【作者】 张磊；

【导师】 董选普；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2012， 博士

【摘要】 传统铸造适合生产形状复杂的零件,但制件力学性能往往较低。传统锻造能生产高性能制件,但难以成形形状复杂的零件,且变形抗力较大。半固态成形介于液态铸造和固态锻造之间,兼具两者的优点,尤其半固态流变成形技术,由于具有短流程、低成本、高性能等优势而成为研究的热点。本文在铸造、锻造和半固态成形工艺的基础上,提出铝合金半固态流变铸锻成形技术,该技术能在同一副模具内实现铝合金的半固态流变铸造和锻造一体化成形,综合了铸造、锻造和半固态成形三种工艺的特点,具有短流程、快速高效、制件组织致密且力学性能高等优点。本文主要设计了旋转气泡搅拌及机械振动铝合金半固态浆料制备装置和铝合金半固态流变铸锻成形装置,研究了铝合金旋转气泡搅拌及机械振动制浆工艺,铝合金半固态流变铸锻成形工艺,并对半固态制浆和流变铸锻成形的机理进行了探讨。首先采用旋转气泡搅拌法、机械振动法的原理研制了铝合金半固态制浆装置,可控制包括搅拌温度、搅拌时间、搅拌转速、气体流速等多个参数。研制了半固态铸锻一体化模具,成形模具和制件均采用垂直分模,凸凹模均采用镶块式结构。设计了驱动成形模具的液压系统,并采用拉线位速传感器来实时显示压射的速度和位移。研究了旋转气泡搅拌制浆工艺,结果表明,要得到非枝晶半固态浆料,应将熔体由液相线以上温度冷却经过液相线并处于液相线以下5℃至20℃的范围内,并在激冷的同时施加搅拌。在本实验条件下,旋转气泡搅拌法的最佳工艺参数为：气流速率2 L/min,搅拌时间3 min,转速200 r/min,静置时间60 s。研究了坩埚中浆料组织的均匀性,发现非枝晶的平均晶粒尺寸在坩埚的上部较小下部较大,晶粒圆整度在坩埚的边缘较中心处圆整。研究了机械振动制浆工艺和复合制浆工艺,结果表明,要获得非枝晶半固态浆料,应使熔体温度由液相线以上激冷经过液相线并处于固液共存区间,在激冷的同时即施加振动。最佳振动时间为5 min。在所选的振动参数范围内,振幅和频率越大振动效果越好。振动速度为150 mms-1时能获得最为细小的非枝晶晶粒,振动速度为80 mms-1时能获得最为圆整的非枝晶晶粒。最佳的振动加速度为8000 mms-2。将垂直方向的振动简化为单自由度系统的简谐强迫振动,推导得出机械振动作用于熔体质点的功率公式,表明振动的效果与f3×A2(即振动速度f×A和振动加速度f2×A的乘积)成正比。直接将最优的旋转气泡搅拌工艺和机械振动工艺耦合得到复合制浆工艺,与单一方法制浆相比,复合制浆能得到最佳的平均晶粒尺寸和晶粒圆整度组合,其平均晶粒尺寸为64.5μm,晶粒圆整度为0.67。研究了液态和半固态金属型浇注试样的组织和力学性能,结果表明,半固态金属型试样的力学性能高于液态金属型(除复合制浆方法所得半固态试样的抗拉强度低于液态金属型外)。机械振动易于导致半固态浆料中出现显微空穴和孔洞,尤其复合气泡搅拌后,试样中易于产生气孔,从而恶化制件的力学性能。由于制浆过程中的搅拌和对流作用,半固态浆料中杂质颗粒难以聚集,其断口中无杂质颗粒存在,且能容许更高的杂质含量而不会恶化力学性能,而液态试样的断口中存在明显杂质颗粒,其延伸率较低。研究了流变铸锻工艺参数对半固态制件组织和力学性能的影响,结果表明,较高的压射压力和压射速度均有利于制件的充型,但压射速度过高易于产生卷气。最佳压射压力为40 MPa,最佳压射速度为92mm/s。压铸制件的最高抗拉强度为189.0MPa,延伸率为12.6%。半固态压铸后的坯料在不同的温度和固相率下进行锻造时,具有不同的力学性能。在低固相率(fs<0.5)锻造时相当于液态模锻,制件力学性能与半固态压铸相当；在接近fs=0.5的固相率锻造时,制件的力学性能恶化,其抗拉强度为170.3 MPa,延伸率为6.9%；在高固相率(fs>0.5)锻造时,制件力学性能与半固态压铸相当；半固态压铸后的坯料随后在固态下进行热锻,能得到最佳的力学性能,其抗拉强度为194.7 MPa,延伸率为14.3%。半固态坯料锻造时存在一个最佳锻造比,坯料在4 mm设定压下量时取得最佳力学性能,其抗拉强度为197.0 MPa,延伸率为11.3%,此时的锻造比为10%。半固态压铸坯料施加锻造后能弥合铸造孔洞类缺陷,得到组织更加致密的制件。在制件的中部为固相颗粒密实的致密组织,在制件的边缘为晶粒被拉长的流线形组织。探讨了半固态锻造及液相偏析的机理。高液相率锻造时,半固态浆料可被视为固相悬浮于液相的单相流体,其流变模型可假设为牛顿流体。公式推导得出半固态浆料的表观粘度与高径比、压强和应变之间的关系,表明锻造或压缩过程中表观粘度与高径比的平方和压强成正比,与坯料的应变成反比。.高固相率锻造时,半固态坯料应被视为两相体,其变形模型可假设为多孔材料。其受力可分为固相部分和液相部分的和,在总应力一定的情况下,固相率越高固相颗粒所受应力越大,固相越易于发生塑性变形,起到强化作用。半固态铸锻件的液相偏析层主要是在流变压铸过程中形成的,由于充型阶段浆料中存在压力梯度而产生液相偏析。通过公式推导得出液相偏析与压力梯度、液相率、粘度、渗流通道数量和曲折因子之间的关系。实验结果显示,枝晶组织制件的表面无液相偏析,Al和Si元素的含量无变化。而球晶组织制件的表面有一层明显的液相偏析,由基体至表面其Al元素出现下降的台阶,而Si元素有小幅的上升,表明液相偏析的存在。更多还原

【Abstract】 Conventional casting is suitable for the production of complex parts. But has the disadvantage of low mechanical properties. Conventional forging can produce high performance parts, while complex one is difficult to be fabricated, and the deformation resistance is larger. Semi-solid forming lies between liquid casting and solid forging, and has advantages of both methods. Semi-solid forming especially rheoforming technology has become a research hotspot for advantages of short process, low-cost, and high-performance, etc. A process of integration of rheoforming and forging was proposed, based on casting, forging, and semi-solid forming processes. This technology which combines features of casting, forging, and semi-solid forming can realize rheoforming and forging of aluminum alloy in one mold. The technology has advantages of short processes, fast and efficient, dense parts, and high mechanical properties. In this paper, a slurry making equipment for applying rotating gas bubble stirring and mechanical vibration was developed. A device for integration of rheoforming and forging was designed. Slurry making process of aluminum alloy using rotating gas bubble stirring and mechanical vibration, and integration of rheoforming and forging process for aluminum alloy were explored. Mechanisms of slurry making, and rheoforming and forging were investigated.The semi-solid slurry making equipment for aluminum alloy was designed applying principles of rotating gas bubble stirring and mechanical vibration. Controllable parameters are stirring temperature, stirring time, rotation speed, and gas flow rate. The mould for integration of rheoforming and forging was designed. Forming molds and specimen are both separated vertically. The convex and concave dies are both insert type structure. Hydraulic system to drive the forming device was designed. A displacement and speed sensor was applied to display the injection velocity and displacement.The slurry making process using rotating gas bubble stirring was investigated. The results show that in order to obtain non-dendritic semi-solid slurry, the melt should be cooled from above liquidus to semi-solid zone, and then has a temperature between 5℃to 20℃below liquidus temperature, and gas bubble stirring should be imposed simultaneously. The optimum parameters of the rotating gas bubble stirring under the experimental conditions are as flow:gas flow rate 2 L/min, stirring time 3 min, rotation speed 200 r/min, and holding time 60 s. The microstructure uniformity in the crucible was examined. The results show that average grain size in the top of the crucible is smaller than in the bottom, and the non-dendritic grains in the edge of the crucible are rounder than in the middle.The slurry making processes using mechanical vibration and combination of gas bubbling and mechanical vibration were studied. The results show that in order to obtain semi-solid slurry with non-dendritic grains, the melt should be cooled from above liquidus to semi-solid zone, and mechanical vibration should be exerted simultaneously. The optimal vibration time is 5 min. In the chosen vibration parameter range, the bigger the amplitude and frequency, the better the mechanical vibration effect. As for the vibration velocity, the most refined non-dendritic grains can be obtained at the vibration velocity of 150 mms-1, and the most spherical non-dendritic grains can be got at the vibration velocity of 80 mms1. The optimal vibration acceleration is 8000 mms-2. The vibration in vertical direction can be simplified as harmonic forced vibration of one freedom system. The formula of mechanical vibration on melt particle was deduced. The results show that the vibration effect is proportional tof3×A2, that is, the product of the vibration velocity f×A and vibration acceleration f2×A. Integrated slurry making process is a combination of the optimal rotating gas bubble stirring process and mechanical vibration process. Compared with the single process, more refined and spherical non-dendritic grains are obtained, with the average grain size of 64.5μm, and grain roundness of 0.67.Microstructure and mechanical properties of liquid and semi-solid metal casting in permanent mold were investigated. The results show that mechanical properties of semi-solid specimens are higher than the liquid casting in permanent mold, except for the tensile strength of the semi-solid specimen corresponding to the integrated slurry making process. Microscopic cavities and holes are easy to be induced in the semi-solid slurry by the mechanical vibration. Stomata are caused in the specimen, especially when the rotating gas bubble stirring is exerted, which will deteriorate mechanical properties of the specimen. There are no impure particles assemble together or distribute in the tensile fractures in the semi-solid specimen, because of the stirring and convection effect in the slurry making process. And high levels of impurities can be tolerated in the semi-solid specimen without deterioration of the mechanical properties. Impurity particles exist in the tensile fractures of the liquid specimen, and the elongation is lower.Effect of rheological casting and forging parameters on microstructure and mechanical properties of the semi-solid specimen were studied. The results show that higher injection pressure and shot velocity are both beneficial to the filling ability of the specimen, but stomata are produced when the shot velocity is too fast. The optimal injection pressure is 40 MPa, and the optimal shot velocity is 92 mm/s. The optimal mechanical properties of semi-solid die-casting are tensile strength of 189 MPa, and elongation of 12.6%. Semi-solid billet has different mechanical properties when forged at different temperature and solid fraction. It is equivalent to liquid forging when the semi-solid billet is forged with the solid fraction lower than 0.5, with its mechanical properties about the same with semi-solid die-casting. When the semi-solid billet is forged at the solid fraction of 0.5, mechanical properties are deteriorated, with the tensile strength of 170.3 MPa, and elongation of 6.9%. When forged at the solid fraction higher than 0.5, the mechanical properties are also close to semi-solid die-casting. The optimal mechanical properties can be obtained, when the semi-solid billet is hot forged in the solid state, with the tensile strength of 194.7 MPa, and elongation of 14.3%. An optimal forging ratio exists in the semi-solid forging process. The best mechanical properties can be obtained at the forging reduction of 4 mm, with the tensile strength of 197.0 MPa, and elongation of 11.3%, corresponding to the forging ratio of 10%. Casting hole class defects in the semi-solid billet can be eliminated after forging, and specimen with denser microstructure is obtained. Solid grains are dense in the middle of the billet, and grains are elongated at the edge of the billet.The mechanism of semi-solid forging and liquid segregation were investigated. When forged at high liquid fraction, semi-solid slurry can be regarded as single-phase fluid with solid grains suspending in liquid, whose rheological model can be assumed to be Newtonian fluid. Through the formula derivation, the relationship between apparent viscosity of the semi-solid slurry and ratio of height to diameter of the billet, and pressure, and strain was obtained. The result shows that the apparent viscosity of the billet in the forging or compression processes is proportional to the square of the height to diameter ratio, and the pressure, and inversely proportional to the strain of the billet. When forged at high solid fraction, semi-solid billet should be considered as two-phase body, whose deformation model can be assumed to be porous materials. The total stress imposed on the billet is the sum of the stress imposed on the solid part and liquid part. In the case of a fixed total stress, the higher the solid fraction is, the bigger the stress on the solid grains is, and the solid grains are easy to be deformed. The liquid segregation in the billet is mainly formed in the rheological die-casting process, in which a pressure gradient exists in the filling stage of the semi-solid slurry. The relationship between liquid segregation and pressure gradient, liquid fraction, viscosity, number of seepage paths, and tortuosity factor of the seepage paths are obtained, through the formula derivation. The liquid segregation is proportional to the pressure gradient. The results show that, there is no liquid segregation in billet with dendritic microstructure, and Al and Si contents have no difference between the edge and central of the billet. Obvious liquid segregation exists in billet with spherical microstructure, and Al element decreases from the central to the edge of the billet, while Si element increases slightly, proving the existence of the liquid segregation.更多还原