挤压AZ91D合金管件组织和力学性能的研究

    

摘要:本文研究了在挤压温度为430℃时,挤压比对镁合金AZ91D挤压管件组织和力学性能的影响,通过分析挤压过程中镁合金AZ91D组织和力学性能的变化,发现挤压比为7.125时,抗拉强度达到最大值306.9MPa,伸长率达到最大值10.1%。当挤压比增大到7.45时,屈服强度达到最大值285.795, 而抗拉强度和伸长率出现下降的趋势。试验得出:在高于再结晶温度下(430℃)挤压镁合金AZ91D,通过调整挤压比,控制再结晶的程度能够获得良好力学性能镁合金AZ91D管材。
Microstructure and Mechanical Properties ofAZ91D Extruded Tube
YU Bao-yi, BAO Chun-ling, SONG Hong-wu, LIU Zheng, YU Hai-peng
(School of Material Science and Engineering, Shenyang University of Technology, Shenyang 110023,China)
Abstract: The influence of extrusion ratio on microstructures and mechanical properties of magnesium alloys AZ91D extruded tube at 430℃ has been researched. After the evolution of microstructures and mechanical properties of AZ91D during extrusion is studied, it shows: tensile strength got the climax value 306.9MPa and elongation peak value 10.1% when extrusion ratio is 7.125, and with extrusion ration increasing to 7.45, yield strength got top value 285.795 MPa with decreased tensile strength and elongation. The conclusion is that mechanical properties of magnesium alloys AZ91D can be enhanced by adjusting extrusion ratio near recrystallization can be got.
Key words:Magnesium; Tube ; Microstructure.; Tensile property

1. Introduction

The application of magnesium alloy tube which has a great application potential such as automobile, aerospace and so on but is so limited, The main reasons is force of extrusion of magnesium alloy tube sometimes is limited by extruding machine ,for its limit of bearing capacity; at the meantime, the production rate is also low. So it is necessary to increase extrusion temperature to get the limitation mean near recrystallization temperature (if it higher than this temperature, magnesium alloy may turn to be burnt) in order to gain the minimum force of extrusion and enhance productivity , at the same time, to get the well synthesis mechanical property by adjusting extrusion ratio. In this paper, to decrease force of extrusion and get low cost, 430℃ is adopted as extrusion temperature.

2. Experimental Procedure

AZ91D Alloy is used in the present study with a chemical component showed in table 1.
Table 1 Chemical compositions of magnesium alloys AZ91D(wt%)
Element Al Zn Mn Si Cu others Mg
Chemical composition 8.3-9.7 0.35-1.0 0.15-0.5 0.10max 0.030max 0.02 remains

Tubes are extruded with as follows processing parameters at YH61-500G hydrostatic machine. Outer diameter of extruded tube is 60mm, Extrusion ratio is: 5.472, 7.125, 7.45 respectively; ingot temperature is: 430℃; die temperature is 20℃ lower than ingot temperature; deforming velocity is 4×10-2S-1; lubricant is powdered +MoS2.
Tensile test is carried our on WES-600D screen display universal test machine at room temperature (rate of extension 0.3mm/s) using specimens cut form extruded tube with linear cutting.
Microstructures are studied under XJL-02 type microscope.

3 Results and discussion

3.1 Microstructure and mechanical properties change during extrusion
Fig.1~4 show cross and longitudinal section microstructure with different extrusion ratio at extruded temperature 430℃. It can be seen that AZ91D alloy is mainly made up of two phases, α-Mg solid solution matrix with some eutectic microstructure and β-Mg17Al12 around. Shape and distribution of β-Mg17Al12 makes contribution to AZ91D mechanical properties in some extent.

Fig. 1 as cast magnesium alloys cross and  Fig.2 magnesium alloys cross longitudinal section
longitudinal section structure with extrusion ratio 5.472 at 430℃

As showed in Fig .1, in as-cast microstructure, the phase of β-Mg17Al12 presents a dispersed net shape distributing surrounding α-Mg matrix to make a apparent dendritic structure. This microstructure determined a low mechanical property, tensile strength only 150.8MPa and yield strength 78.375MPa, elongation 4.1%. This is because that net shape distribution of β-Mg17Al12 destroys the matrix alloy’s continuance, so that its mechanical property can not fully perform due to the discontinuity.

Fig .2 shows the extrusion microstructure with extrusion ratio of 5.472 under 430℃. On cross section, part of the white α-Mg matrix is extruded flat, but most of them are still distributed as block shape, separated by β-Mg17Al12 phase, and the α-Mg and β-Mg17Al12 phases are finer, for the grain distance is shorter than as-cast. The volume of eutectic microstructure in α-Mg solid solution doesn’t change obviously. The main reason is that the external force is not large enough to make the eutectic phase broken. On longitudinal section, the α-Mg solid solution shows obvious lath shape, and most of β-Mg17Al12 is fragment dispersedly distributing on the grain boundary. But the α-Mg lath matrix is not separated completely. The grains gradually grow along the direction of external force due to the extrusion and a lot of dislocations (Fig.5) emerging in grains. The mechanical property is higher than as-cast microstructure, tensile strength 291.4MPa., the yield strength 233.12MPa, And elongation 8.5 %. This is mainly because AZ91D magnesium alloy occur plastic deformation under extrusion. Then β-Mg17Al12 phase is broken. It changed from the whole to small grains distributing on the grain boundary. Eutectic microstructure is also changed from lath distributing to spot distributing in α-Mg solid solution. It can be seen from the picture that the eutectic microstructure is slightly prolonged. Meantime, with hcp structure, AZ91D is uneasily deformed, therefore , the deformation is mainly caused by slip and twin crystal (seen in Fig6), which produced large amount of dislocations and then made the alloy strengthened.

Fig 3 is the extrusion microstructure under 7.125 extrusion ratio. On the cross section, the white α-Mg matrix grains are smaller, and occur distorted deformation, part of them are tangled up each other. under the non-equilibrium freezing condition, the β-Mg17Al12 become even smaller by mean of alien eutectic. Because of the increase of extrusion ratio, the eutectic microstructure occur dendritic broken, dispersed distribution in α-Mg solid solution. on the longitudinal section, the extrusion streamline is finer. The white α-Mg matrix lath gradually become narrow . the broken fine

Fig.3Magnesiumalloyscross longitudinal section  Fig.4 Magnesium alloys cross longitudinal section
Structure with extrusion ratio 7.125 at 430℃ structure with extrusion ratio 7.45 at 430℃

Fig. 5 Dislocation 49000× Fig.6 slip and twin crystal 49000×

granular β-Mg17Al12 is distributing as lath shape. The eutectic microstructure broken to granular shape distribute in α-Mg solid solution. At the same time, the dislocation density increases because of the metallic plastic resistance of deformation with increment of deformation degree. The strength and hardness increase obviously. The tensile strength can reach 306.9MPa,;yield strength is 275.52 MPa; elongation can highly reach10.1%.work-hardening play an important role in this change . In addition, a small amount of equiaxed grains can be found from the picture. The dynamic recrystallization has occurred apparently in the deformed structure, but the grains haven’t grown large which makes contribution to the strengthening of AZ91D microstructure. Consequently, the hardness effect is higher than softening effect.

Fig4 is the extrusion microstructure under 7.45 extrusion ratio. On the cross section ,the white α-Mg matrix grains are extruded flat to become finer. But the white α-Mg matrix and the gray β-Mg17Al12 phase are not continuous. Black α +β eutectic alter obscure, and become small. White α-Mg and gray β-Mg17Al12 exhibit lath shape, two phases distributing alternatively., this structure which is useful to improve mechanical property In theory, decreases the tensile strength making σb 288.2MPa. The increasing extrusion ratio and dump energy are the mainly causes of the phenomena. At the same time, extrusion temperature is higher than recrystallization temperature, and atom activity increasing, so grains begin to change from deforming elongate grain to new equiaxed grain, and dynamic recrystallization grain begin to grow up. The changed structure destroyed original texture structure, decreased piled-up and tangle of dislocation which reduced the mechanical property then soft action was greater than hardening action,. On the other hand, this can be explained by formulation (1):

(1)
Where d is the mean diameter of recrystallization grain, N is nucleation rate, G is linear velocity of growth, K is proportional constant. From the formulation, it can be conclude that recrystallization grain size is determined by G/N. To refine grain , it must reduce G/N. From Fig.4, bigger deform results in the increasing of recrystallization nucleation rate and growth rate, but the increasing of nucleation rate is greater than that of the growth rate, which make the crystal grain finer. According to formulating Holtz-pelt The smaller diameter caused higher yield strength. Therefore, yield strength improve greatly to 285.795MPa. Non-homogeneous deformation and non-uniform stress distributing which lead to residual stress existence and non-uniform metal inner physical properties and mechanical state, make metal plastic descending, presenting as elongation fall down 9%.

3.2 Discussion
Above all, in the structure of extruding tubes, ,the crystal grains of cross section became small and densification; fibre-streamline can be see n obviously on longitudinal section. Then with the increasing of extrusion ratio (deforming quantitative), the deforming degree of metal is increasing; the fibre-streamline is smaller and more compaction; the crystal grains are smaller. When the extrusion ratio is smaller than 7, although dynamic state recrystallization took place, the crystal grains growing finer. At this time work-hardening have the main function. When the extrusion ratio is greater than 7, above critical-deforming-degree, then the storage power enlarge, thus it caused the increasing of the recrystallization nucleation-rate and the velocity of growth, the former is bigger than the latter, which makes the recrystallization-crystal grain finer. And mechanical property got peak value at extrusion ratio 7.125(showed in Fig7). A discussion can be made as follows:

1) Crystal grains refines and grain-boundary increases with the increasing of the extrusion ratio, making the result of grain-boundary strengthening. On one hand, under the same external force, strain of inner fine crystal grain and its boundary is relatively smaller, and the deformation is homogeneous, which causes less crack because of stress concentration and improve the mechanical property.
2) During the process of extrusion, β-Mg17Al12 phase is elongated and broken along with the orientation of deforming, and present as chain shape, which makes α-Mg matrix and β-Mg17Al12 phase continuous on its orientation of stress, thus forms the texture to enhance mechanical property.
3) During extrusion deformation, there are two reverse processes—work-hardening and recrystallization. When extrusion ratio is smaller than 7.125, the function of working–hardening is greater than recrystallization; when extrusion increased to 7.45, the recrystallization become stronger which caused decreasing of dislocation and reduced the effect of work-hardening, producing low tensile strength.

Fig. 7 mechanical properties of different extrusion ratio(1 is as-cast )

4 conclusions

1) During extrusion, when β-Mg17Al12 disperse in cross section, meshy distribute among α-Mg in longitudinal section, it can not break basal body continuity and make two phases fully used, acting as strengthening phase. Grain refinement makes grain boundary increase, performing grain boundary strengthening function.
2) Stereo scan photograph shows during AZ91D extrusion process at 430℃, when extrusion ratio is larger than 7, the decreasing of mechanical properties is the result of work-hardening and dynamic recrystallization.
3) Tensile test of Extruded tube specimens at room temperature shows extrusion of AZ91D ingot at 430℃not only makes resistance of deformation fall down but also can obtain tube with greater mechanical properties by choosing proper extruded ratio to control microstructure. In this experiment the best mechanical properties can be got near extruded ratio 7.125, which is sb 306.3Mpa,σs288.795Mpa,δ10.1%。


摘要:本文研究了在挤压温度为430℃时,挤压比对镁合金AZ91D挤压管件组织和力学性能的影响,通过分析挤压过程中镁合金AZ91D组织和力学性能的变化,发现挤压比为7.125时,抗拉强度达到最大值306.9MPa,伸长率达到最大值10.1%。当挤压比增大到7.45时,屈服强度达到最大值285.795, 而抗拉强度和伸长率出现下降的趋势。试验得出:在高于再结晶温度下(430℃)挤压镁合金AZ91D,通过调整挤压比,控制再结晶的程度能够获得良好力学性能镁合金AZ91D管材。
Microstructure and Mechanical Properties ofAZ91D Extruded Tube
YU Bao-yi, BAO Chun-ling, SONG Hong-wu, LIU Zheng, YU Hai-peng
(School of Material Science and Engineering, Shenyang University of Technology, Shenyang 110023,China)
Abstract: The influence of extrusion ratio on microstructures and mechanical properties of magnesium alloys AZ91D extruded tube at 430℃ has been researched. After the evolution of microstructures and mechanical properties of AZ91D during extrusion is studied, it shows: tensile strength got the climax value 306.9MPa and elongation peak value 10.1% when extrusion ratio is 7.125, and with extrusion ration increasing to 7.45, yield strength got top value 285.795 MPa with decreased tensile strength and elongation. The conclusion is that mechanical properties of magnesium alloys AZ91D can be enhanced by adjusting extrusion ratio near recrystallization can be got.
Key words:Magnesium; Tube ; Microstructure.; Tensile property

1. Introduction

The application of magnesium alloy tube which has a great application potential such as automobile, aerospace and so on but is so limited, The main reasons is force of extrusion of magnesium alloy tube sometimes is limited by extruding machine ,for its limit of bearing capacity; at the meantime, the production rate is also low. So it is necessary to increase extrusion temperature to get the limitation mean near recrystallization temperature (if it higher than this temperature, magnesium alloy may turn to be burnt) in order to gain the minimum force of extrusion and enhance productivity , at the same time, to get the well synthesis mechanical property by adjusting extrusion ratio. In this paper, to decrease force of extrusion and get low cost, 430℃ is adopted as extrusion temperature.

2. Experimental Procedure

AZ91D Alloy is used in the present study with a chemical component showed in table 1.
Table 1 Chemical compositions of magnesium alloys AZ91D(wt%)
Element Al Zn Mn Si Cu others Mg
Chemical composition 8.3-9.7 0.35-1.0 0.15-0.5 0.10max 0.030max 0.02 remains

Tubes are extruded with as follows processing parameters at YH61-500G hydrostatic machine. Outer diameter of extruded tube is 60mm, Extrusion ratio is: 5.472, 7.125, 7.45 respectively; ingot temperature is: 430℃; die temperature is 20℃ lower than ingot temperature; deforming velocity is 4×10-2S-1; lubricant is powdered +MoS2.
Tensile test is carried our on WES-600D screen display universal test machine at room temperature (rate of extension 0.3mm/s) using specimens cut form extruded tube with linear cutting.
Microstructures are studied under XJL-02 type microscope.

3 Results and discussion

3.1 Microstructure and mechanical properties change during extrusion
Fig.1~4 show cross and longitudinal section microstructure with different extrusion ratio at extruded temperature 430℃. It can be seen that AZ91D alloy is mainly made up of two phases, α-Mg solid solution matrix with some eutectic microstructure and β-Mg17Al12 around. Shape and distribution of β-Mg17Al12 makes contribution to AZ91D mechanical properties in some extent.

Fig. 1 as cast magnesium alloys cross and  Fig.2 magnesium alloys cross longitudinal section
longitudinal section structure with extrusion ratio 5.472 at 430℃

As showed in Fig .1, in as-cast microstructure, the phase of β-Mg17Al12 presents a dispersed net shape distributing surrounding α-Mg matrix to make a apparent dendritic structure. This microstructure determined a low mechanical property, tensile strength only 150.8MPa and yield strength 78.375MPa, elongation 4.1%. This is because that net shape distribution of β-Mg17Al12 destroys the matrix alloy’s continuance, so that its mechanical property can not fully perform due to the discontinuity.

Fig .2 shows the extrusion microstructure with extrusion ratio of 5.472 under 430℃. On cross section, part of the white α-Mg matrix is extruded flat, but most of them are still distributed as block shape, separated by β-Mg17Al12 phase, and the α-Mg and β-Mg17Al12 phases are finer, for the grain distance is shorter than as-cast. The volume of eutectic microstructure in α-Mg solid solution doesn’t change obviously. The main reason is that the external force is not large enough to make the eutectic phase broken. On longitudinal section, the α-Mg solid solution shows obvious lath shape, and most of β-Mg17Al12 is fragment dispersedly distributing on the grain boundary. But the α-Mg lath matrix is not separated completely. The grains gradually grow along the direction of external force due to the extrusion and a lot of dislocations (Fig.5) emerging in grains. The mechanical property is higher than as-cast microstructure, tensile strength 291.4MPa., the yield strength 233.12MPa, And elongation 8.5 %. This is mainly because AZ91D magnesium alloy occur plastic deformation under extrusion. Then β-Mg17Al12 phase is broken. It changed from the whole to small grains distributing on the grain boundary. Eutectic microstructure is also changed from lath distributing to spot distributing in α-Mg solid solution. It can be seen from the picture that the eutectic microstructure is slightly prolonged. Meantime, with hcp structure, AZ91D is uneasily deformed, therefore , the deformation is mainly caused by slip and twin crystal (seen in Fig6), which produced large amount of dislocations and then made the alloy strengthened.

Fig 3 is the extrusion microstructure under 7.125 extrusion ratio. On the cross section, the white α-Mg matrix grains are smaller, and occur distorted deformation, part of them are tangled up each other. under the non-equilibrium freezing condition, the β-Mg17Al12 become even smaller by mean of alien eutectic. Because of the increase of extrusion ratio, the eutectic microstructure occur dendritic broken, dispersed distribution in α-Mg solid solution. on the longitudinal section, the extrusion streamline is finer. The white α-Mg matrix lath gradually become narrow . the broken fine

Fig.3Magnesiumalloyscross longitudinal section  Fig.4 Magnesium alloys cross longitudinal section
Structure with extrusion ratio 7.125 at 430℃ structure with extrusion ratio 7.45 at 430℃

Fig. 5 Dislocation 49000× Fig.6 slip and twin crystal 49000×

granular β-Mg17Al12 is distributing as lath shape. The eutectic microstructure broken to granular shape distribute in α-Mg solid solution. At the same time, the dislocation density increases because of the metallic plastic resistance of deformation with increment of deformation degree. The strength and hardness increase obviously. The tensile strength can reach 306.9MPa,;yield strength is 275.52 MPa; elongation can highly reach10.1%.work-hardening play an important role in this change . In addition, a small amount of equiaxed grains can be found from the picture. The dynamic recrystallization has occurred apparently in the deformed structure, but the grains haven’t grown large which makes contribution to the strengthening of AZ91D microstructure. Consequently, the hardness effect is higher than softening effect.

Fig4 is the extrusion microstructure under 7.45 extrusion ratio. On the cross section ,the white α-Mg matrix grains are extruded flat to become finer. But the white α-Mg matrix and the gray β-Mg17Al12 phase are not continuous. Black α +β eutectic alter obscure, and become small. White α-Mg and gray β-Mg17Al12 exhibit lath shape, two phases distributing alternatively., this structure which is useful to improve mechanical property In theory, decreases the tensile strength making σb 288.2MPa. The increasing extrusion ratio and dump energy are the mainly causes of the phenomena. At the same time, extrusion temperature is higher than recrystallization temperature, and atom activity increasing, so grains begin to change from deforming elongate grain to new equiaxed grain, and dynamic recrystallization grain begin to grow up. The changed structure destroyed original texture structure, decreased piled-up and tangle of dislocation which reduced the mechanical property then soft action was greater than hardening action,. On the other hand, this can be explained by formulation (1):

(1)
Where d is the mean diameter of recrystallization grain, N is nucleation rate, G is linear velocity of growth, K is proportional constant. From the formulation, it can be conclude that recrystallization grain size is determined by G/N. To refine grain , it must reduce G/N. From Fig.4, bigger deform results in the increasing of recrystallization nucleation rate and growth rate, but the increasing of nucleation rate is greater than that of the growth rate, which make the crystal grain finer. According to formulating Holtz-pelt The smaller diameter caused higher yield strength. Therefore, yield strength improve greatly to 285.795MPa. Non-homogeneous deformation and non-uniform stress distributing which lead to residual stress existence and non-uniform metal inner physical properties and mechanical state, make metal plastic descending, presenting as elongation fall down 9%.

3.2 Discussion
Above all, in the structure of extruding tubes, ,the crystal grains of cross section became small and densification; fibre-streamline can be see n obviously on longitudinal section. Then with the increasing of extrusion ratio (deforming quantitative), the deforming degree of metal is increasing; the fibre-streamline is smaller and more compaction; the crystal grains are smaller. When the extrusion ratio is smaller than 7, although dynamic state recrystallization took place, the crystal grains growing finer. At this time work-hardening have the main function. When the extrusion ratio is greater than 7, above critical-deforming-degree, then the storage power enlarge, thus it caused the increasing of the recrystallization nucleation-rate and the velocity of growth, the former is bigger than the latter, which makes the recrystallization-crystal grain finer. And mechanical property got peak value at extrusion ratio 7.125(showed in Fig7). A discussion can be made as follows:

1) Crystal grains refines and grain-boundary increases with the increasing of the extrusion ratio, making the result of grain-boundary strengthening. On one hand, under the same external force, strain of inner fine crystal grain and its boundary is relatively smaller, and the deformation is homogeneous, which causes less crack because of stress concentration and improve the mechanical property.
2) During the process of extrusion, β-Mg17Al12 phase is elongated and broken along with the orientation of deforming, and present as chain shape, which makes α-Mg matrix and β-Mg17Al12 phase continuous on its orientation of stress, thus forms the texture to enhance mechanical property.
3) During extrusion deformation, there are two reverse processes—work-hardening and recrystallization. When extrusion ratio is smaller than 7.125, the function of working–hardening is greater than recrystallization; when extrusion increased to 7.45, the recrystallization become stronger which caused decreasing of dislocation and reduced the effect of work-hardening, producing low tensile strength.

Fig. 7 mechanical properties of different extrusion ratio(1 is as-cast )

4 conclusions

1) During extrusion, when β-Mg17Al12 disperse in cross section, meshy distribute among α-Mg in longitudinal section, it can not break basal body continuity and make two phases fully used, acting as strengthening phase. Grain refinement makes grain boundary increase, performing grain boundary strengthening function.
2) Stereo scan photograph shows during AZ91D extrusion process at 430℃, when extrusion ratio is larger than 7, the decreasing of mechanical properties is the result of work-hardening and dynamic recrystallization.
3) Tensile test of Extruded tube specimens at room temperature shows extrusion of AZ91D ingot at 430℃not only makes resistance of deformation fall down but also can obtain tube with greater mechanical properties by choosing proper extruded ratio to control microstructure. In this experiment the best mechanical properties can be got near extruded ratio 7.125, which is sb 306.3Mpa,σs288.795Mpa,δ10.1%。


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     收录时间:2016-03-25 14:51 来源:太空模具网  作者:匿名
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