柔性變極性等離子弧特性及鋁合金橫焊穿孔熔池行為
發(fā)布時(shí)間：2019-06-03 23:03
【摘要】：隨著鋁合金焊接結(jié)構(gòu)大型化發(fā)展,原位焊接制造成為關(guān)注的焦點(diǎn),其中最為典型的應(yīng)用是重型運(yùn)載火箭燃料貯箱的焊接,而鋁合金橫焊技術(shù)成為制約其發(fā)展的瓶頸。鋁合金橫焊難度較大、焊接質(zhì)量和效率較低,主要是重力對(duì)熔池作用和鋁合金本身物理特性導(dǎo)致。為解決上述問題、提高鋁合金橫焊效率和質(zhì)量,開展了變極性等離子弧鋁合金穿孔橫焊技術(shù)的研究。根據(jù)常規(guī)變極性等離子弧(簡(jiǎn)稱“常規(guī)電弧”)穿孔橫焊難點(diǎn)和穿孔熔池穩(wěn)定建立條件,從改變電弧特性角度出發(fā),提出柔性變極性等離子弧(簡(jiǎn)稱“柔性電弧”)。在穿孔焊接前提下,通過改變電弧能量和壓力分布,促進(jìn)穿孔熔池穩(wěn)定建立,最終實(shí)現(xiàn)了8mm厚2A14鋁合金板穿孔橫焊,焊縫成形良好且組織、力學(xué)性能優(yōu)異。首先,采用常規(guī)電弧對(duì)6mm、8mm厚鋁合金板進(jìn)行穿孔橫焊試驗(yàn),明確鋁合金穿孔橫焊的瓶頸問題。板厚6mm時(shí),穿孔橫焊的難點(diǎn)是咬邊和氣孔缺陷的控制;板厚8mm時(shí),穿孔橫焊的難點(diǎn)是如何保證穿孔熔池穩(wěn)定建立。針對(duì)穿孔橫焊難點(diǎn),探究咬邊缺陷的成因和穿孔熔池穩(wěn)定建立的條件。正面咬邊缺陷主要是由較大熱輸入和電弧壓力導(dǎo)致;背面咬邊缺陷主要是由較小熱輸入或較大電弧壓力導(dǎo)致。熔池動(dòng)態(tài)行為觀察結(jié)果表明,熔池背面上、下側(cè)熔化金屬的搭橋連接是穿孔熔池建立的關(guān)鍵,熔化金屬在熔池正面的填充能夠防止穿孔熔池的崩潰,圍繞小孔的流動(dòng)是穩(wěn)定穿孔熔池的標(biāo)志。根據(jù)問題產(chǎn)生原因,提出了初步控制措施。“小規(guī)范+背面預(yù)熱”措施可消除6mm板穿孔橫焊咬邊缺陷;“坡口設(shè)計(jì)”措施可促進(jìn)8mm板穿孔熔池的建立,但未能徹底解決穿孔熔池穩(wěn)定性問題。根據(jù)穿孔熔池穩(wěn)定建立的熱、力條件,提出了從改變電弧特性角度控制穿孔熔池穩(wěn)定性的研究思路。通過使用三孔型水冷噴嘴,改變電弧能量和壓力分布,獲得柔性變極性等離子弧,改善了橫焊縫成形、提高了穿孔過程穩(wěn)定性。良好作用效果源于其獨(dú)特的電弧特性。與常規(guī)電弧相比,柔性電弧形態(tài)擴(kuò)展,壓縮程度降低;靜特性下移,阻抗特性降低。通過分裂陽(yáng)極法測(cè)量電弧能量密度分布,利用水冷銅板小孔法檢測(cè)電弧壓力分布。與常規(guī)電弧相比,柔性電弧能量集中程度較高,源于三孔型水冷噴嘴的使用和較大的離子氣流量;在工件表面柔性電弧能量分布呈橢圓形,改變了穿孔熔池形貌;柔性電弧壓力幅值及梯度都大大降低,增加了電弧穩(wěn)定性。通過穿孔熔池受力計(jì)算和流動(dòng)行為分析,闡釋了柔性電弧的作用機(jī)理:電弧壓力的減小,使焊縫背面最大允許孔徑增大,促進(jìn)了穿孔熔池尾部的填充;其熱源特性提高了熔化金屬的流動(dòng)性,促進(jìn)了熔化金屬的搭橋連接和正面填充,提高了穿孔焊接的穩(wěn)定性。利用流體動(dòng)力學(xué)計(jì)算軟件Fluent,對(duì)穿孔焊接過程進(jìn)行數(shù)值模擬,定量研究電弧形式和電弧壓力對(duì)熔池行為的影響。為準(zhǔn)確描述柔性電弧特性,建立了雙橢圓面-錐體復(fù)合熱源模型和力源模型。通過UDF(User Defined Function)二次開發(fā)和VOF(Volume of Fluid)界面追蹤技術(shù),實(shí)現(xiàn)了三維瞬態(tài)穿孔焊接數(shù)值模擬并驗(yàn)證了模型的可靠性。在穿孔熔池形成過程模擬中,柔性電弧穿孔速度慢,熱傳導(dǎo)在工件寬度方向作用顯著,熔池溫度較高,溫度場(chǎng)分布不對(duì)稱;電弧壓力增加使穿孔直徑增大、小孔周圍金屬液膜變薄、向熔池正面的流動(dòng)速度降低。在穿孔熔池填充過程模擬中,柔性電弧熔池內(nèi)熔化金屬的搭橋連接速度快、填充能力強(qiáng),熔池凝固速度慢。根據(jù)模擬結(jié)果,提出了穿孔熔池填充判定條件:表面張力附加力大于電弧力和重力合力;熔化金屬搭橋連接和正面填充時(shí)間小于熔池凝固時(shí)間。判定條件揭示了橫焊位置穿孔熔池的熱、力平衡機(jī)制。最后,對(duì)鋁合金柔性電弧穿孔橫焊工藝及接頭組織、力學(xué)性能進(jìn)行研究,并與常規(guī)電弧立焊進(jìn)行對(duì)比分析,檢驗(yàn)工藝穩(wěn)定性和接頭可靠性。柔性電弧穿孔橫焊工藝窗口較寬,具有良好的焊接穩(wěn)定性;采用大電流、氧化膜刮削和焊前預(yù)熱的綜合措施可有效控制氣孔缺陷。柔性電弧橫焊接頭抗拉強(qiáng)度和斷后延伸率略高于立焊接頭,斷裂模式為塑性斷裂,接頭中微氣孔缺陷和離異共晶是重要的裂紋源。橫焊接頭上側(cè)熱影響區(qū)組織晶粒細(xì)小、硬度值較高,組織和力學(xué)性能具有不對(duì)稱性。這種不對(duì)稱性是由橫焊縫上、下側(cè)焊接熱循環(huán)的不對(duì)稱性引起的,而根源是重力導(dǎo)致的穿孔熔池金屬流動(dòng)的不對(duì)稱。
[Abstract]:With the development of the large-scale welding structure of the aluminum alloy, the in-situ welding is the focus of attention. The most typical application is the welding of the fuel tank of the heavy-duty vehicle, and the technology of the lateral welding of the aluminum alloy becomes the bottleneck of the development. The transverse welding of the aluminum alloy is difficult, the welding quality and the efficiency are low, mainly due to the effect of gravity on the molten pool and the physical characteristics of the aluminum alloy. In order to solve the above problems, the transverse welding efficiency and quality of the aluminum alloy are improved, and the technology of transverse welding of the variable-polarity plasma arc aluminum alloy is carried out. The flexible variable-polarity plasma arc ("flexible arc") is proposed according to the difficulty of transverse welding of the conventional variable-polarity plasma arc ("conventional arc") and the stable establishment condition of the perforated molten pool. Under the premise of perforation welding, by changing the arc energy and pressure distribution, the stable establishment of the perforated molten pool is promoted, and finally, the through-hole transverse welding of the 8mm-thick 2A14 aluminum alloy plate is realized, and the welding seam is well formed and the mechanical property is excellent. First, by using the conventional electric arc, the 6-mm and 8-mm-thick aluminum alloy plate is used for punching and transverse welding, and the bottleneck problem of the transverse welding of the aluminum alloy is determined. When the plate thickness is 6 mm, the difficulty of the transverse welding of the perforation is the control of the defect of the bite edge and the air hole; when the plate thickness is 8 mm, the difficulty of the transverse welding of the perforation is how to ensure the stable establishment of the perforated weld pool. In view of the difficulty of the cross-welding of the perforation, the cause of the undercut and the conditions for the stable establishment of the molten pool are investigated. The face-edge defects are mainly caused by larger thermal input and arc pressure; the back-edge defects are mainly caused by small thermal input or large arc pressure. The observation of the dynamic behavior of the molten pool shows that the bridging connection of the molten metal on the back of the molten pool is the key to the establishment of the perforated molten pool, and the filling of the molten metal on the front of the molten pool can prevent the collapse of the molten pool, and the flow around the small holes is the mark of the stable perforated pool. According to the cause of the problem, the preliminary control measures are put forward. The "Small Specification + Back Preheating" measures can eliminate the defects of the horizontal welding of the 6-mm plate through the transverse welding, and the "groove design" measures can promote the establishment of the 8-mm plate through-hole molten pool, but the stability of the perforated molten pool can not be thoroughly solved. Based on the heat and force conditions established in the stable molten pool, a new approach to control the stability of the molten pool from the change of the arc characteristic is presented. By using the three-pass water-cooling nozzle, the arc energy and the pressure distribution are changed, the flexible variable-polarity plasma arc is obtained, the transverse welding seam is improved, and the stability of the perforation process is improved. The good effect results from its unique arc characteristics. Compared with the conventional electric arc, the flexible arc shape expands, the compression degree is reduced, the static characteristic is moved down, and the impedance characteristic is reduced. The electric arc energy density distribution is measured by a split-anode method, and the arc pressure distribution is detected by a water-cooled copper plate small hole method. Compared with the conventional electric arc, the flexible arc energy concentration degree is high, the use of the three-pass water-cooling nozzle and the large ion gas flow rate are generated, the flexible arc energy distribution on the surface of the workpiece is elliptical, the shape of the perforated molten pool is changed, and the amplitude and the gradient of the flexible arc pressure are greatly reduced, And the arc stability is increased. The action mechanism of the flexible arc is illustrated by the analysis of the stress calculation and the flow behavior of the perforated molten pool. The arc pressure is reduced, the maximum allowable pore diameter of the back surface of the weld is increased, and the filling of the tail of the molten pool is promoted; and the heat source property of the welding molten pool improves the fluidity of the molten metal, The bridging connection and the front filling of the molten metal are promoted, and the stability of the through-hole welding is improved. The influence of the arc form and the arc pressure on the behavior of the molten pool is quantitatively studied by means of the fluid dynamics calculation software Fluent. In order to accurately describe the characteristics of the flexible arc, a double-ellipsoid-cone composite heat source model and a force source model are established. Through the development of the UDF (User Defined Function) and the Volume of Fluid interface tracking technique, the numerical simulation of three-dimensional transient perforation welding is realized and the reliability of the model is verified. in the process simulation of the punching molten pool, the flexible arc perforation speed is slow, the heat conduction is significant in the width direction of the workpiece, the temperature of the molten pool is high, the distribution of the temperature field is not symmetrical, the increase of the arc pressure causes the diameter of the through hole to increase, and the metal liquid film around the small hole is thin, The flow rate to the front of the bath is reduced. In the process simulation of the filling process of the punching molten pool, the bridge connecting speed of the molten metal in the flexible arc welding pool is high, the filling capacity is strong, and the solidification speed of the molten pool is slow. According to the simulation results, the conditions for determining the filling of the weld pool are put forward: the additional force of surface tension is larger than that of the arc force and the gravity force, and the melting metal bridging connection and the front filling time are less than the setting time of the molten pool. The determination condition reveals the heat and force balance mechanism of the weld pool in the horizontal welding position. In the end, the transverse welding technology and joint structure and mechanical property of the aluminum alloy flexible arc perforation are studied, and compared with the conventional arc welding, the stability of the process and the reliability of the joint are verified. The flexible arc punching transverse welding process window is wide and has good welding stability, and the comprehensive measures of high current, oxidation film scraping and pre-welding pre-welding can effectively control the pore defects. The tensile strength and elongation of the flexible arc transverse welding head are slightly higher than that of the vertical welding head, and the fracture mode is plastic fracture, and the micro-pore defect and the divorced eutectic in the joint are important crack sources. The microstructure and mechanical properties of the lateral heat-affected zone on the lateral welding head are fine, the hardness value is high, and the structure and the mechanical property are asymmetric. This asymmetry is caused by the asymmetry of the heat cycle on the horizontal weld and the lower side, and the root cause is the asymmetry of the metal flow in the molten pool caused by gravity.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2015
【分類號(hào)】：TG456.2
本文編號(hào)：2492288
[Abstract]:With the development of the large-scale welding structure of the aluminum alloy, the in-situ welding is the focus of attention. The most typical application is the welding of the fuel tank of the heavy-duty vehicle, and the technology of the lateral welding of the aluminum alloy becomes the bottleneck of the development. The transverse welding of the aluminum alloy is difficult, the welding quality and the efficiency are low, mainly due to the effect of gravity on the molten pool and the physical characteristics of the aluminum alloy. In order to solve the above problems, the transverse welding efficiency and quality of the aluminum alloy are improved, and the technology of transverse welding of the variable-polarity plasma arc aluminum alloy is carried out. The flexible variable-polarity plasma arc ("flexible arc") is proposed according to the difficulty of transverse welding of the conventional variable-polarity plasma arc ("conventional arc") and the stable establishment condition of the perforated molten pool. Under the premise of perforation welding, by changing the arc energy and pressure distribution, the stable establishment of the perforated molten pool is promoted, and finally, the through-hole transverse welding of the 8mm-thick 2A14 aluminum alloy plate is realized, and the welding seam is well formed and the mechanical property is excellent. First, by using the conventional electric arc, the 6-mm and 8-mm-thick aluminum alloy plate is used for punching and transverse welding, and the bottleneck problem of the transverse welding of the aluminum alloy is determined. When the plate thickness is 6 mm, the difficulty of the transverse welding of the perforation is the control of the defect of the bite edge and the air hole; when the plate thickness is 8 mm, the difficulty of the transverse welding of the perforation is how to ensure the stable establishment of the perforated weld pool. In view of the difficulty of the cross-welding of the perforation, the cause of the undercut and the conditions for the stable establishment of the molten pool are investigated. The face-edge defects are mainly caused by larger thermal input and arc pressure; the back-edge defects are mainly caused by small thermal input or large arc pressure. The observation of the dynamic behavior of the molten pool shows that the bridging connection of the molten metal on the back of the molten pool is the key to the establishment of the perforated molten pool, and the filling of the molten metal on the front of the molten pool can prevent the collapse of the molten pool, and the flow around the small holes is the mark of the stable perforated pool. According to the cause of the problem, the preliminary control measures are put forward. The "Small Specification + Back Preheating" measures can eliminate the defects of the horizontal welding of the 6-mm plate through the transverse welding, and the "groove design" measures can promote the establishment of the 8-mm plate through-hole molten pool, but the stability of the perforated molten pool can not be thoroughly solved. Based on the heat and force conditions established in the stable molten pool, a new approach to control the stability of the molten pool from the change of the arc characteristic is presented. By using the three-pass water-cooling nozzle, the arc energy and the pressure distribution are changed, the flexible variable-polarity plasma arc is obtained, the transverse welding seam is improved, and the stability of the perforation process is improved. The good effect results from its unique arc characteristics. Compared with the conventional electric arc, the flexible arc shape expands, the compression degree is reduced, the static characteristic is moved down, and the impedance characteristic is reduced. The electric arc energy density distribution is measured by a split-anode method, and the arc pressure distribution is detected by a water-cooled copper plate small hole method. Compared with the conventional electric arc, the flexible arc energy concentration degree is high, the use of the three-pass water-cooling nozzle and the large ion gas flow rate are generated, the flexible arc energy distribution on the surface of the workpiece is elliptical, the shape of the perforated molten pool is changed, and the amplitude and the gradient of the flexible arc pressure are greatly reduced, And the arc stability is increased. The action mechanism of the flexible arc is illustrated by the analysis of the stress calculation and the flow behavior of the perforated molten pool. The arc pressure is reduced, the maximum allowable pore diameter of the back surface of the weld is increased, and the filling of the tail of the molten pool is promoted; and the heat source property of the welding molten pool improves the fluidity of the molten metal, The bridging connection and the front filling of the molten metal are promoted, and the stability of the through-hole welding is improved. The influence of the arc form and the arc pressure on the behavior of the molten pool is quantitatively studied by means of the fluid dynamics calculation software Fluent. In order to accurately describe the characteristics of the flexible arc, a double-ellipsoid-cone composite heat source model and a force source model are established. Through the development of the UDF (User Defined Function) and the Volume of Fluid interface tracking technique, the numerical simulation of three-dimensional transient perforation welding is realized and the reliability of the model is verified. in the process simulation of the punching molten pool, the flexible arc perforation speed is slow, the heat conduction is significant in the width direction of the workpiece, the temperature of the molten pool is high, the distribution of the temperature field is not symmetrical, the increase of the arc pressure causes the diameter of the through hole to increase, and the metal liquid film around the small hole is thin, The flow rate to the front of the bath is reduced. In the process simulation of the filling process of the punching molten pool, the bridge connecting speed of the molten metal in the flexible arc welding pool is high, the filling capacity is strong, and the solidification speed of the molten pool is slow. According to the simulation results, the conditions for determining the filling of the weld pool are put forward: the additional force of surface tension is larger than that of the arc force and the gravity force, and the melting metal bridging connection and the front filling time are less than the setting time of the molten pool. The determination condition reveals the heat and force balance mechanism of the weld pool in the horizontal welding position. In the end, the transverse welding technology and joint structure and mechanical property of the aluminum alloy flexible arc perforation are studied, and compared with the conventional arc welding, the stability of the process and the reliability of the joint are verified. The flexible arc punching transverse welding process window is wide and has good welding stability, and the comprehensive measures of high current, oxidation film scraping and pre-welding pre-welding can effectively control the pore defects. The tensile strength and elongation of the flexible arc transverse welding head are slightly higher than that of the vertical welding head, and the fracture mode is plastic fracture, and the micro-pore defect and the divorced eutectic in the joint are important crack sources. The microstructure and mechanical properties of the lateral heat-affected zone on the lateral welding head are fine, the hardness value is high, and the structure and the mechanical property are asymmetric. This asymmetry is caused by the asymmetry of the heat cycle on the horizontal weld and the lower side, and the root cause is the asymmetry of the metal flow in the molten pool caused by gravity.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2015
【分類號(hào)】：TG456.2
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