本文選題：超硬鋁合金　切入點：雙脈沖熔化極氬弧焊　出處：《沈陽工業(yè)大學(xué)》2017年博士論文

【摘要】：Al-Zn-Mg-Cu(7XXX)系鋁合金是20世紀40年代伴隨著航空技術(shù)發(fā)展而產(chǎn)生的超硬鋁合金材料,具有強重比高、機械性能優(yōu)良、耐蝕性較好和環(huán)�？苫厥盏戎T多優(yōu)勢,廣泛應(yīng)用于航空航天、國防軍事、軌道車輛及汽車工業(yè)等部門,成為以上領(lǐng)域最具競爭力的結(jié)構(gòu)材料。目前超硬鋁合金的應(yīng)用主要以擠壓和模鍛工藝成形,很少采用焊接連接,即使采用焊接結(jié)構(gòu),絕大部分也是使用攪拌摩擦焊(FSW)工藝固相連接,其主要原因是超硬鋁在傳統(tǒng)熔焊工藝過程中極易出現(xiàn)熱裂紋、氣孔等缺陷和焊接接頭軟化,以上因素限制了超硬鋁合金的進一步應(yīng)用。雙脈沖熔化極氬弧焊(DP-MIG)有其獨特的高低頻脈沖耦合攪拌作用,對Al-Zn-Mg-Cu系鋁合金實施熔化焊接成為可能。本課題以AA7075-T651超硬鋁合金為研究對象,基于DP-MIG焊接方法,采用“低匹配”的ER5356及ER5556焊絲焊接6.35mm厚軋制板材,連接方式為開V型坡口對接平焊。實驗方案:首先進行DP-MIG焊接參數(shù)優(yōu)化,采用高速攝影分析雙脈沖特性電弧形態(tài)、熔滴過渡行為,研究低頻脈沖頻率F對焊接接頭組織及力學(xué)性能的影響;優(yōu)化焊接參數(shù)基礎(chǔ)上實施機械振動焊接試驗,并確定恰當振動頻率及振幅;最后將優(yōu)化工藝的焊接接頭施加焊后熱處理(PWHT),研究不同焊絲填充焊縫的力學(xué)性能和疲勞性能,焊接接頭不同區(qū)域的納米級微觀組織性能,諸如晶界、孿晶界及偏析等,并觀察時效析出強化相的大小、形狀及分布,進而分析焊后熱處理T6制度對焊縫顯微組織及力學(xué)性能的影響機理。通過DP-MIG焊接參數(shù)優(yōu)化和“雙脈沖MIG-機械振動”聯(lián)合參數(shù)優(yōu)化對AA7075-T651鋁合金焊接,然后對不同試件進行焊后熱處理,得出以下主要結(jié)論:(1)采用DP-MIG焊AA7075-T651超硬鋁合金,分別以ER5356焊絲與ER5556焊絲為填充金屬進行焊接參數(shù)優(yōu)化,所得最佳焊接工藝參數(shù)如下:焊接平均電流I=190A、焊接速度V=0.78cm·s-1、低頻脈沖頻率F=3Hz、電弧挺度參數(shù)為-6,保護氣體流量為18L/min、焊絲干伸長為12mm。試驗結(jié)果為:ER5356焊絲焊接接頭抗拉強度為292.5MPa,達母材抗拉強度的52.0%,接頭最低硬度為80.8HV,伸長率為3.5%,而ER5556焊絲焊接接頭的抗拉強度為342.5 MPa,為母材抗拉強度的60.9%,焊縫最低硬度為90.6HV,伸長率為8.5%;與ER5356焊絲相比,ER5556焊絲接頭拉伸斷口上有明顯的撕裂棱,同時韌窩較深且大小勻稱,韌窩中第二相顆粒很多,呈現(xiàn)為微孔聚集性斷裂模式,而ER5356斷口上撕裂棱較少,韌窩尺寸明顯偏大且淺,韌窩中第二相顆粒較少,斷口表現(xiàn)出混合斷裂形貌,即穿晶斷裂與沿晶斷裂同時存在。(2)“雙脈沖MIG-機械振動”組合工藝的最佳優(yōu)化規(guī)范參數(shù)是:焊接電流I=190A、焊接速度V=0.78 cm·s-1、振動幅度A=0.02mm,振動頻率F=40Hz,組合工藝規(guī)范體現(xiàn)焊接參數(shù)對振動參數(shù)的良好交互作用。施加振動后,以ER5356焊絲作為填充金屬,接頭的抗拉強度為352.6MPa,焊接接頭強度系數(shù)為0.627;接頭近焊縫側(cè)熔合線區(qū)域柱狀晶較細,其方向趨于晶粒長大方向,晶內(nèi)密布細小析出相,焊縫中心組織為細小的等軸樹枝晶和振動方向一致的胞狀樹枝晶組成;顯微金相觀察焊接接頭各區(qū)域沒有發(fā)現(xiàn)明顯氣孔和微裂紋,振動優(yōu)化后焊縫組織比無振動焊縫組織細小,DP-MIG高低頻耦合攪拌與機械振動綜合作用,有效抑制熱裂紋產(chǎn)生,并加速氣孔的溢出。(3)采用焊后熱處理(固溶處理(480℃×50 min)+室溫水淬+人工時效(80℃×24h)+人工時效(120℃×24h))各工藝試驗,對比兩種無振動接頭試驗得知,ER5356焊絲填充接頭的抗拉強度為445.7MPa,焊接接頭強度系數(shù)從0.520提高至0.793,ER5556焊絲填充接頭的抗拉強度為490.0MPa,焊接接頭強度系數(shù)從0.609提高至0.872;在“雙脈沖MIG-機械振動”組合工藝下,ER5356焊絲填充接頭抗拉強度為490.2MPa,焊接接頭強度系數(shù)從0.558提高至0.872。(4)PWHT對AA7075-T651超硬鋁合金焊接接頭性能恢復(fù)的本質(zhì)原因是時效強化析出相的種類、數(shù)量、大小及分布狀態(tài)決定的。TEM顯示,焊接接頭不同區(qū)域有數(shù)量較多深淺不同、體積大小不等的近球形或圓棒狀時效析出相MgZn2彌散分布,促使其力學(xué)性能大幅度恢復(fù)和“軟化”區(qū)得到顯著改善;低周疲勞試驗顯示,ER5556焊絲比ER5356焊絲填充的焊接接頭具有更長的疲勞壽命,ER5356焊絲填充的焊接接頭在較低的0.3-0.5%外加總應(yīng)變幅下呈現(xiàn)較高的循環(huán)變形抗力,而在較大的0.6%和0.7%外加總應(yīng)變幅下,ER5556焊絲填充焊接接頭呈現(xiàn)較高的循環(huán)變形抗力,形變硬化較強�；贒P-MIG焊AA7075-T651鋁合金,采用機械振動工藝及焊后熱處理(PWHT)是減少焊接缺陷、提高焊接質(zhì)量的有效方式,為采用MIG熔焊工藝焊接超硬鋁合金提供理論支撐和指導(dǎo)意義。
[Abstract]:Al-Zn-Mg-Cu (7XXX), Aluminum Alloy is super alloy material in 1940s along with the development of aviation technology and the high strength weight ratio, excellent mechanical properties, good corrosion resistance and Recyclable environmental protection and many other advantages, is widely used in aerospace, military, railway vehicles and automobile industry and other departments, to become the most competitive the above material fields. The structure of aluminum alloy mainly by extrusion and forging process of forming, little use of welded connections, even with the welding structure, the vast majority are also using friction stir welding (FSW) process of solid connection, the main reason is that aluminum hot crack can easily arise in the traditional welding process, porosity and other defects and the welding joint softening, the above factors restrict the further application of super hard aluminum alloy. Double pulsed MIG welding (DP-MIG) has its unique high and low frequency pulse coupling. Mix, implementation of melting welding is possible on the Al-Zn-Mg-Cu system Aluminum Alloy. In this paper the AA7075-T651 alloy as the research object, based on the DP-MIG welding method, the welding of 6.35mm thick plate rolled low, "ER5356 and ER5556 wire connection for V groove butt welding. The experimental scheme: firstly, DP-MIG welding parameters. Optimization of double pulse arc shape characteristics analysis of high speed photography, the droplet transfer behavior of low frequency pulse frequency effect of F on Microstructure and mechanical properties; optimization of welding parameters based on mechanical vibration welding test, and to determine the proper vibration frequency and amplitude; finally the optimized technology of welded joints after welding heat treatment (applying PWHT), mechanical properties and fatigue properties of weld filler of different welding wire, nano microstructure and properties in different regions of the joints, such as grain boundary, twin boundary And segregation, and observe the precipitation hardening phase size, shape and distribution, and then analyzes the mechanism of post weld heat treatment T6 system influence on the microstructure and mechanical properties of welding seam. The parameter optimization and double pulse MIG- mechanical vibration welding joint parameter optimization of AA7075-T651 Aluminum Alloy by DP-MIG welding, then different specimens the heat treatment after welding, the main conclusions are as follows: (1) the DP-MIG welding of AA7075-T651 aluminum alloy, ER5356 and ER5556 respectively by wire welding wire for filler metal for welding parameter optimization, the optimal welding parameters are as follows: the average welding current I=190A, welding speed V=0.78cm s-1, low frequency pulse frequency F=3Hz, arc stiffness parameter -6, gas flow is 18L/min, the wire extension for the 12mm. test results: ER5356 wire welded joint tensile strength is 292.5MPa, up to 52% of the tensile strength of the base metal, the joint Low hardness is 80.8HV, elongation is 3.5%, and the ER5556 wire welded joint tensile strength is 342.5 MPa, the tensile strength of the base metal is 60.9%, the minimum weld hardness is 90.6HV, elongation is 8.5%; compared with ER5356 wire, ER5556 wire has obvious tearing edge joint tensile fracture, while the dimples are deep and well proportioned size the second phase particles, the dimple, showed the micropore aggregation fracture, while ER5356 fracture tearing edge is less, the dimple size was too large and shallow, second phase particles smaller dimples, the fracture showed that mixed fracture, transgranular fracture and intergranular fracture at the same time. (2) "the optimal parameters of double pulse MIG- mechanical vibration combined process is: welding current welding speed of V=0.78 I=190A, CM s-1, A=0.02mm F=40Hz vibration amplitude, vibration frequency, combined process norms embodied welding parameters of the vibration parameters and good interaction Effect of applying vibration. After using ER5356 wire as filler metal, the tensile strength is 352.6MPa, welding joint strength coefficient is 0.627; the joint side near the weld fusion line of columnar crystal is fine, its direction tends to grain growth direction, intragranular filled with small precipitates, the weld microstructure of fine equiaxed branches the crystal and vibration direction cellulardendrite; microscopic observation of the region there is no obvious pores and micro cracks in the welded joints, vibration optimized weld microstructure than non weld tissue vibration small, DP-MIG high frequency coupling stirring and comprehensive effect of mechanical vibration, effectively inhibit the heat crack, and accelerated stomatal overflow. (3) postweld heat treatment (solution treatment (480 c * 50 min) + water quenching + artificial aging at room temperature (80 DEG C * 24h) + artificial aging (120 C * 24h)) the process test, comparing the two kinds of vibration free joint tests, ER5356 The tensile strength of wire filled joints for 445.7MPa welded joint strength coefficient increased from 0.520 to 0.793, the tensile strength of ER5556 wire filled joints for 490.0MPa welded joint strength coefficient increased from 0.609 to 0.872; in the "double pulse MIG- mechanical vibration combined process, ER5356 filler wire joint tensile strength is 490.2MPa, welding joint strength coefficient increased from 0.558 to 0.872. (4) PWHT of AA7075-T651 aluminum alloy welded joint performance recovery is essential reason of hardening precipitates in the type, number, size and distribution of the welded joint decision.TEM display, there is a large number of different shades of different regions, dispersed size ranging from nearly spherical or rod shaped precipitates MgZn2, the mechanical properties of large amplitude recovery and" softening "area has been significantly improved; showed low cycle fatigue test of ER5556 wire, ER5356 wire fill ratio The fatigue life of welded joints filled with longer, welding wire ER5356 filled in the lower 0.3-0.5% total strain amplitudes showed a higher cyclic deformation resistance, and applied in the larger of the 0.6% and 0.7% of the total strain amplitude, ER5556 filler wire deformation resistance of welded joints showed a high cycle, strong deformation hardening of DP-MIG welding. AA7075-T651 Aluminum Alloy based on the mechanical vibration welding process and post heat treatment (PWHT) is to reduce welding defects, effective way to improve the welding quality, provide theoretical support and guidance for the MIG welding process of welding aluminum alloy.

【學(xué)位授予單位】：沈陽工業(yè)大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TG407

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