A look through the microscope: when dendrites resemble fir trees
12. December 2019
The image shows dendrites that resemble fir trees.
“Is it Christmas already?” was the well-known line from a television advert some years ago. Fittingly for the run-up to Christmas, the inspectors at the MaterialtestCenter and ExpertCenter of W.S. Werkstoff Service GmbH were considering an unusual question: from a metallurgical perspective, how do fir trees contribute to the strength of a welded joint?
The short answer is: not at all, as the following investigation shows.
Examining a small spot weld
The investigation concerned the weld analysis of a very small spot weld on a wire that had broken off. Even someone with no knowledge of fractography can see clear differences between the two halves of the fracture surface in Fig. 1.
The right-hand half shows a typical fatigue fracture surface with clearly recognisable fatigue striations, a damage pattern encountered relatively frequently in fractography.
A surprise on the fracture surface
The left-hand half of the fracture surface, by contrast, revealed an unexpected structure. The pores of a hot-crack surface contained numerous dendritic solidification structures whose shape closely resembles little fir trees. These dendritic structures form during the solidification of the weld metal when liquid metal solidifies inside a cavity or crack.
Fig. 4: …which, at higher magnification, prove to be fir-tree-like dendrites formed during solidification.
Hot cracks as the starting point for fatigue damage
We can therefore give a clear answer to the opening, somewhat tongue-in-cheek question about strength. A hot crack obviously does not contribute to the strength of a welded joint. Quite the opposite: it constitutes a geometric notch and therefore acts as a stress concentrator.
In this case, the notch effect evidently played a major part in initiating a fatigue crack that ultimately caused the spot weld to fail.
