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Microstructural and fatigue behavior of Inconel 718 weldments performed using gas tungsten arc welding process (GTAW) were studied. Ti and Nb segregation tends to form brittle phases (Laves, NbC and TiC), which were identified in the interdendritic region of the weld metal (WM). The heat generated to weld Inconel 718 produced a microstructural transformation of the γ´´ (Ni3Nb), decreasing the microhardness in comparison to the base metal (BM). The welded joints were heat treated by a solubilization and aging treatment. Fatigue life behavior of the welded joints was measured. Curves of stress amplitude versus number of cycles to failure were obtained for the welded joints. Nevertheless, considering the difference in tensile properties of the welded joints and BM, similar fatigue life resistance was found. The fatigue crack initiation was located close to the WM, coinciding with softer zone as identified by microhardness. Fatigue crack growth rate da/dN as a function of stress intensity factor range ΔK was determined. The slower da/dN as a function of ΔK corresponds to the WM in the as-welded condition, followed by the HAZ, WM after the heat treatment, and finally the BM. This phenomenon was analyzed in terms of the crack closure effect induced by the plasticity zone ahead of the crack tip.

Partially aged 6061 aluminum plates were welded by the gas metal arc welding process. During the welding process, a cooling system to act as a heat sink was used to extract the heat coming out from the welding process. To determine the experimental weld thermal cycles, K type thermocouples were placed at the heat affected zone. Finite element was used to compute the thermal distribution and isothermal sections generated by welding. The temperature measurements, Vickers microhardness profiles and tensile tests were observed to be related with the microstructural β″ to β′ phase transformation. The over-aging of the heat affected zone tends to diminish when the heat sink is used as compared with the reference welding condition. An increase on tensile strength (about 5.8%) and ductility (close to 29%), for welds performed in partial aging materials with respect to the welds with artificial aging condition (6061-T6 alloy) was also observed.

This research investigates the feasibility of using the thermoelectric power to monitor the thermal embrittlement in 2507 super duplex stainless steel (SDSS) exposed to a temperature between 280 °C and 500 °C. It is well known that the precipitation of Cr-rich α′ phase as a result of the spinodal decomposition is the major cause of the embrittlement and the loss of corrosion resistance in this material. The specimens are thermally aged at 475 °C for different holding times. A series of mechanical testing including the tensile test, Vickers microhardness measurement, and Charpy impact test are conducted to determine the property changes with holding time due to the embrittlement. The mechanical strengths and ferrite hardness exhibit very similar trends. Scanning electron microscopy images of impact-fractured surfaces reveal a ductile to brittle transition in the fracture mode as direct evidence of the embrittlement. It is shown that the thermoelectric power is highly sensitive to the thermal embrittlement and has an excellent linear correlation with the ferrite hardness. This paper, therefore, demonstrates that the thermoelectric power is an excellent nondestructive evaluation technique for detecting and evaluating the 475 °C embrittlement of field 2507 SDSS structures. Keywords: Thermoelectric power, Super duplex stainless steel, 475 °C embrittlement, Nondestructive evaluation

Dual phase steel DP980 sheets were joined by resistance spot welding (RSW) process. Mechanical resistance of the welds was characterized by microhardness, tensile shear and fatigue tests. A significant hardness decreases was observed in the RSW lap joints with respect to the base material, which was attributed to phase transformations during the heating and cooling of the steel. Fatigue Wohler curves using a fixed load ratio of 0.1 were obtained. It was found that the spot weld at the nugget interface close to the fusion zone induced a critical stress concentration factor, which decreased the fatigue life of the joints in the as-welded condition. Failure of the welds was initiated at the interface between welded sheets. Two predominant fatigue fracture modes were observed associated with mode I/III cyclic loading, which were correlated with the fatigue crack initiation and propagation stages. Compressive residual stresses were induced by a loading-unloading cycle on the spot welds, which tends to increase the fatigue life of the joints when compared to the as-welded condition.

Heat moving source models along with transient heat analysis by finite element method were used to determine weld thermal cycles and isothermal sections obtained from the application of a gas tungsten arc welding beads on Inconel 718 plates. Analytical (Rosenthal's thick plate model) and finite element results show an acceptable approximation with the experimental weld thermal cycles. The isothermal sections determined by numerical simulation show a better approximation with the experimental welding profile for double-ellipse model heat distribution than Gauss model. To analyze the microstructural transformation produced by different cooling rates in the fusion and heat affected zones, Vickers microhardness measurements (profile and mapping representation) were conducted. A hardness decrement for the heat affected zone (∼200 HV0.2) and fusion zone (∼240 HV0.2) in comparison with base material (∼350 HV0.2) was observed. This behavior has been attributed to the heterogeneous solubilization process of the γ″ phase (nickel matrix), which, according to the continuous-cooling−transformation curve, produced the Laves phase, δ and MC transition phases, generating a loss in hardness close to the fusion zone.

The fatigue and crack growth behavior of Inconel 718 and AL6XN alloys joined with an ERNiFeCr-2 filler by the gas tungsten arc welding process was evaluated. A comparison between Wohler curves for the monolithic IN718 and AL6XN alloys and the IN718-AL6XN dissimilar welded joint were obtained. Although Vickers microhardness profiles revealed the presence of a soft region in the heat affected zone of the IN718 base material, the fatigue crack nucleated at the fusion zone near to the AL6XN and propagated across the weld bead. The welded joint and the AL6XN base material presented a similar high cycle fatigue behavior despite the presence of an unmixed zone at the AL6XN/weld metal interface. The results of the fatigue crack growth test indicated that the weld metal has the highest growing rate owing to the presence of brittle secondary phases such as Laves and NbC. The fatigue crack growth rates obtained from experimental data were compared with measurements of the distance between striations on the fracture surfaces, both were very close.

Thermal cycles and isothermal sections were determined analytically by using the Rosenthal thin plate model, as well as by means of finite element method. The simulation model provides an acceptable approximation to thermal cycles measured during the welding process. A discrepancy for the analytical model with respect to the experimental results was found. Due to the thermal effect a hardness decrement of 43% (60 HV 0.1 ) with respect to the base material (107 HV 0.1 ) at a distance of 18 mm from the center of the welding bead was observed. The weld thermal cycle measured experimentally at this distance traverses the C-curve for the β′ precipitation phase at a peak temperature of 380 °C, with a dwell-time of approximately 10 s. The presence of rod-shape β′ precipitates was observed on a transmission electron microscopy image. A correlation between isothermal sections and the hardness distribution for different zones of the welded joint was identified.

The dilution effects in welds of a 6061-T6 (Al-Si-Mg) alloy obtained by the modified indirect electric arc (MIEA), using an ER4043 filler metal (Al-Si), and postweld heat treatment (PWHT) were analyzed. The soft zone (55 to 70 HV0.1) formed by the microstructural transformation in the heat-affected zone (HAZ) was eliminated. The hardness measurements were presented on a traditional microhardness profile and mapping representation. A hardening effect of the fusion zone was observed; the hardness values were above 120 HV0.1 and tended to be uniform. This behavior could be attributed to the chemical composition of the filler metal, the Mg migration from the base to the weld metal, and the reversible process of the PWHT, which promotes precipitation hardening. Improvement for yield (260 MPa) and tensile strength (310 MPa) of the MIEA joints was observed; these values were similar to those obtained for the base metal. However, the presence of porosity in the fusion zone limits the ductility of the joints (4.3 pct). Even though the yield and tensile strengths of the base metal and welded joints were similar, the stress concentration due to porosity in the weld metal generated data dispersion in fatigue life. As a consequence, the high-cycle fatigue life decreases with respect to the base metal. In contrast, when the crack propagates under elastic conditions, the crack-tip singularity is affected by the porosity in the weld metal (stress liberator). This aspect, in conjunction with the hardening effect in joints subjected to PWHT, improves the fatigue crack growth rate when compared to the as-welded condition.

The influence of variable load spectra in the fatigue crack growth (FCG) was analyzed. A 4.1% interference was pre-set to some specimens by an expansion operation. As-drilled hole and hole expanded specimens were subjected to FCG tests under a combination of baseline cyclic loading and periodic overloads or underloads applied. Fatigue life was of approximately 1 million of cycles for the as-drilled hole, and 3.5 million of cycles for the hole cold expanded. The pre-setting operation and variable load spectra interaction exhibited a synergy, which was associated with multiple periods of retardation and arresting observed in the crack growth rate.

Ferritic stainless steels are widely used in transportation industry due to their exceptional performance regarding mechanical and corrosion properties. However, after a welding process, joints feature the sensitizing phenomenon because of the heat exchange from the torch affecting mechanical properties and corrosion resistance. This work describes the behavior firstly of mechanical properties of weld joints of ferritic stainless steel as base material without and with filler material (AISI 308L) by gas tungsten arc welding (GTAW). Operating parameters such as arc voltage, welding currrent, welding speed, feed speed, shielding gas flow were evaluated. Secondly, samples of weld joints were coated by flame spraying of composite particles in order to reduce the weight loss induced by corrosion in a salt spray (fog) apparatus. Changes induced from GTAW on the heat affected zone and Thermal Spraying on corrosion resistance, were monitored by optical and scanning electron microscopy, microhardness and longitudinal tensile testing. Results show that GTAW helps to control the size and the microstructure of heat affected zone improving simultaneously the mechanical properties. Meanwhile, welded joints covered by composite coatings feature a lower damage provided that the ceramic phase has been homogeneously dispersed.

Gas Tungsten Arc Welding (GTAW) process by using two different heat input conditions was used to join Inconel™ 718. Welds were performed by applying one and three welding beads with and without post-weld heat treatment (PWHT). Mechanical characterization was performed by microhardness measurements and tensile tests. It was observed an improvement in tensile mechanical behaviour for the welds with three welding beads, however, the ductility tends to decrease compared with the application of only one welding bead. For the welds without PWHT the yield strength increases approximately 157 MPa, but the fracture energy decreased roughly 27% with respect to the application of only one welding bead. This aspect was attributed to the precipitation of brittle phases due to weld thermal cycles during the multi-pass welding process.

Gas tungsten arc welding (GTAW) was used to join Inconel 718 and AL6XN plates. The welding was made in aged condition for the Inconel 718 plates. Mechanical properties were determined by tensile, microhardness and instrumented Charpy impact tests. An ERNiFeCr-2 filler metal was fed by a semiautomatic mechanism. The results show that the mechanical properties of as-welded and hardening recovery provided by post weld heat treatment (HRPWHT) conditions are close. The HRPWHT provide a notorious improvement in mechanical properties in the fusion zone (FZ) as well as in the Inconel 718 side. However, it did not have any effect in the AL6XN side. Due to the aforementioned, the resistance of the weldment (IN718-AL6XN) is similar in both conditions. For the tension tests, the weldments to which heat treatment was not applied exhibit the failure in the weld bead, on the other side, the weldments with HRPWHT fractured along the fusion line at the AL6XN side. The failure in the weld bead was attributed to the presence of detrimental secondary phases (NbC and Laves), which were partially dissolved with the HRPWHT. This provides similar mechanical properties of the dissimilar welded joints. The most evident difference was observed in the elongation percentage, which was higher in the as-welded condition (14.05%) in comparison with the HRPWHT condition (10.8%).

Compact test specimens were extracted from a 6061-T6 aluminum alloy welded plate with a thickness of 9 mm to analyze the cold hole expansion effect on fatigue crack growth tests conducted in mode I cyclic loading. At R = 0.1, a sharp crack in base metal, weld metal and heat affected zone was propagated from 17 to 24 mm. The fatigue crack growth at 24 mm ( α = a / W = 0.3) was delayed by drilling a hole at the crack tip and applying a cold hole expansion of 4.1%. The residual stress fields due to cold hole expansion were determined with the finite element method. The fatigue crack growth testing was continued up to a crack length of 35 mm ( α ∼ 0.43) at the same R , and crack opening displacements of the post-expansion crack were also determined with the finite element method. The results were expressed in terms of crack length versus number of cycles, as well as, fatigue crack growth rate as a function of applied and effective stress intensity factor range. The cold hole expansion contributed to delay the fatigue crack growth in base metal, and to a lesser extent in the weld metal and heat affected zone. A crack closure effect was determined by means of load versus crack opening displacement curves of the post-expansion crack, which was, completely or partially closed, in welded zones with compressive residual stress fields. The fracture surfaces of each welded zone were analyzed to elucidate the crack nucleation zone and its relation with the residual stress field. In all cases the crack was initiated at the surface of the specimen where the residual stresses were positive.

Tensile and hardness values for 7075-T651 aluminum alloy in the as welded and post weld heat treated conditions (solubilization and artificial aging-T6), obtained using GMAW and modified indirect electric arc (MIEA) welding processes are presented. Results showed that the base material along rolling direction exhibited a tensile strength of around 600 MPa and elongation of 11%. For the as welded condition, tensile strength was 260 MPa and elongation percent of 3%. This behavior was attributed to brittleness induced by the microstructural characteristics of the welded alloys, as well as high porosity. Hardness profiles along the welds were obtained and different welded zones were identified. A soft zone (~100 HV0.1) in the heat affected zone for GMAW and MIEA was observed, the minimum hardness corresponding to weld metal (~85 and ~96 HV0.1 for GMAW and MIEA, respectively). The high dilution between filler and base metal during welding in MIEA allows to the Zn and Cu to flow from the base metal into the weld metal, inducing hardening by solution and subsequent artificial aging. In this regard, the hardness of the weld metal for MIEA increases by 56%, while the tensile strength reaches a value close to 400 MPa. For GMAW, non-favorable hardening effect was observed for the weld metal after solution and artificial aging.

International audience; The local mechanical properties of a weld zone, in a 6061-T6 aluminium alloy subjected to the modified indirect electric arc technique have been studied. The mechanical properties of the base metal, the weld metal and the heat affected zone were determined by means of usual and instrumented indentation testing, as well as micro-traction testing. To analyse the heat input effect resulting from the welding process, the evolution of the weld zone size was evaluated by means of classical indentation under a constant applied load. The results were presented using a Vickers hardness map representation. This allows monitoring exact hardness variation while leading to the identification of the different zones of the welded joint. Instrumented indentation testing was carried out to determine the local mechanical properties, such as the yield stress, the bulk modulus and the strain-hardening exponent. Obtained results are compared to those derived from tensile tests conducted on micro-specimen cuts taken from the weld zone. It was observed that yield stress values are directly comparable for indentation and micro-traction experiments. As for the elastic properties, no comparison was possible since the bulk modulus is measured by indentation, whereas it is the Young's modulus by tensile test. The micro-traction testing seems to be more sensitive to represent the work hardening of a material since the corresponding exponent is found to be constant by instrumented indentation.

Plates of Al-6061-T6 with a thickness of 12.7 mm were welded with a non-heat treatable ER4043 electrode in four welding passes using the typical single V groove butt joint (at room temperature) and in one pass using the novel modified indirect electric arc (MIEA) joint (at 50, 100 and 150 C). Microstructural characterization of the weld metal revealed that the MIEA joint design yielded microstructures in which columnar growth was restricted and a grain refining effect occurred without the addition of inoculants, even with preheating of the joints. The self-refining effect in the MIEA joint was discussed in terms of the cooling behavior and the large dilution of base material. Vickers microhardness profiles revealed that the high dilution rates obtained with the MIEA joint caused alloying of the weld pool with magnesium producing a weld metal composition sensible to heat treating due to the precipitation of Mg2Si precipitates. The grain self-refining effect of the MIEA joint along with alloying of the weld metal were found to have a profound impact on the mechanical properties of the welded joints.

Results of temperature measurements during welding of 12.7 mm thick AA6061-T6 alloy plates by modified indirect electric arc (MIEA) are presented. This study describes the thermal cycles of the heat affected zone (HAZ) and also in the fusion zone. Depending upon the position of the transducers, the maximum temperatures measured in the HAZ range from 308 to 693 °C, these measurements were related with the tensile test results, and the failure zone reported previously by the authors [1]. It was observed that, there is a decrease in the mechanical strength of the welded joints, due to the microstructural changes suffered by AA6061-T6 alloy in which formation of the β’ occurs according to the TTT transformation diagram. The inherent cooling conditions of the weld pool observed for the MIEA technique (only one pass of welding), have permitted to establish the characteristics of solidification and microstructure for a specific cooling rate.Se presentan los resultados de medición de temperatura durante la soldadura por arco eléctrico indirecto modificado (AEIM) de la aleación AA6061-T6. Éstas, describen los ciclos térmicos de la zona afectada por el calor (ZAC) y en la zona de fusión. Los resultados muestran que las temperaturas máximas medidas en la ZAC se encuentran en un rango de 308 a 693 °C, dependiendo de la posición de los sensores, estas mediciones fueron correlacionadas con los resultados de resistencia a la tracción y la zona de fallo, reportados previamente por los autores[1]. Se observó que existe una disminución en la resistencia mecánica de las uniones, debido a los cambios microestructurales por la formación de precipitados sobreenvejecidos, β’, de acuerdo con su diagrama de transformación (TTT). Las condiciones de enfriamiento en el baño fundido inherentes a la técnica por AEIM (un solo paso de soldadura), permitieron establecer las características de solidificación y microestructura esperada para una velocidad de enfriamiento determinada.

International audience; This paper reports the effect of the welding profile generated by the modified indirect electric arc (MIEA) technique on the fatigue behavior of 6061-T6 welded joints. The calculations are based on the effect of the stress concentration factor produced by the characteristic geometry of the welding profile formed during the welding process. It was found that the fatigue life of welded samples using this welding technique was larger in comparison with data reported in the literature. Also, measurements of microhardness and tension testing were performed to account for the effect of different levels of fatigue damage on the mechanical properties of these welds. Experimental findings reveal that, with reference to undamaged samples, fatigue damage increases the microhardness in the weld metal and heat affected zone (HAZ) whereas it only produces a moderate increase in yield strength of approximately 14% for a 75% of fatigue damage. Tensile and fatigue tests indicate that the thermal affection undergone by the plates during fusion welding dictates failure in the HAZ under tension stress but not under fatigue. The failure mechanism under fatigue is discussed in terms of theoretical analyses and fractography.

International audience; The characteristics of the fatigue crack growth in the base metal, weld metal and heat affected zone (HAZ) were quantified by testing compact type specimens of 6061-T6 welds obtained by the modified indirect electric arc technique. The fatigue crack growth depends on the microstructure imposed by the welding thermal cycle and it was observed that in the HAZ the crack growth rate is lower than that in the weld metal, but higher than that in the base metal. Microhardness maps revealed that this behaviour is due to the formation of a larger plastic zone around of the crack tip produced by loss of hardening. A comparison of fatigue crack growth of weld metal and HAZ for modified indirect electric arc and friction stir welding shows that the weld metal produced by friction stir welding exhibits better resistance to crack propagation, but both processes behaved similarly in the HAZ.