*Article* **Tool Wear Mechanism in Cutting of Stack CFRP/UNS A97075**

## **Severo Raul Fernandez-Vidal \*, Sergio Fernandez-Vidal, Moises Batista and Jorge Salguero**

Industrial Design Department, Faculty of Engineering, Mechanical Engineering, University of Cadiz, Av. Universidad de Cádiz 10, E-11519 Puerto Real-Cádiz, Spain; sergio.fernandezvidal@mail.uca.es (S.F.-V.); moises.batista@uca.es (M.B.); jorge.salguero@uca.es (J.S.)

**\*** Correspondence: raul.fernandez@uca.es; Tel.: +34-956-48-3460

Received: 1 July 2018; Accepted: 23 July 2018; Published: 25 July 2018

**Abstract:** The aeronautics industry's competitiveness has led to the need to increase productivity with one shot drilling (OSD) systems capable of drilling stacks of dissimilar materials (fibre/metal laminates, FML) in order to reduce riveting times. Among the materials that constitute the current aeronautical models, composite materials and aluminium (Al) and titanium (Ti) alloys stand out. These one-pass machining techniques produce high-quality holes, especially when all the elements that have to be joined are made of the same material. This work has followed a conventional OSD strategy and the same cutting conditions applied to CFRP (carbo-fibre-reinforced polymer), Al and CFRP/Al stacked sheets to know the wear mechanisms produced. With this purpose, results were obtained by using current specific techniques, such as microstructural analysis, monitoring of the shear forces and analysis of macrogeometric deviations. It has been determined that when these drilling techniques are applied under the same cutting conditions to stacks of materials of a different nature, the results of the wear mechanisms acting on the tool differ from those obtained when machining each material separately. This article presents a comparison between the effects of tool wear during dry drilling of CFRP and UNS A97075 plates separately and when machined as stacks.

**Keywords:** wear; drilling; machining; dry drilling; stack; FML; CFRP; UNS A97075

#### **1. Introduction**

The aeronautical sector has always been a benchmark in research, development and innovation. This has been motivated by the intense competitiveness that exists within the sector, generating a continuous need to improve functional, environmental and energetic efficiency in the processes, guaranteeing quality and seeking a direct impact on economic performance [1].

The first challenge the aerospace industry faces in its fourth revolution is to automate processes that nowadays include the extensive use of manual labour, especially in relevant operations such as assembly operations [2].

Among the different joining methods available in the industry, riveting is still most often used, regardless of the materials involved in the assembly. This joining process requires a previous drilling operation. OSD techniques can produce high-quality holes, especially in cases where all the elements to be joined are made of the same material [3,4].

In the construction of the latest aircraft models, the excellent characteristics of the carbon-fibrereinforced thermoset matrix composites (CFRP) have formed a balance with the current light metal alloys (aluminium alloys 2XXX and 7XXX and titanium alloys Ti6Al4V) [3–6]. These structures are known as fibre/metal laminates (FML). The present study focuses on comparisons when facing drilling stacks of diverse types of material, resulting in a very different situation from machining them separately [7].

This case requires us to study the wear mechanisms that affect the tool during the drilling process of composite material stacks with metal alloys, relating them to the final quality of the drill in order to improve the performance of the process. Although some authors use optical techniques to quantify wear, these techniques do not offer a continuous record of wear and tear, and are difficult to integrate into the company's intelligent systems. It has become necessary to monitor continuous variables directly related to tool wear during drilling, such as axial force [7]. This will favour the virtual integration of the company, being one of the main paradigms pursued in industry 4.0. The techniques used to carry out the analysis include microstructural analysis, monitoring of the shear forces and analysis of macrogeometric deviations.
