Liquid Film Migration Research Articles

Metallurgical interactions in solid/liquid diffusion couples of AA4343/AA3xxx/AA4343 layered aluminum sheet

The effect of pre-stretching on solid-liquid interaction, microstructure and brazing performance of AA4343/AA3xxx/AA4343 layered aluminum sheet during brazing at the atomic level (solid solution), nanoscale (dislocation) and micron level (grain boundary) are studied by glow discharge optical emission spectrometry (GDOES), scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The microstructure after brazing largely depends on the forming condition. The driving force of liquid film migration (LFM) is the reduction of stored deformation energy. The interaction between the liquid clad and solid core layers changes the elemental distribution and microstructure, which in turn affects the brazing performance of the sheet. The results reveal the mechanism of solid-liquid interaction of layered aluminum sheet during brazing and clarify the driving force of LFM of layered aluminum sheet during brazing. Consequently, these results provide important insights for the design and theoretical research of functional layered aluminum sheet.

Substantial coercivity enhancement in Dy-free Nd-Fe-B sintered magnet by Dy grain boundary diffusion

To date, the highest coercivity reported for Dy-free Nd–Fe–B sintered magnets under the Dy grain boundary diffusion (GBD) process is ∼2.4 T. Therefore, exploring various approaches for achieving higher coercivity is beneficial for eliminating the dependence on the scarce and expensive heavy rare-earth elements, Dy and Tb. In this study, we report record-breaking high coercivity by separately diffusing Dy70Cu30 and then Pr68Cu32 eutectic alloys through grain boundaries in fine-grained Dy-free sintered magnets. The two-step GBD-processed sample containing a low Dy content (only 0.9 wt%) exhibited a high coercivity of 2.8 T and an excellent temperature coefficient of coercivity of β = -0.447%/ °C. The second GBD process facilitated Dy infiltration from the bulk surface toward the center by chemically induced liquid film migration to form a uniform Dy-rich shell throughout the bulk. Pr was also enriched in the Dy-rich shell near the surface of Nd2Fe14B grains in the two-step GBD-processed sample. However, the Pr-enrichment in the shell did not cause the degradation of the coercivity at high temperatures. This unique shell structure strengthened the anisotropy field to achieve 2.8 T coercivity without alloying Dy to the initial sintered magnet.

Computational simulation of thermo-mechanical field and fluid flow and their effect on the solidification process in TWIP steel welds

The primary austenitic solidification mode of Twinning Induced Plasticity (TWIP) steel weld joints is one of the most important issues limiting its weldability. In order to gain knowledge on this topic, a comprehensive study was carried out through numerical simulation of diffusive-convective phenomena of fluid flow and heat transfer of the melted material in the weld pool of TWIP steel weld joints. Weldments were performed using different heat inputs and constraint conditions. The predictions of convective motion, thermal history and cooling rates estimated by finite element (FE) and finite volume (FV) simulations were correlated with chemical species segregation (Mn, C, Al and Si) and the solidification mode in the fusion zone. The microstructural characterization of weld joints and the numerical results provided a full analysis of the weld pool solidification process. Results revealed a relationship between both the dendrite size and Mn segregation with the estimated velocity field of liquid material in the melted zone. Moreover, the presence of δ-ferrite was experimentally detected in some weld joints and associated with the convective movement of melted material and chemical segregation. The cellular dendritic growth mode found in some weld joints contributed to the formation of δ-ferrite and promoted the transition of the solidification mode from austenitic to ferritic-austenitic. As well δ-ferrite allowed to mitigate hot-cracking and interrupted the Mn-enriched liquid film migration towards the heat-affected zone, avoiding the formation of liquation cracks. The presence of δ-ferrite phase maintained a high mechanical strength providing improved elongation regarding the welded material mechanical properties.

The Effect of Microstructural Evolution on the Brazeability of Two-Layer Al Sheets

In this study, the microstructural evolution and the interaction between the clad and the core alloys that occurs during the brazing process of two-layer Al sheets with equiaxed grains were examined. The study was carried out using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and glow discharge optical emission spectrometry (GDOES). The effects of microstructure on the brazing performances of two-layer sheets were clarified. Although the grains were fine and equiaxed before brazing, three typical microstructural evolutions happened during brazing, corresponding to three kinds of interactions between the clad and core alloys of the aluminum brazing sheets. In the alloys, which had either relatively uniform grain growth or no grain growth, the interaction between the clad alloy and the core alloy was weak; accordingly, they showed a smooth surface, an even microstructure, faint element mutual diffusion, and eventually good brazeability. Meanwhile, in the alloy with obvious abnormal grain growth (AGG), strain-induced liquid-film migration (SILFM) occurred when the energy was too low to cause strain-induced boundary migration (SIBM). This led to rough and uneven surface morphology, significant mutual diffusion, and surface segregation of elements; eventually, this produced the worst brazeability.

Phase field modeling of partial remelting during reheating of a multiphase peritectic solidification microstructure

While they are known for years to be the main source of equiaxed grains in metallurgical processes such as casting, melting and remelting phenomena currently receive an increasing attention owing to the developments of additive manufacturing. In multiphase alloys, i.e. having a eutectic or peritectic microstructure, these phenomena still remain poorly documented. In this work, the solidification and subsequent remelting of peritectic alloys have been studied using phase field simulations, with emphasis on the influence of the peritectic equilibrium, that yields the peritectic reaction L+β→α below the peritectic temperature and the reverse peritectic reaction α→L+β above. During solidification, the growth of the peritectic α phase has little influence on the growth of the pro-peritectic β phase. On the other hand, the evolution during remelting of a previously solidified peritectic microstructure is more complex and involves temperature gradient zone melting and liquid film migration phenomena. Close to the peritectic temperature, the reverse peritectic reaction is driven by liquid film migration, and some scenarii are evidenced depending on whether the growth takes place without triple junction from a pure α phase or with a triple junction along the α/β interface. At the scale of the mushy zone, the Temperature Gradient Zone Melting phenomenon also occurs but remains incomplete at the time scale investigated here. Finally, a new mechanism for fragmentation, induced by the reverse peritectic reaction during remelting, has been identified.

Modelling of liquid film migration in Al-Cu alloys.

Microstructural evolution in the presence of liquid film migration (LFM) is simulated for Al-Cu alloys using a cellular automaton (CA) model. Simulations are performed for the microstructural evolution and concentration distribution in an Al-4 wt.%Cu alloy with initially equiaxed grain structures holding in a temperature gradient. A slight deviation from local equilibrium, estimated from experimental data, is considered to be the driving force for LFM. The direction of LFM is triggered by concentration fluctuations setting a concentration gradient as a further driving force. The simulation successfully reproduces the experimentally observed microstructures generated by LFM accompanied by a particle free zone behind the liquid film. The solid concentration in the particle free zone is found to be the equilibrium solid concentration. The simulated concentration profile across the migrating liquid film agrees well with experimental measurements. The simulated grain structure becomes coarser and highly elongated after holding in the temperature gradient. The results reveal that the increase in transversal grain width is mainly controlled by LFM, while the grain elongation in longitudinal direction is attributed to both LFM and temperature gradient zone melting. The solid concentration decreases from the initial (supersaturated) composition to the local equilibrium solid concentration corresponding to the local temperature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Gibbs-Thomson effect as driving force for liquid film migration: Converting metallic into ceramic fibers through intrinsic oxidation

Liquid film migration is of great practical importance in materials engineering. The phenomenon has been shown to depend on thermal gradients and coherency strain, but no single driving mechanism seems capable of justifying the whole array of experimental observations. On the other hand, the inevitable capillarity effects are often disregarded due to the unknown 3-dimensional geometry of the system. Here, we present evidence of liquid film migration governed primarily by capillarity through a microstructural setup of cylindrical interfaces that allows clear interpretation and modeling. The experiments rely on the strong oxygen-gettering ability of tantalum fibers dispersed in a tungsten matrix and on field-enhanced diffusivity provided by pulse plasma compaction. Tantalum scavenges the residual oxygen present in the W powder and, as a result, oxide films grow around the fibers. These oxide tubes, in liquid state during sintering, migrate toward the fiber axis and eventually become oxide rods surrounded by metallic Ta. The process is driven by the Gibbs-Thomson effect that generates the required composition gradient across the liquid film. An analytical description of the film evolution is implemented by combining the incoming O flux with capillarity-driven migration. Possible contributions from other mechanisms are examined and the relevance of the Gibbs-Thomson effect to the general phenomenon of liquid film migration is established.

Internal Corrosion of Warm Formed Aluminum Alloy Automotive Heat Exchangers

The objective of this study is to evaluate the effects of sheet temper, forming conditions, and the resultant post-braze microstructure on the corrosion behavior of Al alloy heat exchangers. Accelerated corrosion tests were performed by flowing a corrosive water solution through brazed heat exchanger samples fabricated from O and H24 temper sheet formed between room temperature and 250 °C. Sheet perforations within the brazed samples were found to be concentrated at the coolant channel inlets, due to crevice corrosion, or around other formed features within the coolant channel, which was attributed to erosion corrosion. A clear relationship between corrosion initiation sites and local microstructure was not observed in the accelerated test. Potentiodynamic polarization measurements were performed using the corrosive water solution and sheet samples pre-strained to 4 and 10% and subjected to a simulated brazing heat treatment. The average corrosion current density was the same for the two conditions, but liquid film migration occurred during brazing in the sample strained to 4%, resulting in localized corrosion and greater variability in the polarization measurements. It is concluded that warm forming does not impair the corrosion resistance of H24 temper sheet, but the corrosion resistance of brazed samples fabricated from O temper sheet is altered by warm forming.

Gibbs-Thompson Effect as Driving Force for Liquid Film Migration

Liquid film migration is of great practical importance in materials engineering. The phenomenon depends on thermal gradients and coherency strain, but no single driving mechanism seems capable of justifying all experimental observations. On the other hand, the inevitable capillarity effects are often indeterminable due to the unknown three-dimensional geometry of the system. Here, we present evidence of liquid film migration governed primarily by the Gibbs-Thomson effect through a microstructural setup of cylindrical interfaces designed to allow clear interpretation and modeling. The experiment relies on the strong oxygen-gettering ability of tantalum fibers dispersed in a tungsten matrix and on field-enhanced diffusivity provided by pulse plasma compaction. Tantalum scavenges residual oxygen present in the W powder and, as a result, oxide films grow around the fibers. These oxide tubes, in liquid state during sintering, migrate toward the fiber axis and become surrounded by external rims of metallic Ta. An analytical description of the film evolution is implemented by combining the incoming O flux with capillarity driven migration. P ossible contributions from other mechanisms are examined and the r elevance of the Gibbs-Thomson effect to liquid film migration is established .

Microstructure evolution of warm deformed multilayered Al alloy sheet during brazing

Warm forming can improve the formability of Al alloy sheets that are used in heat exchanger production, but the effect of warm forming on the brazing performance of the sheet is not well understood. This investigation evaluates the brazing performance and corresponding microstructure evolution of a warm formed sheet comprised of an Al-Mn core alloy and an Al-Si clad alloy. Understanding the conditions that lead to the detrimental process of liquid film migration (LFM) during brazing is of particular interest. Warm forming was simulated by tensile tests performed between room temperature and 250 °C, and at strain rates between 7E-4 s −1 to 7E-2 s −1 . A brazing treatment consisting of a 3 min hold at 600 °C was simulated using differential scanning calorimetry. Brazing performance was evaluated by the ratio of the clad alloy solidification energy to the initial melting energy ( i.e. ΔH sol /ΔH melt ). The post-braze microstructure and ΔH sol /ΔH melt depended on strain, strain rate, and forming temperature. ΔH sol /ΔH melt was 0.5 or less when LFM occurred during brazing, but improved to 0.7 when the core alloy recrystallized. The dislocation density in the sheet was calculated using the Bergstrom hardening model; dislocation densities over which LFM and full core recrystallization occurred were distinct from one another. When LFM did not occur, isothermal solidification reduced the amount of clad alloy available for brazing by 23 %. It is concluded that the post-braze microstructure strongly depends on the sheet forming conditions, and that the driving force for LFM is the reduction of stored deformation energy.

Formation mechanism of Tb-rich shell in grain boundary diffusion processed Nd–Fe–B sintered magnets

We propose a new mechanism for the formation of Tb-rich shell in the Nd–Fe–B sintered magnet that was subjected to the grain boundary diffusion (GBD) process using TbH2. Tb-enriched liquid films form along grain boundaries during the GBD process, which migrate by chemically induced liquid film migration (CILFM) to form Tb-rich shells behind. As a result, the Tb-rich shell form asymmetrically along a grain boundary with an abrupt concentration change at a core/shell interface with a gradual decrease of Tb concentration toward a grain boundary. These microstructure features can be reproduced with phase-field simulations.

The semi-solid microstructural evolution and coarsening kinetics of AZ80-0.2Y-0.15Ca magnesium alloy

The semi-solid billets of the AZ80-0.2Y-0.15Ca magnesium alloy (AZ80M) were fabricated through the strain-induced melt activation (SIMA) process by hot extrusion and then partial remelting. The effects of isothermal temperature and soaking time on the microstructural evolution of the SIMA processed AZ80M in the semi-solid state were analyzed. The results showed that the average grain size (Deq) and the shape factor (SF) of globular grains increased with the increase of the isothermal temperature. However, the value of Deq has visibly declined trend with the isothermal temperature rising to 600 °C. The results of the coarsening kinetics of AZ80M alloy indicated that the coarsening rate (K) increases first and then decreases with the reduction of solid volume fraction (fS) and the critical solid volume fraction (fS0) is 0.5. The coarsening mechanism plays a dominant role in the growth behavior of the solid-grains when fS ≥ 0.5, which can be explained by the modified liquid film migration (MLFM) model. However, the melting mechanism plays a more critical role than coarsening mechanism when fS ≤ 0.5, which can be explained by the liquid film migration (LFM) model. In addition, the SIMA processed AZ80M alloy has a lower coarsening rate than other magnesium alloys, which most likely due to the addition of rare earth elements.

An assessment of the brazing performance of warm formed automotive heat exchangers

Scaled-down electric vehicle battery cooling plates were formed from multi-layered Al brazing sheets in the O and H24 sheet tempers between room temperature and 250 °C. The formed plates were joined using an industrial furnace brazing process. All brazed assemblies were leak-free when an internal pressure of 0.28 MPa was applied, demonstrating the integrity of the components. Digital radiography of the brazed assemblies revealed only localized defects within individual samples, with no obvious patterns appearing across all samples of a given forming condition. Thus, no clear difference in brazing performance between forming conditions was noted at the component level, although slightly more consistent brazed fillets were noted for the H24-250 °C condition compared to H24-RT. Strain distributions throughout the formed plate were predicted using a finite element model, to facilitate microstructure analysis of the brazed assemblies. The post-braze microstructure in plates formed from O temper sheet depended on the local strain, but did not strongly depend on forming temperature: liquid film migration and a non-recrystallized core were observed at locations of relatively low strain, while coarse recrystallized core grains and a clad residue were found at locations of high strain. Conversely, the microstructure was homogeneous throughout H24 temper plates formed at room temperature and at 250 °C, and consisted of a recrystallized core alloy and a clad residue layer at the sheet surface. It is concluded that warm forming does not adversely affect the brazing performance of Al brazing sheet and, therefore, is a promising manufacturing technique for the production of next-generation heat exchangers.

The microstructural evolution and hardness of the equiatomic CoCrCuFeNi high-entropy alloy in the semi-solid state

The semi-solid slurries of the CoCrCuFeNi high entropy alloy (HEA) were fabricated through the recrystallization and partial melting (RAP) process by cold-rolling and partial remelting. Based on Kim's theory, the solidus-liquidus temperature range and the fraction of the liquid phase as a function of temperature were examined from the Differential Scanning Calorimetry (DSC) curve. The effect of isothermal temperature and soaking time on the microstructural evolution of the CoCrCuFeNi HEA in a semi-solid state were analyzed. The results showed that the globular structure was obtained without the new phase formed for the semi-solid slurries, containing two face-centered-cubic (FCC) solid-solution phases. The average grain size of the globular grain and the globular degree increased with increasing the isothermal temperature and soaking time. The study of the coarsening kinetics indicated that the coarsening process could be described by the modified liquid film migration model. The sluggish diffusion effect of the current HEA leaded to the low coarsening rate, compared with other conventional alloys. However, the existence of the liquid phase in the semi-solid state resulted in a higher grain growth rate for the CoCrCuFeNi HEA with a lower activation energy of 206.9 kJ/mol, compared with the diffusion couple and the grain growth process for the other HEAs at low temperatures. To evaluate the mechanical properties, Vickers hardness was measured to represent the strength of the CoCrCuFeNi HEA. The CoCrCuFeNi semi-solid slurries showed a Hall-Petch type relationship of the average grain size dependence on Vickers hardness.

Sagging resistance of warm formed aluminum brazing sheet

Interrupted tensile tests, performed between room temperature (RT) and 250°C, were used to simulate warm forming of an AA3003/AA4045 brazing sheet. Brazing performance was predicted from sagging distance measurements after a thermal cycle. The sagging distance as a function of strain for sheets strained at 150°C was similar to that of RT strained samples, while the sagging distances were larger at all levels of applied strain for sheets strained at 200°C and 250°C. Large sagging distances were correlated with the occurrence of liquid film migration during simulated brazing and a recovered substructure in the core alloy, while small sagging distances were associated with a coarse, recrystallized core alloy. The poor brazing performance of sheets formed above 150°C was attributed to a reduction in the stored strain energy during forming, resulting in recovery rather than recrystallization during brazing, leaving a microstructure which is susceptible to liquid film migration.

Liquid Film Migration in Warm Formed Aluminum Brazing Sheet

Warm forming has previously proven to be a promising manufacturing route to improve formability of Al brazing sheets used in automotive heat exchanger production; however, the impact of warm forming on subsequent brazing has not previously been studied. In particular, the interaction between liquid clad and solid core alloys during brazing through the process of liquid film migration (LFM) requires further understanding. Al brazing sheet comprised of an AA3003 core and AA4045 clad alloy, supplied in O and H24 tempers, was stretched between 0 and 12 pct strain, at room temperature and 523K (250 °C), to simulate warm forming. Brazeability was predicted through thermal and microstructure analysis. The rate of solid–liquid interactions was quantified using thermal analysis, while microstructure analysis was used to investigate the opposing processes of LFM and core alloy recrystallization during brazing. In general, liquid clad was consumed relatively rapidly and LFM occurred in forming conditions where the core alloy did not recrystallize during brazing. The results showed that warm forming could potentially impair brazeability of O temper sheet by extending the regime over which LFM occurs during brazing. No change in microstructure or thermal data was found for H24 sheet when the forming temperature was increased, and thus warm forming was not predicted to adversely affect the brazing performance of H24 sheet.

On migration of primary/peritectic interface during interrupted directional solidification of Sn-Ni peritectic alloy

The migration of the primary/peritectic interface in local isothermal condition is observed in dendritic structure of Sn–Ni peritectic alloy after experiencing interrupted directional solidification. It was observed that this migration of primary Ni3Sn2/peritectic Ni3Sn4 interface towards the primary Ni3Sn2 phase was accompanied by migration of liquid film located at this interface. The migration velocity of this interface was confirmed to be much faster than that of peritectic transformation, so this migration was mostly caused by superheating of primary Ni3Sn2 phase below TP, leading to nucleation and migration of liquid film at this interface. This migration can be classified as a kind of liquid film migration (LFM), and the migration velocity at the horizontal direction has been confirmed to be much faster than that along the direction of temperature gradient. Analytical prediction has shown that the migration of liquid film could be divided into two stages depending on whether primary phase exists below TP. If the isothermal annealing time is not long enough, both the liquid film and the primary/peritectic interface migrate towards the primary phase until the superheated primary phase has all been dissolved. Then, this migration process towards higher temperature is controlled by temperature gradient zone melting (TGZM).

Liquid film migration in laser welded joint of Inconel 617

Abstract

Simulation of Liquid Film Migration during Melting

Melting of a single-phase polycrystalline material is known to start by the formation of liquid films at the surface and at grain boundaries. The internal liquid films are not necessarily quiescent, but can migrate to avoid/reduce supersaturation in the solid phase. The migration is discussed in the literature to be governed by coherency strains of the solid/liquid interface, by concentration gradients in the liquid or by concentration gradients in the solid phase. A phase transformation model for diffusional phase transformations considering interface thermodynamics (possible deviations from local deviations) has been put up to describe the migration of the solid/liquid (trailing) and the liquid/solid (leading) interfaces of the liquid film. New experimental results on melting in a temperature gradient in combination with simulation calculations reveal that concentration fluctuations in the liquid phase trigger the liquid film migration and determine the migration direction, until after a short time in the order of microseconds the process is governed by diffusion in the solid phase.

Microstructural coarsening of 7005 aluminum alloy semisolid billets with high solid fraction

In the present study, 7005 aluminum alloy billets were supplied in the warm extruded and T6 heat treated state. They were then reheated to the semisolid state and the microstructural evolution and coarsening investigated. This is necessary groundwork if the billets are to be formed to shape by semisolid processing, an innovative manufacturing route which has recently gained wide use for a range of aluminum alloys but which has not yet been applied to 7005. The results showed that the average spheroid size, liquid film thickness and liquid fraction of 7005 aluminum alloy increased with increase in soaking time. Intragranular liquid droplets were present in solid grains, and coarsened with increase in soaking time and isothermal temperature. The roundness of globules of solid in the semisolid state at 600°C–615°C was in the range of 1.5 to 2.0 after soaking for periods up to 12min at 620°C, but for times greater than 15min was more than 2, which is unlikely to be suitable for the semi-solid forming process, where smooth flow is required and therefore relatively round globules. When the isothermal temperatures were 600°C, 610°C, 615°C and 620°C, the coarsening rates were 542μm3s−1, 606μm3s−1, 683μm3s−1 and 688μm3s−1, respectively. The coarsening rate K increased with the increase of isothermal temperature. These values place this alloy on the dividing line between typical coarsening rates for normally cast alloys and typical coarsening rates for normally wrought alloys. This is attributed to the fact that the alloy has a relatively simple precipitation hardening system in comparison with other 7xxx alloys and the hardening precipitate (MgZn2) melts below the temperature the experiments have been carried out at, hence preventing the precipitates inhibiting liquid film migration and diffusion along narrow liquid films at spheroid boundaries. Coarsening tends to occur via Ostwald ripening and coalescence, but Ostwald ripening plays an increasing role with the increase of isothermal temperature.