Yusuf Kaynak

Cryogenic Machining-Induced Surface Integrity: A Review and Comparison with Dry, MQL, and Flood-Cooled Machining

The process of cryogenic machining, due to increased demand for environmentally friendly manufacturing processes, has seen a growing interest in the machining community. This article presents an overview of cryogenic machining and its induced surface integrity characteristics such as surface roughness, topography, grain refinement and machining-induced layer, microhardness, phase transformation, residual stress and fatigue life in machining of various materials including difficult-to-machine materials, aerospace alloys, lightweight materials, etc. The effect of cryogenic machining on surface integrity characteristics is discussed, and compared with dry, Minimum Quantity Lubrication (MQL), and flood-cooled machining processes. In addition to being an environmentally friendly process, this study shows that cryogenic machining significantly contributes to improved functional performance of machined components through its superior and more desirable surface integrity characteristics.

Cutting Speed Dependent Microstructure and Transformation Behavior of NiTi Alloy in Dry and Cryogenic Machining

The effects of cutting speed in cryogenic and dry machining on the surface integrity characteristics (the affected layer, microhardness, transformation response, transformation temperature, and latent heat for transformation) of NiTi shape memory alloys are investigated. It has been found that the cutting speed has remarkable effects on the surface and subsurface properties of machined NiTi alloys. Increased cutting speed results in decreased subsurface hardness and increased latent heat for phase transformation. In general, the depth of affected layers decreases with increased cutting speed in dry and cryogenic machining. Chips show a similar behavior as affected layer in terms of transformation response and microhardness. Cryogenic machining is found to have greater effects on the surface and subsurface properties of the machined work material in comparison with dry machining at all given cutting speeds.

Machining and Phase Transformation Response of Room-Temperature Austenitic NiTi Shape Memory Alloy

This experimental work reports the results of a study addressing tool wear, surface topography, and x-ray diffraction analysis for the finish cutting process of room-temperature austenitic NiTi alloy. Turning operation of NiTi alloy was conducted under dry, minimum quantity lubrication (MQL) and cryogenic cooling conditions at various cutting speeds. Findings revealed that cryogenic machining substantially reduced tool wear and improved surface topography and quality of the finished parts in comparison with the other two approaches. Phase transformation on the surface of work material was not observed after dry and MQL machining, but B19′ martensite phase was found on the surface of cryogenically machined samples.

The Effect of Active Phase of the Work Material on Machining Performance of a NiTi Shape Memory Alloy

Poor machinability with conventional machining processes is a major shortcoming that limits the manufacture of NiTi components. To better understand the effects of phase state on the machining performance of NiTi alloys, cutting temperature, tool-wear behavior, cutting force components, tool-chip contact length, chip thickness, and machined surface quality data were generated from a NiTi alloy using precooled cryogenic, dry, minimum quantity lubrication (MQL), and preheated machining conditions. Findings reveal that machining NiTi in the martensite phase, which was achieved through precooled cryogenic machining, profoundly improved the machining performance by reducing cutting force components, notch wear, and surface roughness. Machining in the austenite state, achieved through preheating, did not provide any benefit over dry and MQL machining, and these processes were, in general, inferior to cryogenic machining in terms of machining performance, particularly at higher cutting speeds.

Surface Characteristics of Machined NiTi Shape Memory Alloy: The Effects of Cryogenic Cooling and Preheating Conditions

This experimental study focuses on the phase state and phase transformation response of the surface and subsurface of machined NiTi alloys. X-ray diffraction (XRD) analysis and differential scanning calorimeter techniques were utilized to measure the phase state and the transformation response of machined specimens, respectively. Specimens were machined under dry machining at ambient temperature, preheated conditions, and cryogenic cooling conditions at various cutting speeds. The findings from this research demonstrate that cryogenic machining substantially alters austenite finish temperature of martensitic NiTi alloy. Austenite finish (Af) temperature shows more than 25 percent increase resulting from cryogenic machining compared with austenite finish temperature of as-received NiTi. Dry and preheated conditions do not substantially alter austenite finish temperature. XRD analysis shows that distinctive transformation from martensite to austenite occurs during machining process in all three conditions. Complete transformation from martensite to austenite is observed in dry cutting at all selected cutting speeds.

Improved surface integrity from cryogenic machining of Al 7050-T7451 alloy with ultrafine-grained structure

Abstract Materials with ultrafine-grained (UFG) structure become the prioritised selections for specific applications where high strength, hardness, wear/corrosion resistance, etc. are sought. Machining materials with UFG structure requires further research as their mechanical and material properties are different from the ones with regular coarse grains. In this study, Al 7050-T7451 aerospace alloy was first processed by friction stir processing (FSP) with UFG structure for use in subsequent machining experiments. Cryogenic machining using liquid nitrogen as the coolant was applied in the experimental work of machining Al 7050-T7451 alloy with UFG structure. Surface integrity characteristics including microstructure, grain size and hardness were investigated in order to understand the differences of these properties induced by dry and cryogenic machining. Smaller grains with the grain size of around 1.6 μm were observed in the machined surface generated by cryogenic machining compared to the grains of bulk materials and the ones acquired by dry machining. On the other hand, higher hardness measured from the surface and subsurface regions of the machined samples show a harder layer formation by cryogenic machining. Forces and temperatures were measured during dry and cryogenic machining. Cutting and thrust force components for both dry and cryogenic condition were found to be almost identical due to the homogeneous UFG structure achieved by the previous FSP while cryogenic machining generated lower cutting temperature. Also, microstructure and hardness of the chips formed by machining were investigated to understand the effects of cryogenic machining on materials with UFG structure from the dynamic recrystallisation and grain growth points of view. Grains with approximately 1 μm grain size existed in the machined chips, and the hardness of the chips was about 42% higher than the bulk material. Overall, the experimental findings demonstrate that the proposed approaches were beneficial to enhance surface integrity characteristics of this material.

Cryogenic Machining of Hard-To-Machine Material, AISI 52100: A Study of Chip Morphology and Comparison with Dry Machining

Cryogenic cooling is a new emerging cooling application in machining processes. Quantitative understanding of the effects of cryogenic cooling on the machining performance is important for continued applications. This study focuses on cryogenic machining of hard-to-machine material, AISI 52100, particularly with an analysis of cooling-induced chip morphology, chip hardness and the effect of workpiece hardness, etc., as these measures reflect the material`s thermo-mechanical behavior during the plastic deformation. AISI 52100 steel, with different initial hardness values, is selected as the work material for orthogonal cutting under dry and cryogenic cooling conditions, and the results are compared. The findings of this study show that cryogenic cooling affects the chip formation process, and the associated hardness produced on the machined surface.