Understanding thermal-mechanical variations and resulting joint integrity of pressure-controlled linear friction welding of thin-steel sheets

Linear friction welding is a solid-state joining technology that bonds materials via friction heating and plastic deformation. This process is being extensively researched for welding metallic sheets with different dimensions; however, it involves difficulties in joining thin cross-sections due to extensive misalignment and unsteady plastic extrusion of softened materials at interfaces. This study introduces novel efforts for joining thin cross-sections through pressure-controlled oscillations and displacements, facilitating localized plastic flow essential for high-strength solid-state bond formation. This method is rare, and the results reveal base metal fractures in tensile-tested welded joints of 2 mm thick S45C steel sheets. The interfacial yielding at specific temperatures is obtained by applying pressure corresponding to the temperature-dependent strength of the material. Accordingly, the welding is attempted using a hydraulic-based clamping system designed to accommodate large sheet lengths while allowing precise control of the interface pressure and temperature to facilitate controlled material plastic discharge. However, the requisite joint evolution tracking remains infeasible due to the intricate weld designs and is instead uncovered through novel numerical investigations. Modeling simultaneous oscillations and forging displacement while maintaining pressure depicted the kinetics of continual interfacial deformation. The transient fluctuations in plastic stress and temperature increments distinguish the stages of forging under different conditions. The computed temperature vs. plastic strain and the measured change in interfacial microstructures from martensite to dynamically recrystallized very fine ferrite with fragmented small cementite explain the lower temperature welding for an increase in applied pressure, enhancing the understanding of linear friction welding of thin steel sections for industrial applications.