An Investigation into Tool Wear and Hole Quality during Low-Frequency Vibration-Assisted Drilling of CFRP/Ti6Al4V Stack

Abstract

The use of lightweight material such as CFRP/Ti6Al4V in stacked structures in the aerospace industry is associated with improved physical and mechanical characteristics. The drilling process of nonuniform structures plays a significant role prior to the assembly operation. However, this drilling process is typically associated with unacceptable CFRP delamination, hole accuracy, and high tool wear. These machining difficulties are attributed to high thermal load and poor chip evacuation mechanism. Low-frequency vibration-assisted drilling (LF-VAD) is an advanced manufacturing technique where the dynamic change of the uncut chip thickness is used to manipulate the cutting energy. An efficient chip evacuation mechanism was achieved through axial tool oscillation. This study investigates the effect of vibration-assisted drilling machining parameters on tool wear mechanisms. The paper also presents the effect of tool wear progression on drilled hole quality. Hole quality is described by CFRP entry and exit delamination and hole accuracy. The results showed a significant reduction in the thrust force, cutting torque, cutting temperature, and flank wear-land.This wear mechanism was attributed to hard and soft abrasion modes [14]. The hard mode is located at WC grains due to the generated dynamic stresses from the broken fiber, material reinforcement, and powder like chips [15]. Consequently, the WC grains suffer crack initiation and propagation fracture [16]. On the other hand, the relatively low hardness Co binder is more easily damaged by carbon fibers (CF) in a process known as soft abrasion mode [14,17,18]. Hence, WC grain spalling is more rapid under the cyclic machining load.High cutting temperature, poor chip evacuation mechanism, and burr formation are of the common issue during the drilling process of Ti6Al4V [19][20][21]. These problems resulted in a higher probability of flute-chip accumulation, progressive tool wear, and poor surface integrity [19,20,22]. Low thermal conductivity and high chemical affinity to the majority of tool materials are due to the adverse titanium characteristics, which resulted in a significant reduction in the tool life with a high probability of catastrophic failure [20,23,24]. This effect contributed to the cutting stress concentration on the tool cutting edge where the maximum cutting temperature is located. Therefore, the machining process of Ti6Al4V is typically associated with the formation of built-up-edge (BUE). The formation of BUE has a significant drawback on the machined surface integrity and the tool life [25,26]. Thus, adding more limitations in the aerospace production procedure [27].Despite, the extensive to optimize separate machining process of both materials, the machining process suffers when it comes to production time, cost, and assembly positional error. Single process drilling of the CFRP and Ti6Al4V materials in situ, also known as stacked drilling shows considerable benefits to overcome separate drilling issues [1][2][3]28,29]. However, the stack drilling o...

The effect of MQL on tool wear progression in low-frequency vibration-assisted drilling of CFRP/Ti6Al4V stack material

“…Compared to the CD at N= 2000 rpm, the analysis of the results showed that the cutting tool temperature under LF-VAD was reduced by 40 % for dry coolant condition [11], and 65 % for MQL coolant condition, respectively, as shown in Figure 4. For high cutting speed (N= 3000 rpm), tool-chip welding has been con rmed as the main issue that resulted in stopping the drilling process at the second hole for CD, and hole number 20 for LF-VAD with A m = 0.25 mm, as reported in [11]. The use of MQL resulted in a successful machining process of 50 drilled holes without any observation of the tool-chip welding process.…”

“…Abrasive wear of the hard carbon ber particles [8][9][10][11], and adhesion wear mechanism during the drilling process of Ti6Al4V. The machining process of Ti6Al4V resulted in a signi cant increases on the cutting tool temperature due to the low thermal conductivity, and the continuous chip morphology of Ti6Al4V [11,12]. The high cutting temperature has a severe effect on the cutting tool condition, particularly during the drilling process where the poor evacuation mechanism located.…”

“…The high cutting temperature has a severe effect on the cutting tool condition, particularly during the drilling process where the poor evacuation mechanism located. Consequently, this process has a high tendency of chip accumulation and the tool-chip welding takes place [11,13]. Furthermore, by increasing the cutting tool temperature, the tool material suffers from the low hardness properties.…”

“…The CFRP has a total thickness of 5.8 mm, while the Ti6Al4V plate has a 6.75 mm thickness. Based on the previous experimental investigation [11,25], Table 1 presents the selected machining parameters, tool, and workpiece materials. The test matrix consisted of two-level of cutting speeds (2000 and 3000 rpm), three levels of vibration amplitudes (0.1, 0.16, and 0.25 mm), and xed feed rate of 0.075 mm/rev.…”

“…The test matrix consisted of two-level of cutting speeds (2000 and 3000 rpm), three levels of vibration amplitudes (0.1, 0.16, and 0.25 mm), and xed feed rate of 0.075 mm/rev. The selected test matrix parameters are targeting to overcome the previously published machining issues, as presented in [11]. The applied minimum quantity lubricant (MQL) has a 5 % concentration of the MECAGREEN 550 with a 100 bar pressure.…”