6系列與7系列鍛造用鋁合金之機械性質及微結構特性之探討

Abstract

本實驗主要在探討7075、6066、6061鋁合金擠製棒材在不同時效之機械性質及顯微組織的特性。利用光學顯微鏡、掃描式電子顯微鏡，穿透式電子顯微鏡、XRD、掃描式熱差分析儀等儀器觀察經熱處理後的材料微結構及析出物。 由實驗結果得知，7075經120℃-72小時時效可得最大抗拉強度(680MPa以上)，若淬火後於室溫放置數天再時效則析出物會較為細小而使強度降低。與T7處理相比，RRA處理與二階段時效的晶界析出物雖然都很粗大，但其無析出帶卻相對較窄，且晶粒內部的擴散組織及析出物仍相對較小，因此能維持較佳的強度。 6066會因含Cu量高，將之重新熔煉鑄造後會產生Q相(Al-Mg-Si-Cu)，其熔點約在535℃左右，固溶溫度過高會導致其熔解產生微孔洞進而弱化強度。若施以適當均質化處理則可將之消除，固溶溫度提高至560℃可使強度提高至500MPa以上。 若對6066及6061在530℃固溶後的時效進行比較，6066最大抗拉強度為460MPa，6061最大抗拉強度則只有353MPa。經成分分析及SEM與TEM觀察可發現6066之合金元素較多且散佈顆粒較緻密，可產生較佳的固溶及散佈強化效果。另外，由DSC分析可知，6066因Si含量較高，在90℃與350℃會分別形成Si-cluster及silicon相，且s”及s’放熱峰都會比6061來的低溫，顯示6066的析出效果會較6061好，而時效結果也證實6066只需8小時即可達到時效尖峰，但6061卻須24小時才可達到時效尖峰。 最後利用極化量測以判斷材料抗應力腐蝕能力，由結果得知7075在T6處理時會有電流反轉現象而產生沿晶腐蝕情形，可能導致應力腐蝕抵抗能力下降，若改以T7、RRA等處理方式則可消除此反轉現象；而6066與6061經過T6也未觀察到此反轉現象，顯示其抗應力腐蝕能力會相對7075來的高。 為探討鋁合金擠製棒材實際鍛造後之特性，本實驗再以旋鍛機進行冷鍛後再固溶時效處理，發現試棒因發生再結晶而破壞原始織構組織，使強度嚴重降低；隨著加工量越大再結晶越小而產生細晶強化進而重新提升其強度。若改以固溶淬火後冷鍛再時效之T8處理，則強度可有效提升，但會由原本之延性破裂轉變為脆性延性共存之破斷模式，伸長率嚴重會下降。

This research mainly discuss that, under different condition of heat treatment, using OM, SEM, TEM XRD, DSC,UTM and hardness tester… to observe and test the mechanical properties of 7075, 6066, 6061 aluminum alloy extruded bars’ microstructure and precipitations. The test result shows that after 120°C-72 hours of artificial aging, can get the best strength (over 680MPa). If after quenching, placed at room temperature for several days, and artificial aging, the precipitations are relatively small, the strength is decreased. Compare to T7 aging treatment, duplex aging treatment and RRA processing both make grain boundary precipitates bulky, too, but PFZ from it grow narrow which makes grain boundary strength increase. Within grain, still maintain and have more diffused organization and precipitation reinforcement. Containing higher Cu, 6066 can produce Q (Al-Mg-Si-Cu) phase after casting. Melting point of Q-phase is about 535°C. Because of such high solid solution temperature, Q-phase will melt with many micro-voids, thereby weakening its intensity. If given appropriate homogenizing treatment, the Q-phase can be eliminated, and the strength will increase to more than 500MPa, after rising its solid solution temperature to 560°C. Compare 6066 and 6061 after solid solution at 530°C and artificial aging. We can find that 6066 has more alloying elements, and dispersed particle. This can provide better solid solution and dispersion strengthening effect. According to DSC analysis, 6066 has higher Si, so under 90°C to 350°C, it will precipitate Si-cluster and silicon phase respectively. s’’ and s’ exothermic peak will lower than 6061. Therefore, 6066 has better precipitation effect. 6061 needs 24 hours to reach its peak aging, while 6066 needs only 8 hours. Using polarization measurement to test material stress corrosion resistance, we can find that 7075 will have current reversal phenomenon and intergranular corrosion phenomenon under T6 treatment. The current reversal phenomenon can be eliminated by T7 or RRA. However, After T6 treatment, 6066 and 6061 will not produce this phenomenon, too. Therefore, their ability to resist stress corrosion will be better than 7075. To explore the material properties of extruded bar after cold forging and heat treatment. In the experiment, bars are given different strains by rotary swaging, and then carry on the solution treatment, aging treatment. We can find that recrystallization occur, the original texture is destroyed and intensity is greatly reduced. When the strain getting higher, the recrystallized grains will become smaller , and the strength will increase. If changed to T8 treatment (solution treatment- quenching- cold forging- aging treatment ), the strength can be effectively improved, but ductile fracture will change to brittle-ductile coexistence mode, and the elongation will greatly decrease.