半固态成形工艺特点及发展现状

半固态成形技术充分利用金属材料在半固态温度区间特殊的微观组织和成形性能,能够在较低的成形载荷下实现复杂构件的高效近净成形,该技术的应用对于提高金属材料的质量利用率和性能利用率、延长模具寿命、节能减排等方面具有重要的作用。主要介绍了半固态成形技术的历史沿革、半固态成形技术的分类、半固态金属坯料浆料的制备工艺、有色金属材料和钢铁材料的半固态成形技术开发以及应用情况,最后提出了半固态成形技术所面临的挑战和机遇。     

半固态流变行为模型及应用

半固态成形是21世纪最具潜力的先进制造技术之一.分析了国内外半固态变形行为研究进展,重点阐述了近似单相本构关系模型、两相本构关系模型、宏观-微观耦合本构关系模型的特点及应用,特别是笔者提出的宏观-微观耦合本构关系模型反映了工艺参数和微观组织参数对半固态流变应力的影响.同时,笔者将新型宏观-微观耦合本构关系模型应用于Al-4Cu-Mg合金半固态反挤压过程的有限元数值模拟,获得了工艺参数对应力应变、温度、晶粒尺寸、液相体分数和挤压载荷等的影响规律,数值模拟结果与半固态实验结果基本一致.     

功率超声振动在金属材料成形中的研究及应用

功率超声振动在金属材料成形领域,如在超声处理液态金属、超声焊接成形、超声金属塑性成形及金属半固态成形等方面有着广泛的研究和应用.结合作者的研究成果,介绍了功率超声振动应用于上述各种成形方法的原理,相对于传统方法的优点及其研究和应用现状.认为,由于超声成形的环保、节能和高效性,功率超声振动在金属材料成形领域的研究和应用,特别是用于半固态成形领域中,将有广阔的发展前景.     

金属半固态成形技术的研究及应用进展

系统介绍了近年来金属半固态成形技术的理论研究及工业应用的进展情况,讨论了半固态成形技术的应用优势、存在问题、研究重点及其发展策略。     

微薄板塑性成形本构关系研究

尺寸效应的影响使得传统的成形理论和变形机制不再适用于微塑性成形.在考虑尺寸效应对微薄板成形性能影响的基础上,对已有的CuZn36黄铜薄板微拉伸实验结果进行处理,提出了一种研究微塑性成形本构关系的方法.根据弹性和塑性变形过程,分阶段分析了t/d(板厚/晶粒大小)对屈服强度和切线模量的影响,修正了双线性弹塑性本构关系,获得了考虑尺寸效应的微塑性成形本构关系.借鉴宏观增量本构关系,结合微拉伸实验,采用Mises屈服准则和随动强化模型,得出适合微塑性成形的弹塑性增量本构方程,为微塑性成形的理论研究和实际应用奠定了基础.     

半固态压力成形工艺的研究进展

半固态成形工艺具有能耗低、制件性能好、可以实现近净成形等特点,近年来倍受人们关注.本文综述了半固态成形工艺在金属及金属基复合材料成形中的应用及其数值模拟技术的研究进展,并对其今后的发展研究方向进行了展望.     

钢铁材料及有色合金构件的多段半固态成形工艺研究

目的为了有效抑制半固态成形过程中的液相偏析,改善半固态成形构件微观组织和力学性能的均匀性。方法提出了包括多段流变成形和多段触变成形在内的多段半固态成形工艺。多段半固态成形工艺均由半固态坯/浆料制备、预成形、控温冷却和终成形4个阶段组成,分别在热模拟试验机和机械伺服压机上开展了SKD11工具钢和6061铝合金的半固态触变成形和半固态流变成形试验。结果在初成形阶段,具有较高液相分数的半固态坯/浆料以较高的应变速率初步充填型腔,限制了液相外流的时间和空间;在控温冷却阶段,半固态坯/浆料的液相分数因部分凝固而降低;在终成形阶段,具有较低液相分数的半固态坯/浆料以较低的应变速率完成型腔的充填,由于固相晶粒在此阶段发生塑性变形而提高了成形构件的力学性能。结论获得了组织均匀性较好的钢铁材料和有色合金构件,验证了多段半固态成形工艺的可行性。     

铜合金半固态流变成形及制备技术研究进展

金属半固态成形已有近50年的发展历史,因其加工优势和市场价值备受关注,并形成了很多加工方法,其中半固态流变成形工艺效率高、成本低,具有较好的发展前景。由于铜合金熔点普遍较高,目前针对其进行的半固态工艺研究较少。简要综述了针对铜合金的半固态流变成形及制备技术的研究新进展,包括各新工艺的加工过程、最优工艺参数选择及工艺过程的机理等,分析了各工艺目前尚存在的问题和原因。阐述了半固态流变制备工艺模型及机理的新进展,并对其发展趋势进行了分析和展望,认为半固态流变成形技术未来的研究方向有半固态浆料制备新方法、新型半固态合金和半固态组织演变机制及其控制方法等。     

Zn-Al钎料固相率及组分对Cu/Al管磁脉冲-半固态复合辅助钎焊接头质量的影响初探

由于节能环保以及轻量化的要求,Cu/Al异种金属复合管件被广泛应用于各工业领域,为此探索一种Cu/Al管件间高效可靠的连接技术具有较大应用价值和研究意义。为了探究Cu/Al管半固态钎焊连接机理并优化相关工艺参数,基于新提出的磁脉冲-半固态复合辅助钎焊方法制备了Cu/Al异种金属管件钎焊接头,利用LS-DYNA分析了半固态钎料不同表观粘度参数下外管内壁的受力情况,采用材料力学拉伸试验机,能谱仪和电子探针显微分析仪研究了半固态Zn-Al系钎料固相率及组分对焊接接头质量的影响。结果表明:外管内壁所受压力幅值随着表观粘度的增大而减小,剪切应力随着表观粘度的增大而增大;半固态钎料固相率为0.8时钎料与母材间没有良好冶金结合,固相率为0.4时会导致钎料填充不足的缺陷。在二次放电电压7 kV,半固态Zn-Al钎料固相率为0.6的条件下,磁脉冲-半固态复合辅助钎焊工艺能够实现Cu/Al管的无钎剂有效连接;与Zn-3Al和Zn-22Al相比,Zn-15Al钎料制得的接头综合性能更好,是较为合适的Cu/Al管磁脉冲辅助半固态钎焊用Zn-Al钎料。     

金属材料半固态凝固及成形技术进展

简要介绍了金属材料半固态凝固及成形技术发展的历史和现状,重点介绍了近年开发的几种金属材料半固态凝固制备技术的代表性方法与特点、铝合金和镁合金等轻金属材料半固态凝固及流变成形的组织与性能、高熔点合金材料半固态凝固制备及成形技术的进展以及半固态凝固成形技术的应用现状,最后对金属材料半固态凝固及成形技术的进展状况及发展前景作了简单的总结和展望。     

Simulation studies on deformation behaviour of AA2017 alloy in semi-solid state using FEA

Abstract

Thixo-forming is one of the near net shaped manufacturing processes in which the final product is made between the liquidus and solidus temperatures. Forming the product at this temperature range provides large benefits compared to conventional forming methods. In the present work, deformation mechanism of AA2017 alloy in the semi-solid state has been studied. As the mechanical behaviour and deformation mechanism of semi-solid metal is completely different from that of the solid state forming, it is necessary to investigate its nature at the semi solid range. To analyse the metal flow, the stress behaviour and the corresponding strain induced, extensive finite element analysis (FEA) based simulation studies have been performed. The commercial software, DEFORM 2D was used for the simulation. The flow behaviour was modelled by formulating an Arrhenius type constitutive relation.     

Microstructural evolution in semi-solid AA7034

The kinetics of semi-solid grain coarsening in a spray-formed AA7034 aluminium alloy were determined by the use of ex situ coarsening experiments. The coarsening rate constant of the alloy decreases with increasing solid fraction for solid fractions greater than 0.65. For solid fractions between 0.6 and 0.65, the coarsening rate constant increases as the solid fraction increases. For solid fractions below 0.6, the coarsening rate constant decreases again with increasing solid fraction. A model of flow-enhanced diffusion was developed to explain the grain-coarsening behaviour at low solid fractions, with flow caused by the macroscopic shape change of the uncontained semi-solid material during coarsening. Together with previous liquid film and modified liquid film migration models, the flow-enhanced diffusion model shows qualitative agreement with available experimental results.     

Influence of Solid Fraction Casting on Microstructure of Aluminum Alloy 6061

Semi-solid processing of AA6061 alloy near liquidus temperature with the addition of micro- and nanoparticles of same alloy may be highly attractive for small intricate shapes due to excellent mechanical properties. The present work utilizes semi-solid behavior of AA6061 alloy, which reduces macro- and nanosegregation of particles, porosity, and forming forces during the shaping process. The experiment utilizes the semi-solid slurry of different solid fractions mixed with a melt at pouring temperature range of 400 ? 640°C. The potential of solid fraction to produce semi-solid slurry has been investigated with the help of microstructure analysis, which is a crucial need for aluminum industries. The result shows that during the stirring, every dendrite modified itself to fine solid grains and dispersion of these grains takes place inside the molten metal. Alloy cooled directly from semi-solid state results in higher relative density with respect to conventional casting. With increase of solid fraction to 25%, the un-melted solid structure inside the pores and cavities in the direction of elongated grains results in the subsequent enhancement in the impact strength, hardness, and compressive strength as 19 kJ/m², 93 BHN, and 550 MPa, respectively, without any alteration in the basic metal matrix composite.     

Mechanics of porous elastic materials containing multiphase fluid

A continuum theory of mixtures for a porous elastic solid saturated by immiscible viscous fluids is presented. The theory includes micro-inertial effects for the local fluctuation in volume fractions of the solid and fluid constituents. Gradients of volume fraction of both the elastic solid and fluid constituents are included in the constitutive variables. Equations governing the macroscopic motion are developed and show that the present theory contains both Biot's equations and multiphase Darcy flow through porous media as special cases.     

Simulation of semi-solid thixoforging using a micro-macro constitutive equation

The constitutive equation of the isothermal steady-state behaviour of semi-solid materials is described with a micro-macro modelling. It is based on the application of the micromechanics concept of “coated inclusion” to semi-solid behaviour and the introduction of an evolving bimodal distribution of the liquid and solid phases. The micro-macro modelling is implemented in the FE code Forge2. The influence of the normal and abnormal namely softening stress–strain rate relationship on thixoforging is analyzed using compression and extrusion tests simulations.     

Rheological Behavior of Semi-Solid AZ91D Alloy

The rheological behavior and microstructure of semi-solid AZ91D was studied using a Couette type viscometer. The results show that the apparent viscosity of semi-solid AZ91D increases with the cooling rate increasing when it is continuously cooled and mechanically stirred and the empiric relation of the apparent viscosity with the cooling rate and solid fraction at shearing rate 93.7 s-1 is expressed as η=[0.66-0.63e(-ε/3.01)]e(fa/0.14), and with the shearing rate and solid fraction at cooling rate 4℃/min is shown as η=0.55e(fa/0.11-γ/87.59), as well as the cooling rate and shearing rate have an important effect on the microstructure of the semi-solid AZ91D alloy slurry, and decreasing the cooling rate and increasing the shearing rate are favorable to the nondendritic evolution of the primary grains.     

Steady State Rheological Characteristic of Semisolid Magnesium Alloy

Isothermal compressive experiments at different temperatures, strain rates and holding time for semisolid AZ91D, Zr modified AZ91D and MB15 alloy with higher solid volume fraction were carried out by using Gleeble-1500D simulator and the true stress-strain curves were given directly. The relationship of apparent viscosity vs temperature, shear rate and holding time of the three kinds of semi-solid magnesium alloys, as well as isothermal steady state rheological characteristic and mechanical behavior were studied. The results show that the three magnesium alloys had the characteristic of shear-thinning. The rheological characteristic of the semi-solid MB15 is different from that of semi-solid AZ91D. The semi-solid MB15 has higher apparent viscosity and deformation resistance.     

半固态ZL201合金本构关系研究

采用Gleeble 3800热模拟试验机对液相线铸造获得的ZL201合金半固态坯料进行了不同工艺参数下的热模拟试验,并进行了变形抗力本构方程拟合,以此为基础根据Stefan方程确定了幂律模型中的表观粘度等参数.采用商用仿真软件ANYCASTING对汽车发动机支架进行了触变压铸过程数值模拟分析,获得了ZL201合金的半固态充填流动特征响应.结果表明:半固态表观粘度随剪切速率增大而下降,在充型过程中呈现剪切变稀的触变成形特点,半固态浆料将以层流方式充填型腔,可获得充型完好的半固态压铸零件.     

一种具有球晶组织的半固态Zn-Al合金钎料

通过熔炼、浇注以及等温热处理的方法制备了具有球晶组织的Zn-Al合金,并采用重熔实验、等温压缩实验和振动辅助半固态钎焊实验研究了半固态Zn-Al合金的固相率与温度的关系、压缩变形行为以及作为钎料的可钎焊性。结果表明:半固态Zn-Al合金在压缩变形时,具有较强的抗变形能力;作为钎料,在钎焊过程中其固相率能控制在(64±5)%范围内,所得钎焊接头中仅在钎缝边缘处界面上有少许氧化膜残留,而其他区域界面上的氧化膜都被彻底去除。     

Recrystallisation in the semi-solid state in 7075 aluminium alloy

There is a strong drive to near net shape process 7000 series aluminium alloys, which are currently machined from the wrought state with much waste. Semi-solid processing is one potential near net shaping route. It relies on the thixotropic behaviour of alloys with non-dendritic, spheroidal microstructures in the semi-solid state. If such alloys are sheared they thin and flow to fill the die. One route to spheroidal semi-solid microstructure is by reheating worked material into the semi-solid state. During the reheating, recrystallisation occurs and as liquid forms it penetrates the recrystallised boundaries to form spheroids. Here we examine the formation of spheroids in as-supplied 7075 aluminium alloy. It is generally known that 7075 is very resistant to recrystallisation in the solid state due to the presence of dispersoid particles pinning grain boundaries. In this work, we have reheated 7075 in the extruded and T6 condition into the semi-solid state. As the temperature rises into the semi-solid regime there is a sudden increase in the appearance of spheroidal grains. The occurrence of recrystallised grains is closely associated with the location of the first liquid to form above the solidus. Fully spheroidal microstructures are obtained on reheating as-supplied material to temperatures in the region of 580 °C, although the fractions of liquid at that temperature are still low at around 5%. Conventionally there is extra cost involved in obtaining spheroidal microstructure feedstock for semi-solid processing because additional steps are introduced into the process route. The advantage here is that as-supplied material can be directly thixoformed once appropriate liquid fractions are attained.