Interface behavior and mechanical properties of 316L stainless steel ...

Int J Adv Manuf Technol (2015) 81:577–583 DOI 10.1007/s00170-015-7237-5

ORIGINAL ARTICLE

Interface behavior and mechanical properties of 316L stainless steel filling friction stir welded joints L. Zhou 1,2 & W. L. Zhou 1 & Y. X. Huang 2 & J. C. Feng 1,2

Received: 10 December 2014 / Accepted: 26 April 2015 / Published online: 10 May 2015 # Springer-Verlag London 2015

Abstract In the present work, the feasibility to repair keyhole left at 316L stainless steel friction stir welding/friction stir processing (FSW/FSP) seam by filling friction stir welding (FFSW) using consumable tools was investigated. Interface behavior and mechanical properties of 316L stainless steel FFSW joints were investigated. The results showed that significant microstructural refinement occurred around the interface of refilled keyhole due to extreme levels of plastic deformation and thermal exposure. No σ phase but few Cr carbides were formed at the refilled joint interface, which would not result in obvious corrosion resistance degradation of 316L stainless steel. Void defects formed at the bottom of the refilled keyhole and the FFSW joint fractured at the interface during the tensile test. Keywords Filling friction stir welding . Austenitic stainless steel . Keyhole repairing . Interface . Mechanical properties

1 Introduction Austenitic stainless steel, AISI type 316 and its modified grades such as 316L, has been widely used in industrial structures for its excellent high-temperature tensile and creep fatigue strengths in combination with good fracture toughness * L. Zhou [email protected] 1

Shandong Provincial Key Laboratory of Special Welding Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China

2

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China

and fabricability [1]. However, the incidence of stress corrosion cracking (SCC) increases due to the continuous service of austenitic stainless steel [2]. The repair of SCC at the external surface of austenitic stainless steel is urgent for extending the service life for the continued safety of industrial structures. Friction stir processing (FSP) is an emerging engineering technology developed based on friction stir welding (FSW) [3]. A rotating tool, consisting of a shoulder and a pin, is plunged into a workpiece and then travels in an expected direction. The material in FSP zone undergoes extreme levels of plastic deformation and thermal exposure, which refines the microstructure and improves homogeneity of the processed zone and, thus, increases its strength, wear property, corrosion resistance, and so on. The technology of FSP also could repair internal or surface defects formed in processes such as casting and welding [4, 5]. In a previous related study, SCC on 316L stainless steel was repaired by FSP using polycrystalline cubic boron nitride (PCBN) tools [6, 7]. However, keyhole remains at the end of the seam with the extraction of the nonconsumable pin. To solve the problems caused by the remaining keyhole, several methods and apparatuses have been developed. The named auto-adjusting or retractable or double-acting FSW equipment is complex and expensive. As for high melting point materials, it is almost impossible due to the limitations of pin tool design [8–10]. Conventional refilling methods by fusion welding technology result in a serious decrease of joint performance. Friction taper plug welding (FTPW) or friction hydro pillar processing (FHPP) is a solid-state joining process developed by the welding institute (TWI) during the 1990s [11, 12], which involves drilling a tapered through hole or blind hole with very thin end wall into a plate. Subsequently, a tapered plug with a similar included angle is friction welded to the matching surface of the hole within a few seconds, by forcing the rotating plug against the drilled hole to fix the

578 Table 1

Int J Adv Manuf Technol (2015) 81:577–583 Chemical compositions and mechanical properties of 316L stainless steel plates and filling tools Chemical compositions, mass—%

Plate Filling tool

Mechanical properties

C

Si

Mn

P

S

Ni

Cr

Mo

Fe

Strength, MPa

Elongation, %

0.019 0.013

0.66 0.33

1.19 1.49

0.033 0.035

0.001 0.018

12.11 12.13

17.41 17.04

2.05 2.09

Bal. Bal.

548 534

59 62

defects in aerospace aluminum and offshore steel structures [13, 14]. However, the specific through hole or blind hole with very thin end wall for FTPW/FHPP is not desirable in some structures due to the limitations of the working environment. Recently, a new technique called filling friction stir welding (FFSW) has been proposed by Huang et al., where a semi-consumable tool consisting of a non-consumable shoulder and consumable joining bit is used to repair the keyholes in aluminum alloy FSW [15, 16]. However, very little attention has been given to keyhole repair in austenitic stainless steel despite their potential industrial importance [17, 18]. In this study, the feasibility to repair remaining keyhole on the friction stir processed (FSPed) 316L stainless steel plates by FFSW using consumable tool was investigated. Behaviors of the bonding interface between the filling tool and keyhole and mechanical properties of FFSW joints were characterized.

2 Experiments The dimension of as-received 316L stainless steel plates was 200×150×10 mm3, and the filling tools were made from 316L stainless steel bar. The chemical compositions and mechanical properties of the as-received plates and filling tools are shown in Table 1. The FSP was performed using PCBN tool based on previous reports [6, 7]. The remaining keyhole was prepared by machining according to PCBN tool for FSP, as illustrated in Fig. 1a. Two types of filling tools were designed based on the remaining keyhole after FSP in previous research [18], as shown in Fig. 1b and c, respectively. The process conditions for FFSW were determined according to preparatory experiments, in which the tool design and welding parameters: the tool rotation speed (ω), applied force (F), and holding time (T), were explored. Tool geometries and features for FSP and FFSW are listed in Table 2. Hereinafter, the filling tools are denoted as 63.6L5D24 and 40L8D24 for the pin cone angles of 63.6 and 40°, respectively. The FSP and FFSW process were performed on a load-controlled FSW system with pure argon shielding employed around the welding zone to avoid surface oxidation. In general, large applied force and tool rotation speed could result in higher heat input to fully plasticize the material and defect-free refilled joints could be achieved. The applied force and tool rotation speed in the present study were chosen as the maximum allowable value

for operation in actual situation, 30 kN and 1500 rpm (revolution per minute), respectively. In addition, the final forging stage in conventional FTPW/FHPP to supply a large force for repair quality also cannot be fulfilled in conventional FSW system for practical application. Varying holding time of 5 and 10 s together with tool design were investigated based on previous research [18]. Hereinafter, the set of parameters are denoted as 30 kN–1500 rpm–5 s and 30 kN–1500 rpm– 10 s for brevity’s sake, respectively. The refilled joints were examined by metallurgical inspections performed on the transverse cross-sections perpendicular to the original FSP direction and coincided with the central axis of keyhole. Microstructural evolution was examined by optical microscopy (OM; Keyence VHX–200/100F) and transmission electron microscopy (TEM; Hitachi H–9000). The transverse joint cross-sections were cut by electrical discharge machining and prepared by standard metallographic procedures. Samples were mounted in epoxy and ground with abrasive paper. Final polishing was conducted with 1 μm diamond paste abrasive. The polished joint cross-sections were electrolytically etched in a solution of 10 % oxalic acid + 90 % water with a power supply set to 15 V for 90 s, and then observed on the OM. TEM specimens were cut from the

Fig. 1 Schematic illustration for FFSW process: a illustration of remaining keyhole, b filling tool-1 for remaining keyhole, and c filling tool-2 for remaining keyhole

Int J Adv Manuf Technol (2015) 81:577–583 Table 2

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Tool geometries and features for FSP and FFSW

Tool code

Tool geometry

Shoulder diameter/mm

Shoulder convex angle/°

Pin diameter /mm

Pin length/mm

Pin cone angle/°

FSP

Threaded conical pin

22

10

10.8

5

30

FFSW–tool-1

Smooth conical pin

24

10

8.9

5

63.6

FFSW–tool-2

Smooth conical pin

24

10

9.2

8

40

typical locations of the joint interface using a focused ion beam instrument (SII SMI3050MS2) and were observed on the TEM at 300 kV. Transverse tensile test samples were cut perpendicularly from the top and bottom of the refilled joint obtained under the same process conditions for metallurgical inspections, as shown in Fig. 2. X-ray inspection was performed for the tensile test samples to check the bonding characteristics around the interface of the refilled joint. Tensile tests were carried out on an Instron–5500 mechanical tester at room temperature with a crosshead speed of 1 mm/min. The strain was measured using an extensometer based on a gauge length of 50 mm (Fig. 2). Furthermore, the facture surface of tensile test sample was examined by scanning electron microscope (SEM; Keyence VE–8800) after the tensile test.

3 Results and discussion Figure 3 is the macroscopic overviews of the refilled joint cross-sections on the original conical and modified spherical keyholes by FFSW under different tool designs and welding parameters. The typical refilled zone (RZ), base metal (BM), and the interface zone (IZ) in the refilled joints were observed, but no obvious heat affected zone (HAZ) was formed due to the relatively low heat input and short high-temperature dwell time. Arc-shape adiabatic shear layers were formed in the RZ as that in conventional FTPW/FHPP joints due to continuous

Fig. 2 Schematic illustration for tensile specimen of refilled joints

Fig. 3 Cross-section appearance of refilled keyhole by FFSW with a filling tool 63.6L5D24 under 30 kN–1500 rpm–5 s, b filling tool 63.6L5D24 under 30 kN–1500 rpm–10 s, c filling tool 40L8D24 under 30 kN–1500 rpm–5 s, and d filling tool 40L8D24 under 30 kN– 1500 rpm–10 s

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Int J Adv Manuf Technol (2015) 81:577–583

Fig. 4 OM microstructure of base metal in 316L stainless steel: a plates and b filling tools

material plasticization [11, 12]. The bonding condition at the IZ depends on the tool design and welding parameters. Some macro void defects were observed at the lower sidewall and bottom of the IZ in the refilled keyhole no matter the tool designs and welding parameters, which could be attribute to the discontinuous plastic flow of filling tool under such high reaction force and tool torque as that in previous study [18]. In

the current case, microstructural features in typical areas at the IZ of the refilled joints (positions A–D shown in Fig. 3a–d) were mainly investigated. Microstructure in the base metal of the as-received plate and filling tool was also observed for comparison. Microstructure of the base metal in the 316L stainless steel plates and filling tools are shown in Fig. 4, which consists of

Fig. 5 Microstructure of typical positions at interface in refilled keyhole shown in Fig. 3a and b by filling tools 63.6L5D24 under holding time of 5 and 10 s: a position A, b position B, c position C, and d position D

Fig. 6 Microstructure of typical positions at interface in refilled keyhole by filling tools 40L8D24 under holding time of 5 and 10 s: a position A, b position B, c position C, and d position D


coarsened grain structures in the range of 30–80 μm along the rolling/extrusion direction based on the linear intercept method [19]. The microstructural variations in typical positions of the FFSW joints using the filling tools 63.6L5D24 and 40L8D24 are demonstrated in Figs. 5 and 6, respectively. As for the refilled keyhole, the top part of the IZ was well bonded, as shown in Figs. 5a and 6a. However, void defects were presented at the lower sidewall and bottom of the IZ regardless of filling tool design and holding time, as indicated in Figs. 5b–d and 6b–d. Grains around the IZ were significantly refined in all the refilled keyholes due to severe plastic deformation-induced dynamic recrystallization (DRX) during the process. Furthermore, it is worth noting that besides the equiaxed DRXed grains, elongated deformed grains along the metal flow direction were also presented in all the FFSW joints due to the dynamic deformation process. It should be noted that black etch pits can be seen in the refilled keyholes, especially at the bottom of IZ where the most serious deformation exposed and grain refinement occurred, which could be explained by increased intergranular corrosion due to grain boundaries increase caused by grain refinement. Besides, the precipitation of Cr carbides at the IZ which result in Cr depletion and degradation of corrosion resistance of austenitic stainless steel also should be considered, as discussed later. As for austenitic stainless steel, σ phase, normally FeCr, precipitates during aging at temperatures between 773 and 1073 K and results in a deleterious effect on toughness, ductility, and corrosion resistance [20]. Generally, the decomposition of austenite to σ phase takes a long time due to the accompanying redistribution of alloying elements by substitutional diffusion in typical fusion welding process [21]. However, σ phase can be rapidly formed in the SZ by the transformation of austenite to δ–ferrite at high temperatures and the subsequent decomposition of the ferrite since FSW introduces high strain and it accompanies DRX [6, 7, 22, 23]. As for the refilled joints by the current FFSW process, microstructure in representative area (bottom part at the IZ) was further characterized by TEM, as shown in Fig. 7. The ferrite phase or sigma phase formation was not evident, but austenite phase remained around the IZ, and the ultra-fine DRXed grains in IZ were observed, as shown in Fig. 7a. In addition, few rod-like types of carbide with the size of several hundreds of nanometers along the grain boundaries were observed. The types of carbides in austenitic stainless steel depend on metallurgical composition and process history. Generally, cubic Cr23C6 with a=10.650 Å is the most common Cr carbide in austenitic stainless steel and is given priority for formation during processing, which was confirmed by selected electron diffraction pattern (SAED), as indicated by Fig. 7b. However, compared with that in the conventional FSW/FSP, the amount of Cr carbides in the IZ is very few and the corrosion resistance of 316L stainless steel decreases insignificantly, due to the relatively

581

Fig. 7 TEM images of representative area (bottom part at the interface) of refilled keyhole shown in Fig. 3: a DRXed austenite phase and b Cr23C6 phase precipitation

short high temperature dwell time during the current FFSW process which has been also verified by previous studies [17, 18].

Fig. 8 X-ray inspection for bonding characteristics at interface in refilled keyhole by filling tools 40L8D24 under holding time of 10 s: a top tensile test sample and b bottom tensile test sample

582

Fig. 9 Stress–strain curve and fracture location during tensile test of refilled joint by filling tool 40L8D24 under 30 kN–1500 rpm–10 s: a top tensile test sample and b bottom tensile test sample

Mechanical properties of the refilled joints by FFSW are closely related to the bonding characteristics at the interface and evaluated by tensile test. Since void defects existed at the lower sidewall and bottom of the IZ regardless of the filling tool design and process parameters, only the typical refilled joint with relatively good interface by filling tool 40L8D24 under 30 kN–1500 rpm–10 s was tested. X-ray inspection was performed on the tensile test samples to check bonding characteristics around the interface for the top and bottom tensile test samples for the refilled joint, as shown in Fig. 8. It was indicated that defects formed around the interface on both the top and bottom tensile test samples, as shown in Fig. 8a and b, respectively, which is consistent with metallurgical inspections performed on the transverse cross-sections of refilled joint.

Fig. 10 Fracture surface morphology of transverse tensile specimens for a upper part of refilled joint and b lower part of refilled joint


Figure 9 shows the tensile properties and fracture locations of the tensile test samples of the refilled joint by filling tool 40L8D24 under process parameters of 30 kN–1500 rpm–10 s. The stress–strain curve for the top tensile test sample was not smooth before fracture (Fig. 9a), and it was more obvious for the bottom tensile test sample (Fig. 9b), which could be explained by the existence of defects at the IZ of the bottom tensile test sample for refilled joint. Both the top and bottom tensile test samples for the refilled joint by filling tool 40L8D24 under process parameters of 30 kN– 1500 rpm–10 s fractured at the interface of refilled keyhole, as shown in Fig. 9. Generally, the fracture surface morphology of the top tensile sample presented plastic fracture characterized by dimples (Fig. 10a), while the fracture surface of the bottom tensile test sample fracture surface showed brittle rupture characteristics at the defective areas (Fig. 10b). The tensile strength is evaluated by nominal stress, and the elongation is determined by an extensometer based on the gauge length. Tensile test results showed that the relative tensile strength and elongation in the refilled joint significantly decreased compared with those of the as-received plate, especially for the bottom tensile test sample of the refilled joint, which could be attributed to the existence of defect at the IZ of refilled joint. As for the current FFSW process based on the basic principles of FSW, the advantages result from the fact that the process takes place in the solid phase below the melting point of the material to be joined, and thus, problems like porosity, grain boundary cracking, and through wall penetration associated with fusion welding repair technology can be eliminated, especially for the materials that are difficult to be joined by conventional fusion welding methods. Compared with previously proposed multi-step self-refilling friction stir welding (SRFSW) to refill the remaining keyhole [17], it can avoid the high-cost and multi-step process in the SRFSW using a series of PCBN tools. However, defects formed at the IZ in the refilled joint without reshaping the remaining keyhole though different filling tool designs and process parameters were used. Therefore, an optimal combination of remaining keyhole after FSP or reshaped keyhole, filling tool design, and


process parameters must be adopted to obtain defect-free refilled joints.

583 4. 5. 6.

4 Conclusions Keyhole in 316L stainless steel plates were repaired by filling friction stir welding technology using consumable tool. The interface behavior and mechanical properties of the refilled joint were investigated. The important findings are summarized as follows: (1) Void defects were formed at the lower sidewall and bottom of the refilled keyhole for all the tool design and process parameters. Microstructure of interface was significantly refined through severe plastic deformationinduced dynamic recrystallization. (2) No σ phase but few Cr carbides was developed at the interface of refilled keyhole due to the relatively short high-temperature retention time. The precipitation of few Cr carbides would not cause obvious Cr depletion and result in the degradation of the corrosion resistance of 316L stainless steel. (3) Tensile test results showed the tensile specimen fractured at the interface of the refilled joint, and the typical relative tensile strength and elongation are significantly lower than those of the as-received plate for defects formed at the refilled joint interface.

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Acknowledgments The work was supported by the State Key Lab of Advanced Welding and Joining, Harbin Institute of Technology (Grant No. AWJ-M13-11), the Natural Scientific Research Innovation in Harbin Institute of Technology (Grant No. HIT.NSRIF.2014131), the Indigenous Innovation and Achievement Transformation Program of Shandong Province (2014CGZH1003), the Production-study-research Cooperative Innovation Demonstration Project Foundation of Weihai City (2014CXY02), and the Science and Technology Development Program of Weihai City (2014DXGJ17).

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