In the aerospace industry, the use of CFRP is gaining ground due to its ability to reduce the weight of aircraft. Apart from its low weight, the material is also characterised by a high level of rigidity and tensile strength in the direction of the fibres.
However, when a tensile load is not applied in the direction of the fibres, tensile strength is considerably reduced. This is a reason why stacks made of CFRP/metal composites are often present in the structure of aircraft that contain a high proportion of CFRP.
Titanium, in particular, is a suitable metal in the manufacturing of stacks. The main advantage of titanium is that its high tensile strength is direction-independent — a feature which reinforces construction. Due to this composite construction, the structure of an aircraft consists of around 20 percent titanium, which is used in door frames, some formers and connecting elements. This type of construction is found in the Airbus A350 and A400M as well as the Boeing 787 Dreamliner.
Layer compositions, known as stacks, made from CFRP and titanium are typically used in high-tech sectors such as the aerospace industry. Titanium combines well with CFRP because the two materials have similar coefficients of expansion.
Temperature change differences on the outside of an aircraft can therefore be better absorbed than with aluminium, for example. Because of aluminium’s low electrochemical compatibility, it can only be connected to CFRP by using an insulating layer.
Drilling On CFRP Stacks
The primary joining technology in aircraft construction is riveting. Before the rivet can be fitted, two or more materials have to be fully drilled through, using a step drill/countersink drill or a Maximiza. Because the drilled holes are part of the connecting elements, they have an influence on symptoms of structural fatigue. The creation of geometrically round drilled holes and perfect countersinks is therefore essential in terms of safety and long service life.
CFRP is highly abrasive to cutting tools, which means that cutting edges wear very quickly when this material is machined. In response to this phenomenon, Polycrystalline diamond (PCD) offers maximum performance and repeatability when drilling through abrasive materials and stacks. PCD technology uses a combination of diamond particles and a metallic binder. PCD drill points also offer the longest tool life and are, in the majority of cases, the more cost-effective solution.
PCD Vein Drills
A development known as PCD vein technology has been around for four years. ‘Vein’ denotes specially designed slots in the carbide head, into which the polycrystalline diamond is incorporated. To do this, PCD powder is poured into the carbide head and sintered in a press at a pressure of 60,000 bar and a temperature of 1,500 deg C.
Solid polycrystalline diamond is formed in the process. This carbide head with the PCD drill point is brazed onto the tool shank. The shank diameter and base diameter, chip clearance, drill back and face geometry of the tool are then ground and eroded, which gives the tool its final geometry.
This CNC-controlled process offers maximum reliability and repeatability, which also guarantees perfect reconditioning. Today, drills with a carbide body and PCD drill point are regarded as the best choice for CNC processes in the aerospace industry.
“With PCD vein technology, we can create geometries for fibre composite materials, plastics and non-ferrous metals for a variety of different machining environments, which would have been impossible with conventional PCD drills,” explains Stefan Benkóczy, innovation manager at Walter.
“A further advantage is that solder failure, which may arise in the case of directly brazed PCD drill points, does not occur with PCD vein technology.”
The process is also an option for reducing costs per drilled hole. “Tool life can now be increased by up to 40 times in comparison to standard carbide tools. PCD vein drills can subsequently be reground up to five times, which fully restores the drills to their original quality,” he says.
While it is true that solid carbide tools with a diamond coating have a long tool life, they are disposable and cannot be reconditioned. Uncoated solid carbide tools can be reground but they have a short tool life. The higher cutting speed with PCD vein tools in comparison to solid carbide tools also contributes to increased cost efficiency.
Determine The Feed Rates
PCD technology uses a combination of diamond particles and a metallic binder. PCD drill points also offer the longest tool life and are, in the majority of cases, the more cost-effective solution. |
A distinction must be made with CFRP between unidirectional and multidirectional fibre arrangement. Because individual fibres cannot hold each other in position, fibre structures which are built up unidirectionally have a greater tendency to delaminate than multidirectional fibres.
Whereas it is possible to achieve feed rates of 0.08 to 0.35 mm with multidirectional CFRP depending on the matrix, a maximum feed rate of only 0.2 mm can be reached with unidirectional CFRP.
The matrix mainly consists of epoxy resin or a thermoplastic, and the interlaminar shear strength determines how well the fibres are bound into the matrix, and how easily they can become detached. Depending on the drill, the cutting speed for the two structures ranges between 80 and 300 m/min.
“Short machining times while still maintaining all quality characteristics and a consistently long tool life when machining CFRP are the result of many years of development,” says Mr Benkóczy.
“The cutting edge that we achieve with PCD vein technology can withstand even the most abrasive of materials very well, and is characterised by slow wear, which is very uniform. The cutting edges on these tools become rounded over time significantly, less than with solid carbide and the tool remains sharp for longer.
