渗碳体石墨化制备无铅易切削石墨黄铜的机理及其切削性能研究

Graphite is believed to be an attractive candidate substituting for lead to produce free-cutting brasses owing to its role in breaking up the swarf into small-sized chips and lubricating property. The major hindrance to developing graphite brasses is an extremely low solubility of carbon in brass melt. In order to solve the solubility problem of carbon in brass melt and suppress the formation of high hardness particles, carbon will be added into brasses by melting cast iron in Cu-Zn melt and graphite brasses will be maded by the liquid phase in-situ formation of cementite particles during casting and the graphitization of cementite particles during annealing in the applying project. The affecting mechanism of cooling rate on the liquid phase in-situ formation of cementite particles during casting will be clarified by analyzing the effects of cooling rate and the adding content of cast iron on the nucleantion and growth of cementite particles. The uniform and fine cementite particles will form in brass melt. The suppressing role of the in-situ forming cementite particles in the phase separation of melt will be studied by analyzing the effects of the size and volume fraction of cementite particles on the composition segregation of brasses. And the brasses without the composition segregation will be prepared. The promoting role of the plastic deformation of cast brasses in the graphitization of cementite particles will be studied by analyzing the effects of the plastic deformation of cast brass on the decomposition of cementite particles and the diffusion of iron atoms. The cementite particles will be fully graphitized by increasing the amount of plastic deformation and controlling annealing process. The role of graphite in breaking up the swarf into small-sized chips and its lubricating property will be studied by analyzing the effects of the size and volume fraction of cementite particles on the machinability of lead-free free-cutting graphite brasses. Graphite brasses substituting for lead brasses will be maded by adjusting casting and annealing process.

石墨因优良的断屑效应和润滑作用而被认为是最适于用来替代铅制备易切削黄铜，但碳在黄铜熔液中极低的溶解度给制备石墨黄铜带来了很大的困难。本项目以铸铁的形式在黄铜中加碳，利用熔铸时渗碳体的液相原位生成和退火时渗碳体的石墨化制备石墨黄铜，从而既解决了碳在黄铜熔液中溶解度极低的问题又不会在黄铜中生成硬质颗粒。 研究冷却速度和铸铁添加量对渗碳体形核和长大的影响，理解冷却速度影响渗碳体原位生成的机理，在熔液中原位生成均匀细小的渗碳体颗粒；研究渗碳体颗粒的尺寸和体积分数对成分偏析的影响，理解原位生成的渗碳体抑制液相相分离的机理，制备出无成分偏析的黄铜；研究塑性变形对渗碳体分解和铁原子扩散的影响，理解塑性变形促进渗碳体石墨化的机理，提高塑性变形量和控制退火工艺促进渗碳体的完全石墨化；研究石墨颗粒的尺寸和体积分数对切削性能的影响，理解石墨的断屑效应和润滑作用，调节熔铸和退火工艺制备出能替代铅黄铜的石墨黄铜。

铅黄铜因为优良的切削性能和低廉的价格而得到广泛应用，但其使用过程中的铅溢出所带来的环境和健康问题使无铅易切削黄铜的研制成为了一个必须攻克的难题。本项目的目的在于研究渗碳体石墨化制备石墨作为弥散相均匀分布于基体的无铅易切削石墨黄铜。.本项目选用石墨作为Pb的替代元素，通过添加共晶铸铁取代原来直接添加石墨粉的方式，并利用石墨化退火工艺来制备无铅易切削黄铜。通过对添加不同铸铁量所制备的石墨黄铜进行成分、组织结构、力学性能、切削性能和腐蚀性能分析，研究不同铸铁添加量对组织和性能的影响。.(1)采用共晶铸铁作为C源，利用渗碳体颗粒在黄铜熔液中的原位生成和退火时渗碳体的石墨化成功制备了石墨黄铜。熔炼时渗碳体在高的冷速下优先形核，黄铜熔液依附核心形核长大，得到渗碳体颗粒弥散地分布在α相（Cu0.64Zn0.36）和β相（CuZn）两相上。.(2)退火时基体上的渗碳体发生石墨化现象，塑性变形加速渗碳体石墨化进程，使渗碳体完全转变成石墨。退火后石墨颗粒的位置和尺寸取决于渗碳体颗粒，得到α相（Cu0.64Zn0.36），β相（CuZn）和弥散石墨颗粒。.(3) 随着铸铁添加量的增加，合金的硬度和强度上升，韧性下降。切屑形貌由螺旋状向C型屑转变，内表面较光滑，切削加工后表面质量较好，切削性能提高。但铸铁添加量达到7%时，切削性能下降。石墨能改善黄铜的切削性能是由于合金中均匀弥散分布的石墨颗粒，类似于空洞或裂缝，在黄铜上相当于微小空洞区域，基体的连续性被切断，产生“切口效应”。这些微区的存在使切削时发生断裂。此外，石墨作为一种良好的润滑剂，在刀具和金属表面形成一层石墨薄膜，减小摩擦力，更易获得光滑的表面。.(4) 石墨黄铜脱锌腐蚀后，腐蚀表面有空洞和裂纹，腐蚀产物间连接一层薄的钝化膜，钝化膜的形成阻碍黄铜中Zn的扩散，提高黄铜的耐腐蚀性能。随着铸铁添加量的增加，石墨黄铜耐腐蚀性能提高，但当铸铁添加量到7%时，耐腐蚀性能降低。石墨黄铜的耐腐蚀性能是石墨黄铜的组织、铁和石墨的含量以及石墨颗粒的分布状态共同作用的结果。.本项目制备出了能替代铅黄铜的石墨黄铜。