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In this work, variation in the dynamic recrystallization (DRX) and dynamic precipitation behavior of AZ80 alloy during extrusion due to changes in extrusion temperature was investigated, and the resultant microstructure and yield asymmetry were analyzed. As the extrusion temperature increases from 250 °C to 350 °C, the primary DRX mechanism changes from twin-induced DRX to discontinuous DRX, resulting in an increase in the DRX area fraction and unDRXed grain size. In addition, as the extrusion temperature rises, Mg17Al12 precipitation during extrusion decreases sharply throughout the extruded material. The reduction in the compressive yield strength (CYS) with increasing extrusion temperature is more pronounced than it is for the tensile yield strength (TYS), which ultimately increases the yield asymmetry of the extruded material. The higher extrusion temperature has less of an influence on the TYS due to the promotion of certain hardening effects. On the other hand, the greater reduction in the CYS is attributed to the increased fraction and size of regions in which { 10 1 ¯ 2 } twins predominantly form and the lower amount of precipitates, which effectively facilitates { 10 1 ¯ 2 } twinning.

The effect of a novel pre-homogenization deformation (PHD) treatment on the microstructure and mechanical properties of extruded AlMg5Si2Mn alloy was investigated. By introducing the cold compression prior to homogenization treatment, the brittle and interconnected eutectic Mg2Si phase in the as-cast AlMg5Si2Mn alloy transformed into fine and spherical particles uniformly dispersed in the aluminum matrix. As the amount of PHD increased, up to 10%, the size of the Mg2Si particles decreased and their sphericity increased in the extruded samples, resulting in a gradual increase in both strength and elongation after heat treatment. The mechanical properties of the PHD-treated AlMg5Si2Mn extrudates were compared with those of materials prepared using various casting and deformation processes, and the potential application to wrought products was discussed.

This study demonstrates that the combined addition of 0.5 wt% Ca and 0.2 wt% Y to a commercial AZ31 alloy leads to the promotion of dynamic recrystallization (DRX) behavior during hot extrusion, which significantly improves the mechanical properties of the extruded alloy. (Mg,Al) 2 Ca and Al 8 Mn 4 Y particles, which are formed during solidification by the addition of Ca and Y, cause an increase in the area fraction of dynamically recrystallized (DRXed) grains through particle-stimulated nucleation. These undissolved particles also cause the material to deform more severely during extrusion, which, in turn, causes a decrease in the size of the DRXed grains and an increase in the residual strain within unDRXed grains. The tensile yield strength of the extruded alloy improves considerably from 248 MPa to 290 MPa by the combined addition of small amounts of Ca and Y, without any loss of ductility. This drastic improvement in strength is attributed mainly to the combined effects of grain-boundary hardening caused by the increased DRX fraction and decreased DRXed grain size, strain hardening caused by the increased dislocation density in the unDRXed grains, and particle hardening caused by the numerous undissolved particles. Despite the significant improvement in strength and the presence of many brittle particles in the extruded alloy containing Ca and Y, the tensile elongation of the alloy hardly deteriorates; this is because a decrease in the area fraction of coarse unDRXed grains suppresses the formation of twins that act as initiation sites for microcracks during tensile deformation. In addition, decreases in the average grain size and texture intensity cause a larger increase in the compressive yield strength than in the tensile yield strength, and this consequently reduces the tension–compression yield asymmetry of the extruded alloy.

A new Mg alloy with the composition of Mg-8Al-0.3Zn-0.1Mn-0.3Ca-0.2Y was successfully fabricated by a low-temperature, low-speed indirect extrusion process. The extruded alloy showed excellent mechanical properties, i.e., a tensile yield strength of 379.3 MPa, ultimate tensile strength of 421.7 MPa, elongation of 11.3%, and yield asymmetry of 0.95, as well as a very high chip ignition temperature of 820 °C. The outstanding mechanical properties are attributed to the microstructural characteristics of the extruded alloy, that is, (i) grain boundary strengthening by fine recrystallized grains, (ii) precipitation hardening by abundant fine Mg17Al12 precipitates, and (iii) strain hardening by the deformed non-recrystallized region with a strong basal texture that is unfavorable for basal slip during tension. Electron backscatter diffraction results showed that no twinning occurred in the recrystallized grains, which had a very small size of approximately 800 nm, whereas {10–12} twins were formed in the non-recrystallized grains even at low compressive stress, which consequently increased the yield asymmetry.

This study investigates the effects of cold pre-forging (CPF) on the extrusion load during hot extrusion and on the microstructure and mechanical properties of extruded AZ80 alloy. For this purpose, a billet is forged to 10% strain at room temperature and then subjected to indirect extrusion at 200 °C. The CPF process causes the formation of a large amount of deformation twins and an increase in the dislocation density of the billet. The cold-forged AZ80 billet exhibits a higher extrusion load during extrusion than an unforged AZ80 billet, owing to the occurrence of twin-boundary hardening and enhanced strain-hardening effects in the former. The initial twins introduced by the CPF process promote dynamic recrystallization behavior during extrusion because they act as nucleation sites for recrystallization. As a result, the CPF process increases the area fraction of fine recrystallized grains and the microstructural homogeneity of the extruded alloy. The AZ80 alloy extruded after CPF shows a higher tensile strength and ductility than the AZ80 alloy extruded without CPF. The improvement in strength is attributed mainly to the decrease in the average grain size caused by an increased recrystallization fraction. The increased ductility is due to the reduced area fraction of coarse unrecrystallized grains, in which microcracks occur during tensile plastic deformation.

This study demonstrates that the application of artificial water cooling during extrusion effectively increases the extrudability of the AZ91 alloy and significantly improves the mechanical properties of the extruded AZ91 alloy. The artificial cooling dramatically reduces the actual temperature of the deformation zone, which results in an increase in the maximum exit speed at which the alloy is extrudable without the occurrence of hot cracking from 4.5 m/min to 7.5 m/min. It also promotes dynamic recrystallization and precipitation behaviors during extrusion, which leads to a reduction in grain size and an increase in the amount of fine Mg 17 Al 12 precipitates. As a result, for the AZ91 alloy extruded at an exit speed of 1.5 m/min, the tensile and compressive yield strengths improve significantly by 51 MPa and 114 MPa, respectively, and its tension–compression yield asymmetry reduces from 0.73 to 1.02 owing to the refinement of the grain size by artificial cooling. In addition, the AZ91 alloy extruded at an exit speed of 7.5 m/min with artificial cooling exhibits a finer grain structure than and superior mechanical properties to the AZ91 alloy extruded at a slower exit speed of 4.5 m/min without artificial cooling. This result indicates that the application of artificial cooling can simultaneously improve the maximum extrusion speed and the tensile and compressive properties of Mg alloys.

This study paid attention to the big difference between the corrosion rates of permanent mold cast and hot-extruded pure Mg. Fe existed as different state according to process history, which affected the corrosion behavior of pure Mg largely. The corrosion rate of high purity Mg was measured extremely low when Fe was dissolved into the matrix as a solid solution while it increased dramatically when Fe was precipitated as a second phase. The precipitation behavior of the second phase containing Fe was affected by other impurities as much as thermal history, which affected the corrosion behavior of high purity Mg. It is suggested from this study that the tolerance limit of Fe is strongly dependent on the content of Fe, the sort and content of other impurities and process history, which should be considered to design the composition and processing route of high corrosion-resistant Mg alloy.

In this study, the effects of processing history on corrosion behaviours of pure Mg with controlled impurity content, were analysed systematically. Results revealed that impurity Fe and Si were precipitated into the Fe-Si phase by heat treatment prior to extrusion, leading to severe micro-galvanic corrosion of as-extruded pure Mg despite the same impurity contents with as-cast pure Mg. This result suggests that control of the actual tolerance limit, considering not only the impurity content but also the processing history, is required in designing alloys.

Non-flammability and high production rate are prerequisite for extending application of Mg extrusion products. In this respect, the present study focuses on microstructure development and mechanical behavior of two novel Mg alloys as compared with a commercial Mg alloy. As a reference, Mg-8Al-0.4Zn (AZ80 bal. wt.%) is extrudable at low speed of 0.5-2.5 m/min. This is due to hot shortness during extrusion, which results from low melting point of secondary phases like Mg 17 Al 12 . Furthermore, Mg alloys are reactive in molten state and rapidly ignited as contacting with oxygen in air. Hence, a non-flammable Mg alloy was developed by a trace addition of 0.3%Ca and 0.2%Y, which form protective oxide layer on Mg grains (AZ80+0.3%Ca+0.2%Y denoted as SEN8). In other way, Mg-5Sn-4Al-1Zn (TAZ541) was developed for high-speed extrusion. Hereby Mg 2 Sn precipitate makes Mg matrix thermally stable due to its high melting point. Three Mg alloys were extruded in the form of 50 mm x 4 mm plate with extrusion ratio of 25:1 at 350 °C with 1.1 m/min. As-extruded microstructures exhibited fully recrystallized structure with equiaxed grains as well as partially elongated grains along the extrusion direction. There was no big difference in grain size with 17.7~19.6 µm. SEN8 showed the weakest texture owing to the effect of Ca and Y. This made yield strength of SEN8 lower and uniform strain higher than AZ80. Mg 2 Sn particles led to higher work hardening of TAZ541 than AZ80 and SEN8. Moreover, SEN8 and TAZ541 had higher hardening capacity due to lower yield strength and higher work hardening rate than AZ80. That is, their relatively weak textures resulted in superior hardening ability. This presented that texture has a more dominant effect on mechanical properties rather than grain size in extruded Mg alloys. Regarding the effects of Ca, Y and Sn, it enables development of next generation Mg alloys with high strength and non-flammability for high-speed extrusion.

Investigation of the microstructural characteristics and compressive properties of extruded AZ31 alloy plates with width–thickness ratios (w/t ratios) of 1, 2, and 3 revealed that a higher ratio promotes dynamic recrystallization (DRX) behavior during hot extrusion. This leads to a decrease in both the fraction of non-DRXed grains and the average grain size. Extruded alloy with a w/t ratio of 1 exhibits a ring basal fiber texture that is homogeneously distributed over its entire cross-sectional area, resulting in identical stress–strain behavior under compression along the normal direction (ND) and transverse direction (TD). In contrast, significant anisotropy appears between the compressive behavior along the ND and TD in alloys with w/t ratios of 2 and 3, which is attributed to a difference in the active dominant deformation mechanism (i.e., slip for the ND and twinning for the TD) caused by the strong ND-oriented basal texture. Consequently, the compressive yield strength is up to ~2.25 times higher along the ND than along the TD.

The effects of microstructural factors, including the fraction of second phase particles and solute content in the matrix, on the corrosion behavior of as-extruded Mg-Sn-Zn alloys were evaluated by immersion and salt spray testing. After immersion testing, the average corrosion rate increased with the increase in Sn content and decrease in Sn:Zn ratio. On the other hand, after salt spray testing, the average corrosion rate increased with the increase in Sn content, but it was minimized at the Sn:Zn ratio of 5:2. Hydrogen evolution appeared to be the dominant controlling factor of corrosion during immersion, while the partial protectiveness of the film also contributed to the corrosion during salt spray testing.

This investigation into the effect of indirectly extruding Mg–7Sn–1Al–1Zn (TAZ711) alloy at high exit speeds on the microstructure and tensile properties found no evidence of surface cracking, not even at the maximum speed tested of 27 m/min. This high-speed extrudability is attributed to the relatively high incipient melting temperature of the alloy (535 °C), which results from the formation of a thermally stable Mg 2 Sn phase. All extruded samples exhibited a completely recrystallized (DRXed) structure consisting of coarse DRXed grains with few particles in combination with relatively fine DRXed grains containing numerous fine precipitates. With an increase in extrusion speed, the total amount of Mg 2 Sn precipitates decreased and the size and quantity of coarse DRXed grains increased due to a rise in temperature during extrusion. This has the effect of reducing the strength of the extruded alloy through a reduction in precipitation and grain-boundary strengthening, yet the elongation of the extruded alloy remains essentially the same due to a loss of ductility caused by the increase in DRXed grain size.

The application of artificial cooling to the high speed extrusion (13.5 m/min) of Mg–7Sn–1Al–1Zn alloy has been found to dramatically reduce the size of recrystallized (DRXed) grains, while also increasing the number of fine Mg 2 Sn precipitates. This is attributed to a decrease in the temperature of the deformation zone, and has the effect of improving the strength of the extruded alloy. The increase in yield strength (YS) and ultimate tensile strength (UTS) is enhanced by increasing the feed rate of the cooling water due mainly to a decrease in the DRXed grain size. Significantly, this increase in strength does not come at the expense of a loss of elongation (EL), as the formation of twins that act as a crack initiation site is suppressed during tensile deformation by the grain refinement. As a result, high-strength TAZ711 Mg alloy (210 MPa YS, 303 MPa UTS, and 10.2% EL) can be obtained even with high-speed extrusion by using artificial cooling.

An Mg–7Sn–1Al–1Zn alloy known to have excellent extrudability and superior strength was subjected to artificial cooling during indirect extrusion by directly spraying water onto the extruded rod at the die exit. The results obtained revealed that this artificial cooling dramatically reduces the temperature of the deformation zone during extrusion, thereby creating a finer grain size, an intensified texture and a greater amount of precipitates when compared to extrusion without artificial cooling. The yield and tensile strength of the extruded alloy is also significantly improved, which is attributed to the effects of grain refinement in combination with an enhanced texture and precipitate hardening.

Grain refinement is an effective method to increase both the strength and toughness of structural materials. Among the various grain refinement methods, carbon inoculation is an effective method for Mg alloys containing Al. SiC is well-known as a carbon inoculation agent; however, conventional methods for using SiC are suitable only for lab-scale experiments. This study investigates the inoculation methods with regard to the use of SiC effectively and economically for grain refinement of Mg alloys, especially for application in large-scale casting processes. Al-SiC refiner was fabricated by extrusion; it demonstrated a uniform structure with well distributed SiC particles on the Al metal matrix. After adding 0.7 wt% extruded SiC refiner, the average grain size of AZ91 alloy decreased from 550 to 90 μm. It demonstrated an effective refining ability in large-scale casting facilities.

The microstructure and tensile properties of a Mg–3Al–1Zn alloy processed by low-temperature indirect extrusion utilizing artificial cooling were investigated. Grain refinement from 5.5 to 1.8 μm was attained by the application of artificial cooling, resulting in an increase in yield strength of as much as 50 MPa at room temperature as well as enhanced plasticity with tensile elongations of 200–320% at strain rates in the range of 3.3 × 10−5–1.0 × 10−4 s−1 at 150 °C.

This paper analyzed the relationships between the extrusion conditions, grain size, and yield strength of an Mg-7Sn-1Al-1Zn alloy, which was extruded at different initial billet temperatures and ram speeds, and developed empirical models to predict the grain size and yield strengths. The results revealed that grain size increases as the extrusion temperature and ram speed increase, resulting in a decrease of the tensile and compressive yield strengths due to reduced effect of grain boundary strengthening. It was also found that the exit temperature is a key determinant of grain size and yield strength, i.e. as the exit temperature decreases, the grain size decreases while the tensile and compressive yield strengths increase. The grain size and yield strength prediction models, which provide results as a function of temperature and strain rate, were developed by considering the extrusion temperature and speed dependencies of the exit temperature, and the predicted results showed a good agreement with the experimental data.

This study demonstrates that cold pre-forging (CPF) conducted before extrusion is a promising means for improving the mechanical properties of extruded magnesium alloys. The CPF process induces numerous twins in the billet, which in turn provides nucleation sites for dynamic recrystallization during extrusion, leading to an increase in the dynamically recrystallized (DRXed) fraction of the extruded alloy. This process increases the uniformity of the DRXed grain structure, thereby improving the strength and ductility of the extruded alloy.

The effect of addition of Sn and/or Zn on corrosion properties of Mg–6Al–0.3Mn alloy was investigated systematically by immersion and salt spray tests. In case of immersion test, average corrosion rate increased by addition of 1 wt% Sn but it decreased by addition of 1 wt% Zn. When the half of the amount of Sn addition was replaced by Zn, the average corrosion rate decreased in comparison with the average corrosion rate of Mg–6Al–0.3Mn–1Sn alloy. In case of salt spray test, the change of average corrosion rate according to composition of the alloy was similar to the result of immersion test. It seemed that the change of corrosion properties according to composition of the alloy was strongly related to the microstructural changes including the sort, fraction and morphology of second phase.

Magnesium alloys exhibit poor ignition resistance after the addition of elements whose solubility limit in the matrix is extremely low, although these elements can thermodynamically bring about the formation of an additional protective oxide layer by alloying. The solute concentration of the alloying elements dissolved in a matrix, in conjunction with conventionally used factors such as the Pilling–Bedworth ratio and the Gibbs free energy change for oxidation, should be considered as an important factor influencing the ignition resistance of magnesium.

The AZ31 magnesium alloys were subjected to indirect and direct extrusion processes, respectively, and the properties of the extruded alloys were compared. It was shown that the indirect-extruded alloy has a lower yield asymmetry between tension and compression than the direct-extruded alloy, which is due to a weaker fiber texture in the former.

The combined addition of calcium and yttrium lead to significant increase in non-flammability and tensile properties, compared to conventional Ca-containing magnesium alloys. It means that we can reduce the amount of calcium addition to get certain amount of non-flammability in the alloys containing both calcium and yttrium together. Because the addition of calcium only can increase the ignition temperature but decrease the tensile properties, it should be minimize not to decrease the tensile properties. Corrosion behavior of calcium and yttrium containing alloys were also investigated systematically, and show that they have better corrosion resistance than the any other commercial alloys or calcium containing alloys. Detailed study on the corrosion behavior confirms that there are optimum amount of calcium and yttrium addition to get best corrosion resistance, and corrosion behavior of these alloys are quite different with other alloys.

Grain refinement involving inoculation of a grain refiner was investigated for application to a commercial Mg alloy. Cylindrical pellets composed of a mixture of MnCO3 and carbon powder with several different ratios were plunged into a commercial AZ91 alloy. The results show that the addition of the mixed pellets can more efficiently refine AZ91 compared to the individual use of carbon powder or MnCO3. In particular, the addition of mixed pellets with a 3:7 ratio between MnCO3 and carbon provides an excellent refining efficiency, decreasing grain size from 460µm to 52µm of the AZ91 alloy. This is attributed to inoculation of MnCO3 and carbon, which brings about the formation of heterogeneous nucleants such as MgO, Al4C3, and Al8Mn5 before solidification of a-Mg and also to melt agitation by the release of CO2 gas