Over the years, the United States Environmental Protection Agency (EPA) has changed the focus of particulate standards from total airborne particles in 1971 to particles size lower than 10 μm (PM
10) in 1987 and then to PM
2.5 (particles size lower than 2.5 μm) standard in 1997 (Ref
1). The shift toward regulating smaller particles is due to their longer residence time in the air, their chemical composition or interaction, and their deeper penetration into the respiratory system. Therefore, the modern metalworking performance cannot be measured based solely on criteria such as productivity, precision, surface quality, or cycle time. Machining shop indoor air quality and environment protection has become another machining process criterion that must be considered. For example, manufacturing industries are being forced to implement strategies to reduce the amount of cutting fluids used in their production lines and to limit the risk of exposition to fine and ultrafine metallic airborne particles. This can be done by controlling the machining parameters and cutting condition, but could also be done by controlling in the material properties and conditions (Ref
2). The properties of various aluminum alloys can be altered by specific designated heat treatment. Some aluminum alloys can be solution-treated to increase their strength and hardness. The heat treatment process can be classified in two process, including solution heat treatment and artificial aging (Ref
3). The A356 aluminum alloy belongs to the Al-Si-Mg system, and is heat-treatable alloys owing to the presence of magnesium content, which combines with Si to form the Mg
2Si precipitation hardening phase (Ref
4). Such alloys are subjected to solution heat treatment at temperatures close to the eutectic temperature, in order to obtain the maximum amount of Mg and Si in solid solution, as well as avoid localized melting at the grain boundaries (Ref
5). The solution heat treatment of Al-Si-Mg alloys is carried out primarily for two reasons: the first is to dissolve Mg and Si to the maximum extent in the aluminum matrix, and the second is to alter the morphology of the eutectic Si particles from their acicular form in the as-cast condition to a finer and more spheroidized form (Ref
6). By heating the solution heat-treated (SHT) alloys to a temperature above room temperature and holding it there, the precipitation accelerates and the strength is further increased compared to natural aging and accompanied by significant drop in ductility. This is called artificial aging, age hardening or just aging and is generally carried out at temperature up to approximately 150 °C for Al-Si-Mg alloys (Ref
7).
The comparison between wet and dry machining reveals that wet machining produces much more aerosol than does dry machining—about 12-80 times in fact (Ref
8). Dry machining is becoming increasingly popular and has been made possible due to technological innovations (Ref
9). However, the problem of the solid aerosols still remains in the dry conditions that is accompanied by emissions of more solid fine dusts (PM
2.5, size is less than 2.5 μm), which are reachable, and penetrate deep into human lungs and cause various diseases (Ref
10). The dust concentrations, up to approximately 300 μg/m
3, are generated when machining dusty materials such as graphite (Ref
11). The dust emission when machining iron, steel, and brass at higher speeds was found to contain large percentage of particles below 5 μm in size (Ref
12,
13). Concentrations of dust emissions generated during machining of metallic parts have been found to depend on the workpiece materials and its conditions as well as cutting parameters applied. Khettabi et al. (Ref
14) also show that most particles generated (number) during orthogonal machining of A6061-T6 aluminum alloy, AISI1018 steel, AISI4140 Steel and gray cast iron are ultrafine particles of about 20 nm in aerodynamic diameter. Similarly, Kouam et al. (Ref
15) found that friction test using brittle aluminum alloys produces more ultrafine particles than microparticles. Balout et al. (Ref
16) studied how the machining parameters influence dust generation during dry machining. They reported that brittle materials generate fewer fine dust particulate (aerodynamic diameter lower than 2.5 μm) than ductile materials during machining process. This behavior was attributable to the difference in tool contact length (zone of secondary deformation and friction) and the high plastic deformation taking place during machining ductile materials. In another study, a correlation was found between chip formation and dust emission; the formation of a discontinuous chip being accompanied by a low emission level of a dust as was compared to the dust obtained when a long chip is formed (Ref
17). Khettabi et al. (Ref
18) conducted similar studies and concluded that the quantity and particle size of dust emission depend on cutting parameters, workpiece material, tool material, and geometry. They also showed that the intensity of dust emission increases with an increase of the cutting speed during turning and milling processes. Songmene et al. (Ref
19) identified several dust generation sources including, primary shear zone, chip surface, and the tool-work piece interface. Some dust generation sources have been evaluated friction at shear plane; the influence of the tribological conditions at the tool-chip interface; influence of tool wear resistance (Ref
20). Little or no work has been carried on studying the dust emission during machining of aged aluminum alloys. The main objective of this study was to evaluate the influence of machining conditions and cutting fluid on the emission metallic particles when machining of A356 aluminum alloy heat-treated at different conditions. In order to study this effect systematically, the aluminum alloy was heat-treated to produce different precipitation states and machined under controlled conditions. This work was carried out in order to minimize dust emission and improve air quality in machining shops.