Friction stir lap welded advanced high strength steels: Microstructure and mechanical properties

https://doi.org/10.1016/j.msea.2011.06.087Get rights and content

Abstract

Friction stir welding was carried out under different heat input and cooling rates to produce lap joints between high strength martensitic steel sheets. The microstructure of the welds was characterized, and microhardness was evaluated. Joint efficiency was determined by lap shear test. Variation in processing conditions governed total heat input, peak temperature and cooling rate during friction stir welding. Weld nugget microstructure depended principally on cooling rate. The slowest cooling rate promoted ferrite–pearlite and the fastest cooling rate resulted in martensite formation in the weld nugget. The weakest region of all the joints was the heat affected zone, which consists of ferrite with small quantities of pearlite. Fracture during shear testing occurred along the heat affected zone of welded joints. The width and grain size of ferrite in heat affected zone controlled the joint efficiency.

Highlights

► For the first time advanced high strength steel was joined by friction stir welding. ► Heat input, maximum temperature, cooling rate were calculated from the process parameters. ► Three distinct microstructural zones developed in welded specimens. ► Weld nugget structure depended on cooling rate after welding. ► The grain size and width of heat affected zone governs the joint efficiency.

Introduction

Steels are widely used in automotive industries. For example, to meet the stringent requirements of safety, formability and reduction of weight for energy saving, interest is shifting from normal dual phase structural steel having medium strength, to multiphase advanced high strength steel (AHSS). AHSS with strength level ≥1000 MPa, contains different combinations of C, Mn, Si, Cr, Mo, V, Ti, Nb, Cu, Ni, etc., to obtain adequate hardenability and high strain hardening capacity. The application of thinner gauge AHSS with respect to conventional high strength low alloy steel (HSLA) and dual phase steel (DPS) depends on formability, in which joining is crucial. The most common joining technique for sheet steels is resistance spot welding (RSW). The spot weldability of high strength including transformation induced plasticity (TRIP) steels with tensile strength in the range of 500–1000 MPa had been studied extensively with emphasis on microstructural evaluation, microhardness distribution, tensile lap-shear strength, cross-tension strength and effect of welding parameters [1], [2], [3], [4]. The main drawbacks of RSW are the development of highly strained brittle microstructure and solidification related defects in weld nugget (WN) because of fast cooling rate [5]. The joint also suffered from softening in the heat affected zone [2], [3].

Friction stir welding (FSW) and friction stir spot welding (FSSW) allow for the possibility of joining advanced high strength steels and reduce problems associated with RSW. However, literature reports on friction stir welded AHSS joints are scanty. The inherent difficulties of FSW of AHSS are high mechanical loading for plunging and high temperature in the range of ∼1000–1200 °C [6], [7]. Apart from this, high carbon equivalent and alloying elements complicate the process. Ueji et al. [8] have studied the FSW of ultrafine grained low carbon martensitic steel. They reported that tool rotation rate influenced microhardness at weld nugget, grain boundary characteristics, morphology of ferrite grains and carbide distribution. Feng et al. [9] used PCBN tool for FSSW of M190 steel and found that increase in welding time from 2.1 to 3.2 s resulted in an increase in the shear strength from ∼7 to ∼11 MPa. FSSW was also investigated for hot stamped boron steel having strength level of ≥1500 MPa using PCBN tool [10].

The present work attempts for the first time to join M190 steel by friction stir welding. The objectives are: (i) to quantify heat input and cooling rate during FSW of M190 steel from available empirical relationships; (ii) to study their influence on the weld microstructure; (iii) to evaluate the mechanical properties of the joints; and (iv) to correlate the microstructure with mechanical properties.

Section snippets

Experimental

The alloy used for the study was commercial M190 martensitic steel with dimension 170 mm length × 38 mm width × 1 mm thickness. The chemical composition and mechanical properties of the alloy are summarized in Table 1, Table 2, respectively.

The lap joints were made using a friction stir welding machine in position control mode under different tool traverse speeds. A composite tool was used for welding. The shank of the tool was made of DENSIMET-180 (W–3.5Ni–1.5Fe) alloy. The shoulder and probe were

Results

The macro observation of the joints revealed ‘bowl’ shaped weld nugget without any discontinuity and defects. Fig. 1 schematically depicts different microstructural regions of the lap welded cross-section with respect to the shoulder and pin positions. WN consisted of regions ‘I’, ‘II’ and ‘III’. Region ‘I’ was the surface/sub-surface of workpiece and in contact with tool shoulder during welding. In region ‘II’, the effect of pin rotation was dominant. Region ‘IV’ was at the periphery of tool

Discussion

WN microstructure depends on peak temperature and cooling rate during welding. Process parameters, on the other hand, controlled the total heat input and cooling rate. Considering FSW as a simple mechanical working, where a circular shaft rotates against plate surface under axial load, the required torque is expressed by [12],U=112πμPd3where μ is the friction between tool and workpiece (and in the present investigation it has been considered as 0.4), P is the contact pressure, d is the

Conclusions

Friction stir welding of advanced high strength, low carbon martensitic steel sheets produced three distinct regions—weld nugget, HAZ-1 and HAZ-2. Peak temperature reached as high as 1080–1090 °C during welding. Total heat input was reduced, and the cooling rate was increased with the enhancement of tool traverse speed. Under normal cooling, the weld nugget consisted of ferrite + pearlite microstructure. When the cooling rate was enhanced, the WN microstructure became martensitic. The change from

Acknowledgement

The financial support provided by INDO_US Science & Technology Forum, N. Delhi, India, for one of the authors (M. Ghosh) under INDO-US Research Fellowship’ 2009 to carry out the investigation at Missouri University of Science & Technology is gratefully acknowledged.

References (27)

  • R. Ueji et al.

    Mater. Sci. Eng. A

    (2006)
  • Y. Hovanski et al.

    Scripta Mater.

    (2007)
  • R. Nandan et al.

    Acta Metall.

    (2007)
  • S.J. Barnes et al.

    Mater. Sci. Eng. A

    (2008)
  • J.R. Patel et al.

    Acta Metall.

    (1953)
  • V.H. Lopez-Cortez et al.

    Welding J.

    (2008)
  • K. Yamazaki et al.

    Welding Int.

    (2000)
  • S. Ferrasse et al.

    Welding World

    (1998)
  • J.E. Gould et al.

    Welding J.

    (2006)
  • W. Peterson
  • R. Ohashi et al.

    Metall. Mater. Trans.

    (2009)
  • H.K.D.H. Bhadeshia et al.

    Sci. Technol. Welding Joining

    (2009)
  • Z. Feng et al.

    SAE Int.

    (2005)
  • Cited by (75)

    • Evolution mechanisms of microstructure and mechanical properties in a friction stir welded ultrahigh-strength quenching and partitioning steel

      2022, Journal of Materials Science and Technology
      Citation Excerpt :

      Fig. 3 displayed distinct hardening behaviors of the SZs and the FG-HAZs which possessed higher hardness than the PM region after the FSW process. Ghosh et al. [39] proposed that the SZ microstructure depended on the peak temperature and cooling rate during welding. In present study, the peak temperature (Fig. 8(c, d)) and cooling rate (Table 3) in the SZs indicated that complete austenitizing and subsequent fully martensitic transformation occurred, causing a microstructural change from multiphase structures of the PM (Fig. 4) to single-phase MF of the SZs.

    View all citing articles on Scopus
    View full text