Plus, things like vibration, nucleating agents, and other process variables influence grain size. Heat input also has a direct effect on the weld metal’s grain size: More heat means larger grain, although there isn’t a fixed relationship. Dendrite arm spacing is an exclusive function of solidification rate, but heat input controls how fast the metal solidifies, with the solidification time proportional to heat input (we’ll skip the math that shows this). So far we’ve talked a lot about solidification rates, but what about heat input? Doesn’t the amount of heat put into the weld influence the dendrite arm spacing? Indirectly, yes. The closer the dendrite arms, the faster the metal solidified. But if you increase the magnification, you’ll see the fine dendritic structure, with the spacing between each arm indicating not only alloy segregation, but also the rate of solidification. That’s because you see only the grain structure. Not to be outdone, the blocked grains simply “grow arms,” and this lateral growth produces the dendrite arms typical of as-solidified metals.įurthermore, a number of dendrites can grow simultaneously from the same grain, and each will have the same crystal orientation.Īll of this can and often does result in a final structure with dendrites existing in a solute-rich environment, which causes the weld metal’s structure to look coarse under low magnification. As it does, these rejected solutes concentrate near the liquid-solid interface, interrupting crystal growth. Keep in mind that grains are growing and the liquid area is getting smaller. That, in turn, lowers the freezing point. These rejected solutes diffuse into the remaining liquid. As the primary dendrites grow into the liquid, these solid areas reject alloying elements that are more soluble in the liquid metal. And because this microsegregation is associated with, and primarily responsible for, the formation of dendrites-crystal growths that resemble pine trees-most of the metals we deal with solidify in dendritic growth mode.Ĭonsequently, dendrites are important. But most commercial metals experience microsegregation of alloying (solute) and residual elements as the weld metal solidifies. In our last column we mentioned that, depending on composition and cooling rate, a weld metal solidifies either in cellular or dendritic growth mode. These growth patterns produce relatively long grains that grow parallel to heat flow and give the solid weld metal’s macrostructure a columnar look. At the same time they tend to block the growth of grains that weren’t so lucky.Īs the grains continue to grow, the amount of liquid decreases until the weld joint is completely solid. This means some grains are in a better position-they’re more favorably oriented-and tend to grow longer and farther than others. These atoms begin to form crystal sites, and these crystals continue growing into grains while maintaining the same crystal orientation.īut metal crystals find it easier to grow in certain crystallographic directions. This means each of the base metal’s solid portion grains provides a nucleus for atoms still in a liquid state. These solid portions act as nucleation sites where the weld metal can solidify. Along this interface, portions of the base metal grains are unmelted. Which grains grow faster depends on their orientation at the point where liquid weld metal meets the solid metal of the base metal. Grains are trying to grow in a variety of directions, and the entire process can get quite competitive, because some grains grow faster and block the growth of others. There’s a lot going on as your weld puddle solidifies.
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