TIG Remelting Process and Its Improvement of Angular Weld Joint Fatigue Strength

Literature Overview

This research published in Journal of Tsinghua University (Science and Technology) in 1990 by Xu Jimin, Wu Su, Yan Bingyi, Shi Yongji, Yang Yaman, and Shi Zhiqiang from Tsinghua University and the Railway Research Institute, investigates the TIG remelting process and its effect on the fatigue strength of angular weld joints. The study addresses a critical engineering challenge in structural welding, where angular weld joints are common in beam-to-column connections, ship structures, and pressure vessel attachments. The fatigue strength of these joints is often limited by the weld toe geometry and residual stress state, which can be significantly improved through post-weld TIG remelting.

Core Technical Points

The TIG remelting process involves re-melting the weld toe region using a TIG torch without adding filler material. The remelting creates a smooth, rounded transition at the weld toe, eliminating the sharp notch effect that serves as a stress concentration site for fatigue crack initiation. The process also modifies the residual stress state at the weld toe, potentially introducing compressive stresses that are beneficial for fatigue performance.

The effectiveness of TIG remelting depends on several process parameters including arc current, travel speed, torch angle, and number of passes. The process parameters must be carefully controlled to achieve the desired toe geometry without excessive heat input that could alter the base metal properties or introduce new defects.

Remelting Mechanism Analysis

Weld Toe Geometry Modification

The primary mechanism by which TIG remelting improves fatigue strength is the modification of the weld toe geometry. Conventional welds typically have a sharp weld toe with a small radius of curvature, which creates a high stress concentration factor. The TIG remelting process melts the weld toe and allows the molten metal to flow and solidify into a smoother, more rounded profile. The resulting toe radius is significantly larger, reducing the stress concentration factor and delaying fatigue crack initiation.

The ideal toe profile after remelting is a smooth, convex curve with a radius of curvature of 1-3 mm. This profile eliminates the sharp notch effect and provides a gradual transition from the weld to the base metal. The remelting process also removes any surface defects such as undercut, porosity, or slag inclusions that may be present at the weld toe, further improving the fatigue performance.

Residual Stress Modification

The TIG remelting process also modifies the residual stress state at the weld toe. The localized heating and subsequent cooling of the remelted zone creates a complex residual stress field. The rapid cooling of the remelted metal creates compressive stresses in the remelted zone, which can be beneficial for fatigue performance. However, the surrounding material may experience tensile stresses due to the constraint from the cooler base metal.

The net effect on fatigue strength depends on the balance between the improved toe geometry and the modified residual stress state. In most cases, the improvement in toe geometry dominates, resulting in a net improvement in fatigue strength. However, excessive heat input during remelting can introduce tensile residual stresses that partially offset the geometric improvement.

Fatigue Performance Improvement

The study demonstrates significant improvements in fatigue strength through TIG remelting of angular weld joints. The fatigue life improvement depends on the stress amplitude range and the number of cycles to failure. At high stress amplitudes, the improvement is primarily due to the reduced stress concentration factor. At lower stress amplitudes, the modified residual stress state also contributes to the improvement.