Abstract
As a promising material, cobalt-chromium alloy has received a lot of attention for its high stiffness and extremely were-resistant property. Cobalt-chromium is a chromium-based alloy containing Cobalt, Nickel, Tungsten and other addictive metal elements. Cobalt-chromium alloy is extensively applied in the field of medical implants, industry and dental implants. Cobalt-chromium alloy is usually manufactured by the technologies including selective laser melting technology and laser engineered net shaping. The processing of cobalt-chromium includes hot isostatic pressing homogenisation heat treatment, etc. In this page, some important information including properties, application and manufacturing related to the cobalt-chromium alloy will be briefly introduced with abundant images and videos attached for illustration.
Introduction
Cobalt-chrome or cobalt-chromium (CoCr) is a metal alloy of cobalt and chromium. Co-Cr alloys are well known for their high wear resistance and corrosion resistance. The promising properties of Co-Cr alloys enable such alloy to be applied in the fields including gas turbines, dental implants, and orthopaedic implants. Compared with other metals and alloys, Co-Cr alloys have excellent wear resistance, strength, impact resistance and low friction whilst both surviving in the environment of the human body and not significantly altering the human physiology, which makes Co-Cr to be commonly applied in the orthopaedic implants. Co-Cr alloy is also extensively applied in the dental implants for it possesses adequate wear and corrosion resistance, fatigue strength and bio-compatibility.
Various manufacturing and processing methods are applied in the treatment of Co-Cr alloy. Selective laser Melting technology and a laser-based additive manufacturing technology as precise manufacturing methods are applied for making dental crowns and other kinds of structures. The Co-Cr alloy is also really suitable for the processing methods including hot isostatic pressing and homogenization heat treatment.
Literature review
The current research related to Co-Cr alloys are mainly focusing on the hip replacement, gas turbine and dental implants [23][25][26][27]. Due to the complexity of the vivo environment, virous researches have been done in different laboratory in the world to investigate the influence of such material on the human body including the concentration of specific elements in the blood [24]. Besides, the wear resistance behaviour affected by the concentration inside which is extremely important in the hip replacement of this material is also deeply studying [19][21].
Property Profile——unstructured information
1.ASTM F75 is one of the superalloys
Superalloys are defined as "alloys developed for elevated temperature service where severe mechanical stressing is encountered and high surface stability is frequently required". There are three classes of alloys that meet this definition - cobalt-base, nickel-based and iron-base [2].
2. The history of Cobalt-based superalloy
Although in terms of properties the hardened nickel-based alloys ("Y" alloy) have taken the majority share of the superalloy market, cast and wrought cobalt alloys continue to be used because of the following characteristics:
- Higher melting points than a nickel (or iron) alloys
- Superior hot corrosion resistance to gas turbine atmospheres
- Superior thermal fatigue resistance and weldability over nickel superalloys
3. Fatigue Test [3]
4. Phase diagram
The alloy consists of two primary phases: a high-temperature γ-face-centred cubic (γ-fcc) phase which shows high elongation and ultimate tensile strength, other is low temperature ε-hexagonal close packed (ε-hcp) phase which shows low elongation and brittle fracture on straining [4].
Manufacturing——selective laser Melting technology
1. Manufacture of Co-Cr dental crowns (selective laser Melting technology）
Additive Manufacturing (AM) technologies are playing a more and more important role in the biomedical field by offering new possibilities for complex shape high-added value parts production [5].
According to functional and biological requirements, different types of materials are currently used in medicine such as stainless steels and Co-Ni-Cr-based, Co-based and
Ti-based alloys. The Co-based alloys possess adequate wear and corrosion resistance, fatigue strength and bio-compatibility. It is known that the most important strengthening methods for Co-Cr alloys are solid solution strengthening and precipitation of carbides.
Micrograph of a non-etched section of Co-Cr framework [6]
Manufacturing——Laser Engineered Net Shaping
Using rapid prototyping (RP) in CAD files to fabricate functional parts directly is a feasible and promising method for manufacturing near-net shape. One process is Lens TM, which USES metal/ceramic powders to make functional parts.
First, a three-dimensional model of the component to be constructed is generated by computer-aided design (CAD). Through the model, the computer program divides the model into several horizontal sections or layers. These cross-sections are created in turn on the substrate on which the three-dimensional object is generated. The deposition process begins by directing a focused Nd-YAG laser beam onto a metal substrate placed on a CNC X-Y bench. The laser first creates a small pool on the substrate. According to the CAD file, a predetermined amount of metal/ceramic powder is injected directly into the bath to increase its volume as the x-y table moves. When the laser beam leaves, the molten material lines rapidly solidify, forming a thin track of solidified material along the laser scanning line that is tightly bound to the substrate or the previous layer. A layer is generated by a succession of overlapping tracks. After each layer is formed, the laser head and the powder feeding nozzle move up one layer to form the next layer. This process is repeated many times until the entire object represented in a three-dimensional CAD model is generated on the substrate and can be adjusted to a solid or vertex object.
Schematic representation of LENSTM [7]
Additively manufactured Co-Cr-Mo alloy impeller using Laser Engineered Net Shaping [8]
Laser Engineered Net Shaping [15]
Process——Hot isostatic pressing
Hot isostatic pressing (HIP) is a manufacturing process, used to reduce the porosity of metals and increase the density of many ceramic materials. This improves the material's mechanical properties and workability. The HIP has emerged as an important manufacturing process in the past decades. It is commercially applied to fabricate fully dense components from different materials such as tool steels, superalloys, certain titanium alloys and a wide range of ceramics.
Flow chart of the computer program for construction of HIP maps [9]
The suggested parameters:
– 1200 °C
– 1000 bar argon
– 240 minutes
Hot isostatic pressing [12]
Process——Homogenisation heat treatment
Homogenization or homogenisation is any of several processes used to make a mixture of two mutually non-soluble liquids the same throughout. This is achieved by turning one of the liquids into a state consisting of extremely small particles distributed uniformly throughout the other liquid. A typical example is the homogenization of milk, where the milk fat globules are reduced in size and dispersed uniformly through the rest of the milk. [10]
Homogenization or diffusion annealing is usually carried out for heavy and intricate castings which are prone to significant intercrystalline segregation. Homogenization annealing is carried out to remove the inevitably occurring chemical inhomogeneity in heavy alloy steel casting.
[11]
Suggested parameters: [3]
– 1220 °C
– 0.7–0.9 mbar argon
– 240 minutes
Homogenisation heat treatment [13]
Process——Casting
The Co-Cr alloy has a good property for casting. Casting is a manufacturing process in which a liquid material is usually poured into a mould, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mould to complete the process. Casting materials are usually metals or various time setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.
SEM micrograph of the cast Co-Cr-Mo-C alloy showing a thermal formation of hcp martensite plates during water quenching after homogenization at 1150℃ for 1 h [16]
Casting
Application——Industry
[22]
Cobalt-based alloys have been in use for several decades in the manufacturing of various components such as vanes or combustion chambers in gas turbines (both the industrial type and aero engines). They can be implemented as wrought or as precision-cast parts.
These materials have been designed primarily to withstand higher stresses at increasing service temperatures. However, because of the variety of aggressive environments encountered, the attention of materials scientists have also been focused on the problem of degradation from oxidation and hot corrosion. In this case, the design trend has been the development of alloys exhibiting a high intrinsic corrosion resistance as well as the development of better coating techniques. The coating process and/or the formation of a layer of corrosion products introduces additional constraints in the design of a particular component because of the surface or interface phenomena and of their evolution during service. [23]
Sputtered S-77 (Co-25Cr-10Ni-7Al-5Ta-0.5Y) on a low activity aluminized coatingThe most severe degradation modes that gas turbine shafts have to face are friction and wear. Surface damages generated by the sliding contact with bearings limit the life of the shafts and therefore reduce their durability and reliability. Hard chromium plating is usually used to restore the original dimensions of worn surfaces of the shafts, pumps and compressors. [24]
Application——Medical implants
Co-Cr alloys are most commonly used to make artificial joints including knee and hip joints due to high wear-resistance and biocompatibility. Co-Cr alloys tend to be corrosion resistant, which reduces complication with the surrounding tissues when implanted, and chemically inert that they minimize the possibility of irritation, allergic reaction, and immune response. The co-cr alloy has also been widely used in the manufacture of the stent and other surgical implants as Co-Cr alloy demonstrates excellent biocompatibility with blood and soft tissues as well. [17]
Hip replacement (total hip arthroplasty) is surgery to replace a worn out or damaged hip joint. The surgeon replaces the old joint with an artificial joint (prosthesis). This surgery may be a choice after a hip fracture or for severe pain because of arthritis [18]. Co-Cr material is typically applied in the hip replacement. These materials must provide high wear resistance, strength, impact resistance and low friction whilst both surviving in the environment of the human body and not significantly altering the human physiology. [19]
There are usually two kinds of structured being used extensively in the past 20 years. A couple of metal or ceramic heads on polymeric acetabular components.
1) Pin-on-disk or hip-simulator wear studies involving the combination of a metallic femoral head component
[20]
The wear evaluation [21]
The wear property may vary with the different concentration of carbon in the Co-Cr alloy.
The material tested
Average Weight-Loss Data for Pin-on-Disk Wear Screening (250,000 Cycles)
Weight-Loss Data for Wear Samples Total Weight Loss over 5,000,000 Cycles
Large metal particle retrieved from metal-on-metal hip-simulator bath serum
Representative smaller metal particles from serum.
Reference
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https://www.youtube.com/watch?v=BsnzgsEXT_A
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[25] SUPPLY. Dental Non-Precious Alloy Porcelain fused to metal Crowns (PFM) supply China (Mainland) Medical Devices[EB/OL]. Weiku.com, 2019. (2019)[2019 -05 -22]. http://www.weiku.com/products/14005766/Dental_Non_Precious_Alloy_Porcelain_fused_to_metal_Crowns_PFM_supply.html.
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[27] Shen lei, xiang dong, zhong lifang. Study on the effect of cobalt-chromium alloy porcelain crown restoration on periodontal tissue of affected teeth [J]. Chinese journal of practical stomatology, 2009, 2(02).
Application——Dental implants
[25]
Cobalt-chromium alloys are widely used in dentistry because of their better biological safety and stability than nickel-chromium alloys. The traditional production method of cobalt-chromium alloy is based on the casting method of "material reduction" method, which includes the procedures of tooth preparation, mold extraction, filling, dressing, wax extraction, embedding and casting.
Surface morphology of casted Co—Cr alloy
Scanning electron microscopy images of casted Co——Cr alloy [27]
Effect of the Co-Cr alloy on the periodontal tissue of the restored teeth.
Application of drilling chrome alloy porcelain umbrella crown repair. The effect on periodontal tissue in short time is small. However, the effect on periodontal tissue increased gradually when the crown was repaired with Cr - PFM for a long time. This requires patients to return regularly for oral health check and periodontal maintenance.