Localized Recrystallization in Cast Al-Si-Mg Alloy during Solution Heat Treatment: Dilatometric and Calorimetric Studies |
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Authors: | S K Chaudhury V Warke S Shankar and D Apelian |
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Affiliation: | (1) Honeywell Turbo Technologies, Torrance, CA 90505, USA;(2) Bodycote Thermal Processing, Andover, MA 01810, USA;(3) Light Metal Casting Research Centre, McMaster University, Hamilton, ON L8S 4L7, USA;(4) Worcester Polytechnic Institute, Metal Processing institute, Worcester, MA 01609, USA; |
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Abstract: | During heat treatment, the work piece experiences a range of heating rates depending upon the sizes and types of furnace.
When the Al-Si-Mg cast alloy is heated to the solutionizing temperature, recrystallization takes place during the ramp-up
stage. The effect of heating rate on recrystallization in the A356 (Al-Si-Mg) alloy was studied using dilatometric and calorimetric
methods. Recrystallization in as-cast Al-Si alloys is a localized event and is confined to the elasto-plastic zone surrounding
the eutectic Si phase; there is no evidence of recrystallization in the center of the primary Al dendritic region. The size
of the elasto-plastic zone is of the same order of magnitude as the Si particles, and recrystallized grains are observed in
the elasto-plastic region near the Si particles. The coefficient of thermal expansion of Al is an order of magnitude greater
than Si, and thermal stresses are generated due to the thermal mismatch between the Al phase and Si particles providing the
driving force for recrystallization. In contrast, recrystallization in Al wrought alloy (7075) occurs uniformly throughout
the matrix, stored energy due to cold work being the driving force for recrystallization in wrought alloys. The activation
energy for recrystallization in as-cast A356 alloy is 127 KJ/mole. At a slow heating rate of 4.3 K/min, creep occurs during
the heating stage of solution heat treatment. However, creep does not occur in samples heated at higher heating rates, namely,
520, 130, and 17.3 K/min. |
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