Microstructure Of Fusion Zone Research Articles

Microstructure and tensile properties of Inconel 718 pulsed Nd-YAG laser welds

Pulsed Nd-YAG laser welding characteristics of Inconel 718 sheet material (2 mm thick) were investigated. Welds were subjected to three different post-weld heat treatments: direct aging, 980 °C solution treatment + aging (980STA), and 1080 °C solution treatment + aging (1080STA). Weld microstructures and room temperature tensile properties were evaluated. Weld tensile properties in direct aged condition were found to be inferior in relation to the base metal due to the presence of Nb-rich brittle intermetallic Laves phase in fusion zone microstructure. Solution treatment at 980 °C was found to result in considerable dissolution of Laves phase, leading to some improvement in weld properties, though not to the level of base metal. The treatment was also found to result in precipitation of needle-like δ-phase around partially dissolved Laves particles. Solution treatment at 1080 °C was found to result in complete dissolution of Laves phase, but also in significant grain coarsening in the base metal, which led to considerable deterioration in base metal properties.

Control of Laves phase in Inconel 718 GTA welds with current pulsing

The presence of Nb rich Laves phase in Inconel 718 weld fusion zones is known to be detrimental to weld mechanical properties. In the present study, an attempt was made to control the formation of Laves phase in alloy 718 gas tungsten arc welds using pulsed current. Welds were produced in 2 mm thick sheets of the alloy with constant current and pulsed current and were subjected to post-weld solution treatment at 980°C followed by aging. Detailed microstructural studies and tensile tests at 650°C were conducted. The results show that the use of current pulsing (i) refines the fusion zone microstructure, (ii) reduces the amount of Laves phase and exerts a favourable influence on its morphology and (iii) improves the response of the fusion zone to post-weld heat treatment and weld tensile properties.

Effect of pulse frequency in gas tungsten arc welding of powder metallurgical preforms

Pulsing of the welding current is one approach for refining the fusion zone microstructure in materials joined by fusion welding. The effect of current pulse frequency on weld bead microstructure, tensile strength, and hardness in joining of powder metallurgical steel preform sheets to wrought copper was studied. Considering weld strength as the quality characteristic in the selection of process parameters, the Taguchi method is used to analyse the effect of each process parameter individually and of their interaction on weld strength, and subsequently to determine the process parameters leading to optimum weld strength. The application of pulsed current causes iron dendrites to become more equiaxed and uniformly distributed owing to dendrite fragmentation. Further, there was an optimum frequency range over which the microstructural refinement was maximal. The same optimum frequency range corresponded to maximum tensile strength. Enhanced fluid flow and reduced thermal gradients are thought to be responsible for refining the solidification structure and the resulting stronger welds.

The microstructural response of delta-stabilized plutonium to pulsed laser welding

The effects of the welding parameters on weld-bead morphology and the microstructural evolution of autogenous conduction-mode melted laser welds in gallium delta-stabilized plutonium were investigated. This investigation demonstrates that delta-stabilized plutonium is easily welded with a pulsed Nd:YAG laser. Variations in the welding parameters created a range of heat inputs that had a significant effect on the size and morphology of the resultant weld and associated heat-affected-zone (HAZ) microstructure. Unlike other low-melting-temperature materials such as aluminum alloys, neither fusion-zone porosity nor hot cracking was observed. The reactivity of plutonium promotes the rapid formation of surface oxides that inhibit complete weld-joint fusion. The high viscosity of the plutonium molten pool impeded the breakup of the surface oxide by fluid motion, thus further impeding joint fusion. Modifications to input parameters to affect changes in the weld-pool flow were only partially successful in improving joint fusion. Analysis of the weld region using optical and electron microscopy and electron microprobe analysis revealed fusion-zone microstructures consisting of fine acicular delta grains, with a limited amount of retained alpha phase. The Pu6Fe, the low-melting-point intermetallic phase, was observed in the base material and the partially melted HAZ. Further investigation into the solid/solid phase transformations, specifically the effect of the cooling rate, during the cooling of the weld will be required to describe fully the microstructural development of the fusion zone.

Microstructural study of weld fusion zone of TIG welded IN 738LC nickel-based superalloy

The weld fusion zone microstructure of a commercial aerospace superalloy IN 738 was examined. Elemental segregation induced interdendritic microconstituents were identified to include terminal solidification product M 3B 2 and Ni 7Zr 2 in association with γ–γ′ eutectic constituent, which require proper consideration during the development of optimum post weld heat treatment.

Fatigue crack growth of AISI 304 stainless steel welds in air and hydrogen

Fatigue crack growth behavior of AISI 304 stainless steel plate and its plasma weld was evaluated in laboratory air and gaseous hydrogen. Observed variation in crack growth characteristics are explained based on the martensitic transformations near the crack tip due to mechanical loading and environmental action. The microstructures of fusion zone consisted of lacy ferrites in the columnar grain and subgrain boundaries within the austenite matrix, versus equiaxial austenite grains with some twins inside the parent metal. The threshold Δ K of the as-welded (AW) specimen required to initiate crack growth was clearly higher than that of the other specimens, regardless of testing environments. The considerable retardation of crack growth vanished for welds subjected to stress-relief treatment. In addition, all specimens tested in gaseous hydrogen exhibited enhanced crack growth. The fatigue-fractured surface of welds was found to be very rough suggesting a zig-zag crack path, unlike the straight path in the base metal. The lack of twin boundary failures in addition to the irregular solidified microstructure could be the causes for the improved resistance to fatigue crack growth in case of the stress-relieved weld. Moreover, the thin martensite layer formed on the cracked surface was correlated with the hydrogen-enhanced crack growth, resulting in inducing extensive quasi-cleavage (QC) fracture for the specimens tested in hydrogen.

Gas tungsten arc welding studies on similar and dissimilar combinations of Al-Zn-Mg alloy RDE 40 and Al-Li alloy 1441

The present study is concerned with gas tungsten arc welding of two high strength aluminium alloys, namely, an Al-Zn-Mg alloy (RDE 40) and an Al-Li based alloy of Russian grade 1441. One of the critical requirements of these two alloys is that they should be weldable. In the present work, weldability aspects of these alloys were studied in terms of solidification cracking tendency, microstructure, tensile properties, and microhardness across the welds. These studies were extended to dissimilar welds between RDE 40 and 1441 produced via conventional gas tungsten arc (GTA) welding as well as pulsed current GTA welding. It was found that RDE 40 welds were less sensitive to solidification cracking and weld metal porosity compared with 1441 alloy. The superior weldability of RDE 40 was related to the equiaxed nature of the fusion zone and a lower sensitivity to moisture pickup. It was possible to produce RDE 40-1441 welds without defects. Pulsed current welding of RDE 40 to 1441 showed improved mechanical properties compared with conventional GTA welding, and these were related to the refinement of the fusion zone microstructure.

Effect of the ratio of peak and background current durations on the fusion zone microstructure of pulsed current gas tungsten arc welded Al-Li alloy

Effect of the ratio of peak and background current durations on the fusion zone microstructure of pulsed current gas tungsten arc welded Al-Li alloy

Microstructure and Mechanical Properties of Weld Fusion Zones in Modified 9Cr-1Mo Steel

Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties. While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern. The design of appropriate welding consumables is important in this regard. In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182. Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h. The joints were characterized in detail by metallography. Hardness, tensile properties, and Charpy toughness were evaluated. Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low. Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions. However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C. The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure.

A Study of Fusion Zone Microstructures of Arc-Welded Joints Made from Dissimilar Aluminum Alloys

Arc welding has proven itself to be an economically affordable and efficient method for the joining of a wide variety of aluminum alloy structures that find extensive use in the industries of ground transportation and building construction. Welded joints, having a “T” configuration, in dissimilar aluminum alloys were produced using the semiautomatic arc welding process. In this study, a combination of a non-heat-treatable aluminum-magnesium alloy and a heat-treatable aluminum-magnesium-silicon alloy was successfully welded. Optical microscopy was used to characterize the fusion zone microstructures of the fillet-welded T joints. The intrinsic microstructural features and the development and presence of defects are highlighted.

Comparison of keyhole and conventional gas tungsten arc welds in commercially pure titanium

Keyhole gas tungsten arc welding (GTAW), a novel process recently developed at CSIRO Manufacturing Science and Technology, Australia, has been used to join commercially pure (CP) titanium. The process enables single pass, out of chamber welding of 12.7 mm thick CP titanium without expensive filler metal addition or joint preparation. The microstructure and mechanical properties of the resultant weld joint have been compared with those of a multipass conventional GTAW joint in CP titanium prepared from plates of the same thickness with a double-V edge preparation and the addition of a matching filler metal. It was shown that the differences in the net heat input and weld thermal cycles associated with the two processes led to similar microstructures, albeit of different degrees of refinement. As a result, the tensile and hardness properties of the two welded joints were similar to each other and comparable to those of the base material. The acicular nature of the fusion zone microstructure in both welds led to a significant increase in their notch toughness over that of the base material. It is concluded that the keyhole GTAW, with its significantly higher productivity combined with the simplicity of proven technology and low capital investment requirements, can be successfully applied in the welding of heavy section CP titanium, without sacrificing the metallurgical quality associated with the GTAW process.

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

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.

An investigation of fusion zone microstructures of welded aluminum alloy joints

Fillet weld “T” joints were produced using dissimilar aluminum alloys by a manual arc welding process. In this study, a heat treatable aluminum–magnesium–silicon alloy and a non-heat treatable aluminum–magnesium alloy were joined. The microstructure of the fusion zone was characterized by optical microscopy. Results reveal that the manual welding process produces adequate fusion, while minimizing weld structure defects.

Fusion zone microstructure and porosity in electron beam welds of an α+β titanium alloy

The effect of electron beam welding parameters on fusion zone (FZ) microstructure and porosity in a Ti - 6.8 Al - 3.42 Mo - 1.9 Zr - 0.21 Si alloy (Russian designation VT 9) has been investigated. It has been observed that the FZ grain width increased continuously with increase in heat input when the base metal was in the β heat-treated condition, while in the α + β heat-treated base metal welds, the FZ grain width increased only after a threshold energy input. The difference is attributed to both the weld thermal cycle and the pinning effect of equiaxed primary alpha on grain growth in the heat-affected zone (HAZ) of α + β heat-treated base metal. Postweld heat treatment (PWHT) in the subtransus and supertransus regions did not alter the columnar grain morphology in the FZ, possibly due to the lack of enough driving force for the formation of new grains by the breaking up of the columnar grains and grain boundary movement for grain growth. As the PWHTs were conducted in a furnace, the role of thermal gradients can be ruled out. Intragranular microstructure in the aswelded condition consisted of hexagonal martensite. The scale of the martensite laths depended on welding speed. The highest porosity was observed at intermediate welding speeds. At low speeds, a majority of pores formed at the fusion boundary, while at high speeds, occurrence of porosity was maximum at the weld center. The trends on porosity can be explained on the basis of solubility of hydrogen in titanium as a function of temperature and the influence of weld thermal cycle on nucleation, growth, and escape of hydrogen gas bubbles. The porosity at slow welding speeds is low because sufficient time exists for the nucleation, growth, and escape of hydrogen gas bubbles, while insufficient time exists for the nucleation of gas bubbles at high welding speeds. The effect of pickling of joint surface, vacuum annealing of the base metal, and successive remelting of the weld metal has also been investigated.

Fusion zone microstructure and porosity in electron beam welds of an α+β titanium alloy

Fusion zone microstructure and porosity in electron beam welds of an α+β titanium alloy

Fusion Zone Microstructures of Laser and Plasma Welded Dissimilar Steel Joints

ABSTRACT Dissimilar steel joints between 13CrMo44 low-alloyed ferritic steel and A1SI 347 austenitic stainless steel were produced using laser beam and plasma arC welding. Both autogenous welding (without Filler) and welding with nickel-based filler wire were employed in each process. Fusion zone microstructures were characterized using both optical and scanning electron microscopy. Compositional analysis was performed using scanning electron microscopy. Hardness was measured to provide data for identifying the microstructures. A clear correlation was observed between the composition, microstructure and hardness. The results show that both autogenous laser and plasma welding produce fusion zones with a mixed martensite/austenite microstructure. Autogenous plasma welds, however, possess less martensite than the laser welds. The formation of martensite was attributed to the material combination and the rapid cooling rate of the welding processes. The results also indicate that both laser and plasma welding with nickel-based filler wire can produce fully austenitic fusion zones. This is mainly due to the high nickel content in the filler wire. According to the results, laser and plasma welding with nickel-based filler wire are recommended as potential industrial processes.

Autogenous gas tungsten arc weldability of cast alloy Ti−48Al−2Cr−2Nb (Atomic percent) versus extruded alloy Ti−46Al−2Cr−2Nb−0.9Mo (Atomic percent)

This study examines procedures for consistently producing sound (crack and void free) welds using the autogenous (without filler metal) gas tungsten arc (GTA) welding process. Cast alloy Ti-48Al-2Cr-2Nb (at. pct) and extruded alloy Ti-46Al-2Cr-2Nb-0.9Mo (at. pct) have been examined to determine if sound welds can be produced using autogenous GTA welding without any preheat. Experimentation consisted of GTA spot welding samples of gamma titanium aluminide at weld current levels of 45, 55, 65, and 75 A for a duration of 3 seconds. For the cast alloy, current levels of 45, 55, and 65 A for 3 seconds produced similar fusion zone microstructures, which consisted of a dendritic solidification structure. The fusion zone microstructure of the 75 A for 3 seconds current level differed significantly from the lower current levels. It also consisted of a dendritic solidification structure; however, the morphology was quite different. For the extruded alloy, current levels of 45 and 55 A for 3 seconds produced fusion zone microstructures similar to the lower current level samples of the cast γ-TiAl, which consisted of a dendritic solidification structure. The fusion zone microstructures of the 65 and 75 A samples were similar to each other, but they had a dendritic solidification structure of a different morphology than that of the 45 and 55 A samples. For both alloys at all current levels, microhardness profiles showed an increase in hardness from the base metal to the fusion zone. There were no significant differences in the average fusion zone hardness as a function of increasing current level. However, nanoindentation testing did show that certain phases and microconstituents in the fusion zone did have significant variations in hardness in relation to the enrichment and depletion of chromium.

Fusion zone microstructure and fatigue crack growth behaviour in Ti-6Al-4V alloy weldments

The fatigue crack growth resistance of α–β titanium alloys can be altered by microstructural modification. During welding, the fusion zone microstructure depends on cooling rate. In the present work, the alloy Ti-6Al-4V was welded over a range of heat inputs, using electron beam and gas tungsten arc welding. The weld microstructure varied from predominantly martensitic under rapid cooling conditions to a mixture of martensite and diffusional products on slower cooling. Post-weld heat treatment resulted in a basketweave α–β aggregate that coarsened with temperature and time. In all welded and heat treated conditions, the fusion zone exhibited a fatigue crack growth resistance superior to that of the base material, which was in part attributed to the lamellar microstructure of the fusion zone. Welding residual stresses also played a beneficial role in the as welded condition. Post-weld heat treatment eliminated the advantage resulting from the welding stresses but not that as a result of microstructure.

Weld metal microstructure prediction from fundamentals of transport phenomena and phase transformation theory

Modelling the evolution of weld metal microstructure requires knowledge of cooling rates at various locations in the fusion zone. In the recent past, significant advances have been made in the calculation of transient three-dimensional temperature fields, considering convective heat transfer and fluid flow in the weld pool. However, very little effort has been made to use these accurate cooling rates to understand fusion zone microstructures. The present paper demonstrates the advantages of microstructure calculations using fundamentals of transport phenomena and phase transformation theory. The velocity and temperature fields, the shape and size of the fusion zone, and the cooling rates at different locations were calculated by solution of the equations for conservation of mass, momentum, and energy in three dimensions. The time-temperature transformation (TTT) diagrams were calculated for a series of steels with varying carbon and manganese contents using a phase transformation model. The TTT diagrams and the computed cooling rates were then used to obtain the continuous cooling transformation (CCT) diagrams and the microstructures. The computed volume fractions of the various microstructural constituents were then compared with the experimental results. Good agreement between the computed and the experimental results indicates significant promise for predicting weld microstructure from the fundamental principles of transport phenomena and phase transformation theory.

Fatigue Crack Growth in Laser Weldments of a Cold Rolled Steel

This paper describes the effect of laser welding process on the fatigue crack propagation in SPCC-CQ1 cold rolled steel. The influences of weld speed and gap width of butt joint on fatigue crack growth rate are considered. Strength of the weld joints is evaluated by a tensile test, while fatigue properties are investigated by fatigue crack propagation test based on linear elastic fracture mechanics. From the results, the added wire filler material had positive effect on the fatigue properties. Increasing welding speed or gap width will improve the resistance to fatigue crack growth because of fast cooling rate and refined equiaxed microstructure in fusion zone. Also, the fatigue crack growth rate of fusion zone and heat affected zone are lower than that of base metal. Transgranular fracture is the typical fracture mechanism and many secondary cracks appear in the fusion zone.