Experimental study on drilling mechanisms and strategies of hybrid CFRP/Ti stacks
Introduction
Hybrid CFRP/Ti stack is an advanced composite structure that has been widely used in modern aerospace industry due to its superior mechanical/physical properties and excellent structural functions including high strength-to-weight ratio, good corrosion/erosion resistance, high design flexibility, etc. The bi-material assembly which consists of two disparate constituents, i.e., carbon fiber reinforced polymer (CFRP) and Ti alloy, can provide combined structural advantages of each stacked phase while their individual weaknesses are significantly avoided [1]. For example, the Ti alloy has good strength-to-weight ratio, high fracture resistance, isotropic behavior, and exhibits good reparability [2], [3], [4], while the CFRP composite shows high specific stiffness, superior corrosion resistance, and excellent fatigue strength [5], [6]. The combination of the metal-to-composite alliance in a hybrid composite structure typically overcomes the lack of fatigue strength and corrosion resistance of metals, and avoids the shortcoming of low bearing/impact strength and reparable problem of composites [7], [8].
The emergence of such hybrid composite stack has greatly revolutionized the conventional material distribution in modern commercial aircraft. Owing to its inherent advantages, the hybrid CFRP/Ti stack has gradually substituted conventional standard CFRP applications and single Ti alloy applications in various aerospace fields. Currently, many giant aircraft manufacturers including Airbus and Boeing are widely employing this multi-phase material in new generation commercial aircraft to produce competitive structural components (e.g., fuselages, skin segments and wing connections) that favor energy saving and benefit system performance improvement. More noteworthy, such an increasing trend is still expected to grow in the next one or two decades.
To satisfy the industrial applications, millions of holes are needed to be drilled out in the stacked materials for assembly purpose [9], [10]. However, this bi-material system is problematic for drilling operations and is often classified as a difficult-to-cut material as reported by Xu et al. [1] in their review work on drilling of hybrid composite stacks. The key causes can be attributed to the disparate properties of the stacked constituents and their respectively poor machinability. During drilling, the problems of severe hole damage including the irreparable delamination in the composite phase and Ti burrs in the metallic phase always prevail in the material removal process and account for a large number of part rejections. Besides, the rapid tool wear progression and premature tool failure are also a key source of increased machining costs. Specifically, it is estimated that the reasons for the majority of the rejected parts are the use of improper cutting parameters, non-optimal drill bits and unfavorable cutting environments [1].
Additionally, in hybrid CFRP/Ti stack drilling, typically two different cutting sequence strategies, i.e., drilling from CFRP → Ti and from Ti → CFRP, exist from the aspect of tool entry and tool exit throughout the chip removal process. To improve the machinability of the bi-material system, a careful selection of cutting sequence strategy is of great importance. Through one previous literature survey [1], it was understood that most scholars [9], [10], [11], [12] have believed that the CFRP → Ti cutting sequence is often a reasonable and efficient strategy for minimizing the severe hole damage when vertical drilling of the bi-material system. Their key supporting arguments were that in such drilling sequence, a lower extent of exit delamination damage can be achieved due to the supporting role of the bottom Ti alloy in preventing laminate deflection and limiting the workpiece dynamics [1], [9]. Further, an analytical model proposed by Qi et al. [13] for drilling FRP/metal stacks also confirmed that the FRP → metal drilling sequence usually yields a higher CTF (critical thrust force) value than that obtained in the metal → FRP drilling sequence, i.e., the FRP → metal drilling can promote a lower delamination extent as compared to its counterpart one. However, this drilling sequence itself has several inherent disadvantages. For instance, it will inevitably result in the serious difficulty of Ti chip evacuation while drilling the bottom Ti phase. Since hot, long and spiral features usually characterize the drilled-out Ti chips, it will cause severe abrasion/erosion actions on the machined CFRP phase and greatly deteriorate the hole quality. Moreover, the spiral Ti chips can easily entangle themselves with the drill margins, and cause premature tool failures like micro chipping or edge fracture. In contrast, the Ti → CFRP drilling sequence is capable of promoting efficient Ti chip ejection and quick heat dissipation due to the short chip evacuation length involved in drilling. However, this drilling sequence is likely to induce a higher extent of delamination damage of the composite phase. To date, the majority of the previous work [9], [10], [11], [12], [14], [15] concerning drilling hybrid CFRP/Ti stacks was performed solely using the CFRP → Ti drilling sequence. A comparative study to clarify the different benefits between the two cutting sequences in drilling has not yet been reported in the open literature.
With respect to the current research advances, a large amount of experimental work has been performed in the past few decades in order to improve the machinability of hybrid CFRP/Ti stacks through the use of superior tool materials [9], [10], [14], [16], [17], optimal tool geometries [11], [18], [19] or favorable cutting environments [18], [20], [21]. At present, the main research hotspots of hybrid composite drilling as surveyed in one recent review work [1] are to (i) investigate the parametric effects on various drilling responses (drilling force, hole quality, etc.), (ii) evaluate different tool performances, and (iii) reveal the tool wear mechanisms and tool failure modes governing the bi-material drilling. In spite of the well-performed studies, the pertinent research work regarding the aforementioned issues, e.g., machinability classification of hybrid CFRP/Ti stacks and the influences of different cutting sequence strategies on drilling, is still rarely reported.
Based on these incentives, this paper aims to carry out a series of experimental studies on drilling hybrid CFRP/Ti stacks by adopting different tool geometries/materials and drilling sequence strategies. The key objectives of present work are to reveal the machinability classification of the bi-material system through the force signal inspection, and clarify the relative effects of cutting parameters and tool geometries/materials on the bi-material drilling. A special focus was made on the evaluation of different tool performances and on the investigation of different cutting sequences’ influences on drilling output. The fundamental machining responses including drilling forces, machined hole quality (e.g., surface roughness, hole diameter, and roundness error) and drilling-induced damage (e.g., delamination extent, and burr defect) were precisely addressed versus the utilized cutting conditions.
Section snippets
CFRP/Ti workpiece details
The studied hybrid CFRP/Ti specimen was provided by VN Composites Company in France, consisting of one annealed Ti6Al4V alloy and one T300/914 CFRP laminate (60 % fiber volume fraction) subjected to the stacking sequence of [45°/−45°/0°/90°]s. Each stacked phase has 4 mm thickness and the entire CFRP/Ti specimen has the total dimensions of 254 mm (length) × 34.5 mm (width) × 8 mm (thickness). The nominal chemical composition of the stacked Ti6Al4V alloy and the basic mechanical/physical properties of
Drilling process and force signal characterization
The drilling force signals developed in function of cutting time can be utilized as an effective method to monitor the status of the on-site tool-work interaction. In hybrid CFRP/Ti stack drilling, the most complicated cutting stage usually takes place in the bi-material interface (aka “CFRP-to-Ti” contact boundary) due to the occurrence of the coupled composite-metal drilling process. Fig. 5 shows the scheme of the drill tip position involved in the CFRP/Ti interface drilling under the cutting
Conclusions
In this paper, the drilling characteristics of hybrid CFRP/Ti stacks have been experimentally studied. The effectiveness of different cutting sequence strategies and different tool geometries/materials in drilling CFRP/Ti stacks has been systematically investigated, and their results are compared. The experimental work has highlighted the vital roles of cutting sequence strategies and tool geometries/materials in affecting the final CFRP/Ti drilling responses. Based on the results acquired, the
Acknowledgements
The authors gratefully acknowledge the financial support of China Scholarship Council (CSC) (Contract No. 201306230091). The authors also would like to thank Mr. Julien Voisin for his technical assistance throughout the drilling experiments.
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