Tribological properties of the rotary friction welding of wood

Highlights

• The influences of welding equipment and wood parameters on friction force, normal force, and temperature were studied.

• The speed of welding into wood had a significant effect on interfacial normal force and friction forces.

• Increased in fiber angle increases friction force, normal force, and welding interface temperature.

Abstract

Wood–wood friction is an ancient subject, and its related mechanisms have been elucidated. However, the tribological mechanism of wood–wood friction at a high speed is still unclear. In this paper, an experimental equipment of wood friction welding was designed and fabricated. The influences of welding equipment and wood parameters on frictional force, normal force, and welding interface temperature were studied. Results show that the speed of welding into wood has a significant effect on interfacial force and normal force. Increase in wood moisture content reduces frictional and normal forces, and increase in fiber angle increases frictional force, normal force, and welding interface temperature. This study provides theoretical and technical basis for the study of wood friction welding.

Introduction

The benefits and disadvantages of wood friction and wear play important roles in production and people’s lives. For instance, the method of making fire by drilling wood was discovered 10,000 years ago [1]. Dowson introduced Darwin's notes of wood friction sketch in his “History of Tribology.” [2] Dowson et al. used a modern technology to verify Darwin's wood friction test [3], [4].

Studies on the tribological properties of wood are mainly concentrated on sliding velocity, wood fiber orientation, and wood fiber density. The friction coefficients of wood and bamboo fiber materials vary with sliding velocity, and the friction coefficients of wood and steel increase with sliding speed [5], [6], [7], [8]. The friction coefficients of wood and steel with rough surfaces at high velocities are considerably higher than those at low velocities [9]. Wood as an anisotropic natural macromolecule material shows viscoelasticity and creep. The friction coefficient of wood mainly depends on adhesive and hysteresis friction. The arrange grain sample, horizontal grain sample, and twill specimen have static friction coefficients of 0.630–0.855, 0.678–0.963, and 0.672–0.930, respectively. The coefficient of friction for static friction coefficient ranges from 59% to 90%. Horizontal grain had the largest dynamic/static friction coefficient, followed by the twill and arrange grain [10], [11].

The tensile strength, young's modulus, bending strength, and density of fiber of bamboo in the outer part with a high density along the thickness direction gradually decreased. The volume fraction of bamboo fiber gradually decreased inward in the radial direction, and the antiwear performance gradually decreased [12]. Wood friction welding is a relatively new wood adhesive technology. Its process is affected by many parameters. On the one hand, the friction welding process parameters, such as welding method, time, pressure, movement frequency, pressure, and pressure time, directly affects the wood tribological properties of friction welding interfaces [13], [14], [15], [16]. On the other hand, the macro- and microstructures of wood anisotropy are layered structures and orderly natural composites [17]. Different kinds of wood with unique chemical composition and macro- and microstructures varying in fiber direction, moisture content, and size show special tribological properties [18], [19]. The different tribological properties of welding interface affect friction welding interface temperature, resulting in interfaces with micro-hierarchical structured and chemical composition changes, which affect interface strength after welding.

Studies about the relationships among various parameters of friction welding, welding pressure, frequency, speed, friction coefficient, and time needed show that the time needed for welding decreases with increasing welding pressure, frequency, and speed [20], [21]. The friction coefficient can reach its maximum rapidly as welding pressure, frequency, and speed increases. Friction heat rises rapidly and provides energy for wood friction welding [22], [23], [24]. However, the frictional force and frictional interface temperature of the rotary friction welding of wood remains unclear. In this paper, a wood rotary friction stress measurement device was manufactured and used in testing interface frictional force, normal force, and temperature at different process parameters of equipment and different wood parameters. The study of the relationships between the tribological behavior and frictional heat in the wood tenon rotating friction welding process has important theoretical and practical significance, increases understanding of the welding interface bonding mechanism, and optimizes wood rotating friction welding process parameters.