Influence of ionic-plasma hardening on the performance of broaches in machining high-temperature nickel alloys

Abstract

In the manufacture of Russian aviation engines, high-temperature granular nickel alloys obtained by hot isostatic pressing with subsequent deformation are widely used in turbine disks and shafts, as well as compressors. The main benefits of this technology include the production of the required granular structure of the alloys; improved mechanical characteristics; and the possibility of using more complex alloys for the manufacture of intricately shaped gas-turbine engines. Airplane engines with turbine disks made of EP741NP high-temperature nickel alloy, characterized by increased long-term strength, are successfully in use at present. Table 1 presents the mean mechanical characteristics of EP741NP alloys produced by hot isostatic pressing. A key operation in the manufacture of gas-turbine engines is the production of herringbone slots in the turbine disk for attaching the gas-turbine blades (Fig. 1). The herringbone joints must ensure simultaneous operation and uniform loading of all the junction components. To this end, the roller-section dimensions and the error in the increment on the working sides of the profile must be maintained within strict limits. There must be no distortion of the working sides of the profile and minimum skewing of one side of the profile with respect to the other. These geometric constraints impose high quality requirements on the machining tool. The herringbone slots in the turbine disk are obtained by broaching, which is currently the most rapid method of machining complex surfaces. The difficulties associated with producing slots in the turbine disk include the need for guaranteed total long-term strength of the broach. Given the high cost of high-temperature alloys and their resistance to machining, we must exercise great care in selecting the tool material. In constrained profile cutting by hard-alloy broaches, it is difficult to guarantee the specified total length with specified surface roughness, as shown by global experience [1‐3]. This may be attributed to the variable temperature loads and forces observed in broaching and also to the brittleness of the hard alloy. The use of hard-alloy tools is also limited by economic considerations, since their production and operation are very expensive.