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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.

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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.

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.

The gas metal arc welding (GMAW) process was used to weld 3003-H14 plates under restricted and unrestricted thermal expansion. Experimental and numerical analysis were conducted to determine the relation between weld thermal cycles and residual stresses. A customized data acquisition system with K-type thermocouples was used to measure the weld thermal cycles, while residual stresses were determined by the hole drilling method. Thermo-mechanical simulation models for the two restricted conditions were implemented from the experimental data obtained. A double ellipse heat distribution geometry was used to model the heat moving source by using the finite element method. Thermal rates and peak temperatures were approximated by the finite element model with 2% difference, with respect to the experimental weld thermal cycles. Longitudinal and transverse normal residual stresses determined by the finite element model showed a good comparison with experimental measurements. The larger residual stresses were in the transverse direction for both clamping conditions, which indicated that working loading paths along the lateral direction of the welded plate are more influenced by the post-welding residual stresses.

The modified indirect electric arc welding (MIEA) and the conventional gas metal arc welding (GMAW) processes were used to weld 7075-T651 aluminum alloy plates. The fatigue life behavior was studied for three different zones in the welded joints (base metal, heat affected zone and weld metal). Compact-type (CT) specimens manufactured from the 7075-T651 aluminum alloy welded plates were subjected to fatigue tests with constant amplitude cyclic loading. The CT specimens corresponding to MIEA weld metal shown the largest fatigue life (number of cycles to reach the stable crack propagation), which was four times larger than the weld metal obtained by GMAW. This improvement was attributed to the high tensile residual stresses induced by the application of three welding beads of the GMAW process in contrast with the stresses induced by the application of only one welding bead in the MIEA. The tensile residual stresses were of no consequence in terms of fatigue life for the heat affected zone (HAZ). At this zone, the microstructural transformation (soft zone formation in the HAZ) induces a similar fatigue life behavior. For the weld metal produced by the MIEA welding process, the fatigue crack growth tends to increase in comparison with the GMAW, whereas for the HAZ no difference was detected. Fracture surfaces of all materials were analyzed by using scanning electron microscopy to correlate the fatigue crack growth condition with the striations formation.

Gas tungsten arc welding (GTAW) was used to join plates of Inconel 718. The mechanical properties were determined by tensile, microhardness, and instrumented Charpy impact tests. An ERNiFeCr-2 filler metal fed by a semi-automatic mechanism was used. Partial dissolution of the strengthening phases, γ′ and γ″, induced a soft region (~ 225 HV1.0) in the heat-affected zone (HAZ) during welding. The yield strength (371.3 MPa) of the as-welded joint is approximately 45% of the base material in aged condition (822.7 MPa). The welds were subjected to a hardening recovery post weld heat treatment (HRPWHT). Impact testing of the hardened welds revealed a reduction of 17% in energy absorbed with respect the aged base material. Hardness measurements showed an increase to ~ 410 HV0.1 in the fusion zone; however, due to segregation of Nb and formation of carbides, the precipitation of γ″ is not fully completed, and the yield strength (719 MPa) of the heat-treated welded joint is lower than the base material in aged condition.

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.

By using an instrumented impact pendulum, the force versus time curves of 7075-T651 aluminum welds were obtained from standard Charpy-V samples. Considering the force–time curves and constant impact velocity, the fracture energies for different zones were quantified. A fracture energy improvement for the HAZ (33.6 J) was observed in comparison with the weld metal (7.88 J), and base metal (5.37 J and 7.37 J for longitudinal and transverse directions, respectively). This toughness increment was attributed to the microstructural transformation caused by the thermodynamic instability of η′ precipitates during the welding. Fracture energy for weld metal was higher than that for base metal, probably due to pores created during solidification. Regarding the dynamic yielding force obtained from the force–time curves, an approximation to the dynamic yield strength for weld, HAZ and base metal was determined. Fracture surfaces revealed an intergranular failure for base metal in longitudinal direction, whereas a predominately brittle failure (cleavage) with some insights of ductile characteristics was observed for the transverse direction. In contrast, a ductile failure was observed for weld metal and HAZ.

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%).

The resistance spot welding (RSW) process was used to join dual-phase steels (DP590 and DP980). Mechanical properties of the welded joints were determined by microhardness measurements, tensile and high cycle fatigue tests. The hardness of different zones of the welded joints were represented by microhardness profiles. It was observed a hardness increment in the spot melting zone of around 150 HV0.5, with respect to the base materials (320 HV0.5 for DP980 and 200 HV0.5 for DP590). A soft region was detected in the heat affected zone of the DP980 material, whereas no soft zone was observed for the DP590 material. The shear strength for the welded joints was similar for both dual phase steels, however, a ductility increment for the DP590 steel was detected in comparison to the DP980. Wohler curves at a constant load ratio were obtained to determine the fatigue life of the welded joints. Failures of the joints were systematically initiated in the nugget zone. The fracture surfaces were analyzed by a scanning electron microscopy to elucidate the fracture mechanism of the welded joints.

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.

This paper is concerned with the effect of strain rate on the tensile properties of 6061-T6 aluminum alloy welds obtained by a novel welding method called the modified indirect electric arc technique. Experimental impact tests in welded joints and base metal were carried out by means of an inertial flywheel at a constant speed of 10 m s −1 . From the force-time evolution, dynamic behavior and impact fracture energy were determined in the case of base metal and welded joints. Additionally, by means of an ultra-high speed camera synchronized to the force signal, and employing digital image correlation method and monitoring markers, the true strain-time curves were obtained. These results enabled determination of the stress-strain behavior under dynamic conditions for the base metal and welded joints, which, in turn, were compared with those reported in quasi-static conditions. The strain rate does not have a strong influence on the mechanical properties of yield and tensile stress, contrasted with energy absorption capacity, which increased significantly. Finally, the effect of strain rate on the fracture surfaces was investigated.

The results of thermal efficiency on welding using the modified indirect electric arc technique [Ambriz RR, Barrera G, Garcia R. SI11:10–17] of the 6061-T6 aluminium alloy are presented. These values are in the range of 90–94%, depending on the preheat temperature and were obtained by means of a heat balance that considers the heat input, the amount of molten mass of the welding profiles, and welding parameters during the joining, giving particular emphasis to the data collected during the digitalization of the arc current. Also, some dimensionless parameters were employed in order to determine the approximation grade of the molten pool, the heat-affected zone, and their corresponding approximations to the experimental results.

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 in 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 ranged from 308 to 693°C, these measurements were related with the tensile test results and the failure zone reported previously by the authors (Ambriz et al., SI11:10–17). It was observed that there is a decrease in the mechanical strength of the welded joints, due to the microstructural changes undergone by the AA6061-T6 alloy in which formation of the β′ occurs according to the time–temperature transformation diagram. The inherent cooling conditions of the weld pool observed for the MIEA technique (single welding pass) have made it possible to establish the characteristics of solidification and microstructure for a specific cooling rate.

The results of the thermal efficiency on welding by modified indirect electric arc technique (MIEA) [1] of the 6061- T6 aluminum alloy are presented. These values are in a range of 90 to 94 %, which depend of the preheating employed. Thermal efficiency was obtained by means of a balance energy which considers the heat input, the amount of melted mass of the welding profiles, and welding parameters during the joining, especially of the arc current data acquisition. Also, some dimensionless parameters were employed in order to determine the approximation grade of the melted pool, the heat affected zone (HAZ), and their corresponding values with the experimental results.En el presente trabajo de investigación, se presentan los resultados de la eficiencia térmica en la soldadura de una aleación de aluminio 6061-T6 por medio de la técnica de arco eléctrico indirecto modificado (AEIM) [1]. Los valores se encuentran en un rango de 90-94 % dependiendo de la temperatura de precalentamiento y fueron obtenidos a través de un balance térmico que considera a la energía aportada por el arco eléctrico, la cantidad de masa fundida de los perfiles de soldadura obtenidos y las variables operativas en el proceso de unión, poniendo especial atención en los datos recogidos en el proceso de digitalización para la intensidad de corriente del arco eléctrico. También se em - plearon algunos parámetros adimensionales para determinar el grado de aproximación de la porción del baño fundido, el ancho de la zona afectada por el calor (ZAC) y sus correspondientes aproximaciones con los resultados experimentales.

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.