双脉冲MIG焊7075超硬铝合金焊接接头组织与性能的研究

发布时间：2018-03-24 09:03

本文选题：超硬铝合金　切入点：双脉冲熔化极氩弧焊　出处：《沈阳工业大学》2017年博士论文

【摘要】：Al-Zn-Mg-Cu(7XXX)系铝合金是20世纪40年代伴随着航空技术发展而产生的超硬铝合金材料,具有强重比高、机械性能优良、耐蚀性较好和环保可回收等诸多优势,广泛应用于航空航天、国防军事、轨道车辆及汽车工业等部门,成为以上领域最具竞争力的结构材料。目前超硬铝合金的应用主要以挤压和模锻工艺成形,很少采用焊接连接,即使采用焊接结构,绝大部分也是使用搅拌摩擦焊(FSW)工艺固相连接,其主要原因是超硬铝在传统熔焊工艺过程中极易出现热裂纹、气孔等缺陷和焊接接头软化,以上因素限制了超硬铝合金的进一步应用。双脉冲熔化极氩弧焊(DP-MIG)有其独特的高低频脉冲耦合搅拌作用,对Al-Zn-Mg-Cu系铝合金实施熔化焊接成为可能。本课题以AA7075-T651超硬铝合金为研究对象,基于DP-MIG焊接方法,采用“低匹配”的ER5356及ER5556焊丝焊接6.35mm厚轧制板材,连接方式为开V型坡口对接平焊。实验方案:首先进行DP-MIG焊接参数优化,采用高速摄影分析双脉冲特性电弧形态、熔滴过渡行为,研究低频脉冲频率F对焊接接头组织及力学性能的影响;优化焊接参数基础上实施机械振动焊接试验,并确定恰当振动频率及振幅;最后将优化工艺的焊接接头施加焊后热处理(PWHT),研究不同焊丝填充焊缝的力学性能和疲劳性能,焊接接头不同区域的纳米级微观组织性能,诸如晶界、孪晶界及偏析等,并观察时效析出强化相的大小、形状及分布,进而分析焊后热处理T6制度对焊缝显微组织及力学性能的影响机理。通过DP-MIG焊接参数优化和“双脉冲MIG-机械振动”联合参数优化对AA7075-T651铝合金焊接,然后对不同试件进行焊后热处理,得出以下主要结论:(1)采用DP-MIG焊AA7075-T651超硬铝合金,分别以ER5356焊丝与ER5556焊丝为填充金属进行焊接参数优化,所得最佳焊接工艺参数如下:焊接平均电流I=190A、焊接速度V=0.78cm·s-1、低频脉冲频率F=3Hz、电弧挺度参数为-6,保护气体流量为18L/min、焊丝干伸长为12mm。试验结果为:ER5356焊丝焊接接头抗拉强度为292.5MPa,达母材抗拉强度的52.0%,接头最低硬度为80.8HV,伸长率为3.5%,而ER5556焊丝焊接接头的抗拉强度为342.5 MPa,为母材抗拉强度的60.9%,焊缝最低硬度为90.6HV,伸长率为8.5%;与ER5356焊丝相比,ER5556焊丝接头拉伸断口上有明显的撕裂棱,同时韧窝较深且大小匀称,韧窝中第二相颗粒很多,呈现为微孔聚集性断裂模式,而ER5356断口上撕裂棱较少,韧窝尺寸明显偏大且浅,韧窝中第二相颗粒较少,断口表现出混合断裂形貌,即穿晶断裂与沿晶断裂同时存在。(2)“双脉冲MIG-机械振动”组合工艺的最佳优化规范参数是:焊接电流I=190A、焊接速度V=0.78 cm·s-1、振动幅度A=0.02mm,振动频率F=40Hz,组合工艺规范体现焊接参数对振动参数的良好交互作用。施加振动后,以ER5356焊丝作为填充金属,接头的抗拉强度为352.6MPa,焊接接头强度系数为0.627;接头近焊缝侧熔合线区域柱状晶较细,其方向趋于晶粒长大方向,晶内密布细小析出相,焊缝中心组织为细小的等轴树枝晶和振动方向一致的胞状树枝晶组成;显微金相观察焊接接头各区域没有发现明显气孔和微裂纹,振动优化后焊缝组织比无振动焊缝组织细小,DP-MIG高低频耦合搅拌与机械振动综合作用,有效抑制热裂纹产生,并加速气孔的溢出。(3)采用焊后热处理(固溶处理(480℃×50 min)+室温水淬+人工时效(80℃×24h)+人工时效(120℃×24h))各工艺试验,对比两种无振动接头试验得知,ER5356焊丝填充接头的抗拉强度为445.7MPa,焊接接头强度系数从0.520提高至0.793,ER5556焊丝填充接头的抗拉强度为490.0MPa,焊接接头强度系数从0.609提高至0.872;在“双脉冲MIG-机械振动”组合工艺下,ER5356焊丝填充接头抗拉强度为490.2MPa,焊接接头强度系数从0.558提高至0.872。(4)PWHT对AA7075-T651超硬铝合金焊接接头性能恢复的本质原因是时效强化析出相的种类、数量、大小及分布状态决定的。TEM显示,焊接接头不同区域有数量较多深浅不同、体积大小不等的近球形或圆棒状时效析出相MgZn2弥散分布,促使其力学性能大幅度恢复和“软化”区得到显著改善;低周疲劳试验显示,ER5556焊丝比ER5356焊丝填充的焊接接头具有更长的疲劳寿命,ER5356焊丝填充的焊接接头在较低的0.3-0.5%外加总应变幅下呈现较高的循环变形抗力,而在较大的0.6%和0.7%外加总应变幅下,ER5556焊丝填充焊接接头呈现较高的循环变形抗力,形变硬化较强。基于DP-MIG焊AA7075-T651铝合金,采用机械振动工艺及焊后热处理(PWHT)是减少焊接缺陷、提高焊接质量的有效方式,为采用MIG熔焊工艺焊接超硬铝合金提供理论支撑和指导意义。
[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.

【学位授予单位】：沈阳工业大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG407

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