本文選題：鎂合金板材 + 織構�。� 參考：《重慶大學》2015年博士論文

【摘要】：鎂合金作為最輕的商業(yè)金屬結構材料,被廣泛應用于航空航天、汽車、電子通信等領域。然而由于鎂合金密排六方的晶體結構,其室溫下的滑移系很少,導致其塑性變形能力很差。傳統(tǒng)的擠壓和軋制鎂合金板材具有很強的基面織構,導致其具有很強的各向異性,限制了鎂合金板材的廣泛使用。本文從改善商業(yè)鎂合金板材組織結構,提高其力學性能及成型性能的角度出發(fā),首先利用晶體塑性有限元模擬了鎂合金板材在拉伸和壓縮過程中基面織構的演變,基于模擬結果,分別采用室溫大變形預拉伸和預壓縮工藝來改善鎂合金板材的組織,研究了變形后及退火后鎂合金板材的組織轉變、織構演變、力學性能及成型性能。最后,通過三種理論模型對比計算了鎂合金板材室溫下的成型極限圖以及n值和r值對成型極限的影響。主要研究內容及成果如下:①通過晶體塑性有限元模型,模擬了AZ31B鎂合金板材在拉伸和壓縮過程中織構的轉變,以及不同的變形方向對織構偏轉的影響。拉伸變形處理后的板材基面織構增強,但是其分布更為發(fā)散。拉伸板材的基面織構沿著與預拉伸方向垂直的方向呈橢圓形分布,且沿該方向其分布更為發(fā)散。沿TD方向預壓縮變形使其晶粒的c軸向TD方向發(fā)生偏轉,導致基面織構強度降低,以及沿TD方向織構的增強。預壓縮板材晶粒的c軸總是平行于預壓縮的方向,導致平行于預壓縮方向基面織構強度的增強。②采用室溫大變形量預拉伸及再結晶退火可以有效提高擠壓AZ31B鎂合金板材的成型性能。經過350°C 1h退火后板材的基面織構強度明顯降低,基面織構由ND方向朝ED方向偏轉,且分布更加彌散。隨著預拉伸變形量的增加,處理后板材的成型性能逐漸提高,屈服強度、塑性應變比r值降低,應變硬化指數(shù)n值增大,其Erichsen值為5.6mm,相對于原始板材的2.5mm提高了~124%。③分別沿ED、45°和TD方向對擠壓AZ31B鎂合金板材進行室溫大變形預拉伸及再結晶退火,研究了預拉伸方向對板材力學性能及成型性能的影響。經過三種不同方向預拉伸的板材,其基面織構強度均得以弱化,但是偏轉方向均垂直于板材預拉伸的方向,板材沿垂直于預拉伸方向的力學性能得到明顯改善,即y沿該方向具有較低的屈服強度、r值以及較高的n值,同時沿預拉伸方向的力學性能降低不明顯。④采用預拉伸速度分別為0.3,3,30,120 mm/min,在準靜態(tài)范圍內研究了預拉伸速度對再結晶退火后AZ31B鎂合金板材成型性能的影響。隨著應變速度的提高,退火后板材的成型性能逐漸提高,當拉伸速度為30 mm/min時,預拉伸板材的erichsen值達到最大值為5.4mm,隨著應變速度的繼續(xù)增大,成型性能呈降低趨勢。⑤采用室溫預壓縮及退火工藝(pca)改善1.0mm厚az31b鎂合金薄板成型性能,并研究了其在變形及退火過程中的組織演變及力學性能的變化。沿td方向預壓縮及退火后,由于拉伸孿晶及靜態(tài)再結晶的緣故,晶粒的c軸∥td方向,使pca板材的基面滑移、c+a滑移和拉伸孿晶變得更加容易,使其具有較低的屈服強度、屈強比,較高的n值和較低的r值,pca板材的erichsen值也由原始的2.35mm提高到4.55mm,提高了~94%。同時,當預壓縮變形量超過4.8%時,鎂合金板材成型性能的改善不再明顯。⑥分別沿rd、45°和td方向對az31b鎂合金板材進行相同變形量的預壓縮,研究了預壓縮方向對鎂合金板材組織及力學性能的影響。通過對比三種不同壓縮方向對鎂合金板材組織、力學性能及成型性能的影響研究發(fā)現(xiàn),pca板材晶粒的c軸由nd方向偏向于預壓縮的方向,同時沿rd及td方向預壓縮可以明顯改善該方向及45°方向的性能,而沿45°方向預壓縮只能改善rd和td方向的性能,此外,沿垂直于板材基面織構發(fā)散的方向進行預壓縮可以最大程度的改善鎂合金板材的成型性能。⑦采用250°c2h和450°c1h的退火工藝研究了去應力退火與再結晶退火對鎂合金板材組織及力學性能的影響。通過對比去應力退火與再結晶退火后預壓縮板材組織、力學性能及成型性能可以發(fā)現(xiàn),兩種退火方式都可以弱化鎂合金板材的基面織構,使其晶粒發(fā)生偏轉,可以降低鎂合金板材的屈服強度、屈強比以及塑性應變比,提高其應變硬化指數(shù),但是保留拉伸孿晶的去應力退火對鎂合金薄板的成型性能改善不大,而再結晶退火可以顯著改善其成型性能,去應力退火板材與再結晶退火板材的erichsen值分別為2.57mm和5.28mm,分別提高了~9%和~125%。⑧研究了150°c下預壓縮及退火對鎂合金板材組織及力學性能的影響。通過150°c沿td方向的預壓縮及退火處理后的板材,pca板材晶粒的c軸平行于td方向,基面織構弱化,并且沿td方向出現(xiàn)峰值,同時具有其較小的屈服強度、屈強比以及r值和較大的n值,pca板材的erichsen值由原始的2.35mm提高到了5.28mm,提高了近125%。⑨分別基于m-k理論、swift分散性失穩(wěn)理論和hill集中性失穩(wěn)理論計算了az31b鎂合金板材在室溫下的成型極限圖(fld)。采用合理的力學性能參數(shù),基于m-k模型的fld可以很好的預測鎂合金板材在雙向拉伸狀態(tài)下的成型極限,基于swift分散性失穩(wěn)理論的fld可以較為準確的預測鎂合金板材在拉-壓狀態(tài)下的成型極限,而基于hill集中性失穩(wěn)理論的fld與實驗數(shù)據(jù)偏差很大,并不適合鎂合金成型極限圖的理論計算。鎂合金板材n值得增大以及r值得降低,都可以有效提高鎂合金板材的室溫成型極限。
[Abstract]:Magnesium alloy, as the lightest commercial metal structure material, is widely used in aerospace, automotive, electronic communication and other fields. However, because of the crystal structure of six sides of magnesium alloy, the slip system at room temperature is very few, resulting in poor plastic deformation ability. The traditional extrusion and rolling magnesium alloy sheet has a strong base texture, which leads to it. With a strong anisotropy, the extensive use of magnesium alloy plates is limited. From the angle of improving the structure of the magnesium alloy sheet and improving the mechanical properties and forming properties of the magnesium alloy, the evolution of the texture of the base surface of the magnesium alloy sheet in the stretching and compression process is simulated by the crystal plastic finite element, based on the simulation results. The microstructure transformation, texture evolution, mechanical properties and molding properties of magnesium alloy sheet after deformation and annealing are studied. Finally, the molding limit diagram and n value and R value of magnesium alloy sheet at room temperature are compared and calculated by three theoretical models. The main contents and results are as follows: 1. Through the plastic finite element model of crystal, the texture transformation of AZ31B magnesium alloy sheet during tension and compression is simulated, and the influence of different direction of deformation on the deflection of texture. The texture of the base surface after the tensile deformation is enhanced, but its distribution is more divergent. The texture of the base surface of the plate is elliptical along the direction perpendicular to the pretension direction, and the distribution is more divergent along this direction. The pre compression deformation along the direction of TD leads to the deflection of the c axial TD direction of the grain, which leads to the decrease of the texture strength of the base surface and the enhancement along the texture in the direction of the TD. The c axis of the grain of the pre compressed sheet is always parallel to the precompression. The direction of contraction leads to the enhancement of texture strength parallel to the base surface of the precompression direction. 2. The pre tensile and recrystallization annealing can effectively improve the forming properties of the extruded AZ31B magnesium alloy sheet. After 350 C 1H annealing, the texture strength of the base surface of the plate is obviously reduced, and the texture of the base surface is deflected from the direction of ND to the ED direction. With the increase of pre tensile deformation, the molding performance of the plate is increased gradually, the yield strength, the plastic strain ratio R value is reduced, the strain hardening exponent n value increases, the Erichsen value is 5.6mm. Compared with the 2.5mm of the original plate, the ~124%. 3 increases the room temperature of the extruded AZ31B magnesium alloy plate along the direction of ED, 45 and TD, respectively. The effects of pre stretching and recrystallization annealing on the mechanical properties and molding properties of the plate are studied. The texture strength of the base surface is weakened by three different directions, but the deflection direction is perpendicular to the direction of the plate pretension, and the mechanical properties of the plate along the direction perpendicular to the pretension are obvious. The improvement is that y has lower yield strength, R value and higher N value along this direction, while the mechanical properties decrease along the pretension direction at the same time. (4) the pre stretching speed is 0.3,3,30120 mm/min respectively. The effect of pre stretching speed on the molding properties of AZ31B magnesium alloy sheet after recrystallization annealing is studied in the quasi static range. The forming performance of the plate is improved gradually after annealing. When the tensile speed is 30 mm/min, the maximum Erichsen value of the pre stretched sheet is 5.4mm. The molding performance is reduced with the strain speed increasing. 5. The molding property of 1.0mm thick AZ31B magnesium alloy sheet is improved by the room temperature precompression and annealing process (PCA). In the process of deformation and annealing, the changes of microstructure and mechanical properties in the process of deformation and annealing are studied. After precompression and annealing along the direction of TD, the TD direction of the c axis of the grain makes the base plane of the PCA sheet slip, and the c+a slip and the tension twin become more easy because of the tensile twins and the static recrystallization. The yield strength is lower and the yield strength is lower. Strong ratio, higher N value and lower R value, the Erichsen value of PCA plate is also increased from original 2.35mm to 4.55mm, and ~94%. is improved. When the pre compression deformation amount is more than 4.8%, the improvement of molding performance of magnesium alloy plate is no longer obvious. 6. The pre compression of the same deformation amount along Rd, 45 degree and TD direction, respectively, is studied. The influence of precompression direction on Microstructure and mechanical properties of magnesium alloy sheet. By comparing the effects of three different compression directions on microstructure, mechanical properties and molding properties of magnesium alloy sheet, it is found that the c axis of the PCA plate is biased in the direction of Nd in the direction of pre compression, and the direction of this direction and the precompression along the direction of RD and TD can obviously improve the direction and 45. The performance of RD and TD direction can only be improved along the direction of 45 degrees. In addition, the precompression perpendicular to the direction of the texture and divergence of the sheet base can improve the molding properties of the magnesium alloy plate to the greatest extent. By comparing the microstructure, mechanical properties and molding properties of the pre compression plate after the stress annealing and recrystallization annealing, the two kinds of annealing methods can weaken the texture of the base surface of the magnesium alloy sheet and make the grain deflect, which can reduce the yield strength, the yield strength ratio and the strength ratio of the magnesium alloy sheet. The strain hardening exponent is improved by plastic strain ratio, but the improvement of tensile twins' stress annealing has little improvement on the forming properties of magnesium alloy sheet, and the recrystallization annealing can significantly improve the molding properties. The Erichsen values of the annealed sheet and the recrystallized sheet are 2.57mm and 5.28mm, respectively, which are improved by ~9% and ~125%. respectively. The effect of pre compression and annealing on the microstructure and mechanical properties of magnesium alloy sheet under 150 C was studied. The c axis of the grain of PCA plate was parallel to TD direction through the pre compression and annealing treatment of 150 degree C along TD direction. The texture of the base surface was weakened, and the peak value appeared along the TD direction. At the same time, the smaller yield strength, the yield strength ratio, the R value and the relative value were compared. The large n value, the Erichsen value of the PCA sheet is increased from the original 2.35mm to the 5.28mm, and the near 125%. is improved based on the m-k theory, the swift dispersion instability theory and the hill centralized instability theory are used to calculate the forming limit diagram (FLD) of the AZ31B magnesium alloy sheet at room temperature. The molding limit of magnesium alloy sheet under biaxial tension is predicted. Based on the swift dispersion instability theory, FLD can accurately predict the forming limit of magnesium alloy sheet under tension and compression, while the FLD and experimental data based on the theory of hill concentrated instability are not suitable for the theoretical calculation of the molding limit diagram of magnesium alloy. Magnesium alloy sheet n is worth increasing and R is worth reducing. It can effectively improve the room temperature forming limit of magnesium alloy sheet.

【學位授予單位】：重慶大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG306;TG146.22

【參考文獻】

相關期刊論文 前1條

1 李瑛,余剛,劉躍龍,葉立元,郭小華;鎂合金的表面處理及其發(fā)展趨勢[J];表面技術;2003年02期

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本文編號：1892512