| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 98 === The objective of this study is to investigate the effect of post-welding heat c processes parameters on the mechanical properties of Mn-Free Al-Zn-Mg-Zr(G7F) weldment. Hardness test, electrical conductivity measurement were conducted to seek for the most suitable heat treatment combinations of process parameters for the T6, T7 and RRA treatment of Al-Mg-Zn-Zr(G7F) weldment. Optical metallographic observation, tensile test, EDS, EPMA and SEM, were conducted to characterize the mechanical properties of these heat-treated weldments.
Experimental results showed that the most suitable process parameters for T6 temper are 470℃/40min. + 80℃/14hr.+120℃/27hr.; for T7 temper are 470℃/40min.+80℃/14hr.+ 107℃/8hr. + 168℃/12hr. ; and for RRA treatment are T6 +180℃/20min.+ 120℃/27hr.。
Welds after T6 temper, weldment with the highest hardness；T7 temper is the most suitable post-weld heat treatment process to produce weldment with right characteristics and the best conductivity, show have best stress corrosion cracking resistance. Weldment after RRA treatment the handness is closed to that of T6 temper, and the conductivity is similar to that of the T7 temper, indicating post-welding RRA heat treatment can improve stress corrosion cracking resistance without the sacrifice of tensile strength.
Appropriate post-welding heat treatment processes parameters can diffuse zinc atoms effectively in the fusion zone. In such a way, the content of alloying elements composition within the fusion zone is change. There are more opportunities for Zn and Mg atoms to form MgZn2 and Mg3Zn3Al2 in the weld to increase the hardness of the weld through precipitation hardening mechanism.
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