發(fā)布時間：2018-07-03 18:53

  本文選題：7×××系鋁合金 + 半連續(xù)鑄造��； 參考：《北京科技大學(xué)》2016年博士論文

【摘要】：高強(qiáng)度7×××系鋁合金因其優(yōu)異的性能在航空航天等工業(yè)領(lǐng)域得到廣泛的應(yīng)用,然而該系鋁合金在半連續(xù)鑄造時往往會面臨開裂缺陷這一嚴(yán)重問題的挑戰(zhàn)。熱裂是合金在凝固時形成的裂紋,與糊狀區(qū)中熱應(yīng)力應(yīng)變、液相補(bǔ)縮及半固態(tài)合金微觀組織有關(guān)。冷裂是鑄錠冷卻到低溫時發(fā)生的脆性開裂,是鑄錠中殘余應(yīng)力和鑄態(tài)合金塑性的惡化導(dǎo)致的。本文結(jié)合Gleeble拉伸實(shí)驗(yàn)、約束桿鑄造、熱收縮測量以及計(jì)算機(jī)模擬等方法對半連續(xù)鑄造開裂現(xiàn)象及相關(guān)判據(jù)進(jìn)行了系統(tǒng)研究。分別采用連續(xù)冷卻法和原位凝固法測量了鑄態(tài)7050鋁合金在固態(tài)和半固態(tài)的機(jī)械性能,研究了合金強(qiáng)度和塑性隨溫度的變化趨勢以及應(yīng)變速率的作用,并根據(jù)所得實(shí)驗(yàn)數(shù)據(jù)擬合了擴(kuò)展Ludwik方程和高溫蠕變方程所需的參數(shù)。在此基礎(chǔ)上建立了模擬鋁合金半連續(xù)鑄造的有限元模型,詳細(xì)研究了大尺寸的圓鑄錠和扁鑄錠中溫度場、流場和應(yīng)力場分布以及工藝參數(shù)的變化所帶來的影響。以Suyitno的SKK判據(jù)為基礎(chǔ),提出了一個適用于7×x×系鋁合金半連續(xù)鑄造的改進(jìn)型熱裂判據(jù)。新判據(jù)引入了兩個重要的參數(shù),分別是描述凝固過程中晶粒聚合情況的參數(shù)fGBWL和熱裂紋在沿聚合晶界擴(kuò)展時所消耗能量的固相能項(xiàng)Us。新熱裂判據(jù)預(yù)測的糊狀區(qū)熱裂敏感性曲線與鑄造經(jīng)驗(yàn)比較一致。測量了幾種具有不同Zn含量和Zn/Mg比的7×××系鋁合金的熱裂敏感性及其在凝固過程中的熱收縮行為。研究發(fā)現(xiàn),合金熱裂傾向并不完全取決于其在凝固過程中的熱收縮量,凝固時形成的非平衡共晶含量也對合金的熱裂敏感性有深刻影響。晶粒細(xì)化可以顯著降低7×××系鋁合金的熱裂傾向,但是過量加入細(xì)化劑又會使合金熱裂敏感性出現(xiàn)明顯上升。后者據(jù)推測可能是由于晶粒過度細(xì)化和在晶界形成大量含Ti一次析出相引起的液相補(bǔ)縮能力降低所致。根據(jù)實(shí)驗(yàn)結(jié)果和有限元模擬提出了一個冷裂指數(shù),發(fā)現(xiàn)7×××系鋁合金半連續(xù)鑄造時,鑄錠冷裂傾向在250。C以上很低,隨著溫度降至200。C以下開始迅速上升。在相同的鑄造工藝條件下,鑄錠半連鑄冷裂傾向大體上與合金的非平衡共晶含量一致。這很可能是因?yàn)?低溫下大量沿晶界分布的共晶極大地削弱了鑄態(tài)合金的塑性,使鑄錠更容易因應(yīng)力集中而開裂。根據(jù)冷裂指數(shù)可知,采用刮水處理使鑄錠中的殘余應(yīng)力降低,可以有效地減小合金在半連續(xù)鑄造時發(fā)生冷裂的傾向。
[Abstract]:The high strength 7 脳 脳 脳 series aluminum alloy has been widely used in aerospace and other industrial fields because of its excellent properties. However, it often faces the challenge of cracking defect in semi-continuous casting. Hot cracking is a kind of crack formed during solidification of the alloy, which is related to the thermal stress and strain in the paste zone, liquid phase feeding and microstructure of the semi-solid alloy. Cold cracking is the brittle cracking of ingot when it is cooled to low temperature. It is caused by the deterioration of residual stress in ingot and the plasticity of as-cast alloy. In this paper, Gleeble tensile experiment, casting of restrained rod, thermal shrinkage measurement and computer simulation are used to study the cracking phenomenon and related criteria of semi-continuous casting. The mechanical properties of as-cast 7050 aluminum alloy in solid and semi-solid state were measured by continuous cooling method and in situ solidification method, respectively. The variation trend of strength and plasticity with temperature and the effect of strain rate on the strength and plasticity of the alloy were studied. The parameters of the extended Ludwik equation and the high temperature creep equation are fitted according to the experimental data. A finite element model for simulating semi-continuous casting of aluminum alloy is established, and the influence of temperature field, flow field and stress field distribution and technological parameters on the temperature field, flow field and stress field in large scale round ingot and flat ingot are studied in detail. Based on Suyitno's SKK criterion, an improved hot cracking criterion for 7 脳 x 脳 series aluminum alloy semi-continuous casting is proposed. The new criterion introduces two important parameters, fGBWL, which describe the state of grain polymerization during solidification, and the solid energy term, U _ s, of the energy consumed by the thermal crack propagating along the polymerized grain boundary. The hot crack sensitivity curve predicted by the new hot crack criterion is in good agreement with the casting experience. The hot cracking susceptibility and thermal shrinkage behavior of several 7 脳 脳 脳 series aluminum alloys with different Zn content and Zn / mg ratio during solidification were measured. It is found that the hot cracking tendency of the alloy does not depend entirely on its thermal shrinkage during solidification, and the non-equilibrium eutectic content formed during solidification also has a profound influence on the hot cracking sensitivity of the alloy. Grain refinement can significantly reduce the hot cracking tendency of 7 脳 脳 脳 series aluminum alloy, but excessive addition of refiner can obviously increase the hot cracking sensitivity of the alloy. The latter may be due to excessive grain refinement and the reduction of the liquid phase feeding capacity due to the formation of a large number of Ti containing primary precipitates at grain boundaries. Based on the experimental results and the finite element simulation, a cold cracking index is proposed. It is found that the cold cracking tendency of the ingot is very low above 250.C when the casting temperature of 7 脳 脳 脳 system aluminum alloy is semicontinuous casting, and it begins to rise rapidly with the temperature falling below 200.C. Under the same casting conditions, the cold cracking tendency of the ingot is approximately the same as the non-equilibrium eutectic content of the alloy. This is probably because at low temperature a large number of eutectic particles distributed along grain boundaries greatly weaken the plasticity of as-cast alloys and make ingots more susceptible to cracking due to stress concentration. According to the cold crack index, the residual stress in the ingot can be reduced by water scraping treatment, which can effectively reduce the cold cracking tendency of the alloy during semi-continuous casting.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TG292

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