Influence of cutting parameters on tool wear and hole quality in composite aerospace components drilling

Abstract

Composite Fiber Reinforced Plastics (CFRP) are characterized by their outstanding mechanical properties combined with reduced density and good resistance to corrosion and fatigue which make them suitable for aerospace components. During assembly procedures, one shoot drilling operations, usually including countersinking cycle, are required to minimize positional errors, enhance tight tolerances and reduce process time.

Countersink drill bits were tested on CFRP test specimens, representative of aircraft components. Along testing, tool wear was monitored with an optical microscope to track its evolution and determine the dominant wear mechanism. On the other hand, hole quality was evaluated since tool life criterion is based on the assessment of machined surface quality.

The influence of cutting speed and feed was analyzed with the objective of looking for extended tool life and more productive cutting parameters.

The information gathered from monitoring tool wear and inspecting hole quality can be used for the enhancement of CFRP drilling and the improvement of the manufacturing process competitiveness, in terms of production cost and time.

Introduction

The use of composite materials by aerospace industry has stood out in the last decade due to their outstanding mechanical properties, combined with reduced density and good resistance to corrosion and fatigue, until today when they shape over 50% of the structural weight of some aircraft. In particular, Composite Fiber Reinforced Plastics (CFRP) have high specific strength and superior fracture toughness [1].

Even though composite materials are characterized by its manufacturing process, which allows to produce near net shape, machining is unavoidable for assembly purposes, being drilling the most common operation. The different components are stacked together to perform the operation in single shot, obtaining the required tolerances of the hole and removing the reaming cycle. Usually, it is included countersinking to avoid interferences of the head of the rivet with the aerodynamic shape. This minimize positional errors, enhance tight tolerances and reduce process time.

Although kinematics of composites machining process remains the same as in metal machining [2], there are several differences due to the fact that composites are inhomogeneous and anisotropic materials made from two constituents: the reinforcements which use to be brittle and the matrix which, for its part, tends to be more ductile. Hence, machining process is based on intermittent fractures and bouncing cutting forces.

The drilling operation is produced by two different material removal processes, related with conventional drill bits geometry. The main cutting edges eliminates most of the material, analogously to an orthogonal cutting, which generates the bulk of the torque or cutting force. On the other hand, the chisel edge behaves as a blunt edge with a high rake angle which work as a punching process and producing most of the thrust force [3]. Regarding chip formation process, it is caused by multiple cutting edges which encounter the fibers at difference orientations despite of the material is made of unidirectional layers [4]. Furthermore, the coefficients of thermal expansion of matrix and fibers can be quite dissimilar which may reduce the dimensional accuracy of the hole and the quality of machined surface.

During composites machining, induced damage on the workpiece has to be taken into consideration since several failure modes can be produced, such as, fiber pullout, delamination, surface damage or burning being delamination most critical for the structural integrity and long term reliability of the component [5]. It is characterized by the separation of the layers caused by the low interlaminate strength of the composite structure and excessive cutting forces which are enhanced by tool wear, inadequate cutting speeds and feeds or improper tool geometry [6].

The level of tool wear is another important feature that have to be monitored on CFRP drilling operations. Most of the wear occurs at the cutting edge corner due to higher cutting speeds, although in the rest of the edges it has to be taken into account [7]. In some cases, the wear may not uniform due to difference mechanical properties of fibers and polymer matrix. The predominant wear mechanisms are abrasion, promoted by the abrasiveness of fibers, and chipping produced by the fluctuation of forces due to material inhomogeneity. These type of wear mechanisms are mitigated with different tool material properties: hardness is required for abrasive wear and toughness for chipping [8]. General recommendation for CFRP drill bits are sharp cutting edges and large positive rake angles to facilitate clean shaving of the fibers.

Several authors have been researching about tool geometry for CFRP drilling and the associated wear produced. Mayuet et al. [9] showed that abrasion was identified to be the dominant wear mechanism on a conventional carbide geometry drill bit under different cutting conditions. The direct influence of tool wear on machining induced damage in woven materials was studied in [10] and correlations between delamination, cutting edge rounding and drilling loads were found. Karpat et al. [11] analyzed the performance of double point angle drill bits on fabric woven CFRP laminates and it was found that feed was the dominant parameter of tool wearing. The influence of the point angle was analyzed on woven CFRP in [12] showing that larger angles produce higher thrust force while the torque remains constant and that the increase of cutting speed does influence hole quality but increase thrust forces [13].

On the other hand, the influence of cutting parameters on the axial and cutting force produced during dry drilling of woven CFRP composites was studied in [14]. Also, Khashaba et al. [15] have analyzed the effect of machining parameters on the cutting force and the quality of the workpiece on woven glass fiber-reinforced epoxy (GFRE) composites in terms of delamination size, surface roughness, and bearing strength. High speed drilling of woven graphite epoxy was investigated by Rawat and Attia in [16] and it was found chipping at the very beginning of the drilling process, followed by abrasion and adhesion of material.

Finally, the behavior of coated versus uncoated carbide drills was studied by Iliescu et al. on [17]. It was shown that thrust force is more influenced by tool wear than torque and that uncoated tools have a power law dependence of tool wear with axial force while coated carbide tools have a linear dependence instead.

The market for composite aerospace materials has duplicated the levels corresponding to 10-years ago, and it is expected to grow another 55% over the next following years [18]. According to Airbus data, the number of holes drilled on composite made aircraft components will be five times bigger in 2020. As a matter of fact, due to the huge number of holes required for the assembly of aeronautical components, machining operations must be designed ensuring competitive levels of productivity and cost per operation, while maintaining the required quality on the machined surface.

So, the main target of this work is to study the wearing process of carbide countersink drill bits, on CFRP test specimens representative of aircraft components, with the objective of looking for extended tool life and more productive cutting parameters. For this purpose, an optical microscope was used to characterize the wear mechanism of the tools and a contact profilometer to assess the quality of the hole, as well as different measurement tools, such as calibers or inside micrometers.