A novel approach in high performance deep hole drilling of Inconel 718

Abstract

Application of deep holes is extensively found in the oil and gas industry to manufacture downhole equipment. These deep holes are fabricated in drill collars to deploy delicate sensors downhole for obtaining vital information like temperature, pressure and other geo-physical data from the bottom of the hole. High temperature and corrosion resistant materials like Inconel 718 is often used for drill collars to shield these sensitive communication equipment from the hostile downhole environment. Gundrilling is an established hole making method known for drilling precise and high aspect-ratio (AR>10) holes in conventional materials such as steels, cast iron and aluminium alloys etc. However, gundrilling of Inconel 718 offers unprecedented challenges like excessive tool wear, hole straightness deviation due to poor thermal conductivity and work-hardening tendency of the alloy. Moreover, as the drilling depth increases the deviation is further exacerbated due to the low rigidity of long gundrill shafts. The present study in the Phase 1 introduces a dynamic model using multispan Euler-Bernoulli beam theory to evaluate the effectiveness of coolant pressure and rotation speed during deep hole fabrication in Inconel 718. In addition, experimental investigations have been conducted to validate the model by studying the effects of coolant pressure and rotation speed on straightness of the fabricated holes. It was found that the straightness deviation was 0.37 mm corresponding to 137.89 bar and 1600 rpm coolant pressure and rotation speed respectively. In Phase 2, a non-contacting EDM process was used to fabricate a straight guide hole followed by conventional gundrilling (CG) with optimum pressure (137.89 bar) and speed (1600 rpm) obtained from the Phase 1 trials. The experimental results revealed that the thrust force reduced drastically from 800 N (CG) to 250 N (EDMG). Moreover, a significant improvement in both rake wear and flank wear was also observed. Finally, a straightness deviation of 0.19 mm was recorded for a maximum drilling depth of 350 mm which is 48.65% lesser than CG. From the study, it was found that this novel EDMG process improved the drilling capability of Inconel 718 by reducing the thrust force generated, enhancing tool life and minimizing the straightness deviation.