**1. Introduction**

Light alloys, mainly aluminum and titanium, are commonly used in different manufacturing fields especially because of their high performance—weight rate, their excellent physical-chemical properties and their advantageous economic cost. In this context, the aluminum market has greatly benefited thanks to its wide application in the aerospace industry, among others.

Continuous growth in the use of these alloys, and constant requirements to improve the performance of their manufacturing processes follow an important tendency to gradually increase the development of different research lines. The aim of these is to find the optimum conditions for the forming procedures of aluminum alloys.

Material removal, or machining, is among the main applications of aluminum alloy forming processes for aerospace applications. However, in the last decades machining processes have been characterized by a reorientation towards less aggressive environmental processes by means of the minimization or elimination of the use of cutting fluids in the cutting process. Under these conditions, cutting tools show an intensification of their wear mechanisms, causing deviations on the initial design specifications in the machined part [1,2]. For this reason, preserving the initial geometry of the cutting tool as long as possible is essential to ensure final tolerances [3]. In this context, a preliminary study of the tribological interference between the material being machined and tools is critical to maintain a precise control of process parameters, obtaining higher performance ranges.

In general terms, when aluminum alloys are machined by chip removal processes, the tool wear process is mainly characterized by the appearance of secondary or indirect adhesion mechanisms [4]. This wear mechanism is specifically based on the incorporation of the machined material over two well-localized areas of the cutting tool: at the edge, giving rise to the Built-Up Edge (BUE); and at the rake face, giving rise to a Built-Up Layer (BUL). Both types of material incorporation may modify the initial cutting geometry, affecting the surface quality of the machined parts [5], as is mentioned previously, and can be seen in the macrographs of Figure 1. In addition, the mechanical instability of the involved effects tends to cause a friction process promoted by the chip, resulting in the lost of particles of the tool surface, which constitutes the main wear effect [6].

**Figure 1.** Cutting tools affected by adhesive wear mechanism in the machining of Al alloys: (**a**) turning insert tool; (**b**) detail of the adhered material thickness; (**c**) drilling tool; (**d**) milling tool for radial operations.

Tool wear mechanisms can be studied under lab conditions using Pin-on-Disk or Pin-on-Flat tests [7–10] reducing the material cost and improving the environmental sustainability of the process. However, only a few studies have been found which investigate the specific tribological pairing of aluminum and tungsten carbide [11]. For this reason, the present work is focused on the study of the wear mechanisms involved in the tribological friction and sliding conditions of the WC–Co (tool material) and UNS A92024-T3 aluminum alloy. Pin-on-Disc test techniques were performed and wear effects were analyzed by volume variation and SEM/Energy Dispersion Spectroscopy (EDS) microscopy in order to obtain a deeper understanding for the detected wear phenomena.

### **2. Materials and Methods**

Pin-on-Disc tribological tests (PoD) were carried out under dry conditions, using a MT/60/NI Microtest Tribometer (Microtest S.A., Madrid, Spain) (motion diagram in Figure 2). The load (N) and linear speed (Ls), track radius (R) and turning speed (ω) were constant, while sliding distance varied, as is listed in Table 1. During the test development, the dynamometric response values, environmental conditions, and temperatures were measured.



**Figure 2.** Pin-on-Disc tribometer motion diagram.

Firstly, Al–Cu UNS A92024-T3 (Ra < 0.4 <sup>μ</sup>m) samples of 90 × 90 mm<sup>2</sup> and thicknesses between 1.6 and 2.0 mm were selected as discs. Their composition is shown in Table 2.

**Table 2.** Composition of aluminum–copper alloy (Weight %).


Then, (WC-6%Co) carbide metal bars with 30 mm length (l) and 4 mm diameter (d) hemispherical ends were used to simulate tool displacement (pins). The average and maximum Hertz contact pressure for the tribological pair were calculated as 0.91 and 1.37 GPa, respectively.

Wear evaluation was carried out following the guidelines of the ASTM G99-04 standard, expressing the friction effects in terms of material volume loss (mm3) as a function of the sliding length. All samples were carefully cleaned using petroleum ether and alcohol (50%). The weight of the aluminum probes were evaluated by a precision scale (Ohaus Pioneer PA214, Parsippany, NJ, USA) before (P0) and after (PF) the development of the tribotests. The precision scale used in the weight evaluation of the samples have a 0.0001 g resolution. This scale resolution is the recommended by the ASTM G99 for the evaluation of results in Pin-on-Disc tribological tests.

In addition, visual inspection was carried out by optical microscopy techniques, using a stereoscopic microscopy device (Nikon SMZ-800, Tokyo, Japan) with the aim of analyzing the effects and consequences of the wear mechanisms involved in the process. The wear track was also measured by a profilometer Taylor Hobson Form Talysurf Series 2 (Leicester, UK).

Specific areas of the carbide pins and aluminum discs were established to perform deeper evaluation with a Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS) microcompositional characterization, by using a FEI Quanta 200 (ThermoFisher Scientific, Hillsboro, OR, USA) with EDAX Phoenix.

#### **3. Results and Discussion**

The friction coefficient behavior and wear mechanisms involved in the process were studied on the contact surface of the carbide pins. The tribological wear effects were evaluated through volume loss of aluminum discs as a function of sliding length. Additionally, a study on the tribological wear behavior between empirical and theoretical models was carried out taking the Archard coefficient as control parameter.
