In friction stir welding, the shapes of final joint are depending on welding conditions and can be one of three cases: onion ring nuggets, zigzag shapes and void defect nuggets. The shape of final joint is the important factor in determining the mechanical properties. The mechanism of material flow during FSW is the most important to analyze how the final joint formed after welding.

Many researchers tried to understand and visualize the effects of the revolutionary pitch on the material flow of the onion ring and void nuggets of Aluminum friction stir lap welded joints. In the case of the onion ring nugget, the interface was bent convexly in such direction, but, as the tool passed. The direction changed. Conversely, in the case of the void nugget. the interface bent to such direction and turned towards new direction as the tool passed difference than onion rings case.

In general, very good information can you find in below article.

https://www.sciencedirect.com/science/article/pii/S0264127515307565

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The effect of onion ring on properties is a topic of research for many years and not yet clearly understood. However, it is believed that these rings can have indirect effect on the properties. Under uni-axial stress field, the uniformity of the ring patterns may have no/less effect on the stress distribution across the weld joint. But, any irregularities in the pattern can create substantial changes in stress flow and increases the probability of stress concentration. Stress concentrated regions are subjected to premature failure under stress field.

Our research shows that in some alloys one can expect differences in the density of precipitates, and thus the material properties (see B. Carter et al. Metallurgical and Materials Trans. A44, 2013, is.4, p.1730-7401s)

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M. Wrobelin