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ORIGINAL PAPER

Clarifying effect of welding conditions on microstructure and mechanical properties of friction stir spot‑welded DH590 automotive high‑strength steel plates Ya‑peng Zhao1,2 · Lian‑feng Chen3 · Shi‑jie Zhu1,2 · Yu‑feng Sun1,2   · Shao‑kang Guan1,2 Received: 12 December 2019 / Revised: 18 April 2020 / Accepted: 20 April 2020 © China Iron and Steel Research Institute Group 2020

Abstract Friction stir spot welding was successfully applied to the 1.2-mm-thick DH590 dual-phase steel plates by using a polycrystalline cubic boron nitride rotating tool. During welding, the rotation speed ranged from 600 to 1000 r/min and the penetration depth ranged from 0.1 to 0.3 mm. In the spot joints, the size of the stir zone increased with the increase in rotation speed as well as the penetration depth of the tool. Comparing with the banded ferrite and martensite structure of the base metal, a mixed microstructure of ferrite and tempered martensite, refined bainite structure and coarse martensite structure were found in the heat-affected zone, thermomechanically affected zone and stir zone of the joints, respectively. Two kinds of interfacial shapes were formed between the upper and lower steel plates, and the formation of the bonded interface was dominated mainly by the penetration depth of the rotating tool rather than the rotation speed. It was revealed that the joints with straight interfaces showed higher shear tensile loads comparing with those with hook-like interfaces. Shear tensile tests showed that the maximum shear tensile load reached about 15.56 kN when the rotation speed and penetration depth were set as 800 r/min and 0.3 mm, respectively. The specimen was fractured through plug failure mode with a total elongation of about 5.6 mm. Keywords  Friction stir spot welding · DH590 dual-phase steel · Microstructure · Joint strength · Microhardness

1 Introduction High-strength steels (HSSs), such as dual-phase (DP) steel, transformation-induced plasticity (TRIP) steel, twinninginduced plasticity (TWIP) steel, complex phase (CP) steel and quenching and partitioning (Q&P) steel, show excellent mechanical properties and have been widely used in automobile industries [1–3]. Since averagely 4000 spot welds are necessary for one modern car in the manufacturing process, the qualities of the welds are therefore becoming very important for the safety of the customers. Nowadays, * Yu‑feng Sun [email protected] * Shao‑kang Guan [email protected] 1



School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China

2



Henan Key Laboratory of Advanced Magnesium Alloys, Zhengzhou University, Zhengzhou 450001, Henan, China

3

Beijing Shougang Co., Ltd., Beijing 100041, China



the main welding or joining methods of HSS in automobile industries include gas shielded welding [4], laser welding [5], resistance spot welding [6], etc. It is widely known that the weldability of HSS is generally poor due to the existence of a large number of alloying elements. During the fusion welding process