Effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE)

Three different mold structures were designed by changing the parameters of mold cavity to study the effect of local strains on the texture and mechanical properties of AZ31 magnesium alloy produced by continuous variable cross-section direct extrusion (CVCDE) with 2 interim dies. Microstructure and texture evolution of AZ31 magnesium alloy after CVCDE were studied by electron backscatter diffraction (EBSD). Mechanical properties were determined by uniaxial tensile tests along extrusion direction (ED) at room temperature. Due to the differences of local strains among the three schemes, the microstructure of Scheme 1 was the most uniform and the average grain size of Scheme 1 was the smallest. Meanwhile, tensile strength and elongation of Scheme 1 were the highest. Different textures had been formed in the three schemes. Lots of extension twins {10–12} (86°< 1–210 >) occurred in the products of the three schemes. The main deformation modes of Scheme 1 and Scheme 2 were slip and twinning. However, slip was dominant in Scheme 3. The deformation modes provided an essential basic for the design of CVCDE mold structure with more interim dies.     

Compressive behaviour of SiC/ncsc reinforced Mg composite processed through powder metallurgy route

In this present work nano coconut shell charcoal (ncsc) and silicon carbide (SiC) particulates were reinforced with AZ31B Mg alloy and suitable magnesium composite was developed by using the powder metallurgy technique followed by hot extrusion. Density measurement of the Mg composites revealed that the addition of ncsc significantly improved the density of the composites and porosity measurement showed minimal porosity. The microstructure of the composites showed even distribution of the ncsc in the AZ31B/3SiC Mg composite. The compressive and impact behaviour of the samples were characterized, the results showed that on increasing the weight percentage of ncsc in AZ31B/3SiC/0.5ncsc Mg composites the mechanical properties such as ultimate compressive strength, 0.2% yield strength, ductility and impact strength decreased. The scanning electron microscope (SEM) analysis of fractured surface of AZ31B Mg alloy and AZ31B/3SiC/0.5ncsc Mg composites showed quasi-cleavage fracture. The presence of ncsc above 0.5 wt% composites revealed mixture of quasi cleavage planes and some dimples.     

等径道角 挤压工艺对铸态 AZ91D 镁合金组织的影响

目的介绍等径道角挤压的原理及其对铸态AZ91D镁合金的组织产生的作用。方法通过确定的试验工艺参数,对AZ91D镁合金进行了等径道角挤压变形试验。使用金相显微镜和扫描电镜(SEM),对变形前后的材料进行了显微组织的观察。结果通过进行ECAE挤压后,AZ91D镁合金中的黑色共晶相(Mg17Al12)产生了回溶,在机械剪切和动态再结晶的综合作用下,晶粒得到了细化。结论通过等径道角挤压,能明显改善铸态AZ91D镁合金的组织。     

Dynamic recrystallization behavior of AZ31 magnesium alloy processed by alternate forward extrusion

One of the important factors that affect the microstructure and properties of extruded products is recrystallization behavior. Alternate forward extrusion (AFE) is a new type of metal extrusion process with strong potential. In this paper, we carried out the AFE process experiments of as-cast AZ31 magnesium alloy and obtained extrusion bar whose microstructure and deformation mechanism were analyzed by means of optical microscopy, electron backscattered diffraction and transmission electron microscopy. The experimental results indicated that homogeneous fine-grained structure with mean grain size of 3.91 μm was obtained after AFE at 573 K. The dominant reason of grain refinement was considered the dynamic recrystallization (DRX) induced by strain localization and shear plastic deformation. In the 573-673 K range, the yield strength, tensile strength and elongation of the composite mechanical properties are reduced accordingly with the increase of the forming temperature. Shown as in relevant statistics, the proportion of the large-angle grain boundaries decreased significantly. The above results provide an important scientific basis of the scheme formulation and active control on microstructure and property for AZ31 magnesium alloy AFE process.     

AZ31镁合金环境友好阳极氧化处理研究

通过正交试验得到AZ31镁合金无铬、无强氧化物的阳极氧化处理配方及工艺,运用扫描电子显微镜、X射线衍射分析及电化学测试系统对阳极氧化膜的组织结构与耐蚀性进行了表征。结果表明,该环境友好阳极氧化工艺可在AZ31镁合金表面形成具有一定耐蚀性的阳极氧化膜;所得阳极氧化膜表面质量好,是一种多孔的陶瓷膜,膜的物相主要是晶态的MgO、ZnO、Mg以及非晶态的MgAl2O4、Mg18Al23等;阳极氧化处理试样的自腐蚀电流密度减小,而腐蚀电位有所提高,说明镁合金表面形成的阳极氧化膜提高了镁合金表面的耐腐蚀性能。     

Processing of AZ31 magnesium alloy by a new noble severe plastic deformation method

In the present investigation a wrought magnesium alloy (AZ31) has been processed applying the accumulative back extrusion (ABE) method. This was performed through different thermomechanical processing routes (different ABE deformation passes at temperatures of 80-380 °C). The results indicate that AZ31 alloy may successfully be deformed through ABE processing even at temperatures as low as 80 °C. Following the ABE processing a sophisticated microhardness testing was conducted and thorough microstructural observations were undertaken using optical microscopy. The results show that the equiaxed submicron size grains have been achieved. As the number of passes was increased, a more homogeneous microstructure with finer mean grain size was obtained. It was also found that increasing the temperature resulted in larger mean grain size and also higher microstructural homogeneity.     

Development of a new integrated severe plastic deformation method

ABSTRACT

A novel one-pass integrated severe plastic deformation method, entitled extrusion compression angular pressing forward extrusion (ECAP-FE), was designed and used for fabricating a fine-grained as-cast AZ31 Mg alloy. Subsequently, mechanical (room temperature compression, tensile and microhardness) and microstructural properties of the processed sample were investigated in detail. In addition, a finite element simulation of the proposed method was carried out for evaluating the equivalent plastic strain and strain rate distribution. The results showed that the ECAP-FE method is a powerful method for processing the ultrafine-grained as-cast AZ31 Mg alloy in a single pass to achieve a uniform and fine microstructure with enhanced mechanical properties. Therefore, it appears that the proposed method has a great potential to a wide range of industrial applications.     

Effect of grain size distribution and texture on the cold extrusion behavior and mechanical properties of AZ31 Mg alloy

An experimental investigation into the cold extrusion and the subsequent annealing processes of an Mg-2.8%Al-0.83%Zn (AZ31) Mg alloy was studied. Microstructures, grain size distribution and texture evolution of the as-extruded and as-annealed specimens were investigated by optical microscopy and electronic backscattered diffraction (EBSD). Tensile tests along the extrusion direction were carried out at room temperature. It is found that firstly, the uniform microstructure with log-normal grain size distribution led to homogeneous cold deformation and good appearance without cracks for the as-extruded rods; secondly, texture was able to soften the yield strength induced by grain refinement and even overcame the refined grain size effect; thirdly, well-distributed microstructure reduced the absolute difference of yield strength for the sub-micrometer-grain Mg alloys.     

Zn层添加AZ31/7075合金复合成形工艺及组织与性能研究EI北大核心CSCD

通过在异种材料界面添加厚度为100μm的Zn箔,采用预挤压与孔型轧制复合工艺成功制备出AZ31/7075复合材料,并利用光学显微镜(OM)、扫描电子显微镜(SEM)、能谱分析(EDS)对复合界面进行表征及显微硬度测试,探究Zn过渡层在挤压复合孔型轧制过程中对产品的影响。结果表明:7075硬质铝合金芯部可细化AZ31镁合金,引入Zn过渡层可减少或者避免镁铝系金属间化合物生成;挤压及变形温升使Mg-Zn互扩散形成的低熔点共晶相熔化,同时加速元素自固相向液相扩散;然而降温冷却使Mg-Zn扩散层易出现不连续裂缝,但后续孔型轧制可显著改善;Mg-Zn扩散层经变形生成的MgZn_(2)金属间化合物具备较高的显微硬度(161HV),但Mg-Zn扩散层变形后厚度则较薄,结合层整体硬度变化不明显。     

Effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy under asymmetrical uniaxial cyclic loading

Magnesium alloys are increasingly used in the automotive and aerospace industries for weight reduction and fuel savings. The ratcheting behavior of these alloys is therefore an important consideration. The objective of this investigation was to study the effects of extrusion ratio on the ratcheting behavior of extruded AZ31B magnesium alloy. The experiments have shown that the extruded AZ31B Mg alloy presented the following characteristic behavior with increasing number of loading cycles: first an apparent cyclic softening was observed, then a cyclic hardening occurred, and finally a stable state was reached. This generic behavior can be explained by the fact that the variation trend of the maximum strain with the number of cycles differs from that of the minimum strain. The extrusion ratio did not influence the cyclic softening/hardening behavior or the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the mean stress and the peak stress. However, the extrusion ratio influenced the final ratcheting strain variation trend of the extruded AZ31B Mg alloy with the stress amplitude. Increasing the extrusion ratio also reduced the ratcheting strain and the effects of the load history on the ratcheting behavior of the extruded AZ31B Mg alloy.     

Effect of glycine addition on the in-vitro corrosion behavior of AZ31 magnesium alloy in Hank's solution

This study reports on the effect of the addition of Glycine to Hank's solution on the in-vitro corrosion behavior of AZ31 magnesium (Mg) alloy at 37 ℃ and a pH of 7.4 studied by using potentiodynamic polarization (PDP),hydrogen collecting techniques and electrochemical impedance spectroscopy (EIS) in combination with surface characterization techniques such as optical microscopy (OM),scanning electron microscopy(SEM),energy dispersive spectroscopy (EDS),X-ray diffraction (XRD) and X-ray photoelectron spectroscopy analysis (XPS).The results reveal that adsorption of glycine initially subdues the dissolution ofAZ31 Mg alloy while in long run it enhances the dissolution of the alloy due to the commencement of the chelation effect ofglycine with Ca2+ released from hydroxyapatite.The chelation ofglycine with Ca2+induces the formation of cracks in the surface film which further promotes the dissolution ofAZ31 Mg alloy thereby forming a porous corrosion products layer on the surface of the alloy.As a result,both the continuous dissolution of AZ31 magnesium alloy and the hydrogen evolution rate (HER) are enhanced with increasing the immersion time in Hank's solution.     

Microstructure and Properties of As-Cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr Alloys at Different Extrusion Ratios

Herein, the effect of hot extrusion with different extrusion ratios (λ = 6, 8, 10, and 12) on the microstructure evolution and properties of as-cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr magnesium alloys, using optical microscopy (OM), scanning electron microscopy (SEM), immersion corrosion and electrochemical corrosion experiment, and tensile testing, is investigated. The results show that the Mg₁₄SnY and Mg₆SnY precipitated phases exist in the alloy before and after extrusion. After hot extrusion, the second phase of the alloy is broken into particles along the extrusion direction, whereas the grain size is significantly reduced, and dynamic recrystallization and deformed grains exist in the microstructure. The mechanical properties of the extruded alloy improve, but the corrosion resistance weakens. When the extrusion ratio is λ = 10, the extruded alloy exhibits relatively good mechanical properties and corrosion resistance. The corrosion behaviors of the extruded alloys are affected by both the grain size and galvanic corrosion. In the initial stage of corrosion, intergranular corrosion plays a major role in reducing the corrosion resistance of the extruded alloys. With prolonged corrosion time, galvanic corrosion has a more significant effect on weakening the corrosion resistance of the extruded alloys.     

Fracture mechanism of AZ31 magnesium alloy processed by equal channel angular pressing comparing three point bending test and tensile test

A commercial magnesium alloy, AZ31 in hot-rolled condition, has been processed by equal channel angular pressing (ECAP) to get microstructure modified. Uniaxial tensile tests were conducted along the rolling/extrusion direction for as-received AZ31 alloy and ECAPed AZ31 alloy. Then, three point bending fracture tests were conducted for specimens with a pre-crack perpendicular to the extruded direction. Digital image correlation (DIC) technique was adopted to determine the deformation field around the crack tip. The fracture surfaces of the failed specimens after tensile tests and fracture tests were observed by Scanning Electron Microscope (SEM). To explore the deformation mechanism, the microstructure and texture of different regions on the deformed specimens were examined through electron backscatter diffraction (EBSD). The results show ECAP process improves both the tensile elongation and fracture toughness of AZ31 alloy. Different from the slip dominated deformation mechanism in the tensile test, deformation twinning presents in the deformation zone adjacent to the crack tip in the three point bending fracture tests. The fracture surface is characterized by co-occurrence of dimple and cleavage features.     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

镁合金表面喷铝防腐蚀层的微观组织分析

在AZ91D镁合金表面热喷铝，用以提高其表面的耐腐蚀性能。采用SEM，EPMA等方法对AZ91D表面喷铝扩散层的组织进行了仔细的观察与分析，发现在防腐蚀层上有大量的Mg17Al12相存在，且呈决状及非连续片状分布。经显微硬度测定，喷铝层硬度较镁合金基体高；腐蚀后这种相凸出于基体，比镁合金基体有较好的耐腐蚀性能。     

Microstructure and mechanical properties of as-extruded AZ80–xSn magnesium alloys

The effect of 0–2?wt-% Sn addition on AZ80 magnesium alloys after 350°C extrusion has been studied by analysing microstructure and mechanical properties. The results indicated that dynamically recrystallised grains were fine and homogeneous with less than 1?wt-% Sn addition. In AZ80–0.5Sn alloy, a large number of Mg₁₇Al₁₂ precipitated phases formed in grains and at grain boundaries during extrusion process. With more than 1?wt-% Sn addition, the size of dynamically recrystallised grains increased and the number of Mg₁₇Al₁₂ phases decreased. The strength of as-extruded AZ80–0.5Sn alloy enhanced largely as compared with that of the as-extruded AZ80 alloy. AZ80–0.5Sn alloy had the outstanding tensile and compressive properties.     

搅拌铸造法制备短碳纤维/AZ91复合材料的组织与性能

采用搅拌铸造法制备了不同体积分数（10vol%、15vol%、20vol%）的短碳纤维增强镁基（CF_s/AZ91）复合材料,并选取了三个挤压比和两个挤压温度对其进行热挤压变形,采用光学显微镜（OM）、SEM和TEM对CF_s/AZ91复合材料的显微组织进行了观察,并测试其室温力学性能及阻尼性能。研究结果表明,热挤压能够有效降低CF_s/AZ91复合材料气孔率;在热挤压过程中,纤维沿挤压方向定向排列,同时基体发生动态再结晶。随着挤压温度及挤压比的增大,晶粒呈现等轴状,组织更加均匀。CF_s/AZ91复合材料经过挤压后,其力学性能得到提高,屈服强度和抗拉强度随挤压比和CF_s体积分数的增大而增大,然而CF_s纤维在热挤压后发生明显断裂,限制了挤压态复合材料强度的进一步提升。低温低挤压比条件下,CF_s/AZ91复合材料具有较好的阻尼性能,随着挤压比及挤压温度的升高,CF_s/AZ91复合材料室温及高温阻尼性能均有所降低。      

Microstructures and mechanical properties of ultrafine-grained Ti/AZ31 magnesium matrix composite prepared by powder metallurgy

The microstructure of ultrafine grain for magnesium alloys can result in drastic enhancement in their room temperature strength, but the issue of low strength at elevated temperature becomes more serious as well due to grain boundary slide. Here ultrafine-grained Ti/AZ31 magnesium matrix composites with high strength at both room and elevated temperature were prepared by vacuum hot pressing and subsequent hot extrusion. The microstructure of the composite samples before and after consolidation processing was characterized, and the mechanical properties of the as-consolidated bulk samples were measured at room and elevated temperatures. The results indicate that after extrusion ultrafine-grained magnesium alloys were obtained and Ti particulates with particulate size of ～310?nm disperse in Mg matrix. The magnesium grain of AZ31-15at.%Ti grows from 66?nm to 800?nm. Meanwhile, the relative densities of Ti/AZ31 composites are higher than 99%. The yield strength (YS) of extruded AZ31-15at.%Ti composite at room temperature is 341?MPa, being 2.4 times higher than original AZ31 alloy. Theoretical estimation shows that remarkably enhanced room-temperature mechanical strength attributes to grain boundary strengthening with the contribution ratio of 74%. In addition, the peak stress of extruded AZ31-15at.%Ti composite at 573?K is 82?MPa and ultrafine Ti dispersions are responsible for the enhanced strength.     

AZ31镁合金无铬阳极氧化新工艺

为了提高镁合金的耐腐蚀性能,采用直流阳极氧化工艺研究了一种新型无Cr环保型镁合金阳极氧化配方及工艺.溶液主要成分包含NaOH、Na3PO4、KF、铝盐和适量添加剂.结果表明,氧化膜主要由MgO和MgAl2O4组成;该环保型阳极氧化新工艺所获得的膜层的耐腐蚀性能等级为9级,优于传统的HAE工艺(8级),对AZ31镁合金能提供更有效的保护.     

T6处理对AZ63镁合金在模拟海水中耐蚀性的影响

为研究热处理工艺对合金耐腐蚀性的影响,采用X射线衍射仪(XRD)、扫描电镜(SEM)、析氢失重以及多种电化学测试等方法研究了经T6处理(固溶处理+人工时效)的AZ63镁合金在模拟海水中的腐蚀行为。结果表明:T6处理(固溶处理+人工时效)镁合金第二相沿晶界呈网状分布,与铸态镁合金相比氢气析出多、腐蚀速度快,并有点蚀产生;T6处理(固溶处理+人工时效)改变了镁合金的组织结构以及分布,增大了稳定相第二相与基体相的接触,降低了合金的耐蚀性。