Microstructure Of Fusion Zone Research Articles

Multiresponse Optimization of Pulsed-Current Gas Tungsten Arc Welding (PCGTAW) for AISI 304 Stainless Steel to St 52 Steel Dissimilar Welds

AISI 304 stainless steel and St 52 steel were dissimilar welded together by pulsed-current gas tungsten arc welding (PCGTAW). The multiresponse Taguchi method was used to optimize the PCGTAW parameters for average microhardness and corrosion potential (E corr) of weld metal. For three factors (pulse current, background current, and pulse frequency) at two levels, an L4 (23) orthogonal array was selected. Microstructure and microhardness of weldment were evaluated by light microscopy and Vickers microhardness (HV0.3). Corrosion resistance of fusion zone in 3.5% NaCl solution was studied using potentiodynamic polarization. The ferritescope was also used to observe the ferrite content on the fusion zone. Multiresponse signal-to-noise ratios and analysis of variance on the measured data were used to identify the optimal levels and relative influence of factors on the variation of the multiple performance characteristics by assigning equal weights to response factors. The results show that the fusion zone microstructure exhibited skeletal delta ferrite in austenite matrix with different ferrite content. Within the selected parameter values, the optimum conditions for microhardness and E corr were found to correspond to the first level of pulse current (130 A), second level of background current (100 A), and second level of pulse frequency (4 Hz), for constant percentage on-time (50%). The pulse frequency, making a 82.436% contribution, was found to be an effective factor for improvement of microhardness and E corr of fusion zone, followed in importance by the background current factor with a 16.47% contribution, while the pulse current had less effect compared with the other factors. In this regard, good agreement between the predicted and experimental output factor results is shown.

Investigation of the Effect of Argon Arc Welding Parameters on Properties of Thin Plate of <i>In Situ</i> Titanium Matrix Composites

The weldability of in-situ titanium matrix composites (TMCs) was studied using the gas tungsten arc welding (GTAW). The effects of GTAW on the microstructure of fusion zone and heat-affected zone were discussed, and the changes of TiB whisker reinforcements in the welded joint were investigated by optical microscope (OM), scanning electron microscopy (SEM), XRD analysis and tension testing at room temperature. Research results show that the GTAW process is a suitable welding method for in-situ TMCs. Under reasonable welding parameters, the welded joints display goo weld seam formation, and TiB whiskers show distinctly smaller sizes and uniform distribution with a special network structure. The maximum tensile strength of welded joints can reach 92% of the base metal under optimum welding parameters.

Investigation of temperature and residual stresses field of submerged arc welding by finite element method and experiments

This article reports on a numerical and experimental investigation to understand and improve computer methods in application of the Goldak model for predicting thermal distribution in submerged arc welding (SAW) of APIX65 pipeline steel. Accurate prediction of the thermal cycle and residual stresses will enable control of the fusion zone geometry, microstructure, and mechanical properties of the SAW joint. In this study, a new Goldak heat source distribution model for SAW is presented first. Both 2D and 3D finite element models are developed using the solution of heat transfer equations in ABAQUS Standard implicit. The obtained results proved that the 2D axi-symmetric model can be effectively employed to simulate the thermal cycles and the welding residual stresses for the test steel. As compared to the 3D analysis, the 2D model significantly reduced the time and cost of the FE computation. The numerical accuracy of the predicted fusion zone geometry is compared to the experimentally obtained values for bead-on-plate welds. The predictions given by the present model were found to be in good agreement with experimental measurements.

Effect of post weld heat treatment on the microstructure and tensile properties of activated flux TIG welds of Inconel X750

This study addresses the effect of post weld heat treatment on the fusion zone microstructure and the mechanical properties of activated flux tungsten inert gas (A-TIG) weldments of Inconel X750. In this study, a compound flux of 50% SiO2+50% MoO3 was used for A-TIG welding of the samples. Comparative studies on the microstructure and mechanical properties have been made on the weldments both in the as-welded and post weld heat treated conditions. Direct ageing post weld heat treatment (PWHT) was carried out at 705°C for 22h on the A-TIG weldment to assess the structure–property relationships. It was inferred that direct ageing post weld heat treatment resulted in better tensile strength (1142MPa) compared to the as-welded coupons (736MPa). The joint efficiencies of the as-welded and post weld heat treated conditions were found to be 60.7% and 94.07% respectively. The impact toughness of the as-welded coupons were found to be greater than the post weld heat treated samples; however the impact toughness of the welds are greater than the parent metal employed in both the cases. This study also attested the detailed structure–property relationships of A-TIG weldments using the combined techniques of optical and scanning electron microscopy, Electron Dispersive X-ray Analysis (EDAX) techniques.

Effect of transverse mechanical arc oscillation on hot cracking (solidification & liquation) and weld metal properties of AA2014 T6 TIG welds

Effect of Transverse Mechanical Arc Oscillation on solidification and liquation cracking in autogenous and filler metals (standard AA4043 and non-standard AA1100 fillers) full penetration heat treatable AA2014 T6 aluminium alloy has been investigated. Both, linear and circular TIG welds were made using specially designed welding setup using Mechanical Arc Oscillation. Arc oscillation was carried out using range of oscillation parameters (amplitude: 0.9–2.0 mm and frequency: 0.28–1.5 Hz). Microstructure and mechanical tests studies were carried out to determine the extent of hot cracking, changes in the microstructure in weld fusion zone and adjacent, partially melted zone and weld mechanical properties respectively. Reduction/elimination of both solidification and liquation cracking due to arc Transverse Mechanical Arc Oscillation in autogenous and filler metal linear and circular TIG welds of AA2014 T6 alloy was significant. Optimum oscillation parameters: amplitude (0.9 mm) and frequency (0.48 Hz), significantly assisted to reduce/eliminate cracking in the fusion zone and adjacent partially melted zone. Interestingly, in these welds, fusion zone grain size was minimum, 12 μm as compared to 21.9 μm in without arc oscillation. Solidification cracking tendency had a non-monotonic dependence while liquation cracking showed monotonic dependence on the combination of arc oscillation parameters with AA1100 filler welds. AA4043 filler welds produced noticeable improvements in mechanical properties. Increments upto 16.6% in YS, 5.6% UTS, 34% ductility, and 17% joint efficiency.

Effect of base metal and welding speed on fusion zone microstructure and HAZ hot-cracking of electron-beam welded Inconel 718

Influences of base metal and welding speed on fusion zone microstructure and heat affected zone (HAZ) hot-cracking of electron beam welded (EBW) Inconel 718 were investigated systematically. Microstructures of fusion zone and HAZ were characterized using the optical microscope and a field emission scanning electron microscope, complemented by the microhardness measurements. It is found that the weld shape transforms from a stemless wine glass shape to a nail head shape with the increase of the grain size of base metal. The largest amount of Laves or the lowest hardness is obtained in the center of the weld pool, which mainly due to the strong microsegregation in this region. The microsegregation of weld pool is significantly affected by welding speed, which is associated with the rate of transference of energy to the weld. The occurrence of HAZ cracking is related to the grain boundary and the generation of porosities is induced by fracture of the brittle carbides. Furthermore, the susceptibility of HAZ cracking is greatly dependent on the grain size of base metal and welding speed.

Studies on microstructure and mechanical properties of keyhole mode Nd:YAG laser welded Inconel 625 and duplex stainless steel, SAF 2205

Abstract

Microstructure and mechanical properties of laser beam welded Al–Li alloy 2060 with Al–Mg filler wire

Abstract Alloy 2060-T8 is a newly developed high-strength Al–Li alloy for applications in aircraft industry. Crack-free welds were obtained in laser beam welding with 5087 filler wire under optimized welding conditions. In this paper, fusion zone microstructure and joint mechanical properties were investigated. Microstructure typical for the weld metal consists of α-Al matrix with a few nanoscale precipitates inside and a coarse icosahedral quasicrystalline T 2 phase at the dendritic and grain boundaries. The quasicrystalline occurred normally in Al–Li–Cu alloys with higher Li contents. Our investigations show that the icosahedral quasicrystalline phase T 2 phase forms in the laser-welded Al–Li alloy 2060 with lower Li content as a result of segregation and replacement of Mg element. The joint tensile strength in as-welded condition is around 317 MPa, about 63% of that of the base metal, and fracture occurs within the fusion zone.

Dissimilar Joining Between Austenitic and Duplex Stainless Steel in Double-Shielded GMAW: A Comparative Study

In the present work, the effect of double-layer shielding and five other process parameters, namely welding voltage, current, primary shielding gas type, its flow rate, and filler material, is studied during dissimilar gas metal arc welding (GMAW) between austenitic and duplex stainless steels (SSs). A simple modification over the GMAW setup is made for additional supply of secondary shielding gas at different flow rates. Two different sets of welding are performed between austenitic and duplex SSs, i.e., AISI 304 with Duplex 2205 and AISI 316 with Duplex 2205, and the contributions of process parameters, their interactions on joint distortion, tensile strength, toughness, and fusion zone microhardness are evaluated. Improvements in joint quality due to the double-shielding environment are also highlighted. Double-layer shielding with secondary shielding by CO2 supply significantly improves tensile strength and toughness and reduces distortion. Fusion and interface zone microstructures are observed by scanning electron microscopy to study the metallurgical behavior of joints fabricated under single- and double-layer shielding environment.

Microstructure and tensile properties of the laser welded TWIP steel and the deformation behavior of the fusion zone

Abstract Microstructure and tensile properties of the laser welded joint of Fe–18.8Mn–0.6C TWIP steel were investigated in this research. The microstructure of fusion zone (FZ) was characterized by means of X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). TEM and in-situ SEM observation were employed to investigate the microstructural evolution and strengthening mechanism of FZ during deformation. The welded joint with a fully austenitic structure was obtained by the laser welding. The granular divorced eutectic phases (Fe, Mn)3C and inclusions formed in the interdendritic regions during the solidification of FZ. The fully austenitic structure and coarse dendrite grains were responsible for the fracture at the weld seam. The FZ exhibited a good combination of strength (e.g. tensile strength up to 1000 MPa) and ductility (e.g. total elongation up to 73%). The microstructural evolution revealed that dislocation slip was the main deformation mechanism at low strains of FZ, while at relatively high strains, mechanical twinning was the domain deformation mechanism and played an important role in improving the strength and ductility as well as the work-hardening effect of FZ.

Mechanical Properties and Microstructure of Resistance Spot Welded Joints of AISI 409M Ferritic Stainless Steel

Resistance spot welding is widely used as a sheet metal joining process, especially in automotive and rail coach manufacturing industries. Ferritic stainless steel (FSS) is used for structural applications in these areas, owing to its superior corrosion resistance compared to conventional carbon steels. In this paper, mechanical properties and microstructure of resistance spot welded FSS joints to grade AISI 409M was investigated. Tensile shear test and micro hardness test were carried out to assess the mechanical properties and failure characteristics of weld joints. Microstructure was evaluated with optical microscopy. The effect of welding current on nugget growth, peak load, fusion zone (FZ) hardness, failure energy and failure mode were analyzed at various current ratings, keeping electrode force, weld cycle time and electrode tip diameter as constants. The results showed that, with the increment in welding current, tensile shear strength increased. Nugget diameter was found to be increasing with increase in current, in expulsion free welds. At low currents, failure mode was found to be interfacial and the pull out mode of failure occurred, at relatively higher current values. Average FZ hardness was found to be more than that of base metal (BM) but less than that of maximum hardness at heat affected zone. Grain enlargement was noticed at high temperature heat affected zone. FZ microstructure consists of predominantly columnar ferrite. Failure occurred at the BM region in all the samples welded above 10 KA current.

A comparative study on fiber laser and CO2 laser welding of Inconel 617

Abstract A comparative study on the influence of fiber laser welding (FLW) and CO2 laser welding (CLW) on the weld bead geometry and the microstructure of fusion zone (FZ) of Inconel 617 was investigated. In CLW joints, the weld bead geometry is Y-type shape. In FLW joints, the weld bead geometry transforms from Y-type to I-type with the decrease of the heat input. The minimum heat input required to achieve the full penetration of the weldment in FLW is lower than the CLW. The melting efficiency in FLW is higher than that in CLW. From the top to the root regions, the secondary dendrite arm spacing (SDAS) in fiber laser welded FZ undergoes a smaller change than that in CO2 laser welded FZ. The elements of Ti, Mo, Cr and Co segregate into the interdendritic regions both in FLW and CLW process. The second phases in CLW with the highest input of 360 J/mm are much larger and more than ones in FLW with the highest heat input of 210.5 J/mm.

Single pass hybrid laser–MIG welding of 4-mm thick copper without preheating

Abstract Laser–arc hybrid welding of highly reflective T2 copper was conducted and significant synergy effect was observed. Effects of primary welding parameters on the shape and integrity of copper joints were studied through bead-on-plate (BOP) welding test. Results of X-ray inspection and infrared photography showed that both porosity susceptibility and crack susceptibility were highly dependent on welding heat input or energy coupling efficiency, but had totally different trend, which narrowed the process window of BOP welding of copper. Hot crack and porosity were successfully suppressed by using a square butt joint configuration with a proper gap size. A 4-mm thick, defect-free butt joint was obtained by single-pass hybrid welding at a speed of 1.3 m/min without preheating the work piece. The microstructure, microhardness, mechanical properties and tensile fracture surface of the butt joint obtained were studied. Tensile strength and elongation rate of the butt joint obtained were about 87% and 56% of that of base metal, respectively. The lowered mechanical properties of butt welded copper joint were related to the softening of fusion zone (FZ) and its vicinity, the coarse microstructure in FZ and heat affected zone (HAZ), and the possible concentration of impurity elements at the central region of FZ.

Sensor based prediction of weld microstructure in pulsed MIG welding

Pulsed gas metal arc welding (P-GMAW) is often used to improve weld quality as well as productivity in today's manufacturing industries. Current pulsing is used in grain refinement of weld fusion zone and uniform microstructure with less heat input. In this work, weld microstructure and its hardness have been studied on low carbon steel in pulsed metal inert gas welding using bead on plate method. The primary objective was to compare the grain morphology and different phase percentage to hardness variation in various welding zones, especially in the interface of weld fusion zone and heat affected zone (HAZ), affected by pulse parameters. Various sensors have been used to monitor the weld microstructural features. The weld peak temperature along with arc heat input were found to be strongly correlated with weld ferrite content, grain size and its hardness. The arc sound is also an essential indicator of weld microstructure affected by arc stability.

Effect of welding polarity on bead geometry, microstructure, microhardness, and residual stresses of 1020 steel

This work examines the effect of welding polarity as a measure of heat input on thebead geometry, microstructure, microhardness and residual stresses of AISI 1020carbon steel that was processed by shielded metal arc welding (SMAW). Single weldbeads were deposited on the steel plate using a constant current but with differentwelding polarities of AC, DC- and DC+. Optical microscopy indicates that weldingby DC- provides the widest weld bead and largest heat affected zone (HAZ) due tothe large heat input subjected to the plate. Nevertheless, similar microstructures in theHAZ and fusion zone (FZ) of the weld were found for all welding polarities. Vickersmicrohardness tests also show that the large heat input by DC- polarity provided theminimum microhardness for all microstructures of the weld. In addition, across-weldmeasurements of the residual stresses by neutron diffraction indicate that the threewelding polarities produced similar profile. High tensile longitudinal residual stresseswere found to extend horizontally from the weld center to the HAZ, which becomecompressive further away from the bead. The highest tensile residual stress in theHAZ occurred for DC- polarity, while it has the lowest value for AC polarity. Throughthicknessmeasurements also indicate that the residual stresses within the HAZ areapproximately constant for AC polarity. This suggests that equal distribution of heatinput by AC polarity to the electrode and plate is not only important for reducingresidual stresses but also on minimizing the differences in residual stresses throughthe weld thickness.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE WELDING JOINT OF A NEW CORROSION- RESISTING NICKEL-BASED ALLOY AND 304 AUSTENITIC STAINLESS STEEL

With the fast development of industry, a serious global problem, pollution, becomes more apparent. A large number of wastewater is discharged, causing the environment pollution. Supercritical water oxidation(SCWO) becomes the most effective method to treat the wastewater within recent years, but the material used in the equipment plays a key role in restricting the application of the SCWO process. Currently, during the SCWO wastewater treatment process, 304 austenitic stainless steel, alloy 625, P91 and P92 steels are the mainly preheater and reactor materials. In order to reduce the serious corrosion and improve economic efficiency of the materials for this process, a new corrosion resistant Ni-based alloy(called X-2# alloy) has been developed with an aim of replacing the previous ones. In particular, it is highly important to the related behavior of this new alloy welding with the original SCWO. Therefore, the microstructure and mechanical properties of the welding joint of the new alloy and304 austenitic stainless steel with manual argon arc welding were investigated. The microstructure and fracture morphologies of the welding joint were analyzed through OM, SEM and EDS, and the detailed analysis of the micro- hardness, tensile strength and other mechanical properties were performed. The results demonstrated that the parent material with the typical 40~65 mm grains size is helpful for dissimilar steel welding, and the microstructurein fusion zone of X-2# side does not show welding defects. However, some ferrites are further formed near the fusion zone of 304 stainless steel sides. There are Cr-rich and Ni-poor distributions in the ferrites. The grain grows seriously in both the areas near the remelt zone and 304 stainless steel side of heat affected zones(HAZs), which affect heavily the performance of welding joint. In addition, the results also uncover that the Vickers-hardness is the minimum in the HAZ. At room temperature, the fracture location of the tensile tests of X-2#/304 is in the welding seam, whereas at 500 ℃ the corresponding position is in the 304 matrix. Due to the strengthening effects of Al, W and Mo elements, the high temperature mechanical properties of X-2# alloy have been found to be even better than those of the 304 austenitic stainless steel.

Microstructure–properties relationships in martensitic stainless steel resistance spot welds

The paper addresses the phase transformations and mechanical response of martensitic stainless steel resistance spot welds. The fusion zone microstructure consists of carbon-rich martensite together with a relatively high amount of retained delta ferrite along the grain boundaries with a transition in solidification mode from equiaxed to columnar dendritic grains across the fusion zone. The heat affected zone microstructure is featured by martensitic matrix together with carbide precipitation. The very high hardness of the fusion zone and the heat affected zone, the sharpness of the notch at sheet/sheet interface, which is located in the hard microstructural zone, and the presence of delta ferrite in the weld nugget play important roles in failure characteristics and mechanical performance of the joint.

An investigation on microstructure and properties of dissimilar welded Inconel 625 and SUS 304 using high-power CO2 laser

Microstructure and properties of laser deep penetration welded Inconel 625 nickel-based alloys and SUS304 stainless steel were investigated. Weld microstructures, room temperature tensile properties and stress rupture properties, impact toughness, and hardness of dissimilar welded joints were evaluated. The experimental results showed that the microstructure of fusion zone near the fusion line in the SUS304 side was mainly cellular, whereas that near the fusion line in the Inconel 625 was predominantly columnar dendrites. Laves phases were precipitated at the grain boundary of cellular and in the interdendritic regions of fusion zone in the different form. The white transition layer was generated, and the signification change of concentrations was occurred at the fusion boundary between weld metal and SUS304. The microstructure at the fusion boundary between weld metal and Inconel 625 was characterized by dendritic boundary melting and thickening and accompanied with a lot of Laves phases precipitated in the interdendritic regions. The tensile strength tests indicated that the dissimilar butt joints ruptured in the fusion zone. The toughness of dissimilar weld metal was declined sharply compared to the two base metals. Fractographic analysis revealed that segregation of Nb and Mo in the interdendritic regions deteriorated the tensile strength and toughness of laser dissimilar weld metal. The corrosion resistance of weld metal was higher than that of SUS304 as a considerable amount of Mo elements in the weld metal inhibiting the generation and development of pitting corrosion.

Microstructure, tensile properties of Ti-6Al-4V by ultra high pulse frequency GTAW with low duty cycle

The paper compares the characteristics of Ti-6Al-4V titanium alloy for conventional gas tungsten arc welding (C-GTAW) process with ultra high frequency pulse gas tungsten arc welding (UHFP-GTAW) on the conditions of 50% and 20% duty cycles. The effects of heat input on average intercept of grain size in the fusion zone (FZ), microstructure, tensile properties and fracture behavior were determined. These findings suggest that a lower heat input by UHFP-GTAW process was responsible for better grain size, microstructure and tensile properties except for the effect of ultra high frequency pulse current. In contrast with C-GTAW process, the average intercept of grain size in FZ decreased by 30% at most with 20% duty cycle. The morphology of basketweave structure predominated in the microstructure of FZ by UHFP-GTAW, moreover, a large density and uniform distribution of resultant basketweave were obtained with 20% duty cycle. The samples obtained by UHFP-GTAW generally displayed better ductility, in particular, when the duty cycle was reduced to 20%, the elongation and percentage reduction of area were respectively increased by 140% and 275%. The fractures with 20% duty cycle were mainly located in the base metal, such fractograph showed development of intense shear and deep dimple aggregation, which might be caused by improvement of the ductility.

Hybrid laser/arc welding of advanced high strength steel in different butt joint configurations

An experimental procedure was developed to join thick advanced high strength steel plates by using the hybrid laser/arc welding (HLAW) process, for different butt joint configurations. The geometry of the weld groove was optimized according to the requirements of ballistic test, where the length of the softened heat affected zone should be less than 15.9mm from the weld centerline. The cross-section of the welds was examined by microhardness test. The microstructure of welds was investigated by scanning electron microscopy and an optical microscope for further analysis of the microstructure of fusion zone and heat affected zone. It was demonstrated that by changing the geometry of groove, and increasing the stand-off distance between the laser beam and the tip of wire in gas metal arc welding (GMAW) it is possible to reduce the width of the heat affected zone and softened area while the microhardness stays within the acceptable range. It was shown that double Y-groove shape can provide the optimum condition for the stability of arc and laser. The dimensional changes of the groove geometry provided substantial impact on the amount of heat input, causing the fluctuations in the hardness of the weld as a result of phase transformation and grain size. The on-line monitoring of HLAW of the advanced high strength steel indicated the arc and laser were stable during the welding process. It was shown that less plasma plume was formed in the case where the laser was leading the arc in the HLAW, causing higher stability of the molten pool in comparison to the case where the arc was leading.