Effect of Residual Stress on the Mechanical Properties of FSW Joints with SUS 409 L

-is study was performed to investigate both the residual stress distribution and the effect of the residual stress formed at the welding region on the mechanical properties of the friction stir welded joints with 409L stainless steel sheets. Residual stress measurement with hole-drilling method; mechanical property evaluation including tensile test, Charpy impact test, and fatigue test; and microstructure observation were conducted. It has got no residual stresses to speak of at the center region of the stir zone because the stored stresses are released in the process of the dynamic recrystallization, while a small quantity of compressive residual stresses is formed at the surface region of the stir zone because of strong compression reaction by the tool shoulder. A considerable amount of compressive residual stresses is formed at the thermomechanical affected zone because of the synergy between the thermal expansion due to the heat conduction from the stir zone and mechanical compression by the tool. -e formation of residual stresses shows a similar tendency between the advancing side and the retreating side. Both the mitigation of residual stress in the stir zone and the formation of compressive residual stress in the thermomechanical affected zone contribute to the improvement of the mechanical properties of the friction stir welded joints.


Introduction
When the materials are welded by conventional fusion welding methods such as tungsten inert gas (TIG) welding, metal inert gas (MIG) welding, and laser welding, several kinds of defect such as solidification cracks, elemental segregation, and brittle phases are generally formed due to melting and solidification reactions in the welds during the welding process.
ese defects significantly degrade the mechanical properties of the welds [1][2][3][4][5][6][7][8][9].Residual stress formed at the welding region is one of the most important parameters determining the mechanical properties of the welds [10,11].Anastassiou et al. reported that residual stresses in the welding region decrease the fatigue and fracture strength of the welds manufactured by the spot welding process [12].ey found that minimum welding residual stresses were achieved by selecting appropriate welding parameters.Nevertheless, conventional fusion welding processes have been generally documented to result in high residual stress and hence poor mechanical properties of the joints.e initial development of friction stir welding (FSW) was focused on the welding of aluminum alloys [13,14].e solid-state process of FSW leads to a number of benefits in terms of the mechanical and metallurgical properties of the joints.With these benefits, FSW has found applications in various industries for joining a variety of materials [15,16].In these days, this technology has only recently been adapted for joining stainless steels.Although some studies [17][18][19] investigated residual stress in the aluminum alloys welded by FSW, none have focused on the residual stress in stainless steels or on the relation between residual stresses and mechanical properties in stainless steels.In the present study, friction stir welding was performed to investigate both the formation of the residual stresses at the welding region and its effect on the mechanical properties of the joints with SUS 409L sheets.e distribution of the residual stresses developed at various welding regions was quantitatively measured by a hole-drilling method.
e formation behavior of the residual stresses at each welding region was also discussed.Effects of residual stress on mechanical properties such as impact and fatigue properties were precisely investigated.

Experimental Procedures
e chemical composition of SUS 409L stainless steel sheets used in the present study is listed in Table 1.e welding specimens were prepared with 400 mm (length) × 150 mm (width) × 2 mm (thickness).FSW was conducted at a tool rotational speed of 800 RPM and traveling speed of 250 mm/min with a convex type PCBN tool having a probe diameter of 5.5 mm and length of 1.6 mm. e tilt angle was maintained at zero degrees.e rotating tool travelled along the butt line between the two welding specimens.Optical microscopy (OM, Olympus, GX51) and electron backscattered diffraction (EBSD) methods were selected to observe the microstructure of the welding region and base metal (BM).Specimens for OM observation were prepared by mechanical polishing using 0.5 μm silica suspension followed by chemical etching using nital solution (40 ml HNO 3 + 60 ml HCl).EBSD specimens were prepared by mechanical polishing followed by electrochemical polishing with a solution of 6% perchloric acid and 94% ethanol.ESBD data were obtained in a field-emission scanning electron microscope (FE-SEM, JEOL, JSM-700F, 200 kV) and analyzed using TSL OIM software to obtain microstructural information.Test specimens were taken from the joints according to the standard and schematically illustrated in Figure 1.Both the gage part of the tensile and fatigue test specimens and the notch of the impact specimens were placed in the center of the stir zone (SZ).Tensile tests were performed using a dynamic material test machine (Instron, 8801, 100 kN) with JIS 2201 subsize specimens (thickness: 2 mm; gage length: 30 mm) at a crosshead speed of 1 mm/min.Impact tests were performed with KS B 0809 specimens (thickness: 2 mm) using a Charpy impact tester (Zwick/Roell, RKP-450) with a hammer degree of 150 °.
Fatigue tests were conducted using a resonance fatigue test machine with ASTM E466 specimens (thickness: 2 mm; gage length: 24 mm).e maximum stress levels used were selected as a function of the yield stress for the FSW joint and the base metal.Values of 40∼80 percent of the yield stress were selected.A stress ratio of R � 0.1 was applied.Fatigue life was defined as the number of cycles to failure.e number of cycles considered as a limit for infinite life was 10 7 cycles.A hole-drilling method was selected for measurement of the residual stresses in the welding region.is method measures residual stress using gages, which respond to the change of electrical resistance.
e value of electrical resistance changes when the residual stresses formed at the welding region are released by drilling a hole.In this study, type B gages were applied according to the ASTM E837 standard, and the holes with 2 mm in diameter were drilled from surface to depth direction.e residual stresses were measured at BM, SZ, TMAZ-AS, and TMAZ-RS per 0.1 mm drilling.

Results
3.1.Microstructure.Figure 2 shows macroscopic and microscopic cross-sectional images of the FSW joints.No macro-and micro-welding defects were found from the optical microscopic observation.e welding region of the FSW joints consisted of stir zone (SZ) and thermomechanical affected zone (TMAZ).SZ is a dynamically recrystallized region characterized by much finer grains than those of BM.Located between SZ and BM, TMAZ, unlike the SZ, retained a deformed and/or elongated structure along the SZ, as is demonstrated by the EBSD analysis that is presented in Discussion.As TIG welding proceeds with melting and solidification of joint materials, a cast structure formed in FZ [6].In contrast, only a recrystallized and deformed structure formed without a cast structure in the FSW joints because of the solid-state welding characteristic of FSW.ese characteristic microstructures formed in the welded joints were considered to be closely related to residual stress evolution and their mechanical properties.e arrows in Figure 3 indicate the measurement positions of residual stress.e residual stresses were measured through the thickness direction from the joints surface up to 1 mm depth so that the measurement positions contained SZ and TMAZ in the joints.

Residual Stress Distribution.
Residual stress measurement positions are schematically illustrated in Figure 3. Residual stress were measured at the BM, SZ, advancing side of TMAZ (TMAZ-AS), and retreating side of TMAZ (TMAZ-RS) as a function of depth from the surface of the joints.As indicated by the insert figure, TMAZ-AS and TMAZ-RS passed through both SZ and TMAZ. Figure 4 shows the residual stress distribution of the base metal 2 Advances in Materials Science and Engineering contiguous to the welding region.Regardless of depth from the surface, residual stress of transverse direction (TD) and welding direction (WD) was nearly not found.It was considered that residual stresses involved from rolling process were removed because the base metal sheets were fully annealed at the step of heat treatment.Figure 5 shows the residual stress distribution at the stir zone of the FSW joint.Almost all areas had low tensile residual stresses.However, some degree of compressive residual stresses was measured at the surface area.It was thought to be that the convex-type tool made an effect on the formation of the compressive residual stresses at the surface area in the stir zone.e residual stresses in both TD and WD presented the same component, and the values of them were generally similar.Figures 6 and 7 show the residual stress distribution at the TMAZ-AS and TMAZ-RS, respectively.e quite distinctive results to that in the stir zone were obtained in these regions.e region corresponding to TMAZ, that is, at depths ranging from 0.7 mm to 0.9 mm, expressed compressive residual stress that exceeded 200 MPa.In TMAZ-AS, the compressive residual stress in the WD was slightly larger than that in the TD.
is trend was the same in TMAZ-RS.In addition, the compressive residual stresses measured in the TMAZ-RS region were also larger than that in the TMAZ-AS region.It was considered that these results were related to imposed force, and material flow phenomena in the welding region include SZ and TMAZ during friction stir welding.ese phenomena could be explained by the previous study.e grain of the TMAZ-AS tends to exhibit more material flow characteristics than that of TMAZ-RS due to the TMAZ-RS's interaction and collision of the flow metals of TMAZ-AS and TMAZ-RS rotating in different directions.With more drastic mechanical force and higher temperature, the metal on the TMAZ-AS could flow more drastically, and grains on the AS are easily elongated and show more flowing characteristics than the TMAZ-RS [20].In summary, it was confirmed that low level of tensile residual stress and considerable amount of compressive residual stress are formed at the center of the stir zone and at the edge of the stir zone, respectively.It is quite different from the results of the TIG joint, which generally forms tensile residual stresses in the fusion zone [6].It was suggested that the difference in the formation of residual stress in the welding region may have an effect on the mechanical properties of the joints.

Mechanical Properties.
Figure 8 shows stress-strain curves obtained from the tensile tests for the FSW joint and the base metal.e yield and ultimate tensile strength of them were similar, although elongation of the FSW joint was slightly lower than that of the base metal.e fracture of the FSW joint occurred in the base metal, not in the welding region.As a result, it was revealed that a hundred percent of Advances in Materials Science and Engineering joint efficiency could be obtained under the optimized process conditions, in case of SUS 409L stainless steel sheets.e Charpy impact test results are shown in Figure 9. e impact properties were evaluated by measuring the crack initiation energy (E i ), the crack propagation energy (E p ), and the total energy (E t ) absorbed up to the fracture.E i of the two specimens was almost the same.However, E p of the FSW joint was higher than that of the base metal.is result is uncommon in welding fields.Generally, the impact properties of the welding joints manufactured by the conventional welding method such as arc and laser welding processes are inferior to the base metal.For this difference, the authors thought that the formation of ultrafine grains in  Advances in Materials Science and Engineering the stir zone, as shown in Figure 2, made an effect on the increase of the crack propagation energy.ese results said that the impact properties superior to the base metal could be obtained by the optimized FSW process condition.e fatigue properties of the FSW joints and the base metal are shown by S-N (stress-number) curves in Figure 10.
e S-N curve of the FSW joint was identical to that of the base metal.For both the FSW joint and the base metal, it was found that the fatigue life is considered infinite when the maximum stress was loaded at the range of 40∼80 percent of the yield strength.
e fatigue limit of the FSW joints manufactured by the present condition was about 150 MPa.
ese results indicate that good fatigue properties equivalent to that of the base metal could be obtained with optimized FSW process conditions in SUS 409L stainless steel sheets.Both the evolution of the characteristic microstructure and residual stress and their possible effects on the mechanical properties of the FSW joints were discussed in the following section.

Residual Stresses Evolution in the FSW Joints.
Residual stresses are stresses that exist in materials although external forces are not applied.To be specific, residual stresses are stresses that exist in structures remaining in the equilibrium state even after the original cause of the stresses has been removed.Residual stresses can occur through a variety of mechanisms including plastic deformations, temperature gradients during thermal cycle, or structural changes with phase transformation.
e heat from welding may cause localized expansion, which is taken up during welding by either the molten metal or the placement of parts being welded.When the finished weld cools, fusion zone cools and contracts more than others.As a result, residual stresses are left in the joints manufactured by conventional welding methods which include fusion and coagulation.However, residual stresses are not formed if the sheets are heated with a uniform temperature distribution.In other words, residual stresses generated by welding are formed due to the nonuniform temperature distribution around the welding region.
erefore, the distribution of residual stresses is decided according to the temperature gradient in each direction.Additionally, the sum of tensile residual stress formed by solidification contraction and compressive residual stress generated by liquidation expansion always comes to zero because the structures with residual stress remain in the equilibrium state.Friction stir welding is one of the typical solid-state welding processes which does not involve melting.erefore, the difference in the distribution of residual stresses could be occurred compare to the conventional welding processes.Based on the result of Figures 4-7, the formation behavior of the residual stress at the welding region during friction stir welding is shown in Figure 11.First, this study is going to consider formation of residual stress at stir zone.σ x which is residual stress of welding direction(length direction, x-direction) is formed as a result of inhomogeneous temperature distribution in width direction(y-direction).In the case of front A-A cross section (Figure 11(b)) close to the tool as a welding heat source, while the temperature around the weld line slightly rises, the temperature of section that remains away toward width direction does not rise.erefore, around the weld line, the temperature tends to expand in the longitudinal direction, and in the section far from the weld line, the temperature remains intact.As a result, the forces in the x-direction are balanced as compression pressure near the weld line and tensile stress away from the weld line.In the case of B-B cross section that is the stir zone, compressive stresses created by initial heating get close to zero because the yield stress and the coefficient of elasticity become very small as the temperature in the stir zone reaches 70% degree of the melting temperature of the base metal.High temperature is enough to recrystallize strain-stored grains in the stir zone.Figure 12 shows the observation result of the microstructure at the time.As shown in Figures 12(a) and 12(b), while SZ is formed with equiaxed fine grains that is considered to be built by dynamic recrystallization during friction stir welding, TMAZ shows shear-deformed grain structure formed with mechanical metal flow.Stress built in the beginning of friction stir process can be assumed to be almost removed during the process of dynamic recrystallization that Advances in Materials Science and Engineering is occurred as a result of frictional heat and plastic flow.
Compressive stress remains at TMAZ, which is the boundary of SZ and the base metal, due to thermal gradient and there will be tensile stress away from the weld line to reach equilibrium of force against compressive stress remaining near the boundary section.After passing the cross section B-B, heating process ends and cooling process begins around the weld line.When cooling process begins, minute tensile stress is considered to be formed at C-C cross section with the weld line as the center because contraction occurs elastically around the weld line.ese results are in contradiction with the result that the tensile residual stress is considerably applied to the fusion zone in the conventional welding method.Stress is formed to satisfy the equilibrium of force in the region away from the weld line.e formation of residual stress at TMAZ which is the boundary of SZ and BM is similar to that of A-A section but the result shows that compressive stress is more developed.
is is due to the synergistic effect of both thermal expansion by conductive heat from SZ and considerable compressive stress formed as a result of mechanical compression by tool's load.In friction stir welding section, unlike weld zone by conventional fusion welding, considerable compressive residual stress is built at TMAZ, and some of compressive residual stress and fine tension residual stress are formed in the surface area and the center area of SZ, respectively.

Relationship between Residual Stress and Mechanical
Properties in the FSW Joints.In the field of fusion welding like arc, resistance, or laser, normally residual stress is formed about similar magnitude of yield stress of the base As a result, it reported that while tensile residual stress close to 200 Mpa is formed in the fusion zone, there is a formation corresponding compressive residual stress in the heat-affected zone.Generally, even if the welding process is optimized and there are no defects in the fusion zone, tensional residual stresses in the fusion zone significantly reduce the mechanical properties.On the contrary, as described in Figures 8-10, tensile strength and fatigue property of friction stir welded joints exceed the level of the base metal, and it is identified that fatigue characteristic is equal to the base metal.ese results contradict with that of the welds manufactured by the traditional fusion welding process mentioned above.It is judged to be resulted from the difference in component of residual stress made from welding region by friction stir welding.In other words, the formation of compressive residual stress near the surface of stir zone, insignificant level of tension residual stress in stir zone, and significant level of compressive residual stress in thermomechanical affected zone contribute to maintaining the level of mechanical feature to an equal degree with the base metal.In case of welds made by traditional way of fusion welding, in which tensional residual stress built in fusion zone is close level of yield stress of its material, there is high possibility of plastic deformation due to cyclic compressive and tensile load or additional static tensile load.However, in case of friction stir welded joints where compressive residual stress as primary residual stress is formed in SZ and TMAZ, plastic deformation and rupture are being delayed when extra loads are applied.It is considered that the mechanical properties of FSW joints are improved due to both the formation of compressive residual stress in TMAZ and the ease of residual stress by dynamic recrystallization in SZ.

Conclusion
is study was performed to investigate both the residual stress distribution and the effect of the residual stress formed at the welding region on the mechanical properties of the friction stir welded joints with 409L stainless steel sheets.e conclusions obtained through the residual stress measurement with hole-drilling method; mechanical property evaluation including tensile test, Charpy impact test, and fatigue test; and microstructure observation are as follows: (1) It has got no residual stresses to speak of at the center region of the stir zone because the stored stresses are released in the process of the dynamic recrystallization, while a small quantity of compressive residual stresses is formed at the surface region of the stir zone because of strong compression reaction by the tool shoulder.(2) A considerable amount of compressive residual stresses is formed at the thermomechanical affected zone because of the synergy between the thermal expansion due to the heat conduction from the stir zone and the mechanical compression by the tool.e formation of residual stresses shows a similar tendency between the advancing side and the retreating side.
(3) Both the mitigation of residual stress in the stir zone and the formation of compressive residual stress in the thermomechanical affected zone contribute to the improvement of the mechanical properties of the friction stir welded joints.

Figure 2 :
Figure 2: Cross-sectional optical micrographs of the joints showing (a) the overall weld region at low magnification and measurement positions of residual stress (marked by white boxes), and high-magnification images showing (b) SZ, (c) boundary region between SZ and BM, and (d) BM.

Figure 9 :F igure 10 :
Figure 9: Force-time graphs and absorbed impact energy of the BM and FSW joints obtained by impact tests.

Figure
Figure Temperature distribution and residual stress formation behavior during friction stir welding.(a) A-A: just before welding, (b) B-B: during welding, and (c) C-C: after welding.

Table 1 :
Chemical composition of SUS 409L stainless steel sheets used in this study (mass %).