WO1998011264A1

WO1998011264A1 - Production of cast products with controlled density by controlling gas concentration in a material

Info

Publication number: WO1998011264A1
Application number: PCT/UA1997/000010
Authority: WO
Inventors: Vitaly Alexandrovich Pereloma; J. Timothy Langan; Vladimir Ivanovich Shapovalov; Yuriy Petrovich Lenda; Sergei Vladimirovich Pan; Vladimir Leontievich Naidek
Original assignee: Ashurst Technology Corporation Ltd.
Priority date: 1996-09-16
Filing date: 1997-09-15
Publication date: 1998-03-19
Also published as: AU4642997A

Abstract

A method for the production of cast products. The method is particulary directed to the production of porous metal products, such as titanium- or zirconium-containing porous metals, by controlling the concentration of gas in the material during melting and solidification of the material. The invention is also directed to products fabricated by such a method.

Description

PRODUCTION OF CAST PRODUCTS WITH CONTROLLED DENSITY BY CONTROLLING GAS CONCENTRATION IN A MATERIAL

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to the field of cast products fabricated with controlled density by controlling the concentration of a gas, such as hydrogen, dissolved in the material during production. The present invention more particularly relates to porous and non-porous materials with controlled density and methods for making such materials and cast products formed from such materials.

2. Description of Related Art

For the production of porous products, methods based on the production and solidification of a foamed . dross, which is a homogeneous suspension of metal powder in a liquid, with subsequent foaming, drying and burning are known. See, for example, U.S. Patent No. 4,569,821 by Duperray et al., German Patent Application No. 1767673 and U.S. Patent No. 3,71 1 ,279 by Galmiche, et al.

Methods based on slip casting are also known. In this case, the material is formed by filling a porous mold with a dross and subsequently drying and burning. After burning, the porous foundry mold is removed by chemical or mechanical methods. These methods enable the production of highly porous cellular materials with a porosity of up to about 90%. See, for example, Japanese Patent Application No. 62-26402 and German Patent Application No. 3640586. There are also production processes for the production of impenetrable porous metal materials having closed porosity. These methods include pouring of melted metal into a foundry mold previously filled with hollow granules of a refractory material. However, these methods are very sophisticated, since the spheres tend to float up and are non-uniformly distributed in the material. Methods for the production of porous materials based on metals and metal alloys, for example titanium, are also known. These methods are usually based on powder metallurgy techniques, wherein metal powders are compacted and sintered. Chips, wire and gauze are sometimes used instead of powders. The porous structure and properties of such materials are controlled by the morphol- ogy of the powders, and the compaction and sintering parameters. One common disadvantage of all of these methods is that it is difficult to produce a gas-proof (nonpenetrable) or liquid-proof (nonpermeable) material since the porosity is open and interconnected. Further, with a porosity of more than 25 to 35 percent, these materials have relatively low mechanical properties, especially a low ductility. Therefore, they are often used just as filtering elements and rarely as a light-weight structural material. Examples of such methods are disclosed in Japanese Patent Application Nos. 61 -48566, 61 -50121 , 51 -33044, 51 -33485, and U.S. Patent No. 4, 162,914 by Cremer.

Methods based on the foaming of a liquid alloy with subsequent solidification of the foam have also been proposed. For example, see U.S. Patent Nos. 3,790,367 by Niebylski; 3,816,952 by Niebylski et al.: 4,933,358 by Jin et al., 4,973,358 by Jin et al.; 5, 1 12,697 by Jin et al. and Japanese Patent Application Nos. 60-32542 and 51-44084. To modify the surface tension of the melt, various thickeners are introduced. Despite the measures undertaken, there are difficulties because of low stability of the metal foam and lack of control over the final porous structure. These factors considerably complicate the casting procedure. In addition, thickening agents almost always decrease the mechanical properties of the final product.

U.S. Patent No. 5, 181,549 by Shapovalov discloses a process wherein a metal base material is heated to the liquid phase with contact between the liq- uid phase and gaseous hydrogen. Hydrogen must have a solubility in the initial base material which decreases with decreasing temperature of the initial base material and increases with increasing hydrogen pressure. During contact between the liquid and hydrogen, it is necessary to maintain a given pressure of hydrogen and provide for hydrogen dissolution in the melt. Following comple- tion of the above operations, it is necessary to cool the base material for solidification, with simultaneous pressure control, in such a manner that it results in simultaneous formation of gaseous and solid phases. Due to the decreasing hydrogen solubility, gas bubbles nucleate and grow simultaneously with a solid phase and do not leave the solidification front, forming a porous structure in the solidified material. The foregoing method is effective for some metals and alloys which have decreasing hydrogen solubility with decreasing temperature, such as nickel, cobalt, manganese, copper, tungsten and molybdenum.

This method, however, is not applicable to some metals, such as zirconium, titanium, vanadium, palladium, and others, which form stable hydrides and in which the hydrogen solubility increases with decreasing temperature, both in liquid and solid phases. SUMMARY OF THE INVENTION

Precipitation of the gas phase from the melt of the foregoing metals in which the hydrogen solubility increases with decreasing temperature can be achieved only by providing unique conditions to control the gas concentration in the molten material and to control the solidification of the molten material.

For these metals, it has been found that it is necessary to provide a high rate of hydrogen dissolution in the molten metal. This can be achieved by means of a mass transfer intensification process. It has been found that the transfer of hydrogen through the "gas-metal" interface will be substantially accelerated when hydrogen is in one of its activated forms. One form of gas molecule activation is achieved by the partial or complete ionization of the gas. To provide such an ionization, sources of concentrated energy such as plasma, electric discharge (arc), laser excitation or others are used. Application of these energy sources for heating and melting of a metal results in substantial overheating of the surface layers of the metal. In the case of metals forming stable hydrides, such an increase of temperature causes a significant drop of hydrogen solubility (see Fig. 1). To avoid this problem and also to accelerate the transfer of hydrogen from the overheated surface layers towards the bulk molten metal, it is helpful to apply a simulta- neous forced agitation to homogenize the composition and temperature of the molten metal. It is also desirable to provide a controlled change of pressure in the course of melt solidification and to create the conditions for gas phase nucle- ation and precipitation. This can be achieved by introducing special additives to the melt which alter hydrogen solubility in the melt and form the sites for gas evolution.

According to the present invention, a combination of the above options is applied to the method of production of porous and non-porous cast materials and products formed therefrom.

DESCRIPTION OF FIGURES Fig. 1 illustrates the calculated dependence of hydrogen solubility in titanium on the partial pressure of hydrogen at different temperatures.

Fig. 2 illustrates a plot of pressure as a function of time during a production process according to one embodiment of the present invention.

Fig. 3 illustrates a plot of porosity as a function of solidification pressure according to another embodiment of the present invention.

Fig. 4 illustrates an apparatus useful for carrying out a method according to an embodiment of the present invention.

Fig. 5 illustrates a cross-section of a porous product according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for controlling the concentration of a gas, such as hydrogen, in a material and the subsequent precipitation of pores in the material to form a cast material that can be based on a metal (e.g. substantially pure metals or metal alloys) or non-metal. The invention particularly relates to the formation of cast products such as cast metal products, and in particular relates to the formation of porous (i.e., at least about 5 percent porosity) and non-porous metal products with controlled density. Porous materials, particularly those with closed porosity, are advantageous in a number of structural applications where light weight is desired while maintaining sufficient mechanical properties.

The present invention is particularly applicable to materials, such as metals, in which the solubility of the pore-forming gas, for example hydrogen, increases with decreasing temperature. An example of such a material is titani- um. Figure 1 illustrates the calculated dependence of hydrogen solubility in titanium on the partial pressure of hydrogen and is plotted at five different temperatures. Line AS is calculated for solid titanium at 1665°C, Line AL is calculated for liquid titanium at the same temperature, 1665°C. Line B is calculated for liquid titanium at 1800°C, Line C is calculated for liquid titanium at 2000°C, Line D is calculated for liquid titanium at 2300°C and Line E is calculated for liquid titanium at 2700°C. As can be seen from these curves, the solubility of hydrogen in titanium increases as the temperature decreases for all hydrogen pressures. Therefore, under normal solidification parameters, hydrogen gas would not precipitate from the titanium during cooling and no substantial amounts of porosi- ty would form. While the foregoing is preferred, the method of the present invention can also be applicable to metals for which hydrogen solubility decreases when temperature is decreased.

According to the present invention, it has been found that if activated (ionized) hydrogen is contacted with the molten metal, such as in the form of hydro- gen-containing plasma, simultaneously with forced agitation of the metal, the rate of hydrogen dissolution in the metal substantially increases and porous materials can be formed.

According to another embodiment of the present invention, porosity can be formed in the material by the evolution of gas from the melt under unique conditions of melting and solidification. These conditions preferably include pres- sure changes during the solidification of the melt and careful control of the solidification rate. It is preferred to conduct the solidification of the melt by including a substantially isothermal hold of the melt during the initial stage of solidification or by providing a sufficiently low solidification rate (slow cooling), or both. To obtain a highly porous structure in the case of metals in which hydrogen solubility increases with a decrease of temperature, the process also preferably includes the use of one or two types of additives that enhance the nucle- ation and growth of pores. The use of such additives for metals in which hydrogen solubility increases with a decrease of temperature significantly simplifies the process of pore formation. The two types of additives that can be utilized are: 1) active additives; and 2) gas evolution promoters. As is discussed in more detail below, the active additives alter the hydrogen solubility in the melt while the gas evolution promoters act as the sites for nucleation of gas bubbles.

The initial base material for use in the present invention can be virtually any metal, metal alloy, ceramic or the like. Particularly preferred are metals such as titanium (Ti), vanadium (V), zirconium (Zr), niobium (Nb), tantalum (Ta), palladium (Pd), scandium (Sc), hafnium (Hf), uranium (U), thorium (Th), lanthanum (La), cerium (Ce), molybdenum (Mo), rhodium (Rh), copper (Cu), iron (Fe), nickel (Ni), chromium (Cr), aluminum (Al), magnesium (Mg). and alloys of these metals. The present invention is particularly applicable to hydride-forming metals and metal alloys. In a particularly preferred embodiment, the initial base material is a titanium-based or zirconium-based metal, such as substantially pure titanium or a titanium alloy. The initial base material can be in virtually any form, such as a charge of powders, flakes, billets, etc. Although the fol- lowing discussion of the present invention refers mainly to titanium and titanium alloys, it is to be understood that the process can be applicable to all of the foregoing base materials or combinations thereof.

As is discussed above, the present invention preferably includes the addition of additives to the charge of initial base material. Preferably, two types of additives are utilized, the first being referred to as an active additive and the other being referred to as a gas evolution promoter. Active additives alter the solubility of the pore-forming gas, e.g. hydrogen, in the melt. The active additive can be either soluble or insoluble in the melt. For example, when titanium is used as the base material it is preferred to add silicon, such as in the form of silica (Si02), to the molten material. The use of silica will also lead to the formation of TiO or Ti02, which acts as a thickening agent and increases the viscosity of the melt, thereby enhancing pore formation. The active additives can optionally be added: (a) directly to the melt of initial base material while in a melting crucible or casting mold; (b) into the stream of molten base material during the pouring of the melt into a mold; (c) directly in the melting crucible or casting mold prior to the addition of base material: or (d) with a charge in the form of a mixture with the initial base material. Preferably, from about 0.05 weight percent to about 5.0 weight percent of the active additive is added as a percentage of the initial base material.

The present invention also preferably includes the use of gas evolution pro- moters which are solid particulates that are substantially insoluble in the molten base material and that form gas evolution sites upon interaction with the molten base material. The gas evolution promoters are preferably selected from particulate materials that have poor wettability with the molten base material. That is, the wetting angle between the gas evolution promoter and the molten base mate- rial should be greater than about 90°. Metal or non-metal refractory elements and compounds meeting the foregoing requirements can be used. The gas evolution promoter can be interactive or passive with respect to the initial base material and they can either be formed by chemical reaction of the initial base material with any other additives in the melt or can be separately introduced into the melt. Materials useful as gas evolution promoters can preferably be selected from materials having a melting point that is substantially higher than that of the initial base material. Particularly preferred as materials for gas evolution promoters are zirconia (Zr02), titania (Ti02), magnesia (MgO), alumina (A1203), and mixtures thereof, particularly when the initial base material is titanium or a titani- um alloy. The particulate gas evolution promoters preferably have a mean size of greater than about 0.002 mm and more preferably have a mean size of from about 0.002 mm to about 2.0 mm. Preferably, from about 0.05 to about 5.0 weight percent of gas evolution promoters are added to the initial base material. The gas evolution promoters can be added: (a) into the stream of molten base material during the pouring of the melt into a mold; (b) directly into the casting mold prior to the addition of base material; or (c) directly into the melting crucible just before pouring the melt into the mold. Any combination of these procedures can also be used. In a preferred embodiment, when the initial base material is titanium or a titanium alloy, the gas evolution promoters are added in the form of powder or consolidated bars placed in the casting mold before the molten base material is cast into the mold. It has been found that adding the gas evolution promoters in this way leads to better controlled porosity characteristics.

Thus, the present invention can include the use of either the active additives or the gas evolution promoters. Preferably, at least the gas evolution pro- moters are utilized. More preferably, a combination of the active additives and gas evolution promoters are used. When both types of additives are used, it is preferred that the total amount of such additives is from about 0.05 to about 5.0 weight percent as a percentage of the initial base material.

According to the present invention, it is also preferred to carefully control the temperature and pressure of the molten base material during melting and also during solidification of the base material. For example, it is preferred to overheat the molten base material, that is, to melt the base material at a temperature that is appreciably higher than the actual melting temperature of the base material. Preferably, the initial base material is heated to a temperature at least about 5% higher than the melting temperature and more preferably at least about 10% higher than the melting temperature. For example, substantially pure titanium, which has a melting temperature of about 1668° C, is preferably heated to a temperature that is at least about 150° C to 170° C higher than the melting temperature, for example between about 1810° C and 1830° C. When the molten base material is cooled down to the solidification temperature, for example, by pouring it into a casting mold, the temperature of the base material is preferably held at the solidification temperature for a period of time to permit the solidification of the molded article to proceed with a low cooling rate.

The control of the pressure during the process is also important according to the present invention. For example, for titanium or a titanium alloy, it is preferred during melting to maintain a hydrogen partial pressure of from about 0.001 MPa to about 5 MPa. The total pressure is preferably from about 0.001 MPa to about 5 MPa and the pressure during the melting phase is preferably higher than the pressure during the solidification phase. After the molten base material which is saturated with a pore-forming gas is poured into a casting mold and prior to the actual start of solidification, the pressure is changed as compared to the pressure during the melting phase. The overall pressure at the start of solidification directly influences the porosity and average pore size in the final material. Referring to Figure 2, the overall pressure change during solidification as a function of time for the production of porous titanium is illustrated. Region I illustrates a decrease in pressure from the melting pressure to the pressure at the beginning of solidification (Point A). The pressure at the beginning of solidification influences the formation of an outer solid skin on the cast porous product. Region II illustrates a gradual reduction in pressure during the solidification phase. The rate of this reduction in pressure defines the level of porosity in the cast porous product. Region III illustrates the substantially isobaric holding of total pressure until solidification of the material is completed. If the initial base material is titanium or a titanium alloy, it is preferred to maintain an overall pressure at the beginning of solidification (Point A) of from about 0.01 MPa to about 3.5 MPa. During the initial solidification (Region II), the pressure is slowly reduced, for example at a rate of from about 0.001 MPa/sec to about 0.5 MPa/sec, after the initial pressure control prior to the start of solidification, which takes from about 0.5 to about 5 seconds. For titanium or titanium alloys, the slow pressure release (Region II) takes from about 5 to about 50 seconds. These parameters are generally applicable only for a given dimension of a cast porous product (e.g. with a thickness of about 30 mm) and for a given porosity level in the final material, and are not applicable to all of the possible ranges of porosity. These conditions advantageously permit the formation of pores within the metal structure. The level of porosity can be decreased and, optionally, a solid object can be produced, by increasing the solidification rate (cooling rate) to essentially quench the molten material and by decreasing the rate of pressure change.

Figure 3 illustrates a percent porosity as a function of the pressure at the beginning of solidification (Point A on Fig. 2). As can be seen from Figure 3, the porosity increases to a certain pressure and then decreases again as that pres- sure is increased. Line I on Figure 3 represents the total porosity of the core of the material while Line II represents the total porosity of the material, that is, the porous core plus the solid outer wall.

According to another embodiment of the present invention, activated (ionized) hydrogen is contacted with the molten base material in order to enhance dissolution of the hydrogen into the material. The activated hydrogen can be in the form of a flow or stream of ionized hydrogen, such as one formed by a hydrogen-containing plasma. The positive ions of hydrogen are protons, i.e., elementary particles. Protons, which have very small dimensions and high physical and chemical activity, dissolve in the liquid material more intensively than molecular hydrogen. Ionized hydrogen can also be supplied by other means, such as by the application of electrical discharge to hydrogen gas, by laser excitation and other means. It is preferred that the atomic concentration of protons in the plasma is from about 0.5 to about 60 atomic percent. Thus, the present invention includes the following preferred production method for porous and non-porous products with controlled density. .An initial base material, such as in the form of a metal charge, is placed in a hermetically sealed installation. After sealing the installation, air is evacuated. The installation is then filled with a working gas mixture which preferably includes Hydrogen gas and an inert gas such as argon, helium or nitrogen under a predetermined pressure. The initial base material is then heated to at least the melt- ing temperature of the base material to form a molten base material, and preferably is heated to a temperature in excess of the melting temperature. A plasma jet is directed toward the surface of the metal charge and can advantageously provide both heating of the base material and substantial saturation of the melt with activated hydrogen. During the heating and melting of the base material, a mold is heated to a predetermined temperature. After the base material is melted, the molten base material is held at a pre-determined temperature for a pre-determined time and the melt is intensely agitated to achieve melt homogenization, that is, a substantially homogeneous temperature and a substantially homogeneous concen- tration of hydrogen. Then the melt is poured into the mold, where it is cooled to a predetermined temperature, at which isothermal holding is performed. From the time that the molten base material is transferred into the mold, the additives interact with the melt, and the total gas pressure in the installation is controlled and is either reduced or increased. The reduction or increase of pressure is ceased after the substantial completion ofsolidification. An increase in pressure during solidification generally results in a lower porosity, and possibly a dense material. The total porosity (ratio of pore volume to total volume of material) depends on a combination of a number of process parameters: average temperature of the melt, total gas pressure in the installation during heating and melting of the charge, partial hydrogen pressure in the installation, rate of gas pressure change in the installation, time and temperature of isothermal holding or solidification rate, the chemical nature of the initial base material and the composition and amount of additives. These parameters generally affect the porosity as follows. The total porosity increases with an increase of hydrogen concentration in the material, an increase in the amount of additives introduced, an increase of the duration of isothermal holding and a decrease of pressure during solidification of the melt. A reverse change of these parameters reduces porosity. The average diameter of the pores depends mainly on the composition and amount of additives, the pressure before the beginning of solidification and the character of the pressure change during solidification. The average diameter of the pores decreas- es with an increase of the solidification pressure.

A production process for forming a porous material, as preferred according to an embodiment of the present invention, includes the following sequential operations:

(a) placing of a base material (metal, alloy, or ceramic) and additives in a special apparatus (Fig. 4);

(b) hermetic sealing of the internal space of the apparatus; (c) evacuation of air from the apparatus; (d) filling of the apparatus with a working gas mixture including an inert gas and hydrogen under a given pressure: (e) heating and melting of the initial base material; (f) ionization of the working gas mixture or hydrogen: (g) heating of a mold;

(h) holding of the melt parameters under temperature and pressure with agitation of the melt;

( 1 ) turning of the installation for the melt pouring into the mold; (j) contacting the melt and additives: (k) controlled change of total gas pressure in the installation: (1) slow cooling of the melt or isothermal holding of the melt at a given temperature;

(m) completion of solidification with simultaneous cessation of pressure control;

(n) cooling of the produced product: (o) evacuation of the installation; (p) filling of the installation with air or inert gas; and (q) removing the product from the mold.

One of the possible variants of the apparatus for implementation of the present invention is illustrated in Fig. 4. It consists of a case 1 with hermetically sealed plasma generator 2 for the generation of plasma 3, peephole 4 in cover 5 for process observation, tube 6 for connection to the system of working gas mixture supply, tube 7 for connection to an evacuation system, tube 8 for connec- tion to a pressure control system capable of stabilization and automatic changing of pressure with a given rate. A crucible 9 for heating and melting of the base material and a source of heat 10 for heating and melting of the base material are located within the pressurized case. The source of heat can be induction, plas- ma, plasma-induction, arc, plasma-arc, and laser heat source as well as a resistance furnace. Mold 11 , designed either for volume or directional solidification, and an installation for heating of the mold is also included in the pressurized case. Infrastructure, including standard evacuation systems and pressure control, means for introduction of additives, means for agitation of the metal in the cru- cible, cooling system for the case and plasma generator and a power supply system for the plasma generator, are not illustrated in Fig. 4.

The preferred process of porous product production is as follows. Cover 5 is opened and the source material and additives are placed into crucible 9. .After the source material and additives are placed in the crucible 9, cover 5 is closed and the installation is sealed. Then evacuation system is switched on and air is evacuated through tube 7.

The preferred operating parameters, such as hydrogen partial pressure, total pressure and rate of pressure change, as well as operating temperatures, will vary depending on the metal or alloy being formed. Generally, the hydrogen partial pressure will vary from about 0.001 to about 5.0 MPa, preferably from about 0.001 to about 2.5 MPa, and the total pressure will vary from about 0.001 to about 5.0 MPa and the rate of pressure change during solidification will vary from about 0.001 to about 0.5 MPa/sec.

When a given pressure is achieved, evacuation is ceased and the system is filled with a gas mixture, preferably hydrogen and an inert gas, with a predetermined total pressure and partial pressure of hydrogen. Then a heat source is activated to melt the base metal. During the melting process it is necessary to provide an intensive agitation of melted base material for homogenizing of the temperature and hydrogen concentration in the metal. For metals in which the rate of hydrogen absorption is small because of low diffusion mobility on the metal-gas boundary, it is preferable to activate the plasma generator 2. The plasma generator 2 is switched on, and plasma flow 3 is directed to the base material 10. Then a pressure stabilization system connected to the installation through the tube 7 is switched on. When the base material is molten and the temperature of the melt 6 reaches a given value, the system is held for a short time under stationary conditions. After this, the installation is turned by about 90 degrees in a vertical plane, and melt 6 is cast to mold 11. After the turn of the installation, the system of gas pressure control, connected to the installation through the tube 8 is switched on to increase or decrease the pressure. Active additives and gas evolution promoters are inserted into a stream of the melt or into the mold. Cooling of the melt in the mold 11 at a rate of about 1 to about 100°C per second and/or an isothermal holding of the melt at a selected temperature is performed. During the cooling or isothermal holding, pressure control in the installation is performed to provide the desirable porosity and aver- age pore size in the product. After solidification is complete, pressure control in the system and temperature control of the system and of the mold are switched off. Then, the natural reduction of temperature with the product occurs. After the installation is evacuated, it is filled with inert gas or air and the cover is opened to remove the porous product from the mold 11. As is discussed above, the materials produced according to the present invention can be metals, metal alloys or ceramics. Fig. 5 illustrates a cross-section of a metallic product according to one embodiment of the present invention. The cast product 50 includes a substantially solid outer surface portion 52 surrounding a porous central core portion 54. The two portions have substan- tially different levels of porosity. The porosity of the central core portion 54 can be, for example, from about 10 percent to about 80 percent. The porosity of the central core is also referred to as the integral porosity. The pores in the central core portion 54 typically have a mean size of from about 0.005 mm to about 5.0 mm. The solid outer surface portion 52 preferably comprises less than about 5 percent porosity and the mean pore size of any pores present in the outer surface portion 52 have a mean pore size of from about 0.0001 mm to about 0.01 mm. Preferably, the outer surface portion is substantially solid.

The materials produced according to the present invention are useful in a number of applications. For example, the porous metallic products, particularly porous titanium and zirconium, and their alloys, can be useful for aerospace applications such as wing structures, automotive applications, sound and heat isolating, damping and absorbing elements, heat exchangers, elements of heat protective shell, elements of a piston assembly, low lubrication bearing materials etc. Particularly, some of these porous materials can be used as structural mate- rials to replace honeycomb like and other light weight structures. Both metal, and non-metal (particularly ceramic) porous materials as well as composite materials based on them can be useful for such automotive applications like fuel cells, exhaust neutralizer substrates etc. In the chemical industry these materials can be useful as catalyst carriers, coarse filters etc. The products can also be useful in biomedical applications, such as for artificial bone implants or artificial teeth. The process advantageously permits the formation of a product having a substantially solid core and a porous exterior, which is useful for bone implants. The products can also be useful for use as electrodes or electrical contacts. On the other hand, non-porous products with enhanced density compared to the initial base materialfor instance so-called hydrogen reinforced materials and, in par- ticular zirconium and zirconium based alloys, can be used in connection with nuclear power plants and for other applications involving radioactive sources.

The present invention also provides the advantage that the products can be cast as near-net shapes. This feature is useful for the application of these materials in sporting goods and other consumer products, high-strength and light weight components of ammunition, and the like.

EXAMPLES

A titanium charge in the form of rods weighing 1.7-2.0 kg is placed into the installation. A mixture of MgO and Si02 with a ratio of 4:5 and a weight of 0.015 -0.020 kg is placed into the mold as a pre-formed bar. Cover 5 is closed and the installation is evacuated to a pressure of 10- 1 mm of mercury. The installation is then filled with a mixture of 10% hydrogen and 90% argon with a total pressure of 0.4 MPa. Then the plasma generator is switched on and plasma flow is directed to the titanium charge located in a crucible. Simultaneously, the system of metal agitation is also switched on.

.After complete melting of charge and reaching a given metal temperature, as determined by means of a thermocouple, the metal is held for about 60-90 seconds under stationary conditions. Then the installation is turned by 90 degrees and the metal is poured into a mold. Simultaneously, the plasma generator power supply and the electromagnetic agitation systems are switched off. .After completion of pouring, pressure control in the device was carried out. Overall pressure inside the installation was lowered to 3.5 MPa at a rate of 0.001 to 0.008 MPa/second. The solidification rate was 5-7°C per second. After solidification completion, the pressure control and mold heating were switched off. When the temperature of the product reached 300-350°C, the evacuation system was switched on. The evacuation sys- tern pumped out gas from the installation to a pressure of 0.001 MPa. The installation was filled by inert gas or by air and cover 5 was then opened and the casting was withdrawn. The casting had a dense skin 3-6 mm in thickness and a porous core. The total porosity of the casting was about 37-55% and the central core portion of the casting had a porosity (integral porosity) of 48-72% with an average pore diameter of 0.1 to 2 mm.

In a similar fashion, a number of examples were carried out using titanium as the base material (which is a typical representative of the metals where hydrogen solubility increases when temperature decreases) and utilizing different additives to enhance porosity of the materials. Results of these examples are listed in Table 1. TABLE I

While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.

Claims

What is claimed is:

1. A method for the production of a cast product with controlled density by controlling the gas concentration in the material, comprising the steps of: a) heating an initial base material up to a temperature sufficient to provide elt- ing and overheating of said initial base material and form a molten base material; b) contacting said molten base material with hydrogen to provide hydrogen in said molten base material; c) contacting said molten base material with active additives which modify the solubility of hydrogen in the molten base material; d) contacting said molten base material with gas evolution promoters which enhance gas evolution in said molten base material; and e) cooling said molten base material to form said product.

2. A method as recited in Claim 1, wherein said initial base material is selected from the group consisting of metals, metal alloys, ceramics or mixtures thereof.

3. A method as recited in Claim 1, wherein said initial base material is a metal.

4. A method as recited in Claim 3, wherein said metal is a metal for which hydro- gen solubility increases when temperature is decreased, or an alloy based on such a metal.

5. A method as recited in Claim 3, wherein said metal is a metal for which hydrogen solubility decreases when temperature is decreased, or an alloy based on such a metal.

6. A method as recited in Claim 1 , wherein said initial base material is a metal selected from the group consisting of titanium, zirconium, hafnium, vanadium, niobium, molybdenum, tantalum, palladium, rhodium, copper, iron, nickel, chromium, scandium, aluminum, magnesium, lanthanum, cerium, uranium, thorium and alloys thereof.

7. A method as recited in Claim 1 , wherein said initial base material is a metal selected from titanium, titanium alloys, zirconium and zirconium alloys.

8. A method as recited in Claim 1 , wherein said heating step comprises the step of heating said initial base material using a heating method selected from induction heating, plasma heating, arc heating, laser heating, resistance heating, radiation heating and combinations thereof.

9. A method as recited in Claim 1 , wherein said step of contacting said molten base material with hydrogen comprises the step of contacting said molten base material with an operating gas comprising hydrogen.

10 A method as recited in Claim 9, wherein said operating gas further comprises a gas which is chemically inert to said initial base material.

11. A method as recited in Claim 1, wherein said step of contacting said molten base material with hydrogen comprises the step of forming a flow of activated hydrogen and directing said flow onto said molten base material.

12. A method as recited in Claim 11, wherein said flow of activated hydrogen is formed by a plasma.

13. A method as recited in Claim 12, wherein said plasma comprises a mixture of positive ions of hydrogen and ions of.a gas which is chemically inert to a said initial base material.

14. A method as recited in Claim 12, wherein said plasma comprises an atomic concentration of protons of from about 0.5 to about 60 atomic percent.

15. A method as recited in Claim 1, wherein said step of contacting said molten base material with hydrogen comprises maintaining a hydrogen partial pressure over said molten base material of from about 0.001 MPa to about 5.0 MPa.

16. A method as recited in Claim 1, further comprising the step of agitating said molten base material to enhance homogeneity of said molten base material and promote hydrogen dissolution therein.

17. A method as recited in Claim 16, wherein said steps of heating and contacting said molten base material with hydrogen comprises contacting said molten base material with a hydrogen-containing plasma.

18. A method as recited in Claim 1 , wherein said steps of contacting said active additives and said gas evolution promoters with said molten base material comprises the steps of introducing active additives and introducing and dispersing particulate gas evolution promoters in a said molten base material.

19. A method as recited in Claim 18, wherein said active additives and gas evolution promoters comprise chemical elements, their oxides, other chemical compounds, and mixtures thereof.

20. A method as recited in Claim 1 , wherein the total concentration of a said active additives and said gas evolution promoters is from about 0.05 to about 5.0 weight percent of said initial base material.

21. A method as recited in Claim 1 , wherein said active additives are introduced as constituent components of the initial charge of said initial base material.

22. A method as recited in Claim 1, wherein said active additives are introduced into the melt of said initial base material while it is in a melting crucible.

23. A method as recited in Claim 1, wherein said active additives and said gas evolution promoters are introduced into .a stream of said molten base material during pouring of said molten base material into a casting mold.

24. A method as recited in Claim 1 , wherein said active additives and said gas evo- lution promoters are placed into a mold prior to pouring of said molten base material therein.

25. A method as recited in Claim 1, wherein said gas evolution promoters comprise particulates having a mean size in the range of from about 0.002 mm to about 2.0 mm.

26. A method as recited in Claim 1, wherein said gas evolution promoters comprise particulates which are substantially insoluble in said molten base material and have a wetting angle with said molten base material of greater than about 90 degrees.

27. A method as recited in Claim 1, wherein said gas evolution promoters comprise particulate materials having a melting temperature substantially higher than the melting temperature of said initial base material.

28. A method as recited in Claim 1, wherein said step of cooling of said molten base material comprises the step of cooling at a sufficiently low rate to form a porous product.

29. A method as recited in Claim 28, wherein said step of cooling comprises cooling and holding of said molten base material under substantially isothermal conditions for a time sufficient to nucleate and precipitate pores in said molten base material.

30. A method as recited in Claim 28, wherein said molten base material is cooled at a rate of from about 1 to about 100°C per second.

31. A method as recited in Claim 28, wherein said cooling step comprises the step of using heating means to control the cooling rate of said molten base material to faciii- tate the precipitation of pores therein.

32. A method as recited in Claim 1 , wherein said method further comprises the step of controlling the total pressure applied to a surface of said molten base material during said cooling step.

33. A method as recited in Claim 32, wherein said step of controlling the total pressure comprises the step of reducing the total pressure.

34. A method as recited in Claim 32, wherein said step of controlling the total pres- sure comprises the step of increasing the total pressure.

35. A method as recited in Claim 32, wherein said step of controlling the total pressure begins upon pouring of said molten material into a mold and ends when solidification of said molten base material in the mold is substantially complete.

36. A method as recited in Claim 32, wherein the total pressure applied to a surface of said molten base material is in the range of from about 0.001 MPa to about 5.0 MPa and the absolute rate of pressure change upon casting is from about 0.001 MPa to 0.5 MPa per second.

37. A method as recited in Claim 32, wherein said step of controlling the total pressure applied to a surface of said molten base material during said step of cooling is carried out using a gas which is chemically inert to said initial base material or by a mixture of said inert gas with hydrogen.

38. A method as recited in Claim 1 , wherein said product is a porous product and wherein the rate and method of controlling the pressure during a said step of cooling of said molten base material is determined by the desired level of porosity and the desired mean pore size in said product.

39. A method as recited in Claim 1 , wherein said product is a porous product.

40. A method as recited in Claim 1 , wherein the total porosity of said porous product is proportional to the amount of said gas evolution promoters.

41. A method as recited in Claim 39, wherein said porous product has an integral porosity of from about 10 to about 80 percent.

42. A method as recited in Claim 39, wherein said porous product comprises pores having a mean size of from about 0.005 mm to about 5.0 mm.

43. A method as recited in Claim 39, wherein said porous product comprises at least two different portions having substantially different levels of porosity.

44. A method as recited in Claim 43, wherein said porous product comprises a sub- stantially solid outer surface portion and a porous core portion.

45 A method as recited in Claim 44, wherein said outer surface portion comprises less than about 5 percent porosity and has a mean pore size of from about 0.0001 mm to about 0.01 mm.

46. A method for forming a product, comprising the steps of: a) heating an initial base material to at least the melting temperature of said base material to form a molten base material; b) contacting said molten base material with activated hydrogen to transfer hydro- gen into said base material; c) agitating said molten base material to enhance dissolution of hydrogen into said base material; and d) cooling said base material to form a product.

47. A method as recited in Claim 46, wherein said heating step comprises the step of heating said base material using a heating method selected from induction heating, plasma heating, arc heating, laser heating, resistance heating and combinations thereof.

48. A method as recited in Claim 46, wherein said contacting step comprises the step of forming an activated hydrogen stream with a plasma and directing said stream at said molten base material.

49. A method as recited in Claim 46, wherein said contacting step comprises contacting said molten base material with a working gas mixture comprising an inert gas.

50. A method as recited in Claim 46, wherein said contacting step comprises maintaining a partial pressure of hydrogen over said molten base material of from about 0.001 MPa to about 5 MPa.

51. A method as recited in Claim 46, wherein said cooling step comprises the step of quenching said molten base material.

52. A method as recited in Claim 46, wherein said base material is a metal.

53. A method as recited in Claim 52, wherein said metal is selected from titanium, aluminum, iron, zirconium, copper, nickel, manganese, vanadium, palladium, niobium, tantalum, magnesium, scandium, hafnium, uranium, thorium, lanthanum, yttrium, cerium and alloys thereof.

54. A method as recited in Claim 52, wherein said metal is selected from titanium and alloys thereof.

55. A method as recited in Claim 46, wherein said product is a porous product.

56. A method as recited in Claim 55, wherein said porous product has a porosity of at least about 10 percent.

57. A method as recited in Claim 55, wherein said cooling step comprises the step of using heating means to control the cooling rate of said base material to precipitate pores therein.

58. A method as recited in Claim 57, wherein said cooling step comprises cooling and holding said molten base material under substantially isothermal conditions for a time sufficient to solidify said molten base material.

59. A method as recited in Claim 46, wherein said heating step and said contacting step comprise contacting said molten base material with a hydrogen-containing plas- ma.

60. A method as recited in Claim 46, further comprising the step of dispersing active additives in said molten base material.

61. A method as recited in Claim 46, further comprising the step of dispersing gas evolution promoters in said molten base material.

62. A method for forming a product, comprising the steps of: a) heating a base material to at least the melting temperature of said base material to form a molten base material: b) contacting said molten base material with hydrogen; c) agitating said molten base material for a time sufficient to permit diffusion of hydrogen into said base material: d) solidifying said molten base material under substantially isothermal conditions for a time sufficient to form solidified porous product; and e) cooling said solidified base material.

63. A method as recited in Claim 62, wherein said base material compnses titanium.

64. A method as recited in Claim 63, wherein said solidifying step comprises holding said base material at a temperature of from about 1590°C to about 1670°C for a time sufficient to precipitate pores in said base material.

65. A method as recited in Claim 64, wherein said porous product has a porosity of at least about 10 percent.

66. A method as recited in Claim 62, wherein said contacting step comprises the step of forming an activated hydrogen stream and directing said stream at said molten base material.

67. A method as recited in Claim 62, wherein said contacting step compπses maintaining hydrogen in contact with said molten base material at a hydrogen partial pressure of from about 0.001 MPa to about 5 MPa.

68. A method for forming a titanium product, comprising the steps of a) heating a base metal comprising titanium to at least the melting temperature of said base metal to form a molten base metal. b) contacting said molten base metal with hydrogen; c) solidifying said molten base metal at a temperature that is sufficient to form a solidified product comprising at least about 10 percent porosity; and d) cooling said base metal to form a porous titanium product

69. A method as recited in Claim 68, wherein said titanium product has a mean pore size of from about 0.005 mm to about 5.0 mm.

70. A method as recited in Claim 68, wherein said contacting step compπses contacting said molten base metal with activated hydrogen.

71 A method as recited in Claim 68, further compπsing the step of agitating said molten base metal dunng said contacting step.

72. A method for forming a metal mateπal, compπsing the steps of a) heating a base metal to at least the melting temperature of said base metal to form a molten base metal; b) contacting said molten base metal with activated hydrogen with simultaneous agitation of said molten base material; c) dispersing active additives and gas evolution promoters in said base mateπal; and d) cooling said molten base metal to form a porous metal product

73 A method as recited in Claim 72, wherein said gas evolution promoters comprise particulate metal oxides having an average size of from about 0 002 to about 2 0 mm

74 A method as recited in Claim 73, wherein said metal oxides are selected from the group consisting ofzirconia, titania, magnesia, alumina, and mixtures thereof

75. A porous titanium product, comprising: a) a central core portion having a porosity of from about 10 to about 80 percent and a mean pore size of from about 0.005 to about 5 mm: and b) a substantially solid outer surface portion surrounding said central core portion; wherein said product has less than about 5 percent porosity.

76. A porous titanium product as recited in Claim 75, wherein said product is a component in an aerospace structure.

77. A porous titanium product as recited in Claim 75, wherein said product is a biomedical component selected from an artificial bone implant and an artificial tooth.

78. A porous titanium product as recited in Claim 75, wherein said product is a component in an automotive structure.