Wear mechanisms of silicon carbide-whisker-reinforced alumina (Al2O3-SiCw) cutting tools when high-speed machining aged Alloy 718

The paper is aimed at the identification and characterization of wear mechanisms of SiC whisker-reinforced alumina when turning aged Alloy 718 under different cutting conditions and when machining dry and with coolant. Secondary and backscatter electron microscopy accompanied by focus ion beam milling and EDX techniques were used for analysis of worn-out tools. Notch wear on the major cutting edge was found to consist of two notches: depth-of-cut notch and secondary notch located outside the chip area. The last was found to be governed by adhesion and attrition associated with adverse chip flow conditions. Formation of a minor notch was related to attrition by the defects found on the machined surface. Diffusion of Ni, Fe, and Cr into SiC whiskers was found to degrade them and facilitate adhesion. Chemical wear mechanisms were found to be responsible for degradation and decomposition of whiskers and formation of tribolayer on tool surfaces, which in turn was related to the reduced adhesion of Alloy 718 on the tool. Cracking on the tool rake and localized plastic deformation were found to further accelerate tool deterioration

Machinability variations in alloy 718 turbine components

Machinability Variations in Alloy 718 Turbine Components PAJAZIT AVDOVIC Department of Materials and Manufacturing TechnologyChalmers University of Technology AbstractAlloy 718 has a reputation for being very troublesome during the cutting process. It was suspected that this material was too hard and having too fine grain size and that these characteristics caused damage to cutting tools during processing. Sometimes it was difficult to machine in different regions in the same detail. In some cases it was possible to machine some discs without any problems whilst at the same time a number of discs were very difficult to treat. For Alloy 718 machined with Al2O3SiCW ceramic tools the main types of wear are mainly notch wear, flank wear and top slice wear. From retrieving spent round C670 tools in production, it was found that an optimum effective cutting angle range existed in terms of insert wear. This means that if the effective cutting edge length is controlled through programming for each feed rate lower wear rates should be obtained. To study the influence of different grain sizes, Alloy 718 work-piece material was heat treated. The results showed that the limiting wear type is the notch wear that occurred at the end of depth of cut, but that, no valid difference between rings with small (ASTM 8) and large grain sizes (ASTM 3.5) in terms of flank wear or notch depth were observed. However, burr formation was observed for both materials, but of different appearance depending primarily on grain size. The large grain size gave burr formation on the surface perpendicular to the cut surface, while the small grain size gave burr formation on the cut surface. Moreover, the wear characteristics and type of burr were accordingly obtained for the material with smaller grain size (ASTM 8) as in production with the same grain size. The implication of these results is that it may be possible to select more aggressive cutting parameters as long as the flank-wear remains below 0.3 mm and the notch wear is limited either through path programming (by varying depth of cut) or by reducing feed rate t

Machinability variations in Alloy 718 —With focus on machining of turbine components

In gas turbine engine, nickel base alloy, such as Alloy 718, accounts for almost half of the total material requirement. Nickel-base superalloys are used for gas turbine components mainly because of their outstanding strength and resistance to oxidation at high temperatures (> 550°C). The massive cost involved with the machining of nickel alloys has driven continued research and development of cutting tool materials as well as of cutting techniques that ensure higher metal removal rate with minimum surface and sub-surface damage on the machined components. In machining of Alloy 718, one of major issues is the machinability variations as the result of variation in the mechanical properties and microstructures from workpiece. Sometimes it was difficult to machine different regions of the same component and when that happened, machining issues arose with chip entanglement and tool failures. The aim of the research addressed in this thesis is to explain machinability variation in Alloy 718 turbine discs with respect to material parameters such a grain size, hardness, deformation and work hardening; all of which are believed to have great influence on tool wear. The investigation was based on the large amount of production data retrieved from the production scene and the experimental data from laboratory. With this work it is expected to refine and adjust the gap between the theory and the production practice to optimize the manufacturing process in general and machining process in particular. For Alloy 718 machined with Al2O3-SiCW ceramic tools the main types of wear are mainly notch wear, flank wear and flaking wear. From retrieving used round C670 tools in production, it was found that an optimum effective cutting angle range existed in terms of insert wear. This means that if the effective cutting edge length is controlled through programming for each feed rate lower wear rates should be obtained. A polar diagram method for describing and evaluating the machinability of Alloy 718 was developed. Five key parameters of the work material, representing the mechanical and physical properties which have strongest influence on its machinability, were employed in the construction of polar diagrams. Work materials of Alloy 718 in which the polar diagrams of machinability were similar in size and shape exhibited very similar behavior during the cutting process. A tool life model, named “ShortCut-Wear-Model”, was developed with consideration of work-hardening effect in the machining of Alloy 718 for tool life prediction. In addition, the statistic based model, Weibull model, was also used for tool life predication makes it possible to derive an optimal replacement strategy which will minimize the unit production cost and other costs associated with machining of Alloy 718

Performance and wear mechanisms of whisker-reinforced alumina, coated and uncoated PCBN tools when high-speed turning aged Inconel 718

Abstract in Undetermined Inconel 718, an efficient superalloy for energy and aerospace applications, is currently machined with cemented carbide tools at low speed (vc≈60 m/min) due to its unfavorable mechanical and thermal properties. The article presents results of a study of superalloy machinability with whisker reinforced alumina, uncoated and coated PCBN tools. Turning of age hardened Inconel 718 (45 HRC) was done under high speed machining conditions (vc=250…350 m/min). Aspects of tool life, tool wear and generated surface quality were studied. Application of uncoated PCBN tools resulted in surface quality and force level superior to other tool materials. Considerable side-flow of workpiece material was found to affect surface quality, especially for coated PCBN and ceramic tools. It was found that protective function of the coating, which increases the tool life up to 20%, is limited only to low cutting speed range. EDX and AFM analyses suggested dominance of chemical and abrasive wear mechanisms. EDX mapping of worn tools pointed absence of diffusional wear for PCBN tools and intensive degradation of whisker reinforcement in ceramic tools due to diffusion of Ni, Fe and Cr

Evaluating the Machinability of Inconel 718 Using Polar Diagrams

The use of a polar diagram method for describing and evaluating the machinability of Inconel 718 was explored. Five key parameters of the work material, representing the mechanical and physical properties, which have the strongest influence on its machinability, were employed in the diagrams. These five parameters were integrated into a single polar diagram, used to describe the machinability of Inconel 718. Variations in the machinability of Inconel 718 products or components of a given type produced in different batches were analyzed. Industrial experiments were conducted to test the relationship between the polar diagram of the work material, its carbon content, and the tool wear of the ceramic cutting tools used in machining it. Work materials of Inconel 718 in which the polar diagrams of machinability were similar in size and shape exhibited very similar behavior during the cutting process. The polar diagram method employed appeared to be useful for selecting suitable cutting data for the machining of new materials. [DOI: 10.1115/1.4002679