Fast processing speeds without carbon fiber damage are possible
RAJESH PATEL, JAMES BOVATSEK, and MASAYUKI FUJITA
A strong drive to increase fuel efficiency and lower carbon emissions in transportation industries is driving the use of carbon fiber-reinforced polymer (CFRP) material in the fabrication of various aircraft and automobile parts. As the material's cost continues to decline, demand and use for it will further increase. CFRP is a lightweight, strong, and durable material with good corrosion and vibration resistance, making it a good candidate to replace many metal parts. An optimally designed CFRP part can be up to 70% lighter than steel and 30% lighter than aluminum—attributes that make CFRP attractive for use in many non-transportation-related industries such as components for wind energy production, sports equipment, oil exploration equipment, and consumer electronics products.
The attributes that make CFRP a unique and useful material also make it difficult to machine with high quality. Manufacturers using CFRP in their products are also looking to decrease fabrication costs. Conventional mechanical and abrasive waterjet cutting techniques are costly because of high tool wear and operating costs, and fiber fracture and delamination of material during machining is common and results in yield loss. The use of lasers for machining provides the advantages of a non-contact process and ease of automation in manufacturing environments. Laser machining eliminates tool wear and gradual degradation in quality associated with mechanical techniques, and reduces operating costs. Furthermore, fiber damage and delamination of material during machining can be reduced or eliminated. However, there is a key challenge for laser machining of CFRP, which is to machine it with both high throughput and with minimal heat-affected zone (HAZ) formation in the material.
Pulsed UV laser capabilities
High-power continuous-wave (CW) infrared (IR) wavelength lasers with multi-kilowatt power levels can machine CFRP at higher speeds, but leave the material with unacceptably large HAZ [1-3]. On the other hand, ultrafast lasers can provide low HAZ, but usually machine materials at slow speeds [4-6]. So, the challenge is to find a laser source and process that deliver a good balance of speed and quality. Pulsed nanosecond lasers have shown moderate processing speeds with reasonable quality, with the wavelength often having a significant impact on results achieved. In particular, stronger absorption at the ultraviolet (UV) wavelength results in good-quality machining. A laser such as Spectra-Physics' high-power Quasar UV laser with up to >60W UV and TimeShift pulse-shaping technology is uniquely suited to ablate, cut, and drill CFRP material without damaging the fibers, and deliver both high speed and quality.
To demonstrate the capability of the Quasar UV laser, we machined 250μm-thick polyacrylonitrile (PAN)-based CFRP plate material (FIGURE 1). We varied the pulse width, power, repetition rate, and scanning speed. We also tested the burst machining capability provided by Quasar's technology. The cutting speed and HAZ—here defined as the average length of exposed fibers along the cut line—were characterized for various conditions.
|FIGURE 1. The Spectra-Physics logo cut in ~1mm-thick CFRP plate.|
Results show that both speed and quality are achieved with this laser. The smallest HAZ of ~15μm was achieved using 2ns pulses (FIGURE 2). This is an average over a number of process conditions, and in some cases the HAZ was effectively zero. Burst machining proved to be advantageous, achieving 20–50% higher cutting speeds for the same average power (FIGURE 3). Through additional process development and optimization efforts, we have shown that this laser can cut 250μm-thick CFRP plates at 70mm/s with HAZ of < 15μm (FIGURE 4).
|FIGURE 2. The effect of pulse duration on HAZ.|
Besides cutting and drilling, bonding and joining of CFRP parts are very important processes since traditional riveting and other types of fastening techniques require drilling holes in the material, which can be detrimental to the strength of the part because of fiber damage during the drilling process. Thus, adhesive bonding is desirable and a commonly used technique for joining of CFRP parts. However, the parts need to be cleaned of various residues and debris imparted to the surface by the molding process. A thorough cleaning and surface texturing of the part without damaging the fibers is crucial to achieve higher joint strength.
|FIGURE 3. The effect of power, repetition rate, and pulse duration (including burst of pulses) on cutting speed.|
Painting of CFRP parts is also challenging because of low surface wettability and poor surface adhesion, both of which are improved with laser processing. A thorough laser cleaning and texturing of the surface is necessary prior to painting to improve wettability of the surface. Research shows that parts textured with a UV laser have higher lap shear strength compared to those textured with an IR laser . Using the Quasar with >60W average power, we have demonstrated an area cleaning and surface texturing rate of 80mm2/min without any visible damage to the fiber (FIGURE 5).
|FIGURE 4. A scanning electron microscopy (SEM) image of a CFRP sample showing excellent laser cut quality.|
We demonstrate that pulsed UV nanosecond lasers are promising for CFRP processing. With the Quasar laser, the high power and programmable pulse width/shape result in both high quality and speed for CFRP processing, including cutting, drilling, surface texturing, and cleaning. With this high-power laser, we demonstrate that low HAZ without damaging the carbon fibers can be achieved with fast processing speeds. Further work is actively being pursued to extend to other configurations of processes and thicker materials, and to expand the study of the effect of laser parameters on machining CFRP.
|FIGURE 5. Surface texturing and hole drilling in CFRP without any visible fiber damage using the Quasar UV laser.|
Quasar is a registered trademark of Spectra-Physics, and TimeShift is a trademark of Spectra-Physics.
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