mg and its alloys, their challenges and opportunities for ...non-degradable implant material is used...

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056

Volume: 07 Issue: 09 | Sep 2020 www.irjet.net p-ISSN: 2395-0072

© 2020, IRJET | Impact Factor value: 7.529 | ISO 9001:2008 Certified Journal | Page 2763

Mg and its Alloys, their Challenges and Opportunities for Implants: A

Review

Rohit1, Munish Gupta2, Puneet Katyal3, Vijender Gill4

1Student, Department of Mechanical Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India

2Professor, Department of Mechanical Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India

3Professor, Department of Mechanical Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India

4Scholar, Department of Mechanical Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India

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Abstract: Magnesium (Mg) and its alloys are lightweight metals with low density and elastic modulus, and having good mechanical properties and good biocompatibility. With these beneficial properties there are several challenges in magnesium alloy implants. Corrosion rate or degradation rate of mg and its alloys is fast due to which they mislays their mechanical integrity and fail to perform their function prior to complete healing of bone. There are many opportunities, to improve the corrosion resistance, mechanical strength, ductility etc., like alloying, mechanical working, surface modification, machining etc. Key words: Magnesium Alloys, Biocompatibility, Hydrogen Evolution, Corrosion, Ductility, Machining, Mechanical Working, Surface Treatment and Coating

Introduction: Magnesium and its alloys are the degradable biomaterials having mechanical properties same as the human bones. Mg is a non-toxic tenor and in an environment of the human body, it degrades completely [1]–[3]. Degradable biomaterial for an implant should have good biocompatibility, sufficient mechanical strength, high corrosion resistance, high wear resistance and degradation rate should be matched with bone healing rate [4]. For many enzymes, magnesium is a cofactor and it is also stabilized the structures of RNA and DNA. In the human body, magnesium is the fourth most abundant element essential for human metabolism [5]–[8]. Machining of magnesium and its alloys is simple due to their lower density, higher specific strength and elastic modulus nearly similar to bone [9]–[11]. Mg and its alloys eliminating the stress shielding problem due to their low modulus of elasticity, it’s about 45GPa that is approximately close to the elastic modulus of natural bone (10-30GPa) [12]. Biomaterials play a crucial role to repair or replace the damaged or diseased bone tissue of human being and animals, and also improve life quality [13], [14]. Implant materials are of two types, degradable and non-degradable implant material. Non-degradable implant material is used for permanent implant. Such similar problems occurs in permanent implants are inflammatory reactions, physical irritation, impact of stress shielding due to larger modulus of elasticity or young’s modulus and material issues like corrosion, wear and bacterial formation [15]–[17]. Now a day, magnesium and magnesium alloys are highly used as a biodegradable implant due to their good properties, stimulated bone growth, good bio-resorption and non-inflammatory reactions [18], [19]. An eligible biodegradable implant is that in which corrosion rate is matched with the bone healing rate, properties of implant material are enough to supply the expected endorsement during the period of bone healing, when the objective bone is healed completely then implanted material completely degrades in the body [20].

Challenges in Mg-alloy implants: For the implantation of mg in the human and animal body many types of biodegradable mg alloys are developed, but approximately all suffering with fast degradation rate and mechanical support is also relatively insufficient [7]. Mg alloys show poor workability and low ductility at room temperature due to its hexagonal closed pack crystal structure. These properties make challenges for stent applications. The large challenges for magnesium and their alloy implantations are higher rate of degradation, rapid rate of corrosion, hydrogen gas evolution, poor mechanical integrity, low ductility, alloying elements toxicity and biocompatibility of the implants.



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Rapid corrosion:

The rapid corrosion of mg and mg alloys has been prior limits to use the materials for an application range in which there is exposure to a corrosive environment [21]. Form aluminium, zinc and other elements, Mg are most active element as it easily loses their electrons. In an aqueous environment mg and its alloys rapidly corrode. Environment of human body act as aqueous environment. In the mechanism of corrosion such reactions happen when the Mg is exposing in ionic media or in aqueous environment [22]–[24].

Mg→ Mg2++ 2e- − (1)

2H2O+ 2e- → H2+ 2OH- − (2)

Mg (s) +2H2O (l) →Mg (OH)2 (s) +H2 (g) − (3)

According to 2nd equation, the pH value of the environment increases around mg surface [1]. As per pH value of the environment, if the pH value is greater than 11.5 than Mg (OH)2 makes an oxide film on the surface of mg alloy implants which is reacting as a protective layer on the implant and reduces the corrosion [25]. And if the pH value is less than 11.5 than mg alloys easily degrades in the aqueous solution [26]. Normally the pH value is approx. 7.4 at the interface of bone and implant, the surface of an implant cannot cover by the Mg (OH)2 film [27].

According to 3rd equation, there is magnesium hydroxide {Mg (OH)2} and hydrogen gas (H2) produced due to mg corrosion [28]. These reactions take place when Mg(s) and Mg (OH)2(s) layer react with chlorine ions in the aqueous solution

Mg(s) + 2Cl-(aq.) →MgCl2 + 2e- ..... (4)

Mg (OH)2 (s) + 2Cl-(aq.) → MgCl2+ 2OH- ..... (5)

Because of exposure to the large chloride environment and corrosion accelerates in physiological concentrations, MgCl2 produced when Mg (OH)2 reacts chloride ions which is highly soluble [7], [20], [25]. Mechanical stability impacts by the rapid corrosion of mg implants. A crucial point about mg alloys is that rate of corrosion decreases as per time [20]. In mg and its alloys the most probable corrosion mechanism happens are pitting and localized corrosion.

As compares by magnesium, Biodegradable polymer materials such as poly lactic acid (PLLA) displays low corrosion rate in the beginning but material degrades quickly in final phase and becomes mechanically feeble [108]. As per displayed in figure 1, Magnesium screw displays better Osseo integration and higher density of mineral as comparison to PLLA and after operation it stay undamaged with bones until 24 weeks, while after operation PLLA screw damaged in 16 weeks.

Biocompatibility:

Implants of mg alloy degrades rapidly due to which OH-, Mg2+ and alloying element ions are exposed with fast concentration that oppositely effects the biocompatibility or viability of the cell. Interaction of biomaterial or cell is determined by the cell culture method [29]. As per ISO10993-5:2009, if the viability of the cell decreases greater than 30% than it is counted as cytotoxic effect [30]. If the alloying elements are added in uncontrollable concentration or out of adequate limit than it causes toxicity and disturb the ordinary molecular functions of proteins, DNA and enzymes [12]. So, it’s essential to obtain the requirement of implantation materials which should be non-toxic in the environment of the human body [2]. Different affects causes in corrosion response due to the existence of extra amount of mg. In the formation of bone mg plays an important role. The interfacing power of implants increased as a result of the existence of mg in Hap (hydroxy-apatite) along with aluminium oxide (Al2O3) [31], [32]. Mg is contemplated as non-toxic and biocompatible [33]–[38].

The bulk of bone mineral make up by the biological apatite which is an important factor in the formation of new bone and in the formation of biological apatite magnesium plays an essential role [39], [40]. Mg is also known for its positive effect on the bone strength and gentleness [36], [41], [42]. For the purpose of implantation, mg must have good mechanical properties, corrosion resistance and should be non-toxic. Due to these conditions less number of compatible elements remains which gives corrosion or mechanical profits when alloyed with magnesium. Two elements may be the most biocompatible are Zinc (Zn) and calcium (Ca) [43].




Hydrogen gas evolution: In vivo mg alloy corrodes/degrades quickly under implantation due to which production of hydrogen gas in high volume. This makes crucial risks for patients [2]. The process of bone healing will delay due to the generation of hydrogen gas in high volume between the bone and implant interface [44], [45] as displayed in figure 2 after implantation at 4weeks and 8 weeks. Punchering process is used for removing the bubbles of hydrogen gas [19], [46]. Hydrogen gas evolution growths with a rise in anodic polarisation of magnesium and hydrogen embitterment due to stress caused by the pressure of hydrogen, lead to brittle fracture of the implant. For every 1mg of magnesium, hydrogen gas evolution rate of 1mL is complex to be exposed from sites of corrosion and also creates toxic effect in the tissue. The pH in the surrounding tissue increases by the existence of hydroxyl groups and due to this existence inhibits cell proliferation and formation of tissue [14].

Different issues cause by the expose of hydrogen gas and formation of forthcoming cystis due to implantation of magnesium (mg). Tissues or tissue layers severances due to the formation of bubbles of hydrogen gas after implanting [47], [48]. Necrosis of surrounding tissues causes delay of healing of bone at the place of surgery caused by hydrogen gas bubbles [49]. In some bad cases, blood stream blocks by the bubbles of hydrogen gas, cause death [24]. Due to rapid degradation of implant, H2 produces and collected where it cannot diffuse by the soft tissues of surrounding at an enough rate [51].

Figure 1: Scanning electron microscopy (SEM) morphology of magnesium (Mg) screw in (a) 4weeks, (b) 24 weeks after operation [20].

Poor mechanical integrity:

For applications of orthopaedic, the mechanical integrity of biomedical implant is also an important component as well as corrosion rate or degradation rate. Although the materials of implant are permitted to decompose, the implanted material still wants to obtain the expected power until the complete healing of bone [52]. With rapid decrease of bending strength the mechanical integrity of the implanted material go down by the degradation of magnesium alloys [53], [54]. Anti-thrombosis and flexibility is of prior essential in the case of stent application/coronary application. In real practices, that type of stents are released to shear stress caused by blood fluid whose have an important effect on the process of degradation of absorbable metallic stents [55], [56].

Bone implant is come below to the static corrosion displays high accumulation as a comparison to the dynamic corrosion release by stents [57]. The entire rate of corrosion (inclusive pitting, erosion, uniform and localised corrosion) is accelerated in metallic stents due to increase in shear stress. The scanning electron microscopy (SEM) picture of degraded AZ31 stent after 7 days is displayed in figure 3. Due to the impact of both mechanical stress and corrosion of surrounding

High purity

magnesium screw

Poly lactic

acid screw




tissue, magnesium based implantations proved high sensitivity to stress corrosion cracking (SCC) in an ionic environment [58], [59], so, high possibility of the collapse of that type of implants.

Poor ductility:

Low ductility is the biggest obstacle in that area where use of magnesium alloys in wide applications. Intrinsically magnesium alloys are brittle in nature and having hexagonal closed packed crystal structure due to which they have low ductility [60]. Occupationally existed alloys of magnesium like WE43, AZ91 and AZ31 having finite ductility as compared with Cr-Co and stainless steel alloys which are usually used in coronary applications. At room temperature, the critical resolved shear stress on the basic slip order of solo-crystal magnesium is nearly 1/100 of other slip orders [61]–[63]. Twins usually exist in magnesium alloys to adapt critical resolute stresses due to hexagonal closed packed (HCP) lattice [64]. At temperature of room, magnesium alloys display finite ductility and low/poor workability due to its hexagonal closed packed crystal structure that forms it challenging for stent applications. The ductility of the implant material is very essential quality of stent applications that could be developed by using mechanical treatments such as cyclical bending, extrusion, hot rolling and use of renovated implantation designs and geometries.

Opportunities for improvement of the efficiency of biodegradable magnesium alloy implants: Improvement of the functionality of magnesium alloys based bio-degradable implantations can be possible via coating, surface treatment, optimizing machining process, alloying, mechanical working, curb composition & metallurgy of the magnesium alloy. Optimized geometry and design of the implant control the rate of degradation through appropriate alloying and surface modifications. The ductility of the implant material is crucial qualities for stent implantation, which can be improved by mechanical treatment such as hot rolling, extrusion, cyclic bending and using optimized design & geometry of the implant.

Surface treatment and coating:

To preserve mechanical support for long time, surface treatment is an influential method to curb the corrosion rate of magnesium alloy [65]. Additional, issues such as hydric bubble aggregation, osteolysis, soon rapid loss of mechanical property and the formation of gap among the tissue and the implant can be overcome by using the surface treatment method. For the improvement of the mechanical stability and cyto-compatibility of magnesium alloy surface treatment is an imposing way to govern surface properties [66].

Implant integration, cell morphology and cell growth can be affected by surface roughness, it is cleared by the study of permanent implant materials such as Ti- alloy and stainless steel. Various methods can be used to achieve surface treatment like self-assembled multi-monolayer coating, chemical conversion coating [67]–[69], Ca-P or HA coating [70], alkaline heat treatment, micro-arc, oxidation and polymeric coating [3], [30], [65]. Reduction of the rate of degradation in the ionic environment is the supreme goal of surface treatment of the magnesium alloy. In the field of biomaterials, coatings or surface treatments are firstly applied to substrates to renovate the biocompatibility or viability of the basic material. In spite of, in the growth of magnesium like a biomaterial some modifications of surface have been calibrated firstly to develop the corrosion resistance of the substrates, either it be mg alloys or pure mg [71]. These corrections can be classified as physical, chemical or admixture of both. Chemical modifications of surface comprise, alkaline treatment,

Figure 2: Hydrogen bubble at the interface of

bone and implant after implantation in (a) 4

weeks (b) 8 weeks [50].

Figure 3: Picture of scanning electron

microscopy displaying the corrosion products

peeled off from AZ31 stent [55].




anodization, acid etching, fluoride treatment and implantation of ion, all these are allied with the substitution of the basically forming and not especially corrosion resistant oxide layer on the surface of the magnesium substrates [72].

Several techniques are utilised in physical surface modification or deposition coating to form vindicatory coatings on the magnesium substrates which can be inorganic, organic, or metallic in nature and commonly provides a physical obstacle between the corrosive environment and the metal [71], [73], [74]. Often, admixture of both physical and chemical surface modification technics are calibrated, with the opening chemical pre-treatment used-up for development of the adhesion of the secondary physical coatings [72].

Possibly, the use of CaP (calcium phosphate) coatings with pre- treatment steps or without pre-treatment steps is the much commonly researched surface treatment technique in the zone of magnesium biomaterials [71], [75]. And also solid evidences that calcium phosphate (CaP) coatings can amplify the corrosion resistance of mg based materials [76]–[80]. In spite of, it had reported that the insufficient control of phase manufacturing, cracking, poor compliance stay problems inside the field [71]. In spite of these problems with calcium phosphate coatings, this system of working, and many other surface modifications mentioned in brief display obligation as a medium to curb the rate of corrosion of magnesium and its alloys for the application of orthopaedic biomaterials.

Suitable preference of size of pore can consequence in considerably improved unification of the implantation with natural tissue. Figure 4 displays an instance of a mg material with a porous microstructure suitable for application of orthopaedic [7].

Alloying: Tenor alloying performs the most crucial role to manipulate the resistance to corrosion and mechanical properties of bio-medical Mg alloys [1], [30]. To curb the properties of corrosion and for the usage of implantation, many of the alloying tenors should be used in magnesium based alloys. The mechanical and physical properties of the alloys can sufficiently improve by using the alloying tenors like Ca, Mn, Zn, RE, Al, Li, Y and Zr in magnesium alloys by – a) improve the corrosion resistance; b) Refine the grain structure; c) make inter-metallic forms which can improve the strength; and d) helps in shaping and production of magnesium alloys [81]. In spite of, it’s inspected that many these types of compositions were fundamentally modelled for other uses where viability was not a prime design parameter [2], [82]. It is reasonable to utilise those alloying tenors which exists in the body of human being or have display advantageously effects on regeneration of tissues and healing, tenors like zinc (Zn), calcium (Ca), zirconium (Zr) and strontium (Sr) in order to minimize averse tissue reactions or cytotoxicity from implantation materials [81].

Corrosion properties, ductility and strength of the substance influence by the conspicuous composition of alloys [83], [84]. Improvements in the corrosion properties and strength of the mg alloys are mainly associated with modification in micro structural features, special reduction happens in the grain size when compared with pure Mg [85], [86]. Zn, Mn and Ca are important for the human body; all these tenors should be the primary preference as alloying tenor for biomedical magnesium alloys. It was calibrated that magnesium alloy expressed good mechanical property and corrosion resistance when it maked alloying with 1.5wt% of manganese (Mn). In the case of magnesium alloys, Zn and Ca should not be adding higher than 6wt% and 2wt% [87]. Some of the essential alloying tenors are discussing which are helping to develop the performance of magnesium alloys for the applications of biodegradable implantations.

Zinc: Zinc (Zn) is an important tenor in the body of human being and it exists in all tissues of the human body. Corrosion resistance and mechanical properties should be improved by the addition of Zn (Zinc) in Mg (magnesium) [30], [88]. The major crucial impact of Zn is the deficiency of hydrogen gas evolution [2], [45]. In binary magnesium alloys, adding of Zn up to 3% detract the grain size and amplifying the mechanical properties, yield strength ranging between 20-130 MPa, young’s modulus ranging between 41-45 GPa and ultimate tensile strength ranging between 90-230 MPa [1]. It is manifest that Mg-3Zn-1Ca is best appropriate alloy for use in biomedical field. Bio-corrosion resistance, elongation and tensile strength, improve by adding 1.6% Zn into Mg -0.6 Si [89]. Exorbitant quantities of Zn beyond a definite range, if ingested, have the potential range, if ingested; have the potential to be corrosion in nature [90]. Above 3wt% of Zn, considerably decreases strength remaining same. So, for magnesium alloys concentration of Zn below 3% is recommended.

Zirconium: Zirconium (Zr) is principally added to magnesium alloys because of its highly impressive grain refining capability [91], [92]. Zr amplifies the alloys strength and the precipitation of united Zr-Fe particles, improves the corrosion




resistance prior to the casting of an alloy [21], [93]. Zr is antecedently used in a broad range in the medical field for the purpose of implantations including dentistry orthopaedic implants and it is broadly acknowledged as biocompatible [94], [95]. Zr displays good Osseo-integration and biocompatibility in both in vivo and in vitro [30], [96]. Zr is very properly used in ternary magnesium alloys. In Mg adding of 1wt% of Zr resulted in the considerable reformation in ductility and mechanical strength of an alloy and the rate of degradation detracted by 50% [97]. The conspicuous damping capacity of magnesium alloys can be help in the absorption of stresses& vibrations at the interface of bone implant and which can be improved with the addition of Zr. Zr is considered as a micro-alloying content of which is lower than 0.8wt% for the biomedical magnesium alloys [95]. So, Zr can be used as a grain refiner and also for the improvement of corrosion and mechanical properties, since indubitable toxicity is not found.

Calcium:

Calcium (Ca) is the very abundant element in the human body and specially stored in the teeth and boners of humans. The limitation of solubility of calcium is 1.34wt% in magnesium. With Mg, calcium can be used as a ternary or binary alloying tenor [30], [98]. Mechanical properties, microstructure, degradation kinetics and electrochemical behaviour of Mg-Ca alloy can be affected by the addition of calcium [63]. Magnesium with 0.6 to 0.7% Ca displays best outcome in terms of mechanical properties, growth of bones and corrosion resistance. By enlarging the calcium material, coarse and thicker Mg2Ca phase sediments along the boundaries of grain, thereby weakens the corrosion resistance and mechanical property of Mg-Ca alloys. Exorbitant quantity of Ca becomes a reason of stone in kidney [2]. Calcium has the capability to detract the grain size of the alloys, so increasing the mechanical properties when compared with the pure Mg [28]. So, concentration of Ca is below 1% is recommended for the magnesium alloys.

Figure 4: Scanning electron microscopy (SEM) view of mg material with porous microstructure created using space holding particles [7].

Strontium: Strontium (Sr) parts identical biological, metallurgical and chemical properties with Mg. Nearly 99% of the strontium is found in the bones of the human body. Sr helps to encourage the formation of bone and it also help to the medication of osteoporotic patients to enhance the mass of bone & detract the phenomenon of fractures [99], [100]. By adding Sr, the performance of the magnesium alloys can be improved by enhancing mechanical strength and corrosion resistance [101]. By adding the Sr in magnesium alloys, grain size can be refined and it can also help to display the corrosion. Increasing of strontium materials more than 2% decreases the corrosion resistance of the magnesium-strontium alloys [102]. So, for the magnesium based biocompatible implants below than 1wt% of Sr is recommended.




Lithium: Magnesium (Mg)-lithium (Li) based alloys are mainly used for the coronary applications because they provides better ductility to accomplish necessity of the radially extendable stents. By adding lithium in magnesium alloy, the axis ratio (c/a) of the lattice detracted, a prismatic slip easily activates and improves the deformability [103], [104]. So, mechanical working such as extrusion, rolling is broadly applied to magnesium, lithium alloys for amplifying the microstructure of as- casting alloys [105]. The existence of Lithium in magnesium enhances the PH and stabilizes the hydroxide film on the corroded surface [106]. A methodical study on biocompatibility, mechanical property and bio corrosion behaviour of Mg-Li premised alloys are still to be detected in the applications of cardiovascular stents. Thereby, this is the high eagerness to design and generate magnesium-lithium premised alloys for the vascular implantations.

Silicon: Silicon (Si) is believable crucial for human being and mainly the SiO2 powder is responsible for their toxicity [107]. The micro-alloying with silicon maybe also a better preference because the strength of Mg alloy can be improved by alloying with Silicon. In spite of, by the addition of even 1wt% of Si can impact on the poor corrosion resistance [97].

Rare earth elements: In the periodic table the rare earth elements (REEs) are counted 15 in digits and these are lies between lanthanum (La) and lutetium (Lu), also scandium (Sc) and yttrium (Y) are included in rare earth elements [109]. Rare earth elements are very crucial for amplifying the resistance to corrosion and mechanical properties of the magnesium alloys [110]. Basically, these elements are added in Mg alloys as master alloys/hardeners which are used to increase the ductility, creep resistance, strength of the metal [111], [112]. Precipitation hardening and solid solution both are the ways by which REEs can enhance the strength like aluminium, and in high chloride environments the corrosion resistance can be improved due to the creation of an oxide rich passivation layer [113], [114]. In spite of, the problem that rises with the usage of the rare earth elements as a portion of Mg alloys for uses in the biomedical field is its comparatively unaware impacts on the physiological system [115].

Aluminium: Aluminium (Al) is commonly added to magnesium alloys for amplifying both the corrosion resistance and the strength of the metal [91]. While the former is broadly because of both precipitation reinforcement and solid solution [92]. The rule of procedure allied with the enhanced corrosion resistance is not properly understood [116], [117]. In spite of, it does appear to be founded that the concentration of aluminium increasing up to a limit detracted the rate of corrosion, but if the concentration of Al is too high then the corrosion resistance should be detracted because of the existence of the Mg17A12 phase in large amount and due to which then galvanic corrosion increases [117], [118]. An solicited rule of procedure for the improved resistance to corrosion of aluminium included magnesium alloys, is the formation of an Al203 flake on the materials surface, which is dissimilar to Mg(OH)2, is insoluble in chloride contains solution [118].

Manganese: Manganese (Mn) is a crucial element in the body of the human being [119], [120]. Without converting the mechanical property, Mn can improve the corrosion resistance of mg alloys [121]–[123]. Mn is basically utilized as a ternary alloy with Mg and other tenors. Mn containing Mg alloy gives excellent anticorrosion properties by adding 1% of Zn [30]. By adding manganese in magnesium alloy the resistance of corrosion can improve and the detrimental impacts of impurities can reduce [21], [124]. Better mechanical properties provided by Mg-Mn-Zn alloys [89]. The ductility and yield strength can also be improved by the addition of Mn in Mg alloys [92], [125]. It is usually associated as portions of other alloy systems, especially those materials which included Zn and Al [21]. Mn in Mg alloys less than 1wt% is recommended.

Machining: Biomedical implants want good geometrical precision and high surface integrity for superior mechanical stability, cell adhesion and less peril of inflammatory reactive [126]. Production technics straight impact sub-surface and surface properties of the bi-metallic materials and also impact the fatigue life and the bio functionality of the bio-medical implant [126]–[130]. Therewith the rate of corrosion, the composition, and so, the degradation behaviour of magnesium premised alloys considerably dependent on subsurface and surface properties after surface machining. Though the magnesium alloy is simple to machining with the use of conventional machining procedures are inadmissible when peal free surface with




better surface integrity are requisite. And at more cutting speeds, inappropriate machining criterion & tool design outcomes in larger friction, enlarged risk of inflammation of chip and infrequently Mg get inflamed, when the melting point is surpassed around the volume of materials [130]–[132]. Though, using proper cutting tools and procedure parameters combination, detracted the complication in machining of magnesium and its alloys. For superior machine ability and to avert ignition of chip at the time of turning of magnesium and its alloys with low cutting speed is recommended. During turning adhesion takes place due to larger cutting speed, on another observation describe that the plausibility of flank built-up (FBU) to the cutting tool [130]. Cryogenic machining of magnesium alloy with the use of liquid nitrogen spray, can reach to very dependable surface integrity together with lower surface roughness, massy basal structure and grain refinement [131]. The regions existent in material can considerably impacts to the machine ability of AZ91 and AZ31 on the process of milling, discontinuous chips created by AZ91 and larger cutting forces were introduced as comparison to AZ31 at all the procedure criterion [132]. Manufacturing methods like laser engineered net shaping (LENS), selective laser melting (SLM), electron beam machining (EBM), etc., are presently obtaining high notability in manufacturing net shape and largely exact implant structures but because of too much cost, these procedures are not be commercialized yet. Un-contacted manufacturing techniques like electric discharge machining (EDM) and laser beam machining (LBM) are very profitable over the conventional machining procedure in case of low wastage of material, non-appearance of mechanical forces and manufacture of complicate geometry [133], [134]. EDM is most prevalent and cast effective manufacturing technique for the machining of conductive metallic materials unrelated of its hardness. In EDM, the large heat generate among the tool electrode and the work materials due to repeated spark generate, due to which the material is deflected through evaporation and melting [128].

Figure 5: Microstructure of the (a) as-cast and (c) extruded Mg-Li alloys, the (b) as-cast and (d) extruded Mg-3Li-2Zn alloys [64].

Mechanical working: Magnesium alloys are generated in large amount via casting within a controlled environment. The as-cast magnesium alloy consists of thick grains with big sediments or 2nd phase particles. Impact of various mechanical procedure operations such as extrusion, rolling, heat treatment at the time of fabrication have the capacity to largely impact microstructural and surface properties [135]–[140] and therefore impact the performance of magnesium alloy, by applying extrusion and rolling process, detracted size of grains and exact orientation assistances to improve the ductility, resistance of corrosion and the strength of magnesium alloys. It was manifest that a considerable decrease in the rate of corrosion of the Mg-Ca alloy which was deep rolled, comparison by the similar alloy which was machined [127]. The existence of residuary compressive stresses since rolling also have the benefit of decreasing micro crack creation from pre




available crack nucleation dots under the substrates. The repression of creation of crack is also a crucial factor in amplifying the materials fatigue life cycle that has been supposed for the applications of biomedical implantations [135].

Pan et al. [64] studied the mechanical and microstructural properties of extruded and as-cast magnesium-lithium alloys. The microstructure of extruded and as-cast magnesium alloys is displayed in figure 5. It was analysed that the elongation and the ultimate tensile strength of as-cast Mg-Ca alloy greatly increased after hot extrusion and rolling. And also the corrosion rate had reduced by hot extrusion and hot rolling [9]. It was investigated that the process of backward extrusion has largely increased the corrosion properties and the mechanical performances of the as-cast and backward emitted Mg-Zn alloys [138]. It was concluded that the detracted corrosion rate, uniform corrosion and enhanced properties can be obtained by cyclic extrusion compression [139]. It was manifest that the double extrusion reaches to refine grains, homogenous structures, enhances corrosion and mechanical properties of biodegradable Mg alloys [81].

Concluding annotations: Magnesium and its alloys having mechanical properties same as bones of human beings, encourage bone regeneration, higher biocompatibility and completely degrades in the environment of the human body. As a result, for vascular and orthopaedic applications, it is a vital component as material of implantation. In spite of, these implants are not reaching for success which are based on magnesium alloys due to constraints such as high degradation rate or corrosion rate, hydrogen gas evolution, toxicity of alloying tenors and poor mechanical integrity which impacts the workability of implants of Mg alloys. Intense corrosion in implants of magnesium alloy discharges the corrosion by-products which can accumulate in the human body. Due to this, hydrogen gas generated in the human body that forms a considerable risk for patients. Also, the mechanical integrity of implants contorts by the degradation of magnesium alloys with a rapid decrease in mechanical properties. Performance of biodegradable implants which are based on magnesium alloys can be enhanced through surface characteristic modifications, microstructure refining, choosing the best manufacturing method with optimal procedure parameters and appropriate alloying with controlled composition. Mechanical working like extrusion and rolling processes detracts the size of grain and creates defined orientation, which helps to amplify the ductility, resistance to corrosion and the strength of magnesium alloys. Surface treatment is an impressive technique to control the rapid loss of mechanical properties, hydric bubble concentricity, corrosion rate, etc. Manufacturing methods straight impact sub-surface and surface properties of the bi-metallic materials and also impact the fatigue life and the bio functionality of the bio-medical implants.

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mg and its alloys, their challenges and opportunities for ...non-degradable implant material is used...

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