【作者】 金玉花；

【导师】 王希靖；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2012， 博士

【摘要】 7050铝合金是航空工业广泛应用的一类重要结构材料,用传统弧焊焊接时,焊缝经常会产生气孔、裂纹、咬边等缺陷。搅拌摩擦焊(FSW)作为一种新型的固相连接方法,它的问世被证实能够避免熔焊焊接易产生的热裂纹、脆性相、残余应力高等缺陷,尤其在以前所谓不可焊接的2000系列、7000系列高强铝合金方面更受亲睐。7050铝合金搅拌摩擦焊接头横截面由焊核区(Nugget)、热-机影响区(TMAZ)、热影响区和母材区四部分组成,焊核区由细等轴的再结晶晶粒组成,热-机影响区由变形梯度较大的弧形晶粒组成,热影响区由粗大的母材晶粒组成。焊接参数：如搅拌头尺寸、搅拌头形状、搅拌头材质、搅拌针旋转速度、搅拌针前进速度、下压力的大小任何一个因素的改变都会影响接头的微观组织。本文侧重对搅拌针旋转速度,旋转速度与前进速度的比值这两个重要参数对搅拌头组织的影响进行研究。采用光学显微镜、扫面电镜、透射电镜等研究了无缺陷理想接头中焊核区、热-机影响区、热影响区各区的组织特征。试验证明：焊核区组织高倍下观察由细等轴晶构成,低倍下为一簇簇的环组成,类似洋葱的截面,取名“洋葱环”,环的出现是第二相粒子再分布的结果,是由粒子的富集带和粒子的贫乏区间隔构成的。各区位错密度的变化非常明显,析出相的分布、数量、形态差异明显。焊核区主要为热循环过程中未溶解的高温稳定相,热-机影响区既有粗化的析出相又有再析出的细小圆盘状析出相,热影响区晶界析出相粗化且呈连续分布,晶界无析出区数量增多,总之,这样的微观结构决定了其接头的硬度分布特征近似为“W”型。热-机影响区、热影响区都有可能成为焊接接头的薄弱区,热-机影响区根部出现“树根”型粒子贫乏区时,该区域成为接头的最薄弱区；当然,热-机影响区无缺陷时,热影响区的粗化成为接头性能低的主要因素,此时热影响区成为接头的最薄弱环节。热-机影响区成为最薄弱区时,焊接态接头的强度系数(接头强度与母材强度的比值)较低,焊后的固溶时效处理难以提高接头强度。热影响区成为最薄弱环节时,接头的强度系数较高,焊后的固溶时效处理可进一步提高接头的强度。7050铝合金属时效强化铝合金,焊后的固溶时效处理显得尤为重要。但是,铝合金属于典型的热的不稳定材料,加热到高温时很容易出现晶粒的异常长大现象,影响这种异常长大的因素除了常规的保温温度和保温时间外,因焊接参数的改变,接头组织不同,热的不稳定性不同。搅拌针旋转速度的平方与行走速度的比值更能代表铝合金7050接头热的不稳定性,比值小,接头固溶时稳定性差,焊核区易出现晶粒的异常长大；比值大,接头固溶时相对稳定。提高固溶加热速率可抑制或降低接头热的不稳定性,加热速率高于900℃/h,固溶处理时接头不再出现晶粒的异常长大。再次,固溶保温后,提高冷却液的冷却速度、细化强化相的粒子尺寸也是进一步提高接头强度的途径。焊接态的接头其拉伸实验中的断裂位置与焊接参数有关,转速与焊速比约为10的接头,热影响区成为最薄弱的区域,拉伸断裂都发生在热影响区,经过合理有效的固溶时效处理后,断裂位置移向焊核区。提高转速与焊速比,该比值等于15时,不论焊接态、固溶时效态的接头断裂始终在焊核与热-机影响区的交界处。该处始终是接头的薄弱区。综上所述,本论文在前人研究的基础上,对高强铝合金搅拌摩擦焊理想接头的组织演化及薄弱区做了分析,对焊后热处理中影响组织的因素做了详尽的调查,找到了提高接头性能的有效措施,论文的研究对于推动铝合金7050的搅拌摩擦焊在航天及其它工业应用中的应用有重要的理论和实际意义。更多还原

【Abstract】 Aluminium alloy AA7050is one of the strongest aluminium alloys in aviation industrial use today. Some several problems is present during fusion welding, such as porosity, lack-of-fusion, and solidification cracking that renders them very useful in aerospace applications and other industries.Friction stir welding(FSW) is a new kind of solid-phase welding process which is used for butt and lap joints, Relative to fusion welds, the FSW joint may avoid thermal crack, lower residual stresses and eliminating brittleness phase. This new technology can join difficult to weld aluminium alloys by traditional fusion techniques, for example, alloys belonging to the2XXX series and the7XXX series with limited weldability. Friction stir welding has received considerable attention since it is invented.Microstructure of transverse cross section of friction stir weld joint of aluminium alloy7050consists of nugget zone, thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ) and base material. The microstructure of nugget zone is greatly refined due to dynamic recrystallization.crystal grain becomes small. Crystal grains of TMAZ show an elongated shapd due to severely plastic deformation during FSW. The grain structure in the HAZ region, which has not been disturbed mechanically by FSW, is similar to that of the base metal, but it is larger than that of the base metal.All the welding parameters, for example,pin size,pin shape,material of pin, the pin rotation speed, the pin travel speed,axial pressure,which of them influences microstructure when one of these parameters changes..In this paper, FSW test is done under different rotation speed and the different ratio of rotation to travel speed. For defect-free welding joint, The microstructure features of the nugget zone and TMAZ zone and HAZ zone have been characterized by optical microscopy and scanning electron microscopy.grain size have been measured by mean linear intercept. Distribution of precipitation have been analyzed by transmission electron microscopy.Micrograph feature of nugget is equiaxed recrystallized grains, however, macrograph of nugget demonstrate that a segregated, banded, microstructure consisting of alternating hard particle rich and hard particle poor regions is developed.This band structure is called "onion ring".In addition, Dislocation density is different in each zone. Most grains in TMAZ contained a high dislocation density, is similar to that of the base metal, even higher that of the base metal, Grains in nugget zone contained low density of dislocation. The size, morphologies and distribution of precipitation in different regions are also discussed. In nugget zone, the phase was composed of dissolved high temperature stable phase. In TMAZ zone, strengthening precipitates were severely coarsened and partly dissolved while a small amount of very fine η particles were re-precipitated.In HAZ zone, Precipitates of both intragranular and intergranular coarsen. the precipitate-free zone increases in width and more phase forming. Such a microstructure leads to "W type" the hardness distribution characteristics for the joint.Either the HAZ zone or the TMAZ zone will be weakest zone for FSW joints, the TMAZ zone will be weakest zone for FSW joints while hard particle poor regions is developed at the bottom of TMAZ zone. Of course, the HAZ zone will be weakest zone because of grain coarsening when there is not defeat in the TMAZ zone. The strength coefficient of joint is low, at the same time, the strength of joint can not be increased by post-weld solution and aged treatment. When the TMAZ zone will be weakest zone. But if the HAZ zone will be weakest zone, Not only the strength coefficient of joint is high, but also the strength of joint can be increased significantly by post-weld solution and aged treatment.Aluminium alloy7050belongs to age hardening aluminium alloys, In order to develop the high mechanical properties of aluminium alloy7050, reduce the negative impact of welding process.Post-weld heat treatment is particularly important.But aluminium alloy is unstability at high temperature, It is easy to appear the abnomal grain growth when it is heated to a high temperature. Temperature and time are two factors that affects grain heat stability.besides, welding parameters also determine stability of the microstructure. For FSW joint of aluminium alloy7050, the ratio of the square of the pin rotating speed to welding speed is on behalf of instability at high temperature. Stability of the joints is low at high temperature for some joints welded with the small ratio of the square of rotation speed to travel speed, It is prone to abnormal grow for nugget grain. thermal stability of the joints welded with the high ratio of the square of rotation speed to travel speed is improved, abnormal grain growth can be resisted or reduced by high heat rate during solution treatment. It is stable during solution treatment at the heat rate of900℃/h.On the other hand, Aluminum alloys containing a high concentration of alloying elements are considered "quench sensitive"where the cooling rate from thermal exposures has an influence on the mechanical properties by fine precipitation. In general, for ideal as-FSW joint, Increasing cooling rate after solution treatment, the abnormal growth of grain is inhibited, the strength of the joint can increased significantly.For the as-welded joint, facture location depends on welding parameters. facture in tensile test occurs in the HAZ because of the HAZ zone is weakest zone when the joint is friction stir welded with the parameter,which the ratio of rotation speed to travel speed equals to10.The facture location shifted from the HAZ zone to the stir zone after solution and aged treatment. When the ratio of rotation speed to travel speed tended to reach15, the weakest zone of the joint shifted from the HAZ zone to the TMAZ zone. For this kind of joint, facture happened in the TMAZ zone both as-welded and post heat treatment joint. The TMAZ zone is the weakest zone all the time.In summary, this paper has obtained some new ideas and achieved some research results based former studies. microstructure character of defect-free FSW joint and weak zone was analysed in detail, the effect of welding parameter on microstructure and property was investigated, the factor that influence propertier of FSW joints was found during solution and aged treatment.some measures that increase the property of the FSW joints were put forward, which could promote development of FSW process in the connection field of7050aluminium alloy.更多还原