触变注射成形Mg-9%Al-1%Zn合金组织特性和凝固行为

为了研究触变注射成形AZ91D合金中固相颗粒的形貌演变和液相的凝固行为,对该合金的组织和凝固行为进行了试验观察和理论分析。典型触变注射成形AZ91D合金由α-Mg和β-Mg17Al12两相构成,α-Mg相又可分为未熔固相和初生固相。未熔固相主要有形貌较为接近球状的固相、形貌不规则的固相、内部含有小液池的固相以及包裹液相的固相4种形貌。形貌不规则的固相被认为是球状固相和包裹液相的固相的中间发展形貌,内部含有小液池的固相可能是包裹液相的固相的初级形貌,包裹液相的固相则可能发生破裂形成不规则固相,最终发展成球状固相。球状固相被认为是最理想的也是最终的固相形貌。初生固相在液相合金中形核并长大,直至有不稳定长大行为发生为止,较为细小、圆整,主要受冷却速率的影响。Mg-Al合金二元相图的分析结果与试验观察到的组织相吻合。     

Mechanical properties of thixomolded AZ91D magnesium alloy

Tensile properties of thixomolded AZ91D alloy were studied to investigate the fracture mechanism by the microstructure and decohesion surfaces of tensile specimens which were manufactured at different processing conditions including barrel temperature, shot velocity, mold temperature and screw rotation speed. The results revealed that mechanical properties of thixomolded AZ91D mainly depended on porosity level, the size and volume fraction of primary solid phase and the size of α-Mg and β-Mg₁₇Al₁₂ in liquid phase. The increase in barrel temperature and shot velocity would cause the increase of both strength and ductility, while increasing mold temperature or improving screw rotation speed was coupled by the reduction of tensile properties. The tensile behaviors for different processing parameters were reasonably interpreted through the dependant factors during the deformation.     

应用连续性方法模拟枝晶生长

为了能够从微观尺度上准确的描述凝固过程中枝晶生长情况，采用连续性模型使界面区域内的物理性质不连续现象变成连续型过渡，此外在界面区域内用平均溶质浓度来保证溶质守恒，模拟结果表明这种方法能够很好的处理固／液两相物理性质的差异，显示凝固过程中枝晶的生长、界面失稳、二次枝晶臂、三次枝晶臂的演化和显微偏析。     

Thixomolded AZ91D and MRI153M magnesium alloys and their enhanced corrosion resistance

AZ91D and MRI153M alloys were produced by thixomolding. Their corrosion resistance is significantly higher than that of similar materials produced by ingot or die-casting. A corrosion rate smaller than 0.2 mm/year in 5 wt% NaCl solution is measured for the thixomolded AZ91D alloy. The corrosion behaviour was evaluated using immersion tests, electrochemical impedance spectroscopy, hydrogen evolution, glow discharge optical emission spectroscopy, and atomic emission spectroelectrochemistry. A bimodal microstructure is observed for both alloys, with the presence of coarse primary α-Mg grains, fine secondary α-Mg grains, β-phase, and other phases with a minor volume fraction. The amount of coarse primary α-Mg is significantly higher for the AZ91D compared with the MRI153M. The network of β-phase around the fine secondary α-Mg grains is better established in the thixomolded AZ91D alloy. A combination of several factors such as the ratio of primary to secondary α-Mg grains, localised corrosion or barrier effect due to other phases, as well as regions of preferential dissolution of the α-Mg due to chemical segregation, are thought to be responsible for the high corrosion resistance exhibited by the thixomolded AZ91D and MRI153M.     

镁合金半固态触变注射成形过程中固相形成机制

触变注射成型是目前唯一用于镁合金工业化生产的半固态成形技术,适合形状复杂、致密度高的零部件的生产。作者主要研究了镁合金触变成型过程中的固相形成机制。触变注射成型过程中主要固相的形成与传统的半固态方法不同,组织中的主要固相不是由螺杆搅拌打碎枝晶形成,而是由未完全熔化的镁合金切屑形成;成型过程中螺杆剪切的主要作用是提供镁合金切屑熔化所需要的热量、混合浆料,并且细化固相;在上述研究基础上,给出了镁合金半固态触变成型过程中固相形态的演变模型。     

Solidification and segregation behaviors in 6061 aluminum alloy

The characteristics of a solid/liquid interface established during directional solidification were investigated in a 6061 aluminum alloy (Al6061 alloy) in which interface patterns, such as planar, cellular and dendritic, form in the mushy zone. Directional solidification experiments were carried out in alumina tubes with an inner diameter of 5 mm to obtain the different microstructures for varying growth rates under an imposed thermal gradient of 9.45 K/mm. As a consequence, primary arm spacing, secondary arm spacing and the length of primary arm were measured for steady-state growth conditions. Furthermore, the solute concentration profiles for the final solidification region were obtained using a SEM-EDS system in order to examine the segregation behaviors of alloying elements. Therefore, it was found that the degree of segregation in the Al6061 alloy was dependent on the growth rate under given solidification conditions, that is, the elemental segregation became more severe with increasing growth rate in the interdendritic region.     

On the generation of thixotropic structures during melting of Mg-9%Al-1%Zn alloy

The microstructural evolution of Mg-9%Al-1%Zn alloy in the form of mechanically comminuted chips and rapidly solidified granules under the sole influence of external heat was investigated. The mechanisms governing phase and morphological transformations in solid and semisolid states were assessed and verified by a comparison with the behaviour of both conventionally cast and cold-deformed ingots, subjected to similar thermal exposures. It was revealed that the cold deformation introduced during the mechanical fragmentation of chips and chemical microsegregation inherited from a rapid solidification by granules, represent key features which control the generation of equiaxed structures during the very initial stages of melting. Ostwald ripening and coalescence were operative in the semisolid mixture within the entire range of the liquid fraction. As a result, an increase in a volume fraction of the liquid was primarily accompanied by the reduction in the number of solid particles, while the reduction in the particle size was significantly lower than anticipated from a change in the solid/liquid ratio. Some implications for engineering the microstructure during semisolid processing are discussed.     

Die-filling process during the thixoforming of a ZA27 alloy cylindrical rod

The die-filling process during thixoforming of a ZA27 alloy cylindrical rod was deduced by analyzing microstructural characteristics in the semi-solid ingot prior to and after forming, and by analyzing the phenomena occurring during this thixoforming. These characteristics referred to some constituent segregation, such as a liquid-phase segregation and an inhomogeneous distribution of primary solid particles. The results indicated that the die-filling process could be properly deduced by using the method developed in this paper. The inhomogeneous distribution of primary solid particles in the formed rods mainly resulted from the inhomogeneous distribution in the semi-solid ingot. The detailed distribution was determined by the sequence of the die-filling. However, the aforementioned liquid segregation was mainly attributed to the die-filling process. The sequence of the die-filling process was from the bottom of the cylindrical cavity to the top and from the edge to the inner part of the cavity.     

垂直定向凝固条件下通道偏析形成过程的数值模拟

建立了描述合金凝固过程热－溶质对流和宏观偏析形成过程的数学模型。模型中耦合求解了凝固过程中质量、动量、能量和溶质守恒方程；同时，基于固液两相区中温度和成分的耦合关系建立了固相分数场的更新方法。利用该模型模拟了底部冷却的二维矩形区域内Fe-C合金凝固过程中通道偏析的形成和发展过程。模拟结果表明，垂直定向凝固条件下通道偏析形成于液相线前沿附近，而不是两相区内部。这一结果很好地支持了文献中基于实验观察提出的通道偏析形成机理。     

S对Hastelloy C-4合金凝固偏析及热塑性的影响

采用金相探针法和高温拉伸试验研究S对HasteuoyC-4合金凝固偏析及热塑性的影响。结果表明，随着S含量的增加，导致固液两相区、温度范围和偏析程度明显加大，引起合金的热塑性降低。因此，降低合金S的偏析有利于合金的锻造性能。     

304不锈钢车削的高压断屑技术研究北大核心

针对304不锈钢的车削过程中铁屑折断难,经常导致铁屑缠刀、加工表面划伤、排屑困难、影响自动化生产线的稳定运行等问题,设计开发7 MPa高压供液系统为车削过程提供高压冷却液。在高压冷却环境下,试验比较304不锈钢在粗、精加工的不同车削参数、刀片情况下的铁屑状态。研究结果表明,7 MPa高压冷却可以解决304不锈钢精加工铁屑不易折断的问题。铁屑的长度与冷却压力、切削参数、刀片有关,通过匹配适当的切削参数和刀片,铁屑长度可以被控制在较短的范围内。在此基础上,提出了一套针对304不锈钢车削加工的高压断屑方案,实现铁屑的可靠折断。     

S355钢焊接温度场和应力场有限元分析

S355钢作为低合金钢,在焊接过程中会伴随着固态相变现象。在考虑S355钢的固态相变效应基础上,建立了焊接过程的热弹塑性有限元模型。通过引入相变转变模型、相变塑性和相变体积模型,计算获得焊后组织分布云图,并分析了焊缝和热影响区典型节点的组织演变规律。结果表明,与不考虑固态相变效应相比,紧邻焊缝两侧的热影响区的Mises应力大幅降低,计算得到的残余应力值与实测值吻合较好。     

铸造镍基高温合金IN792的凝固和偏析行为研究

采用等温凝固淬火试验(ISQ)与差热扫描量热(DSC)分析的方法对铸造镍基高温合金IN792的凝固过程和元素在固、液相中的分配进行研究,得到了合金的等温凝固组织、相析出顺序图、凝固特性曲线以及元素在不同温度下的偏析特性。ISQ结果表明:IN792合金的液相线温度为1328℃,开始析出MC碳化物的温度为1310℃,开始形成(γ+γ’)共晶的温度为1225℃,于1190℃开始析出二次γ’相。由DSC测得的合金宏观固相线温度(尚存体积分数5%残余液体)和由ISQ测得的微观固相线温度(终凝温度)分别为1250和1180℃,两者相差达70℃,这一温度范围往往是热裂缺陷形成的敏感区间。IN792合金在液相线下30℃范围内液体量锐减,析出了体积分数约85%的γ枝晶。1300～1270℃是合金枝晶间液体由连通转为断开的温度范围,它与铸件疏松形成密切相关。在1325～1190℃温度区间,IN792合金中元素W、Co分配系数大于1,倾向富集于枝晶干固相,为负偏析元素;Zr、Mo、Ti、Ta、Cr分配系数小于1,倾向富集于枝晶间液相,为正偏析元素;Al在凝固早期富集于液相而后来倾向于分布在枝晶干固相区,随着凝固温度降低逐步转变为负偏析元素。在1325～1210℃温度范围内,随着凝固温度降低,Al、Ni的分配系数升高,而Mo、Cr的分配系数降低。相反,在1210～1180℃温度区间,随着凝固温度降低,Ni、Al的分配系数降低,而Mo、Cr的则升高。     

Influence of arc constriction current frequency on tensile properties and microstructural evolution of tungsten inert gas welded thin sheets of aerospace alloy

The main objective of this investigation is to study the influence of arc constriction current frequency (ACCF) on tensile properties and microstructural evolution of aerospace Alloy 718 sheets (2 mm in thickness) joined by constricted arc TIG (CA-TIG) welding process. One variable at a time approach of design of experiments (DOE) was used, in which ACCF was varied from 4 to 20 kHz at an interval of 5 levels while other parameters were kept constant. The joints welded using ACCF of 4 kHz exhibited superior tensile properties extending joint efficiency up to 99.20%. It is attributed to the grain refinement in fusion zone leading to the evolution of finer, discrete Laves phase in interdendritic areas. An increase of ACCF above 12 kHz caused severe grain growth and evolution of coarser Laves phase in fusion zone. Alloy 718 welds showed more obvious tendency for Nb segregation and Laves phase formation at higher levels of ACCF due to the slower cooling rate. The volume fraction of Laves phase was increased by 62.31% at ACCF of 20 kHz compared to that at 4 kHz, thereby reducing the tensile properties of joints. This is mainly due to the stacking of heat input in weld thermal cycles at increased levels of ACCF.     

Formation of ultra-fine grained materials by machining and the characteristics of the deformation fields

Formation of ultra-fine structure in several materials by severe plastic deformation has been studied by plane strain machining. The microstructure generated in machined chips was characterized by optical microscopy and transmission electron microscopy as ultra-fine grains. A theoretical model was adopted to evaluate large plastic deformation in the primary deformation zone, the results show that the typical shear strains generated at the shear plane are in the range of 2–10. A more realistic finite element model was developed to characterize the deformation field associated with chip formation in plain orthogonal machining. The numerical results show that most of the grain refinement associated with the formation of ultra-fine grained chip can be attributed to the large shear strain imposed in the deformation zone. It could be feasible to take machining as a method to preparing ultra-fine grained materials and a type of experiment method to study severe plastic deformation.     

侧向凝固通道偏析的数值模拟

建立了二元系凝固过程通道偏析形成的数学模型,给出了描述焓与温度及固相分数耦合关系的表达式.在实验验证的基础上,对亚共晶与过共晶成分的NH4Cl-H2O侧向凝固通道偏析的形成位置与生长方向进行了数值模拟研究.模拟计算结果表明,偏析通道起源于糊状区,偏析通道中的富集溶质从糊状区流向液相区.为了维持偏析通道中的液体流动,枝晶间液体可通过糊状区从液相区得到补充.糊状区中富集溶质的流动方向取决于析出溶质的密度.NH4Cl-70%H2O侧向凝固时,析出的溶质密度较小,偏析通道倾斜向上生长,在糊状区上部形成A偏析.NH4Cl-90%H2O侧向凝固时,析出的溶质密度较大,在糊状区下部偏析通道倾斜向下生长.     

Correlating the microstructure and tensile properties of a thixomolded AZ91D magnesium alloy

Thixomolding¹, an emerging semisolid technology, was used to process an AZ91D magnesium alloy under experimental conditions designed to yield from 5 to 60% of the primary solid particles. The thixotropic microstructures obtained were characterized in detail and linked to the corresponding tensile properties. An increase in primary solid content was accompanied by its larger microchemical and microstructural inhomogeneity expressed by Al and Zn segregation, sub-micron precipitates of Mg₁₇Al₁₂ and eutectic islands. At the same time, the size of α-Mg grains within the eutectic mixture was reduced. For the volume fraction of the primary solid up to about 20%, the tensile strength and elongation remained at the level of 240 MPa and 4.5%, respectively. A further increase of the primary solid caused a reduction in both strength and ductility. The fractographic analysis revealed a correlation between the primary solid content and the morphology of the decohesion surface. It is concluded that for alloys with a solid fraction below approximately 20%, the internal structure of the primary solid and the eutectic mixture control the properties. For a large volume of unmelted fraction, the interface between the primary solid and the eutectic mixture is a key factor which controls the tensile properties of the thixomolded alloy.     

Microstructures forming in friction welding of Inconel 718 alloy – joint performance and its controlling factors in friction welding of Inconel 718 alloy

Microstructures forming in the friction welding of Inconel 718 alloy have been investigated in order to understand the phenomena occurring during the welding process and to determine the factor controlling the joint performance from a metallographic point of view. In the interfacial zone, liquation microstructures characterized by a eutectic structure consisting of γ and Laves phases, and Nb-rich microstructures along the grain boundary (Nb-rich GB microstructures) were observed, and their amounts increased with the friction time and pressure. The volume fractions of these liquation structures were greater in the flash than in the interfacial zone, suggesting that the liquid phase was preferentially expelled into the flash by friction pressure, compared with the solid phase. Since the liquid phase was rich in Nb, this preferential expulsion of the liquid phase caused the depletion of Nb, a major element for the precipitation hardening of the alloy. The depletion of Nb brought about a decrease in the hardness in the interfacial zone after a post-weld heat treatment for precipitation hardening. Thus, although the friction bonding is a solid state welding process, the liquation occurs in the weld of Inconel 718 alloy having a wide solid–liquid coexisting temperature range, and has a significant influence on the joint performance.     

FH96高温合合金的凝固过程及元素平衡分配系数的研究

基于Scheil-Gulliver凝固模型,采用热力学计算软件Thermo-Calce及相应的Ni基数据库,对FGH96高温合金的凝固过程进行了热力学模拟计算,以揭示FGH96合金凝固过程中各相的析出规律,及凝固过程中各元素的偏析行为;并用Scheil-Gulliver方程对残余液相中各合金元素的质量分数随固相分数的变化进行回归,得到了各元素的平衡分配系数.结果表明,在FGH96合金凝固过程中,当固相分数达到0.7、0.94、0.98时由残余液相中分别开始析出MC型碳化物、TCP相、γ′相;B、C、Mo、Nb、Ti、Zr、Si的平衡分配系数小于1,而Co、Cr、W、Fe、Al的平衡分配系数大于1.     

Phase-field simulation of solidification dendritic segregation in Ti-45Al alloy

The microstructures and mechanical properties of Ti Al alloys are directly linked to micro-segregation which cannot be avoided during solidification. So a thorough understanding of the micro-segregation should be a great help to further enhance the mechanical properties of the cast products. Theoretical analysis and experiments have been used to predict the micro-segregation, but it is very difficult to observe and determine the dendritic segregation in the micro region. Phase-field method has been employed for the simulation of dendritic growth. However, due to the complicated quasi-sub regular solution model for Ti-45Al(at.%) alloy, the classic phase-field models have difficulty to deal with the free energy. In this work, a phase-field model by linking thermodynamic calculation was used to simulate solidification dendritic segregation of Ti-45 Al alloy for Liquid→Liquid+β(Ti). The free energies of solid phase and liquid phase for Ti-45 Al alloy were calculated by Thermo-Calc and then coupled with the phase-field equations. The simulation results show the dendritic morphology and Al content variations between liquid and growing solid phase for Ti-45 Al alloy. With the growth of the β(Ti), dendritic segregation is formed in the liquid and solid phases due to the solute partitioning and rejection into the liquid. As a result, the dendrite arms are depleted of Al element, while the inter-dendrites are enriched. The dendritic tip growth velocity decreases with the progress of solidification, whereas the segregation ratio increases.