Small Heat Input Research Articles

Evolution of phase ratio and its effect on residual stress for 2205 duplex stainless steel multipass welded joints by thermo-metallurgical-mechanical model

The control of phase ratio and residual stress is a crucial subject for the structural integrity of duplex stainless steel welded components. Therefore, this paper aims to clarify the evolution of phase ratio and its effect on residual stress for duplex stainless steel multipass welded joints by a thermo-metallurgical-mechanical coupled welding model, alongside the crucial experiments which contribute to the development and validation of the coupled model. The developed model takes both the precipitation and dissolution behavior of austenite under repeated thermal cycles into consideration, thereby demonstrating a remarkable ability to predict the distribution of phase ratio and residual stress. The influencing mechanism of solid-state phase transformation on residual stress is discussed extensively. The results reveal that, for the duplex stainless steel welded joints by gas tungsten arc welding, excessive austenite is often formed within the welding zone particularly for the middle passes, while it is the occurrence of ferritization at the heat affected zone. The effect of phase transformation on residual stress is mainly presented by the change of mechanical properties and phase volume. The overtransformation from ferrite to austenite tends to induce a higher tensile residual stress, vice versa, compressive stress. In addition, it is challenging to achieve phase balance through heat input alone. However, a smaller heat input is recommended to minimize residual stress while ensuring welding penetration. This paper serves as a theoretical foundation for controlling the phase ratio and mitigating residual stress during the welding of duplex stainless steel, thereby contributing to the enhancement of weld quality and structural reliability.

Numerical simulation and experimental study on the heat and mass transfer behavior during the pulsed laser cladding process

Compared with continuous wave (CW), pulse wave (PW) perturbation results in a larger temperature change rate inside the cladding layer, which leads to a higher quality cladding layer. In this paper, a three-dimensional numerical model of laser cladding Fe60 powder was established. The multiphysics field coupling transient evolution and solute distribution under PW and CW operating modes were studied. The problem of lack of theoretical support for laser cladding process under different working modes in heat source was solved. Calculations show that at 1970 ms when the temperature is stable, the maximum temperature, flow rate, and stress under the PW mode are 2660 K, 0.34 m/s, and 354 MPa, respectively, which are smaller than those under the CW mode. The four solute distributions at stable temperature are as follows: The minimum mass fractions of Fe and Mn are 87% and 0.49% in the CW mode, and the maximum mass fractions of Cr and Ni are 11% and 5%. In the PW mode, the minimum mass fractions of Fe and Mn are 82% and 0.44%, and the maximum mass fractions of Cr and Ni are 14% and 5%. PW has a smaller heat input than CW, resulting in a smaller molten pool, so the elements in the molten pool are more fully mixed with other alloying elements. Metallographic experiments were performed using a scanning electron microscope to examine the mechanical properties on the fusion cladding layer, and the accuracy of the model was verified by combining numerical simulations with experimental studies for comparative analysis.

Effect of cold metal transfer (CMT) on the unmixed zone formation in Stellite 6 overlays on ferritic steels

ABSTRACT The segregation of elements and formation of a UMZ is an unfavorable but inevitable phenomenon in dissimilar welding. The present study employed conventional GMAW welding and the CMT process with the same welding parameters to clad a flux-cored Stellite 6 electrode on an A516 Grade 60 steel. The effect of the CMT welding process on the formation of UMZ in the Stellite 6 overlay was investigated. OM and SEM were used to observe the cross-section of the samples. The distribution of the alloying elements across the interface between the weld and base metals was measured using EDS analysis. It was observed that the UMZ in the CMT welding process was thinner and more uniform which was attributed to the steady arc energy and convection flow during welding. Smaller heat input, lower penetration, and higher build-up were the main reasons for uniformity and reduced UMZ width.

Study on Porosity Defect Detection in Narrow Gap Laser Welding Based on Spectral Diagnosis.

As an advanced connection technology for large thick-walled components, narrow gap laser welding has the advantages of small heat input and high efficiency and quality. However, porosity defects are prone to occur inside the weld due to the complex welding environment. In this study, the influence of the process parameters and pollutants such as water and oil on the porosity defect were explored. The action mechanism of water on the electron temperature and spectral intensity of the laser-induced plasma was analyzed. The results showed that the spectral intensity during narrow gap laser welding was weaker than that of flat plate butt welding. Under the optimal welding process conditions, the electron temperature during narrow gap laser self-fusion welding was calculated as 7413.3 K by the Boltzmann plot method. The electron density was 5.6714 × 1015 cm-3, conforming to the thermodynamic equilibrium state. With six groups of self-fusion welding parameters, only sporadic porosity defects were observed according to the X-ray detection. When there was water on the base metal surface, a large number of dense pores were observed on the weld surface and in the weld through X-ray inspection. Compared with the spectral data obtained under the normal process, the relative light intensity of the spectrometer in the whole band was reduced. The electron temperature decreased to the range of 6900 to 7200 K, while the electron density increased. The spectrum variation during narrow gap laser wire filling welding was basically the same as that of laser self-fusion welding. The porosity defects caused by water and oil pollutants in the laser welding could be effectively identified based on the intensity of the Fe I spectral lines.

Study on joint characteristics in laser butt welding of AMed and wrought Ti6Al4V plates

Titanium alloy Ti6Al4V has been widely applied to medical, automotive, and aerospace industries due to its excellent properties such as high strength and excellent corrosion resistance. On the other hand, additive manufacturing (AM) technology can give the freedom of design of the products. In order to spread the AMed products, the joining of AMed and wrought products are required, and it is important to understand the joint characteristics. In this study, butt welding of Ti6Al4V plate was conducted by fiber laser in argon shielding, and the joint characteristics of laser weld wrought/wrought, AMed/AMed, and AMed/wrought Ti6Al4V plates were experimentally investigated. The AMed plate has higher tensile strength than wrought plate but the elongation of AMed plate is smaller, since AMed plate has α’ martensite due to rapid cooling during laser irradiation in AM process. Then, the laser weld joint of AMed/AMed plates has higher tensile strength, but smaller elongation than that of wrought/wrought plates. The weld joint of AMed/wrought plates shows good welding state, since small heat input leads to formation of small weld bead with higher hardness between wrought and AMed plates.

Experimental Study on Electro-Spark Additive/Subtractive Repair for Worn Cemented Carbide

Worn cemented carbide tool bits are often discarded because of the difficulty of their repair, resulting in a great deal of waste. Surface strengthening technology often extends the service life of worn tools. Electro-spark deposition (ESD) coating and matrix materials are metallurgically and closely bonded, and the approach has the characteristics of small heat input, a small heat-affected zone, and low repair cost, so it is suitable for strengthening the surface of cemented carbide tools. As the surface of cemented carbide tools is often not flat, which affects the uniformity of the deposited layer, the surface needs to be polished before ESD. Therefore, this paper proposed a method involving the electro-spark additive and subtractive repair of worn cemented carbide. Experiments involving the ultrasonic-assisted EDM grinding (UEDG) of cemented carbide were carried out. The effect of brass, 45 steel, and tungsten electrode materials on the removal rate, tool wear, and surface roughness were investigated. The results showed that the material removal rate of the tungsten electrode could reach 3.27 mm3/min, while the electrode loss was only 8.16%, and the average surface roughness was only 2.465 μm, which was better than the other two electrodes. Thus, the tungsten electrode exhibited a high material removal rate, low electrode loss, and good surface quality. The effects of the TiC, TiN, and TC4 electrodes on cemented carbide ESD were studied using optical 3D surface topography and other instruments, and the surface roughness, thickness, and hardness of the deposited layer were compared. The results showed that the surface roughness of the TC4 material reached 52.726 μm, which was better than that of the TiN and TiC materials. The thickness of the TiC deposition layer was 172.409 μm and the hardness value was 2231.9 HV; thus, the thickness and hardness of the TiC material’s sedimentary layer were better than those of the TiN and TC4 materials.

Effect of weld geometry on irradiation damage resistance of 316L stainless steel weld at 400 ℃

ABSTRACT To explore the effect of weld geometry on irradiation damage and irradiation hardening, three different groove types of 316 L austenitic stainless steel welds were exposed to H+ irradiation. Testing consisted of exposure to 5-MeV to a dose of 6.2 × 1017 ions/cm2 at 400°C by means of atomic force microscope (AFM), scanning electron microscope (SEM), grading incident X-ray diffraction (GIXRD), transmission electron microscope (TEM), tensile testing and Nano-indentation. The results show that after irradiation, defects such as voids, dislocation loops, and precipitates were produced in the three weld metals. The double U-groove weld has the characteristics of small heat input, smaller grain structure, and higher grain boundary density, which hinders the movement of dislocations, vacancy clusters, precipitates, and other defects, making the defect distribution inside the weld more uniform and smaller, resulting in better mechanical properties before and after irradiation. It is a feasible idea and method to improve the irradiation damage resistance and irradiation hardening resistance of weld by changing the shape of welding groove to refine the grain.

Numerical Analysis of Microstructure Anomalies during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Favorable Dendrite Tip Undercooling

Two important thermometallurgical factors, i.e. aluminum redistribution and dendrite tip undercooling, are numerically analyzed to better understand growth kinetics of microstructure development with favorable and unfavorable crystallographic orientations in three dimensions during advancement of Ni-Cr-Al ternary single-crystal melt-pool solidification interface to optimize microstructure stability within gamma γ phase. Inappropriate growth crystallography dominates profiles of aluminum redistribution and undercooling of dendrite tip in the crack-susceptible region, simultaneously. For beneficial (001)/[100] growth crystallography, the profiles of solid aluminum concentration and undercooling ahead of dendrite tip are clearly symmetrical throughout solidification interface. For detrimental (001)/[110] growth crystallography, the profiles of solid aluminum concentration and undercooling ahead of dendrite tip are quite asymmetrical within two sides of weld, and this asymmetrical phenomenon is of particular interest. The same energy of heat input is accessible to each half of molten pool, however, the difference of dendrite growth driving forces between right side and left side of weld pool kinetically exacerbates microstructure development. The complexity of (001)/[110] welding configuration is consistently attributable to larger overall solid aluminum concentration and undercooling of dendrite tip in [100] dendrite region than that of [010] dendrite region. In the two welding configurations, the size and shape of [001] dendrite growth region modifies growth kinetics of dendrite tip to extend columnar morphology of epitaxial growth for crack-resistant microstructure development with smaller solid aluminum concentration and narrower dendrite tip undercooling by either decrease of laser power or further increase of welding speed, while [100] dendrite growth region preferentially aids columnar/equiaxed transition (CET) with severe aluminum redistribution and wider dendrite tip undercoolig. The smaller heat input is imposed, the smaller solid aluminum concentration and narrower dendrite tip undercooling are crystallographically induced that is capable of elimination of metallurgical contributing factors for microstructure anomalies and vice versa. Optimum (001)/[100] growth crystallography and low heat input with decrease of laser power and increase of welding speed minimize columnar/equiaxed transition and stray grain formation, improve resistance to solidification cracking and ameliorate weld integrity, while (001)/[110] growth crystallography and high heat input with increase of laser power and decrease of welding speed significantly contribute to weldability exacerbation and microstructure degradation. The crystallography-dependent mechanism of efficient microstructure anomalies reduction, which is attributed to decreasing aluminum enrichment and undercooling ahead of dendrite tip, is therefore proposed. Consequently, comparison between calculation results and experiment results is plausible and acceptable. The usefulness application of this numerical analysis improves predictive capability and enables attractive microstructure control of single-crystal superalloys with similar materials properties.

Study on applicability of temper bead repair welding in bottom-mounted instrumentation using bead formation - temperature field - hardness prediction model

To rationalize the evaluation process of temper bead repair welding, a numerical model that can predict hardness during repair welding was developed by combining a coupled bead shape-heat conduction analysis model with a hardness prediction model based on many experimental results. To confirm the usefulness of the developed model, bead-on-plate welding was performed experimentally and numerically to compare the measured and calculated distribution of hardness. As a result, measured and calculated hardness were in good agreement. In addition, the developed model was applied to temper bead repair process for a bottom-mounted instrumentation, and the appropriate welding conditions for reducing the hardness of the repair welded zone were discussed. Numerical results revealed buttering and small heat input could be effective to reduction of hardness for temper bead welding. Since these results was consistent with previous findings, the developed model was found to be applicable to the temper bead welding and it enables quantitative evaluation of hardness according to temper bead repair welding conditions.

Experimental study on fracture toughness of quenched and tempered and TMCP high strength steels

In this study, the fracture toughness of five kinds of high strength steel (HSS) with the nominal yield strength of 550 MPa, 690 MPa and 890 MPa were experimentally studied. Two steel delivery conditions, i.e., quenched and tempered HSS and thermo-mechanical control process HSS, were included and compared. The effect of heat input on the fracture toughness of HSS was investigated. A comparative study was organized for the fracture toughness of base material (BM), heat-affected zone (HAZ) and weld metal (WM). Results indict that the fracture toughness of the heat-affected zone of QT steels (QT550 and QT690) first increases and then decreases slightly with the increase of heat input. There is an optimal heat input for quenched and tempered HSS welding to achieve satisfactory fracture toughness. Either too small or large heat input is not beneficial to the resistance to crack propagation. With increase of steel strength, it seems the optimal heat input for quenched and tempered HSS becomes smaller. The effect of heat input on fracture toughness of TMCP HSS is different with quenched and tempered HSS: higher heat input would normally produce higher fracture toughness for HSS TMCP550. With the increase of yield strength, the fracture resistance increases for both quenched and tempered HSS and TMCP HSS.

Numerical Analysis of Solidification Behavior during Laser Welding Nickel-based Single-crystal Superalloy Part III: Auspicious Control of Dendrite Tip Undercooling

Location-dependent dendrite tip undercooling is numerically elucidated to predict crystallography-assisted resistance to centerline grain boundary formation and morphology transition of stray grain formation ahead of dendrite tip in the ternary Nickel-Chromium-Aluminum molten pool during course of nonequilibrium solidification for explanation arduous solidification behavior control of microstructure melioration. Heat input is not so salient as welding configuration for auspicious solidification behavior and beneficial microstructure development. Advantageous symmetry of welding configuration efficiently lessens dendrite tip undercooling for prevalent dendrite morphology stability of planar interface with alleviation of columnar/equiaxed transition (CET) phenomenon. The bimodal distribution of undercooling ahead of dendrite tip is symmetrically dominant for (001)/[100] growth crystallography with capability of increasing morphology of interface kinetics for epitaxial growth and guarantees single-crystal potential. Alternatively, the distribution of undercooling ahead of dendrite tip is asymmetrically prevalent for (001)/[110] growth crystallography with inefficiency of nonhomologous solidification behavior for discontinuous intersection of solidification interface. Undercooling ahead of dendrite tip inside [010] growth region is not so wide as inside [100] growth region, where thermometallurgically initiates unstable solidification interface and inferior solidification behavior, with unfavorable crystallography in the case of asymmetrical (001)/[110] welding configuration. The smaller heat input is applied, the narrower undercooling ahead of dendrite tip is acquired to significantly mitigate microstructure anomalies with favorable solidification conditions, meliorate metallurgical properties and potentially improve weldability with viability of epitaxial columnar morphology and vice versa. Optimum heat input, especially low laser power and high welding speed together, is a viable and robust way to limit plethora of undercooling and easily decrease solidification behavior anomalies. When low laser power or rapid welding speed is chosen, low heat input not only lessens [100] dendrite growth region, where is spontaneously vulnerable to columnar/equiaxed transition, as ramification of prominent dendrite tip undercooling, but also metallurgically ameliorates [001] dendrite growth region, where morphologically aids epitaxial growth and activates stable planar interface, with achievable diminution of dendrite tip undercooling. Symmetrical (001)/[100] welding configuration, in which undercooling ahead of dendrite tip is preferably narrower than asymmetrical (001)/[110] welding configuration, is one of the most important ingredient for auspicious control of dendrite tip undercooling, once other welding conditions are similar. The main reason, why welding conditions (both low heat input and (001)/[100] welding configuration) is quite superior to welding conditions (both high heat input and (001)/[110] welding configuration), is attributable to favorable crystallography-dependent thermometallurgical factors to suppress inhomogeneous microstructure as long as solidification conditions within marginal stability range. Satisfying crack-free microstructure development is strongly interdependent on kinetics-related solidification behavior through scrupulous control of dendrite tip undercooling to balance between microstructure amelioration and weld depth requirement. The mechanism of columnar/equiaxed transition elimination, by which kinetic driving forces of abnormal microstructure development within high-undercooling region on either left or right side of weld pool is diminished through challenging method of crystallography-dependent dendrite tip undercooling control, is therefore proposed. Finally, there is reasonable consensus between numerical analysis results and experiment results. The numerical analysis provides credible insight into where is liable to microstructure anomalies and why dendrite tip undercooling suppresses stray grain formation for successful laser surface modification of Ni-based single-crystal superalloy.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part II: Multicomponent Dendrite Growth

Multicomponent dendrite growth is theoretically predicted to optimize solidification cracking susceptibility during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification. The distribution of dendrite trunk spacing along the weld pool solidification interface is clearly symmetrical about the weld pool centerline in beneficial (001)/[100] welding configuration. The distribution of dendrite trunk spacing along the weld pool solidification interface is crystallography-dependent asymmetrical from bottom to top surface of the weld pool in detrimental (001)/[110] welding configuration. The smaller heat input is used, the finer dendrite trunk spacing is kinetically promoted by less solute enrichment and narrower constitutional undercooling ahead of solid/liquid interface with mitigation of metallurgical contributing factors for solidification cracking and vice versa. Vulnerable [100] dendrite growth region is predominantly suppressed and epitaxial [001] dendrite growth region is favored to spontaneously facilitate single-crystal columnar dendrite growth and reduce microstructure anomalies with further reduction of heat input. Optimum low heat input (both lower laser power and higher welding speed) with (001)/[100] welding configuration is the most favorable one to avoid nucleation and growth of stray grain formation, minimize both dendrite trunk spacing and solidification cracking susceptibility potential, improve resistance to solidification cracking, and ameliorate weldability and weld integrity through microstructure modification instead of inappropriate high heat input (both higher laser power and slower welding speed) with (001)/[110] welding configuration. The dendrite trunk spacing in the [100] dendrite growth region on the right side of the weld pool is considerably coarser and grows faster than that within the [010] dendrite growth region of the left side in the (001)/[110] welding configuration to deteriorate weldability, although the welding conditions are the same on the either side. Furthermore, the alternative mechanism of crystallography-dependent solidification cracking as consequence of asymmetrical microstructure development and diffusion-controlled dendrite growth of γ phase is therefore proposed. The theoretical predictions are comparable with experiment results. The reliable model is also useful for welding conditions optimization for crack-free laser processing.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-Based Single-Crystal Superalloy Part III: Crystallography-Dependent Solidification Behavior

The solidification temperature range was numerically analyzed to optimize nonequilibrium solidification behavior during ternary Ni-Cr-Al nickel-based single-crystal superalloy weld pool solidification with variation of laser welding conditions (either heat input or welding configuration). The distribution of solidification temperature range along the fusion boundary is beneficially symmetrical about the weld pool centerline in the (001)/[100] welding configuration. The distribution of solidification temperature range along the fusion boundary is detrimentally asymmetrical about the weld pool centerline in the (001)/[110] welding configuration. The stray grain formation and solidification cracking are preferentially confined to [100] dendrite growth region. [001] epitaxial growth region with columnar dendrite morphology is favored at the expense of undesirable [100] growth region with equiaxed dendrite morphology to facilitate essential single-crystal solidification with considerable reduction of heat input. The smaller heat input is used, the narrower solidification temperature range is thermodynamically promoted to reduce nucleation and growth of stray grain formation with decrease of constitutional undercooling ahead of dendrite tip and mitigate thermo-metallurgical factors for morphology instability and microstructure anomalies. Potential low heat input(both decreasing laser power and increasing welding speed) with (001)/[100] welding configuration decreases solidification temperature range to significantly minimize columnar/equiaxed transition (CET) and stray grain formation, and improve resistance to solidification cracking through microstructure control. On both sides of weld pool are imposed by the same heat input, while the solidification temperature range along the fusion boundary inside of [100] dendrite growth region on the right part of the weld pool is spontaneously wider than that of [010] dendrite growth region on the left part to increase solidification cracking susceptibility in the (001)/[110] welding configuration. Furthermore, another mechanism of solidification cracking as consequence of severe solidification behavior and anomalous microstructure with asymmetrical crystallographic orientation is therefore proposed. The theoretical predictions are well verified by experiment results. The useful and satisfactory numerical modeling is also available for other single-crystal superalloys during successful laser repair process without stray grain formation.

Influence of boss-backing welding to ERW pipe

Station and valve chamber design often encounter the situation of drilling hole at the main pipeline and welding boss-backing to connect the branch pipe. Boss hole location should generally be at least 100 mm away from the longitudinal weld or spiral weld. However, because the electric resistance weld (ERW) is difficult to distinguish in practice, some bosses mounting position coincide with ERW or close to. In this paper, the influence of boss-backing welding directly on the longitudinal weld to the original residual stresses of ERW pipe was studied. The microstructure of pipe body and longitudinal weld after welding was also analysis. The testing results showed that the overall residual stresses of ERW pipe were relatively small. Residual stress at the longitudinal weld region were smaller than those at the pipe body region. After the boss-backing welding, the axial residual stress at the longitudinal weld and the circumferential residual stress at the pipe body region near the intersection increased sharply to 2.5 (444 MPa) and 3.8 (433 MPa) times, respectively. The invaded width and depth to the ERW pipe after welding were about 15.167 mm and 3.376 mm. Granular bainite with necklace type M-A constituents could be observed at the invaded zone. It is suggested that small welding heat input should be adopted for boss-backing welding.

Investigation on solidification structure and temperature field with novel processing of synchronous powder-feeding underwater laser cladding

Synchronous powder-feeding underwater laser cladding(SULC) has a great application prospect in underwater in-situ manufacturing/remanufacturing due to its advantages of small heat input to the substrate and adjustable of additive materials. In this study, the iron-based planar coating with no phase transformation and dilution zone less than 8 μm was prepared on A32 deck steel by SULC using self-designed powder-feeding nozzle and optimizing process parameters. The powder-feeding gas flow rate directly determines the size and stability of the local dry region above the deposition point, and the laser power and powder-feeding rate are the key factors to compensate the laser energy dissipation. Due to the influence of local temperature gradient in SULC, the coating has obvious solidification structure distribution. Columnar grains are mainly distributed upon the trough of fusion line, while dendrites and equiaxed grains are distributed on both sides caused by the temperature gradient becomes smaller and the direction of heat flow changes due to the external environment. In the upper region of SULC coating the equiaxed grains tend to grow up with the increase of laser power. To gain better scientific understanding on differences between atmospheric laser cladding and SULC, the numerical simulation was used to reveal the temperature field changes and coating formation under SULC working parameters. The results show that under the same laser power, due to the forced cooling of larger gas flow and external water environment in SULC process, there is a larger temperature gradient at the deposition point, and the temperature drop rate of the coating and the substrate is greater. In the microstructure, it is shown as columnar grains has a greater directional growth trend. With the increase of laser power, this phenomenon becomes more obvious.

Numerical Analysis of Stray Grain Formation during Laser Welding Nickel-based Single-crystal Superalloy Part I: Columnar/Equiaxed Morphology Transition

The thermo-metallurgical modeling of stray grain formation was further developed by couple of heat transfer model, dendrite selection model, multicomponent dendrite growth model, nonequilibrium solidification model and minimum undercooling model during three-dimensional nickel-based single-crystal superalloy weld pool nonequilibrium solidification over a wide range of welding conditions (laser power, welding speed and welding configuration). Welding configuration simultaneously influences distributions of stray grain formation and columnar/equiaxed transition (CET). The stray grain formation and dendrite morphology ahead of solid/liquid interface are symmetrically distributed about the weld pool centerline in the (001)/[100] welding configuration. The stray grain formation and dendrite morphology ahead of solid/liquid interface is asymmetrically distributed in the (001)/[110] welding configuration. Vulnerable [100] dendrite growth region is suppressed in favor of epitaxial [001] dendrite growth region to predominantly facilitate single-crystal dendrite growth with further reduction of heat input. Stray grain formation and solidification cracking are preferentially confined to [100] dendrite growth region. The smaller heat input is used, the less nucleation and growth of stray grain formation with decreasing constitutional undercooling ahead of solid/liquid interface is incurred with mitigation of metallurgical driving forces for solidification cracking and columnar dendrite morphology is increased and vice versa. Symmetrical crystallographic orientation of dendrite growth spontaneously ameliorates microstructure development, and improves resistance to solidification cracking. The mechanism of asymmetrical solidification cracking because of crystallography-dependent stray grain formation and morphology instability is therefore proposed. Optimum low heat input (low laser power and high welding speed) with (001)/[100] welding configuration essentially minimizes both stray grain formation and columnar/equiaxed morphology transition and is beneficial to weldability and weld integrity through morphology control, while undesirable high heat input (high laser power and slow welding speed) with (001)/[110] welding configuration leads to microstructure anomalies and worsens solidification cracking susceptibility. The stray grain formation and morphology transition in the [100] dendrite growth region on the right side of the weld pool are more severe than that in the [010] dendrite growth region on the left side, although the same heat input imposes on both sides of the weld pool in the (001)/[110] welding configuration. The theoretical predictions agree well with the experiment results. Moreover, the promising and reliable model is also applicable to other single-crystal superalloys with similar metallurgical properties for successful crack-free laser welding or laser cladding.

Quantitative analysis of thermal performance and flow characteristics of oscillating heat pipes with different initial pressure

Because of the importance of saturation condition to the operation of the oscillating heat pipe (OHP), the initial pressure of the OHP is a key parameter that affects its thermal performance. In the present work, thermal performance and flow characteristics of OHPs with different initial pressure (0.007 MPa, 0.018 MPa, 0.036 MPa and 0.065 MPa) under different heating power (0–140 W) and inclination angle (30°, 45°, 60° and 90°) were experimentally tested. Meanwhile, a regression model was proposed to quantitatively describe the variation of thermal resistance with heat input and initial pressure. The results showed that both the average temperature of evaporation section and the thermal resistance of OHP increased with the increase of initial pressure. The thermal performance could be 493% deteriorated with the increase of initial pressure from 0.007 MPa to 0.065 MPa. Compared with the OHP with lower initial pressure, the OHP with higher initial pressure had a smaller velocity of working fluid movement and a bigger temperature oscillation amplitude under the same amount of heat input. Based on the regression model, thermal resistance of OHP was more sensitive to heating power under small heat input, while more sensitive to initial pressure under big heat input. We hope the present work could provide some insights into the effects of initial pressure on the thermal performance and flow characteristics of the OHPs.

Laser powder bed fusion of Inconel 718 on 316 stainless steel

The joining of Inconel 718 and 316 stainless steel has notable industrial importance. However, traditional arc-welding has many issues. In particular, the formation of the detrimental Nb-rich Laves phase in the heat affected zone can promote crack initiation and propagation, which leads to poor mechanical properties. The small heat input and the rapid solidification of laser powder bed fusion (PBF) has the potential to suppress this problem. Despite the infancy of the multi-material PBF, it is of interest to investigate the relationship among the process parameter, the microstructure at the materials joining point, and the mechanical performance. In this study, we used high-speed x-ray imaging to in-situ monitor the printing of Inconel 718 tracks on a 316H SS substrate via laser powder bed fusion, followed by post-build characterization and testing with SEM, XRD and nanoindentation. We observed that a higher linear energy density resulted in a more diffused interface and a smaller compositional gradient, but as a trade-off, generated more keyhole pores and thermal cracks. We also showed that the built layer exhibited chemical inhomogeneity at two length scales: a tens-of-μm-scale compositional vortex due to the melt flow, and a sub-μm-scale Nb-rich dendritic structure due to the non-equilibrium solidification that led to the formation of the Laves phase. It was inferred that the chemical inhomogeneity may benefit the mechanical properties at the interface by providing more interlocking between the two materials and also by limiting the effect of the brittle Laves phase to potentially short range.

The effect of pulsed cold-wire feeding on the performance of spray GMAW

The application of pulsed cold-wire (P-CW) feeding has been one of the new approaches for arc welding over the past years. Such technique applied to GTAW shows that it can influence the process performance. However, P-CW feeding in GMAW has not been explored yet. Thus, the current work aimed at introducing and evaluating the effects of P-CW on the performance of GMAW, particularly with the spray metal transfer. The P-CW pulsing frequency and the angle between the cold-wire and the plate were varied in bead-on-plate and V-groove weldments. The effects of the P-CW on the weld bead geometry and metallurgical texture were, then, examined through cross-sections. The metal transfer from the wire tip to the weld pool and the arc behavior during wire feeding were analyzed backed by high-speed imaging. A thermal camera was used to monitor the back of the plates to examine the heat behavior with both the P-CW and non-pulsed feeding conditions. On the one hand, the results revealed that, although the pulsation affects the arc and the electric signal behavior and the metal transfer, no sensitive influence could be noticed regarding the weld bead geometries. On the other hand, the pulsed cold-wire feeding interfered in the thermal profile outside the weld pool. For the highest frequency explored, regardless the feeding angle, less heat was transferred to the plate. The smaller heat input in these cases limited grain growth, reducing the coarse grain HAZ area and resulting in a more refined microstructure. Therefore, P-CW showed to interfere in the GMAW process performance.

Tensile strength and fatigue crack growth rate of chamfered and clamped A6061 friction weld joints

Friction welding is a solid-state joining technique. It is suitable to be used to join a round bar of aluminum that has problems in joining. This paper reports measurements of the tensile strength and fatigue crack growth rate of a continuous drive friction welding (CDFW) joint of aluminum alloys A6061. The CDFW process was conducted by using the round bar A6061 machined to form a chamfer angle and applying a clamping process before the upset stage. Various chamfer angles of 0, 30, 45, and 60 degrees were machined on the stationary round bar. In order to increase the tensile strength and to reduce the fatigue crack growth rate of the CDFW joint, round clamps were applied on the CDFW joint. CDFW process was conducted with the revolution speed of 1,100 rpm, the initial compression force of 3.9 kN during friction stage for 4 seconds, and an upset force of 28 kN for 60 seconds. The specimens of friction weld joints were machined to shape the specimens of tensile strength testing and fatigue crack growth testing. Fatigue crack growth testing was performed using a cantilever rotary bending machine. The testing results show that using a small chamfer angle together with the round clamp produced a CDFW joint that exhibited higher tensile strength than the joint without chamfer or clamping. The specimen created with a chamfer angle of 30 degrees and the clamping method had the highest tensile strength and the lowest fatigue crack growth rate among the samples studied. This result was caused by smaller heat input as a result of using a small one-sided chamfer together with two stages of plastic deformation from the clamping process and upset process during CDFW. The fatigue crack growth rate is also confirmed by macro and scanning electron microscope imaging of the fracture surfaces. The area of fatigue crack growth of the specimen with high tensile strength is wider than the specimen with lower tensile strength. The striations are also observed more clearly in the fracture surface of the specimen with the highest tensile strength and the lowest fatigue crack growth rate, namely the specimen, which has a chamfer angle of 30 degrees with clamping