What is ultra-low temperature processing? Why hasn’t it become popular? In the tool wear mechanism, wear itself is not the only reason for tool failure; high temperature is the real enemy of tools. In conventional wet cutting processes, the main function of cutting fluid is lubrication and/or splashing on the tool and workpiece to remove heat. In low-temperature cutting processes, liquid nitrogen is used as the coolant instead of cutting fluid. The temperature of the coolant poured may be 20°C, while the temperature of liquid nitrogen is -196°C. Such a large temperature difference is sufficient to turn the tool into a heat absorber. This article delves into the frigid universe of these remarkable substances, exploring their properties, applications, and the pivotal role.
Benefits of Liquid Nitrogen Ultra-Low Temperature Processing
- Extended Tool Life:
Low-temperature cooling can effectively control the heat generated during the machining process. Excessive heat is one of the main causes of tool wear. By using extremely low-temperature cooling media (such as CO2 or liquid nitrogen), heat can be quickly removed from the cutting area, significantly reducing tool thermal wear and thus extending tool life. This not only reduces the frequency of tool replacement but also reduces the decline in machining quality caused by tool wear. - Improved Processing Efficiency:
Although not directly mentioned in the link, improved processing efficiency is an important advantage of low-temperature cooling. Longer tool life means less time spent on tool replacement. In addition, better cooling effects allow for higher cutting speeds and feed rates without sacrificing machining quality. These factors combined can significantly improve overall machining efficiency. - Reduced Residual Stress:
This is a key advantage of low-temperature cooling technology, especially critical for the nuclear industry. Heat generated during the machining process can cause residual stress inside the workpiece. Low-temperature cooling can effectively control this heat, thereby reducing the formation of residual stress. The article specifically points out that this technology has the potential to produce favorable compressive stress rather than tensile stress, which can help prevent crack initiation and propagation, extending the service life of nuclear components. - Improved Surface Quality:
Low-temperature cooling can significantly improve the roughness of the machined surface. This is because better cooling effects can reduce thermal deformation and allow for more precise control of the cutting process. Better surface quality not only improves the aesthetics of the parts but may also improve their functional performance. - Reduced Thermal Damage:
In traditional machining, excessive heat can cause thermal damage to the surface or subsurface of the workpiece, such as microcracks or microstructural changes. Low-temperature cooling can effectively prevent this thermal damage, maintaining the original properties of the material. - Environmentally Friendly:
Compared to traditional oil-based or water-based coolants, using CO2 or liquid nitrogen as cooling media is more environmentally friendly. These gases naturally evaporate after use, leaving no harmful residues and requiring no complex treatment processes. This not only reduces environmental impact but also simplifies cleaning and waste disposal procedures. - Controllability:
Especially when using CO2, the cooling effect is easier to control than liquid nitrogen. The article mentions that although CO2 is not as cold as liquid nitrogen, it is “more controllable and reduces the risk of adverse material effects”. This controllability allows researchers to more precisely optimize processing parameters to achieve the best machining results.
Cooling Forms of CNC Machining Ultra-Low Temperature Processing
Liquid nitrogen ultra-low temperature processing technology in CNC machining mainly adopts two cooling forms: internal tool cooling and external spray cooling.
Internal tool cooling involves delivering liquid nitrogen through cooling channels inside the tool, spraying out from near the tool tip or cutting edge, directly acting on the cutting zone. This method provides precise cooling effects, especially suitable for deep hole machining or situations where external cooling is difficult, effectively controlling cutting temperature and reducing thermal deformation. It is commonly used in drilling, milling, and other operations, particularly suitable for machining difficult-to-cut materials such as titanium alloys and high-temperature alloys.
External spray cooling involves adding nozzles near the tool or workpiece to spray liquid nitrogen onto the cutting area or workpiece surface. This method can be direct application (liquid nitrogen sprayed directly onto the cutting area) or indirect application (liquid nitrogen sprayed onto the tool or workpiece). External spray cooling is simple to implement, easy to retrofit existing equipment, has a wider cooling range, and offers high flexibility. It is widely used in various machining methods such as turning, milling, and grinding, especially effective when machining difficult-to-cut materials like stainless steel and high-strength steel.
Regardless of the cooling form used, liquid nitrogen ultra-low temperature processing technology can significantly improve machining efficiency, enhance surface quality, extend tool life, and improve the machinability of difficult-to-cut materials. This technology utilizes material properties at low temperatures, such as low-temperature brittleness, to reduce cutting forces while effectively controlling cutting temperature and reducing thermal damage. Compared to traditional cutting fluids, liquid nitrogen naturally evaporates after use, making it more environmentally friendly. However, in practical applications, factors such as safety and cost-effectiveness need to be considered, and research on the performance changes of different materials at low temperatures is necessary to optimize processing parameters. With continuous technological development, liquid nitrogen ultra-low temperature processing is becoming an important method for high-precision, high-efficiency machining, especially playing an increasingly important role in high-end manufacturing fields such as aerospace and nuclear industries.
Gases Used in CNC Machining Ultra-Low Temperature Processing
| Gas | Chemical Symbol | Boiling Point (K) | Properties | Main Applications |
|---|---|---|---|---|
| Liquid Nitrogen | N₂ | -195.8 | Colorless, odorless, inert | Commonly used for cryogenic processing, good cooling effect |
| Carbon Dioxide (Dry Ice) | CO₂ | -78.5 (sublimation point) | Sublimates from solid to gas | Easy to control, environmentally friendly |
| Helium | He | -268.9 | Extremely low boiling point, inert | Ultra-low temperature applications, e.g., superconductor cooling |
| Hydrogen | H₂ | -252.9 | Extremely low boiling point, flammable | Industrial low-temperature applications, safety precautions needed |
| Argon | Ar | -185.7 | Inert, colorless, odorless | Welding, cutting, coolant |
| Krypton | Kr | -153.4 | Rare gas, inert | Special cryogenic processing and cooling |
| Xenon | Xe | -108.1 | Rare gas, inert | Special cryogenic processing and cooling |
| Liquid Oxygen | O₂ | -183.0 | Strongly oxidizing | Cryogenic processing and cooling, safety precautions needed |
| Liquid Argon | Ar | -185.7 | Inert, colorless, odorless | Cryogenic processing and cooling |
| Liquid Hydrogen | H₂ | -252.9 | Extremely low boiling point, highly flammable | Efficient cryogenic cooling, strict safety measures required |
| Nitrous Oxide | N₂O | -88.5 | Colorless, slightly sweet taste | Cryogenic processing, also used for medical anesthesia |
Technical Challenges and Equipment Introduction for Low-Temperature Processing
Ultra-low temperature machining technology has important applications in modern manufacturing, but its technical challenges mainly focus on the following aspects:
- Material Processing Difficulty: Under ultra-low temperature environments, the physical properties of materials change significantly, such as increased hardness and decreased toughness. For example, aluminum alloys increase in both strength and plasticity under ultra-low temperature conditions, leading to increased processing difficulty. Additionally, difficult-to-machine materials like titanium alloys experience rapid tool wear and challenging quality control during ultra-low temperature processing.
- Cooling System Design: Ultra-low temperature processing requires precise temperature control, making the design of an appropriate cooling system crucial. For example, ultra-low temperature cutting technology using liquid nitrogen as a cutting medium requires the design of a temperature-controllable cooling system. Moreover, low-temperature minimum quantity lubrication technology has significant effects on reducing cutting temperature, cutting force, and improving workpiece surface quality in turning, milling, and grinding operations.
- Tool Wear and Lifespan: In ultra-low temperature environments, tool wear rate accelerates, and tool life shortens. For instance, the “competitive mechanism” of material hardening and friction reduction in low-temperature environments affects the trend of cutting force changes, which is directly related to the cooling degree of the workpiece and depends on the supply method of the low-temperature medium.
- Equipment and Material Adaptability: Operating machinery in extremely low-temperature environments brings unique challenges, especially when selecting suitable sealing materials. These materials must withstand low temperatures without affecting their functionality. Additionally, casing materials need to maintain good toughness and cold resistance to prevent brittle fracture issues due to excessively low temperatures.
- Processing Precision and Stability: Ultra-low temperature processing demands extremely high precision and stability from equipment. For example, in low-temperature environments, equipment engines may have difficulty starting, drive performance may decrease, and damage may occur due to freezing. Furthermore, while the strain hardening ability of materials significantly enhances during low-temperature deformation, this may also lead to work hardening issues.
- Technical Complexity and Cost: Ultra-low temperature processing technology is complex and costly. For example, the direct cooling method of dry refrigeration machines lacks heat transfer through cooling media, leaving the superconducting magnet of the cooling system in a near-adiabatic state, making it sensitive to ambient temperature. Moreover, the machining process itself involves cutting raw materials, resulting in low material utilization and high costs.
The main challenges faced by ultra-low temperature machining technology include material processing difficulty, cooling system design, tool wear and lifespan, equipment and material adaptability, processing precision and stability, and technical complexity and cost. These challenges need to be overcome through technological innovation and process improvements.
CNC ultra-low temperature processing can improve machining efficiency and precision in multiple aspects by lowering the temperature of materials and tools during the machining process, specifically including the following points:
- Reduced Thermal Deformation:
In traditional cutting processes, a large amount of heat is generated, causing workpieces and tools to deform due to thermal expansion and contraction. Ultra-low temperature processing can significantly reduce this heat generation, thereby reducing thermal deformation and maintaining higher dimensional stability and machining precision. - Improved Tool Wear Resistance:
In low-temperature environments, the wear rate of cutting tools usually decreases. Ultra-low temperatures can change the physical properties of materials, effectively improving the wear resistance of tools, extending tool life, and reducing downtime and tool replacement frequency. - Enhanced Cutting Performance:
Low temperatures can reduce material cutting forces, decrease cutting resistance during the cutting process, and therefore increase machining speed and feed efficiency. This efficient cutting process can significantly improve overall machining efficiency. - Enhanced Material Cutting Characteristics:
Some materials exhibit better cutting performance at low temperatures, such as increased hardness and toughness, making the cutting process smoother under these conditions and reducing the occurrence of machining defects. - Reduced Cutting Fluid Requirements:
As low-temperature processing can reduce frictional heat during the cutting process, it may reduce or even eliminate the need for cutting fluids, thereby lowering production costs and reducing the complexity of the machining process. - Improved Surface Quality:
Under low-temperature cutting conditions, the surface finish and dimensional accuracy of workpieces are usually better, thereby improving the quality of the final product.