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Materials 2015, 8(12), 8424-8436; doi:10.3390/ma8125467

An Evaluation of Global and Local Tensile Properties of Friction-Stir Welded DP980 Dual-Phase Steel Joints Using a Digital Image Correlation Method

1
Department of Energy System Engineering, Korea National University of Transportation, 50 Deahak-ro, Chungju-si, Chungbuk 27496, Korea
2
Advanced Welding and Joining R&D Group, Research Institute of Advanced Manufacturing Technology, Korea Institute of Industrial Technology, 156 Gaetbeol-ro (Songdo-dong), Yeonsu-Gu, Incheon 21999, Korea
3
Metal Forming R&D Group, Research Institute of Advanced Manufacturing Technology, Korea Institute of Industrial Technology, 156 Gaetbeol-ro (Songdo-dong), Yeonsu-Gu, Incheon 21999, Korea
*
Author to whom correspondence should be addressed.
Academic Editor: Jung Ho Je
Received: 25 August 2015 / Revised: 18 November 2015 / Accepted: 25 November 2015 / Published: 4 December 2015
(This article belongs to the Section Structure Analysis and Characterization)
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Abstract

The effect of the microstructure heterogeneity on the tensile plastic deformation characteristic of friction-stir-welded (FSW) dual-phase (DP) steel was investigated for the potential applications on the lightweight design of vehicles. Friction-stir-welded specimens with a butt joint configuration were prepared, and quasi-static tensile tests were conducted, to evaluate the tensile properties of DP980 dual-phase steels. The friction-stir welding led to the formation of martensite and a significant hardness rise in the stir zone (SZ), but the presence of a soft zone in the heat-affected zone (HAZ) was caused by tempering of the pre-existing martensite. Owing to the appearance of severe soft zone, DP980 FSW joint showed almost 93% joint efficiency with the view-point of ultimate tensile strength and relatively low ductility than the base metal (BM). The local tensile deformation characteristic of the FSW joints was also examined using the digital image correlation (DIC) methodology by mapping the global and local strain distribution, and was subsequently analyzed by mechanics calculation. It is found that the tensile deformation of the FSW joints is highly heterogeneous, leading to a significant decrease in global ductility. The HAZ of the joints is the weakest region where the strain localizes early, and this localization extends until fracture with a strain near 30%, while the strain in the SZ and BM is only 1% and 4%, respectively. Local constitutive properties in different heterogeneous regions through the friction-stir-welded joint was also briefly evaluated by assuming iso-stress conditions. The local stress-strain curves of individual weld zones provide a clear indication of the heterogeneity of the local mechanical properties. View Full-Text
Keywords: friction-stir welding (FSW); dual-phase (DP) steel; microstructure; global and local tensile behavior; digital image correlation (DIC) friction-stir welding (FSW); dual-phase (DP) steel; microstructure; global and local tensile behavior; digital image correlation (DIC)
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Lee, H.; Kim, C.; Song, J.H. An Evaluation of Global and Local Tensile Properties of Friction-Stir Welded DP980 Dual-Phase Steel Joints Using a Digital Image Correlation Method. Materials 2015, 8, 8424-8436.

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