Numerous epidemiological studies have linked PM_2.5 with significant health problems. These studies have linked exposure to submicron particle to a variety of problems including aggravated asthma, decreased lung function, increased respiratory symptoms such as irritation of the airways, coughing or difficulty breathing [1,2]. Several particles, such as dust, smoke have been routinely measured to determine their toxic and potential carcinogenic effects [3,4]. The size of particles, particulate matter (PM_2.5) and ultrafine particles (<0.1 μm), is directly linked to their potential for causing health problems [5-8]. Small metallic particles, less than 2.5 micrometers in diameter, pose the greatest problems, because they can get deep into lungs, and some may even get into bloodstream. Exposure to particulate matter (PM_2.5) is largely beyond the control of individuals and requires action by public authorities at the national and international levels [9].

Most machining processes, in addition to producing the part desired shape, also generate chips, metallic particles and wet aerosols if the process is lubricated (Figure 1). In the air, both aerosols forms (wet and dry) can be inhaled and therefore threaten human health. Solid aerosols are generated from workpiece material during dry and wet machining, while liquid aerosol production is caused by the use of cutting fluids [10,11]. The primary mechanisms responsible for wet aerosol production include fluid impact on the workpiece and also evaporation [12-14]. Cutting fluids are not only toxic [15,16], but are also costly (initial purchase and treatment of used fluids). The emission of liquid aerosols caused by cutting fluids can be eliminated by using dry machining. However, the problem of the solid aerosols still remains in this case [17]. The potential health hazards associated with the machining have become another machining process performance indicator to consider [18]. The machining processes can be classified in terms of particle emission [19]. For processes such as grinding, drilling and turning, some remarkable works on particle emission were produced [19-27]. Reference [21] indicates that the majority of dust generated during machining is composed of ultrafine particles (<1 micron), the most dangerous ones. Reference [22] showed that, during grinding, most metallic dust generated is breathable, and, without a dust suction system, the level of dust particle concentration in the air is higher than the legal limit regulations in North American Occupation Safety and Health.

Reference [23] measured the dust distribution in a full scale workshop. A sampling of the full range of particle sizes was carried out on 16 processes. The particle size distribution was shifted to smaller particles, with 16 processes selected. Reference [24] explained the dust generation by the friction phenomenon, and identified five particle emission sources during drilling: shearing action, deformation and friction of chips, deformation and friction on the tool-chip interface, friction on toolworkpiece, and friction of chips in drill flutes. Reference [17] demonstrated that speed, feed, and depth of cut are key variables in the formation of dust during the machining of cast iron. Fine dust emission was found to decrease with increasing in cutting speeds [25]. It was also demonstrated experimentally [27] that it is possible to reduce the dust generation (PM_2.5) during dry machining without negative effects on the productivity. The strategies for particle emission reduction at the source included the selection of appropriate tool geometry, the use of good combination of cutting speed and feed rate, and the adoption of specific machining strategies [27].

Many particle physical factors can influence their toxicity (i.e., composition, size, size distribution, shape and specific surface area). The metric generally regarded as a better indicator of health risks rather than the particle mass is the specific surface [26]. However, there is currently a lack of information available in the literature on the emission rates of particles from machining in terms of their specific surface. Little or no work has been carried on studying the relationship between specific surfaces and machining conditions.

The main objective of this study was to evaluate the influence of machining conditions on the emission of metallic particles when machining different aluminum alloys (6061-T6, 2024-T351 and 7075-T6). The work was carried out in order to minimize dust emission and improve air quality in machining shops.

The operation selected for this study was a full slot milling process (Figure 1). The experiments were performed on a HURON-K2X10 machine-tool (Power: 50 kW, Speed: 28000 rpm, Torque: 50 Nm). A compartment was used to enclose the volume of air containing particles emitted by the machining process. A Scanning Mobility Particle Sizer (SMPS 3936) was used to quantify the particles produced. The SMPS was connected to limps of collection by a suction pipe at a flow rate of 1.5 l/min from the cutting area (or near this area). A ten-stage micro-orifice uniform deposit impactor (MOUDI, MSP Corp) was used for the sample collection. One set of samples was collected with polycarbonate substrate and the other with aluminum substrates. The dust sampling collection was carried out before, during and after each cutting process to return to initial ambient concentration (Figure 1).

A computer equipped with a data acquisition and analysis system was also connected to the measuring device. To evaluate the effects of the cutting parameters a full factorial design was used in this study. The factors studies and their levels are summarized in Table 1. The total number of experiments performed was 108 experiments. The specific surface area of particles (Cs) was computed from the particle concentration and used as output responses. Cs is calculated using the following equation:

The cutting tool used was a carbide end mill (Φ19 mm). The inserts had the following characteristics:

• IC328: 0.5 mm nose radius, TiCN coating

• IC4050: 0.5 mm nose radius, TiCN/Al₂O₃/TiN coating

The combined influence of the cutting speed and the feed on the specific surface of particles depends on the tools and workpiece material. During slot milling process the experiment shows the relative low influence of the cutting speeds tested on the specific surface of particles generated. Eventually, the response surface identified a region of the experimental field in which the specific surface of the particle emitted is minimal (to be avoided). This minimum is given by the combination of a critical cutting speed and feed rate. It can be admitted for low cutting speed and high feed rate there is a decrease the size of dust generation (Figure 8). The influence of cutting speed on the size of dust generation remains quietly similar to the influence of the tools coating.

Figure 9 shows the size for the specific surface of particle generated according to the number concentration. In the experimental field, the critical zone for the PM_2.5 generated is located at low size of specific surface. The observations were explained in terms of friction effects of the tools. Some researchers have explained the dust generation sources by the friction phenomenon [17-25]. These friction zones are not the only dust generation sources. The chip formation is the main element that can be studied to better understand the dust emission phenomenon. The chip formation identifies zones in which the stress modes are different and each zone has an influence on the dust formation. At the secondary shear zone, there is a velocity gradient on a thin chip. This gradient inside the shear zone secondary leads deformation and formation chip segments. The chip segmentation emits particles to discharge the surface tension due to the creation of new surfaces (Figure 10). The chip will be formed by micro-segments that undergo a local work hardening due to the action of some asperities of the tool rake face, after which the hardened small part is separated by a local brittle fracture. The size of the particles separated depends on the tool rake face roughness, the cutting conditions, and the workpiece material.