Procedia Engineering 68 ( 2013 ) 405 – 410

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1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysiadoi: 10.1016/j.proeng.2013.12.199

ScienceDirect

The Malaysian International Tribology Conference 2013, MITC2013

Low strain rate upset forging of preformed CoCrMo powder alloy for load bearing application: A review

M.F. Faziraa*, M. Mohammada, N. Roslania, M.H. Saleha, M.A. Ahmada aAMREC SIRIM Berhad, Lot 34, Jalan Hi-Tech 2/3, Kulim Hi-Tech Park, 09000 Kulim, Kedah, Malaysia

Abstract

CoCrMo alloy are known to be widely applied as biomedical implant materials. They have been used for artificial hip and knee joints because of their excellent corrosion and wear resistances as well as good mechanical properties and biocompatibility. Although as-cast or annealed CoCrMo alloys are frequently subjected to practical use, there are possibilities to exhibit insufficient mechanical properties due to coarse microstructure and solidification defects that often lead to premature failure of implants. Therefore, the review suggested that an improvement of tensile properties as well as fatigue properties shall be made for further mechanical reliability by applying low strain rate upset forging. Grain-structure or grain-flow optimization is a sufficient area to be studied to produce parts with enhanced mechanical properties, impact strength, and fatigue endurance limits. Forgings deliver strengths were the most needed while maintaining a good balance of 3-demensional properties. By optimizing grain-flow characteristics during the forging process, we should be able to tailor the directional strength, which permits forging to meet the requirements of the parts with the desired configurations.

© 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia. Keywords: CoCrMo, forging, low strain rate, load bearing

1. Introduction

CoCrMo alloys are known to be widely applied as biomedical implant materials. They have been used for artificial hip and knee joints because of their excellent corrosion and wear resistances as well as good mechanical properties and biocompatibility [1-4]. As-cast or annealed CoCrMo alloys are frequently subjected to practical use; however, there are possibilities to exhibit insufficient mechanical properties due to coarse microstructure and solidification defects such as microvoids, segregation of solute atoms, and so on. These shortcomings often lead to premature failure of implants [5,6].

Hence, the improvement of tensile properties as well as fatigue properties is eagerly anticipated because further mechanical reliability is required for providing long-term use as hard tissue replacement. [5] Cobalt-base alloys posess high mechanical property capability. The alloys were first used as cast components followed by wrought

* Corresponding author. Tel.: +6044017100; fax: +6044033225. E-mail address: fazira@sirim.my

© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

406 M.F. Fazira et al. / Procedia Engineering 68 ( 2013 ) 405 – 410

alloys. Alloys produced for structural applications such as hip prostheses shall be forged for optimal properties. The forging process results in maximum strength and toughness for CoCr alloys but may not produce uniform grain size. [7]

Prior heat treatment provides an improved microstructure to compensate the limitation of the degree of homogenization that may be achieved by a single step forging operation; this is proved by Immarigeon et. al. They also suggested that the limitations on attaining microstructural homogeneity through isothermal forging may be overcome through the use of powder fabricated material rather than castings. Forging of consolidated prealloyed powders might be a better processing route for producing mechanically sound surgical implants. [6] The same author found that the plastic flow was considerably more inhomogeneous at high strain rate hence suggested for low strain rate forging so that the deformation shall appear more homogeneous [6].

Since CoCr alloy are difficult to machine, closed-die forging can minimize machining requirements. Additional improvements have also being done by using controlled strain rate isothermal forging. In this type of forging, the dies are kept at the forging temperature. This eliminates die chill and allows microstructures in forgings to be controlled more effectively. The deformation appears more homogeneous at slower strain rates. However, there were some limitations on attaining microstructural homogeneity through this method [6].

In critical load-bearing applications, the superior structural integrity of forgings makes them the preferred choice over components made by other metalworking processes. Forgings deliver strength where most needed while maintaining a good balance of 3-dimensional properties. Achieved by optimizing grain-flow characteristics during the forging process, the ability to tailor this directional strength permits forgings to meet different performance requirements for an almost unlimited number of part configurations [8].

Grain-structure or grain-flow optimization can produce enhanced mechanical properties, impact strength, and fatigue endurance limits, often boosting service life several times over castings and other competitive processes. In addition, resultant higher strength in thinner sections facilitates weight reduction and permits higher design limits. Consequently, such advantages are responsible for renewed emphasis on optimizing grain flow in forged components [8].

It is estimated that approximately 1 million hip replacements and 250,000 knee replacements are carried out per year. This number is expected to double between 1999 and 2025 as a result of aging populations worldwide and growing demand for a higher quality of life. [9] Another statistical data estimated that by the end of 2030, the number of total hip replacements will increase by 174% and total knee arthoplasties is predicted to grow by 673% from the present rate [10].

2. CoCrMo as biomedical implant materials

CoCrMo has been used for biomedical implants for a number of years for its combination of mechanical

properties, corrosion resistances and biocompatibility [11]. They are widely used in load bearing application parts, namely total hip and knee replacement. They are preferred alloys with metal-on-metal contact since the tribological properties; high corrosion resistance and wear performance; are superior in comparison with those of titanium alloys [12].

2.1. Microstructure of CoCrMo powder and the formulation

The raw material suggested for the work is a CoCrMo alloy powder, delivered by Sandvik Corporation, produced

by gas atomization. The alloy powder particles have a spherical shape (Fig. 1a) sized 22 m. Lithium stearate (LiS) (2%) was added as a lubricant and mixed during 1-h in a tubular mixer. This additive appears in the micrograph (Fig. 1b) as non-spherical impurities. Since the size of the powder are very small and lead to lamination after compaction, tungsten ball, sized 10mm, were added during the mixing. The ratio of ball to powder is 3:1. The milling process grounded the spherical powder to irregular shape (Fig. 1c), hence allow better binding properties.

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Fig 1. Microstructure of (a) CoCrMo alloy powder , (b) CoCrMo alloy powder added with 2% LiS and (c) Milled CoCrMo alloy powder + 2% LiS with tungsten ball

2.2. Physical metallurgy and chemical composition of CoCrMo

Cobalt-based alloys have two possible crystal structures: close packed hexagonal (CPH) at low temperatures

(below 417°C) and face centered cubic (FCC) at high temperatures (above 417°C) [12]. Fig. shows the XRD pattern of the CoCrMo powder alloy as compared to the one with LiS as additive.

(1a)

(1c)

(2a)

(1b)

408 M.F. Fazira et al. / Procedia Engineering 68 ( 2013 ) 405 – 410

Fig. 2 XRD pattern of (a) CoCrMo alloy powder and (b) CoCrMo alloy powder added with 2% lithium stearate (LiS) as binder

Commonly, there are two types of CoCrMo alloys used for biomedical applications, which depend on the level of

carbon added. However, both alloys have a balance of cobalt, which can be as low as 60 wt%. There is approximately 28% chromium. Typically 5-7 wt% molybdenum is used to improve the mechanical properties of the alloy as it provides solid solution strengthening and good localised corrosion resistance. [13,14]

CoCrMo alloys can be termed ‘high carbon’ (usually 0.15-0.25 wt%) or ‘low carbon’ (usually less than 0.06 wt%) depending on the amount added in the gas atomizing process [15]. Carbon additions between 0.1 and 0.3 wt% have been shown to favour the formation of carbides which increase wear resistance [16]. These micron-scale carbides that form at the surface of the alloy are much harder than the alloys matrix and so they will protect the surface from wear [17].

The composition of the CoCrMo powder alloy is shown in Table 1.

Table 1. Composition of CoCrMo alloy powder

Element Sandvik Co212-c ASTM F75-92 requirement

Chromium, Cr 27-30 % 27-30 %

Molybdenum, Mo 5-7 % 5-7 %

Nickel, Ni 0.5 % max < 0.5 %

Carbon, C 0.35 % max < 0.35 %

Iron, Fe 0.75 % max < 0.75 %

Silicon, Si 1 % max < 1%

Manganese, Mn 1 % max < 1 %

Cobalt, Co Balance Balance

3. Forging at low strain rate

3.1. Powder forging

Powder forging is a natural extension of the conventional press and sinter (powder metallurgy) process, which has long been recognized as an effective technology for producing a great variety of parts to net or near-net shape. [18] The shape, quantity, and distribution of porosity in powder metallurgy and powder forging parts strongly

(2b)

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influenced their mechanical performance. Powder forging therefore is a deformation processing technology aimed at increasing the density of powder metallurgy parts and thus their performance characteristics.

According to Kuhn and Ferguson [19], in order to develop full density of the parts whilst achieving the desired part shape, the process of powder forging involves axial compression of the powder preforms. A trap die consisting of a through-hole die cavity, a movable upper punch for load application, and a lower punch of ejection of the part is used to carry out the process. Proper design of the preform is very crucial to ensure the success of the powder forging, so that the preform will densify, fill the die completely, and avoid cracking during forging. Basic principles of metal flow mechanics will offer sufficient guidelines and concepts to design the preform.

The sintered preforms may be forged directly from the sintering furnace, stabilized at lower temperatures and forged, or cooled to room temperature, reheated, and forged. All cooling, temperature stabilization, and reheating must occur under protective atmosphere to prevent oxidation. 3.2. Low strain rate forging and grain deformation

High-energy-rate forging, sometimes known as high-velocity forging, is a closed-die hot- or cold-forging process

in which the stored energy of high-pressure gas is used to accelerate a ram to unusually high velocities in order to effect deformation of the workpiece. The velocity of the ram generates the major forging force. Though the process have several advantages, such as ability to forge complex parts and can successfully forge many metals that have low forgeability, it also have some drawbacks; the process is generally limited to symmetrical parts, the production rate is slower than in mechanical forging and the dies must be carefully designed and fabricated in order to withstand the high impact. [18]

Low strain rate forging technique is suggested as an energy saving process while providing improved mechanical properties of parts by promoting control of grain flow. Orientation of grain flow, alignment of the metal microstructure with the geometry of the part being forged, is directly liable for developing maximum tensile strength, toughness or impact strength, fatigue resistance and greater service-life expectancy. [8]

In addition to enhanced properties, grain-flow control offers other benefits by permitting orientation of minor inclusions and other microstructural features to accentuate performance gains. In effect, good grain flow or a specific type of grain flow can actually improve the material itself. [8]

4. Load bearing application and wear behavior

Forgings have the superior structural integrity that makes them the preferred choice over components made by other metalworking processes for load-bearing applications. [8] The mechanical property requirement of CoCrMo as outlined by the ASTM F799 is shown in Table 2 [20].

Table 2. Mechanical requirement according to ASTM F799

CoCrMo

Property F799

Tensile strength (MPa) 1172

Yield strength (MPa) 827

Elongation (%) 12

Reduction of area (%) 12

Hardness (HRC) 35

Cobalt-based alloys have their wear resistance to the hard macroscopic carbides present in the microstructure.

Carbides are harder than the surrounding alloy and so are more resilient to the two and three body abrasive wear that can be experienced in a metal-on-metal contact [13]. The wear rate depends on the carbide volume fraction as well as their size and distribution, which is known to be affected by the thermal processing of the alloy. Carbide size

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distribution refers to the different morphologies that can exist such as blocky, particulate agglomerated or lamella eutectoid carbides depending on the thermal treatment undergone [21].

5. Conclusions

This work was initiated with the aim to reduce the high energy applied as compared to low energy requirement while using low strain rate. The approach is expected to provide better properties of CoCrMo as biomedical material, specifically for load bearing analysis while saving the energy of producing the parts. The work is still at the beginning stage, where the formulation of the powder is yet to be optimized. The process will be followed by compaction of the formulated powder and sintering of the green body at optimum temperature to obtain a preform with a desired sintered density. The preform shall then be heated to a forging temperature and while red hot, the workpiece shall be forged at a low strain rate to promote control of isotropic grain flow. The forged part shall be analyzed for its tribology properties – wear, tensile strength, toughness and hardness. The microstructure of the grain flow shall be studied by transmission electron microscopy (TEM). Prior to the analysis, the bulk sample shall be prepared by ion milling. Acknowledgements

This research work is an ongoing project funded by Ministry of Science, Technology and Innovation (MOSTI) of Malaysia and supported by Advanced Materials Research Centre (AMREC), SIRIM Berhad for the experimental and characterization facilities.

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