To curtail the negative effects of traditional flood machining, dry cutting using carbide tools has emerged as a prominent alternative for manufacturers, owing to its low cost and phenomenal surface qualities. In line with this view, high-speed machining of high-strength AISI 4340 alloy steel was carried out using multilayer Al₂O₃/TiCN-CVD and TiAlN/AlCrN-PVD carbide tools in a dry environment. The experimental scheme was adopted, as per Taguchi’s L₁₈ orthogonal array, to investigate the two most crucial machinability aspects, namely tool life and surface roughness. An analysis of variance (ANOVA) was performed on the obtained data, and it was inferred that the feed rate exhibited the strongest effects on both the tool life and surface roughness, with corresponding percentage contributions of 46.22% and 68.96%, respectively. The longest tool lives of 14.75 min and 10.08 min were obtained at a low cutting speed and feed rate for CVD and PVD tools, respectively. However, the lowest surface roughness of 0.276 µm and 0.307 µm was achieved at a high cutting speed and low feed rate for PVD and CVD tools, respectively. The evolution of tool wear, studied through the microscopic images of the worn tools, revealed that a high cutting speed and feed rate accelerated the flank wear for both types of tools. Nevertheless, the CVD tool exhibited better results due to the thick and effective Al₂O₃/TiCN coating layer, which protected the carbide substrate against thermal–mechanical loads. Moreover, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) performed on the worn tools revealed that adhesion, oxidation, diffusion, and abrasion were the main wear mechanisms for both types of tools.

Hard turning is a precision machining process used to cut materials with hardnesses exceeding 45 HRC using single-point tools. It offers an efficient alternative to traditional grinding for finishing operations in manufacturing. This paper explores the machinability of hardened AISI 4340 steel for a hard turning process utilizing dry and cryogenic (Cryo) plus minimum quantity lubrication (MQL) (Cryo+MQL) techniques, focusing on critical machinability aspects such as cutting force, surface roughness, and tool life. The orthogonal dry turning was performed with a cutting speed (V) ranging from 300–400 m/min, a feed rate (f) between 0.05 and 1 mm/rev, and a depth of cut (doc) from 0.1 to 0.3 mm. A statistical analysis of the obtained results revealed that the feed rate was the most influential parameter, contributing 50.69% to the main cutting force and 80.03% to surface roughness. For tool life, cutting speed was identified as the dominant factor, with a contribution rate of 39.73%. Multi-objective optimization using Grey relational analysis (GRA) identified the optimal machining parameters for the hard turning of AISI 4340 alloy steel as V = 300 m/min, f = 0.05 mm/rev, and doc = 0.1 mm. The Cryo+MQL technique was subsequently applied to these parameters, yielding significant improvements, with a 48% reduction in surface roughness and a 184.5% increase in tool life, attributed to enhanced lubrication and cooling efficiency. However, a slight 4.6% increase in cutting force was observed, likely due to surface hardening induced by the low-temperature LN₂ cooling. Furthermore, reduced adhesion and tool fracture on the principal cutting edge under Cryo+MQL conditions justify the superior surface quality and extended tool life achieved. This research highlights the industrial relevance of hybrid lubrication in addressing challenges associated with hard turning processes.

Hardened layers are commonly required for automotive components after their production using a machining process in order to enhance the service life of these components. This study investigates the possibility of producing a nanostructured machined surface which can increase the hardness of the machined surface by varying the machining parameters under cryogenic conditions in end milling of AISI 4340. The end milling experiments were performed using multi-layered TiAlN- and AlCrN-coated carbide. Prior to the experiment, a finite element method (FEM) was used to simulate the cutting temperature generated and it had been found that at cutting speed of 200–300 m/min, feed rate of 0.15–0.3 mm/tooth, axial depth of cut of 0.3–0.5 mm, and radial depth of cut of 0.2–0.35 mm, the temperature generated can be sufficiently high to cause austenitic transformation. A field emission scanning electron microscope (FESEM) equipped with angle selective backscattered (AsB) detection analysis was used to investigate the microstructure and machined-affected layers of the machined surfaces. The crystallographic orientation/phase change and nano-hardness were analysed through X-ray diffraction (XRD) and a nano-hardness testing machine. The results showed that the cryogenic machining had significantly affected the surface integrity characteristics of the AISI 4340 alloy due to refined microstructure, favourable phase structure, and higher hardness near the surface layer. The results of this study may be useful in providing an insight into a potential technological shift from conventional surface case hardening processes to the present technique.

Magnesium alloy AZ91D is used extensively in the automotive industry because of its high strength-to-weight ratio. Typically, components produced using the alloy are required to have good surface finish and to contribute to high productivity but require long cutting times. Cryogenic cooling is an environmentally friendly technology which has been proven to improve cutting tool life and surface finish. This paper presents an investigation on the effects of dry and cryogenic cutting conditions at a high cutting speed regime for milling of magnesium alloy. This study focused on a high-speed regime due to the chips of magnesium alloy being highly combustible and an effective means of decreasing the temperature in the cutting zone was of great concern. The machining experiment was carried out using uncoated carbide end milling utilizing a full factorial design (L16) with cutting speeds of 900 m/min and 1300 m/min, feed rate of 0.02 mm/tooth and 0.05 mm/tooth, axial depth of cut at 0.2 mm and 0.3 mm, and radial depth of cut at 10 mm and 40 mm. For dry machining, the longest tool life at flank wear (VBmax) of 0.21 mm was at 30 min, which was obtained at cutting speeds of 1300 m/min, feed rate of 0.02 mm/tooth, axial depth of cut at 0.2 mm, and radial depth of cut at 40 mm. Using this cutting condition, a mirror-like surface of 0.106 µm was produced. For machining under cryogenic condition at VBmax of 0.2 mm, the maximum tool life of 1864 min was achieved at a cutting speed of 900 m/min, feed rate of 0.02 mm/tooth, axial depth of cut of 0.3 mm, and radial depth of cut of 40 mm. Under this cutting condition, a lower surface finish of 0.091 µm was obtained. It can be concluded that the application of liquid nitrogen (LN2) is very effective in enhancing the tool life and in obtaining a better-machined surface, especially at a lower cutting speed of 900 m/min. A longer tool life and high-quality machined parts will significantly improve the productivity and cost savings in the related industry.