conditions in the contact zones but above all to the lower thermal conductivity of the oxidic coating system. As shown in figure 6, this leads to more heat being led away via the chip, so that the substrate is subject to less thermal stress. As the graphics in figure 6 effectively show, the performance potential of the in- serts coated with TiAlN/γ-Al2O3 is by far not exhausted even after a milling path of 12 m. 5. MILLING INCONEL 718 For milling nickel-based alloys, e. g. Inconel 718, cutting materials with great wear resistance and toughness are required due to the extremely high mechanical and thermal load on the cut- ting tool. In the interrupted cut the tools are subject to extraordinarily high mechanical and ther- mal alternating stress and the pronounced high-temperature strength of the material. This means that both HSS and cemented-carbide tools may only be used at comparatively low cut- ting speeds and feeds, and thus that only low chip removal rates may be achieved. The milling of these difficult to machine materials is extremely time-intensive and costly. That is why there is a great need for higher-performance cutting tools. The milling tests of Inconel 718 for assessing the performance of inserts coated with TiAlN/γ- Al2O3 were carried out during a down-cut inserted-tooth end milling operation with a circular arc tool entry /5/. Indexable cemented carbide inserts coated with different coating systems were used. An emulsion was used. Characteristic features of the wear formation on the coated cemented-carbide indexable inserts include high levels of chipping on the tool face and tool flank, see figure 7. This type of wear affects tool life to a certain degree in the case of all coated and uncoated inserts. The reason for this is the fracturing of the cutting material caused by the thermal-mechanical alternating stress, which leads to the formation of cracks and chipping on the cutting edge. Figure 7: Milling Inconel 718: Effect of the coating system on wear behaviour. 106 7th Coatings – 2008 6. CONCLUSIONS During milling operations on the nickel-based alloy, Inconel 718, the austenitic steel X6CrNiMoTi17-12-2 and the quenched and tempered steel, SAE4140+QT, crystalline γ-Al2O3 as coating system showed high performance due to a range of properties which predestine its use as a coating material. The necessity of a TiAlN-Interlayer when using γ-Al2O3 as coating material on cemented carbide was pointed out. The range of the thermal load on the cutting tool when turning austenitic steel, Inconel 718 and the quenched and tempered steel 42CrMo4+QT was measured. The deposition of TiAlN/γ-Al2O3 as multilayer coating systems provide a multitude of new possi- bilities for PVD coatings that are dedicated to the specific machining task. Enormous potential is thus available for increasing the performance of cutting tools. 7. ACKNOWLEDMENTS The work presented in this paper has been supported within the framework of the Collaborative Research Centre (Sonderforschungsbereich - SFB 442) entitled “Environmentally Friendly Tri- bological Systems” of the German Research Foundation (Deutsche Forschungsgemeinschaft - DFG). The authors gratefully acknowledge the financial support of the German Research Foun- dation (DFG) within the project number Kl 500/60-1. The authors thank for the generous sup- port. 8. REFERENCES 1. 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