1. Introduction
Milling is a commonly used processing technology in modern manufacturing. Aluminum alloy is known for its low density, high strength, high temperature resistance, and sturdy load-bearing capacity, and has become a key material for manufacturing important structural components in the aerospace industry [
1,
2]. During the milling process, accompanied by intense friction and heat release, if not cooled in a timely manner, it can cause local temperatures to be too high, resulting in thermal damage to the surface of the workpiece [
3]. Traditional pouring cooling processing of aluminum alloys often leads to problems such as excessive cutting force, severe tool wear, and poor surface quality of the workpiece [
4,
5]. The chemical additives contained in traditional coolants not only pollute the environment, but also pose a threat to workers’ health. Therefore, finding an efficient and environmentally friendly cutting fluid and processing method has become particularly important.
As a near-dry processing technology, micro lubrication (MQL) can effectively protect the environment and worker health. According to reports, compared to traditional pouring lubrication, MQL can reduce human toxicity by 87% and slow down the global warming trend by 21%. Compared with dry-cutting methods, micro lubrication can improve the surface quality of workpieces [
6,
7,
8,
9]. Khettabi et al. investigated the MQL and dry milling properties of 7075, 6061, and 2024 aluminum alloys. They found that MQL milling can reduce cutting force and improve surface finish [
10]. Micro lubrication uses environmentally friendly vegetable oil and animal oil as lubricants, which will not cause pollution to the environment. Due to the high temperature and high speed involved in processing certain materials, traditional vegetable and animal oils often undergo oxidation, thereby reducing the lubrication effect during high-speed and high temperature processing. According to reports, adding nanoparticles to basic animal and vegetable oils can further enhance the thermal conductivity of the base fluid, improve its lubrication ability, and do no harm to the environment [
11].
Singh et al. elaborated on the research progress of nano-cutting fluids. Research has found that compared to dry and traditional wet lubrication machining, the use of MQL technology with the addition of nanoparticles can significantly reduce cutting force and cutting temperature, while improving surface quality [
12]. Albert et al. added graphite nanoparticles to the base solution and subsequently ground D-2 tool steel. Compared to traditional MQL grinding, this reduces grinding force and improves surface quality [
13]. Rahmati et al. compared the surface quality changes during the milling process of AI6061-T6 alloy and evaluated the effects of using mineral oil and adding different mass fractions of MOS
2 nanoparticles on lubrication performance. The research results show that lubricants containing the minimum amount of MOS
2 nanoparticles perform better in improving surface quality compared to mineral oils [
14]. Anuj et al. configured multi-walled carbon nanotubes and Al nanoparticles into cutting fluids with 0.25%, 0.75%, and 1.2% mass fractions, and subsequently conducted AISI 304 stainless steel turning experiments. As the mass fraction increases, the lubrication performance of the cutting fluid gradually improves significantly. Compared to a single Al nanofluid, the mixed nanofluid has a certain reduction in tool wear and peak processing temperature, which are 11% and 27.36%, respectively [
15]. Sharma prepared a nanofluid cutting fluid using carbon nanotubes (CNTs) and conducted turning experiments on AISID-2 steel. The results showed that the temperature in the cutting area decreased significantly [
16]. Aref added different qualities of MoS
2 and CuO nanoparticles to rapeseed oil and soybean oil, and ground AISID-2 tool steel. The research results show that adding 4% CuO nanoparticles and 2% MoS
2 nanoparticles to soybean oil can reduce the normal force of soybean oil by 19% and the tangential force by 35% [
17]. Guan et al. proposed the use of a composite filled with oleic acid inside multi-walled carbon nanotubes to prepare nanofluids. Compared to ordinary multi-walled carbon nanotubes, the composite filled with oleic acid has better heat transfer and wettability within the multi-walled carbon nanotubes [
18].
Multi-walled carbon nanotubes (MWCNTs) are nanotubes formed by winding multiple layers of graphite sheets around the centerline, with a hollow inner cavity structure inside [
19]. The thermal conductivity of multi-walled carbon nanotubes can reach 6 kW/(m·k), which inherently has good lubrication properties and can play a role similar to “micro bearings”. Therefore, multi-walled carbon nanotubes are suitable as additives for cutting fluids [
20]. MOS
2 nanoparticles have a layered structure, formed by covalent bonding between sulfur and molybdenum atoms, forming a hexagonal crystal structure [
21,
22]. The MOS
2 molecular layers are connected to each other through weak van der Waals forces, which endow MOS
2 with flexible structural by sliding the molecular layer, it can transform to the dual surface to form a transfer film, endowing MOS
2 with the characteristic of reducing friction.
This article uses soybean oil as the base fluid, combined with multi-walled carbon nanotubes with excellent thermal conductivity and molybdenum disulfide nanoparticles with excellent lubrication performance, to prepare three types of nanofluids: MOS2 nanofluids with mass fractions of 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.%, MWCNTs nanofluids, and MOS2/MWCNTs mixed nanofluids. Nanofluid-assisted milling experiments were conducted on 7050 aluminum alloy to evaluate the effects of nanofluid on cutting force F, cutting temperature °C, and surface roughness Ra. A nanofluid with a significant lubrication effect was selected and 16 orthogonal milling experiments were conducted to obtain an empirical formula for cutting force. The multi-objective genetic algorithm and TOPSIS comprehensive evaluation method were used to optimize various parameters, providing important reference values for practical nanofluid micro lubrication milling machining.