Friction stir welding was performed at four different welding parameters (rpm: 508, 708, 931, 1216 at traverse speed 41 mm/min). But at some parameters (931-41) microhardness was less whereas the UTS value was more than the others. How is it possible?
This could be resulted due to re-precipitation. For UTS, the probable reason may be the accumulation of dislocation densities during the tensile deformation, which can further be validated by factogrpahy analysis. i.e location and behavior of fracture surface. Additionally, grain size measurement in stir zone could help for establishing relations for hardness and UTS values. After having all these data the final comment can be made.
- I think, for FSW joint, it is better to try to measure the overall hardness using Rockwell or Brinell rather than microhardness using Vickers because the thin and small indenter used in measuring microhardness may indent one of the material's constituents either precipitates or matrix which may misled the test results.
Below the papers you may refer for your conclusion in related to Microhardness effect:
http://www.tandfonline.com/doi/abs/10.1080/13621718.2016.1152748
Article Effect of process parameters on microstructure and mechanica...
the microhardness decreases when the alloy is not protected by the oxide layer
In precipitation hardening aluminium alloys, one does not look for any direct relationship between hardness and a mechanical property such as UTS. The exact way in which the material goes through thermal history will decide the precipitation parameters that will impact the hardness, but may not at all affect the UTS
I would suggest to measure the overall hardness by using Rockwell or Brinell methods instead micro-hardness using HV at low load.
I agree with Prof Al-Badrawy A. Abo El-Nasr' answer.
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