Manufacturing, characterization and applications of lightweight metallic foams for structural applications: Review

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 5 (2018) 20391–20402

www.materialstoday.com/proceedings

ICMPC_2018

Manufacturing, characterization and applications of lightweight metallic foams for structural applications: Review Parthkumar Patela, P.P.Bhingoleb*, Dhaval Makwanac a

b

M Tech student,Mechanical Engineering Department, IITRAM,, Ahmedabad - 380026, India. Assistant Professor,Mechanical Engineering Department, IITRAM,, Ahmedabad - 380026, India. c PhD student,Mechanical Engineering Department, IITRAM,, Ahmedabad - 380026, India.

Abstract Metallic foam becomes most promising class of material in industrial and scientific area due to advantages of unique combination of mechanical properties like lower weight, high energy absorbing capacity, high stiffness and high damping capacity. Metal foams are being used in many industrial applications such as light weight structural applications, bone implants, filters, electrodes, heat exchangers etc. The purpose of this study is to summarize the fabrication and characterization of metal foam for bone implant applications. Various biomaterials, fabrication methods of metal foam, aluminium and its alloy foam and characterization techniques are reviewed in present work. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Metallic foam; aluminum; structural applications; biomaterials

1. Introduction New class of solid metallic foams finding its importance because of exceptional combinations of mechanical and physical properties. Foams can be defined as uniform dispersion of gaseous phase inside solid or liquid [1]. Highly porous materials along with foam having cellular structures possess unique and attractive combinations of mechanical and physical properties which is the main reason that nature frequently use them in various functional and structural applications like bone, wood etc.[2]. With the rapid growth of aerospace, defence and automotive field there is need of materials having properties like low weight for better fuel efficiency and high specific strength

* Corresponding author. Tel.: +91-8266990912; E-mail address: [email protected]

2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.

20392

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

along with high energy absorption capacity to sustain impact forces [3,4]. Lightweight alloys as well as their foams have very sound mechanical, electrical, acoustic, chemical and thermal properties so their structural and functional applications are increasing [4,5]. Metallic foams can be recycled without any waste management issue [6]. One of the unique advantage of foams is that mechanical properties can be controlled by controlling pore size, density, geometry and foaming material. Foam structures can be of several types which includes open foams (pores connected to each other so that matter can pass through them), partially open foams and closed foams (isolated closed pores filled with any gas). All aforesaid types of foams have different applications. Open cell foams are used in functional applications (filters, heat exchangers, catalyst supports) while closed cell and partially open cell foams are used in structural applications (silencer, sound and energy absorbers, bearings, automobile, aerospace, defence, construction etc.) [2]. Al foam structures possess comparatively high specific strength and stiffness because of less density than that of solid structures, easy recycling, non-toxic outstanding properties for sound absorption, vibration and impact energy [7]. Increase in effectiveness of porous materials in biomedical application focused attention of researchers towards this field. Foam structures of bioinert (materials which do not having any effect on or response to any biological activity), bioactive (materials which have effect on or draw response from any living tissue), biodegradable (can be broken down into simpler forms by biological process), bioresorable (materials which can be broken or absorbed by living body) or biocompatible (materials having no harmful or toxic effect on any part of living body) are extensively used in biomedical applications like tissue engineering, scaffolds, body implant etc.[8]. One of the main reason behind application of foam structures in implants could be facility of adjustment of mechanical properties of foam structure to compare with that of natural bone by controlling various parameters during foam manufacturing [8,9]. Porous materials found effective in biomedical applications for drug adsorption and delivery inside body of living being [10,11]. Application of biodegradable porous polyester based materials were suggested for bone substitution applications. But they have limitations in terms of strength and relatively low moduli making them unsuitable for aforesaid load bearing applications [12]. So it is important to develop new material for load bearing bone substitution application having similar mechanical properties as that of natural bone [8]. Foam structures of materials like Ti, Ni, SS316, Fe, Zn, Mg and Cu have been used for biomedical applications. In lightweight materials foam category Ti, Al and Mg are widely used. Al foam structures absorb appreciable impact energy under quasi-static loading and dynamic loading by large plastic deformations making it ideal candidate for energy absorbing and construction protecting applications like in sacrificial crash boxes, which are deformable energy absorbing devices used in automobile systems between chassis and bumper to reduce impact of accidents to passengers within velocity range of 3-5 m/s [7,13–17]. Al foams are best suitable to be used in non-flammable constructions for sound and thermal insulation, mechanical damping, acoustic absorption, sandwich cores, lightweight panels for building and transport designed to resist buckling and impact, vibration control, strain insulation [19,20,21]. Light Al foam sandwiched between denser outer Al sheets is important application of metallic foams [9]. Mg have various exceptional properties like high specific strength, high shock absorption capability, low density, good cast ability and good attributes for recycling makes it favourable option for structural applications [20]. Properties like vibration damping, sound and energy absorbing capacity makes Mg suitable for functional applications [21]. In current times, the growth rate of Mg is increasing at about 15pct per year [22]. Mg has exciting properties of biodegradation and bioresorption [23,24]. So there is large scope of use of Mg foam structures in bone substitute applications. This review mainly focused on selecting economic production method, optimizing parameters for selected manufacturing method with consideration of strength and density and characterization of foam structure of lightweight material for structural and biomedical applications. There is need to study resultant foam properties matching with that of natural bone. Al and Mg is considered as lightweight materials. Among these two aforesaid materials, Al is selected as target material because of easy and safe methods of Al foam manufacturing. Also there is chance of explosion in manufacturing of Mg foam. Research can be done in future in this field in developing optimum parameters for manufacturing of Mg foam structures. After introduction of foam structures concept with their properties and applications, main materials used in biomedical field are discussed in next section. Then production methods for foam structures are discussed. As this review is focused on Al so a whole section is dedicated to Al as foam material. In this section methods and processing parameters used in past literature are reviewed. Last section discusses the characterization methods used in past literature for characterization of foam structures.

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

20393

2. Materials used in biomedical field Bone is bioceramic composite of collagen, non-collagenous proteins, minerals, other organic matters and water with unique combination of fracture toughness and high strength due to it organic-inorganic structure [25–27]. Bone is identified as either cortical bone (hard outer layer with 10-30% porosity) or cancellous bone (internal spongy layer with 30-90 % porosity) [26]. Biomaterials are artificial materials used to replace part of any living system and they function with remaining in contact with living tissue [28]. Bone implants made of biomaterials are used to replace any part in living system. Bone implants can be of temporary type which degrades over time or it can be of permanent type. Later type of implants have advantages that it reduces number of surgical operations need to be done [29]. Scaffolds are temporary implants made of porous biodegradable material [26]. Properties of ideal porous metal used in scaffolds is discussed in previous literature [30]. Those properties involve biocompatibility [31], non-immunogenic; osteoconductive (adhesion and growth of bone cells around scaffold); osteoinductive (ability to heal bone) [26]; mechanical strength and properties like Young’s modulus (Young’s modulus of cortical bone is around 15 to 20 GPa and that of cancellous bone is around 0.1 to 2 GPa), compressive strength (compressive strength of cortical bone is around 100 to 200 MPa and that of cancellous bone is around 2 to 20 MPa) should be similar to natural bone; scaffold should allow to form blood vessels around it to support transport of nutrient, waste, oxygen [25]; porosity of scaffolds should be in the range of 250-300 µm as optimum value and should be at least greater than 100 µm for better diffusion of nutrients and oxygen for cell requirements [34,35]; scaffolds should contain both micro and macro porosities because it was recently concluded that those scaffolds perform better than scaffolds having only one type of porosity [33]. All solid biomaterials are divided into four groups: biometals, bioceramics, biopolymers and biocomposites [34]. Biomaterials from each group are discussed in subsequent paragraphs. Biometals are widely used among all other group of biomaterials. Because of high strength to weight ratio titanium is used more widely in biomedical implants. Titanium is used in various forms such as pure titanium, Ti6Al4V alloy, Ti50Ta alloys [38,39]. Most of alloys of titanium used in biomedical implants are β-stabilized alloys because they aid in bone growth since they have lower elastic modulus [36]. In earlier times vanadium stainless steel was used for biomedical implants before 1920s. Than various new alloys of stainless steel like SS304, SS316L, ultra-clean high nitrogen austenitic stainless steels were used according to required properties ad applications [37–40]. Cobalt alloys generally used for their corrosion resistance properties in biochemical environment include CoCrMo alloys and CoCrNiMo alloys in which F1058 (40Co20Cr15Ni7Mo) has been used for long time as permanent implant [39,41,44]. Various amalgams are used for dental fillings which includes silver tin amalgams, high copper amalgams (Cu6Sn5), silver and gold based alloy amalgams [37]. Rare earth magnets like NdFeB family are used because of their strong magnetic properties for specialized medical applications [37]. Rare earth magnets are used as dental keepers because the strong magnetisation helps to keep dental fixtures in place [37]. NiTi alloys because of their special properties like psuedoelasticity and shape memory effect with superior corrosion resistance, wear resistant, biocompatibility makes them best suitable for biomedical implants like synthetic bone plates, wire for the bone marrow cavity, dental implants, hip prostheses, blood filters, and endovascular prostheses [45,46,47]. Precious metals Pt, Au, Ag and their alloys often used in dentistry applications because of their good ductility, castability and corrosion resistance [40]. Tantalum used for X-ray marker and stents because of its low magnetic susceptibility and excellent X-ray visibility [40]. Biodegradable alloys found their potential application in biomedical applications. Among them Mg based alloys (including Ca-, Al-, RE- based alloys) [23,43–48] and Fe based alloys (including pure iron, Mn- based alloys) were proposed [11,49–51]. Biodegradable materials degrade inside body environment along with bone healing process. So main advantage of using biodegradable material in implant application is that it reduce surgery needed to remove implant material after bone healing process. Open porous structure allow body fluids transport and growth of new tissue as shown in Fig.1 [52]. Bioceramics have high corrosion resistance, low cytotoxicity and high biocompatibility so they are best suitable to be used in biomedical implants. Bioceramics can be categorized into bioinert (alumina, zirconia), biodegradable (tricalcium phosphate), bioactive (hydroxyapatite, bio-active glasses, glass ceramics) and porous for tissue ingrowth (hydroxyapatite coated metals, alumina) [53]. Amorphous phase alloys with non-ferrous system or Bulk Metallic

20394

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

Glass (BMG) found their importance in biomedical applications because of their excellent biocompatibility properties, high wear resistance, high strength and superior corrosion resistance [54]. Various types of BMG systems like Ti-based, Zr-based, Fe-based, Mg-based, Zn-based, Ca-based and Sr-based alloying systems are used as per requirements [6,60]. BMG foams fulfill the requirements related to physical and mechanical properties for biomedical applications [55]. Biopolymers are used in biomedical applications because of their close similitude to natural polymeric tissue components. Various polymers used in biomedical applications include polyamides (nylons), cellophane, polyethylene, polypropylene, cuprophane, polyacrylates, fluorocarbon polymers, rubbers (silicon, natural and synthetic) and high strength thermoplastics [28,56]. Biocomposites used in biomedical applications are dental filling composites, carbon fiber-reinforced methyl methacrylate bone cement, ultrahigh molecular weight polyethylene and porous materials orthopedic implants [28]. Porous structure assist growth of bone or other tissues through pores as shown in Fig. 1. Biomaterials used in various biomedical applications as shown in Table. 1 [40,57] 3. Production of foam structures Research on foam fabrication methods started in 1920s and is going on till today’s date, for fabrication of good quality foams [9]. Production methods can be classified according to state of material from which foam structures are made or initial state of materials for foam manufacturing [2,9,19]. Production methods of foams can be classified as production from liquid metal; solid metal powder; vapour or gaseous phase metal or metal ions. Among these classification production of foams from liquid metal and solid metal powder are the most described methods in previous literature. Methods from each classification are discussed in subsequent sections. Table 1. Application of various biomaterials at various body parts of human body [40,57] Devices

Material used

Bone plate, screw, pin, wires

316L SS, Ti, Ti6Al4V, Co-Cr alloys

Orthodontic wire filling

316L SS, CoCrMo, TiNi, TiMo, AgSn(Cu), amalgam, Au

Cranioplasty

Polyethylene, PMMA, Ti, HAP, HAP cements, TCP

Artificial joint in orthopedic

CoCrMo, Ti6Al4V, Ti6Al7Nb

Spinal fusion

Bioglass, Hydroxyapatite (HAP), Tricalcium Phosphate (TCP)

Artificial hip, knee, elbow, shoulder

316L SS, Ti, Ti6Al4V, Co-Cr alloys, High density Alumina, Polyethylene (UHMWPE)

Intramedullary nails

316L SS, Ti, Ti6Al4V, Co-Cr alloys, polysulphone-carbon fibre composites

Artificial eardrum

316L SS

Artificial valve in cardiovascular

Ti6Al4V

Stent

316L SS, CoCrMo, Ti

Alveolar & Mandibular bone replacements

Polytetrafluoroethylene (PYFE) – carbon composite, Porous Al2O3, Bioglass, HAP

Craniofacial plate and screw

316L SS, Ti, Ti6Al4V, CoCrMo

3.1 Foam manufacturing from liquid metal Foam can be manufactured from liquid metal by direct foaming or by indirect foaming with the help of casting around solid space holding fillers. Some methods of foam manufacturing are discussed in subsequent sections 3.1.1 Direct foaming by blowing agents This method is most explored in literature among all methods of foam manufacturing [3,6,13,14,58–62]. This method mainly used for foaming of Al and its alloys, though other metals can be foamed by this method. In this method, metal is melted and suitable viscosity enhancing material is added to increase viscosity of melt. Graphite or stainless steel stirrer can be used for uniform mixing. Then Blowing agents are added for blowing of gas for foaming. Metal hydrides, metal carbonates or metal oxides are generally used as blowing agents. Blowing agent

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

20395

with decomposition temperature matching with melting point of metal is generally chosen for better quality of resultant foam [16]. After uniform mixing of blowing agent with molten metal, temperature of mixture is increased. As soon as the temperature is reached to decomposition temperature of blowing agent, it starts to blow bubbles of gas which are trapped inside melt. After gas entrapment in liquid melt, it is suddenly cooled to avoid any effect of gravity during solidification. Shinko wire Co., Japan has patent of this method and it commercially manufacture Al foam under trade name ‘ALPORAS’[63]. Various parameters which can affect foam quality in direct foaming method are amount of viscosity enhancing agent, amount of blowing agent, stirring time, holding time after blowing agent addition, temperature at which viscosity enhancement agents and blowing agents are added, solidification method and time etc. Titanium hydride powder is generally used for fabrication of porous aluminum and other alloy and it is most popular process for melt processing routes and powder metallurgy routes [64]. By adding titanium hydride (1.6 %wt) at 680°C in aluminiun alloy melt, Decomposition of blowing agent will occurs at high temperature and it releases the hydrogen [64–68]. During decomposition of titanium hydride, chemical reaction as per equation (1) may occurs [64,69]. Dong-Hui Yang et. al. carried out the fabrication of Mg foam by use of calcium carbonate as a blowing agent [70,71]. Exothermic reaction as per equation (2) occurs within the temperature range of 575-750 ºC [71]. TiH2 = Ti + H2 (1) Mg + CaCO3 = MgO + CaO + CO (2) Zirconium hydride, Magnesium hydroxide and dolomite can also be used as a blowing agent [64,70]. Chromium nitride and strontium carbonate is also used as a foaming agent to manufacturing of porous steel [72]. Foam structures made by direct foaming are shown in Fig.2. Foams manufactured by direct foaming technique have superior energy absorbing capacity and have highly isotropic and homogeneous cellular structure [16]. So it can be concluded that direct foaming technique is excellent candidate for studying mechanical variability [17].

Fig. 1. Schematic diagram showing growth of bone into porous structure [9]

Fig. 2 Cross-section of an aluminium foam manufactured by direct foaming method [73]

3.1.2 Gas injection method

Foam can be manufactured by injecting pressurized gas in melt directly. Control of viscosity of melt in this process is critical. Viscosity can be controlled by varying temperature of melt or adding ceramics. Gas injection method is extensively used for foaming of Al and its alloys. This method is not suitable for foaming of materials which oxidize rapidly like Ti and Mg. Gases which can be used in this method are air, nitrogen, carbon dioxide, inert gases, oxygen and even water [10,20,83]. This method for foaming of Al and its alloys is being used by Cymat Al Co.(originally by ALCAN) in Canada and Hydro Al in Norway [63,74,75]. Advantages of this method includes large volume of foam can be produced continuously and complex shapes can be produced by selecting proper mould [9,76]. 3.1.3 Solid gas eutectic solidification Some liquid metal make a eutectic system in hydrogen gas. When material melt into hydrogen atmosphere under high pressure around 50 atm, homogeneous melt charged with hydrogen is obtained [63,65]. When liquid metal cooled by reducing temperature and pressure, the melt will be transformed into two phase region i.e. mixture of

20396

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

solid and gas [63]. Solidification will starts by removal of heat. This method can be used for nickel, copper, Al and Mg [2,77–80]. Main disadvantage of this method is poor homogeneity [80]. 3.1.4 Powder Compact melting technique (Isostatic pressing) This is powder metallurgical method in which metal powder is used as a material and foaming takes place in liquid state. Metal powders or alloy powders are mixed. Then powder is mixed with blowing agent such that blowing agent is homogeneously distributed. Compaction of powder metal is done by hot uniaxial isostatic pressing [81,82]. After that, temperature is increased up to melting point of metal matrix. Homogeneously distributed blowing agent start to decompose and it release the gas. Thus, uniform highly porous structure is obtained. The benefits of this method is many metals and alloy like Al and its alloy, tin, lead, brass, gold can be foamed. Generally pure Al and its alloys such as 2xxx and 6xxx, AlSi7Mg and AlSi12 are foamed by this method. Composites of foam and ceramics can also be foamed [2]. AlSi12 can be made by diffusion bonding to alumina plates at 500°C temperature and pressure of 100 kPa. Al 6061 foam has been bonded to alumina plates by allowing the foam to expand between layers of ceramics at particular distance [2]. 3.1.5 Investment casting method In this method, a mould template of polymer is fabricated and coating of wax slurry is applied on it. Then infiltration of molten metal is done. Porous structure is obtained after removal of mould. Al, Mg, AlSi7Mg and copper can be foamed by this method [83]. From this method, 80-97% porosity can be achieved. Main disadvantage of this method is higher manufacturing cost. 3.1.6 Spray forming In spray forming method, liquid metal is first atomized. Spray of fast flying atomized metal is created which is sprayed on substrate of required shape to obtain dense solid form of metal. If blowing agent powders are added in atomized spray, it results in release of gas which upon solidification becomes reason of porosity in resultant foam structure. Main advantages of spray forming method are low oxide content, fine grain size. This method can be used for steel and copper alloy. Porosities obtain from this method is around 60% s [2,84]. Main disadvantage of this method is less uniformity in porosity. 3.2 Foam manufacturing from metal powder In this method, metal powder remains in solid state during processing. Generally sintering and other solid state operations are used to fabricate metallic foams. 3.2.1 Sintering of metal powders and fibres This method includes powder manufacturing, compaction of moulding and sintering. For bronze material, sintering temperature is around 820ºC and porosities can be obtained around 20-50% [2]. Disadvantage of this method is very low strength of resultant foam structures. Al and its alloys contain oxide layer. So fabrication metallic foam of Al from powder or granules is difficult. It can be deformed during pressing operation and break the oxide layers which create metallic bonding between particles. 3.2.2 Space holder method In this method, metal powder is alloyed with materials like carbamide, ammonium bicarbonate, dolomite, titanium hydride etc. Pellets can be made by compaction of powder without using binder material. These pellets are heated upto melting temperature of space holder material [9,85]. During sintering process, blowing agents are removed leaving behind space holder particles from the pores. So, highly porous material can be fabricated. From this method, 90% porosity can be achieved with good strength [9,86]. Advantage of this method is that complex shape can be fabricated and it is economical. This method is generally used for fabrication of porous steel. 3.2.3 Foaming of slurries Slurry of metal powder is first made and blowing agents are added. Some additives can also added to improve flow properties of slurry. This slurry is poured into mould and heated at high temperature. At high temperature, slurry expands and releases the gas due to decomposition of blowing agents [2]. So, porous structure can be

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

20397

obtained. Al foam can be made by slurry of Al powder and orthophosphoric acid with aluminum hydroxide or hydrochloric acid as a blowing agent. Upto 7 pct relative density can be achieved by this method [2]. Open cellular metal can be made by this method. Disadvantages of this method include that, cracks may form in porous structure and poor strength is obtained. 3.2.4 Foaming by rapid prototyping technique Foaming by rapid prototyping includes 3D printing process for producing sacrificial template. This template can be used for metallic slurry to produce porous metal structure. Sacrificial template is made layer by layer with a binder. After one layer of binder, another layer of metallic/ceramic power is created by spray. In this way, entire part is completed by layers [9,87]. From this method, porous Mg can be generated. Open cellular Mg can be fabricated by this method [9,88]. Main disadvantage of this method is higher cost. 3.3 Electro deposition technique In this method, metal used is in form of ions in an electrolyte. The metal is galvanically deposited on polymer foam by electro-deposition technique [63]. Then it is removed by thermal treatments. Foamed polymer is replaced by metallic material. So, actual foaming is not occurring in solid state. Also polymer foam require some electrical conductivity for galvanic deposition initially [2]. From this method, Nickel-Chromium foams can be fabricated. Density of 0.4 to 0.65 g/cm3 can be achieved by this method, which is independent of average pore size [2]. 3.4 Vapor deposition technique In this method, gaseous metals are used. Polymer precursor is used to define geometry of metallic foam. Metal vapours are produced in vacuum chamber and allow to condense in precursor. Nickel foams, Nickel carbonyl foams can be fabricated by this method [2] Densities obtained 0.2 to 0.6 g/cm3 range. There are different methods for fabricating metallic porous material structures including metallic foam from liquid metal, powdered metal and metallic ions in electrolyte etc. Metallic foam can supposed to replace conventional parts with respect to its physical and mechanical properties and also to its economical factors by selecting suitable production methods [63]. 4. Aluminum as foaming material Al foams have attractive combinations of mechanical properties like high efficiency in absorbing energy of impact due to its high specific strength, lightweight and high specific stiffness [59]. Most commonly used Al alloy as foaming material as pure Al, 2xxx series and 6xxx series [72,89]. Al foams made from AlCu4 alloy (fractional porosity 83pct) showed stress amplitude 30 times higher than that of polyethylene (PE, fractional porosity 87pct) in stress-strain curve [72]. It has been shown that Al foams with addition of alloying elements such as Mg exhibited higher compressive strength than Al foams with pure Al (99.5 pct purity) [72]. Also because of good foaming properties and low melting points cast Al-Si alloys are used frequently in foaming process. Baumgartner and Gers studied comparison of expansion rates of AlSi12 and AlMgSi alloy foaming [90]. The expansion rate of AlSi12 is high because of very low viscosity of melt while that of AlMgSi expansion rate is low. But temperature range between pore initiation and foam collapse is high in expansion of AlMgSi as compared to that of AlSi12. So timetemperature control is not necessarily important in case of AlMgSi alloy. It can be conclude from this observation that foaming of AlMgSi alloy is easy as compared to foaming of AlSi12 alloy. So AlMgSi is the best choice for foaming in terms of ease of foaming as well as its superior mechanical properties and applications [90,91]. AlMgSi alloy is most explored and widely used alloy because of superior corrosion resistance and excellent mechanical properties. AlMgSi finds its applications in the field of aerospace, automotive (pistons, cylinder liners, bearings) and constructional applications [91]. Also strength of ultrafine-grained AlMgSi alloy can be enhanced by combination of cryogenic rolling with warm rolling [92] as shown in below Fig 3 and Fig 4 . Direct foaming by blowing agent method is best suitable on the basis of better control of process parameters for foaming of Al. Some concepts and process parameters used in past literature in direct foaming of Al and its alloys is reviewed here. Foaming with metal hydrides as blowing agent give better value of mechanical properties as compared to foaming with metal carbonates [6,93]. TiH2 has been used as blowing agent among metal hydrides in past literature and resulted in better quality foams. TiH2 powder starts to blow with release of hydrogen at 450⁰C, atmospheric pressure 1 bar and ends at 1668°C (melting point of Titanium) [90]. In direct foaming of Al by casting

20398

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

Fig. 3. Optical micrograph of closed cell Al foam [13]

Fig. 4. SEM micrograph of Al foam sample showing cell [7]

route technique with addition of Ca for viscosity enhancement and TiH2 as blowing agent, Ti and Ca remains inside foam and forms Al4Ca and TiAl3 precipitates [13] . Literature suggest the effect of pct TiH2 addition on expansion of AlSi12 foam that there is no effect of addition of TiH2 beyond 1pct on porosity or expansion of foam [16]. So in practical applications pct addition of TiH2 taken varies from 0.5-1pct. Maximum porosity achieved in literature with TiH2 as blowing agent in Al was 80 pct and for AlSi12 was 85pct. It was mentioned that porosity can be increased by adding reinforcement particles or by changing alloying compositions but not by increasing amount of foaming agent [16]. Foam expansion ratio is around 18.3 (assuming no loss) in Al foaming with 1.5 wt pct TiH2 addition [60]. But observed foam expansion ratio of only 6.3 found in literature [60]. 5. Characterization of foam structures Various techniques are used to characterize foam structures to get useful information of their microstructure, morphology, mechanical properties etc. Table 2 summarizes various methods used for microscopically characterization of foam structures in past. Application of some of the methods are described in this review. Table 2. Methods of characterization used for foam structures Method Optical microscopy Scanning Electron Microscopy (SEM) Transmission Electron Microscopy (TEM) X-Ray diffraction (XRD) X-ray micro tomography Selected area electron diffraction (SAED) analysis

Reference [2–6,13,38,39,62] [2,4,6,7,25,33,38,39] [2,25,35] [6,25] [2,29,33] [25]

Optical microscopy helps to analyse cell microstructure and morphology. Optical microscopy needs sample preparation which includes cutting, polishing (on emery paper and/or on polishing clothe) and etching with suitable etchant (depends on foam material) of samples. Pore size and pore distribution can be determined by optical microscopy [1]. Optical microscope image of one such foam is shown in Fig.4. Average pore diameter is calculated by considering the pore diameter at different locations on the transverse direction as well as on foaming direction using the formula L = 1.5/N, where N is the number of cells per unit length [59]. The cell aspect ratio is ratio of mean cell diameter in perpendicular direction to foaming direction to the mean cell diameter in the foaming direction [59]. Scanning Electron Microscopy is used to analyse surface and grain structures which could not be visible by optical microscopy directly with large depth of field. Potential variations on surface can be detected by SEM [94]. SEM micrograph of Al foam sample is shown in Fig.4. SEM is often considered as bridge between TEM and optical microscopy. Three dimensional micrograph at all magnifications can be obtained of rough samples because of high depth of focus and large working distance of SEM [95]. Electron Back Scatter Diffraction (EBSD) used to determine structure, crystal orientation and phase of material in SEM.

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

20399

Immense range of signals can be obtained which infer diffraction patterns, images and many different types of spectra from very small area of sample by Transmission Electron Microscopy [96]. Transmission Electron Microscopes come in different types: HRTEMs, HVEMs, IVEMs, STEMs, and AEMs. X-ray tomography used to determine three-dimensional density distribution in sample [1]. X-ray diffraction method used to determine crystalline structure of all components present in sample [6,25]. Various tests are performed on foams to determine mechanical properties like compressive strength, tensile strength, flexural strength, hardness etc. Tests done in particular research work depends on target application. Table 3 summarizes various test used in past. Almost all mechanical properties of foam structure were discussed in literature [19]. Those tests include tensile, compression, shear, multi-axial, fatigue, creep, hardness, surface strain mapping tests. Also non-destructive resonance frequency technique for tensile test and damping measurements has proposed in literature [72]. Table 3.Various mechanical tests used in literature Test Compression test Tensile test Hardness test (Vicker’s hardness) Flexural test Bending test Fatigue test Friction wear test

Reference [2,3,6,7,13,16–19,33,59,61,62] [2,5,19,36] [2,3,5,19,35] [97] [18,19] [41] [36]

As foam structures are mostly used for energy absorbing applications, compression test is most important and necessary test for foam structures. Also literature reflects this thing as can be seen in Table 3. Compression tests were carried out in literature with different dimensions of specimens as per material availability and required accuracy of results. Standards were followed in testing wherever required to do so. One of the main reason behind use of foam structures in structural application is because of its unique shape of compressive stress-strain curve. The stress-strain curve consist of three distinct reasons: linear elastic region, plateau region, densification region [7]. Some of the important things to be keep in mind during mechanical testing are noted here. Young’s modulus of Al foam was determined in literature and it was defined as unloading modulus in stress-strain curve of compression testing of Al foam [69,70]. Unloading modulus was considered as double that of loading modulus, whereas loading Young’s modulus was calculated from initial loading region of stress-strain curve of compression curve [59]. Mechanical properties of Al foam mainly depends on cell wall material and presence of gas/fluid inside cells [13– 16]. One of the main reason behind use of foam structures in structural application is because of its unique shape of compressive stress-strain curve. The stress-strain curve consist of three distinct reasons: linear elastic region, plateau region and densification region. Plateau stress was found as main parameter in literature for strength consideration which can be defined as mean stress value form 5pct - 30pct strain during compression [59]. Effect of strain rate on energy absorption and plastic strength in Al foam manufactured by direct foaming technique was studied in literature by uniaxial compression tests with sample size with cross section of 25 X 25 mm2 and height of 50 mm for strain rate sensitivity experiments [13]. 6. Conclusions Metallic foam structures are becoming interesting and important field of research in recent times. Metallic foams have wider scope in terms of structural, functional, biomedical and chemical applications. Applications of foam structures in biomedical is increasing due to their open cell structures and blood vessels as well as body fluids can pass through foam structure. Also artificial bone implants can be made from closed foam structure such that weight and strength of artificial bone implant made from foam material mimic the strength and weight exactly as of natural bone. Finally, developing suitable production method with optimal process parameters and suitable characterization methods for foaming of Al for biomedical application is the further field of research. Acknowledgement Authors acknowledge the financial support provided by Department of Science and Technology, Science and engineering research board (SERB), Government of India (Grant No. ECR/2016/001518).

20400

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

[34] [35]

J. Banhart, Foam Metal:The Recipe, Europhys. News. (1999) 17–20. J. Banhart, Manufacture , characterisation and application of cellular metals and metal foams, 46 (2001) 559–632. V.K. Jeenager, V. Pancholi, B.S.S. Daniel, The effect of aging on energy absorption capability of closed cell aluminum foam, Adv. Mater. Res. 585 (2012) 327–331. S. Ozan, M. Taskin, S. Kolukisa, Application of ANN in the prediction of the pore concentration of aluminum metal foams manufactured by powder metallurgy methods, Int J Adv Manuf Technol. 39 (2008) 251–256. W. Jiang, Z. Fan, D. Liao, A new shell casting process based on expendable pattern with vacuum and low-pressure casting for aluminum and magnesium alloys, Int J Adv Manuf Technol. 51 (2010) 25–34. R.E. Raj, B.S.S. Daniel, Aluminum Melt Foam Processing for Light-Weight Structures, Mater. Manuf. Process. 22 (2007) 525–530. D.K. Rajak, L. a. Kumaraswamidhas, S. Das, An Energy Absorption Behaviour of Foam Filled Structures, Procedia Mater. Sci. 5 (2014) 164–172. C.E. Wen, M. Mabuchi, Y. Yamada, K. Shimojima, Y. Chino, T. Asahina, Processing of biocompatible porous Ti and Mg, Scr. Mater. 45 (2001) 1147–1153. S. Singh, N. Bhatnagar, A survey of fabrication and application of metallic foams (1925–2017), J. Porous Mater. (2017). B. Bhargava, N.K. Reddy, G. Karthikeyan, R. Raju, S. Mishra, S. Singh, R. Waksman, R. Virmani, B. Somaraju, A novel paclitaxeleluting porous carbon-carbon nanoparticle coated, nonpolymeric cobalt-chromium stent: Evaluation in a porcine model, Catheter. Cardiovasc. Interv. 67 (2006) 698–702. H. Hermawan, H. Alamdari, D. Mantovani, D. Dubé, Iron–manganese: new class of metallic degradable biomaterials prepared by powder metallurgy, Powder Metall. 51 (2008) 38–45. D.W. Hutmacher, Scaffolds in tissue engineering bone and cartilage, 21 (2000) 2529–2543. A. Paul, U. Ramamurty, Strain rate sensitivity of a closed-cell aluminum foam, Mater. Sci. Engng A. 281 (2000) 1–7. S. Ramachandra, P. Sudheer Kumar, U. Ramamurty, Impact energy absorption in an Al foam at low velocities, Scr. Mater. 49 (2003) 741–745. J. Banhart, H. Berlin, Aluminium foams for lighter vehicles, Int. J. Vehicle Design 37 (2005) 114–125. J. Banhart, J. Baumeister, M. Weber, Powder Metallurgical Technology for the Production of Metallic Foams Experimental procedure Preparation of metal foams, 1995 (1995) 201–208. B.A. Fuganti, L. Lorenzi, Aluminium Foam for Automotive Applications, Adv. Eng. Mater. 2 (2000) 200–204. C. Yu, J. Banhart, Mechanical Properties of Metallic Foams, 37–48. M.F. Ashby, A.G. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams : A Design Guide, ButterworthHeinemann publications (2000). Y. Wang, D. Li, Y. Peng, X. Zeng, Numerical simulation of low pressure die casting of magnesium wheel, Int J Adv Manuf Technol 32 (2007) 257–264. G.A. Lara-rodriguez, I.A. Figueroa, M.A. Suarez, O. Novelo-peralta, I. Alfonso, Journal of Materials Processing Technology A replication-casting device for manufacturing open-cell Mg foams, J. Mater. Process. Tech. 243 (2017) 16–22. A.A. Luo, Magnesium : Current and Potential Automotive Applications, (2002) 42–48. F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, H. Windhagen, In vivo corrosion of four magnesium alloys and the associated bone response, Biomaterials. 26 (2005) 3557–3563. M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials : A review, 27 (2006) 1728–1734. M.J. Olszta, X. Cheng, S.S. Jee, R. Kumar, Y.Y. Kim, M.J. Kaufman, E.P. Douglas, L.B. Gower, Bone structure and formation: A new perspective, Mater. Sci. Eng. R Reports. 58 (2007) 77–116. S. Bose, M. Roy, A. Bandyopadhyay, Recent advances in bone tissue engineering scaffolds, Trends Biotechnol. 30 (2012) 546–554. Tadashi Kokubo, Bioceramics and their clinical application, Woodhead publishing limited Cambridge England (2008). R.S. Lazarus, C.G. Jung, Biomaterials An introduction, Plenum Press, New York (1992). K. Rezwan, Q.Z. Chen, J.J. Blaker, A.R. Boccaccini, Biodegradable and bioactive porous polymer/inorganic composite scaffolds for bone tissue engineering, Biomaterials. 27 (2006) 3413–3431. F. Matassi, A. Botti, L. Sirleo, C. Carulli, M. Innocenti, Porous metal for orthopedics implants, Clin. Cases Miner. Bone Metab. 10 (2013) 111–115. D.F. Williams, Biomaterials On the mechanisms of biocompatibility, Biomaterials 29 (2008) 2941–2953. J. Rouwkema, N.C. Rivron, C.A. van Blitterswijk, Vascularization in tissue engineering, Trends Biotechnol. 26 (2008) 434–441. J.R. Woodard, A.J. Hilldore, S.K. Lan, C.J. Park, A.W. Morgan, J.A.C. Eurell, S.G. Clark, M.B. Wheeler, R.D. Jamison, A.J. Wagoner Johnson, The mechanical properties and osteoconductivity of hydroxyapatite bone scaffolds with multi-scale porosity, Biomaterials. 28 (2007) 45–54. S. V Dorozhkin, Calcium orthophosphate bioceramics, Ceram. Int. (2015). E.A. Trillo, C. Ortiz, P. Dickerson, R. Villa, S.W. Stafford, L.E. Murr, Evaluation of mechanical and corrosion biocompatibility of TiTa alloys, J. Mater. Sci. Mater. Med. 12 (2001) 283–292.

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402 [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

[50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67]

20401

M. Niinomi, D. Kuroda, K.I. Fukunaga, M. Morinaga, Y. Kato, T. Yashiro, A. Suzuki, Corrosion wear fracture of new type biomedical titanium alloys, Mater. Sci. Eng. A. 263 (1999) 193–199. D.J. Blackwood, Biomaterials: Past Successes and Future Problems, Corros. Rev. 21 (2003) 97–124. J. Stewart, D.E. Williams, The initiation of pitting corrosion on austenitic stainless steel: on the role and importance of sulphide inclusions, Corros. Sci. 33 (1992). J. Pan, C. Karlén, C. Ulfvin, Electrochemical study of resistance to localized corrosion of stainless steels for biomaterial applications, J. Electrochem. Soc. 147 (2000) 1021–1025. H. Hermawan, D. Ramdan, J.R.P. Djuansjah, Metals for Biomedical Applications, in: Biomedical Engineering – From Theory to Applications (2011). F.X. Gil, J.M. Manero, J.A. Planell, Relevant aspects in the clinical applications of NiTi shape memory alloys, J. Mater. Sci. Mater. Med. 7 (1996) 403–406. C.M.Wayman, T W Duerig, K N Melton, D Stöckel, Engineering Aspects of Shape Memory Alloys, Butterworth-Heinemann publication (1990). B. Heublein, R. Rohde, V. Kaese, M. Niemeyer, W. Hartung, A. Haverich, Biocorrosion of magnesium alloys: a new principle in cardiovascular implant technology?, Heart. 89 (2003) 651–656. Y. Xin, C. Liu, X. Zhang, G. Tang, X. Tian, P.K. Chu, Corrosion behavior of biomedical AZ91 magnesium alloy in simulated body fluids, J. Mater. Res. 22 (2007) 2004–2011. P. Peeters, M. Bosiers, J. Verbist, K. Deloose, B. Heublein, Preliminary results after application of absorbable metal stents in patients with critical limb ischemia., J. Endovasc. Ther. 12 (2005) 1–5. A.C. Hänzi, A.S. Sologubenko, P.J. Uggowitzer, Design Strategy for Microalloyed Ultra-Ductile Magnesium Alloys for Medical Applications, Mater. Sci. Forum. 618–619 (2009) 75–82. E. Zhang, L. Yang, J. Xu, H. Chen, Microstructure, mechanical properties and bio-corrosion properties of Mg-Si(-Ca, Zn) alloy for biomedical application, Acta Biomater. 6 (2010) 1756–1762. Z. Li, X. Gu, S. Lou, Y. Zheng, The development of binary Mg-Ca alloys for use as biodegradable materials within bone, Biomaterials. 29 (2008) 1329–1344. M. Peuster, P. Wohlsein, M. Brügmann, M. Ehlerding, K. Seidler, C. Fink, H. Brauer, a Fischer, G. Hausdorf, A novel approach to temporary stenting: degradable cardiovascular stents produced from corrodible metal-results 6-18 months after implantation into New Zealand white rabbits., Heart. 86 (2001) 563–569. M. Peuster, C. Hesse, T. Schloo, C. Fink, P. Beerbaum, C. von Schnakenburg, Long-term biocompatibility of a corrodible peripheral iron stent in the porcine descending aorta, Biomaterials. 27 (2006) 4955–4962. M. Schinhammer, A.C. Hänzi, J.F. Löffler, P.J. Uggowitzer, Design strategy for biodegradable Fe-based alloys for medical applications, Acta Biomater. 6 (2010) 1705–1713. A. Nouri, P.D. Hodgson, C. Wen, Biomimetic Porous Titanium Scaffolds for Orthopedic and Dental Applications, Biomimetics learning from nature (2010) 415–450. L.L. Hench, Bioceramics : From Concept to Clinic, J. Am. Ceram. Soc. 74 (1991) 1487-1510. M.Z. Ibrahim, A.A.D. Sarhan, F. Yusuf, M. Hamdi, Biomedical materials and techniques to improve the tribological, mechanical and biomedical properties of orthopedic implants – A review article, J. Alloys Compd. 714 (2017) 636–667. H.F. Li, Y.F. Zheng, Recent advances in bulk metallic glasses for biomedical applications, Acta Biomater. 36 (2016) 1–20. M. Niinomi, Recent metallic materials for biomedical applications, Metall. Mater. Trans. A. 33 (2002) 477–486. S. Velayudhan, Processing and evaluation of hydroxyapatite-ethylene vinyl acetate copolymer composites for cranioplasty, (2004). R.E. Raj, B.S.S. Daniel, Prediction of compressive properties of closed-cell aluminum foam using artificial neural network, 43 (2008) 767–773. R.E. Raj, B.S.S. Daniel, Customization of closed-cell aluminum foam properties using design of experiments, Mater. Sci. Eng. A. 528 (2011) 2067–2075. S.N. Sahu, A.A. Gokhale, A. Mehra, Prediction of Bubble Size Distribution in Aluminium Foam as a Function of % Titanium Hydride Addition, Trans. Indian Inst. Met. (2016). U. Ramamurty, A. Paul, Variability in mechanical properties of a metal foam, Acta Materialia 52 (2004) 869–876. J. Meyer, On the mechanical performance cell al alloy foams, Acta Materialia 45 (1997) 5245–5259. J. Banhart, J. Baumeister, Production methods for metallic foams, Mat. Res. Soc. Symp. Proc 521 (1998) 121–132. Svyatoslav, V. Gnyloskurenko, T. Koizumi, K. Kita, T. Nakamura, Aluminum Metallic Foams Made by Carbonate Foaming Agents, 12 (2013) 5–12. D.S. Schwartz, D. S. Shih, A.G. Evans, H. N. G. Wadley, Porous and Cellular Materials for Structural Applications, Material reseacrh society (1998). M. Mukherjee, F. García-Moreno, C. Jim_enez, A. Rack, J. Banhart, Microporosity in aluminium foams, Acta Mater. 131 (2017) 156– 168. A. Gokhale, S. Sahu, V. Kulkarni, B. Sudhakar, N. Rao, Effect of Titanium Hydride Powder Characteristics and Aluminium Alloy Composition on Foaming, (2006) 247–256.

20402 [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97]

Parthkumar Patel et al./ Materials Today: Proceedings 5 (2018) 20391–20402 H. Yu, Z. Guo, B. Li, G. Yao, H. Luo, Y. Liu, Research into the effect of cell diameter of aluminum foam on its compressive and energy absorption properties, Mater. Sci. Eng. 455 (2007) 542–546. V. Duz, M. Matviychuk, A. Klevtsov, V. Moxson, Industrial application of titanium hydride powder, Met. Powder Rep. 0 (2016) 1–9. N.V. Pulagara, S. Saini, R.S. Dondapati, A Study of Manufacturing and Mechanical Properties of Mg-foam Using Dolomite as the Blowing Agent : A Review, 4 (2015) 7–10. D. Yang, B. Hur, S. Yang, Study on fabrication and foaming mechanism of Mg foam using CaCO 3 as blowing agent, 461 (2008) 221– 227. C. Yu, H.H. Eifert, Metal foaming by a powder metallurgy method : Production , properties and applications, (1998) 181–188. L. Ma, Z. Song, Cellular structure control of aluminium foams during foaming process of aluminium melt, Scr. Mater. 39 (1998) 1523– 1528. Iljoon Jin, Lorne D. Kenny, Harry Sang, Method of producing lightweight foam metal, United States Patent 4973358 (1990). Lorne D. Kenny, Martin Thomas, Process for shape casting of particle stabilized metal foam, United states patent 5281251 (1994). N. Babcsán, J. Banhart, D. Leitlmeier, Metal foams – manufacture and physics of foaming. J.M. Apprill, D.R. Poirier, M.C. Maguire, T.C. Gutsch, GASAR Porous Metals Process Control, MRS Proc. 521 (2011) 291. C.J. Paradies, A. Tobin, J. Wolla, D.S. Schwartz, D.S. Shih, A.G. Evans, H.N.G. Wadley, The Effect of Gasar Processing Parameters on Porosity and Properties in Aluminium Alloy, 1 (1998) 297–302. C. Park, S.R. Nutt, Metallograpiuc study of gasar porous magnesium, Mat. Res. Soc. Symp. Proc., 521 (1998) 315–320. Z. Li, T. Yang, Q. Jin, Y. Jiang, R. Zhou, Influence of structural parameters on tensile property of gasar porous copper, Rare Met. Mater. Eng. 43 (2014) 2609–2613. J.E. Pashak, Cellularized light metal, United states patent 2935396 (1960). B.C. Allen, M.W. Mote, Method of Making Foamed Metal, United states patent 3087807, (1963). Y. Yamada, K. Shimojima, Y. Sakaguchi, M. Mabuchi, M. Nakamura, T. Asahina, T. Mukai, H. Kanahashi, K. Higashi, Processing of cellular magnesium materials, Adv Eng Mater. 2 (2000) 184–187. P. Kelley, Method of producing closed spherical porosity in spray formed metals, United States Patent 5266099 (1993). A. Kennedy, Porous Metals and Metal Foams Made from Powders, Powder metullargy (2016) 31-46. I.H. Oh, N. Nomura, N. Masahashi, S. Hanada, Mechanical properties of porous titanium compacts prepared by powder sintering, Scr. Mater. 49 (2003) 1197–1202. J.P. Li, J.R. De Wijn, C.A. Van Blitterswijk, K. De Groot, Porous Ti6Al4V scaffold directly fabricating by rapid prototyping: Preparation and in vitro experiment, Biomaterials. 27 (2006) 1223–1235. M.P. Staiger, I. Kolbeinsson, N.T. Kirkland, T. Nguyen, G. Dias, T.B.F. Woodfield, Synthesis of topologically-ordered open-cell porous magnesium, Mater. Lett. 64 (2010) 2572–2574. I. Duarte, M. Oliveira, Aluminium Alloy Foams : Production and Properties, Powder Metullargy, (2003) 47-72. F. Baumgartner, H. Gers, Industrialisation of P / M foaming process, (1999) 73–78. G. B.V. Kumar, C.S.P. Rao, N. Selvaraj, M.S. Bhagyashekar, Studies on Al6061-SiC and Al7075- Al2O3 Metal Matrix Composites, Journal of Minerals and Materials Characterization and Engineering 9 (2010) 43-55. U. G. Kang, H. J. Lee, W.J. Nam, The achievement of high strength in an Al 6061 alloy by the application of cryogenic and warm rolling, J Mater Sci 47 (2012) 7883–7887. D. P. Papadopoulos, H. Omar, F. Stergioudi, S. A. Tsipas, H. Lefakis, N. Michailidis, A novel method for producing Al-foams and evaluation of their compression behavior, Journal of Porous Materials 17 (2010) 773–777. C.W. Oatley, W. c. Nixon, R.F.W. Pease, Scanning Electron Microscopy, Adv. Electron. Electron Phys. 21 (1965) 181–247. P.F. Kane, G.B. Larrabee, Characterization of Solid Surfaces, Plenum press, Texas (1920) D.B. Williams, C.B. Carter, The Transmission Electron Microscope, Transm. Electron Microsc. (1996) 3–17. C.O. Clerc, M.R. Jedwab, D.W. Mayer, P.J. Thompson, J.S. Stinson, Assessment of wrought ASTM F1058 cobalt alloy properties for permanent surgical implants, J. Biomed. Mater. Res. 38 (1997) 229–234.