Review Regarding the Influence of Cryogenic Milling on Materials Used in the Aerospace Industry

Abstract

In the aerospace industry, an important number of machined parts are submitted for high-performance requirements regarding surface integrity. Key components are made of materials selected for their unique properties and they are obtained by milling processes. In most situations, the milling process uses cooling methods because, in their absence, the material surface could be affected by the generated heat (temperatures could reach up to 850 °C), the residual stress, the cutting forces, and other factors that can lead to bad integrity. Cryogenic cooling has emerged as a pivotal technology in the manufacturing of aeronautical materials, offering enhanced properties and efficiency in the production process. By utilizing extremely low temperatures, typically involving liquid nitrogen or carbon dioxide, cryogenic cooling can significantly enhance the material’s properties and machining processes. Cryogenic gases are tasteless, odorless, colorless, and nontoxic, and they evaporate without affecting the workers’ health or producing residues. Thus, cryogenic cooling is also considered an environmentally friendly method. This paper presents the advantages of cryogenic cooling compared with the classic cooling systems used industrially. Improvements in terms of surface finishing, tool life, and cutting force are highlighted.

1. Introduction

The milling process is a machining method used for producing different parts of various shapes and sizes. This process is realized by removing material, in the form of chips, from a bigger part, called the workpiece. This removal is accomplished by the controlled movement of a rotary tool with multiple cutting edges, known as a cutting tool. This operation can be performed on different types of milling machines, from simple manual machines to sophisticated computer numerical control (CNC) machines [1]. The milling process is widely used across the manufacturing industry to create intricate shapes with precise dimensions in metals [2], plastics [3], and composite materials [4]. In the automotive industry, the milling process plays a crucial role in the production of engine components or chassis parts [5]. In the aerospace industry, it is used for manufacturing all kinds of parts, from the fuselage to engine components, with high precision and reliability [6], and medical equipment, like implants, prosthetics, and surgical instruments, where precision and biocompatibility are critical [7]. The electronic industry also uses the milling process for manufacturing circuit boards, connectors, and housings [8].

In most situations, the milling process uses a cooling or lubricating liquid. Without a cooling/lubricating method, various problems could arise on the surface of the material, which might be damaged by the temperature (e.g., AISI 4340—850 °C) [9]; the residual stress on the surface of the material [10]; the higher cutting forces [11,12], leading to a shorter life for the cutting tool [13]; and other factors that could lead to the bad surface integrity of the material [14,15,16,17,18,19].

The literature presents various types of lubricants or coolants that have been used to improve machinability, like oil-based fluids [20], gases and vapors [21], solid lubricants [22], and nano-cutting fluids [23]. Among these methods, cryogenic machining is a new eco-friendly technology, which uses cryogenic liquids sprayed onto the cutting zone to cool down the cutting tool and the workpiece [24]. Cryogenic liquids have boiling temperatures below −90 °C, meaning that, when sprayed onto the cutting zone, they immediately transform into gases. Most cryogenic liquids are colorless, tasteless, odorless, and nontoxic and are used in many industries, like aerospace, electronics, the automotive industry, and medical services [25,26,27,28,29,30,31].

The milling process in the aeronautical industry presents several unique complexities due to the stringent requirements for precision, material properties, and safety standards. The material selection, like a high-strength alloy, poses challenges in terms of its hardness, heat resistance, and machinability. The tolerance and surface finish must ensure optimal characteristics for the safety and reliability of the aircraft. Because of these complex factors, cryogenic cooling should be the “green” solution for optimizing the process in terms of efficiency, productivity, quality, and reducing production costs.

The aim of this review is to present the advantages of cryo-milling compared with the dry method and also the contribution of cryogenic liquids combined with the MQL method for materials used in the aerospace industry.

2. Main Materials Used in the Aeronautical Industry

The aeronautical industry utilizes a wide range of materials, each selected for its unique properties that contribute to the performance, durability, and, most importantly, the safety of the aircraft [32,33,34,35]. These materials are chosen based on their strength-to-weight ratio, corrosion resistance, thermal stability, and ease of manufacture, among other criteria. Furthermore, we describe some of the key materials [36] used in the aeronautical industry.

2.1. Aluminum Alloys

Historically, aluminum alloys have been the backbone of the aerospace industry, prized for their low weight, good corrosion resistance, and excellent mechanical properties [37]. Alloys such as 2024, 7075, and 6061 (

Table 1. Chemical compositions of aluminum alloys mostly used in aerospace engineering.

	Cu	Zn	Mg	Mn	Fe	Si	Cr	Ti	Al	Refs.
Al 2024	3.8–4.9	<0.25	1.2–1.8	0.3–0.9	<0.5	<0.5	<0.1	<0.15	Bal.	[45]
Al 6061	0.15–0.4	<0.25	0.8–1.2	<0.15	<0.7	0.4–0.8	0.04–0.35	<0.15	Bal.	[46,47]
Al 7075	1.2–2.0	5.1–6.1	2.1–2.9	<0.3	<0.5	<0.4	0.18–0.28	<0.2	Bal.	[48]
Al 7150	1.2–1.9	7.2–8.2	2–2.9	<0.2	<0.2	<0.15	0.1–0.22	<0.1	Bal.	[49]
Al 7155	2–2.6	7.6–8.4	1.8–2.3	<0.1	<0.25	<0.25	<0.05	<0.1	Bal.	[50]

) are commonly used in airframes and components [38]. Aluminum alloys are often encountered in structural applications in the aeronautical industry because of the exceptional balance between their strength-to-weight ratio, low density, and good maintainability at a low cost [37,39]. Over 70% of the structure and shell of an aircraft is made of aluminum alloys [34,35,40,41,42,43,44].

2.2. Titanium Alloys

In the aerospace industry, titanium is preferred for its strong strength-to-weight ratio, exceptional corrosion resistance, and excellent weldability. It also has good microstructure stability and can withstand high temperatures [36]. Table 2 presents the chemical composition of the most commonly used Ti alloys found in airframes [51], turbine engines [52,53,54] (Figure 1), and structural components [55] of the aircrafts, as well as in motorsport, compressor disks, blades, and hush kits [56].

Table 2. Chemical composition of titanium alloys.

	C	Fe	N	Al	O	V	H	Sn	Y	Zr	Mo	Si	Nb	Other	Ti	Refs.
Titanium Ti6Al4V	<0.05	<0.3	<0.02	2.5–3.5	<0.12	2–3	<0.015	-	<0.005	-	-	-	-	0.4	Bal.	[55]
Titanium Ti3Al2.5V	<0.05	< 0.2	<0.05	2.5–3.5	< 0.15	2–3	<0.015	-	-	-	-	-	-	0.35	Bal.	[57,58]
Titanium Ti5Al2.5V	<0.1	<0.5	<0.03	4–6	<0.2	-	<0.015	2–3	-	-	-	-	-	0.35	Bal.	[51]
Titanium Ti6-2-4-2S	5.5–6.5	<0.25	-	-	-	-	-	1.8–2.2	-	3.6–4.4	1.8–2.2	0.06–0.12	-	-	Bal.	[59]
IMI 834	-	-	-	<5.8	-	-	-	<4	-	<3.5	<0.5	<0.35	<0.7	-	Bal.	[60]

Table 2. Chemical composition of titanium alloys.

	C	Fe	N	Al	O	V	H	Sn	Y	Zr	Mo	Si	Nb	Other	Ti	Refs.
Titanium Ti6Al4V	<0.05	<0.3	<0.02	2.5–3.5	<0.12	2–3	<0.015	-	<0.005	-	-	-	-	0.4	Bal.	[55]
Titanium Ti3Al2.5V	<0.05	< 0.2	<0.05	2.5–3.5	< 0.15	2–3	<0.015	-	-	-	-	-	-	0.35	Bal.	[57,58]
Titanium Ti5Al2.5V	<0.1	<0.5	<0.03	4–6	<0.2	-	<0.015	2–3	-	-	-	-	-	0.35	Bal.	[51]
Titanium Ti6-2-4-2S	5.5–6.5	<0.25	-	-	-	-	-	1.8–2.2	-	3.6–4.4	1.8–2.2	0.06–0.12	-	-	Bal.	[59]
IMI 834	-	-	-	<5.8	-	-	-	<4	-	<3.5	<0.5	<0.35	<0.7	-	Bal.	[60]

Figure 1. Main materials and temperatures in jet engines [61,62].

Figure 1. Main materials and temperatures in jet engines [61,62].

2.3. Nickel-Based Superalloys

These alloys are essential for components that operate at high temperatures, such as jet engine parts (Figure 1), as well as other high-temperature alloys [63]. Nickel-based superalloys primarily consist of nickel and additional elements, including chromium, cobalt, molybdenum, aluminum, and titanium (

Table 3. Chemical composition of nickel-based superalloys.

	Ni	Cr	Fe	Nb	Mo	Ti	Al	Mn	Si	C	Co	Other	Ref.
Inconel 718	53	17	18.7	5.05	3.05	1.05	0.9	0.23	0.1	0.05	-	Bal.	[66]
Waspaloy	Bal.	19.5	-	-	4.3	3	1.3	-	-	0.08	13.5	0.006B 0.06Zr	[67]
Hastelloy X	Bal.	22	18.5	-	9	-	-	0.5	0.5	0.1	-	-	[68]
Inconel 625	Bal.	22	3	3.5	9	0.2	0.1	-	-	0.01	0.1	-	[67]
Haynes 230	Bal.	22	-	-	2	-	-	1	0.5	0.075	-	12W	[69]
Rene 41	Bal.	19	-	-	10	3.1	1.5	-	-	0.09	11	0.05B	[67]

). The specific composition varies depending on the required properties of the alloy, such as strength, corrosion resistance, and temperature stability. These materials are renowned for their excellent mechanical strength, resistance to thermal creep deformation, good surface stability, and resistance to oxidation and corrosion at high temperatures [63,64,65].

2.4. Steel Alloys

Although heavier than aluminum and titanium, certain high-strength steels are used for critical components like landing gear due to their toughness and fatigue resistance. Steel alloys used in aircrafts are often heat-treatable high-strength alloys, which may include additional elements like chromium, nickel, molybdenum, and manganese (

Table 4. Chemical composition of steel alloys.

	Fe	Ni	Cr	Mn	C	Mo	Si	S	P	Co	Other	Ref.
AISI 4340	95–96	1.6–2	0.7–0.9	0.6–0.8	0.37–0.43	0.2–0.3	0.15–0.3	0.04	0.035	-	-	[75]
AISI 4037	98–98.6	-	-	0.7–0.9	0.35–0.4	0.2–0.3	0.15–0.35	<0.04	<0.035	-	-	[76]
AMS 6514	Bal.	18.5	-	0.1	0.03	4.8	0.1	-	-	9	0.6Ti 0.1Al	[77]
AMS 6512	Bal.	18.5	-	0.1	0.03	4.8	0.1	-	-	7.5	0.4Ti 0.1Al	[78]
AMS 6515	Bal.	18.5	-	0.1	0.03	4.8	0.1	-	-	12	1.4Ti 0.1Al	[79]

) [70]. These elements improve the material’s mechanical properties, including tensile strength, hardness, and resistance to fatigue and corrosion [71]. Maraging steel, a low-carbon, nickel-rich variety, is known for its extreme strength and toughness without malleability loss. Common uses for steel alloys include (Figure 2) landing gears [72,73], engine components [74], actuators, and hydraulic systems [72,73].

2.5. Other Materials Used in Aircrafts

Thermoplastics, thermoset plastics, ceramics, and ceramic matrix composites (CMCs) are used in interior cabins and nonstructural components; these materials are selected for their light weight, design flexibility, and fire resistance. Examples of thermoplastics used in aerospace include Polyetheretherketone (PEEK) and polyphenylene sulfide (PPS) [80]. Ceramic materials are used in high-temperature applications such as turbine blades and thermal protection systems, offering excellent heat resistance and strength-to-weight ratios [81]. Ceramics are employed in various aerospace applications due to their ability to withstand extreme temperatures and corrosive environments while maintaining structural integrity. The use of ceramics and ceramic matrix composites in aeronautics represents a shift towards more efficient, reliable, and sustainable air travel. Their ongoing development and integration into aerospace technologies demonstrate the industry’s commitment to innovation and performance improvement [82]. We will not further develop this section since thermoplastic and ceramic materials are not within our research scope, mainly because they are not manufactured using the milling process.

3. Cooling and Lubricating Fluids

The aerospace industry often processes hard-to-cut materials like titanium alloys (e.g., Ti-6Al-4V) [83], nickel-based superalloys (e.g., Inconel 718) [21], and high-strength aluminum alloys (e.g., 7075, 6061) [48] that require the most efficient cooling methods [84]. Depending on the process and the material, there are multiple types of coolants/lubricants available (Figure 3). The most common and efficient are emulsions [85], oil-based coolants/lubricants for the MQL method (minimum quantity lubrication) [86], and liquid nitrogen [87].

4. Cryogenic Cooling

Cryogenic cooling refers to the process of reducing temperatures in a system or material using cryogenic fluids such as liquid nitrogen (LN₂) [28], liquid helium (LHe) [88], liquid argon (LAr) [89], or liquid carbon dioxide (LCO₂), which is not really considered a cryogenic liquid because of its liquefaction temperature [90]. These fluids have boiling points significantly below those of conventional cooling agents. By exploiting the extremely low temperatures of these substances, often reaching down to -196°C in the case of liquid nitrogen, cryogenic cooling can achieve effects that are not possible with traditional cooling methods. Cryogenic liquids can be applied by spraying onto the surface of the workpiece and the cutting tool or can be internally injected through channels practiced inside the cutting tool with minimal cooling impact and lower liquid consumption [24]. Cryogenic cooling is used across a wide range of applications, from industrial processes to scientific research. In manufacturing, it is employed to improve the mechanical properties of metals and alloys, enhance machining processes, and increase the durability of cutting tools. The significant decrease in temperature can cause changes in the material’s microstructure, leading to increased hardness and strength, reduced thermal expansion, and minimized wear [91] and tear during processing [92]. The use of cryogenic liquids has created new possibilities in fields such as materials science, electronics, and other areas where extreme cold is necessary to change material properties, increase efficiency, or enable new technologies [93]. Cryogenic cooling is a critical technology with wide-range implications across various sectors, offering unique solutions to challenges in manufacturing [94], electronics [95], medicine [96], and scientific research [97]. Its continued development is essential for supporting innovation and advancement in these fields [98]. Cryogenic milling, also known as cryo-milling, is a process that uses liquid nitrogen (LN₂) or another cryogenic liquid such as liquid oxygen (LOX), liquid argon (LAr), liquid helium (LHe), or liquid hydrogen (LH₂). The abundance of nitrogen in the atmosphere (78%) [99] makes it relatively easy to produce in liquid form. In the milling process, some materials, especially hard-to-cut materials used in the aerospace industry, such as titanium alloys (e.g., Ti-6Al-4V) [83], nickel-based superalloys (e.g., Inconel 18) [21], and high-strength aluminum alloys (e.g., 7075) [48], cannot be machined without cooling. Cryogenic cooling is recognized as an exceptionally effective method for reducing temperatures when machining these materials. LN₂ is the most common cryogenic liquid; it is eco-friendly and has one of the lowest temperatures, with a boiling point of -196 °C [21]. This makes it very efficient for cooling hard-to-cut materials, resulting in good-quality manufactured parts [100].

Although cryogenic liquids take quite a long time to produce, the purpose is to discover the benefits and advantages of using cryogenic cooling in machining these hard-to-cut materials. Cryogenic cooling emerged as a pivotal technology in aeronautical materials machining, offering enhanced properties and efficiency in the production process. By using extremely low temperatures, typically involving liquid nitrogen or carbon dioxide, cryogenic cooling can have a significant impact on material properties and machining processes. Studies have found that cryogenic cooling can improve the mechanical properties of metals and alloys used in the aerospace industry. The process can increase hardness [47], wear resistance [101], and tensile strength by altering the material’s microstructure during cooling. As a result, materials become better suited for the challenging conditions of aerospace applications, such as high speeds, temperatures, and stresses [39].

4.1. Delivery Methods in Cryogenic Cooling

Two common cooling methods are used to deliver cryogenic liquids. The first method is internal injection (Figure 4), where the fluid is delivered into the cutting zone through internal channels of the cutting tool and the holder [12,24,102]. The second method is the external spray method (Figure 5), where the cryogenic fluid is sprayed directly onto the cutting zone using nozzles [83,103,104,105,106,107]. The internal injection cooling method typically uses a smaller amount of cryogenic liquid and is focused on cooling the cutting tool with less efficiency in cooling the cutting zone compared to the spray method [24].

Considering that the best results in terms of surface quality of machined parts, tool life, and cutting forces were achieved when the MQL cooling technique was combined with the cryogenic cooling technique (see Section 4.2.), O. Pereira’s team [109] developed an innovative and easy-to-manufacture system that offers a quick solution by combining the MQL jet with four external CO₂ jets (Figure 6). The innovation includes a nozzle that can mix the cryogenic liquid and the lubricating fluid, directing it to the cutting zone. The design of the system was based on the one originally patented by David P. Jackson in 2008 (Figure 7) [110].

4.2. Benefits and Features—Improved Machinability

Promising results have been reported using the combination of LN₂/CO₂ and MQL when machining Ti-6Al-4V in shoulder milling [111] and turning [112].

-

Cooling materials to cryogenic temperatures can make them more brittle, which facilitates their cutting or machining, especially for tough or elastomeric materials but also for other materials such as titanium, inconel, or aluminum. Table 5 summarizes the papers that focus on the cryo-machining of materials used in the aerospace industry included in this review. The term “machinability” can be represented by the following criteria:

-

Surface finish;

-

Tool life;

-

Cutting force;

-

Chip shape;

-

Limiting rate of removed material [113].

Table 5. Summary of the papers focusing on the cryo-machining of materials used in the aerospace industry.

Reference	Workpiece Material	Machining Outputs	Cooling Method	Obs.
[84]	Al 6082	Flank wear Crater wear Surface roughness Topography and texture of the machined surface	Dry machining MQL LCO2 MQL + LCO2
[102]	Al 6082-T6	Cutting temperature Surface roughness Cutting forces Surface morphology Chip morphology Tool wear	LCO2 LN2 conventional wet machining
[106]	Al 8011	Cutting temperature Surface roughness Tool wear Chip morphology Microstructure analysis Hardness	Dry machining conventional wet machining LCO2 MQL MQL + LCO2
[114]	Al 7075	Surface roughness Cutting temperature Cutting force	LN2 conventional wet machining	turning
[115]	Al 7075	Cutting force Surface roughness Chip morphology Tool wear Surface roughness	Dry machining LCO2 MQL MQL + LCO2
[12]	Ti-6Al-4V	Cutting force	Dry machining LN2
[24]	Ti-6Al-4V	Cutting force Tool wear Tool performance	conventional wet machining nano-MQL LN2 (internal and external) nano-MQL + LN2
[83]	Ti-6Al-4V	Workpiece temperature Resulting forces Microhardness of the workpiece surface Surface topography	Dry machining LCO2 Sub-zero MWF
[104]	Ti-6Al-4V	Cutting temperature Surface roughness Chip morphology Cutting forces Tool wear Microhardness	Conventional wet machining LCO2 MQL + LN2
[116]	Ti-6Al-4V	Surface roughness Tool wear Power consumption	Dry machining conventional wet machining cryogenic machining
[117]	Ti-3Al-2.5V	Power consumption	Dry machining MQL LN2 MQL + LN2
[11]	Inconel 718	Tool wear Cutting forces	MQL Conventional wet machining MQL + LCO2
[13]	Inconel 625	Surface roughness Tool wear Cutting temperature Chip morphology	MQL LN2 MQL + LN2
[21]	Inconel 718	Cutting force Tool wear	Conventional wet machining LCO2 (internal and external) MQL MQL + LCO2
[66]	Inconel 718	Tool wear Surface roughness	Dry machining LCO2
[105]	Inconel 718	Tool wear Energy consumption Chip reduction coefficient Surface roughness	Dry machining conventional wet machining LN2	turning
[118]	Inconel 718	Surface roughness Residual stress Chip morphology Cutting force Tool wear	Dry machining High pressure jet LN2 MQL Nano-MQL	turning
[119]	Inconel 718	Surface roughness Tool wear Microhardness	Conventional wet machining MQL + LN2	turning
[120]	Inconel 718	Tool wear Surface roughness Chip morphology	Dry machining Conventional wet machining LCO2 MQL + LCO2	turning
[121]	Inconel 718	Tool wear Cutting forces Tool temperature	LCO2 internal and external
[107]	AISI-4037	Surface roughness Cutting temperature Dimensional deviation Pattern and chip formation mode Cutting force	Dry machining Conventional wet machining LN2	turning
[122]	AISI 4340	Surface roughness Cutting temperature	Dry machining LN2
[91]	AISI 4041	Tool wear Cutting temperature	Dry machining LN2 LCO2 MQL MQL + LCO2	turning
[103]	SiCp/Al composites	Cutting forces Chip morphology Surface topography	Dry machining LN2

Table 5. Summary of the papers focusing on the cryo-machining of materials used in the aerospace industry.

Reference	Workpiece Material	Machining Outputs	Cooling Method	Obs.
[84]	Al 6082	Flank wear Crater wear Surface roughness Topography and texture of the machined surface	Dry machining MQL LCO2 MQL + LCO2
[102]	Al 6082-T6	Cutting temperature Surface roughness Cutting forces Surface morphology Chip morphology Tool wear	LCO2 LN2 conventional wet machining
[106]	Al 8011	Cutting temperature Surface roughness Tool wear Chip morphology Microstructure analysis Hardness	Dry machining conventional wet machining LCO2 MQL MQL + LCO2
[114]	Al 7075	Surface roughness Cutting temperature Cutting force	LN2 conventional wet machining	turning
[115]	Al 7075	Cutting force Surface roughness Chip morphology Tool wear Surface roughness	Dry machining LCO2 MQL MQL + LCO2
[12]	Ti-6Al-4V	Cutting force	Dry machining LN2
[24]	Ti-6Al-4V	Cutting force Tool wear Tool performance	conventional wet machining nano-MQL LN2 (internal and external) nano-MQL + LN2
[83]	Ti-6Al-4V	Workpiece temperature Resulting forces Microhardness of the workpiece surface Surface topography	Dry machining LCO2 Sub-zero MWF
[104]	Ti-6Al-4V	Cutting temperature Surface roughness Chip morphology Cutting forces Tool wear Microhardness	Conventional wet machining LCO2 MQL + LN2
[116]	Ti-6Al-4V	Surface roughness Tool wear Power consumption	Dry machining conventional wet machining cryogenic machining
[117]	Ti-3Al-2.5V	Power consumption	Dry machining MQL LN2 MQL + LN2
[11]	Inconel 718	Tool wear Cutting forces	MQL Conventional wet machining MQL + LCO2
[13]	Inconel 625	Surface roughness Tool wear Cutting temperature Chip morphology	MQL LN2 MQL + LN2
[21]	Inconel 718	Cutting force Tool wear	Conventional wet machining LCO2 (internal and external) MQL MQL + LCO2
[66]	Inconel 718	Tool wear Surface roughness	Dry machining LCO2
[105]	Inconel 718	Tool wear Energy consumption Chip reduction coefficient Surface roughness	Dry machining conventional wet machining LN2	turning
[118]	Inconel 718	Surface roughness Residual stress Chip morphology Cutting force Tool wear	Dry machining High pressure jet LN2 MQL Nano-MQL	turning
[119]	Inconel 718	Surface roughness Tool wear Microhardness	Conventional wet machining MQL + LN2	turning
[120]	Inconel 718	Tool wear Surface roughness Chip morphology	Dry machining Conventional wet machining LCO2 MQL + LCO2	turning
[121]	Inconel 718	Tool wear Cutting forces Tool temperature	LCO2 internal and external
[107]	AISI-4037	Surface roughness Cutting temperature Dimensional deviation Pattern and chip formation mode Cutting force	Dry machining Conventional wet machining LN2	turning
[122]	AISI 4340	Surface roughness Cutting temperature	Dry machining LN2
[91]	AISI 4041	Tool wear Cutting temperature	Dry machining LN2 LCO2 MQL MQL + LCO2	turning
[103]	SiCp/Al composites	Cutting forces Chip morphology Surface topography	Dry machining LN2

4.2.1. Surface Finish

The surface finish is represented by the roughness and the cleanliness of the machined surface. This is mostly affected by the feed rate and the cutting speed [123], but the adhesion of the material to the cutting tool also has a significant impact [39]. In a study on the surface finish of AISI 4340, machining tests were conducted at a depth of cut ap = 0.3 mm with two different cutting speeds, Vc = 180 and 220 m/min, and two feeds, fz = 0.10 and 0.15 mm/tooth, under both dry and cryogenic conditions [122]. In these experiments, only the surface roughness, Ra, was measured after the tests, and the best results were obtained for processing the material under LN₂ cooling. Under dry conditions, at a cutting speed Vc = 180 m/min, the roughness was Ra = 0.33 µm, while, under LN₂ cooling, the roughness was improved by approximately 25%, resulting in a value of 0.25 µm. For Vc = 220 m/min, the improvement was reduced to only 14%, from Ra = 0.35 µm under dry condition to Ra = 0.3 µm under LN₂ cryogenic spray [122] (Figure 8). The results revealed that the feed rate had the most important influence on the surface roughness, with the depth of cut having a much smaller impact.

These results were confirmed by the tests made on AISI 4037 by Dhar and Kamruzzaman, who compared cryogenic machining to wet and dry cooling procedures. Even though the process involved turning and not milling, the surface roughness (Ra) for cryogenic cooling was smaller than for dry turning conditions, with Ra = 3 µm and Ra = 4 µm, respectively, which represented a 25% improvement (Figure 9) [107].

Good improvement in the surface finish was also observed when milling aluminum alloys under the cryogenic cooling method. Using a cutting speed of 5000 rpm, feed rates of 90 and 150 mm/min, and a depth of cut of 3 mm on 6061-T6 aluminum, cryo-milling and dry machining resulted in an improvement of almost 50% between the two methods. Surface roughness Ra = 1.8/1.9 µm was measured for dry and flooded milling, while, for cryogenic milling, roughness was Ra = 1.1 µm, all for 90 mm/min feed rate. For the 150 mm/min feed rate, the values were almost the same, with Ra = 1.6/1.8 µm for dry milling and Ra = 0.9 µm for cryogenic milling [39] (Figure 10).

Similar findings were reported by V. Sivalingam et al. [84] for the milling of aluminum alloy 6082, which focused on surface morphology. Two sets of parameters were used to machine the alloy: v_c = 90 m/min, f = 0.1 mm/rev and v_c = 120 m/min, f = 0.2 mm/rev under dry, MQL, cryogenic, and MQL + cryogenic conditions. The results after machining the alloy revealed that, for the first set of parameters, the surface roughness improved from Ra = 1.9 µm for the dry milling to Ra = 0.62 µm for the cryogenic milling, resulting in an improvement over 200%. When increasing the Vc while maintaining the same feed, the difference was smaller but still significant (158%), with variation from 1.65 µm for dry milling to 0.58 µm for cryogenic milling. Increasing the feed rate to f = 0.2 mm/rev resulted in similar results. For those parameters, cryogenic milling also yielded better results in terms of surface roughness compared to MQL (53% lower). Only in the case of the combined MQL and cryo-cooling, a small improvement in roughness (only 20%) was obtained compared to cryo-cooling [84] (Figure 11).

Cryogenic milling of aluminum alloys offers an important advantage by reducing the adhesion of the chips to the cutting tool and improving the cleanliness of the surface. The reduced temperature creates a hardening effect on the chips and prevents the chemical reactions between the aluminum and the cutting tool [39], thus reducing the adhesion phenomena. Lower adhesion reduces chip–tool friction, resulting in a decrease in the cutting temperatures and cutting forces, which are tightly connected to the residual stress on the workpiece surface. N. H. A. Halim et al. [66] also found that using LN₂ while milling resulted in achieving a better surface roughness in the case of Inconel 718 machining. In their experiments, cryogenic cooling was compared to dry milling of this hard-to-cut material, using PVD multi-coated carbide milling inserts.

Table 6. Cutting parameters [66].

	Runs
	1	2	3	4	5	6	7	8
Cutting speed, Vc (m/min)	120	120	130	130	140	140	140	140
Feed rate, fz (mm/tooth)	0.25	0.20	0.15	0.25	0.25	0.20	0.15	0.25
Depth of cut, ap (mm)	0.5	0.7	0.5	0.3	0.7	0.5	0.5	0.7
Width of cut, ae (mm)	0.4	0.2	0.2	0.2	0.4	0.4	0.2	0.4

presents the values of the controlled parameters.

The results showed an improvement of roughness (Ra) by 75% and 142.8% compared to dry milling, proving that the cryogenic milling managed to extend the tool life (Figure 12)

Overall, we can state that the cryogenic milling approach offers important benefits regarding the surface finish, improving the surface roughness, with percentages varying from 14 to 158% depending on the machining parameters and the working material. These findings might be explained by the fact that the machining temperature was significantly reduced by the presence of the cryogenic agent and also the lubricating effect being induced in the case of MQL and cryoMQL leading to a lower adhesion of the material to the cutting tool and lower friction between the two.

4.2.2. Tool Life

Cooling and lubricating the work tool leads to an increased tool life [24,124]. However, liquid nitrogen, a gas-based coolant, is not known for its lubricating properties. Nevertheless, under certain conditions, it can improve the tool life when milling specific materials.

Research conducted by V. Sivalingam et al. [84] showed that cryo-milling of aluminum alloy 6082 leads to improved tool life compared to dry milling. Tests performed with two cutting speeds (90m/min and 120 m/min) over a length of cut of 350 mm revealed significant improvements in flank wear (46% and 55%) compared to the dry milling (Figure 13).

In a study by M. A. Suhaimi et al. [24], the milling of grade 5 titanium alloy, Ti-6Al-4V, under the influence of liquid nitrogen compared to flood coolant and MQL was presented. The study found that the flank wear of the cutting tool was smaller when milling with cryogenic cooling compared to dry milling but larger compared to MQL milling. The improvement from using cryogenic cooling compared to flood coolant milling was about 5%, which was not considered significant. However, combining MQL with liquid nitrogen spray showed a substantial improvement of 90% compared to flood cooling [24]. It was also noted that the spray method of LN₂ was more efficient compared to internal injection (Figure 14).

Another research study conducted by A. Shokrani et al. [116] on the same titanium alloy (Ti-6Al-4V) focused on the tool wear and surface quality. The experiment used a cutting speed (Vc) of 70 m/min, a feed rate (fz) of 0.03mm/tooth, a depth of cut (ap) of 1 mm, and an immersion rate of 50%. The study compared the effects of cryogenic cooling and dry milling. The results showed that cryogenic cooling improved the tool life compared to the other two methods. The flank wear for the cryo-milling was VB = 0.065 mm compared to 0.15 mm and 0.17 mm for dry and wet milling, respectively. Using LN₂ improved the tool life with 130% compared to dry milling [116] (Figure 15).

An improvement of the tool life was also observed while milling Inconel 718 under the influence of the cryogenic liquid. Depending on the milling parameters, improvements between 20 and 90% have been highlighted [66]. All the cutting parameters used by Halim et al. are presented in Table 6.

Tests made on nickel-based superalloy, Inconel 625, showed that lubrication is required during machining to decrease tool wear [13]. For a cutting depth of 0.5 mm and a feed rate of 0.12 mm/rev, three cutting speeds (50, 75, and 100 m/min) were tested. As expected, the higher the cutting speed, the higher the tool wear, with 25% more wear for the cryogenic cooling compared to the MQL. Thus, the simple cryogenic cooling was not adequate to increase tool life but improvement was obtained when it was combined with MQL. However, combined cryogenic cooling and MQL led to a 333% improvement in tool life compared to using only MQL [13] (Figure 16). Similar results were reported by O. Pereira [11] for Inconel 718, demonstrating that cryoMQL can be a reliable alternative to wet machining.

4.2.3. Cutting Force and Surface Stress

Overall, the flank wear was reduced by 4 to 333% when cryogenic cooling was involved compared to dry milling due to the reduction in the thermal effect and the mechanical stress.

In most experiments, cutting forces are closely linked to tool wear. The force needed to machine the workpiece increases significantly once the insert flank starts to wear [24]. Flood cooling and nano-MQL produced similar cutting force values, whereas catastrophic failure occurred when machining Ti-6Al-4V with either internal or external cryocooling due to the lack of lubrication. Nevertheless, the best results in terms of cutting forces were achieved when nano-MQL was combined with indirect cryogenic cooling (Figure 17).

Cutting forces were also measured in the articles of O. Pereira et al. [21,121]. In [20], the cutting force while milling Inconel 718 was compared among wet, cryogenic, MQL, and cryoMQL machining. The experiments showed that wet machining with oil emulsion (10% synthetic oil) yielded the best results compared to the other cooling/lubricating methods. Nevertheless, the increase in the cutting forces is not quite significant when using cryoMQL or internal and external CO₂ cryogenic cooling (Figure 18). Considering that chemical-based cooling fluids raise important risks for human health and the environment [125], it can be concluded that cryocooling could represent a viable option for Inconel 718 machining.

Yuan Ma et al. [126] have developed a new prediction model of residual stress based on three directional cutting forces and cutting temperatures under different feed rates and depth of cuts. They concluded that the cutting forces, temperature, and the value of tensile surface residual stress increased with the increase in feed rate and depth of cut. In end milling, the thermal effect leads to tensile residual stress, while the mechanical effect, represented by the cutting forces, results in compressive residual stress along peripheral direction. G. Li et al. [127] proposed a prediction method for residual stress in the turning of Ti-6Al-4V. They determined that three parameters have the greatest influence on residual stress: friction coefficient, tool edge radius, and cutting speed. The friction coefficient and tool edge radius affect the thickness of the residual stress layer, while the increase in the cutting speed determines the transformation of residual stress to tensile stress. The residual stress prediction model, based on cutting force and cutting temperature, produced good results, with errors lower than 6%. Jiang et al. [128] built a model to quantitatively analyze the influence of cutting force and temperature on residual stress generation. The model showed that the cutting force played a leading role compared to the influence of temperature. The tangential residual stress is mainly caused by tangential force and temperature, while radial residual stress is mainly influenced by the radial force, with temperature having a smaller influence than its effects on the tangential residual stress. The article by X. Zang et al. demonstrated a direct proportionality between cutting force and residual stress [10]. They found that machining the aluminum–lithium alloy under the influence of LN₂ cooling improved structural fatigue. The test results showed that the aluminum–lithium alloy part suffered compression stress on the surface. All the values were negative but, under the LN₂ influence, the values showed 100 MPa difference in favor of the cryogenic liquid (Figure 19) [10].

Dhananchezian and Kumar [129] prepared modified multi-coated carbide tools for the turning process of steel (AISI 1045). They observed that cryogenic cooling reduced cutting force by 27% and surface roughness by over 25% compared to wet machining. Kaynak et al. [130] and Deshpande et al. [131] used cryogenically treated and untreated tools in machining Inconel 718—a nickel-based alloy. They reported that the metallurgy of the tool material along with the mechanical and thermal properties are highly improved by cryogenic treatment of the tool. The treated tool improved product quality, tool life, and material removal rate, while also reducing cutting temperature, cutting forces, and tool vibration.

5. Conclusions

Cryogenic cooling in machining gained popularity in the early 2000s, and the number of published papers on this matter has constantly increased ever since. In this context, the present review was intended as a summary of the most important advantages of cryocooling over the other available cooling methods for the processing of materials used in the aerospace industry.

The most efficient cryogenic cooling method is spraying, as it cools down the cutting tool and the cutting zone but, most importantly, it cools down the workpiece before and after it is manufactured. The area of cryogenic spray is usually larger compared to the internal cooling method, similar to the flooding method or the MQL method. In most cases, cryogenic cooling, either alone or combined with MQL, increases tool life and reduces cutting forces compared to dry machining. It also results in better surface quality and allows machining at a lower temperature. Reduction in cutting force is associated with lower power consumption, making cryocooling an eco-friendly cooling method. Additionally, cryogenic liquids are eco-friendly because they are colorless, tasteless, odorless, and nontoxic, and they evaporate without leaving any residue. Therefore, we can state that cryo-milling is, in fact, “green manufacturing”. Increasing tool life is very important as it allows more parts to be manufactured with the same cutting tool before it needs to be replaced, contributing to reducing the manufacturing costs.