Sustainability – according to definition, is the development that meets the needs of the present, without compromising the ability of future generations to meet their own needs. For fulfilling this definition’s aim, the ideas have to be implemented in all the fields and levels of production and in this way, contribute to the idea of global sustainability.
At the moment, the manufacturing industry is under increasing pressure of financial crisis, new sustainable development regulations, supply-chain and customer demands. One way to gain an advantage in this situation is to focus on competitive sustainable manufacturing (CSM). CSM has the dimension of economy, ecology, sociology and technology and calls for practices and decisions that will assure high-adding-value solutions.
The global environmental problems caused by the consumption of natural resources and the pollution resulting from the life of technical products, have led to increasing political pressure and stronger EU regulations being applied to both the manufacturers and users of such products. The industries involved in production are additionally under economic pressure, and are attempting to compensate for increasing costs and create added value for their products. The adoption of sustainable development in production offers industry a cost effective route for improving economic, environmental, and social performance (i.e. the three pillars of sustainability).
Heat resistant alloys (high-temp alloys) with high-melting temperatures are nowadays important materials used in the manufacture of aero-engine components. These super-alloys can be grouped into four major categories: nickel-based alloys; cobalt-based alloys; iron-based alloys (e.g. high chromium stainless steel), titanium alloys and tungsten. The ability to retain high mechanical and chemical properties at elevated temperatures makes super-alloys ideal materials for use in both rotating and stationary components in the hot end of jet engines. The components produced with super-alloys are smaller and lighter than those made from conventional steel. This results in significant fuel savings and, therefore, a reduction in environmental pollution.
Super-alloy materials are also used by the chemical, medical and structural/construction industries in applications requiring extraordinarily high temperature properties and/or corrosion resistance. Power plants and the aerospace industry use higher proportions of machined components made from super-alloys, such as turbine engine components. The machining of nickel-based, tungsten, and titanium-based alloys and other high-tech materials are very expensive because of their high-temperature properties as well as their ability to retain a high strength-to-weight ratio, which are essential for the economic exploitation of aerospace engines.
However, these properties, on the other hand, lead to shorter tool-life, which significantly increases tooling waste and machining costs. It is envisaged that the aerospace sector will witness a doubling of the aircraft fleet over the next 20 years in response to the replacement of older aircraft and growth opportunities. Therefore, the same trend of development/usage is indirectly expected in innovative machining/production systems, which additionally prove the high potential for innovative sustainability-oriented machining processes.
The fact that the volume of aerospace-alloy usage is continually increasing has resulted in huge pressure to reduce machining cost by developing efficient technologies. From the initial investigations it is possible to state that innovative sustainable machining process (cryogenic machining, high pressure jet assisted machining, etc), in combination with appropriate tool material(s) and technology, has resulted in enhanced performance when machining aerospace (hard-to-machine) alloys.
Conventional cooling lubrication fluids (CLFs) in these FOCUSapplications are not as effective as the proposed alternatives in terms of decreasing the cutting temperature and improving environmental sustainability. In some cases, they even cannot fulfill the machining or final part requirements related to its functionality. Additionally, one of the most fundamental concerns is the use of CLFs, which has a direct influence on the environment, performance of machined surface and machining economics.
In reality, there are always some losses of CLF in the production process. This occurs through vaporisation, the loss with chips and parts as they leave the machine tool, the loss of machine components such as handling devices, as well as through leakage. Taking into account that up to 30% of the annual total CLF consumption can be lost from the system by the above means, it becomes clear that technologies employing CLFs are unsustainable, and by avoiding of their usage through applying dry, near-dry, etc machining alternatives, there would be a huge process gain from the sustainability point of view. With regard to this, cryogenic machining is proposed as alternative ahead of conventional machining processes.
Cryogenics is the field related to technology at deep freezing temperatures. Traditionally, the field of cryogenics is taken to start at temperatures below 120 K (~ – 150 °C). The definition includes the more common cryogenics such as helium, hydrogen, neon, nitrogen, oxygen, argon, krypton, xenon, methane, ethane, and propane. Carbon dioxide is commonly added to the list even though a pressure over 50 kPa is required to maintain it in liquid form. Even the term ‘cryogenics’ seem like an esoteric field, it plays a major role in modern industry and science. Some of the applications are – air separation plants for breaking it down into its components for industrial and medical uses, liquefied helium has become unavoidable cooling element of magnetic resonance imaging systems in modern hospitals, in space technology, where cryogenics are through the liquefied hydrogen and oxygen used as fuels, food freezing and cooling, purging and blanketing, etc.
The application of cryogenic fluid to cool the metal cutting process started as early as the 1950s. The cryogenic fluids used were CO2, freon, or solvenlene. They were sprayed in the general cutting area or were applied to the workpiece before cutting in a prechill. This method, however, consumed excessive amounts of cryogenic fluid and had no lubrication effect. Additionally, this reflects high costs and present high complexity in delivering of cryo fluid to the cutting zone. Today, the process of liquefaction and storage system becomes more affordable, and there is a need to develop and rise the cryogenic machining on an industrial level.
Cryogenic machining presents a method of cooling the cutting tool and/or part during the machining process. More specifically, it relates to delivering of cryogenic CLF (instead of an oil-based CLF) to the local cutting region of the cutting tool, which is exposed to the highest temperature during the machining process, or to the part in order to FOCUSchange the material characteristics and improve machining process performance.
Generally, nitrogen fluid is used as the cryogenic coolant. Using it in machining process, when delivered to the cutting zone, it immediately evaporates and returns back to the atmosphere, leaving no residue to contaminate the part, chips, machine tool, or operator. Thus, it is eliminating disposal costs related to CLF usage. This represents completely clean process in contrast to conventional oil- based CLFs.
Potential benefits of cryogenic machining includes sustainable machining methods (cleaner, safer, environment friendly, more health acceptable, etc) to eliminate numerous costs associated with conventional cutting fluids and clean-up operations. It also increases material removal rate without increase in worn tool and tool change over costs, thereby, increasing productivity. It increases tool life due to lower abrasion and chemical wear. Additional benefits include machining of hard parts and hard-to-machine alloys, which in the past, could have been produced only via expensive grinding operations; surface roughness of machined workpiece improvement; produced parts quality improvement by preventing mechanical and chemical degradation of machined surface. It also enables potentially lower investment costs due to reduction in number of machine tools required and improvement of manufacturing flexibility for reduced production times & high output, etc.
All these benefits occur due to lower cutting temperatures in cutting zone, improvement of chip breakability, decreased BUE formation probability, decrease of burr appearance probability, inert environment assurance, no oil-based emulsion used, no additional processes needed, liquid nitrogen specifications and changes in material characteristics at lower temperatures, etc.
The role of cryogenics is pointing out the reasons and needs for practices in the field of sustainable development on all levels and fields, even machining process, for assuring our common goal of global sustainability. The focus is on novel machining technologies that are meeting environmental and social regulation, while still being competitive.
Additionally, the main pillars of sustainability are under the scope, i.e. reduced energy consumption, prolongation of tool-life, improved final product functionalities through improved machined surface integrity, etc. Experimental analysis shows that as an alternative to cost infectivity, health and environmental problems of plastic infiltrant machining procedure, cryogenic machining can be used for machining of porous tungsten for dispenser cathodes.
Cryogenic machining is able to keep unsmeared surface — keeps open pore structure that satisfies the industry standard. This accomplishment has never been achieved with using of any other cooling/lubrication techniques. As a result, it can be claimed that cryogenic machining as sustainable machining alternative can essentially provide: (i) improved environmental friendliness, (ii) reduced cost, (iii) reduced energy consumption, (iv) reduced waste and more effective waste management, (v) enhanced operational safety, and (vi) improved personnel health.