Porosity In Castings Research Articles

X-ray tomography studies on porosity and particle size distribution in cast in-situ Al-Cu-TiB2 semi-solid forged composites

X-ray computed tomography (XCT) was used to characterise the internal microstructure and clustering behaviour of TiB2 particles in in-situ processed Al-Cu metal matrix composites prepared by casting method. Forging was used in semi-solid state to reduce the porosity and to uniformly disperse TiB2 particles in the composite. Quantification of porosity and clustering of TiB2 particles was evaluated for different forging reductions (30% and 50% reductions) and compared with an as-cast sample using XCT. Results show that the porosity content was decreased by about 40% due to semi-solid forging as compared to the as-cast condition. Further, XCT results show that the 30% forging reduction resulted in greater uniformity in distribution of TiB2 particles within the composite compared to as-cast and the 50% forge reduction in semi-solid state. These results show that the application of forging in semi-solid state enhances particle distribution and reduces porosity formation in cast in-situ Al-Cu-TiB2 metal matrix composites.

Improving the quality of die castings through optimal plunger motion planning: analytical computation and experimental validation

High-pressure die casting is a process widely used to manufacture components with high productivity and dimensional accuracy. Its main disadvantage is the high percentage of scraps, due to a high amount of defects. Thus, the identification of the parameters affecting quality of castings is the current challenge towards an efficient and effective production. In their previous work, the authors have found and statistically validated a novel parameter explaining and forecasting both static mechanical properties and porosity of castings. Such a parameter, defined as the root mean square value of the plunger acceleration in the fast shot stage, represents a measure of the average force transmitted by the plunger to the melt and has been proved to be very effective in predicting casting quality. In order to provide a practical tool for the use of this parameter, this work proposes an analytical method for its computation, starting from the plunger displacement curve or just some notable points. The method formulation takes advantage of the analytical development of the typical motion primitives adopted and also accounts for limitations due to the machine. Hence, the optimization of the process can be achieved by selecting in advance the most suitable plunger motion profile, among those feasible, that allows to improve the casting quality. Besides the theoretical formulation, a meaningful experimental validation is provided to demonstrate the correctness of the proposed parameter and of the analytical approach, as well as its ease of implementation that makes it suitable for industrial applications.

Solidification Kinetics of Graphite Nodules in Cast Iron and Shrinkage Porosity

The shrinkage porosity of castings made from cast iron with spherical graphite (SGI) depends on a combination of intrinsic (density and volume of phases, solidification kinetics) and extrinsic conditions related to casting-mold thermo-mechanical interactions. Precipitation of graphite nodules increases the specific SGI volume, and control of the nucleation rate in solidified castings can be used for improving casting soundness. In this article, the method of structural reconstruction of solidification kinetics was used to link the nucleation rate of graphite nodules to experimentally observed shrinkage porosity in a specially designed test casting. An automated SEM/EDX system was used to determine the “true” two-dimensional graphite nodule distributions in the casting sections. These two-dimensional distributions were converted into the volume particle distribution functions (PDF), and the solidification kinetics were reconstructed by applying inverse simulations. The experiments were performed with variations in inoculation and pouring temperature. The shrinkage porosity was compared to the restored sequence of graphite nodule nucleation in the specific casting volumes. It is shown that the second nucleation wave in low-temperature poured and inoculated SGI eliminated interdendritic microporosity. The suggested method could be used in industry to improve the soundness of SGI castings.

On the Effect of Hot Isostatic Pressing on the Creep Life of a Single Crystal Superalloys

The creep behavior of a single‐crystal Ni‐base superalloy in two microstructural states is compared. One is obtained by casting followed by a conventional heat treatment. The other results from the same nominal heat treatment integrated into a hot isostatic pressing process. The microstructure after HIP differed from that in the conventional route in two respects. First, the γ′ particles are smaller and the γ channels are narrower. Second, after HIP, the number density of pores is lower and the pore sizes are smaller. The HIP microstructure improves creep in two respects: the finer γ/γ′‐microstructure results in lower minimum creep rates. Moreover, the shrinkage of cast porosity during HIP delays the nucleation and growth of micro cracks and results in higher rupture strains in the low‐temperature high stress regime.

Investigation of Oxide Bifilms in Investment Cast Superalloy IN100: Part II. Characterization

Oxide bifilms are a proposed casting inclusion reported to have been observed in vacuum investment cast polycrystalline Ni-based superalloys. Ongoing research seeks to determine if current superalloy casting practices can result in the formation of oxide bifilms, and subsequently if it is possible to observe and characterize this phenomenon. The effect of casting atmosphere, turbulence, filtering, hot isostatic pressing, and heat treatment has been investigated to identify the critical parameters that have been reported to result in bifilm formation in Ni-based superalloy IN100. Scanning Auger microscopy (SAM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were utilized to characterize samples from each casting condition. In situ ultrahigh vacuum Auger fractography did not indicate the presence of bifilms on the fracture behavior of IN100 in any processing condition. SAM analysis identified a sulfur-enriched monolayer on the surface of dendritic casting porosity, and identified heterogeneous Ti oxycarbide inclusions in air cast IN100. SEM analysis also indicated the presence of Ti oxycarbide inclusions in air cast IN100, and determined that these inclusion structures consist of fine blocky external M(Ti, Mo)C carbide enveloping an internal core of alumina. HR-TEM analysis indicated that none of the oxycarbide inclusion interfaces exist as discontinuous unbound interfaces, and that the internal alumina core is an ultra-fine polycrystalline structure.

Multiaxial high cycle fatigue damage mechanisms associated with the different microstructural heterogeneities of cast aluminium alloys

This paper is dedicated to the high cycle fatigue (HCF) behaviour of cast Al–Si alloys. In particular, three similar alloys with different microstructural characteristics are investigated. The result of an experimental campaign is presented, in order to characterise the fatigue behaviour, and more specifically the fatigue damage mechanisms related to the different microstructural heterogeneities (i.e. casting porosity, dendrite size, SDAS, non-metallic inclusions and silicon particles), observed under different multiaxial loading conditions: pure tension, plane bending, pure torsion and combined tension–torsion with a load ratio R=−1.It is shown that casting porosity has a very detrimental influence on the uniaxial and combined tension–torsion fatigue strengths. However, a much lower influence is observed for the torsional fatigue strength.For the porosity-free alloy, it is observed that the formation of persistent slip bands (PSB) in the aluminium matrix is the major fatigue crack initiation mechanism regardless of the loading modes, at a load ratio of R=−1. It is also shown that the aluminium matrix has a large role in the formation of PSB and that the Si particles facilitate the formation of PSB.

Study of the Effect of Shrinkage Porosity on Strength Low Carbon Cast Steel

Today there are many computer systems for modeling of the casting technology processes. All of them allow calculating the availability and distribution of the shrinkage porosity in the test casting, but this information allows only making changes in existing casting technology. In this paper you obtain the information about changes in the local and structural mechanical properties of the casting in the presence of its volume shrinkage porosity. Article presents the results of direct experimental studies of technological defects (shrinkage and gas porosity) impact on the mechanical properties of low carbon steel castings. Methods of investigation are also disclosed, including the methods for producing of molded samples obtained at different process conditions and the crystallization apparatus which is described for the measuring of the density of the samples. There are the mathematical relationship for the elastic modulus, yield stress, elongation and fatigue characteristics fracture cast steel with low carbon content in the presence of the volumetric shrinkage porosity.

Numerical simulation and experimental verification of gravity and centrifugal investment casting low pressure turbine blades for high Nb–TiAl alloy

Gravity and centrifugal investment casting processes of low-pressure turbine blades with high Nb–TiAl alloy were simulated by Procast software. Actual blade components were poured by vacuum induction suspended furnace with Ar protection. The experimental verification indicated that the simulation results were in good agreement with the experimental results. Comparative results had shown that the surface of centrifugal casting blade was more complete than that of gravity casting one. In gravity casting process, molten metal filled the thinnest trailing edge at last, resulting in the generation of misrun defects. Furthermore, the shrinkage porosity and crack defects of gravity casting were much more and dispersive. The internal and external quality of centrifugal casting was much better than that of gravity casting. Microstructures from edge to center of gravity casting blade had no significant change. The microstructure for centrifugal casting blade is finer than that for gravity casting blade, however, a large number of dentritic γ segregation appeared in the blade edge of centrifugal casting, which resulted from the fast cooling rate of centrifugal casting surface.

Fabrication and wear characterization of an A413/Ni surface metal matrix composite fabricated via friction stir processing

Abstract In the present study, friction stir processing (FSP) was used for the incorporation of Ni particles into the surface of an A413 alloy to fabricate a surface composite. FSP parameters were the rotation speed of 2000 rpm, the traverse speed of 8 mm/min and the tilt angle of 2°. Single pass and three-pass FSP were conducted on the samples. For the evaluation of microstructures, optical microscopy and scanning electron microscope were utilized. Also, for the investigation of intermetallic formation, energy dispersive spectroscopy was used. The wear resistance of different composites was investigated at ambient and elevated temperatures. Microstructural observations revealed that the FSP led to significant breakup of acicular Si particles, elimination of α­Al dendrites and heal the casting porosity. It was found that the hardness and wear behavior of A413 cast alloy were strongly influenced by applying FSP. Also, the in situ formation of Al3Ni particles during FSP was a critical factor controlling the wear mechanism. Sliding wear tests revealed that the increase in the number of passes created a homogeneous distribution of Al3Ni intermetallic particles and thereby resulting in a significant improvement in wear resistance at both room and high temperatures.

Investigation of Porosity Changes in Cast Ti6Al4V Rods After Hot Isostatic Pressing

The porosities of cast Ti6Al4V rods were investigated nondestructively using x-ray microcomputed tomography (microCT) before and after HIP. This allowed the visualization and quantification of porosity changes in the same samples, which indicate excellent pore closure for large pores. This is the first reported application of x-ray microCT for direct analysis of investment casting porosity before and after HIP. The method shows promise for further investigations using delayed HIP treatments or monitoring of pore closure at further varying HIP cycle settings. The nondestructive nature of the analysis has resulted in the interesting observation of small subsurface pores (<250 μm) unaffected by hot isostatic pressing in two of the samples.

Frequency analysis of volumetric porosity in aluminum castings at high and low cooling rates

Abstract Three dimensional porosity analysis was conducted using computed tomography for Al-Si alloy castings. Statistical analysis was performed to compare the porosities of the castings at high and low cooling rates. It was observed that the cooling rates strongly affect the formation of pores for both small and large pores. The incidence of small pores measuring 0.00003 mm3 to 0.0004 mm3 can be up to six times higher in castings formed at low cooling rates. For large pores measuring 300 μm to 360 μm, it was observed that lower cooling rates give rise to larger pores, which are known to be detrimental to castings as potential failure initiation sites.

Contribution to the Identification of Porosity Type in AlSiCu High-Pressure-Die-Castings by Experimental and Virtual Way

Abstract The paper concerns the problem of discontinuity in high pressure die castings (HPDC). The compactness of their structure is not perfect, as it is sometimes believed. The discontinuities present in these castings are the porosity as follow: shrinkage and gas (hydrogen and gas-air occlusions) origin. The mixed gas and shrinkage nature of porosity makes it difficult to identify and indicate the dominant source. The selected parameters of metallurgical quality of AlSi9Cu3 alloy before and after refining and the gravity castings samples (as DI - density index method), were tested and evaluated. This alloy was served to cast the test casting by HPDC method. The penetrating testing (PT) and metallographic study of both kinds of castings were realized. The application of the NF&amp;S simulation system allowed virtually to indicate the porosity zones at risk of a particular type in gravity and high-pressure-die-castings. The comparing of these results with the experiment allowed to conclude about NF&amp;S models validation. The validity of hypotheses concerning the mechanisms of formation and development of porosity in HPDC casting were also analyzed.

Porosity of Castings Produced by the Vacuum Assisted Pressure Die Casting Method

Abstract The paper presents the results of investigations concerning the influence of negative (relative) pressure in the die cavity of high pressure die casting machine on the porosity of castings made of AlSi9Cu3 alloy. Examinations were carried out for the VertaCast cold chamber vertical pressure die casting machine equipped with a vacuum system. Experiments were performed for three values of the applied gauge pressure: -0.3 bar, -0.5 bar, and -0.7 bar, at constant values of other technological parameters, selected during the formerly carried initial experiments. Porosity of castings was assessed on the basis of microstructure observation and the density measurements performed by the method of hydrostatic weighing. The performed investigation allowed to find out that - for the examined pressure range - the porosity of castings decreases linearly with an increase in the absolute value of negative pressure applied to the die cavity. The negative pressure value of -0.7 bar allows to produce castings exhibiting porosity value less than 1%. Large blowholes arisen probably by occlusion of gaseous phase during the injection of metal into the die cavity, were found in castings produced at the negative pressure value of -0.3 bar. These blowholes are placed mostly in regions of local thermal centres and often accompanied by the discontinuities in the form of interdendritic shrinkage micro-porosity. It was concluded that the high quality AlSi9Cu3 alloy castings able to work in elevated temperatures can be achieved for the absolute value of the negative pressure applied to the die cavity greater than 0.5 bar at the applied set of other parameters of pressure die casting machine work.

Reference Dies for the Evaluation of Tensile Properties of Gravity Cast Al-Si Alloys: An Overview

Aluminium-Silicon (Al-Si) alloys are the most extensively used Al foundry alloys and are widely used in gravity die-casting (GDC) of automotive components. Reference dies are used to characterize the tensile properties of the castings. Among the various die configurations, the ASTM B-108 (also called the Stahl Mold), and the AA Step Mold are most popular in the foundries. Several modifications to the standard dies have been suggested in the scientific literature to obtain better mechanical properties in minimizing casting defects. This work reviews the scientific literature on the effect of different reference dies on the porosity and tensile properties of the Al alloy castings.

Influence of Chemical Composition on Porosity in Aluminium Alloys

Abstract Porosity is one of the major defects in aluminum castings, which results is a decrease of a mechanical properties. Porosity in aluminum alloys is caused by solidification shrinkage and gas segregation. The final amount of porosity in aluminium castings is mostly influenced by several factors, as amount of hydrogen in molten aluminium alloy, cooling rate, melt temperature, mold material, or solidification interval. This article deals with effect of chemical composition on porosity in Al-Si aluminum alloys. For experiment was used Pure aluminum and four alloys: AlSi6Cu4, AlSi7Mg0, 3, AlSi9Cu1, AlSi10MgCu1.

Optimization of low-pressure die casting process parameters for reduction of shrinkage porosity in ZL205A alloy casting using Taguchi method

In order to reduce serious tendency of the shrinkage porosity in ZL205A alloy, the design of experiment technique, Taguchi method is introduced to optimize the process parameters of cylindrical shell ZL205A alloy parts produced by low-pressure die casting in this article. The effect of pouring temperature, filling time and packing pressure on the shrinkage porosity in ZL205A castings is investigated using the orthogonal array and Advanced Porosity Model in ProCAST to calculate the volume of shrinkage porosity. The effectiveness of selected casting parameters in reducing the shrinkage porosity in cylindrical shell ZL205A casting is verified through analysis of signal-to-noise ratio and analysis of variance. Numerical results indicate that the optimized process parameters, including a pouring temperature of 725 °C, filling time of 20 s and packing pressure of 175 kPa, can be used to reduce the formation of shrinkage porosity in ZL205A castings.

Effects of Multi-electromagnetic Field on the Solidification Structure of IN100 Superalloy Castings

AbstractIN100 superalloy castings were vacuum cast with the application of multi-electromagnetic field. The effect of multiple-electromagnetic field on the solidification structure of IN100 superalloy castings has been studied. The results show that with the application of the multiple-electromagnetic field composed of the main frequency rotary electromagnetic field with 60A current and the steady direct current electromagnetic field with 290A current, the shrinkage porosity of IN100 superalloy castings is greatly improved and the fine equiaxed grain fraction of IN100 superalloy castings is increased to 100%. Moreover, the action mechanism of multiple-electromagnetic field is discussed to explain its effect on improving the solidification structure of IN100 superalloy castings.

Fatigue crack initiation and damage mechanisms during ultrasonic fatigue testing of cast aluminium alloy AlSi7Mg

During the last years, an increased interest in in situ-damage characterization during ultrasonic fatigue testing evolved. Different non-contact in situ-surveillance systems have been developed or applied to the present ultrasonic fatigue testing systems, e.g. analysis of the nonlinearity parameter and the resonant frequency [1, 2] as well as thermal measurements [3–5]. In addition, Heinz et al. [6] performed noncontact strain measurements during ultrasonic fatigue testing. The aim of the present investigations is to evaluate the fatigue crack initiation in the cast aluminium alloy AlSi7Mg (A356) in sand and die cast condition under high cycle and very high cycle fatigue loading conditions. Therefore, a combined analysis of the development of the specimen temperature, the resonant frequency fres and the nonlinearity parameter rel was performed. For such investigations it is essential to keep the ambient temperature strictly constant and the increase of temperature due to the resonant conditions as low as possible. Therefore, a spot cooler using a vortex tube was applied to the ultrasonic fatigue testing system. With this special cooling device it was possible to keep the ambient temperature constant. An infrared thermo-camera system was used for the in situ-examination of the specimen temperature and the surveillance of the ambient temperature during ultrasonic fatigue testing. The determination of the resonant frequency and the nonlinearity parameter was realized online during fatigue tests according to Mayer et al. [2]. After failure, the fracture surfaces of the specimens were investigated intensively by scanning electron microscopy (SEM). According to Krewerth et al. [5], the correlation of the results from thermographic and fractographic investigations can shed some light on the crack initiation point. Figure 1a shows the S-N curves of the fatigue tests on the aluminum specimens produced by sand casting (batch 1) and die-casting (batch 2) in hot isostatically pressed condition, respectively. Specimens of both batches failed due to casting porosity. However, the casting porosity in the sand cast material was significantly higher leading to a lower fatigue strength. Several specimens were investigated by thermographic measurements accompanied by simultaneous analysis of the resonant frequency as well as the nonlinearity parameter. Exemplarily, specimen 1 (sand casting, see Fig. 1a) is explained here in more detail. The development of the resonant frequency (fres), the nonlinearity parameter ( rel), the ambient temperature Tamb, the maximum temperature Tmax and the temperature variation T is shown in fig. 1b. This specimen failed due to an internal defect at Nf = 2.16 × 106 cycles. The dashed black

Surface roughness and internal porosity of partial removable dental prosthesis frameworks fabricated from conventional wax and light-polymerized patterns: A comparative study

Surface roughness and internal porosity are 2 potential risk factors that threaten the biocompatibility and long-term mechanical stability of cast partial removable dental prostheses frameworks. The purpose of this study was to investigate the influence of the material, of conventional wax patterns, and the recently introduced light-polymerized patterns on the initial surface roughness and internal porosity of the cobalt-chromium castings of partial removable dental prostheses. Twenty-eight identical partial removable dental prosthesis frameworks were fabricated on a cast of a maxillary Kennedy Class III modification 1 partially edentulous patient. Fourteen frameworks were fabricated from each pattern material. The initial surface roughness, Ra (μm), of the resultant castings was measured with a profilometer. Radiographs of the castings were made on dental occlusal films with a dental x-ray unit with the following exposure conditions: tube voltage 70 kV; exposure time 1.0 seconds; tube current 7 mA; and source-film distance 35 cm. The number and size of porosities in the 3 main components (clasp units, major connectors, denture base meshwork) were assessed. Statistical analysis was conducted with the independent sample t test for surface roughness and the Mann Whitney test for internal porosity (α=.05). The statistical analysis of mean surface roughness and internal porosity revealed no significant difference between the 2 groups. The surface roughness and internal porosity of frameworks fabricated from conventional wax and Liwa patterns were comparable, with no significant differences between the 2 pattern materials.

Influence of silicon carbide chills on solidification process and shrinkage porosity of castings made of nickel based superalloys

The paper presents the manufacturing methodology of IN713C castings by application of investment casting method and silicon carbide (SiC) chills.On the basis of numerical simulation and conducted microstructure analysis, it was established that the application of SiC chills results in significant decrease in shrinkage porosity, in comparison to castings without chills. The temperature measurement was carried out, and the influence of chills on the kinetics of solidification process was established. It was determined that the application of SiC chills causes the increase in cooling rate (∼0·1 K s−1), in comparison to the casting without chills. The authors carried out numerical optimisation with the use of ProCAST software and established which parameters have the largest influence on the solidification process of castings with and without chills. In addition to SiC, the graphite and Al2O3 were analysed as chill material in terms of influence on the predicted kinetics of cooling process and the shrinkage porosity of castings. The conditions of solidification process for castings, which are equipped with chills and are free from shrinkage porosity, were determined on the basis of obtained results.