Investigating the Ability of Process Parameters to Reduce Defects during the Drilling of Carbon-Fiber-Reinforced Plastic (CFRP) and Aluminum Stack ^†

Abstract

The effects of drilling process parameters, such as drill diameter and feed rate, on the delamination of holes has been examined while drilling Carbon-Fiber-Reinforced Plastics and aluminum (CFRP/Al2219-T6). The stack method was employed while drilling in order to reduce the defects in the drilling process and enhance the quality of the holes produced. A total of nine experiments were performed using the central composite design. An analysis of variance (ANOVA) revealed that feed rate and drill size have significant effects on the delamination of holes. The experimental results show that the minimum delamination value (1.018) was obtained at a feed rate of 26.5 mm/min and with a drill diameter of 6.5 mm.

1. Introduction

Carbon-Fiber-Reinforced-Plastic (CFRP) is typically manufactured as “make-to-shape” parts through various molding techniques and utilized in the grouping of lightweight metal stacks such as titanium and aluminum alloys [1]. Once composite parts incorporating a combination of lightweight metal stacks, for example, aluminum alloys, have been manufactured, they need to be integrated into the main assembly or system using mechanical fasteners, such as rivets and screws, and this requires the drilling of holes. However, drilling these materials can be challenging, and defects such as delamination can occur due to differences in the properties of CFRP and stacked metals. Delamination is a type of failure. When the amount of thrust force exceeds a certain threshold, this can lead to the delamination of the layers in a multilayer material. This can cause a considerable reduction in mechanical durability and a decrease in interlaminar strength. Preventing delamination is of utmost importance when drilling CFRP and aluminum stack, particularly as they are increasingly utilized in the aerospace and automobile industries, where safety is a paramount concern.

Researchers have optimized different process parameters, such as feed rate, drill diameter, and spindle speed, to reduce the delamination of drilled holes. For instance, D’Orazio et al. [2] reported the effects of drill diameter and feed rate on delamination and thrust while drilling CFRP/AA 7075 stack using a nanocoated TiAlN drill bit. In this study, a total of 170 holes were drilled in the CFRP/Al stack, and the results show that the observed increment in delamination was due to increasing the number of holes. The delamination associated with nanocoated TiAlN was greater compared to that of a DLC-coated drill bit. The impact of spindle speed, feed rate, and drill diameter on the circularity, surface roughness, and torque has been studied by Redouane Zitoune et al. [3]. The authors concluded that feed rate has a greater impact on hole quality than spindle speed. In another research article, Alessio et al. [4] evaluated delamination and thrust during the taping of CFRP/Al7075/CFRP stack. With an increasing number of tapped holes, delamination and thrust forces increased due to the wear of the tool.

A literature review of [1,2,3,4] revealed that researchers have focused on optimizing cutting parameters to enhance the hole quality and trust force in the drilling process using high-speed steel tools. However, the effects of drilling process parameters on the stack of CFRP with Al2219-T6 must still be explored due to its application in the aerospace industry. Hence, the objective of this article is to evaluate the influence of drill diameter and feed rate on delamination while drilling CFRP/Al2219-T6 stack. Furthermore, a central composite design has been used in the experiment, and an ANOVA table has been used to analyze the significance of the process parameters.

2. Materials and Method

XRF spectrometer was used to perform a spectrometry test, and the result regarding the chemical composition of Al2219-T6 obtained from the spectrometry test is shown in

Table 1. Spectrometry results of Al2219-T6.

Composition (wt. %)	Cu	Fe	Al	V	Si	Mn	Ti	Mg
Al2219-T6	6.3	0.23	Bal.	0.10	0.18	0.35	0.06	0.02

. The stack method was used in the drilling process because Alessio et al. revealed that the stack method helped to reduce the defects in the drilling process [4]. In this method, Al2219-T6 plate was machined to achieve the required dimensions (425 × 225 × 3) in mm. Afterward, shot peening was applied to the surface of Al2219-T6, which would later be bonded with CFRP. A chemical agent (Trichloroethylene) and primer (EW-5000) were applied to polish the surface of the Al2219-T6. Finally, prepreg carbon fabric was placed in an autoclave at a pressure of 3 bar to increase the resulting material’s bonding capability.

After the preparation of the stack plate, drilling was performed using the HSS TiN Coated Twist Drill tool on the Taiwan-made universal milling machine center, as shown in Figure 1c. Delamination describes the damage inflicted on the surface of a composite workpiece either at the entry or exit point during the drilling process [2]. Therefore, the output response delamination was measured after performing experiments at each experimental setting. By dividing the damaged zone diameter by the drill bit diameter, the degree of delamination was calculated. The diameter of the damaged zone was measured using an optical microscope (VZM-200), and diameter of the drill bit was measured using a digital caliper.

3. Experiment Design

The lower and upper ranges of two dominant factors, namely, drill diameter and feed, in the drilling of CFRP and Al2219-T6 were selected based on trial runs and a literature review [2,3,4], as shown in

Table 2. Ranges of drilling process parameters.

Machining Parameters	Unit	Lower Level	Upper Level
Feed rate	mm/min	40	80
Drill diameter	mm	4	9

. According to the central composite design, nine experiments were performed, and two process parameters and one center point were considered, as depicted in

Table 3. Experiment design.

Experiment No.	Input Parameters		Output
	Feed Rate (mm/min)	Drill Diameter (mm)	Delamination
1	40	4	1.020
2	26.5	6.5	1.018
3	72.5	2.7	1.025
4	105	9	1.045
5	72.5	6.5	1.027
6	118.5	6.5	1.045
7	72.5	10.1	1.030
8	40	9	1.022
9	105	4	1.035

.

4. Results and Discussion

The experimental results show that the minimum value of delamination (1.018) was achieved in experimental run 2 (feed rate 26.5 mm/min and drill diameter 6.5 mm). The significance of the selected factors and their contribution to the output, i.e., delamination, were determined using analysis of variance (ANOVA) with 95% confidence intervals, as shown in

Table 4. ANOVA results for selected factors.

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	0.0008	2	0.0004	51.35	0.0002	Significant
A-Feed (f)	0.0007	1	0.0007	96.64	<0.0001	Significant
B-Drill Size	0	1	0	6.06	0.0491	Significant
Residual	0	6	7.51 × 10−6
Cor Total	0.0008	8
R-sq. 94.48%, R-sq.(adjusted) 0.9264, R-sq.(predicted) 0.8751

. ANOVA indicated that the model of process parameters with the de-lamination is significant, as the p-value of model (0.0002) is lower than 0.05. p-values below 0.05 indicate that a model is significant[5]. The ANOVA table also shows that the reduction in delamination is significantly impacted by the drill diameter and feed rate, and the feed rate has a greater impact on delamination than the drill size, as evidenced by its lower p value (0.0001). Furthermore, the adjusted R-square [6] and predicted R-square [7] helped to determine the adequacy of the mathematical model. Therefore, R-square adjusted (0.9264) and R-square predicted (0.8751) depicted a high degree of model accuracy, as shown in Table 4.

A three-dimensional mesh plot was created to analyze the combined effect of the drill diameter and feed rate, as shown in Figure 1a. It was clearly observed that delamination increases with an increasing drill diameter and feed rate. This is due to the increase in thrust force that accompanies an increase in the drill diameter and feed rate, which leads to the breaking of the inter-laminar layers of carbon fabric, which, in turn, leads to an increase in the delamination of holes [3]. Similarly, the single-factor plot showed that delamination gradually increases with an increasing feed rate up to a certain level, and then the increment of delamination slightly reduces when further increasing the value of the feed rate, as shown in Figure 1b. On the other hand, delamination increases when increasing the value of the drill diameter, but after a certain level, delamination starts to reduce with the increment in the drill diameter, as evident in Figure 1b. This happens due to the damage that starts to occur in the inter-laminar layers at higher values of the drill diameter [4].

5. Conclusions

It has been determined that the optimal value of delamination (1.018) was achieved under experimental conditions of a feed rate of 26.5 mm/min and a drill diameter of 6.5 mm. The ANOVA table concluded that feed rate and drill size have a significant impact on delamination during the drilling of CFRP/Al2219-T6. It was also concluded from the single factor plot and 3D Mesh plot that delamination increases with an increasing feed rate and drill size. Furthermore, it has been revealed that the feed rate’s contribution to increasing delamination is significantly higher than that of drill size.