Formation Of Shrinkage Porosity Research Articles

A mechanistic explanation of shrinkage porosity in laser powder bed fusion additive manufacturing

This work focuses on the occurrence of shrinkage porosity and its manifestation in Alloy 718 during laser-powder bed fusion (PBF-LB) processing. Shrinkage porosity is a common defect in metal castings that degrades part performance. In traditional metal castings, the Niyama criterion is a reliable heuristic for predicting the formation of shrinkage porosity. However, the Niyama criterion’s applicability in the PBF-LB process remains unexplored, and there is no known and evaluated heuristic to predict shrinkage porosity in the PBF-LB manufactured parts. This work employs microstructure characterization and an analytical heat transfer model to develop a mechanistic explanation for the formation of shrinkage porosity in the PBF-LB process. The results show that the Niyama criterion could not effectively predict the occurrence of shrinkage porosity. Further, the formation of shrinkage porosity is primarily driven by secondary dendrite arm growth in the solidifying microstructure, where the transition to cellular growth at high cooling rates during solidification mitigates porosity by removing locations for pore formation. This means a heuristic based on solidification cooling rate could reliably predict the occurrence of shrinkage porosity. For practical use, shrinkage porosity process maps are presented for use in process design and control to directly aid shrinkage porosity mitigation in process planning. The process-shrinkage porosity relationship results also indicate that trends in PBF-LB manufacturing toward higher deposition temperatures and higher throughput are likely to aggravate the conditions for shrinkage porosity formation and further elevate the importance of the mitigation strategies presented in this work.

Casting of film-cooling holes for single-crystal blades

The flawless formation of film-cooling holes (FCHs) is critical for manufacturing single-crystal blade. Presently, high-energy beam drilling is mostly used to process gas film holes, which introduces the limitations of complex hole structures that are difficult to form and processing thermal defects that are difficult to eliminate. This study demonstrates an approach to form FCHs using casting, where an indirect additive manufacturing technology using a core/shell integral ceramic mold is applied for casting. Cylindrical 0.4, 0.8, and 4.0 mm-diameters hole are cast. The influence of fine ceramic cores on the microstructure of single-crystal blades during directional solidification is investigated using simulations and experiments. The fine ceramic cores of the FCHs affect the growth morphology of dendrites around the hole, which is determined by the hole diameter, primary dendrite spacing, and local temperature filed. Owing to the thermal conductivity difference between the alloy and ceramic, the local temperature filed around the holes changes abruptly, leading to the formation of shrinkage porosity and cavities. Furthermore, the process window for the casting of FCHs is determined using the longitudinal temperature gradient (GZ) and solidification rate (VZ). The proposed approach is of practical significance for the defect-free casting of tiny structures in single-crystal blades.

Numerical simulation of shrinkage porosity defect in billet continuous casting

Abstract Shrinkage porosity is a typical internal defect in the continuous casting billet, which occurs frequently and is difficult to solve. To explore the influence factors of central shrinkage porosity, a novel unsteady thermomechanical coupling analysis algorithm is developed based on the billet solidification characteristics, and the central shrinkage behavior during the ending solidification process is simulated. Results show that when the casting speed increases from 1.6 to 2.8 m·min−1 and the center outward displacement is reduced from 9.20 × 10−2 mm to 5.8 × 10−2 mm, it means casting speed has a significant effect on the formation of shrinkage porosity, and for this caster, the higher casting speed is more suitable for the secondary cooling zone. Without the changes in the solidification structure, when the superheat degree of molten steel increases from 10 to 40°C, the center outward displacement value decreases from 7.12 × 10−2 mm to 6.91 × 10−2 mm. In that case, the superheat degree has no obvious effect on the center displacement value.

Optimization of centrifugal casting process parameters by Taguchi method to reduce shrinkage porosity ratio of K417 superalloy

The process parameters were optimized by simulation and verification experiments using orthogonal experimental design and the Taguchi method to lessen the tendency of significant shrinkage porosity in the centrifugal cast ring parts of K417 nickel-based superalloy. Advanced Porosity Model (APM) in ProCAST was used to predict the shrinkage porosity of centrifugal castings, and the effects of centrifugal speed, pouring speed, pouring temperature and preheating temperature of the mold were investigated on the shrinkage porosity ratio of the castings. According to the results, the parameter that has the greatest influence on the shrinkage porosity ratio of centrifugal casting is the centrifuge speed, followed by the preheating temperature of the mold, and the pouring temperature and pouring speed have relatively small effects on it. The optimized parameters were proposed as follows: centrifugal speed of 500 r/min, pouring speed of 225 mm/s, pouring temperature of 1400[Formula: see text], and mold preheating temperature of 50[Formula: see text], which could effectively reduce the formation of shrinkage porosity of the K417 centrifugal casting rings.

Quantitative Estimation of Formation of Shrinkage Porosity by the Niyama Criterion

Quantitative Estimation of Formation of Shrinkage Porosity by the Niyama Criterion

Mechanisms of gas and shrinkage porosity formation in solidifying shear bands

In specialised solidification processing techniques such as High Pressure Die Casting, Twin-Roll Casting and others, an additional external deformation load is applied to achieve the required shape, leading to the formation of microstructural features such as shear bands. The mechanism for forming these features is believed to be dependent on dynamically evolving strain fields, which are dependent on the local solid fraction, applied strain rates and casting geometry. To investigate this, a semisolid (∼ 50 % solid fraction) Al-10 wt.% Cu alloy is isothermally injected into a bespoke die using a custom-designed thermo-mechanical rig. The semisolid deformation, formation of Cu-rich dilatant bands and subsequent pore nucleation and growth are captured using fast synchrotron X-ray radiography. The local normal and shear strains acting on the mush are quantified using digital image correlation to identify the dilatant shear bands and the dominant local strain component. Correlating the radiographs with strain maps reveals that gas pores within the dilated interstices grow, while those in compressed regions are squeezed out. A linear correlation between accumulated volumetric strain and porosity volume fraction demonstrates that higher dilations give rise to a local increase in both gas and shrinkage porosity.

Geometry-Driven Criterion Function for Predicting Shrinkage Porosity in Stainless Steel Castings with T-Junction

Metal castings with the presence of shrinkage porosity are often recycled or rejected and in turn reduce productivity of the process as well as increase energy cost involved in the process. This can be overcome by prevention of its occurrence using a suitable prediction technique. A detailed study of literature reveals that several Criterion Functions (an empirical model that connects solidification phenomena with formation of shrinkage porosity) have been employed to predict the location of shrinkage porosity in castings manufactured using particular process-alloy combination by values of process parameters (mostly thermal gradient, cooling rate, and velocity of molten metal). However, criterion function considering the effect of geometric variation in stainless steel castings on an extent of shrinkage porosity need to be established. In the present work, a benchmark casting, a combination of three T junctions, has been cast and used for the development of geometry-driven criterion function for stainless steel castings. Real experimental results with the presence of shrinkage porosity were used for superimposing on virtual experimental results (simulated results) for establishment of local simulation conditions. These conditions are further used in extrapolating results using casting simulation. The developed geometry-driven criterion function was further validated and found to be effective in prediction of shrinkage porosity.

X-ray Computed Tomography Studies on Porosity Distribution in Vacuum Induction Cast Al-7Si Alloys

Porosity in aluminum alloys is a great concern to the casting and automotive industry. In this publication, porosity formation in air-melted and vacuum induction melted (VIM) aluminum alloys was studied and compared to understand its effect on microstructure and mechanical properties of Al-7Si alloys. Al-7Si alloys were cast at 700°C and 900°C in a muffle furnace and VIM furnace. Microstructural results show that the alloys cast in muffle furnace refined the eutectic silicon compared with the cast samples prepared in VIM furnace. X-ray computed tomography (XCT) was used for three-dimensional (3D) visualization and quantification of porosity in these alloys. The volume fraction of pores was observed to be higher in alloy air-melted at 900°C compared with 700°C. XCT results from VIM alloy samples showed no significant porosity when cast at either 700°C or 900°C. The morphology of large pores in alloys air-melted at 700°C represents the formation of shrinkage porosity due to the incomplete flow of molten metal during solidification. Tensile test results show that the elongation property of VIM alloy was increased by more than 20% compared with air-melted alloy. The tensile strength and elongation were observed to be higher for alloy samples cast at 700°C compared with 900°C for both air-melted and VIM alloys. The findings from microstructure, XCT, and tensile tests show that vacuum induction melting improves the mechanical properties of the alloy compared with air-melted alloy.

Modeling Ablation Casting

Research published in open literature on ablation casting has been mainly experimental. Modeling of this innovative technology is scarce. This article describes an initial attempt in modeling ablation casting process. Material properties and boundary conditions were defined for calculating cooling curves and microstructural length scales in A356 alloy castings made using sand casting and ablation casting processes. Model predictions were compared with published data on cooling curves and solidification times in an A356 alloy casting. The validated model was then used to predict unique solidification features associated with the ablation casting process, including the eutectic solidification time, shrinkage porosity formation, eutectic front morphology, the advancing rates of the eutectic front and the associated eutectic silicon morphology. Simulation results indicate that the progressive advancing ablation cooling is capable of reducing secondary dendrite arm spacing, modifying the eutectic silicon phase owing to the water quench effect, and eliminating shrinkage porosity using much smaller risers than usual if the ablation is correctly driven towards the risers.

Internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio

Steel ingot with a large height-to-diameter ratio is utilized to produce multiple products by one stock in practice. Water cooling is a usual way to enhance production efficiency. However, the combination of the two factors will generate internal defects, such as shrinkage porosity and hot crack. The characteristic of internal shrinkage crack in a 10 t water-cooled steel ingot with a large height-to-diameter ratio was examined by an ultrasonic test. A slice was sectioned from the ingot middle part where billets containing star-like crack were further extracted. The billets were examined by X-ray high energy industrial CT, and the compactness was reconstructed in three dimensions. Microstructure near the crack was observed using scanning electron microscopy, and the solidification process and grain size were studied by high temperature confocal microscopy. Moreover, thermomechanical simulation and post-processing were carried out to analyze the formation of shrinkage porosity and hot crack. A new criterion considering mushy zone mechanical behavior in brittle temperature as well as grain size distribution was proposed to evaluate hot cracking potential in the ingot. The results show that a deep shrinkage porosity band easily forms in the center line of such an ingot with a large height-to-diameter ratio, and water-cooling further generates excessive tensile stress tearing the liquid films around the porosities. Then, hot cracks begin to propagate along grain boundaries. The grain size in the upper and center of the ingot is large, which leads to an inverted cone shape defects zone in the ingot center.

Optimization of Shrinkage Porosity in AlSi5Cu1Mg Alloy using Response Surface Methodology

Zero defects are the ultimate aim of any manufacturer casting aluminium alloys. shrinkage porosity is the major defect in casting AlSi5Cu1Mg alloy using Gravity Die Casting (GDC) process. This paper deals with development of effective approach to describe the optimal condition to reduce the shrinkage porosity formation. Solidification time, metal temperature, and preheat molten temperature are varied using Response Surface Methodology (RSM) based Box-Behnken Design (BBD)and conducted experiments. Solidification time and pouring metal temperature are directly proportional and solidification time and preheat temperature of the mould are indirectly proportional. Theoretical approach is validated with the experimental approach and the predicted values from RSM based BBD is useful and effective in determining the optimized process conditions in GDC process.

Simulation of Shrinkage Porosity Formation During Alloy Solidification

Porosity due to solidification shrinkage is a troublesome defect in metal casting. It results in low yields and increased costs in production and limits the performance of cast components in service. By reliably predicting porosity in casting process simulation, porosity can be minimized or eliminated. Here, a new model for simulating the formation of shrinkage porosity during solidification is presented. The model is based on the recent discovery that shrinkage porosity nucleates and grows in regions of a casting where the solid fraction is the lowest. It calculates the feeding flows and pressure distribution in the liquid while accounting for the density variation during cooling and solidification. It predicts the location, extent and amount of all types of shrinkage porosity in a casting, including riser pipes and large internal holes, surface sinks, and distributed micro-shrinkage. Porosity predictions are presented for simple casting geometries and for a more complex Mn-steel experimental casting. The comparisons to the observations made in the experimental casting demonstrate the capability of the model to accurately predict the various types of shrinkage porosity. Numerical studies are performed to investigate the sensitivity of the predictions to various model parameters.

Computational Modeling of Shrinkage Porosity Formation in Spheroidal Graphite Iron: a Proof of Concept and Experimental Validation

Prediction of microporosity (microshrinkage) formation in casting alloys continues to be a subject of high interest to metalcasters because failure to avoid this defect results in many instances in rejection of the casting with associated financial loss. It is a complex problem involving multiple material and process variables. Spheroidal graphite (SG) iron castings are particularly vulnerable to this type of defects because of the low mushy zone permeability associated with the late phases of solidification of this iron. A previously developed model that captures some important elements of the physics of the problem, including gas evolution in the melt with increasing fraction of solid and decreasing mushy zone permeability during solidification, was used to calculate the porosity distribution in L-shaped SG iron castings. The calculation results compared well with experimental observation on L-shaped castings. Some disagreements between calculations and experiments are attributed to gas pore migration during solidification, a phenomenon that is not included in the physics described by the model.

Optimization of shrinkage porosity in AlSi5Cu1Mg alloy using response surface methodology

Casting of aluminium alloys with zero defects would be ultimate aim of any manufacturer. In this present study, an effective approach has been developed to describe the optimal condition identified to reduce the shrinkage porosity formation in AlSi5Cu1Mg alloy using Gravity Die Casting (GDC) process. Experiments were conducted by varying solidification time, metal temperature, pre-heat molten temperature using response surface methodology (RSM) based Box-Behnken Design (BBD). The results revealed that solidification time increases with increase in pouring metal temperature and decrease with pre heat temperature of the mould. Experiments were conducted in order to validate the theoretical approach. The predicted values from RSM based BBD with experimental data, was proposed that the developed method is useful and effective method to determine the optimized process conditions in GDC process.

Effect of mould design on the Niyama criteria during solidification of CH3C 80t ingot

ABSTRACT Eighty tons of CH3C steel ingots used as backup rollers which should have high wear resistance and fatigue strength. Centreline Shrinkage porosity of 80 tons steel ingot reduces resistance to fatigue. The shapes of the mould and the feeding design have the greatest impact on the formation of shrinkage porosity around the central axis. Therefore, parameters such as the ratio of height to average ingot diameter, the polygon number of ingot and concave radius of the ingot were studied on the formation of shrinkage porosity. For this purpose, the simulation of finite element method was used. Simulation results were used to select the optimal ratio of the H/D, the number of polygon and the concave radius of the ingot to produce a new CH3C 80t ingot mould in Esfarayen Industrial Complex (EICO).

Study on Reduction in Shrinkage Defects in HPDC Component by Optimization of Localized Squeezing Process

In high-pressure die casting component, shrinkage defects plays a major role in leakage of fluid from components; therefore, it becomes necessary to predict the exact location of the shrinkage defect to reduce its intensity to an acceptable level. Nowadays, a localized squeezing process is one of the popular ways of reducing the shrinkage defect in high-pressure die casting components. Squeeze pins can be used to compensate for shrinkage defects in these components. The main reason for the formation of shrinkage porosity at the critical location of a given component is large and poorly fed hot spot. In this paper, shrinkage defects are reduced from level III to level I by determining optimum values of squeeze pin parameters by DOE and flow simulation, obtained results are implemented in order to test and verify effectiveness of the method. An excellent agreement is indicated for the simulation result and the experimental results.

Real-time X-ray Radiography and Computational Modeling of Shrinkage Porosity Formation in Aluminum Alloy Castings

The formation of shrinkage porosity in an A356 aluminum alloy wedge casting is investigated using real-time video radiography. An image processing technique is developed to quantify the amount of porosity observed in the video. The transient temperature field is obtained using casting simulation. The results show that initially, sinks develop in the low solid fraction regions on the surface of the wedge. Once the surface becomes sufficiently solid to be rigid, the subsequent solidification shrinkage is accommodated by internal porosity. The internal porosity is found to nucleate and grow in the central part of the wedge where the solid fraction is the lowest. A computational model is developed to predict the formation of the surface sinks and the internal porosity. The model is validated using the experimental measurements and a parametric study is performed to investigate the sensitivity of the predictions to changes in the model parameters.

Reducing Shrinkage Porosity in High-Performance Steel Castings: Case of an ASME B16.34 Gate Valve Body: Part 2—Simulation and Experimental Verification

This study presents a systematic framework for reducing shrinkage porosity in high-performance steel castings, based on a simulation-driven optimisation process coupled to micro- and macrostructural evaluation of the castings. For the test case, the macro- and microstructural investigation of a Class 300 gate valve body, based on the ASME B16.34 standard and made from A216 WCB cast steel, using X-ray computed tomography and scanning electron microscopy, was presented in Part 1: Analysis, Techniques and Experimental Approach. This forms the basis for introducing appropriate design and process improvement measures to reduce shrinkage porosity severity levels, based on parameters that influence shrinkage porosity formation, particularly in terms of temperature gradient and solidification rate. Based on the improved design, and using a patternless sand mould manufacturing technology, a trial casting of the valve body was produced, showing a significant improvement in shrinkage porosity levels.

Simulation of casting process: case study on the gating and feeding design for outlet diaphragms of iron ore ball mill

In this paper, the sand casting process was evaluated using ProCAST simulating software and practical experience for manufacturing of the high chromium stainless steel outlet diaphragms used in iron ore ball mill. In the simulation practice, the effects of feeding and gating designs on the liquid flow, heat transfer and solidification phenomenon were evaluated to minimize the liquid surface turbulence during filling of the mold cavity and the shrinkage porosity formation during the liquid solidification in the mold. The effects of different gating ratios and number of feeders were numerically studied using different indicators in the software such as linear velocity, niyama criteria, fraction solid and shrinkage porosity. Further, the optimal design obtained by simulating process was empirically examined. The simulation results showed that the 1:3:4 gating ratio is the proper gating system that it obtains an unpressurized fluid flow without any surface turbulent of the liquid steel in the filling step. In addition, the simulation of solidification phenomenon was indicated that four feeders are required to eliminate of the shrinkage porosities. The empirical casting examination shown the optimal design by the simulation is proper for manufacturing of the steel part without critical defects.

Suppression of shrinkage porosity in laser-joining of CFRP and steel using a laser surface modification process “Surfi-Sculpt ®”

The emergence of shrinkage porosity is a significant issue in the laser joining of carbon fiber reinforced polymer (CFRP) and steel. The joining process uses laser heating to melt a thermoplastic CFRP matrix in contact with a metal substrate to create a physical bond upon cooling. During the cooling process, melted CFRP at different location has different solidification rate, leading to the shrinkage porosity in the CFRP. In this study, the laser Surfi-Sculpt® is used to prevent the formation of shrinkage porosity in CFRP. The effects of the morphology characteristics (protrusion height and density) on shrinkage porosity are investigated, and the thermal distribution and history during the joining is simulated to examine mechanism of shrinkage porosity suppression. The results show that formation of shrinkage porosity can be suppressed with the laser Surfi-Sculpt®, which is attributed to the change of heat conduction path. The protrusions conduct the heat from the interior of melted zone to steel, decreasing the heat accumulation at the interface and suppressing the formation of shrinkage porosity. After adopting this technique, the porosity can be reduced from 7.13% to 1.26% and the shear strength of the joint increases from 9.3 MPa to 30.8 MPa.