Papers by Keyword: Cryogenic Cooling

Abstract: Ultrasonic Vibration-Assisted Machining (UVAM) has been investigated as one of the most suitable methods to imprint patterns on metal surfaces. Actually, UVAM, as a consequence of the superimposition of a high-frequency and small-amplitude vibration to the cutting tool motion, makes possible the generation of micro-textured surfaces. On the basis of the vibration pattern and parameters, cutting process and tool characteristics, micro-dimples of different morphology and dimension can be obtained. However, the literature review has evidenced that none of the available studies considered the influence of the cooling conditions as well as the workpiece material on the obtainable dimples geometry. To this aim an UVAM experimental campaign was carried out at varying cutting speed, workpiece material and cooling conditions. Then, the surface finish was evaluated in terms of dimples size and surface roughness. Experimental results showed that deeper dimples are obtained for materials characterized by higher elastic modulus and machined under cryogenic cooling.

Abstract: Due to their poor machinability, titanium alloys need to be worked at low cutting speeds to prevent a fast tool failure caused by the very high temperatures that are reached at the tool-chip interface. As demonstrated by previous works by the authors, an improvement of productivity for Titanium alloys can be obtained by adopting cryogenic cooling during the machining operations.The present work shows the features of a toolholder specifically designed for cryogenic adduction in turning operations, following Hong’s design guidelines. The paper compares tool life results between traditional and cryogenic rough turning by adopting Grade 5 titanium as the working material. Rough turning is economically more relevant to the machining industry, especially in the aerospace field where generally a large quantity of rough material has to be removed due to the very high buy-to-fly ratio of aerospace components. A full factorial experimental plan was performed basing on typical rough turning parameters. Machining outputs such as forces, roughness, temperatures, friction coefficients were calculated in order to define statistical differences between cryogenic cooling method using the special toolholder and traditional oil water emulsion cooling system.Furthermore, thanks to tool life results the Taylor’s law for cryogenic and traditional cases was calculated and an hypothetical production scenario for Ti6Al4V parts was analysed. An analytical model to calculate production costs and time was built for both cooling methods. A 4-turrets turning centre was considered and cooling methods and costs per hour of machine tool were taken into account in a cost model. The results show that the benefits in terms of tool life offered by liquid nitrogen cooling allows to improve productivity by adopting higher optimal cutting parameters. This improvement, coupled to an increase of tool life, is very significant and allows not only to reduce time of production but also to cover the major costs of liquid nitrogen and have a slight reduction of machining total costs.

Abstract: In the area of tribology, surface roughness has become one of the most important factors that contributed to the evaluation of part quality during machining operation. In order to understand the behavior of cryogenic cooling assistance in machining Inconel 718, this paper aims to provide better understanding of tribological characterization of liquid nitrogen near the cutting zone of this material in milling process. Experiments were performed using physical vapor deposition (PVD) - coated carbide inserts under cryogenic and dry cutting condition. The cryogenic results of the cutting temperature, cutting forces and surface roughness of the ball nose cutting tool have been compared with those of dry machining. Finally, experimental results proved that cryogenic implementation can decrease the amount of heat transferred to the tool up to almost 70% and improve the surface roughness to a maximum of 31% when compared with dry machining.

Abstract: Residual stress is one of the most important surface integrity parameter that can significantly affect the service performance of a mechanical component, such as: contact fatigue, corrosion resistance and part distortion. For this reason the mechanical state of both the machined surface and subsurface needs to be investigated. Residual stress induced by dry and cryogenic machining of hardened AISI 52100 steel was determined by using the X-ray diffraction technique. The objective was to evaluate the influence of the tool cutting edge geometry, workpiece hardness, cutting speed, microstructural changes and cooling conditions on the distribution of the residual stresses in the machined surface layers. The results are analysed in function of the thermal and mechanical phenomena generated during machining and their consequences on the white layer formation.

Abstract: Titanium alloys, mainly because of their poor thermal conductivity, need to be cut at relatively low cutting speeds to avoid a severe diffusion wear, with obvious negative consequences on the profitability of machining. An important amount of research activities has been done in order to increase productivity in titanium machining operations and one of the most promising solutions is represented by the use of liquid nitrogen as a coolant during the machining operation. The aim of this paper is to compare traditional and cryogenic turning of Ti6Al4V in a region of cutting parameters particularly relevant to the aerospace industry where no previous data are available. The cutting parameters are those typical of titanium alloys rough machining which is considered, cost-wise, the most important operation because, for aerospace components, the so-called Buy-To-Fly ratio can reach values up to 20:1. The experiments have been performed using a full factorial design in order to statistically evaluate, using ANOVA and regression analyses, the significance of the input factors on the process most interesting outputs. The considered input factors are: type of cooling method, cutting speed and feed rate. The main analysed responses are: tool wear, surface roughness, cutting forces, coefficient of friction and chip morphology. The results show the significance of the cooling method on the tool life and that cryogenic machining is able to increase the tool life with respect to wet cutting. On the other hand, the beneficial effect of the liquid nitrogen cooling is reduced at high cutting speed and feed rate. Besides, the results showed that a small but significant reduction can be achieved for both the repulsion force and the coefficient of friction at the tool-workpiece interface.

Abstract: Titanium and its alloys are hard materials, wear resistant, high strength to weight ratio. Therefore this material become very promising, especially in aerospace application. However, its application restrict when face machining processes. This material is very hard which is very difficult to manufacture by machining. Its low Youngs modulus tends to springy and creates vibration or chatter. Moreover, it has low heat dissipation rate that make the heat concentrate in the tool tip especially in the friction surface between tool and chip. Those phenomena result in very low tool life and low quality of machined surface, in term of surface roughness, surface integrity. This article describes some efforts to overcome those problems. Categorically, there are some groups of effort, i.e. varying machining parameters, modification the tool, treatment of the material, and different method of applying the coolant. It seems that using cryogenic cooling upon the tool is the most promising new technology to machine the titanium alloy.

Abstract: One of the most important characteristics of the element bearings behaviour evaluation is the dimensional stability in time. The structures resulting in conventional thermal treatments are characterized by a major instability along time. Structural changes cause dimensional changes for both austenite and martensite transformations. Residual austenite transformation generates an increase in volume while the martensite transformation decomposes when volume decreases. Research has shown that cryogenic treatment and steel tempering in ultrasonic field applied to bearing steels have a positive influence on the dimensional stability. In what the dimensional stability concerns, the heat treatment below 0°C acts specifically on those sources of strain that have a maximum contribution to further parts dimension modification, especially on the transformation of austenite into martensite. It would have been better if the amount of austenite that remained after quenching at room temperature had been higher and the steel contained more carbon and alloying elements [.

Abstract: Besides developments in the area of dry machining and minimum quantity lubrication, the use of coolant lubricants is still essential when machining high alloyed steels or heat resistant materials like titanium and nickel based alloys. Experts agree that this fact will not change in the next decade. For this reason it is necessary to use coolant lubricants as effectively as possible to maximise their positive effect on productivity and process stability. High-performance cooling strategies like high-pressure cooling and cooling with cold gases (cryogenic cooling) have received increased attention in the last years. Through the targeted supply of coolant lubricants to the cutting site it is possible to decrease tool wear, increase cutting speeds, guarantee defined chip breakage and chip transport and – in terms of cryogenic cooling – waive part cleaning. This paper shows current research results in the above mentioned field. Since the performance of a high-pressure coolant lubricant supply in turning difficult to cut materials has been shown in many previous papers, this paper focuses on the quantification of the potential in turning different steels, namely quenched and tempered but also stainless steel in comparison to the conventional flood cooling. Since energy efficiency is very crucial, pressure and flow rate have to be adjusted carefully and in accordance with the cutting parameters to guarantee best results with less energy. Moreover the effects of cryogenic cooling will be evaluated in comparison to high-pressure cooling and conventional flood cooling. In latter field, cutting tests were carried out under variation of the flow rate in order to find the minimum required value for a certain machining task with the overall aim to prevent waste of the media used. Especially in cryogenic cooling technologies, many fundamental research regarding the working mechanisms but also further developments in cutting tool and machine tool technology are still necessary to make this technology ready for industrial use.

Abstract: The paper discusses sensitization characteristics in medium chromium Ferritic Stainless Steel (FSS) welds processed at two different grain refining conditions namely metal powder addition and cryogenic cooling. Tungsten Inert Gas (TIG) torch welded samples were exposed to 10% oxalic acid electrolytic etch for evaluation of sensitization. Empirical analysis indicated that, though both the grain refining conditions reduced the sensitization width relative to the conventional weld, the width was virtually nil with cryogenic cooling suggesting the elimination or reduction of sensitization. Similarly, the microstructure of cryogenically cooled welds showed no ditched grain boundary but aluminum powder treated welds revealed extensive ditching and none in titanium treated welds while knifeline attack was observed in welds treated with the mixture of aluminum and titanium powders.

Abstract: 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.