590MPa级高强钢轮辋接头组织性能与失效分析

发布时间：2018-09-01 10:54

【摘要】：汽车行业中采用轻质材料如高强钢板、铝合金是实现轻量化的有效途径,轮辋,作为汽车的重要组成部件,考虑到其承载能力和安全性能,低合金高强钢依旧是轮辋生产使用最多的材料。闪光对焊,是一种典型的电阻焊接方法,由于具有高效率、高速率以及高强度接头等优点一直是轮辋焊接的主流方法。随着高强钢板强度的逐渐提高,轮辋闪光对焊接头的断裂频率也相应增加。因此本文选取590CL轮辋高强钢板作为母材,首先研究高强钢轮辋的闪光对焊接头组织性能特点;其次在单因素变化下探讨闪光对焊参数对接头组织性能的影响规律;然后对轮辋在扩口与扩张工序中接头的断裂机制进行分析;最后采用焊后空冷而非水冷的方式研究接头组织性能,并利用ARGUS应变测量系统对轮辋扩口成形的应变进行实测。研究结果表明590CL高强钢板成分符合轮辋用钢标准,闪光对焊接头焊缝由魏氏铁素体和少量的贝氏体构成,母材主要由珠光体、铁素体及轻微带状分布组织组成。对焊接头的抗拉强度值为642MPa,冲击吸收功为96.5J,焊缝处的显微硬度值最大,这与焊缝中的针状铁素体和贝氏体有关。闪光对焊工艺参数中的闪光留量、顶锻留量、顶锻压力对闪光对焊接头组织性能影响显著。当闪光留量由4mm增加至8mm时,焊缝中的魏氏组织严重化,且焊缝夹杂物数量增加,对焊接头力学性能下降,这与闪光对焊中的热输入有关。当顶锻留量由2.5mm增加至5.5mm时,焊缝处夹杂物数量减少。当顶锻压力由6MPa增加至9MPa时,接头焊缝中魏氏铁素体增加,贝氏体细化,夹杂物数量减少。结合实际轮辋生产断裂数据,单因素变化时,闪光留量为4mm、顶锻留量为3.5mm、顶锻压力为8MPa时,接头力学性能达到最优,实际轮辋接头断裂频率最低约为2%。从宏微观的角度对高强钢轮辋扩口与扩张工序的接头断裂机制进行分析,轮辋接头断裂的方式主要表现为脆性断裂和韧性断裂。焊缝中粗大片状的魏氏铁素体组织是造成轮辋扩口接头断裂的主要原因,裂纹扩展到轮辋中部由于粗大的晶粒造成脆-韧转变,造成裂纹由焊缝向HAZ偏转;HAZ拉薄现象、焊缝处夹杂物、微裂纹数量以及焊后的残余应力集中等综合因素是造成轮辋扩张接头断裂的原因,裂纹沿着整个轮辋闪光对焊接头发生全部开裂。闪光对焊后采用空冷而非水冷的方式有利于提高接头性能。采用空冷的对焊接头焊缝由粒状贝氏体组成,夹杂物数量减少且未出现魏氏铁素体。闪光对焊接头抗拉强度为652.7MPa,冲击吸收功为101.1J,弯曲试样中未出现弯曲裂纹,整体力学性能要优于经水冷的闪光对焊接头。采用ARGUS应变测量系统对轮辋扩口工序时的主应变、次应变以及厚度减薄率进行测量。轮辋扩口变形整体不大,主要集中在轮辋边缘部位,闪光对焊接头区域的轮辋边缘处的应变要小于其他区域,在扩口作用力下易造成轮辋接头边缘处断裂。
[Abstract]:Lightweight materials such as high-strength steel plate and aluminum alloy are used in automobile industry. Rim, as an important component of automobile, is an effective way to achieve lightweight. Considering its bearing capacity and safety performance, Low-alloy high-strength steel is still the most used material for rim production. Flash butt welding is a typical resistance welding method. Because of its high efficiency, high speed and high strength joint, it has been the mainstream method of rim welding. As the strength of high strength steel plate increases gradually, the fracture frequency of rim flash butt welding joint increases accordingly. In this paper, the high-strength steel plate of 590CL rim is selected as the base material. Firstly, the characteristics of microstructure and properties of high-strength steel rim flash butt welding joint are studied; secondly, the influence of flash butt welding parameters on the microstructure and properties of the joint is discussed under the change of single factor. The fracture mechanism of the joints in the process of flaring and expanding was analyzed, and the microstructure and properties of the joints were studied by air cooling instead of water cooling after welding, and the strain of the flares was measured by ARGUS strain measurement system. The results show that the composition of 590CL high strength steel sheet conforms to the standard of rim steel. The weld of flash butt welding joint is composed of ferrite and a little bainite, and the base metal is mainly composed of pearlite, ferrite and slightly banded microstructure. The tensile strength of the butt welded joint is 642 MPA, the impact absorption energy is 96.5J, and the microhardness of the weld is the largest, which is related to the acicular ferrite and bainite in the weld. The effect of flash retention and top forging force on the microstructure and properties of flash butt welding joint is significant. When the flash retention is increased from 4mm to 8mm, the microstructure of the weld is serious, the amount of inclusions in the weld increases and the mechanical properties of the butt joint decrease, which is related to the heat input in the flash butt welding. The amount of inclusions in weld decreases with the increase of top forging retention from 2.5mm to 5.5mm. When the top forging force is increased from 6MPa to 9MPa, the ferrite in the weld increases, the bainite is refined and the number of inclusions decreases. Combined with the fracture data of actual rim production, when the single factor changes, the flash retention is 4mm, the forging retention is 3.5mm, and the forging pressure is 8MPa, the mechanical properties of the joint reach the optimum, and the lowest fracture frequency of the actual rim joint is about 2. The fracture mechanism of high strength steel rims during the expansion process was analyzed from macro and micro angle. The fracture modes of rim joints were mainly brittle fracture and ductile fracture. The coarse ferrite structure in the weld is the main reason for the fracture of the expanding joint of the rim. The crack spread to the middle of the rim resulting in brittleness and ductile transition, which results in the crack deflecting from weld to HAZ. The inclusions in the weld, the number of microcracks and the residual stress concentration after welding are the reasons for the fracture of the expanded joints of the rims, and all cracks occur along the whole rim of the flash butt welded joints. Air cooling instead of water cooling after flash butt welding is helpful to improve joint performance. The welds of air-cooled butt welded joints are composed of granular bainite, the amount of inclusions is reduced and no ferrite is found. The tensile strength of flash butt welding joint is 652.7MPa, and the impact absorption energy is 101.1J. there is no bending crack in the bending specimen, and the overall mechanical property is better than that of water-cooled flash butt welding joint. ARGUS strain measurement system was used to measure the main strain, secondary strain and thickness thinning rate. The strain at the rim edge of the flash butt welding joint is smaller than that of the other regions, and the edge fracture of the rim joint is easy to be caused by the flaring force.
【学位授予单位】：华侨大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG407

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