Abstract
Although substantial research work has been conducted in order to understand and improve the drilling of multi-material aerospace stacks, some key aspects related to process and tool parameters and their impact on the interface quality still need to be addressed. This paper reports on the research conducted to investigate the impact of tool point angle and interlayer gap width on borehole quality, focussing on the interface region. A number of drilling tests were carried out using tools with different point angles and CFRP/titanium stacks with different interlayer gap widths. The results show that the damage on the CFRP interlayer surface is caused by the drilling of the titanium layer, as some of the upwards-travelling titanium chips penetrate into the stack interface. An increase in tool point angle results in larger entry burrs on the titanium interlayer surface, which is attributed to the correlation between tool point angle and thrust force and the capability of tools with low point angles to remove damage generated by surface skidding. The introduction of an interlayer gap promotes the ingress of titanium chips into the stack interface, thereby leading to more pronounced interface damage.
Highlights
In the aerospace industry, stacked structures comprising layers made of different materials, most commonly carbon fibrereinforced polymers (CFRP), aluminium and titanium, are gaining popularity due to their advantageous mechanical and Highlights: Drilling one of the most challenging stack combinations in the aerospace industry Drilling CFRP/Ti stacks with tools exhibiting a wide range of tool point angles Drilling CFRP/Ti stacks with accurately pre-set interlayer gap widths Identification of damage in CFRP due to titanium chip flow and interface ingress Establishing relationship between interlayer gap width and CFRP interface damage physical properties when compared with traditional singlemetal structures
This would have resulted in significant variations in the interlayer gap thickness during different drilling cycles [28], which would have made it difficult to establish a relationship between tool point angle and interlayer borehole quality [24]
Bearing in mind that the previous research was carried out on aluminium/aluminium stacks, it was assumed that the drilling of CFRP/titanium stacks would have led to significantly more damage, as a result of the softer nature of the entry layer, in this case CFRP, and the considerably harder and more abrasive nature of the chips produced when drilling into the lower layer, in this case titanium
Summary
Xu and Mansori [11] carried out experiments to determine factors affecting the drilling of CFRP/ titanium stacks They observed that tool features such as chisel edge length, point angle and helix angle play a more dominant role in the resulting borehole quality and tool life than tool substrate and coating. Xu et al [15] assessed the impact of through-tool minimum quantity lubrication (MQL) when drilling CFRP/titanium stacks According to their findings, the use of MQL results in substantial improvements in terms of composite borehole surface morphology (reduced fibre pull-out and fibre/matrix debonding) and reduces the extent of tool wear, but does not affect the thrust force and, the delamination damage.
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