【作者】 孙涛；

【导师】 沈以赴；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2021， 硕士

【摘要】 本文针对航空、航天领域铝合金与镁合金异种金属T型接头焊接成型的实际需求,采用搅拌摩擦焊接技术对2024-T4铝合金与AZ31B镁合金组成的T型接头的结构设计、成形工艺进行了实验研究,并围绕焊接接头完整性及接头的显微组织、力学性能开展研究,优化接头设计和工艺参数。通过改变翼板和腹板位置设计了四种T型接头结构,包括Al_W/Mg_S-B、Mg_W/Al_S-B对接型T型接头和Al_W/Mg_S-L、Mg_W/Al_S-L搭接型T型接头。对四种接头结构的T型接头成形性的研究发现两种铝镁对接型T型接头的成形性能较差,焊缝存在明显的缺陷;铝镁搭接型T型接头的成形性能较好,焊缝表面质量较佳,且角焊缝被充分填充。Al_W/Mg_S-L型T型接头在旋转速度为800r/min,焊接速度为30mm/min的工艺参数下,下压量为0.3mm时焊缝表面成形质量最好,旋转速度在1200r/min时焊缝横截面发现隧道缺陷,其余焊接参数下获得的焊缝较为致密。焊接接头横截面容易出现弱结合、钩和冷搭接缺陷。翼板上显微组织分布特征类似于普通对接接头,腹板上热机影响区(TMAZ)处的晶粒发生了动态再结晶,形成均匀细小的等轴晶,晶粒无塑性变形痕迹;热影响区中的晶粒长大,降低了组织的均匀性。焊缝显微硬度在翼板方向呈“W”形分布,在腹板方向上呈“N”型分布,在铝镁混合区硬度值迅速增大,远高于母材硬度,随后硬度在腹板热影响区达到最低值。翼板抗拉强度达到最高为375MPa,约为2024-T4铝合金母材的79.8%;腹板抗拉强度最高为102MPa,约为AZ31B母材的53%;弱结合缺陷容易作为裂纹源导致断口的脆性断裂。Mg_W/Al_S-L型T型接头在旋转速度为1000r/min,焊接速度为15mm/min时焊缝中出现较小的隧道缺陷,所有焊接参数下的T型接头角焊缝中均出现弱结合,钩和冷搭接缺陷。焊缝焊核区晶粒尺寸从翼板镁的上部到底部铝板依次降低;TMAZ发生部分动态再结晶,前进侧的TMAZ晶粒被明显拉长,后退侧TMAZ部分晶粒出现塑性变形迹象;角焊缝区晶粒被挤压发生明显的塑性变形。焊缝原始连接面附近形成混合区,混合区两相界面可发生Al、Mg元素的互扩散,形成金属间化合物相。焊缝显微硬度分布特征与Al_W/Mg_S-L型T型接头类似。翼板的抗拉强度最大值为178.24MPa,达到母材的91.9%。腹板最大抗拉强度为120MPa,达到镁合金母材的62%。翼板断口分为三个区域,表现为不同的断裂机制,上部断口为混合断裂,下部角焊缝处为机械粘合,中部表现为脆性断裂模式;腹板断裂机制包括脆性断裂和准解理断裂;拉伸过程中主要的裂纹源是弱结合缺陷。更多还原

【Abstract】 In this paper,according to the actual requirements of welding formability of dissimilar metal T-joint between aluminum alloy and magnesium alloy in aviation and aerospace field,the structure design and forming process of 2024-T4/AZ31B magnesium alloy T-joint were studied by using friction stir welding technology.The integrity,microstructure and mechanical properties of the welded joint were studied to optimize the joint design and process parameters.Four kinds of T-joint structures are designed by changing the position of wing and stringer plate,including butt(Al_W/Mg_S-B Mg_W/Al_S-B)and lap(Al_W/Mg_S-L、Mg_W/Al_S-L)T-joint.The formability of T-joints with four kinds of joint structures is studied.It is found that the formability of two kinds of butt T-joints is poor,and the weld has obvious defects.The formability of two kinds of T-joints is better,and the weld surface quality is better.The fillet weld is fully filled.At the rotation speed of 800r/min and the welding speed of 30mm/min,the weld surface forming quality of Al_W/Mg_S-L T-joint is the best when the down pressure is 0.3mm.When the rotation speed is1200r/min,tunnel defects are found in the weld cross section,and the weld is denser under other welding parameters.The cross section of welded joint is prone to weak bonding,hook and cold lap defects.The microstructure distribution of the wing plate is similar to that of the common butt joint.The dynamic recrystallization of the grains at TMAZ on the steinger forms uniform and fine equiaxed grains,and the grains have no traces of plastic deformation.The grain growth in heat affected zone reduces the uniformity of the structure.The microhardness of the weld is"W"shaped in the direction of the wing plate and"N"shaped in the direction of the stringer.The microhardness of the weld increases rapidly in the Al-Mg mixed zone,which is much higher than that of the base metal,and then reaches the lowest value in the heat affected zone of the stringer.The maximum tensile strength of flange is 375MPa,which is about 79.8%of 2024-T4 base metal.the maximum tensile strength of stringer is 102MPa,which is about 53%of AZ31B base metal.Weak bonding defects are easy to cause brittle fracture as crack source.At the rotation speed of 1000r/min and the welding speed of 15mm/min,small tunnel defects appear in the weld of Mg_W/Al_S-L T joints.Weak bonding,hook and cold lap defects appear in the fillet weld of T-joint under all welding parameters.The results show that the grain size of the weld nugget zone decreases from the upper part of magnesium to the lower part of aluminum plate.Partial dynamic recrystallization occurs in the TMAZ,and the grain in the forward side of TMAZ is obviously elongated.The grain in the backward side of TMAZ shows signs of plastic deformation;the grain size in the heat affected zone is significantly higher than that of the base metal.The grain size in the corner joint is higher than that of the base metal.The grain in the weld zone is extruded to produce obvious plastic deformation.A mixed zone is formed near the original joint surface of the weld,and the intermetallic compound phase can be formed by the mutual diffusion of Al and Mg elements at the two-phase interface of the mixed zone.The distribution of microhardness of weld is similar to that of Al_W/Mg_S-L T-joint.The maximum tensile strength of the wing plate is 178.24MPa,which is 91.9%of the base metal.The maximum tensile strength of the stringer is 120MPa,which is 62%of the base metal of magnesium alloy.The fracture of wing plate is divided into three regions,showing different fracture mechanisms.The upper fracture is mixed fracture,and the lower fillet weld is mechanical bonding.The middle part is brittle fracture.The fracture mechanism of stringer includes brittle fracture and quasi-cleavage fracture.The main crack source in tensile process is weak bonding defect.更多还原