Friction extrusion of AZ91/bioactive glass gradient composite

Abstract

The effect of the pre-friction extrusion microstructure of AZ91 billet on the microstructure, mechanical properties, and corrosion resistance of AZ91-bioactive glass surface composite rods was investigated. The results show that friction extrusion on AZ91 alloy billet with cast microstructure results in agglomerated bioactive glass particles in the composite rod microstructure. Compared to as-cast AZ91 alloy, friction extrusion on AZ91 alloy billet with cast microstructure results in 93% more corrosion resistance. Also, friction extrusion on AZ91 alloy billet with extruded microstructure leads to a gradient AZ91-bioactive glass composite rod with ultimate tensile strength, yield strength, and corrosion resistance of 9, 2, and 74% higher than AZ91 alloy rods, respectively.

Keywords

Friction extrusion AZ91 alloy bioactive glass particles gradient composite

Introduction

Magnesium and magnesium-based alloys have attracted the attention of various industries, including the medical industry, due to their properties, such as low density (1.74–1.85 g cm ⁻ ³), high strength-to-weight ratio, and good biocompatibility. Nevertheless, Despite these properties, their use is relatively limited compared to other engineering materials due to disadvantages such as reduced tensile and creep strengths at high temperatures, low wear resistance, and the high rate of destruction as implantable [1,2]. In the past few years, many efforts have been made to make biodegradable implants, develop different methods, and reduce the corrosion of metals as much as possible to restore body tissues without having to undergo secondary surgery. The criteria for evaluating the efficacy of biological materials are ideal biodegradability under load, proper mechanical support during the process of rebuilding the damaged tissue, and dissolution at a rate appropriate to the host tissue. By virtue of their biodegradable properties, the products of degradation can be received by body tissues and should assist in the regeneration of the host tissue. Famous biodegradable bioceramics, such as calcium phosphates, have low mechanical stability, lack favourable degradation when used under load, and have been rendered ineffective due to sudden degradation. In contrast, magnesium-based alloys, like AZ91, are known to be biodegradable metallic biomaterials with better mechanical properties and are closer to bone tissue. Integrating these biocompatible materials through the fabrication of magnesium-matrix composites provides biomedical applications in tissuesunder load [3].

There has been great interest in fabricating magnesium matrix composites using bio-ceramic reinforcement in various manufacturing methods. In a study, hydroxyapatite was used as reinforcement in AZ91D. It was found that the properties of the fabricated composite were very similar to the bone, and during the immersion test, bone cells could adhere, proliferate, and survive on the surface of the composite [4]. Chen et al. [5] made an AZ91-hydroxyapatite composite by compression moulding. It was observed that the composite showed high compressive strength, and the presence of hydroxyapatite promoted cell adhesion and proliferation. Campo et al. [6] used the powder metallurgy method and punching to improve the mechanical and microstructural properties of a magnesium matrix composite reinforced with hydroxyapatite (HAP). They found that the increase in HAP increased hardness and compressive strength. Also, due to the porous surface and change in texture intensity, the Mg-5HAP composite demonstrated good corrosion resistance.

Li et al. [7] made a magnesium matrix composite reinforced with carbon nanotubes by stir casting. In the range of 0.65 wt-% of carbon nanotubes, the tensile strength and elongation of the composite increased by 150% and 30%, respectively. However, the mechanical properties were slightly reduced due to the agglomeration of carbon nanotubes. In another study, adding 1 wt-% of multi-walled carbon nanotubes increased yield strength, fracture compressive strength, and final compressive strength by 10, 36, and 20%, respectively [8]. Owing to their superior solvability and biodegradability, bioactive glass (BG) reinforcements such as 64SiO₂-31CaO-5P₂O₅ had a significant advantage over other particles [9]. Huan et al. [9] reported that adding BG to the ZK30 matrix can increase corrosion resistance compared to ZK30 alloy. BG reinforcements could potentially improve the tensile strength, bioactivity, and degradation rates of Mg implants [1,10]. Dutta et al. [11] showed that adding 10 wt-% of BG to the magnesium matrix enhanced corrosion resistance. Also, Yin et al. [12] studied the fabrication of the ZK30-BG composite using selective laser melting. They observed that encapsulating BG in the ZK30 alloy enhanced the corrosion resistance of the Mg-matrix matrix and its capacity to produce apatite.

According to different sources, adding bio-reinforcement particles enhances magnesium's biological properties and controls its degradability rate. Importantly, The addition of biocompatible reinforcement particles negatively affects the magnesium matrix strength. Therefore, it is important to maintain magnesium matrix strength and create biocompatibility in the surface areas. Making a magnesium matrix composite with a gradient distribution of reinforcing particles in the surface areas can be an effective solution. Among the proposed methods for making metal matrix composites, the friction extrusion (FE) method is one of the solid-state material processing methods that can make gradient composites. This method follows the principles of the friction stir welding process, whereby the rotation of the tool and the generation of heat cause local softening of the material. The material is subjected to a thermo-mechanical process. Studies have shown that this method can produce sound and hole-free rods and tubes in which the microstructure of the raw material is modified through a dynamic recrystallisation mechanism. The significance of this method as a solid-state process lies in characteristics such as creating a fine equiaxed microstructure, controlling the chemical composition during the process, using simple equipment, and performing the process in one step [13]. The main parameters in this process are rotational speed, extrusion speed, axial force, and punch diameter. The rotational speed plays the most important role in reducing grain size, and the punch diameter has the least contribution [14].

Although the effect of FE process variables on the microstructure and properties of magnesium and its alloys has been studied by various researchers [15–17], there have been relatively few studies in the field of metal matrix composite manufacturing, particularly magnesium [18–21]. Therefore, it is necessary to conduct further research to assess the potential of this process to make gradient composites. Since magnesium has low formability, the initial microstructure of magnesium billets can significantly affect material flow during magnesium extrusion during FE, which has not been studied previously. Composite making with bio-ceramic reinforcements constitutes one of the most effective approaches to improving magnesium alloys’ mechanical properties and biocompatibility [22]. More specifically, one of the most important bio-ceramics is bioactive glass, which has significant biological properties, excellent biocompatibility, the ability to bond with soft and hard tissues, and angiogenesis [9]. The formation of hydroxyapatite between bioactive glass and hard tissue causes higher protein absorption, cell differentiation, and acceleration of bone formation [23].

Since the distribution of bioactive glass in the surface areas and the formation of the gradient structure can play an important role in the bio-efficiency of the magnesium matrix composite, this study examines the effectiveness of the FE process to achieve this objective. The composite manufacturing process was conducted using the as-cast and extruded AZ91 alloy to examine the effect of the initial microstructure of magnesium on the distribution of bioactive glass particles and the formation of the gradient structure. Then, the effect of the process on the microstructure, mechanical properties, and corrosion of the magnesium matrix composite was investigated.

Experimental process

This research used AZ91 alloy with chemical composition (wt-%) of 8.55% Al, 0.66% Zn, 0.13% Mn, 0.002% Fe, 0.001% Cu, and balance Mg was used to make a magnesium matrix composite. The as-cast and extruded AZ91 alloy rod was employed as the primary billet for FE. Before the FE process, extruded AZ91 billet was made by homogenising the as-cast billet for 24 h at 415°C and then extruding from 77.5 to 52 mm in diameter and then from 52 to 20 mm in two stages. The mould and billet temperature was kept constant at 240°C in the extrusion stage.

Before FE, the as-cast and extruded AZ91 billet was machined to a 20 mm diameter and cut into 25 mm lengths. Then, according to Figure 1, the rod's cross section was drilled to insert the bioactive glass particles. The FE process was carried out using the die and punch shown in Figure 1. According to preliminary investigations, the best conditions for FE were a rotational speed of 600–1200 rev min⁻¹ and an extrusion speed of 20–40 mm min⁻¹. Optimal rotational and extrusion speeds were determined to be 1200 rev min⁻¹ and 20 mm min⁻¹, respectively. Accordingly, the extrusion operation was conducted using the parameters outlined above. Table 1 shows the names of the different samples. The sol–gel process was used to synthesise bioactive glass (BG) particles with a chemical composition of 64SiO₂-31CaO-5P₂O₅, which served as composite rod reinforcement. A detailed description of the synthesis process can be found in [24]. Figure 2 shows a scanning electron microscopy (SEM) image of the BG particles. After the FE operation, different samples were extracted from the extruded rod to investigate microstructure, mechanical properties, and corrosion resistance. A tool system designed according to Figure 3 was used to measure the rod surface temperatures during the FSBE process. Figure 3 illustrates a schematic view of the temperature measuring tool attached to the punch used for experiments.

Table 1

The FSBE processed samples labelling.

Sample label	Rotational speed (rev min⁻¹)	Traverse speed (mm min⁻¹)	Initial condition of AZ91 rod	Bioactive glass content (vol.-%)
As-cast	–	30	As-cast	–
Extruded	–	30	Extruded	–
C-FE	1200	20	As-cast	3
E-FE	1200	20	Extruded	3
C-FE-12/2	1200	20	As-cast	–
E-FE-12/2	1200	20	Extruded	–
C-FE-12/4	1200	40	As-cast	–
E-FE-12/4	1200	40	Extruded	–
C-FE-9/2	900	20	As-cast	–
E-FE-9/2	900	20	Extruded	–

Metallography samples were subjected to grinding and polishing procedures. A solution containing 150 mL of ethanol, 10 mL of distilled water, 10 mL of acetic acid, and 4.26 g of picric acid was used to etch the samples’ microstructure. The microstructure was examined using a Dewinter DEW/780 optical microscope and a Philips-XL30FSEM SEM equipped with an EDX detector. Phase analysis was performed using the X-ray diffraction Rigaku Ultima IV instrument.

The samples’ hardness was determined using a Koopa Universal (UV1) microhardness tester with a 100 g load applied at 10-second intervals. A tensile test was performed according to ASTM E8 using a crosshead speed of 1 mm min⁻¹. The tensile test samples had a length of 100 mm. The corrosion behaviour of AZ91 rods and FSBE-processed AZ91s was evaluated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). Specifically, a potentiostat galvanostat (Metrohm Autolab, Model PGSTAT204) and a potentiostat galvanostat (Wonatech, Model Zive SP1) were used in a simulated body fluid solution (pH = 7.4) at 37°C for the tests. The SBF was prepared using Kokubo's methodology [25]. Details of corrosion resistance evaluations in the SBF solution are reported in previous work [24].

Friction extrusion simulation

Using the procedure reported in [26], the plastic strain and temperature during FE predicted using simulation method. This study employed the finite element model to simulate the FE process. The arbitrary Lagrangian-Eulerian (ALE) method was used to enable the workpiece to maintain a high deformation rate during the FE. As a result, the FE is precisely modelled using ABAQUS software in explicit mode. Rod, punch, and die dimensions were assumed to be the same as in the experimental work. As a Lagrangian rigid body, the rotating punch and container were meshed with 4-node thermally coupled 3D quadrilateral bilinear rigid elements. Also, the rod was meshed with a 4-node thermally coupled tetrahedron, linear displacement, and temperature (C3D4T). The number of elements considered in the rod, punch and die was 254103, 84354, and 99456, respectively. All punch movement conditions were assigned based on the punch reference point to ensure precise punch movement. The convective heat transfer coefficient was used to model convection heat loss at the container and punch exterior surfaces. The Johnson-Cook model was utilised to model magnesium matrix material flow as follows:

\begin{aligned} σ_{y} & = (1 - D) [A + B (1 - D) ε_{P}^{n}] \\ \times [1 + C \ln ((1 - D) \frac{{\dot{ε}}_{P}}{{\dot{ε}}_{0}})] [1 - {(\frac{T - T_{h}}{T_{s} - T_{h}})}^{m}] \end{aligned}

(1)

The Johnson-Cook constant reported in the [27] used in this study. In the simulation, the reference strain rate $({\dot{ε}}_{0})$ is considered as 1 s⁻¹. Furthermore, the damage value (D) is considered in the Johnson-Cook equation and calculated according to the method described in [28,29]. The thermo-physical constant of AZ91 rod and die material was imported into the model according to values reported in [30,31].

Results and discussion

Figure 4 shows the microstructure of various zones of composite rods with different magnifications. As can be seen, the FE process with an as-cast rod does not cause uniform distribution in the surface areas of the FE-processed rod. However, the FE operation causes a more uniform distribution of bioactive glass powder in the surface areas. Bioactive glass particles are not present in the central areas of both composite rods. This phenomenon is due to the nature of the FE process and the insertion of bioactive glass particles into the holes near the surface of the AZ91 rod. A gradient distribution of bioactive glass particles can be observed in the cross section of extruded rods in the composite sample fabricated using extruded AZ91 rod.

The holes embedded in the primary rod are situated near the surface, so the bioactive glass particles are primarily distributed in these surface areas. According to [17,32], the material at the centre of the initial AZ91 rod is directly extruded from the die orifice without significant lateral deformation, as it does not experience the shear force applied by the rotating die. In contrast, the bioactive glass particles and material away from the centre of the rod are subjected to pressure and pushed against the die. During die rotation, frictional contact between the material and the die generates high shear forces, which force the bioactive glass particles and material away from the centre of the rod to rotate with the die. As the die advances, the materials closest to the die orifice are extruded first, while the bioactive glass particles and material continue to rotate until they are extruded. Since the holes containing bioactive glass particles are located near the surface of the AZ91 rod and far from the centre, they will only be present in the surface areas of the extruded rod under the influence of the shear force and pressure applied by the die. Consequently, there will be no glass phase in the central areas.

Simulation and temperature measurement results indicate that the amount of plastic strain and temperature of samples E-FE and C-FE are 25.2 and 430°C and 26.8 and 442°C, respectively. Since the sample C-FE was extruded at lower temperatures and lower plastic strain, it can be argued that these two factors reduce the material's flow ability during FE, resulting in a non-uniform distribution of bioactive glass particles within the microstructure. The distribution factor (DF) of bioactive glass particles in the composite was calculated according to the procedures described by Yourdkhani and Hubert [33]. As reported by the researchers [33], a distribution factor of 1 indicates a uniform distribution. In samples C-FE and E-FE, the distribution factor at the zone near the surface was 0.56 and 0.79, respectively, and a lower amount of agglomerated bioactive glass particles was formed at the surface of sample E-FE.

The optical image of the microstructure of the initial AZ91 rods is shown in Figure 5. By comparing Figures 4 and 5, the FE process in samples C-FE and E-FE led to the formation of fine equiaxed grains in the extruded samples. Nevertheless, when the extruded rod was used, a significant increase in grain size was observed compared to the initial AZ91 rod. Of course, it should be noted that dynamic recrystallisation occurred in the microstructure in both cases of the FE-processed rods due to the effect of heat and plastic deformation during the FE process. These results indicate that if extrusion is performed before FE, the extruded rod will have an average size larger than the initial AZ91 rod. According to [27], the dynamic recrystallised grain size decreased with decreasing temperature or increasing plastic strain. Plastic strain and temperature results demonstrate that sample C-FE exhibited a smaller grain size than sample E-FE due to a lower temperature and higher plastic strain. Importantly, in both cases, most of the beta phase in the initial AZ91 microstructure was dissolved after FE, and its amount decreased in the microstructure.

The SEM image of the cross section of different samples is shown in Figure 6 in order to provide a more accurate investigation of the microstructural changes during the FE process. The microstructure of the as-cast and extruded AZ91 rod includes α-Mg, Al-Mn intermetallic compounds, and beta ( $β$ -Mg₁₇Al₁₂) phase. However, the obvious difference in the microstructure of the as-cast and extruded AZ91 rod is the presence of the α + β eutectic structure in the as-cast rod and the formation of the beta phase as a coarse and continuous phase. Meanwhile, in the extruded AZ91 rod, a large part of the beta phase was dissolved or broken into smaller particles than in the as-cast AZ91 rod. Furthermore, there was no trace of eutectic structure in the extruded AZ91 rod, even though the majority of the beta phase was resolved after FE. Also, the eutectic structure was formed in some areas of the composite samples. The amount of $β$ -Mg₁₇Al₁₂ and $α + β$ eutectic phases in the zone near the surface of the FE-processed samples was more than in the centre, which is attributable to the higher strain and heat generation during the FE process in the near-surface zone [34,35]. These factors promoted the dissolution of the $β$ -Mg₁₇Al₁₂ phase in the near-surface zone, leading to the formation of a solid solution. However, it is essential to note that the presence of bioactive glass particles and areas with high strain in the near-surface zone can increase the likelihood of precipitation of the $β$ -Mg₁₇Al₁₂ phase and α + β eutectic in this area. Other studies have also reported that the presence of second-phase particles and high-strain regions can affect the nucleation of the $β$ -Mg₁₇Al₁₂ phase and α + β eutectic [36–39].

The X-ray diffraction results of different samples are shown in Figure 7. Except for the as-cast AZ91 rod, the beta phase peak has decreased drastically in other samples. According to the procedure reported in [40], the amount of beta phase as-cast AZ91 rod, extruded AZ91 rod, sample C-FE, and sample E-FE is 1.00, 0.78, 0.38, and 0.31, respectively. The conditions for diffusion and dissolution of the beta phase in the magnesium matrix were provided under the influence of applied heat and plastic strain during the process, resulting in a substantial decrease in the amount of beta phase in the extruded rods. It should be noted that, regardless of the initial conditions of the initial AZ91 rod, the value of the beta phase after the FE process remained relatively unchanged. However, in the sample containing the bioactive glass phase, the amount of beta phase was slightly higher than in the sample without the bioactive glass phase. This phenomenon is caused by the presence of the bioactive glass phase and the provision of beta-phase nucleation sites during cooling and reprecipitation of the beta phase. As a result of applying the FE process, the XRD peaks shifted compared to the initial AZ91 rod. According to the X-ray diffraction results and Equation (2), the amount of cumulative (residual) strain in the microstructure of the extruded rods was calculated [40]:

ε = Δ θ \times \cot θ = \frac{Δ d}{d}

(2)

where d represents the interplanar spacing, θ denotes the diffraction pattern angle, and Δd and Δθ stand for the variations in interplanar spacing and diffraction pattern angle between the FE-processed samples and the initial AZ91 rod, respectively. The calculated values for samples C-FE and E-FE is 0.0075 and 0.0071, respectively. Accordingly, the FE operation on the as-cast AZ91 rod led to a less cumulative strain in the extruded rod.

In Figure 8, the hardness changes of different samples are illustrated. Evidently, the hardness decreased after FE processing compared to the initial AZ91 rods. Also, the hardness deviation decreased. Dissolution of the beta phase during the FE process decreased hardness compared to the initial AZ91 rod. On the other hand, the formation of the equiaxed recrystallised grains in the microstructure after the FE process and the uniform distribution of second-phase particles resulted in more uniform changes in hardness and a decrease in hardness deviation. It should be noted that although most of the beta phase dissolved in the magnesium matrix after FE, the presence of bioactive glass phase particles and the solid solution formed in the matrix prevented a sharp drop in hardness in FE-processed rods. However, a higher amount of beta phase and smaller grains in the composite rod fabricated from as-cast AZ91 rod led to higher hardness in the composite rods.

Figure 9 shows the stress–strain curves and results for tensile strength and elongation of different samples. Also, the fracture surface of different samples is shown in Figure 10. The minimum strength and elongation are related to the as-cast AZ91 rod. The beta phase with continuous morphology at the grain boundary led to the weakening of the structure, resulting in a decrease in strength and an increase in elongation. There were no dimples on the fracture surface of this sample, indicating a ductile fracture. Also, most of the fracture surface had cleavage fractures, which are a sign of a brittle fracture. By performing the FE process, the contribution of ductile fracture to the fracture surface increased.

Compared to the as-cast initial AZ91 rod, the extruded samples exhibit a significant increase in elongation, as indicated by the tensile strength results. By breaking the integrated structure of the beta phase and distributing the second-phase particles in the microstructure, the tensile strength of the extruded rod increased. Meanwhile, the presence of agglomerated glass phase particles caused stress concentration and separation at the bioactive glass particles-matrix interface. This phenomenon can decrease strength and elongation. This phenomenon may explain the lower strength and elongation of the composite rod fabricated from the as-cast AZ91 rod. According to the results of the tensile test, although composite rods have a 4–14% reduction in elongation when compared to rods without bioactive glass particles, the tensile strength of the composite rod is 22–34% higher than extruded rods without bioactive glass particles, depending on the conditions of the initial AZ91 rod (as-cast or extruded).

A polarisation analysis and electrochemical impedance spectroscopy (EIS) were conducted to evaluate the samples’ corrosion resistance. The results of the polarisation test on different samples are shown in Figure 11. The values of i_corr and E_corr in the as-cast AZ91 rod were 125.45 µA cm⁻² and −1.48 V, respectively, and the values of i_corr and E_corr in the extruded AZ91 rod were 94.13 µA cm⁻² and −1.50 V, respectively. The corrosion current density of the FE-processed rods was significantly lower than that of the initial AZ91 rods. In the FE-processed rods fabricated from the as-cast AZ91 rod, E_corr moved towards nobler values, and i_corr decreased. The value of i_corr in two composite samples obtained from as-cast and extruded AZ91 rods was 8.32 and 9.89 µA cm⁻², respectively. The values of polarisation resistance and corrosion rate were obtained according to the procedure proposed in [27]. The results of the polarisation test are reported in Table 2. The composite sample fabricated from the as-cast AZ91 rod had the highest Rp (1403.21 Ω.cm²), the lowest i_corr (8.32 µA cm⁻²), and the lowest CR (0.21 mm year⁻¹). As a result, it had the highest corrosion resistance. The polarisation resistance and corrosion current density exhibit an inverse relationship. Moreover, with the increase in Rp, the value of i_corr decreases. Also, regardless of the conditions of the initial AZ91 rod, the corrosion resistance of the composite rod is at least 64–87% higher than that of extruded rods without bioactive glass particles.

Table 2

Corrosion resistance parameters achieved of potentiodynamic polarisation curves.

sample	E_corr (V)	i_corr (µA cm ⁻ ²)	Rp (Ω.cm²)	CR (mm year ⁻¹ )
As-cast AZ91 rod	−1.48	125.45	89.71	2.86
Extruded AZ91 rod	−1.50	94.13	98.32	2.05
C-FE	−1.45	8.32	1403.21	0.21
E-FE	−1.47	9.89	1356.98	0.53
C-FE-12/2	−1.55	35.12	284.87	1.50
E-FE-12/2	−1.58	37.60	295.23	1.57
C-FE-12/4	−1.49	25.78	358.90	1.12
E-FE-12/4	−1.51	26.98	341.03	1.24
C-FE-9/2	−1.48	32.13	305.89	1.29
E-FE-9/2	−1.52	33.67	301.32	1.48

EIS curves of different samples are shown in Figure 12. Loop impedance and corrosion resistance both increased with increasing loop radius. According to the Nyquist plots, the composite sample fabricated from the as-cast AZ91 rod had the largest capacitive loop in Figure 12(a). A larger radius of the capacitor ring shows higher corrosion resistance. The similar shapes of capacitance curves demonstrate that charge transfer is the only effective factor in corrosion behaviour. In Bode plots (Figure 12(b)), |Z| is the impedance modulus; at low frequencies, the larger |Z| means higher corrosion resistance.

It is clear that the composite sample fabricated from the as-cast rod has the highest value of |Z|. The equivalent circuit reported in [24] used in the corrosion analysis. In the equivalent circuit, solution resistance, surface layer resistance, charge transfer resistance, and surface homogeneity are indicated by Rs, R1, R2, and n, respectively. The double and corrosive layer capacities are related to the stationary phase components CPE1 and CPE2 at the interface between the substrate and the electrolyte [41]. Y and n variables specify the CPE impedance [42]:

Z_{CPE} = \frac{1}{Y {(j ω)}^{n}}

(3)

The dispersion index and non-ideal capacitance are represented by Y and n, respectively. When n = 1, CPE equates to ideal capacitance. When n = 0, CPE is the same as pure resistance. According to the fitting parameters in Table 3, the total resistance value of RT (RT = R1 + R2) for the as-cast AZ91 rod, extruded AZ91 rod, sample C-FE, and sample E-FE is 49.19, 64.32, 119.51, and 108.45 Ω.cm², respectively. Applying bioactive glass particles increases the resistance of the corrosion product layer (R1). The phase curves of the samples are shown in Figure 12(c). The phase angle of each sample decreased continuously, indicating the presence of a passive layer and the deterioration of the protection quality with time and frequency. A high phase angle with a significant electrochemical resistance capability promoted the formation of a high-density oxide coating [43]. Similar patterns can be observed in the phase diagrams for all samples, as shown in Figure 12(c). A larger frequency loop is the result of the charge transfer interaction between the sample surface and electrolyte solution, represented by the charge resistance and the stationary phase element.

Table 3

The measurements made after fitting the experimental EIS data.

sample	Rs (Ω.cm²)	R1 (Ω.cm²)	CPE1 (F.cm⁻²)	n1	R2 (Ω.cm²)	CPE2 (F.cm⁻²)	n2	R_T (Ω.cm²)	$χ$ -squared
As-cast AZ91 rod	29.33	3.02E-5	4.16E-11	0.96	49.18	6.03E-5	0.77	49.19	0.0752
Extruded AZ91 rod	25.59	4.53E-5	4.09E-11	0.91	70.01	5.73E-5	0.80	64.32	0.0453
C-FE	8.45	14.78	1.97E-4	0.76	178.23	2.54E-4	0.89	119.51	0.0029
E-FE	9.73	11.35	2.21E-4	0.80	170.12	2.87E-4	0.84	108.45	0.0034

The SEM image of the corroded surface is shown in Figure 13. A layer of Corrosion products is evidently present on the surfaces of the samples. Cl⁻ diffusion pathways may weaken the α-Mg matrix due to microcracks. The as-cast AZ91 rod surface has more cracks, weaker surface bonding, a more unstable surface, and a more corrosion-related material. All of this indicates that the surface is less resistant to corrosion. Because of more stable surfaces, stronger interfacial bonds, and less formation of corrosion products after the corrosion test, the composite rods corrode at a slower rate than the initial AZ91 rod.

Comparing the mechanical and corrosion properties with those of other researchers who studied the friction extrusion of magnesium and its alloys [15–17,44] revealed that the use of bioactive glass particles and the application of the friction extrusion process on the AZ91 rod improved corrosion resistance by at least 74% compared to the initial AZ91 alloy. It is possible, moreover, to achieve mechanical properties similar to or even greater than those obtained in other research by using bioactive glass particles.

One of the challenges in this study was the effect of higher volume percentages of bioactive glass particles, which could affect their distribution during extrusion. Further research is needed on the effects of higher volume percentages of bioactive glass particles on the microstructure, mechanical properties, and corrosion resistance. Furthermore, the effect of heat treatment after extrusion is one of the important factors influencing the corrosion behaviour and distribution of beta-phase particles, which requires further investigation.

Conclusions

This study investigated the fabrication of AZ91-bioactive glass surface composite rods by a friction extrusion process using an AZ91 alloy billet with different microstructures. The main findings are as follows:

Friction extrusion on AZ91 alloy billet with extruded microstructure led to a larger grain size and gradient distribution (with a distribution factor of 0.79) of bioactive glass particles in the composite rod cross section.

Under the influence of plastic deformation and heat generation during the FE process, the beta phase content in the extruded rods fabricated from as-cast and extruded AZ91 rods decreased by 62 and 60%, respectively. Regardless of the initial condition of the initial AZ91 rod, the beta phase value after the FE process does not differ much.

The presence of second-phase particles and high-strain at the near-surface zone of the composite rod resulted in the nucleation of the $β$ -Mg₁₇Al₁₂ phase and α + $β$ eutectic.

The presence of bioactive glass phase particles and the solid solution formed in the matrix prevented a sharp drop in hardness in FE-processed rods. The higher amount of beta phase and smaller grains in the composite rod fabricated from as-cast AZ91 rod led to higher hardness (74.5 ± 6.8 HV) in the composite rods.

The composite rod fabricated from the as-cast AZ91 rod showed the highest Rp (1403.21 Ω.cm²), the lowest i_corr (8.32 µA cm⁻²), and the lowest CR (0.21 mm year⁻¹). As a result, it had the best corrosion resistance.

Friction extrusion on AZ91 alloy billet with cast microstructure produced an AZ91-bioactive glass composite rod with ultimate tensile strength, yield strength, and corrosion resistance of 28, 29, and 93% higher than AZ91 alloy rod.

This research showed that the fabrication of AZ91-bioactive glass composite using the friction extrusion process could significantly improve the strength and corrosion resistance of the SBF solution. Therefore, it can be concluded that AZ91-bioactive glass composite is a suitable replacement for magnesium alloys in making implants.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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