Reduced Pressure Test Research Articles

Influence of reduced pressure on density of aluminium extrusion alloy

Hydrogen has high solubility in molten aluminium during solidification, causing porosity, low tensile strength, low elongation and poor surface finish. Porosity is enhanced by inclusions and particles suspended in the melt. The popularity of the reduced pressure test (RPT) stems from its simplicity. It is basically a comparative and semiquantitative test. The reduced pressure encourages pore formation and lower density than if obtained under atmospheric conditions of solidification. The RPT samples of 6063 aluminium alloy were solidified under different vacuum conditions and correlated to its specific gravity. Visual examination of the samples shows that the standard RPT is less sensitive to hydrogen concentration when used with the 6063 alloy than in the case of aluminium foundry alloys. The results reveal that the use of a reduced pressure of 28″Hg (−95 kPa) instead of the suggested standard vacuum of 25″Hg (−84·5 kPa) gives a better visual assessment of the gas content in the melt.

Effect of Melt Quality and Quenching Temperature on the Mechanical Properties of SIMA 2024 and 7075

Spherical grains can be obtained by several semisolid processes in aluminium alloys. One of these methods is called Strain Induced Melt Activated (SIMA). In this work, commercially available 2024 and 7075 alloys were subjected SIMA process. First, optimised process parameters were investigated for the homogeneous spherical grain structure. This was followed by solution heat treatment of the parts. For the quenching medium, room temperature and 80°C boiling water was selected. The effect of different quenching temperatures over the mechanical properties was tested. Hardness, tensile and fatigue tests were applied. In addition, reduced pressure test was used to assess alloy quality and the results were compared with the mechanical tests.

A Novel Technology of Hydrogen Measurement Based on Directional Solidification in Molten Aluminum Alloy

Hydrogen is a harmful element in aluminum. How to accurately measure hydrogen content in molten aluminum alloy has been significant for improving the metallurgical qualities of aluminum products. Currently, Reduced Pressure Test (RPT) based on the famous Sievert’ law is the most commonly used method to measure the hydrogen in aluminum. However, due to the changes in solidification morphology of alloys under different solidification conditions, hydrogen will be closed more or less in the sample by the developed dendrites, which results in the dispersion of test results. According to the principle of solute redistribution, an improved reduced pressure technology under directional solidification was proposed in this paper. The solidification interface was pushed on along the single direction by controlling the temperature gradient of solid-liquid interface. Hydrogen is released in front of the solid-liquid interface through the solute redistribution. The result shows that the hydrogen content increases and the mean variance decreases with the increasing of the temperature gradients. The method was proved to be of higher accuracy.

Investigations of Alloying Elements and Rare Metals on the Hydrogen Content and Inclusion in Aluminum Alloy

The influences of alloying elements Mg, Zn and Cu and rare metals Ce, Er and mix rare metal (MRE) on the hydrogen content and inclusion in 1050 aluminum alloy were studied by optical microscope, scanning electronic microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results show that when adding 1wt.%Mg, 5wt.%Zn, 0.5wt.%Er and 0.5wt.%MRE into 1050 aluminum alloy, respectively, the decrements of density of the reduced pressure test (RPT) samples are 0.36g/cm3, 0.24g/cm3, 0.72g/cm3, and 0.24g/cm3, respectively. Mg and Er significantly increase the tendency of hydrogen adsorption in Al melt. The results of XRD analysis and microstructural observation indicate that the alloys adding Er contain more oxides and gas porosity.

Degassing of LM24 Al alloy by intensive melt shearing

Hydrogen can dissolve to a significant extent in molten aluminium and is the main cause of gas porosity in castings. The effect of intensive melt shearing on hydrogen concentration in molten aluminium alloy has been studied by both reduced pressure test and direct hydrogen measurement, and the results show that intensive melt shearing has a significant degassing effect for the LM24 aluminium alloy melt. By applying intensive melt shearing, the hydrogen concentration in the liquid alloy was reduced from 0·16 to 0·08 mL/100 g, and the density index Di was reduced from 12·40 to 2·96%. It was also found that shearing speed, shearing time and holding time after shearing could influence the degassing efficiency. In addition, pressurised melt filtration was also carried out to understand the mechanism of reduced porosity by intensive melt shearing, particularly the role of oxide in the formation of porosity, and the microstructure refinement.

Porosity, hydrogen and bifilm content in Al alloy castings

In Al alloy castings, it is often not microstructure, but entrained defects in the form of bifilms that dominate mechanical properties. To assess their effect, this study targets a new quantification of metal quality, measuring the lengths and number of bifilm defects as determined from the microstructural section of a reduced pressure test (RPT) sample.

Degassing, hydrogen and porosity phenomena in A356

The decrease in the solubility of hydrogen in liquid aluminium with temperature has been believed to be a major source of porosity. Therefore, degassing treatment is carried out in foundries to decrease porosity in castings. However, it was shown that it is difficult to nucleate hydrogen porosity in the absence of bifilms where bifilms simply aid the growth of pores. For this purpose, several casting experiments were carried out with commercial A356 alloy. In the first series of testing, the melt was degassed and then upgassed to three different levels. In the second series, the melt was first upgassed and then degassed gradually. In another test, hydrogen level was kept constant and sample collection was carried out. In all the trials, 10 bars were cast into sand mould to produce cylindrical samples for tensile testing. A step mould was used to investigate the porosity distribution and tensile samples were also collected from the same casting. For each casting experiment, a reduced pressure test sample was taken to check metal quality by using bifilm index. It was found that the turbulence and vortex (i.e. increase in bifilm population) during rotary degassing has a more significant effect on mechanical properties and porosity than the hydrogen content.

Characterization of the Melt Quality and Impurity Content of an Lm25 Alloy

The melt quality of an LM25 aluminum casting alloy has been examined using reduced pressure test (RPT) measurements, porous disc filtration analysis (PoDFA), and fatigue and tensile tests. The aim of this study was to determine the existing melt quality and thus to evaluate methods used with respect to monitoring and improving melt cleanliness. Special emphasis was given to the influence of oxides. It was found that the melt quality has varying degrees of effect on the tests used. The results indicate in particular that it was necessary to distinguish between “new” oxides (bifilms) and “hard” inclusions in the melt, as new oxides impact on porosity, whereas hard inclusions impact on ductility. Based on the results of this study, suggestions for the measurement of the melt quality have been proposed.

The influence of processing parameters on the ultrasonic degassing of molten AlSi9Cu3 aluminium alloy

The effect of high intensity ultrasound on the degassing of AlSi9Cu3 alloy using the novel MMM (Multi-frequency Multimode Modulated) technology was studied. Different ultrasonic parameters (power and frequency), melt temperature and processing times were tested and their influence on the degassing efficiency evaluated. RPT (Reduced Pressure Test) was used to evaluate the hydrogen content in the alloys and the samples porosity level and density. For the experimental conditions used in this research, it was found that ultrasonic frequency has no influence on the hydrogen removal rate that, in turn, strongly depends on the ultrasonic power, the processing time and the melt temperature. The experimental results suggest that the MMM ultrasonic technology is an important improvement to the fixed-frequency ultrasonic systems by significantly decreasing the processing time to achieve a quasi-equilibrium hydrogen concentration in aluminium melts.

Study of fluxing temperature in molten aluminum refining process

Aluminum melts are often contaminated with non-metallic inclusions. Large amounts of these inclusions and oxides accelerate the trend of porosity formation in casting parts. They significantly decrease the corrosion resistance and mechanical properties of the aluminum castings. Fluxing is one of the conventional methods for removing inclusions from melts. Efficiency of this process is affected by several parameters, such as: chemical composition, morphology, granulation, the amount of flux, fluxing temperature, fluxing time, and the fluxing method. In this paper, the effect of fluxing temperature on the refining process of molten aluminum alloy has been studied. They have been investigated using tensile test, reduced pressure test (RPT), K-mould test, metallography, SEM, radioscopy, and the optimum temperature of fluxing has been determined.

Use of bifilm index as an assessment of liquid metal quality

There is growing evidence that, in general, aluminium castings under-perform by a large margin. This is not only because of shrinkage or gas porosity, but particularly because of the existence of extremely thin but detrimental defects called bifilms. Bifilms are the initiators of porosity and failures. The opening of a bifilm to make a pore or a crack involves negligible energy, being so easy that it can be assumed that this process will be overwhelmingly favoured. The discriminating use of the Reduced Pressure Test (RPT) clearly reveals the existence of bifilms. Until now, the unpredictable behaviour of bifilms has made the RPT hard to quantify. In the present study, a new fundamental parameter is proposed to assess melt quality called the Bifilm index which is defined as the total length of bifilms estimated on a polished RPT section. Typical total bifilm lengths vary from 3 to 300 mm. The effect of the bifilm index on the mechanical properties was studied using several common Al–Si alloys cast under different conditions. The total bifilm length plus perhaps the total bifilm number seems a rational basis for the quantification of the quality of liquid Al alloys. For the first time, evidence has been uncovered that high oxide content not only reduces ductility, but, acting as a composite material, can simultaneously increase strength.

Critical assessment of reduced pressure test. Part 2: Quantification

The presence of bifilms in aluminium alloys has not so far been widely accepted, because there is no single metal quality test that is capable of identifying and classifying them, despite them appearing to be among the most serious defects that can exist. The reduced pressure test (RPT) is widely known and already used throughout the industry. However, the purpose of the present study was to evaluate its potential for the assessment of metal quality in terms of hydrogen content and bifilm content, and assess whether it has potential to be developed into a quantitative technique. To investigate these effects, two alloys from the previous work were used: LM4 (Al–5Si–3Cu) and LM24 (Al–8Si–3Cu–Fe). Various quality index concepts were explored in an effort to quantify the results of the reduced pressure test. The 'Bifilm Index', the total length of bifilms on the sectioned surface of the RPT sample solidified at 100 mbar, appears to have promise as a fundamental parameter to quantify quality. Good products appear to have a typical total bifilm length up to 1 mm, whereas possibly tolerable products are up to 30 mm. Poorly handled alloy can contain bifilms of total length approaching 300 mm. In terms of area of bifilms in a standard weight of melt, for a RPT sample weighing about 90 g, these values correspond approximately to 1.0, 1000 and 100 000 mm2 per 100 g.

Critical assessment of reduced pressure test. Part 1: Porosity phenomena

During the production of aluminium ingots and castings, the surface oxide on the liquid may be folded into the bulk liquid to produce crack-like defects (bifilms) that are extremely thin, but can be extensive, and so constitute seriously detrimental defects. In this work, it has been found that bifilms are the initiator and hydrogen is only a contributor to the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The discriminating use of the RPT clearly reveals the existence of bifilms, and the effect of hydrogen on porosity formation. However, it seems that the RPT is of little use to evaluate the hydrogen content of the alloy. To investigate these effects, two alloys were studied in laboratory experiments LM0 (99.5%Al), LM4 (Al– 5Si–3Cu) and one in an industrial environment LM24 (Al–8Si–3Cu–Fe).

The Effect of SR-Modification Treatment on Porosity Formation of Reduced Pressure 319 Al Alloy Castings

The effect of Sr-modification on pore formation in reduced pressure and atmospheric conditions was studied. It was found that in thin wall steel cup conventional specimens, the Sr-modification had no significant effect on pore formation. A new type of metallic cup with a riser was designed for use in a reduced pressure test as a mould. It was designed to improve directional solidification so that no macro shrinkage occurred in test samples. Using this mould, the effect of Sr-modification on pore formation was investigated. The effect of Sr-modification on the increased pore number density and pore volume fraction became significant in both reduced pressure and atmospheric conditions. Also, the results indicate that the effect of Sr-modification on pore formation of reduced pressure samples is not considerable in the melt with a hydrogen content lower than 0.1cc/100g Al.

Liquid metal quality

Castings are now becoming commonplace in highly demanding and safety critical applications, for example in the aerospace and automotive industries. It is, therefore, imperative that internal defect structures and potential failure mechanisms are fully understood if reliable castings are to be produced consistently by the foundry. Many alloy systems, such as those based on aluminium, magnesium, zinc and certain copper- and iron- and steel-based alloys suffer from the entrainment of double oxide films and dissolved gases. However, the presence of damaging oxide films is often only detected when a casting leaks or fractures at such inclusions. The ubiquitous presence of these very thin double films has not been widely accepted because no single metal quality test has been able to resolve such thin but extensive defects. We have developed a novel reduced pressure test that allows a detailed assessment of metal quality through the simultaneous visualisation of double oxide films and gas content. The technique will be demonstrated for use on aluminium alloys as this system provides clear examples and contains only a single dissolved gas.

An evaluation of calcium as a eutectic modifier in A357 alloy

Modification of the eutectic silicon in Al-Si alloys is traditionally performed by sodium and strontium addition to alter the form of the silicon from an acicular shape to a fine fibrous one. This modification results in a considerable improvement in mechanical properties, especially elongation. Although not commonly used as a modifier, calcium also has the potential to modify the eutectic silicon in Al-Si alloys. Metallographic observation and thermal analysis studies in A357 alloy indicate that, when the cooling rate is quite low, Ca modifies the eutectic silicon, but the effect of Ca as a modifier is not as strong as is Na or Sr. When the cooling rate is high, a fully modified structure can be obtained with calcium. Recovery of Ca in the melt is high and certain. It does not fade with long holding times, and retains its effect after repeated remeltings.Calcium addition does increase porosity in the cast alloys. The reduced pressure test (RPT) shows that the density of samples decreases with calcium addition, and observation of polished sections confirms the porosity increase. Tatur tests indicate that Ca modification increases microporosity, and decreases macroporosity.Calcium and strontium interact together to reduce the effectiveness of each other as a modifier. This interaction can be seen by thermal analysis as well as by microstructural studies.

Evaluation of Inherent Distortions in the IIST Facility Using the RELAP5/MOD3 Code

The Institute of Nuclear Energy Research (INER) integral system test (IIST) facility is a reduced-height, reduced-pressure test facility constructed at INER that is used to simulate the thermal hydraulics of the Maanshan nuclear power plant (NPP). A small-scaled facility is not capable of simulating all the physical phenomena of an NPP because the behavior of an NPP during accidents is very complicated. Proper scaling then plays an important role in the design of a test facility to ensure the usefulness and applicability of experimental data obtained from a small-scaled facility. However, distortions caused by necessary compromises in the design and construction of a small-scaled test facility exist. The analysis here evaluates whether the inherent distortions in the IIST facility will distort the thermal-hydraulic behaviors of a natural-circulation experiment and influence the usefulness and applicability of the experimental data. Based on the current calculations, the IIST experimental results are found to be partially distorted. Appropriate consideration of and correction for these distortion effects are needed before the results of the IIST natural-circulation experiments can be used to reliably investigate the Maanshan NPP behavior expected by way of an appropriate scale-up procedure.

The Reduced Pressure Test as a Measuring Tool in the Evaluation of Porosity/Hydrogen Content in Al-7 Wt pct Si-10 Vol pct SiC(p) Metal Matrix Composite

Porosity is one of the important factors critical to the production of optimum aluminum alloy castings. Hydrogen is mainly responsible for the “gas porosity” in such castings, which is also affected by other factors including melt cleanliness. The importance, therefore, of obtaining a reliable estimate of the melt hydrogen level prior to casting has led to the development of several techniques, among which the reduced pressure test (RPT), basically a comparative, qualitative test, appears to be the one popularly used in foundries due to its simplicity and easy adaptation to the foundry floor. Attempts have been made to quantify the test by correlating the densities of reduced pressure samples with the hydrogen contents of their melts. In the present study, the RPT was tested as a means of determining the hydrogen content in Al-7 wt pet Si-10 vol pet SiC composite melts as part of an on-going study being carried out in our laboratories on such composites. The results reveal that rather than indicating the hydrogen content of the melt, the RPT is a better indicator of the porosity content of the cast sample and can be employed as a melt quality measuring tool, provided the sample density is correctly related to said po- rosity. Qualitative analysis is substantiated throughout by pore size and distribution data ob- tained from image analysis. It is also found that compared to the unreinforced A356 matrix alloy, the composite material has a beneficial effect on the formation of porosity due to the tendency of the SiC reinforcement particles to restrict the growth of the pores. This, coupled with the microporosity associated with the presence of the SiC particles, results in the skewed pore size distribution curves typically observed for the composite samples.

オゾンを含む酸素ジェットの可視化

The preliminary visualization technique for a low density gas flow is discussed. The optical absorption phenomena of ultraviolet ray by ozone are applied. Ozone has a region of continuous absorption between 2000A and 3000A of an ultraviolet light wave length, with a noximium at 2537A. Therefore, the parallel ultraviolet light from a low pressure mercury lamp is injected from one side wall of a test section. In the reduced pressure test section, oxygen-ozone mixture jets are forced into from a nozzle. The ultraviolet light passing through the jets are absorpted. The radiation emerging, after absorption in the jets is transformed into visible light by fluorescent screen.