鋁含量對鎂合金銲接熱裂縫與銲後熱處理影響之研究

Abstract

本論文旨在探討鎂合金AZ31、AZ61與AZ80三種鎂合金之鋁含量對銲接熱裂縫與銲後熱處理之影響，研究共分為兩部分：第一部份為鋁含量與鎂合金銲接熱裂縫之關係，探討外加應變量與熱循環次數對AZ31、AZ61與AZ80鎂合金之熱裂縫敏感性之影響。研究結果發現：鎂合金鋁含量越高，其熱裂縫總長度越長；經過多次熱循環後，銲道金屬熱影響區之熱裂縫總長度亦會會隨著增加。隨著外加應變之增加，AZ61與AZ80之熱裂縫敏感性也隨之增加。研究中發現，Al含量在熱裂縫行為中扮演重要角色。因在凝固時的非平衡凝固過程中，Al成分逐漸被析出，越接近凝固完全時，Al含量會越高。因此容易在晶界處有Mg17Al12之低熔點共晶物出現。在銲接過程中，產生的熱量會使Mg17Al12液化形成晶界液化區。隨外加應變作用，該區會被拉開形成熱裂縫。因此，Al含量可以評估鎂合金熱裂敏感性。AZ31的熱裂縫敏感性最低，AZ80之熱裂縫敏感性最高。第二部分為鎂合金銲後熱處理，將AZ31、AZ61與AZ80三種鎂合金以惰性氣體鎢電極銲進行對接銲，並填入AZ61當填料金屬，並將三種鎂合金施以T6熱處理(固溶＋人工時效)。在經過T6熱處理後，AZ61與AZ80之銲道與母材均能看到Mg17Al12在基地中析出。隨著時效時間增加，Mg17Al12析出量亦為之增加，其中以AZ80析出效果最佳，AZ61次之。由實驗結果可觀察到，Al含量達8wt%的AZ80之Mg17Al12析出量最多。而Mg17Al12的多寡影響試片之硬度與抗拉強度，因此Al含量越高的AZ80之T6熱處理效果最好，可以達到微克氏硬度值98。但時效時間超過16小時，則開始產生析出物顆粒變大，造成過時效之影響。AZ31之Al含量較低，無法利用T6熱處理來加強抗拉強度與硬度，需使用其他強化機構來達成硬化。AZ61與AZ80鎂合金在T6熱處理會有連續型與非連續型的析出物，此現象以AZ80最為明顯。研究亦發現AZ80經8小時的時效可以將塊狀Mg17Al12析出物細化，可取代合金元素Zn的功用。

The aims of this study are to investigate the effect of aluminum content on hot cracking and post-weld heat treatment of AZ series magnesium alloys. Part one, the AZ series magnesium alloys' hot cracking susceptibility is affected by aluminum content. The spot varestraint testing is used to evaluate the hot cracking susceptibility of AZ series magnesium alloys, namely AZ31, AZ61 and AZ80. The effect of augment strain and the numbers of thermal cycles on the cracking susceptibility are investigated, and the total crack length is used to evaluate the hot cracking susceptibility. The results indicate that both the increase of aluminum content in magnesium alloys and multiple thermal cycles cause the total length of W.M.HAZ's cracks to become longer. Al content plays an important role in the hot cracking mechanism. The non-equilibrium solidification process precipitates Al to unsolidified alloy liquid during alloys solidification. When there is more Al content, the more precipitation (Mg17Al12) with low melting point is produced at grain boundaries. The heat produced in the welding process causes Mg17Al12 liquefaction to form grain boundaries liquefaction. Pulled by the augment strain, the grain boundaries liquefaction shows hot cracking. To sum up, the Al content of AZ series magnesium alloys can be used to assess the hot-cracking susceptibility. The hot cracking susceptibility of AZ80 is higher than that of AZ61 and AZ31. The second part investigates the post-welding heat treatment of AZ series magnesium alloy weldment. AZ31, AZ61 and AZ80 are welded in butt welding by semi-GTAW, using AZ61 as filler metal. T6 heat treatment, solution treatment + artificial aging, are imposed on three types of magnesium weldment. The Mg17Al12 precipitates in grain and grain boundaries at fusion and base metal after T6 treatment. With more aging time, the more Mg17Al12 is produced in grain and grain boundaries. AZ80's precipitation hardening effect is the best among AZ series magnesium alloys. The results indicate that Mg17Al12's quantity affects the Hv hardness and tensile strength of weldment. Thus, AZ80 has the best hardening effect after post-welding heat treatment, because it has more Al content that it can precipitate much Mg17Al12 to improve mechanical properties. It is noteworthy that over aging effect occurs during 8 to 16hours of aging time. Experimental results show that the aging time reaches over 8 hours, Mg17Al12 will be refined. So long aging time can replace the function of alloying elements Zn of AZ80.