Solidification Growth Rate Research Articles

Effects of ultrasonic vibration on microstructure and mechanical properties of 1Cr12Ni3MoVN alloy fabricated by directed energy deposition

Due to the rapid melting and solidification during directed energy deposition (DED) process, the defects and columnar crystals are likely to generate in the deposition layers, which reduce the quality and performance of the whole parts. Therefore, in order to improve the microstructure and mechanical properties of 1Cr12Ni3MoVN alloy manufactured by DED method, ultrasonic vibration (UV) has been employed to assist directed energy deposition process in this work. The results indicate that the high-intensity ultrasonic vibration can weaken the epitaxy growth tendency of crystal grains, and significantly improve plasticity while keeping an approximate strength. In addition, a two-dimensional numerical model is established to simulate the effect of ultrasonic vibration in the molten pool. The simulation results show that ultrasonic vibration remarkably improves the flow velocity and pressure in the molten pool, inducing the cavitation effect that breaks dendritic crystal and affects crystal characteristics. Meanwhile, the acoustic streaming effect changes the thermodynamic conditions and promotes high-temperature diffusion, which uniforms temperature distribution and reduces the temperature gradient in the molten pool. Thus the reduced temperature gradient G and raised solidification growth rate R promote the formation of fine equiaxed crystal characteristics after UV treatment. The product G × R increases and the ratio G/R decreases after UV treatment, resulting in the formation of fine equiaxed crystals.

Influence of growth velocity on fragmentation during directional solidification of Al – 14 wt.% Sn alloy studied by in-situ synchrotron X-radiography

Directional solidification of Al – 14 wt.% Sn alloy was investigated by means of synchrotron X-radiography at the European Synchrotron Radiation Facility (ESRF). The dynamics of the solidification microstructure formation was studied in-situ and in real-time. In the present manuscript, a special attention is given to the influence of increasing growth velocity on the fragmentation phenomenon of dendrite arms, which has an important role and impact on the final grain structure. Fragmentations of both secondary dendrite arms and more strikingly part of primary trunks have been observed. The experiments show that the number of fragments increases concomitantly with the solidification growth rate until it reaches a limit value after which fewer or no more fragments are observed. The possible origins of the fragmentation are discussed in detail and the likelihood of each of these potential mechanisms is estimated to understand the reason of both primary and secondary arm detachment. It is shown that the fragmentation in a transient regime is correlated to the variations of the solute accumulation, latent heat rejection and the curvature of the attachment neck during solidification. Moreover, during these experiments, Columnar-to-Equiaxed Transition (CET) induced by the fragmentation phenomenon is observed. The velocity of the dendrite fragments due to the buoyancy force is a major parameter to control the CET and our results show that a reverse Equiaxed-to- Columnar Transition can even be observed. Once the growth velocity is of the same order or higher than the fragment flotation velocity, the fragments are trapped within the columnar dendritic microstructure, and CET cannot take place or the equiaxed regime cannot be sustained.

Effect of Surface-Active Element Oxygen on Heat and Mass Transfer in Laser Welding of Dissimilar Metals: Numerical and Experimental Study

The effects of the surface-active element oxygen on the laser welding of 304 stainless steel (304SS) and nickel were numerically and experimentally studied in pure argon and argon–oxygen mixed gas atmospheres containing 21% oxygen (AMO). In this study, the molten pool morphology, thermal behavior, solidification phenomenon, correlation between dilution and convection flow, and microhardness of welding joints were analyzed. As a result of oxygen effects, the molten pool was deeper, the maximum temperature was higher, and the maximum flow velocity was lower in the AMO. The cooling rate (GR) and combination parameter (G/R) were studied by the direct simulation of temperature gradient (G) and solidification growth rate (R). Combined with the solidification microstructure, it was found that oxygen had little effect on grain size. The major elements Fe, Cr, and Ni within the solidified molten pool in the AMO were uniformly diluted, while the distribution of the above elements was non-homogenous in pure argon. Stronger flow and multiple directions of convection inside the molten pool contributed to uniform dilution in the AMO. The distribution of microhardness was similar to the content of Cr, and the microhardness at the substrate interface of the joint was higher in the AMO than in pure argon. The preliminary conclusions of this study provide in-depth insights into the effects of surface-active element oxygen on heat and mass transfer in laser dissimilar welding.

Laser Powder Bed Fusion Additive Manufacturing of a Low-Modulus Ti-35Nb-7Zr-5Ta Alloy for Orthopedic Applications.

Laser powder bed fusion (L-PBF) was attempted here to additively manufacture a new generation orthopedic β titanium alloy Ti–35Nb–7Zr–5Ta toward engineering patient-specific implants. Parts were fabricated using four different values of energy density (ED) input ranging from 46.6 to 54.8 J/mm3 through predefined laser beam parameters from prealloyed powders. All the conditions yielded parts of >98.5% of theoretical density. X-ray microcomputed tomography analyses of the fabricated parts revealed minimal imperfections with enhanced densification at a higher ED input. X-ray diffraction analysis indicated a marginally larger d-spacing and tensile residual stress at the highest ED input that is ascribed to the steeper temperature gradients. Cellular to columnar dendritic transformation was observed at the highest ED along with an increase in the size of the solidified features indicating the synergetic effects of the temperature gradient and solidification growth rate. Density measurements indicated ≈99.5% theoretical density achieved for an ED of 50.0 J/mm3. The maximum tensile strength of ≈660 MPa was obtained at an ED of 54.8 J/mm3 through the formation of the columnar dendritic substructure. High ductility ranging from 25 to 30% was observed in all the fabricated parts irrespective of ED. The assessment of cytocompatibility in vitro indicated good attachment and proliferation of osteoblasts on the fabricated samples that were similar to the cell response on commercially pure titanium, confirming the potential of the additively manufactured Ti–35Nb–7Zr–5Ta as a suitable material for biomedical applications. Taken together, these results demonstrate the feasibility of L-PBF of Ti–35Nb–7Zr–5Ta for potentially engineering patient-specific orthopedic implants.

Crack free metal printing using physics informed machine learning

Process parameters and thermophysical and mechanical properties of alloys affect cracking which remains a major challenge in metal printing. Cracks occur because of multiple mechanisms and currently, there is no unified mitigation strategy. Here we evaluate the effects of variables related to the physics of cracking computed by a mechanistic model and independent experimental data using machine learning to prevent cracking. The computed solidification stress, the ratio of the vulnerable and relaxation times, ratio of the temperature gradient to solidification growth rate, and cooling rates and experimental data are used to generate a cracking susceptibility index that predicts crack formation before printing. Computed values of these four variables when used in a decision tree, support vector machines, and logistic regression can predict crack formation with exceptional accuracy. Information gain, information gain ratio, and Gini index-based feature selection calculations provide the same comparative influence of these four variables. Results are presented as easy-to-use cracking susceptibility maps. Our approach can help in process optimization, designing new alloys, and solving problems in manufacturing beyond metal printing.

Effect of Heat Treatment and Electric Discharge Alloying on the Tribological Performance of Selective Laser Melted AlSi10Mg

Abstract Selective laser melting (SLM) is an emerging additive manufacturing (AM) technology for fabrication of complex lightweight components along with improved mechanical properties. However, the properties are highly influenced by the continual heating and cooling during deposition, variation in local temperature, size, and shape of melt pool, and solidification growth rate. Therefore, postprocessing is very often required to control various properties of additive manufactured components. The present work investigates the influence of various postprocessing methods such as heat treatment and electric discharge alloying (EDA) on ambient and elevated temperature wear behavior of selective laser melted AlSi10Mg alloy and compared with its tribological behavior with cast AlSi10Mg. The dry wear tests were conducted using a pin on disk (POD) tribometer with EN-31 as counter body. The EDA treated SLM AlSi10Mg showed the least wear-rate and coefficient of friction (COF) at both ambient and elevated temperatures (1.05 × 10−4 mm3/Nm and 0.434 and 3.12 × 10−5 mm3/Nm and 0.531, respectively) due to its higher hardness (189.8 HV) as compared with other samples. The wear-rate and COF of cast specimen are found to be highest among all specimens at both ambient and elevated temperatures (1.34 × 10−4 mm3/Nm and 0.528 and 4.49 × 10−5 mm3/Nm and 0.724, respectively). Lower wear-rate and higher COF are observed at elevated temperature due to the excessive formation of wear-resistant oxides (Al2O3, SiO2, and MgO) and glaze layers for all samples compared with ambient temperature wear behaviors of its counterparts. Abrasive wear, adhesive wear, oxidation wear, and surface delamination are the prominent wear mechanisms observed for ambient and elevated temperatures for all the specimens.

Numerical and Experimental Investigations of Laser Metal Deposition (LMD) Using STS 316L

This study aimed to understand the effect of heat accumulation on microstructure formation on STS 316L during multilayer deposition by a laser metal deposition (LMD) process and to predict the microstructure morphology. A comprehensive experimental and numerical study was conducted to quantify the solidification parameters (temperature gradient (G) and growth rate (R)) in the LMD multilayer deposition process. During deposition, the temperature profile at a fixed point in the deposit was measured to validate the numerical model, and then the solidification parameters were quantified using the model. Simultaneously, the microstructure of the deposit was investigated to confirm the microstructure morphology. Then, a relationship between the microstructure morphology and the G/R was proposed using a solidification map. The findings of this study can guide the design of scanning paths to produce deposits with a uniform structure.

Thermal behavior and microstructure evolution mechanism of W-20%Fe alloy fabricated by laser metal deposition

A three-dimensional finite element method (FEM) model was established to study the effects of laser processing parameters on the thermal behavior, melting/solidification mechanism, and microstructure evolution during laser melting deposition (LMD) of W-20%Fe alloy, considering the temperature-related thermal physical properties, multiple heat transfer, and latent heat of phase change. It was shown that there was a positive correlation between the maximum temperature and the laser powers. As the laser power increased, the cross-sectional configuration of the molten pool became deeper and narrower. The solidification characteristics, dependent on the solidification growth rate, R, and the temperature gradient, G, were obtained to predict the morphology and scale of the solidified microstructure. The maximum temperature gradient in the molten pool was slightly increased from 1.52×106°C/m to 2.09×106°C/m, as the laser power increased from 800 W to 1100 W. When the laser power was 1000 W and the scanning speed was 400 mm/min, the G/R elevated considerably from top to bottom region of the molten pool about 5.542°Cs/mm to 1829°Cs/mm. The columnar dendrites and the equiaxed dendrites were obtained at the bottom and top regions of the molten pool, respectively. The columnar dendrites were observed at the edge of the molten pool, which was attributed to the high G/R (2.09×109°Cs/mm). The corresponding LMD experiment was carried out, which demonstrated that the established physical model was reliable and accurate.

Electrochemically prepared Ni3Si with controllable areal capacity

Lamellar Ni3Si prepared by the combination of directional solidification and selective corrosion was proved to be a battery-type material. Its areal capacity is decreased with the increasing directional solidification growth rate, which is due to decreasing the lamellar space hinders the matter exchange between electrodes and the electrolyte. When the discharge current is 5 mA, the areal capacity of Ni3Si electrode with growth rate of 8 μm/s is 0.2 mAhcm−2, meanwhile the areal capacity of Ni3Si electrode with growth rate of 10 μm/s is only 0.16 mAhcm−2. After 1000 cycles of consecutive charge and discharge, the capacity loss of Ni3Si electrode with growth rate of 10 μm/s is the smallest, only 16.4%. Therefore, 10 μm/s is the optimal choice for preparing Ni3Si electrodes, after synthetically considering both the areal capacity and cyclic stability. Hopefully, this paper can provide a candidate material preparation of Ni-based battery or Li+ and Na+ batteries.

Effect of Microstructure Features on the Corrosion Behavior of the Sn-2.1 wt%Mg Solder Alloy

The Sn–Mg eutectic alloy is a potential replacement for the traditional Sn-38.1 wt%Pb solder alloy, since lead has been banned because of its risk to human health and the environment. However, studies in the literature related to the Sn-2.1 wt%Mg alloy are restricted to mechanical, electrical and thermal properties. Therefore, this study aims to evaluate the corrosion behavior of this alloy, as a function of the microstructural arrangement obtained from directional solidification performed in a transient heat extraction regime. The thermal solidification parameters (solidification growth rate and cooling rate) were determined along the length of the casting for correlations with microstructural features. The resulting microstructure was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. Subsequently, the electrochemical impedance spectroscopy, linear polarization, equivalent circuit and evolution of hydrogen release analyzes were performed to evaluate the corrosion behavior of the samples in a 0.5 M NaCl solution at 25 °C. Interphase spacing and Mg2Sn fraction were found to influence the corrosion behavior, showing higher corrosion resistance for a more refined microstructure.

Modeling of thermal behavior and microstructure evolution during laser cladding of AlSi10Mg alloys

An improved three-dimensional finite element model has been proposed for studying the thermal behavior and microstructure evolution during laser cladding of AlSi10Mg alloys. Different material properties between AlSi10Mg powders and AlSi10Mg alloys are distinguished from the experiment and theoretical calculation to provide more reliable material parameters for simulation. In order to investigate the melting and solidification process during the formation of cladding layers, a temperature selection judgment mechanism is established to simulate the evolution of AlSi10Mg powders from the powder state to melting state and alloy state. In addition, to simulate the complex thermal behavior associated with powder particles and the voids between particles, a simplified exponential attenuation model is used for correcting the heat source. A complex asymmetric heat source considering about the different material properties and laser absorptivity on both sides of the remelting zone is used for multi-track cladding process.By simulating the temperature distribution of molten pool, the improved FEM could be used to predict the geometric shape of cladding layers (ignoring the effect of melting flow) and the temperature history. The simulation results show that the heat tends to diffuse to the unmelted powder owing to the asymmetric heat source during multi-track cladding, which leads to the asymmetry of cladding layers along the width direction. Based on the results of the temperature field simulations and the solidification characteristics of AlSi10Mg powders, the temperature gradient (G), solidification growth rate (R), cooling rate (G*R) and G/R are investigated to predict the morphology and size of the solidification microstructure under different laser scanning parameters. The scanning speed mainly determines the cooling rate during the laser cladding process, which results in different microstructures. Higher scanning speed leads to higher cooling rate, corresponding to a finer microstructure. Coarse dendrites are generated at the bottom of the molten pool, while finer dendrites are formed at the top.

Effects of three-dimensional vibration on laser cladding of SS316L alloy

As a widely used approach for surface modification, laser cladding plays a significant role in surface hardening and repairing of metallic parts. However, the defects existing in laser cladding parts, such as cracks, tensile residual stress, and pores, significantly affect the performance and the application of the cladding parts. In this study, an SS316L layer was fabricated on a 45 steel substrate using a laser cladding process assisted by three-dimensional vibration. The microstructures and the properties of the coating and the bonding interface with various vibration parameters were analyzed and compared. The microstructures of the cladding layer and the matrix were analyzed by an optical microscope and a scanning electron microscope. The results show that the three-dimensional vibration is able to refine the columnar dendrites in the cladding layer. The oscillation accelerates both the liquid flow and the thermal transfer of the molten pool, which not only reduces the temperature gradient but also increases the solidification growth rate. Meanwhile, the porosity and the maximum pore size in the cladding layer were significantly reduced by the vibration. The average microhardness of the cladding layer significantly increased assisted by the three-dimensional vibration. Experimental results demonstrate that the effects of the three-dimensional vibration are more significant than the single vertical vibration. The proposed approach provides a potential way to improve laser additive manufacturing and remanufacturing.

Resistance element welding of magnesium alloy and austenitic stainless steel in three-sheet configurations

The Mg alloy and two austenitic stainless steel sheets were joined together by a metallurgical bond across the rivet and two austenitic stainless steel sheets. More heat was generated at the austenitic stainless/austenitic stainless interface than at the rivet/ austenitic stainless steel interface, leading to larger nugget size at the austenitic stainless steel /austenitic stainless steel interface at all welding currents. Thus, the nugget size at the austenitic stainless steel /austenitic stainless steel interface mainly influenced the transition from interfacial to pullout failure modes. The fusion zone microstructure consisted of ferrite and austenite. The microstructure in the edges of the nugget (both in the rivet and ASS) consisted of fine columnar dendritic grains. Owing to variation of temperature gradient and solidification growth rate, the grains morphology changed from columnar dendritic to equiaxed dendritic in the nugget center. The fine grains resulted in high fusion zone hardness. Digital image correlation analysis revealed that the joints could experience joining zone rotation/out-of-plane displacement during lap-shear tests, which reduced the magnitude of strain sustained by the joints in the loading direction. The joint configuration that did not undergo joining zone rotation and failed via pullout failure in the austenitic stainless steel sheet exhibited superior lap-shear performance.

Three-dimensional grain growth during multi-layer printing of a nickel-based alloy Inconel 718

Heterogeneous grain structure is a source of the inhomogeneity in structure and properties of the metallic components made by multi-layer additive manufacturing (AM). During AM, repeated heating and cooling during multi-layer deposition, local temperature gradient and solidification growth rate, deposit geometry, and molten pool shape and size govern the evolution of the grain structure. Here the effects of these causative factors on the heterogeneous grain growth during multi-layer laser deposition of Inconel 718 are examined by a Monte Carlo method based grain growth model. It is found that epitaxial columnar grain growth occurs from the substrate or previously deposited layer to the curved top surface of the deposit. The growth direction of these columnar grains is controlled by the molten pool shape and size. The grains in the previously deposited layers continue to grow because of the repeated heating and cooling during the deposition of the successive layers. Average longitudinal grain area decreases by approximately 80% when moving from the center to the edge of the deposit due to variable growth directions dependent on the local curvatures of the moving molten pool. The average horizontal grain area increases with the distance from the substrate, with a 20% increase in the horizontal grain area in a short distance from the third to the eighth layer, due to competitive solid-state grain growth causes increased grain size in previous layers.

Fusion zone geometries, cooling rates and solidification parameters during wire arc additive manufacturing

Structure, properties and serviceability of components made by wire arc additive manufacturing (WAAM) depend on the process parameters such as arc power, travel speed, wire diameter and wire feed rate. However, the selection of appropriate processing conditions to fabricate defect free and structurally sound components by trial and error is expensive and time consuming. Here we develop, test and utilize a three-dimensional heat transfer and fluid flow model of WAAM to calculate temperature and velocity fields, deposit shape and size, cooling rates and solidification parameters. The calculated fusion zone geometries and cooling rates for various arc power and travel speed and thermal cycles considering convective flow of molten metal agreed well with the corresponding experimental data for H13 tool steel deposits. It was found that convection is the main mechanism of heat transfer inside the molten pool. Faster travel speed enhanced the cooling rate but reduced the ratio of temperature gradient to solidification growth rate indicating increased instability of plane front solidification of components. Higher deposition rates could be achieved by increasing the heat input, using thicker wires and rapid wire feeding.

Relation of thermal behavior and microstructure evolution during multi-track laser melting deposition of Ni-based material

A three-dimension finite element model was proposed to understand thermal behavior and microstructure evolution in multi-track laser melting deposition (LMD) of Inconel 625. The latent heat of phase change, multiple heat transfer, temperature dependent thermal physical properties were considered to ensure the accuracy of the simulation. Based on the simulated results, solidification characteristics, including temperature gradient (G), solidification growth rate (R), cooling rate (G × R) and G/R, could be obtained to predict the morphology and scale of the solidification microstructure. The results showed that the G/R was increased from 7.3 × 105 °C s/m2 at the top of the molten pool to 1.22 × 107 °C s/m2 at the bottom. As a result, columnar dendrites were generated at the bottom of the molten pool, while equiaxed dendrites were formed at the top. Simultaneously, columnar dendrites were observed at the edge of the molten pool, which was attributed to the high G/R (1.18 × 108 °C s/m2) at the edge of the molten pool. Furthermore, due to the lower cooling rate at the overlapping region than that at the bottom region, columnar dendrites generated at the overlapping region were coarser compared with those at the bottom of the molten pool. Specially, it should be noted that as increasing the number of deposited track, the G/R at the top of the molten pool exhibited with a slight increase and the G/R at the bottom presented with no obvious change. However, the G/R at the edge of the molten pool had an apparent decrease. The above simulation results showed a good agreement with the experimental results.

Numerical and experimental investigations of variable polarity gas tungsten arc welding

Variable polarity gas tungsten arc welding (VP-GTAW) has been suggested as a feasible method for welding aluminum alloys due to the enhanced surface cleaning of the oxide film. However, the principles governing the correlations between the electrode positive (EP) ratio, weld geometry, and weld microstructure are not well understood. Therefore, experiments with different EP ratios but same welding current were conducted. A three-dimensional heat transfer and fluid flow model of VP-GTAW was developed with validation against experimental results. Solidification parameters such as the temperature gradient, solidification growth rate, and cooling rate were computed from the model. The results indicate that VP-GTAW produces nearly defect-free joints. Both the top weld width and bottom weld width decrease with increasing EP ratio due to the decreased heat input. The bottom weld width is greater than the minimum weld profile width in the middle of the plate, which indicates that the Marangoni stress has a significant effect on the convective heat transfer and weld geometry. Variable grain dimensions are produced by different EP ratios at the same welding current. Weld pool oscillation originated from the alternating arc force plays a dominant role in the grain dimension.

Effect of welding wire and groove angle on mechanical properties of high strength steel welded joints

AbstractMechanical properties of high strength steel welded joints strictly depend on the welding process, the filler material composition and the welding geometry. This study investigates the effects of using cored and solid welding wires and implementing various groove angles on the mechanical performance of weld joints which were fabricated employing the gas metal arc welding process. It was found that weld joints of low alloy, high strength steels using low alloy steel cored welding wires exhibited higher tensile strength than that of low alloy steel solid wire and chromium‐nickel steel bare welding wire when the method of gas metal arc welding is employed. The effect of groove angle on the strength and toughness of V‐groove and double V‐groove butt‐joints was investigated. V‐groove joints, with higher tensile strength than double V‐groove joints in the whole range of groove angles, were superior in toughness for small groove angles, but impact toughness values of both joints were comparable for large angles. The effect of heat input and cooling rate on the weld microstructure and weld strength was also investigated by performing thermal analysis employing the commercial software ANSYS. It was concluded that cooling rate and solidification growth rate determined the microstructure of the weld zone which had great consequences in regard to mechanical properties.

Perspective on Double Pulsed Gas Metal Arc Welding

Aluminum alloy welding suffers from problems such as solidification cracking and hydrogen-induced porosity, which are sufficiently severe to limit its potential applications. Because mitigated porosity incidence and solidification cracking are observed in aluminum welds using double pulsed gas metal arc welding (DP-GMAW), a comprehensive review of the mechanism is necessary, but absent from the literature. The oscillation of arc force and droplet pressure causes a weld pool stir effect. The expansion and shrinkage of the weld pool cause unusual remelting and resolidification of the previously solidified metal. DP-GMAW has an increased solidification growth rate and cooling rate, compared with conventional pulsed welding at same heat input. Both numerical and experimental results reveal the remarkable concept that refined microstructure in the fusion zone is obtained by using DP-GMAW. The mechanism of microstructural refinement is revealed as a weld pool stir effect and increased cooling rate. Hydrogen bubbles easily float out and then release from the weld pool originated from the weld pool stir effect. Reduced solidification cracking is achieved due to the refined solidification structure that originated from the increased cooling rate. The advantages, evolution process, and future trend of DP-GMAW are discussed.

A pathway to microstructural refinement through double pulsed gas metal arc welding

In fusion welding, variations of cooling rate and microstructure are commonly achieved by changing heat input. Here we show that the cooling rates can be adjusted and microstructural features could be refined at constant heat input while changing the pulsing parameters. The temporal variations of cooling rates, solidification growth rate and weld geometry during both single and double pulsed welding of an aluminum alloy are examined. Furthermore, microstructures of both single and double pulsed gas metal arc welding are presented to demonstrate significant microstructural refinement of double pulsed gas metal arc welds for the same heat input.