Mold compositions for additive casting of metal objects

Abstract

The disclosure concerns printable refractory compositions, more particularly ceramic-based pastes for 3D printing of molds for additive metal casting. In particular, the present disclosure concerns composition for forming mold regions having controlled thermal conductivity and dissipation and controlled release of gaseous products therefrom during heating to mitigate mechanical failure risks in an additive casting process of metal objects.

Claims

1. A paste composition for manufacturing of a mold for additive casting of a metal object, in a process of subsequent formation of production layers, each production layer comprising at least one mold region and at least one metal object region, the mold region comprising at least one paste composition, the paste composition comprising: between about 65 wt % and about 75 wt % of at least one ceramic material in particulate form; between about 3 wt % and about 8 wt % of at least one inorganic binder, wherein said at least one inorganic binder is different from said at least one ceramic material; between about 4 wt % and about 14 wt % of at least one porosity controlling additive; between about 0.05 wt % and about 3 wt % of at least one of an energy absorbing additive and an energy conductive additive, wherein said energy absorbing additive is different from said at least one ceramic additive and said energy conductive additive is different from said at least one ceramic material, and wherein said energy absorbing additive is different from said energy conductive additive; and between about 10 wt % and about 20 wt % of at least one carrier liquid; the paste composition having a mass loss of at least about 10 wt % out of the total mass of the paste composition when heated to about 220 C. for a period of time of no more than about 30 minutes; wherein said at least one ceramic material comprises a mixture of at least one first ceramic material having a particle size of between about 50 m and about 300 m and at least one second ceramic material having a particle size of between about 1 m and about 15 m.

2. The paste composition of claim 1, wherein said at least one porosity controlling additive is selected from one or more types of microspheres.

3. The composition of claim 2, wherein said microspheres are made of aluminum oxide, aluminum silicate, silicon dioxide, borosilicate glass, yttria-stabilized zirconia, and mixtures thereof.

4. The composition of claim 2, wherein said microspheres have a particle size of between about 50 m and about 500 m.

5. The paste composition of claim 1, wherein said at least one ceramic material is selected from zirconia, alumina, silica, zirconium silicate, yttria-stabilized zirconia, silicon carbide, tungsten carbide, and mixtures thereof.

6. The paste composition of claim 1, wherein said at least one inorganic binder has a polymerization temperature of between about 150 C. and about 850 C.

7. The paste composition of claim 1, wherein said at least one inorganic binder is selected from silicate-based binders, phosphate-based binders, aluminosilicate binders, alumino-silico-phosphate binders, magnesium oxide, colloidal silica, and mixtures thereof.

8. The paste composition of claim 7, wherein said at least one inorganic binder is at least one silicate-based binder.

9. The paste composition of claim 8, wherein said silicate based binder is selected from sodium silicate, potassium silicate, colloidal silica, and mixtures thereof.

10. The paste composition of claim 7, wherein the at least one inorganic binder is at least one phosphate-based binder.

11. The paste composition of claim 10, wherein said phosphate-based binder is selected from alkali metal trimetaphosphate, alkali metal monophosphate, aluminum phosphates, sodium tripolyphosphate, silico-aluminophosphate, monoaluminium phosphate, polyphosphates, dihydrogen aluminophosphate, polyphosphazene and mixtures thereof.

12. The paste composition of claim 1, wherein said energy absorbing additive is selected from carbon powder, carbon black, graphene, graphite, carbon nanotubes, UV-absorbing pigments, visible spectrum absorbing pigments, IR absorbing pigments, magnesium-aluminum oxides, ceramic fibers, and mixtures thereof.

13. The paste composition of claim 1, wherein said energy conductive additive is selected from carbon powder, carbon black, graphene, graphite, carbon nanotubes, boron nitride, carbon nitride, silicon nitride, silicon carbide, metal particles, metal fibers, metal oxides and mixtures thereof.

14. The paste composition of claim 1, wherein the paste composition comprises both said energy absorbing additive and energy conductive additive.

15. The paste composition of claim 1, wherein said at least one liquid carrier is selected from water, C.sub.1-C.sub.6 alcohols, C.sub.5-C.sub.12 alkanes, mineral oils, natural oil, synthetic oils, and any mixture thereof.

16. The paste composition of claim 1, further comprising at least one surfactant.

17. The paste composition of claim 1, further comprising at least one mechanical reinforcing element.

18. A paste composition for manufacturing of a mold for additive casting of a metal object, in a process of subsequent formation of production layers, each production layer comprising at least one mold region and at least one metal object region, the mold region comprising at least one paste composition, the paste composition comprising: between about 65 wt % and about 75 wt % of at least one ceramic material in particulate form; between about 3 wt % and about 8 wt % of at least one inorganic binder, wherein said inorganic binder being different from said at least one ceramic material; between about 4 wt % and about 14 wt % of at least one porosity controlling additive in the form of microspheres, said microspheres being made of aluminum oxide, aluminum silicate, silicon dioxide, borosilicate glass, yttria-stabilized zirconia, or mixtures thereof; between about 0.05 wt % and about 3 wt % of at least one of an energy absorbing additive and an energy conductive additive, wherein said energy absorbing additive is different from said at least one ceramic additive and said energy conductive additive is different from said at least one ceramic material, and wherein said energy absorbing additive is different from said energy conductive additive; and between about 10 wt % and about 20 wt % of at least one carrier liquid; wherein the paste composition has a mass loss of at least about 10 wt % out of the total mass of the paste composition when heated to about 220 C. for a period of time of no more than about 30 minutes.

19. The paste composition of claim 18, wherein said at least one ceramic material has a particle size of no more than about 300 m.

20. The paste composition of claim 18, wherein said at least one inorganic binder is selected from the group consisting of silicate-based binders, phosphate-based binders, aluminosilicate binders, alumino-silico-phosphate binders, magnesium oxide, colloidal silica, and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1A-1B are schematic representation of an additive casting process of a metal object according to an embodiment of this disclosure, in which paste compositions of this disclosure are used for construction of the mold regions in the production layers of the mold-metal fabricated structure.

(3) FIGS. 1C-1D are schematic top view of production layers: comprising a mold region and a metal object region (FIG. 1C), and a mold region and multiple metal object regions (FIG. 1D).

(4) FIGS. 2A-2B are schematic representation of an additive casting process of a metal object, in which a complex mold region is applied in the production layers of the mold-metal fabricated structure, according to another embodiment of this disclosure.

(5) FIG. 2C is a schematic top view of a production layer of the embodiment of FIGS. 1A-1B, comprising a first mold section, a second mold section and a metal object region.

(6) FIGS. 3A-3B are schematic representation of an additive casting process of a metal object, in which a complex mold region is applied in the production layers of the mold-metal fabricated structure, according to another embodiment of this disclosure.

(7) FIGS. 4A-4B are schematic representation of an additive casting process of a metal object, in which a complex mold region is applied in the production layers of the mold-metal fabricated structure, according to another embodiment of this disclosure.

(8) FIG. 4C is a schematic top view of a production layer of the embodiment of FIGS. 2A-2B or FIGS. 3A-3B, comprising a barrier wall, a complex mold region (constructed out of first and second sections) and a metal object region.

(9) FIG. 5 shows small-scale mass loss tests for various paste compositions.

(10) FIGS. 6A-6D are images of dried samples of paste compositions Form. 1-Form 4, respectively, as detailed in Table 1.

(11) FIGS. 7A-7D are optical microscopy pictures of cross-sections of paste compositions with various porosity controlling additives.

DETAILED DESCRIPTION OF EMBODIMENTS

(12) Reference is first being made to FIGS. 1A-1C, showing steps of an exemplary additive casting process of a metal object, in which paste compositions of this disclosure are used for the construction of mold-metal structures fabricated from a stack of production layers.

(13) Process 100 comprises deposition 110 of a paste composition according to a desired mold contour in a production layer 200i (i being an integer (i>0) defining the number of the production layer). The deposited paste composition is then heated at step 120, continuously or in several heating intervals, to obtain at least partial polymerization of the inorganic binder and a reduction of at least 10 wt % of its total mass of its total mass within no more than 30 minutes, to form the mold region 210 of the production layer 200.sub.1.

(14) At this stage, the mold region 210 is stable enough to carry out one or more post-deposition surface treatments (not shown), if desired, in order to smoothen the surface texture (e.g. the inner walls 212 of mold region 210) or define surface features of the mold region.

(15) At step 130, molten metal is being cast into the cavity 220 defined by the mold region. The metal 225 then cools at step 140 to at least partially solidify, thereby forming the metal object region 230 and completing the manufacture of the first production layer, before deposition of fresh paste to form the mold region of the ensuing production layer 2002. The cycle of steps 110-140 is repeated, such that a stack of production layers is obtained, by consecutive cycles of production layers fabrication, until the end of the casting of the entire metal object.

(16) Step 130 comprises the deposition of molten metal (represented in FIG. 1B by molten metal drops 222) into cavity 220. For example, molten metal 222 is deposited while a metal depositor (not shown) is positioned above cavity 220. Step 130 may comprise pre-deposition heating of the previously-cast metal region(s) 225 such that the top surface of metal region 225the previously deposited metalmay be in at least partially molten state during molten metal deposition of the subsequent quanta of molten metal. Step 130 may further comprise post-deposition heating of the deposited object region to modify its cooling profile. In some embodiments, step 130 is implemented in a sequential manner on a plurality of working areas (not shown) that constitutes object region 220. For example, a metal head (not shown) composed of molten metal depositor and working area heater, travels over object region 220 during step 130.

(17) As seen in FIGS. 1A-1B, prior to deposition of the first mold region, a base paste layer 200b can be deposited (at optional step 105), as a substantially continuous base layer (i.e. without object regions), thereby functioning as a solid mold base layer.

(18) As can be seen in FIG. 1D, several metal object regions 230 can be formed in a single mold region 210, thereby forming a nested configuration of several, separate metal objects to be produced.

(19) FIGS. 2A-2C show the steps of another exemplary additive casting process of a metal object, in which two paste compositions of this disclosure are used for manufacturing the mold region in a production layer.

(20) In process 1000 comprises deposition 1100 of a paste composition according to a desired mold contour in a production layer 2000i (i being an integer (i>0) defining the number of the production layer), to form a second mold section 2100.

(21) Another paste composition of this disclosure is then deposited, at step 1200, adjacent the second mold section 2100, to form a first mold section 2160 that laterally encloses the second mold section 2100. The first mold section 2160 is deposited such as to form a substantially continuous inter-mold interface 2140 (to ensure physical contact between the first and second mold sections along this interface). The mold region is then heated at step 1300, to permit at least partial polymerization of the inorganic binder, as well as mass loss of at least 10 wt %, to thereby mechanically stabilize of the mold region. As noted above, in such configurations, the first and second mold sections differ in at least one property (e.g. in their drying rate, polymerization temperature, polymerization rate, etc.), or differ in composition. Due to this difference, one of the mold sections mechanically stabilizes before the other, hence providing mechanical support to said other mold section until said other mold section is sufficiently stable for the next manufacturing step of the production layer.

(22) The mold region is then optionally further heated, at step 1400, for further mechanical stabilization, followed by deposition of molten metal 2220 into cavity 2200 formed by the mold region 2100, at step 1500. The molten metal is then permitted to cool and at least partially solidify, at step 1600, thereby forming the metal object region 2300. In this manner, a production layer, e.g. layer 20001, that includes the first mold section 2160, the second mold section 2100 and the metal region 2300 is formed. Deposition of the next production layer, e.g. layer 20002, can then commence by repeating the steps 1100-1500 to form a stack of production layers, until the entire object is manufactured in an additive process.

(23) An optional step 1450 can be carried out between steps 1400 and 1500. After step 1300 (or 1400, if applied), the mold region 2100 is stable enough to carry out one or more post-deposition surface treatments (step 1450), if desired, in order to smoothen the surface texture of metal-facing surface 2120 or define desired surface features of the mold-facing surface.

(24) Further, step 1500 can comprise pre-deposition heating of the previously-cast metal object region(s) 2250 such that the top surface of metal object region 2250the previously deposited metalmay be in at least partially molten state during molten metal deposition of the subsequent quanta of molten metal. Step 1500 may further comprise post-deposition heating of the deposited metal object region to modify its cooling profile. In some embodiments, step 1500 is implemented in a sequential manner on a plurality of working areas (not shown) that constitutes metal object region 2200. For example, a metal head (not shown) composed of molten metal depositor and working area heater, travels over metal object region 2200 during step 1500.

(25) As seen in FIG. 2A, prior to deposition of the first mold region, a base mold layer 2000b can be deposited (at step 1050), as a substantially continuous base layer (i.e. without object regions), thereby functioning as a solid mold base layer.

(26) It is of note that while in this example the second mold section is formed before the first mold section, it will become apparent to any person of skill that the first mold section can also be formed before the second mold section. Further, it will become apparent that in some production layers the first mold section is formed before the second section, and in other production layers of the same mold-metal object the order of deposition can be reversed.

(27) Alternatively, the first and second paste compositions can be deposited simultaneously, e.g. from adjacent deposition nozzles.

(28) The embodiment shown in FIGS. 1A-2C is exemplary for application of a paste composition according to this disclosure that is self-standing, namely, sufficiently viscous to maintain its deposited shape without further support. Examples of applications in which the paste composition requires initial mechanical support prior to polymerization are provided in FIGS. 3A-3B and 4A-4C.

(29) In the embodiment of FIGS. 3A-3B, a process 1000 similar to process 1000 of FIGS. 2A-2B is demonstrated, however including a step 1150 of forming a barrier wall 2400 prior to deposition of the first paste composition in step 1200. Thus, the first paste composition is deposited to the space formed between barrier wall 2400 and surface 2140 of the second mold section 2100. In process shown in FIGS. 3A-3B, the barrier wall is constructed in an additive mannernamely, for each production layer 2000i, another vertical section of barrier wall 2400 is deposited.

(30) In the embodiment of FIGS. 4A-4B, process 1000 is similar to process 1000, however in this process a vertically continuous barrier wall 2420 is deposited at step 1030, such that the barrier wall 2420 has a height that is at least the same as the height of the final mold-metal object to be produced.

(31) As can be seen in FIG. 4C, the processes of FIGS. 3A-3B and 4A-4B result in a construction of production layers that are constituted by a barrier wall, a mold region constituted by first and second mold sections, and a metal object region.

(32) In the embodiments of FIGS. 3A-4C, it is sometimes desirable to utilize a paste composition that is self-leveling for forming the first mold section, such that the top surface of the mold region in a given production layer is substantially flat (i.e. substantially parallel to the receiving surface), such that a metal-over-mold construction can be obtained.

(33) As noted, the paste compositions of this disclosure are designed to be suitable for such mold-metal additive casting processes, particularly tailored to maintain their mechanical stability and mold integrity during the cycles of extensive heat shocks and applied stresses (e.g. due to volume changes of the melting and solidifying metal in the object region).

(34) The paste composition of the present disclosure is deposited to form mold regions having controlled gaseous products release rate and improved heat absorbance/conductance, such that once exposed to heat (e.g. resulting from contact with the molten metal or from external heating), the paste composition functions to provide mechanically stabilized mold regions, improved heat dissipation through the mold region bulk and reduced risk of mechanical failure. In other words, the paste compositions of this disclosure are characterized by controlled energy absorbance and conduction, as well as controlled gas release during their polymerization and drying, thereby enabling obtaining mold regions that reach thermal and mechanical stability relatively fast with reduced risk of mechanical failure.

(35) The paste compositions of this disclosure are designed to have efficient energy absorption and dissipation. This permits for effective energy uptake and transfer through the voluminous mold region during casting, to minimize the thermal shock experienced by the mold region during casting and thermal cycling. In addition, efficient and controllable gas release from the paste composition during absorption of such heat enables minimizing the mechanical damage that can be formed due to uncontrolled and abrupt gas release from the mold region (e.g. due to boiling liquid or gaseous chemical reaction products), hence, maintaining the mechanical integrity of the mold (and particularly the metal-mold interface) throughout the casting process.

(36) The inventors have surprisingly found that in paste compositions of this disclosure, effective energy (e.g. heat) uptake and effective heat dissipation through the mold region, together with controlled evacuation of gaseous products, can be obtained to permit a relatively quick cycle time in the additive manufacturing process. By controlling overall porosity formed during heating and drying of the paste composition, it was found that a balance between energy uptake and dissipation with control over the gaseous products evacuation can be obtained. Without wishing to be bound by theory, by forming paths for gaseous products release, porosity prevents internal pressure buildup during drying of the paste (which is carried out by applying heating conditions). Further, by temporarily physi-sorb molecules of the carrier liquid, e.g. water, a delay in moisture and/or the rate of vaporization of the carrier liquid from the paste composition can be obtained, thereby assisting in preventing rapid shrinkage and/or capillary collapse. Addition of an energy absorption and/or conductance additive improves heat uptake and dissipation through the bulk of the mold region. The balance between the fast energy absorption and dissipation, that cause quick increase in temperature within the bulk of the mold region, together with the careful control over the porosity formed during the drying process, results in optimal rate of drying and/or gaseous products evacuation from the mold region. Such balance permits a significant reduction in the risk of cracking or popping effects at the bulk of the mold region as well as at its surface, while also permitting high resistance of the mold region to thermal shock, thereby permitting relatively short fabrication cycles at elevated temperatures and cost-effective process.

Example 1

(37) In order to demonstrate the combined effect of the porosity controlling additive and the energy absorption/conductivity additive, paste compositions were prepared according to Table 1 and 1-2:

(38) TABLE-US-00001 TABLE 1-1 Tested paste compositions (wt %) Component Function Comp. 1 Comp. 2 Comp. 3 Comp. 4 Zirconia Ceramic 72-80 65-75 70-80 65-75 powder Sodium Inorganic 3-6 3-6 3-6 3-6 silicate binder Alumina Porosity- 5-10 5-10 microspheres controlling additive Carbon-based Energy 0.5-3 0.5-3 additive absorbance & conductance additive Additives Surfactants, 1-5 1-5 0.5-5 0.5-5 solvents, etc. Water Carrier liquid 11-17 11-17 11-17 11-17

(39) TABLE-US-00002 TABLE 1-2 Tested paste compositions (wt %) Component Function Comp. 5 Comp. 6 Zirconia Ceramic 65-75 65-75 powder Aluminum Inorganic 3-6 3-6 phosphate binder Alumina Porosity- 5-10 microspheres controlling additive Energy Energy 0.5-5 0.5-5 absorbance & absorbance & conductance conductance additive additive Additives Surfactants, 1-5 1-5 solvents, etc. Water Carrier liquid 11-17 11-17

(40) Small scale samples were prepared to assess the effect on the mass loss, drying profile and the mechanical stability of the compositions.

(41) Small scale mass loss and drying profile was assessed using Radwag MA50R halogen lamp moisture scale, using 3-7 g samples, at heating to 220-230 C., at ambient atmosphere.

(42) The results for the small scale mass loss tests are shown in FIG. 5.

(43) Further, the compositions were deposited in cylindrical forms and dried at 230 C. over a hot plate. Development of surface and bulk artifacts were monitored and captured, as shown in FIGS. 6A-6D.

(44) As can be seen from FIG. 5, addition of the microspheres and/or carbon-based additive significantly shortened the overall drying time, as well as increased the drying rate. While from FIG. 5 it seems that the combination of microspheres and carbon-based additive (Comp. 4) does not provide significant change in drying rate of a sample that contained carbon black without microspheres (Comp. 3), the significance of the combination of Comp. 4 is clearly evident from FIGS. 6A-6D.

(45) As can be seen in FIGS. 6A-6D, in all of Comps. 1, 2 and 3, significant mechanical artifacts, such as cracks and voids are observable, while in Comp. 4 no such artifacts can be seen under the same drying conditions.

(46) The combination of results of FIG. 5 and FIGS. 6A-6D provides evidence of the careful balance obtained by the combination of porosity controlling additives and energy absorbance/conductance additives in compositions of this disclosure. Utilization of energy absorbance/conductance additives (Comp. 3) increases the absorbance and dissipation of energy, i.e. thermal energy, through the composition-however due to the fast increase in temperature, the liquid components in the composition boil faster and more violently, causing development of internal stresses in the paste during its drying, resulting in significant cracking of the drying paste. Utilization of porosity controlling additives (Comp. 2), while accelerating to some extent the drying process due to formation of evacuation paths for the gaseous products, also causes results in cracking, typically due to capillary collapse during heating without sufficient heat dissipation means.

(47) When combining the microspheres with carbon black (Comp. 4), a balance is obtained between the quick uptake and dissipation of energy within the paste during heating, and the control over the porosity which provides means for controlling the gradual release of gaseous products from the paste during heating. Hence, the development of internal stresses and surface stresses is kept to a minimum, preventing formation of cracks, voids or other surface artifacts that reduce the mechanical stability of the mold region produced from the paste.

(48) It is of note that when the samples were utilized to produce large-scale samples, e.g. of a sample having an area of 100 cm.sup.2 at a thickness of 4-8 mm, Comp. 1 remained moist even following 30 minutes of heating, while Comp. 4 dried within 6-7 minutes (to a moisture degree of <2%). In other words, in large scale, Comp. 1 showed significantly low mass loss, while Comp. 4 showed a mass loss of over 10 wt % within 6-7 minutes.

(49) The compositions of Table 1-2 have shown similar behavior.

Example 2

(50) The effect on porosity utilizing different porosity was assessed for paste compositions utilizing various porosity controlling additives, as detailed in Table 2. No energy absorbance/conductance additives were used for these samples in order to permit better visualization of porosity artifacts. All samples were dried under the same conditions. Optical microscopy images of cross-sections of the samples are shown in FIGS. 7A-7D.

(51) TABLE-US-00003 TABLE 2 Effect of various porosity controlling additives on porosity formation Sample Porosity control additive Porosity formation mechanism FIG. 7A None Macropores formed during mixing of ingredients and deposition FIG. 7B Alumina microspheres Hollow microspheres FIG. 7C Sodium dodecyl sulfate Foaming agent FIG. 7D Poly(diallyldimethylammonium Sacrificial organic additive that chloride) (PDAC) decomposes during heating

(52) As can be seen, without addition of porosity controlling additives (FIG. 7A), uneven and uncontrolled macropores form prior to and/or during drying. Such uncontrolled macropores constitute voids in the mold region and are locations for stresses development during drying and thermal cycling, that may eventually cause mechanical failure of the mold.

(53) Addition of microspheres (FIG. 7B), e.g. alumina microspheres, resulted in evenly distributed fine porosity, mainly due to the utilization of substantially monodisperse hollow microspheres that have relatively high mechanical stability.

(54) Foaming agents, such as sodium dodecyl sulfate (SDS) and the like (FIG. 7C), resulted in formation of air bubbles trapped in the paste composition, that collapse to form porosity during heating of the paste. This additive resulted in significant increase in overall porosity, however also to the formation of relatively large pores.

(55) Sacrificial materials undergo at least partial decomposition during heating and drying of the paste. Addition of fine powders of PDAC (FIG. 7D), resulted in finer pores than those obtained with SDS.

(56) It should be noted that the present disclosure is not limited to the production conditions and operational parameters provided in the above-discussed Examples, and the present disclosure may be implemented in various production conditions and operational parameters. The present disclosure is not limited to the casting of gray iron objects and various metals and/or metallic alloys which are suitable for melting and casting, can be used, for example, other iron types, steel and other ferrous alloys, aluminum alloys, copper alloys, nickel alloys, magnesium alloys, and the like.

(57) Aspects of the present disclosure were illustrated with reference to the deposition of the mold material in the form of a solid paste tube with circular cross-section. The present disclosure is not limited by the cross-section of the mold paste as well as by the cross-section shape and dimension of the mold paste.