Current Projects: FHWA/PSI

The Problem

Currently, most of the processes commonly used in bridge fabrication are fusion welding processes. The problem with fusion welding is that molten and resolidified metal in the weld deposit remains as-cast microstructure in the structure and has mechanical properties greatly dependent on chemistry/dilution and cooling rates.

These processes inherently have a potential for creating weld discontinuities such as solidification cracking in the fusion zone [figure 1] or gas and slag entrapment. Generic concerns remain in HAZ grain coarsening and loss of toughness in fusion welds.

The other option for bridge fabrication is solid-state welding in which the material in the weld has never actually reached its melting point, thus eliminating or greatly reducing the chance for problems associated with fusion welds however, the disadvantages with solid-state welding processes are:

  • High capital cost
  • Relatively low welding speeds and high heat input (except for high frequency welding)
  • Need for high production volumes for similar shapes to justify investment
  • Lack of mobility - inability to make field welds or repairs
  • Insufficient technical data comparing solid-state welds with fusion welds


Many common fusion arc welding processes are currently used in bridge fabrication. Partial or full-penetration Submerged Arc Welding in T-joints have been the most commonly used fusion welds in I-beam fabrication for decades. Emerging high productivity technologies such as hybrid Laser Beam Welding/Gas Metal Arc Welding have also been introduced to improve weld quality, productivity, weld properties and reduce residual stresses.

What We Are Doing

This research is aimed at filling the knowledge gap of data comparing fusion welds with solid-state welds as well as build on the advantages of solid-state welding over fusion welding. The objective of this study is to find alternate solid-state processes for welding modern bridge steels for possible inclusion in the AWS D1.5 Bridge Welding Code.

We are comparing fusion welds to solid state welds and gathering data using several different methods of non-destructive and destructive testing including hardness testing, root and face bends, toughness testing, and COMSOL multi-physics modeling.

Solid-state weld samples are made using Thermatool's experimental High Frequency Induction welder, while the new NASA Thermal Stir Welder is used to make samples of different similar and dissimilar combinations of materials. Friction Stir Welding samples are made at the Navy Surface Warfare Center.

LeTourneau University Research and Development is currently developing a hybrid High Frequency + Friction Stir Welder for an additional type of solid-state welding from which we can make samples for testing and data collecting.


FHWA R&D: $220,000 / 2 years


Federal Highway Administration (FHWA)



Carderock Division, Naval Surface Warfare Center

For a look at LeTourneau's work increasing the FSW tool life
with hybrid FSW + HFI welding, click here.