Ni-based Filler Metal Research Articles

Experimental-theoretical study of a novel approach for dissimilar welding of ferritic steels in power plants: An investigation of morphology-composition-property

Carbon migration is one of the major challenges in the dissimilar welding of ferritic steels, which severely affects the mechanical and structural integrity of the welds and must be prevented at all costs. A novel procedure for dissimilar welding of T22 and T91 steels was developed considering four different welding configurations. The configurations were investigated using optical and electron microscopy, energy dispersive spectroscopy, and mechanical testing to perform a comprehensive study. In one of the four configurations, a Ni-based filler metal was used, and as expected, no hardness gradient was observed between the filler metal and the base metals. A new theoretical qualification method was also introduced to compare the overall performance of the desired welding consumables.

Phase diagram-guided composition design and property investigation of Ni-based filler metals

The fundamental mechanism underlying the formation of brazed joints involves the diffusion of elements between the base metal (BM) and the brazing filler metal (FM). The formation of brittle compounds in FM during the brazing process frequently results in a significant deterioration of the mechanical properties of the brazed joints. Ni and Cu elements exhibit infinite solubility, and the incorporation of Cu element results in a decrease in the liquidus temperature of FM, thereby enhancing its surface wettability on the BM. The present investigation involves the preparation of as-cast and amorphous FMs with varying Cu element concentrations to examine their material properties, such as weldability, wettability, thermophysical behavior, and creep resistance. The thermophysical and mechanical properties of the nickel-based filler metal BNi-2 were evaluated using the JMatPro software for material thermodynamic simulation and compared with data obtained from existing literatures. An analysis is conducted on the creep properties of a novel FM comprising of Ni, Cr, Fe, Si, B, and Cu. The results suggest that the thermophysical properties of BNi-2 filler are minimally impacted by the presence of Cr (6–8 wt%), Fe (2.5–3.5 wt%), Si (4–5 wt%), and B (2.75–3.5 wt%). Furthermore, alterations in the composition of Si and B elements have a significant impact on the distribution of γ and γ′ phases, consequently influencing the creep properties of the material. At 25 °C, the equilibrium phase of BNi-2 with different Cu contents indicates a gradual decrease in the content of γ phase, while the content of γ′ and Cu–Ni phases increases with an increase in Cu content. The BNi-2 filler with 3 % Cu exhibits the most favorable properties in terms of steady creep rate and fracture life compared to other fillers. This is attributed to the competitive equilibrium between the γ phase and Cu–Ni phase. The addition of 6 % Cu element results in the optimal wettability of the filler metal and creep resistance of the brazed joints. It is also observed that as the concentration of Cu reaches 9 %, there is a gradual increase in the FeB phase, a rise in the proportion of brittle compounds, and a decline in the amorphous forming ability.

Effect of wire composition on microstructure and penetration crack of laser-cold metal transfer hybrid welded Cu and stainless steel joints

In this paper, Cu-based and Ni-based filler metals were used to obtain laser-CMT hybrid welded copper/stainless steel (Cu/SS) joints. The effect of wire compositions on the microstructure and mechanical properties of T2 Cu/304 SS dissimilar metal joint was studied. The results showed that Fe–Cu liquid phase separation occurred in the fusion zone of the Cu-based weld, and Cu penetration cracks appeared in the heat-affected zone (HAZ) on the steel side. The Cu penetration cracks formation is derived from supersaturated Cu atoms penetrating, gathering and merging at the steel side fusion line along the austenite grain boundaries. The penetration cracks can be eliminated effectively by the Ni-based filler metal, because the diffusion of Cu into the fusion zone and the HAZ on the steel side is limited significantly by the increase of Ni content. The Ni-based weld consisted of dendrite grains in the fusion zone, and Fe-rich blocks and Ni-rich islands nearby the HAZ of Cu-side.

Retaining Mechanical Properties of GMA-Welded Joints of 9%Ni Steel Using Experimentally Produced Matching Ferritic Filler Metal

Motivated by the loss of tensile strength in 9%Ni steel arc-welded joints performed using commercially available Ni-based austenitic filler metals, the viability of retaining tensile strength using an experimentally produced matching ferritic filler metal was confirmed. Compared to the austenitic Ni-based filler metal (685 MPa), higher tensile strength in gas metal arc (GMA) welded joints was achieved using a ferritic filler metal (749 MPa) due to its microstructure being similar to the base metal (645 MPa). The microstructure of hard martensite resulted in an impact energy of 71 J (-196 °C), which was two times higher than the specified minimum value of ≥34 J. The tensile and impact strength of the welded joint is affected not only by its microstructure, but also by the degree of its mechanical mismatch depending on the type of filler metal. Welds with a harder microstructure and less mechanical mismatch are important for achieving an adequate combination of tensile strength and notched impact strength. This is achievable with the cost-effective ferritic filler metal. A more desirable combination of mechanical properties is guaranteed by applying low preheating temperature (200 °C), which is a more practicable and economical solution compared to the high post-weld heat treatment (PWHT) temperature (580 °C) suggested by other research.

Dynamic formation characteristics and mechanism of hybrid laser arc welding surface layer by Ni-based filler metal based on rotating laser induction

This study used a nickel-based filler metal to fabricate a surfacing layer on a low-carbon steel substrate by a hybrid welding process with a rotating laser and arc. The surfacing layer's weld formation was studied using optical microscopic observation (OM). The microstructure of the layers was studied by using the scanning electron microscope (SEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The arc, metal transfer, molten pool flow behaviors, and U–I characteristics during hybrid laser arc welding (HLAW) were analyzed using the arc analyzer and high-speed camera. The results show that a rotating laser improves the weld formation of gas metal arc welding (GMAW). The microstructure of the two surfacing layers consists of dendritic γ phase and a small amount of Laves and NbC phases. The high dilution rate of the HLAW surfacing layer results in relatively more Laves and fewer NbC phases. The arc length of HLAW is dynamically deflected with the rotation of the laser, which expands the arc action and heating area of the molten pool. The droplet transfer mode changed from globular transfer dominant in GMAW to the projected transfer dominant in HLAW. The location where the droplet transfer into the molten pool is dynamic changed, forming a multi-point impact on the molten pool. Combined with the deflected arc with the laser rotating laser, the HLAW has a broad and deep molten pool. Understanding the metal transfer behavior of Ni-based alloys in rotating laser-induced arc hybrid surfacing is of great significance for improving surfacing forming.

Ni/TiC composite electrodeposition on the surface of Ni-based superalloy

Composite electrodeposition technology was used to form Ni/TiC composite coatings on a Ni-based filler metal to introduce dispersive and uniform particles during the welding process. Scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy were used to study the effects of composite electrodeposition parameters (current density, electrodeposition temperature and TiC particle concentration) on coating microstructure (the porosity, deposition percentage of particle, surface morphology and preferred orientation of the coating). Finally, an arc welding was performed to study the effect of TiC particle introduction on the weld microstructure. By observation, the increase in current density from 6 A/dm2 to 8 A/dm2 reduced the deposition percentage of TiC particle from 12.7% to 6.0%. The maximum deposition percentage of TiC particle was 14.2% at the optimum deposition temperature of 45 °C. According to the result, the ratio of nickel sulfate concentration to TiC particle concentration was proved to have main effect on surface flatness and porosity of coating, the coating with the best flatness and lowest porosity was prepared with TiC particle concentration of 30 g/L in electrolyte. In addition, the composite coating deposited on the smooth surface showed good bonding with the substrate. Welding tests indicated that the introduction of TiC particles increased the tortuosity of the grain boundaries and refined the grains. The average grain size of the weld decreased from 224.24 to145.88 μm at a particle content of 1.0%. The average grain diameter decreased by 34%. The strain fracture (STF) test shows that at the most sensitive temperature of DDC (1050 °C), the critical strain of DDC is increased to 3.5% after the introduction of TiC particles (which is close to the performance of FM-82), while the DDC critical strain of the original welding material FM-52M is 2.4%. The result shows that the DDC resistance of the weld has been improved.

Macrosegregation induced interface structure and its effect on creep failure in dissimilar metal welds between Ni-based alloy and 10% Cr martensitic steel

This study reported the evolutionary behavior of macrosegregation induced interface structure during post weld heat treatment (PWHT) and its effect on creep failure in dissimilar metal welds (DMWs) between Ni-based alloy and 10%Cr martensitic steel with Ni-based filler metal. During welding, macrosegregation led to formation of partially mixed beach-like lath martensite interface structure on the steel side. The beach-like structure transformed to rod-like Ni-rich and C-rich martensite accompanied by Ni-depleted and C-depleted martensite/ferrite after PWHT due to partial austenization and diffusion of austenite-stabilizing elements like Ni and C. Interfacial creep failure occurred along the interface between steel and the beach-like interface structure, which was related to creep voids nucleation at the BCC/FCC interface between steel and the beach-like structure during creep at 600 °C. Hence, the relationship between macrosegregation, interface structure and creep failure in the DMWs was clarified.

Wire-Feeding Laser Welding of Copper/Stainless Steel Using Different Filler Metals.

Ni-based filler metal and Ni-Cu-based filler metal were used to obtain copper/stainless steel (Cu/SS) joints through wire-feeding laser welding. Along the SS/weld interface, there exist different grain sizes (from coarse columnar grains to fine equiaxed grains). The heat affected zone (HAZ) on the copper side consisted of two areas with different grain sizes and the size of the grain in the Cu-HAZ of the Ni-Cu-based filled joint was much smaller than that of the Ni-based filled joint. Our results showed that grain refinement at the copper/weld (Cu/weld) interface of the Ni-Cu-based filled joint was observed through high-resolution electron backscattered diffraction (EBSD). There was a hardness elevation at the Cu/weld interface of the Ni-Cu-based filled joint due to the grain refinement on the weld of the copper side. The maximum tensile strength of the Ni-Cu-based filled joint was obtained and reached 91.2% of the tensile strength of the copper base metal (Cu-BM). Joints in this study were observed to fracture in a ductile mode. Furthermore, the Ni-Cu-based filled joint exhibited a higher plastic deformation, which was primarily caused by the large deformation of the weld zone and the large deformation of the Cu-BM due to the high plasticity of the weld, which alleviated the stress concentration, as indicated by 2D-digital image correlation (DIC) test results.

Development of a Novel Ni-Based Multi-principal Element Alloy Filler Metal, Using an Alternative Melting Point Depressant

Brazing is a crucial joining technology in industries where nickel-superalloy components must be joined. Nickel-based brazing filler metals are extensively employed, possessing excellent mechanical properties, corrosion resistance, and retained strength at elevated temperatures. To function as a filler metal, the alloy melting point must be reduced to below that of the materials being joined, but the addition of melting point depressants (MPDs) such as boron, silicon, and phosphorus can, however, lead to the formation of brittle intermetallics, potentially compromising the joint performance. In the present work, a novel multi-principal element brazing alloy (in the style of a high entropy alloy), utilizing Ge as an alternative MPD along with a reduced B addition, is investigated. The design process considered binary phase diagrams and predictions based on Thermo-Calc software and empirical thermodynamic parameters. The alloy was used to vacuum braze nickel-superalloy Inconel-718, and microstructural and mechanical investigations are reported. The maximum shear strength achieved was 297 MPa with a brazing temperature of 1100 °C and 60-minute hold time, with isothermal solidification completed. Shear strength was only slightly reduced with increased joint width. Assessments are made of the ability to accurately predict properties of multi-principle element alloys using Thermo-Calc software and empirical thermodynamic parameters.

Corrosion Resistance Evaluation of a Stainless-Steel Brazed Joint in HCl Solution

To clarify the effect of the acid solution type on corrosion resistance, the corrosion behavior of stainless steel brazed joints in HCl aqueous solution was evaluated through electrochemical measurements. Anodic polarization curves of a ferritic stainless-steel base metal, Ni-based brazing filler metals, and a brazed joint were recorded. In addition, in situ observations were conducted to observe the corrosion behavior of each structure of the brazed joint. Corrosion potentials of the brazing filler metal were lower than that of the base metal. In situ observations of the brazed joint revealed the order of corrosion in aqueous hydrochloric acid. According to the electrochemical measurements, under an actual corrosive environment, the brazing filler metal can function as an anode and selectively corrode. In addition, the anodic polarization curve of the brazed joint showed values between those of the polarization curves of the brazing filler metal and the base metal, indicating that the corrosion resistance could be electrochemically evaluated in HCl aqueous solution.

Effect of controlled microstructure brazed with foil-type Ni-based filler metal on corrosion resistance of stainless steel joint

Effect of controlled microstructure brazed with foil-type Ni-based filler metal on corrosion resistance of stainless steel joint

High-temperature tensile properties and interface structure of Ni–Fe dissimilar butt joints welded using the cold metal transfer process

Ni–Fe dissimilar butt joints (Inconel 718 and SUS316 sheets) were welded with an Ni-based filler metal using the cold metal transfer (CMT) process. The morphology of the joints was analyzed to elucidate the formation characteristics of the low-dilution-rate welding joints. The tensile strength of the joints was tested at different temperatures and with different macro morphologies to locate the weak area. The finite element method (FEM) was utilized to calculate the stress concentration of the Ni–Fe fusion line (FL) region at high temperatures. It was found that the stress was reduced and was not concentrated in the Ni–Fe FL region, if the WM reinforcement width was wider than 8 mm (original top gap of welding butt joint). The micro morphology and element distribution of the fracture surface and the interface cross section demonstrate that the interface of the Ni–Fe FL with insufficient diffusion is the region where the solidification crack occurs. Thus, this interface is the source of the FL crack and can substantially undermine the high temperature tensile properties of the joints. The metallurgical mechanism of solidification crack formation was also discussed by the phase diagram and solidus-liquidus temperature calculation. The surface of the solidification crack is Fe-based, indicating that it was formed during the melting and re-solidification of the SUS316 base metal surface. Furthermore, the formation temperature was higher than the initial solidification temperature of the welding metal.

Temperature effects on the tensile strength of brazed Fe-Cr-Al alloy (Fecralloy®) with Ni-based filler metal and corresponding image processing based analysis

Brazing bonding of iron-chromium-aluminum alloy (Fecralloy) with nickel based filler metal (MBF20) were performed at different maximum temperatures, 1000 °C and 1100 °C by varying the rate of temperature rise, 5 °C/min and 10 °C/min. And for comparison, diffusion bonding of Fecralloy-Fecralloy was also done with a condition, 1000 °C and 10 °C/min. For testing the tensile strength, the prepared tensile strength specimens were just bonded for diffusion bonding, and filler metal was inserted between the half specimens for brazing bonding. And in order to visually investigate about the bonding conditions, channeled heat plates were also prepared. The dimension of Fecralloy plates was 49 mm × 24 mm × 2 mm thick with three 2 mm diameter channels and 60 urn thick MBF20 was used as filler metal. The cross section parts of the test specimens were analyzed with SEM (XRD, EDS, EPMA, BSE) images, which were image processed to quantify the penetration depth of both filler and base metals to counter materials and components distribution using image processing. And such quantified results were compared with the tensile strength. From the both analysis, the tensile strength results were correlated with the change of thickness of filler metal as well as the distribution of components at the different bonding conditions.

Phase transformation study during diffusion brazing of γ-TiAl intermetallic using amorphous Ni-based filler metals

The paper aims at understanding the microstructure evolution of the diffusion brazing of γ-TiAl intermetallic compound using two Ni-based filler metals, a ternary Ni-Si-B, and a quinary Ni-Cr-Fe-Si-B. It was found that the joint region comprised of two distinct zones: analogous reaction layers adjacent to the TiAl substrates and a solidified zone at the middle of the joint region. The reaction layers consisted of separate intermetallic phases regions including ternary and binary Ti-Ni-Al intermetallic compounds. The solidified zone mainly composed of the Al1-xNi3Six isostructural solid solution and athermally Ni-boride or Ni-Cr rich borides containing eutectic microstructures at the joint centerline. The maximum shear strength was 240 MPa, and obtained by using Ni-Cr-Fe-Si-B filler metal. Fracture of the joint with Ni-Cr-Fe-Si-B filler occurred in the reaction layer/solidified zone interface, but joint with Ni-Si-B filler was fractured at solidified zone.

Microstructure and creep fracture behavior in HR3C/T92 dissimilar steel welds

This paper investigated the microstructure and creep fracture characteristic of the HR3C/T92 dissimilar steel welded joint using Ni-based filler metal. After crept for 2415.75 h, the joint preferentially fractured at the FGHAZ of T92 side in welded joint, not at the interface between T92 steel and Ni-based WM, which exhibited type IV cracking. The type IV fracture was triggered by the accumulation of cavity damage, the number of cavities was relatively few within the 0.4 tf in the FGHAZ of T92 side, but increased significantly from the 0.6 tf to 0.8 tf. After 0.8 tf, the creep cavities rapidly coalesced into cracks and growth, leading to brittle fracture. In the CGHAZ of T92 side close to the fusion line, the formation of blocky δ-ferrite phases was observed, which is responsible for the diffusion of carbon atom from the CGHAZ to WM/CGHAZ interface during welding. The hardness of δ-ferrite is much lower than that of martensitic matrix of CGHAZ, however, the creep damage preferentially takes place in the FGHAZ instead of the CGHAZ with formation of blocky δ-ferrite. It can be recognized that the CGHAZ with a few isolated δ-ferrite blocks near the interface is not the weakest zone for the early failure of the HR3C/T92 dissimilar steel welded joint compared with the FGHAZ.

Alloying-dependent microstructure influence on corrosion resistance of AISI 321 cell joints brazed by Ni-based filler metals

Alloying-dependent microstructure influence on corrosion resistance of AISI 321 cell joints brazed by Ni-based filler metals

Microstructural Evolution of Dissimilar Metal Welds Involving Grade 91

Dissimilar metal weld failures between low alloy Cr-Mo ferritic steels and austenitic stainless steels made with Ni-base filler metals are typically observed along the fusion line. Such DMW failures often exhibit the onset of damage well before their expected service life. Failure is typically associated with a carbon-depleted region in the ferritic steel and formation of creep voids along a row of so-called Type I carbides. More recently, the formation of Type I carbides adjacent to a carbide-free ferrite band has also been observed in DMWs where the ferritic steel was a 9 pct Cr creep strength-enhanced ferritic steel. However, it has not been completely clarified whether these microstructural features formed during welding, post-weld heat treatment or service. In this study, single pass bead-on-plate welds were prepared on a Grade 91 steel substrate using commonly specified Ni-base weld metals; ENiCrCoMo-1, ENiCrMo-3, ENiCrFe-3, ENiCrFe-2 and EPRI P87 (ENiFeCr-4) and were characterized in the as-welded, post-weld heat treatment (PWHT) and aged conditions. The evolution of the ferrite band was found to form by a diffusion-controlled process and was observed after PWHT and aging associated with regions exhibiting steep concentration gradients in the partially mixed zone (PMZ). In this study, steep concentration gradients were observed along the toe of the weld whereas relatively shallow gradients were observed at the weld bottom-center, resulting in a discontinuous ferrite band along the fusion line during PWHT and aging. The variation in chemical gradients in the PMZ corresponded to different chemical potential gradients, which lead to differences in carbon diffusion during high-temperature PWHT and aging. The results of this study provide insight into potential solutions for minimizing the risk of failure in future applications.

Preparation of Ni-based pasty filler and its application in C/C composites brazing

A spherical Ni-based filler metal powder with fine particle size and a uniform distribution was prepared by vacuum melting inert gas atomization method. At the same time, an organic binder carrier suitable for pasty filler metal was prepared. A Ni-based pasty filler was prepared by mixing the filler metal powder with a binder and stirring at a high speed. The C/C composites were jointed using the prepared pasty filler. Subsequently, the strength of the joint was tested, and the interface characteristics of the brazed joint were analyzed by scanning electron microscope, EDS spectrum analyzer, x-ray diffractometer and metallographic microscope. The results show that pasty filler can increase the contact area before brazing, which is beneficial to increase the metallurgical bonding strength of the joint. Under suitable brazing conditions, a gradient buffer structure can be constructed to reduce thermal mismatch and increase joint strength.

Image Processing-Based Analysis of Temperature Dependent Interlayer in Brazed Fe-Cr-Al Alloy (Fecralloy®) with Ni-Based Filler Metal

This paper presents the result of the brazed Iron-chromium-aluminum alloy (Fecralloy) with nickel filler metal (MBF20). The heat plates with the machined 2 mm diameter channels were brazed using 60 μm thick MBF20. In the experiment, the brazing bonding was performed at three different temperatures, and the SEM-EDS images of the cross sections were acquired to analyze. By performing image processing, the change of the filler metal thickness was estimated, and the thickness increased from 53.8 μm, 64.1 μm, and 115.8 μm at 900 °C, 950 °C and 1000 °C, respectively. In addition, the image-processed distributions of CrxBy and NixAly phases with the change of bonding temperatures were quantified. The image processing methods in this work are proposed to be used for quantitative analysis.

Retention of Delta Ferrite in the Heat-Affected Zone of Grade 91 Steel Dissimilar Metal Welds

This study aimed to determine the mechanism of δ-ferrite retention in the coarse-grained HAZ (CGHAZ) of Grade 91 steel dissimilar metal welds (DMWs) with Ni-based filler metals. This phenomenon was investigated in four DMWs made with cold-wire gas tungsten arc process using Alloys 625, 617, 82, and P87 filler metals. A narrow band of δ-ferrite grains was identified in the CGHAZ in all welds. It was hypothesized that δ-ferrite retention was caused by local carbon depletion in the CGHAZ, which was validated through extensive thermodynamic and kinetic simulations and metallurgical characterization. Carbon diffusion across the fusion boundary was driven by the carbon chemical potential gradient between Grade 91 steel and the Ni-based filler metals, which was facilitated by long high-temperature dwell times resulting from a difference in heat capacity and thermal conductivity between the base and filler metals. A linear relationship was established between the amounts of retained δ ferrite and the predicted carbon depletion in the CGHAZ of each DMW. Alloy 625 filler metal generated the largest extent of carbon depletion and the most retained δ ferrite, followed by Alloys 617, 82, and P87. The carbon depletion resulted in local softening of the CGHAZ martensite.