^ Cross Section of a Composite Blade (Source: http://classroom.materials.ac.uk/caseRoto.php)
Are two heads are better than one? Well, only so long as too many cooks do not spoil the broth. Composite Materials or Composites perfectly manifest this precarious balance. A combination of two or more separate materials, composites possess properties distinct from those of its components. And yes, most composites include only two elements.
Made of the matrix and reinforcement phases, composites blend the reinforcement’s strength with the matrix’s toughness. Individual materials cannot offer such synergy. We use composites mainly because they offer high-strength and high-stiffness combination with low-weight.
Carbon-Fiber composites weigh only 20% of 1020-Grade Steel but are five times stronger. Carbon-Fiber composites offer better strength-to-weight ratio over glass-reinforced composites. Composites comprise over 20% of Airbus A380’s (the world’s largest passenger airliner) material.
Heterogeneity, low thermal-conductivity, abrasiveness, and anisotropy of composites produce excessive tool wear and workpiece deformation when you use conventional machining processes. You have to modify traditional processes or use unconventional processes to machine composites.
Why Machining Composites is Tough?
Wood and bone are natural composites. Even our humble reinforced-concrete is a composite. Fiberglass was the first modern-day composite, strong glass-thread reinforcement cemented by a plastic matrix.
Composites have two phases that do not mix completely:
- Reinforcing Phase: strong, low-density material
- Matrix or Binder Phase: tough or ductile materials
Matrix can be of epoxy, polyimide, phenolic, and polyethretheketone (PEEK). Fibers can be of carbon, ceramic, graphite, tungsten, glass, and polymer (polyethylene, Kevlar). Matrices shield reinforcements from chemical and environmental attack while orienting them appropriately to bear load.
Based on the reinforcing mechanism, composites can be:
- Fiber Reinforced
- Particle Reinforced
- Dispersion Strengthened
Challenges in machining composites due to:
- Anisotropy: different characteristics along different directions
- Low Thermal Conductivity
- High Fiber Abrasiveness
- Extreme Tool Wear
- Inferior Machining:
- Flawed Workpieces
- Breakout: stress-induced expansion of a hole’s cross-section (in drilling)
- Poor Tolerance
- Substandard Surface Finish
- Uncut Fibers
- Fiber Pullout
- Delamination: failure due to layer-wise separation of constituents
- Low Cutting Rates
- Excessive Dust Generation
Unidirectional fiber composites are particularly delamination-prone. Fiber strength augments their abrasiveness. Larger diameter fibers demand greater cutting forces. Composites with greater percentage of fibers (by volume) are tougher to machine.
Drilling is the most common operation for composites. Industries use Laser and Waterjet Machining extensively. Electro Discharge Machining (EDM), Electro Chemical Spark Machining (ECSM), and Ultrasonic Machining are yet to mature. Lasers and waterjets provide quality cuts on a numerous materials and are automation-friendly.
Laser Cutting: lack of contact between workpieces and tools is the prime benefit. This eliminates mechanical cutting forces enabling chatter-and-vibration-free machining of thin-small parts. Complications stem from wide variance in heat capacities, thermal conductivities, and vaporization temperatures of the matrix and the reinforcement.
Lasers cut 0.08mm thick composites only. Cutting thicker parts requires greater power that expands the Heat Affected Zone (HAZ). Kerf Width and HAZ determine a laser cut’s quality.
HAZ is the area where temperature exceeds the matrix’s vaporization temperature. Use least-possible cutting power to minimize HAZ. There is a unique feed-rate for every power-level that gives minimum HAZ.
Select proper power-levels and feed-rates to prevent changes in material properties. Thermal Diffusivity, Energy Absorption, and Reaction Temperature are important here. Toxic CO and CO2 fumes are an issue, delamination is not.
Waterjet Cutting: removes material by localized shearing and erosion using very thin waterjets of 0.08-0.5mm diameters at 400MPa (maximum) pressure. Waterjets cut composites with metal, organic, and ceramic matrices via turning, cutting, drilling, and milling.
Cutting-Speed and maximum cut-able thickness varies with material. Fiber orientation does not influence cutting speed. High cutting-speed, unchanged surface microstructure and the absence of HAZ and dust are major merits. High speeds and moisture can however cause delamination.
Higher pressures provide better material removal-rates and smoother surfaces but introduce surface waviness. The process offers acceptable 0.5-2.5mm cut-widths, ±0.4mm tolerances, and 1.3-3mm corner radii. Appropriate design minimizes the inherent safety hazards, aerodynamic noise, and mechanical noise of the process.
- Piercing Small-Diameter Holes with 200-400MPa Pressure
- Milling Blind Holes
- Circular Kerf Cutting of Large-Diameter Holes
Piercing ceramics requires higher pressures than for brittle materials. Greater pressure lowers piercing time but creates fractures, delaminations, and cracks. Piercing rate decreases with increasing hole-depth. Pierced holes taper with top diameters being larger.
Electro Discharge Machining (EDM): uses controlled, localized sparks at 8,000-12,0000C between electrodes and conducting materials to shape the latter through controlled melting. Sparks affects the surface only. Types:
- Trailing Electrode/Wire: electrode traverses the path creating the desired shape
- Die Sinking: electrode is a die shaped exactly like the final part. It is guided systematically into the workpiece. The latter melts and gives the desired shape
Increasing current-levels hikes the electrode-wear-rate and material-removal-rate. Excessively-high currents can fracture workpieces. Uniform electrical conductivity gives stable EDM. Machining time increases with fiber content. EDM is also used for ceramics.
Electro Chemical Spark Machining (ECSM): similar to EDM but can fabricate non-conducting composites. ECSM involves an electrolytic apparatus. The cathode is the shaping die or the trailing wire as in EDM. Hydrogen bubbles form when direct current is passed. These bubbles generate heat and sparks.
Workpiece is maintained at a fixed distance from the anode and brought closer to the cathode. Melting and vaporization shape the workpiece. The anode-cathode distance and the cathode-workpiece gap are critical parameters.
Ultrasonic Machining: is applied to brittle materials with poor electrical conductivity that are not machined with EDM or ECSM. Stainless-Steel tools perform superior than those of High-Speed-Steel (HSS) or Mild-Steel.
Tools vibrate at high-frequency (19-25kHz) and very-low amplitudes (few thousandths of an inch). This mobilizes the abrasive-liquid slurry between the tool and the workpiece and removes material by erosion. This produces shapes and cavities that traditional methods cannot.
- Simpler to cut composites along the length of fibers. Transverse cutting fractures and chips fibers
- Use Positive Rake Angle for tools irrespective of the tool material. Such tools smoothly slice into composites and shear fibers
- Blunt tools drag fibers, not break them as required
- Diamond-Tipped tools are most effective and long-lasting as also the most expensive. Carbide-Tipped and HSS tools come next
- Use sharp tools at low feed-rates and high spindle-speeds for milling composites
- Diamond-Tipped tools with polished flutes drill best quality holes when operating at slow feed-rates and high spindle-speeds
- Coolants check overheating of tools and workpieces, prevent tool-clogging, and enable dust control
- Bounce-Back or tool-pressure-induced deformation is a minor issue for composites
- turning long, thin tubes, use headers few thousandths of an inch smaller than tube diameter at both ends
- drilling, facing, or turning shorter tubes, chuck-held steel extensions are enough
- preventing scratching of workpieces, add duct-tape over chuck jaws
- Change electronic cabinet filters on machines more frequently than you would do when machining non-composites
With progressive expansion in the application of composites, we will witness the development of better composite materials and superior machining processes for shaping them.
To know more on material and technology developments in the world of machining, visit our blog. Contact Kemplon Engineering for large scale custom metal fabrication and marine fabrication services, and marine pipe fitting using a diverse range of materials.