Abstract
Laser machining is an advanced technology that provides efficiency and precision for the processing of wood. In this paper, the ablation mechanism of wood processed via a water-jet assisted nanosecond laser was analyzed. The influences of cutting speed and laser power on the cutting width of northeast China ash wood (NCAW) (Fraxinus mandshurica Rupr.) with and without the water-jet assisted system were evaluated. The surface morphology of the kerf of processed NCAW was observed via scanning electron microscopy (SEM). Furthermore, a factorial design experiment was carried out to analyze the effects of process parameters on the cutting width. Additionally, the experimental results were processed by multilinear regression analysis. The results showed that with the water-jet assisted system, the minimum value of the cutting width was 0.18 mm when the cutting speed was 50 mm/s and the laser power was 6 W, and good surface quality was obtained. The experimental results were processed by an analysis of variance and multilinear regression analysis. The predicted model, effectively validated by the experiments, had good prediction accuracy, which provided a theoretical basis for predicting the cutting width of NCAW processed by a water-jet assisted laser.
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Water-jet Assisted Nanosecond Laser Microcutting of Northeast China Ash Wood: Experimental Study
Chunmei Yang,a,b Ting Jiang,a,b,* Yueqiang Yu,a Yan Bai,a Mingliang Song,a,b Qian Miao,a,b Yan Ma,a,b,* and Jiuqing Liu a,*
Laser machining is an advanced technology that provides efficiency and precision for the processing of wood. In this paper, the ablation mechanism of wood processed via a water-jet assisted nanosecond laser was analyzed. The influences of cutting speed and laser power on the cutting width of northeast China ash wood (NCAW) (Fraxinus mandshurica Rupr.) with and without the water-jet assisted system were evaluated. The surface morphology of the kerf of processed NCAW was observed via scanning electron microscopy (SEM). Furthermore, a factorial design experiment was carried out to analyze the effects of process parameters on the cutting width. Additionally, the experimental results were processed by multilinear regression analysis. The results showed that with the water-jet assisted system, the minimum value of the cutting width was 0.18 mm when the cutting speed was 50 mm/s and the laser power was 6 W, and good surface quality was obtained. The experimental results were processed by an analysis of variance and multilinear regression analysis. The predicted model, effectively validated by the experiments, had good prediction accuracy, which provided a theoretical basis for predicting the cutting width of NCAW processed by a water-jet assisted laser.
Keywords: Nanosecond laser; Water-jet assisted system; Northeast China ash wood; Factorial design experiment; Micromorphology
Contact information: a: Northeast Forestry University, College of Mechanical and Electrical Engineering, Harbin, 150040, China; b: Forestry and Woodworking Machinery Engineering Technology Center, Northeast Forestry University, Harbin 150040, China;
* Corresponding authors: jiangting1112@163.com; ycmnefu@126.com; nefujdljq@163.com
INTRODUCTION
Wood is a natural and environmentally friendly material that is widely used in the construction, packing, furniture, and flooring industries (Fleming et al. 2003; Seo et al. 2011; Fukuta et al. 2016a). Because of its widespread use, improving the utilization ratio of wood resources has become the focus of researchers. The tools used in traditional processes have a certain geometric shape, which leads to the waste of wood resources in the process of wood processing. The saw dust left over after processing also pollutes the environment. Laser processing technology has advantages over traditional processes, including high efficiency, no noise, and narrow cutting width, which makes up for the deficiency of traditional processes. Therefore, it is widely used in fields such as cutting, engraving, and surface treatment of wood and wood composites (Barcikowski et al. 2006; Leone et al. 2009; Eltawahni et al. 2011; Vidholdová et al. 2017).
Recently, researchers have gradually focused on fine processing of wood. Nukman et al. (2008) investigated CO2 laser cutting of Malaysian hardwood. They outlined the relationship between processing parameters and types of wood with different properties in terms of optimum cutting conditions. Additionally, they showed that an acceptable surface was obtained when nitrogen was used to assist the cutting process. Fukuta et al. (2016b and 2016c) used solid-state lasers, short-wavelength lasers, to process wood to obtain precise machining because short wavelengths allowed for a smaller theoretical focal diameter. In addition, short-pulse lasers allowed heat effects to the surrounding area to be controlled while increasing the peak power. Hernández-Castañeda et al. (2011) concluded that cutting wet pine wood with a fiber laser produced narrower cutting kerf and a smoother surface in comparison with dry samples. However, after the wood was cut with a laser, assisting gas could not fully keep residues away from the wood surface, which affected the surface quality. Therefore, in the author’s previous research, a water jet assisted nanosecond laser was used to process Korean pine to improve its surface quality. This was mainly because the cooling and washing effects of water-jet had reduced heat-affected zone and washed away the residues on the surface of the Korean pine, which avoided the process defect of traditional laser (Yang et al. 2018). However, different species of wood have different physical and chemical properties that directly affect the surface quality of the wood. The authors found no research that has investigated the machining of hardwood processed by a water-jet assisted nanosecond laser. Northeast China ash wood (NCAW) is a type of precious hardwood tree in northeast China with a high density. It is widely applied in the furniture industry because of its beautiful texture and good resistance to decay.
Therefore, this paper analyzes the ablation mechanism of wood processed by a water-jet assisted nanosecond laser. A single factor experiment and a factorial design experiment were performed to evaluate the influences of laser power and cutting speed on the cutting width of NCAW with and without the water-jet assisted system. Furthermore, the surface quality of NCAW after cutting was assessed, and the prediction model of multilinear regression was established. This research provided a foundation for the prediction of the cutting width of NCAW processed by a water-jet assisted nanosecond laser.
PROCESS MECHANISM
The ablation mechanism diagram of wood processed by a water-jet assisted nanosecond laser is shown in Fig. 1. In the machining process, the high energy density of the laser beam rapidly focuses on the processing region of the wood. The decomposition of lignin starts at approximately 280 °C (Nassar and MacKay 1984). Therefore, the internal moisture within the wood cell evaporated quickly, which resulted in empty tracheids. Due to a rapid change of temperature, a great thermal stress gradient occurred around the processing region. At the same time, local expansion and contraction took place in the substrate of the wood. Hence, microcracks caused by residual stress appeared. Furthermore, due to the lower ignition temperature of wood, part of the laser energy density in the processing zone was always lower than the laser energy density for the vaporization of wood, causing the burning of wood. Thus, carbonization and ablation of the heat-affected zone (HAZ) are observed, along with residuals, such as carbon granules, charring, and ash content, which has a negative impact on the surface quality of wood (Panzner et al. 1998; Yue et al. 2013; Kubovský and Kačík 2014; Yu et al. 2015). However, water jets not only can reduce the heat-affected zone and microcracks, they also effectively take away residuals, thus improving the surface quality of wood processed by water-jet assisted nanosecond lasers.
Fig. 1. The ablation mechanism diagram of wood processed by water-jet assisted nanosecond laser
EXPERIMENTAL
Materials and Experimental Setup
The NCAW, belonging to a typical hardwood, was used as the experimental material (Haicheng Machining Factory, Harbin, China). The NCAW has an air-dry density of 0.70 g/cm3 and a moisture content of 12.32%. A NCAW with the same size was obtained after processing, and the size was 100 mm × 60 mm × 2 mm (length × width × thickness).
The experimental setup is shown in Fig. 2.
Fig. 2. Experimental setup
The laser system contained a Nd:YAG (Neodymium Doped Yttrium Aluminum Garnet) laser (Dongjun Laser Co., Ltd., Chengdu, China), a JDW3-250 laser power supply (Dongjun Laser Co., Ltd., Chengdu, China), and a PH-LW06-BLP laser cooling system (Dongluyang Co., Ltd., Shenzhen, China). The water-jet assisted system consisted mainly of annular nozzles. The feeding system moved through the X-axis, Y-axis, and Z-axis. The focusing system consisted of a combination of optical lenses.
The wavelength of the laser was 1064 nm, the pulse width was 20 ns, and the pulse repetition rate was in the range of 1 kHz to 10 kHz. The focal length of the focusing lens was 100 mm. The focused beam spot size was 0.05 mm. The angle of water jet could be adjusted to control the range of the water ring. The adjustable nozzle flow pressure was set as 0.13 MPa. The water flow of the water-jet system was 8 m3/ h, and the water-jet speed was 5.75 m/s.
Methods
The process direction was along the NCAW fiber. The specimen and process method are shown in Fig. 3. The focused beam spot was located on the top of the surface of the NCAW. The cutting width was the average of the widths on the top and bottom surfaces along the thickness direction, which were measured using an optical microscope (JNOEC XS213, Tianlong Instrument Factory, Nanjing, China). The tangential section of the processed NCAW with and without water-jet assist was observed using a FEI Quanta200 scanning electron microscope (FEI Instruments, Hillsboro, OR, USA).