Microstructures and mechanical properties of a welded CoCrFeMnNi high-entropy alloy

The response of the CoCrFeMnNi high-entropy alloy to weld thermal cycles was investigated to determine its applicability as an engineering structural material. Two processes were used: high-energy-density, low-heat-input electron beam (EB) welding and low-energy-density, high-heat-input gas tungsten arc (GTA) welding. Weldability was determined through comprehensive microstructural and mechanical property characterisation of the welds. The welds did not develop solidification cracking or heat-affected zone cracks. The microstructures in weld fusion zones are similar to that in the as-cast materials, consisting of large columnar grains with dendrite. The dendrite arm spacing and the extent of elemental segregation were less in the welds than in the cast ingot, and also were less pronounced in the EB weld than in the GTA weld. Compositional microsegregation between dendritic cores and interdendritic regions of the welds was insignificant. Both welds exhibited slightly higher yield strengths than the base metal. The EB weld possessed comparable tensile strength and ductility to that of the base metal. In comparison, the GTA weld maintained ～80% of the base metal’s tensile strength and 50% of the ductility.     

Effects of base and filler chemistry and weld techniques on equiaxed zone formation in Al–Zn–Mg alloy welds

Abstract

Equiaxed zone (EQZ) formation in Al–Zn–Mg alloy welds as affected by base metal, filler metal chemistry and weld techniques is studied. Filler metal chemistry and welding techniques have great influence on the formation of EQZ microstructure as base metal composition has. In an effort to characterise the equiaxed grain zone formation in Al–Zn–Mg alloy welds two commercial Al alloys AA7018 and RDE40 were selected. Gas tungsten arc welding in continuous current, pulsed current and arc oscillation mode were applied to weld the base materials. The influence of Sc containing fillers have been studied and compared with the commercial filler material. Mechanical and metallurgical characterisation were carried out in the EQZ. Intergranular corrosion in EQZ was studied according to ASTM G 110-92. Results reveals that RDE40 with low solute contents showed wider EQZ but relatively better corrosion and mechanical properties compared to AA7018 EQZ. Gas tungsten arc welding in pulsed and arc oscillation mode fusion boundary region exhibits better corrosion and mechanical properties compared to continuous current mode welds. Addition of Sc to the AA5556 filler combined with pulsed mode resulted in elimination of EQZ, better corrosion and mechanical properties compared to welds made with conventional AA5556 filler and also the presence of Sc within the EQZ so called unmixed zone has been observed.     

Search for filler metal for welding of ferrous alloys to titanium

Abstract

The use of a filler metal to facilitate the gas tungsten arc (GTA) welding of ferrous alloys to titanium alloys has been investigated. Semi-empirical rules have been applied to identify alloying elements that would resolve the important problems of brittle intermetallic formation and weld cracking. Vanadium was found appropriate. The GTA welds between a low carbon steel and Ti–15V–3Cr–3Sn–3Al made with a vanadium filler wire resisted cracking better than comparable autogenous welds. However, the presence of a hard, brittle eutectic microconstituent along the ferrous side of the fusion boundary drastically limited the gain in weldability. As anticipated, analysis of GTA welds produced with vanadium filler wire suggested the presence of a ternary (Fe,Ti,V) single phase. Although cracking was reduced with vanadium, the practical benefit of a vanadium filler wire for GTA welding is small because the weld metals remain very hard and brittle.     

Response surface approach to optimize the pulsed current gas tungsten arc welding parameters of Ti−6Al−4V titanium alloy

Titanium alloy (Ti−6Al-−4V) is very widely used in the fabrication of advanced industrial equipment, combat vehicles, gas turbines, spacecraft and so on. The preferred welding process for titanium alloy is gas tungsten arc (GTA) welding due to its comparatively easier applicability and better economy. However, welding of titanium alloy leads to grain coarsening at the fusion zone and the heat-affected zone, and this often results in inferior weld mechanical properties and poor resistance to hot cracking. Hence, in this investigation an attempt has been made to refine the fusion zone microstructure of titanium alloy by using a pulsed current GTA welding process instead of a constant current GTA welding process. Further mathematical models were developed by means of a response surface method, which enabled the process parameters to be optimized to achieve a minimum grain size and maximum hardness in GTA welding of the alloy under study. The parameter optimization involved the use of a response surface, contour plots and Kuhn-Tucker conditions.     

Influence of scandium on weldability of 7010 aluminium alloy

Abstract

The commercial 7000 series aluminium alloys are based on medium strength Al–Zn–Mg and high strength Al–Zn–Mg–Cu systems. The medium strength alloys are weldable, whereas the high strength alloys are non-weldable. This is because the amount of copper present in these alloys gives rise to hot cracking during solidification of welds. As a result, the high strength Al–Zn–Mg– Cu base alloys are not used for applications where joining of components by welding is an essential step. In the present study, using a combination of qualitative Houldcroft test and quantitative Varestraint test, it is shown that a small addition of scandium to the commercial 7010 alloy reduces the hot cracking susceptibility during solidification of welds produced by the gas tungsten arc welding process. The improvement in weldability is found to be the result of the considerable grain refinement in the weld structure following the scandium addition. The results of microhardness and tensile tests are further described within the context of the present work to demonstrate that the 7010+Sc welds also exhibit a combination of improved strength and ductility.     

Solidification and microstructure modelling of welds in aluminium alloys 5754 and 6111

Abstract

A solidification and microstructure modelling approach has been developed to predict weld metal and heat affected zone (HAZ) characteristics. The freezing range and phase evolution in the weld metal region were predicted using thermodynamic and diffusion controlled growth calculations. The calculated freezing range was correlated with the weld solidification cracking tendency. A simplified analytical model was suggested to describe thermal cycles that are experienced by the HAZ. This analytical model was coupled with a published microstructure model for age hardenable alloys to predict the hardness variations across the HAZ. The above integrated approach was evaluated using experimental welds made on non­age hardenable 5754 (Al–Mg) and age hardenable 6111 (Al–Mg–Si) alloys using gas tungsten arc, electron beam, and gas metal arc welding processes.     

The Weldability of an Al-Li-Cu Alloy

The development of low-density aluminum-lithium alloys for aerospace applications provides industry with an attractive alternative to fiber-reinforced composites for reducing weight while maintaining the fabrication cost advantages of aluminum. In this study, gas metal arc (GMA), gas tungsten arc (GTA), and electron beam (EB) welding processes were used to evaluate the potential fusion weld-ability of alloy 2090 (Al-2.2 Li-2.7 Cu-0.12 Zr). The results indicate that, with proper filler alloy selection, 2090 may be easily welded with a low sensitivity to weld solidification cracking. The weld porosity of 2090, the source of which is hydrogen enriched surface oxides, may be eliminated by chemically or mechanically milling the surface prior to welding. The dominant modification in the heat-affected zone of 2090 weldments is dissolution of strengthening phases. This degradation is reduced by the use of high energy density heat sources, such as an electron beam, or post-weld thermal treatments.     

Effect of pulsed current GTA welding parameters on the fusion zone microstructure of AA 6061 aluminium alloy

AA6061 aluminium alloy (Al−Mg−Si alloy) has gathered wide acceptance in the fabrication of food processing equipment, chemical containers, passenger cars, road tankers, and railway transport systems. The preferred process for welding these aluminium alloys is frequently Gas Tungsten Arc (GTA) welding due to its comparatively easy applicability and lower cost. In the case of single pass GTA welding of thinner sections of this alloy, the pulsed current has been found beneficial due to its advantages over the conventional continuous current processes. The use of pulsed current parameters has been found to improve the mechanical properties of the welds compared to those of continuous current welds of this alloy due to grain refinement occurring in the fusion zone. In this investigation, an attempt has been made to develop a mathematical model to predict the fusion zone grain diameter incorporating pulsed current welding parameters. Statistical tools such as design of experiments, analysis of variance, and regression analysis are used to develop the mathematical model. The developed model can be effectively used to predict the fusion grain diameter at a 95% confidence level for the given pulsed current parameters. The effect of pulsed current GTA welding parameters on the fusion zone grain diameter of AA 6061 aluminium alloy welds is reported in this paper.     

Fusion zone grain refinement in aluminum alloy welds through magnetic arc oscillation and its effect on tensile behavior

Grain size reduction in weld fusion zones confers the advantages of an increased resistance to solidification cracking and an improvement in mechanical properties. Oscillation of the welding arc through an imposed alternating magnetic field is one of several approaches to modify weld solidification structures. In this study, gas tungsten arc welds were produced in two high strength, age hardenable aluminum alloys with and without an external magnetic field. Metallographic characterization revealed the degree of structural refinement produced by magnetic arc oscillation. The decrease in grain size was found to increase tensile elongation, while the effect on strength and age hardening response was only meager. The improvement in ductility was partially maintained in the peak aged condition also.     

Retention of austenite in the welded microstructure of a 0.16C–1.6Mn–1.5Si (wt.%) TRIP steel

In this work, the retention of austenite in post-welded microstructures of a 0.16C–1.6Mn–1.5Si (wt.%) TRIP steel is investigated. Fully penetrated welds are produced by means of gas tungsten arc (GTA) welding and laser beam (LB) welding. The microstructure, particularly retained austenite, is analyzed using optical microscopy, Vickers hardness measurements, X-ray diffraction and saturation magnetization. It is found that the GTA welded TRIP steel contains a relatively large fraction of retained austenite, which may benefit the weldability of this steel. A minimum hardness is found in the heat-affected zone (HAZ) next to a high hardness plateau after both LB and GTA welding as a result of a large fraction of ferrite. It is suggested that for TRIP steels, proper control of the formation and decomposition of retained austenite in the HAZ is important to prevent weld failure. The hardness is therefore not a sufficient indicator for the weldability.     

Metallographic and OIM study of weld cracking in GTA weld build-up of polycrystalline,directionally solidified and single crystal Ni based superalloys

Abstract

The repair of gas turbine components is of importance both commercially and scientifically to ensure cost effective repair schemes that will extend the lives of hot end components such as blades and stators. The present communication reports the results of a metallographic and orientation imaging microscopy study of weld cracking observed in the gas tungsten arc repair welds of a polycrystalline (IN738LC), a directionally solidified (Rene 80) and a proprietary single crystal (SX) alloy. The three alloys were welded with low, intermediate and high strength weld fillers, using a weld build-up approach rather than a conventional weld repair of a through thickness crack. This procedure would be applicable for example to worn area on the tips of turbine blades. Inhomogeneous initial microstructures and those from solidification processes led to extensive heat affected zone microfissuring in the IN738LC alloy, associated with MC carbide liquation, liquation of gamma prime (γ′), segregation of boron and strain effects from precipitation of γ′ in both single and double pass welds. As observed previously in a V shaped weld preparation, the extent of microfissuring in alloy IN738LC increased substantially from the use of the low and intermediate strength weld fillers, to extensive heat affected zone microfissuring by using the high strength IN738 filler. In the directionally solidified Rene 80 welds, due to the reduction in grain boundary area per unit volume, only minor heat affected zone cracking was observed, while the SX alloy did not crack at all. The absence of any cracks in the SX alloy welds despite the presence of stray grains in the fusion zone appears to be related to reduced stress levels in the welds due to the choice of welding technique and the welding parameters.     

Observations on welding of  2+O+&bgr; titanium aluminide

Abstract

The weldability characteristics of an  ₂+O+&bgr; titanium aluminide of nominal composition Ti–24Al–15.5Nb (at.-%) have been investigated. Conventional gas tungsten arc (GTA) and electron beam (EB) welds exhibited columnar fusion zone grains. Pulsed current and arc oscillated GTA welds exhibited predominantly equiaxed fusion zone grains. The microstructure of GTA welds and pulsed current GTA welds exhibited  ₂+O phases, whereas arc oscillated GTA welds and EB welds contained  ₂+O+&bgr; ₀/&bgr; ₂; however, &bgr; ₀/&bgr; ₂ is predominant in EB welds. The EB welds, which contained  ₂+&bgr; ₀/&bgr; ₂ microstructure, exhibited high strength and ductility compared with GTA welds. The observed microstructural variations are explained on the basis of possible weld thermal cycles and convective currents in the weld pool.     

Combined effect of inoculation and magnetic arc oscillation on microstructure and tensile behaviour of type 2090 Al–Li alloy weld fusion zones

Abstract

In the development of Al–Li alloys for aerospace structures, their behaviour during welding plays an important role. One way of improving weldability is to refine weld solidification structures, which can be achieved by a variety of means. In this work, a type 2090 Al–Li alloy was gas tungsten arc welded with two different filler materials corresponding to types 2319 (Al–6.3Cu) and 4043 (Al–5.2Si). Inoculation with titanium together with arc oscillation through an imposed alternating magnetic field was used to refine the weld fusion zone microstructures. Post-weld aging and tensile testing were employed to assess possible improvements in performance. It was found that the combined treatment of inoculation and magnetic oscillation resulted in fully equiaxed, fine grained structures and that this led to a noticeable increase in aging response and tensile properties, especially ductility.     

Grain refinement and hydrogen embrittlement in iron aluminide alloy FA129

Abstract

Iron aluminides are susceptible to hydrogen cold cracking during gas tungsten arc welding. Fine grained base materials have been shown to be more resistant to environmental embrittlement when tested in the presence of water vapour than coarse grained base materials. To study the effect of fusion zone grain size on cold cracking susceptibility, welds were produced using magnetic arc oscillation to refine the fusion zone grain structures. Tensile tests were conducted in varying water vapour atmospheres, on weldments with average fusion zone grain sizes ranging between 115 and 530 μm. Fracture strength data followed Hall–Petch behaviour and the effect of water vapour concentration was also incorporated into the traditional plot. The results of the tensile tests showed the finer grain size fusion zones were less susceptible to hydrogen cracking and more tolerant of high hydrogen concentration than coarse fusion zone grain structures.     

Gas-tungsten arc welding of AZ91 magnesium alloy

The gas-tungsten arc (GTA) welding behaviors of the commercial AZ91 magnesium alloy were examined in terms of process efficiencies and microstructure characteristics. This study focused on the effects of GTA welding process parameters (like welding current in the range of 100/300 A and welding speed in the range of 3.33/13.33 mm/s) on energy absorption by the substrate material. The dependences of arc and welding efficiency on the used process parameters were presented. The measurements revealed that the arc efficiency values ranged from 0.63 to 0.88. Melting efficiency was found to rise with both increasing welding current and speed. The analyses revealed a strong influence of the GTA welding process on the width and depth of the fusion zone and also on the refinement of the microstructure in the fusion zone. The results of dendrite arm size (DAS) measurements were presented. Additionally, the presence of a partially melted zone (PMZ) was disclosed.     

Improvement of Microstructure and Mechanical Behavior of Gas Tungsten Arc Weldments of Alloy C-276 by Current Pulsing

Superalloy C-276 is known to be prone to hot cracking during fusion welding by Gas Tungsten Arc method.Microsegregation occurring during cooling of fusion zone with consequent appearance of topologically close-packed phases P and l has been held responsible for the observed hot cracking. The present work investigated the possibility of suppressing the microsegregation in weldments by resorting to current pulse. Weldments were made by continuous current gas tungsten arc welding and pulsed current gas tungsten arc welding using ERNi Cr Mo-4 filler wire. The weld joints were studied with respect to microstructure, microsegregation, and mechanical properties. Optical microscopy and scanning electron microscopy were employed to study the microstructure. Energy-Dispersive X-ray Spectroscopy was carried out to evaluate the extent of microsegregation. Tensile testing was carried out to determine the strength and ductility. The results show that the joints fabricated with pulsed current gave rise to narrower welds with practically no heat affected zone, a refined microstructure in the fusion zone, reduced microsegregation, and superior combination of mechanical properties.     

Electrochemical corrosion behavior and microstructural characteristics of electron beam welded UNS S32205 duplex stainless steel

Different electrochemical techniques were used to study the corrosion behavior of UNS S32205 duplex stainless steel (DSS) welded autogenously using a single-pass by electron beam welding process, supplemented by microstructural characterization. Furthermore, a comparative study was also performed between multipass gas tungsten arc (GTA)-welded and EB-welded DSS for their microstructure and corrosion behavior. The differences in weld thermal cycle and chemical composition influenced the fusion zone microstructure of both the welds and eventually their corrosion properties. The general corrosion resistance of the EB weld was lower than the base metal and higher than the GTA weld despite its weld zone being characterized by a relatively unbalanced phase ratio (α/γ) in comparison to the GTA weld. However, the EB weld showed relatively higher susceptibility to pitting corrosion than the base metal and GTA weld due to its poor repassivation characteristics and poor resistance to pit growth.     

Effect of modified AA4043 filler on partially melted zone cracking of Al-alloy gas tungsten arc welds

Abstract

The effect of grain refiners (zirconium, Tibor and scandium) added to the fusion zone through AA4043 filler on the partially melted zone (PMZ) cracking in gas tungsten arc (GTA) welded AA6061 alloy was investigated. Welds of AA6061 in thermal tempers of artificially aged condition (T6) were made with continuous current and pulsed current techniques. Varestraint testing was carried out to study PMZ cracking in welds, and optical SEM examination performed to evaluate it. Addition of grain refiners to the fusion zone improved the PMZ cracking resistance very significantly. Pulsed current technique was also found to improve the resistance to PMZ cracking, as a result of the possible reduction in the strain in the PMZ owing to the ductile fine grained fusion zone. Severe PMZ cracking in the welds of AA4043 filler without grain refiners was attributed to a greater amount of silicon rich eutectic at the grain boundaries of the PMZ.     

Studies on weldability of iron-based powder metal alloys using pulsed gas tungsten arc welding process

The extended use of powder metal components can be improved by the use of welding joining methods. This work investigates the weldability of iron-based powder metal alloys (Fe–Ni, Fe–Ni–P alloys) using the pulsed gas tungsten arc welding process (GTAW) with three different filler metals (AWS R 70S-6, AWS R 309L, AWS R Fe–Ni). Results revealed that the Fe–Ni powder metal alloy does not present any metallurgical difficulty concerning the weldability for all types of filler metal studied. The Fe–Ni–P powder metal alloy, microstructural examinations showed that, despite its high content of phosphorus (0.25 wt%), the utilization of pulsed GTAW process with stainless steel 309L filler metal resulted in welds free of porosities and solidification cracks. Metallographics examinations suggest that the absence of solidification cracks in this alloy can be mainly attributed to the presence of delta ferrite in the stainless steel weld metal which absorbed part of the phosphorus and significantly reduced the formation of the Fe₃P low-melting eutectic in the weld pool during cooling. In contrast, solidification cracks were observed when joining the Fe–Ni–P powder metal alloy using RFe–NI and R70S-6 filler metals. Hardness tests carried out indicated a heat affected zone (HAZ) with no excessive hardening for all alloys studied. Furthermore, tensile tests showed that the fractures always occurred in the base metal with tensile strength slightly superior to the value of unwelded samples. As a result, this investigation showed the feasibility of joining iron-based powder metal alloys by the pulsed GTAW process since a rigid control of the heat input is implemented together with an adequate choice of the filler metal, especially when welding the Fe–Ni–P alloy.     

Friction welding of Cr-Zr copper alloy

Copper and copper alloys have a higher thermal conductivity than even aluminium, itself regarded as having good heat conduction. When materials with high thermal conductivity are joined, the heat rapidly diffuses in the base metal in a way that generally results in poor weldability due to the difficulty of concentrating heat in welds. To obtain sound welds with satisfactory penetration during fusion welding of copper alloys, it is necessary to ensure high-temperature preheating and high interpass temperatures. Copper alloys are also reported to face problems such as solidification cracking and blowhole formation.¹ To solve these problems, it is necessary to perform welding at low heat input for a short time, which are conflicting requirements because of the high heat conduction.