大塑性变形的AM60镁合金半固态等温处理研究

为了制备晶粒细小且球化程度高的的AM60镁合金半固态坯料,对铸态和等径道角挤压态的AM60镁合金半固态等温处理过程进行了研究.借助金相显微镜对AM60镁合金铸坯和等径道角挤压后的铸坯在半固态等温处理中的微观组织演变进行了观察.研究结果表明:对于AM60镁合金,直接等温处理获得的半固坯晶粒很粗大,其平均晶粒尺寸都在100μm以上,晶粒球化效果不理想,很难获得合格的半固态坯;新SIMA法是一种非常理想制备AM60镁合金半固态坯的方法,利用该方法制备的AM60半固态坯的微观组织晶粒十分细小,平均晶粒尺寸在8～22μm,晶粒球化程度高;随着保温时间的延长,新SIMA法制备的AM60半固态坯的微观组织出现长大现象;随着等温处理温度的升高,固相晶粒的平均尺寸先增加后减小,晶粒球化程度越来越高.     

Microstructure evolution of SIMA processed Al2024

In this paper, the strain induced melt activated (SIMA) process of A2024 was investigated systematically in order to provide a basis for semi-solid forming of the alloy. To delete the hydrostatic component of stress, the uniform uniaxial compression process is performed. The relation between the grain size and strain before heating is presented and a microstructure evolution model for SIMA is proposed on the basis of experiment. There include two steps in SIMA process. In the first step of plastic deformation, there are two mechanisms to control the deformation, slip or cross-slip or twinning in grain and grain rotation. The deviatory stress will benefit to slip or cross-slip or twinning in grains, and the hydrostatic stress will benefit to rotation of gain. In the second step of heating, the grain boundary will melt first and the shape of the grains will be globe.     

Evolution of microstructure and mechanical properties for 9Cr18 stainless steel during thixoforming

Hot compression tests were carried out in the semi-solid state of 9Cr18 stainless steel on Gleeble-1500 thermal simulation testing machine to investigate the effects of thixoforming parameters on its microstructure and mechanical properties. In this paper, microstructure was observed by scanning electron microscopy (SEM) and analyzed using energy dispersive spectrometer (EDS), and true stress–stain curves of the specimens with different initial microstructures after thixoforming were obtained to study the deformation mechanism. The results showed that thixoforming parameters such as reheating temperature and the strain rate had a significant influence on microstructure and mechanical properties evolution of 9Cr18 semi-solid billet. With increasing reheating temperature or decreasing strain rate, average size of carbides decreased from 2 μm to 0.5 μm, and the phenomenon of liquid extrusion during thixoforming became more obvious. During thixoforming, carbon atoms diffused to molten metal from austenite in the centre of specimens. When thixoforming temperature reached 1300 °C, martensitic transformation occurred after rapid cooling. Flow stress of semi-solid billet was lower than traditional ingot casting and hot rolled state steel, when reheated to the semi-solid range, due to their different original microstructure.     

基于SIMA法的铝合金复杂构件触变锻造成形技术

采用半固态触变锻造技术成形铝合金构件,容易实现轻量化、低成本、短流程制造,因此在汽车、航空航天等领域应用广泛,但是对于高强铝合金复杂形状构件触变锻造存在半固态坯料制坯工序复杂、制件固液偏析严重和力学性能较弱等问题。基于SIMA制坯方法,提出了分级热处理、快速感应重熔和梯度等温处理等重熔工艺以及触变-塑变复合成形等新成形技术,优化了变形铝合金二次重熔半固态组织调控和触变锻造技术,获得了良好的半固态球晶组织并成形出合格的制件,最后提出了铝合金触变锻造成形中仍需解决的问题和发展方向。     

Microscopic observation of cold-deformed Al–4Cu–Mg alloy samples after semi-solid heat treatments

Wrought aluminum alloys can be effectively fabricated by a strain-induced, melt-activated (SIMA) process. The SIMA method involves plastic deformation of an alloy to some critical reduction point and a semi-solid heat treatment in the solid–liquid temperature range. The semi-solid heat treatment is a key process to control the semisolid microstructures. In this paper, the microscopic morphology of a cold-deformed SIMA treated Al–4Cu–Mg alloy has been investigated, and the effects of microstructural evolution, precipitation behavior and dislocation morphology on the mechanical properties are discussed. The experimental results show that the number of CuAl₂ (θ phase) precipitates and the dislocation density of Al–4Cu–Mg alloy decreased gradually by the semi-solid heat treatment. Moreover, unique dislocation morphologies including helical dislocations and dislocation loops appeared and evolved to reduce the stored energy. With an increase of the holding time in the semi-solid heat treatment, the ultimate strength and yield strength decreased. The reduction of these mechanical properties of the SIMA treated Al–4Cu–Mg alloy is mainly due to the decrease of refinement strengthening, solution strengthening, and dislocation strengthening in the semi-solid heat treatment.     

Microstructural evolution and mechanical properties of back-extruded Al 7075 alloy in the semi-solid state

The thixoforming process is a new method for manufacturing complicated and net shape components through which high strength materials can be formed more easily. In this study 7075 Al alloy which has low extrudability has been thixoformed by backward extrusion process. The recrystallisation and partial melting (RAP) route was used to obtain the semi-solid feedstocks for thixoforming. Microstructural evolution during partial remelting was studied at temperatures for times. Results showed that a fine and globular microstructure can be obtained by the RAP route. The results showed that high semisolid isothermal temperature would increase the liquid volume fraction and accelerate the spherical processing of the solid particles. Furthermore at long holding time, the globular grains coarsened slightly and the average grains size are increased. The experimental results showed that when the semisolid billet is hold at 580°C with the holding time, less than 30 min, the microstructure of the billet is composed of spherical grains and remnant liquids, the average grain size are smaller than 100 μm. So the remelted billet is suitable for thixoforming. In this paper, a back-extruding of 7075 Al alloy with a high solid fraction in the semi-solid state at 580°C for 10 min was performed. Mechanical properties of thixoformed components at room temperature were examined. Tempering treatment T6 has been applied after thixoforming to investigate the effects of heat treatment on mechanical properties of thixoformed parts. The tensile properties and low hardness values in the as-thixoformed 7075 Al alloy were improved by subsequent heat treatment. Post-forming heat treatment is one of the key parameters for improving the mechanical properties of thixoformed parts.     

Effects of Al–5Ti–1B on the structure and hardness of a super high strength aluminum alloy produced by strain-induced melt activation process

In this study the effect of Al–5Ti–1B grain refiner on the structural characteristics and hardness of Al–12Zn–3Mg–2.5Cu aluminum alloy has been investigated. The alloy was produced by modified strain-induced melt activation (SIMA) process. Reheating condition to obtain a fine globular microstructure was optimized. The specimens subjected to deformation ratio of 40% (at 300 °C) and various heat treatment times (5–40 min) and temperature (550–620 °C) regimes were characterized in this study. Microstructural study was carried out on the alloy by the use of optical and scanning electron microscopy (SEM) in both unrefined and Ti-refined conditions. The results showed that for the desired microstructures of the alloy during SIMA process, the optimum temperature and time are 575 °C and 20 min respectively. The hardness test results of the alloy also revealed that T6 heat treatment is more effective in hardness enhancement of all specimens in comparison with SIMA processing.     

电磁搅拌对半固态AZ91D镁合金组织的影响

研究了电磁搅拌参数对连续冷却条件下AZ91D镁合金组织的影响.结果表明:当电磁搅拌的频率达到或高于50 Hz时,半固态AZ91D镁合金浆料或坯料组织中的球状初生固相越来越多,越来越圆整;在电磁搅拌频率为200 Hz和冷却速率较低的条件下,搅拌的功率越大,半固态AZ91D镁合金组织中的球状初生固相越多,晶粒也越细小.在电磁搅拌条件下,AZ91D镁合金熔体的激烈流动导致较为均匀的温度场和溶质场、更加剧烈的温度起伏,促进了半固态AZ91D镁合金球状组织的形成.半固态重熔加热使半固态AZ91D镁合金坯料初生固相的形态发生进一步的球化.     

变形的AZ91D镁合金半固态等温压缩的力学行为

为了了解等径道角挤压(ECAE)的AZ91D镁合金在半固态压缩变形中的力学特征,利用半固态温压缩实验、Gleeble1500实验机和金相显微镜对其在半固态压缩变形中的力学行为进行了研究.结果表明:ECAE的AZ91D镁合金在半固态等温压缩中变形由固相晶粒本身的塑性变形、液相包围着固相晶粒的滑动和转动构成;该材料的真应力-真应变曲线由应力激增阶段、应力下降阶段、稳态阶段和应力增加阶段组成;随保温时间的增加或变形温度的升高,获得相同应变量的真应力明显下降,稳态应力和峰值应力也明显下降;随应变速率的增加,稳态应力增加.     

新SIMA法制备铝合金3A21半固态坯料

利用有限元模拟软件模拟不同路径下等径角挤压过程,分析材料等效应变分布情况,通过等径角挤压实验、半固态等温处理实验、金相显微镜、金相检验软件系统等实验方法和分析设备,研究铝合金3A21半固态坯料显微组织晶粒尺寸同等效应变的关系,分析不同工艺参数对晶粒等积圆直径和形状系数的影响。结果表明,随着材料经过ECAE后等效应变的增大,半固态坯料晶粒尺寸减小;随着保温时间的延长,晶粒尺寸增大、圆整度增加。     

Microstructural Evolution of 6061 Alloy during Isothermal Heat Treatment

The semi-solid billet of 6061 aluminum alloy was prepared by the near-liquidus semi-continuous casting(LSC) with rosette or near-spheroide grains.The pre-deformation processing was applied before partial remelting to further improve the microstructure and properties of the semi-solid alloy.The effects of different processing parameters,such as holding temperature and holding time,on the semisolid microstructures during partial remelting have been investigated.It was found that the optimal partial remelting parameters should be 630℃ and 10-15 min for 6061 alloy cold rolled with 60% reduction in height of pre-deformation.The coarsening rates were anasysed by Lifshitz-Slyozov-Wagner(LSW) theory.The pre-deformed 6061 alloy exhibits lower coarsening rate constants than that of the as-cast one,and also lower than other alloys processed by different method found in previous literature.It is because the coarsening rate is associated with the initial microstructure and composition of the alloy.The secondary phases in the alloy inhibit the migration of the liquid film grain boundaries.The microstructure obtained by using the combination of near-liquidus semicontinuous casting and pre-deformation treatment is better than that without pre-deformation processing,which demonstrates that the used method is promising for fabricating high quality semi-solid alloys.     

Study on Multiple Electromagnetic Continuous Casting of Aluminum Alloy

To obtain semi-solid Al alloy billet with high quality, an investigation was carried out by imposing a multiple magnetic field from the outside of a copper mold in the continuous casting. AISi6Mg2 alloy designed for semi-solid metal （SSM） processing was continuously cast through a submerged entry nozzle under various conditions. Effects of multiple magnetic field on meniscus motion, temperature distribution and billet quality were examined. The experimental results showed that meniscus disturbance caused by electromagnetic stirring could be controlled effectively and the surface quality of semi-solid AI alloy billet was improved greatly, and an uniformly fine, globular microstructure across the transverse section of the billet was achieved by optimizing the distribution of multiple magnetic field.     

Solid–liquid structural break-up in M2 tool steel for semi-solid metal processing

The success of semi-solid metal processing mostly depends on the formation of suitable starting microstructure, which must consist of solid metal spheroids in a liquid matrix. Various methods of obtaining this structure have been established; they include recrystallisation and partial melting (RAP), strain-induced melt-activated (SIMA), or simple mechanical stirring, to name a few. These methods, as widely discussed, have mostly been applied with light alloys, mainly aluminium based. This article discusses solid–liquid structural break-up in M2 tool steel subjected to a direct re-melting procedure from the as-annealed condition. The role of carbide dissolution in the grain boundary liquation of the steel is described. This leads to the production of near spheroidal solid grains in a liquid matrix, a microstructure suitable for the thixoforming process. Microstructural examination revealed that carbide particles contained in bands at 1220 °C slowly disappeared with temperature. At 1300 °C, the solid grains seemed to be free from carbides. Most of the carbides had now re-precipitated at the grain boundaries. Thixoforming carried out at 1340 and 1360 °C revealed the thixotropic properties of the semi-solid metal slurries. The results indicate a widening of the range of potential routes to thixoformable microstructures.     

Influence of fiber content on mechanical,morphological and thermal properties of kenaf fibers reinforced poly(vinyl chloride)/thermoplastic polyurethane poly-blend composites

Kenaf (Hibiscus Cannabinus) bast fiber reinforced poly(vinyl chloride) (PVC)/thermoplastic polyurethane (TPU) poly-blend was prepared by melt mixing method using Haake Polydrive R600 internal mixer. The composites were prepared with different fiber content: 20%, 30% and 40% (by weight), with the processing parameters: 140 °C, 11 min, and 40 rpm for temperature, time and speed, respectively. After mixing, the composite was compressed using compressing molding machine. Mechanical properties (i.e. tensile properties, flexural properties, impact strength) were studied. Morphological properties of tensile fracture surface were studied using Scanning electron microscope (SEM). Thermal properties of the composites were studied using Thermogravimetric Analyses (TGA). PVC/TPU/KF composites have shown lower tensile strength and strain with increase in fiber content. Tensile modulus showed an increasing trend with increase in fiber content. Impact strength decreased with increase in fiber content; however, high impact strength was observed even with 40% fiber content (20.2 kJ/m²). Mean while; the 20% and 30% fiber contents showed higher impact strength of 34.9, 27.9 kJ/m²; respectively. SEM showed that there is poor fiber/matrix adhesion. Thermal degradation took place in three steps. In the first step, composites as well as the matrix had a similar stability. At the second step, matrix showed a slightly better stability than the composites. At the last step, composites showed a better stability than the matrix.     

Evaluation of the thixoformability of the A332 Alloy (Al–9.5?wt%Si–2.5?wt%Cu)

The main purpose of this article is to develop and/or use a commercial conventional low-cost raw material as thixoforming material in order to diminish the costs of the thixoforming process. Semi-solid technology usually uses aluminium low-silicon alloys such A356 (Al–7.0 wt%Si) as raw materials. High silicon content alloys with a quasi-eutectic composition diminish the semi-solid range, making it difficult to control the thixoforming temperature, although present excellent mechanical properties. This article reports on the semi-solid behaviour of Al–9.5 wt%Si–2.5 wt%Cu (A332). Thermo-Calc simulations and experimental DSC techniques were used to map the temperature transition from solid to liquid in order to achieve the best semi-solid behaviour and hence the best thixoforming temperature. Samples were reheated at three temperatures to 30, 45 and 60% of the solid fraction applying holding times of 0, 30, 90 and 210 s. The morphological evolution and semi-solid behaviour of the samples at these temperatures were determined via the fixed platen compression test. The structure showed the best semi-solid behaviour at 572 °C, with an apparent viscosity of up to 1.5 × 10⁵ Pa s. The results indicated that the semi-solid behaviour of the commertial Al–9.5 wt%Si–2.5 wt%Cu alloy is similar to that of the alloy A356. Despite its large dendritic structure it is possible, with the correct combination of temperature and time, to use this alloy as raw material for the thixoforming process. Furthermore, semi-solid parts can be produced by thixoforging using this low-cost material without any special preparation.     

Investigation of the effect of Al-8B master alloy and strain-induced melt activation process on dry sliding wear behavior of an Al–Zn–Mg–Cu alloy

This study was undertaken to investigate the influence of Al–8B master alloy and modified strain-induced melt activation process on the structural characteristics and dry sliding wear behavior of Al–12Zn–3Mg–2.5Cu aluminum alloy. The optimum amount of B containing master alloy for proper grain refining was selected as 3.75 wt.%. The alloy was produced by modified strain-induced melt activation (SIMA) process. Reheating condition to obtain a fine globular microstructure was optimized. The optimum temperature and time in strain-induced melt activation process are 590 °C and 10 min, respectively. T6 heat treatment was applied for all specimens before wear testing. Significant improvements in wear properties were obtained with the addition of grain refiner combined with T6 heat treatment. Dry sliding wear performance of the alloy was examined in normal atmospheric conditions. The experimental results showed that the T6 heat treatment considerably improved the resistance of Al–12Zn–3 Mg–2.5Cu aluminum alloy to the dry sliding wear. The results showed that dry sliding wear performance of globular microstructure specimens was a lower value than that of B-refined specimens without strain-induced melt activation process.     

Compressive deformation and forging behavior of Ti14 alloy in semi-solid state

Deformation behavior and formability of Ti14, a new typical α + Ti₂Cu alloy in the semi-solid state, were investigated by compressive tests in temperature range between 1223 and 1473 K and by forging tests between 1223 and 1373 K. Tensile tests of the forged alloys were also performed to study the effect of forging on mechanical properties. The results show that the maximum compressive stress was greatly dependent on the fraction solid, and that a transition in stress occurred at a solid fraction between 0.94 and 0.98 (corresponding to the temperature between 1323 and 1373 K). This transition was related to the decrease in amount of solid bridges between grains. Excellent formability and grain refinement due to dynamic recrystallization were achieved during the semi-solid forging. Furthermore, tensile tests showed a high ultimate tensile strength and yield strength after semi-solid forging at more than 1273 K, which was attributed to the grain refinement caused by the semi-solid forging. Ductility was also improved by changing the forging ratio. These results indicate that both good formability and tensile properties can be simultaneously attained by the compressive processing in the semi-solid state.     

Effect of Pouring Process on the Microstructures of Semi-Solid AlSi7Mg Alloy

The effect of different pouring temperatures and different pouring heights, the distance between the mouth of the pouring ladle and the top of the mold, on the microstructure of AlSi7Mg alloy have been researched in the paper. When the pouring temperature is close to the liquidus temperature, the primary α-Al in 'the billets of AlSi7Mg alloy solidified into spherical and nodular fine grains distributed homogeneously. The optimum pouring temperature for semi-solid AlSi7Mg billet with spherical or nodular primary α-Al is 615℃. At the same pouring temperature, the higher the pouring ladle, the more easily the spherical and nodular primary alpha -Al obtained in the semi-solid AlSi7Mg billet. When the pouring temperature is close to the liquidus temperature and the pouring ladle is relatively high, it is the great cooling rate, the flow of the molten allay caused by pouring and the large simultaneous solidification region induced by the near liquidus temperature, that promote the formation of spherical or nodular primary α-Al.     

Plastic anisotropy and strain-hardening behavior of Mg–6%Li–1%Zn alloy thin sheet at elevated temperatures

An Mg–Li–Zn (designated as LAZ61) alloy containing about 6 wt% of Li has been prepared by melting and solidification in a carbon steel crucible, and extruded at a billet preheating temperature of 200 °C. The extruded plate was then cold-rolled to a final thickness of 0.6 mm with a total reduction of approximately 82%. Tensile tests were carried out in the rolling and transverse directions and at various temperatures to explore the effects of anisotropy and temperature on mechanical properties and strain-hardening behavior. Kocks–Mecking type plots were used to illustrate different stages of strain hardening. Anisotropic behavior of LZ61 sheet were observed in the mechanical properties at all test temperatures due to the development of texture in α phase during cold-rolling and a low content of BCC β phase. The cold-rolled LZ61 alloy sheet showed stage II and stage III strain-hardening behavior at test temperatures of room temperature and 100 °C. The specimens tested at 200 °C did not show stage II strain hardening. Higher initial strain-hardening rates were observed in the transverse direction as a result of the cold-rolled fibrous structure providing more strong barriers to the dislocation movement.     

Numerical analysis on near net shape forming of Al–Al3Ni functionally graded material

The results of numerical studies on the near-net shape forming of Al–Al₃Ni functionally graded material (FGM) are presented and are compared with the experiments. FGM billets at an elevated temperature of semi-solid condition are set in a container and are subject to backward extruding, and FGM cups are obtained. Due to the composition gradient of FGM the effective viscosity of semi-melt FGM billet varies spatially. The flow/deformation of semi-melt FGM billet is strongly influenced by the spatial variation of effective viscosity. Some characteristic behaviors of flow/deformation of FGM during the semi-solid process are presented and discussed.