【作者】 胡志力；

【导师】 苑世剑； 王小松；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 博士

【摘要】 航空航天领域结构轻量化的发展，对薄壁构件的需求越来越多，铝合金异形截面薄壁构件在飞机及航天器结构制造中的应用范围不断扩大。此类构件形状复杂，径厚比大，对管材尺寸和成形性能提出了新的要求。采用传统铝合金管材制造方法，很难获得大直径薄壁铝合金管材，因此迫切需要研发面向内高压成形的薄壁铝合金管材制造技术。为了解决该难题，本文提出了铝合金管材搅拌摩擦焊(FSW)旋压复合成形工艺。以2024铝合金为研究对象，揭示了FSW管材制备过程中组织演变规律与塑性变形行为，给出了螺旋焊缝管材内高压成形特征及壁厚分布规律，为解决薄壁铝合金管材制备及后续成形的难题提供新方法和理论依据。采用FSW板材接头，研究了接头组织及塑性变形行为，获得了热处理对接头延伸率的影响规律。焊态接头组织性能差异，导致变形不均匀，延伸率下降44%。热处理可以改善接头变形均匀性，提高延伸率。接头延伸率随热处理温度升高，先增加后降低。300℃时，达到最高值，为焊态的1.6倍。通过铝合金管材FSW旋压成形实验，给出了旋压减薄率对管材组织及力学性能的影响规律。结果表明：旋压对母材晶粒细化比焊缝更明显，减薄率达70%时，母材晶粒从200μm降至3.5μm。旋压减薄率越大，管材晶粒细化效果越显著，管材强度也越高。通过热处理及自由胀形实验，揭示了加热温度对管材组织及胀形性能的影响规律。300℃时，旋态管材变形组织转变为细小等轴晶和大角度晶界的再结晶组织，焊缝晶粒约为1.7μm，母材晶粒约为4μm，管材组织均匀性显著改善。350-400℃时，管材晶粒异常长大，并沿厚度分层，外层晶粒为内层的4-6倍。管材膨胀率随温度升高，先增加后降低。300℃管材膨胀率最高，为旋态的2.1倍。热处理控制管材组织稳定性，决定了管材成形性能。通过自由胀形实验，获得了焊缝强度匹配对FSW管材环向壁厚分布的影响规律。发现等组配管材距焊缝圆心角30°和180°母材壁厚减薄严重，最大减薄为20.6%，这与高组配管材壁厚分布规律类似；低组配管材，破裂于焊缝，最大减薄为23.7%，焊缝减薄明显高于母材。塑性理论分析表明：焊缝强度差异，导致高组配管材截面焊缝相邻及对向母材曲率半径较大，等效应力高于其他母材，从而壁厚减薄严重。通过变径管内高压成形实验，获得了螺旋焊缝管材壁厚减薄规律。发现管材轴向壁厚减薄呈M形：距对称面1/4膨胀区长度处减薄严重，而两端及对称面减薄较小。管材环向壁厚减薄规律与自由胀形管材类似，但壁厚分布更均匀。这主要是通过预成形，使焊缝相邻母材先贴模，缓解过度减薄。利用数值模拟，揭示了螺旋焊缝管材自由胀形时壁厚分布与应力状态的关系。发现FSW管材胀形时截面发生畸变。高组配时，焊缝相邻母材曲率半径大于其他母材，受到环向及轴向双拉应力较大，应变较高，壁厚减薄严重。低组配时，管材截面曲率半径分布与高组配相反，焊缝相邻母材受到环向及轴向双拉应力较小，应变较低，壁厚减薄较小。更多还原

【Abstract】 Due to the increasingly urgent lightweight demand in the aerospace industry,the application of thin-walled aluminum alloy tube fittings continue to expand inthe aircraft and aerospace industry. Such fittings with complex shape have alarge radius-to-thickness ratio, which put forward more strict requirements fortube geometry and formability. It is difficult to obtain such large diameter thin-walled aluminum tube by the conventional aluminum tubing manufacturingmethod. Thus an urgent need for research and development of hydroformingthin-walled tube is required. In this paper, large diameter thin-walled aluminumwere produced using a hybrid process combining friction stir welding andspinning.2024aluminum alloy was studied as the base materials. Therelationship between the microstructure and plastic deformation behavior of theFSW joints were revealed during FSW tube preparation and the manufacturingproblem of the thin aluminum pipe was solved. The hydroforming characteristicsand wall thickness distribution law of spiral weld pip were given, aiming toprovide theory foundation and technology support to the application of the newprocess.The relationship between the microstructure and the plastic deformationbehavior of the FSW joints was examined. The effect of the post-weld heattreatment on the plastic deformation characteristics of the FSW joints wasrevealed. The microstructure heterostructure of the joints lead to non-uniformmechanical properties and plastical deformation, and the elongation of the jointdecreases44%. Deformation heterogeneity of the joint is improved by the post-weld heat treatment and the joint showed high ductily. The plasticity of the jointincreases firstly and then decreases with increasing the annealing temperature.When the temperature is300℃, the plasticity has a maximum value, which is1.6times higher than that of the welded joint.Combined spinning composite forming process of aluminum alloy FSWtube experiments, the effect spinning thinning rate on microstructure andmechanical properties of FSW pipe was given. It is found that the grain refiningeffect is more obvious with increasing the reduction of spinning. The grain sizeof base metal (BM) is decreased from200μm to3.5μm, with spinning thinning rate of70%. The more uniform distribution of grains in the weld and BM isgiven. The strength of the tube increases significantly with increasing thereduction of spinning.The microstructure of the FSW tube was controlled by heat treatment, andthe effect the annealing temperature on the microstructure of the tube wasobtained. The bulging performance variation of the FSW tube after heattreatment was revealed during the hydroforming. When the temperature is below300℃, the deformed microstructure of the spin state of the pipe change into therecrystallized microstructure of fine equiaxed grain and large-angle grainboundaries. The grains of the weld are approximately1.7μm and the grains ofbase material are4μm. The organization uniformity of the tube has beensignificantly improved. When the temperature is350-400℃, the abnormalgrowth of the tube occurs, and the delamination along the thickness of the tubeis obvious in which the outer layer grain is4-6times higher than that of theinner layer. With increasing temperature, the expansion coefficient of the FSWtube first increases and then decreases. When the temperature is300℃, theexpansion coefficient of the tube has a maximum value, which is1.6timeshigher than that of the spinning. The expansion coefficient of the FSW tube canbe significantly increased by heat treatment, which is due to that the heattreatment can control the organizational stability and further improve theformability of FSW tube.The effect strength matching coefficient on the wall thickness distributionof spiral weld was obtained during the hydroforming. It is found that the BMnear the weld (nearly30°and180°) combined with the area opposite the weldshows a great thinning of23.7%for the tube, which is consist with the wallthickness distribution of the tube with a high-strength matching coefficient. Thetube with a low-strength matching coefficient ruptures in the weld with themaximum thinning of23.7%, which is significantly higher than that of the BM.The mechanical analysis points out that the BM near the weld combined with thearea opposite the weld show larger equivalent stress for the tube with a high-strength matching coefficient, thus above region show seriously thinning.Base on the experimental research of hydroforming multidiameter tube withspiral weld, the thickness distribution law of the FSW tube could be obtained. It is found that the axial wall thinning shows M-shaped distribution. Severethinning is found in the expansion zone of1/4from symmetry plane alonglongitudinal direction, and the thinning in the ends of tube including symmetryplane is smaller. Hoop thinning of tube is similar to the free bulging tube, but itshows more uniform thickness distribution. By pre-forming, the contacting-dieof the cross-section is controlled, and base metal adjacent weld contacts diefirstly. Therefore, the uniformity of thickness distribution is improved.The numerical simulations were conducted to reveal the relationshipbetween thickness distribution, cross-sectional shape and stress state of spiralweld tube during free bulging. It is found that cross-section of the FSW tube isno longer circular. The radius of the base metal adjacent weld is greater thanother BM for η=1.2tube, and suffer higher hoop and axial tensile stress, whichlead to severe thinning. The distribution of the radius for η=0.9tube is oppositeto the η=1.2tube. The hoop and axial tensile stress of base metal adjacent weldis smaller, which lead to less thinning of the tube.更多还原