THE POWER OF ANY NATION owes much to her manufacturing prowess. Our living standard owes much to the ingenuity of humanity. For example, welding was, is and will be an intimate partner in the history of manufacturing. From submarines to rockets, these vehicles are possible because of the advancement of welding technology. Submarines can dive deeper and quieter because the understanding of the crystalline integrity of welds continues to improve. Rockets carry bigger payloads and more fuel partly because of friction stir welding. One method works above melting point, the other below melting point—a critical distinction. Each one has its advantages and disadvantages.
Conventional welding that involves melting goes through similar steps. Metal is heated to the melting point, metal is joined either with or without filler and metal is cooled—which is when the conflict starts, with differing crystalline structures in front and behind the melt zone.
Different sizes of crystals form along different areas of the joint. Coarser crystals form on the melted metal side and finer grains on the base metal side because the base never reached the melting point, so it retains most of the characteristics of the base metal. The contact face—where molten metal meets host material and where larger crystals solidify and grip the finer crystals of the base metal—is where stress resides. Millions of dollars are spent relieving this residual stress, this boundary. The majority of cracks and repairs are focused at this juncture.
And then, once in a while in the history of welding, something elegant yet radical comes along. The technique is simple to operate, and results are stunning. Yet we all missed it for the last 19 years. There was no lineup. No one was beating a path to this door.
Solid-state fusion, or dynamic recrystallization, uses all the old techniques, yet arranges crystals in a slightly different manner. Voila! New science. The crystalline structure is even and fine grained across base metal, transition zone and fusion area. This technique incorporates heat, rotation and forge, all in tandem. In dual-combination spin and forge, heat and forge are old hat. Been there, done that.
In this new technique, one side is stationary; using induction, heat works both faces to below melting (this supplies 95% + of the required energy input). The second workpiece is rotated and forged. By rotation, the crystals are sheared as they are forming. This motion inhibits formation of large crystals as they cool—dynamic recrystallization.
Once shear and forge forces are introduced, there could be, most likely but not proven, a 3D volume where crystals form in the conventional time versus temperature chart. Of course, there is a heat transition zone, but this author argues that there is no heat-affected zone. Heat affected implies permanent change. In dynamic recrystallization, the crystals might grow to a coarser grain size but transition back to fine grain. This is a critical point—we do not know if coarser grains are ever formed or if crystals transition directly to fine grained. We can argue whether to label this “transition” or “affected.” Regardless, this is a rich area for future studies.