基于微喷射粘结的铸型直接成形关键技术基础研究

Compared with laser sintering forming technology, using bonding particles layer upon layer to manufacture casting mold has some excellent advantages, such as small equipment investment and low operation cost, and it is more suitable for rapidly casting metal parts. At present, the main problems of the technology is that the precision and strength made by it are low. The strength of casting mold needs to be improved by post-processing if the mold is used for pouring, which will reduce the properties of casting, and extend the manufacturing cycle. This project studys the basic problems of key technology of direct forming high performance casting mold based on micro-jetting and bonding, and establishs the theoretical foundation for rapidly manufaturing low cost casting of metal parts. This paper studys the integrated mechanism in interface between spray droplet and powder,and the curing behavior and permeate dynamic characteristics of binder droplet in the layered powder,and also the morphology characteristics and formation mechanism of bond bridge between binder and powder, as well as studys the change of bulk density of powder and the force interaction between powder feeder and formed layered powder under different feeding powder conditions. In order to solve the problems of low precision and strength of casting mold, we will propose to eliminate the formed layered powder shifting and cracking in the spreading powder process by using tilting forming table, at the meantime, through micro droplets and mulitiple injections to improve the jetting precision and control the curing time of binder to eliminate the overflowing of binder droplets on the edge of the outline, and we optimize the particle size distribution of powder, add indirectly submicron powder to the forming powders to increase the density of layered powder, and constructing coating on the surface of casting mold are taken to enhance the strength of casting. The results of this project have a large value for theory and application.

采用粘结剂分层粘结粉末成形铸型，与激光分层烧结成形比较，具有设备投资小、运行成本低等突出优点，更适合金属零件的快速铸造。目前，该工艺成形铸型的精度和强度低，需经后处理才能使用，这既降低铸件性能，又延长制造周期。本项目将研究阻碍微喷射粘结直接成形高性能铸型的若干关键技术基础问题，为实现金属零件快速低成本铸造奠定理论基础。研究喷射微滴与粉末界面结合机理、粘结剂流体在粉末层中的固化行为和铺展渗透动力学特征、粘结桥的形貌特征及形成机制，研究不同铺粉条件下粉体堆积特性、铺粉器与已成形粉末层相互作用特征。拟采用倾斜式成形台消除铺粉过程中已粘结粉体的位移和开裂现象，同时通过小微滴多次喷射提高喷射精度和控制粘结剂固化时间消除轮廓边缘粘结剂溢出等措施解决成形精度偏低问题，并通过优化粉体粒径分布和间接加入亚微米级粉末提高粉体致密度及铸型表面构建涂层等措施提高铸型强度。本项目的研究成果具有较大的理论和应用价值。

为解决微喷射粘结直接成形铸型精度和强度偏低难题，研究了喷射过程中粘结剂液滴落点位置的影响因素及机理；研究了铺粉过程中已粘结粉层所受力的演化规律及其对铸型成形精度和强度的影响机制；自主研制了具有倾斜铺粉方式的微喷射粘结成形设备及控制系统；研究了粘结过程中粘结剂液滴在粉末层中的渗透动力和固化行为；研究了不同工艺参数对铸型成形精度和强度的影响规律及其机理；提出了以纳米分散液为粘结剂的“孔隙填充”成形新方法，研究了纳米分散液在粉层中的分布特征及其对型芯性能的影响机理。研究结果表明：喷射的粘结剂液滴沿抛物线轨迹飞行而使液滴落点位置产生误差，通过位置补偿算法并使喷头安装高度小于1.5mm时，其喷射成形误差小于0.2mm。在铺粉过程中已成形粉末层受到新粉末施加的切应力和压力应作用而易发生移动和开裂，采用发明的随动下落式倾斜铺粉方式使已成形粉层所受压应力和剪应力趋于0，从而提高成形精度和强度。影响铸型成形质量的主要因素有粉末角形系数和粒径、粘结剂性能（表面张力、粘度、固化速率和固化强度）和喷射量、分层厚度等因素有关，这些因素交互影响粘结剂在粉层中的渗透动力、固化分布特征以及单位粘结桥数量和强度，从而影响成形精度和性能；较佳的铸型微喷射粘结成形工艺为型砂角形系数小于1.1、粒径范围为70-140目，酚醛树脂粘结剂喷射量为25%，分层厚度0.32mm获得的砂型X、Y、Z轴向误差分别为0.11mm、0.09mm、-0.03mm，抗拉强度为1.21MPa，透气率为158.5，发气量为16.9ml/g。喷射的纳米氧化锆颗粒填充氧化钙粉层孔隙并包裹氧化钙颗粒表面，使坯体致密度增大，降低烧结收缩率，且烧结形成的中间化合物锆酸钙可阻隔氧化钙与水蒸气的接触，大幅提升其抗吸湿能力。当纳米氧化锆分散液喷射量为20%，成形的氧化钙陶瓷型芯坯体综合质量最好，三个轴向尺寸误差都小于0.15mm，致密度为0.4497；经1300℃烧结后的收缩率为2.11%、抗弯强度为14MPa、48小时吸湿率为1.18%。本项目的研究成果较适合金属零件的低成本快速铸造，具有重要的理论和实际意义。