https://orcid.org/0000-0002-5102-2459
Abstract
The behavior of fatigue crack growth(FCG) plays a vital role in the design and performance of modern structural materials like Al 6061-T6 alloy. Al 6061-T6 is an alloy of Al-Mg-Si which is artificially aged under T6 condition. In the present study, Al 6061-T6 plates of 6 mm thickness welded by a gas metal arc welding(GMAW) technique using an ER-5356 as filler wire. The microhardness profile was obtained through Vickers microhardness test. The pre-qualification tests such as X-ray radiography, tensile, and bending tests were carried out on base metal (BM), heat-affected zone (HAZ), and weld zone (WZ)along rolling direction to study the mechanical behavior. The fatigue crack growth rate (FCGR) tests on the three zones were carried out as per ASTM E-647 standards under stress ratios of 0.1, 0.3, and 0.5. The WZ possessed highest FCGR compared to BM and HAZ. The lowest fracture toughness (Kc) was found for WZ, which is due to fusion of filler wire with BM. The microstructural transformation in HAZ from very fine needle shape β'' precipitates to coarse bar shape β' precipitates produced by the thermal effect during the welding process resulting in moderate FCGR compared to WZ and BM. Mechanical strength and microstructure regeneration during the welding thermal cycle are the two important factors that affect the FCGR in all zones. Fractographic analysis was carried out using a scanning electron microscope (SEM) to study the crack initiation, final fracture, nature of the fracture, and causes of failure under various stress ratios.
REVIEWING EDITOR:
1. Introduction
Aluminum alloys are in direct competition with composite materials in the aviation manufacturing industry due to considerable improvements in mechanical strength, good weldability, and low production costs (Miller et al., Citation2000). The addition of Mg, Si, and Mn mainly strengthens the Al 6061 alloy. The T-6 thermal treatment includes homogenization and aging, which result in precipitation-based hardening (Zhang et al., Citation2020). Al 6061-T6 has excellent fracture toughness, high mechanical strength, and stress corrosion resistance, resulting in its use as an ideal material for application in the aviation and marine industries (Fan et al., Citation2021). Aluminum alloys are generally welded together by using the gas metal arc welding (GMAW) technique because of their quick welding, minimal deformation, and narrow heat-affected zone (Ambriz et al., Citation2010). The pressure vessels, aircraft structure, automotive body parts, and pipelines are welded by the GMAW technique. The mechanical properties and hardness value are enhanced by the post-weld heat treatment, which additionally reduces residual stress imposed by the precipitation of the β′ phase (Ahmad & Bakar, Citation2011). The BM, HAZ, and WZ were identified by hardness values. The temperature of materials in WZ is high enough to coarsen the grain which soften the WZ (Harara, Citation2015). The HAZ has two subzones in addition to the WZ. The first is located close to the WZ and is subject to solution temperature and the second is located close to BM and is subject to over aging temperature. The popular reaction occurs in the second subzone and it replaces the semi-cohesive needle and rod shape of the Mg and Si combination with the discontinuous Mg2Si (Almanar et al., Citation2012). This shows that in GMAW the grain growth and precipitation reaction in the HAZ has a significant impact on the joint’s microstructural characteristics (Almanar et al., Citation2012). The mechanical resistance of the joints decreased after welding as a result of an over-aging occurrence at the HAZ imposed on by the weld thermal cycle. Al 6061-T6 alloy weld hardening loss has been investigated in terms of precipitate transformation from β'' to β' phase (Nikseresht et al., Citation2015). High cycle loading is applied to these components, which causes fatigue failure. Researchers are very interested in the study of how cracks nucleate and propagate in aluminum and its alloys, and in order to comprehend crack propagation, they used a vast quantity of experimental data as well as theoretical approaches (Sashank et al., Citation2018). The FCGR is significantly faster in the final stages of crack formation compared to the earlier stages, and the final crack size has less of an effect on fatigue life (Svensson et al., Citation2000). This makes the entire life highly dependent on the size of the original crack. Using fracture mechanics concepts, the ability of a material to withstand crack growth under cyclic loading conditions is expressed as either the FCGR (da/dN) v/s crack tip stress intensity factor range (ΔK) or the crack length (a) v/s the number of cycles (N) (Živojinović et al., Citation2013). The FCGR predictions are usually obtained under cyclic loading on pre-cracked specimens at constant load amplitude. Pre-existing cracks do not grow below a certain fatigue stress intensity threshold value, and the crack growth rates with decreasing ΔK become comparatively small, less than 10-5mm/cycle (Henkel et al., Citation2015). A linear relationship between loge da/dN vs. loge ΔK is commonly used to analyze the characteristics of FCGR in the Paris region. FCGR behavior is of significant practical importance in the Paris region because it is frequently related to damage sizes for in-service inspection of high-performance parts under cyclic loading. The final stage in the Paris curve defines the final stages of crack propagation in terms of ΔK (or more accurately, Kmax), which approaches the critical stress intensity (Kc) (Abdulstaar et al., Citation2017). The key factors affecting FCGR include stress ratio (R), microhardness value, alloy toughness, and specimen thickness (John et al., Citation2003). They also depend on the effects of several physical and microstructural factors that affect the fatigue life of aluminum alloys. The metallurgical microstructures that disperse plastic strain and minimize strain concentration contribute to lowering crack initiation (Hagihara et al., Citation2007). At relatively high ΔK levels, the metallurgical factors leading to an increase in fracture toughness frequently also contribute to an increase in resistance to crack propagation (Pao et al., Citation2003). The FCGR test includes two subtests: crack initiation and crack propagation. Stage I, which is connected to many microstructural characteristics, happens when the fracture has nucleated and begun to propagate (Hanlon et al., XXXX). As a result of their cracks growing longer and their deformation systems becoming active, stage II is characterized by an increase in the stress intensity range. Stage III is associated with unstable crack growth and unpredictable behavior (Masuda et al., Citation2021). In an Al-Mg-Si alloy, crack nucleation, and crack propagation through the aluminum matrix occur when the ΔK is increased near the crack tip for the longer crack (Ilman, Citation2010). The compact tension (CT) specimens for tension-tension fatigue testing can be loaded using a clevis loading device. The stress ratio R = Pmin/Pmax is in the 0 < R > 1 range (Borrego, Citation2009). For CT specimens, the thickness should be in the range of W/20 < B < W/4, according to ASTM requirements (Vikram & Kumar, Citation2015). The literature survey study reveals that not much work has been done on the study of fatigue crack growth of Al 6061-T6 welds along the rolling direction in the weldments, heat-affected zone, and base metal welded by the gas metal arc welding (GMAW) process. In the present paper, fatigue crack growth of Al 6061-T6 welds obtained by gas metal arc welding along the longitudinal direction is studied, and fractographic analysis was carried out on the FCG test samples by scanning electron microscopy (SEM).
2. Experimental details
The GMAW technique was employed to weld the Al 6061-T6 alloy plate in a butt joint configuration as shown in . In this welding technique ER-5356 as a filler rod along with pure argon as a shielding gas was used. The detailed chemical composition of ER 5356 and Al 6061-T6 is shown in . Preheating was carried out before gas metal arc welding, and it involves heating the entire part to a specified temperature before welding. This reduces the cooling rate of the weld and drives out moisture. This in turn helps prevent hydrogen build-up and the potential for cracking (Liu et al., Citation2016). The welding was carried out along a longitudinal direction as per the welding procedure specification (WPS) as shown in . Post-weld heat treatment (PWHT) is required for Al 6061-T6 welded alloy because it improves the mechanical and microstructural properties of the weld. Welding can cause Al 6061-T6 to lose 30-40% of its strength compared to the parent material. PWHT can improve these properties by increasing joint efficiency and minimizing softening in the heat-affected zone (HAZ) (Jogi et al., Citation2008).
Figure 1. (a) Single V-groove GMAW welded butt joint (b) Al 6061-T6 welded plate.
Table 1. Chemical composition of ER-5356 and Al6061-T6.
Table 2. Welding Parameters of GMAW welding for Al 6061-T6 as per WPS.
3. Results and analysis
The details of X-ray radiography, Vickers microhardness test, tensile and bending test, and FCGR test are discussed below.
3.1. X-ray radiography and Vickers microhardness test
The X-ray radiography inspection was performed as per ASME sec IX standard to determine the welded joint’s quality and defects like flaws and pores (Liu et al., Citation2015). The X-ray radiography films are shown in . To characterize the microhardness profile near the weld location, Vickers microhardness tests were conducted. illustrates the variation of hardness values from the weld region to the base metal on the welded plate along the longitudinal direction. The unique hardness regime can be used to distinguish between the WZ, HAZ, and BM regions on the welded plate.
Figure 2. Microhardness profile of the GMAW welded Al 6061-T6 specimen.
Table 3. X-ray radiography films.
3.2. Tensile and bending test
The tensile and bending tests were performed as per ASME sec IX and ASTM A-370 on WZ, HAZ, and BM samples of GMAW welded along the longitudinal direction using the universal testing machine as shown in . Tensile tests were performed on BM, HAZ, and WZ samples to evaluate various mechanical characteristics such as yield strength, ultimate tensile strength, and breaking strength. Bending tests were carried out to ensure that the welded structures performed satisfactorily under root and face bending at an angle of 180°. shows the detailed specification of tensile and bending test specimens as per ASME standards.
Figure 3. Universal testing machine.
Figure 4. (a) The detailed specification of tensile and bending test specimens and (b) tensile and bending test specimens.
The tensile test results for the BM, HAZ, and WZ of Al 6061-T6 GMAW welded samples along the longitudinal direction are shown in . According to the results, the weld region may be considerably weaker than other zones. The joint strength is influenced by the microstructure and chemical composition of the weld region. The presence of alloying elements such as silicon and magnesium that combine and go through precipitation processes to generate a stronger precipitate of Mg2Si, is the key factor leading to improved base metal strength (Serrano PeRez & Ambriz, Citation2016).
Table 4. The tensile test result of Al 6061-T6 GMAW welded samples.
The weld zone possesses a yield strength of 60% of base metal and an ultimate tensile strength of 82% of base metal. Similarly, the ultimate tensile and yield strengths of HAZ are 80% and 82% of BM respectively. The mechanical strength of the weldment is improved by post-weld heat treatment, and the mechanical strength of HAZ is significantly higher than that of WZ due to the Vickers microhardness value (Fahimpour et al., Citation2013; Guzmán et al., Citation2019; Li et al., Citation2006; Malopheyev et al., Citation2016). The fractured surface of the tensile test specimen as shown in . The three-point bending test is an essential technique for evaluating the weld quality and bend strength of a structure. The results of the face and root bending tests reveal that neither the base metal nor the HAZ exhibited any signs of microcracks. It exhibits no imperfections, and the structure has good ductility and better bend strength without failure (FadilIslamović, Citation2009; Mohsein, Citation2015; Xu et al., Citation2016).
Figure 5. Fractured surface of tensile test specimens.
3.3. Fatigue crack growth (FCG) test
The fatigue crack growth (FCG) tests on the WZ, HAZ, and BM were carried out using servo-hydraulic controlled fatigue crack growth testing equipment with a 250 KN capability according to ASTM E-647 standards. shows the detailed specification of the FCG test specimen. CT specimens of WZ, HAZ, and BM were taken directly from the GMAW-welded plate as shown in . The specimens had initially fatigue pre-cracked at 10 Hz, a 0.1 load ratio, and a 3.5 KN load amplitude (Ambriz et al., Citation2010; D’Urso et al., Citation2014). The crack growth rates at various ΔK values were evaluated using the load shedding approach (ΔK decreasing) to find the threshold stress intensity ΔKth value. The fundamental constants C and m in the equation da/dN = CΔKm were found using the constant amplitude approach (increasing ΔK method) (Salam et al., Citation2010). The FCG tests were carried out at ambient temperature with a constant amplitude cyclic loading at a frequency of 10 Hz. The parameters used in the FCG test are presented in .
Figure 6. Compact tension test specimen for FCG test.
Figure 7. Location of WZ, HAZ and in Al 6061-T6 GMAW welded plate.
Table 5. The parameters used in the FCG test.
The experimental data of da/dN v/s ΔK for BM, HAZ, and WZ were used to generate the FCG curves for different R values of 0.1, 0.3, and 0.5. The Paris law was fitted for all stress ratios (R) with high correlation coefficients. A comparison was made for BM, HAZ, and WZ of Al 6061-T6 GMAW welded samples along the longitudinal direction. The experimental results for BM, HAZ, and WZ for different stress ratio (R) values are shown in . Considering the Paris region of the FCGR curve, the Paris equation is given as da/dN = CΔKm, where the exponent m and the coefficient C are the material constants.
Table 6. FCG Test result of Al 6061 T-6 GMAW welded specimen.
shows the da/dN for BM as a function of ΔK for various R values of 0.1, 0.3, and 0.5. The FCGR curves for fatigue threshold (Kth), linear Paris regimes, and high fracture toughness (Kc) are shown in .
Figure 8. da/dN v/s ΔK for BM.
The values of Kth and Kc of BM under different R values are tabulated in . The threshold fatigue crack growth behavior in Al 6061-T6 alloys is influenced by the microstructure, such as grain size, matrix precipitates, and surrounding grain boundary precipitates (Ambriz et al., Citation2010; Dai et al., Citation2013; Jata et al., Citation2000; Jian et al., Citation2017). The Kth and Kc of BM gradually decrease by increasing the R values.
The results reveal that the Kth value decreased as the R-value increased when da/dN ≤ 10−5 mm/cycle. If da/dN ≥ 10−5 mm/cycle, the R-value did not have much effect on how quickly fatigue cracks grew (Fan et al., Citation2021). The Kth and Kc generally tend to decrease as the R-value increases. As a crack propagates, the crack tip stress intensity factor range increases until it eventually exceeds the critical stress intensity (Kc), which results in failure.
The comparison of da/dN vs. ΔK of BM under different R values is shown in . The FCGR is more significantly impacted by the R values. The fatigue threshold (Kth) in stage I decreases with an increasing R-value, which causes da/dN to be high at a high R-value at the same ΔK. The value of da/dN for the highest R-value is a consequence of the critical condition, and the maximum stress intensity Kmax, which can be calculated as ΔK/(1 − R), approaches the fracture toughness Kc in Stage III. The crack length (a) vs. number of cycles (N) is shown in . The BM specimen has pre-cracks up to 13.012 mm under R = 0.1.
Figure 9. Crack length (a) v/s Number of cycles(N) for BM.
When the number of cycles increased, the crack started propagating and reached the critical crack length (ac)= 33.45 mm under 59,350 cycles. The corresponding ΔK value for R = 0.1 was found to be 17.769 MPa√m. Similarly, for R = 0.3 and 0.5, the specimen is pre-cracked up to 13.02 mm and 13.09 mm, respectively. The crack starts to propagate and reaches the critical crack length (ac)= 32.87 mm and 31.158 mm under 90,166 and 107,504 cycles, and the corresponding stress intensity range was found to be 15.078 MPa·√m and 13.691 MPa√m, respectively.
The da/dN vs. ΔK in a logarithmic scale of HAZ for different stress ratios (R) is shown in . The Kth for HAZ under R = 0.1 is 6.554 MPa√m, and the fracture toughness of HAZ was 16.6457 MPa√m. The Kth and Kc tend to decrease with increasing R values, as observed in . It reveals that the load amplitude and R-value significantly impact crack propagation.
Figure 10. da/dN v/s ΔK for HAZ.
shows a v/s N for HAZ under R = 0.1, 0.3, and 0.5. The curves appear to be smooth throughout the critical crack length, then start to fluctuate with the varying R values. As the R-value increased, the number of cycles for the crack growth rate increased simultaneously. It also suggests that the primary variable affecting the crack growth rate is the R-value. The specimen was pre-cracked up to 13 mm under an R = 0.1. The crack started to grow as the number of cycles increased, eventually attaining the critical crack length (John et al., Citation2003; Li et al., Citation2018).
Figure 11. a v/s N for HAZ.
The crack growth rate for WZ under R = 0.1, 0.3, and 0.5 is shown in . The microstructure, mechanical strength, microhardness value, and filler material all influence the threshold FCGR behavior. The threshold fatigue strength and fracture toughness normally decrease as the stress ratio increases, as shown in .
Figure 12. da/dN v/s ΔK for WZ.
shows a vs. N for WZ under R = 0.1, 0.3, and 0.5. The curves were continuous and smooth; however, a clear fluctuation was seen between the curves as the load ratio varied. The number cycle for crack propagation increases along with the stress ratio, which indicates a major factor influencing fatigue crack growth is exerted by the stress ratio (Lados & Apelian, Citation2006; Lakshminarayanan et al., Citation2009).
Figure 13. a v/s N for WZ.
to show a vs. N for BM, HAZ, and WZ under R = 0.1, 0.3, and 0.5. The number of cycles from the beginning to the end of a crack is counted, and these numbers are compared with data from other zones. In contrast to the HAZ and BM, the WZ exhibits a faster growth rate. The WZ grows more quickly than the HAZ and BM. As the R-value increases, the fatigue crack growth rate increases. In comparison to HAZ and WZ, it has been found that BM experiences more cycles of fatigue crack propagation.
Figure 14. a v/s N of cycles for R = 0.1.
Figure 15. a v/s N for R = 0.3.
Figure 16. a v/s N for R = 0.5.
The GMAW welded joint’s crack growth behaviour is uniform throughout, although there are some noticeable variations in crack length between the zones. Along the propagation zone, the BM and WZ show uniform crack length and exponential behavior. In HAZ, the crack length exhibits quasilinear growth at first and quick growth near the end of the fracture (Ambriz et al., Citation2010; Vikram et al., Citation2014). It was concluded that the microstructure of this zone tends to slow down the spread of cracks by favoring the formation of a large plastic zone near the crack tip. The critical crack size (ac) for each area of the welded joints was to be determined using the graph. The critical crack size correspondingly correlates to ac, where the crack tends to expand quickly until catastrophic failure is detected (Vikram & Kumar, Citation2015). The BM specimen, followed by the HAZ and WZ specimens, displays the highest value of ac (Ilman, Citation2010; Vasco-Olmo et al., Citation2016). The base metal withstood more cycles compared to HAZ and WZ to reach ac.
The WZ revealed that the minimum cycles were necessary to get the critical crack size (ac) and also noted that it is roughly 66% of the BM. This is because WZ is brittle and has a poor yield strength due to the filler metal’s high silicon concentration.
shows the da/dN v/s ΔK for BM, HAZ, and WZ materials at R = 0.1. There were noticeable differences in the fatigue crack growth rate behaviour in each zone. The BM showed slower crack propagation than the HAZ and WZ when subjected to R = 0.1. The yield strength of the BM is higher than that of HAZ and WZ, despite the fact that the microstructural characteristics of the material have a substantial impact on the FCGR and the crack tends to propagate more quickly in these two zones than in the BM (Huang et al., Citation2004; Yang et al., Citation2018). The results shown in show that for BM, HAZ, and WZ, Kth and Kc values differ significantly. Under a stress ratio of 0.1, the Kc for WZ is 79.2% of the BM, and the Kc for HAZ is 89.2% of the BM.
Figure 17. da/dN v/s ΔK for R = 0.1.
The comparison of da/dN vs. ΔK of WZ, HAZ, and BM at R = 0.3 as shown in . The ΔK of HAZ is 80% of that of BM. The welding thermal cycle influenced the microstructural transition from fine needle-shaped precipitates to coarse bar-shaped precipitates. It also significantly affects the rate at which cracks develop in HAZ (Ambriz et al., Citation2010). This demonstrates that the temperature achieved in the HAZ and WZ typically higher than the alloy’s ageing temperature directly influences the crack growth condition. The filler wire and base metal are fused during GMAW welding. Due to the high percentage of eutectic Si, the temperature in the welding process affects the microstructure with refined grain size, resulting in poor toughness, a faster rate of crack propagation, and a smaller stress intensity range compared to BM (Borrego, Citation2004; Mayén et al., Citation2017).
Figure 18. da/dN v/s ΔK for R = 0.3.
Internal stress exists in the WZ, which slows down the crack propagation rate due to residual stress distribution in the welded joints. When ΔK is low, the crack growth rate in WZ is slower than in BM because the compressive residual stress in WZ prevents the crack from growing further. The crack propagation rate can be considerably increased to a level even higher than BM at high ΔK due to metallurgical flaws. A similar trend was also observed in R = 0.3 and R = 0.5, as shown in and . The experimental data from the FCGR test were fitted into the Paris equation for all regions. Consider two different points in da/dN vs. ΔK in the Paris region of the sigmoidal curve to evaluate the material constants C and m. shows the material constants C and m in the linear region (Paris region) of the sigmoidal curve of BM, WZ, and HAZ of Al 6061-T6 GMAW welded samples respectively.
Figure 19. da/dN v/s ΔK for R = 0.5.
Table 7. Fitting constant obtained from experimental data.
3.4. Fracture characteristics
The macrograph of fractured surfaces for Al 6061-T6 GMAW-welded samples along the longitudinal direction is shown in . The transcrystalline mechanism of failure in the threshold regimes is observed. The fracture characteristics of compact-type specimens were studied along the crack propagation direction. The HAZ and BM show a significantly straight crack path along the welding direction. The transition between the crack initiation zone and the final fracture zone is visible in BM and HAZ. On the macrographs, multiple stages of fatigue threshold, crack formation, and fractured surface path propagation can be observed.
Figure 20. Macro graph of fractured surfaces.
3.5. Fractographic analysis
SEM was used to examine the fractured surface of the FCG test samples. Flat fracture surfaces are observed in BM, HAZ, and WZ. All fracture surfaces were classified according to the cleavage type, as shown in to .
The SEM of the fatigue fractured surfaces of WZ, HAZ, and BM near the threshold region and fractured region are shown in to . The fractography of BM under R = 0.1, 0.3, and 0.5 shows transgranularity with evidence of elongated dimples (Jogi et al., Citation2008). The crack path is extremely smooth, and the transgranular mode of crack propagation is observed for R = 0.1, 0.3, and 0.5 according to the BM fractured profiles. This is mostly due to the fact that, like in Al-Mg-Mn-Si alloys, the incoherent dispersoids of the α-phase inhibit intergranular fracture by reducing strain concentration at grain boundaries (Henkel et al., Citation2015). The facets appear near the threshold region, as shown in , due to the crystallographic nature of crack growth. On the fracture surfaces, fatigue striations were observed during the steady-state crack propagation phase. For all R values, it was discovered that the striation spacing was equivalent to the crack propagation rates. In the fracture zone, a dimple rupture was observed, as shown in , which represents a change towards a ductile type of fracture at the later phases of crack propagation. Brightly reflecting facets caused by the crystallographic mode of crack propagation appeared on the fracture surfaces in the near-threshold region. Intergranular failure can be observed in Region III due to the separation of the dendritic arms.
Figure 21. Micrograph of BM. (a) R = 0.1 at Kth region, (b) R = 0.1 at Kc region, (c) R = 0.3 at Kth region, (d) R = 0.3 at Kc region, (e) R = 0.5 at Kth region, (f) R = 0.5 at Kc region.
On the fractured surfaces of HAZ and WZ, striation formation was observed. The striation path’s alignment changes on the cracked surface of HAZ, as shown in . The primary cause of this characteristic is attributed to coarse precipitates in the microstructure, which alter the pace at which local cracks form and the distance between fatigue striations (Al-Wajidi et al., Citation2019).
Figure 22. Micrograph of HAZ. (a) R = 0.1 at Kth region, (b) R = 0.1 at Kc region, (c) R = 0.3 at Kth region, (d) R = 0.3 at Kc region, (e) R = 0.5 at Kth region, (f) R = 0.5 at Kc region.
The fracture morphology of the WZ has changed from transgranular to mixed mode (transgranular and intergranular), as evidenced by various weld nugget grain patterns on the fracture surfaces (Brahami et al., Citation2018; Serrano PeRez & Ambriz, Citation2016). Region III shows an intergranular failure, which leads to the brittle type of failure, and fatigue striation is difficult to reveal, as shown in .
Figure 23. Micrograph of WZ. (a) R = 0.1 at Kth region, (b) R = 0.1 at Kc region, (c) R = 0.3 at Kth region, (d) R = 0.3 at Kc region, (e) R = 0.5 at Kth region, (f) R = 0.5 at Kc region.
The crack path is independent of the microstructure feature in a transgranular failure. In this area, well-defined striations and localized ductile failure modes can be observed in . For various stress ratios, the same behaviour was observed. Regardless of the stress ratio, every zone exhibits a ductile type of fracture in the final fracture region.
4. Conclusion
The mechanical and fatigue crack growth (FCG) behaviour in WZ, HAZ, and BM of Al 6061-T6 welded joints obtained by the GMAW were quantified as below.
The mechanical strengths of BM, HAZ, and WZ of the Al 6061-T6 GMAW welded specimen along the longitudinal direction were analyzed. The WZ results show a reduction in mechanical strength due to the fusion of filler electrode ER-5356 with the BM. The HAZ shows moderate tensile strength among WZ and BM, which is due to the coarse and uniformly distributed precipitates of Mg2Si.
The behavior of threshold crack growth throughout the welding thermal cycle is influenced by the microstructure regeneration. The temperature attained in the HAZ and WZ is often higher than the alloy ageing temperature, which can directly affect the crack growth condition.
The WZ region showed poor fatigue crack growth resistance compared to HAZ and BM. The brittle microstructure characteristics of the WZ due to a high concentration of eutectic Si have been attributed to the higher fatigue crack growth rate.
It has been found that the fatigue crack growth rate is significantly affected by the stress ratio(R). At a given applied cyclic load, an increase in the stress ratio leads to an increase in the fatigue-crack growth rate in all the 3 zones.
The fractography of BM at R = 0.1, 0.3, and 0.5 shows the presence of elongated dimples in addition to transgranular fracture modes. The WZ fracture morphology changed from transgranular to mixed mode fracture (combination of transgranular and intergranular) as a result of different weld nugget grain morphologies.
The final fracture remains ductile irrespective of the stress ratio in all 3 zones.
Author contribution statement
All authors take public responsibility for the content of the work submitted for review. The contributions of all authors have been described in the following manner:
The authors confirm contribution to the paper as follows: Study conception and design: Dr. P C Arunakumara, Sagar H N, Bimal Gautam. Data collection: Sagar H N. Analysis and interpretation of results: Dr. P C Arunakumara, Sagar H N, Dr. S Rajeesh. Draft manuscript preparation: Dr. P C Arunakumara, Sagar H N. All authors reviewed the results and approved the final version of the manuscript.
Novelty and application
The literature review revealed a lack of research on the fatigue crack growth in Al 6061-T6 welds obtained by gas metal arc welding methods in three regions such as the base metal, the heat-affected zone, and the weldment.
The present study investigates the characteristics of fatigue crack growth under stress ratios of R = 0.1, 0.3, and 0.5 in base metal, weldment, and the heat-affected zone. The results suggest that while the stress ratio increases, there is a substantial variation in the growth of fatigue crack. The objective of the present work is to study the FCG in the 3 zones of the welded plate along the longitudinal direction, the change in microstructure during the welding thermal cycle, and the effect of the stress ratio on the fatigue crack growth rate.
Acknowledgments
The authors acknowledge the DRDO, ARMREB, and India for their financial support. The authors express their gratitude to Dr. Bhupender Singh Rawal and Mr. Bimal Gautam for their constructive advice. The authors also thank the principal and head of the mechanical engineering department at RIT, Bangalore, for their unwavering support.
Disclosure statement
All authors have seen and agree with the contents of the manuscript and there is no financial interest to report. We certify that the submission is original work and is not under review at any other publication.
Data availability statement
The author confirmed that the data supporting the findings of this study are within the article. Raw data supporting the findings of this study are available from the corresponding author upon reasonable request.
Additional information
Funding
Notes on contributors
Arunakumara P. C.
Dr. P. C. Arunakumara, PhD (Fracture Mechanics), MS Ramaiah Institute of Technology, Research Interest: Fatigue and Fracture, Mechanical Vibrations.
Sagar H. N.
H. N. Sagar, MTech (Machine Design), MS Ramaiah Institute of Technology Research Interest: Fatigue and Fracture.
Bimal Gautam
Bimal Gautam (Mechanical Engineering) MTech, R & D Engineers, Pune, Research Interest: Material Characterization.
Rajeesh S.
Dr. S. Rajeesh, MTech (Mechanical Engineering), MS Ramaiah Institute of Technology Research Interest: CFD, Fatigue and Fracture.
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