A novel pressure-controlled joule-heat forge welding method to fabricate sound carbon steel joints below the A₁ point

Highlights

• A novel “Pressure-controlled Joule-heat Forge Welding (PJFW)” method was developed.

• The welding temperature is determined uniquely by the applied pressure in PJFW.

• Uniform temperature and hardness distributions are achieved over weld interfaces.

• A sound S45C steel joint was successfully fabricated by PJFW under the A₁ point.

• This method shows potential to well join carbon steel regardless of carbon content.

Abstract

A novel solid-state joining method, that we called Pressure-controlled Joule-heat Forge Welding (PJFW), in which electric resistance heat is utilized as the heat source, was successfully developed. Medium carbon steel, S45C, rods were joined using this PJFW method under various conditions. The welding temperature can be determined uniquely by the applied pressure in the PJFW. The fabricated joints show homogeneous temperature distribution and hence hardness distribution along the weld interfaces. A sound S45C joint showing superior tensile properties comparable with the base material was successfully fabricated by providing an appropriate high pressure to lower the welding temperature below the A₁ point in order to prevent the brittle martensitic transformation, and applying a sufficiently large faying-surface deformation, not only to introduce significant microstructural refinement and large strains to prevent the softening, but to also sufficiently fragment the oxide layers, generate fresh surfaces, promote their atomic bonding and eliminate weld interface defects.

Introduction

Extensive efforts have been made to strengthen structural materials to improve the collision safety of transportation vehicles, and lower their weight to improve fuel efficiency [[1], [2], [3], [4]]. Since carbon steels can provide a good strength-ductility combination with low cost by carbon content increasing and appropriate microstructural tailoring, they are considered as promising structural materials for use in the automotive industries. However, for medium and high carbon steels, it is hard to obtain sound weld joints by conventional fusion welding methods due to the brittle martensitic transformation undergoing upon cooling from the high-temperature field (above the phase transformation point, A₁) that may cause an occurrence of cracking in the weld zone, and other typical fusion-welding-associated issues, such as coarse columnar grains, high residual stresses, severe welding distortions and solidification voids/defects, which significantly deteriorate the joint qualities [[5], [6], [7]]. Solid-state joining techniques with a low welding temperature below the A₁ point are, therefore, promising and strongly required to join medium and high carbon steels [8,9].

Rotary friction welding (RFW) is one of the solid-state joining techniques in which the friction heat is generated between the faying surfaces of two components to soften the interface materials, then they are plasticized out to the outer region as flashes to produce the joint. Since RFW allows a fast joining with a welding temperature much lower than that of the fusion welding, it has been extensively adopted to join carbon steels [[10], [11], [12], [13], [14]]. However, the large difference in peripheral velocities between the weld interface center and periphery induces an inhomogeneous heat generation over the weld interface, making the welding temperature and the associated interface microstructure also inhomogeneous and hard to control in the RFW [15,16].

In light of these issues, we propose a novel joining concept in which the electric resistance heat is utilized to soften the interface materials, then they are plasticized out as flashes by a high pressure to produce the joint. Since the electric resistance heat is utilized as the heat source rather than the rotary friction heat, the heat generation is expected to distribute homogeneously over the weld interface. Moreover, since a high pressure is applied, the interface materials can be deformed at a relatively low temperature, thus allowing the welding temperature possibly lower than the A₁ point. This view has been well verified in our previous studies on linear friction welding (LFW) and RFW [[17], [18], [19]].

In this study, a novel welding apparatus was designed in-house for this novel joining method that we named “Pressure-controlled Joule-heat Forge Welding (PJFW)”. Medium carbon steel rods were joined by this method, and the effects of the processing parameters on microstructure and mechanical properties of the joints were systematically investigated in order to obtain a sound carbon steel joint below the A₁ point.