rus
Issue #1/2025
A.V.Botkin, E.P.Volkova, G.D.Khudododova, O.B.Kulyasova, R.K.Islamgaliev, R.Z.Valiev
RESEARCH AND DEVELOPMENT OF MAGNESIUM ALLOY Mg-1%Zn-0.06%Ca FOR APPLICATION IN MEDICINE
RESEARCH AND DEVELOPMENT OF MAGNESIUM ALLOY Mg-1%Zn-0.06%Ca FOR APPLICATION IN MEDICINE
DOI: https://doi.org/10.22184/1993-8578.2025.18.1.70.79
Biodegradable and biocompatible magnesium alloy materials show a promising future in medical applications and are currently the subject of active research. This paper presents the results of using combined thermomechanical processing by means of equal-channel angular pressing (ECAP) and subsequent extrusion, to produce the long-sized rods from magnesium alloy Mg-1%Zn-0.06%Ca with an ultrafine-grained structure and enhanced mechanical properties. The thermomechanical conditions have been determined through the use of computer modeling, with specific attention paid to intervals of strain rates, the degree of deformation, and the stress-strain state during the ECAP and extrusion processes. An experimental deformation was conducted, and the structure of the rods obtained through combined processing was investigated. The results demonstrate that the combined processing of the initial homogenized alloy, comprising ECAP and subsequent extrusion, enabled the formation of UFG structure with a grain size of approximately 1 µm and the creation of nano-sized particles, which led to a significant increase in the mechanical properties of the alloy in the rod-shaped samples intended for the manufacture of promising implants in maxillofacial surgery.
Biodegradable and biocompatible magnesium alloy materials show a promising future in medical applications and are currently the subject of active research. This paper presents the results of using combined thermomechanical processing by means of equal-channel angular pressing (ECAP) and subsequent extrusion, to produce the long-sized rods from magnesium alloy Mg-1%Zn-0.06%Ca with an ultrafine-grained structure and enhanced mechanical properties. The thermomechanical conditions have been determined through the use of computer modeling, with specific attention paid to intervals of strain rates, the degree of deformation, and the stress-strain state during the ECAP and extrusion processes. An experimental deformation was conducted, and the structure of the rods obtained through combined processing was investigated. The results demonstrate that the combined processing of the initial homogenized alloy, comprising ECAP and subsequent extrusion, enabled the formation of UFG structure with a grain size of approximately 1 µm and the creation of nano-sized particles, which led to a significant increase in the mechanical properties of the alloy in the rod-shaped samples intended for the manufacture of promising implants in maxillofacial surgery.
Теги: computer finite element modeling ductility equal-channel angular pressing extrusion magnesium alloy microhardness ultimate tensile strength ultrafine-grained structure компьютерное конечно-элементное моделирование магниевый сплав микротвердость пластичность предел прочности равноканальное угловое прессование ультрамелкозернистая структура экструзия
INTRODUCTION
It is well known that many people suffer from bone fractures or cardiovascular diseases every year due to accidents or diseases [1, 2]. As a consequence, implants, artificial joints and walls are in great demand [3, 4]. Magnesium is one of the most promising metals for making biodegradable implants for biomedical applications [5, 6]. However, when developing the new implants the urgent task is to increase the strength properties and corrosion resistance of magnesium alloys, to preserve their biocompatibility at the choice of alloying elements. Magnesium alloys of the Mg-Zn-Ca system attract special attention as structural materials for implant manufacturing due to their unique advantages: low Young’s modulus, good biocompatibility and biosolubility [7–9]. It is known that alloying of magnesium is a necessary step to improve its mechanical and corrosion properties [10, 11]. Non-toxic calcium and zinc are often used as alloying elements to improve the magnesium alloys properties. Moreover, zinc is a co-factor for specific enzymes in bone [8] and calcium is useful for bone growth/healing [10]. As a consequence, Zn and Ca have good biocompatibility. At the same time, Mg-Zn-Ca alloys have insufficient strength for their use as implant materials. As it was established that one of the most effective ways to increase the strength of metals and alloys can be the formation of UFG structure by methods of severe plastic deformation (SPD) [12, 13]. At the start of this work, only a small number of publications were available on the refinement of grain structure in Mg-Zn-Ca alloys using SPD methods, due to the challenges associated with the deformation processing of these alloys [8, 10, 14, 15]. Additionally, there was a lack of publications on the production of UFG magnesium alloys in the form of bars for use in the manufacture of implants for maxillofacial surgery.
The aim of this work was to formation of ultrafine-grained structure by SPD methods and to analyse the mechanical properties enhancement in billet bars made of magnesium alloy Mg-1%Zn-0.06%Ca.
RESEARCH METHODS
Initial cylindrical billets of magnesium alloy Mg-1%Zn-0.06%Ca with a diameter of 10 mm and a length of 70 mm were produced on a lathe from a round casting obtained by gravity casting. The Mg-1%Zn-0.06%Ca alloy ingot was cast at the Solikamsk Experimental Metallurgical Plant (Russia). In order to equalise chemical composition over the volume of the billet and eliminate the effects of dendritic liquation, the cast cylindrical billets were subjected to homogenisation annealing in a Nabertherm muffle furnace at 450 оC for 24 hours with cooling in water [16]. This state of the billets was taken as the initial state.
Deformation processing was performed in two stages. At the first stage, cylindrical specimens with a diameter of 10 mm were subjected to 4-pass equal-channel angular pressing along the route Bc with the angle of intersection of the inlet and outlet channels 120о. Deformation by ECAP method was performed at a temperature of 350 оC. For this purpose, the ECAP tooling was preheated to the deformation temperature using two electric heating elements in the form of clamps mounted on the die. The details of this processing are described in our work [17]. At the second stage, extrusion was carried out in the die with the angle of inclination of the deforming section’s forming section to the symmetry axis of 60о and draw 4. The extrusion temperature did not exceed 200 оC.
Finite element computer modelling of the stress-strain state of billets during ECAP and extrusion was carried out using the Deform-3D software product under the assumptions described in [17].
The microstructure was examined on a JEM-6390 scanning electron microscope (SEM) at an accelerating voltage of 20 kV. The fine structure was studied on a transmission electron microscope (TEM) JEM-2100 with an accelerating voltage of 200 kV.
The Vickers method was chosen to measure the microhardness, and measurement was carried out in cross section along the diameter of the specimen on an Emco-TestDurascan 50 microhardness tester with a load of 0.49 N and a dwell time of 10 s. Mechanical tensile tests at room temperature were performed on flat specimens with the dimensions of the working part 4 × 1 × 0.65 mm3 in uniaxial tension on an Instron 5982 testing machine with a loading rate of 10-3 s-1. The flat specimens were cut from the centre region of the deformed bar after extrusion in the longitudinal direction.
RESULTS
Results of computer modelling
Thermomechanical conditions including strain rate intervals, degrees of deformation and stress-strain state during ECAP and extrusion were determined using computer modelling (Figs.1–3).
The average normal stress (see Fig.1) in the centre of plastic deformation during ECAP and extrusion is not uniformly distributed. The average value of the mean normal stress calculated for ten points evenly taken from the graph (Fig.1) for ECAP is –59.23 MPa, and for extrusion –378.73 MPa.
The averaged value of the deformation degree calculated for ten points uniformly taken from the graph (Fig.2a) after ECAP is 2.8. The distribution of the deformation degree in the cross-section of the extruded bar, which was previously subjected to four-pass ECAP, is non-uniform (Fig.2b) and not symmetrical. In the central region the strain is equal to 3.57, in the surface layer the strain value in the circumferential direction corresponds to the range from 4.64 to 5.
The strain rate at ECAP is distributed unevenly over the deformation centre and corresponds to the interval (0.018–0.15) s-1. The strain rate at extrusion is also non-uniformly distributed over the deformation centre and corresponds to the interval (1.01–7.8) s-1 at a punch speed of 0.83 mm/sec, the same as at ECAP. From the comparison of strain rate intervals, it follows that at extrusion with drawing 4 the strain rate is approximately 50 times higher than at ECAP.
Fig.4 shows a photo of the original billet and a bar with a diameter of 5 mm and a length of 280 mm after deformation treatment.
Structure and properties of samples.
The structure of the homogenised state consisted of large grains of 220 μm in size (Fig.5a). In this structure, both at the boundaries and in the body of grains, large particles of 6 µm size were also observed, but their volume fraction was less than 1%. It was previously found that Ca2Mg6Zn3 particles are formed in magnesium alloys in which the atomic percentage Zn/Ca ratio exceeds 1.2–1.4 [16]. In the case of our alloy Mg-1%Zn-0.06%Ca, the ratio of Zn/Ca in atomic percentages is 10 and, therefore, Ca2Mg6Zn3 particles are also formed in the structure at homogenisation annealing. After deformation by ECAP+extrusion, the structure is not completely homogeneous – both 500 nm and 3 µm grains are present (Fig.5b). As it was observed bt SEM, larger grains, about 1 µm in size, were irregularly shaped and contained a high density of dislocations (Fig.5c). Whereas smaller grains, less than 500 nm in size, were equiaxed, with clear boundaries and triple junction with 120о angle and were free of dislocations (Fig.5d). Nanodispersed particles smaller than 10 nm were also observed in the structure throughout the volume, their volume fraction was about 1% (Fig.5e, f).
Mechanical tensile tests of homogenised samples showed a tensile strength of 140 MPa, with tensile elongation of 15%. After ECAP+extrusion treatment, the alloy ultimate strength increased to 335 MPa with 13% ductility retention. The hardness values were 44 HV and 81 HV, respectively (Table 1).
DISCUSSION
The results of computer modelling have shown that the deformed state of a cylindrical billet is close to the shear pattern during ECAP, while during extrusion the deformed state is close to the tensile pattern, which causes a non-monotonic deformation that contributes to structure refinement [18].
According to the results of computer modelling, the averaged value of the mean modulus stress during extrusion is approximately 6.4–7 times greater than the mean normal stress during ECAP (see Fig.1). As is known [19], the increase in the modulus of the mean normal stress promotes the healing of defects in the crystal structure of the metal that appear during deformation.
The average value of the deformation degree calculated for ten points uniformly taken from the graph (Fig.2a) after ECAP is 2.58. Analytical estimation of the degree of deformation during ECAP for 4 passes was performed according to the formula given in [20]:
(1)
where N – number of passes, Ψ – outside angle, Φ – inner angle.
In the calculation we took the external angle equal to 20° and the internal angle – the angle of channel intersection – 120°. The value of deformation 2.52 for four passes of ECAP calculated by formula (1) agrees well with the average value obtained by modelling.
When examining the homogenised state structure, individual large particles of 6 μm in size were found at the grain boundary and in the grain body (Fig.5a). However, after ECAP+extrusion, mainly fine particles smaller than 10 nm were found in the structure (Fig.5e, f). It is evident that during the SPD prosessing, there was an intensive refinement and possibly dissolution of the coarse particles found in the initial structure, with their gradual transformation into nanoparticles. Such unusual phase transformations are known for SPD processing [12, 20].
As in [21], we found an increase in the mechanical properties of magnesium alloy compared to the homogenised state after ECAP and extrusion: tensile strength increased to 335 MPa, microhardness to 81 HV. This is obviously due to contributions from both grain size reduction and hardening due to nanodispersed particles. At the same time ductility decreased insignificantly.
Combined treatment by equal-channel angular pressing (ECAP) and subsequent extrusion usually allows achieving high physical and mechanical properties of materials, which is important for obtaining hardened billets with complex profiles [22]. In the present work, using ECAP and extrusion at lower temperature, bars with a diameter of 5 mm and a length of 200–300 mm were obtained from magnesium alloys for the first time. At further upgraiding of the technology it is planned to obtain the rod-shaped billets exceeding 1 m in length, which will be used for manufacturing implants – plates and screws for maxillofacial surgery using CNC machines [23].
CONCLUSIONS
The finite element modelling of the stress-strain state of the billet during ECAP and extrusion allowed us to determine the mechanical conditions of processing of the Mg-1%Zn-0.06%Ca alloy, under which an ultrafine-grained structure was formed (mean normal stress, strain rate and degree of deformation) and bars with a diameter of 5 mm and a length of (200–300) mm were obtained. The combined treatment including ECAP and extrusion significantly improves mechanical properties – microhardness from 44 to 81 HV, ultimate strength from 140 to 335 MPa while maintaining plasticity. The enhancement in strength properties was evidently attributable to contributions from grain size reduction and hardening resulted from the formation of nanodispersed particles. Obtaining of bars from magnesium alloys with UFG structure opens up the possibility of effective manufacturing of implants in the form of plates and screws for maxillofacial surgery.
ACKNOWLEDGEMENTS
The work was supported by RSF No. 24-43-20015. The experimental part of the work was carried out with the use of the equipment of the Nanotech Centre of the Federal State Budgetary Educational Institution of Higher Education "UUST".
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
It is well known that many people suffer from bone fractures or cardiovascular diseases every year due to accidents or diseases [1, 2]. As a consequence, implants, artificial joints and walls are in great demand [3, 4]. Magnesium is one of the most promising metals for making biodegradable implants for biomedical applications [5, 6]. However, when developing the new implants the urgent task is to increase the strength properties and corrosion resistance of magnesium alloys, to preserve their biocompatibility at the choice of alloying elements. Magnesium alloys of the Mg-Zn-Ca system attract special attention as structural materials for implant manufacturing due to their unique advantages: low Young’s modulus, good biocompatibility and biosolubility [7–9]. It is known that alloying of magnesium is a necessary step to improve its mechanical and corrosion properties [10, 11]. Non-toxic calcium and zinc are often used as alloying elements to improve the magnesium alloys properties. Moreover, zinc is a co-factor for specific enzymes in bone [8] and calcium is useful for bone growth/healing [10]. As a consequence, Zn and Ca have good biocompatibility. At the same time, Mg-Zn-Ca alloys have insufficient strength for their use as implant materials. As it was established that one of the most effective ways to increase the strength of metals and alloys can be the formation of UFG structure by methods of severe plastic deformation (SPD) [12, 13]. At the start of this work, only a small number of publications were available on the refinement of grain structure in Mg-Zn-Ca alloys using SPD methods, due to the challenges associated with the deformation processing of these alloys [8, 10, 14, 15]. Additionally, there was a lack of publications on the production of UFG magnesium alloys in the form of bars for use in the manufacture of implants for maxillofacial surgery.
The aim of this work was to formation of ultrafine-grained structure by SPD methods and to analyse the mechanical properties enhancement in billet bars made of magnesium alloy Mg-1%Zn-0.06%Ca.
RESEARCH METHODS
Initial cylindrical billets of magnesium alloy Mg-1%Zn-0.06%Ca with a diameter of 10 mm and a length of 70 mm were produced on a lathe from a round casting obtained by gravity casting. The Mg-1%Zn-0.06%Ca alloy ingot was cast at the Solikamsk Experimental Metallurgical Plant (Russia). In order to equalise chemical composition over the volume of the billet and eliminate the effects of dendritic liquation, the cast cylindrical billets were subjected to homogenisation annealing in a Nabertherm muffle furnace at 450 оC for 24 hours with cooling in water [16]. This state of the billets was taken as the initial state.
Deformation processing was performed in two stages. At the first stage, cylindrical specimens with a diameter of 10 mm were subjected to 4-pass equal-channel angular pressing along the route Bc with the angle of intersection of the inlet and outlet channels 120о. Deformation by ECAP method was performed at a temperature of 350 оC. For this purpose, the ECAP tooling was preheated to the deformation temperature using two electric heating elements in the form of clamps mounted on the die. The details of this processing are described in our work [17]. At the second stage, extrusion was carried out in the die with the angle of inclination of the deforming section’s forming section to the symmetry axis of 60о and draw 4. The extrusion temperature did not exceed 200 оC.
Finite element computer modelling of the stress-strain state of billets during ECAP and extrusion was carried out using the Deform-3D software product under the assumptions described in [17].
The microstructure was examined on a JEM-6390 scanning electron microscope (SEM) at an accelerating voltage of 20 kV. The fine structure was studied on a transmission electron microscope (TEM) JEM-2100 with an accelerating voltage of 200 kV.
The Vickers method was chosen to measure the microhardness, and measurement was carried out in cross section along the diameter of the specimen on an Emco-TestDurascan 50 microhardness tester with a load of 0.49 N and a dwell time of 10 s. Mechanical tensile tests at room temperature were performed on flat specimens with the dimensions of the working part 4 × 1 × 0.65 mm3 in uniaxial tension on an Instron 5982 testing machine with a loading rate of 10-3 s-1. The flat specimens were cut from the centre region of the deformed bar after extrusion in the longitudinal direction.
RESULTS
Results of computer modelling
Thermomechanical conditions including strain rate intervals, degrees of deformation and stress-strain state during ECAP and extrusion were determined using computer modelling (Figs.1–3).
The average normal stress (see Fig.1) in the centre of plastic deformation during ECAP and extrusion is not uniformly distributed. The average value of the mean normal stress calculated for ten points evenly taken from the graph (Fig.1) for ECAP is –59.23 MPa, and for extrusion –378.73 MPa.
The averaged value of the deformation degree calculated for ten points uniformly taken from the graph (Fig.2a) after ECAP is 2.8. The distribution of the deformation degree in the cross-section of the extruded bar, which was previously subjected to four-pass ECAP, is non-uniform (Fig.2b) and not symmetrical. In the central region the strain is equal to 3.57, in the surface layer the strain value in the circumferential direction corresponds to the range from 4.64 to 5.
The strain rate at ECAP is distributed unevenly over the deformation centre and corresponds to the interval (0.018–0.15) s-1. The strain rate at extrusion is also non-uniformly distributed over the deformation centre and corresponds to the interval (1.01–7.8) s-1 at a punch speed of 0.83 mm/sec, the same as at ECAP. From the comparison of strain rate intervals, it follows that at extrusion with drawing 4 the strain rate is approximately 50 times higher than at ECAP.
Fig.4 shows a photo of the original billet and a bar with a diameter of 5 mm and a length of 280 mm after deformation treatment.
Structure and properties of samples.
The structure of the homogenised state consisted of large grains of 220 μm in size (Fig.5a). In this structure, both at the boundaries and in the body of grains, large particles of 6 µm size were also observed, but their volume fraction was less than 1%. It was previously found that Ca2Mg6Zn3 particles are formed in magnesium alloys in which the atomic percentage Zn/Ca ratio exceeds 1.2–1.4 [16]. In the case of our alloy Mg-1%Zn-0.06%Ca, the ratio of Zn/Ca in atomic percentages is 10 and, therefore, Ca2Mg6Zn3 particles are also formed in the structure at homogenisation annealing. After deformation by ECAP+extrusion, the structure is not completely homogeneous – both 500 nm and 3 µm grains are present (Fig.5b). As it was observed bt SEM, larger grains, about 1 µm in size, were irregularly shaped and contained a high density of dislocations (Fig.5c). Whereas smaller grains, less than 500 nm in size, were equiaxed, with clear boundaries and triple junction with 120о angle and were free of dislocations (Fig.5d). Nanodispersed particles smaller than 10 nm were also observed in the structure throughout the volume, their volume fraction was about 1% (Fig.5e, f).
Mechanical tensile tests of homogenised samples showed a tensile strength of 140 MPa, with tensile elongation of 15%. After ECAP+extrusion treatment, the alloy ultimate strength increased to 335 MPa with 13% ductility retention. The hardness values were 44 HV and 81 HV, respectively (Table 1).
DISCUSSION
The results of computer modelling have shown that the deformed state of a cylindrical billet is close to the shear pattern during ECAP, while during extrusion the deformed state is close to the tensile pattern, which causes a non-monotonic deformation that contributes to structure refinement [18].
According to the results of computer modelling, the averaged value of the mean modulus stress during extrusion is approximately 6.4–7 times greater than the mean normal stress during ECAP (see Fig.1). As is known [19], the increase in the modulus of the mean normal stress promotes the healing of defects in the crystal structure of the metal that appear during deformation.
The average value of the deformation degree calculated for ten points uniformly taken from the graph (Fig.2a) after ECAP is 2.58. Analytical estimation of the degree of deformation during ECAP for 4 passes was performed according to the formula given in [20]:
(1)
where N – number of passes, Ψ – outside angle, Φ – inner angle.
In the calculation we took the external angle equal to 20° and the internal angle – the angle of channel intersection – 120°. The value of deformation 2.52 for four passes of ECAP calculated by formula (1) agrees well with the average value obtained by modelling.
When examining the homogenised state structure, individual large particles of 6 μm in size were found at the grain boundary and in the grain body (Fig.5a). However, after ECAP+extrusion, mainly fine particles smaller than 10 nm were found in the structure (Fig.5e, f). It is evident that during the SPD prosessing, there was an intensive refinement and possibly dissolution of the coarse particles found in the initial structure, with their gradual transformation into nanoparticles. Such unusual phase transformations are known for SPD processing [12, 20].
As in [21], we found an increase in the mechanical properties of magnesium alloy compared to the homogenised state after ECAP and extrusion: tensile strength increased to 335 MPa, microhardness to 81 HV. This is obviously due to contributions from both grain size reduction and hardening due to nanodispersed particles. At the same time ductility decreased insignificantly.
Combined treatment by equal-channel angular pressing (ECAP) and subsequent extrusion usually allows achieving high physical and mechanical properties of materials, which is important for obtaining hardened billets with complex profiles [22]. In the present work, using ECAP and extrusion at lower temperature, bars with a diameter of 5 mm and a length of 200–300 mm were obtained from magnesium alloys for the first time. At further upgraiding of the technology it is planned to obtain the rod-shaped billets exceeding 1 m in length, which will be used for manufacturing implants – plates and screws for maxillofacial surgery using CNC machines [23].
CONCLUSIONS
The finite element modelling of the stress-strain state of the billet during ECAP and extrusion allowed us to determine the mechanical conditions of processing of the Mg-1%Zn-0.06%Ca alloy, under which an ultrafine-grained structure was formed (mean normal stress, strain rate and degree of deformation) and bars with a diameter of 5 mm and a length of (200–300) mm were obtained. The combined treatment including ECAP and extrusion significantly improves mechanical properties – microhardness from 44 to 81 HV, ultimate strength from 140 to 335 MPa while maintaining plasticity. The enhancement in strength properties was evidently attributable to contributions from grain size reduction and hardening resulted from the formation of nanodispersed particles. Obtaining of bars from magnesium alloys with UFG structure opens up the possibility of effective manufacturing of implants in the form of plates and screws for maxillofacial surgery.
ACKNOWLEDGEMENTS
The work was supported by RSF No. 24-43-20015. The experimental part of the work was carried out with the use of the equipment of the Nanotech Centre of the Federal State Budgetary Educational Institution of Higher Education "UUST".
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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