半固态加工制浆技术的研究进展

概述了半固态金属加工制浆工艺的发展以及研究现状,介绍了具有代表性的几种半固态加工技术及其相应的制浆技术,总结了强制对流条件下非枝晶组织的形成机制.流程简洁、成本低廉的流变成形工艺路线成为当前的发展方向,形成具有高密度晶核的半固态浆料是获取高质量半固态加工部件的必要条件.     

Dynamical Solidification Behaviors and Metal Flow during Continuous Semisolid Extrusion Process of AZ31 Alloy

In this paper, a novel near-net-shape forming process, continuous semisolid extrusion process (CSEP) of AZ31alloy was proposed, and the dynamical solidification behaviors and metal flow during the process were firstly investigated.During casting AZ31 alloy by this process, non-uniform microstructure distributions and nonequilibrium solidification region near the roll surface were found in the roll-shoe gap.Microstructural evolution from dendrite to rosette and spherical grains was observed during the casting by CSEP.Casting temperature,roll-shoe gap width and cooling ability have great effect on casting process and metal flow, so these factors should be carefully controlled, a proper casting temperature of 710-750℃ is suggested.The white a phases were strongly stretched during the processing, and the remnant liquids are correspondingly distributes along the solid phase boundaries and also show stripped lines.     

双螺旋搅拌法制浆工艺参数对AZ91D合金半固态组织的影响

双螺旋搅拌工艺制备了AZ91D合金半固态浆料,获得金属半固态加工要求的细小、等轴分布均匀的非枝晶组织。研究了熔体温度、搅拌速度和剪切时间对该组织的影响规律。结果表明:采用合适的搅拌工艺,可获得金属半固态加工要求的细小、等轴分布均匀的非枝晶组织。即降低降体温度、搅拌速度及适当剪切时间有利于细小球形晶粒的形成。加大搅拌速度及合理搅拌时间可以细化晶粒,主要是由于是高剪切作用下熔体获得一个非常均匀温度场,从而增加有效形核的缘故。     

Globular Structure of M2 High Speed Steel by Thermomechanical Treatment in the Semisolid State

The globular structure of M2 high speed steel in the rolled ‐ annealed and as cast conditions was investigated in the semisolid state. Metallographic observations resulted in globular austenite particles that were surrounded by a liquid phase. Dissolution of various carbides in the austenite phase at semisolid temperatures led to grain boundary liquation and formation of near‐spherical solid grains in a liquid matrix. Therefore, at the semisolid state, the solid particles were free from carbides. MC‐ type and M₆C‐ type eutectic carbides re‐ precipitated at the grain boundaries during cooling of the samples from the semisolid temperature. The variation of shape factor versus holding time and holding temperature was examined. A transition value for shape factor changes in high speed steels was achieved. The growth rate constants of the Ostwald ripening and the coalescence mechanisms were calculated by using the experimentally determined rate constant. It was observed that less liquid droplets were enclosed inside the solid particles compared with non‐ferrous alloys. Besides, it has been shown that at high solid fraction, the Ostwald ripening mechanism plays a prominent role in the coarsening phenomenon in comparison with the coalescence mechanism. Grains can rotate and arrange low misorientation with each other at high liquid contents, therefore low energetic grain boundaries form between these grains. These grain boundaries play an important role in the coalescence mechanism.     

7A04合金半固态等温压缩的力学行为

通过半固态等温压缩实验,研究了7A04合金在半固态压缩过程中的力学行为。结果表明,半固态压缩时的峰值应力受变形温度和应变速率的影响较大,而稳态应力值受变形温度和应变速率的影响较小,且该合金具有应变速率敏感性。以此为基础,研究了半固态等温压缩过程中的微观组织变化及金属流动规律,揭示了半固态加工中可用较小的力实现较大变形的真正原因,为合理制定半固态加工工艺参数奠定了理论基础。     

半固态锻造A356铝合金轮毂的组织与性能

对低温浇注A356铝合金的组织和二次加热组织转变规律及半固态锻造铝合金轮毂的组织与力学性能进行了研究.结果表明:在635～ 655℃下浇注,可获得具有细小均匀、近球形晶粒的A356铝合金圆棒坯,圆棒坯在600℃下加热60 min,晶粒进一步球化;在750kN锻压力下可锻造成铝合金轮毂,经T6热处理后,轮毂的抗拉强度、屈服强度和伸长率分别为327.6MPa,228.3MPa和7.8％.表明,低温浇注法制备半固态坯料与半固态锻造工艺相结合,可生产出高性能的铝合金轮毂.     

Primary Carbide Formation in Tool Steels: Potential of Selected Laboratory Methods and Potential of Partial Premelting for the Generation of Thermodynamic Data

To predict the solidification and product properties of tool steels with complex chemical compositions, an understanding of the transformation behavior is crucial. Therefore, the quaternary Fe–C system with 10 wt% Cr and 3 wt% W (a subsystem of cold work steels, with M₇C₃ and M₂₃C₆ carbides) and the Fe–C system with 6 wt% W and 5 wt% Mo (simplified high-speed steel, with M₆C and MC carbides) are selected. The motivation for this study is to develop a methodology for the safe and fast production of model alloys and the close to equilibrium performance of differential scanning calorimetry (DSC) measurements. Regular diffusion annealing of as-cast carbidic steels is time-consuming, but with an additional heat treatment during the DSC measurement in the semisolid zone (30–50% liquid phase fraction), a status close to equilibrium can be achieved within minutes due to the high diffusion. To prove the potential of the equilibration by partial premelting in the DSC, additional equilibration and quenching experiments are performed in a Tammann furnace and investigated using a scanning electron microscope and X-ray diffraction analysis. By combining these methods, carbide types and the transformation temperatures can be verified to evaluate and construct complete phase diagrams.     

Efficient fabrication of semisolid nondendritic Al alloy slurry with high quality

Subjecting a normal mechanical vibration to a cooling slope plate,is a proposed method for preparing semisolid nondendritic slurry,named shear-vibration coupling sub-rapid solidification(SCS).Taking Al-8Si alloy as model material,the temperature field and distribution field of solid or liquid phase during SCS were simulated using COMSOL Multiphysics software to primarily choose the optimal processing parameters.Subsequently,the slurries were prepared with the parameters selected according to the simulation results and the microstructures of the slurries were experimentally investigated.Results indicate that the simulation results could provide a basis for roughly choosing the processing parameters,although the calculated solid fractions are always higher than the experimental ones.The processing parameters affect the primary grain size,shape factor and solid fraction mainly through altering the contact duration of melt on the plate,and thus affecting the cooling effect on the melt,nucleation rate,and grain dissociation and proliferation.Experiments with optimized processing parameters show that the primary grains in the slurry have an average size of about 32μm and shape factor of 1.38,and are quite uniform,even at the highest pouring rate of 2.81 kg·s^-1,the size and shape factor are about 46μm and 1.7,respectively,which implies that the proposed SCS is a promising technology for efficient fabrication of high-quality Al slurry available for engineering applications.