Microstructure Of Fusion Zone Research Articles

Microstructural Analysis of K-TIG-Welded New Ni-Based Superalloy VDM Alloy 780

The fusion zone microstructures in K-TIG-welded and post-weld solution heat-treated new superalloy VDM Alloy 780 were examined. In addition, the kinetics of the base metal grain growth during solution heat treatments were analyzed. (S)TEM analyses show that major interdendritic microconstituents formed in the fusion zone due to elemental microsegregation are MC carbides and coarse irregularly shaped Laves phase. Additionally, minor secondary interdendritic phases are found to include γ′, γ″, and tiny plate-like Laves particles. To prevent any potential deterioration of mechanical properties caused by the irregular Laves phase, post-weld solution heat treatments (PWSHTs) at 954 °C to 1060 °C/1 hours were performed to remove the Laves phase. PWSHT at 954 °C only partially eliminates the Laves particles while forming an abundance of interdendritic δ/η phase. Laves phase is dissolved entirely without forming δ/η platelets after PWSHT at 1060 °C. It is proven that Laves eutectics in VDM Alloy 780’s fusion zone can be eliminated through PWSHT without significantly coarsening the base metal’s grain size in comparison to Alloy 718 as a result of substantial grain growth inhibition likely caused by solute segregation at grain boundaries.

Synergistic effects of Monel 400 filler wire in gas metal arc welding of CoCrFeMnNi high entropy alloy

Weldability plays a crucial role in the journey of high entropy alloys towards their engineering applications. In this study, gas metal arc welding was performed to join an as-rolled CoCrFeMnNi high entropy alloy using Monel 400 as the filler wire. The present research findings demonstrate a favorable metallurgical chemical reaction between the Monel 400 filler and the CoCrFeMnNi high entropy alloy, resulting in compositional mixing within the fusion zone that promotes a solid-solution strengthening effect, counteracting the typical low hardness associated to the fusion zone of these alloys. The weld thermal cycle induced multiple microstructure changes across the joint, including variations in the grain size, existing phases and local texture. The grain size was seen to increase from the base material toward the fusion zone. An FCC matrix and finely sparse Cr-Mn-based oxides existed in both base material and heat affected zone, while in the fusion zone new FCC phases and carbides were formed upon the mixing of the Monel 400 filler. The role of the filler material on the fusion zone microstructure evolution was rationalized using thermodynamic calculations. Texture shifted from a γ-fiber (in the base material) to a strong cubic texture in the fusion zone. Digital image correlation during tensile testing to fracture coupled with microhardness mapping revealed that, stemming from the process-induced microstructure changes, the micro and macromechanical response differed significantly from the original base material. This study successfully established a correlation between the impact of the process on the developed microstructural features and the resultant mechanical behavior, effectively assessing the processing-microstructure-properties relationships towards an improved understanding of the physical metallurgy associated to these advanced engineering alloys. In conclusion, this work provides an important theoretical framework and practical guidance for optimizing the engineering applications of high entropy alloys.

Improvement of microstructure and mechanical properties of laser welded Al-Si coated 22MnB5 steel joints by austenitic element copper

In this experiment, the effects of different copper foil thicknesses on the morphology, structure, and mechanical properties of laser lap welded 22MnB5 steel joints were studied. The aim was to reduce the damage of Al-Si coating and increase the mechanical properties of welded joints. As the thickness of copper foil increases from 0 to 100μm, there was a gradual decrease in the presence of δ-ferrite in the fusion zone (FZ) and fusion boundary (FB), while the copper-rich content of lath martensite (LM) gradually increased. The reason was that Cu is an austenite element which can expand the γ phase area and play a stabilizing and strengthening role. The result of JmatPro software can support it. It found peritectic reaction (L+δ→γ) range expanded from 1414 to 1422°C to 1436–1453°C which led to more γ phases formed during the solid-state phase transition (δ→γ) at high temperature. Based on this phenomenon, as the copper foil increased (0–100μm), the joint tensile strength increased from 361 MPa to 706 MPa. However, after adding too much copper foil (200μm), the microstructure of FZ varied to a copper-rich phase (ε-Cu) with low hardness and brittleness which adhered to the martensite grain boundaries and reduced the bonding force of the crystal boundaries, so the the mechanical properties of the weld joint seriously reduced (232 MPa). When the copper foil was thin or non-existent (0–30μm), there was still more delta ferrite in FB (low strength and brittle), and the fracture type of the joint mainly appeared as a cleavage fracture. When adding 100μm copper foil, the fracture expressed ductile fracture due to the interfacial fracture due to the lack of δ-ferrite in FB. If the thickness of Cu increased to 200μm, the fracture changed back to FB which expressed a quasi-cleavage fracture.

Effect of Wire Preheat and Feed Rate in X80 Steel Laser Root Welds: Part 1 — Microstructure

Laser welding with cold versus hot wire feed was employed as a root pass to weld X80 pipeline steel. The influences of wire feed rate and preheat on the fusion zone microstructure were investigated. Increasing the wire feed rate helped generate acicular ferrite in the weld metal, and preheating the wire further suppressed the formation of bainite. The acicular ferrite in the upper region of the fusion zone was finer than that in the lower region, which was due to an increase in nucleation sites available. Five fill and cap passes were applied by gas metal arc welding to fill the remaining top part of the groove. Compared to arc welding with a higher heat input, laser welding led to finer prior austenite grains and smaller bainite packet size in the coarse-grained heat-affected zone and limited the formation of martensite-austenite constituents in the intercritically reheated coarse-grained heat-affected zone.

Effect of Wire Preheat and Feed Rate in X80 Steel Laser Root Welds: Part 2 — Mechanical Properties

In Part 1 of this study, the influences of feed rate and preheat of the wire on the fusion zone microstructure of X80 steel laser root welds were investigated. Part 2 of this study reveals the effect of cold versus hot wire feed on mechanical properties. Although strength overmatching with the X80 base metal was maintained, the wire alloying elements reduced the hardness of the upper region of the laser fusion zone but had limited effect on the bottom of the fusion zone due to the incomplete mixing of the filler material with the base metal in the bottom root region of the joint. The addition of ER70S-6 filler wire improved the impact toughness of the weld metal at 0°C, due to the formation of an acicular ferrite microstructure. As the temperature decreased to –20°C (–4°F) and –45°C (–49°F), ductile to brittle transition occurred, leading to lower weld metal toughness.

The impact of interfacial characteristics on the interfacial properties of 316 L/CuSn10 multi-material manufactured by laser powder bed fusion

Multi-material additive manufacturing (MMAM) enables the simultaneous creation of multi-material components in a single process, accomplishing customized different performances in specific regions of the same components. In particular, the copper-steel multi-material structure can provide considerable strength and high thermal conductivity, thus playing a crucial role in electronic components, heat exchangers and injection molds. However, few studies have explored the relationship between the microstructure and properties of the copper-steel fusion zone. Therefore, this study aims to reveal the heterogeneous evolution of the microstructure in the copper/steel fusion zone and its impact on the interfacial properties of 316 L/CuSn10 multi-material structure fabricated by laser powder bed fusion (L-PBF). We characterized the multi-layer specimens using laser confocal microscopy and SEM to obtain the macroscopic and microscopic microstructure, liquid metal embrittlement (LME) microcrack characteristics and precipitation evolution of the copper-steel fusion zone. The generation of liquid phase separation (LPS) was explored based on block specimens, and the textural characteristics of the fusion zone were analyzed using EBSD. As a result, because of multiple element precipitation and the high thermal conductivity of copper alloy, abrupt grain size changes occur in the fusion zone, affecting the fusion zone's corrosion resistance and nano-hardness. In tensile tests analyzing the anisotropy, the results show that LME microcracks have little impact on the interfacial bonding properties of the copper-steel bimetallic structure, among which the ultimate tensile strength of the specimens formed at 30° to the substrate reached a maximum of 562.9 MPa. These findings add substantially to our understanding of the relationship between the microstructure and properties of copper-steel bimetallic interface, and provide a theoretical reference for additive manufacturing copper-steel multi-material components.

Effect of laser beam wobbling and welding speed in X80 steel wire-fed laser root welds

A wobble laser was combined with a wire feed during laser root welding of X80 pipeline steel. The enlarged laser irradiation area and molten pool resulting from the laser beam wobbling provided a better gap-bridging ability, which is critical during laser welding of thick pipes. Applying laser beam wobbling enabled better control of filler metal dilution, which determined the weld metal microstructure and mechanical properties. In the present work, more bainite along with reduced acicular ferrite was formed in the wobble laser weld, due to the effect of dilution resulting from increased base material melting during wobbling beam, along with an inhomogeneous fusion zone microstructure. The instrumented indentation testing was utilized for the first time to distinguish the local yield strength variation of different regions of the fine weld zones, allowing one to verify if the weld metal strength meet the target value in industrial standards. As the productivity being improved when the welding speed increased from 1.0 to 1.5 m/min, the presence of more low temperature transformation products generated with a faster cooling led to a 26 HV higher hardness and a 70 MPa higher yield strength in the upper region of the fusion zone.

Comparison of V groove design on mechanical performance and fracture behaviour in TIG welding of 5754-H111 aluminium alloy

The aim of this study is to enhance the performance and effectiveness of welding 5mm thick plates with different groove designs and investigate their post-effects on material properties. Specifically, the study compares single V and double V butt joints of 5mm thick AA5754-H111 strain-hardened aluminium alloy using Tungsten Inert Gas (TIG) welding with ER5356 (Al-Mg) filler rod. Two types of weld grooves, single V groove and double V groove were utilized to examine the microstructure and character of the weld and to study the effect of heat distribution. The average Vickers hardness values were observed to be 78.2HV in the double V groove of weld zone compared to the 76.6HV in single V groove weld zone. The tensile strength of single V and double V groove were 210 and 207 MPA respectively. The results showed that the failure location of all weld samples was in the fusion zone (FZ) and exhibited brittle fracture. The microstructure of the weld fusion zone revealed the presence of porosity in various sizes, which was also observed in the fractography images of tensile failed samples. Moreover, the microstructure studies exhibited clear variations in the fusion, intersection, and heat-affected zones. The formation of particles was found to differ in the weld fusion zone at various locations due to changes in the V groove welding method.

Investigation the effect of dissimilar laser welding parameters on temperature field, mechanical properties and fusion zone microstructure of inconel 600 and duplex 2205 stainless steel via response surface methodology

This study focused on dissimilar welding characterization of Inconel 600 and duplex 2205 stainless steel using central composite design (CCD) of experiments the response surface methodology (RSM). This study determined the effect of laser welding parameters and the reactions of the temperature field on the melt pool, the mechanical characteristics of the weld joint, and the geometry of the melt pool. According to the ANOVA results, the power of laser and focal distance were found to be the most influential factors on the temperature of both Inconel 600 and duplex stainless steel. The weld joint's tensile strength and elongation were significantly influenced by laser power and focal distance. Increasing the laser power from 250 to 450 W raised the tensile strength from 250 to 550 MPa. The Mo rich phases formed at the inter-dendritic region according to the EDS phase analysis results in loss of ductility and the resultant tensile strength of the samples failure from the fusion zone adjacent to the duplex stainless steel. At high laser power levels, the samples fractured from fusion zone while at lower laser powers below 350 W, the samples fractured from the HAZ and the areas adjacent to the duplex steel fusion line. The micro-hardness value of the weld joint at different laser power of 525 W and 375 W was increased to the maximum values of 370 and 325 HV, respectively from the fusion line of Inconel 600 to the center of the fusion zone. Further, molten pool microstructure of the dissimilar joint zone was mainly composed of a cellular and columnar dendritic structure Variations in melt flow, temperature gradient and solidification rate from the molten scan line to the weld center clearly changed the grain growth and the resultant microstructure in different areas of the fusion zone. By transferring the laser light to the center of the Inconel 600 and duplex stainless steel joint, the molten pool depth was increased from 0.2 to 1.5 mm.

MICROSTRUCTURE AND PROPERTIES OF LASER-WELDED BUTT JOINTS OF AlSi1MgMn ALUMINIUM ALLOY

The process of autogenous laser welding of butt joints of aluminium alloy AlSi1MgMn plates with thickness of 3.0 mm was investigated. The Yb:YAG disk laser was used in the study. The aim was to determine the influence of the basic parameters of laser welding on the joint quality, fusion zone geometry, microhardness distribution across the butt joint, structure and microstructure of fusion zone (FZ) and heat affected zone (HAZ) was studied and determined. The chemical composition of the base metal was determined by spark emission spectroscopy, while the phase constituents were determined by XRD analysis. Results of the study have shown that the geometry of welds is proper without excessive imperfections. No voids were found the FZ. However, weld face of the joints are slightly convex, while the roots are slightly concave. Microscopic observations showed no discontinuities in the fusion zone of the welds. A significant decrease in hardness in the FZ was observed. The hardness in the FZ of the test joints is approx. 67÷70 HV0.1. The fracture of the samples during tensile tests occurred in the FZ or at the boundary FZ/HAZ, and the tensile strength was ranged from 200MPa to 220MPa.

Studying the Effects of Annealing on the Mechanical Properties and Microstructure of Ultrasonic Vibration Welds

The impact of heat treatment on the mechanical characteristics and microstructure of an ultrasonic vibration metal inert gas (MIG) weld is examined in this study. The findings show that the heat treatment method has a significant influence on the mechanical characteristics and structure of the welding specimens. The annealing will increase the grains’ size, thereby reducing the hardness and increasing the ductility of the joint. Due to the impact of ultrasonic during welding, the microstructure in fusion zone of both annealed and non-annealed specimens showed small and fine grain structure (with the grain size being measured at less than 40µm). The result also demonstrates that during annealing, the carbon tended to diffuse from the area of high density to the region of low density, resulting in a reduction in hardness compared to the initial sample. Also, it was discovered that elements that lower weld strength (such as the irregularity of the grain structure and the dendritic structure produced by metal crystallization) significantly decreased.

Effects of heat input on microstructure evolution, mechanical and corrosion properties of TC4 alloy by keyhole TIG welding

In this study, Keyhole tungsten inert gas (TIG) process, a new variation of traditional TIG process, was performed to weld TC4 titanium alloy with a thickness of 6 mm. The effect of heat input on microstructure and their correlation with mechanical properties and corrosion behavior was studied. The results showed that the microstructure of fusion zone and heat-affected zone changed obviously due to the change of heat input. The microstructure in the joints was transformed from equiaxed primary α and α+β lamellae to complete primary β grains, and then α′ phases were precipitated in the primary β grain. With the increase of heat input, the content and length of the acicular α′ phase grains were increased, resulting in a decrease in the number of high angel grain boundaries (HAGBs). Due to the phase strengthening effect of α′ phases, the microhardness of the joints increased, and the tensile strength increased, reaching a maximum value of 1030.1 MPa when the heat input was 1118 J/mm. The impact toughness of the joints increased firstly and then decreased. The corrosion behavior of the joints was worse than that of the base metal (BM). The best corrosion resistance was obtained when the heat input was 1046 J/mm. It concluded that the excellent comprehensive properties can be obtained when the heat input was in the range of 1046–1118 J/mm.

Monitoring of thermo-cycles in fibre laser welding of duplex stainless steel 2205 sheets and its correlation with microstructures and mechanical properties

The study reports the influence of change in the heat supplied (43 J mm−1 to 18.5 J mm−1) on the microstructures as well as mechanical properties of weld joints obtained by welding of Duplex stainless steel 2205 using fibre laser. In-process thermal monitoring of the molten weld pool was carried out using IR pyrometer. Cooling rates (i.e. solidification and solid) were calculated from the thermo-profiles of weld pool, and it increases with the decrease in heat input. From the optical images, it is observed that columnar grains originated from the fusion zone walls and merged at the center. Since, solidification front velocity is comparable on both sides’ leads to a central edge. Ferrite phase content observed in fusion zone microstructure, increases with the increase in solid cooling rate. The result suggests that the joints fabricated at lowest heat input displayed highest tensile strength. The maximum tensile strength been reported to be 872.5 ± 10.8 MPa, and failure occurred at parent metal. Tensile strength of weld joints of DSS 2205 was found to have improved with increasing cooling rate. Higher cooling rate results in the formation of fine dendritic grains as well as higher ferrite content in the weld metal.

Study on Al migration, microstructure and hardness in fusion zone of Al-Si coated press-hardened steel by various laser powers

In this paper, the influence of Al distribution in the fusion zone (FZ) on microstructures and properties of welded joints was studied under 13 different laser powers (300 W→3000 W). The results show that Al can quickly accumulate at the upper surface of the FZ and the fusion line. Due to the stirring action of the beam and the dilution of the molten pool metal, Al becomes gradually uniform with the increase of laser power. This work guides the welding process parameters of lap welding the second layer steel plate to obtain uniform weld. In addition, with the decrease of Al content, the microstructure and hardness of the FZ change as follows: Al-based solid solution (Al > 60.2 wt%, 80 ∼ 120 HV) →Fe-Al intermetallic compound (IMC) (10 wt% < Al < 60.2 wt%, 340 ∼ 980 HV) →Fe-based solid solution (Al < 10 wt%, 350 ∼ 530 HV). Therefore, this work revealed the relationship between the distribution-microstructure-hardness of Al and gave the possible microstructures and properties database of Fe/Al welding joints.

Microstructural evolution and precipitated phase characteristics in the fusion zone for the as-repaired Inconel 718 alloy by directed energy deposition additive manufacturing

In this study, five laser powers were employed to repair Inconel 718 alloy by directed energy deposition additive manufacturing technology. Under various solidification conditions, microstructural evolution and precipitated phase characteristics in the fusion zone (FZ) of the as-repaired Inconel 718 specimens were investigated. The results showed that the main dendritic morphology in the FZ changes from equiaxed dendrites to columnar dendrites with increasing laser power, accompanied with the morphological evolution of numerous Laves phases from a long-chained shape to a blocky shape. However, these morphological changes for the Laves phase are not only relevant to the solidification structure characteristics but also affected by the repetitive vertical thermal cycles induced by layer-by-layer deposition because multiple thermal cycles could be regarded as conducting mild heat treatment to the FZ microstructure. On the one hand, the gradually increasing heat accumulation effects are conducive to partially dissolving Laves phase with increasing laser power; on the other hand, there are a number of γ′′ phase, γ′ phase and δ phases precipitated in the Nb-rich zone. When laser power is 1400 W, the volume fraction of the Laves + δ phases is the highest but the volume fraction of the γ′ + γ′′ phases is the lowest among the five as-repaired specimens, resulting in the lowest micro-hardness.

Microstructural study of keyhole TIG welded nickel-based superalloy G27

The weld fusion zone (FZ) microstructure obtained after keyhole tungsten inert gas welding and post-weld solution heat treatments (PWSHTs) of a new nickel (Ni)-based superalloy called G27 is studied, and the grain growth behavior in the base material (BM) during PWSHTs is characterized. Microsegregation-induced interdendritic microconstituents in the FZ of as-welded G27 are identified by analytical (scanning) transmission electron microscopy ((S)TEM) as niobium (Nb)-rich MC carbides, Nb-rich Laves eutectic constituents, γ’ and η phases. The Laves eutectics are generally considered brittle and can adversely affect the mechanical properties of the weldment; thus, an hour PWSHTs were performed at 954 °C–1060 °C to eliminate the γ/Laves eutectics. PWSHT up to 1010 °C results in only partial removal of Laves eutectics with an excessive formation of η phase surrounding the Laves phase. Complete dissolution of Laves eutectics with no η phase formation is achieved after a PWSHT is performed at 1060 °C. Relative to INCONEL® alloy 718, the complete elimination of the γ/Laves eutectic constituents in the FZ of G27 through a PWSHT was proven to be achieved without causing excessive grain growth in the BM, which could be due to the pinning effect of the fine molybdenum (Mo)-rich precipitates, that are formed during solution heat treatment and identified as hexagonal close-packed phase particles through extensive (S)TEM analyses.

MICROSTRUCTURE AND PROPERTIES OF LASER-WELDED BUTT JOINTS OF X2CRTINB18 STEEL

The process of autogenous laser welding of stainless steel X2CrTiNb18 was investigated. The Yb:YAG disk laser was used in the study for welding of 1.5mm thick butt joins. The influence of basic laser welding parameters on the joint quality, fusion zone geometry, microhardness distribution across the butt joint, structure and microstructure of fusion zone (FZ) and heat affected zone (HAZ) was studied and determined. The quality level of the test joints was determined according to the PN-EN ISO 13919-1 standard. The chemical composition of the base metal was determined by spark emission spectroscopy. Results of the study have shown that laser welding parameters have a significant influence on the quality of butt joints. However, it is possible to produce joints that meet the high requirements and criteria of quality. The width of HAZ of the butt joints welded in the investigated range of parameters was narrow and did not exceed 0.2 mm. It was found that the most common imperfections of the analysed joints are concavity of the weld face and insufficient penetration. The HAZ region of joints is characterized by mainly ferritic microstructure with complex carbides precipitates. In turn, the fusion zone is dominated by a ferritic microstructure but also precipitates were observed, which were identified as fine dispersion carbides. Hardness of the base metal is round 160÷170HV0.3. A tendency to increase in hardness in the HAZ and FZ was observed. The maximum hardness measured in HAZ was approx. 220HV0.3, while in the FZ 190HV0.3.

Minimizing intermetallic Laves phase evolution and enhancing precipitation strengthening of Superalloy-718 joints using InterPulse magnetic arc constriction and high frequency pulsation

The techniques of InterPulse magnetic constriction and high frequency pulsation of arc in gas tungsten arc welding (GTAW) were deployed to minimize the detrimental intermetallic laves phase evolution in fusion zone (FZ) of Superalloy-718 welds and enhance the precipitation strengthening of joints. The Superalloy-718 joints were developed by using an advanced variant of GTAW popularly known as InterPulse gas tungsten constricted arc welding (IP-GTCAW). The joints were subjected to the post weld heat treatment (PWHT) cycles of direct aging (DA), solution annealing at 980 °C and 1065 °C followed by aging (980STA and 1065STA) respectively. The tensile properties and hardness of joints were evaluated. The microstructures of joints were analyzed using optical (OM) and scanning electron microscopy (SEM). The elemental analysis of Laves phase and dendrite core of FZ of joints was performed by using energy dispersive spectroscopy (EDS). The tensile fractured surfaces were analyzed using SEM. Results showed that the Superalloy-718 joints developed using IP-GTCAW showed better response to PWHT than GTAW joints because of the greater refinement of FZ microstructure and evolution of finer and discrete Laves phases in FZ. The 980STA joints exhibited superior tensile properties than DA and 1065STA joints. It is correlated to the greater dissolution of Laves phases in nickel austenitic (γ) matrix of FZ resulting in more niobium (Nb) accessible for the precipitation strengthening of FZ. The 1065STA joints showed slightly higher hardness of FZ than 980STA joints because of the almost complete dissolution of Laves phases in FZ. However, the tensile properties of 1065STA joints are lower than 980STA joints because of severe grain growth in FZ and BM. The evolution of microvoids at the interface of Laves phase/weld matrix leading to the development and coalescence of microcracks in FZ on tensile loading is the main mechanism responsible for the premature fracture of joints.

Microstructure and corrosion resistance of fusion zone in Ti-6Al-4V alloy welded using pulsed- and continuous-wave lasers

In this study, continuous wave (CW) and pulsed wave (PW) lasers (pulse frequencies of 5, 50, and 500 Hz) were used to weld TC4 alloy joints, and the microstructure and corrosion behaviours of the FZ were studied. The results showed that the PW played a role in refining the prior β grains, with the best grain refinement achieved at a pulse frequency of 50 Hz. The main factors affecting the corrosion resistance are the grain size and KAM values. A mathematical model was established to investigate the effect of grain size/KAM value and pulse frequency on the corrosion resistance.

Comparative Study on Laser Welding Thick-Walled TC4 Titanium Alloy with Flux-Cored Wire and Cable Wire

In the welding process of thick-walled titanium alloys, the selection of the wire type is one of the critical factors affecting the welding quality. In this paper, flux-cored and cable wires were used as filler materials in the welding of thick-walled titanium alloys. The macrostructure, microstructure, texture, and grain size of both welded joints were compared by employing an optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM), and the tensile and impact properties were also evaluated. The comparison result showed that the fusion zone microstructure of both welded joints was dominated by a basketweave structure composed of interwoven acicular α' martensite, whereas the microstructure of flux-cored wire welded joints was finer, and the degree of anisotropy was low. The strength of both welded joints was higher than that of the base metal, ensuring that fracture occurred in the base metal area during tension. The Charpy impact energy of the flux-cored wire welded joint was 16.7% higher than that of the cable wire welded joint, indicating that the welded joint obtained with the flux-cored wire performed better in the welding process of thick-walled titanium alloys.