For decades, the aircraft industry has utilised composite materials in multiple applications, including flight surfaces and some internal cabin parts. Unfortunately, these materials are unique to each design in their fibre layering techniques, resins & curing processes, making it difficult to achieve consistency in manufacturing & assembly. Composite materials are bonded together to form complex structural sub-assemblies that must be either assembled together or attached to other structural components, such as aluminium or titanium. This presents a unique set of challenges that requires radical new technologies. One of the newest materials using carbon fibre and resins is called Carbon-Fiber Reinforced Polymer (CFRP). Due to its attractive properties, such as weight-to-strength ratio, durability and extreme corrosion resistance, CFRP is used mostly in primary structure applications, like the aircraft hull and wings.
Composite materials are generally composed of a soft, tough matrix with strong, stiff reinforcements; fibre-reinforced polymers are the broad class of composites usually targeted.
Fibre reinforcements
Carbon fibre/graphite fibe (high strength or high modulus)
Glass fibres
Ceramic fibres
Polymer fibres (kevlar, polyethylene)
Tungsten fibres
Polymer matrix
Epoxy
Phenolic
Polyimide
Polyetheretherketone (PEEK)
Misconceptions related to defects in composite manufacturing
There are, however, some misconceptions associated with the defects and variability in composite manufacturing. For example, many of us mistakenly think that all composites are the same. In reality, except for similar machining performances, they differ to a great extent. To prevent issues arising from this, gaining a clear understanding of customers’ quality requirements is crucial so that the performance can adequately match up to it. What’s equally important is getting as much information regarding customers’ materials as possible in order to avoid slowing down cutting tool development. Since this potentially wide specification range of composite materials could restrict machining optimisation, we have to focus on developing machinability indexes to improve the understanding of materials. Metals are generally governed by standard specifications like DIN/ ASTM/JIS, etc, but there are no global standards for composites yet.
Challenges associated with composite machining
The physical and chemical properties of CFRPs differ based on several aspects, like the number of carbon fibres, the ratio of fibre to resin, direction of fibres, method of layering & equipment used, fixturing & process conditions, etc, to name a few. High strength-to-weight ratio leads to a widespread acceptance in structural aerospace components, and corrosion resistance & radiolucent properties have made CFRP/carbon fibre attractive in the medical industry.
Drilling is the most widely used machining operation in CFRPs. The defects in drilled holes are understood based on the acceptable limits for quality depletion and wear. A majority of customers rely on delamination (separation of layers) as the primary cause of ‘end of life’ and use visual methods to detect this. So, the typical challenges associated with composite machining comprise –
Surface quality
Delamination
Fibre pullout
Uncut fibres
Breakout
Rapid tool wear
Rapid flank wear due to abrasive nature of composites
To overcome these challenges, the tool design should be developed with regards to the failure modes observed. Development can be divided into two streams:
1. Geometry
Positive geometry to minimise stresses that can cause delamination
Sharp geometry to cut fibres with localised, induced strain
Chip evacuation is not essential but dust needs to be evacuated
2. Material
Sufficient hardness to resist abrasion wear
Strength to support sharp geometries
Machining of composites
While the machining of ductile metals is based on shearing, the machining of composites involves several mechanisms:
Compression-induced fracture of fibre (buckling)
Bending-induced fracture of fibre
Shearing, yielding and cracking of the matrix
Interfacial debonding
Sub-surface damage
Practical tooling solutions
Currently, only three types of conventional tools in the market can address both geometry & material design:
1. Veined PCD drills
Drills with PCD sintered directly onto carbide
Enable complex shaping of geometry
2. Diamond-coated drills
CVD diamond coating with higher hardness than PCD
Any geometry is possible
Orbital drills
Helical milling of hole reduces thrust, and therefore, breakout/delamination
Diamond coating shows a tool life improvement of nearly 10x that of an uncoated solid carbide drill. Diamond coatings require specific carbide substrates (low Co, coarse grain structure) for best adhesion. Such substrates sometimes lack the toughness required for heavy-duty applications.
Tool design for composite routing (milling)
The standard style end mills generate cutting forces in only one direction. With a positive helix cutter, this will have the tendency to lift the workpiece while causing damage to the top edge. As against this, the compression style router generates cutting forces into the top and bottom surfaces of the workpiece. These forces stabilise the cut while eliminating damage to the workpiece edges. Some crucial requirements include:
Aggressive ramping rates, high RPM capabilities and a superior surface finish — time after time
Varying axial depth of cut, meeting the challenges of a wide range of applications
No material breakout or burr formation upon entry or exit of the workpiece
Conventional push drilling versus orbital drilling
Conventional push drilling
Rotating the tool around its own axis
Zero cutting speed at cutter centre
Continuous contact with hole edge
Cutter diameter same as hole diameter
Continuous chips
Orbital drilling
Rotating the tool around its own axis
Revolving (orbiting) the tool around hole centre
Cutting edge intermittently in contact with hole edge
Cutter diameter less than hole diameter
Due to the high quality holes generated by orbital drilling, the following manufacturing steps might be eliminated:
Disassembly
Deburring
Cleaning
Reassembly
Repair
Use of composite materials in the future
As the industry explores new ways to reduce structural weight to increase fuel efficiency, studies predict that the use of composite materials will increase by more than 40%. Kennametal is helping aerospace manufacturers prepare for these future changes through continuous innovation.
Courtesy: Kennametal
