【作者】 吴和保；

【导师】 黄乃瑜； 樊自田；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2005， 博士

【摘要】 镁合金密度小,比强度、比刚度高,减振性、导热性、电磁屏蔽和抗辐射能力强,易切屑加工,易回收,资源丰富等一系列优点,是继钢铁和铝合金之后发展起来的第三类金属结构材料,被喻为21 世纪的绿色工程材料,在汽车、电子电器、航空航天等领域具有重要的价值和广阔的应用前景。本研究针对复杂薄壁镁合金铸件的特点,将真空压差铸造和消失模铸造技术集成,力求解决镁合金成形过程中的氧化燃烧和充型能力不足的问题,开发了一种设备简单、生产效率高、铸件质量好的精确成形的新工艺,具有较高的学术价值和广阔的应用前景。本研究揭示了可控气压下镁合金消失模铸造的工艺原理及其设备的设计原则,研制了适用于镁合金的可控气压下消失模铸造的首台实验样机,开创了一种非常适合于镁合金液态成形的可控气压下消失模铸造新方法,与压铸相比,它具有投资少、生产成本低、适应范围广、铸件可热处理等优点; 与砂型铸造相比,具有尺寸精度高、表面质量好、充型能力强、补缩效果好等优点,使金属液的充型速度由重力消失模铸造的50 mm/s 提高到80～150mm/s,能够满足壁厚3mm 甚至更薄的复杂镁合金铸件的生产。通过对可控气压下镁合金消失模铸造工艺的系统研究,基本掌握了充气流量、真空度、涂层厚度、浇注温度、泡沫密度、浇注系统最小阻流面积等工艺参数对镁合金充型能力的影响规律,得出了影响镁合金流动性的非线性数学模型: L₁=-5.107+2.736Q^1.6+55.645T^0.01-1.4322δ^3.64-1.9685ρ^1.69 为进一步制定工艺规范奠定技术基础,运用优化的工艺参数,成功地浇注出了壁厚3mm 的汽车排气管等薄壁复杂镁合金零件。本文采用电极接触测试法和摄影法对可控气压下消失模铸造镁合金的流动性、充填形态、充型速度变化规律进行了系统的研究,基本掌握了可控气压下消失模铸造镁合金的流动性、流动前沿形貌、充型速度及其变化规律,建立了低真空度条件下液态镁合金拱形推进的物理模型。更多还原

【Abstract】 Magnesium alloy could be used in automobile, vehicle, electric appliances, communication, space and aviation and military industries because of its lower density, high strength-to-weight ratio, high modulus, good heat conductivity, well electromagnetic shielding, superior damping characteristic, good machinability and rich in natural resources. It was entitled as new metal materials friendly to environment in the 21st century following iron and aluminum alloy, but high oxidative combustion and absence of formation technology holdback its widely application. In this paper, a new precision casting process being known as lost foam casting (LFC) process under controllable pressure was presented, which takes advantages of LFC process, low-pressure casing process, and vacuum suction casing process, so as to solve the oxidative combustion and lower flowability of magnesium alloy. It is very important of the investigation on LFC process under controllable pressure for academic sense and applications of magnesium alloy. First of all, the processing principle and facility design guideline of LFC process under controllable pressure had been investigated, and first facility for experiments which includes system of SF6 gaseous mixture was developed. Contrasted with die-cast process, this new technology for the formation of magnesium castings possess multiplicate advantages such as low investment and productive cost, good adaptability, and castings were able to be strengthened by heat treatment. The casting of Mg-alloy produced by the new process would possess high precision, smooth surface, good mold-filling and feeding ability. The rate of filling mold can reach 80～150mm/s which is much higher than the rate of 50 mm/s in gravitational LFC process, so that the production of complicated thin-walled Mg-alloy castings with 3mm or more thinner section thickness would be made possible. The effects of gas flux, vacuum degree, coating thickness, pouring temperature, pattern density, and critical gate area on the mold-filling ability of magnesium alloy were investigated. According to the data obtained by orthogonal experiments, a nonlinear regression equation about the influences of processing parameters on flowability of magnesium alloy was created shown below. L1=-5.107+2.736Q1.6+55.645T0.01-1.4322δ3.64-1.9685ρ1.69 It was indicated that the filling length of Mg-alloy increased with the increase of gas flux and pouring temperature, and decreased with increase of coating thickness and pattern density. The flowability, front pattern and filling velocity of Mg-alloy in LFC process under controllable pressure were studied by electrode contact test method and photogrammetric procedure. The physic model with convex filling front was posed. The solidification characteristics of magnesium alloy was studied using electrical probes connected to data acquisition controlled by computer. The maximal temperature and front actual temperature of Mg-alloy melts along the filling course were detected. Based on the cooling curse of Mg-alloy melts, the mode and range of solidification, solidification time, and heat transfer in the dry sand mould were investigated. A thermal equilibration equation about filling length of Mg-alloy has been deduced, as shown below. dtdvAHHTTCTTfHgL QkEEEmLm= δ(1 .62?E )?ρL(0?+ 2K )+(s?Mg)? The results shown that the temperature range of solidification is 160~180℃, and the range of solidification time is 66~338s. The melting processing and heat treatment processing were opted. The microstructure, mechanical properties and fraction characteristics were measured. The results shown that the tensile strength reached 180.8 MPa, yield strength reached 113.2 MPa, elongation was 4.4% for as-cast Mg-alloy, and after heat-treating, the tensile strength can rise to 258.70MPa and elongation to 10.3 percent in T4 heat-treating process, yieldstrength rose to 161.21 MPa in T6 heat-treating process,which are much higher compared with other magnesium alloy produced in other casting process. Lastly in the paper, pouring practices of three kind of Mg-alloy castings with medium complicated shape were tried. The pouring application shown that low-pressure EPC process was much applicable for the complicated thin-walled and common casting defects and its formation mechanics were analyzed.更多还原