Scripta Materialia 57 (2007) 873–876 www.elsevier.com/locate/scriptamat

Friction stir spot welding of hot-stamped boron steel Y. Hovanski,a,* M.L. Santellab and G.J. Granta a

Pacific Northwest National Laboratory, Energy Materials and Manufacturing, MSIN: K2-03, 902 Battelle Blvd., P.O. Box 999, Richland, WA 99352, USA b Oak Ridge National Laboratory, Oak Ridge, TN, USA Received 11 June 2007; revised 19 June 2007; accepted 20 June 2007 Available online 6 August 2007

Hot-stamped boron steel was successfully joined via friction stir spot welding using polycrystalline cubic boron nitride tooling. The resulting microstructure, microhardness and mechanical properties are reported, including a brief look into failure mechanisms. Relationships between the unique mechanical mixing, phase transformations and failure initiation sites associated with joining martensitic steels are characterized.  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: FSW; Spot welding; Boron steel

Friction stir spot welding (FSSW), a relatively new solid-state joining process receiving substantial scrutiny from the automotive industry, has proven to be a costeffective and productive means for joining lightweight structural alloys such as aluminum. Similar in concept and appearance to its predecessor, resistance spot welding (RSW), FSSW avoids the severe heating and cooling cycles induced during the resistance method. Furthermore, additional advantages and successes of the solidstate approach have made FSSW the preferred method for spot joining aluminum alloys [1,2]. Recent enhancements in US automotive rollover standards require vehicle designers to meet more stringent safety requirements. While these standards could be met by simply increasing the amount of material used to construct the passenger compartment, such action would lead to substantial weight gain and decreased fuel efficiency. Achieving strict safety requirements without adding weight necessitates increased utilization of materials with high-strength-to-weight ratios in structural component design and fabrication. Several obvious material choices emerge including the use of advanced high-strength steels (AHSS) with tensile strengths greater than 1000 MPa. Such escalation in the use of AHSS only emphasizes the importance of providing viable joining techniques that both preserve and protect the unique microstructural balance required to maintain * Corresponding author. Tel.: +1 509 375 3940; fax: +1 509 375 4448; e-mail: [email protected]

their desirable mixture of mechanical properties. While many joining methods are available for high-strength, low-alloy steels, AHSS with higher carbon and alloy percentages have proven to be more difficult [3]. Electric resistance spot welding, typically employed by the automotive industry when joining structural steel components, has yet to prove a completely effective joining method for AHSS due to microstructural embrittlement created by extreme post-weld thermal gradients [4,5]. However, with recent successes in FSSW of several AHSS, including DP600, DP800 and M-190 [6–8], an effort was commenced to evaluate this joining process for more problematic AHSS. Martensitic hot-stamped boron steel (HSBS) was selected with a nominal tensile strength of more than 1500 MPa and elongations beyond 9%. The analyzed chemical composition of this alloy, supplied by SSAB HardTech, was 0.20 C–1.26Mn–0.27Si–0.057Al–0.22Cr –0.04Ni–0.034Ti–0.019V–0.002B–0.007N (wt.%). This steel was finished to 1.4 mm thick sheets that were heat treated to produce the martensitic microstructure with hardness of 46 Rockwell C, nominal yield strength of 1150 MPa, and tensile strength of 1550 MPa. Welding trials were performed on a friction stir welding research and development system (MTS ISTIR) designed to maintain spindle runout of less than 0.01 mm in accordance with recommendations from toolmakers developed while friction stir welding steels [9]. Lap welds were formed between two 1.4 mm thick strips each with dimensions of approximately 100 mm in length and 31 mm in width. The strips were fixtured in a lap

1359-6462/$ - see front matter  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.06.060

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Plunging

Stirring

Drawing out

Figure 1. Visual schematic of the three-step friction stir spot welding process.

configuration with an overlap of 35 mm. Welds were made at the center of the overlapping section following a three-step approach outlined in Figure 1. The FSSW process, similar to the plunge phase of linear friction stir welding, provides frictional heating between the interface of the tool pin and the top sheet that effectively increases the hot workability of the HSBS specimens. With sufficient reduction in the flow stress at temperature, a combination of forging and stirring joins the two sheets without melting the base materials. Heat generation during the plunge phase softens material immediately adjacent to the tool pin and shoulder, thus providing a working volume of material that is mixed both axially about the tool and vertically through the thickness of the sheet interface. The selected tool material was polycrystalline cubic boron nitride (PCBN) and was fabricated into a 20 tapered three-flat conventional tool pin with a pin length of 2.3 mm. The tool shoulder was a 10.2 mm diameter concave design. This tool material was selected based on its well-documented success in friction stir welding various other high-strength materials [10], and with the intent of avoiding a tool material development study. Tools used in this study performed hundreds of welds in several AHSS without significant wear, yet clearly they must survive many more in order to be compatible with a mass production, automotive environment. All welds were produced with a two-stage plunge process, initiated with a rapid plunge and followed by a slow plunge that acted as a dwell period. Total weld cycle time (initial touch down to the moment of tool retraction) ranged from 1.9 to 10.5 s. Variation in cycle time resulted from changing plunge velocity and dwell time. Initial plunge rates from 0.4 to 3 mm s 1 were explored in conjunction with dwell plunge rates of 0.2 and 0.07 mm s 1. The influence of rotational velocity was also investigated; speeds of 800, 1200, 1600 and

2000 rpm were examined in combination with numerous plunge schemes. Each condition yielded a fully consolidated joint with measurable differences in the final cross-sectional area of the bonded region. Individual welds were evaluated via optical microscopy and microhardness on transverse cross-sections, allowing metallurgical characterization of the effects on welds made with varied process parameters. A representative transverse cross-section and associated micro-indentation map of a FSSW in HSBS are presented in Figure 2, showing a weld made at 800 rpm with a 2.7 mm plunge depth. Distinct microstructural regions are shown in Figure 2A, including both a fully transformed weld nugget and heat-affected zone (HAZ). Mixing around the tool as well as through the thickness of the stack is demonstrated by axial symmetry and the clear upturned edge of the lower sheet. The microhardness data presented in Figure 2B exhibit a uniform hardness distribution within the weld nugget, with the exception of a narrow horizontal band at the nugget mid-plane. The hardness data also reveals a softened region in the HAZ adjacent to the nugget that grades outward to base metal properties 4–7 mm outside the weld nugget. Further investigation into the specific microstructures developed during the FSSW process and the post-weld transformation was carried out via optical and scanning electron microscopy. Figure 3 presents optical micrographs of several regions within the weld and surrounding HAZ. Figure 3A maps out locations at a lower magnification, and Figures 3B and C presents more detailed views of the representative microstructures in the nugget and HAZ. The majority of the transformed nugget retained characteristics comparable to the original martensitic base metal microstructure with the exception of a thin region of ferrite originating from the interface of the two sheet stack to a location near the pinhole at the center of the nugget. This distinctive band of ferrite remains along both sheet surfaces as well as throughout the weld nugget. The property gradient within the HAZ shows increasing hardness radially outward from the nugget, which largely corresponds to the tempered condition of the martensite shown in Figure 3B. The effective property depression is greatest immediately outside the weld nugget in the surrounding region of ferrite, although clearly such structural characteristics also penetrate the weld nugget as shown via microhardness in Figure 2B. The ferritic microstructure inherent in the narrow, softer region of the weld nugget is shown

Figure 2. (A) Transverse photo micrograph of a friction stir spot weld in HSBS; the weld was produced at 800 rpm with an initial plunge depth of 2.7 mm in 3.9 s. (B) A microhardness map of the same friction stir spot weld.

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Figure 3. Transverse photo micrographs of welded cross-sections in HSBS. (A) As-welded condition at 20· magnification. (B) Prominent tempered martensite characteristic of the HAZ shown at 1000· magnification. (C) A narrow region of ferrite within the mostly martensitic nugget shown at 1000· magnification.

Figure 4. Transverse photo micrographs of friction stir spot welds in HSBS. (A) Crack propagation along the narrow band of ferrite in the weld nugget of a specimen pulled to partial failure, shown at 5· magnification. (B) Ferritic band in the weld nugget of a friction stir spot weld in HSBS shown at 200· magnification.

in Figures 3C and 4B, and is similar to the microstructure developed at the interface of the nugget and HAZ shown by the darker region at this interface in Figures 2A and 3A. Every weld condition was tested in unguided lap shear showing only small deviations in strength, joined area and associated energy for samples with like parameters. Variation in weld parameters led to a much greater variance in joint properties with lap shear tensile strengths ranging from 6 to 12 MPa associated with changes in rotational velocity, plunge rate and dwell time. Plunge depth was an important variable affecting bonded area and strength; however, other variables also proved crucial in developing higher strengths. Deviations in the initial plunge rates showed only a modest affect on the overall weld size or lap shear strength, but alterations in the dwell times or rotational velocity created vastly different reactions in all categorized responses. The importance of the dwell time was emphasized under every condition, with an increased dwell yielding between a 40% and 90% direct increase in lap shear strength for all plunge rates reported. Rotational velocity also clearly affected weld strengths, but the response differed for different tools and plunging conditions. To better understand the failure mechanisms, several weld samples were pulled to partial failure by stopping standard lap shear tensile tests at a set load drop and prior to ultimate failure of the sample. A transverse cross-section of a partially failed weld specimen is presented in Figure 4A showing crack propagation from the sheet interface along the thin ferritic region within the weld nugget. Examination of partially failed HSBS FSSW specimens show consistent failure along this

softened zigzag region of ferrite within the weld nugget. Crack propagation throughout the nugget tracing this narrow region and ultimately traversing the nugget follows the path of depressed strength values associated with this ferritic region of the nugget. Failure ultimately takes places at a direction perpendicular to the inner wall of the pinhole between the propagating crack and the free surface. The origin of this softened ferritic region is not clear. The ferrite could have been stabilized either from dissolution of Al–Si surface oxides or from a slight decarburization of sheet surfaces during processing and heat treating. Another possibility exists relating surface kinetics and reactivity of the boron steels with potential coatings or die lubricants used during the hot stamping process. Regardless of the origin of the ferrite zigzag band, clearly a more homogeneous weld nugget would dramatically change the current failure path, possibly leading to an increase in overall bond strengths of friction stir spot welds in HSBS. In conclusion, FSSW was successfully applied to hotstamped boron steels. Commercially available tool materials were effectively demonstrated to withstand the thermal and mechanical conditions inherent to FSSW of HSBS steels at the laboratory scale. PCBN tools were used for hundreds of welds in several AHSS without showing visual signs of deleterious wear. While the current approach was an initial study showing the applicability of FSSW to AHSS, further development of tool design and process parameters needs to be done before a more critical examination of mechanical properties is warranted. Additionally, an evident need to eliminate the narrow band of ferrite within the nugget must be addressed in future work.

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This work was funded by the US Department of Energy Office of FreedomCAR and Vehicle Technologies as part of the Automotive Lightweight Materials Program under Dr. Joe Carpenter. [1] R. Sakano, K. Murakami, K. Yamashita, T. Hyde, M. Fujimoto, M. Inuzaka, Y. Nagao, H. Kashiki, 3rd Int. FSW Symp. (2001). [2] R. Hancock, Weld. J. 83 (2) (2004) 40. [3] J. Gould, W. Peterson, Fabricator 35 (8) (2005) 34–35. [4] S. Ferrasse, P. Verrier, F. Meesemaecker, Weld. World 41 (3) (1998) 177–195.

[5] K. Yamazaki, K. Sato, Y. Tokunaga, Weld. Int. 14 (7) (2000) 533–541. [6] W.J. Kyffin, P.L. Threadgill, H. Lalvani, B.P. Wynne, 6th Int. FSW Symp. (2006). [7] Z. Feng, M.L. Santella, S.A. David, R.J. Steel, S.M. Packer, T. Pan, M. Kuo, R.S. Bhatnagar, SAE 2005 Trans. J. Mat. Manuf. (2005) 592–598. [8] M.P. Miles, J. Sederstron, K. Kohkonen, R. Steel, S. Packer, T. Pan, W.J. Schwartz, C. Jiang, MS&T 2 (2006). [9] S. Packer, T. Nelson, C. Sorensen, R. Steel, M. Matsunaga, 4th Int. FSW Symp. (2003). [10] S. Packer, T. Nelson, R. Steel, M. Mahoney, ISOPE (2005) 1–4.