【作者】 鄢勇；

【导师】 张大童；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 搅拌摩擦焊接是一种固态连接技术,已成功应用于同种铝合金与镁合金的焊接。对于铝合金和镁合金的异种焊接,搅拌摩擦焊焊接过程温度低,不产生熔化现象,是实现铝合金和镁合金异种焊接的有效途径。搅拌摩擦焊过程中搅拌区得到细小的等轴晶组织,因此进一步了解搅拌摩擦焊过程晶粒组织变化规律十分有必要。本文对6 mm厚的5052铝合金和AZ31镁合金同种和异种搅拌摩擦焊接,考察了不同焊接参数对同种和异种焊缝组织和力学性能的影响,并采用急停和快速淬火的方法探讨了AZ31镁合金搅拌摩擦加工过程中晶粒长大的机制,得到如下主要结论:(1)研究了搅拌摩擦焊工艺参数对6 mm厚的5052铝合金板材接头组织和力学性能的影响。在150 mm/min的焊接速度下,旋转速度在600 rpm-1500 rpm的范围内,均得到了高质量的焊缝。在搅拌区产生了细小的等轴晶组织,最小晶粒尺寸为6.3μm。搅拌头的旋转速度越高,搅拌区的晶粒尺寸越大。硬度曲线呈“W”型,焊缝中心硬度与母材相当,在距焊缝中心大约3 mm的位置硬度最小值约为HV52左右。在旋转速度为600 rpm与焊接速度为150 mm/min焊接参数下得到的接头强度为236.2 MPa,延伸率为22.4%,分别达到母材的92.9%和96.1%。(2)研究了板厚为6mm的AZ31镁合金搅拌摩擦焊工艺,并对接头的断裂机制进行了考查。在转速为1000rpm,焊接速度为60 ~ 300mm/min条件下获得表面平整,无缺陷的焊缝。与母材相比,搅拌区的晶粒得到明显细化,随焊接速度增加,搅拌区的晶粒尺寸减小。搅拌区的硬度高于其他区域。焊接速度为150mm/min时,接头拉伸强度最高,达到母材的92.7%。断裂多发生在热影响区,热影响区晶粒粗大且分布不均,显微硬度最低,是焊接接头的薄弱环节。(3)采用急停和快速淬火技术获得了各个不同加工时间的焊缝组织,分析了焊缝搅拌区中心晶粒尺寸与加工时间的关系以及搅拌区晶粒大小的分布状态,研究表明:搅拌区发生动态再结晶,晶粒为非常细小的等轴晶组织。在0-5秒时,即接近匙孔区域,搅拌区平均晶粒尺寸大约为1.46μm,其中晶粒尺寸小于1μm的微细晶组织占80%左右。随着加工时间的延长,搅拌区晶粒不断长大,微细晶数量减少,大晶粒数目增加,到15 s时,晶粒尺寸小于1μm的微细晶组织占75%左右,25 s时,平均晶粒大小增大到2.06μm。再结晶初始阶段,晶粒长大迅速,在搅拌摩擦焊结束后,晶粒持续长大,直到最终得到稳定尺寸。(4)对6 mm厚5052铝合金与AZ31镁合金板材进行了异种搅拌摩擦焊接。讨论了异种搅拌摩擦焊接过程中出现的搅拌头磨损现象,研究了不同参数下异种焊缝的成形性能,并对600 rpm-40 mm/min条件下异种焊接接头的组织和力学性能进行分析,研究结果表明:采用低硬度搅拌头时,搅拌头易有含金属间合物Al12Mg17和Al3Mg2的粘附层产生,搅拌头磨损较严重,不能形成完整焊缝。提高搅拌头硬度时,异种焊缝在600 rpm-800 rpm和40 mm/min、60 mm/min的条件下外观成形良好,没有明显的裂纹产生。在600 rpm-40 mm/min条件下获得无宏观缺陷的焊缝。焊缝上部分为简单的交接,下部分两种金属却呈现了相互混合交接,并存在由Al合金带和Mg合金带组成的洋葱环结构。搅拌区组织发生动态再结晶,为细小的等轴晶,但Mg侧和Al侧搅拌区晶粒大小分布不均匀。Mg侧搅拌区Mg的平均晶粒大小约为5.4μm,Al侧搅拌区不同位置获得Mg的平均晶粒大小约为2.8μm。搅拌区得到Al的平均晶粒大小约为3μm。TEM、XRD分析得出搅拌区有金属间化合物Al12Mg17的存在。焊缝硬度分布波动大,硬度最大值存在于搅拌区,大约为母材硬度的2倍。拉伸断裂为脆性断裂,断裂位置位于硬度变化最大出,大约为焊缝前进侧距中心2.5 mm处。更多还原

【Abstract】 Friction stir welding is a solid-state joining technique and has achieved the monolithic welds of aluminum and magnesium alloys. Friction stir welding is a potential candidate for the joining of dissimilar welds between aluminum alloys and magnesium alloys due to the lower processing temperature. Microstructural evolution was complicated and fine equixed grains were obtained in stir zone. Therefore, it is essential to reveal the mechanism of the grain growth during friction stir welding process. In this paper, microstructure and mechanical properties of the monolithic and dissimilar weld between 5052 aluminum alloy and AZ31 magnesium alloy produced by friction stir welding were studied. Microstructural evolution during friction stir processing of AZ31 magnesium alloy was observed with the employment of stop action and“rapid quench”technique. The main conclusions are summarized as follows.(1) Effects of friction stir welding parameters on the microstructure and mechanical properties of 5052 aluminum alloy joints were investigated. Sound welds were achieved under a range of rotation rate 600 rpm-1500 rpm at a constant welding speed 150 mm/min. Fine equiaxed grains were obtained in stir zone and the minimum grain size was 6.3μm. The grain size of stir zone increased with the rotation speed increasing. The profile of hardness distribution presented a“W”shape and the maximum value of stir zone was equal to the base metal. Furthermore, the minimum value of hardness, about 52 HV, located at a distance of 3 mm from the weld center on the advancing side. Joints with its tensile strength of 236.2 MPa and elongation of 22.4% were achieved with a rotation speed of 600 rpm and a welding speed of 150 mm/min, which were 92.9% and 96.1% of the base material, respectively.(2) Friction stir welding of AZ31 magnesium alloy plate with its thickness of 6 mm was studied, and the tensile fracture mechanism of FSW joints was investigated. At a constant rotation rate of 1000rpm, defect-free joints could be obtained under a range of welding speed 60~300mm/min. Coarse grains in base material were changed into fine equiaxed grains in stir zone after FSW. The grain size of stir zone decreased with the welding speed increasing. Micro-hardness measurement showed that the hardness of stir zone was higher than that of the other zones. Joints with its tensile strength of 92.7% of the base material were achieved under the welding speed of 150 mm/min. Heat affected zone was the weakest zone in the whole weld, due to the inhomogeneous distribution of coarse grains and the lowest hardness in the heat affected zone. (3) Stop action and“rapid quench”technique was adopted to understand the microstructural evolution during friction stir processing of AZ31 magnesium alloy. The relationship between the processing time and grain size was investigated. Analysis of microstructure revealed that grain refinement occurred and the DRX grain structure was obtained in the stir zone. The minimum average grain size in the whole weld was about 1.46μm, which located near the keyhole, and the proportion of fine grain smaller than 1μm was up to about 80%. The number density of fine grain decreased and grain became coarser along the centerline of weld from the keyhole. The proportion of fine grain was reduced to 75% in location of 15 second and the average grain size was up to 2.06μm in 25 second. The experimental results also showed that the process of grain growth would be continued after the stirring of the tool.(4) Dissimilar friction stir welding between 5052 Al alloy and AZ31 Mg alloy with the plate thickness of 6 mm was investigated. Abrasion of shoulder was found during dissimilar friction stir welding and materials composed with intermetallic compound of Al12Mg17 and Al3Mg2 were found to envelope the rotational pin. Dissimilar joints were produced under the rotational speed of 600 rpm-800 rpm with welding speed of 40 mm/min and 60 mm/min and no cracks were found in the appearance of the joints. Sound weld was obtained at 600 rpm rotation speed and 40 mm/min welding speed. A simple bond interface was formed on the top of joint and an intermixed structure existed in the bottom of the joint. Complex flow pattern characterized by intercalation lamellaes was formed in the stir zone. Structure of Mg alloy presented different refined grains in different region of stir zone. The average grain size of Mg alloy was 5.4μm in stir zone near the Mg side and the minimum was about 2.8μm in stir zone near the Al side. Aluminum alloy was also refined in stir zone and the average grain size was about 3μm. Analysis of TEM and XRD revealed that intermetallic compound of Al12Mg17 was formed in stir zone. Microhardness measurement of the dissimilar welds presented an uneven distribution due to the complicated microstructure of the weld, and the maximum value of microhardness in the stir zone was twice higher than that of the base materials. The tensile fracture position located at the advancing side (aluminum side), where the hardness distribution of weld showed a sharp decrease from the stir zone to 5052 base material.更多还原