What is the reason that some welds made via Friction Stir Welding, show similar or higher yield point compared to base material but very little elongation may be less than 2-3%?
If it is defective then it should not be able to sustain so much load (up to yield point) in the first place.
Thanks.
Some more infomation are required to further delve into the causes. What was the material welded? What thickness? What was the range of process parameters? What kind of defects did you observe.
During my Ph.D work on friction stir welding of cast aluminum matrix composites, I had experienced breaking of tensile samples on the base metal in spite of small tunnel or pin hole defects in the weld nugget. In that case the reason was as follows. The friction stir welding process induces more compressive force which might have closed micro pores and caused more densification of the composite to exceed the strength of parent cast composite.
Normally that means that the problem of the weld concerns the microstructure of the metal and not the macrostructure. Check the hardness values inside the weld nugget and compare them to the base metal. I expect very high microhardness, Moreover, there may exist brittle intermetallics inside the weld nugget.
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