Effect of Hot Extrusion on Microstructure, Texture, and Mechanical Properties of Mg-Zn-Mn-0.5Ca Alloy

Abstract

This paper investigates the microstructure, texture, and mechanical properties of the Mg-4Zn-1Mn-0.5Ca alloy subjected to hot extrusion under varying conditions of temperature (260 °C, 300 °C, 340 °C) and extrusion speed (0.01 mm/s, 0.1 mm/s, 1 mm/s). The primary objective is to determine the optimal extrusion parameters within the selected experimental range for achieving superior mechanical properties. The results indicate that, when extruded at a constant speed of 0.1 mm/s, the alloy exhibits optimal performance at 340 °C, with a yield strength of 202 MPa, ultimate tensile strength (UTS) of 306 MPa, and elongation at fracture of 18.9%. A decrease in extrusion temperature leads to an increase in yield strength but a reduction in ductility. Specifically, the UTS reaches its peak at 342 MPa at 300 °C, while it drops slightly to 329 MPa at 260 °C. The final results show that the comprehensive mechanical properties of the Mg-4Zn-1Mn-0.5Ca alloy obtained by hot extrusion treatment with an extrusion temperature of 300 °C and extrusion speed of 0.1 mm/s are the best and can effectively improve the mechanical properties of the alloy and provide a good choice for the preparation of other biodegradable magnesium alloy products.

1. Introduction

Thermal extrusion stands as a prominent processing method for magnesium alloys. During extrusion, the material experiences triaxial compressive stress, which suppresses microcrack formation, thereby enhancing material ductility. Furthermore, thermal extrusion plays a crucial role in improving the morphology and distribution of second phases within magnesium alloys, reducing casting defects, and enhancing material density.

Different types of thermal extrusion, including direct extrusion, indirect extrusion, and lateral extrusion, involve controlling parameters such as extrusion ratio, temperature, and speed during the deformation process. Various extrusion processes exhibit distinct effects on alloy grain size, precipitate phases, texture, and mechanical properties. Park et al. [1] studied the influence of reverse extrusion rate on the microstructure and properties of Mg-7Sn-1Al-1Zn alloy. The results showed that an increase in extrusion speed led to a decrease in the volume fraction of precipitated phases due to the generation of significant extrusion heat at higher speeds, increasing the solubility of the alloy element Sn in magnesium and reducing the precipitation of second phases. Moreover, elevated extrusion temperatures accelerated the growth rate of dynamically recrystallized grains, resulting in grain coarsening and collectively causing a decline in the alloy’s tensile strength. Guo [2] investigated four different thermal extrusion processes for homogeneous Mg-5Li-3Sn-2Al-1Zn alloy, revealing finer grains due to dynamic recrystallization occurring during the extrusion process compared to the grains in the homogeneous alloy. Post thermal extrusion processing, TYS, UTS, and EL of Mg-5Li-3Sn-2Al-1Zn alloy were significantly increased to varying degrees. Wang et al. [3] studied the dynamic precipitation and dynamic recrystallization behavior of Mg-7 Gd-2Nd-0.5Zr alloy during the hot compression process and found that under appropriate hot compression conditions, dynamic precipitation and dynamic recrystallization occur in the alloy, and dynamic precipitation precedes dynamic recrystallization. Higher temperature and strain rate promote DRX, and dynamic recrystallization mainly occurs in the deformation zone.

In hot extrusion processes, it is essential to control key deformation parameters such as extrusion ratio, extrusion temperature, and extrusion speed. Significant research has already been conducted on the hot extrusion of Mg-Zn-Ca alloys. Most previous studies, however, have focused primarily on the high-temperature fast extrusion of highly alloyed Mg-Zn-Ca systems, as well as on the effects of extrusion parameters on the recrystallization behavior and texture evolution in Mg-Zn-Mn-Ca alloys [4,5]. In contrast, studies investigating low-alloy Mg-Zn-Mn-Ca under conditions of low-temperature slow-speed extrusion remain relatively scarce. Therefore, this study aims to address this gap by employing a Mg-4Zn-1Mn-0.5Ca alloy, known for its favorable mechanical properties, in a series of hot extrusion experiments conducted at different temperatures and speeds. The objective is to investigate how variations in extrusion temperature and speed affect the microstructure and mechanical properties of this alloy. Through analyzing the changes in microstructure, this study highlights the influence of hot extrusion conditions on grain structure and dynamic recrystallization behavior in magnesium alloys. Additionally, tensile tests were performed at room temperature on extruded samples from each set of conditions to evaluate how these microstructural changes impact the strength and ductility of the alloy. The findings of this research provide valuable insights into the comprehensive performance of calcium-containing magnesium alloys. Moreover, they offer promising options for the development and fabrication of other biodegradable magnesium alloy products [6,7].

2. Experimental Procedure

The experimental material used in this study is an as-cast magnesium–zinc–calcium alloy. During the casting process of magnesium alloys, fast thermal conductivity and steel molds with high heat exchange efficiency lead to rapid cooling rates, resulting in chemical composition variations, element segregation, and dendritic structures within the alloy, reducing its thermal plasticity and workability. Therefore, a homogenization treatment is essential before conducting hot extrusion.

In Mg-Zn-Mn-Ca alloy systems, the melting point of the second phase MgZn is 347 °C, while the thermal stability of the second phase Ca₂Mg₆Zn₃ is relatively good. To ensure significant solid solution of the second phases into the magnesium matrix, the chosen homogenization treatment parameters involved heat preservation at 380 °C for 12 h in the box furnace and air cooling once out of the oven.

In this experiment, a constant extrusion ratio of 16:1 was used to squeeze the billet from a 40 mm diameter to 10 mm diameter. In order to reduce friction during the extrusion process, a lubricant was added. With reference to previous research on the high-temperature extrusion of rare-earth-free magnesium alloys [8,9], a constant extrusion speed at 0.1 mm/s was selected, and the extrusion temperature was selected at 260 °C, 300 °C, and 340 °C; when the extrusion temperature was constant at 300 °C, the extrusion speed was 1 mm/s, 0.1 mm/s, and 0.01 mm/s, respectively, and the temperature deviation in the extrusion process did not exceed 10 °C. This experiment verified the proposed research content, that is, the influence of extrusion temperature and extrusion speed on the texture and mechanical properties of MG-Zn-Ca alloy in the selected parameter range. Ultimately, an oil-based graphite lubricant was chosen due to its excellent performance at high temperatures. The extrusion experiments were conducted using the Zhonglu Chang ZLC-300 extrusion machine (St. Zhongluchang, Jinan, Shandong, China), which has a maximum extrusion force capacity of 3000 kN, as shown in Figure 1. This setup ensured that the extrusion process could be carried out efficiently while maintaining control over the key parameters affecting material flow and microstructural evolution.

By carefully selecting both the extrusion conditions and the equipment, this experimental design provides a robust foundation for examining how variations in temperature and speed influence the microstructure and mechanical properties of magnesium alloys. The choice of lubricant, extrusion machine, and process settings reflects the importance of fine-tuning each aspect of the experiment to ensure accurate and reliable results.

The extrusion process for the Mg-Zn-Mn-Ca alloy was as follows:

• Preheating and insulation of the mold: Heat the furnace to 50 °C above the extrusion temperature to preheat the mold. Once the internal temperature of the extrusion cylinder reaches the extrusion temperature, adjust the furnace temperature to maintain insulation at the extrusion temperature for 2 h.
• Billet insulation: After the mold insulation time elapses, apply lubricant to the inner wall of the extrusion cylinder and the surface of the billet. Sequentially place the billet and spacer into the extrusion mold for heating and insulation, maintaining it for 1 h.
• Hot extrusion deformation: Insert the extrusion rod into the mold, set the stroke speed of the extrusion machine, and commence extrusion, ensuring temperature control during the process.
• Unloading and mold removal: Upon completion of extrusion, saw the extruded rod for air cooling, label it appropriately, then remove the mold. Clean the spacer and interior of the extrusion cylinder for subsequent extrusion processes.

Tensile tests were conducted on a DNS200 universal testing machine at a strain rate of 10⁻³ s⁻¹. Dog bone-shaped tensile specimens measuring 10 mm × 3 mm × 1 mm were cut along the extrusion direction of the rods for each alloy, with three samples tested for each alloy. Furthermore, microhardness measurements were performed on the surface of thin-walled microtubes. Observation of the samples’ microstructures was carried out using an optical microscope (OM; Leica DM270, St. leica, Wiesbaden, Germany) and a scanning electron microscope (SEM; TESCAN-S800, Brno, Czechia) equipped with an HKL electron backscatter diffraction (EBSD) system. For OM observations, the samples were initially ground on silicon carbide papers of varying grit sizes, followed by etching in a solution of 1 mL glacial acetic acid, 6 mL ethanol, and 0.6 g picric acid. The samples were then cut and ground along the longitudinal direction, followed by electrochemical polishing at −25 °C in InAc2 solution at 20 V voltage and 0.03 A current for 90 s. Additionally, data acquisition and analysis were performed using Channel 5 software. Phase analysis was conducted using an X-ray diffractometer.

3. Results and Discussion

3.1. Microstructure of Extruded Rods

Extrusion temperature and extrusion speed are the two key variable parameters in this study. The microstructural evolution of the Mg-4Zn-1Mn-0.5Ca alloy extruded at a constant speed of 0.1 mm/s, under varying temperatures (260 °C, 300 °C, and 340 °C), is shown in Figure 2, At the same time, in order to observe the change in the whole grain and the local coarse grain, sampling observation was carried out at different magnifications. From the optical microscopy (OM) images, it can be observed that as the extrusion temperature increases, the fraction of recrystallized regions within the alloy also increases. Simultaneously, the size of the recrystallized grains in these regions is gradually enhanced. The reduction in grain size correlates with an improvement in the alloy’s yield strength. During plastic deformation, fine grains allow dislocation slip to occur more evenly across multiple grains, which helps distribute deformation uniformly and reduces the likelihood of stress concentration. Additionally, grain boundary sliding can further aid in accommodating deformation, contributing positively to the material’s ductility [10]. Grain size, therefore, not only influences the yield strength of the magnesium alloy but also affects its tensile strength [11]. As the grain size decreases, both the tensile strength and elongation at fracture improve, indicating that fine grains have a beneficial effect on enhancing the overall strength and plasticity of the magnesium alloy. Furthermore, with increasing extrusion temperature, the number of non-recrystallized deformed coarse grains decreases. The secondary phase in the alloy, consisting of particles and bands, shows no significant change in morphology or distribution with varying extrusion temperatures.

Figure 3 displays OM images from different regions of the extruded bar. It was found that closer to the surface of the bar, the number of large, non-recrystallized grains decreases, and the microstructure is primarily composed of uniform, fine recrystallized grains. Conversely, near the center of the bar, fewer fine recrystallized grains are present, and the number of elongated, non-recrystallized coarse grains increases, leading to a more heterogeneous microstructure.

This microstructural non-uniformity is mainly caused by uneven deformation during the hot extrusion process. In the extrusion process, the center of the bar experiences less deformation compared to the edges, resulting in a higher degree of deformation toward the periphery. Consequently, the grains at the edges of the bar are finer than those at the center. This inherent difference in deformation levels leads to the observed variation in microstructure between the center and outer regions of the extruded bar.

Extrusion temperature stands as the most crucial parameter in the hot extrusion process, significantly impacting the smooth progression of extrusion and exerting a substantial influence on the microstructure and properties of the extruded rod [12]. The dynamic recrystallization of magnesium alloys is profoundly influenced by the deformation temperature. The driving force behind dynamic recrystallization lies in the energy difference between grains in the alloy, represented by the difference in dislocation density. This indicates that both the nucleation and growth of dynamically recrystallized grains are controlled by the ability of dislocation movement, also relating to atomic diffusion along grain boundaries. Higher deformation temperatures facilitate dynamic recrystallization occurrence, leading to more complete dynamic recrystallization with increased extrusion temperature. Consequently, the degree of dynamic recrystallization in magnesium alloys intensifies with rising extrusion temperatures, resulting in further grain growth during recrystallization.

According to the Zener–Holloman parameter, also known as the Z-parameter [13]:

$$Z=\stackrel{•}{\epsilon }\mathit{exp}(Q/\mathit{RT})$$

(1)

• $\stackrel{•}{\epsilon }$: strain rate;
• Q: activation energy, Mg activation energy of 135 KJ/mol;
• R: gas constant;
• T: deformation temperature.

d_DRX = AZ⁻ⁿ

(2)

• d_DRX: recrystallization grain diameter;
• A: constant;
• n: power law exponent.

According to Formula (1), when the deformation level remains constant, meaning a consistent strain rate, reducing the extrusion temperature will result in an increase in the Z value. As per Equation (2), the diameter of recrystallized grains varies inversely with the Z value, indicating that the diameter of recrystallized grains decreases as the extrusion temperature lowers. Lowering the deformation temperature leads to a finer recrystallized grain structure.

Extrusion speed is another significant parameter in the hot extrusion process. At higher extrusion speeds, the thermal effects induced by deformation raise the billet’s temperature, reducing flow stress [14]. However, further acceleration of the extrusion speed raises flow stress due to the hardening rate during metal deformation surpassing the softening rate of recrystallization, hindering material formability. Extrusion speed also impacts the surface quality of magnesium alloys, with slower speeds generally resulting in better surface quality for the material.

Figure 4 displays OM images of the Mg-4Zn-1Mn-0.5Ca alloy extruded at 300 °C using different speeds (0.01 mm/s, 0.1 mm/s, 1 mm/s). It can be observed that dynamic recrystallization occurs in all Mg-4Zn-1Mn-0.5Ca alloys extruded at different speeds, with varying degrees of completion. The alloy grains exhibit significant refinement. As the extrusion speed increases gradually from 0.01 mm/s to 1 mm/s, the volume fraction of recrystallization in the alloy gradually rises, recrystallized grains grow, and the area of non-recrystallized regions decreases. Furthermore, it is noted that the extrusion speed does not significantly affect the morphology and distribution of second phases within the alloy.

In the case of extrusion at 0.01 mm/s, the finer recrystallized grains may be attributed to the following reason: at lower and constant extrusion temperatures, lower extrusion speeds lead to a decrease in the critical stress required for recrystallization nuclei formation [15]. This facilitates the nucleation process of recrystallization, resulting in grain refinement. In contrast, for extrusion at 1 mm/s, the larger recrystallized grains can be explained as follows: as extrusion speed increases, a significant amount of extrusion heat is generated during the hot extrusion process, and intensified friction between the alloy and the die results in additional frictional heat. These thermal effects elevate the billet’s temperature, promoting dynamic recrystallization. According to Formulas (1) and (2), an increase in temperature leads to larger diameters of recrystallized grains.

3.2. Recrystallization Distribution Texture Evolution of Extruded Rods under Different Extrusion Conditions

3.2.1. Extrusion at Different Temperature Conditions

Figure 5 illustrates the EBSD analysis of the Mg-4Zn-1Mn-0.5Ca alloy extruded at 0.1 mm/s under different temperatures, with Figure 5a–c corresponding to extrusion temperatures of 260 °C, 300 °C, and 340 °C, respectively. At 260 °C extrusion, the recrystallized grain size is relatively small, resulting in a lower indexing rate in the recrystallization region. In Figure 5b,c, most grains exhibit a basal plane parallel to the extrusion direction. In Figure 5a, some non-recrystallized large grains show a certain degree of deviation in orientation, where their c-axis forms a specific angle with the extrusion direction. The non-recrystallized deformed large grains in Figure 5b display the same orientation as their recrystallized grains, with recrystallized small grains and deformed grains around these large grains maintaining similar orientations, reflecting characteristics of dynamic recrystallization.

Observations reveal that as extrusion temperature increases from Figure 5a–c, and the recrystallized grain size gradually enlarges, while orientations become more random. Examination of the orientation difference distribution maps in Figure 5b,c shows a decrease in average orientation difference with the rise in extrusion temperature, and in the orientation difference distribution map in Figure 5c, an increase in the proportion of high-angle grain boundaries above 80° is observed. Additionally, Figure 4 presents the distribution of Schmid factors for the Mg-4Zn-1Mn-0.5Ca alloy extruded at three different temperatures. Notably, at 260 °C extrusion, there is a distinct peak in Schmid factor distribution, indicating the presence of non-recrystallized deformed large grains. With a Schmid factor of 0.09, sliding systems are less likely to initiate, leading to deformation challenges. As the extrusion temperature rises, the Schmid factor increases, with an escalation in grains colored green, yellow, and red in the Schmid plot, representing the alloy’s orientation softening and easier initiation of slip systems [2], which is beneficial for the alloy’s plasticity.

The recrystallization distribution of the Mg-4Zn-1Mn-0.5Ca alloy extruded at 0.1 mm/s under different temperatures is depicted in Figure 6. The white areas represent non-recrystallized grains, while the blue regions signify recrystallized grains. Texture analysis for three regions is provided on the right side, with Figure 6a–c corresponding to extrusion temperatures of 260 °C, 300 °C, and 340 °C, respectively. At 260 °C extrusion, due to the smaller size of recrystallized grains, the EBSD indexing rate is not notably high. Nonetheless, it is evident that non-recrystallized deformed grains occupy a significant proportion within the selected area, contributing to a high texture intensity of approximately 40 in the non-recrystallized region [16,17], which aids in enhancing alloy strength.

Figure 6b,c illustrate that as the extrusion temperature rises, the basal plane texture intensity in the recrystallized region of the alloy decreases, whereas it exhibits a slight increase in the non-recrystallized region. However, the overall texture intensity in the non-recrystallized region remains relatively low, with no significant changes observed in the microtexture of the selected area. It is also noted that, with increasing extrusion temperature, the influence of texture intensity in the recrystallized region on the alloy’s selected area becomes more pronounced, while the contribution from the non-recrystallized region weakens. In summary, extrusion temperature affects how much the recrystallized and non-recrystallized regions contribute to the microtexture of the selected area.

Figure 7 displays the EBSD analysis results of the volume fractions of recrystallized and non-recrystallized regions of the Mg-4Zn-1Mn-0.2Ca alloy after extrusion at 0.1 mm/s speed and different temperatures. Due to the significantly low indexing rate at 260 °C, it is not discussed. It can be observed that with increasing extrusion temperature, the volume fraction of the recrystallized region in the alloy gradually increases, which aligns with the findings from metallographic observations.

3.2.2. Different Speed Extrusion Conditions

Figure 8 shows the EBSD analysis of Mg-4Zn-1Mn-0.5Ca alloy extruded at different speeds at 300 °C. In the previous experiments of extrusion temperature, the comprehensive performance of the sample obtained at 300 °C was the best, so a temperature of 300 °C was selected for the experiment with different extrusion speeds. Figure 8a–c correspond to extrusion speeds of 0.01 mm/s, 0.1 mm/s, and 1 mm/s, respectively. The grain size displayed in the IPF map follows the same pattern as that seen in the microstructure image. It is observed that at all three extrusion speeds, most grains in the alloy have their basal planes parallel to the extrusion direction [17]. Non-recrystallized large grains exhibit the same orientation as their recrystallized counterparts, with recrystallized small grains and deformed grains around these large grains maintaining consistent orientation, still exhibiting characteristics of dynamic recrystallization. In the orientation difference distribution map, it can be noted that with increasing extrusion speed, the proportion of low-angle grain boundaries decreases. At an extrusion speed of 1 mm/s in Figure 8c, there is an increase in the proportion of high-angle grain boundaries above 80° in the orientation difference distribution map. Additionally, Figure 8 illustrates the distribution of Schmid factors for the alloy at the three extrusion speeds. It is observed that at an extrusion speed of 0.01 mm/s, there are more grains in a biased blue color, indicating a prominent plateau region in the Schmid factor distribution, representing non-recrystallized deformed large grains. With Schmid factors below 0.08, slip system activation is challenging, making deformation difficult. As the extrusion temperature increases, the Schmid factor also increases. The number of grains leaning towards green, yellow, and red colors in the Schmid map increases, indicating a softening of the alloy’s orientation, making slip system activation easier and beneficial for the alloy’s ductility. When the extrusion speed reaches 1 mm/s, another higher plateau region appears on the left side of the alloy’s Schmid factor distribution, corresponding to two large, deformed grains in the Schmid map.

The non-recrystallized distribution of the Mg-4Zn-1Mn-0.5Ca alloy is shown in Figure 9, where the white areas represent non-recrystallized grains and the blue areas signify recrystallized grains [18]. The right side presents texture analyses for three regions corresponding to extrusion speeds of 0.01 mm/s, 0.1 mm/s, and 1 mm/s (Figure 9a–c). It is evident that the basal texture strength of the recrystallized areas in all three alloys is relatively weak. When the extrusion speed reaches 1 mm/s, the basal texture strength of the recrystallized area minimizes at 4.13, while the texture strength of the non-recrystallized area becomes the highest among the three alloys, nearing 49. In general, the trend in basal texture strength in the recrystallized areas of the three alloys shows an initial increase followed by a decrease, with a relatively modest increase. Conversely, the texture pattern in the non-recrystallized areas decreases before rising, mirroring the microstructural texture evolution in the selected alloy regions. This consistency suggests that the alloy’s texture is predominantly influenced by non-recrystallized deformed large grains.

Figure 10 presents the EBSD analysis results of the volume fractions of recrystallized and non-recrystallized regions in Mg-4Zn-1Mn-0.2Ca alloy after hot extrusion at different speeds and the same temperature. Combined with the previous microstructural images, it can be observed that with increasing extrusion speed, the volume fraction of recrystallized regions in the alloy gradually increases. This increase is more pronounced when transitioning from a speed of 0.01 mm/s to 0.1 mm/s, while the increment in the volume fraction of recrystallized regions is less noticeable when going from 0.1 mm/s to 1 mm/s.

3.3. Mechanical Properties

Figure 11 illustrates the tensile mechanical properties of the Mg-4Zn-1Mn-0.5Ca alloy extruded at speeds of 0.1 mm/s at different temperatures (260 °C, 300 °C, 340 °C). The various values summarized from Figure 12 are shown in

Table 1. Various mechanical properties of Mg-4Zn-1Mn-0.5Ca extruded at different temperatures.

Samples	Yield Strength/MPa	Tensile Strength/MPa	Elongation/%
260 °C extrusion	281 ± 2	329 ± 4	11.6 ± 0.1
300 °C extrusion	254 ± 3	342 ± 2	15.8 ± 0.2
340 °C extrusion	202 ± 2	306 ± 3	18.9 ± 0.1

. It is observed in the graph that when the extrusion speed remains constant at 0.1 mm/s, after extrusion at 340 °C, the yield strength of the Mg-4Zn-1Mn-0.5Ca alloy is 202 MPa, the ultimate tensile strength is 306 MPa, and the elongation at break is 18.9%. As the extrusion temperature decreases, the yield strength of the alloy increases, the elongation decreases, and the ultimate tensile strength initially rises to 342 MPa, then drops to 329 MPa at 260 °C.

The crystal phase structure of the Mg-4Zn-1Mn-0.5Ca alloy extruded at different temperatures consists of fine recrystallized grains and coarse non-recrystallized deformed grains. The strength and plasticity of magnesium alloys are jointly determined by these two types of grains [19]. With decreasing extrusion temperature, the reason for the increased strength of the alloy is as follows: as the extrusion temperature decreases, the size of recrystallized grains in the recrystallization region of the alloy decreases [20]. According to the Hall–Petch formula, the strengthening effect of fine grains is enhanced. Simultaneously, with the decrease in extrusion temperature, the volume fraction of large non-recrystallized deformed grains in the alloy increases, resulting in hindered dislocation motion within them. Dislocations pile up and accumulate, leading to a greater deformation strengthening effect. The combined effect of these two mechanisms causes the strength of the alloy to increase as the extrusion temperature decreases. Moreover, the increasing number of large non-recrystallized deformed grains with decreasing extrusion temperature reduces the deformation coordination ability of the alloy during tension, deteriorating its plasticity, and lowering the elongation at break.

Figure 12 depicts the room temperature tensile mechanical properties of Mg-4Zn-1Mn-0.5Ca alloy extruded at 300 °C with different speeds (0.01 mm/s, 0.1 mm/s, 1 mm/s). The various values summarized from Figure 12 are shown in

Table 2. Various mechanical properties of Mg-4Zn-1Mn-0.5Ca extruded at different speeds.

Samples	Yield Strength/MPa	Tensile Strength/MPa	Elongation/%
0.01 mm/s extrusion	224 ± 3	322 ± 2	19.1 ± 0.2
0.1 mm/s extrusion	254 ± 3	342 ± 2	15.8 ± 0.2
1 mm/s extrusion	206 ± 1	331 ± 4	12.7 ± 0.2

. It is evident from the graph that when the extrusion temperature is constant, lower extrusion speeds result in higher yield strength and ultimate tensile strength overall, along with lower elongation at break for Mg-4Zn-1Mn-0.5Ca. When the extrusion speed is 0.01 mm/s, the alloy’s strength is lower than that at 0.1 mm/s, possibly due to excessively prolonged extrusion leading to abnormal furnace temperature control, affecting the mechanical performance of the alloy.

The increase in alloy strength with decreasing extrusion speed is attributed to the following factors: as the extrusion speed decreases, the size of recrystallized grains in the recrystallization region of the alloy reduces, resulting in fine grain strengthening and enhancing the alloy’s strength [8]. Additionally, as the extrusion speed decreases, the volume fraction of large non-recrystallized deformed grains in the alloy increases. This hinders internal dislocation motion within these grains, leading to dislocation pile-ups and enhanced deformation strengthening effects. The combined effect of these mechanisms causes the alloy’s strength to rise with decreasing extrusion speed. The increased presence of large non-recrystallized deformed grains reduces the deformation coordination ability of the alloy during tension, deteriorating its plasticity and lowering the elongation at break.

4. Conclusions

In this study, extrusion of the Mg-4Zn-1Mn-0.5Ca alloy with good comprehensive mechanical properties was carried out at different temperatures (260 °C, 300 °C, 340 °C with extrusion speed of 0.1 mm/s and extrusion ratio of 16:1) and different speeds (0.01 mm/s, 0.1 mm/s, 1 mm/s, extrusion temperature 260 °C, 300 °C, 340 °C, extrusion ratio 16:1). The effects of extrusion temperature and extrusion speed on the microstructure, texture, and mechanical properties of Mg-4Zn-1Mn-0.5Ca alloy extrusion bars were studied. The microstructure and mechanical properties of the extruded Mg-4Zn-1Mn-0.5Ca alloy were investigated. The following conclusions are drawn:

(1)

With the decrease in extrusion temperature, the recrystallization region of the alloy decreases and the recrystallization grains become finer. Tensile strength and yield strength increase with the decrease in extrusion temperature, and the elongation decreases. At the same time, the texture of the recrystallization zone of the alloy decreases, while the overall texture of the alloy is slightly enhanced. With the decrease in extrusion speed, the non-recrystallized region expands and the recrystallized grain size decreases. In general, the tensile strength and yield strength increase with the decrease in extrusion speed, while the elongation tends to decrease. The texture strength of the recrystallization zone of the alloy decreases with the increase in the extrusion speed, and the texture of the alloy increases with the increase in the extrusion speed.

(2)

On the basis of observing the microstructure and recrystallized grains of the sample, the mechanical properties of the sample obtained under the selected parameters were tested. It was found that the comprehensive mechanical properties of the sample were the best when the temperature was 300 °C and the extrusion speed was 0.1 mm/s, which is expected to become a reference in the actual production process.