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    Machinability and surface integrity investigation during helical hole milling in AZ31 magnesium alloy
    (KeAi Publishing Communications Ltd., 2023) Adhikari, Raviraja; Bolar, Gururaj; Shanmugam, Ragavanantham; Köklü, Uğur
    Conventional drilling has been widely used for making holes in structural materials. However, drawbacks like high cutting forces, poor surface finish, high cutting temperatures, excessive tool wear, and undesirable burr formation while drilling magnesium alloys have necessitated the development of alternative hole-making methods. Lately, the helical milling process has attracted interest in facilitating hole-making for assembly applications. However, the machinability of magnesium alloys using the helical milling process needs more investigation. Therefore, the presented work analyzed the influence of axial pitch, tangential feed, and spindle speed on cutting forces and surface integrity while milling AZ31 magnesium alloy. Axial feed was the most crucial factor contributing to the thrust force (71.8%), followed by tangential feed (13.2%). All three process variables impacted the radial force. Spindle speed was the most influential variable affecting the surface roughness (48.7%), followed by axial pitch (31.4%) and tangential feed (12.5%). Microhardness closer to the free surface of the hole was higher than the subsurface hardness. Moreover, microhardness showed an upward trend with the rise in axial pitch and tangential feed; however, it reduced with increased spindle speed.
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    Machining Temperature, Surface Integrity and Burr Size Investigation during Coolant-Free Hole Milling in Ti6Al4V Titanium Alloy
    (Mdpi, 2023) Shanmugam, Ragavanantham; Baloor, Satish Shenoy; Koklu, Ugur; Polishetty, Ashwin; Bolar, Gururaj
    Modern Aircraft structures use titanium alloys where the processing of holes becomes essential to assemble aerospace parts. Considering the limitations of drilling, the study evaluates the helical milling for hole processing in Ti6Al4V. The experimental evaluation was conducted by considering burr size, surface roughness, machining temperature, and microhardness under coolant-free conditions. The axial feed and cutting speed were varied at three levels, and nine experiments were conducted. The results exhibit a lower machining temperature during helical milling than during drilling. In addition, the helical milling helped to lower the surface roughness and size of the exit burrs. However, helical-milled holes showed higher subsurface microhardness than conventionally drilled holes. The process variables were influential on machining temperature magnitude. The highest recorded temperature of 234.7 & DEG;C was observed at 60 m/min of cutting speed and 0.6 mm/rev feed. However, the temperature rise did not affect the microhardness. Strain hardening associated with mechanical deformation was the primary mechanism driving the increase in microhardness. Helical-milled holes exhibited an excellent surface finish at lower axial feeds, while chatter due to tool deformation at higher feeds (0.6 mm/rev) diminished the surface finish. The surface roughness increased by 98% when the cutting speed increased to 60 m/min from 20 m/min, while a moderate increment of 28% was observed when the axial feed increased to 0.6 mm/rev from 0.2 mm/rev. Furthermore, the formation of relatively smaller burrs was noted due to significantly lower thrust load and temperature produced during helical milling.
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    Tool wear and surface roughness evaluation during drilling and helical milling in ti6al4v titanium alloy
    (Springer, 2024) Hiremath, Anupama R.; Köklü, Uğur; Bolar, Gururaj
    Conventional drilling operation is extensively employed for hole-making in structural assemblies made of titanium alloys. However, low thermal conductivity and work hardening behavior make it a difficult material to machine. Moreover, these properties can lead to temperature build-up and material adhesion, accelerating tool wear and failure. Also, high temperatures during drilling can alter microstructure and decrease fatigue and stress corrosion resistance. The study, therefore, investigates the utility of helical milling as an alternative for processing holes in Ti6Al4V titanium alloy. The two processes were evaluated by studying tool wear and its effect on surface roughness. Holes were processed using cutting speed and feed conditions considering parity in machining time. The twist drill and helical mill showed signs of tool wear in form of coating loss. In addition, work material adhesion at cutting edges of the two tools was observed. Moreover, helical milling displayed more serious built-up edge formation as compared to drilling process. However, severity of tool damage was significantly lower during the helical milling operation. Tool wear influenced surface roughness. Surface roughness increased as the tool wear progressed. But, magnitude of surface roughness was lower in helically milled holes than in drilled holes. Overall, initial assessment indicates helical milling as an adept process for making bores in titanium alloys. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024.