铝铁合金半固态变形的组织演变

对电磁搅拌的铝铁合金在半固态温度条件下用INSTRON-5500R实验机进行单向压缩实验,实验结果表明,铝铁合金在半固态温度变形时,随着变形温度的升高,真应力峰值降低;随着应变速率增加,真应力峰值也增加,随后会发生应变-软化现象。应变速率愈小,软化过程愈长,应力下降愈缓慢。变形试样中心和边缘区域的变形情况不同,中心区域固相颗粒的变形程度大于边缘区域。在不同的变形量、变形温度和变形速率下,其内部固-液两相的流动方式和变形机理不同。固相主要集中在试样的心部发生塑性变形,而液相则流向边部,其组织形貌与变形温度、变形量和变形速率都有直接的关系。     

On the microstructure formation in chromium steels rapidly cooled from the semi-solid state

Semi-solid metal forming requires precise knowledge concerning the microstructural parameters in the mushy state. For many light metals, the liquid and solid fraction and the size, shape and contiguity of the solid phase can be evaluated easily in the “quenched from the freezing range” condition. For iron-based alloys, however, determining them is more difficult or even impossible because steels may undergo different phase-transformations during cooling. Due to the high processing temperatures, diffusion during quenching is also of more importance. Here we describe the phase formation during rapid cooling from the semi-solid state of two different steel grades, tool steel X210CrW12 and bearing steel 100Cr6. For both of these steels the microstructure in the as-quenched state does not directly reflect the condition in the semi-solid state, and thus no metallographic evaluation of the microstructural parameters is possible. It is also shown here that the microstructure of semi-solid processed steels is completely different from that of the conventionally treated species.     

Microstructure evolution of semi-solid Mg-14Al-0.5Mn alloys during isothermal heat treatment

A new Mg-14Al-0.5Mn alloy that exhibits a wide solidification range and sufficient fluidity for semi-solid forming was designed.And the microstructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. The mechanism of the microstructure evolution and the processing conditions for isothermal heat treatment were also discussed.The results show that the microstructures of cast alloys consist ofα-Mg,β-Mg17Al12 and a small amount of Al-Mn compounds.After holding at 520℃ for 3 min,the phases ofβ-Mg17Al12 and eutectic mixtures in the Mg-14Al-0.5Mn alloy melt and the microstructures ofα-Mg change from developed dendrites to irregular solid particles.With increasing the isothermal time,the amount of liquid increases,and the solid particles grow large and become spherical.When the holding time lasts for 20 min or even longer,the solid and liquid phases achieve a state of dynamic equilibrium.     

60Si2Mn半固态浆料的制备和流变轧制的组织形貌

以弹簧钢（60Si2Mn)为研究对象，采用电磁搅拌的方式制备半固态浆料，研究了搅拌参数对半固态浆料组织形貌的影响；同时对半固态浆料进行了流变轧制，观察和分析了半固态浆料在轧制过程中的组织演变及轧制产品可能存在的组织缺陷。结果表明：在变温连续搅拌条件下，随着搅拌时间的延长，半固态浆料的固相率提高，初生固相颗粒的直径减小，组织更均匀；经过1道次的轧制，半固态浆料中的液相从固相颗粒间挤出，流向轧件的表面，大部分固相集中在轧件的中心，产生宏观偏析现象，提高固相率可减小偏析程度。此外，轧件中还可能出现缩孔、分层组织等缺陷。     

等温热处理对Mg-Gd-Zn-Zr合金半固态组织的影响

选择稍高于共晶反应温度作为等温热处理温度,对铸态Mg-15Gd-2Zn-0.6Zr合金进行等温热处理,获得了半固态球化组织。研究了热处理温度和保温时间对半固态组织的影响,探讨了半固态组织演变机制及适用于低温等温热处理的半固态Mg-Gd-Zn-Zr合金成分设计。结果表明,液相组织具有低的温度敏感性,其组织演变主要机制为α-Mg表面熔化和α-Mg动态再析出,而固相颗粒球化机制为:α-Mg树枝晶→枝晶臂粗化→枝晶臂合并、不规则多边形化→球化。     

Metastable structure of austenite base obtained by rapid solidification in a semi-solid state

Material processing in a semi-solid state with rapid solidification is an innovative technology, which enables us to produce complex-shaped semi products in one operation. Unconventional properties and microstructures can be obtained in this way. Material processing in a semi-solid state has been used for materials with lower melting temperatures, particularly for Al alloys.This paper concentrates on the development of new technologies for production of miniature thin-walled steel components with complicated shapes. Ledeburitic steel with 1.8% of carbon and 11% of chromium was chosen for this experimental study. This material was used to produce very small thin-walled semi products. From the initial structure consisting of primary and secondary carbides distributed in a ferrite matrix was obtained a microstructure with over 90% of metastable austenite after cooling from the semi-solid state. The main aim of this experimental program was to describe the effect of two different methods of heating to the semi-solid state. The first method used unique heating equipment, combining high frequency and resistance heating. The second method consisted of conventional heating in a furnace. The influence of the cooling rate on the development of the microstructure was investigated. If was found that both heating and cooling rates influence grain size and the size and the morphology of carbide network placed between the globular austenite grains.Structure analysis was performed with the help of light microscopy, laser scanning confocal microscopy and scanning electron microscopy. EDX analysis was applied to determine chemical profiles of phases and X-ray diffraction phase analysis was used to establish volume fraction of austenite in the final microstructure. Characterisation of individual structural components was further completed by hardness measurements.     

A three-phase simulation of the effect of microstructural features on semi-solid tensile deformation

A direct finite-element microstructure model for prediction of the deformation behavior of semi-solid metallic alloys is presented. The 2D model geometry is based on a modified Voronoi tessellation, and includes rounded corners to approximate an equiaxed-globular grain structure, liquid surrounding the grains, and micro-porosity. An elasto-plastic empirical constitutive equation is derived for the solid grains, while the liquid is approximated as a perfectly plastic material with a very low yield stress. The resulting three-phase model was used to investigate the effects of fraction solid, porosity, and grain size on the constitutive behavior of a semi-solid aluminum alloy, AA5182. The model predictions were validated against experimental data at high fraction solid. These simulations reveal a strong correlation between semi-solid grain size and yield stress, and between porosity and strain localization. The application of direct finite-element simulations is shown to be an effective technique for examining the effects of microstructure phenomena on the macro constitutive behavior of semi-solid materials.     

Microstructure evolution of semi-solid Mg- 14A1-0.5Mn alloys during isothermal heat treatment

A new Mg-14Al-0.5Mn alloy that exhibits a wide solidification range and sufficient fluidity for semi-solid forming was designed. And the rnicrostructure evolution of semi-solid Mg-14Al-0.5Mn alloy during isothermal heat treatment was investigated. The mechanism of the microstructure evolution and the processing conditions for isothermal heat treatment were also discussed. The results show that the microstructures of cast alloys consist of α-Mg,β-Mg17Al12 and a small amount of Al-Mn compounds. After holding at 520 ℃ for 3 min, the phases of β-Mg17Al12 and eutectic mixtures in the Mg-14Al-0.5Mn alloy melt and the microstructures of α-Mg change from developed dendrites to irregular solid particles. With increasing the isothermal time, the amount of liquid increases, and the solid particles grow large and become spherical. When the holding time lasts for 20 min or even longer, the solid and liquid phases achieve a state of dynamic equilibrium.     

Deformation Behavior and Constitutive Equation Coupled the Grain Size of Semi-Solid Aluminum Alloy

Isothermal compression tests on Al-4Cu-Mg alloy were carried out in semi-solid state. Deformation behavior and microstructural evolution are discussed in this paper. Meanwhile, a new constitutive equation, which couples the grain size and liquid volume fraction, has been established for the semi-solid deformation behavior. The results show that the maximum difference between the calculated with the experimental data is less than 15%. The present equation is satisfactory for describing the correlation between the flow stress and microstructure of semi-solid Al-4Cu-Mg alloy. The coupled simulation of the deformation behavior and microstructural evolution during the semi-solid forming would be conducted easily through writing the present equation in a FE code.     

Three-dimensional granular model of semi-solid metallic alloys undergoing solidification: Fluid flow and localization of feeding

A three-dimensional (3-D) granular model which simulates fluid flow within solidifying alloys with a globular microstructure, such as that found in grain refined Al alloys, is presented. The model geometry within a representative volume element (RVE) consists of a set of prismatic triangular elements representing the intergranular liquid channels. The pressure field within the liquid channels is calculated using a finite elements (FEs) method assuming a Poiseuille flow within each channel and flow conservation at triple lines. The fluid flow is induced by solidification shrinkage and openings at grain boundaries due to deformation of the coherent solid. The granular model predictions are validated against bulk data calculated with averaging techniques. The results show that a fluid flow simulation of globular semi-solid materials is able to reproduce both a map of the 3-D intergranular pressure and the localization of feeding within the mushy zone. A new hot cracking sensitivity coefficient is then proposed. Based on a mass balance performed over a solidifying isothermal volume element, this coefficient accounts for tensile deformation of the semi-solid domain and for the induced intergranular liquid feeding. The fluid flow model is then used to calculate the pressure drop in the mushy zone during the direct chill casting of aluminum alloy billets. The predicted pressure demonstrates that deep in the mushy zone where the permeability is low the local pressure can be significantly lower than the pressure predicted by averaging techniques.     

半固态钢铁材料轧制产品的力学特性

将弹簧钢（60Si2Mn)和不锈钢（1Cr18Ni9Ti)在半固态下1道次轧制成形，对轧制产品进行拉伸实验，研究其在室温条件下的力学性能，并对拉伸变形过程中的塑性变形机理进行了分析，研究结果表明：不同的固相率对轧制产品的力学性能有明显的影响，在研究范围内随着固相率的提高，其力学性能亦提高，半固态轧制过程中所产生的液固相分离，导致半固态轧制产品组织分布差异，使轧制产品不同区域的力学性能不同，同时由于半固态轧制产品特有的球形固相颗,其伸拉时的塑性变形机理也有自身的特点。     

Deformation characteristic of semi-solid ZCuSn10 copper alloy during isothermal compression

The compression tests were carried out by Gleeble-1500 thermo-mechanical simulator with samples of semi-solid ZCuSn10 alloy prepared by strain-induced melt activation(SIMA) process. The original microstructure and the deformation temperature of semi-solid ZCuSn10 alloy are different. The strain is 0.2, and the strain rate is 1 s~(-1) for the compression test. The results show that when the semi-solid ZCuSn10 alloy was prepared by SIMA process, the liquid fraction of semi-solid microstructure increases, and the solid grain is smaller,more uniform and more inclined to be round as the rolling pre-deformation increasing. The results also indicate that the deformation resistance of ZCuSn10 alloy in semi-solid state decreases with the deformation temperature increasing or the solid fraction of original microstructure decreasing. The stress–strain curves of the isothermal compression can be divided into quasi-elastic deformation stage and plastic deformation stage, and there are three deformation zones in the samples after isothermal compression, namely the difficult deformation zone, the large deformation zone and the free deformation zone. In the three deformation zones, the main deformation mechanism is flow of liquid incorporating solid particles(FLS)mechanism, plastic deformation of solid particles(PDS)mechanism and liquid flow(LF) combining with FLS mechanism, respectively.     

Al-Zn对称成分Spinodal分解合金的组织、结构与相成分

利用组织分析方法和X射线衍射分析, 研究了Al-Zn对称成分Spinodal分解合金的组织、结构与成分. 结果表明, Al-Zn对称成分合金经固溶处理后可获得单相组织, 但该组织在室温下将发生Spinodal分解和Zn的析出. 在钢模铸造的快速冷却条件下, 该合金的相组成已经接近于平衡态, 但存在较多的缺陷和较大的成分不均匀性. 利用实测晶格常数的方法可以近似确定Spinodal分解和不连续析出等高分散度两相组织中富Al固溶体的相成分.     

铸轧对半固态镁合金组织的影响

实验研究了镁合金半固态铸轧的组织。铸轧试验是在一台自制的铸轧试验机上进行。试验结果表明,铸轧对半固态镁合金组织有显著的影响。铸轧前的半固态镁合金是由机械搅拌制备,组织中先结晶的固相颗粒形状不规则,有枝晶的痕迹;然而铸轧后组织中,先结晶的固相颗粒形状变得非常圆整,接近球形。基于试验结果,提出了半固态铸轧对一次相颗粒的圆整化机制。认为铸轧对半固态组织的圆整化作用是通过固液两相相对流动摩擦消除尖角而圆整化或固相颗粒翻滚破碎重叠合并而圆整化两种方式实现的。此外,在一定的工艺条件下,凝固的金属在进一步的轧制变形中,圆整的球形一次相颗粒被轧制变形,变成为椭球状。     

添加0.5%富铈混合稀土AZ91D镁合金半固态组织的形成

半固态浆料的固相颗粒尺寸、形态和分布主要取决于熔化过程中液相的形成与演化过程。采用添加0.5%富铈混合稀土来改善AZ91D镁合金的铸态组织,研究在半固态等温热处理中的组织演变以及非枝晶组织制备与控制的机理。结果表明:稀土合金化处理可促进初生相在等温热处理过程中由枝晶向粒状晶的转变,可获得更加细小、均匀的球状固相颗粒,并且其粗化速度较慢。半固态等温热处理过程中,整个系统处于熔化和结晶的动态平衡,铸态组织中枝晶根部高溶质浓度区或系统的温度、浓度起伏是固相颗粒内液相形成的内在和外在条件。     

Liquid formation and microstructural evolution during re-heating and partial melting of an extruded A356 aluminium alloy

Thixoforming processes require feedstock having a non-dendritic, equiaxed microstructure, which in some cases can be obtained by a deformation–recrystallisation technique followed by partial melting. From the study of an extruded, re-heated and partially re-melted A356 alloy, a number of phenomena were observed and analysed: grain and particle growth, texture and low angle grain boundary formation, primary phase coarsening during isothermal holding in the semi-solid state and uneven distribution of Si particles and liquid phase. Both grain growth (solid state) and particle growth (semi-solid state) obey a R³ against time relationship. It was also found that microstructural coarsening follows an Ostwald ripening type of growth and that coalescence was also present, although exerting a minor role. Finally, phenomena such as the evolution with time of the proportion of low angle grain boundaries, and the relationship between recrystallisation, the onset of liquid formation and the coarse Si particle precipitation and dissolution are presented and discussed.     

钢/铝管磁脉冲辅助半固态钎焊界面行为

结合电磁成形技术和半固态钎焊技术,提出了一种钢/铝管磁脉冲辅助半固态钎焊工艺,利用电磁脉冲产生的洛伦兹力使铝外管高速碰撞半固态钎料,通过半固态钎料中固相颗粒对母材表面径向压缩和轴向剪切作用去除母材表面氧化膜,实现钢铝异种管材的无钎剂钎焊. 在不同工艺参数下进行了钢/铝管磁脉冲半固态钎焊试验,研究了钎焊接头界面元素的扩散行为和金属间化合物的生长机理. 结果表明,焊缝组织主要为α-Al以及富锌相,铝侧界面处的Al₂O₃氧化膜破碎与去除情况良好,钢侧界面处有薄层FeAl₃金属间化合物形成,各部位均获得较好的冶金结合.     

半固态Al-Si-Cu-Mg合金压缩变形特征的研究

用Gleeble-1500热模拟实验机研究了不同变形速率、不同变形量下AlSi15Cu3MgFe铝合金的半固态应力与应变的变化规律。研究结果发现：变形速率不同时,应力随着变形速率的增加而增加;变形量不同,随着变形量的增加,流体的流动趋于更稳定的流动形态。     

SEMI-SOLID MICROSTRUCTURE AND ITS EVOLUTION OF WROUGHT ALUMINUM ALLOY BY DIRECTLY HEATING-ISOTHERMAL TREATMENT

Microstructure evolution of wrought aluminum alloy extruded rods and the mechanism of liquid phase formation during reheating were investigated. And the relation between the volume fraction of liquid phase and the recrystallization microstructure was proposed. The results show that increase in reheating temperature and time can augment the volume fraction of liquid phase and accelerate the grain spheroidization, as a result of which the requirement of semi-solid forming can be satisfied. Due to the higher aberration energy of grain boundary, the melting point is lowered as a result of the easy diffusion of atoms. At higher reheating temperature the grain boundary melts, the growth of the recrystallized grain is inhibited and the grain is refined. The composition of the low melt-point phase along the recrystallized grains was determined using EDS. It can be seen from the experimental results that when the extrusion rod of the wrought aluminum alloy is reheated at 610℃ for 20min, perfect fine equiaxial grains can be obtained, the average grain size is about 66.34μm and the volume fraction of solid phase is about 68%.     

Induction heating process of an Al–Si aluminum alloy for semi-solid die casting and its resulting microstructure

During induction heating, the relationship between time and temperature must be controlled exactly to obtain a homogeneous temperature distribution over the entire cross-sectional area. Because the initial solid fraction in the semi-solid die casting (SDC) process is the main parameter to achieve a homogeneous flow behavior of the liquid and solid phases and to prevent macro-segregation effects in the SDC process, an accurately controllable induction heating method must be selected for the reheating process. The purpose of this work is not just to obtain the desire solid fraction, generally about 50%, but also to ensure the optimal induction heating conditions of A356 alloy to reduce the temperature gradient of a 76 mmdiameter×90 mmlength billet and to obtain a fine globular microstructure without grain coarsening (resulting microstructure). This work shows that, in the case of a three-step reheating process for the SDC process, the final holding time is the most important factor to maintain a fine globular microstructure without grain coarsening.