The Influence of Processing Variables on the Dimensional Integrity of Spheroidal-graphite Iron Castings

Abstract

This investigation was carried out to identify the major factors and their degree of influence on the dimensional accuracy of spheroidal-graphite iron castings produced in chemically- bonded sand moulds. Test castings were poured into furan-resin-bonded zircon sand and silica sand moulds and sodium silicate/ CO₂ bonded silica sand moulds. A comparison of casting sizes with those of the mould cavity into which they were poured showed considerable scatter and overlap. From these data the size that each casting would have had, had it solidified without graphite formation, was calculated and found to depend on mould cavity size for each type of mould. By isolating the differences in casting size due to graphite it was possible to identify the influencing factors. Thus castings poured into furan-resin-bonded zircon sand have the highest contraction and their size depends primarily on the amount of graphite present. The dimensions of castings poured into silica sand moulds show more variation and depend not only on the amount of graphite present and the structure of the metal but also on the thermal expansion of the silica sand moulds.     

Enhanced cooling for solidification of aluminium alloys in permanent moulds using advanced heat pipe solutions

Abstract

In order to obtain sound cast components with good properties, a number of measures must be taken to make parts defect free. It is well recognised by the casting industry that it is essential to control cooling rates of permanent mould castings in order to speed up solidification and control the solidification pattern. Each of the traditional controlled cooling techniques (air or water cooling passages and chill inserts) presents certain disadvantages and none offers optimum thermal management. A new cooling method for permanent moulds is proposed. This new technique is based on heat pipe technology that was developed specifically for the cooling of permanent moulds in the casting of light metals where high heat fluxes are normally encountered. The influence of the conductivity of mould coatings on casting solidification and dendrite arm spacing with heat pipe cooling was investigated. Typical experimental results are also presented.     

塑料模具的快速制造技术

开发了一种塑料模具快速制作的工艺，该工艺利用薄材叠层（LOM）快速成形机制作原型，结合硅橡胶转制和精密铸造方法，实现模具的快速制造。模具材料使用锌基合金，模具制作周期短，成本低，质量好，利用该工艺制造了汽车塑料灯罩的模具。     

Gas absorption in grey cast iron during mould filling

Abstract

The effect of mould filling on the hydrogen and nitrogen concentration in grey cast iron has been investigated. A special mould was designed and the influence of several process and mould parameters, such as degree of turbulence, permeability, core binder and coatings, were studied. The hydrogen concentration increased during mould filling and the absorption was favoured by turbulent filling, low permeability and larger gas evolution from the mould and cores. In some cases, the hydrogen concentration in the iron after mould filling almost reached the solubility limit, which increases the risk that gas precipitates during solidification.

Exact values of the influence of mould filling on the nitrogen concentration could not be obtained, but some observations could still be conducted. The absorption of nitrogen seemed to be favoured by the same parameters as hydrogen.     

Properties of silica sand and GBF slag moulds practiced by Nishiyama process

ABSTRACT

Investigations were carried out to use Granulated Blast furnace (GBF) slag as mould material for either full or partial replacement of existing silica sand in foundry industry. Nishiyama process was adopted for evaluating the same. A series of sand tests were carried out on sand and slag individually and also combinations of these two. Three types of moulds were made with sand, slag individually and combination of these two. Both laboratory and industrial castings of ferrous and non ferrous materials were performed. Results of mould permeability, compression and shear strength of GBF slag reveal that is a suitable candidate for either partial or full replacement of molding sand. During casting of both laboratory and industrial, neither fuse, dripping nor collapse of the mould walls was observed; this is true for both ferrous and non ferrous castings. Castings with good surface finish, no surface defects and porosity were made by slag moulds.     

Application of aeration sand filling,squeeze balance control and synchronised pattern drawing to flaskless moulding process

Abstract

In recent years, global warming and environmental deterioration problems have drawn public attention. There is an ever increasing demand for production equipments in foundries that are more human friendly and environmentally cleaner. To meet such needs, a new type green sand moulding machine employing 'aeration' sand filling process has been developed. In order to make further improvements in moulding performance, new technologies, squeeze balance control and synchronised cope and drag pattern drawing technology, applied in squeezing and mould stripping process of the flaskless moulding equipment has been developed. Adoption of the squeeze balance control system achieves uniform mould strength and prevents distortion of pattern plates by automatically controlling the squeeze pressure applied to the cope and drag moulds in the squeeze process. This optimises the squeeze pressure according to the shape of the pattern. In this study the effectiveness of these technologies was proved by executing performance tests with an actual machine.     

Valid mould and process design to cast tensile and fatigue test bars in tilt pour casting process

Abstract

Tilt pour gravity casting technology is increasingly being used for shape casting various components with aluminium alloys. The ASTM B108/B108M-08 standard exists for a metal mould to evaluate the mechanical properties of castings made by gravity permanent mould process, yet there is no standard mould for the tilt pour process. We have designed, developed, tested and validated a standard mould to cast tensile and fatigue test bars in a tilt pour casting process. The new mould has demonstrated abilities to cast sound castings of A356·2 aluminium alloy, and the uniaxial tensile properties were superior to those obtained from conventional direct pour gravity casting process.     

Effect of cooling rate on air gap formation in aluminium alloy permanent mould casting

Abstract

Displacements of the casting surface and the mould surface at the casting/mould interface were experimentally measured during the solidification of aluminium alloys in a permanent mould. Temperatures of the casting and mould surfaces at this interface were also recorded and correlated with displacement measurements. Four different commercial Al–Si alloys were investigated at varying cooling rates. These results are compared with available data on the effect of cooling rate on solid fraction evolution and consequently strength development during solidification. The temperature of the casting surface at the moment of air gap initiation was found to decrease with increasing cooling rate, although this relationship was confirmed at the 95% confidence level for only one of the alloys, AC601, for which sufficient data points were available. The solid fraction at the casting surface at gap initiation in this alloy is shown not to change with cooling rate. In all hypoeutectic alloys, the gap formed before the solid fraction at the casting surface reached 1·0 at slow cooling rates. For the near eutectic alloy BA401 it occurred at almost 1·0. Casting surface contraction rates following gap formation are also presented both as a function of time and casting surface temperature. It is shown that contractions predicted using the linear thermal expansion coefficient provide a reasonable approximation.     

Sequence control of pressing velocity for pressure in press casting process using greensand mould

Abstract

Recently, a press casting process using a greensand mould was actively developed by the authors' group to improve productivity and casting quality. This process consists mainly of two parts: in the first, molten metal is poured in the lower mould by a ladle, and in the second, the upper mould falls down toward the lower mould to press the molten metal. The feature based advantages of this greensand casting are an astonishing increase in casting yield (over 90%) and energy saving. A rapid pressing process should be realised as a vital task to achieve an increase in productivity. In addition, the advanced technique of this process will also suppress casting defects such as a cold shut and the inclusion of generated oxide film due to the temperature decrease in molten metal. As the technical problem on high speed pressing, the other defects are caused by increased pressure. Metal penetration, in which the solidified molten metal is soaked among the sand particles in greensand, often occurs. Hence, the authors propose controlling the pressing velocity in order to realise high production competence and to suppress the rapid increase in pressure. In this paper, a pressure suppression method in high speed pressing is proposed in which the sequential velocity control of the press can be applied to various casting moulds. The control design is conducted simply and theoretically, and a brief mathematical model of the fluid pressure is built using a complicated, exact model of CFD. Then, the permitted range of pressure fluctuation is given for the production of high quality casting. Considering the upper limit of the pressure constraint for defect free pressing, the switching velocity pattern of multisteps is derived using the proposed mathematical model without trial and error adjustment. The control performance for pressure fluctuation using the obtained velocity reference is checked by CFD simulations.     

Fracture toughness of ceramic moulds for investment casting with ice patterns

Abstract

Ice patterns can be used to make ceramic investment moulds for metal castings. Owing to the characteristics of ice, the ceramic mould must be made at subzero temperatures and consequently, requires a different formulation than shells built at room temperature. Success of this process depends greatly on the fracture toughness of mould materials. The present paper describes the experimental results of fracture toughness of mould materials processed from different compositions. The Taguchi method was used to reduce the trial runs. The parameters considered included the ratio of fibre containing fused silica and aluminosilicate powders, the volume of binder and the volume of catalyst. The microstructure and green fracture surface of test bars were also examined to understand the underlying mechanism. While conducting the four point bend test on ceramic mould samples, some samples had exceedingly low strengths appearing as outliers in the Weibull analysis. Examination of these low strength ceramic samples improved understanding of failure of mould materials. Sound moulds have been made for the investment casting process with ice patterns based on the analysis of experimental results. The casting of an M8 bolt is used to demonstrate that metal castings of complex geometry can be fabricated using ice patterns. The measured tolerances are within the required tolerance range.     

The tilt casting process

Tilt casting is a process with the unique feature that, in principle, liquid metal can be transferred into a mould by simple mechanical means under the action of gravity, but without surface turbulence. It therefore has the potential to produce very high quality castings. Even so, the process is not often optimised in the industrial environment. This investigation represents an attempt to investigate some fundamental problems associated with the process.

A computer controlled, programmable roll-over casting wheel with a diameter of 1 m was used to produce sand castings in an Al-4.5% Cu alloy. The filling of the mould was studied using realtime X-ray radiography. Real-time X-ray radiography revealed that the molten metal could exhibit tranquil, turbulent or chaotic flow into the mould during tilt casting, depending on (i) the angle of tilt of the mould at the start of casting, and (ii) the tilting speed. Essentially horizontal transfer of the melt could achieve tranquil filling of the mould with minimum surface turbulence by a tilt starting position above the horizontal. The tensile properties of castings made using various starting conditions and rotation rates were measured and the results analysed using Weibull statistics to quantify reliability. Results are summarised on a map of the various operating regimes for tilt casting. An operational window for the production of reliable castings has been defined for the first time.     

Effect of particle size variations of gypsum bonded investment powders on metallurgical quality of investment castings

Abstract

In the present work, the effect of particle size distribution parameters of investment powder on the metallurgical characteristics of castings produced by flask mould investment casting process was investigated. The main aim of this study is to establish the relationships between the casting quality and investment powder characteristics. For this purpose, models for tensile, density and surface roughness specimens were prepared in accordance with the standards and these models were moulded by gypsum bonded investment powders. Specimens of sterling silver (92·5Ag–7·5Cu alloy) were subsequently cast into these moulds by centrifugal casting process. After the completion of solidification process, the specimens were tested and results were analysed. The preliminary results illustrate that fineness of investment powders play an important role in determining the metallurgical quality of castings produced by investment casting process.     

Heat transfer characteristics of lost foam casting process of magnesium alloy

1 Introduction The growing demand for mass reduction in aerospace and automotive industries has greatly increased the magnesium application. Currently, casting is the main industrial forming method for magnesium alloys, but the lag of research and develop…     

Mullite shell mould for casting of advanced CG and SX components in nickel based superalloys

Abstract

Mullite base shell mould system is used for casting of superalloys in columnar and single crystal grains in temperature range of 1480–1550°C. The colloidal silica gel+mullite filler with and without fine alumina slurries was prepared followed by two shell moulds: one without alumina (shell system I) and the other with alumina (shell system II). Shell made with slurry system II resulted in increase in green strength by 10% and fired (950°C) strength by 125% respectively. Sag resistance capability was observed more for shell system II for tested temperatures from 1500 to 1650°C. Dimensional stability of low pressure turbine blade cast at 1550°C was also studied. No shell bulging effect was observed for both shell moulds. The importance of mullite filler material for shell mould to be used for investment casting and its capability to withstand high temperature without metal mould reaction have been highlighted.     

Effect of colloidal silica addition on compressive strength of frozen mould

Abstract

The frozen mould which is produced by refrigerating the mixture of sand and water has the possibility to improve both the casting cost and the poor working environments such as noise, vibration and dust. The transition of mechanical properties of the frozen mould when it was transforming into thawed and dried states was examined. The compressive strength of the standard frozen mould consisting of only sand and water dramatically decreased when it was thawed and the strength was not recovered by drying. In contrast, the frozen mould produced by substituting colloidal silica solution for water showed some compressive strength even in thawed state and the strength was elevated by subsequent drying. The reusability of the sand mixture with colloidal silica solution was also evaluated. The compressive strengths did not deteriorate with any reclamation. The above results prove that the frozen mould with colloidal silica solution has an excellent industrial potential.     

Towards more reliable investment castings

Abstract

The need for more reliable investment castings to meet the expectations of end users is outlined and the research undertaken during the Fundamentals of Investment Casting (FOCAST) project to meet this requirement is summarised. The traditional gravity poured, top gated mould designs used widely by the investment casting industry are shown to produce the least reliable aluminium alloy and steel castings. Changing to a bottom gated design to minimise surface turbulence during mould filling leads to a significant improvement in reliability, although the mould designs may not be particularly easy to implement in practice. It has been shown that a correctly used tilt pouring technique can also reduce surface turbulence and thereby improve reliability, and it is considered that this process is worthy of further development and evaluation by the investment casting industry. Countergravity mould filling has also been shown to be capable of producing more reliable castings than conventional gravity casting. The three techniques are compared and their industrial implementation discussed.     

Experimental study of sand filling process of aeration filling and high pressure squeezing flaskless moulding machine

Abstract

Aeration filling is a new flask (or moulding chamber) filling method, which has been developed from the shooting/blowing process. As a sand filling stage, aeration filling determines the ultimate quality of mould in a sense, such as integrality and uniformity. If the moulding sand can be filled in every part of the mould cavity, an accurate shape would be replicated after the finial compaction stage, otherwise the mould may have weak regions. This paper focuses on the study of the sand filling process and its influential factors. From experiments and comparison with conventional shooting (or blowing) process, it can be seen that this method has the advantages of better sand filling effect and less energy consumption.     

Graphite degeneration in surface layer of ductile iron castings

Abstract

In the furan resin moulding technique sulphur in P-toluenesulphonic acid (PTSA), usually used as the hardener, has been identified as an important factor causing graphite degeneration at the metal/mould interface, especially at lower graphite nodularity levels. The greatest surface layer thickness and the lowest graphite nodularity, and shape factors, were obtained with irons solidified in moulds coated with an S bearing material. Uncoated moulds provided better results, but employing a MgO type coating effectively neutralised the sulphur migrating from the mould. In the present solidification conditions, the application of an active mould coating also influenced the graphite phase characteristics in the entire section of the casting, up to its centre. Negative effects were observed using an S bearing coating and positive effects from an MgO based coating.     

Fluid Flow,Mould Filling,and Gating

Abstract

It is a well known and accepted fact that the mould-filling process plays an important role in achieving a quality casting. In this paper, the author presents a review of the state-of-the-art regarding the understanding of the process. It is also shown that experimental methods as well as numerical fluid-flow modelling and simulation techniques, can be used to improve the understanding of the casting process. Finally, it is revealed how new numerical simulation techniques can be used successfully to achieve the correct gating dimensions which will ensure optimum mould filling.