The ground vehicle and aerospace industries use carbonfiber-reinforced plastic (CFRP) because it is more durable and lighter than other materials. CFRP is used in components such as aircraft fuselages, wing skins, door panels, automotive body panels, and many other areas that are trickling down to mainstream production vehicles from racing applications. Multiple layers of CFRP and another material, such as titanium (Ti), are called a stackup.
The use of composite materials for applications demanding strength, flexibility, and elimination of corrosion is well documented. Using composites rather than aluminum in aircraft structures saves weight on the order of 20%. This weight reduction is combined with the ability to achieve aerodynamically more advantageous shapes through precision molding, resulting in better payload fractions and reduced fuel requirements.
For the Boeing aircraft line, the 777 consists of 11% composite by weight, while the 787 Dreamliner contains more than 50% composite by weight and about 80% by volume. The Bell/Boeing V-22 Osprey uses composites for the rotor blades, the rotor hubs, the wings, the interconnect drive shafting, the nacelle structure, the fuselage, and empennage, except for the fuselage frames. Carbon fiber manufacturing is also used in Formula 1 racers, high-end bicycles, and the Bugatti Veyron sports car.
All of these factors combined explain the ever-increasing popularity of composite materials in many high-performance applications. This is in spite of their significantly higher raw material costs and relatively involved processing—frequently requiring expensive autoclaves or other pressure- and heat-producing devices to compact and cure the materials.
A key machining/drilling challenge for CFRP applications is that traditional liquid coolants, particularly petroleum-based coolants, should not be used for cooling during machining operations. For this reason, nearly all composite manufacturing operations are performed completely dry. Since CO2 cooling is a dry process, it is a particularly suitable solution for heat buildup in this difficult, expensive, and time-consuming operation of composite stackup machining.
Drilling and Milling
One of the major challenges of using CFRP or CFRP layered with titanium (CFRP-Ti) is drilling holes in this stackup material for bolts or rivets to secure the CFRP-Ti panels in place. The carbon fibers in the CFRP-Ti are very abrasive to drill, plus significant heat is created in the drilling process, which damages the resin binders of the composite material.
The addition of a Ti layer introduces an even more damaging chip that can erode the corners and inner diameter of the drilled hole and, therefore, create a sloppy fit between the panel and fastener.
CFRP-Ti stackups, where a layer of CFRP is either glued or fastened to a Ti panel, are most commonly used for the fuselage of an aircraft. Thousands of holes that are typically 0.64 cm diameter need to be drilled through the stackup to fasten the panels to the frame of the aircraft.
The manufacturing process of an aircraft wing involves starting with oversized sheets of CFRP that have been formed into the approximate size and shape required. An overhead gantry mounted CNC milling spindle is then often used to trim the sheet to the finished dimensions.
Dust-like chips created in this machining operation are very abrasive and there is a large amount of heat generated. The heat generated in the milling process, just like in drilling, can soften the binders in the resin that holds the layers of carbon-fiber fabric together. This softening can lead to the layers delaminating and eventually failing. This downside can be eliminated by using CO2 through-tool cooling, which significantly reduced the heat in the cut zone, while maintaining a dry machining environment.
Composite Stackup Drilling Issues
Focusing on the drilling of CFRPs, it is necessary to contend with three primary issues.
First, CFRPs have very low heat conductive and storage properties. This issue forces relatively slow cutting speeds to maintain an acceptable equilibrium between heat generation and heat disposal.
Second, CFRPs are very abrasive. This issue makes diamond-reinforced tools desirable, but they are heat sensitive by themselves.
And lastly, CFRP layups can easily delaminate on the exit side of a drilled hole. This issue must be handled in part by maintaining low matrix material temperatures. Other additional measures can include sacrificial backing material, the addition of an outer scrim cloth layer, and controlling drill exit forces.
When drilling a CFRP-Ti stackup, a frequent challenge for airframe manufacturers is to hold a consistent hole size in the softer composite. Usually the composite is the top layer and is drilled first, then the titanium is drilled with the titanium chips passing through the composite. Coolants such as water and oil, or misting of oil, are often used with stackup structures.
These conventional coolant approaches do not provide optimal cooling for machining of these stackup materials, which often results in poor hole surface quality or holes out of dimensional tolerance.
Composite-titanium (CO-Ti) stackups are utilized in the fuselage of most new aircraft. In part due to the size of aircraft frame assemblies, it is not feasible to clean liquid coolants or oils from them during manufacturing. Therefore, the drilling operations performed to secure fuselage panels to the frame are typically done dry. Drilling through Ti without any coolant is a difficult process that is very time consuming. The feed rate of the drill bit through the Ti layer must be kept very slow to keep the created chips light, fluffy, and relatively cool, so they do not damage the CO layer.
CO2 Cooling Testing
Through work supported by the National Science Foundation and Department of Energy, researchers from Cool Clean Technologies have shown that CO2 through-tool cooling can significantly increase productivity while maintaining required hole tolerances in both the composite and Ti layers.
A series of tests were conducted to compare the effectiveness of CO2-based cooling sprays over traditional machining, both dry and flooded, of CFRP and CFRP stackups. The key metrics to be evaluated were drilling temperature, tool life, hole quality, productivity, and energy savings.
The experimentation and testing performed utilized an EFCS (environmentally friendly coolant system) to provide the CO2 coolant. The EFCS, designed and manufactured by Cool Clean Technologies, is capable of delivering CO2 coolant to the tool-work piece interface in two methods. The first method is used for drilling and delivers CO2 coolant through coolant ports in the tool. The second method, which delivers CO2 through an external nozzle, is used for milling.
The tooling used was carbide drills 6.337 mm diameter with a WD1 coating. The carbide drill went through a stackup of composite and titanium, with the composite component being 10.16 mm thick on the top and the 12.7-mm thick titanium component being on the bottom. Two panels with 32 holes each were drilled, equaling a total of 1463 mm of drill travel.
Work was performed on a vertical CNC mill that ran one panel of 32 holes over the course of about 8.5 minutes, with each hole taking about 16 seconds. The drill and stackup conditions were inspected at the end of each panel.
During the drilling process the temperatures of the drill, the composite, and the titanium were recorded. Temperature data was measured with a laser IR gun as the drill broke through the bottom of the part. This temperature measurement process was not ideal as it likely overestimated the temperature and thus underestimated the cooling benefit. Nevertheless, the data did show the general trend of lower temperatures, ranging from 20°C for the titanium layer measurements to 27°C for the drill and composite temperatures.
The EFCS process produced composite hole diameters in close tolerance with the hole size in the titanium. The composite hole diameter drilled with the flood coolant was significantly larger than the titanium hole, thus causing the composite hole size to increase above the 6.35 mm +0.0762 mm customer specification.
The EFCS system provided a significantly better hole size tolerance within specification, as compared to the holes drilled with conventional coolant. The flood coolant produced a significantly larger hole in the composite during the first half of its tool life. Improvement occurred as the drill cutting edge was dulled resulting in smaller chips causing less damage to the composite hole size. The EFCS system produced a hole with 0.0254 mm variability vs. 0.1524 mm for conventional coolants, or 6x less variability.
Productivity and Tool Life
The researchers verified that the CO2 coolant is able to provide productivity increases of 30% or more when compared to conventional coolants in drilling CFRP-Ti stackups and composite milling. This processing time reduction combined with an increased tool life of 10% or higher, an improved surface finish by up to 2x, and a dry work environment to contribute to energy savings on the order of 17%, reduce carbon emissions by 100% and improve the bottom line of the users by 22.5%.
Based solely on the productivity and tool life cost savings achievable with the CO2 over flood coolants, not even taking into account the dollars related to energy and flood coolant maintenance and cleanup, the typical payback on a CO2 coolant system is less than one year.
CO2 coolant provides a significant reduction in tool and work piece temperature when compared to a dry process. The ability to put your hand on the tooling immediately after milling is proof that the work piece and cutter are kept at a temperature below 43°C. This is impossible to do when using traditional liquid coolant.
By keeping the temperature of both the composite work piece and tooling down near ambient conditions, no degradation to the resin binders occurs. This significant temperature reduction also leads to an increase in feed rate and therefore productivity. Finish cuts with CO2 are smoother than when run dry, almost polished looking. Under a microscope, no inclusions or gouging caused by the milling process are evident.
This article is based on SAE International technical paper 2014-01-2234 by Nelson W. Sorbo and Jason J. Dionne, Cool Clean Technologies.