鍶改良劑、旋壓成型及熱處理對A356鋁合金磨耗腐蝕性質之影響;Effect of strontium modifier, spinning deformation processing and heat treatment on the wear-corrosion properties of A356 alloy

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Title:	鍶改良劑、旋壓成型及熱處理對A356鋁合金磨耗腐蝕性質之影響;Effect of strontium modifier, spinning deformation processing and heat treatment on the wear-corrosion properties of A356 alloy
Authors:	鄭穎駿;Yin-Chun Cheng
Contributors:	機械工程研究所
Keywords:	磨耗腐蝕性質;旋壓加工;熱處理;A356鑄鋁合金;共晶矽顆粒;鍶改良劑;wear-corrosion property;spinning deformation processing;heat treatment;Sr modifier;eutectic Si particle;A356 alloy
Date:	2012-05-31
Issue Date:	2012-06-15 19:39:15 (UTC+8)
Abstract:	含Mg之亞共晶Al-Si合金（Al-7Si-0.3Mg, A356 alloy）具有質輕、耐磨、耐蝕且鑄造性佳等優點，廣泛應用於航空、汽車等運輸工業。生產鋁鑄件需先將鑄錠熔解並精煉後，將鋁液注入模穴予以成型。成型後鑄胚先切除冒口再經過熱處理、機械加工等步驟方可進行各項品質檢驗。近年來因應節能減碳需求，運輸工業對輕量化且具高強度之零組件需求提升，發展鑄鋁合金塑性成型技術之研究亦與日俱增。基於鑄件製程對於A356合金微結構與性質的影響，以及考慮實際應用的層面，本研究首先探討熔煉中常用之鍶改良劑及熱處理條件對於鑄件微結構、磨耗腐蝕特性之效應。再深入研究旋壓成型（Spinning Deformation Processing, SDP）改善鑄鋁合金機械強度、耐磨及抗蝕等特性的機制，以提升工業應用的效益。實驗結果發現，以鍶改良之A356合金具有纖維狀共晶矽組織，細小地集中於樹枝間區域，達到完全改良狀態，導致乾磨耗速率及磨耗腐蝕速率降低。其腐蝕速率卻因伽凡尼腐蝕敏感性增加而提高，降低合金抗蝕性。熱處理球化共晶矽形貌、粗化其尺寸，亦藉析出相強化初晶鋁，提高合金硬度。乾磨耗速率與伽凡尼腐蝕敏感性因此降低，同時增強抵抗磨耗腐蝕的能力，減低合金磨耗腐蝕速率。以改善平衡電位下磨耗腐蝕速率為例，熱處理的改善效果高於鍶改良劑，其中以T6熱處理改善效果最顯著，可達29 %。以SDP加工A356合金，其鑄造微結構隨壁厚縮減率增加有明顯受擠壓、拉長之現象，且其變形具方向性。相對於未進行加工部位，其鑄造缺陷明顯減少，共晶矽被細化且散布於初晶鋁基地中。隨著壁厚縮減率增加，微結構與硬度有逐漸均勻之效果，導致材料受拉力後裂口成長不易，因而提高材料延展性、拉伸強度以及品質指標。材料耐磨特性亦受其影響，乾磨耗速率明顯下降。此外，加工後組織減低材料腐蝕敏感性，避免腐蝕集中攻擊樹枝間區域，提高合金抗蝕性。耐磨、抗蝕的提升有助於增強抵抗磨耗腐蝕的能力，因此SDP加工後材料具有較低之磨耗腐蝕速率。以改善平衡電位下磨耗腐蝕速率為例，A356合金經SDP加工其改善效果達31 %。Hypoeutectic Al-Si alloy containing Mg (Al-7Si-0.3Mg, A356 alloy) is extensively used for aerospace and automobile industry due to its lightness, excellent abrasion and corrosion resistance, high mechanical strength and good castability. The aluminum castings have precise dimension and good quality that had gone through the melt treatment, pouring to the mold for designing shape as well as subsequent heat treatment and mechanical machining. Due to constantly increasing ecological concerns and demands for higher performance, lightweight construction is a key factor to success mainly in the transportation sector. The development of metal forming technology on the aluminum casting becomes attractive for the foundry manufacturer. As design, materials, and manufacturing processes have to be considered integratively, the modifier and heat treatment as well as spinning deformation processing (SDP) related to microstructure and various properties of A356 alloy are investigated accordingly. The results showed that the addition of Sr modifier changed the morphology of eutectic Si particles and further improved the wear and wear-corrosion properties. Compared to unmodified A356 alloy, the corrosion resistance decreased with addition of Sr. The deterioration in corrosion property triggered the reduction in improvement of wear-corrosion rate when the applied potential increased. The heat treatment of A356 alloy spheroidezed and coarsened eutectic Si particles after the solution treatment as well as strengthened the ?-Al matrix after the aging treatment. The enhanced microstructure reduced the wear rate and improved the corrosion susceptibility of modified A356 alloy, which further enhanced the wear-corrosion resistance. When the A356 alloy wear-corroded at open circuit potential, the improvement from heat treatment was higher than that caused by Sr modifier. T6 heat treatment significantly reduced wear-corrosion rate for 29 %. The cast structure of A356 alloy became elongated with increasing reduction of thickness caused by SDP. Compared to absence of SDP, the casting defects were reduced, the eutectic Si particles were refined and distributed throughout the Al matrix. The hardness reached a steady value due to uniformity of microstructure increasing aggressively with the reduction of thickness. The enhanced microstructure retarded crack nucleation and coalescence resulting in improvement of elongation, tensile strength and quality index as well as reduction of dry wear rate. In addition to improvement of mechanical properties, the corrosion susceptibility of interdendritc region was reduced by dispreading eutectic Si particles and eliminating casting defects due to SDP resulting in a much reduced pitting profile in comparison with A356 alloy absence of SDP. As wear-corrosion test, the strengthening of wear resistance and reduction of corrosion susceptibility caused by SDP diminished the synergistic attack of wear and corrosion, which led to improvement of wear-corrosion rate of A356 alloy. When the A356 alloy wear-corroded at open circuit potential, SDP significantly reduced wear-corrosion rate for 31 %.
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