Low Heat Input Conditions Research Articles

Microstructures and mechanical behaviour of bimetallic structures of tungsten alloy (90WNiFe) and nickel alloy (In625) fabricated by wire-arc directed energy deposition

ABSTRACT This study reported the fabrication of bimetallic structures (BS) from tungsten alloy (90WNiFe) and nickel alloy (In625) via wire-arc directed energy deposition (DED)-based additive manufacturing (AM). Its microstructures and mechanical properties were investigated for three different heat input conditions: low (180A), medium (200A), and high (220A). The highest average ultimate tensile strength of 618 MPa was achieved in the low heat input condition, with an average elongation of 49%. Although the ultimate tensile strength of the produced BS is lower than that of 90WNiFe and In625 individually, its yield strength closely resembles that of In625 processed by wire-arc DED. Tensile samples from each heat input condition experienced different fracture locations and exhibited distinct fracture morphologies. It can be concluded that the bonding strength is attributed to the diffusion of chromium, nickel, molybdenum, and niobium from In625 into the γ-(Ni, Fe, W) binding matrix of the 90WNiFe substrate.

Joint Characteristics of Cu-Ni Alloy Fabricated by GTAW and MPAW Processes: A Comparative Study

The current work presents a comparative analysis of the joint behavior of Cu-Ni alloy weldments fabricated by Micro-Plasma Arc Welding (MPAW) and Gas Tungsten Arc Welding (GTAW) processes. The Cu-Ni alloy thin sheets are fabricated at different values of heat input (~40-135 J/mm) by MPAW and GTAW processes, respectively. Further, to evaluate their characteristics, joints are subjected to metallurgical, mechanical, and electrochemical testing. The joints fabricated with a higher magnitude of heat input resulted in deteriorated surface quality with a value of Ra~ 6.13 μm. The increased surface roughness value of the joints resulted in a higher corrosion rate (1.273 mm/year). A finer microstructural morphology is achieved for lower heat input condition. Accordingly, the weldment exhibited higher joint efficiency of ~91%.The prominent reason for achieving higher joint strength is related to the presence of lower secondary dendritic arm spacing (SDAS), which enhances the joint strength and ductility for the joints as compared to higher SDAS value. Further, the micro-fractography analysis reveals the presence of micro/macro-voids for high heat input, whereas the existence of numerous dimples of varying size and depth is observed for low heat input condition, implying the role of heat input of utmost importance.

Enhanced mechanical properties and corrosion resistance of friction-stir processed ultralight LA141 magnesium-lithium-aluminum alloy

This study focuses on conducting friction-stir processing (FSP) on 3 mm thick LA141 alloy using different tools. Under low heat input conditions of 600 rpm and 200 mm/min, the use of the WC-Co cemented carbide tool successfully produced a defect-free stir zone. This stir zone exhibited an ultrafine grain structure resulting from continuous dynamic recrystallization. Additionally, the stir zone introduced massive grain boundaries, nanoscale MgAlLi2 particles, and stacking faults. The stir zone displayed a favorable balance of strength and ductility. The enhanced strength is attributed to the inhibitory effect of nanoscale MgAlLi2 particles, grain boundaries, and Lomer-Cottrell locks on dislocation movement. The improved work hardening ability and ductility can be attributed to the increase in stacking faults, Lomer-Cottrell locks, and deformation twins during deformation. Furthermore, the stir zone showed improved corrosion resistance, characterized by lower corrosion current density and larger polarization resistance. This improvement in corrosion resistance can be attributed to multiple factors. These include the reduced defect density, refined and evenly distributed MgLiAl2 particles that weakened galvanic corrosion effects, and the presence of more stable Li2CO3 and Al2O3 compounds that resulted in a compact and protective film.

Effect of reducing heat input on autogenous TIG welding of Ti–6Al–4V alloy

3 mm-thick Ti–6Al–4V alloy plates were welded in butt joint configuration through autogenous TIG welding method using a trailing gas shielding arrangement. After attaining full penetrated weld joint with appropriate heat input, subsequent experiments were conducted by reducing the heat input, which was obtained through combination of higher welding current and scan speed. An in-depth microstructural analysis of the weld joint executed under different heat input conditions shows a strong correlation with the microhardness, tensile strength, and flexural strength of the weld joint. A significant improvement in the mechanical properties of the weld joint was perceived for employing lower heat input. Alteration in the primary-α, primary-β, and martensite-α phases, due to the variation in heat input, amended the mechanical properties of the weld joint under lower heat input condition. With the reduction of heat input from 0.406 to 0.232 kJ/mm, the tensile and flexural strengths of the weld joint increased from 1020 to 1070 MPa, and from 1015 to 1950 MPa, respectively. The tensile strength of the joint reached up to 98% of the base material at minimum heat input (0.232 kJ/mm). The experimental results show the potential of TIG welding for joining Ti–6Al–4V alloy under low heat input condition.

Enhancing microstructure and mechanical properties of laser powder bed fusion-fabricated AlSi10Mg alloy through tailored friction stir processing and post-heat treatment

In this study, a series of advanced processing techniques is employed to control the internal structure and improve the mechanical properties of AlSi10Mg alloy fabricated via laser powder bed fusion (L-PBF). In the L-PBF, AlSi10Mg sheets are subjected to friction stir processing (FSP) under low heat input conditions. The main objective of this study is to eliminate microstructural defects and refine the microstructure while maintaining a high tensile strength. FSP effectively refines the grain structure but decreases the strength significantly owing to the thermal cycle. Various post-heat treatment strategies have been proposed to optimize the microstructure and strength of AlSi10Mg alloys as well as to address the strength reduction issue. Samples in different processing stages are characterized using optical microscopy (OM), field-emission scanning electron microscopy (FE-SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), as well as tensile and hardness tests. Notably, the FSP + direct aging (FSP + DA) treatment significantly enhanced both the ultimate tensile strength (UTS) and yield strength (YS) without compromising the ductility achieved after FSP. By applying the strengthening model, the YS contributions of the main strengthening mechanisms in the Al matrix and Si phase at different processing stages are quantitatively analyzed. The estimated YS values are in good agreement with the experimentally measured values.

The influence of shoulder-workpiece clearance on channel formation during friction stir channeling at low and high heat inputs

Friction stir channeling (FSC) is a newly developed manufacturing process that can replace the traditional fabrication processes of cooling channels for heat exchangers. Regardless of its application, the size and shape of a cooling channel are fundamental to its efficiency. In FSC, clearance between the shoulder of the tool and the workpiece top surface is a prime factor that significantly influences channel size and shape. This study examined the effect of clearance on channel characteristics using two different clearance values, 0.8 mm and 1.2 mm, respectively. The process parameters are chosen in such a way that the effect of clearance can be understood under low and high heat input conditions on several characteristics. These include channel geometry, dimensions, roof thickness, the roughness of the internal surface of the channel walls, and their topography. A rectangular channel geometry with a larger height and width was found to form at 1.2 mm clearance with high heat input parameters. Lower clearance at low heat input resulted in a successful channel, but a higher clearance value produced a defective channel for the same parameters. In conclusion, the results demonstrate how clearance and the amount of heat input are co-dependent to produce a channel with enhanced characteristics that may directly influence the performance of a cooling channel in various heat exchanger applications.

Method of Suppressing Solidification Cracking by Laser Surface Melting and Epitaxial Growth Behavior for Directionally Solidified 247LC Superalloy

In this study, the relationship between solidification cracking and epitaxial growth behavior with the high-speed laser surface melting of a directionally solidified 247LC superalloys was fundamentally and metallurgically investigated, to develop a successful welding procedure for the next generation of gas turbine blades. Under typical laser surface melting conditions (scan speed: 50 mm/s, heat input: 40 J/mm), severe solidification cracking phenomena occurs. The key metallurgical factors of solidification cracking have been identified as solidification segregation-assisted pipeline diffusion behavior at the solidification grain boundary, and in the randomly formed polycrystalline melting zone microstructure. In addition, under extremely low heat input and high-speed laser beam scan conditions (scan speed: 1000 mm/s, heat input: 2 J/mm), an effective surface melting zone can be obtained within a single directionally solidified grain under a relatively high-energy beam density (65 J/mm2) using the characteristics of single-mode fiber lasers. Results reveal that the laser melting zone successfully shows a 99.9% epitaxial growth achievement ratio. Because of the superior epitaxial growth ratio within the laser surface melting zone, and the rapid solidification phenomena, formation of a solidification grain boundary and solidification segregation-assisted pipeline diffusion behavior can be suppressed. Finally, a solidification crack-free laser melting zone can thus be achieved.

Investigations of Microstructure and Mechanical Properties in Wire + Arc Additively Manufactured Niobium–Zirconium Alloy

Herein, the feasibility of the gas tungsten arc welding‐based wire + arc additive manufacturing process for fabricating thin wall structures of niobium‐1 wt% zirconium (NbZr1) alloy is investigated. Three different heat input conditions (low, medium, and high) have been selected for fabricating it. The microstructure is characterized by using optical microscopy, scanning electron microscopy, X‐ray diffraction, energy‐dispersive spectroscopy, and electron backscattered diffraction (EBSD). The microstructure shows the columnar dendritic structure elongated in the build direction. No cracks or porosity are observed in the structure. Average Vickers hardness for low, medium, and high heat input conditions are 146.6, 162.1, and 163.5 HV, respectively. There is an increasing trend of microhardness value along the deposition height, which can be attributed to the difference in secondary dendritic arm spacing and the formation of precipitates. The tensile strength of the specimen is comparable to the conventional and additively manufactured structures. EBSD analysis confirms that possible subgrains are responsible for good mechanical properties at room temperature. In the majority of the tensile samples, the failure mechanism has been identified as a ductile fracture. The mechanical characteristics fluctuate with locations in each of the thin walls, suggesting anisotropy in the deposits.

Microstructure and mechanical properties of friction stir welded ultralight Mg-14Li-1Al alloy

In this paper, commercially cold-rolled ultralight Mg-14Li-1Al (LA141) alloy plates having a thickness of 2 mm were successfully joined by friction stir welding (FSW). Compared with the traditional H13 steel tool, it can easily produce sound joints at low heat input condition using WC-Co cemented carbide tool. The grain size of the joints exhibited a clear inverse proportional relationship with the Zener-Hollomon ( Z ) parameter as ln d = 8.41–0.32 ln Z . The grain refinement mechanism was attributed to the combined effect of continuous dynamic recrystallization and geometric dynamic recrystallization. Ultrafine grained joint with a mean grain size of 1.3 μm can be achieved at extremely low heat input condition of 600 rpm and 200 mm/min. The joint had the highest strength of 184 MPa and a welding efficiency of 95.8%. The strengthening mechanism of the joint was considered as the synergic effect of grain boundaries, dislocations, and nanoscale MgAlLi 2 precipitations. A small amount of stacking faults and deformation twins also contributed to the strength improvement. The grain boundary strengthening was considered as the dominant strengthening mechanism. This study demonstrated that the LA141 alloy exhibited a good weldability when it was subjected to the FSW technology. It also provided a practical guidance for the application of FSW LA141 alloy in the field of lightweight manufacturing. • Ultralight LA141 alloy plates of 2 mm in thickness were successfully joined by FSW. • It was easier to obtain a sound joint using WC-Co cemented carbide tool. • Ultrafine grained joint with a mean grain size of 1.3 μm was achieved. • The dominant grain refinement mechanism was attributed to continuous dynamic recrystallization. • The FSW LA141 alloy joint exhibited a highest welding efficiency of 95.8%.

Dynamic recrystallization, Laves phase evolution and mechanical performance of nuclear-grade Nb containing FeCrAl alloy joints fabricated by friction stir welding

Nb containing FeCrAl alloys are a new class of promising candidates for accident-tolerant-fuel (ATF) materials used in nuclear power plants. The macrostructure, dynamic recrystallization, Laves phase evolution, tensile properties, and their correlations of the Nb containing FeCrAl alloy after friction stir welding (FSW) were systematically investigated at different welding parameters. FSW led to grain refinement and the stir zones (SZs) were mainly composed of fine equiaxed grains. The second-phase particles in the base materials (BM) were C14-type Fe2(Nb,Mo) with a diameter range of 500–900 nm. Under low heat input conditions, the majority of the Fe2(Nb,Mo) Laves phase particles in the SZs were dissolved into the matrix. With heat input increasing, very fine Fe2(Nb,Mo) Laves phase dispersions and Y2O3 in the nanoscale were re-precipitated. Microhardness distributions of Nb containing FeCrAl alloy joints obtained at different welding parameters exhibited identical “U-shaped” profiles. The average hardness of the base material (BM) was ∼352HV, while that in the stir zone (SZ) was reduced to 224-229HV. The joints obtained at 400rpm-80 mm/min held optimal yield strength (389 ± 11 MPa) and ultimate tensile strength (492±6 MPa). Micron-scale Fe2(Nb,Mo) Laves phase particles in the BM helped accumulate dislocation pile-ups. In contrast, the re-precipitated nanoscale Fe2(Nb,Mo) and Y2O3 precipitates could not pin dislocations as effectively as those larger particles in the BM. As a result, the mechanical properties of FSW joints were lower than those of the BM.

Friction stir welding of CoCrNi medium-entropy alloy: Recrystallization behaviour and strengthening mechanism

Dynamic recrystallization (DRX), deformation-induced mechanical twins, and strengthening mechanism of CoCrNi equi-atomic medium-entropy alloy (MEA), welded by friction stir welding (FSW) at different welding speeds, were systematically investigated in this study. The results indicated that FSW led to grain refinement in the stir zones (SZs) with diameters fluctuating in the range of 2.1–9.6 μm. The hardness was improved from ∼154HV (base material, BM) to 238–263HV (SZ) by FSW. The yield strength and ultimate tensile strength of the optimal SZs were 601 MPa and 844 MPa, respectively, which were 260% and 134% higher than those of the as-received material, respectively. Analyses of DRX behaviour indicated that discontinuous DRX (DDRX), continuous DRX (CDRX), and geometric DRX (GDRX) were successively activated with increasing strain during FSW to facilitate grain refinement. The larger the grain size, the higher the fraction of mechanical twins. However, the thickness of the twins had a negative correlation with the grain size. Although a few HCP structures were formed on the twin boundaries, grain refinement and dislocation hardening mechanisms remained dominant in enhancing the mechanical properties of the SZs. The strength-ductility synergy by FSW, under low heat input conditions, was caused by the formation of thin mechanical twins in fine equiaxed grains. Thinner twins were more effective in transferring and homogenising plastic deformation, thereby contributing to the postponing of plastic instability and further promoting an excellent combination of strength and ductility. These findings confirm that FSW is a promising approach for strengthening the MEA for superior performance.

On the interaction of cooling rate with thermal-microstructural-mechanical characteristics of laser-welded α + β titanium alloy

ABSTRACT The joining of thin sheet Ti-6Al-4V alloy has potential application as an encapsulation of miniature components. However, it is quite challenging to achieve a defect-free joint in a fusion welding process since the complexity increases many folds with reduced thickness of the materials. In the present investigation, thin sheet Ti-6Al-4V alloy is welded under different process parameters (26, 45, and 74 J/mm), using a controlled and regulated pulse laser welding technique. The weld joints are further analysed using an experimental and numerical approach to explore the interplay of thermal-microstructural and mechanical characteristics. The outcome of the present investigation reveals better surface quality, desirable microstructural changes, and enhanced joint strength for the joints fabricated under the least heat input condition (26 J/mm). Such overall improvement in the joint characteristics is associated with the high value of cooling rate (2834 K/s), leading to the formation of fine needle-shaped α’-martensite, which corresponds to maximum joint efficiency of 90%. Surface finish is another crucial factor that is affected by the amount of heat input. Joints fabricated under low heat input conditions (26 J/mm) exhibits a lower roughness value (Ra~5.67 µm) as compared to joints fabricated under high heat input conditions (74 J/mm).

Analysis of heat and mass transfer in ultrasonic vibration-enhanced friction stir welding of 2195 Al–Li alloy

Ultrasonic vibration (UV)-enhanced friction stir welding (UVeFSW) is used for welding of 2195 Al–Li alloy plates. A model is developed to study the heat and mass transfer in UVeFSW of 2195 Al–Li alloy. It reveals that additional UV in FSW could reduce the heat flux and temperature at the contact interfaces when compared to those in conventional FSW. Although the temperature is slightly decreased with additional UV, the acoustoplastic effect of the UV could reduce the flow stress and viscosity in the shear zone, resulting in an enhanced fluidity of the plastic material with a higher flow velocity. It is demonstrated that the variations of viscosity, strain rate and flow stress around the tool at the same radial distance are more severe at low heat input condition, such as with a low rotation speed or a high welding speed. The model is validated by the experimental data.

Welding Process Improvement of 347H Stainless Steel Boiler Tube for Mitigating Reheat Cracking

Many failures at the welded joint of 347H tubes have recently occurred during the short-term operation of the boiler tubes. To mitigate reheat cracking or strain induced precipitation hardening, the finite element analysis and experiment for optimization of welding procedure were performed. Preheating, high inter-pass temperature, low and high heat input conditions were compared with existing welding procedure. According to mechanical analysis, high inter-pass temperature and high heat input condition showed the lower plastic strain and tensile residual stress at the inner area of heat-affected zone. At high temperature tensile test, the high inter-pass temperature condition had lower yield strength and higher elongation, while the high heat input condition showed higher yield strength and smaller elongation. In the specimen of high heat input condition, the dissolution of most precipitates and coarse grain were observed in the heat-affected zone. Relatively small amount of dissolution and smaller grain got shown in high inter-pass temperature condition. Therefore, high inter-pass temperature condition can be an alternative. Through X-ray diffraction analysis, it was confirmed that nitrogen was added to Super304H and carbon was substituted for the existing crystal structure of Nb(C,N). This will mitigate reheat cracking by increasing the solvus temperature of Nb(C,N) and reduce the grain coarsening. Key words: Strain induced precipitation hardening, Carbide dissolution, Plastic strain, Heat input, Inter-pass temperature

Thermal sensing and heat input control for thin-walled structure building based on numerical simulation for wire and arc additive manufacturing

In the wire and arc additive manufacturing process, the cumulative temperature increase caused by the repeated deposition process reduces the dimensional accuracy of the build structure. When the temperature of the build structure increases, the deposited metal width increases, while the height decreases. In this case, using a lower heat-input condition is effective because the temperature near the molten pool can be locally lower and the molten pool size can decrease. Thus, the height of the build structure can increase, and the uniformity of the deposited metal width can be improved. In this paper, a heat-input condition control system is proposed. This system controls the voltage—a heat input condition—which is applied from the welding power source according to the temperature of the build structure. The temperature of this structure is measured using a radiation thermometer during the deposition process. To determine the optimal voltage value, the relationship between the build structure temperature and the deposited metal shape is estimated by a numerical simulation. The optimal heat input condition is derived based on simulation results with varying preheat temperatures and heat input conditions. By using numerical simulations, the number of experiments can be minimized. To evaluate the proposed method, a thin-walled structure was built, and its profile was measured. The accuracy of the build was successfully improved by using the proposed method.

Relationship between FSW parameters and hardness of the ferritic steel joints: Modeling and optimization

The response surface method was used to study the influence of friction stir welding on the hardness of the ferritic carbon steel joints. The input parameters were tool rotational speed, tool traverse speed, and tool axial force. In addition, FSW parameters were optimized to obtain the highest value of the hardness. Microstructure and hardness of the joints were characterized using optical microscopy, orientation imaging microscopy, transmission electron microscopy, Vickers hardness, and tensile test. The results showed that among the parameters, axial force had the most effect on the hardness of the joints. The best situation for FSW was obtained at rotational speed, traverse speed, and the axial force of 980.4 rpm, 66.6 mm/min, and 2.95 kN, correspondingly. The optimum condition during FSW caused a maximum hardness of 197 Hv. The most important result of this study was the fact that the hardness of the joints does not follow the Hall-Petch relationship, which was due to the change of ferrite morphology and formation of the pearlite phase in the joints at higher heat input conditions. The lower heat input condition caused higher dislocation densities. Moreover, the influence of residual gas contamination is disclosed for the joints welded with and without Ar shielding gas.

Superior mechanical properties in high-Ni transformation induced plasticity steels after friction stir welding

The high-Ni TRIP steel plates were butt joined by friction stir welded. For this aim, the rotational and traverse speeds were set at 600rpm-100 mm-min−1 and 950rpm-50 mm-min−1. The optical microscopy, X-Ray diffraction (XRD, orientation image microscopy, and tensile test were employed for the characterization of the joints. The results showed that friction stir welding replaced the large grains of the base metal with fine and equiaxed grains by continuous dynamic recrystallization (CDRX). In addition, friction stir welding lead to the stabilization of the austenite. The lower heat input condition caused finer grain sizes. Using lower heat inputs resulted in more retained austenite by means of both work hardening and finer grain size mechanisms. The friction stir welded joints showed superior UTS and elongation of 1130 MPa-115 % and 1050 MPa-76 % respectively for the low and high heat input conditions compared to that of the base metal i.e., 880 MPa-60 % due to stabilization of austenite and formation of more transformation induced plasticity (TRIP) effect. The low heat input joint showed superior tensile properties thanks to more austenite stabilization.

Experimental investigation of microstructure and mechanical properties of IS2602-EN31 MIG welded joint

In this paper, Experimental investigation is carried out on IS2062-EN31 dissimilar MIG weldments and influence of different heat inputs on the properties different zones of welded structures. Low heat input (5.7238 kJ/mm), high heat input (6.714 kJ/mm) conditions are selected and with help of these heats input conditions made of 8 mm thickness defect free weldments are prepared. It is found that the joint fabricated at a low heat input condition showing excellent metallurgical and mechanical properties. Both Mechanical and metallurgical properties of the joint improved with low heat input condition, fine grain size was measured with at welded zone and it decreased. Tensile and hardness is improved due to grain size was observed to be Eight with high and Nine low heat input condition.

Friction Stir Processing of Beta C and Ti-185: A Unique Pathway to Engineer Microstructures for Exceptional Properties in β Titanium Alloys

Metastable β titanium alloys offer a wide range of attractive property combinations. Conventional processing options have practical limitations in achieving maximum attainable properties and utilizing them to their fullest potential. In the current study, friction stir processing (FSP) of β alloys is explored as a unique path for microstructural engineering. Two metastable β titanium alloys, Ti-1Al-8V-5Fe (Ti-185) and Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C), subjected to FSP with two different tool rotation rates at a constant traverse speed were analyzed in different microstructural conditions. Fully retained β grain structures with grain sizes in the range 4 to 7 µm in Ti-185 and 10 to 15 µm in Beta C were obtained in the as-FSP condition. Post-FSP duplex aging treatment of the low heat input condition resulted in better tensile properties compared to those of high heat input condition, attributable to high number density of nucleation sites generated during FSP. Transmission electron microscopy observations of high-strength Ti-185 sample revealed fine α platelets of length in the range 50 to 130 nm and an aspect ratio in the range 3 to 10, providing significant strengthening contribution. Approximate nose temperatures and nose time calculations of the continuous cooling transformation of β to α were made for various commercial Ti alloys to assess the potential of achieving ultra-high strength levels via FSP.

On the role of sliding load and heat input conditions in friction stir processing on tribology of aluminium alloy–alumina surface composites

ABSTRACTAluminium (AA5083)-alumina surface composites are prepared by friction stir processing in two conditions of heat input. The low heat (LH) input conditions is achieved at a rotational speed of 710 rpm and a traverse speed of 100 mm/min, and high heat (HH) input conditions are achieved at a rotational speed of 1400 rpm and a traverse speed of 40 mm/min. The tribological characteristics of aluminium alloy, friction stir processed (FSPed) alloy and FSPed surface composites against steel ball are studied at 5, 10 and 20 N load. While no significant influence is found on frictional behaviour, wear resistance of FSPed composites is superior to FSPed alloys. FSPed composites fabricated at HH input conditions exhibited improved wear resistance as compared to LH input condition. Adhesion and delamination are dominant wear mechanisms at 20 N. Debris particles are reduced in size and hydroxidated in sliding of surface composites.