Analysis of dry and cryogenic drilling on hole quality in AA2024-T6 using HSS and HSS-Co tools

Abstract
Machining, particularly drilling, plays a significant role in various industries, including aerospace and automotive, where efficient drilling is a critical factor in their operational success. However, low-quality holes can result in structural failures and increase production costs. Achieving high-quality holes necessitates appropriate selection of drilling tools, process parameters, and machine setup conditions. This study investigates the impact of drilling parameters, specifically feed rate and spindle speed, on chip formation and hole quality, including deviations from normal size, circularity, cylindricity, perpendicularity, and surface roughness. Experiments were conducted on aluminum alloy 2024-T6 (AA2024-T6), a widely used material in the aerospace industry. This paper investigated the drilling process parameters using uncoated high-strength stainless steel (HSS) and cobalt-coated high-strength stainless steel (HSS-Co) drill bits, with a 30° helix angle and 140° point angle, under both dry and cryogenic conditions with liquid nitrogen (boiling point: − 195.8 °C). Results demonstrated that HSS drill bits, in cryogenic conditions, produce holes with lower surface roughness, minimal deviation from nominal size, and reduced errors in circularity, cylindricity, and perpendicularity compared to those made under dry conditions. Conversely, HSS-Co drill bits yielded high-quality holes under dry conditions across all feed rates and spindle speeds. Furthermore, short and segmented chips were formed at high feed rates and low spindle speeds. No significant variations were observed in the chip’s formation for both HSS and HSS-Co drills. Additionally, scanning electron microscopy (SEM) was used to examine the internal defects of the drilled holes, providing further insight into the impact of drilling conditions on hole quality.