Some Recent Developments in the Aluminium Casting Industry

The author shows that the call for high-integrity accurate thin-walled castings in light alloy has resulted in a reappraisal of sand-based moulding methods which might provide designers with an economic answer to their needs. It is suggested that several sand-based processes can now produce results equal or superior to traditional diecasting routes based on gravity or pressure fill. Amongst subjects discussed are low-pressure-filled zircon and silica sand moulds; core-assembly techniques; the use of expendable expanded polystyrene patterns and the V-Process.     

Effect of antimony on the eutectic reaction of heavy section spheroidal graphite castings

Abstract

There is a strong demand for heavy section castings made of spheroidal graphite with a fully ferritic matrix, e.g. for manufacturing hubs for windmills. Such castings with slow solidification process are prone to graphite degeneration that leads to a dramatic decrease of the mechanical properties of the cast parts. Chunky graphite is certainly the most difficult case of graphite degeneracy, though it has long been known that the limited and controlled addition of antimony may help eliminate it. The drawback of this remedy is that too large Sb additions lead to other forms of degenerate graphite, and also that antimony is a pearlite promoter. As part of an investigation aimed at mastering low level additions to cast iron melts before casting, solidification of large blocks with or without Sb added was followed by thermal analysis. Comparison of the cooling curves and of the microstructures of these different castings gives suggestions to understand the controlling nucleation and growth mechanisms for chunky graphite cells.     

Effect of bismuth on refinement of graphite in thin wall spheroidal graphite cast iron

Abstract

Although it is known that the addition of bismuth refines the graphite nodule in spheroidal graphite cast iron, the refinement mechanism has not yet been clarified. In this research, the effect of bismuth on the refinement has been investigated by examining the microstructure of the spheroidal graphite cast iron containing a small amount of bismuth. Bismuth was added at 0˙01 mass% to the spheroidal graphite cast iron melt containing 3˙5–3˙7 mass% carbon and 2˙0– 2˙8 mass% silicon, then the melt was poured into the mould to obtain the stepped test bar with 2, 3, 5 and 10 mm thicknesses. The graphite nodule increased as the bismuth content increased. The diameter of the graphite nodule decreased as the thickness decreased, namely, as the cooling rate increased. The graphite nodule was further refined by the addition of bismuth. The increase in silicon content increased the graphite nodule count and the ferrite in the matrix. It postulated that bismuth exists as simple substance or a compound in the vicinity of the nucleus of the graphite.     

The Effect of Phosphorus on the Microstructure and Properties of Compacted Graphite (CG) Irons

Abstract

The effect of up to 0.15% phosphorus on the microstructure and mechanical properties of step-block castings made from compacted- graphite iron has been examined. There was no major effect on the microstructure, but two minor effects were observed. First, the higher-phosphorus irons showed a small increase in the amount of carbides and pearlite within the microstructure, but there was no effect on the depth of surface chill. Second, at low magnesium contents the higher-phosphorus irons demonstrated a marked increase in the amount of flake graphite. Phosphorus had no adverse effect on tensile strength, and a reported lowering of tensile elongation at elevated phosphorus content was not found in this work. There was, however, a decrease in Charpy impact toughness at higher phosphorus levels.     

The influence of internal chills on the structure and properties of aluminium alloy castings

Many of the drawbacks associated with the production of conventionally cast ingots and castings, such as the presence of pipes, centre-line segregation and columnar grains, can be attributed to the manner and extent of cooling inside the mould cavity and the problems of heat transfer from the centre to the outside of the casting. These result in lowering of the average mechanical properties and the yield of the casting. In the past, the use of external chills to reduce the above defects has had limited effects. This is because the influence of external chills becomes marginal beyond a certain distance. One of the potential methods of overcoming such problems is to employ heat sinks in the form of internal chills. Despite reported work by Russian and Japanese investigators on the use of internal chills, in the form of powder or strip, in iron and steel castings, no detailed information on the use of such chills in aluminium or other non-ferrous alloys is available in the literature.

This paper presents details and findings of an investigation carried out with Al-4.5% Cu (LM11) alloy using chills of cylindrical form of the same composition. The influence of such internal chills, placed centrally and non-centrally was assessed in terms of changes in solidification time, temperature gradient, percentage melting, microstructure, density, and the ultimate tensile strength of the castings.

The investigation has shown that solidification time decreases linearly with the percentage volume of the chills. Progressive structural refinement, corresponding to this reduction in solidification time, has also been observed. Distribution of the chills is seen to play an important role. One centrally-placed chill is essential in the refinement of the structure of the central region of an ingot, as well as to reduce the size of the central open pipe. The use of microchills in the form of turnings and powder has also been found to refine the structure considerably. The density and ultimate tensile strength of castings has been found to increase up to the optimum volume of chill, i.e. 2.5% at 75–115 K and 0.63% at 35 K superheat, and then decrease.     

Nucleation of graphite in cast iron melts depending on manganese,sulphur and oxygen

Abstract

The form and distribution of graphite in grey iron influences the mechanical properties and depends on numerous factors, such as nucleation and cooling speed conditions. The main focus of the present work is the influence of manganese, sulphur and oxygen on the nucleation of graphite in lamellar cast iron melts. Previous studies showed that the nucleation in GJL melts is initiated by a MnS particle. For investigations in the field of nucleation in grey iron the authors examined several EN-GJL-200 specimens. The specimens were cast with and without inoculant. The studies of the specimens were realised using light microscopy, SEM-EDS (energy dispersive X-ray analysis), WDS (wavelength dispersive X-ray analysis). The experimental results were compared with the software Thermo-Calc calculations. From the experimental results and the Thermo-Calc calculations it can be concluded, that Mn and S contents and the Mn/S ratio respectively plays a very important role for the nucleation. A direct influence of oxygen on the nucleation of graphite could not be observed.     

Modelling the effect of graphite morphology on the modulus of elasticity in cast irons

Abstract

Nine grades of pearlitic cast iron containing different graphite morphologies (from flake, compacted and spheroidal) have been studied. The parameters investigated include the graphite aspect ratio, nodularity, graphite size and modulus of elasticity. These parameters have been correlated and compared with different existing bound and model equations. It has been found that the modulus of elasticity of the graphite phase increases as the aspect ratio and nodularity of the graphite increases, i.e.flake graphite gives a lower modulus of elasticity than spheroidal graphite. The experimental values of the modulus of elasticity show good agreement to bound and model equations, although flake graphite cast irons show higher deviation from the modelled values. An equation for the correlation between the graphite modulus of elasticity and the nodularity is presented. Introducing this linear correlation into an existing model for the determination of the effective modulus of elasticity gives a continuous function, including all grades of cast irons, with a very good agreement with experimental values. The modulus of elasticity of cast irons can be accurately predicted from both bound and especially model equations, using the aspect ratio and nodularity of the contained graphite particles. The fit is improved by using a modulus of elasticity of the graphite phase that is based on the graphite morphology, considering that the modulus of elasticity of the graphite is different in the basal and prismatic planes.     

Studying elastic deformation behaviour of cast irons by acoustic emission

Abstract

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.     

Advances in the Technology of Titanium Castings

Abstract

This paper describes the techniques used for making titanium casting and how current mould-making methods can produce pieces with good dimensional tolerances and surface finish. Both rammed sand and lost wax investment methods can be used for mould-making, the choice depending on piece complexity, weight, required finish and economic considerations. Hot isostatic pressing is normally applied to castings for aerospace and results are presented showing the beneficial effect of this process in terms of reduction of internal porosity. The effect of HIP on mechanical properties is also presented.     

Effects of nodularity on thermal conductivity of cast iron

Abstract

The thermal transport properties of five predominately pearlitic grades of grey, compacted graphite and spheroidal graphite iron have been investigated by the laser flash technique. Samples have been taken from cylinders cast in controlled thermal environments designed to produce three dissimilar cooling rates. Digital image analysis has been utilised in order to characterise the different graphite morphologies. The results indicated linear relationships between the thermal transport properties and the roundness of the graphite and the nodularity for compacted graphite and spheroidal graphite iron. A pronounced decrease in the thermal conductivity occurred when the lamellar graphite structure was transformed into compacted graphite. The thermal conductivity of compacted and spheroidal graphite iron has been recalculated with good accuracy over a temperature range of 25–500°C by means of regression analysis.     

Control of graphite formation in solidification of white cast iron

Abstract

Graphite formation should be strictly suppressed for the most abrasion resistant white cast irons, since austenite (γ)+graphite eutectic structure shows lower hardness and selectively wears thus deteriorates the abrasion resistance even though the austenite transform to hard phase such as martensite. On the other hand, a small amount of fine graphite is desired to distribute in rolls for hot steel mills to suppress the scoring. However, strong carbide formers such as Cr, V, Nb have been increasingly added to rolls, in order to crystallise more harder carbides. As γ+carbide eutectic grows, the residual liquid among eutectic cells becomes poor in carbide formers and rich in elements which promote graphite formation. Therefore an appropriate alloy design is essential for the hot steel milling rolls. In this study, the graphite formation mechanisms are discussed for chromium cast iron, high speed steel type cast iron and Ni hard type cast iron.     

Influence of graphite morphology on dry sliding wear of flake graphite cast irons

Abstract

Four flaky graphite cast irons of different graphite structures with a pearlitic matrix were prepared to clarify the graphite structure's influence on the dry sliding wear property. Two melts of cast iron with different carbon contents were solidified at two different cooling rates. The four resultant samples had type A flaky graphite or type D eutectic graphite structure with different graphite volume fractions and fully pearlitic matrixes. A pin on the disc type wear test evaluated the four samples' dry sliding wear properties. Results showed that the type D graphite structure wore down faster than the type A graphite structure did. The type of graphite morphology influenced the specimen wear rate as strongly as the graphite volume fraction did in flaky graphite cast irons of this experimental range.     

Mechanical characteristics of spheroidal graphite cast irons containing Ni and Mn with mixed ferrite and bainitic ferrite microstructure

Abstract

This study aims to clarify the influence of additive elements of Ni and Mn on tensile and impact properties of three kinds of spheroidal graphite cast irons (SG irons), which are as cast, annealed and austempered samples. Spheroidal graphite cast irons with Ni (0–4˙5 mass-%) and Mn (0–0˙5 mass-%) melted by a high frequency induction furnace and cast into a Y block CO₂ mould with 30 mm in thickness. From the viewpoint of heat treatment, tensile strength and hardness of SG irons become larger in the order of ferritised<as cast<austempered ones. Matrix structures of SG irons, which are conducted to austempering treatment from α and γ mixture range, consists of bainitic ferrite with high toughness. Austempered SG iron with 3%Ni in 0˙1%Mn series is found to become higher tensile strength compound with elongation and toughness of 901 MPa, 17% and 915 kJ m^?2.     

Fatigue crack growth behaviour of 4130X steel in H2S environment

Abstract

Fatigue crack growth rates of 4130X steel used for compressed natural gas vessels were investigated in this paper. Considering the operating conditions, corrosion fatigue tests at a low frequency of 0·0067 Hz, in H₂S saturated, H₂S unsaturated and air environments were conducted on modified wedge opening load specimens by using a home made low cycle fatigue test system. Curve fitting was applied to the fatigue test data of da/dN–ΔK according to Paris formula. A correlation study between fracture surface and stress intensity factor range was conducted and K values for three stages in different environments were characterised quantitatively. The results show that da/dN in H₂S environment is more than 20 times faster than in an air environment. When the H₂S concentration reaches a certain range, the increase of da/dN becomes slower than that of the H₂S concentration. da/dN differs by 2·4 times while the concentration differs by 11 times. The corrosive environment accelerates the fatigue failure.     

Flow analysis of semi solid processing for grey cast iron

Abstract

Fluidity tests for semi solid processed grey cast iron were conducted at different degrees of primary fraction solid using fluidity strips with 2·5, 5 and 14 mm wall thickness. The microstructure of the fluidity strip castings was examined. Fluidity parameters, such as slurry velocity, critical solid concentration and choking range were studied. It can be concluded that critical solid concentration increases with the increase of wall thickness. The slurry velocity drops suddenly at a certain fraction solid very near to the critical solid concentration. Choking range zone can be measured by observing microstructure. A solidification model for fluidity has been developed based on the literature and microstructure study of semi solid processed grey cast iron. A mathematical equation has been obtained from the solidification model by modifying Flemings' fluidity model in order to analyse cavity filling of semi solid grey cast iron.     

Effects of transition from lamellar to compacted graphite on thermal conductivity of cast iron

Abstract

Different levels of magnesium were added to a standard grey iron alloy in order to obtain a range of graphite morphologies from lamellar to compacted graphite. The thermal conductivity/diffusivity of samples, solidified at different cooling rates, was investigated by means of the laser flash technique. There is a significant decrease in the thermal conductivity as the morphology transits from lamellar to compacted graphite. The thermal conductivity of grey iron decreases considerably at elevated temperatures, whereas the thermal conductivity of compacted graphite iron is less sensitive to changes in temperature. At increased nodularities, compacted graphite irons exhibit a maximum thermal conductivity at ～400°C. The influence from the cooling conditions on the thermal conductivity decreases as the morphology alters from lamellar graphite to compacted graphite. The effective thermal conductivity of cast iron is modelled by means of existing models for composites.     

Fabrication of SiC particle dispersed spheroidal graphite cast iron composite by vacuum assisted casting and its microstructure

Abstract

SiC particle preforms were infiltrated with spheroidal graphite cast iron melt by vacuum assisted casting in the sand mould, and spheroidal graphite cast iron composites in which the particles were dispersed in the surface region were fabricated. Although the melt infiltration was not accomplished when the melt was poured under atmospheric pressure, the infiltration was accomplished by the vacuum assisted casting when the SiC particle volume fraction and preform thickness were optimised. When the Si content of the cast iron was 2˙5 mass%, the phase consisting of mainly Fe₃Si was formed at the particle/matrix interface due to the reaction between the cast iron melt and the particles during the infiltration. The matrix of the composite consisted of fine spheroidal graphite particles, ferrite, pearite and chill crystal. Although the increase in the Si content suppressed the reaction and chill, no infiltrated area was observed in the composite.     

Microstructure and properties of austempered ductile cast iron with refined graphite nodules

Abstract

The present study examined the influence of refinement of graphite nodules on microstructure and tensile properties of austempered ductile cast iron (ADI). A casting technique using deoxidation treatment enabled manufacture of thin walled castings made of ductile cast iron without ledebulite. The thin walled casting (t=2 mm) was subjected to extreme refinement of graphite nodules, where the number of graphite nodules was 1750 mm^?2. Decrease in graphite nodule diameter resulted in refinement of ausferrite and γ-pool structures in ADI and rapid reaction of austempering. The significant increase in the number of graphite nodules resulted in a remarkable drop in the tensile strength and elongation of ADI. These results can be explained by the graphite nodule distance.     

高强度铸造铝合金凝固过程中的元素偏析

通过液淬的方法，研究Al-4.5Cu合金及添加Si,Mn,Ni,Zr元素后合金的凝固过程及溶质分布。结果表明，将Si,Mn,Ni,Zr元素的加入，都能使Al-Cu合金的结晶温度间隔不同程度地减少，其中Al4.5Cu3Si的结晶温度间隔最小，Cu的分布,特点是：晶界高于晶内，一次晶界高于二次晶界”，并随着温度的降低Cu向晶界偏析的趋势加剧，Ni的分布，晶界高于晶内，Mn的分布，晶界高于晶内，但差别不大，Zr在晶界与枝晶间的偏析处的含量较高。