Machining Difficult-to-Cut Materials

History of human being has always been closely tied to the materials they have had available. Ancient civilizations commenced their existence using the naturally available materials such as soil, stone, plants, and even bones. For a very long period of time during the history of mankind, humans survived using only these naturally occurring and easily obtained materials. Hence, searching for new materials with diverse range of characteristics and acquiring knowledge about their application have been a fundamental basis for human development and innovation since the early days. Challenged by the daily life, human has always been curious to discover new materials to achieve broader range of his ambitious desires. From this perspective, the appropriate material of choice was and still is determined not only based on availability, cost, efforts, and even the ease of implementation, but also based on the specific properties offered by the material to satisfy the part functionality and design requirements. These requirements include but are not limited to a broad spectrum of properties such as strength, toughness, heat resistance, corrosion resistance as well as essential tools, equipment, and manufacturing processes. Thus, materials and materials science have been the fundamental factors to the development of civilizations. The importance of materials to the development of civilizations is to the extent that the anthropologists classify the historical eras based on the materials used during that era such as the stone, copper, bronze, and iron.

Metals, specifically steels, have transformed from their limited use by early blacksmiths to the current state of industrial mass production. The gradual progression of steelmaking processes has led to advancements in manufacturing processes, the quality and performance of products, as well as improving economies. The current chapter covers the machining of hardened steels, which is also known as hard machining. Hard machining refers to the process whereof a cutting tool removes the material from the surface of a workpiece with hardness value over 45 HRC and it can reach even up to 70 HRC. Hard machining can be achieved by almost all of the conventionally used machining operations such as hard turning, hard milling, hard boring, and hard broaching. The main objective of this chapter is to present important information about hardened steels. It briefly provides general information about hardened steels, their history of evolution, and a description of their unique mechanical and metallurgical characteristics. More importantly, it discusses the problems associated with manufacturing hardened steel parts and possible ways to overcome their machining difficulties. In this chapter, the machining operations that can be utilized for machining hard materials are investigated with a main focus on the application of hard turning and hard milling. The main challenges in the machining of hard materials, particularly hardened steels, applicable tool materials, required machine tool specifications, and attainable surface integrity will also be reviewed.

Titanium and its alloys are outstanding materials of choice, and their applications are rapidly growing worldwide in high-value markets such as aerospace, marine, power generation, heat exchangers, automotive and biomedical industries. They owe their popularity to their superior characteristics such as high strength to density ratio, also known as high strength to weight ratio or specific strength, as well as high corrosion resistance. Titanium alloys are used in aerospace industry to protect the fuselage, especially in military aircraft, from corrosion and heat damages caused by air friction in supersonic and hypersonic speeds. They are also widely utilized in marine industry to prevent corrosion from seawater or surrounding environment. Although titanium and its alloys are preferable materials by many design engineers, their poor machinability introduces a major drawback that plays a discouraging role in material selection decision. Machining titanium and its alloys requires extra care and attention to machine tool, cutting tool, and cooling strategy as the key elements of each machining system. The main objective is to prevent or minimize vibration, protect the tools from overheating and failure, and also achieve the desired dimensional accuracy and surface quality on the part. This chapter provides the readers with a brief review of the history of titanium, metallurgical aspects of titanium and the effect of alloying elements as well as their mechanical characteristics, and their industrial applications. This chapter also studies titanium and its alloys from machinability prospective in terms of mechanical behavior during machining, mechanics of chip formation, and appropriate cutting tools. The challenges and issues during machining titanium alloys will also be discussed in this chapter.

The term “superalloys” refers to a group of alloys that are capable of maintaining their mechanical characteristics after prolonged exposure to elevated temperatures. This category of material was primarily developed for applications such as turbo-superchargers and aircraft turbine engines. However, their applications have been expanded over the time to many other industrial sectors such as gas turbines, rocket engines, petroleum refineries, and chemical plants. From composition standpoint, among all of the metallic alloys ever developed for industrial, commercial, and military applications, superalloys are one of the most complex ones. This complexity enables metallurgists to develop different alloys and tailor their characteristics for wide range of applications. In addition to their modifiable features, the growing demand of industry for heat-resistant materials has further boosted superalloys development to the present level of sophistication. This chapter provides the readers with a brief review of superalloys, history of evolution, and their current industrial applications. It also presents the opportunities and challenges that may raise during machining superalloys. Applicable cutting tools, manufacturing processes, and other influential parameters on the machining and machinability of superalloys will also be discussed in this chapter.

Composite materials, or in their short-form composites, are a specific category of materials in which two or more materials, as constituents or ingredients, with considerably dissimilar physical or chemical properties are combined together to achieve unique characteristics that might be quite different from those of individual components. In composite materials, the constituent materials remain distinct within the final structure. One of the constituents acts as the main body, which forms the bulk of composite. The main body surrounds and supports the other constituent that usually acts as the strengthening or reinforcing element. Between the two elements, the former is called matrix while the latter is named reinforcement. If the matrix is made of a metallic material, the resultant composite is called metal matrix composite or in short MMC. The reinforcement can be of any metal or other types of materials such as ceramics or organic compounds. Composite materials usually have distinguished physical properties that cannot be found combined in traditional materials. The current chapter explores the composite materials, especially MMCs with the main focus on the challenges that might be encountered during machining of these advanced materials. It presents a brief review of composites’ history of evolution, their unique characteristics, and their mechanical properties. Cutting characteristics, appropriate tool materials, modes of tool wear, and other influential factors that must be taken into consideration when machining composite materials will also be presented in this chapter. The chapter ends with an overview of the challenges associated with machining reinforced fiber composites.

Ceramics are quite varied in terms of industrial applications while they are unique in terms of mechanical properties and manufacturing methods. Ceramic materials and their unique characteristics are one of the main fields in which a great rate of research progress and development have been done in the past, yet a greater progress can be foreseen in future. Due to their exceptional physical properties, components made from ceramic need to be produced using advanced and well-controlled manufacturing processes. The present chapter covers topics such as description of ceramic materials, history of evolution, and their current applications in industry. It also discusses the machining and machinability of ceramic materials. This chapter can be divided into five main sections: introduction, application in industry, manufacturing process, challenges, and a brief overview of non-traditional machining of ceramics. Relevant background information about ceramics as well as their applications in different industrial sectors such as automotive, aerospace, medical, military, textile, and more is also reviewed in this chapter. The current chapter also presents the current challenges in machining ceramics, applicable tools and manufacturing processes, and other influential contributing factors to the machining and machinability of ceramics.

The previous chapters covered different types of difficult-to-cut materials. Despite superior material properties and physical characteristics, the poor machinability is a common feature among all of these materials. Consequently, their exceptional characteristics and their candidacy as the paramount option for utilization in various applications are hindered by several challenges; most of them are directly associated with poor machinability. The main challenge is short tool life because of accelerated tool wear, which in turn lowers the productivity and increases the production cost. Other issues such as lack of dimensional accuracy and poor surface integrity are also commonly observed during machining difficult-to-cut materials. Among several factors that can be listed as the causes of poor machinability of some of these materials, the root cause is believed to be low thermal conductivity. This feature leads to the concentration of heat in the cutting zone and eventually rapid tool deterioration. In machining some difficult-to-cut materials, the heat induces other issues such as thermal errors and low dimensional accuracy. As a result, effective dissipation of heat from the cutting zone leads to improved machinability and better tool life. Heat dissipation in machining is commonly achieved by the utilization of cutting fluids. However, concern has recently grown due to the possible environmental and health hazards caused by cutting fluids. The growing concern has forced the governments and other associated agencies to impose strict policies and regulations to govern the application, recycle, and disposal of cutting fluids. Complying with governments regulations to reduce or eliminate the cutting fluids and thus minimize their associated hazards has dramatically increased the production cost. This is of particular importance when machining difficult-to-cut materials since excessive coolant is traditionally used due to the inherent characteristics of these materials. As a result, the industry aims at shifting from using flood cooling toward more economical yet environmentally friendly options. These options include minimum quantity lubrication techniques (MQL), environmentally friendly cutting fluids, nano-cutting fluids, self-cooling rotary tools, and eventually dry cutting. The current chapter presents a brief description of cutting fluids, different cooling strategies and their effectiveness, application of nano-cutting fluids in machining difficult-to-cut materials, and dry machining of these materials using self-propelled rotary cutting tools.