Gas Porosity Formation Research Articles

Optimizing Factors affecting gas porosity in GDC of Al-Si (A356) Alloy using Taguchi’s DOE

Aluminium alloys find extensive use in various applications because of its advantage of strength-to-weight ratio,. However, the casting process introduces susceptibility to defects, and managing all parameters proves to be a complex task. Casting rejections often stem from one or more defects, with mold parameters, filling, and solidification processes being key contributors. When the molten metal of the Al-Si (A356) alloy is exposed to the atmosphere in a holding furnace, it develops an oxide layer on its surface. Turbulence in the molten metal leads to the formation of a double oxide layer, also known as bifilms. Gas entrainment occurs due to the turbulent movement of metal, resulting in the entrapment of gas in the molten metal and ultimately causing gas porosity in the final casting. Gas entrapment during the filling process leads to gas porosities during the solidification phase.This study aims to comprehend the factors influencing the formation of gas entrapment during the casting of the aluminium alloy Al-Si (A356). The experimental investigation utilizes Taguchi’s Design of Experiments (DOE) to explore the impacts of parameters such as pouring temperature (PoT), degassing time (DgT), and volume in the holding furnace (VoH) on the formation of gas porosity in casting [8].An experimental setup was created to produce tensile test rods and reduced pressure test (RPT) samples. The results were validated using the mechanical properties of the tensile test samples and the density of the Reduced Pressure Test (RPT) samples. Better the mechanical properties, fewer will be the defects in castings. Taguchi’s DOE is applied to analyse the results. The experimental findings indicate that a pouring temperature at level 1 (710°C), degassing time at level 1 (10 minutes), and volume in the holding furnace at level 1 (>450 kg) yield favourable castings. Confirmatory tests were conducted to validate the results, and they aligned with the experimental data.

Simulation of gas porosity formation and interaction with dendrite and eutectic structures during solidification of Al-Si alloys

A two-dimensional (2-D) multi-phase cellular automaton (CA)-finite difference (FD)-lattice Boltzmann (LB) coupled model is developed to simulate the hydrogen pore formation during dendritic and eutectic solidification of hypoeutectic Al-Si alloys. In this model, the dendrite and eutectic structures are simulated using a CA technique. The solute diffusion is solved by the FD method. The nucleation, growth, and motion behavior of gas pore, and hydrogen transport are simulated by a LB Shen-Chen scheme. It is found that the pores can grow fully but compete in the primary dendrite growth stage. Conversely, pores newly nucleated in residual inter-dendrite liquid during the eutectic solidification stage grow restrictedly and independently. Pore shape severely deformed by the compression of the surrounding solid structures. Moreover, the effects of the initial Si composition, initial hydrogen concentration and cooling rate on the pore formation are qualitatively and quantitatively studied. It is found that the initial Si composition significantly impacts on the pore size and distribution, but little effect on the porosity percentage which decreases markedly with the decreasing initial hydrogen concentration and increasing cooling rate. Comparison reveals that the simulated microstructure morphological characteristics and the predicted porosity percentage agree well with the experimental observation reported in the literature.

Effect of process parameters on the porosity in laser-directed energy deposition of Al2O3 reinforced Inconel-based composite coating

Metal matrix composites (MMC) possess an excellent combination of mechanical and metallurgical properties but are difficult to process. Laser-directed energy deposition (DED) technique can be effectively employed to fabricate MMC coatings, producing dense coatings with sound metallurgical bonding. In this investigation, Inconel 718/ alumina (Al2O3) MMC coating is manufactured using powder-fed laser DED process. At first, different wt. % of alumina particles (i.e., 1, 3, 5, and 7 wt%) were mixed with Inconel 718 powder, and after that, four different types of MMC coatings were fabricated. By analyzing the mechanical properties and microstructure of those MMC coatings, the optimum amount of Al2O3 in MMC coating is determined. It is observed that IN718 −5 wt% Al2O3 composite coating exhibits better mechanical properties (i.e., hardness = 625 ± 6 HV), so further experiments were conducted with this coating. In the second phase of this investigation, the effect of laser power, scanning speed, and powder feed rate on porosity is analyzed. Beyond a laser power of 1700 W, porosity increases significantly due to the formation of gas porosity. On the other hand, beyond a scanning speed of 500 mm/min, the energy input is insufficient to melt the powder, which leads to the enhancement of lack-of-fusion porosity. With an increase in the powder feed rate, the lack of fusion porosity grows due to a higher laser power attenuation. The mid-range of laser power and scanning speed and lower range of powder feed rate are suitable for creating MMC coatings with better mechanical properties.

The effect of wettability on gas porosity formation during directional solidification of alloys: Insights from lattice Boltzmann-cellular automata simulations

The wettability between solid and porosity plays an important role in the growth of dendrites and pore morphology during solidification of alloys. In this original article we use a two-dimensional coupled lattice Boltzmann-cellular automata-finite-difference (LB-CA-FD) model to simulate the effect of wettability on the dendritic and pore morphologies. Wettability can be adjusted by adjusting the fluid-solid interaction coefficient. The predicted results show that the wettability can strongly influence the formation of the dendritic and bubble structures. The solid-gas coupled growth occurs for the poor wettability, while the bubbles are engulfed by the growing dendrite for the good wettability. It can be found that the cooling rate can strongly affect the area ratio of the preexisting bubbles although it has little effect on the morphologies of the bubbles and dendritic structures. Moreover, morphological evolution of multidendritic growth encountering multiple bubbles during directional solidification is also investigated.

Numerical simulation of dendritic growth and porosity evolution in solidification of Al-Cu alloy with lattice Boltzmann – Cellular automata method

A multi-phase lattice Boltzmann method-cellular automata (LBM-CA) model is developed for the microstructural morphologies of dendritic growth and porosity evolution in the solidified Al-Cu alloy. The proposed model can simulate the dendritic growth and porosity evolution during solidification taking account for the solidification conditions such as cooling rates and initial hydrogen concentration. LB and CA were adopted to simulate the diffusion of alloy solute and hydrogen in liquid melt and phase transition during solidification, respectively. Microstructural morphologies including the formation of gas porosity and the growth of solid phase were visualized with different solidification conditions. The simulation results were validated by comparing with experiment data reported in the literature in terms of the percentage of porosity and morphology tendency with respect to the solidification conditions. The pore size decreased when applying the higher cooling rate due to the insufficient growth time. Apparently, the higher initial hydrogen concentration led to the higher percentage of the porosity. The proposed model can be utilized to optimize the solidification conditions for reducing porosity defect in solidified material including 3D printing and welding process as well as casting process.

A computational study of porosity formation mechanism, flow characteristics and solidification microstructure in the L-DED process

Laser-assisted directed energy deposition is an additive manufacturing process used to manufacture metallic parts. The gas porosity is one of the prominent deposition defects in the processed parts. This influences the mechanical properties which can cause the part failure. In this work, the mechanism of gas porosity formation at low energy density is addressed using computational modeling. An investigation is carried out to capture the powder particles interaction with the melt pool and resulting porosity formation, molten pool hydrodynamics, and solidification microstructure in the L-DED process. The numerical results reveal that the stagnant zone in the melt pool leads to entrapment of bubbles which eventually forms porosity. This bubble entrapment phenomenon is studied by varying the powder mass flow rate, and it is found that increasing the mass flow rate results in rapid bubble formation which increases the chances of gas porosity formation. The temperature gradient and cooling rates are used for solidification analysis and prediction of as-solidified grain morphology. Using the empirical relation, the effect of local thermodynamic solidification conditions on the size of the dendritic microstructure is analyzed. The predicted melt pool geometry and porosity morphology agree with the experimental results.

A comparative study between the addition of NH4Cl and KCl in NaNO3 and NaF-based degassers in aluminum casting

The quality of Al-Si casting product is important for its applications, however the molten aluminum has high solubility of H2 at melting temperature in the casting process, so the formation of gas porosity during aluminum solidification is inevitable. This can cause a major drawback to the mechanical properties. The most common method to remove hydrogen gases in the molten metal is degassing. In this experiment, a conventional degassing method using Sodium Nitrate-Sodium Fluoride based degasser is conducted. Al-Si12 was melted at 720 °C then the degasser was plunged into the furnace bottom and held for three minutes. The results show better mechanical properties by the addition of degasser. This is because the degasser binds H2 gases which is formed in the melting process. The mechanical properties between casting products that use NaF-NaNO3 degasser and NaF-NaNO3 with addition of NH4Cl or KCl were measured and compared. The lowest mechanical properties were obtained when using degasser without NH4Cl or KCl. This is because NH4Cl or KCl is reactive and helps with the efficiency of degasser.

Features of Gas Porosity Formation Along Helium Ion Trajectories in Vanadium Alloys

The results of an investigation of the development of porosity and swelling in the alloys V–Cr, V–W, V–Ta, and V–W–Ta irradiated by 40 keV helium ions to fluence 5·10 20 m – 2 at 650°C are presented. The investigations were conducted by means of transmission electron microscopy along the travel path of ions; this afforded some idea about the total swelling of the samples and the character of the depth distribution of the porosity in the targets. It was found that the gas swelling in binary alloys is identical to within the measurement error. Multicomponent alloying is effective from the standpoint of the suppression of helium swelling – the ternary alloy V–1%W–1%Ta is subjected to significantly less helium swelling. It observed for the first time in experiments that the distribution and penetration depth of helium ions different significantly from the calculations, and the effect depends strongly on the chemical composition of the irradiated alloy. Among the alloying elements tantalum promotes deeper penetration of helium ions. In addition, the effect increases with increasing concentration of tantalum in the alloy: in the alloys V–1%Ta and V–2%Ta, pores were discovered at depth 450–500 and 850–900 nm, respectively, which is significantly greater than the computed travel distance of 48 keV helium ions in vanadium (~300 nm).

Effective Degassing for Reduced Variability in High-Pressure Die Casting Performance

The reduced solubility of hydrogen in molten aluminium is believed to be a key factor influencing the formation of gas porosity, which adversely affects the mechanical properties. In this study, two crucibles of AlSi10MnMg alloy were degassed using conventional rotary degassing and high shear melt conditioning (HSMC) respectively and then cast into tensile specimens using the high-pressure die casting (HPDC) process. An optimal holding time of 10 min was established for both processing techniques corresponding to reduced density index (DI) and reduced variation in tensile performance. After rotary degassing, DI values were found to increase with increasing holding times, rising to 4.1% after 70 min. For HSMC, a quasi-steady state was observed with a maximum DI value of 1.4% after 190 min. The pore size in HPDC cast specimens was observed to be considerably lower after degassing with the HSMC device compared with rotary degassing.

THE PROBLEM OF MODIFYING AN ALUMINUM-SILICON EUTECTIC ALLOY OF SILUMINS. WAY OF SOLUTIONS

It is shown that modern industrial modifiers of silumin eutectic contribute to the saturation of the melt with hydrogen. Its micro bubbles block the branching of eutectic crystals and promote the formation of gas porosity in castings.To improve the efficiency of the process of modifying the eutectic and reduce the gas porosity, a method of hereditary modification is proposed. A special technology has been developed to solve the problem of modification of silumin eutectic.

Improving the Quality of Cast Ingot for the Production of Defect-Free Rolled and Polished Blanks of Silver-Copper Coinage Alloy

The causes of defect generation in Ag-7.5 wt% Cu coinage alloy billets and in rolled and polished blanks were evaluated in this paper. Microstructural and compositional study of the as-cast billets indicated that excessive formation of gas-porosity and shrinkage cavity was responsible for crack formation during rolling. Carbon pick-up from charcoal flux cover used during melting, formation of CuS inclusions due to high-S content and rapid work-hardening also contributed to cracking during rolling. In order to prevent the defect generation, several measures were adopted. Those measures significantly reduced the defect generation and improved the surface luster of the trial rolled strips.

Factors Affecting the Formation of Gas Porosity in A356 Alloy Wheel by Gravity Die Casting

Factors Affecting the Formation of Gas Porosity in A356 Alloy Wheel by Gravity Die Casting

Research of gas release during crystallization of aluminium and its alloys

The problem of gas porosity formation in the manufacture of aluminum structural or cast porous alloys is discussed and some results of our research in this area are shown in the article. The main aim of the research is to investigate the influence of various technological, physical and chemical factors on the process of hydrogen gas release during crystallization of aluminum and its alloys. Using system simulation allows a scientist to investigate fully the flow of a process. In this paper to simulate the process of hydrogen gas release it was considered the simplest and, therefore, the most discussed in the literature option when the crystallization front moves in an unlimited amount of melt. The presented method allows determining what factors and how affecting the process of hydrogen gas release during production of aluminum castings and its alloys. The research results can be applied by experts in the field of metal technology, metallurgy, foundry, production of aluminum castings for mechanical engineering.

Gas induced semi-solid process effects on microstructure and mechanical properties of 319 aluminum alloy

Abstract The exclusive effects of gas induced semi-solid parameters such as rheocasting temperature, agitation intensity and flow rate of gas injection into the melt on the average size and shape factor of primary α-Al particles and subsequently tensile properties of 319 aluminum alloy have been studied. Applying vigorous agitation and rapid heat extraction simultaneously in the early stages of solidification in order to achieve a slurry with fine and dispersed primary α-Al particles is essential. Hence, in order to investigate these two factors separately, three different types of graphite diffusion in three proportional gas flow rates have been used. It was found that a reduction in rheocasting temperature leads to a decrease in the average size of the primary α-Al particles and an increase in their sphericity and improvement of tensile properties as well. The average size of primary particles slightly decreases as the agitation intensity increases as expected; while a decrease in agitation intensity in a given gas flow rate leads to the formation of gas porosity. Also the gas was trapped due to an increase in gas flow rate at constant agitation intensity whereas the decrease in gas flow rate has no significant effect on morphology of primary α-Al particles.

Gas porosity evolution and ion-implanted helium behavior in reactor ferritic/martensitic and austenitic steels

The peculiarities of gas porosity formation and helium retention and release in reactor ferritic/martensitic EP-450 and EP-450-ODS and austenitic ChS-68 steels are investigated by transmission electron microscopy and helium thermal desorption spectrometry (HTDS). The samples were irradiated by 40keV He+ ions up to a fluence of 5·1020m−2 at 293 and 923K. An nonuniform distribution of helium bubbles and high-level gas swelling in ferritic/martensitic steels were found at high-temperature helium implantation. The same irradiation conditions result in formation of uniformly distributed helium bubbles and low-level swelling in ChS-68 steel. Temperature range of helium release from EP-450-ODS steel was considerably wider in comparison to HTDS-spectra of the EP-450 steel. A considerable quantity of helium is released from ODS steel in the high-temperature range after the main peak of the HTDS-spectrum.

Gas Porosity Formation in the Vanadium Alloys V–W, V–TA, V–ZR During Helium-Atom Irradiation at 650 °C

Transmission electron microscopy is used to study the influence of alloying vanadium with tungsten, tantalum, and zirconium on the natural mechanisms of helium porosity formation and gas swelling during 40 keV He+ ion irradiation to dose 5 · 1020 m–2 at 650 °C. The formation of a bubble microstructure and gas swelling of vanadium alloys depends on the chemical and impurity composition of the irradiated materials. It was determined that in vanadium large facetted helium pores form and high gas swelling is observed, and in addition the swelling is significantly greater in high-purity vanadium. Alloying with tungsten increases swelling while tantalum has virtually no effect and zirconium and its analog titanium greatly reduce swelling. A definite correlation is observed between swelling and the number of electrons participating in the interatomic bond in alloys.

Role of Double Oxide Film Defects in the Formation of Gas Porosity in Commercial Purity and Sr-containing Al Alloys

The role of double oxide film (bifilm) defects in the formation of gas porosity in commercial purity and Sr-containing Al alloys was investigated by means of a reduced pressure test (RPT) technique. The liquid metal was poured from a height into a crucible to introduce oxide defects into the melt. The melt was then subjected to different “hydrogen addition” and “holding in liquid state” regimes before RPT samples were taken. The RPT samples were then characterized by determining their porosity parameters and examining the internal surfaces of the pores formed in them by scanning electron microscopy. The results indicated oxide defects as the initiation sites for the growth of gas porosity, both in commercial purity and Sr-containing Al alloys. The results also rejected reduction of the surface tension of the melt, increase in the volumetric shrinkage and reduction in interdendritic feeding as the possible causes of an increase in the porosity content of the Al castings modified with strontium. The change in the composition of the oxide layers of double oxide film defects was suggested to be responsible for this behaviour.

Effects of Strontium, Magnesium Addition, Temperature Gradient, and Growth Velocity on Al–Si Eutectic Growth in a Unidirectionally-solidified Al–13 wt% Si Alloy

Al–Si eutectic growth mechanism was investigated in a directionally-solidified Al–13 wt% Si alloy with different strontium (Sr) and magnesium (Mg) additions, growth velocities and temperature gradients. Macro- and micro-scale metallographic analyses revealed that addition level of Sr and Mg, temperature gradient and growth velocity are important factors affecting stability of solidifying Al–Si eutectic front and the final morphology of eutectic grains in the solidified Al–13 wt% Si alloys. By varying (tailoring) these factors, a variety of eutectic grain structures and morphologies such as planar front, cellular structure, a mix of cellular and columnar, or equiaxed dendrites, can be obtained. Increasing temperature gradient, reducing growth velocity, or decreasing Sr and Mg contents is beneficial to stabilizing planar growth front of eutectic grains, which is qualitatively in accordance with constitutional supercooling criterion for binary eutectic growth. In contrast, adding more Sr and Mg, increasing growth velocity, or decreasing temperature gradient produces large constitutional supercooling, leading to columnar-equiaxed transition (CET) of eutectic structure, which can be interpreted on the basis of Hunt's Model. It is also found that both solute concentration and solidification variables have significant impact not only on eutectic growth, but also on gas porosity formation.

Hydrogen-induced gas porosity formation in Al–4.5 wt% Cu–1.4 wt% Mg alloy

The effects of low (0.067 cm3/100 g) and relatively high (0.19 and 0.27 cm3/100 g) initial melt hydrogen concentration, solidification processing conditions, and grain refining on the formation of hydrogen-induced gas porosity in Al–4.5 wt% Cu–1.4 wt% Mg alloy have been quantitatively investigated. The study was conducted with unidirectionally cooled laboratory-size ingots solidified at 0.2–37 K/s. An optical microscope-based image analyzer and precision density measurement based on the Archimedes’ principle were used to quantify the characteristics of the hydrogen-induced porosity in the ingots. Predictably, increase in melt hydrogen concentration and decrease in solidification rate increased the amount of porosity and average pore size. However, the effect of solidification rate was greater at the very low melt hydrogen concentration (0.067 cm3/100 g). These results are consistent with reported effects of solidification rate and melt hydrogen content on porosity formation in other aluminum alloys. Addition of grain refiner slightly increased the amount of porosity and the average pore size, especially at solidification rates above 1 K/s.

Examples of New Models Applied in Selected Simulation Systems with Respect to Database

Abstract The tolerance of damage rule progressively meets the approval in the design casting parts procedures. Therefore, there were appeared the new challenges and expectations for permanent development of process virtualization in the mechanical engineering industry. Virtualization is increasingly developed on the stage of product design and materials technologies optimization. Increasing expectations of design and process engineers regarding the practical effectiveness of applied simulation systems with new proposed up-grades modules is observed. The purpose is to obtain simulation tools allowing the most possible realistic prognosis of the casting structure, including indication, with the highest possible probability, places in the casting that are endangered with the possibility of shrinkage- and gas porosity formation. This 3D map of discontinuities and structure transformed in local mechanical characteristics are used to calculate the local stresses and safety factors. The needs of tolerance of damage and new approach to evaluate the quality of such prognosis must be defined. These problems of validation of new models/modules used to predict the shrinkage- and gas porosity including the chosen structure parameters in the example of AlSi7 alloy are discussed in the paper.