Compressive Load-bearing Capacity of Laser-arc Hybrid Welded Corner Joint Members

Steel bridges are marvels of engineering, but their strength relies on something invisible and often troublesome: the welds that hold them together. When metal plates are joined, the intense heat of welding causes the material to expand and then contract as it cools. This process leaves behind hidden stresses and can warp the metal, creating slight bends or twists that were not there before the work began. For the massive steel members used in bridges, these tiny imperfections matter. They can weaken the structure's ability to carry heavy loads, particularly when the bridge is being pushed down by the weight of traffic. Engineers have long used standard arc welding, a reliable method that involves melting metal with an electric arc. However, for thick steel plates, this process often requires multiple passes, layering heat over and over again, which can accumulate significant distortion and stress. A newer technique, laser-arc hybrid welding, promises to solve this by combining the deep, focused power of a laser with the gap-filling ability of an electric arc, potentially creating stronger joints with far less heat.

A team of researchers from the University of Tokyo, Osaka University, and IHI Corporation set out to test this promise on a specific type of high-performance steel used in Japanese bridges, known as SBHS500. They focused on corner joints, the L-shaped connections where two steel plates meet, which are critical components in bridge construction. The team compared the traditional method, which required four separate passes of arc welding to fully join the thick plates, against the new hybrid method, which aimed to do the same job in a single pass. They built dozens of these steel corners, subjected them to precise measurements, and used powerful computer simulations to track exactly how heat moved through the metal and how it changed shape. Their goal was not just to see if the new method worked, but to understand exactly how it changed the hidden stresses and the physical shape of the steel, and whether those changes made the bridge members stronger or weaker under pressure.

The results revealed a dramatic difference between the two methods. The traditional arc welding process required a massive amount of energy, delivering a total heat input of 7,652 joules per millimeter of weld length across its four passes. In stark contrast, the laser-arc hybrid welding achieved the same full penetration of the steel plates using only 614 joules per millimeter, a single pass that utilized less than one-tenth of the energy. This reduction in heat had an immediate and visible effect on the physical shape of the steel. The traditional joints warped significantly, bending out of their intended flat plane by as much as 2.37 millimeters on one side and 1.87 millimeters on the other. The hybrid-welded joints, however, remained remarkably straight, with their maximum bending reduced by approximately 70 percent, measuring only 0.67 millimeters and 0.47 millimeters respectively. The researchers found that the intense, localized heat of the laser created a narrow, deep weld that cooled quickly, preventing the large-scale shrinking and twisting that plagued the slower, multi-pass arc method.

Beyond the visible shape, the researchers also measured the invisible forces trapped inside the metal, known as residual stress. These are the internal pressures that remain after the welding is finished, caused by the metal trying to shrink but being held back by the surrounding cooler material. In the traditional arc-welded corners, the surface of the steel was under significant compressive stress, reaching levels as high as 307.8 newtons per square millimeter. The hybrid-welded corners showed a much gentler internal state, with the maximum compressive stress reduced by about 50 percent, dropping to 143.3 newtons per square millimeter. The computer simulations, which modeled the flow of heat and the movement of the metal atoms, confirmed these findings, accurately reproducing the temperature changes and the resulting distortions observed in the physical experiments. The simulations showed that the hybrid process created a simpler, more uniform thermal history, which allowed the metal to settle into a more stable shape with fewer internal conflicts.

To understand how these physical and internal changes affected the actual strength of the bridge members, the team looked at data from previous compression tests where these same types of joints were squeezed until they reached their limit. The joints made with the hybrid method were stiffer, resisting deformation with an elastic stiffness that was about 24 percent higher than the traditional joints. This increased stiffness is a direct result of the hybrid joints being straighter and having fewer initial bends to correct under load. In terms of the maximum weight they could hold before failing, the hybrid joints performed just as well as, or slightly better than, the traditional ones, carrying an average maximum load of 1,343 kilonewtons compared to 1,280 kilonewtons for the arc-welded versions. The researchers concluded that while the hybrid method did not drastically increase the ultimate breaking point, it significantly improved the precision and stability of the joints. By reducing the heat input, the hybrid process minimized the warping and internal stress that can weaken a structure, offering a way to build steel bridge members that are more dimensionally accurate and mechanically reliable without sacrificing their load-bearing capacity.