Local melting and tool slippage during friction stir spot welding of Al-alloys |
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Authors: | Adrian Gerlich Motomichi Yamamoto Thomas H North |
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Affiliation: | (1) Department of Materials Science & Engineering, University of Toronto, 184 College St., Rm. 140, Toronto, ON, Canada, M5S 3E4 |
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Abstract: | Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a
combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by
either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received
base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone
temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation.
In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate
values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain
values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature
following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination
of rapid quenching, high plunge rates, and extremely short dwell time settings. |
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Keywords: | |
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