Investigation of strand surface defects using mould instrumentation and modelling

Abstract

The surfaces of continuously cast steel blooms exhibit a variety of surface features and defects, which were investigated to reveal the interactions at the meniscus between the steel shell and interfacial flux layers that caused them. One such defect formed at periodic intervals along the surface of first and second blooms in a sequence. It was characterised by gradually deepening oscillation marks, followed immediately by longitudinal striations or 'glaciation marks'. In severe cases, deep depressions were clearly visible within the glaciated region. These defects were investigated through plant trials and both physical and mathematical modelling. The defects were found to exhibit a characteristic temperature history: temperature troughs that move down the mould at the casting speed. These defects may be monitored in much the same way as sticker breakouts, thereby allowing existing thermocouple based breakout detection systems to be modified to include a quality alarm. This study attributes these defects to high amplitude, low frequency, mould level fluctuations. A mechanism is proposed which ascribes the generation of these defects to the interaction of the meniscus with the slag rim at peaks in the mould level cycle. Installing an improved mould level control system eliminated the defects.     

The Fluidity of Sodium and Strontium Modified Sand-cast Aluminium-Silicon Foundry Alloys

The fluidity of A319, A356 and A413 alloys has been measured using a sand-cast spiral in the 700°C to 750°C temperature range. Modification by either sodium or strontium has no detectable effect on the fluidity and does not change the temperature dependence of fluidity within this temperature range. Measurements at 745°C indicate that sodium levels up to 0.037 wt% or strontium levels up to 0.024 wt% have no effect on the fluidity of sand-cast A356 alloy.     

Curing kinetics of ceramic slurries used in investment casting with ice patterns

Abstract

Ice patterns can be used to make ceramic investment moulds for metal castings. Using ice means that the ceramic slurries must be poured around the pattern and cured at sub freezing temperatures. Success of this process depends significantly on the curing kinetics of the slurries. This paper describes the experimental results of an investigation into the curing kinetics of slurries with differing compositions. The variables considered include volume percent solids, temperature and the volume of the catalyst. Spindle speed was also included as an experimental variable to see if the viscosity exhibited shear rate dependency. The Taguchi method of experimental design was used to reduce the number of trial runs. The curing kinetics of the slurries was studied by measuring the viscosity variation of the slurries with time. The data suggests that there is an initiation and propagation stage and a reactant depletion stage during the slurry gelation process. Three models are evaluated for fitting the time dependence of the viscosity data: a simple exponential in time, a Dougerty–Krieger exponential where there is a defined end point time, and a two stage model developed by superposing a simple exponential for the second stage with an Avrami kinetics model for the first stage. The results indicate that the two stage model can best describe the curing. Significance and effects of the process variables are discussed. For stage one, the characteristic time is strongly dependent upon the amount of catalyst, the spindle speed and the solids loading. For stage two, the characteristic time is strongly dependent upon the amount of catalyst, as well as temperature. A cylindrical ice part is used as an example to investigate the dimensional accuracy. The measured outer diameters of the castings are very close to the nominal outer diameter.     

TiAl ALLOYS FOR INDUSTRIAL USE

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Production of High Quality Castings by the Centrifugal Electroslag Method

Abstract

The Ukrainian-based E. O. Paton Electric Welding Institute has developed new technology for the production of high-quality castings using the centrifugal electroslag casting process (CESC). The technology consists of preparing molten metal in an electroslag crucible furnace from where it is poured, together with the slag, into a rotating casting mould. The authors show that the initial metallic materials may consist of rolled off-cuts, worn-out parts and tools, lumpy waste, and cast stock from continuous casting plants. The use of electro-slag melting technology ensures that there is no loss of alloying elements from the basic charge materials and that there is an absence of harmful contaminants. It is said that the energy characteristics of the electroslag crucible furnace are not inferior to those of induction or arc furnaces. The process technology facilitates the production of high-quality close-to-form castings. The paper presents results of integrated studies of CESC casting quality and describes the method employed in the effective refining of the metal structure. The results of the authors' studies, together with industrial testing and evaluation, show that CESC castings can compete with forgings and stampings in the manufacture of special-purpose components.     

Calculation of thermophysical properties of iron casting alloys

Abstract

A simple and easy calculation tool to determine thermophysical properties, such as density, heat capacity, liquidus and solidus temperature, thermal conductivity, latent heat, and volume contraction, of iron casting alloys based on equilibrium phase diagrams is introduced. It is well known that accurate thermophysical properties of iron casting alloys are necessary for a valid simulation of the casting process. While there are a number of thermophysical calculation programs, a specific knowledge of thermodynamics is required to operate them. The calculation method proposed in the present study does not require any special knowledge of thermodynamics, only information regarding the composition of the alloy. The proposed calculation tool is based on the CALPHAD approach for the modelling of multicomponent alloys using experimental published data. Comparison of calculated thermophysical properties for several conventional iron casting alloys with experimental results showed good agreement.     

Microstructure and mechanical properties of Alborex+SiCp/AS52 hybrid metal matrix composites

Abstract

AS52 based hybrid composites reinforced with aluminium borate whiskers and SiC particulates were fabricated by a squeeze casting method. In a hybrid preform, the volume fraction of the aluminium borate whisker was 15% and that of the SiC particulate was 5%. Microstructures of the specimens were observed using OM and SEM. In the hybrid composites, the Alborex whiskers were uniformly distributed in the matrix and a few agglomerations or clusters of SiC particulates were observed. Hardness and three point bending tests were carried out to measure the mechanical properties of the specimens. The mechanical properties of the hybrid composites were higher than those of whisker reinforced composites and the hybrid composite with finer size particulates showed higher strength than the one with coarse particulates.     

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.     

Application of fuzzy PID self-adaptive controller in pressurisation control system of counter gravity casting

Abstract

Good counter gravity casting process control is very important to the production of high quality light metal parts. However, due to uncertain dynamic properties, large delay and interactive influencing factors, this kind of process control is quite complex and it is hard to build a precise mathematic model to describe its characteristics. PID control dose not depend on mathematic model, but once the parameters are given, it cannot regulate itself and cannot get satisfied control effect. In order to satisfy the higher control performance requirement, fuzzy logic control is introduced, which is used to revise three coefficients of PID controller. It can greatly overcome the defects of conventional parameter fixed PID algorithm and get a good self-adaptive ability. Compared with the commonly used classical PID controller, the proposed fuzzy PID controller shows excellent robustness against system parameter variations and external disturbances in the counter gravity casting machine.     

Macrosegregation of ternary Al–4·5wt-%Cu–1·0wt-%Mg alloy in horizontal direct chill casting: implementation of non-equilibrium microsegregation model

Abstract

An extended and modified continuum mixture model for macrosegregation in solidifying alloys is applied to the case of horizontal direct chill casting (HDC) of a ternary Al–4·5Cu–1·0Mg (wt-%) alloy. A thermodynamic closure model for a Al–Cu–Mg alloy is coupled with an extended mixture continuum model to obtain steady state macrosegregation fields for the different casting parameters (e.g. casting speed, inlet superheat and inlet plenum position) in this study. Macrosegregation calculations were performed assuming that the solid forms a rigid and stationary network in all parts of the mushy zone. A microsegregation non-equilibrium model is used to calculate phase fraction compositions and phase distributions. The Cu exhibits more segregation compared to the previous study of a binary Al–Cu alloy, while Mg has a similar segregation profile in all cases but less pronounced. The phase compositions show that Cu is mostly distributed in secondary and ternary phases, while Mg is uniformly configured in the primary (Al) and ternary phase (Al₂CuMg). The primary Al phase is dominant (over 90%) in all cases, with highest peaks in depleted regions below the centreline of slab, except in the case where the inlet plenum is positioned on the bottom of the refractory plate due to a different flow pattern.