Laser cutting high performance materials

The growing use of carbon fibre reinforced plastic (CFRP) composites as high performance material in aerospace and automotive industry has prompted studies in developing technologies for machining of composites in recent years. Use of lasers for cutting and drilling of composites is attractive due to its flexibility and its being free from tool wear.

CFRP composite materials are constructed by two materials, carbon fibre and polymer matrix, that have significant different properties. The two constituents work together to give the composite unique properties, such as high strength and lightweight, which are markedly superior to those comparable metallic alloys. The nature of inhomogeneous in property and structure make the composite more prone to damage during machining than conventional metals.

Lasers as non-contacting and abrasionless machining tools exhibit unique advantages in materials processing, e.g. they eliminate tool wear, vibrations and cutting forces, while high processing speed and automation are attainable. Because of the large differences, e.g. one or two orders, in vaporisation temperature and thermal conductivity between the two constituents, a challenge in laser processing is to minimise thermal damage to the epoxy matrix and to retain high processing speed simultaneously. Process qualities, such as heat affected zone (HAZ), charring, delamination and epoxy recession due to intense thermal damages, are major obstacles for industrial applications of laser machining composites in the past.

Drilling composites of various thicknesses

In the research conducted by the Singapore Institute of Manufacturing Technology team of experts, the group focused on the results of process development in drilling CFRP composites using UV laser.

CFRP composites with thicknesses of 0.3 mm to 7 mm were used for the study. An Avia-X high power Q-switched third-harmonic Nd:YVO 4 diode pumped solid state laser (DPSS) system was used for the experiments. The laser is operated at a wavelength of 355 nm and repetition rates ranging from 10 kHz to 100 kHz. The pulse duration of the output beam is 20 to 35 ns depending on the repetition rate. The maximum average output power is 10 W. The output beam profile is Gaussian in shape. The laser beam was delivered to the CFRP composite substrate using a galvanometric scanner with an f-theta lens achieving a spot size of 25 μm (1/e 2). The focal plane for drilling thicker sample (>1 mm) was first set at the surface. As drilling progress, the focal plane was periodically moved towards beam incident direction. The drilling quality was examined using optical microscopy and scanning electron microscopy (SEM).

An array of nine holes of 2 mm diameter was drilled to offset the possible effects of non-uniform fibre distribution and orientation in the laminate. The spacing between holes was set at 4 mm. Laser power of 10 W at repetition rate of 40 kHz was used for all experiments. The laser beam scanned along the outline of each hole and multi passes were needed to drill through the laminate. The drilling was stopped for every four passes for about 1~2 seconds to reduce heat accumulation. The nine holes were drilled through with different number of passes.

Also noted is the number of passes required to drill through a 0.3 mm thick laminate as a function of laser beam scanning speeds. The number of passes required to drill through the nine holes was found to increase with laser beam scanning speed. Enormous heat was accumulated in the laser irradiated area when the scanning speed was below 50 mm/s, which resulted in material burning after a few laser beam scans. On the other hand, at a laser scanning speed of 1000 mm/ s or above, not all nine holes were drilled through uniformly even after prolonged drilling.

The experiments were stopped after 5Nike Air Flight Huarache