HAZ Characterization and Mechanical Properties of QP980-DP980 Laser Welded Joints

Junliang Xue,Zhandong Wan,Caiwang Tan,Hongqiang Zhang,Wei Guo,Peng Peng,Mingsheng Xia,Yongqiang Li

doi:10.1186/s10033-021-00596-x

Abstract

The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding. The weld zone (WZ) was fully martensitic structure, and heat-affected zone (HAZ) contained newly-formed martensite and partially tempered martensite (TM) in both steels. The super-critical HAZ of the QP980 side had higher microhardness (~ 549.5 Hv) than that of the WZ due to the finer martensite. A softened zone was present in HAZ of QP980 and DP980, the dropped microhardness of softened zone of the QP980 and DP980 was Δ 21.8 Hv and Δ 40.9 Hv, respectively. Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain, leading to the formation of low angle grain boundaries (LAGBs). Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs, which led to significant dislocation interaction and formation of cracks. The electron back-scattered diffraction (EBSD) results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing, resulting in the failure of joints located at the sub-critical HAZ of DP980 side. The QP980-DP980 dissimilar steel joints presented higher elongation (~ 11.21%) and ultimate tensile strength (~ 1011.53 MPa) than that of DP980-DP980 similar steel joints, because during the tensile process of the QP980-DP980 dissimilar steel joint (~ 8.2% and 991.38 MPa), the strain concentration firstly occurred on the excellent QP980 BM. Moreover, Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value (~ 5.92 mm) and the peak punch force (~ 28.4 kN) due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.

Highlights

In order to reduce the weight of automobiles and increase requirement of passenger safety, advanced high strength steels (AHSSs) have been widely applied in automotive industry [1, 2]
Image quality (IQ) map, inverse pole figure (IPF) map, Kernel average misorientation (KAM) map and Taylor Factor (TF) map can be obtained by electron back-scattered diffraction (EBSD)
Ferrite and martensite both are difficult to distinguish by phase map because of their body centered cubic (BCC) α crystal structure [22], martensite islands are dark grains, as shown Figure 4b, owing to its faint Kikuchi patterns due to its highly distorted lattice compared with ferrite

Summary

Introduction

In order to reduce the weight of automobiles and increase requirement of passenger safety, advanced high strength steels (AHSSs) have been widely applied in automotive industry [1, 2]. The retained austenite (RA) in QP steel can transform into martensite during deformation due During the manufacturing processes, jointing of dissimilar or similar AHSSs using fusion welding techniques is inevitable [8]. Heat input of fusion welding of AHSSs is associated with some problems such as heat affected zone (HAZ) softening, formation of porosities as well as solidification cracks. Guzman-Aguilera et al [9] assessed the effect of the welding heat input on the microstructure and mechanical properties of Si-alloyed TRIP steel. They found that the welded joints occurred softening at the sub-critical HAZ and the failure location of the arc welded samples was the sub-critical HAZ. Farabi et al [10] suggested that the degree of softening was

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