1. Introduction
Natural fiber reinforced polymer (NFRP) composites are used in technical area such as construction, automotive parts, and sports’ goods due to their many advantages, for instance, low density, high specific strength and stiffness, low hazard manufacturing process, and low cost [
1]. Manufacturers are expected to use NFRPs as replacement of synthetic fiber reinforced polymer (SFRP) composites when producing automobile parts in order to minimize environmental impacts and reduce vehicle weight [
2]. Products made with composite materials need to be machined (for example, drilled) in order to obtain the final dimensions or for assembly purposes. Conventional drilling is a popular hole making method, and an indispensable process for creating holes for parts assembly [
3].
Many publications have covered the machining of SFRPs, while little research work has been done on the machining of NFRPs. Results relating to the machinability of SFRPs cannot be directly applied to NFRPs because of the differences in the mechanical properties of these two composites [
4]. Debnath et al. [
5] investigated the drilling behavior of nettle/polypropylene composite with various drill bit geometries. They found that a specific drill bit geometry is only suitable for cutting one material, but is not appropriate for cutting others. Consequently, studying the cutting mechanism is necessary so that each material’s machinability can be fully understood. Bajpai and Singh [
6] studied the machinability of sisal/polypropylene composite during the drilling process. They observed that the thrust force created with the standard drill tool is significantly bigger than that obtained with the trepanning tool. Maleki et al. [
7] carried out an investigation on the machining of woven jute fabric reinforced polymer composite with different drill tool materials. They [
7] showed that the delamination factor and thrust force are lower with the HSS drill compared to those obtained with carbide drills (CoroDrill 854 and CoroDrill 856). The surface roughness generated with the CoroDrill 856 was higher than that made with the HSS tool and the carbide CoroDrill 854. They recommended using the HSS twist drill when drilling jute fiber reinforced polymer composites [
7].
The type of fibers used in composite materials has a great influence on the machinability of NFRPs since the tool used will cut both the fibers and the matrix. Therefore, in addition to machining cutting parameters, the properties of the fibers must be taken into account when selecting the cutting tool [
4]. Ismail et al. [
8] investigated the drilling of sustainable and conventional composites (HFRP—hemp fiber reinforced polycaprolactone, and CFRP—carbon fiber reinforced polymer). They observed that cutting parameters can significantly affect the damage sustained by both composites. The thrust force, the roughness, and delamination increased with feed rate increase for these materials. Their results also showed that CFRP presents a lower surface roughness value, and that HFRP exhibits a smaller delamination factor during drilling process in the same cutting conditions. Abilash and Sivapragash [
9] studied the influence of cutting condition on delamination during drilling of polyester composite reinforced with bamboo fiber. They found that the feed rate and the drill bit diameter play a larger role in delamination failure. Venkateshwaran and ElayaPerumal [
10] presented that an increase in the feed rate and the spindle speed contributed to the rise of the delamination factor during drilling of epoxy composite reinforced with banana fibers. Chegdani and Mansori [
11] found that the use of coated tools generated an increase in specific cutting energy in the drilling of bidirectional flax fiber reinforced polypropylene. The study [
12] explored the machinability of two types of green composites (Sisal/PLA and Grewia Optiva/PLA). They noticed that drilling with a twist drill and a parabolic drill produces a better surface quality and a lower thrust force than does the Jo drill tool.
Debnath et al. [
13] investigated the drilling of sisal reinforced epoxy and polypropylene composites. They found that the machinability of sisal composite depends not only on the tool geometry, but also on the matrix material. The thrust force and torque generated are lower for the parabolic drill than for the step and four-facet drills. Higher torque values were observed for sisal/epoxy than for to sisal/polypropylene at the same cutting condition. No delamination was observed for both materials. Palanikumar and Valarmathi [
14] studied the drilling of MDF (Medium Density Fiberboard, which is a wood-based composite) and discovered that the thrust force increased with the feed rate. Prakash et al. [
15] observed that the roughness increased with the drill bit diameter and feed rate during drilling of MDF. Szwajka et al. [
16] noted that the tool coating type and the feed rate significantly influenced the thrust force and the roughness when machining MDF.
Hybrid composite materials are made from a common matrix reinforced with more than one type of reinforcements. Fu et al. [
17] defined hybrid composites as having two or more reinforcing materials mixed in a common matrix. Recently, some studies have looked at the machinability of hybrid composites. Jayabal et al. [
18] investigated the machinability during drilling of glass-coir/polyester hybrid composite. They found that the feed rate has a more significant effect on the thrust force than the drill diameter and spindle speed do. Navaneethakrishnan and Athijayamani [
19] observed the effect of cutting conditions on the cutting force during drilling of vinylester reinforced with sisal fiber and coconut shell powder. They discovered the drill point angle and the feed rate have the significant impact on thrust force. Increases in the point angle and the feed rate contribute to increases in the thrust force. Vinayagamoorthy [
20] observed that a decrease in the spindle speed and/or an increase in the feed rate lead to the rise of the thrust force and the roughness during drilling of polyester composite reinforced with jute and steel fiber. They also discovered that the thrust force increases with a drill diameter and drill point angle increase.
The dust generated during machining of metallic materials has been studied by several researchers [
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31], who observed that the relationship of the dust emission with cutting conditions such as the workpiece material, tools, and cutting parameters [
21,
28,
31]. Small size dust emitted during machining has a significant impact on the environment and on the health of machine operators [
22,
29]. Djebarra et al. [
24] observed that during machining, the greatest proportion of dust generated is less than 2.5 microns, for different workpiece materials at different cutting conditions. The quantity of particles emitted is a function of the spindle speed and the feed rate [
22,
26]. Kouam et al. [
27] found that friction has a great impact on dust formation. Zaghbani et al. [
25] noted that the deformation conditions in the chip formation zone greatly influence the dust generated, while the cutting conditions do not significantly affect the nanoparticle generation rate. The result from [
26] indicates that the tool coating materials influence the dust generated during the machining of aluminum alloys. The study [
29] investigated the effect of the initial temperature of the workpiece material on fine dust emission during dry drilling and found that the initial workpiece temperature greatly affects the dust generation. For pre-cooled workpiece materials, the equivalent generation was low.
To date, only a few studies have focused on dust generated during machining of composites. Marani et al. [
32] found that the cutting parameters and the microstructure of the workpiece material directly affect the dust generated during metal matrix composite milling. Songmene et al. [
33] found that fine dust generation is significantly reduced by using water MQL (Minimum Quantity Lubrication) during polishing of granite, while ultrafine particle generation is insensitive to water use. Kremer and Mansori [
34] observed that a smooth coating tool generates more dust than a rough one during machining of metal matrix composite. The study Kremer and Mansori [
35] showed that the dust created during cutting of metal matrix composite is affected differently depending on the tool type. Haddad et al. [
36] revealed that dust emission is a function of cutting parameters and the tool geometry during the trimming of polymer composite reinforced with carbon fibers.
The aim of this study is to investigate the effects of machining conditions on the machining process performance indicators: Specific cutting energy, thrust force, surface roughness, and fine dust and ultrafine dust emission during dry drilling of a new hybrid biocomposite material made of miscanthus fibers and biochar reinforced polypropylene.
2. Experimental Setup
2.1. Workpiece Material
The hybrid biocomposite material used consists of a matrix (polypropylene (PP)/polyolefin elastomer (POE)) randomly reinforced with biochar and chopped miscanthus fiber, and mixed with a MAPP (Maleic Anhydride grafted Polypropylene) compatibilizer. The composition of the hybrid biocomposite is presented in
Table 1.
PP (trade name, 1350N) is produced by Pinnacle Polymers LLC, LA, USA. POE (trade name, Engage 7487); MAPP (trade name, Fusabond 613). The biochar is the result of the pyrolysis operation of natural miscanthus fibers (average length of 4 mm) invented in Southern Ontario, Canada. Competitive Green Technologies supplied four millimeters long miscanthus fibers that were used for the manufacturing of the hybrid composite.
Figure 1 describes the miscanthus fibers and biochar that were made from miscanthus fibers by pyrolysis [
37]. The mechanical properties of hybrid biocomposite are described in
Table 2 [
38].
The hybrid biocomposite material was produced through the press molding process, using a French Press USA machine, TMP, Model EHV, max tonnage 57 tons. The following steps represent the process for producing compression-molded materials. Firstly, the mold platens are preheated to 180 °C for 30 min. Then, the material is loaded into the press and pre-heated with closed platens, but without raising the pressure in order to melt material before pressing. This operation takes about 10 min. Then vacuuming is conducted to degas for 3 min, and platens are closed at a pressure of 2 tons for 10 min. Finally, the platens are cooled to below 50 °C and then removed from the press.
The hybrid biocomposite material was developed in 2017 by the University of Guelph Bioproducts Discovery and Development Center for internal automobile parts production. The result obtained from [
38] showed this hybrid biocomposite exhibited better stiffness than the matrix (PP/POE). The Young’s modulus of the matrix was increased by 70% as reinforced with biochar (15%wt) and miscanthus (15%wt). Hybrid biocomposite has higher tensile and flexural strength than commercial Talc/PP composite (RTP132UV). As a result, hybrid biocomposite is considered the most likely material to replace the commercial Talc/PP composite.
2.2. Experimental Procedure
The dry drilling of hybrid biocomposite was conducted on a 3-axis CNC machine-tool (HURON—K2X10) with the following main characteristics: Maximum power, 50 KW; and spindle speed, 28,000 rpm. Standard HSS twist drill bits (6 mm, 8 mm, 10 mm diameters) were used to drill holes on the workpiece. The cutting parameters were selected based on the tool manufacturer’s catalogue and literature.
The workpiece sample (300 mm × 120 mm × 5 mm) was screwed to an aluminum back plate support. The support (300 mm × 120 mm × 30 mm) had 80 drilled holes (12 mm diameter). The subsystem (workpiece and back plate support) was placed and tightened onto the dynamometer with screws. The responses measured and analyzed were the drilling force, the surface roughness, and the particle emission. The drilling forces were measured using a dynamometer (type Kistler 9255B) clamped on the machine table and connected to the charge amplifiers (type Kistler 5010) (Kistler Instrument Corporation, New York, NY, USA) that generated output signals, which were transmitted to a data translation card (type DT 9836, Data Translation Inc., Marlborough, MA, USA) and then connected to a personal computer.
A Scanning Mobility Particle Sizer (SMPS, model #3080, TSI Inc., Shoreview, MN, USA) equipped with a nano DMA (Differential Mobility Analyzer) was used to measure ultrafine particles generated with sizes ranging from 7 nm to 100 nm during drilling. An Aerodynamic Particle Sizer (APS, model 3321, TSI Inc., Shoreview, MN, USA) was used for measuring of fine particles, with diameter ranging from 0.5 to 10 μm. For both pieces of equipment, the dust samples were sucked by a pump (1.5 L/min) through a suction tube, with the end of a tube placed near the machining area. The suction tube was connected to the dust measurement system, which consisted of APS and SMPS. The experimental scheme is illustrated in
Figure 2.
The roughness profilometer (Mitutoyo, model SJ–410) (Mitutoyo America Corporation, Aurora, IL, USA) was used to measure the surface roughness. This equipment was connected to a computer with the help of the SURFPAK–SJ software for recording and analyzing the roughness data. The measurements of the surface roughness of a drilled hole were carried out in the feed direction, and were repeated three times for each tested condition.
The experiments were based on a full factorial design, with 3 input parameters at 3 levels. In order to obtain reliable and accurate results, each test was repeated three times, and the mean of the measured values was selected for analysis.
Table 3 summarizes the factors investigated and their respective levels.
4. Conclusions
In this investigation, the effects of cutting parameters and drill bit diameter on machinability in the dry drilling of a new hybrid biocomposite were studied. Based on the experimental data and statistical technique employed, the following conclusions are drawn:
The machining conditions significantly influence the thrust force. The feed rate was found to have a higher effect on the thrust force than did the spindle speed and drill bit diameter. An increase in thrust force results from an increase in cutting parameters and drill bit diameter.
The specific cutting energy for the thrust force considered as a material property was investigated. This energy increased with an increase in spindle speed and decreased with an increase in feed rate and/or drill bit diameter.
The drill bit diameter was observed to have a greater impact on the surface roughness than did the cutting parameters. The surface roughness decreased with a decrease in the drill’s diameter, the spindle speed, and the feed rate.
During drilling of this new composite material, both fine particles (PM10) and ultrafine particles (diameters ranging from 7–100 nm) were generated. The total number concentration of fine particles reached 1300 #/cm3, while the ultrafine particle generation reached 9000 #/cm3 depending on machining conditions used and on the particle size studied. The drill bit diameter and the feed rate have significant effects on the fine dust generation, while the spindle speed is not statistically significant. The total number concentration of fine particles decreased with an increase in feed rate, spindle speed, and/or drill bit diameter. More fine particles emitted had aerodynamic diameters less than 2.5 µm. The cutting parameters and the drill bit diameter did not show significant statistical effects on ultrafine particle generation during drilling of the hybrid composite at the 95% confidence level. Ultrafine particle generation was therefore difficult to predict.