Effects of dynamic recrystallization in extruded and compressed AZ31 magnesium alloy

Commercial AZ31 billets were extruded to round rods at three extrusion velocities of 0.8~m/min, 1.4 m/min and 2.2 m/min, at 643 K, 673 K and 703 K, respectively. The microstructure and texture evolution at different conditions were investigated by optical microscopy (OM) and electron backscattered diffraction (EBSD) techniques. The average grain size is reduced significantly after the hot extrusion, and the microstructure is obviously inhomogeneous due to dynamic recrystallization, which is confirmed by uni-axial compression experiments at elevated temperatures with different strain rates. The uni-axial compression leads to a fibre texture, and {1120}<0110> texture is the major texture in extruded rods due to non-basal slip. As the extrusion temperature is elevated, the main texture becomes weak and other texture components appear.     

Effect of deformation speed on the microstructure and mechanical properties of AA6063 during continuous extrusion process

Experiments and numerical simulations were conducted to analyze the continuous extrusion of AA6063 aluminum alloy under extrusion wheel angular velocities of 0.52, 0.78, 1.04 and 1.3 rad/s. Simulation results indicate that variations in extrusion wheel velocity directly affect material deformation and significantly influence the maximum extrusion temperature. This work also reveals that deformation and temperature have opposing effects on the microstructure of the resulting product. A greater wheel velocity causes a higher strain rate and extrusion temperature. Increasing the wheel velocity, at an initially low speed, causes a large increase in strain rate. This results in a decrease in grain size. In contrast, at high wheel velocities, further increases to wheel velocity have much less effect on the strain rate, leading to an increase in grain size as the increased extrusion temperature dominates the mechanics of grain growth. Tensile test results demonstrate that the tensile strength of the resulting aluminum extrusions mainly depends on the exit temperature, which is decided by the deformation speed. Tensile strength and hardness slightly increase with increased deformation speed. Extremely high extrusion temperature results in brittle failure and low mechanical properties of the resulting product when the extrusion speed reaches 1.3 rad/s. This paper suggests that an optimum extrusion wheel velocity, which will generate products with good mechanical properties, exists.     

An extrusion−shear−expanding process for manufacturing AZ31 magnesium alloy tube

A new severe plastic deformation method for manufacturing tubes made of AZ31 magnesium alloy with a large diameter was developed, which is called the TCESE (tube continuous extrusion?shear?expanding) process. The process combines direct extrusion with a two-step shear?expanding process. The influences of expanding ratios, extrusion temperatures on the deformation of finite element meshes, strain evolution and flow velocity of tube blanks during the TCESE process were researched based on numerical simulations by using DEFORM-3D software. Simulation results show that the maximum expanding ratio is 3.0 in the TCESE process. The deformation of finite element meshes of tube blanks is inhomogeneous in the shear?expanding zone, and the equivalent strains increase significantly during the TCESE process of the AZ31 magnesium alloy. A extrusion temperature of 380 °C and expanding ratio of 2.0 were selected as the optimized process parameters from the numerical simulation results. The average grain size of tubes fabricated by the TCESE process is approximately 10 µm. The TCESE process can refine grains of magnesium alloy tubes with the occurrence of dynamic recrystallization. The (0001) basal texture intensities of the magnesium alloy tube blanks decrease due to continuous plastic deformation during the TCESE process. The average hardness of the extruded tubes is approximately HV 75, which is obviously improved.     

挤压参数对生物植入应用镁合金降解性能的影响：综述（英文）

作为新一代临时生物材料,镁合金具有良好的生物相容性和生物可降解性,也有助于损伤骨组织的修复。但是,其在人体体液中不具备所要求的耐腐蚀性能。挤压等热机械加工对镁合金的力学性能和生物腐蚀行为均有影响。本文综述挤压参数(挤压比和温度)对镁合金生物腐蚀性能的影响。它们的影响主要归因于挤压合金显微组织的改变,包括最终的晶粒尺寸和均匀度、织构以及第二相的尺寸、分布和体积分数。挤压过程中的动态再结晶和晶粒细化使组织更均匀,并导致基面织构的形成,从而提高镁合金的强度和耐腐蚀性能。挤压温度和挤压比是影响降解的重要因素。随着挤压比的增加和/或挤压温度的降低,镁合金的晶粒尺寸减小,与挤压方向平行的样品两侧的基面织构增强,析出相体积分数降低,晶粒尺寸减小,这些都有助于提高镁合金植入物的耐腐蚀性能。     

Microstructure and properties of AZ80 magnesium alloy prepared by hot extrusion from recycled machined chips

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Role of Cu film texture in grain growth correlated with twin boundary formation

To understand the role of Cu film texture in grain growth at room temperature (RT) in relation to twin boundary formation Cu films were deposited on various barrier materials and the Cu film texture was investigated by X-ray diffraction. Cu grain growth was rapid on a barrierless SiO₂/Si substrate and very slow on a Ta barrier due to strong (1 1 1) texture. The growth rate and the average grain diameter after being kept at RT for up to ∼60 days were maximum at a (2 0 0)_Cu peak to (2 2 2)_Cu peak area ratio of ∼1.0, where {1 1 1}, {1 0 0} and {5 1 1} grains coexisted. Such coexistence of three or more orientations of grains is essential in facilitating Cu grain growth at RT. Similarly, the average twin boundary (TB) density was maximum when Cu grain growth was facilitated. TB formation in nano-sized Cu grains was not controlled by grain size, but due to grain growth. The TB could be annealing twins caused by irregularities in the stacking sequence during relatively fast grain growth. The Cu film texture is concluded to be determined at the beginning of deposition, and the wettability of various barrier materials by the Cu films plays a key role in determining the film texture.     

Effects of deformation parameters on microstructure and mechanical properties of magnesium alloy AZ31B

Plastic deformation and dynamic recrystallization (DRX) behaviors of magnesium alloy AZ31B during thermal compression and extrusion processes were studied.In addition, effects of deformation temperature and rates on the microstructure and mechanical properties were investigated.The results show that the DRX grains nucleate initially at the primary grain boundaries and the twin boundaries, and the twinning plays an important role in the grain refinement.The DRX grain size depends on the deformation temperature and strain rate The average grain size is only 1 μm when the strain rate is 5 s-1 and temperature is 250 ℃.It is also found that the DRX grain can grow up quickly at the elevated temperature.The microstructure of extruded rods was consisted of tiny equal-axis DRX grains and some elongated grains.The rods extruded slowly have tiny grains and exhibit good mechanical properties.     

Some studies on hot extrusion of rapidly solidified Mg alloys

Rapidly solidified magnesium alloys show great potential for application in automotive and aerospace industries. In this study, Mg-Al-Zn alloys (AZ91) were rapidly solidified by a melt-spinning process to form ribbons. Pulverized ribbons were cold-compacted and then hot-extruded to form rods. During extrusion, a specially designed die with constant strain rate profile was used and found to be advantageous. By properly establishing the complete process, extruded rods of rapidly solidified AZ91 alloys exhibiting good combination of room temperature strength and ductility were produced. Microstructural investigations were carried out on melt-spun ribbons and extruded rods. Effects of extrusion die shape, extrusion ratio, and extrusion temperature on mechanical properties of the extruded rods were also investigated.     

Effect of the extrusion conditions on the texture and mechanical properties of indirect-extruded Mg–3Al–1Zn alloy

The effects of the processing conditions on the texture and mechanical properties of indirect-extruded Mg–3Al–1Zn alloy are investigated. During the extrusion process, the alloy was subjected to various initial billet temperatures and ram speeds, exhibiting different extrudate exit temperatures depending on the extrusion conditions. The Zener-Hollomon parameter acquired from the exit temperature of the extrudate and the ram speed was found to have a significant effect on the grain size, texture, and yield asymmetry of the extruded alloy. Among the processing conditions investigated, a higher Zener-Hollomon parameter resulted in a finer grain size and weaker fiber texture, thereby decreasing the yield asymmetry of the extruded Mg alloy.     

Effect of Extrusion Temperature on the Microstructure and Mechanical Properties of Mg–5Al–2Ca Alloy

In this work, the Mg–5Al–2Ca alloy was extruded at 573, 623 and 673 K, with a ratio of 16:1 and a constant speed of 3 mm/s. Results demonstrate that the Al2Ca particle is formed in Mg–5Al–2Ca alloy. The size, amount and distribution of Al2Ca particles are influenced evidently by extrusion temperature. Unlike previous reports, the intensity of basal texture increases with increasing extrusion temperature, and the reasons are analyzed and given. Even though the average grain size increases as the extrusion temperature increased from 573 to 623 K, the YS, UTS and elongation of asextruded Mg–5Al–2Ca alloy are almost kept the same at 573 and 623 K. The reason is speculated as the balance of grain size, Al2Ca phase and texture at the two temperatures. The work hardening rate depends on extrusion temperature, and the largest θ value of Mg–5Al–2Ca alloy is obtained when the extrusion was performed at 623 K.     

Friction and wear behaviors of reciprocatingly extruded Al–SiC composite

This study focuses on the friction and wear behaviors of reciprocatingly extruded Al–SiC composites. To increase the strength of metal matrix composites and refine the grains of the matrix some deformation processes can be applied, such as reciprocating extrusion (RE). For this reason, RE was carried out on a 6061 Al matrix by a SiC (20 μm) reinforced composite one. The billets were extruded under a pressure of 17.5 MPa at 573 K with a 10:1 extrusion ratio. The reciprocating extrusions were carried out by using up to 15 passes.     

Effect of β grain growth on variant selection and texture memory effect during α → β → α phase transformation in Ti-6 Al-4 V

In the present study, the texture evolution and the role of β grain growth on variant selection during β → α phase transformation have been investigated in Ti-6 Al-4 V with and without 0.4 wt.% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate similar starting microstructures and crystallographic textures. Subsequently, both materials were solution-heat-treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Additional interrupted slow cooling experiments were carried out to identify the α lamellae that nucleate first from β grain boundaries. Detailed electron backscatter diffraction analysis was carried out and it was found that the β heat treatment did not generate new texture components although the intensities of the individual components changed dramatically depending on the alloy/β grain size. Variant selection was assessed by comparing measured α texture components with predicted α texture components based on the high-temperature β texture assuming equal variant selection. It was found that with increasing β grain size variant selection intensified favouring the {φ1, Φ, φ2} {90°, 30°, 0°} texture component. Interrupted cooling experiments revealed that α nucleates first on β grain boundaries that are formed by two β grains having a common (1 1 0) normal and that these α lamellae display almost exclusively a {φ1, Φ, φ2} {90°, 30°, 0°} orientation. Consequently, the dominance of this variant with increasing β grain size can be related to the relative free growth of this particular α texture component into an “empty” β grain.     

Extrusion of silicon-rich AlMgSi alloys

As a part of on-going research on phase transformations during the deformation of light alloys, the effect of silicon excess on the extrudability and mechanical properties of the standard AlMgSi1 alloy within AA6082 alloy is investigated in this study. The AlMgSi1 alloy and three experimental aluminum alloys with a silicon content of 1.98%, 3.73% and 5.51% were direct-chilled cast into billets 95 mm in diameter, homogenized at 540 °C for 4 h and extruded into 12 mm diameter rods at different extrusion speeds. The results showed that an increase in the silicon content reduced the extrudability of the AlMgSi1 alloy by lowering the limiting extrusion speed. However, the extruded alloys with 3.73% and 5.51% silicon, generally characterized by a fine grained microstructure, exhibited higher strength levels compared with the 1.98% silicon alloy. Nonetheless, the mechanical properties of these alloys, in the T6 temper condition, were below those of the AlMgSi1 base alloy.     

Influence of extrusion temperature on deformation behavior of Csf/AZ91D composite during semi-solid extrusion

Deformation behavior and formability of C_sf/AZ91D magnesium composite were investigated by semi-solid extrusion between 695 K and 728 K, including temperatures below and above the partial melting temperature. A method of constructing kinematically admissible velocity fields for axisymmetric extrusion based on the theory of flow function was proposed. Flow lines were analyzed in C_sf/AZ91D composite after extrusion at elevated temperatures. Based on an analytic flow function, the deformation field was obtained. The results show that when the composite is extruded in a semi-solid state containing a small volume of liquid, the presence of the liquid reduces deformation resistance by relaxing the stress concentrations, and improves the formability of composites as lubricant. However, the gradient of velocity field is increased and deformation uniformity is aggravated at temperatures greater than partial melting point at 701.3 K. A more uniform deformation field was attained at the temperature close to or slightly below the partial melting temperature.     

Evolution of Microstructure,Residual Stress,and Tensile Properties of Mg–Zn–Y–La–Zr Magnesium Alloy Processed by Extrusion

The microstructure, texture, residual stress, and tensile properties of Mg–6 Zn–2 Y–1 La–0.5 Zr(wt%) magnesium alloy were investigated before and after extrusion process, which performed at 300 °C and 400 °C. The microstructural characterizations indicated that the as-cast alloy was comprised of α-Mg, Mg–Zn, Mg–Zn–La, and Mg–Zn–Y phases. During homogenization at 400 °C for 24 h, most of the secondary phases exhibited partial dissolution. Extrusion process led to a remarkable grain refi nement due to dynamic recrystallization(DRX). The degree of DRX and the DRXed grain size increased with increasing extrusion temperature. The homogenized alloy did not show a preferential crystallographic orientation, whereas the extruded alloys showed strong basal texture. The extrusion process led to a signifi cant improvement on the compressive residual stress and mechanical properties. The alloy extruded at 300 °C exhibited the highest basal texture intensity, the compressive residual stress and hardness, and yield and tensile strengths among the studied alloys.     

Evolution of microstructure and texture of AZ61 Mg alloy during net-shape pressing of extruded perform

The micro orientation theological behavior of AZ61 Mg alloy during net-shape forming of tensile specimens via close-die pressing of extruded preformed and the effect of the press deformation rate on the microstructure characteristics were characterized with electron back-scattering diffraction (EBSD) orientation imaging microscopy and metallography. The results indicate that the intensity distribution of basal {0001}<10(1)0 > texture on the cross-section of the extruded perform is uniform and parallel to the extrusion direction. Subjected to pressing in extrusion direction, deformation shear stress leads to grain rotation and basal texture {0001}<10(1)0> deviation from the extrusion direction, spreading in the direction perpendicular to pressing direction. The texture intensity increases with the press deformation rate and reaches its peak value at 50%, which is considerably lower than the value reached in extrusion deformation. Then, the texture intensity decreases with the press deformation rate reversely.     

Kinetics and driving forces of abnormal grain growth in thin Cu films

The abnormal growth of individual (1 0 0) oriented grains is monitored by the in situ electron backscatter diffraction technique for more than 24 h at three different annealing temperatures (90 °C, 104 °C and 118 °C) in 1-5 μm thick Cu films on polyimide substrates. The (1 0 0) grain growth velocity increases with higher film thickness and annealing temperature, as suggested by an earlier model by Thompson and Carel. As a result, the final (1 0 0) texture fraction becomes more dominant for higher annealing temperatures and larger film thicknesses. The Thompson-Carel model, however, predicts that the (1 1 1) grains will preferably grow at temperatures up to 118 °C. Our calculations of the driving forces revealed that in addition to minimization of the strain energy (due to the thermal mismatch between film and substrate) and of the surface energy, the energy stored in the dislocations plays a decisive role in grain growth. Our observations can be understood by the notion that initially available (1 0 0) grain nuclei start to grow very rapidly, due to dislocation annihilation, and thus “overrun” the (1 1 1) grains in size.     

Fabrication of 93W–Ni–Fe alloy large-diameter rods by powder extrusion molding

A powder extrusion molding (PEM) process has been used for the manufacturing of tungsten heavy alloy rods with large length to diameter ratio. An improved wax-based multi-component binder was developed for PEM of 93W–Ni–Fe alloy. The miscibility of its components and the characteristics of the binder were evaluated and good thermal–physical properties were obtained. Also, the feedstock rheological properties, extrusion molding and debinding process were studied. The feedstock exhibited a pseudo-plastic flow behavior. The large length to diameter ratio rods, with diameters up to 36 mm were extruded at 65 °C by optimizing the extrusion process. A two-step debinding process was employed to remove the binder in the extruded rods. Solvent debinding was carried out in n-heptane at 45 °C to extract the soluble components. A process of repeated short time immersion and drying of the extruded rods (called short-period solvent debinding) was developed and using this novel technique the binder removed was raised from 45% to 60%. SEM analyses indicated that a large volume of pores was formed in debound rods, but had not created interpenetrating pore channels yet. The rest of the binder could be thermally extracted at a high heating rate without defects.     

Characterization of the High-Strength Mg–3Nd–0.5Zn Alloy Prepared by Thermomechanical Processing

Magnesium alloys based on Nd and Zn are promising materials for both aviation industry and medical applications.Superior mechanical properties of these materials can be achieved by thermomechanical processing such as extrusion or rolling and by aging treatment, which can significantly strengthen the alloy. The question remains especially about the connection of texture strength created in the alloys based on the specific conditions of preparation. This work focuses on the Mg–3 Nd–0.5 Zn magnesium alloy prepared by hot extrusion of the as-cast state at two different temperatures combined with heat pre-treatment. Extrusion ratio of 16 and rate of 0.2 mm/s at 350 and 400 °C were selected for material preparation. The structures of prepared materials were studied by scanning electron microscopy and transmission electron microscopy. The effect of microstructure on mechanical properties was evaluated. Obtained results revealed the strong effect of thermal pre-treatment on final microstructure and mechanical properties of extruded materials. The Hall–Petch relation between grain size and tensile yield strength has been suggested in this paper based on the literature review and presented data. The observed behavior strongly supports the fact that the Hall–Petch of extruded Mg–3Nd–0.5 Zn alloys with different texture intensities cannot be clearly estimated and predicted. In addition, Hall–Petch relations presented in literature can be sufficiently obtained only for fraction of the Mg–3Nd–0.5 Zn alloys.     

Dependence of Microstructure, Texture and Tensile Properties on Working Conditions in Indirect-Extruded Mg-6Sn Alloys

通过光学显微镜（OM）、扫描电子显微镜（SEM）、X射线衍射仪（XRD）和电子材料试验机研究了反向挤压Mg-6Sn合金的组织、织构和拉伸性能与挤压条件（挤压速度和初始坯料温度）间的依赖关系。研究结果表明：反向挤压 Mg-6Sn 合金的晶粒尺寸、再结晶体积分数和动态析出第二相体积分数在很大程度上取决于反向挤压参数,随着挤压速度和初始坯料温度的升高,合金的织构强度减弱。通过控制挤压速度和初始坯料温度,经高温挤压后的合金表现出最高的抗拉强度。对于商业 AZ31 镁合金而言,合金的强度随着挤压温度的升高而降低。本研究的 Mg-6Sn 合金经高温挤压后的抗拉强度和经低温挤压后的 AZ31 合金相当。上述研究结果表明 Mg-6Sn 合金是一种适合于高速度挤压的新型合金系。