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Plunge Depth Control and Hook Formation Analysis with Friction Stir Welding in Lap Jointed Al/Fe

Title
Plunge Depth Control and Hook Formation Analysis with Friction Stir Welding in Lap Jointed Al/Fe
Author
Jin Young Yoon
Alternative Author(s)
윤진영
Advisor(s)
이세헌
Issue Date
2019. 8
Publisher
한양대학교
Degree
Doctor
Abstract
The main purpose of this study is to deal with two issues arising from welding of a lap joint of Al/Fe dissimilar materials using friction stir welding. The first issue, despite increasing need for friction stir welding for high-temperature softening materials, system deflection due to relatively high plunging force remains an obstacle. System deflection results in the vertical position error of a welding tool and insufficient plunge depth. In this study, the deflection compensation control to maintain plunge depth, was used the plunging force was coaxially measured, and the position error was estimated using a force-deflection model. A linear relationship was confirmed between the force and deflection
this relationship is dependent on the stiffness of the welding system while independent of process parameters and base materials. The plunge depth control was evaluated during the friction stir spot welding of an Al 6061-T6 alloy sheet and a dissimilar metal combination of Al 6061-T6 alloy/dual phase (DP) 590 steel. Under varying process parameters, the deflection compensation control maintained a plunge depth with an error of less than 50 μm. Conventional position control has a maximum error of nearly 300 μm. In addition, comparison with conventional friction stir welding was carried out. Three control methods were evaluated, namely conventional position control, offset position control, and deflection compensation control in the friction stir lap welding of 3.0 mm-thick Al 5083-O alloy over 1.2 mm-thick DP 590 steel. The desired plunge depth was 0.2 mm into the steel sheet. However, the pin did not reach the steel surface under conventional position control due to deflection of the vertical axis of the welding system. In offset position control, an additional offset of 0.35 mm could achieve the desired plunge depth with considerable accuracy. Nevertheless, a gradual increase of the plunge depth along the longitudinal direction was unavoidable, due to an in-situ decrease of the material yield strengths. In deflection compensation control, the deflection is estimated by the coaxially measured plunging force and the force-deflection relationship and then corrected by deflection compensation control. Thus, the plunge depth is stabilized along the longitudinal direction and is precisely controlled with a 3.3 μm standard deviation of error during the tool traverse stage. There is also a consistent bias of 32 μm caused by the resolution of the measuring system, and it can be easily calibrated in the deflection compensation control system. The second issue, the formation of defects called hook at the interface of the dissimilar materials. In this study, the mechanism of hook formation was show and the effect of hook shape on the mechanical properties of welding was analyzed. The hook starts to be created at the position where the pin is inserted into the Fe material, and the final shape is determined at the lower end of the backside shoulder. Although the hooks are generally known as defects, the test results show that the tensile strength of weld with hooks is higher in the formed condition than in the case where the hook is not formed. Also, in the case of hooks, there is a difference depending on the pulling direction of the tension. The hooks tend to lower the tensile strength when placed in the stressed flow but tend to increase the tensile strength when not in the stressed flow. The mechanical properties of the Al/Fe dissimilar material friction stir spot welding were improved by intentionally generating the hook by controlling the tool path.
URI
http://dcollection.hanyang.ac.kr/common/orgView/000000110605https://repository.hanyang.ac.kr/handle/20.500.11754/109755
Appears in Collections:
GRADUATE SCHOOL[S](대학원) > MECHANICAL ENGINEERING(기계공학과) > Theses (Ph.D.)
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