The joining efficiency of bolted joints, or rivets, highly depends on the quality of the machined holes. However some specific characteristics of composite laminates such as inhomogeneity, anisotropy, the layered structure and the extreme abrasiveness of fiber reinforcements present a considerable challenge for processing operations like milling and drilling.
In particular, the highly abrasive nature of fiber reinforcements leads to an extensive tool wear during drilling operations of composites. The tool wear is mainly characterized by a steadily increasing cutting edge rounding (Figure 2) which leads to a higher thrust force.
Figure 2: Tool wear during drilling of FRP
Since the thrust force is acting perpendicular to the single layers of the composite laminate, process- induced part damages like delamination, fiber fraying or spalling can occur (Figure 3).
Figure 3: Typical process-induced damages for drilling of FRP
Finally, the damage extent can be enhanced by the increased friction. Since thermal conductivity of FRP is generally very low, the result is a significantly higher process temperature at the cutting zone. This may damage additionally the highly heat-sensitive polymer matrix of composites.
Using liquid cooling lubricants, as it is applied for metal cutting, is problematic due to possible swelling of the polymer and the formation of a viscous dust/chip-oil-compound, resulting in high cleaning costs because of machine contamination, for instance. Therefore, almost 80% of all composite cutting processes are operating without any coolant or air coolant which, however, is not very effective.
In the following, the current main problems of composite drilling processes are listed:
- Fast proceeding tool wear (insufficient low tool life)
- Locally concentrated thermal tool load (lack of efficient cooling solutions)
- Process induced work piece damages (additional rework of rejection of parts)
Considering the drilling process of hybrid assemblies (stacks) which comprise a sequential layer of metallic (e.g. titanium, aluminum) and FRP materials, the above mentioned challenges for sole fiber reinforced composites are even more pronounced.
For economic reasons and reasons related to tolerance requirements, processing of those materials is realized in one step, namely "one shot" drilling. Accordingly, composite-based materials and metals have to be machined with the same drilling tools, so tool geometry/material is a compromise to meet the requirements of both material classes in the best possible way.
The used drilling tools are affected by two different types of wear mechanism, which are both critical in their effect. Beside the already mentioned abrasive wear caused by the fiber reinforcements, the cutting tool is subjected to high mechanical and thermal loads that are generated during the cutting process of metals. This makes the use of cooling fluids inevitable since the generated process heat has to be dissipated from the cutting zone as fast as possible. An application of cooling fluids is limited, though, due to the sensibility of polymer matrix material. This conflict of interest is faced by the application of Minimal Quantity lubrication (MQL), which is widely used for stack cutting processes. However, the cooling or the lubricating effect is not sufficient, so tool life is still very limited. In addition, the damaging of expensive components is risked since the generated hot metallic chips may damage or unintentionally remove the polymer matrix material, which mainly has a melting point below 200°C. Consequently, the manufactured bore holes usually show damages of different diameters within the single layers, leading to a non-compliance of the quality requirements. Ultimately, this leads to low process performances and the need for expensive and time-consuming rework.
In the following, the current main problems of stack drilling processes are listed:
- Very fast proceeding tool wear (higher than for sole composite drilling)
- Occurrence of process induced work piece damages as for sole composites
- Damage of FRP layers and FRP metal interfaces due to overheating of polymer or unintentional material removal caused by hot metallic chips
- Limited process parameters due to low cooling efficiency by the use of MQL
Work piece damages and quality deviations can drastically reduce the strength against fatigue, thus degrading the long-term performance of composite laminates. Drilling-induced damages that result in a component rejection represent an extensive economic loss as well as an environmental burden since drilling is often the final machining operation within the entire manufacturing chain. Especially in view of the prognosticated growth rates, a process reliable and economical drilling system/process has to be developed in order to overcome the former described challenges.