本文選題：鑄態(tài)Ti60合金　切入點(diǎn)：熱變形行為　出處：《西北工業(yè)大學(xué)》2015年博士論文

【摘要】：熱塑性加工技術(shù)具有生產(chǎn)效率高、材料利用率高、產(chǎn)品質(zhì)量穩(wěn)定等優(yōu)點(diǎn)，而且還能有效改善工件的力學(xué)性能，在金屬零件制造過程中占據(jù)了重要的地位。在熱塑性加工成形領(lǐng)域內(nèi)，金屬材料成形性能的研究一直是人們關(guān)注的重點(diǎn)之一，而金屬的韌性斷裂是影響成形性能的重要因素。隨著計(jì)算機(jī)硬件和軟件技術(shù)的發(fā)展以及損傷斷裂力學(xué)理論的日臻完善，利用數(shù)值模擬技術(shù)，將損傷斷裂力學(xué)引入有限元計(jì)算成為先進(jìn)熱塑性加工領(lǐng)域的前沿研究方向之一。然而，現(xiàn)有國內(nèi)外的韌性開裂準(zhǔn)則均基于室溫變形而建立，對于熱變形開裂的預(yù)測有一定的局限性，而且高溫高應(yīng)變速率條件下臨界開裂參數(shù)難以測定。因此，建立熱變形開裂準(zhǔn)則以迅速、準(zhǔn)確地預(yù)測可能出現(xiàn)的開裂行為，成為了熱塑性加工技術(shù)發(fā)展迫切需要解決的瓶頸問題之一。為此，本文以鑄態(tài)Ti60鈦合金為研究對象，研究了合金的熱變形行為，揭示了合金的開裂行為及其機(jī)制，建立了熱變形開裂準(zhǔn)則，并結(jié)合有限元模擬分析技術(shù)，系統(tǒng)分析了合金大規(guī)格棒材開坯工藝過程，優(yōu)化了開坯工藝參數(shù)，并進(jìn)行試驗(yàn)驗(yàn)證。主要研究內(nèi)容和結(jié)果如下： 基于流動(dòng)應(yīng)力應(yīng)變數(shù)據(jù)，揭示了不連續(xù)屈服程度與晶粒尺寸成反比關(guān)系，構(gòu)建了考慮應(yīng)變因素的本構(gòu)方程，采用動(dòng)態(tài)材料模型，繪制了Ti60合金的熱加工圖，優(yōu)化了Ti60鈦合金的加工窗口，理想加工參數(shù)為變形溫度1120℃，應(yīng)變速率為0.01~0.1s-1和變形量為45-60%。 基于熱壓縮試驗(yàn)樣本，研究了Ti60合金的開裂行為及機(jī)制，其表面主要表現(xiàn)為45°剪切開裂和縱向自由表面開裂兩種宏觀斷裂形貌，其微觀形貌分別表現(xiàn)為拉長型的韌窩和等軸韌窩，且合金開裂程度隨著變形溫度的降低、應(yīng)變速率和變形程度的增加而增加。裂紋易于在α/β相界、β晶界或氧化層中形核，并且傾向于沿著β晶界或者沿著β晶粒內(nèi)的α/β叢域擴(kuò)展。 提出了一種采用雙鏡頭、強(qiáng)補(bǔ)光以及專業(yè)攝影架等方式進(jìn)行修正了的高速攝影新方法，有效解決了裂紋觀測時(shí)裂紋產(chǎn)生位置隨機(jī)、觀測視場亮度較暗以及拍攝畫面不穩(wěn)定而存在臨界裂紋捕獲困難的問題，準(zhǔn)確測量了合金熱壓縮過程的臨界開裂變形量，提高了觀測裂紋出現(xiàn)位置及時(shí)間的精度。 通過對現(xiàn)有典型開裂準(zhǔn)則的理論分析以及所預(yù)測的開裂位置和臨界損傷變化規(guī)律可知，F(xiàn)rudenthal準(zhǔn)則可以預(yù)測熱壓縮過程發(fā)生的心部開裂，而CL、OK、Brozzo、McClintock、Oyane、RT準(zhǔn)則均可預(yù)測赤道處自由表面開裂，這與實(shí)際完全吻合。其中，OK準(zhǔn)則能較好綜合考慮建立開裂準(zhǔn)則時(shí)所考慮的應(yīng)力因素，且在一定程度上等價(jià)于Kuhn經(jīng)驗(yàn)準(zhǔn)則和McClintock空洞合并模型。基于引起Ti60合金韌性開裂的誘發(fā)應(yīng)力為環(huán)向拉應(yīng)力，本文選用累積塑性能模型的OK準(zhǔn)則進(jìn)行修正以期建立Ti60合金的熱變形開裂準(zhǔn)則。 綜合累積塑性能模型OK準(zhǔn)則力能函數(shù)形式和溫度補(bǔ)償應(yīng)變速率Zener-Hollomon因子，最終建立了用于預(yù)測Ti60合金高溫?zé)嶙冃蔚捻g性開裂準(zhǔn)則。采用FORTRAN語言二次開發(fā)子程序?qū)嶙冃伍_裂準(zhǔn)則嵌入商用有限元軟件DEFORM-3D中，實(shí)現(xiàn)了合金變形-傳熱-損傷耦合分析，預(yù)測結(jié)果與試驗(yàn)結(jié)果吻合較好，驗(yàn)證了模型的有效性。 最后，借助已嵌入開裂模型子程序的DEFORM-3D有限元軟件，以實(shí)現(xiàn)充分破碎鑄態(tài)組織和防止開裂為目標(biāo)，，對大尺寸Ti60合金鑄錠進(jìn)行不同砧型(平砧、120oV型砧和U型砧)拔長、單道次下不同壓下量、壓下速度和進(jìn)給量以及多道次下拔長開坯、鐓拔開坯和翻轉(zhuǎn)角度等不同工藝參數(shù)下的有限元模擬。 平砧拔長坯料端面開裂損傷嚴(yán)重及產(chǎn)生“端面窩心”缺陷，但實(shí)際開坯過程中無需更換模具，開坯效率較高。U型砧和120oV型砧具有較高的心部應(yīng)變和較好的防開裂的效果，但型砧尺寸隨試樣截面尺寸變化而變化，開坯效率較低。 單道次變形條件較優(yōu)的變形量、壓下速度以及進(jìn)給量分別為30%、10-15mm/s和130mm。基于單道次優(yōu)化的工藝參數(shù)，拔長開坯或鐓拔開坯時(shí)，四個(gè)道次后不僅可實(shí)現(xiàn)鑄錠組織被充分打碎，且表面未出現(xiàn)明顯的開裂。且增加45°翻轉(zhuǎn)而進(jìn)行的六方拔長比90°翻轉(zhuǎn)進(jìn)行的四方拔長具有更好的截面應(yīng)變積累和更加均勻的應(yīng)變分布。實(shí)驗(yàn)證實(shí)，選擇優(yōu)化的開坯工藝參數(shù)，大尺寸Ti60鈦合金鑄錠變形均不會(huì)產(chǎn)生明顯開裂。
[Abstract]:Thermoplastic processing technology has the advantages of high production efficiency, high material utilization rate, the advantages of stable product quality, but also can effectively improve the mechanical properties of the workpiece, occupy an important position in the metal parts manufacturing process. In the hot plastic forming field, research on the forming properties of metal materials has been one of the focus of attention however, fracture toughness of metal is an important factor affecting the forming performance. With the development of computer hardware and software technology development and fracture mechanics theory, numerical simulation technology, the damage and fracture mechanics into the finite element method has become one of the frontier research field of advanced thermoplastic processing. However, the current domestic and overseas toughness fracture criterion is based on room temperature deformation and the establishment, has some limitations for prediction of thermal deformation and cracking, high temperature and high strain rate under the condition of critical cracking The parameters are difficult to measure. Therefore, the establishment of thermal deformation and cracking criterion to quickly and accurately predict the cracking behavior may occur, become one of the urgent need to solve the bottleneck problem of the development of thermoplastic processing technology. Therefore, the as cast Ti60 alloy as the research object, studied alloy hot deformation behavior, reveals the cracking the behavior and mechanism of the alloy, hot deformation and fracture criterion is established, combined with the finite element simulation analysis, system analysis of large size bar alloy billet process, optimized process parameters, and test. The main research contents and results are as follows:
Based on the flow stress and strain data, reveals the discontinuous yielding degree and grain size is inversely proportional to the relationship, construct constitutive equation considering strain factors, using the dynamic materials model, drawing the hot processing map of Ti60 alloy, the optimization of the processing window of Ti60 titanium alloy, the ideal processing parameters for the deformation temperature of 1120 DEG C, should be the transmission rate of 0.01~0.1s-1 and 45-60%. deformation
Hot compression test sample based on the cracking behavior and mechanism of Ti60 alloy was studied, the surface is mainly 45 degrees and the longitudinal shear crack free surface cracking of two macroscopic fracture morphology, the microstructure were elongated type of dimples and dimple, and alloy cracking degree with the temperature decreasing, strain rate and deformation degree increases. Cracks in the alpha / beta phase boundary, nuclear beta grain boundaries or oxide, and tend to along grain boundaries or along the grain in the beta beta alpha / beta bundle domain extension.
Put forward a new method using dual lens, high-speed photography was fixed and fill strong professional photography frame etc., effectively solve the crack observation when crack location is random, observation field brightness is dark and image instability and critical crack capture difficult problem, accurate measurement of the critical cracking alloy hot compression process the amount of deformation, improve the crack observation time and the accuracy.
Through the theoretical analysis of the existing typical fracture criterion and crack position prediction and critical damage variation shows that the Frudenthal criterion can predict the heart of cracking, hot compression process of CL, OK, Brozzo, McClintock, Oyane, RT criterion can predict the equator free surface cracking, which are consistent with the real one., the OK criteria can better integrated into considering crack criterion of the stress factor, and equivalent to a certain extent in the experience of Kuhn criterion and the McClintock cavity with model induced by Ti60. The toughness of the alloy cracking stress for tension based on the cumulative plastic OK criterion performance model was modified to establish Ti60 alloy thermal deformation Cracking Criterion.
Comprehensive cumulative plastic performance model OK criterion can function form and temperature compensated strain rate factor Zener-Hollomon, finally established for predicting ductile fracture criterion of thermal deformation of Ti60 alloy at high temperature. Using FORTRAN language to develop two times the thermal deformation and fracture criterion for embedded subroutine of the commercial finite element software DEFORM-3D, the alloy thermoviscoplastic damage coupling the analysis and prediction results are in good agreement with the test results, verify the validity of the model.
Finally, with the help of finite element software DEFORM-3D has been embedded in cracking model subroutine, to achieve full of broken cast structure and prevent cracking as the goal, different type of anvil of large size ingot of Ti60 alloy (flat anvil, anvil type 120oV and type U anvil) pull long, single pass under different pressure, and multi pass pull long billet under speed and feed pressure, finite element simulation of upsetting cogging and flip angle under different process parameters.
Flat anvil and blank end cracking damage serious "face." defects, but the actual opening without the need to replace the mold blank in the process of high efficiency billet type.U and type 120oV anvil high heart strain anvil and good anti cracking effect, but the anvil with the size of cross section size of samples changes billet, low efficiency.
Single pass deformation deformation conditions. The optimum pressure speed and feed rate were 30%, 10-15mm/s and 130mm. single optimization based on process parameters, drawing billet or upsetting billet, four times after not only can realize the ingot microstructure was fully broken, and surface cracking of Ming Dynasty the section strain and increase the pull. Four 45 degree turn and six party pulled longer than the 90 degree turn for long has better accumulation and more uniform strain distribution. Experiments proved that the optimized blooming process parameters, large size Ti60 titanium alloy ingot deformation are not obvious cracking.

【學(xué)位授予單位】：西北工業(yè)大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2015
【分類號】：TG146.23

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