銲接對Al-3.7 wt%Zn - 2.5 wt%Mg合金應力腐蝕性質的影響

Influence of Welding on SCC Resistance of an Al-3.7wt%Zn-2.5 wt%Mg Alloy

本研究以十字型銲件試驗、U型試件試驗、定負荷試驗、以及自創的銲道雙懸臂樑試驗等四種方法，探討過時效之Al-3.7 wt%Zn - 2.5 wt%Mg合金的母材、銲件或熱影區（Heat Affected Zone，HAZ）的應力腐蝕破裂（Stress Corrosion Cracking，SCC）性質。實驗結果顯示，只有銲接殘留應力而無外加應力的十字型銲件在3.5% NaCl水溶液中不會有SCC發生；U型試件試驗和定負荷試驗結果亦顯示長軸和長橫軸方向的抗SCC性質十分優良，且銲接後仍能保有此優良性質，第一型雙懸臂樑試驗得知短橫軸方向受力時，裂縫會發生，但因為母材與熔填金屬的性質差異，裂縫會被銲道阻擋而停止；第二型雙懸臂樑試驗被證實可以適用於熱影響區的SCC性質探討，而其獲得的區域Ⅱ速率與應力腐蝕臨界應力強度值顯示HAZ的抗SCC性質優於母材，原因是銲接過程中HAZ因為熱影響而產生再次過時效現象，使得晶界析出物粗化、數目變少及間距變大，造成其抗SCC性質提升。

Four testing methods were performed to investigate the stress corrosion cracking (SCC) resistance of the base metal, weldment and heat affected zone (HAZ) of an overaged Al-3.7wt%Zn-2.5 wt%Mg Alloy in 3.5% NaCl solution. Cruciform weldment test results reveal that the residual stress from welding cannot result in SCC. The results of U-bend and constant load tests also show that this alloy loaded in longitudinal and long transverse direction has excellent SCC resistance even after welding. Furthermore, type I double cantilever beam (DCB) test shows the crack occurs in specimens loaded in the short transverse direction but cannot propagate through weldment due to the better SCC resistance of ER5183 Al-Mg filler metal. Type II DCB test indicates that the SCC resistance of HAZ is better than that of base metal because of reoveraging which results in coarsening and reducing amount of precipitates and increasing space between precipitates in grain boundaries during welding.