Comprehensive Analysis of the Microstructure and Mechanical Properties of Friction-Stir-Welded Low-Carbon High-Strength Steels with Tensile Strengths Ranging from 590 MPa to 1.5 GPa

Hyeonjeong You,Sung Yi,Soongkeun Hyun,Minjung Kang,Cheolhee Kim

doi:10.3390/app11125728

Abstract

High-strength steels are being increasingly employed in the automotive industry, requiring efficient welding processes. This study analyzed the materials and mechanical properties of high-strength automotive steels with strengths ranging from 590 MPa to 1500 MPa, subjected to friction stir welding (FSW), which is a solid-phase welding process. The high-strength steels were hardened by a high fraction of martensite, and the welds were composed of a recrystallized zone (RZ), a partially recrystallized zone (PRZ), a tempered zone (TZ), and an unaffected base metal (BM). The RZ exhibited a higher hardness than the BM and was fully martensitic when the BM strength was 980 MPa or higher. When the BM strength was 780 MPa or higher, the PRZ and TZ softened owing to tempered martensitic formation and were the fracture locations in the tensile test, whereas BM fracture occurred in the tensile test of the 590 MPa steel weld. The joint strength, determined by the hardness and width of the softened zone, increased and then saturated with an increase in the BM strength. From the results, we can conclude that the thermal history and size of the PRZ and TZ should be controlled to enhance the joint strength of automotive steels.

Highlights

Friction stir welding (FSW) is a well-known solid-state welding process with minimum thermal effect on weldments
The entire region was fully austenized when the temperature rose and recrystallized because of phase transformation when cooled; (2) partial recrystallized zone (PRZ): the peak temperature is between Ac1 and Ac3; the region cooled down after partial austenization at the p4eoafk12 temperature; (3) tempered zone (TZ): Since the peak temperature is lower than Ac1, the initial microstructure is not transformed to austenite, and only grain growth and tempmeraerdtemnsairtetecnosuitledcboeuoldbsbeervoebdsberevcaeudsbeeocfatuhseetoemf tpheeritnegmepfefercint;ganefdfe(c4t); baansde m(4)etbaal s(BeMm)e:ttahle (BinMit)i:atlhme iicnriotisatlrumcitcurroestirsumctauirnetaisinmeda.inEtaaicnhezdo. nEeacwhazsoindeenwtaifiseiddeunstiinfigedthuesminigcrtohsetrmuicctruor-e, starnudctaulrseo, tahnedpaelasok ttheme ppeearaktuterme wpearsateustriemwaatesde.stimated
In the case of the DP590 steel, stress concentration occurs in the base metal (BM), whereas the stress is concentrated in the PRZ

Summary

Introduction

Friction stir welding (FSW) is a well-known solid-state welding process with minimum thermal effect on weldments. FSW was initially applied to nonferrous metals and later to high-strength steels. Polycrystalline cubic boron nitride, W-based materials, and Si3N4 have been proposed as tool materials instead of steel used for the FSW of Al alloys [2]. Another important feature of friction stir-welded steels is the welding system deflection due to the high applied load. Accurate depth control has been successfully achieved in the FSW of steels using high-stiffness welding systems [3,4] and compensation methods such as offset control, constant force control [5,6], and force-deflection models [7,8]. Hybrid processes using an additional heat source can help reduce the yield strength of base materials and enhance the process capability [9,10,11,12]

Methods

Results

Conclusion