Machinability variations in alloy 718 turbine components

Licentiate thesis, 2009

Machinability Variations in Alloy 718 Turbine Components PAJAZIT AVDOVIC Department of Materials and Manufacturing Technology Chalmers University of Technology Abstract Alloy 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 to <0.2 mm/rev. Finally, a polar diagram method for describing and evaluating the machinability of Alloy 718 was developed. To do this five key parameters that included adhesion, strain hardening, thermal conductivity, hardness and abrasiveness were selected to construct a polar diagram for different batches of Alloy 718 from data for similar production parts. Thereafter, related production experiments were conducted to test the relationship between the polar diagram, tool wear and the role of carbon content when machining with ceramic cutting tools. The results showed that materials like Alloy 718 with similar machinability polar diagrams in size and shape exhibited very similar behavior during cutting processes. Thus, the polar diagram method is useful and helpful to choose the suitable cutting data for a new material in machining processes. Keywords: Machining, Alloy 718, flank wear, notch wear, top slice, grain size, hardness, burr formation, disturbances, interruptions, variation in material, variation in tool, instability