| Summary: | 碩士 === 國立中央大學 === 機械工程學系 === 85 === The project proceeds to find out the effects of process and
heat treatmenton the strength and stress corrosion cracking
(SCC) susceptibilities of AA7050aluminum alloys for aerospace.
By means of optical microscope(OM), transmissi-on electron
microscope (TEM), tensile test and slow strain rate test(SSRT),
the variations of microstructure due to those processes and
their effects on strength, ductility and SCC resistance can be
clarified. The procedures involved are : 1. altering the quench
rate after solution treatment and investigate its effects on
the strength and SCC resistance. 2. research the variations of
coherency of dispersoid Al3Zr due to recrystallization and its
effects on quench sensitivity. 3. for the intermediate
thermomechanical treatment (ITMT) proces investigate the
influences of fine-grain structure on the strength and SCC
resistance. After all, design an ideal process to obtain the
7050 aluminum alloys with high strength and high SCC resistance.
The results show that : step-quench after solution treatment
will develop coarse and separated grain boundary precipitations,
and such kind of structure do improve the SCC resistance of the
7050 alloys. Recrystallization not only transforms the interface
between dispersoids Al3Zr and matrix to an incoherent one, but
also increases the area of high-angle grain boundary.Both of
the structures can raise the quench sensitivity of 7050
alloys.Otherwise, the strength canbe raised by fine-grain
structure with ITMT process. The effective processes to
obtain high strength and high SCC resistance are : ITMT treated,
then step-quench in oil bath(220℃*10sec), water quench,
7daysnatural aging and artificial aging(120℃*24hr) or hot
rolled, then step-quench (120℃*24hr). in oil bath(200℃*30sec),
water quench, 7days natural aging and artificial aging.
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