灰铸铁激光熔区石墨-环境多相耦合机理及形态可控优化研究

Surface modification on cheap gray cast iron material is one of important ways to reduce material costs of mechanical equipment and improve their mechanical properties. At present, laser surface modification is an effective means to realize this approach. However, local combined with bad state and the frequent micro crack initiation are often caused in molten area by complex and coupling phases effect and undesirable graphite morphology. It has become a bottleneck to hinder further development of laser modification technology on gray cast iron. Therefore in this project, investigations will be taken such as (1) establishing a numerical model of laser surface melting in considering the influence of graphite phase, and the phases coupling thermal response process in molten area can be cleared combined with experimental analyses, the microscopic analysis model of graphite phase is established to study the the coupling action law of graphite - environmental phases and the driving conditions of morphological changes; (2) based on the analysis model and experiment, researching the effects of graphite phase morphology on the distributions of the thermal stress and thermal deformation, analyzing bonding strength between these phases, revealing the thermal stress - driven micro crack induction mechanism by graphite phase in molten area; (3) investigating the controllable parameters of graphite phase morphology and process factors influence law, deriving the intrinsic relationship between the parameters and factors, and giving the optimization model, thus the optimization scheme of graphite phase morphology can be obtained. Through the above researches, the basic key problems of laser modification on grey cast iron material will be solved, and the important significance exists for improving the surface modification quality as well as for reducing the equipment material cost.

廉价灰铸铁材料的表面改性是降低机械装备材料成本、提高装备性能的重要途径，激光表面改性正是实现这一途径的有效手段，然而激光熔区复杂的相间耦合作用和不良的石墨相形态常造成局部结合状态差、微裂纹萌生频繁等缺陷，成为阻碍灰铸铁激光改性技术进一步发展应用的瓶颈。为此本项目①考虑石墨相的影响建立激光熔凝宏观数值模型，配合实验分析，明确熔区多相耦合热响应过程，建立石墨相微观分析模型，研究的石墨-环境相的耦合作用规律和形态变化的驱动条件；②基于分析模型和实验，研究石墨相形态对热应力和热变形分布的影响，分析相间结合状态，揭示熔区相间热应力驱动下的石墨相微裂诱导机制；③研究石墨相形态可控参数和工艺环节因素影响规律，建立可控参数和工艺因素间的内在联系，给出优化模型，从而得到石墨相形态优化方案。通过本项目的研究，解决灰铸铁材料激光改性的基础性关键问题，对于提高表面改性质量、降低装备材料成本具有重要意义。

本项目利用激光熔覆技术对灰铸铁进行表面改性，重点对灰铸铁熔覆层制备过程中的物相变化规律和优化方法进行研究，其中石墨相作为灰铸铁中的重要物相，在激光熔覆过程中的行为变化对组织转变、裂纹萌生都有重要影响。首先以石墨相在激光熔覆过程中的行为变化和控制方法为研究目标，深入揭示了石墨相对组织、裂纹的影响规律，设计合适的熔覆工艺提升熔覆层质量。沿激光扫描方向上的应力主要分布于熔覆层中心线两侧，垂直于激光扫描方向上的应力分布均匀，在熔覆层边界和结合处的应力较大，易导致裂纹的出现；扫描速度降低导致结合区石墨相上的热作用强度变大，碳原子扩散增多，石墨相细化更为显著，而激光功率对石墨环境温度和石墨细化的影响较小；激光二次扫描可以使熔覆层的残余拉应力获得较好释放，提高了熔覆层的韧塑形，降低石墨尖端裂纹的开裂几率。在冷却过程中基体的石墨相区域拉应力逐渐增大，结合区石墨相区域富集片状马氏体和残余奥氏体组织，石墨尖端微裂纹的萌生是微观组织和拉应力集中共同作用的结果。进一步明确了石墨相尖端微裂纹萌生机理，建立了灰铸铁激光熔覆热力耦合模型和石墨-环境相微观模型，研究石墨长度、尖端角度等形态参数和对接、平行、垂直等聚集方式对石墨相尖端应力影响规律，石墨中段长度较小时，石墨两尖端应力会相互影响，造成两尖端应力增大，随着石墨中段长度的增加，两尖端相互作用减弱；石墨尖端对接距离会导致尖端应力有一定幅度的增加且集中范围增大，两石墨相平行和垂直相互作用微弱，对应力改变作用较小。然后基于激光熔覆热响应特征分析及石墨相行为规律分析，提取石墨的主要可控形态参数，构建了石墨形态的参数化表征方法。并且以激光参数和元素成分为设计变量进行激光熔覆实验，对激光熔覆过程中工艺参数与石墨相形态参数的影响关系进行了分析，以为石墨形态的优化提供理论借鉴。