Lightweight materials have come to play an important role in modern aircraft because of the need to reduce fuel consumption. For example, in the structure of the Boeing 787 Dreamliner, the operating empty weight of carbon fiber reinforced polymers (CFRP) and titanium is 50% (57 ton) and 15% (17 ton), respectively. Due to CFRP and Ti’s superior properties, their rate of usage is increasing in the aerospace industry.
However, the materials are difficult to machine. Specific problems are the excessive abrasive nature of the composite material and the poor thermal conductivity of titanium (about one-sixth that of steels). Previous research has shown that carbide tools with low cobalt content are recommended for composites drilling due to their increased tool hardness and thus increased abrasion resistance. For Ti, they possess high hot hardness to withstand high stresses, wear durability, and good thermal resistance and high thermal conductivity to endure the thermal gradient and thermal shock, as well as the high tensile and shear stress.
Because each of the materials requires extremely different types of drill geometry and cutting conditions, machining CFRP-Ti stacks is particularly challenging. Burrs and chips, which are produced when drilling the titanium, damage the composite layers and produce holes outside the required size accuracy; good-quality holes, where diameter tolerances reach 30 µm or even less, are necessary to obtain good fastening and guarantee safety in use.
Current techniques and tools for drilling in CFRP-Ti stacks are relatively slow and expensive. Although such stacks have been used in industry for at least 20 years, more research is required regarding the influence of different drill geometry features on tool performance and hole quality. Tool life needs to be increased and the quality of the hole must be improved to reduce finishing operations.
Researchers from the University of Sheffield Advanced Manufacturing Research Centre (AMRC) with Boeing and Sandvik Tooling have taken on the stack-up challenge, with the goal being a one-shot drilling operation.
They evaluated four different drill designs, comparing them in terms of cutting forces, temperature, and hole quality. Material composition (carbide—10% cobalt with a 3-µm coating of FuturaNano [TiAlN]) and several dimensional parameters (point angle of 140° and 6.35-mm diameter) of the prototype drills were the same. The two variables were number of flutes (2 and 3) and flute helix angle (20° and 40°).
For each drill, five holes were made in 32 separate runs.
The drill with a high helix angle (40°) and two flutes offered decreased temperature in the cutting zone and lower cutting forces. The three-flute drill led to higher cutting temperatures because of two factors: first, the cutting edges are the main source of friction, leading to the generation of the heat, and they also have a significant impact on torque and power consumption; second, it has a smaller flute volume than is the case for the two-flute drill, which led to clogging of Ti chips in the flutes, in turn causing problems with chip evacuation and preventing heat dissipation from the cutting zone.
In general, the drills with a higher helix angle suffered from chipping of the primary cutting edges when used at a higher feed rate. This was due to the fact that the higher helix angle produces a sharper cutting edge because the included angle between the rake angle and the primary clearance angle is lower. The drills with a lower helix angle have a stronger cutting edge that is less prone to chipping; however, this also resulted in higher cutting forces and temperatures.
All the holes produced in the CFRP were oversized and had very high surface roughness (up to 10 µm Ra) due to erosion resulting from the titanium chip evacuation. Further work is needed to improve hole size and surface roughness. That work will be based on the 40° helix angle, two-flute design. Additional testing is also needed to evaluate other factors.
It is clear that tool performance and hole quality are largely controlled by titanium.