Conventional drilling and helical milling of aircraft stacked structures - a comparative study

由碳纤维增强聚合物复合材料（CFRP）和钛合金（Ti4 Al 6V）组成的叠层结构由于其上级力学性能和减轻的重量而越来越多地用于飞行器中。在飞机装配中，CFRP/Ti叠层是在多次加工过程后紧固的，该过程涉及钻孔、铰孔、拆卸、去毛刺和重新组装操作。不幸的是，CFRP和金属的机械和热性能之间的巨大差异给这些材料的钻孔带来了巨大的挑战。钻孔过程中遇到的其他问题包括快速工具磨损、CFRP损坏、表面光洁度差和毛刺形成。螺旋铣削（或轨道钻削）是一种新兴技术，由于其较低的切削力/温度和高效率，被认为是在堆叠结构上进行孔加工的最佳选择之一。然而，螺旋铣削过程中产生的分层和毛刺的形成，除了几何误差（孔径尺寸不足）外，还会影响孔的加工质量。本课题的主要目的和目标是：1）探索和比较不同的螺旋铣削和常规钻削加工工艺在航空航天CFRP/Ti叠层结构中的加工机理。2)建立加工参数与孔的几何精度、表面完整性和材料可靠性（例如，带开孔结构的疲劳寿命）之间的关系。CFRP/Ti叠层的常规钻孔/螺旋铣削工艺的基本特征将阐明表面损伤的机制，从而有助于告知/改进技术，以避免钻孔或螺旋铣削工艺中完全不可接受的事件。方法学：为了验证加工孔的几何精度和表面完整性，将部署一系列先进的表征设备，例如扫描电子显微镜、白色光表面轮廓仪、X射线断层扫描仪和原子力显微镜等。然后将结果与从带开孔试样中获得的疲劳试验数据相关联。提出了评价孔加工质量的综合性能指标作为评价标准，以钻削效率最大化为目标进行加工参数优化。这将是第一个研究比较传统的钻孔和螺旋铣削加工的航空航天结构，并与孔的制作过程中产生的材料的可靠性。该项目与EPSRC的“制造未来”战略密切相关。本课题的研究成果将为自动化钻井设备和工艺的发展提供具体的理论和技术基础，并带来显著的效益。

Stacked structures consisting of carbon fibre reinforced polymer composites (CFRP) and titanium alloy (Ti4Al6V) are increasingly used in aircrafts nowadays due to their superior mechanical properties and reduced weight. In aircraft assembly, CFRP/Ti stacks are fastened following a multi-shot machining process which involves drilling, reaming, disassembly, deburring and reassembly operations Unfortunately, the vast difference between the mechanical and thermal properties of CFRP and metal has created enormous challenges in drilling of these materials. Other problems encountered during drilling include rapid tool wear, CFRP damage, poor surface finish and burr formation. Helical milling (or orbital drilling) is an emerging technology and is considered as one of the best choices to perform the hole making on stacked structures due to its lower cutting forces/temperature and high efficiency. However, delamination and burr formation resulted from the helical milling process can affect the hole making quality, in addition to the geometric error (insufficient size of hole diameter). Although some research efforts have been directed to this research area, most of them are empirical and the helical milling machining mechanism and the resulting hole surface integrity / reliability are still unclear.The aim and objectives of this project are:1) to explore/compare the mechanisms of different hole making processes (i.e. conventional drilling and helical milling) in machining of aerospace stacked CFRP/Ti structures. 2) to establish the relationships between machining parameters and the hole geometrical accuracy, surface integrity and the material reliability (e.g. fatigue life for structures with open hole). Fundamental characteristics of conventional drilling / helical milling process for CFRP/Ti stacks will clarify the mechanism of surface damage and thus help to inform / improve the technologies in an effort to avoid completely unacceptable occurrence in drilling or helical milling processes. Methodology: To characterise the geometric accuracy and surface integrity of the machined holes, a range of advanced characterization facilities will be deployed, such as Scanning electron microscope, white light surface profilometer, X-ray tomography, and atomic force microscope etc. The machined surface will be further characterized for its residual stress using nanoindentation, supplemented by digital image correlation technique. The results will then be related to the fatigue test data obtained from coupons with open holes. After comprehensive performance indices for evaluating the quality of a machined hole are proposed as the criteria, the machining parameter optimization will be performed by maximizing the drilling efficiency. This will be the first study to compare conventional drilling and helical milling in machining of aerospace structures, and to correlate the hole making process with the resulting material reliability. The project is closely in line with the EPSRC "Manufacturing the future" strategy. The outcomes of the project will provide the concrete theoretical and technical foundations and bring significant benefits to the development of automated drilling equipment and processes.