NEW IN THE THEORY AND PRACTICE OF LOST-FOAM CASTING
Dr. Igor O. Shinsky
Foundry Association of Ukraine
During 35 years in Institute of foundry problems (IFP, Kiev, Ukraine) laws of mass-heat-exchange process, gas-hydrodynamics, solydification conditions and conditions of structure-quality formation of castings made of ferrous and non-ferrous alloys by Lost-foam technology are investigated, and new types of Lost-foam technology are created on this basis.
Recently in IFP new theoretical visions and representations about gas-hydrodynamic processes, structure and usage characteristics foramtion of cast products and interaction of liquid, solydifying metal with thermal-destruction products of polystyrene model's (with presence of macro- and fine reinforcement elements in it) are proved, even with applying of the high control pressure on liquid and solydifying metal [1,3,4].
For example, it is created thermal-physical model of Lost-foam casting with presence of macro-reinforcement phase (MRP) in polystyrene model (PM), last defines resistance to matrix alloy (МA) movement which depends on geometry and weight of MRP, a place of its arrangement in PM, temperature conditions of mould filling by MA, pressure influencing on it, and also wettability of MRP by matrix alloy.
Thus it was taken into account, that at dense packing of MRP, MA goes on the "capillary" channels, created by MRP particles, the heat-exchange factor с is determined in view of a movement mode of MA in porous channel (laminar, turbulent), and the contact surface of heat-exchange in system "MA-MRP" was determined in view of integrated diameter of porous channels.
Using developed thermal-physical model it is possible to describe, for example, kitetics of MA temperature changes (bronze Si3Mn1, Sn5ZN5Pb5) on height of mould (Тb2) and at presence of MRP in it (steel balls of 1 mm). The analysis of the received data, which graphic interpretation it is submitted on fig.1, has allowed to establish advantage of imposing of pressure on MA in comparison with gravitational pouring. As the height of MA penetration in porous cavitiy formed by MRP at imposing pressure with the required contact temperature (Ткmin) reaches 850-1100 mm, that in 15-20 times is higher than values gravitational pouring (fig.1), and it allows to approve about opportunities of reception of the reinforced cast designs without restriction of their geometrical sizes and weight .
On the basis of this thermal-physical model have been investigated the big file of MA systems (various non-ferrous and ferrous alloys), MRP (metal and non-metallic materials with various geometry), that has allowed to create a line of technologies of reception of the cast reinforced designs with application to methods of Lost-foam casting tehnology.
The new reinforced cast designs possess a complex of functional properties earlier not inherent traditional mono-castings and to cast alloys, namely: - reception of cellular structure in a cast design which keeps strength characteristics equal or exceeding similar in mono-castings, but allows to lower their weight on 40-50%; - increasing of strength characteristics of cast designs made of traditional cast materials by reinforcing a body of a product by reinforcement phase (fibres, spheres, cores, tubes, etc.); - changing of properties of a superficial layer (tribotechnical, corrosion, wearproof, tightening) by introducing of metal and ceramic materials reinforcement phases in a superficial layer of PM .
As it has been established by the researches, determining factor at formation of the cast reinforced designs (CRD) with a required level of operational properties is formation of reliable connection in system "MRP-MA".
With this purpose of confirmation of the data of MRP influence on formation of structure and properties of the cast reinforced designs (CRD), on fig.2а are submitted micro-structures in these CRDs made on the basis of iron-carbonaceous alloys where in quality MA have been used: grey, ductile iron with the spherical graphite type and carbonaceous steel, and in quality of MRP-carbonaceous steel (cores, fraction) and grey iron (fraction) .
Metallographic analysis of reinforced castings micro-structures made of grey cast iron (GCI) and ductile iron with the spherical graphite type (DISG) with reinforcement phase as cores in diameter of 2 and 5 mm made of Steel 20 testifies (fig.2a - A, B, C, D) about presence of significant diffusion of carbon from cast-iron in steel MRP and rather deep it casrbon saturation in a transitive zone, which makes 0,4-0,6 mm: from side of MA represents a layer in width 0,1-0,16 mm with perlite structure (П100), and from the side of MRP - 0.2-0.4 mm.
In samples of cast designs of system "Steel 35 - Steel 20". where last is MRP (fig.2a - Е) is precisely observed MRP fusion on depth up to 0,2 mm, mechanical and diffusion mixing of MA and MRP meltings at formation of a transitive zone which width does not exceed 0,4-0,5 mm.
With the purpose of MRP influence confirmation on formation of structure and properties of the cast reinforced designs (CRD) on the basis of copper alloys various combinations of MA (bronzes, brasses) and MRP (fraction, chips, cores and plates), some from micro-structures of researched systems are submitted on fig.2b. The created new kinds of cast macro-composites allow to use them, first of all, for products of tribotechnical purposes (sliding bearings) that allows to increase a resource of products not less than in 2 times with simultaneous reduction in their cost price on 40-60% .
In the modern theory and practice of casting processes change of castings' properties by the mean of economical and effective modifying, alloying of liquid metal directly in the mold have great perspectives. Disperse active components introduction directly in PM allows to expand this technology direction and to create conditions for reception of cast products with new characteristics not only on all volume, but also in superficial layers and it local parts [5,6].
On the basis of the lead experimental researches of technology of moulding with PM filled by disperse components (DC) with the account heat-mass exchange processes, hydrodynamics and structure foramtion began possible to establish the basic laws and advantages of this method and to create corresponding physical and mathematical model of this technological process [14,15,16].
The mathematical model and software product created on its basis allows to determine conditions of metal movement with DC, their interaction with liquid, solidifying metal, structure foramtion and properties development of cast products.
With the confirmation purpose of the data received with use created mathematical models about influence of a DC (modifying, alloying) on structure and properties formation of cast products on a basis ferro-carbon and non-ferrous alloys have been investigated various combinations of alloys (cast-iron, steel, bronze, aluminium) and DC (FeSi, FeCr, FeMn, FeMo, FeV, FeSiMg, CuNi), microstructures of experimental systems are shawn on fig.3. So input of DC centers as FeSi allows to receive predetermined structure (94-96% perlite and up to 4% cementite) in thin-walled castings (3-5 mm) made of grey cast-iron (fig.3 - А), and without application of DC the structure obviously corresponds to white cast-iron which contains 40% cementite (fig.3 - В). DC introduction as FeCr allows to receive structure of wearproof chromium cast-iron (45-50% perlite and up to 50% cementite) in castings with wall thickness 20 mm (fig.3 - C) from initial grey cast-iron (fig.2 - D).
Application of DC for aluminium alloys allows to create composites with new characteristics. So use FeCr allows to change obviously structure (fig.3 - Е) of aluminium alloy (12% Si) (fig.3 - F) and accordingly wear resistance which is higher in 2,2-3,0 times than initial.
On the basis of polystyrene models' with DC casting technology new kinds of cast macro composites and castings with functional superficial properties have developed, which can be used, first of all, for wearproof , corrosion-resistance and tribotechnical applications, and it allows to increase their resource not less than in 2 times with simultaneous reduction in their raw cost on 40-60% [14,15].
For definition of expediency of application in practice of polystyrene models' castings with focused open porosity, as means of struggle against formation of specific defects connected to interaction of polystyrene thermal-destruction products with liquid, solidifying metal had been established laws and distinctions in interaction of thermal-destruction products with liquid and hardening metal at presence in the mould model with "zero" and focused porosity.
On the basis of set of the carried out natural experiments new physical and mathematical model of Lost-foam casting process has been created, having taken for a basis physical model of process of filling of the mould with PM of "zero" porosity [1,14,4].
Such mathematical model can be used for definition of the basic technological parameters, as counter-pressure of vapor-gaseous phase (VGP Рf), a gap "metal-model" d, velocity of metal rise in mould W1 and volume of defects on a castings' surface D at casting on PM with focused porous channels.
With the purpose of data confirmation of focused porosity influence on formation of gas pressure and superficial defects a series of natural and settlement experiments has been realized. For presentation of influence of porosity on the specified parameters on Fig.4 the data are submitted at casting of copper-base alloys. It agrees these data (fig.4а) it is obviously visible, that volume of specific defects (D) in comparison with application of monomodels are reduced at 2-20. Similar influence renders porosity and on size of gas pressure (Pf) which is reduced at copper-base alloys casting at 2-16 time in comparison with application of monomodels (fig.4b).
It is important to note, that at presence of porosity in polystyrene model which exceeds 25%, conditions of mould filling and formation of castings' quality does not depend on presence of polystyrene model in the mould.
Taking into account advantages of methods of molding in porous moulds under excessive (mechanical, gas) pressure important was to investigate laws of influence of a high pressure on conditions of interaction of PM thermal-destruction products with a liquid, solidifying alloy and formation of structure-application properties of cast products, to establish features heat- mass- exchange process and gas-dynamics.
For this purpose the new circuit of Lost-foam casting  under controlled pressure which allows to create pressure upon liquid metal up to 10-15 МPа has been realized, to ensure velocity moving of the squeezing chamber (metal) with accuracy of 0,5 mm. And to make casting from ferro-carbon and non-ferrous alloys with wall thickness from 2 mm, dimensions up to 500х500х1500 mm and weight 0,1-1000 kg.
Use of the created metal pouring in mould circuit with polystyrene model, i.e. under a high pressure, when P1>>Pf, where P1 - pressure upon liquid metal, alters representation about features of Lost-foam casting [1,2], namely: the pressure influencing on liquid metal P1 considerably exceeds counter-pressure VGP in a gap "metal-model" Pf and thermomechanical resistance of PM to alloy movement pm; and speed of metal rise in mould W1 does not depend from metallostatical pressure H1 and can exceed speed of model's fusion W5 and to be a constant during all pouring period, irrespective of variable horizontal section of the mould since it is possible to observe dependence Wk x Fk = W1 x F1 where Wk, W1-speed of the squeezing chamber and metal in the mould; Fk, F1 - section of the squeezing chamber and the current section of model in the mould and then it is possible: at F1-rotor - W1 - const.
Developing of the generalized physical model of alloy movement in the mould with PM under controlled pressure has allowed to develop corresponding mathematical model of Lost-foam process under controlled high pressure which allows to optimize the basic technological parameters of molding for reception mono-reinforced cast products and castings with differential properties made of ferrous and non-ferrous alloys.
With the data confirmation purpose of influence thermal-force parameters of Lost-foam casting on formation of structure, mechanical and casting properties a series of natural experiments has been realized. In all cases imposing pressure on liquid metal conducts to growth (decrease) in all researched characteristics on 15-40%. For presentation of influence of pressure on strength of ferro-carbon alloys it is submitted on Fig.5. In accordance to these data (fig.5) it is obviously, that tensile strength rises on 15-30% in comparison with gravitational pouring.
For realization of the created new technologies of reinforced castings production, cast designs with differential properties, functional layers made of grey cast iron, ductile iron, wearproof cast iron, steels, copper- and aluminium-base alloys by Lost-foam casting technology disperse components, macro-reinforce phase, and also focused porosity at use of low and a high pressure it has been created new and earlier developed equipment adapted for molding with applying of polystyrene models, which part are submitted on fig.6 and it allows to design and develop casting equipment complexes for castings' production in weight range 0,1-1000 kg with production capacity
100-25000 metric tons of quality castings (fig.7) per one year with a high level of economic efficiency and ecological-environment safety.
List of references