Dual-beam laser drilling process for thick carbon fiber reinforced plastic composites plates

Abstract

The laser ablation has been proved to be an effective way to machine carbon fiber reinforced plastic composites (CFRPs). Previous experimental studies on laser drilling of CFRPs are usually limited to thin plates. For thick (above 10 mm) CFRPs plates, the studies on feasibility, efficiency and dimensional accuracy of laser drilling processes are still missing. This study presents a novel dual-beam opposite dislocation (DBOD) laser drilling process developed for thick CFRPs machining. By utilizing DBOD process, for the first time, the thickness of the CFRPs specimen reaches up to 10 mm, which has been significantly improved compared to other studies. In order to investigate the material removal mechanisms and heat-affected zone (HAZ) effect of laser drilling process, a three-dimensional heat flow model was established by using finite element method (FEM). Comprehensive ablation experiments on fiber layers of CFRPs were conducted to validate the theoretical models. In addition, two types of drilling experiments have been performed on thick CFRPs plates to study the feasibility, efficiency and quality of laser drilling by DBOD process as compared to the single-beam process. It was found that for single-beam process, material removal efficiency could be divided into three stages: rapid removal stage (Stage I), stationary removal stage (Stage II), slow removal stage (Stage III), and DBOD laser drilling process could maintain the drilling process on the first two stages (Stage I &II), which improve the manufacturing efficiency. The experimental results showed that DBOD drilling process can improve the processing efficiency by almost two times with a much smaller HAZ at the hole entrance or exit compared to single-beam process. Hence, the DBOD laser drilling has the potential to be implemented on thick CFRPs drilling process.

Introduction

In a decade, the demand for light-weight composite materials has strongly increased in aerospace and automobile industries. Carbon fiber reinforced plastic composites (CFRPs) offer a high strength-to-weight ratio, high modulus-to-weight ratio, excellent damage and corrosion tolerance, and good fatigue resistances, which makes them a top candidate in medical, aerospace, and automotive applications (Dandekar and Shin, 2012). However, as reported by Shyha et al. (2009) and Sorrentino et al. (2019), since the connection of CFRP components still remains as a challenge, drilling holes on CFRPs is inevitable and difficult in the assembly process due to the complex structures of the CFRPs. For example, in order to obtain uniform and isotropic mechanical properties, CFRPs are usually woven with multidirectional fiber orientations (MD CFRPs). Additive manufacturing technique enables the fabrication of continuous CFRP composites with complex carbon fibers layout and customized MD CFRPs structures (Yu et al., 2019). Fu et al. (2018) found that high drilling qualities are more difficult to obtain from MD CFRPs than unidirectional CFRPs due to the thermal effects. Hence, some of their applications have often been hindered due to the tremendous difficulties in the drilling processes with low manufacturing costs and high machining quality, especially for thick CFRPs plates (Kumar et al., 2018).

There are two categories of CFRPs drilling processes including conventional methods (e.g. twist-drilling, etc.) and unconventional methods (e.g. water-jet, laser-assisted methods, etc.). For twist-drilling, drilling-induced defects can be frequently generated (such as delamination, burr or cracks) and special drill bit geometries are applied to obtain high drilling quality. Tsao (2007) used a core-saw drill to obtain smaller delamination of CFRP laminates. Marques et al. (2009) and Feito et al. (2018) demonstrated that a special step drill had encouraging results in terms of delamination reduction and low thrust force on drilling CFRPs. Jia et al. (2016) designed an intermittent-sawtooth structure drill to minimize the possibilities of drilling-induced defects of CFRPs. However, owing to the rapid tools wear during drilling of CFRPs, the repeatable high quality of CFRPs drilling still remains a big challenge. Alberdi et al. (2015) conducted abrasive water-jet drilling experiments on CFRP/Ti6Al4V stacks, and better surface roughness of specimen was observed than mechanical drilling. However, the disadvantages of water-jet drilling include difficulties of wastewater handling, moisture absorption of CFRPs, as well as the low processing speed as reported by Sala (2000); Moeller (2007) and Wang et al. (2018).

Laser beam machining techniques provide an excellent alternative for processing CFRPs due to tool wear-free and high machining flexibility as reported by El-Hofy and El-Hofy (2018). However, excessive heat affected zone (HAZ) is one of the main limitations of laser ablation, which affects mechanical and thermal properties of CFRPs. To minimize the HAZ, several investigations have been conducted. Riveiro et al. (2012) used a continuous wave CO₂ laser to cut CFRPs plate with a 3 mm thickness. A minimum HAZ of 540 um was obtained. Kalyanasundaram et al. (2018) performed fatigue tests after fiber laser drilling of CFRPs with a 2.7 mm thickness. They found that the damage growth of the HAZ was similar to that of conventional machined. To improve single photon energy, Takahashi et al. (2016) conducted CFRPs cutting experiments of a 2 mm thickness with IR and UV laser. They found that the UV laser produced higher processing quality than IR laser. To reduce the laser-matter interaction time, picosecond pulsed laser was applied for the cutting/drilling of CFRP sheets with a 1.5−2 mm thickness. It was demonstrated that ultra-short pulse laser has significantly improved the laser processing quality with no visible HAZ as shown by Wolynski et al. (2011) or Hu and Zhu (2018). Staehr et al. (2019) used a new nanosecond pulsed laser to drill a 3 mm thick MD CFRPs, and a borehole diameter of 4.8 mm was obtained with a visible HAZ.

However, while using a laser cuts thick CFRPs (above 10 mm) plates, the processing efficiency and quality are ruined by the ejection of a plume, which is generated by a large amount of plasma and evaporated materials. Hence, a parallel pass process of laser cutting is necessary and proposed by Herzog et al. (2016). It used a scanner system to move the laser beam shifted in parallel to broaden the kerf width and CFRPs plates with a thickness of up to 12.7 mm had been cut.

It can be seen that, previous studies for CFRPs with laser ablation is limited on cutting. For CFRPs drilling, it is usually limited to thin plates of 3 mm thickness or less. For thick CFRPs (above 10 mm), the feasibility, efficiency and dimensional accuracy of laser drilling are still unknown. Hence, laser drilling for thick CFRPs plate is of great urgent to be investigated in detail. This paper presents a novel opposite dislocation dual-beam (DBOD) laser drilling process on thick MD CFRPs with 100 W picosecond laser of 1064 nm wavelength. It uses two scanner head systems to move the laser beam and two varioSCAN systems to retrace the focus position automatically. For the first time, the specimen thickness of laser drilling reaches up to 10 mm. In order to investigate the material removal mechanisms and HAZ of laser drilling, a three-dimensional heat flow model was established by finite element method (FEM). Comprehensive ablation experiments on fiber layers of CFRPs were conducted to validate the theoretical models. Two types of drilling experiments have been performed on thick CFRPs to study the drilling the feasibility, efficiency and quality by DBOD process as compared to single-beam process. It was found that for single-beam process, material removal efficiency could be divided into three stages: rapid removal stage (Stage I), stationary removal stage (Stage II), slow removal stage (Stage III), and DBOD laser drilling process could maintain the drilling process on the first two stages (Stage I &II), which improve the manufacturing efficiency. The experimental results showed that DBOD drilling process can improve the processing efficiency by almost two times with a much smaller HAZ at the hole entrance or exit compared to single-beam process. Hence, the DBOD laser drilling has the potential to be implemented on thick CFRPs drilling process.