<p>Gamma Titanium Aluminide (γ-TiAl) alloys are metal matrix composites mainly consisting of titanium, aluminum, small amounts of niobium and chromium together with traces of silica. The resulting intermetallic structure of γ-TiAl alloys provides exceptional properties that include low density (~3.8 g/cm3) nearly 45% that of nickel superalloys, high yield strength (400-650 MPa) up to 800 ºC, high stiffness (E = 175 GPa at 20 ºC; 150 GPa at 700 ºC), high specific stiffness (45 GPa cm3/g) which is 50% greater than structural materials used in aircraft engines, high thermal conductivity (22 W/mK) compared to 7.3 for Ti-6Al-4V, relatively low coefficient of thermal expansion (10.8 x 10-6/ºC) compared to 14.8 for nickel-based superalloys, and good creep resistance in the temperature range of 600-700 ºC. Furthermore, at elevated temperatures in particular, γ-TiAl alloys outperform titanium alloys in terms of oxidation resistance, specific strength, and resistance to ignition. Due to these favorable properties, γ-TiAl alloys are being considered for replacing nickel, cobalt, and titanium-based superalloys for various applications in the temperature range of 400-800 ºC. In addition, for substantial weight reduction and performance enhancement, γ-TiAl alloys are being cogitated as an alternative material to manufacture complex three-dimensional components in automotive, aerospace and biomedical industries. Justifiably, these heat-resistant lightweight intermetallic alloys are ideally suited for the manufacture of aerospace engine parts including turbine and compressor blades subjected to the severe mechanical and thermal environment. However, γ-TiAl is considered to be very difficult to machine due to its low ductility (&lt;2%) at room temperature, which results in surface cracks, low dimensional accuracy, and poor machinability. Surface cracks adversely affect the performance of machined parts, especially in high-temperature applications. These shortcomings have hindered the widespread application of γ-TiAl; which has been further exacerbated due to the lack of cutting parameters that can be used in the industry. Cutting parameters such as speed, feed rate, as well as depth-of-cut can not only influence forces, energies, tool wear, surface finish, dimensional tolerances but also the overall quality of the part. In this paper, a three-factor, five-level rotatable central composite design (CCD) of experiments with cutting speed, feed rate, and depth-of-cut as independent machining parameters is adopted to investigate various turning process responses such as forces, energies, chip morphology, and surface finish. Turning experiments are performed on an Okuma Captain-L370 lathe with Levi-Cast Daido (RNT650) γ-TiAl cylindrical workpieces, and hexagonal shape CNMX 43A2-SM 1105 turning inserts with PVD TiAlN coating. Results show that the depth-of-cut factor has a major impact on the cutting forces and resultant force Fr, while the feed rate factor has a dominating effect on specific cutting energy Uc and surface finish Ra. Polynomial and logarithmic equations are developed, which are further transformed into logarithmic models with R2 in the range of 92-99.5% for cutting forces and energy responses, and R2 ~72% for surface roughness responses Ra and Rz. Subsequently, the corresponding 2-D contour plots, and 3-D surface plots are drawn to identify optimal regions. By means of regression analysis and response surface methodology (RSM), optimal turning parameters of 23 m/min cutting speed, 0.39 mm/rev feed rate, and 1.4 mm depth-of-cut are obtained, which renders the minimum machining energy, leading to increased tool life and improved machinability. Furthermore, it is practically possible to obtain a smooth surface finish in the range of Ra &lt;0.4 μm, which is well accepted in the industry. This work provides results of machinability investigation of γ-TiAl using hexagonal cutting inserts, in which CCD and RSM are used to determine optimal machining parameters that can find application in automobile and aerospace industries for machining of high-temperature resistance parts.</p>

Process Parameter Optimization in Machining of γ-TiAl With Hexagonal Inserts

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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