The effects of cryogenic cooling on surface integrity in hard machining: A comparison with dry machining
Introduction
Traditionally and historically, manufacturing processes are planned and developed in order to achieve either maximum productivity or minimum cost. In contrast, present trends push manufacturers to develop new methodologies incorporating sustainability concepts [1]. Sustainable manufacturing processes are those which demonstrate improved environmental impact and energy and resource efficiency, generate minimum quantity of wastes, provide operational safety and personal health, while maintaining or improving the product quality [2]. In this context, hard machining has become a much more desirable finish-manufacturing process, compared to the traditional grinding process because of its ability to reduce production costs, increase productivity, and especially enhance product quality. However, there are several issues related to this process that require further investigation, and the major issue among these is the high temperatures at the tool-chip and tool-workpiece interfaces in conjunction with the plastic deformation, both strongly affecting the surface integrity and the quality of the machined product. In fact, the deformation process is concentrated in a very small zone, and the local high temperatures due to rapid heat generation have important consequences on the machined surface layer such as the microstructural alterations and white layer formation.
Although, numerous studies have been conducted on the white layer formation in machining of hardened steels [3], [4], [5], [6], [7], only a few studies investigate the effects of cooling, particularly the minimum quantity of lubricant (MQL), on this affected layer [8], [9]. The effects of cryogenic cooling on the surface integrity of the machined component have recently been shown [10], while no specific studies as yet reported the effectiveness of cryogenic cooling in hard machining. Therefore, the primary objective of this paper is to investigate the effects of cryogenic cooling on surface integrity (surface roughness, white layer thickness, residual stresses, grain size, etc.) in hard machining of AISI 52100 and compare the performance with dry machining at varying process parameters (cutting speed, workpiece hardness, tool edge conditions, etc.).
Section snippets
Cryogenic machining
Cryogenic machining presents a method of cooling the cutting tool and/or workpiece during material removal processes. The coolant is usually nitrogen fluid (LN) that is liquefied by cooling to −196 °C. Nitrogen is a safe, non-combustible, and noncorrosive gas. In fact, 78% of the air we breathe is nitrogen. The liquid nitrogen in a cryogenic machining system quickly evaporates and returns to the atmosphere, leaving no residue to contaminate the workpiece, chips, machine tool, or operator, thus
Surface roughness
The surface roughness values, Ra, of the machined sample were measured three times for each set of cutting process parameters and cooling conditions to evaluate the characteristic of the machined surface (Fig. 2), and averaged to obtain mean values. The obtained Ra measurements reflect the surface quality in machining with cryogenic coolant, and were found to be consistently superior to that obtained in dry machining. Fig. 2 also shows a mapped region called “turning replaces grinding” where
Evaluation of hard machining performance
Fig. 9 shows the overall evaluation of hard machining performance carried out under dry and cryogenic cooling conditions. In these charts, each parameter has been equally weighted, and its disposition was made by considering that the values on the outer layers offer improved and better surface integrity. In dry machining (Fig. 9(a)), higher cutting speeds and initial workpiece hardness provide major benefits in term of the parameters related to the fatigue life (i.e., deeper surface residual
Conclusions
Experimental observations reported in this study suggest that the use of cryogenic coolant in machining of hardened AISI 52100 steels significantly affects the surface integrity. In particular, cryogenic cooling conditions limit the white layer thickness and offer better surface roughness. In contrast, dry machining offers better performance on residual stress profiles and, therefore would contribute to improved fatigue life, although it produces a thicker white layer which is detrimental for
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