Abstract
Rotary friction welding of wood typically uses dowels made from the same material as the base wood or involves specific modifications to the dowels, but these methods have practical limitations and are complex. This study focused on commonly used dowel materials (softwood: Scots pine, hardwood: birch), with moisture content adjusted to 7 to 10%, and examined the welding performance and micro-mechanisms. Through orthogonal experiments, the influence of process parameters on the welding strength of both wood types was systematically investigated. The microstructures of the welded areas were analyzed using a depth-of-field microscope and scanning electron microscope (SEM) to explore the friction mechanisms. The results indicated that both Scots pine and birch dowels can be effectively welded using rotary friction. The optimal parameters were identified as follows: Scots pine dowels—hole diameter ratio of 8/12, rotational speed of 3000 r/min, feed rate of 25 mm/s; birch dowels—hole diameter ratio of 8/12, rotational speed of 2500 r/min, feed rate of 20 mm/s. Depth-of-field microscopy revealed larger weld areas and well-preserved surface structures. SEM images showed that during welding, the materials between the dowels and base wood melted, flowed, and re-solidified into a tightly bonded structure, ensuring a durable connection.
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Investigation of Joining Performance and Microstructural Mechanisms of Softwood and Hardwood Dowel Joints via Rotary Friction Welding
Jian Zhang 
,a Yuelong Li ,a,* Hai Zhu,a Feng Zhang,a Yuchen Zhang,a Zhenhe Li,a
Yanfeng Li,b and Yujia Liu b
Rotary friction welding of wood typically uses dowels made from the same material as the base wood or involves specific modifications to the dowels, but these methods have practical limitations and are complex. This study focused on commonly used dowel materials (softwood: Scots pine, hardwood: birch), with moisture content adjusted to 7 to 10%, and examined the welding performance and micro-mechanisms. Through orthogonal experiments, the influence of process parameters on the welding strength of both wood types was systematically investigated. The microstructures of the welded areas were analyzed using a depth-of-field microscope and scanning electron microscope (SEM) to explore the friction mechanisms. The results indicated that both Scots pine and birch dowels can be effectively welded using rotary friction. The optimal parameters were identified as follows: Scots pine dowels—hole diameter ratio of 8/12, rotational speed of 3000 r/min, feed rate of 25 mm/s; birch dowels—hole diameter ratio of 8/12, rotational speed of 2500 r/min, feed rate of 20 mm/s. Depth-of-field microscopy revealed larger weld areas and well-preserved surface structures. SEM images showed that during welding, the materials between the dowels and base wood melted, flowed, and re-solidified into a tightly bonded structure, ensuring a durable connection.
DOI: 10.15376/biores.20.3.6267-6285
Keywords: Wood rotary friction welding; Orthogonal experiment; Process parameters; Melting and flow; Welding mechanisms
Contact information: a: College of Mechanical and Electrical Engineering, Northeast Forestry University, No. 26 Hexing Road, Xiangfang District, Harbin, Heilongjiang, China; b: Qiqihar Heping Heavy Ind Grp Co Ltd, Qiqihar 161000, Heilongjiang, Peoples R China;
* Corresponding author: endlesszhang@163.com
INTRODUCTION
Wood is composed of natural polymers, mainly cellulose, hemicellulose, and lignin. Rotary friction welding of wood dowels involves the generation of heat through friction between a high-speed rotating dowel and a pre-drilled hole in the solid wood, leading to the softening and fusion of lignin and hemicellulose, which form a structure. After cooling, this process results in a high-strength welded interface. Due to its rapid processing, absence of additives and harmful gases, high strength, and high material recovery rate, this technique is considered a green and environmentally friendly method, positioning it as an innovative technology in wood joining. It has made substantial progress, particularly in the development of glue-free bonding in Europe. In comparison with traditional adhesive and metal joining methods, wood welding avoids problems such as formaldehyde release, poor durability, and metal corrosion, making it widely used in applications such as construction and furniture manufacturing (Zhou et al. 2014). In wood rotary friction welding, key factors influencing the welding process include the intrinsic properties of the wood (species, grain direction, etc.) and process parameters (dowel rotational speed, feed rate, and the hole diameter ratio between the dowel and the pre-drilled hole in the base material). Additionally, the pre-treatment of both the dowel and the base material significantly affects the welding performance (Luo et al. 2017). Current research on the rotary friction welding of wood dowels mainly has explored changes in welding process parameters, physical and chemical transformations during welding, as well as the heat treatment and water resistance of the materials. Budhe et al. (2017) demonstrated that at elevated temperatures, resin matrices and adhesives undergo softening, leading to an increase in viscoelastic response. However, the fibers themselves do not degrade. Additionally, temperature fluctuations contribute to progressive debonding and weakening at the material and fiber/matrix interface, primarily due to differences in thermal expansion coefficients between the fibers and resin.
Leban et al. (2004) utilized X-ray microdensitometry to examine vibration-welded hardwoods (beech and oak) and softwoods (spruce), revealing a significant increase in wood density at the adhesive interface. The disappearance of intercellular structures at the interface, reduction in voids, and increase in density were positively correlated with bonding strength, with more uniform and pronounced densification yielding stronger joints. Rodriguez et al. (2010) investigated the feasibility of high-speed rotary dowel welding in Canadian hardwoods (sugar maple and yellow birch). The study assessed variables such as wood species, grain orientation, rotational speed, and receiving hole diameter. Results indicated that this technique was suitable for both wood types, with average tensile strength comparable to polyvinyl acetate (PVAc) adhesive-bonded joints. Gfeller et al. (2004) demonstrated that mechanically induced wood flow welding enables the rapid formation of structurally sound joints without the need for adhesives.
The underlying mechanism primarily involves the melting and flow of amorphous polymers, mainly lignin and partially hemicellulose, facilitating interconnection between wood cells. During welding, long wood cells and fibers partially separate, forming an entangled network within the molten lignin matrix, which subsequently solidifies into a fiber-reinforced composite. Simultaneously, some unbound wood fibers are expelled, while crosslinking reactions occur between lignin and furfural. Zhang et al. (2017) welded birch dowels to domestic larch and European spruce substrates, investigating the impact of pre-drilled hole-to-dowel diameter ratios on pull-out resistance and comparing these with direct insertion methods. The study showed that the optimal hole diameter ratio was 9.5/10 for larch substrates and 8.5/10 for spruce substrates. It was emphasized that the pre-drilled hole should be smaller than the dowel to ensure effective friction between the dowel and the wood substrate (Wang and Jin 2019; Zhu et al. 2020; He et al. 2024). The addition of certain additives or solutions (Pizzi et al. 2005; Kubovsky et al. 2020) has been shown to improve welding strength within specific ranges. Kubovský et al. (2020) found that heat-treated wood exhibited excellent resistance to corrosion, pests, and moisture, enhancing its dimensional and shape stability. Roszyk et al. (2020) highlighted the potential of heat-treated wood in construction and humid environments. Li et al. (2024) used bamboo dowels instead of wood dowels for rotary friction welding with beech, achieving an adhesive strength of 6.42 MPa, surpassing that of white glue (PVAc).
Scots pine and birch are commonly used wood species in daily life and are high-quality materials frequently utilized in furniture and construction. Heat-treated Scots pine exhibits improved dimensional stability, reducing moisture-induced swelling and shrinkage, and enhancing the wood’s resistance to decay and microbial degradation. This treatment significantly boosts the wood’s durability and creates a more stable chemical structure, which increases its resilience to environmental factors such as ultraviolet light and precipitation, making it more suitable for outdoor applications. Additionally, the heat treatment deepens the wood’s color, giving it a darker brown or reddish-brown hue, which enhances its aesthetic appeal (Zhang et al. 2024). In this study, thermally treated Scots pine, a physically modified wood material processed at high temperatures (ranging from 188 to 220 °C) to enhance its properties, was uniformly used as the substrate. Given that the experimental substrate was thermally treated Scots pine, the dowels were also selected from Scots pine to ensure compatibility in terms of thermal expansion coefficient. In contrast, materials such as metal or adhesive have significantly different expansion coefficients from the substrate, which may lead to deformation at the joint due to fluctuations in temperature, humidity, and moisture content, ultimately compromising welding strength. Although using wood with a matching expansion coefficient in the mortise-and-tenon structure can effectively mitigate deformation issues, the machining process is relatively complex and incurs higher manufacturing costs. Furthermore, as Scots pine is a softwood, this study also aims to investigate hardwood (birch wood.) as a dowel material to examine the impact of using materials with different vs. identical expansion coefficients on welding strength. Notably, both dowel materials are widely used in the furniture manufacturing industry and are well-accepted in the market. They offer advantages such as affordability, aesthetic appeal, and stable composition, making them highly promising for practical applications. Using orthogonal experiments, the effects of different dowel species on the tensile strength and welding performance of wood rotary welding are investigated. This research aims to provide reference data for optimizing wood welding techniques.
EXPERIMENTAL
Materials
The dowel materials used in this experiment were Scots pine and birch, both of which were smooth dowels with a grain orientation parallel to the wood fibers. The dowels had diameters of 10, 11, 12, and 13 mm, with a uniform length of 100 mm. To facilitate welding, the welding section of each dowel was pre-machined with a chamfer of 8 mm in length at a 45° angle.
The substrate material was thermally treated Scots pine with dimensions of 100 mm × 50 mm × 20 mm (length × width × height). Prior to welding, through-holes with an 8 mm diameter were drilled into the substrate. The moisture content of the dowels was first measured to ensure that it met the target range. If the moisture content deviated from the required range, manual adjustments were made. Ultimately, the moisture content of both the dowels and the substrate was controlled within 7% to 10%. Detailed material parameters are presented in Table 1, while the specific structure of the welded specimens and their through-holes are illustrated in Fig. 1.
Table 1. Specific Parameters of Each Material