Influence of Different Offsets on the Mechanical and Fatigue Properties of Friction Stir Weld Lap Joints
- Vol. 52, Issue 8, Pages: 95-100(2022)
DOI: 10.7512/j.issn.1001-2303.2022.08.13
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In the welding process, due to the deformation of the material to be welded or the operation error, the stirring needle may be misaligned with the center line of the joint, which may cause an offset, which will adversely affect the quality of the joint. Therefore, it is necessary to study the offset. Use friction stir welding to weld 6005A-T6 hollow aluminum profiles, prepare butt-lap friction stir welding (FSW) joints with different offsets, perform tensile tests and fatigue tests under constant load, and explore the effect of different shaft shoulders on the tensile strength and fatigue performance. The results show that the tensile strength values of the specimens under the offset of the shaft shoulder of the six stirring needles are distributed around 200±10 MPa, which indicates that the influence of the tensile strength is not obvious. By calculating the joint strength coefficients, it is found the butt-lap joint strength coefficients under the shoulder offset are all higher than 0.7, which can be judged as qualified; for the two groups of test pieces with shoulder offset of AS3 mm and RS1.8 mm, when the maximum load value is 55 MPa ,there is no failure under the specified life, and the fatigue performance can be judged to be qualified. Under a larger load, the test specimens all fail within the specified life. The fracture position is the junction of the heat-affected zone and the fusion line, indicating that the position is the stress concentration is greater.
friction stir welding;
axial offset;
tensile strength;
fatigue performance
随着我国轨道交通装备制造水平的不断提高,对高速轨道列车的车体制造材料有了更高的要求,制造轻量化、更加节能环保的高速列车势在必行[
目前已有许多搅拌摩擦焊接头的力学性能和疲劳性能的研究成果。Tsutomu Tanaka等[
基于为企业生产和结构设计与评估提供数据基础的目的,本文针对不同偏移量对FSW接头力学性能及疲劳性能的影响进行了重点研究。对6005A-T6中空铝型材进行搅拌摩擦焊工艺试验,制备不同偏移量缺陷的焊接试件,通过拉伸、扫描、恒定载荷下的疲劳试验研究不同偏移量的对FSW接头力学性能及疲劳性能的影响。
试验材料为4.5 mm厚度的6005A-T6中空铝型材板,尺寸500 mm×300 mm×4 mm,供货状态为T6态,其化学成分和力学性能如
Si | Fe | Cu | Mn | Mg | Cr | Zn | Ti |
---|---|---|---|---|---|---|---|
0.59 | 0.18 | 0.09 | 0.12 | 0.57 | 0.01 | 0.02 | 0.01 |
屈服强度Rp0.2/MPa | 抗拉强度Rm/MPa | 伸长率A/% |
---|---|---|
≥200 | ≥255 | ≥6 |
图1 搅拌摩擦焊接头形式
Fig.1 Friction stir welded joint form
理论上焊前需要将搅拌针对准接头的中心线,但在实际生产过程中,搅拌针会不可避免地向前进侧(AS)或者后退侧(RS)发生偏移,造成搅拌针无法对齐焊缝中心线。其次,在焊接过程中如果出现焊接变形以及夹具设计不合理,也可能导致搅拌针无法对齐焊缝中心线造成偏移。本次试验通过设计不同方向的偏移量,判断不同尺寸和不同方向的偏移量对焊缝力学性能以及疲劳性能的影响。焊前不倒角、不打磨,焊接工艺参数如
试板编号 | 偏移量/mm① | 搅拌头倾角/(°) | 组装间隙/mm | 压入量/mm | 搅拌头转速ω/(r·min-1) | 焊接速度v/(mm·min-1) |
---|---|---|---|---|---|---|
PYA-1 | AS0.5 | 2 | 0 | 0.25 | 1 500 | 1 100 |
PYR-2 | RS0.5 | 2 | 0 | 0.25 | 1 500 | 1 100 |
PYA-3 | AS1.8 | 2 | 0 | 0.25 | 1 500 | 1 100 |
PYR-4 | RS1.8 | 2 | 0 | 0.25 | 1 500 | 1 100 |
PYA-5 | AS3.0 | 2 | 0 | 0.25 | 1 500 | 1 100 |
PYR-6 | RS3.0 | 2 | 0 | 0.25 | 1 500 | 1 100 |
注: ①AS为前进侧,RS为后退侧。
焊接完成后,根据标准ISO4136-2001在WDW-300 kN 型电子万能试验机上进行拉伸试验,拉伸速度为5 mm/min。拉伸试件尺寸如
图2 拉伸试件尺寸示意
Fig.2 Schematic of the tensile specimen dimension
图3 疲劳试件尺寸示意
Fig.3 Schematic of the dimension of the fatigue specimen
6种轴向偏移尺寸下的接头抗拉强度如
试件号 | 轴向偏移尺寸 /mm | 试件实际尺寸 /mm | 抗拉 强度 Rm/MPa | 平均值 /MPa | 接头强度系数① | 断裂 位置 |
---|---|---|---|---|---|---|
PY-1-1 | AS0.5 | 20×4.5 | 209.1 | 210.25 | 0.82 | HAZ |
PY-1-2 | 211.4 | |||||
PY-2-1 | RS0.5 | 20×4.5 | 201.3 | 199.60 | 0.78 | HAZ |
PY-2-2 | 197.9 | |||||
PY-3-1 | AS1.8 | 20×4.5 | 210.2 | 206.15 | 0.81 | HAZ |
PY-3-2 | 202.1 | |||||
PY-4-1 | RS1.8 | 20×4.5 | 217.9 | 212.85 | 0.83 | HAZ |
PY-4-2 | 207.8 | |||||
PY-5-1 | AS3.0 | 20×4.5 | 204.9 | 205.55 | 0.81 | HAZ |
PY-5-2 | 203.2 | |||||
PY-6-1 | RS3.0 | 20×4.5 | 195.3 | 198.7 | 0.78 | HAZ |
PY-6-2 | 202.1 |
注: ①接头强度系数为接头强度与母材强度之比。
图4 抗拉强度与偏移量关系
Fig.4 Relationship between tensile strength and offset
可以看出,6种轴向偏移尺寸下的抗拉强度稳定在(200±10) MPa范围内,表明这6种搅拌针轴向偏移尺寸对试件的抗拉强度值影响较小。此外,对于搅拌摩擦焊铝合金接头来说,接头强度系数T=0.7可定义为合格。由表可知,6种轴向偏移尺寸下的试件对搭接接头强度系数均高于0.7,接头强度合格。值得指出的是,上述接头失效位置均出现在HAZ附近,这是因为HAZ没有搅拌针的机械搅拌作用且受到较多的焊接热输入,在焊接热的作用下HAZ发生过时效现象,析出相长大,导致HAZ力学性能相对于其他区域显著下降。文献[
采用JEOL-6360LV型SEM电镜对断口中心及边缘进行扫描,断口微观形貌如
图5 后退侧轴向偏移3 mm试件拉伸断口微观形貌
Fig.5 Microscopic morphology of the tensile fracture of the specimen with an axial offset of 3 mm on the receding side
为节省试验成本,取AS 3 mm和RS 1.8 mm两种偏移量下的焊接接头进行疲劳试验,并进行断口形貌分析。AS 3 mm的疲劳试验结果如
试验类别 | 试件 数量 | 最大载荷/MPa | 断裂 情况 | 断裂时循环次数/×107 | 断裂 位置 |
---|---|---|---|---|---|
恒定载荷下脉动 拉伸疲劳试验 | 10 | 55 | 未断 | 1 | — |
1 | 130 | 断 | 0.160 11 | HAZ |
图6 AS 3.0 mm的断裂试件
Fig.6 Fracture specimen with AS 3.0 mm shoulder offset on theforward side
图7 AS 3.0 mm大载荷下疲劳试验断口微观形貌
Fig.7 Microscopic morphology of the fatigue test fracture of the specimen with a 3 mm axial offset on the forward side under heavy load
试验 类别 | 试件 数量 | 最大载荷/MPa | 断裂 情况 | 断裂时循环/×107 | 断裂 位置 |
---|---|---|---|---|---|
恒定载荷下脉动 拉伸疲劳试验 | 10 | 55 | 未断 | 1 | — |
1 | 130 | 断 | 0.182 03 | HAZ |
图8 RS 1.8 mm的断裂试件
Fig.8 Fracture specimen with a 1.8 mm shoulder offset on receding side
图9 RS 1.8 mm大载荷下疲劳试验断口微观形貌
Fig.9 Microscopic morphology of the fatigue test fracture of the specimen with a 1.8 mm axial offset on the receding side under heavy load
由
RS 1.8 mm疲劳断口同样存在启裂区以及扩展区,并且疲劳辉纹清晰可见。不同于AS 3.0 mm疲劳断口,RS 1.8 mm未出现未填满缺陷;由于热影响区是整个焊接接头最为薄弱的部分,因此,启裂区位于此区域。断口终断区呈纤维状且分布着大量大小不一的等轴韧窝,终断区断裂形式为韧性断裂。
(1)拉伸试验结果表明:不同尺寸及不同方向偏移量对6005A-T6对搭FSW接头抗拉强度影响不明显,接头的强度系数均高于0.7,拉伸性能良好。
(2)对AS 3 mm和RS 1.8 mm两种工艺下的试件进行疲劳试验,结果表明:55 MPa恒定载荷下,两组试件均未在指定寿命内发生断裂,可判定FSW接头疲劳性能合格。
(3)断口扫描试验结果表明:拉伸断口整体呈纤维状,中心及边缘分布着大量的等轴韧窝,可判定断裂形式为典型的韧性断裂;接头疲劳断口启裂区位于试件热影响区表面,扩展区疲劳辉纹清晰,终断区存在着大量韧窝,最终断裂形式为韧性断裂。
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