Investigation of Dimensional Accuracy on Slant Type Taper Cutting By Wedm of Ni-Based Superalloy for High-Temperature Applications

Abstract

The demand for the usage of superalloys has increased due to their good mechanical properties in extreme atmospheric conditions. The aerospace industry uses about 60-70% of superalloys. They have an austenitic face-centred cubic having advantages such as better mechanical properties, higher modulus, higher solubility of alloying elements and systems of gliding plane. They can be classified as nickel, iron and cobalt-base superalloys. As nickel-based superalloys pose a higher fraction of melting temperature, therefore more favourable than cobalt-based and iron-nickel-based superalloys. Nickel-based superalloys have mechanical properties like good surface stability, high- temperature mechanical strength, resistance to corrosion or oxidation, and resistance to thermal creep deformation. These alloys can be used over a wide temperature having an operational range from -217°C to 700°C. Nickel-based superalloys can be strengthened through solid solution and precipitation. They can be used in different application as they have excellent fabricability, high-temperature corrosion resistance, and weldability but lower mechanical strength. Hastelloy-X is a nickel-chromium-iron-molybdenum alloy named after ‘Haynes Stellite Alloy’. As the Matrix stiffening is delivered by the molybdenum content results in high strength in a solid-solution alloy having good fabrication characteristics. It is one of the nickel-based superalloys which has applications in afterburners, gas turbine engines components, tailpipes, cabin heaters and flame holders etc. Nickel-based superalloys pose a major challenge during machining due to low thermal diffusivity and high-temperature strength. The conventional process leads to many surface defects like tearing, feed mark, surface crack, burr and tool defects like flank wear, creator wear, edge chipping, welding and adhesion of coated surface. Non-conventional machining techniques are the alternate methods used for machining such hard to cut materials with minimal surface and tool damage. From the literature, it was noted that nickel-based alloys can also be machined Laser Beam Machining, Abrasive Jet Machining and Electric Discharge Machining. Among different non-conventional machining process wire electric discharge machining (WEDM) was found to be the most precise and high-quality finishing. Although wire electric discharge machining is most accurate there will be minute errors during machining due to the complexity of the component. These errors are caused due to the flexible nature of the wire (electrode). The complexity of the component also demands accuracy in tapering operation which is achieved by making the wire slant or taper to the required angle through the movement of guideways during machining. This method has many drawbacks like insufficient flushing, wire bending, guide wear, wire break, angular inaccurate, surface damage. In the literature, it was seen that to achieve tapering was performed using convention tapering, change in wire circulation mechanisms and use hard materials as wire guides which avoids guide wear. For the present studies, an economic aluminium slant type fixture was employed to achieve tapering which is eaiser to manufacture. It was developed and fabricated to achieve tapering eliminating the major disadvantage i.e. bending of wire during conventional tapering in WEDM. This fixture provided the required angle to the workpiece (Hastelloy-X) where the required taper component can be machined. The cutting speed parameters like pulse on time, servo voltage, wire feed and servo feed were fixed based on the different performance parameters like cutting speed, surface roughness, surface crack density, angular error and kerf width for different taper angles (0°, 15°, 30°, 45° and 60°). As most of the application have taper angle ranging between 0-30° for different profiles i.e. triangular, square and cicular profiles were considered for three different angles such as 0°,15° and 30°. Further, the profiling parameters i.e. wire guide distance (WGD), corner dwell time (CDT), wire offset (WO) and cutting speed override (CSO) were explored in the machining of triangular, square and circular profiles at different taper angles (0°,15° and 30°). Output response parameters such as profiling speed, surface roughness, profile area, angular error, corner errors, recast layer thickness and microhardness were studied in detail to explore the effects of profiling parameters on triangular, square and cicular profiles and also the effect of taper angles on triangular, square and cicular profiles was also analyzed. Artificial neural network (ANN) and adaptive neuro-fuzzy interference (ANFIS) system were used to predict the profiling speed and surface roughness. Finally, different responses during taper profiling of Hastelloy-X using WEDM were explored and discussed in detail.