Key Engineering Materials Vol. 955

Abstract: Recently, there has been a significant interest in the research and industrial application of titanium aluminides. These alloys present great relevance in the aerospace and automotive sectors, where machining processes for the manufacture of high-dimensional precision parts are a fundamental aspect. The interest in the use of these alloys is since they combine a very low density, lower than the Ti6Al4V alloy typically used in these applications, and exceptional mechanical properties at high temperatures. However, titanium aluminides present great difficulties to be machined, being classified as a difficult-to-cut material, as they have high hardness and brittleness, low thermal conductivity, and high chemical reactivity. In the last decade, research on the machining processes of these intermetallic composites has been carried out, although few published works can be found. In particular, research on milling processes is incipient, and further progress is needed in the study and characterization of the process. In this work, a review of the state of the art is carried out to establish the achievements made concerning technological parameters, improvements with the application of sustainable lubrication systems, and the efficiency of the process in terms of tool wear and cutting temperature. In this way, future lines of research in this field would be established. In short, the results obtained aim at a further definition of the tribological system involved in titanium aluminide milling, the use of cryogenic and mixed MQL-cryogenic lubrication, and the definition of tool configurations that allow higher machining speeds to be achieved.

Abstract: The Heat Transfer Coefficient (HTC) in casting is critical input in numerical simulations. However, it depends on alloy composition, melt temperature, mold preheating temperature and local casting modulus, hence it is difficult to determine a priori. This work uses temperature measurements obtained by thermocouples during investment casting of a multiple section reference part. HTC was determined by three different methods: (a) analytical, formulating a detailed expression for the HTC in time steps for a one-dimensional approximation (b) inverse, following a well-known iterative algorithm proposed in literature and (c) "trial-and-error" simulation runs for different HTCs based on the judgment of the analyst. In conclusion, the analytical method proved to be fast, but yields very approximate HTC. The inverse method achieves accurate temperature evolution, but it might be unrealistic in physics terms. The trial-and-error method is flexible and may be accurate yet cumbersome and uncertain, unless automated, for instance through a Genetic Algorithm.

Abstract: Abrasive waterjet cutting is a valuable method for removing material without causing thermal damage, making it suitable for machining materials of different thicknesses and minimising waste. However, machining thicker materials requires higher flow rates and pressure, resulting in increased energy consumption and surface defects that increase costs. This study proposes a multi-pass strategy to improve the performance of abrasive waterjet machining. The study aims to investigate the impact of the number of passes on the efficiency of machining a thick UNS A92024 alloy. Surface integrity will be evaluated from two perspectives: macrogeometry (such as machining depth and taper) using image processing, and microgeometry (surface roughness). The study will also analyse the relationship between the number of passes and traverse speed to identify the optimal combination and develop a predictive model to enhance overall process performance.

Abstract: Laser Surface Texturing (LST) has demonstrated to be the most reliable technique for the micro-modification of surfaces, allowing to obtain taylored surfaces. These modifications, depending on the basic micro-geometry and its repetition pattern, can provide special functionalities to a surface, such as hydrophilicity, hydrophobicity, reflectance, anti-bacterial, ostheo-integrability, as well as custom aesthetic, among others.Nevertheless, when a laser irradiates metallic surfaces, the micro-structure can be modified due to the heat induced, changing the mechanical properties of the surface. To avoid these effects, cold or ultra-short pulsed lasers must be used.A cold laser emits optical pulses with a duration below 1 ps (ultra-short pulses), in the domain of femtoseconds (fs=10^-15 s). These ultra-short pulses, combined with high frequencies, in the megahertz region, leads to pulse trains with high repetition rates. This allows the sublimation of the material, keeping it relatively cold due to the short exposition time to irradiation.Ti6Al4V is the most used Ti alloy, thanks to its excellent weight/mechanical properties ratio. Nevertheless, its tribological behavior is very poor. Although there is intense research to improve it by using LST, the study of the influence of femtosecond laser parameters in the desired micro-geometries is still a gap in the scientific literature.In this research, a study of the influence of power (up to 50 W) and frequency (up to 2 MHz) in the fs-laser texturing of Ti6Al4V is presented. Local pulse repetition, linear and surface textures have been studied by combining power and frequency in these ranges, evaluating the geometry obtained by variable focus microscopy. The study carried out has allowed to determine the optimal set of parameters as a function on the target texture geometry, as well as the range in which the LST removal process changes from sublimation (for texturing) to melting (for micro-machining).

Abstract: The present work aims to optimize the processing parameters to minimize the geometric deviations of thin-walled parts machined from 6061-T651 aluminum alloy by high-speed milling (HSM). The experimental tests were carried based on a factorial design of experiments, which included as input factors axial cutting depth, cutting speed, and feed per tooth, resulting in shape deviations but also roughness and hardness of machined surfaces. After machining, the residual stresses were determined to establish, if possible, a cause-effect relationship between parts deviations and the magnitude of stresses involved. The experimental tests allowed us to obtain the optimum machining parameters under maximum productivity conditions that ensure the required geometric precision of parts.

Abstract: In machining, the heat flow into the workpiece during the cutting process is often a major concern. The temperature rise can lead to substantial residual stresses or elastic in-process deformations which may result in the dimensional tolerance requirements being violated. In the present study a modelling strategy is developed for determination of the heat load during indexable drilling. The heat load on the workpiece is determined from 3D thermomechanical Coupled Eulerian Lagrangian analyses of orthogonal turning for various chip thicknesses and cutting speeds. The determined heat load is then transferred to a 3D transient heat transfer analysis of the indexable drilling process for the determination of the temperature field. Thereby, this modelling technique avoids the complex cutting process that is performed in real cutting simulations and thereby reducing the computational complexity of the problem considerably. The simulated temperatures are compared with experimentally measured temperatures and some conclusions are drawn regarding the modelling approach.

Abstract: The continuous demand focused on optimizing titanium machining techniques in the aerospace industry, makes improving machining processes in this area of great interest to the industry. The contamination produced by the coolants used to machine titanium is a major problem to be addressed, since it is a material that requires cooling due to its strength, physical qualities and low thermal conductivity. That is why the implementation of a RHVT cooling system can improve the current situation. The aim of this work is to compare the final quality of the drilling by applying the system of RHTV (Ranque Hilsch Vortex Tube) cooling techniques and to see the advantages of its application with the dry machining process. This cooling system is expected to reduce drilling temperatures, thereby increasing the environmental performance of the manufacturing process. It is expected to set up a preliminary study based on a comparison between dry drilling and drilling assisted by the application of RHTV. Macro and microgeometric defects will be evaluated to determine the cooling system efficiency, as well as the machining temperatures reached.

Abstract: Surface marks, such as scratches or cosmetic marks, commonly appear during the manufacturing phase of metallic components, because of the contact between tools and sharp edges with the surface of the parts. Scratches, depending on their width, depth, and root radius, cause a decrease in the fatigue life of metallic alloys. In particular, the presence of scratches with a size comparable to the grain size favors the generation of fatigue cracks in these features. In the aerospace industry, the presence of surface marks is a common cause of rejection. The low hardness of aluminium, a material widely employed in the manufacture of aerospace structures, contributes to the generation of surface marks. In this paper, a preliminary geometrical characterisation of scratches is established. It aims to define a set of parameters to characterise exhaustively the different scratches and to generate different behavior models for each type of scratch. Parameters such as scratch length, path radius, and burr height are considered in addition to the well-known parameters such as scratch depth, root radius, and open angle.

Abstract: Ceramic tools are a feasible solution for machining hard steels due to their excellent performance in high-temperature environments, which improves productivity. However, the high cutting speeds achieved with ceramic tools can generate cutting temperatures so high that they damage the surface roughness of the component. Therefore, this study aims to analyze the influence of different texturing inclinations on the final roughness of the component and the cutting forces during cutting on Al₂O₃ with TiC for the turning of 1.2990 cold work tool steel. The experimental setup involved testing five different texture grooves inclinations on the rake face of CNGA 120408 inserts, with a recommended cutting speed of 180 m/min, feed of 0.15 mm/rev, and a depth of cut of 0.5 mm. The tests were carried out in dry conditions. Results showed that the textured tools produced lower cutting forces (8%) and better surface roughness (improvement of 16% for R_a and 18% for R_z) than the reference tool without texture. Therefore, texturing inclinations can be used to mitigate the impact of high cutting speeds on the component during ceramic tool machining.