Summer 2022 Contents:                                                                                                Issue No. 55


Root ConFusion

This nugget discusses potential confusion in correctly identifying weld root in fillet welds.

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In a fillet weld, the theoretical root is defined as the point where lines drawn along the part surfaces will intersect.  Under ideal circumstances, a fillet weld fusion zone will include the theoretical root and have sufficient leg fusion to form a fillet of sufficient size to meet weld specifications.  While such a weld looks great in schematic, it often looks a lot confusing once the actual weld is sectioned for metallographic assessment of weld throat.   In some cases, the root is not fused and in others parts have gap which makes it difficult to identify the root.   Situation gets worse in lap-fillet welds, where it is often not possible to identify the theoretical root, adding to the considerable root confusion.  In this newsletter, we make an attempt to simplify and clarify the basics of root fusion.


Theoretical weld root is shown in schematic in Figure 1a for a typical T-joint fillet weld.  Under ideal circumstances, the weld fusion zone will include the theoretical root and extend a bit further along the weld interface, as shown in Figure 1b.  The actual weld root is now used to draw a line perpendicular to the weld face (excluding reinforcement in convex welds) which is essentially the shortest line segment and defines the weakest link in the weld when loaded in shear parallel to the weld length; this line segment is the weld throat.  One of the first sources of confusion is the temptation to use the deepest point of the weld as the weld root.  Such a measurement will likely produce a larger weld throat that seems to meet spec, but in reality, does not identify the weakest link of the weld which is measured from the actual weld root.


In situations where the parts have a gap, which is quite common in automotive applications, the weld now has two roots, as show in Figure 2a.  Here again the principal of weakest link has to be used to identify which of the roots will produce the smallest weld throat, and that root should be labelled as the actual root.  Similar situation arises when the weld fusion zone does not include the theoretical root, as shown in Figure 2b.  Here again, the user can identify two roots, and has to pick one that produces the shortest weld throat.



Figure 1: Schematic (a) shows theoretical root and weld interface.  Schematic (b) shows actual root location.



Figure 2: Schematic (a) shows location of two roots on parts with gap, while (b) shows two roots on a weld that lacks root fusion.


That brings us to another contentious and confusing topic: Is it essential for the fusion zone to include the theoretical root?  Not necessarily.  As long as the weld fusion zone is large enough and meets weld size requirements, root fusion is not necessary.  The downside is that in the absence of root fusion, a greater amount of weld metal has to be deposited to meet weld specs of leg length.  Secondly, when an inspector is measuring weld size with gauges, the assumption is that the theoretical root is fused, and hence the external size measurements provide an indirect measurement of minimum weld throat – extending from theoretical root to weld face.  If the theoretical root is not fused, then those correlations no longer work, and external measurements are then suspect.


Situation becomes even more complicated in interpretation of the lap-fillet welds where the face of the lap component is often completely consumed in the fusion zone (Figure 3a) and with that we lose all reference to identify the theoretical root.  In such situations only option is to correctly identify the actual root, and then drop a perpendicular to the weld face to measure weld throat.  Now the user is faced with a fusion zone that looks more like an obtuse triangle (Figure 3b), not the typical right triangle.   Here again, the main aspect to keep in mind is to identify the weakest link, or the shortest line, from the actual root to the weld face; type of triangle is not important.


Figure 3.  Schematic (a) shows a lap fillet weld, while (b) shows photo of a lap fillet weld with location of the root.


In addition to measurement of physical geometry of the weld fusion zone, the issue of weld metallurgy also has to be considered.  In situations where a deep penetration weld is made (Figure 1b), the fusion zone chemistry will have significant contribution from the parent metal which may not be optimal for avoiding weld cracks.  In comparison, weld that does not include root fusion (Figure 2b), will likely have a fusion zone that is primarily composed of the filler metal.


As with everything in welding, a welding engineer should take care to review all aspects of the weld before signing off on validation of the weld process, or else will have to deal with root confusion when the quality gremlins are up in arms.