The WC-5wt.% TiC compacts, which was fabricated by pulsed current activated sintering process (PCAS), were cryogenically treated to improve the mechanical performance. The densely consolidated specimens were exposed to liquid nitrogen for 6, 12, and 24 h. All cryogenically treated samples exhibited compressive stress in the sintered body compared with the untreated sample. The cryogenically treated samples exhibited significant improvement in mechanical properties, with a 9% increase in Vickers hardness and a 52.6% decrease in the fracture toughness compared with the untreated samples. However, excessive treatment of over 12 h deteriorates the mechanical properties due to tensile stress in the specimens. Therefore, the cryogenic treatment time should be controlled precisely to obtain mechanically enhanced hard materials.
Titanium alloys are used in various industries due to their superior mechanical strength and corrosion resistance. However, titanium is classified as a difficult-to-machine material due to its low thermal conductivity that consequently causes poor tool life. In this study, cryogenic+MQL milling was performed to improve the machinability of Ti-6Al-4V; a cryogenic coolant and a minimum quantity fluid were sprayed simultaneously. The machinability was analyzed according to the cooling and lubrication conditions, focusing on the cutting force and tool wear. When the minimum quantity fluid was injected using two nozzles during cryogenic machining, the cutting force remained low despite the increase in machining distance due to the effective lubrication. The average cutting force at the long machining distances (82-86 passes) was 14.8% lower than that under the wet condition. The tool wear progressed without chipping, and the flank wear length was 55.5% lower than that of the wet machining because the cryogenic cooling and minimum quantity lubrication reduced the tool temperature, friction, and thermal shock.
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The objective of this study was to perform surface hardening experiments of titanium alloy using laser. The surface hardness value after laser hardening treatment was observed to increase with respect to the inflow of laser energy. However, when the laser energy exceeded the critical value, damage and cracks were observed on the surface of the material. The relationship between surface hardness values and process variables such as laser energy, scan speed, and number of laser scans was quantitatively modeled through the design of experiments and analysis of variance. Using the established mathematical model, the surface hardness value of the material can be predicted accurately with an average of 10% error over various process conditions. Analysis of the surface composition of the material using energy dispersive spectrometry showed that titanium oxide was the main cause of the increasing surface hardness. Further studies will be conducted to improve the accuracy and predictability of the model using nonlinear modeling techniques.
Lightweight parts are necessary to improve fuel efficiency and reduce environmental impacts in transportation industry. As a result, there has been a shift away from using conventional metals toward using lighter materials with superior mechanical strength. These new materials typically include titanium alloys, nickel alloys, carbon fiber reinforced plastics (CFRPs), and CFRP-metal stacks, which are classified as advanced materials. However, due to the unique properties of these materials (e.g., high strength, low thermal conductivity, carbon fiber-induced hardness, etc.), the cutting process can be difficult. As a result, various manufacturing issues can occur during the cutting process, such as high tool wear, surface quality deterioration, delamination of the CFRP layer, fiber pull-out, and thermal deformation. In this paper, difficult-to-cut advanced materials were reviewed with regard to the influence of the physical properties of the materials and various defect issues that can occur during the mechanical cutting process. In addition, various approaches to improve the cutting process are introduced, including protecting tools with coatings, altering tool features, using high pressure or cryogenic cooling, extending tool life via ultrasonic vibration machining, and improving product quality and machinability.
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This paper presents a numerical study on the thermal characteristics of a milling process of titanium alloy with nanofluid minimum-quantity lubrication (MQL). The computational fluid dynamics (CFD) approach is introduced for establishing the numerical model for the nanofluid MQL milling process, and estimated temperatures for pure MQL and for nanofluid MQL using both hexagonal boron nitride (hBN) and nanodiamond particles are compared with the temperatures measured by thermocouples in the titanium alloy workpiece. The estimated workpiece temperatures are similar to experimental ones, and the model is validated.
Titanium alloy has been widely used in the aerospace industry because of its high strength and good corrosion resistance. During cutting, the low thermal conductivity and high chemical reactivity of titanium generate a high cutting temperature and accelerates tool wear. To improve cutting tool life, cryogenic machining by using a liquid nitrogen (LN2) jet is suggested. In cryogenic jet cooling, evaporation of LN2 in the tank and transfer tube could cause pressure fluctuation and change the cooling rate. In this work, cooling uniformity is investigated in terms of liquid nitrogen jet pressure in cryogenic jet cooling during titanium alloy turning. Fluctuation of jet spraying pressure causes tool temperature to fluctuate. It is possible to suppress the fluctuation of the jet pressure and improve cooling by using a phase separator. Measuring tool temperature shows that consistent LN2 jet pressure improves cryogenic cooling uniformity.
Cryogenic machining uses liquid nitrogen (LN2) as a coolant. This machining process can reduce the cutting temperature and increase tool life. Titanium alloys have been widely used in the aerospace and automobile industries because of their high strength-to-weight ratio. However, they are difficult to machine because of their poor thermal properties, which reduce tool life. In this study, we applied cryogenic machining to titanium alloys. Orthogonal cutting experiments were performed at a low cutting speed (1.2 – 2.1 m/min) in three cooling conditions: dry, cryogenic, and cryogenic plus heat. Cutting force and friction coefficients were observed to evaluate the machining characteristics for each cooling condition. For the cryogenic condition, cutting force and friction coefficients increased, but decreased for the cryogenic plus heat condition.
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Recently, titanium alloys have been widely used in aerospace, biomedical engineering, and military industries due to their high strength to weight ratio and corrosion resistance. However, it is well known that titanium alloys are difficult-to-cut materials because of a poor machinability characteristic caused by low thermal conductivity, chemical reactivity with all tool materials at high temperature, and high hardness. To improve the machinability of titanium alloys, cryogenic cooling with LN2 (Liquid Nitrogen) and nanofluid MQL (Minimum Quantity Lubrication) technologies have been studied while turning a Ti-6Al-4V alloy. For the analysis of turning process characteristics, the cutting force, the coefficient of friction, and the surface roughness are measured and analyzed according to varying lubrication and cooling conditions. The experimental results show that combined cryogenic cooling and nanofluid MQL significantly reduces the cutting forces, coefficients of friction and surface roughness when compared to wet condition during the turning process of Ti-6Al-4V.
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