超声振动强化搅拌摩擦焊工艺及接头疲劳性能的研究

发布时间：2018-06-20 18:09

本文选题：超声振动强化搅拌摩擦焊 + 疲劳性能　；参考：《山东大学》2017年硕士论文

【摘要】：超声振动强化搅拌摩擦焊工艺(UVeFSW),将超声能量作用于搅拌工具前方的待焊接工件上来改善接头的成形质量,获得了焊接载荷降低、焊接缺陷减少、焊接速度提高、焊接工艺窗口拓宽等工艺效果。然而,有关超声振动对于搅拌摩擦焊(FSW)接头疲劳性能影响方面的研究尚属空白。本文在这方面开展了研究,对于促进UVeFSW工艺的发展和应用,具有重要的工程实用价值。开展了 6 mm厚2024-T3铝合金板的FSW和UVeFSW工艺试验。对比了FSW和UVeFSW两种工艺条件下,接头宏/微观组织、力学性能和焊接载荷等方面的差异。结果表明,超声振动可以增加焊核区宽度,细化第二相粒子,但对晶粒尺寸影响不大;后退侧热力影响区晶粒的变形程度变大,前进侧热力影响区边界流线趋于光滑。通过高频恒幅疲劳试验和疲劳断口的SEM分析,研究了超声振动对接头疲劳性能的影响。采用国际焊接学会(IIW)阐明的方法对试验结果进行了分析。在所用实验条件下,所得数据点和拟合所得的疲劳寿命(S-AO曲线都高于FAT45线,常规FSW和UVeFSW两种工艺所得接头均具有高于IiW标准熔焊接头的疲劳性能。试验结果表明,在50%存活率时,超声振动使得接头的疲劳强度得到大幅提高,由常规FSW接头的86.2 MPa提高到了 UVeFSW接头的103.74 MPa,达到了母材的96.13%。当按照95%存活率和75%置信度对数据进行处理时,疲劳数据较大的离散性使得施加超声振动对于疲劳性能的影响减弱。在接头横断面上,根据材料的流动定义了三个边界和一个特征区域。在轴肩作用的区域,后退侧向前进侧和前进侧向下流动的两股材料,形成交界1;轴肩作用区域向下流动的材料和搅拌针作用区域向上流动的材料,形成交界2;前进侧焊核区与热力影响区之间组织变化剧烈,形成交界3。这三个边界相互连接,在前进侧形成一个相对薄弱的特征区域,应力集中程度达到最大,通常会成为疲劳裂纹萌生的位置。在所用试验条件下,常规FSW接头全部起裂于特征区域。而超声振动使得特征区域的应力集中程度减弱,UVeFSW接头73%起裂于交界3,其他则起裂于特征区域之外。SEM断口分析发现,在裂纹扩展区,超声振动使得疲劳条带更加明显,间距变小。在瞬断区,超声振动使得第二相粒子的尺寸更小、数量更多、分布更均匀,与此对应的韧窝连接更加紧密。常规FSW接头的断口呈现脆性和韧性的混合断裂特征,UVeFSW接头的断口呈现韧性断裂特征。
[Abstract]:Ultrasonic vibration strengthened friction stir welding (FSW) technology is used to improve the joint forming quality by applying ultrasonic energy to the workpiece to be welded in front of the stirring tool. The welding load is reduced, the welding defect is reduced, and the welding speed is increased, the welding load is reduced, the welding defect is reduced, and the welding speed is increased. Welding process window widening and other process effects. However, the influence of ultrasonic vibration on fatigue properties of FSW joints is still blank. The research in this field is of great practical value for promoting the development and application of UVeFSW process. FSW and UveFSW process tests of 6 mm thick 2024-T3 aluminum alloy plate were carried out. The differences of macro / microstructures, mechanical properties and welding loads between FSW and UVeFSW were compared. The results show that ultrasonic vibration can increase the width of the nugget zone and refine the second phase particles, but has little effect on the grain size, and the deformation degree of the grain in the thermal affected zone of the receding side becomes larger, and the boundary streamline of the thermal affected zone on the forward side tends to smooth. The influence of ultrasonic vibration on fatigue properties of joints was studied by means of high frequency constant amplitude fatigue test and SEM analysis of fatigue fracture. The test results are analyzed by the method described by the International Welding Society (IIW). Under the experimental conditions, the fatigue life of the joints obtained by FSW and UVeFSW is higher than that of IiW standard welded joints. The fatigue life of the joints is higher than that of FAT45. The fatigue properties of conventional FSW and UVeFSW joints are higher than those of IiW standard welded joints. The experimental results show that the fatigue strength of the joint is greatly improved by ultrasonic vibration at 50% survival rate, from 86.2 MPA of conventional FSW joint to 103.74 MPA of Uve FSW joint, reaching 96.13 MPA of base metal. When the data were processed according to 95% survival rate and 75% confidence, the influence of ultrasonic vibration on fatigue performance was weakened due to the large dispersion of fatigue data. On the cross section of the joint, three boundaries and a characteristic area are defined according to the material flow. Two strands of material flowing downwards and backward in the axial shoulder region, forming a junction 1; the material flowing downward in the axial shoulder action area and the material flowing upward in the agitated needle area, The formation of the junction 2, the forward side of the nugget zone and the thermal impact zone between the microstructure changes dramatically, forming the junction of 3. 5%. The three boundaries are connected with each other and form a relatively weak characteristic area on the forward side. The stress concentration reaches the maximum and usually becomes the position where the fatigue crack initiation occurs. Under the test conditions, all the conventional FSW joints start to crack in the characteristic region. However, ultrasonic vibration weakens the stress concentration in the characteristic region by 73% and starts at the junction of 3%, while the other crack begins outside the characteristic region. SEM analysis shows that in the crack propagation zone, ultrasonic vibration makes the fatigue band become more obvious and the spacing becomes smaller. In the transient fault region, ultrasonic vibration makes the size of the second phase particles smaller, the number more, the distribution more uniform, and the corresponding dimples are more closely connected. The fracture surfaces of conventional FSW joints show brittle and ductile mixed fracture characteristics. The fracture surfaces of UVeFSW joints show ductile fracture characteristics.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG453.9

【相似文献】