Al-5C中间合金对AZ31镁合金的细化机理

采用粉末原位合成工艺制备Al-5C中间合金,研究Al-5C中间合金对AZ31镁合金晶粒细化的影响及细化机理。结果表明：Al-5C中间合金由α（Al）和 Al 4 C 3两相组成,Al 4 C 3颗粒的尺寸分布由烧结时间控制。Al-5C中间合金能显著地细化AZ31镁合金晶粒尺寸,当Al-5C中间合金添加量低于2%时,随着Al-5C中间合金添加量的增加,AZ31镁合金晶粒尺寸减小。晶粒细化机理是由于 Al4C3与 Mn 反应生成的Al-C-Mn 颗粒能作为初生α-Mg晶粒的异质形核基底,从而细化晶粒。     

Grain refinement of AZ31 magnesium alloy by new Al-Ti-C master alloys

New Al₄C₃-containing Al-Ti-C master alloys (Al-0.6Ti-1C and Al-1Ti-1C) were developed by introducing Ti element into Al-C melt using melt reaction method, in which most of the TiC particles distribute around Al₄C₃ particles. It is believed that most of the C firstly reacts with Al melt and form Al₄C₃ particles by the reaction Al(l)+C(s)→Al₄C₃(s), and after adding Ti into the Al-C melt, the size of Al₄C₃ particles is decreased and the distribution of Al₄C₃ is improved through the reaction Ti(solute)+Al₄C₃(s)→TiC(s)+Al(l). With the addition of 1% Al-1Ti-1C master alloy, the average grain size of AZ31 is reduced sharply from 850 μm to 200 μm, and the grain morphology of α-Mg transits from a fully-developed equiaxed dendritic structure to a petal-like shape. Al-C-O-Mn-Fe compounds are proposed to be potent nucleating substrates for primary Mg. Appropriate addition of Ti is believed to increase the grain refinement efficiency of Al₄C₃-containing Al-Ti-C master alloys in AZ31 alloy.     

Effect of Vanadium Nitride (VN) Particles on Microstructure and Mechanical Properties of Extruded AZ31 Mg Alloy

The effect of vanadium nitride (VN) particles additives on microstructure and mechanical properties of the extruded AZ31 Mg alloy was systematically investigated. The experimental results revealed that the addition of 0.5 wt% VN decreased the average grain size of AZ31 Mg alloy from 6.4 to 4.9 µm. With the increase in VN content, the refining effect would weaken because excessive VN particles would negatively affect the dynamic recrystallization process of the alloys. The scanning electron microscopy and energy-dispersive spectroscopy indicated that AlN, VN and Al-V-N particles with different morphologies were distributed in the streamline along the extrusion direction during the extrusion process. The mechanical properties of AZ31 Mg alloy vary with the addition of VN. The extruded AZ31 + 0.5 wt% VN Mg alloy possesses an excellent combination of high strength and ductility. The yield strength and ultimate tensile strength of the extruded AZ31 + 0.5 wt% VN Mg alloy were increased without sacrificing ductility. This is mainly due to the grain refinement caused by double-heterogeneous nucleation particles. With a further increase in VN content, the presence of excessive VN particles increases the stress concentration, and the initiation source of microcracks in the alloy during alloy deformation makes the cracks more easily propagated and results in a decrease in the ductility of the extruded alloy.     

Al4 C3细化AZ91合金晶粒的现象和机理

通过向熔体中直接添加Al4C3颗粒,研究了Al4C3对AZ91合金的晶粒细化现象和机理的影响.结果表明,Al4C3颗粒对AZ91合金的晶粒细化效果与孕育参数直接相关,孕育温度越高孕育时间越久,Al4C3的细化效果越明显.研究发现,实验添加的Al4C3颗粒并没有直接成为镁的晶核,而是被推开到了晶粒生长界面前沿,Al4C3的细化作用主要是因为Al4C3发生了微量的溶解,从而向熔体中提供了碳,碳的存在对熔体起到了碳质孕育法处理的作用.     

AZ31镁合金差温热轧过程的晶粒细化

实验研究了经不同道次差温热轧AZ31镁合金的金相组织,结合对轧制过程,尤其是轧件温度场的数值模拟结果,分析了AZ31镁合金差温热轧过程晶粒细化机制与主要影响因素,获得了通过轧制过程动态再结晶,使轧材晶粒尺寸随轧制道次增加,而持续细化的工艺参数,并制备出平均晶粒尺寸为5μm左右的细晶AZ31镁合金板材。     

Effects of C<Subscript>2</Subscript>Cl<Subscript>6</Subscript> addition on grain refinement and mechanical properties of AZ31 magnesium alloy

Grain refining effects of C₂Cl₆ addition were investigated using AZ31 magnesium alloy. The addition of C₂Cl₆ to AZ31 magnesium alloy significantly reduced the grain size from 280 μm to about 120 μm, and the tensile properties were also increased due to grain refinement. EPMA analysis showed that apparent segregation of carbon element occurred in AZ31 castings treated with carbon. It was considered that the carbon might also have strong segregating power that restricts the grain growth and affects the constitutional undercooling, which in turn results in grain refining effects.     

累积叠轧温度对AZ31镁合金组织和性能的影响

通过光学显微镜、室温拉伸试验、显微硬度计、X射线衍射仪、扫描电镜等方法研究了累积叠轧温度对AZ31镁合金晶粒尺寸、基面织构、界面结合情况及力学性能的影响。结果表明:3道次累积叠轧后的AZ31镁合金晶粒细化效果明显,硬度增大,随着累积叠轧温度的升高,晶粒细化效果减弱,硬度增加趋势减弱。累积叠轧温度升高有弱化基面织构的作用。AZ31镁合板材在450 ℃累积叠轧3道次,综合力学性能最佳,为显微硬度70.64 HV0.05,抗拉强度288.64 MPa,屈服强度203.76 MPa,伸长率16.96%,界面结合强度21.53 MPa。     

Al-3Ti-3B中间合金对AZ31镁合金晶粒细化的影响

利用OM、XRD、SEM和EPMA等手段研究了Al-3Ti-3B细化剂对AZ31镁合金微观组织的影响。结果表明,添加适量的Al-3Ti-3B细化剂能使铸态AZ31镁合金粗大的树枝晶转变为均匀的等轴晶;加入量为0.4%时取得了较好的细化效果,固溶处理后的AZ31镁合金平均晶粒尺寸由300μm减小到50μm。TiB2和AlB2粒子的异质形核作用是促使晶粒细化的主要机制,且TiB2粒子在晶界上的偏聚可进一步阻碍晶粒长大。     

坯料细化处理对AZ31镁合金挤压组织的影响

对超声细化和未细化的AZ31镁合金棒料进行均匀化退火后热挤压,并对热挤压后的组织和硬度进行了对比分析。结果表明,与未经过晶粒细化处理棒料的热挤压组织相比,预先经过晶粒细化处理的AZ31镁合金棒热挤压组织更加均匀。当挤压比λ为16、挤压料温度为380℃、挤压速度为0.9 m/min时,组织发生回复再结晶。与未经晶粒细化处理棒料的挤压组织相比,经过晶粒细化处理的挤压组织更加细小;挤压速度增加到10 m/min时,经过晶粒细化处理后的AZ31镁合金挤压变形后棒料边缘容易发生二次再结晶现象,形成一条宽约75μm的粗晶组织,边缘附近区域组织中有孪晶形成。同时,经过晶粒细化处理后的AZ31镁合金挤压棒的硬度较高。     

Effect of Micro- and Nano-SiC Particulate Reinforcements in Magnesium-Based Metal Matrix Composites

The effects of the volume ratios of micro and nano-SiC particles (SiCp) on the grain refinement, distribution of SiCp particle, and tensile properties of the as-cast AZ31B (Mg-3Al-1Zn-0.3Mn) magnesium-based metal matrix composites have been investigated. As the volume fraction of micron SiCp decreases to 9 vol.% and the nano-SiCp increases to 1 vol.%, excellent grain refinement effect is achieved. This is due to both the uniform distribution and refining effects of micro- and nano-SiCp. Moreover, the micron SiCp distribute along the grain boundaries, while nano-SiCp is mainly distributed around the micron SiCp. The room-temperature tensile results show that the optimal room-temperature yield and tensile strengths are achieved with a 9/1 ratio of micro to nano-SiCp, while the 9.5/0.5 ratio yielded the highest elongation.     

Effect of Carbon Nanotube on High-Temperature Formability of AZ31 Magnesium Alloy

Room-temperature tensile properties of AZ31 alloy have significantly been improved when reinforced with carbon nanotube via ingot metallurgy process. However, high-temperature (up to 250 °C) elongation-to-failure tensile test of the developed nanocomposite revealed a considerable softening in the AZ31 alloy matrix accompanied by an incredible ductility increment (up to 132%). Microstructural characterization of the fractured samples revealed that the dynamic recrystallization process has induced a complete recrystallization in the AZ31 alloy at a lower temperature (150 °C) followed by substantial grain growth at a higher temperature used in this study. Fractography on the fractured surfaces revealed that the room-temperature mixed brittle-ductile modes of fracture behavior of AZ31 alloy have transformed into a complete ductile mode of fracture at high temperature.     

Microstructure and Mechanical Properties of Friction Stir-Welded Mg-2Nd-0.3Zn-0.4Zr Magnesium Alloy

A 2 mm thick Mg-2Nd-0.3Zn-0.4Zr (NZ20K) and AZ31 plates were friction stir welded. The microstructures of joint were compared and the tensile properties at room temperature and 200 °C were measured. The fracture features and the microhardness of joints were investigated. The effect of the strengthening phases in NZ20K joint was discussed compared with AZ31 joint. The results indicate that NZ20K shows better property especially at high-temperature environment. The grain of NZ20K in the nugget zone (NZ) is refined obviously with uniform distribution of strengthening phase particles and it shows clear boundary between NZ and thermo-mechanically affected zone (TMAZ). The grains of TMAZ are elongated because of the stir action of tool pin. The heat-affected zone is narrow with coarse grains. Mg₁₂Nd is the main strengthening phase in NZ20K joint through XRD analysis. The ultimate tensile strength of NZ20K joint decreases a little from room temperature to 200 °C for its main strengthening phase particle-Mg₁₂Nd being stable when the temperature goes up. On the contrast, the ultimate tensile strength of AZ31 joint decreases a lot at 200 °C for its strengthening phase soften or dissolve at high temperature. The hardness of NZ20K joint is higher than AZ31 joint and the lowest hardness of both joints is achieved on the advancing side where the fracture occurred.     

Mechanism of grain refining in AZ91 Mg welds by arc oscillation

Grain refining can improve the solidification-cracking resistance and mechanical properties of welds. Mg alloys are increasingly used for vehicle weight reduction, AZ91 Mg being the most widely used Mg casting alloy. The present study was conducted to study grain refining in gas–tungsten arc welding of AZ91 Mg, focusing on identifying the mechanism of grain refining. Significant grain refining was obtained by transverse arc oscillation. Dendrite fragmentation was identified as the grain refining mechanism, instead of heterogeneous nucleation or grain detachment. This was done by using an overlap welding procedure in which the weld made with transverse arc oscillation (test weld) overlapped with another weld made previously without arc oscillation (preweld). It was found that repeated prewelding followed by welding with transverse arc oscillation resulted in very fine grains.     

Deformation Mechanism and Hot Workability of Extruded Magnesium Alloy AZ31

Using the flow stress curves obtained by Gleeble thermo-mechanical testing, the processing map of extruded magnesium alloy AZ31 was established to analyze the hot workability. Stress exponent and activation energy were calculated to characterize the deformation mechanism. Then, the effects of hot deformation parameters on deformation mechanism,microstructure evolution and hot workability of AZ31 alloy were discussed. With increasing deformation temperature, the operation of non-basal slip systems and full development of dynamic recrystallization(DRX) contribute to effective improvement in hot workability of AZ31 alloy. The influences of strain rate and strain are complex. When temperature exceeds 350 °C, the deformation mechanism is slightly dependent of the strain rate or strain. The dominant mechanism is dislocation cross-slip, which favors DRX nucleation and grain growth and thus leads to good plasticity. At low temperature(below 350 °C), the deformation mechanism is sensitive to strain and strain rate. Both the dominant deformation mechanism and inadequate development of DRX deteriorate the ductility of AZ31 alloy. The flow instability mainly occurs in the vicinity of 250 °C and 1 s-1.     

Machining characteristics of fine grained AZ91 Mg alloy processed by friction stir processing

AZ91 Mg alloy was considered and friction stir processing (FSP) was adopted to achieve grain refinement to investigate the effect of grain size and secondary phase on machining characteristics during drilling at various speeds and feeds. Super saturated AZ91 Mg alloy was obtained after FSP and the grain refinement was achieved from (166.5±8.7) µm to (21.7±13.5) µm. Surprisingly, hardness reduced for FSP AZ91 Mg alloy (88.95±6.1) compared with AZ91 alloy (108.2±15.6), which was attributed to the reduced secondary phase. However, the mean cutting force for FSP-treated (FSPed) AZ91 Mg alloy was marginally increased. The edge damage of the drilled holes was lower for FSPed AZ91 Mg alloy compared with unprocessed AZ91 Mg alloy. Hence, it can be understood that the grain refinement may slightly increase the cutting forces during drilling but better edge finishing can be achieved in machining of AZ91 Mg alloy.     

挤压AZ31B镁合金板材高温轧制变形行为研究

通过单道次轧制试验,研究了AZ31B挤压镁合金板材在温度为365℃和450℃时的轧制性能,其变形量范围为10%～60%,应变速率为2.1s-1～5.0s-1。通过光学显微镜和扫描电镜观察了轧制变形中的微观组织及其演变。结果表明,在变形的初始阶段,孪生为主要的变形机理和硬化机制。由孪生变形积聚的畸变能和非基滑移的启动,导致了动态再结晶的形核与长大,增大变形速率可以抑制晶粒长大,使平均晶粒尺寸细化到7μm～10μm。365℃温轧制变形使板材晶粒明显细化,温度较高时,晶粒细化作用有限。在同一变形量下,随着轧制温度的升高,板材的晶粒呈长大趋势,在365℃轧制温度下,随着道次变形量的加大,细晶百分含量随之迅速增加。当轧制温度提高到450℃时,晶粒细化有限,晶粒尺寸保持在20μm以上。     

Effect of electromagnetic casting process on microstructure and properties of AZ31 magnesium alloy

ABSTRACT

In this paper, AZ31 magnesium alloy is cast by applying the semi-continuous casting process with a low-frequency electromagnetic field. By studying the influence of electromagnetic field frequency, excitation current intensity and casting velocity on the microstructure and mechanical properties, the optimum process under oil-slip electromagnetic casting conditions was determined to improve the degree of grain refinement, yield strength, elongation and tensile strength of AZ31 alloy. An improved microstructure refining effect and higher hardness can be obtained with a current intensity af 60 A. The microstructures and mechanical properties obtained for different casting velocities of V = 200 mm/min and V = 230 mm/min at processing parameters of f = 30 Hz and I = 120 A were compared. Our results suggest that a higher casting speed does not lead to grain refinement or improved mechanical properties. Frequency     

Effects of Er on the microstructure and properties of AZ31 magnesium alloy prepared via the EMS process

The effects of small amounts of the rare-earth element erbium on the microstructure and the mechanical properties of AZ31 magnesium alloy via the electromagnetic stirring(EMS) process have been studied.It has been shown that AZ31-Er alloys are mainly composed of α-Mg solid solution and β-Mg17Al12 phases.When the Er content reaches 0.12 wt.%,the characteristic peaks of Al2Er can be observed.The micro-structure is obviously refined and the tensile strength of the AZ31-based alloy at ambient temperature is significantly improved by contents of 0.03 wt.% Er,especially the elongation(δ=19%).More addition of Er obviously decreases the tensile strength and elongation of the AZ31-based alloy because of the grain coarsening and the reduction of β-Mg17Al12 phases.     

Effect of Ca addition on microstructure of twin-roll cast AZ31 Mg alloy

A twin-roll cast (TRC) AZ31-0.7Ca alloy sheet has been subjected to thermo-mechanical treatment (TMT) followed by annealing and its microstructure was evaluated. The as-cast microstructure of TRC AZ31-0.7Ca alloy is essentially similar to that of TRC AZ31 alloy, except for the presence of Al₂Ca dispersoid particles. The presence of Al₂Ca particles imparts significant changes to the microstructure of TMT-ed TRC alloy such as the refinement of grain size and evolution of textures different from those of ingot cast and TRC AZ31 alloys.     

AZ31镁合金压缩变形微观织构演变规律

采用电子背散射衍射(EBSD)原位跟踪实验方法研究了AZ31镁合金压缩变形微观织构演变规律。在温度为170℃条件下,研究了AZ31镁合金轧制板材经过3次连续真空压缩(变形量分别为11%、17%和23%)时,其相同观察区域的微观织构演变。研究结果表明,AZ31镁合金轧制板材的微观织构为典型的(0001)基面织构。当温度为170℃、变形量为11%时,晶粒取向发生显著改变,大部分晶粒都发生了完全孪生,只有少数发生部分孪生,原始的基面轧制织构大幅减弱,孪生变体符合60°/1010和86.3°/1210取向关系。随着变形量的增加,滑移开始启动,孪晶晶界减少,织构变化不明显。压缩变形过程微观织构演变机理主要以拉伸孪生为主,基本上没有压缩孪生出现。