Despite the growth of composites and other lightweight materials, aluminium alloys remain an attractive choice for the aerospace industry due to their distinct manufacturing processes, good resistance to fatigue crack growth and superior damage tolerance. In the aerospace industry, the drilling process is widely used among all the machining processes as millions of holes are required to produce riveted and bolted joints in the assembly operation of the aircraft's structures. The major challenges which arise from the drilling of these alloys are characterized by the low hole quality, which might initiate cracks within the airframe structure and reducing their reliability. This results in the rejection of parts at the assembly stage, which directly impacts the manufacturing cost. Hence, an appropriate selection of tool geometry, tool materials and coatings, cutting speed, feed rate, and drilling machines is required to meet the requirements of machined parts. The large number of holes required for riveting means that their installation must be carried out in a fast and precise manner. This can be achieved by using multi-head drilling tools that can drill several holes simultaneously. Therefore, in this study, a simultaneous multi-hole drilling approach was used to investigate the important drilling output parameters, such as the thrust force, chips formation, post-drilling tool conditions and hole metrics including surface roughness, deviation of hole from the nominal size, circularity, cylindricity, perpendicularity, and burrs formation under dry conditions. Moreover, the inside hole surface defects and top and bottom hole edges were examined using scanning electron microscopy. The investigations were based on different cutting parameters, the maximum and minimum possible center-to-center spindle distances of the multi-spindle head, tool geometry, tool materials and four types of tool coatings (TiN-, TiCN-, TiAlN-, and TiSiN). Furthermore, analysis of variance was employed for estimating the relationships between the input parameters (spindle speed, feed, and tool coating) and the studied hole quality metrics. The focus was mainly on Al2024 alloys, which is commonly used as an aerospace structural material. The results show that uncoated carbide drills with high point angle and smaller diameter generated less thrust force, produced higher quality holes, and formed a lower built-up edge due to short chips. The common surface damage found on the inner hole surface was smearing, feed marks, and metal debris adhesion. The results also show that the uncoated carbide drills performed better at low spindle speeds, while TiCN-coated drills produced better hole quality at higher spindle speeds. Regarding the coated drills, TiCN-coated drills produced holes with the least deviation, circularity, cylindricity and perpendicularity at high spindle speeds. TiSiN-carbide coated drills produced the most oversized holes and noticeable damage and deformations on their surface following TiAlN and TiN. Besides, tools of the multi-spindle head can be adjusted in any position without affecting the hole quality, which is useful for increasing productivity at a higher rate in manufacturing industries.
|Date of Award||2021|
|Supervisor||Majid Tolouei-Rad (Supervisor), Ana Vafadar (Supervisor) & Khaled Giasin (Supervisor)|
Simultaneous machining of aerospace structural materials using poly-drills
Aamir, M. (Author). 2021
Student thesis: Doctoral Thesis