Abstract
Nickel-base alloys are proven materials in the fields of the aerospace and oil industry, which is due to their characteristic material properties of high temperature strength, high toughness and good oxidation resistance. These properties are beneficial to applications in technical components in general. However, they also represent challenges for machining. Especially while drilling Inconel 718, high temperatures occur in the chip-formation zone that implicate high thermal load in the material and thus, influence the surface integrity, for example, by causing white layers. Hence, the development of strategies to improve the ability to supply cutting edges with cooling lubricant is becoming increasingly important. In this context, an alternative process design, the discontinuous drilling, takes place, characterized by a periodic interruption of feed motion and thus, chip formation. A minor retraction movement from the contact zone enables the cooling lubricant to reach the cutting edges and to reduce their thermal load. In comparison to the conventional process of drilling Inconel 718, the effects of discontinuous drilling with varying numbers of interruptions on the resulting surface integrity and further parameters of drilling qualities are analyzed. Thereby, the prevention of process-related phase transformations due to thermal impact was discovered when a discontinuous drilling strategy was implemented.
Highlights
In contrast to developing optimized tool geometries used in conventional drilling processes, the approach of discontinuous drilling was designed as a new process strategy to improve the machining of nickel-base alloys by inserting targeted interruptions into the feed motion and permitting a reliable coolant supply to the cutting edges
Microscopic images of longitudinal sections separated from bore hole walls illustrate the appearance of white layers during the conventional drilling of Inconel walls illustrate white layers during the conventional drilling
The microstructure examined at the drilling depths subsequent to the point of feed interruption is free from thermal-induced phase transformations in both discontinuous processes
Summary
Inconel 718 is considered to be one of the most widely used alloys for high-temperature applications in the aerospace industry due to its high tensile strength, fatigue strength and temperature resistance, which ensure that the alloy’s capability is maintained at temperatures up to 700 ◦ C [1]. The mentioned properties qualify the material for applications in turbine blades, exhaust gas components, high-performance engines, reactors and turbochargers [2,3,4,5]. The nickel-base alloy is a proven material in the building of power plants and is applied in the oil and chemical industries. The described advantageous properties are equivalent to extensive challenges in machining. From the interaction of low thermal conductivity and high toughness, accelerated tool wear and reduced surface quality can be derived, which has to be emphasized in particular for drilling [1,6]
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