發(fā)布時間：2018-06-16 19:57

  本文選題：鎂合金板材 + 電致塑性　； 參考：《山東大學》2016年博士論文

【摘要】：作為21世紀的“朝陽金屬”,鎂合金具有低密度、高比強度／剛度、強阻尼、易回收等諸多優(yōu)點,有著非常廣闊的發(fā)展前景。近年來,結(jié)構(gòu)輕量化和環(huán)境保護需求的加劇,更加促進了鎂合金在航空航天、交通運輸、電子通訊等制造業(yè)領域的推廣應用。然而,鎂合金晶格的密排六方結(jié)構(gòu)導致其室溫延展性較差,嚴重制約了鎂合金的產(chǎn)業(yè)化發(fā)展。板材漸進成形作為一種柔性近凈成形加工工藝,通過對板材逐次局部成形代替整體成形以提高板材成形性能,是當今先進制造技術的重要發(fā)展方向之一。作為高能量場在塑性成形中的應用,電致塑性效應能夠顯著提高難成形材料的成形性能,被廣泛關注,但其電流密度閾值要求高,限制了其應用。單點漸進成形工藝中,工具頭與板材接觸面積小,能有效滿足電致塑性高電流密度閾值的要求。電致塑性效應與漸進成形技術的結(jié)合,能夠顯著提高鎂合金板材的成形性能和制件成形質(zhì)量,推動鎂合金成形工藝的發(fā)展,具有重要的理論意義與工程價值。本文以提高鎂合金板材成形性能為目的,將電致塑性效應引入板材漸進成形工藝,提出鎂合金板材電致塑性漸進成形技術,并對其成形工藝、成形機理以及電致塑性效應作用機制進行了實驗研究與機理分析,主要工作和研究成果如下：(1)提出了鎂合金板材電致塑性漸進成形方法,論證了其可行性與技術優(yōu)勢�；诶碚摲治�,對成形系統(tǒng)進行自主設計研究,搭建了電致塑性漸進成形實驗平臺。針對具體工況,對成形過程摩擦機理進行實驗研究與理論分析,研究結(jié)果表明,電致塑性漸進成形摩擦方式為載流混合摩擦,二硫化鉬涂層是其較理想潤滑劑,在實現(xiàn)穩(wěn)定導電的前提下能起到良好的潤滑作用。結(jié)合具體實驗平臺,對成形實施過程和成形質(zhì)量控制進行了理論分析,為后續(xù)試驗研究提供了理論指導。(2)基于響應曲面分析方法,研究了電致塑性漸進成形各關鍵工藝參數(shù)對板材成形性能的影響規(guī)律及顯著性。研究結(jié)果表明,電脈沖參數(shù)對板材成形極限有著最顯著的影響,隨著電脈沖參數(shù)的增加,板材成形極限明顯增大；進給速率和層間步進量均與成形極限響應值呈負相關關系,且顯著性水平相近,工具頭直徑對板材成形極限無顯著影響；電脈沖參數(shù)與進給速率／工具頭直徑、進給速率和層間步進量存在較顯著的交互作用。基于正交試驗,研究了電致塑性漸進成形工藝參數(shù)對制件成形幾何精度和表面硬度的影響規(guī)律及顯著性。研究結(jié)果表明,電脈沖參數(shù)是制件成形精度的最顯著影響因素,隨著電脈沖參數(shù)增加,制件幾何精度明顯提高；較低的進給速率、合適尺寸的中等層間步進量與工具頭直徑可明顯減小制件成形高度誤差,而較小的工具頭直徑可導致斜壁過多彈性變形積累,明顯增大成形回彈角度；對于制件表面硬度,只有選擇合適尺寸的中等工具頭直徑時可見其較明顯提高,其他因素對其無顯著影響。經(jīng)綜合分析,選取電脈沖參數(shù)、進給速率、層間步進量和工具頭直徑分別為80V/400Hz、800mm/min、0.2mm和8mm的參數(shù)組合時,板材成形性能和制件成形質(zhì)量較好。相對室溫不加電漸進成形,此工藝參數(shù)下的電致塑性漸進成形可大幅提高鎂合金板材成形能力。(3)基于成形工藝研究結(jié)果,選擇合理成形參數(shù),開展了電致塑性漸進成形實驗,結(jié)合理論分析,從成形過程溫度、成形力、應變狀態(tài)三方面對電致塑性漸進成形機理進行了研究。成形溫度研究結(jié)果表明,成形板材定點溫度由工具頭實時位置與工具頭實時溫度決定,僅在該點處于當前成形區(qū)附近時溫度較高,且高溫階段持續(xù)時間很短；成形溫升速率受電脈沖參數(shù)、工具頭直徑、制件成形角度影響,與進給速率和層間步進量無關,與電流值呈正相關,與工具頭尺寸和制件成形角度呈負相關。成形力研究表明,板材軸向成形力隨加工深度增加,先逐漸增大,后逐漸趨于穩(wěn)定,最后在電脈沖持續(xù)作用下呈現(xiàn)出降低趨勢；橫向成形力只在工具頭運動軌跡的切向產(chǎn)生,無徑向力存在,因此,橫向力在X,Y方向的分力隨工具頭圓周運動過程呈正弦規(guī)律交替變化且總相差π/2相位；電脈沖參數(shù)、層間步進量以及成形角度是影響成形力的較顯著因素,其次是進給速率,工具頭直徑對成形力影響不顯著；最大軸向力與電脈沖參數(shù)變化呈負相關,與進給速率、層間步進量、工具頭直徑以及成形角度成正相關。通過試驗手段獲取了成形板材不同加載路徑下的應變場分布以及極限應變點的應變歷史,研究結(jié)果表明,漸進成形過程板材成形區(qū)應變歷史為快速急劇變形過程,對于方錐臺形件模型直線加載區(qū)域,其應變狀態(tài)基本為平面應變,而圓錐臺形件模型為圓周加載,其應變狀態(tài)向雙向拉伸形式發(fā)展；較優(yōu)參數(shù)下的電致塑性漸進成形可使鎂合金板材達到很高的成形極限。(4)基于控制電脈沖參數(shù)的漸進成形試驗和單向拉伸試驗,研究了不同電脈沖參數(shù)對鎂合金電致塑性效應的影響規(guī)律,并結(jié)合理論模型,對電致塑性效應模式進行了分析。研究結(jié)果表明,鎂合金板材的電致塑性效應主要取決于脈沖有效值電流密度,隨著脈沖有效值電流密度的增加,板材漸進成形性能明顯提高。對于電致塑性單拉變形,板材流動應力隨脈沖有效值電流密度的增加逐漸降低,而板材的斷裂延伸率在達到一定值后不再增加,甚至呈現(xiàn)出降低趨勢,表明鎂合金板材的電致塑性拉伸變形存在一個最優(yōu)電脈沖參數(shù)范圍。另外,脈沖有效值電流密度對電致塑性效應的決定作用,以及各工藝參數(shù)與電流密度的關系,是引發(fā)各工藝參數(shù)對電致塑性效應存在交互影響作用的原因。電致塑性效應存在非熱效應,且其大小由脈沖峰值電流密度決定。在脈沖有效值電流密度相同或相近時,具有高峰值電流密度的電脈沖能產(chǎn)生更加顯著的電致塑性效應。電致塑性存在閾值效應,當脈沖有效值電流密度低于閾值時,板材成形性能隨脈沖有效值電流密度變化較小,電致塑性效應不明顯,而當其高過閾值后,電致塑性效應會顯著增加。(5)針對電致塑性漸進成形制件與電致塑性單拉試樣,開展了微觀組織研究,探討了電脈沖提高鎂合金板材成形性能的微觀機理。研究結(jié)果表明,電脈沖可降低鎂合金板材動態(tài)再結(jié)晶溫度,加速動態(tài)再結(jié)晶過程,同時,可抑制斷裂孔洞的產(chǎn)生,從而提高鎂合金板材成形性能。電致塑性漸進成形過程快速加熱、高溫持續(xù)時間短的特點,抑制了晶粒長大,促進鎂合金超細等軸晶粒的產(chǎn)生,為晶界滑移創(chuàng)造了先決條件,促使晶界滑移在較低溫度下發(fā)生。電致塑性拉伸變形過程中,大參數(shù)電脈沖可引發(fā)縮頸變形區(qū)瞬間高溫,使材料發(fā)生逆共晶反應(α+β=L),從而在材料內(nèi)部變形區(qū)引發(fā)一定量液相的產(chǎn)生。合適數(shù)量的液相可優(yōu)化材料變形機制,使材料延展性提高,但當液相數(shù)量過多時,可使晶界弱化,從而導致材料整體延展性的降低。
[Abstract]:As the "sunrise metal" in twenty-first Century, magnesium alloy has many advantages, such as low density, high specific strength / stiffness, strong damping, easy recovery and so on. It has a very broad prospects for development. In recent years, the demand for structure light weight and environmental protection is aggravated, and it has promoted magnesium alloy in aerospace, transportation, electronic communication and other manufacturing fields. However, the six square structure of the lattice of magnesium alloy leads to the poor ductility of the magnesium alloy, which seriously restricts the development of the magnesium alloy. As a flexible and near net forming process, the plate progressive forming is an advanced manufacturing technology by replacing the whole forming of sheet metal to improve the forming performance of the sheet. As one of the important development directions, as the application of high energy field in plastic forming, the electroplastic effect can significantly improve the formability of the refractory material, and it is widely concerned, but its current density threshold is high and its application is limited. In the single point progressive forming process, the contact area of the tool head and the plate is small, and the high electroplasticity can be effectively met. The requirement of the current density threshold. The combination of the electro plastic effect and the progressive forming technology can significantly improve the forming properties of the magnesium alloy sheet and the forming quality of the parts, and promote the development of the magnesium alloy forming process. It has important theoretical significance and engineering value. In this paper, the purpose of improving the forming performance of the magnesium alloy sheet is to make the electroplastic effect. The progressive forming process of sheet metal should be introduced, and the electroplastic progressive forming technology of magnesium alloy sheet is put forward, and its forming process, forming mechanism and the mechanism of electroplastic effect have been studied and analyzed. The main work and research results are as follows: (1) the method of progressive forming of magnesium alloy sheet is put forward. Feasibility and technical advantages. Based on theoretical analysis, the independent design of the forming system is carried out and an experimental platform for the progressive forming of electric plastic is set up. The experimental research and theoretical analysis of the friction mechanism of the forming process are carried out according to the specific working conditions. The results show that the friction mode of the progressive forming process is the mixed friction of the carrier flow and the molybdenum disulfide. The coating is an ideal lubricant, and it can play a good lubrication effect on the premise of stable conduction. Combining the concrete experimental platform, the forming implementation process and the forming quality control are analyzed theoretically. (2) based on the response curved surface analysis method, the electroplastic progressive forming is studied. The influence of the key process parameters on the sheet forming performance is found. The results show that the electric pulse parameters have the most significant influence on the sheet forming limit. With the increase of the electric pulse parameters, the forming limit of the sheet is obviously increased, and the feed rate and the interlayer step are all negatively related to the forming limit response value, and the significance is significant. The diameter of the tool head has no significant influence on the sheet forming limit, and there is a significant interaction between the electrical pulse parameters and the feed rate / tool head diameter, the feed rate and the interlayer step. Based on the orthogonal test, the influence of the technological parameters of the electroplastic progressive forming process on the geometric precision and the surface hardness of the parts is studied. The results show that the parameters of electric pulse are the most significant factors affecting the forming precision of parts. With the increase of the parameters of the electric pulse, the geometric precision of the workpiece is obviously improved. The lower feed rate, the suitable size of the middle step and the diameter of the tool head can obviously reduce the height error of the workpiece, and the smaller diameter of the tool head. It can lead to the accumulation of over elastic deformation of the inclined wall and obviously increase the springback angle of the forming. For the surface hardness of the workpiece, it is obvious that it is obviously improved when the diameter of the medium tool head is suitable for the selection of the suitable size, and the other factors have no significant influence on it. When the parameters of 80V/400Hz, 800mm/min, 0.2mm and 8mm are combined, the sheet forming properties and the forming quality of the parts are better. The forming ability of the magnesium alloy sheet can be greatly improved by the incremental forming of the electroplastic forming under this process. (3) based on the results of the forming process, the reasonable forming parameters are selected and the electroplasticity is carried out. The progressive forming mechanism of the forming process is studied from three aspects: the forming process temperature, the forming force and the strain state. The results of the forming temperature study show that the fixed point temperature of the formed sheet is determined by the real time position of the tool head and the real time temperature of the tool head, and the temperature is only at the time temperature near the current forming zone. The temperature rise rate is very short. The forming temperature rise rate is influenced by the parameters of electric pulse, the diameter of the tool head and the forming angle of the workpiece. It is not related to the feed rate and the interlayer step. It has a positive correlation with the current value. It has a negative correlation with the size of the tool head and the forming angle of the workpiece. In addition, it gradually increases, then gradually tends to stabilize, and finally shows a decreasing trend under the continuous action of electric pulse. The lateral forming force is only produced by the tangential direction of the tool head movement path, and there is no radial force. Therefore, the force of lateral force in the direction of X and Y is alternately changed with the sine law of the tool head circle movement and the difference of the phase of PI /2 is always different. The parameters of electric pulse, interlayer step and forming angle are the most significant factors affecting the forming force, followed by the feed rate, the diameter of the tool head has no significant influence on the forming force; the maximum axial force is negatively correlated with the change of the electrical pulse parameters, and is positively related to the feed rate, the interlayer step, the diameter of the tool head and the forming angle. The strain field distribution under different loading paths and the strain history of the limit strain point are obtained. The results show that the strain history of the sheet forming zone in the progressive forming process is a rapid and rapid deformation process. For the linear loading area of the square cone platform model, the strain state is basically plane strain, while the cone model is the model. For circumferential loading, the strain state of the alloy develops to the form of biaxial tension, and the electroplastic progressive forming under the superior parameters can make the magnesium alloy sheet reach a very high forming limit. (4) based on the progressive forming test and uniaxial tensile test, the influence of different pulse parameters on the electroplastic effect of magnesium alloys is studied. According to the theoretical model, the model of the electroplastic effect is analyzed. The results show that the electroplastic effect of the magnesium alloy sheet mainly depends on the current density of the pulse effective value. With the increase of the current density of the pulse effective value, the progressive forming performance of the plate is obviously higher. The increase of the current density of the pulse effective value gradually decreases, and the elongation at break of the plate no longer increases, or even shows a decreasing trend. It shows that there is an optimal electric pulse parameter range for the electroplastic tensile deformation of the magnesium alloy sheet. In addition, the effect of the current density of the pulse effective value on the electroplastic effect is determined. The relationship between the process parameters and the current density is the cause of the interaction of the process parameters on the electroplastic effect. The electroplastic effect has a non thermal effect and its size depends on the pulse peak current density. The electric pulse energy with the peak current density is the same or the phase of the pulse current density. There is a more significant electroplastic effect. There is a threshold effect on the electroplasticity. When the current density of the pulse effective value is lower than the threshold, the forming performance of the plate varies with the current density of the pulse effective value, and the electroplasticity effect is not obvious, and the electroplastic effect will increase significantly after the high threshold. (5) the progressive forming of the electroplastic forming process is made. The microstructural study was carried out and the microstructure of the magnesium alloy sheet was studied. The results showed that the electric pulse could reduce the dynamic recrystallization temperature of the magnesium alloy sheet and accelerate the dynamic recrystallization process. At the same time, it could inhibit the formation of the fracture cavity and thus improve the formation of magnesium alloy sheet. With the characteristics of rapid heating and short duration of high temperature, the characteristics of rapid heating and short duration of high temperature can inhibit grain growth and promote the production of superfine equiaxed grains in magnesium alloys. It creates a prerequisite for grain boundary slip and leads to the occurrence of grain boundary slip at a lower temperature. The high temperature in the shape area makes the material reverse the eutectic reaction (alpha + beta =L), which leads to the production of a certain amount of liquid phase in the internal deformation area of the material. The appropriate amount of liquid phase can optimize the deformation mechanism of the material and improve the ductility of the material, but when the amount of liquid phase is too large, the grain boundary can be weakened and the overall ductility of the material is reduced.
【學位授予單位】：山東大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：TG306

【引證文獻】

相關期刊論文 前1條

1 文懷興;劉桂芳;史鵬濤;;AZ31B鎂合金熱漸進成形實驗研究[J];鍛壓技術;2017年07期

，

本文編號：2027923