本文選題：再制造 + 脈沖電流　； 參考：《大連理工大學(xué)》2014年博士論文

【摘要】：再制造工程作為一項(xiàng)戰(zhàn)略型新興產(chǎn)業(yè)不僅能夠節(jié)省資源、能源,保護(hù)環(huán)境,而且具有顯著的經(jīng)濟(jì)效益。再制造毛坯損傷形式復(fù)雜多樣,其中裂紋損傷所占比重很大,在以往的再制造過程中,無論什么形式的裂紋損傷一經(jīng)被檢測出來直接報(bào)廢,因此,金屬材料裂紋止裂與損傷愈合技術(shù)是機(jī)械裝備再制造領(lǐng)域急需解決的關(guān)鍵問題之一,尤其對于含有裂紋損傷的高附加值機(jī)械裝備(如工程機(jī)械、船舶、飛機(jī)、大型壓縮機(jī))的核心部件實(shí)施再制造時(shí),首先要修復(fù)裂紋損傷才能保證后續(xù)再制造工藝的有效性。脈沖電流由于其自身的一些特性被應(yīng)用于材料制備及物理機(jī)械性能改善領(lǐng)域。關(guān)于脈沖電流裂紋止裂、愈合技術(shù).目前國內(nèi)外缺少系統(tǒng)的理論分析和實(shí)驗(yàn)研究。本文圍繞脈沖電流的繞流效應(yīng)和焦耳熱效應(yīng)在裂紋止裂、愈合中的應(yīng)用,從理論分析、實(shí)驗(yàn)研究和數(shù)值模擬三個(gè)角度開展研究工作。本文的主要研究內(nèi)容和研究成果如下： (1)基于復(fù)變函數(shù)理論,建立脈沖電流裂紋止裂過程理論模型。根據(jù)自由邊界處電流密度為零的邊界條件,將帶有貫穿橢圓形切口的無限大薄板中通入電流問題的求解過程劃分為無橢圓孔和存在橢圓孔擾動(dòng)兩種狀態(tài)的疊加,簡化了裂紋邊界條件處理過程。當(dāng)橢圓孔短軸趨于零時(shí),可以將其近似看作裂紋,電流密度、焦耳熱源功率和熱應(yīng)力在裂紋尖端具有奇異性,仿照斷裂力學(xué)中應(yīng)力強(qiáng)度因子概念,引入強(qiáng)度因子的概念描述裂紋尖端附近區(qū)域電、熱、熱應(yīng)力場,為設(shè)計(jì)實(shí)驗(yàn)方案和選擇工藝參數(shù)提供理論基礎(chǔ)。 (2)基于脈沖功率技術(shù),自行設(shè)計(jì)并搭建強(qiáng)脈沖電流放電裝置HCPD-I。通過對電容器放電回路的分析,確定實(shí)驗(yàn)研究的工藝參數(shù)為電容量和電容充電電壓；增加電容量能夠提高回路電流密度和延長脈沖電流作用時(shí)間；增大電容充電電壓只能夠提高回路電流密度。 (3)選用工業(yè)中應(yīng)用廣泛的AISI316L和壓縮機(jī)轉(zhuǎn)子葉輪材料Inconel625和FV520B為研究對象,通過實(shí)驗(yàn)研究材料種類、試樣尺寸和工藝參數(shù)對裂紋尖端熔化區(qū)尺寸和微觀組織的影響規(guī)律,探討利用脈沖進(jìn)行裂紋止裂的機(jī)理。脈沖電流處理過程,由于兩裂紋面間距較大電流無法通過,電流在裂紋前端繞流集中,在焦耳熱作用下,裂紋尖端局部溫度升高甚至瞬間熔化,裂紋尖端曲率半徑增大,改善了裂紋前端的應(yīng)力集中狀態(tài)。由于脈沖電流作用時(shí)間非常短暫,裂紋前端在快速加熱和冷卻作用下,熔化區(qū)周圍局部組織發(fā)生固態(tài)相變,組織均勻細(xì)化。而遠(yuǎn)離裂紋尖端的基體電流密度相對較低,焦耳熱作用不明顯,表明脈沖電流具有選擇作用,對無裂紋部位影響很小。 (4)采用拉伸方式預(yù)制裂紋的尖端組織發(fā)生嚴(yán)重的塑性變形,沿著拉力軸線方向呈纖維狀分布。以AISI316L材料為例探討脈沖電流裂紋止裂同時(shí),對裂紋尖端局部塑性變形區(qū)的恢復(fù)機(jī)理。利用掃描電鏡和透射電鏡分析脈沖電流處理前后裂紋尖端形貌、顯微組織和位錯(cuò)形態(tài)的變化,研究結(jié)果表明,脈沖電流處理后,尖銳的裂紋尖端變?yōu)闄E圓形熔孔,裂紋尖端曲率半徑增大,降低了裂紋尖端的應(yīng)力集中,并且裂紋前端局部發(fā)生再結(jié)晶,形成細(xì)小等軸晶粒,局部塑性變形組織發(fā)生恢復(fù)。 (5)采用鉆孔壓縮法預(yù)制內(nèi)部微裂紋,初步探索脈沖電流用于裂紋愈合的可行性。分析工藝參數(shù)和脈沖電流處理次數(shù)對裂紋愈合效果的作用規(guī)律,并從能量的角度探討脈沖電流作用對微裂紋愈合的機(jī)理。脈沖電流處理過程,在繞流集中和焦耳熱效應(yīng)的作用下,裂紋周圍形成壓應(yīng)力,使裂紋面間距變窄,裂紋周圍原子在電流作用下向缺陷部位移動(dòng)填充,將長裂紋分隔成斷續(xù)短裂紋,短裂紋在愈合過程產(chǎn)生孔洞,孔洞尺寸縮小,數(shù)量減少,直至裂紋全部愈合。小工藝參數(shù)、多次處理更有利于延長裂紋愈合區(qū)域尺寸,提高愈合效率。 (6)分析裂紋尖端形貌和微觀組織對力學(xué)性能演變趨勢的影響。納米壓痕硬度測量結(jié)果表明,脈沖電流處理后裂紋前端熱影響區(qū)硬度高于基體。X射線殘余應(yīng)力分析結(jié)果表明,脈沖電流處理后裂紋前端產(chǎn)生了強(qiáng)大的壓應(yīng)力。脈沖電流處理后,AISI316L、 Inconel625和FV520B三種材料的抗拉強(qiáng)度均呈現(xiàn)不同程度的提高,而延伸率變化趨勢各異,脈沖電流對塑性的影響與材料種類有關(guān)。研究工藝參數(shù)對拉伸性能和疲勞性能的作用規(guī)律,增大放電回路的電容量和電容充電電壓均可以擴(kuò)大裂紋尖端的熔化區(qū)面積,但裂紋尖端熔化區(qū)尺寸過大反而會降低其力學(xué)性能。通過建立熔化區(qū)尺寸與拉伸性能和疲勞性能關(guān)系曲線,確定本實(shí)驗(yàn)條件下裂紋止裂效果最佳的熔化區(qū)尺寸區(qū)間。含有拉伸裂紋的試樣經(jīng)過脈沖電流處理后,拉伸性能和疲勞性能均得到改善。尖銳的裂紋尖端變鈍,降低了裂紋尖端的應(yīng)力集中,尤其是裂紋前端局部塑性變形恢復(fù),組織細(xì)化、位錯(cuò)密度增大是提高拉伸裂紋力學(xué)性能的主要原因。 (7)基于電-熱-結(jié)構(gòu)耦合理論,應(yīng)用ANSYS有限元分析軟件直觀展示脈沖電流處理過程電流在裂紋前端繞流集中和焦耳熱釋放現(xiàn)象。以AISI316L試樣為例,按照實(shí)驗(yàn)過程進(jìn)行建模、施加載荷和邊界條件,模擬計(jì)算試樣厚度、裂紋長度和工藝參數(shù)對裂紋前端熔化區(qū)尺寸的影響規(guī)律,并對比實(shí)驗(yàn)測量結(jié)果,兩者誤差小于12%；計(jì)算了脈沖電流處理過程中試樣的應(yīng)力場分布和殘余應(yīng)力應(yīng)力場分布,模擬計(jì)算的殘余應(yīng)力場與實(shí)驗(yàn)測量結(jié)果數(shù)量級相同。
[Abstract]:As a strategic emerging industry , it can not only save resources , energy , protect the environment , but also have obvious economic benefits . In the process of remanufacturing the core components of mechanical equipment , such as engineering machinery , ship , aircraft and large - scale compressor , it is necessary to repair the crack damage in order to ensure the effectiveness of the subsequent re - manufacturing process .

( 1 ) Based on the theory of complex function , a theoretical model of the crack arrest process of the pulse current is established . According to the boundary condition of zero current density at the free boundary , it is divided into two states : no elliptical hole and existence of elliptical hole disturbance . It can be considered as crack , current density , Joule heat source power and thermal stress . The concept of stress intensity factor in fracture mechanics can be considered . The concept of strength factor is introduced to describe the electrical , thermal and thermal stress fields near the crack tip . The theoretical basis is provided for designing experimental scheme and selecting process parameters .

( 2 ) Based on the pulse power technology , HCPD - I was designed and built . By analyzing the discharge circuit of capacitor , it is determined that the process parameters of the experimental study are capacitance and capacitor charging voltage .
increasing the electric capacity can improve the current density of the loop and prolong the action time of the pulse current ;
increasing the charge voltage of the capacitor can only improve the circuit current density .

( 3 ) In the process of pulse current treatment , the local temperature of the crack tip is increased and the stress concentration state of the front end of the crack is improved . The current density of the crack tip is relatively low due to the large current of the crack tip , and the thermal effect of the Joule heat is not obvious .

The results show that the sharp crack tip becomes elliptical hole , the radius of curvature of the crack tip is increased , the stress concentration of the crack tip is reduced , and the crack tip is locally recrystallized to form fine equiaxial crystal grains , and the local plastic deformation structure is restored .

( 5 ) The effect of pulse current on the healing of cracks is investigated by means of drilling compression method . The effect of pulse current on the healing of cracks is analyzed .

( 6 ) The influence of the morphology of the crack tip and microstructure on the evolution tendency of the mechanical properties is analyzed . The results of the nano indentation hardness measurement show that the hardness of the front end of the crack is higher than that of the matrix after the pulse current treatment . The results show that the tensile strength and the fatigue property of the crack tip are improved after the pulse current treatment . The sharp crack tip becomes dull , the stress concentration of the crack tip is increased , and the microstructure is refined . The increase of dislocation density is the main reason to improve the mechanical properties of the tensile crack .

( 7 ) Based on the theory of electro - thermal - structural coupling , ANSYS finite element analysis software is applied to show the flow concentration and Joule heat release phenomenon of the pulse current in the front end of the crack . Taking the AISI316L specimen as an example , the influence law of the thickness , crack length and process parameters on the size of the crack tip melting zone is simulated according to the experimental procedure , and the experimental results are compared with the error of less than 12 % .
The stress field distribution and residual stress field distribution of the sample during the pulse current processing are calculated . The residual stress field of the simulated calculation is of the same order of magnitude as the experimental measurement result .

【學(xué)位授予單位】：大連理工大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2014
【分類號】：TH16;TG661

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