Performance of Dowel-Welded T-Joints for Wood Furniture

Comparison of two types of furniture joints namely step butt T-joints and mortise and tennon T-joints held together either by one or two welded or glued dowels showed that the shear strength results of welded dowel and glued dowel joints were comparable. For mortise and tennon T-joints there is, in general, no difference if the dowel is inserted at a 45° or 90° angle. Also there is no significant difference between welded and glued dowel joints stiffness values in both step butt and mortise and tennon T-joints of the same geometry. Also, there is no significant difference between dowels inserted at 45° or 90° for mortise and tennon T-joints. Glued dowel and welded dowel step butt T-joints behave quite differently from linear joints. Thus, in step butt T-joints a higher shear strength is obtained if a single dowel is inserted at 45° for both glued and welded dowels. Both shear strength and stiffness increase as the number of dowels increases, namely from one to two. The application of the welded joint technique to joints where the number of dowels is limited by the limited space in which they can be inserted, such as in furniture, can give shear strength results comparable to those obtained by gluing the same dowels. This is particularly the case for mortise and tennon T-joints.     

Wood Blockboards Fabricated by Rotational Dowel Welding

Holding wood pieces together by rotationally welded dowels can be used to make blockboard panels with the dowels inserted in the side of the wood substrate slats. The average results for both tensile and three-point bending tests indicate that a 20° dowel insertion angle yields strength results better than 10° and 0° insertion angles. Applied load vs deformation in the two types of tests showed that blockboard panel stiffness was greater for the panels with 20° dowel insertion angle.     

Zig-zag rotational dowel welding for exterior wood joints

A zig-zag pattern of rotationally welded dowels across the interface of a butt joint between two wood planks was shown to yield strong joints without any adhesive. Tests after 2 h in boiling water showed that the joints were able to resist both 2 h immersion in boiling water, as well as the subsequent oven drying. Some unusual densification patterns were observed by X-ray microdensitometry. X-ray microdensitometry analysis of densification patterns in the dowel and at the dowel/substrate interface indicate that further improvement and optimisation of this process should consider minimizing the compression rate at the end of dowel rotation as well as minimization of the insertion impact of the dowel to avoid interfacial microfissures that can sometimes develop.     

Performance of Dowel-Welded Wood Furniture Linear Joints

Comparison of three types of furniture joints such as scarf joints, step butt joints and dovetail joints held together either by one or two welded dowels, glued dowels and steel nails showed that the dowels always gave better shear strength and greater stiffness than the steel nails. The results of welded dowels and glued dowel joints were found to be comparable. The application of the welded joint technique to joints where the number of dowels is limited by the narrow space in which they can be applied, such as in furniture, can give results comparable to those obtained by gluing the same dowels.     

Wood blockboards for construction fabricated by wood welding with pre-oiled dowels

Holding wood pieces together by rotationally welded dowels can be used to make blockboard panels with the dowels inserted in the side of the wood substrate slats. Pre-oiling the dowels with sunflower oil: (i) eases their insertion in the pre-drilled substrate, (ii) allows the insertion of dowels to a much greater depth by its lubricating action for welding, thus allowing more layers of wood to be joined and (iii) provides an improved water resistance to the welded joint. The strength of the joints prepared with this system is comparable to those already obtained with other welded dowel systems but with improved water resistance.     

Parameters influencing wood-dowel welding by high-speed rotation

Oven-dry dowels, insertion of hot dowels, cross-cut dowels, substrate holes of step-decreasing diameter as a function of depth, use of ethylene glycol or other compounds able to decrease the glass transition temperature of wood components have all been shown to contribute to improving weld joint strengths in a variety of less drastic conditions than the 10 mm/8 mm dowel/substrate hole diameter difference. The results show that once the depth of the dowel is much greater than 15 mm, then almost all the conditions used improve the weld strength. This means that the proportion of area welded in relation to the tensile strength of the dowel itself is a determining factor. The greater this area the higher the strength, irrespective of the application conditions used. Thus, over a certain welded area the dowel breaks when tested in tensile, i.e., the joint is stronger than the dowel. Temperatures > 180°C are reached during the quick welding step with the temperature decreasing in less than 1 min to 60–70°C. The same chemical reactions as occurring in vibrational welding have been shown by solid-state ¹³C-NMR analysis to also occur in dowel rotation welding. In dowel rotation welding the production of carbohydrate-derived furanic aldehydes is higher (a) from the wood material of the substrate in which the hole is pre-drilled rather than from the material of the wood dowel itself, (b) when the weld joint strength is good, and (c) when the rate of dowel insertion is higher.     

Shrink-Fitting and Dowel Welding in Mortise and Tenon Structural Wood Joints

Shrink-fitting, a common bonding technique in metal assemblies, was used for mortise and tenon wood joints. The joints had considerable strength. Shrink-fitting yielded joint strengths comparable to those obtained by using several welded dowels. Increasing the number of welded dowels, however, produced joints of higher strength than those bonded just by shrink-fitting. Combining in the same joint both dowel welding and shrink-fitting yielded joint strengths higher than those obtained by the individual techniques alone, and at the same time allowing decreasing the number of welded dowels needed.     

Performance of Dowel-Welded L-joints for Wood Furniture

Rotationally welded wood dowels were used to hold together step butt and mortise and tenon L-joints and these joints were tested against European Norms for these types of joints. A factorial experiment was carried out in which (1) these two types of joints were tested, (2) 4 or 6 dowels per joint were used, (3) dowels were inserted at either 45° or 90° to the surface of the wood, (4) two types of wood were used, beech and European ash, and (5) two types of tests were carried out, namely compression and tension tests. The strength results were used to find out which of these were the most important individual parameters and their interactions, namely type of joint, number of dowels, and type of test. The strength results were well in excess of the relevant L-joints standard specifications for wooden windows.     

Parameter interactions in two-block welding and the wood nail concept in wood dowel welding

The interactions between parameters found to be determinant in wood dowel welding by high speed rotation have been evaluated. Of these, the interactions that proved to be the most significant, in descending order, were rotation rate/dowel moisture content, followed by rotation rate/ethylene glycol, and finally, at a lower level of significance, the interactions rotation rate/dowel temperature, wood grain direction/wood species and dowel temperature/wood species. Of the individual factors, once the most determinant factor already optimized in previous studies, namely the dowel/hole diameter difference, was fixed, the most significant were wood grain direction, dowel moisture content (dryness) and wood species. The optimized process yielded excellent strength results. The regression equations developed were able to predict the strength obtainable. The torque for insertion of the dowel in the substrate hole has been measured for several cases and the results are presented. In no cases the value of the torque needed for insertion was excessive and insertion was, therefore, easy. Wood joints composed of two pieces of timber held together by a dowel welded to both of them were assembled for the first time. Two further new concepts have also been advanced and tested: (i) the conical dowel, to maximize welding area and (ii) the concept of the wood nail in which a slightly conical fast-rotating hardwood dowel is inserted rapidly into a softwood substrate into which no hole has been pre-drilled. X-ray densitometry of the samples prepared with the latter approach showed some interesting mechanical interlocking features that might contribute to dowel bonding in softwoods.     

Accelerating vs Constant Rate of Insertion in Wood Dowel Welding

Dowel insertion rates whether accelerating or constant are determining parameters for the tensile strength of welded dowel joints. Dowel insertion at acceleration up to 4–7 m/s² appears to yield the best results at the lower dowel insertion speed, characteristic of welding by manual drills. Strength results for dowel welded joints of 24 mm depths of up to 2700 N/mm² have been obtained. Constant dowel insertion rates appear instead to yield best results when the insertion rate is much higher and can be better controlled, as it is the case for a computer controlled dowel insertion equipment. Strength results for dowel welded joints of 24 mm depths of up to 4700 N/mm² have been obtained for insertion rates of 18–20 mm/s and welding time as short as 1.2 s. The results indicate that at low to medium insertion rates, the greater is the acceleration the better are the results. At fast insertion rates the acceleration has only little effect. The predominant effect is that a short welding time yields a high strength joint.     

Comparative potential of alternative wood welding systems,ultrasonic and microfriction stir welding

Two alternative welding systems were evaluated for wood welding. Ultrasonic welding produces joints of good strength but it appears to be applicable only to thin wood pieces. It does not appear that further possible process improvement could bring the joint strength to a structural level. Microfriction stir welding does show potential for welding continuously wooden plates without any limitation on length of wood pieces. The strength of the weld obtained was low due to the limited depth of the weldline. Optimisation of parameters is necessary. A drawback at present appears to be the limited thickness of the wooden pieces that can be welded. X-ray micro densitometry, scanning electron microscopy and optical microscopy showed that the main difference compared to the other techniques is that in the microfriction stir weld, there is a veritable welded line of molten material. This molten material comes from the wood in contact with the rotating steel cylinder, which has flowed down in the micro gap between the two pieces of timber where it has bonded by solidifying.     

Application of Numerical Modelling to Dowel-Welded Wood Joints

Numerical models using a 3-D finite element analysis method and the behaviour of dowel-welded wood joints are presented. Simulation results for step butt wood joints with two welded wood dowels under shear are analyzed and a good agreement with the experimental results is shown. Anisotropic elasto-plastic constitutive law with hardening associated with material densification, without distinction between radial and tangential properties, was used for the compressive behaviour of wood. The good coherence of the results obtained demonstrates clearly the capability of the model developed to simulate accurately the non-linear behaviour of dowel-welded wood joints to failure.     

The effect of edge banding thickness of white oak bonded with different adhesives on withdrawal strengths of beech dowels in composite materials

Dowel joints are widely used in furniture frame construction as a load-bearing connection structure, as well as a simple locator for parts. Joints constructed with dowels were subjected to withdrawal, bending, shear, and tensile forces. The aim of this study was to determine the withdrawal strengths of 6, 8, 10 mm diameter beech dowels embedded into matching holes drilled into the edges of medium-density fiberboard (MDF) and particleboard (PB) with solid wood edge banding of white oak with 5, 10 and 15 mm thickness, bonded with hot-melt, poly(vinyl acetate) (PVAc) and Desmodur-VTKA (D-VTKA), a polyurethane-based one-component adhesive. The effects of edge banding thickness, dowel dimension, type of composite material and type of adhesive used for edge banding on the withdrawal strength were determined. According to the interaction results from the Duncan test the highest withdrawal strength (7.019 N/mm²) was obtained in beech dowels with 6 mm diameter for MDF with solid wood edge banding of white oak with 10 mm thickness bonded with the hot-melt adhesive. Should the dowels be subjected to withdrawal, it is advised that a beech dowel should be used for MDF with solid oak edge banding with 10 mm thickness bonded with a hot-melt adhesive in furniture production and decoration applications.     

Enhancing the Exterior Performance of Wood Joined by Linear and Rotational Welding

Application of rosin, a wood derived, non-toxic, natural, inexpensive and easily and abundantly available natural material, to the wood faces to be joined by either linear vibration welding or rotational dowel welding has shown to greatly enhance the water resistance of welded wood joints. The method of application has been shown to have a marked effect on the results, with the application and drying of a diluted rosin solution to the wood surfaces before welding yielding the best results. The considerable improvement in water resistance does not still allow classification of the joints as fully exterior grade. However, dowel welding can now be used for protected exterior joints due to a combination of rosin waterproofing and joint geometry. Welded dowel joints holding together for longer than 455 days immersion in water indicate this to be the case. Rosin-treated linear vibration joints held together well in excess of 30 days but retained a measurable strength, in the best case, only up to 18 days water immersion. The wood anatomy and chemical reasons for the effect of rosin were determined by X-ray microdensitometry and CP–MAS ¹³C-NMR analysis.     

Wood joints by through-dowel rotation welding: microstructure, 13C-NMR and water resistance

Two-block wood joints were obtained by insertion and welding without adhesives of dowels by high-speed rotation. Their strengths were better than that obtained by poly(vinyl acetate) (PVAc) gluing. X-ray-microdensitometry analysis showed that a complete welding of the dowel to the substrate occurred and that a perfectly tight joint was formed. Isolation of the flow material allowed CP-MAS ¹³C-NMR analysis of its composition with possibly low interference from the constituents from the substrate. The flow material appeared to be composed of hemicelluloses, apparently xylans, and lignin. Scanning electron microscopy coupled with the NMR analysis results showed that microfibrils of cellulose, in both amorphous and crystalline states, torn from the wood surface during welding, as well as very small proportions possibly of recrystallized xylans and furanic compounds formed by heat transformation of the carbohydrates, were present. The geometry of the dowel joint allowed the joint to maintain up to 88% of its initial tensile strength after 24-h immersion in cold water.     

Influence of Wood Grain Direction on Linear Welding

Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strengths in linear wood welding. End-grain-to-end-grain welds of good strength were obtained for both beech and oak woods. The tendency to defibration in end-grain-to-end-grain welding indicated that for wood densities higher than or equal to the density of beech wood, end-grain-to-end-grain welding is possible and yields sufficient joint strength. A higher density seemed to yield a stronger joint. Wood pieces having other grain directions were also vibration welded. These were: (1) with the grain perpendicular to the wood longitudinal grain direction, (2) with the grain of both wood pieces at 45° to the wood longitudinal grain direction and (3) with the grain of both wood pieces at 45° to the wood longitudinal grain direction but at 90° to each other to form a fishbone-like pattern. The first of these yielded results comparable to end-grain-to-end-grain welding. The other two yielded much lower strength of the joints, indicating that fibre orientation in the interphase composite formed during welding had considerable influence on joint strength. These differences in joint strength have been explained by the very marked effect that anisotropy of the interphase composite has on fibre/matrix composites.     

Joining of wood layers by friction welding

A welding technique has been developed to join wooden work pieces by frictional heat without any wood glue. During the welding process 'melting' of the surfaces, resulting from the influence of pressure and frictional heat, was observed. Solidification and bond formation between the welded pieces happened during cool down of the heat-affected zone. The solidification of the heat-affected zone is explained. Tests were carried out to determine the resistance of the joint as a function of the interfacial temperature and the solidification time. The results show that after 20 s of cooling, the shear strength of the welded bonds amounted to 70% of the ultimate shear strength of the connection after 15 min of solidification. This behaviour allows continuous welding of multilayered wood laminates. Due to the use of natural materials only, the welded products are environmentally compatible. In case of disposal, the welded components can be burned in the same way as natural wood without the evolution of toxic compounds.     

Vibration welding of heat-treated wood

Vibration welding of wood that has been preheated according to an industrial two-step process indicates that such wood can be welded and can yield welded joints of good strength. The joint strength is, however, markedly lower than obtained when welding non-heat-treated timber. In general, weld strength of the timber is poor if welding is done on hydrothermolyzed wood. The strength results are instead much better if welding is done at the end of the complete heat treatment process, i.e., after the dry heat step. The weld lines of heat-treated wood show entangled cells where there is none or very little of the molten matrix intercellular material usually observed in welded timber. Furthermore, in weldlines obtained after hydrothermolysis an increase in rigidity and brittleness of the wood cells is observed. Hence, the wood cells are not entangled at all or very little. Both observations indicate that heat treatment has affected the main melting region of the wood, namely the intercellular material. As most of this material is already either lost or heavily cross-linked during heat treatment, only little of it is now available to melt and bind the wood surfaces during vibrational wood welding.     

Wood Welding: Chemical and Physical Changes According to the Welding Time

Wood welding using linear friction is a technique that has been developed in the past five years. The goal of this study was to analyze the microstructure development in the interphase enabling the wood-to-wood adhesion without any adhesive. Chemical and physical analyses have been carried out using infrared thermography, mechanical shear tests, transmitted light microscopy and X-ray densitometry. They have been considered as efficient to qualify the characteristics of the welded joints. The aim of this paper is to present a study using these analysis methods to observe the physical modifications of the wood in the interphase according to the welding time. The welding process of beech wood (Fagus sylvatia) with a welding time between 0 and 11 s could be divided into three different phases. The first phase describes changes in the physical and chemical characteristics of the wood. Densification and anatomical modifications occur in this phase. The second phase represents stabilization of the welded joint. The last phase of the cycle is a conditioning phase. All phases are controlled by the heat spread in the interphase and the time of heat exposure. Various parameters such as welding time, shear strength, temperature and width of the welded joint have been correlated and a hypothesis on the chemical reactions occurring in the interphase has been put forth. This study allowed discovering a window of parameters in which the quality of the welded joint is quite stable. Improving the quality of manufactured welded wood products without adhesive can now be done more easily due to this method.     

Enhancing water resistance of welded dowel wood joints by acetylated lignin

Low molecular mass acetylated organosolv lignin from wheat straw and from depolymerised low sulphur organosolv wood lignin have been shown to markedly improve both the water resistance and the mechanical performance of welded dowel wood joints. The acetylated oligomers distribution and extent of acetylation of the two lignins were determined by Matrix assisted laser desorption ionization-time-of-flight mass spectrometry. Extensive acetylation was confirmed by CP-MAS ¹³C NMR spectrometry. Force–displacement measurements on welded dowel joints to which acetylated wood lignins were added showed a ductile behaviour. This is due to the interpenetration of the elastic acetylated lignin network into the more rigid composite network of the welded interphase.