The aerospace industry is a key area in advanced materials research. The increasing temperatures in aircraft engines lead to the use of new materials with higher capabilities, especially titanium alloys and nickel-based superalloys [1
]. Titanium alloys are often selected for the plane fuselage (10% in weight) to prevent crack growth as in motor parts (35–40%), followed by nickel-based superalloys (40%), but with a higher volume percentage. The focus here is on titanium aluminides, also called γ-TiAl alloys or intermetallic alloys.
The development of TiAl alloys started in 1950 but industrial interest continues to grow [2
]. Indeed, some parts of the turbine engine could be manufactured out of γ-TiAl instead of nickel superalloys (Inconel 718) and conventional titanium alloys (such as Ti64). The new gamma TiAl are in transition between the 2nd and 3rd generation, with several applications proposed for the latter. Thus, NASA [3
] is proposing the use of the alloys in the Revolutionary Turbine Accelerator/Turbine-Based Combined Cycle (RTA/TBCC) Program for the next-generation launch vehicle, with gamma TiAl as a potential compressor and structural material. The problem with manufacturing real parts is the post-processing required to achieve the tight tolerances defined in the blueprints, because alloy fragility and rapid tool wear are two key problems. An experimental campaign was the only way to achieve data sound enough to work in real applications [4
]. The difference between γ-TiAl alloys and other alloys such as Ti6Al4V lies in the aluminum percentage, 43–48% in γ-TiAl to 6% in Ti6Al4V, which highly improves the thermal conductivity in γ-TiAl but reduces the ductile transition temperature, which occurs in the range 600–800 °C.
Specifically, hole making in this kind of piece is a critical step during the manufacturing process [5
] due to the close tolerances imposed and considering the accumulated added value before this stage.
The available literature referring to titanium-based alloys is quite extensive. However, the underlying physical‒chemical phenomena need to be further investigated. Over the last few decades, an important increase in productivity has been observed [6
]. A great effort was made to increase the mass removal rate (MRR), with higher cutting speeds and feed rates [7
]. Some authors focused on the chip formation process. Komanduri and von Turkovich [9
] explained the serrated chip morphology using the thermoplastic shear model. A second theory states that chip generation is due to a periodic crack initialization over the primary zone because of the high stress levels [10
]. Many authors studied the metallurgical microstructure of the chip by X-ray diffraction and SEM [11
]. Tool wear is another subject of concern. Some authors found that wear mechanisms are also very dependent on the cutting tool materials [13
]. Hartung et al. [14
] gave a complete picture of wear in titanium alloys. Nouari et al. [15
] studied the influence of the cutting parameters and tool geometry over the wear phenomena, stating the importance of an adequate characterization of the temperature, pressure and contact length. They used FE modeling to predict the temperature, recognized as a key parameter in controlling tool wear. More recently, Bermingham et al. [16
] studied the effect of cryogenic techniques on tool life in titanium alloy Ti6Al4V. They found that the greatest improvement in tool life using a cryogenic coolant occurs for high feed rates combined with low depths of cut. Polvorosa et al. [17
] studied wear mechanisms in nickel-based alloys and discussed the effect of using different coolant pressures. Beranoagirre et al. discussed the machinability of gamma TiAl in EDM [18
] and in drilling processes [19
], where a mechanistic model was proposed for cutting force and torque prediction. Regarding soft titanium alloys, Kuczmaszewski et al. [20
] demonstrated that ultra-fine-grain SC milling cutters are the tools most resistant to chipping while leaving a good surface finish. Jozwik et al. analyzed the tribological properties of Ti6Al4V [21
] and the effects of the technological parameters of the selective laser melting (SLM) process on the morphological properties of the manufactured surfaces [22
Uddin et al. [23
] evaluated the influence of the key process parameters on drilling performance when using high-speed steel (HSS) bits. They studied the process mechanics in terms of dimensional accuracy, burr formation, surface finish, etc. Some authors used ANOVA analyses and multi-objective optimization to find optimum drilling parameters using response surface analyses in aluminum [24
] and titanium alloys [25
Although the existing research sheds some light on the effect of different cutting parameters of machining holes in gamma TiAl alloys, two main aspects are still to be clarified. First, these analyses consider only one kind of gamma TiAl alloy in each case; second, the study of the wear process of drilling of these alloys is a very expensive task because it requires the waste of many cutting tools over their whole lifetime to test different cutting conditions, and the destruction of large amounts of expensive workpieces that are systematically drilled. Collaboration with industries will not easily solve this restriction because they may not be open to testing many different drilling conditions or allow testing on real workpieces up to the tool’s breakage limit. Therefore, the identification of the best cutting conditions with a very limited number of different experimental tests is a must for wear analysis of gamma TiAl alloys.
The present work is framed in the study of wear when machining holes in gamma TiAl. In particular, the work presents the results from drilling tests on three types of gamma TiAl, TNB-type, extruded MoCuSi, and ingot MoCuSi, to define an optimal set of cutting parameters. The paper proposes a comprehensible model to identify the relationships between cutting conditions and resulting wear that is useful to estimate tool life and to save part tolerances.
In this work ANalysis Of VAriance (ANOVA) is proposed as an optimal solution for the identification of the best cutting conditions with a limited number of different experimental tests of hole drilling in gamma TiAl alloys. ANOVA is a statistical technique that allows for extracting the percentage of influence of each input variable in a defined output of a complex process. ANOVA, as its name suggests, focuses on the analysis of the variance of a defined output, trying to split it according to the variance in the dataset of each input in the output behavior. This technique has been successfully used to evaluate the influence of input parameters in many complex manufacturing tasks like the dimensional precision of single tracks produced by selective laser melting [26
], the fatigue properties of fiber-reinforced additively manufactured specimens [27
], the surface roughness of drilled holes in the maintenance of stacked hybrid magnesium‒titanium components [28
] and the wear process of drilling tools in hard drilling of AISI D2 [29
]. Compared with these works, in this research, instead of a Taguchi Design with a very limited number of experimental conditions, a full factorial is tested to assure an extensive dataset that could provide significant conclusions.
3. Experimental Procedure
This stage develops the main results of the experimental drilling operations, i.e., the evaluation of the cutting conditions in terms of tool wear, chip morphology, and surface finishing. The operations were performed with a tungsten carbide drill (see Figure 1
) to a hole depth of 20 mm (L/D = 5), using an internal coolant (8.5 bar). The FU 70 W Rhenus® (Rhenus, Mönchengladbach, Germany) coolant was used in all the tests, free of ammonia and boron, but especially for aerospace applications. Tools are coated with (Al,Ti)N type (Miracle, Mitsubishi© Mitsubishi®
, Tokyo, Japan), and then polished to reach a very low surface roughness. The micro-structure of TF15 carbide grade, ISO S grade (WC90%‒Co10%, grain size < 0.9 µm), has a Young’s modulus of 580 GPa. The machining tests were performed in a vertical CNC machining center, Kondia© model B640 (Kondia, Elgoibar, Spain), with maximum rotational speed of 10,000 rpm and 25 kW. It is worth mentioning that the machining of low-machinability materials could lead to problems in terms of the available nominal power in some milling operations, but not in this case.
For the experimental tests, a vertical machining center Kondia®
B640, with nmax
= 10,000 rpm and Pnom
= 5 kW was used (Figure 2
). The cutting parameters were selected according to the values recommended by the tool manufacturer and the past experience of the authors [19
These parameters, cutting speed Vc
and feed per revolution fn
, are shown in Table 2
. The high cost of the materials (>€500/kg) is a constraint that limits the number of tests. Due to economic reasons, only two tests were carried out for each of the cutting conditions defined. The results were obtained from the mean values of these two tests. Table 2
summarizes the cutting speed vc
, the feed per revolution fn
, the spindle speed n, and the feed speed vf
After the experiments, tool wear was analyzed using the digitized images from an optical microscope (Motic SMZ Microscope, Hong Kong, China) equipped with a digital camera. The process of tool wear in a drill bit is shown in Figure 3
. Some important parameters are: We
, chisel edge wear width; Wf
, flank wear at flank face of the cutting edge; Wo
, outer corner wear width; Wm
, margin wear width and Wm’
, margin wear (second) width. Additionally, to describe tool rake wear, the ISO norm (ISO TC29/WG22) uses KT and KB to measure the rake wear depth and length. The mechanisms of heat generation, friction, chemical reactions, and their relative amount differ according to the work materials and the selected cutting conditions. In this case, the authors measured the wear on the rake face KB as they observed it was the most trackable, recognizable, and easy-to-measure parameter. On the contrary, flank wear here was difficult to measure as it appears in a chiseled narrow edge and looks irregular due to the brittle behavior of the tool.