局部轧制及热处理对AZ系镁合金焊接组织与性能的影响研究

发布时间：2018-12-14 07:53

【摘要】：镁合金作为最轻的金属结构材料,具有比强度高,减震性、电磁屏蔽性强,易切削加工、易回收等优点,在汽车、电子、航空航天和国防军工领域具有较大的应用潜力。然而,镁合金的熔沸点、燃点都比较低,化学性质活泼,焊接过程中容易出现气孔、粗晶等问题,特别是焊接组织与母材组织之间的显著差异,导致镁合金焊接接头的力学性能普遍较差,与母材相差甚远,严重限制了镁合金的发展与应用。本文针对此问题,分别以双面TIG焊的AZ31镁合金和AZ80镁合金为研究对象,对其进行焊后加工,主要包括余高轧制和热处理。通过实验结合模拟的方式,分析了焊缝余高轧制工艺的变形特点,研究了不可热处理强化的AZ31镁合金和可热处理强化的AZ80镁合金,在焊后处理过程中的显微组织、焊接气孔以力学性能演化特点,以及强化机理。主要研究结论如下:(1)模拟结果显示,余高轧制过程中,焊接板材上的应变分布并不均匀,分为咬入区、稳定变形区和板材离辊区,应变分布与轧制压下量紧密相关。余高轧制不仅使焊缝区发生变形,还会使焊缝周边区域发生间接变形,对变形区实现形变强化,热变形时还会激发动态再结晶,细化组织。(2)AZ31镁合金焊接接头的抗拉强度和延伸率,都随着余高轧制压下量的增加而显著升高。当压下量达到一定值时,拉伸断裂位置由焊缝转移至母材,此时焊接接头的拉伸性能几乎与母材相同。焊接区的局部形变强化是AZ31镁合金余高轧制过程中的最主要强化机制,室温轧制即可大幅提升AZ31焊接接头的力学性能,但是轧制压下量超过20%后焊缝区容易开裂。变形温度升高,可显著提升焊接接头的塑性,降低焊缝轧制开裂倾向,但同时会削弱形变强化效果,需要更大的变形量才能使焊缝区的强度高于母材,从而使断裂位置转移。(3)AZ80镁合金经过TIG焊后,焊缝区产生大量Mg17Al12相,会在后续的余高轧制过程中增强形变硬化效果,影响再结晶行为,而热影响区的第二相溶解,失去了Mg17Al12相对晶界的钉扎作用,晶粒在后续固溶处理过程中很容易发生异常长大,导致该区域强度降低。合理的复合处理工艺,可在提高焊缝区强度的同时有效缓解热影响区晶粒长大,充分改善镁合金焊接接头的组织性能。(4)铝含量较高的AZ80镁合金在焊接过程中更容易产生气孔,余高轧制工艺可有效消除焊缝中的气孔。随着轧制压下量的增加,气孔逐渐闭合。轧制温度偏低时,可能会在气孔周围产生裂纹,不利于焊接接头的力学性能,轧制温度升高可提高镁合金塑性,避免裂纹的出现。(5)AZ80镁合金的变形抗力较大,在低于300℃进行余高轧制时焊缝很容易开裂,轧制过程难以进行。在合理的轧制温度范围内,AZ80焊接接头的综合力学性能随着轧制压下量的增加而显著升高,主要强化机制为形变强化、细晶强化以及焊缝中气孔的闭合,几种强化机制在350℃左右得到最好的配合。(6)AZ80焊接接头在余高轧制后进行时效的过程中,Mg17Al12相在孪晶界、孪晶内外、变形区和未变形区的析出形貌、尺寸与分布都存在差异。采用余高轧制与热处理相结合的焊后复合处理,可以控制各个区域的显微组织,从而影响焊接接头的力学性能。存在焊接气孔的AZ80焊接接头经过焊后复合处理,屈服强度达到200MPa(母材的85%),抗拉强度314MPa(母材的88%),延伸率4.5%(母材的45%)。(7)无法得到焊缝余高或余高不足时,可对镁合金焊接板材进行无余高的整体热轧,通过动态再结晶细化各区域显微组织,缩小晶粒尺寸差异,提高焊接接头强度。低温短时间的去轧制应力退火可使焊接接头的塑性提升,得到更好的综合力学性能。
[Abstract]:As the light metal structure material, the magnesium alloy has the advantages of high specific strength, shock absorption, strong electromagnetic shielding property, easy cutting and processing, easy recovery and the like, and has great application potential in the fields of automobile, electronics, aerospace and national defense and military industry. however, the melting point and the ignition point of the magnesium alloy are relatively low, the chemical property is active, and the problems of air holes, coarse crystals and the like in the welding process are easy to occur, in particular, the mechanical property of the magnesium alloy welding joint is generally poor, the mechanical property of the magnesium alloy welding joint is generally poor, and the welding joint of the magnesium alloy is far from the parent material, The development and application of the magnesium alloy are severely restricted. In this paper, the AZ31 magnesium alloy and the AZ80 magnesium alloy with double-side TIG welding are used as the research object, and the welding is carried out on the AZ31 magnesium alloy and the AZ80 magnesium alloy, which mainly comprises the following high rolling and heat treatment. In this paper, by means of experiment and simulation, the deformation characteristics of the weld residual high rolling process are analyzed, the non-heat-treated reinforced AZ31 magnesium alloy and the heat-treated reinforced AZ80 magnesium alloy are studied, and the microstructure and the welding hole in the post-welding process are characterized by mechanical property evolution, and the strengthening mechanism. The main research results are as follows: (1) The simulation results show that the strain distribution on the welded plate is not uniform during the high rolling process, and it is divided into the bite zone, the stable deformation zone and the plate-to-roll zone, and the strain distribution is closely related to the rolling reduction. The residual high rolling can not only deform the weld zone, but also cause indirect deformation of the peripheral area of the weld, and the deformation zone can be deformed and strengthened, and the dynamic recrystallization and the refining of the tissue can also be stimulated in the case of thermal deformation. (2) The tensile strength and the elongation of the AZ31 magnesium alloy welded joint increase with the increase of the rolling reduction. When the pressing amount reaches a certain value, the tensile fracture position is transferred to the mother material by the welding line, and the tensile property of the welded joint is almost the same as that of the mother material. The local deformation strengthening of the welding area is the main strengthening mechanism in the process of high-temperature rolling of the AZ31 magnesium alloy, and the mechanical property of the AZ31 welding joint can be greatly improved by rolling at room temperature, but the weld area is easy to crack after the rolling reduction exceeds 20%. the deformation temperature is increased, the plasticity of the welding joint can be obviously improved, the rolling and cracking tendency of the welding seam is reduced, the deformation strengthening effect is weakened at the same time, the strength of the weld zone is higher than that of the parent material, and the fracture position is transferred. and (3) after the AZ80 magnesium alloy is subjected to TIG welding, a large amount of Mg17Al12 phase is generated in the weld zone, the deformation hardening effect is enhanced during the subsequent high-high rolling process, the recrystallization behavior is influenced, and the second phase of the heat-affected zone is dissolved, and the pinning effect of the Mg17Al12 relative to the grain boundary is lost, The crystal grains tend to grow abnormally during the subsequent solid-dissolving process, resulting in a reduction in the strength of the region. The reasonable composite treatment process can effectively relieve the grain growth of the heat affected zone while improving the strength of the weld zone, and fully improve the tissue performance of the magnesium alloy welding joint. and (4) the AZ80 magnesium alloy with higher aluminum content can more easily generate air holes during the welding process, and the residual high rolling process can effectively eliminate the air holes in the welding seam. with the increase of the rolling reduction, the air holes are gradually closed. When the rolling temperature is low, cracks can be generated around the air holes, which is not beneficial to the mechanical property of the welding joint, and the rolling temperature is increased, so that the plasticity of the magnesium alloy can be improved, and the occurrence of cracks is avoided. and (5) the deformation resistance of the AZ80 magnesium alloy is large, and the welding seam is easy to crack when the residual high rolling is carried out at lower than 300 DEG C, and the rolling process is difficult to carry out. In the reasonable rolling temperature range, the comprehensive mechanical properties of the AZ80 welded joint are significantly increased with the increase of the rolling reduction, the main strengthening mechanism is the deformation strengthening, the fine grain strengthening and the closing of the air holes in the welding line, and several strengthening mechanisms are best matched at the temperature of about 350 DEG C. (6) The microstructure, size and distribution of the Mg17Al12 phase in the grain boundary, the inner and outer, the deformation zone and the undeformed zone of the AZ80 welded joint are different in the process of aging after the residual high rolling. The microstructure of each region can be controlled by the post-welding composite treatment combined with the combination of the residual high rolling and the heat treatment, so as to influence the mechanical property of the welded joint. The tensile strength of the AZ80 welded joint with the welding hole is 200MPa (85% of the parent material), the tensile strength is 314MPa (88% of the mother material), and the elongation rate is 4.5% (45% of the parent material). and (7) when the residual height or the residual height of the weld can not be obtained, the magnesium alloy welding plate can be subjected to a non-residual high-temperature hot rolling, and the microstructure of each region can be refined by dynamic recrystallization, so that the grain size difference can be reduced, and the strength of the welding joint can be improved. the low-temperature short-time rolling stress annealing can improve the plasticity of the welded joint and obtain better comprehensive mechanical property.
【学位授予单位】：重庆大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG457.1;TG339

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