Solidification Mode Research Articles

Effect of Ni addition on microstructure evolution Keyhole-GTAW WM of a duplex stainless steel

To improve the phase balance in the weld metal of keyhole gas tungsten arc welding (K-GTAW) joint, the Ni element was added in a form of pasting a certain width and thickness (depending on the added content) of nickel band on the I-type groove. The results show that three different convection zones along thickness direction, which are the Marangoni convection, a convection driven by Lorentz force, as well as a transition layer. And then the chemical composition (Ni element content) and solidification mode transform along thickness direction. Therefore, the lathy, acicular and Widmanstätten austenite, in turn, appear along thickness direction. Meanwhile, the intensity of Kurjdumov-Sachs (K-S) orientation relationships ORs dominants gradually.

On the interaction of microstructural morphology with residual stress in fiber laser welding of austenitic stainless steel

The development of compressive residual stress is governed by the metallurgical transformation and associated microstructure, which in turn influences the mechanical properties of a welded structure. In the present work, the microstructural interaction with the thermo-mechanical performance of Yb-Fiber laser-welded austenitic stainless steel (SS304) is evaluated at different heat inputs and varying defocus distances. The interrelation between microstructural morphology and the typical pattern of residual stress is systematically investigated. The development of the thermal-metallurgical-mechanical model particularly considers the effect of solid-state phase transformation (SSPT) such that the model predicts the phase fraction and its influence on residual stress distribution. The present study exclusively focuses on the mechanism of residual stress generation as a consequence of dual-phase microstructural evolution during solidification. Low heat input (45J/mm) leads to a high cooling rate, excellent tensile strength, and low tensile residual stress. The finite element-based thermo-mechanical model predicts the residual stress with a maximum deviation of ±50 MPa compared to the experimental value. The Ferritic-austenitic (FA) mode of solidification results in the combination of skeletal and lathy δ-ferrite morphology. Increasing peak intensity of δ {110} with a cooling rate confirms the enhancement of δ-phase within the austenite matrix and restricts the complete transformation to the austenitic phase. A relatively high amount of δ-ferrite due to enriched Cr and lean Ni content at the dendritic core with fine or acicular morphology at a high cooling rate reduces the longitudinal and transverse residual stress significantly. Besides, the increasing trend of primary dendritic arm-size of δ-ferrite yields a gradual increase in residual stress.

Microstructural Evaluation of Copper Brazed Joints Using Silver-Based Filler Metal

The oxy-acetylene torch brazing of copper tubes using silver-based filler metals (Ag-Cu-Zn-Cd) was investigated in this study. Filler metals containing silver content of 15% and 35%, designated by BR15 and BR35, were used to assess the influence of the silver content in the microstructure and mechanical performance of brazed joints. The brazed joints microstructure was analyzed by means of optical microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The microstructures of the BR15 and BR35 samples were composed mainly of Ag–Cd-rich phase, solid solution of Cu–Zn, and a eutectic structure of Ag and Cu. The increase in the Ag content changed the solidification mode from cellular to dendritic and caused an increase in the eutectic phase volume fraction. The tensile strength of the Ag–Cu–Zn–Cd filler and the phos–copper filler metals was compared. The tensile strength of silver-based filler metal was slightly higher than the phos–copper filler metal brazed joints.

Influences of cooling rates on delta ferrite of nuclear power 316H austenitic stainless steel

High temperature confocal melting was conducted to study the volume fraction variation of as-cast δ-ferrite at cooling rate from 5 °C/min to 6000 °C/min of 316H austenitic stainless steel (316H). The results demonstrate that the solidification mode of 316H was ferrite-austenite type (FA mode) regardless of the cooling rate. The peritectic reaction occurs between ferrite and liquid phase during solidification. The higher the cooling rate is, the higher the δ-ferrite volume fraction is. The δ-ferrite fraction variation can be explained by the combination of FA solidification mode during solidification and ferrite to austenite solid-state transformation during cooling after solidification.

Investigation of solidification modes of high Manganese steels

ABSTRACT The solidification modes of high manganese steels were measured by experiments and comparatively calculated by different predicted methods. The DSC experiments indicate the same phase transformation sequence as the isothermal solidification experiments. It is demonstrated that the solidification modes of the Fe–0.028C–21.1Mn–5.72Al, Fe–0.28C–21.0Mn–5.80Al and Fe–0.64C–21.0Mn–5.80Al alloys are F mode, FA1 mode and FA2 mode. The change of heating/cooling rate influences the position and shape of the peaks in the DSC curves but has no effect on the solidification mode. By comparison with different calculation methods, the equilibrium model is shown to be a valuable tool to predict the solidification sequence of the high-Mn steel, from which the critical carbon range for the Fe–C–20Mn–5Al alloy is calculated to be 0.062∼0.427 wt-%.

Combining CaO–SiO2–TiO2 and CaO–SiO2–Al2O3 ternary phase systems for design of bimetallic welds

The bimetallic welds are frequently utilized for pipeline transport system of the nuclear power plants. The occurrences of welding defects generally depend on the filler electrode as well as the electrode coatings during shielded metal arc welding process. This study involves the design of austenitic stainless steel welding electrodes for SS304L–SA516 bimetallic welds. The objective of research work includes the novel design of Al2O3–TiO2–CaO–SiO2 coatings by combining two ternary phase systems using extreme vertices mixture design methodology to analyze the effect of key coating constituents on the weld metal chemistry and mechanical properties of the welds. The significant effect of electrode coating constituent CaO on weld metal manganese content is observed which further improves the toughness of bimetallic weld joints. Various regression models have been developed for the weld responses and multi objective optimisation approach using composite desirability function has been adopted for identifying the optimized set of electrode coating compositions. The role of delta ferrite content in promoting the favourable solidification mode has been studied through microstructural examination.

Influence of Assembly Gap Size on the Structure and Properties of SUS301L Stainless Steel Laser Welded Lap Joint.

The microstructure and properties of laser welding lap joints with different assembly gap sizes are experimentally investigated. The laser weld joint is composed of γ-austenite and δ-ferrite, and the strip ferrite phase is mainly distributed at the austenite grain boundary. The weld metal presents the austenitic-ferritic (AF) solidification mode. When there is a gap between the two plates, a triangular region composed of similar equiaxed crystals can be found, and the size of the cellular crystals in this region decreases significantly. When the assembly gap size increases from 0.1 mm to 0.4 mm, the weld penetration state of the joint changes from full penetration to semi-penetration, and the surface collapse increases. The excessive size of the gap leads to a decrease in the tensile-shear force and fatigue strength of laser welded joints. In order to ensure that the surface morphology and properties of the welded joint meet the quality standard and requirement, the assembly gap should be less than 0.1mm.

Effect of backing gas mixtures on corrosion properties of stainless steel grade 304 weld metal by autogenous GTAW

Pitting corrosion would pose difficulty for the cleaning of equipment used in food processing, even when using the Cleaning in Place (CIP) process. It tends to occur at a location requiring welding. In general, Gas Tungsten Arc Welding (GTAW) is the most applicable process for food and drug equipment. To improve the properties of the weldment, filler metals are commonly used. However, the use of thin-sheet stainless steel in the application of filler metals would be difficult in food processing. In this research, the weld metal properties would be improved only by using N2 mixing to the backing gas during welding. Experiments were conducted using various mixtures of N2 on 1.5, 2.0, and 3.0 mm thicknesses of 304 stainless steel. The roots of weld metal taken from the samples were evaluated for corrosion properties in 3.5%NaCl from the polarization curves. The results showed the improvement of pitting corrosion from 379 mV to 757 mV, while the general corrosion rate decreased from 0.00477 mm/y to 0.00099 mm/y with 100%Ar and 100%N2, respectively. Both Nitrogen content and solidification rate together affected the solidification modes and ferrite contents that would affect the corrosion properties. At least 15%N2 mixing would be sufficient for weld metal property improvement. Due to the decrease of ferrite content from N2 addition, solidification cracking susceptibility test by fishbone test was acceptable with no cracking observed, showing the potential to recommend N2 mixtures for actual production.

Weld Metal Microstructure Prediction in Laser Beam Welding of Austenitic Stainless Steel

In the present work an approach to weld metal microstructure prediction is proposed, based on an analytical method that allows the evaluation of the thermal fields generated during the laser beam travel on thick plates. Reference is made to AISI 304L austenitic steel as a base material, with the aim to predict the molten zone microstructure and verify the best condition to avoid hot cracking formation, which is a typical issue in austenitic steel welding. The “keyhole” full penetration welding mode, characteristic of high-power laser beam, was simulated considering the phenomenological laws of conduction by the superimposition of a line thermal source along the whole thickness and two point sources located, respectively, on the surface and at the position of the beam focus inside the joint. This model was fitted on the basis of the fusion zone profile, which was experimentally detected on a weld seam obtained by means of a CO2 laser beam, in a single pass on two squared edged AISI 304L plates, that were butt-positioned. Then the model was applied to evaluate the thermal fields and cooling rates, the fusion zone composition and the solidification mode.

Efficient thermal management of the photovoltaic/phase change material system with innovative exterior metal-foam layer

This research proposes an innovative configuration for modifying the thermal management system of the photovoltaic (PV) panel, integrated with a phase-change material (PCM) with an attached exterior metal-foam layer to the rear part of the PCM container. The configuration enables faster heat dissipation from the system leading to longer thermal-management duration for the PV panel during the energy storage (PCM charging) mode. Also, it supports faster energy recovery (PCM discharging) rate by decreasing the amount of heat trapped in the melted part of the PCM during the solidification mode. The effects of the foam-layer inclusion, PCM thickness, and PV tilt angle are numerically simulated and revealed. A mathematical model that accounts for natural convection effects of the melted PCM and non-Darcy effects of the metal foam, is formulated and validated via previous experiments. The results show that inclusion of an exterior foam layer could increase the PCM melting time, and the corresponding PV thermal-management duration, by up to 32% and 55% respectively, depending on the PCM thickness and PV tilt angle. The results also show that reducing the tilt angle from 90° to 30° increases the melting time by about 18%. Thus, a superior PCM-based thermal management is achieved. Moreover, utilizing PCM of thickness in the range of 20–30 mm lengthens the melting time by up to 54%, enhancing the potential for improved thermal management of PV panels.

Analyzing and Predicating PH value in Nature Rubber Solidification Process Using Multiple Regression Method

Acid coagulation is the most common process in nature rubber(NR) processing and the acid concentration in latex is a key factor that will decide the quality of processed rubber. In order to invalid the change of PH value processed with acid concentration on latex clots, a group division experiment is designed to obtain the data of PH value, smell and color in latex clots under different solidification time, mode, and temperature. Then a multiple regression model based on Statistical Product and Service Solutions (SPSS) is used to analyse and predicate the PH value in different conditions. The result shows that predication method can have a good result and the average relative error of PH value is 4.60%.

Solidification Cracking Susceptibility of the Weld Metal of Additively Manufactured 316L Stainless Steel

In this study, the solidification cracking susceptibility of the weld metal of additively manufactured 316L stainless steel was investigated. Laser powder bed fusion was employed to fabricate rectangular specimens with a thickness of 4 mm, and Trans-Varestraint tests were performed to evaluate the weldability of the specimens. Subsequently, the maximum crack lengths were measured and used as indices of susceptibility. The results showed that the weld metal of additively manufactured 316L stainless steel had higher susceptibility to solidification cracking than that of conventional wrought 316L stainless steel. The high susceptibility was considered mainly due to the primary austenite solidification mode in the weld metal of the additively manufactured specimen having a relatively higher content of nitrogen. Key words: Additive manufacturing, Powder bed fusion, Solidification cracking, 316L stainless steel

Banding and microstructural features in laser cladding of a 304 substrate using 316 powder

The aim of the cladding process is to improve mechanical, chemical, and physical features of the surface of metals. This laser process is frequently used to promote such alterations via deposition of metals/alloys with mechanical behavior superior to that of the substrate. In this contribution, an AISI 304 austenitic stainless steel is covered with an AISI 316 as powder filler metal using multiple laser deposition parameters. In the produced samples, the effect of the studied parameters on the geometry, surface appearance, dilution, and microstructural features was evaluated. The authors focus on metallurgical characteristics of cladding, seeking to explain the formation of three attributes: banding, columnar structure, and equiaxial grains. It was determined that banding is composed of a cellular-dendritic structure and is generated by changes in the microstructure size, derived from local variations in the solidification rate. The same applies for the columnar and equiaxial zones, but the appearance of equiaxial geometry is due to the change of the growth direction of the aforementioned columns, in the middle of the fusion zone. All the samples confirmed the formation of F/MA-type solidification mode, which indicates that the cooling rate is low and similar to those recorded in conventional fusion welding.

Effect of Fillers and Autogenous Welding on Dissimilar Welded 316L Austenitic and 430 Ferritic Stainless Steels

This experimental study examines the effects of dissimilar welding of 316L austenitic and 430 ferritic stainless steels, welded with gas tungsten arc welding process with (ER316L and ER309L) and without fillers (autogenous). Microstructural examination was performed by optical and scanning electron microscopy. ER316L and ER309L fillers welds illustrated equiaxed and columnar grains; ER309L weld showed higher Creq/Nieq ratio than that of ER316L weld, produced vermicular ferrite, while autogenous weld shows lathy ferrite and martensite. Chromium carbide precipitate was also observed in autogenous weld. The mechanical properties were measured and higher hardness was obtained in autogenous weld compared to ER316L and ER309L filler weld. Higher tensile and impact strength was measured in ER316L filler weld as compared to ER309L filler and autogenous weld. The corrosion resistance of the joints was carried out by double loop electrochemical potentiokinetic reactivation and potentiodynamic polarization test to assess the sensitization and pitting resistance, respectively. Higher degree of sensitization and pitting were observed in autogenous weld as compared to ER316L and ER309L filler weld due to precipitation formation, but the pitting corrosion was found to be lower for ER309L filler weld as compared to ER316L filler and autogenous weld due to the effect of mode of solidification.

Microstructure and Fatigue Behavior of 2205/316L Stainless Steel Dissimilar Welded Joints

The relation among microstructure and fatigue behavior of 2205/316L stainless steel dissimilar welded joints was investigated. Plates of 6.35 mm in thickness with a single-V joint configuration were gas metal arc welded (GMAW) in a single pass by feeding at 6 m/min an ER2209 filler wire with a heat input of 1.2 kJ/mm. Grain growth in the high temperature-heat affected zone (HT-HAZ) occurred mostly at the mid-height of the plates, delimiting the width of this region up to ~1.28 and ~0.73 mm of the 2205 and 316L plates, respectively. Dilution of the 316L plate with the ER2209 filler altered the solidification mode in this side of the weld and led to a significant content of austenite along the fusion line. Fatigue tests were performed using sinusoidal waveform at room temperature applying uniaxial cyclic loading, between constant stress limits within the elastic deformation of tension and compression (Δσ) with stress ratio R = −0.3. With stress ranges of 98% and 95% the fatigue specimens rapidly failed in much less than 106 cycles. The failure crack initiated at the surface of the 316L in the HT-HAZ near the weld toe. Surface analyses of unbroken specimens before and after fatigue testing revealed a significant increment in roughness of the 316L base material owing to the formation of intrusions and extrusions.

Microstructure and mechanical properties of AlNiCuCoX high-entropy Alloy

AlNiCuCoX (X=Cr, CrTi, CrFe, FeTi and CrFeTi) series high-entropy alloys were prepared by vacuum arc furnace. The microstructures and properties of the alloys were analyzed. The results show that the microstructures of AlNiCuCoX alloys are composed of FCC solid solution and BCC solid solution, and the number of phases formed in the alloys is far lower than that predicted by equilibrium phase law. AlNiCuCoCrTi shows chrysanthemum like microstructure, while AlNiCuCoFeTi and AlNiCuCoCrFeTi alloys shows petal-like, chrysanthemum like and reticular (intercrystalline) white microstructure. The principal element is the key factor affecting the solidification mode (hypoeutectic, eutectic and hypereutectic solidification) of AlNiCuCoX series alloys. If the interdendritic phase is not considered, AlNiCuCoCr and AlNiCuCoCrFe high-entropy alloys are solidified in the end-solid solution solidification mode. When Ti is added, the alloy is solidified in the hypoeutectic solidification mode, and the equilibrium microstructure is composed of primary phase and eutectic. The microhardness of the most alloys increased with the increase of the number of principal elements, and the hardness of AlNiCuCoCrFeTi alloys is the highest (548HV). The compressive strength and strain of most AlNiCuCoX alloys are low (especially AlNiCuCoCrFeTi alloy). Both AlNiCuCoCr and AlNiCuCoCrFe alloys have better comprehensive mechanical properties.

Welding of nitrogen containing austenitic stainless steel using GMAW process

In this study, the welding aspects of 21–4-N Nitronic Steel has been considered. It was found that the clusters of carbide were dissolved after solution annealing of the as-received base material at 950 °C and 1150 °C temperatures for 120 min, followed by water quenching. The weld solidification mode was changed from austenite-ferrite at the weld-HAZ interface to ferrite–austenite at the weld center as per their Cr/Ni equivalent ratio. Weld center has minimum austenite and maximum ferrite and this ferrite content decrease towards HAZ (heat affected zone). Further, grain coarsening and carbides dissolution took place due to the heating effect of the weld thermal cycle in multipass welding. The tensile strength was improved after solution annealing for different base metal conditions and weld made on respective base metals. The welding made on 1150 °C solution annealed samples were outperformed.

Effect of weld metal composition on impact toughness properties of shielded metal arc welded ultra-high hard armor steel joints

Abstract Nowadays, ultra-high hard armor (UHA) steels are employed in armor tracked vehicle (ATV) construction because of their high hardness, high strength to weight ratio, and excellent toughness. UHA steels are usually welded using austenitic stainless steel (ASS) welding consumables, to avoid hydrogen-induced cracking (HIC). The use of ASS consumables to weld the above steel was the only available remedy because of higher solubility of hydrogen in the austenitic phase. In this investigation, an attempt was made to investigate the effect of ASS consumables (with different Cr eq /Ni eq ratio) on solidification mode, impact toughness and microstructural characteristics of shielded metal arc (SMA) welded UHA steel joints. The welded joints were characterised based on impact toughness properties, hardness, and microstructural features. As the ferrite number increases with an increase in Cr eq /Ni eq ratio result in different solidification mode (A, FA, F). It is also found that ferrite number of weld metal has appreciable influence on impact toughness and has inversely proportional relationship with impact toughness of the welded joints.

Electrostatic levitation processing and microscopic hardness property of hyperperitectic Ti60Ni40 alloy

The solidification mechanism of hyperperitectic Ti60Ni40 alloy was systematically investigated by using the electrostatic levitation (ESL) technique, and the maximum undercooling achieved was up to 373 K (0.25 TL). When the undercooling is below a threshold of 281 K, typical peritectic solidification prevailed within the levitated liquid alloy, and the double recalescence processes were observed by a high-speed camera. In this case, the primary TiNi compound preferentially grew as well-defined dendrites, and the interdendritic Ti2Ni phase was subsequently produced by the peritectic reaction and transformation. TEM analyses showed that an incomplete martensitic transformation occurs in the primary TiNi phase, which results in the complicated structure of B19’ martensite and R-phase. Once the undercooling exceeded 281 K, the coupled-growth mode between TiNi and Ti2Ni compounds was induced, leading to the lamellar and anomalous eutectic-like microstructures. The enhancement of undercooling promoted the migration speed of liquid-solid interface in both typical peritectic solidification and metastable coupled-growth modes, but it exhibited an abrupt reduction in critical undercooling. The microscopic hardness of the primary TiNi phase processed by ESL was 48.1% higher than that of the Ti60Ni40 master alloy, while it increased by 29.0% for the peritectic Ti2Ni phase. These indicated that the peritectic-type superalloys with advanced performances could be designed by modulating the solidification process and solid solubility through different undercoolings.

Addition of Silver to an Mg–Al–Zn Alloy Treated by Conventional and Chilled Solidification: A Microstructural Approach

AZ91 alloy was produced under different solidification modes and the effect of various amounts of silver on the microstructure of the samples was studied. The results showed that addition of silver modified the morphology of Mg17Al12 intermetallic phase. The continuous grain boundary network of Mg17Al12 was broken. In addition, silver altered the morphology and distribution of Mn–Al intermetallic compound. Changing the mode of solidification from conventional to chilled solidification with simultaneous addition of silver had a significant effect on the phase distributions within AZ91. The morphology and distribution of Mg17Al12 and Mn–Al were a sensitive function of these parameters. Chilled solidification changed the microstructural constituents in a manner in which the effect of silver was approximately omitted. Addition of 0.4 wt % of silver altered the phases within the microstructure in an opposite way from the samples which contained 0.1 wt % of silver.