Radial direction ultrasonic-vibration and laser assisted turning of Al3003 alloy

The utilization of various energy sources to assist the machining process has become prominent to obtain significant improvement in the machining performance. These energy sources have been utilized without using any cutting fluids which makes them eco-friendly. The combined action of laser and ultrasonic vibration energies during the turning process has shown significant achievement in machining process capabilities. Therefore, an attempt has been made to provide ultrasonic vibration in the radial direction and laser to preheat aluminium 3003 alloy simultaneously during the ultrasonic-vibration-laser assisted turning (UVLAT) process. Machining performance has been analyzed in terms of machining forces, machining temperature, chip morphology, surface damage, and surface roughness. A comparative machining performance analysis has been performed among the conventional turning (CT), ultrasonic vibration assisted turning (UVAT), laser assisted turning (LAT), and UVLAT processes. The outcomes of the present study revealed significant improvement in the machining performance for aluminium 3003 alloy during the UVLAT process. However, surface damage and surface roughness have been affected negatively during the UVLAT process due to the pin-point hammering and particle adhesion on the workpiece part. Hence, it can be said that the selection of vibration direction is a critical factor during the vibration machining processes.


Introduction
Manufacturing operations consume approximately 40% of the overall energy and about 25% of the global natural resources [1]. The manufacturing sector utilizes different processes to manufacture the part and the machining process is the most commonly utilized in the manufacturing industries such as automotive, aviation, marine, etc [2]. It is an energy consuming and waste generating process apart from creating environmental pollution and occupational health hazards [3]. Moreover, cutting fluids are used significantly during the machining operations in the manufacturing industries which enhances manufacturing costs and environmental hazards [4]. Therefore, to overcome the ever-increasing challenges of natural resource consumption and environmental pollution, countries are required to implement a sustainable development strategy for better product quality and productivity. Hence, various machining techniques have been developed such as dry machining [5], minimum quantity lubrication (MQL) [6], cryogenic machining [7], and hybrid machining [8] to minimize energy consumption and environmental pollution.
Hybrid machining processes have gained significant importance in the manufacturing industries due to their enhanced machining performance in comparison to the CT process. Hybrid machining processes are based on the simultaneous and controlled application of several energies or tools which significantly affects the performance characteristics as well as surface integrity [9,10]. Besides, the development of hybrid machining processes should be in such a manner that the combined benefits should be more than double the individual technique and their disadvantages should be minimized [11]. Several energies such as ultrasonic vibration, thermal, magnetic, electric, water-jet, pressure, etc, have been utilized to assist the machining process. Among

Experimental procedure
This section describes the experimental setup for the UVAT, LAT, and UVLAT processes. Besides, the cutting tool material, workpiece material, experimental conditions, performance measurement parameters are also described. First, the experimental setup will be discussed followed by other sections.

Experimental setup of UVAT process
A fixture was developed for mounting the transducer-booster assembly on the lathe machine (NH-22, HMT Ltd) and it was synchronized with the lathe for adequate movement in the radial direction. The vibratory unit consists of a frequency generator, transducer, and booster. The frequency generator generates an electric signal and these signals were transformed into vibration form by the transducer. The amplitude of vibration was enhanced by the booster and sent the vibrations on the cutting tool. The EN8 carbon steel was used as the booster material and it was fabricated in a cylindrical shape. The length of the cylindrical booster was calculated as [30] ( ) where L was the booster length, C was the sound velocity of the material (5930 m s −1 ), and f was the frequency of the vibration (20000 Hz). The length and diameter of the booster were designed as 120 mm and 25 mm, respectively to ensure targeted amplitude on the cutting tool. The amplitude was achieved by the cylindrical booster which comes out to be 20 μm. The cutting tool was clamped at the bottom of the booster by a standard screw. The ultrasonic vibrations were provided to the cutting tool at a frequency of 20 kHz and an amplitude of 20 μm. The laser displacement sensor was used to measure the vibration amplitude of the cutting tool and the vibration amplitude was found within the range of 20 ± 2 μm. The slenderness ratio was defined as the diameter of the input side of the booster to the length of the booster. The slenderness ratio significantly affects the amplification factor. Nad described that the amplification factor was reduced due to the increase in the slenderness ratio [31]. Besides, Wang et al reported that stepped booster had high stress concentration due to the sudden change in diameter which might lead to failure [32]. Moreover, Amin et al revealed that cylindrical booster was limited by their amplification. The diameter of the input side of the booster should be between kept 25-75 mm to avoid damping of the amplitude [33]. In the present work, the diameter was considered as 25 mm.

Experimental setup of LAT process
Similarly, a rectangular bracket with slots was fabricated to adjust the laser cutting head. This bracket was rigidly supported by two vertical plates which were fixed on an angle. The angle was positioned on the cross slide so that the laser head fixture could move appropriately in the cutting direction during the experiments. A fiber laser source with specifications as 1 kW maximum power, 1080 nm wavelength, and 1.3 beam quality factor (M 2 ) was used to preheat the aluminium 3003 alloy. The specifications of the fiber laser source were provided by the supplier. Moreover, the laser power could be selected up to maximum 1 kW at an interval of 10 W depending on the requirement. The laser beam head was connected to the output of the laser source through an optical fiber cable. It was positioned in such a manner that the laser beam was always focused on the workpiece surface but not on the cutting tool. Circumferentially, the laser beam was 45°ahead of the cutting tool, i.e., the laser approach angle. The laser beam irradiated the workpiece surface with a 1 mm spot size. The laser beam was focused by a laser cutting head which had a focusing lens with a focal length of 125 mm. The distance between the nozzle tip of the cutting head and the workpiece surface was measured by slip gauges, which were 18 mm. With the help of the laser cutting head, the laser beam was focused 8 mm below the nozzle tip to get a defocused laser beam of 1 mm on the workpiece surface. The diameter of the laser beam was calculated, according to Kant and Joshi [34]. The axial distance between the center of the laser beam spot and the cutting edge of the cutting tool was adjusted to 4 mm. It was kept constant during the experiments to ensure the preheating of the workpiece surface and to avoid damaging the cutting tool edge. Also, the generated chips did not block the interaction between the workpiece and the laser. No sign of a heated region or any heat-affected zone was observed on the chips. A compressed air pressure (0.4 bar) was used to protect the focusing lens during the experiments.

Experimental setup of UVLAT process
To facilitate the UVLAT process, the laser head along with the transducer was mounted on a lathe machine by developing separate fixtures. The tool post of the lathe machine was replaced by the developed fixtures. The ultrasonic vibration was made to coincide with the radial cutting direction whereas the laser heating was focussed on the workpiece surface. Figure 1 shows the experimental setup of the UVLAT process.

Experimental conditions
The experiments were performed on the aluminium 3003 alloy with the chemical composition as per the literature [29]: Mn-1.26%, Si-0.61%, Fe-0.40%, Cu-0.13%, Zn-0.10, and Al-balance. The cylindrical workpiece with a diameter of 20 mm was taken and the length of the workpiece was 150 mm. The tungsten carbide inserts CNMG 120408 was used as the cutting tool with specifications such as rake angle = 0°, clearance angle = 5°, and nose radius = 0.8 mm. The machining parameters and conditions are shown in table 1. The low cutting speed was selected in the present study because at low cutting speed, the influence of the UVAT process had shown significant improvement in machining performance due to a lower tool workpiece contact ratio (TWCR) [35]. Besides, Nath et al reported that lower cutting speed, feed rate, and depth of cut had yielded enhanced machining performance during the UVAT process [36]. Moreover, the frequency of generators that were used in the present study only operate at a particular frequency and the amplitude was kept constant because the booster will work only at a particular frequency. Hence, the frequency and amplitude were not considered as a variable. The laser power was considered as 100, 150, and 200 W during the LAT and UVLAT processes whereas other laser parameters were described in earlier section. Low laser power was considered because of the low cutting speed used in the present study. High laser power could affect the thermal properties of the cutting tool because of the high machining temperature and also could melt the workpiece surface. Besides, the laser approach angle (45°) was selected for effective laser heating on the workpiece surface, according to Rebro et al [37]. In addition, the distance between the center of the laser beam spot and the cutting edge of the cutting tool (4 mm) was selected to avoid the thermal degradation of the cutting tool. Furthermore, the distance between the nozzle tip of the cutting head and the workpiece surface (18 mm) was considered due to some restrictions possessed by the experimental fixtures. Apart from that, the diameter of the laser beam (1 mm) was selected based on the focal length of the focussing lens and distance between the nozzle tip of the cutting head and the workpiece surface (18 mm).

Machining performance evaluation
The machining forces data was recorded by a Kistler 9257B dynamometer with a sensitivity of ±0.1 N and the recorded data were analyzed by Dynoware software. The machining temperature was measured by a FLIR A315 thermal imaging camera and the data was analyzed by FLIR Tools+ software. The emissivity of the aluminium 3003 alloy was considered as 0.06, as per the literature [23]. The chip morphology and surface damage of aluminium 3003 alloy were analyzed by a Jeol JSM-6610LV scanning electron microscope. The surface roughness was examined by a Zeiss Handysurf surface roughness tester. The surface roughness was evaluated at  three different places of the workpiece and the average value was taken for analysis. It was calculated in the feed direction with a 4 mm length profile and 0.8 mm cut-off length.

Machining forces
The average machining forces were measured and analyzed for the CT, UVAT, LAT, and UVLAT processes (figure 2). A noteworthy reduction in machining forces (in all three directions) was observed when ultrasonic vibration and laser energies were utilized individually and simultaneously compared to the CT process. For the UVAT process, the reduction in machining forces for the cutting, radial, and feed forces were 46%, 48%, and 49%, respectively than the CT process. The advantage of the UVAT process for machining forces was quite obvious and it was well explained in the literature. Kim and Lee described that periodic engagement and disengagement of the tool led to the generation of a lower average machining force for the UVAT process compared to the CT process [38]. Besides, Nath and Rahman explained that the lower TWCR, lower built-up edge on the cutting tool, reduction in surface tearing, aerodynamic lubrication, and generation of thinner and even chips were the main influential factor in reducing the machining forces during the UVAT process [35]. The cutting, radial, and feed forces were decreased by 18%-62%, 35%-51%, and 12%-53%, respectively during LAT for all laser power than those to CT. The benefits of the LAT process for machining forces were mainly arised because of the material thermal softening. Sun et al revealed that the softening of the material because of localized heating by the laser source resulted in a reduction in friction force and led to lower machining forces in LAT [39]. Additionally, the reduction in cutting, radial, and feed forces was more pronounced with an increase in laser power due to the higher thermal softening of material and lower yield strength at higher laser power.
Rajagopal et al described that the material yield strength was lowered at higher laser power due to the more thermal softening of material which led to a reduction in machining forces during the LAT process [40]. Simultaneous application of laser and ultrasonic vibration energies (UVLAT) resulted in the reduction of machining forces by approximately 84%, 55%, and 68% in comparison to the CT, UVAT, and LAT processes, respectively for all three directions. The cutting tool was engaged and disengaged periodically due to the ultrasonic vibration energy, and the workpiece material was thermally softened due to the localized melting by the laser heating, simultaneously during the UVLAT process which led to a higher reduction in machining forces. Deswal and Kant explained that the combined interaction of laser and ultrasonic vibration energies led to a reduction in machining forces because of the cutting tool periodic separation characteristics and thermal softening of workpiece material, simultaneously during the UVLAT process [23]. Apart from that, the reduction in machining forces was more pronounced for higher laser power during the UVLAT process. This may be ascribed due to the higher material thermal softening at higher laser power and frequent vibration preventing the generation of built-up edge on the tool, leading to a higher reduction in machining forces [25]. Thus, it can be stated that the utilization of laser and ultrasonic vibration energies simultaneously during the turning process is better for reducing the average machining forces when compared with the utilization of these energies individually.

Machining temperature
The machining temperature was measured during the experimental study of the CT, UVAT, LAT, and UVLAT processes. The thermal images and maximum machining temperature obtained during the CT, UVAT, LAT, and UVLAT processes are represented in figure 3. Higher maximum machining temperature was observed for the UVLAT process compared to the CT, UVAT, and LAT processes. The increment in maximum machining temperature for the UVAT process was observed around 24% when compared with the CT process. The results regarding the machining temperature were debatable in the literature due to the nature of friction in the ultrasonic machining processes. Moreover, the influence of friction was found to be less for enhancing the machining temperature during the UVAT process. However, the vibro-impacts imposed by the cutting tool on the workpiece surface led to energy dissipation which led to an increase in maximum machining temperature during the UVAT process, according to Muhammad et al [41]. Babitsky et al also reported higher machining temperature for the UVAT process than the CT process. They described that additional energy supplied into the system due to ultrasonic vibration, different chip formation mechanism, and increased yield stress of the material due to higher strain rates were responsible for the increase in maximum machining temperature for the UVAT process than the CT process [42]. Apart from that, the maximum machining temperature during the LAT process was increased by 58%-141% when compared to the results achieved for the CT process. It was expected that the maximum machining temperature would be higher for the LAT process in comparison to the CT process during the experiments. The amount of heating by the laser source on the workpiece material created additional energy on the workpiece surface and led to an increment in maximum machining temperature during the LAT process than that of the CT process. Wei et al also reported higher machining temperature during the LAT process than that in the CT process due to higher energy creation by the laser heating source [43]. Besides, the machining temperature was observed to be increased with an increase in laser power for the LAT process due to the creation of excess thermal energy at the workpiece surface. The volume of the material to be removed would be increased with an increase in cutting speed due to this some additional energy was required which led into an increase in maximum machining temperature [44]. Furthermore, Rashid et al explained that higher thermal energy was generated on the workpiece surface at higher laser power which led to an increase in the machining temperature for the LAT process [45]. The maximum machining temperature during the combined laser and ultrasonic vibration energies (UVLAT process) was obtained higher than the CT, UVAT, and LAT processes by approximately 77%-174%, 59%-135%, and 25%-74% respectively. The additional energy was created by the tool due to ultrasonic vibration, and laser heating increases the workpiece temperature during the simultaneous interaction of vibration and laser for the UVLAT process. Kim et al explained that the combination of additional energy from the laser beam and tool vibro-impacts resulted in higher machining temperature during the UVLAT process [25]. Furthermore, the increase in machining temperature was more obvious at higher laser power due to the excess intensity created by the laser heating at the workpiece surface. Deswal and Kant described that more energy was transferred by the laser heat source at higher laser power which was responsible for higher machining temperature in the UVLAT process than that of the CT, UVAT, and LAT processes [24]. In addition, they revealed that higher machining temperature was responsible for higher material thermal softening due to which lesser machining forces were obtained during the UVLAT process. Similar types of finding regarding the machining forces were obtained in the present work with respect to the machining temperature. Therefore, it can be stated that the higher maximum machining temperature was obtained during the UVLAT process in comparison to the CT, UVAT, and LAT processes which resulted in lower average machining forces for the UVLAT process.

Chip morphology
The chip morphology was analyzed during the CT, UVAT, LAT, and UVLAT processes, as shown in figure 4. The outcomes revealed that the UVLAT process resulted in crack-free chip edges compared to the CT, UVAT, and LAT processes. Chip segmentation during the CT process was observed and it was initiated due to the evolution of strain and temperature localization, according to Zhang and Choi [46]. They reported that an adiabatic shear band was generated not only from the chip root but also from the free chip surface which led to the chip segmentation during the machining of aluminium alloys. Chip segmentation and continuous lines on the chip surface were obtained due to the tool harmonic motion for the UVAT process. The vibratory marks of the tool on the chip surface were arised due to the rapid vibratory motion of the tool and it was an indication of the tool harmonic motion during the UVAT process, as described in the work of Deswal and Kant [12]. Babitsky et al also reported chip segmentation and continuous lines on the chip surface due to the vibration impacts and incremental continuous chip formation process. However, negligible chip segmentation was observed for the LAT process and the chip thickness was found to be increased with an increase in laser power. Luan et al described that the strain strengthening rate and rheological stress of the workpiece material were decreased due to the laser heating effect, which suppresses the adiabatic shear formation and leads to the minimization of chip segmentation [47]. Besides, Zhang et al explained that the higher chip thickness was generated for higher laser power because of the more material thermal softening and more removal of material from the workpiece surface [48]. Minimum chip segmentation, crack-free chip edges, higher chip thickness, and continuous lines on the chip surface were obtained for the UVLAT process. The combined action of laser and ultrasonic vibration energies, simultaneously led to better chip generation because of the different chip generation phenomena for both the laser and ultrasonic vibration energies. The vibratory nature of the cutting tool and thermal softening of the workpiece material simultaneously led to the generation of lower machining forces and higher machining temperature. The workpiece material became softened due to this reason crack-free chips and higher chip thickness were generated for the UVLAT process. Moreover, the cracks were found to be reduced with an increase in laser power due to the ductile chip formation during the UVLAT process, according to Deswal and Kant [23]. They illustrated that the chip generation phenomenon was improved during the UVLAT process because of the generation of lower machining forces and higher machining temperature in the UVLAT process. Therefore, it can be concluded that the minimum chip segmentation, crack-free chip edges, higher chip thickness, and ductile chips were obtained during the UVLAT process compared with the CT, UVAT, and LAT processes.

Surface damage
Surface damage on the machined surface for the CT, UVAT, LAT, and UVLAT processes was analyzed based on the SEM images which can be seen in figure 5. The defects were higher for the UVLAT and UVAT processes whereas lesser defects were obtained during the LAT process, and the minimum defect was observed in the CT process. The unexpected outcomes were observed regarding the machined surface damage for the UVAT, LAT, and UVLAT processes. During the CT process, fewer particle adhesions, no scratches, no microholes, no cracks, and no micropits were found on the machined surface. Jomma et al reported that lesser defects on the machined surface damage would be due to the lower machining temperature and lower microchipping on the cutting tool edge which led to fewer particle adhesion on the machined surface during the CT process [49]. Fewer scratches, particle adhesion, no micropits, no cracks, and no microholes were found on the finished surface for the LAT process at all laser power. During the LAT process, the machining temperature became high and aluminium particles would have stuck on the cutting tool edge. Due to the sticking of the particles, these could have ploughed on the machined surface which led to the generation of some scratches and particle adhesion. Scratches and particle adhesion were observed on the machined surface during the LAT process in the work of Deswal and Kant [17]. They explained that the material adhered on the cutting tool could have ploughed on the machined surface due to the ductile nature of the aluminium alloy, resulting in scratches and particle adhesion on the machined surface during the LAT process. Lesser scratches, fewer particle adhesions, and a lamella type structure were found on the finished surface for the UVAT process. The ultrasonic vibration employed in the radial direction might have impacted on the workpiece surface like pin-point hammering due to which defects appeared on the machined surface. The hammering action of the tool might have impacted the machined surface due to the ductile nature of aluminium alloy which resulted in higher damage on the machined surface. Nestler and Schubert also observed lamella type structures and higher damage on the machined surface when vibration was employed in the radial direction during the UVAT process [50]. According to them, vibration in the radial direction led to the generation of micro structured surface which could be beneficial if some predefined structures were to be generated. The generation of the micro structured surface would be unavoidable when vibration was employed in the radial direction. Fewer scratches, large particle adhesion, and a lamella type structure were generated on the finished surface for the UVLAT process. The vibration employed in the radial direction and laser heating on the workpiece surface, simultaneously led to the generation of higher damage on the machined surface. Due to the vibration in the radial direction, the pin-point hammering of the cutting tool led to the generation of lamella type structure. Besides, the thermal softening of the material due to the laser heating would have enabled higher damage due to the ductile nature of the aluminium alloy. Moreover, the higher maximum machining temperature during the UVLAT process would have led to the more sticking of the particles on the cutting tool edge which could have ploughed on the machined surface, resulting in higher damage on the machined surface. Kim et al reported that the surface temperature of the workpiece increased which led to the higher heat affected zone with severe thermal damage and the thermal damage would have surpassed the machining depth of cut, resulting in higher damage on the machined surface [26]. They also indicated that careful assessment would be required before commencing any machining during the UVLAT process. Therefore, it can be stated that the vibration in the radial direction could be beneficial if some predefined structure were desired on the machined surface otherwise it will led to higher damage on the machined surface during the UVLAT process as compared to the CT process.

Surface roughness
The surface roughness was evaluated and analyzed for the UVLAT process and then compared with the CT, UVAT, and LAT processes. The surface roughness evaluated during the CT, UVAT, LAT, and UVLAT process is shown in figure 6. Best surface roughness was observed in the CT process followed by the LAT process and the UVLAT process, whereas worst surface roughness was observed for the UVAT process. The results obtained in the present work were found to be unexpected with the published literature. The surface roughness in the UVAT process was increased by 66% compared to the CT process. The pin-point hammering by the cutting tool on the workpiece surface led to the generation of damage on the machined surface ( figure 5) and this might be the possible reason for higher surface roughness in the UVAT process than that of the CT process. Nestler and Schubert reported that the cutting in the radial direction during the UVAT process led to the generation of micro structured surface which was responsible for higher surface roughness than the tangential and feed direction of the UVAT process [50]. They also revealed that cutting in the radial direction would be beneficial for predefined structures on the machined surface during the UVAT process. The surface roughness during the LAT process was found to be increased around 5%-16% than that the CT process. The scratches and particle adhesions were observed on the machined surface (figure 5) during the LAT process due to the ductile nature of the aluminium alloy and the thermal softening of the workpiece material by laser heating. Higher machining temperature and sticking of the aluminium particles led to the generation of higher damage on the workpiece surface and higher surface roughness. Deswal and Kant reported higher surface roughness during the machining of aluminium alloy for the LAT process than the CT process [17]. They revealed that the adhesion of aluminium alloy on the tool rake face and built-up edge could have occurred because aluminium alloys possess a higher chemical affinity towards the carbide cutting tool materials to form an adhesive layer. The adhered material could have ploughed on the machined surface regularly which led to the higher surface roughness during the LAT process than the CT process. The surface roughness for the UVLAT process was increased up to 27%-57% in comparison to the CT process. The pin-point hammering of the cutting tool due to the vibration and thermal softening of the workpiece material due to the laser heating led to the generation of higher surface roughness. The higher damage on the machined surface was observed during the UVLAT process than that of the CT process (figure 5) and the reason was explained in section 3.4 which corelates well with the surface roughness. Kim et al and Dominguez-Caballero et al also reported higher surface roughness during the UVLAT process when compared with the CT process [26,28]. According to them, higher machining temperature during the UVLAT process led to higher heat affected zone with severe thermal damage, resulting in higher damage on the machined surface and higher surface roughness for the UVLAT process than the CT process. Hence, it can be stated that the vibration in the radial direction and laser heating on the workpiece surface, simultaneously resulted in higher surface roughness during the UVLAT process in comparison to the CT process.

Conclusion
In the present study, the combined application of laser and ultrasonic vibration energies in the radial direction during the UVLAT process was investigated to analyze the machining performance of aluminium 3003 alloy. The machining performance was analyzed in terms of machining forces, machining temperature, chip morphology, surface damage and surface roughness, and compared among the CT, UVAT, LAT, and UVLAT processes. From the findings of the study, it can be stated that the UVLAT process could enhance the machining performance of aluminium 3003 alloy than that of the CT, UVAT, and LAT processes. The conclusions drawn from the above study are summarized as follows: 1. The separation characteristics of the cutting tool and thermal softening of the workpiece material resulted in lower machining forces for the UVLAT process compared to the CT, UVAT, and LAT processes.
2. Maximum machining temperature was observed to be higher during the UVLAT process than the CT, UVAT, and LAT processes.
3. Lower machining forces and higher machining temperature were responsible for minimum chip segmentation, crack-free chip edges, and higher chip thickness for the UVLAT process in comparison to the CT, UVAT, and LAT processes.
4. Pin-point hammering by the vibratory tool, and particle adhesion and sticking by the laser heating led to higher machined surface damage and higher surface roughness during the UVLAT process when compared with the CT, UVAT, and LAT processes.
The present study reveals that the UVLAT process is an eco-friendly machining process, because no coolants are used which makes it a sustainable manufacturing process. Besides, the vibration direction plays an important role to predict the machining performance during the UVLAT process. The outcomes of the present work can guide future studies to achieve the desired machining performance for different workpiece materials and for different vibration directions in various manufacturing industries.