Influence of hydrogen charging on mechanical properties of gas tungsten arc weldments of aluminium-lithium alloy 8090

The effect of hydrogen charging on the mechanical properties of gas tungsten arc welds (GTAW) of aluminium-lithium alloy 8090 (2 mm thick rolled sheets) was studied using cathodic hydrogen charging. To stimulate an increased amount of hydrogen into welds, the charging current density was increased through a galvanostatic circuit. The deleterious effect of hydrogen on ductility is documented in terms of degradation in tensile ductility (reduction in area and elongation-to-failure). Microscopic analysis was performed to characterize the microstructure and grain morphology of the weldments. Hardness measurements revealed an increase in hardness of the charged welds over the uncharged counterpart. Scanning electron microscopy observations of uncharged welds revealed a mixed mode failure with predominantly ductile rupture. Although, the charged welds exhibited a near similar mode of failure to that of the uncharged welds, extensive planar slip deformation was observed near the outer surface of the uncharged welds. The change in fracture mode from the outer surface to the central portion of the charged welds is attributed to intrinsic differences in hydrogen densities. An attempt has been made to rationalize the role of hydrogen on tensile properties and quasi-static fracture behaviour of the GTAW welds.     

Dynamic fracture toughness of heavy section, narrow gap, gas tungsten arc weldments

Dynamic fracture toughness tests were performed on three, ASME SA533 Gr A Cl 2 narrow gap, gas tungsten arc weldments (minimum yield strength equals 70 ksi, 485 MPa). Linear elastic K_Id results were obtained at low temperatures while J-integral techniques were utilized to evaluate dynamic fracture toughness over the transition and upper shelf temperature ranges. Loading rates in terms of K averaged 4.41 × 10⁴ksi√(in.)/sec (4.88 × 10⁴MPa√(m)/sec). Tensile, Charpy impact and drop weight nil ductility transition (NDT) tests were also performed. The dynamic fracture toughness of both stress relieved (24 hr at 1125°F, 607°C) plus quenched and tempered SA533 Gr A Cl 2 narrow gap, gas tungsten arc weldments: (a) easily transcended the ASME specified minimum reference toughness K_IR curve, and (b) significantly exceeded the fracture toughness demonstrated by lower strength, stress relieved (3/3.5 hr at 1125°F, 607°F) SA533 Gr A Cl 2 automatic submerged arc weldments.     

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

Abstract

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.     

Abnormal distribution of microhardness in tungsten inert gas arc butt-welded AZ61 magnesium alloy plates

In this study, the effects of heat input on the distribution of microhardness of tungsten inert gas (TIG) arc welded hot-extruded AZ61 magnesium alloy joints were investigated. The results show that with an increase of heat input, the distributions of microhardness at the top and bottom of the welded joints are different because they are determined by both the effect of grain coarsening and the effect of dispersion strengthening. With an increase of the heat input, the microhardness of the heat-affected zone (HAZ) at the top and bottom of welded joints and the fusion zone (FZ) at the bottom of welded joints decreased gradually, while the microhardness of the FZ at the top of welded joints decreased initially and then increased sharply. The reason for the abnormal distribution of microhardness of the FZ at the top of the welded joints is that this area is close to the heat source during welding and then large numbers of hard β-Mg₁₇(Al,Zn)₁₂ particles are precipitated. Hence, in this case, the effect of dispersion strengthening dominated the microhardness.     

Study on the temperature measurement of AZ31B magnesium alloy in gas tungsten arc welding

This paper is based on the Finite Element Analysis (FEA) to study the AZ31B Magnesium Alloy welding temperature filed, using a convenient, non-contact and fast response measured temperature method—Infrared Radiation (IR), the welding temperature field of AZ31B magnesium alloy plate in Gas Tungsten Arc Welding (GTAW) is measured by IR, the isothermal map of magnesium alloy plate is measured using IR device. The cooling curves are measured by thermocouple. Experiments and simulations by FEA are carried out to investigate the welding temperature field. The simulated results showed good agreement with the experiment ones.     

Investigations on the microstructure and mechanical properties of multi-pass pulsed current gas tungsten arc weldments of Monel 400 and Hastelloy C276

This research article reported the weldability, microstructure and mechanical properties of the dissimilar combinations of nickel alloys such as Monel 400 and Hastelloy C276. Multi-pass pulsed current gas tungsten arc (PCGTA) welding was employed for joining these dissimilar metals using ERNiCrMo-3 filler. Interface microstructures showed the absence of unmixed zone at the HAZ of both the sides. It was evident from the studies that all the tensile failures occurred at Monel 400 side. The average impact toughness portrayed by these dissimilar weldments was found to be 41 J. Bend test results showed that these dissimilar combinations offer augmented ductility. The outcomes of the study substantiated the use of current pulsing for the successful joints of Monel 400 and Hastelloy C276 by correlating the mechanical and metallurgical properties.     

TiB2-reinforced composite coating by gas tungsten arc welding

In situ synthesized TiB₂-reinforced Fe-based coating was fabricated by gas tungsten arc welding (GTAW) on AISI-4340 steel substrate using cheaper Fe–Ti, Fe–Cr, Fe–W, Fe–B alloys and B₂O₃ powders. The effects of processing parameters on the coating were investigated experimentally. Primary dendrites of ferrite (α) phase and complex TiB₂, Fe₂B borides were detected at the coated surface. The experimental results show that either coated surface or interface microstructures were formed by the distribution of particularly boron and titanium concentration. The difference in hardness of the microstructures is specifically attributed to the type of borides. The type, dimension, and the volume concentration changes of borides were correlated with the parameters as the concentration of additives and the dilution from the base material. The surfaces were subsequently characterized by scanning electron microscopy (SEM), the energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), and differential thermal analysis (DTA).     

Microstructural characteristics of gas tungsten arc synthesised Fe-Cr-Si-C coating

Abstract

The gas tungsten arc (GTA) method was used to synthesise Fe-Cr-Si-C alloy coatings, and processing effects on the coating were investigated experimentally. Coatings were developed on an AISI type 1040 steel substrate. Four different regions were obtained in the surface coating; and in these regions either a hypoeutectic or a hypereutectic microstructure was found. The hypoeutectic microstructure consisted of primary dendrites of austenite (γ) phase and eutectic M₇C₃ (M=Cr,Fe) carbides. On the other hand, the hypereutectic microstructure consisted of M₇C₃ primary carbides and eutectic. A hypoeutectic or hypereutectic microstructure was determined by the combination of particularly carbon concentration, solidification rate, and extent of substrate melting. The higher hardness of the hypereutectic microstructure is attributed especially to the formation of M₇C₃ primary carbides. The lower hardness of the hypoeutectic microstructure is related to three effective parameters: first, the presence of γ phase in the primary dendrites; second, excessive dilution from the base material; and third, relatively low concentrations of chromium and carbon.     

Synthesis of Fe based ZrB2 composite coating by gas tungsten arc welding

Abstract

ZrB₂/Fe composite coating was in situ synthesised by gas tungsten arc welding cladding process on AISI 1020 steel. Zr, B₄C and Fe–B alloy powders were used as precursor powders. The phase composition and microstructure were investigated by X-ray diffraction analysis, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Microhardness of ZrB₂/Fe composite coating at room temperature was examined. Main phases obtained from Zr and B₄C precursor are ZrB₂ and α-Fe, and those obtained from Zr and Fe–B precursor are ZrB₂ and FeB. In the upper part of these composite coatings, ZrB₂ phase mainly grows along temperature gradient direction. The middle part of these composite coatings has the highest ZrB₂ content and highest microhardness. Gradient dispersions of ZrB₂ reinforcements appeared in the composite coating from the middle to the bottom, leading to gradient dispersions of microhardness. With decreasing dilution rate, ZrB₂ content and microhardeness increase.     

Corrosion behavior of aluminum 6061 alloy joined by friction stir welding and gas tungsten arc welding methods

Wrought aluminum sheets with thickness of 13 mm were square butt-welded by friction stir welding (FSW) and gas tungsten arc welding (GTAW) methods. Corrosion behavior of the welding zone was probed by Tafel polarization curve. Optical metallography (OM) and scanning electron microscopy together with energy dispersive spectroscopy (SEM-EDS) were used to determine morphology and semi-quantitative analysis of the welded zone. FSW resulted in equiaxed grains of about 1–2 μm, while GTAW caused dendritic structure of the welded region. Resistance to corrosion was greater for the FSW grains than the GTAW structure. In both cases, susceptibility to corrosion attack was greater in the welded region than the base metal section. T6 heat treatment resulted in shifting of the corrosion potential towards bigger positive values. This effect was stronger in the welded regions than the base metal section.     

The properties of gas tungsten arc deposited SiCP and Al surface coating on magnesium alloy AZ31

SiC particles and aluminum powders were used to deposit on the surface of magnesium alloy AZ31 by pulse square-wave alternating current gas tungsten arc (GTA) processing. This method is an effective technique in producing a high performance surface modified composite layer. The microstructure, microhardness, wear resistance and corrosion behavior of the GTA surface modified composite layer were evaluated. It was proved that no reaction products were formed at the SiC-matrix interface and no melting or dissolution of the SiC particle occurred during GTA surface modification. The microhardness of GTA surface modified composite layer was between 100 and 150 HV according to the variation of the GTA processing parameters. The microhardness, wear resistance and corrosion behavior of the GTA surface modified layer were superior to that of the as-received AZ31. The optimum processing parameters of the GTA surface modification of magnesium alloy AZ31 with SiC + Al for the formation of a homogeneous crack/defect-free and grain refinement microstructure were established.     

Rapid prototyping of 5356-aluminum alloy based on variable polarity gas tungsten arc welding: process control and microstructure

This paper concentrates on rapid prototyping of a 5356-aluminum alloy based on a new deposition process of variable polarity gas tungsten arc welding (VPGTAW), and describes the microstructure and geometrical properties of the deposited layers. The wettability and distortion tendency of the deposited layers is effectively improved by preheating the substrate up to 118°C, monitoring the arc-length, and adjusting the arc current during the deposition process. The relationships between the geometry of the deposited layers and the welding parameters are developed. The surface roughness of the deposited parts is found to be in the order of 2 μm. The deposited layers exhibit equiaxed dendrites at the top layer, fine equiaxed grains at the middle, and bottom of a deposited wall together with some precipitates distributed at the grain boundary regions, and coarse columnar grains at the bonding zone between the deposited wall and the substrate. The residual microstructure such as grain size and distribution of precipitates is highly dependent on the related locations in the deposited wall. The deposited samples possess a maximum hardness at the top layer and exhibit a slight decreasing trend towards the middle and bottom of the fabricated part due to the heat effects of the material that occurs during the deposition. By understanding these relationships between parameters and their effect on the process output, the process can be used more effectively and the quality be improved as well.     

Optimization of magnetic arc oscillation process parameters on mechanical properties of AA 5456 Aluminum alloy weldments

The present work pertains to the improvement of mechanical properties of AA 5456 Aluminum alloy welds through magnetic arc oscillation process. Taguchi method was employed to optimize the magnetic arc oscillation welding process parameters of non-heat treatable AA 5456 Aluminum alloy welds for increasing the mechanical properties. The same optimum condition was observed in all the properties. Regression models were developed. The effect of welding current, welding speed, amplitude and frequency on mechanical properties was also studied. Analysis of variance was employed to check the adequacy of the developed models. Microstructures of all the welds were studied and correlated with the mechanical properties.     

Underwater repair of fatigue cracks by gas tungsten arc welding process

A fatigue crack repair method for offshore jacket tubular joints is presented. This method is applicable for fatigue cracks detected and sized by regular inspection techniques such as magnetic particle inspection up to a crack depth in the range of 5 mm. The main advantage of the method relies on its easy implementation allowing to be applied immediately after the crack is detected because equipment required is easy to transport and operate; so, it can be on board of a regular inspection campaign ship. A welding procedure specification has to be prepared and qualified prior to implementation of the repair method, considering particular parameters of the jacket fleet and selecting the welding equipment to be used. The proposed method could reinstall the original fatigue life in early stages of growth avoiding the need of more expensive procedures of repair such as joint clamping.     

Keyhole gas tungsten arc welding of AISI 316L stainless steel

Keyhole gas tungsten arc welding (K-TIG) was used to weld AISI 316L stainless steel of mid-thickness (thickness ranging 6–13 mm). 316L plates of 10-mm thickness were jointed using an I-groove in a single pass without filler metal. The effects of welding parameters on the fusion zone profile were investigated. The weld properties, including mechanical properties, microstructure, and corrosion resistance, were analyzed. The primary weld microstructures were austenite and δ-ferrite. The tensile strength and impact property of the weld were almost the same as those of the base metal, while the corrosion resistance of the weld was even better than that of the base metal. High-quality 316L stainless steel joints can be realized through K-TIG welding with high productivity and low processing cost. The practical application of K-TIG welding to join mid-thickness workpieces in industry is well demonstrated and an ideal process for welding AISI 316L of mid-thickness with high efficiency and low cost is presented.     

Optimization of gas tungsten arc welding process by response surface methodology

Gas tungsten arc welding is widely used for connecting of boiler parts made of A516-Gr70 carbon steel. In this study important process parameters namely current, welding speed and shielding gas flow rate were optimized using response surface methodology (RSM). The simultaneous effects of these parameters on tensile strength and hardness were also evaluated. Applying RSM, simultaneous effects of welding parameters on tensile strength and hardness were obtained through two separate equations. Moreover, optimized values of welding process parameters to achieve desired mechanical properties were evaluated. Desired tensile strength and hardness were achieved at optimum current of 130 A, welding speed of 9.4 cm/min and gas flow rate of 15.1 l/min.     

Effects of Gravitational orientation on the microstructural evolution of gas tungsten arc welds in an Al-4 wt% Cu alloy

Gas tungsten arc (GTA) welds on Al-4 wt% Cu alloys were investigated to determine effects of gravitational orientation on the weld solidification behavior. A bead-on-plate welding was performed by varying the relation between the arc translation direction and gravity vector, e.g., parallel-up, parallel-down, and perpendicular orientations. A solidification rate (V _S) was calculated from the measured grain orientation, and a thermal gradient (G _L) was estimated from the observed weld pool shape following a linear relation. A primary dendrite spacing (₁) decreased continuously from the s-l boundary to the weld pool surface regardless of the gravitational orientations. Larger ₁ for the parallel-up weld was observed near the boundary and surface than that of the perpendicular and parallel-down welds, which is believed to be associated with a smaller G _L due to larger weld pool dimension and with different solidification morphology. A solidification morphology and orientation in the perpendicular and parallel-up welds was comparable with a loss of columnar directionality near the weld surface and a continuous grain orientation. However, the parallel-down weld exhibited more columnar structure near the surface, which might be associated with the larger G _L and relatively mild convection flows. Outward convection flows in the parallel-down weld might be inhibited because of its reverse direction with respect to the gravity vector. This resulted in abnormal S shape of the trailing s-l interface and the V _S, which was receded toward the weld pool center. Based on these findings, significant influence of gravitational orientation resulted in the variation on the weld pool shape associated with convection flows, which in turn affected solidification orientation/morphology and the primary dendrite spacing.     

Microstructure and mechanical performance of pulsed current gas tungsten arc surface engineered composite coatings on Mg alloy reinforced by SiCp

A new pulsed current GTA surface-modified process was used to fabricate composite layer on the surface of Mg alloy AZ31. Current pulsing enhances fluid flow, reduces temperature gradients and causes a continual change in the weld pool size and shape, so that it is responsible for refining the solidification structure in the composite layer. The observed grain refinement was shown to result in an appreciable increase in composite layer bend strength. Composite layers with lower scan speed have higher bend strengths and they also seem to have “good” metallurgical bond with the substrate thus showing better mechanical behavior than the other higher scan speeds used in this present study. The wear rate of the composite layer decreases linearly with increase in SiCp volume fraction and the wear resistance of composite layer varies inversely with square of the reinforcement size. Composite layers with higher H/E have smaller accumulative strain, smaller accumulative strain energy, and thus better wear resistance. The wear mechanism was oxidation at low-applied load levels and adhesion/delamination at high-applied load levels.