Fracture Surface Facets and Fatigue Life Potential of Castings

Fatigue potential has been studied in cast aluminum alloys with regard to the fatigue crack initiation mechanism at the casting defects, particularly surface and subsurface defects. The significance of facets is interpreted as the presence of defects in the interior of castings. Furthermore, two varieties of facets have been identified, one originating as a dendrite-straightened bifilm (type I facet) and the other originating from a slip plane mechanism around casting defects (type II facet). The fatigue life potential of castings is reexamined based on the involvement of defects during the formation of both types of facets. It is proposed that the true fatigue life potential of defect free castings has yet to be observed, i.e., if castings can be produced without defects, then their fatigue performance will be significantly higher than even the best performances observed so far.     

Making castings of nonferrous alloys in bulk investment molds

The theoretical aspects and practical findings of the development and commercial implementation of new engineering processes for making art castings of nonferrous alloys are presented. The castings weigh from 0.1 to 100 kg and reach 1.5 m in size. They are cast into bulk filler molds of pouring mixtures based on gypsum and cement binders and can be produced individually and in batches.     

Determination of the Lifetime of a Double-Oxide Film in Al Castings

One of the most important casting defects in Al alloys is thought to be the double-oxide film defect (bifilm) which has been reported to have a deleterious effect on the reproducibility of the mechanical properties of Al castings. Previous research has suggested that the atmosphere inside such bifilms could be consumed by reaction with the surrounding melt, which might decrease the size of the defects and reduce their harmful effect on mechanical properties. In order to follow the change in the composition of the interior atmosphere of a bifilm, analog air bubbles were held inside Al alloy melts, for varying lengths of time, and subjected to stirring, followed by solidification. The bubble contents were then analyzed using a mass spectrometer to determine the changes in their compositions with time. The results suggested that initially oxygen and then nitrogen inside the bubble were consumed, and hydrogen dissolved in the melt diffused into the bubble. The consumption rates of O and N as well as the rate of H diffusion were dependent upon the type of oxide, which was dependent on the alloy composition. The reaction rates were the fastest with MgO (in an Al-5Mg alloy), slower with alumina (in commercial-purity Al alloy), and the slowest with MgAl₂O₄ spinel (in an Al-7Si-0.3Mg alloy). It was estimated that the times required for typical bifilm defects in the different alloys to lose their entire oxygen and nitrogen contents were about 345 seconds (~6 minutes), in the case of Al-5Mg; 538 seconds (~9 minutes), in the case of a commercial purity alloy; and 1509 seconds (~25 minutes), in the case of the Al-7Si-0.3Mg alloy (2L99) due to the different oxides that the different alloys would be expected to form.     

Macrosegregation in a copper alloy directionally cast with exudation of liquid

Cu-8 pct Ag alloys containing radioactive silver were cast into a heated mold with a water-cooled copper base producing a columnar cast structure. A short time after casting, the chill was removed causing local reheating and exudations to occur at the chill face. The exudations were found to be solute-rich, and the casting adjacent to the chill solute-poor. A series of castings were made with different time intervals between pouring and removal of the chill. Quantitative measurements were made of the macrosegregation of each of the castings both in the exudations and in the casting proper. The results are accounted for on the basis of a simple model which assumes back flow of the interdendritic liquid during solidification.     

The research and development of new type of ferro-base heat-resistant alloy

Nickel-base superalloys are high performance materials subject to severe operating conditions in the high temperature turbine section of gas turbine engines.Turbine blades in modern engines are fabricated from Ni-base alloy single crystals which are strengthened by ordered g’ precipitates.Turbine disks are made from polycrystal line Ni-base alloys because these components have higher strength requirements(due to higher stresses).By increasing the upper temperature limit for the next generation of disk materials,the aviation industry will see significant environmental as well as cost benefits. Researchers in the High Temperature Materials Center of the National Institute of Materials Science of Japan have recently completed their work on a new kind of disk alloys.The new disk alloys,a kind of nickel-coble-base superalloys processed by a normal cast and wrought(C & W) route,can withstand temperatures in excess of 725 degree centigrade,a 50-degree increase over C&W disks currently in operation. In this presentation,the author shows the design idea,workability and properties of these Ni-Co-base superalloys. Furthermore,the evaluation of the processing and microstructure on a full-scale processing of Ni-Co-base superalloy turbine disk are described,which demonstrated the advantages and possibility of the Ni-Co-base disc alloys at the component level.     

Effect of Holding Time Before Solidification on Double-Oxide Film Defects and Mechanical Properties of Aluminum Alloys

Double-oxide films (bifilms) have been held responsible for the variability in mechanical properties of aluminum castings. It has been suggested that the air entrapped inside a bifilm can react with the surrounding melt, leading to its consumption, which might improve the mechanical properties of the castings. In this work the effect of holding the melt before solidification on the distribution of mechanical properties, and by implication on entrained double oxide films, was investigated for several different aluminum alloys. The Weibull moduli of plate castings were determined under tensile conditions, and their fracture surfaces were examined for evidence of oxide films. The results suggested the occurrence of two competing mechanisms during the holding treatment: (1) the consumption of air inside the bifilms by reaction with the surrounding molten metal that may lead to improvements in mechanical properties and (2) the accompanying diffusion of hydrogen into the bifilms, which would be expected to have a deleterious effect on properties.     

Influence of temperatures of melt overheating and pouring on the quality of aluminum alloy lost foam castings

Lost foam casting (LFC) is currently one of the most efficient and promising methods of fabricating high-quality thin-wall castings possessing specified dimensional accuracy, required surface roughness, and other properties. This technology is widely used in the production of aluminum alloy products. To minimize costs in the fabrication of wares and to fabricate high-quality castings, it is reasonable to use an increased amount of secondary materials in the charge, herewith paying attention to the melt overheating temperature and holding time. The results of studying the temperature modes of smelting pouring aluminum alloys in the LFC are presented. The most efficient modes in manufacturing conditions under consideration which provide the best quality characteristics of leak-tight castings by dimensional accuracy and surface roughness were as follows: the melt overheating temperature is 880–890°C and the melt pouring temperature into the casting mold is 820–830°C. The influence of various variants of temperature parameters of smelting and pouring the melt of the AK7 composition during the LFC on the content of nonmetallic inclusions in the cast state is investigated. It is revealed that the minimal γ-Al₂O₃ content in the final alloy is provided by a melt overheating temperature of up to 880–890 or 940–950°C and a melt pouring temperature into the casting mold of 820–830°C.     

Discussion of Six Papers From May 2013 Issue

The May 2013 issue of Metallurgical and Materials Transactions A is a typical issue, containing a number of papers, all of which would be explicable assuming the near-universality of the presence of oxide bifilms in metals as a result of the entrainment of the surface oxide film during pouring. The unbonded interface between the necessarily doubled-over surface film, often only a few nanometers thick, and often occurring at grain boundaries, appears to be the cause of grain boundary failures and the initiation of cracks. The precipitation of second phases on the outer surfaces of the bifilm, in good atomic contact with the matrix, explains the apparent brittleness of many intermetallics and second phases. The number of papers from the May issue of MMTA illustrates the ubiquitous presence of bifilms, up to now almost unknown, and their central importance in physical metallurgy, and their possible control, in turn, by up-to-date process metallurgy.     

Influence of orbital shaking on microstructure and mechanical properties of A380 aluminium alloy produced by lost foam casting

Using vibration for refining microstructure and improve mechanical properties of aluminium alloys castings are in the interest of researchers for many years. Within the framework of these studies mechanical, ultrasonic and electromagnetic vibration applications were carried out. Results of these processes can be summarized as grain refining and changing the dendritic structure into globular. Accordingly increasing in density and mechanical properties were reported. In this work, orbital shaking technique was used alternatively to conventional mechanical vibration in lost foam casting (LFC) of A380 aluminium alloy. In the experiment castings, effects of shaking movement and speed during pouring were investigated. First of all orbital shaking movement has not damage LFC parts and any shape disorder was not occurred. Optical microstructure observations show that, the increase in shaking speed, decrease secondary dendrite arm spacing (SDAS) and partial dendrite arm fractures were determined at 150 rpm shaking. Density and hardness of as cast specimens were increased with shaking and rising shaking speed as well.     

Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification

Unmodified, Na-modified, and Sr-modified castings of Al-7?pct Si and Al-12.5?pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification.     

Microstructure and Corrosion Characterization of Squeeze Cast AM50 Magnesium Alloys

Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior.     

Influence of melting conditions of aluminum alloys on the properties and quality of castings obtained by lost foam casting

The development of modern foundry production is characterized by a constant increase in requirements for the quality of fabricated casting and rational use of material resources, which determines the search for new technical and process solutions, making it possible to acquire the required properties of cast wares along with resource saving. Herewith, the question of revelation and investigation into the regularities of the influence of thermal-temporal parameters of smelting and pouring of aluminum alloys into the casting mold during the lost foam casting on tightness and mechanical and qualitative characteristics of thin-wall castings remain poorly known and complex for implementation, especially allowing for the performance of resourcesaving measures. In this publication, the influence of process parameters of smelting on the strength, tightness, and content of nonmetallic inclusions in castings of the gas-analyzer case made of AK7 alloy during the lost foam casting is considered. The data set acquired based on the experimental investigations has been subjected to statistical processing. The use of statistic models makes it possible to acquire the results of the influence of the holding time and content of secondary materials in the charge on strength and tightness of mentioned castings. The results of an investigation into the influence of holding the AK7 melt at the overheating temperature of 880–890°С on the content of nonmetallic inclusions in castings show that it can be regulated varying the holding time. This procedure decreases the melt microinhomogeneity and provides the acquisition of numerous castings with a minimal content of nonmetallic inclusions.     

Investigation of spherisation in microstructures of aluminium casting alloys for thixoforging process

Thixoforging combined with low superheat casting (LSC) is a promising shaping process for aluminium casting alloys. LSC process is based on rapid solidification of an alloy which cast with low pouring temperature. With this method, a feedstock material is produced with non-dendritic microstructure that ready for spherisation in reheating sequence of further semi-solid process. Al-Si alloys are still castable even at low temperatures due to their excellent fluidities. This study subjects to present spherisation of A356 and A380 alloy billets cast with LSC process that provides appropriate beginning material with relatively high sphericity. Obtained billet parts were reheated for different times at a semi-solid state temperature. Some of these billets were directly quenched for observing the effects of reheating and the others were thixoforged. With sufficient reheating time, deformation of thixoforging process did not significantly affect on the spherical microstructure. Unnecessarily long reheating period caused excessive grain growth. A356 alloy had higher spherisation tendency than A380 alloy under similar process conditions.     

Formation of defect bands in high pressure die cast magnesium alloys

Die cast magnesium components are being increasingly used worldwide because of the excellent castability and properties that magnesium alloys offer. High pressure die casting of thin-walled components is particularly suitable because of the excellent flow characteristics of molten magnesium alloys. Typical automotive applications for thin-walled castings include components such as instrument panels, steering wheels, door frames and seat frames. These applications require optimisation of the quality and performance of the castings. It has been found that bands of porosity or segregation which follow contours parallel to the surface of the casting are formed under certain casting conditions in thin-walled magnesium high pressure die castings. The presence of this type of defect can have a significant effect on the mechanical properties. This paper investigates the effect of varied casting conditions on casting integrity and the appearance of the bands. A rationale for understanding the origin of these defects is related to the solidification behaviour, the mushy zone rheological properties and the filling pattern of the casting with associated shearing of the mushy zone. Methods to optimise the process parameters to control the occurrence of the banded defects, and thereby optimise the quality of high pressure die cast magnesium components, are outlined.     

熔炼工艺对高铬铸铁性能的影响

对高铬铸铁的熔炼速度，过热温度、浇注温度及变质处理进行了研究。结果表明，合理的熔炼工艺及Ｒｅ－Ｂ变质处理能明显地提高高铬铸铁的机械性能。     

Formation of Nondendritic Primary Aluminum Phase in Hypoeutectic Alloys in Controlled Diffusion Solidification (CDS): A Hypothesis

Controlled diffusion solidification (CDS) is a novel process wherein specific Al alloys can be cast by mixing two precursor alloys of specific compositions and temperature and subsequently casting the resultant mixture. This process enables a nondendritic morphology of the primary Al phase in the cast samples, which is beneficial in mitigating hot tearing tendencies and enabling castability of dilute Al (wrought) alloys to obtain castings with superior mechanical and performance properties. In this study, a hypothesis is proposed to describe the mechanism of the CDS process, specifically the processes of mixing two precursor alloys and a subsequent solidification process. Al – 4.5 wt pct Cu was used as an example alloy system to propose a hypothesis and to verify the various features in the mechanism of mixing two alloys. Experimental results show that the mixing process naturally causes copious nucleation of one of the alloys mixed and that the turbulence energy during mixing distributes these nuclei uniformly to enable a favorable solidification condition for a nondendritic cast microstructure. It is critical that the alloy with the higher thermal mass (mass and temperature) is mixed into the alloy with lower thermal mass to obtain a valid CDS process and that the reverse will not yield a favorable homogeneous cast sample. Certain critical parameters during the CDS process have also been identified and quantified for a favorable microstructure of the casting.     

The Effect of Holding Liquid Aluminum Alloys on Oxide Film Content

Melts of commercially pure liquid aluminum, and an Al-7Si-0.3Mg alloy, were cast into molds designed to produce entrainment of oxide film defects. The melts were held for periods of up to 20 minutes to investigate whether changes in the oxide film defects in the melt could occur, once sufficient time had elapsed for consumption of their internal atmosphere. The alloys were characterized by the determination of their Weibull modulus, examination of fracture surfaces under scanning electron microscopy (SEM), and determination of their porosity characteristics. The Weibull moduli of the ultimate tensile strength (UTS) values of the Al-7Si-0.3Mg alloy were reduced initially by holding in the liquid state for 10 minutes, but then the values increased after holding for 20 minutes. This high Weibull modulus was found despite oxide films being observed on the fracture surfaces. In the case of the commercial purity Al, the UTS Weibull moduli increased only slightly with holding for 20 minutes. The results suggested that holding of Al alloys in the liquid state influenced the scatter of mechanical properties by influencing the porosity content of the castings, which was related to their oxide film content. Some evidence for healing of a double oxide film defect with time was also found in the commercial purity Al alloy.     

The air-gap formation process at the casting-mold interface and the heat transfer mechanism through the gap

The formation process of the air gap at the casting-mold interface and the heat transfer mechanism through the gap were investigated by measuring the displacement of, and the temperature in casting and mold for cylindrical and flat castings of aluminum alloys. The thickness of the air gap was measured as the difference between the location of the casting surface and that of the mold inner surface. For cylindrical castings, the mold began to move outward immediately after pouring, while the casting stayed until solidification progressed to a great extent. For flat castings, the mold began to move greatly toward the casting pushing the casting immediately after pouring and moved reversely after a maximum appeared. It was possible to calculate the displacement of the mold by thermal expansion. It was found that when the thickness of the air gap was not large, the heat through the gap was transferred mainly by heat conduction.     

Microstructural and Statistical Study of Semisolid Casting of 6061 Alloy Using a Miniature Cooling Slope

Preparation of metallic semisolid slurries using the cooling slope method is increasingly becoming popular because of the simplicity of design and control of the process. Microstructural features of the resultant semisolid castings such as size and sphericity of the primary particles are affected by several processing parameters such as pouring rate, cooling slope surface angle and length as well as the melt superheat. In this work, a miniature cooling slope for semisolid casting of small parts was built and attempts were made to develop an empirical relationship showing the correlation between the sphericity of the microstructure of semisolid cast 6061-aluminum alloy and the processing variables. The relationships were developed by a two-level factorial method. The results showed that the interaction of cooling slope length and pouring rate factors had the most effect on the sphericity of the final semisolid cast microstructure.     

Cracking in γ-TiAl due to high speed particle impact

Cracking due to small particle impacts on prospective γ-TiAl blades in jet turbine engines has been studied experimentally and theoretically. Flat rectangular specimens of two γ-TiAl alloys, with edges chamfered to resemble the taper of blades, were subjected to small particle impacts. The forms of damage observed resembled those revealed by earlier studies using specimens that were cast closely to the shape of blade leading edges. Numerical simulations of the impact events were carried out using finite element analysis, using uniaxial stress-strain behavior that was obtained in compression at high strain rates. A criterion for cracking is proposed that requires critical levels of plastic strain and tensile stress. Predictions of crack extent that were based on such a combined criterion were found to be in the range of observations. The predictive methodology, together with information on the fatigue strength of various damage states, potentially offers designers the opportunity to examine the risk associated with small particle damage on contemplated blades.