Effect of various rotational speeds on the inhomogeneous distribution of microstructure and toughness in friction stir weld joint of Q1100 ultra-high strength steel

IF 2.4 4区 材料科学 Q2 METALLURGY & METALLURGICAL ENGINEERING
Yawen Hu, Zheng Wang, Tingfang Tao, Shuai Chen, Hongbo Cui
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Abstract

Using different rotational speeds in friction stir welding is a new way to improve the microstructure and the impact toughness of the weld. However, the specific speed most suitable for Q1100 ultra-high strength steel has yet to be discovered. Here, friction stir welding of 8 mm thick Q1100 ultra-high strength steel was carried out at constant welding speeds of 375 rpm, 475 rpm, and 600 rpm. With the aid of an optical microscope, a microhardness tester, an impact tester, a tensile tester, and a field emission scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) system, the microstructure characteristics such as the carbide precipitation phase in the original austenite grains, the dislocation pattern within the martensitic lath, and the fracture morphology of the impact specimens were analyzed. The results show that the overall hardness and impact toughness of the advancing side and retreating side of the stirring zone show a trend of first increasing and then decreasing with the increase of the rotational speed, in which the hardness value of the advancing side of the same speed is higher than that of the retreating side. In contrast, the impact toughness is higher on the retreating than on the advancing side. At 375 rpm, the carbide is needle-like, the martensite bundle is narrow, and there are more high-angle grain boundaries, the hindrance to crack expansion is significant, and the toughness is higher relative to the base material. At 475 rpm, the martensite bundle widens, the number of high-angle grain boundaries increases, both the advancing and retreating sides are ductile fractures with small tearing dimples of varying sizes distributed around the large tearing dimples, and the toughness increases relative to 375 rpm. When the speed at 600 rpm, the carbide is coarser, the martensite bundle is more expansive than at 475 rpm, the number of high-angle grain boundaries is reduced, the tearing toughness dimples are unevenly distributed, and the toughness is reduced. Compared to 375 rpm and 600 rpm, the joint cooling rate is either faster or slower, and the 475 rpm cooling rate is just right in between. Meanwhile, when the rotational speed at 475 rpm, the average hardness of the joint is 375 HV, the impact work at − 40℃ is 51 J, and its tensile strength and elongation are 998 MPa and 3.09%, respectively, with the best comprehensive mechanical properties.

Abstract Image

不同转速对 Q1100 超高强度钢搅拌摩擦焊缝微观组织不均匀分布和韧性的影响
摘要 在搅拌摩擦焊中使用不同的转速是一种改善焊缝微观结构和冲击韧性的新方法。然而,最适合 Q1100 超高强度钢的特定转速尚未发现。在此,我们以 375 rpm、475 rpm 和 600 rpm 的恒定焊接速度对 8 mm 厚的 Q1100 超高强度钢进行了搅拌摩擦焊接。借助光学显微镜、显微硬度计、冲击试验机、拉伸试验机和配备电子背散射衍射(EBSD)系统的场发射扫描电子显微镜(SEM),分析了冲击试样的微观结构特征,如原始奥氏体晶粒中的碳化物析出相、马氏体板条内的位错模式和断口形态。结果表明,随着转速的增加,搅拌区前进侧和后退侧的整体硬度和冲击韧性呈先增大后减小的趋势,其中相同转速下前进侧的硬度值高于后退侧。相反,后退侧的冲击韧性高于前进侧。375 rpm 时,碳化物呈针状,马氏体束狭窄,高角度晶界较多,裂纹扩展受阻明显,韧性相对母材较高。转速为 475 rpm 时,马氏体束变宽,高角度晶界增多,前进侧和后退侧均为韧性断裂,在大撕裂窝周围分布着大小不等的小撕裂窝,韧性相对于 375 rpm 有所提高。当转速为 600 rpm 时,碳化物更粗,马氏体束比 475 rpm 时更膨胀,高角度晶界数量减少,撕裂韧窝分布不均,韧性降低。与 375 rpm 和 600 rpm 相比,接头冷却速度或快或慢,而 475 rpm 的冷却速度正好介于两者之间。同时,当转速为 475 rpm 时,接头的平均硬度为 375 HV,-40℃时的冲击功为 51 J,抗拉强度和伸长率分别为 998 MPa 和 3.09%,综合力学性能最佳。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Welding in the World
Welding in the World METALLURGY & METALLURGICAL ENGINEERING-
CiteScore
4.20
自引率
14.30%
发文量
181
审稿时长
6-12 weeks
期刊介绍: The journal Welding in the World publishes authoritative papers on every aspect of materials joining, including welding, brazing, soldering, cutting, thermal spraying and allied joining and fabrication techniques.
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