Machinability of Cobalt Chromium Molybdenum Alloys—Ⅰ

Cobalt chromium molybdenum alloys are considered advanced materials and are popular in a variety of engineering and medical applications. However, it is classified as a difficult material to machine due to its unique combination of properties, including high strength, toughness, wear resistance, and low thermal conductivity.

These properties often hinder the machinability of such alloys, resulting in rapid tool wear and short tool life. The review cited in this paper provides a review of the characteristics and properties of these materials and their machinability evaluations under various machining conditions and also provides a full discussion of machining trends and future research in cobalt-based and cobalt-chromium-molybdenum alloys.

Cobalt-chromium alloys are important in many engineering fields, such as aerospace engines, nuclear energy, biomedicine, and gas turbines. It is mainly attributed to its excellent properties, such as corrosion resistance, wear resistance, high creep resistance, heat resistance, and good biocompatibility. In biomedical applications, cobalt chromium is widely used in the manufacture of orthopedic implants, especially for heavy-duty joints, such as knee and hip joints, because of its excellent wear resistance and corrosion resistance.

The presence of molybdenum in the composition of cobalt alloys reduces the grain size, thereby enhancing the strength of the solid solution and subsequently improving the mechanical properties of these alloys.

In addition, chromium (Cr) particles form a protective oxide layer on the surface, providing better corrosion resistance in the human environment. In terms of machining, these alloys remain difficult to machine as they retain their strength and hardness at high temperatures.

Low thermal conductivity, high strain hardening, high hardness at high temperatures, and high wear resistance are the reasons for the poor machinability of cobalt-chromium-molybdenum alloys.

Cobalt-based alloys were first proposed by E. Hayes as cobalt-chromium (Stellites) in the early twentieth century. The main structure of cobalt-based alloys depends on the form of carbide in the Co matrix and grain boundaries to improve mechanical strength.

Cobalt-chromium-molybdenum alloys are usually manufactured by casting processes (cobalt-chromium-molybdenum alloy (F75), forging processes (cobalt-chromium-tungsten-nickel alloy (F90), cobalt-nickel-chromium-molybdenum alloy (F562), cobalt-nickel-chromium-molybdenum-tungsten alloy (F563) and powder metallurgy techniques.

As a result, parts produced by casting processes exhibit better creep strength and toughness, while parts produced by forging and forging processes have higher strength and enhanced fracture resistance. In addition, powder metallurgy is used in the production of complex and near-net products.

The castable CoCrMo alloy has been used extensively in dentistry for decades, and more recently, it has been used to make artificial joints. The forged CoNiCrMo alloy is relatively new and is now being used to make the stems of heavily loaded joint prostheses such as knees and hips. According to the American Standards for Testing and Materials (ASTM), there are four types of CoCr alloys that are recommended for surgical implants, they are. (1) cast CoCrMo alloys (F75), (2) forged CoCrWNi alloys (F90), (3) forged CoNiCrMo alloys (F562), and (4) wrought CoNiCrMoWFe alloys (F563).

Cited paper: Zaman H A, Sharif S, Kim D W, et al. Machinability of cobalt-based and cobalt chromium molybdenum alloys-a review[J]. Procedia Manufacturing, 2017, 11: 563-570.

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