US20180345362A1

US20180345362A1 - Tool and method for direct squeeze casting

Info

Publication number: US20180345362A1
Application number: US15/611,821
Authority: US
Inventors: Richard Osborne; Qigui Wang; Herbert W. Doty
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2017-06-02
Filing date: 2017-06-02
Publication date: 2018-12-06
Also published as: CN108971449A; DE102018113057A1

Abstract

A casting tool for a direct squeeze casting process that includes a cast mold tool with a contoured internal passage for better die thermal management. This enables the use of a grey cast iron mold material. A durable mold surface may also be formed through a nodular cast iron reaction with a Magnesium addition in either sand core or sand core coating.

Description

INTRODUCTION

This introduction generally presents the context of the disclosure. Work of the presently named inventors, to the extent it is described in this introduction, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against this disclosure.
Low volume and prototype cast parts are generally limited to sand cast processes because the tooling costs for permanent mold processes may be prohibitive. The part designers who rely upon these low volume sand cast parts are faced with a significant problem: the characteristics of sand cast parts may not generally exhibit the same characteristics as those which are created using high volume permanent mold processes. This places a significant limitation on the ability of a part designer to verify a design early in the design process.
Current high volume manufacturing processes for producing engine components of a motor vehicle, for example, cylinder blocks, include high pressure die casting (HPDC) processes. Although the tools used for a HPDC process are more expensive and require a much longer lead time to produce, these costs may be distributed over the large number of parts that are generated by the HPDC process so that the individual part cost may be lower. However, the HPDC process also has some problems. Typically, as molten metal is introduced into a mold, the HPDC high velocity fill processes entrain air, generate oxides, and have difficulty addressing metal shrinkage from certain regions within the mold.
Co-pending, co-assigned U.S. patent application Ser. No. 15/223,911, the disclosure of which is hereby incorporated by reference in its entirety, discloses a novel direct squeeze casting process that addresses many of the issues of the above-described casting processes. The novel direct squeeze process benefits from a slower pour velocity which results in a quiescent fill of the mold that reduces or eliminates turbulence in the flowing molten metal. This reduces the entrainment of air in the resultant casting which reduces the possibility of gas porosity and enables the possibility of heat treating. Further, the direct squeeze process provides the ability to better compensate for the shrinkage of the metal as it cools and solidifies. Pressure may be strategically applied only to those portions which may most benefit from that pressure, such as, for example, thicker sections, bulkheads and the like. This provides the ability to produce high integrity cylinder block castings that can be heat treated to optimum tensile, fatigue strength, and other material characteristics.
FIG. 1 is an interior view of the novel direct squeeze casting system 100 for forming a casting 128 within an interior cavity 130. The system 100 includes a top mold 102 and a bottom mold 104. A set of inserts or slides 106, 108, 110, and 112 are positioned in the top and bottom molds 102 and 104. The slides 106, 108, 110, and 112 are configured to reciprocate along channels in both the top and the bottom molds 102 and 104 as indicated by the arrows 114, 116, 118, and 120. The slides 106, 108, 110, and 112 may move outward along the channels to accommodate an overfill volume and/or move inward to compensate for metal shrinkage during casting solidification while applying direct pressure along the arrows 114, 116, 118, and 120.
FIG. 2 schematically illustrates pressure being directly applied in a controlled manner from six directions (top and bottom and from the sides) to mold a mechanical component 122. Specifically, the top mold 102 can be moved up and down as indicated by the arrow 124 and the bottom mold 104 can be moved up and down as indicated by the arrow 126, in addition to the direct pressure applied by the slides 106, 108, 110, and 112 along the lines 114, 116, 118, and 120. Further, the applied pressure can be controlled with the use of one or more pressure punches and a vent (not shown) to apply and control the pressure to regions of interest of the solidifying casting. The slides and the one or more pressure punches can operate simultaneously or independently of each other.
The novel direct squeeze process enables the use of lower casting pressures which reduce tooling and press rigidness requirements, which enables the use of simpler casting machines, hydraulic systems and controls compared to HPDC machinery. As such, simpler casting machines, hydraulics and controls and improved tool life lowers the cost per component compared to components made with HPDC systems.
Both the HPDC and direct squeeze processes rely upon the use of tools that are typically made from a forged and machined slab of steel, such as, for example an H13 steel, which is known for its toughness and thermal stability. These forged and machined tools typically also include internal passages which may be used to manage heat transfer within the mold. A liquid flows through the passages to either heat or cool the mold tool. Proper thermal management is necessary to improve the properties of the cast material and production rates. In addition, thermal management is necessary to increase tool life by reducing thermal fatigue and lowering reactions with molten metal, such as soldering or fusing to the die surface. Further, thermal management is important to manage the solidification pattern of the cast metal especially with cast parts having a variation in thickness. Current tool fabrication techniques severely limit the shape of the internal passages. Machining techniques require direct, “line-of-sight” access from an external surface to provide access by a machining tool to enable the machining of an internal passage. Therefore, these machined internal passages are limited to only having a straight shape. This further severely limits the route that the resultant machined internal passage may take through the mold tool. Additionally, H13 steel, which is commonly used, has a relatively low thermal conductivity and it is desirable to have a higher thermal conductivity to improve the heat transfer capacity to better manage heat transfer during the casting process.
As explained above, during casting, metal is introduced into a shaped mold cavity and held in place while the metal solidifies. The shape of the mold is then replicated on the surface of the casting and the material properties of the casting are determined by at least the following three factors: 1) the alloy being cast; 2) the presence of inclusions; and 3) the solidification conditions. With respect to the solidification conditions, the higher the solidification rate, the more the properties of the casting may be enhanced. In addition, the pattern of the solidification may influence the solidification shrinkage arising from the reduction in specific volume of the alloy as it transitions from liquid to solid. The solidification pattern of the casting is directly dependent upon the configuration of the internal passages within the mold tool. The fact that these passages are limited to only having a straight shape that originates from an external surface, as a result of the fact that they are machined, severely limits the capability of the mold tool designer to design internal passages which may better manage the solidification conditions and solidification pattern of the casting. In order to machine these passages, the tool must be relatively thick and heavy. Additionally, any passages which result from the machining include sharp edges at the intersections which act as stress raisers or concentrators which reduce the resistance to thermal fatigue.

SUMMARY

In an exemplary aspect, a casting tool for a direct squeeze casting process includes a contoured internal passage.
In another exemplary aspect, the contoured internal passage forms a thermal management passage for the direct squeeze casting process.
In another exemplary aspect, the contoured internal passage forms a venting passage for the direct squeeze casting process.
In another exemplary aspect, the contoured internal passage forms a gate passage for the direct squeeze casting process.
In another exemplary aspect, the tool includes a gray iron material.
In another exemplary aspect, the tool includes a mold surface having transition layers with a ductile and compacted graphite structure
In another exemplary aspect, the tool includes a mold surface comprising a pressure sensitive die coating.
In this manner, the contoured shape of the internal passage improves the management of heat transfer, the venting, and the movement of molten metal into the mold during the casting and solidification process. In particular, the contouring of the internal passage ensures that the internal passages more closely approach an optimized shape that better fits with the shape of the casting and provides improved heat transfer characteristics for the casting. As a result, materials having better thermal properties may be used for the mold tool and the properties of those mold tools may be improved. This improves the solidification pattern of the castings, reduces cycle times, reduces the cost of the mold tool, reduces the time required to produce the mold tool, and reduces the cost of the overall mold system.
Further, the contoured shape of the internal passages reduce or eliminate the presence of stress risers or concentrators in comparison to the conventional passage, thereby improving resistance to thermal fatigue and tool life. The contoured shape of the internal passage also may improve the flow of molten metal through the tool. Additionally, the improved surface of the contoured internal passages may provide a micro-rough surface which increases the thermal transfer capacity between the die/tool and the molten metal.
The advantages provided by these exemplary embodiments enable the use of a coring technique that more closely shapes the internal passages to an optimized shape. The shape, volume, size and surface texture of the contoured internal passages may be optimized to provide enhanced thermal transfer characteristics in those regions where high heat flux or heat extraction is desired. In comparison to conventional machined tools, the contoured internal passages in the exemplary embodiments may also result in a more compact and lighter tool. These and other advantages will become evident from the following detailed description.
Further areas of applicability of the present disclosure will become apparent from the detailed description provided below. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.
The above features and advantages, and other features and advantages, of the present invention are readily apparent from the detailed description, including the claims, and exemplary embodiments when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is an interior view of the top and bottom molds for a system for an exemplary direct squeeze casting system;

FIG. 2 is a schematic view of the direct squeeze casting system of FIG. 1;

FIG. 3 is an interior view of an exemplary cast top mold tool for a direct squeeze casting system;

FIG. 4 is a cross-section view of an exemplary cast gray iron mold tool surface and a sand mold surface which has been modified to provide the exemplary cast gray iron mold tool surface; and

FIG. 5 is a perspective view of portions of direct squeeze casting (DSC) process tooling which includes cored, near net shape cast gray iron tooling.

DETAILED DESCRIPTION

FIG. 3 is an interior view of an exemplary top mold tool 300 for a direct squeeze casting (DSC) system. The top mold tool 300 includes a curved internal passage 302. Unlike internal passages in conventional direct squeeze mold tools which are limited by machining methods to only including straight shaped sections, the internal passage 302 includes multiple curved sections. As is clearly illustrated, the internal passage 302 is quite complex and its shape has been optimized for the specific mold design to provide the highest quality molded part. In an exemplary embodiment, the complex and optimized internal passage is provided by forming the top mold tool 300 using a casting process which relies upon the use of a core to form the internal passage. A core is a device used in casting processes to provide internal passages having shapes and locations within the mold tool which may otherwise not be achieved through other methods, such as by a machining process.
FIG. 4 is a cross-section view of an exemplary cast gray iron mold tool surface 406 and a sand mold surface 404 which has been modified to provide the exemplary cast gray iron mold tool surface 406. The sand mold 400 includes conventional sand mold materials 402 but also includes a sand mold surface 404 which may incorporate Magnesium particles or granules. The molten gray iron that is introduced into the sand mold 400 during casting may react with the Magnesium and will form a different structure at the surface interface 406. The use of the sand mold 400 having these Magnesium particles may result in thin transition layers 406 on a surface of a gray iron mold tool 408 which may then include a ductile and compacted graphite structure which provides improved mechanical and durability properties.
In an exemplary embodiment, the surface 406 may be known in the art as a “Duraface” which is more durable than other gray iron surface which may include crack-like graphite flakes. The ductile iron contains spheroidal graphite particles while the conventional gray iron surface has flake-shaped graphite particles and the compacted graphite iron may further contain graphite intermediate particles that are rod and flake shaped in their respective microstructure. In contrast, the spheroidal graphite particles are round and smooth in comparison with the graphite flakes, which resemble flakes having sharp edges. The spheroidal graphite particles provided by the exemplary embodiment results in a surface having reduced susceptibility to thermal stress and cracking which promotes increased tool life.
FIG. 5 is a perspective view of portions of an exemplary DSC process tooling 500 which includes exemplary cored, near net shape cast gray iron tooling 502. The DSC process tooling 500 may be used with a direct squeeze casting process to, for example, produce aluminum inline open deck cylinder blocks using mold cavity pressures ranging between about 15 to 3000 PSI. The optimum pressures depend upon a number of factors including, for example, casting geometry, feed distance, and the rate of heat extraction of the mold tool. With the improved heat extraction that is enabled by contoured internal passages, the pressures may be further optimized which results in a high integrity aluminum cylinder block casting.
Further, exemplary embodiments enable the use of mold tool materials which have vastly improved characteristics over H13 steel which may be typically used for DSC processes and or other high volume casting processes. Die pieces are generally machined from solid blocks of hot worked or forged H13 steel. In contrast, the use of a gray iron mold tool material is enabled by the present invention. Gray iron has a significantly improved (higher) thermal conductivity in comparison with H13 steel. In some instances, gray iron alloys may have between about 50% to 150% greater thermal conductivity than an H13 steel. This may enable vastly improved control over the solidification of the part which are cast by the gray iron mold tool in comparison with an H13 steel mold tool. Further, the improved and increased thermal conductivity may provide a significantly reduced solidification time of the casting which may reduce the cycle times of the DSC process.
Faster solidification may further reduce the adverse effects from gases that may be in solution with the molten metal. Any reduction in gas entrainment reduces the likelihood of blistering or other adverse effects that might otherwise occur during a subsequent heat treatment process. Thus, the improved thermal management, gating, and venting that is provided by the inventive curved, cored internal passage within the cast mold tool improves the capability to further optimize heat transfer, thereby further improving the quality and material properties of the casting.
The combination of the greatly improved thermal conductivity with the optimized contoured internal passages of the cast mold tools improves the tool heat exchange rate, may further minimize thermal fatigue of the mold tool, and provide much improved control over the solidification pattern of the casting. This not only improves the quality and properties of the casting, but further improves the cycle times of DSC process to approach those of HPDC casting processes.
Further, the amount of solidification shrinkage of gray iron in comparison with H13 steel, as a result of their differing microstructures and coefficients of thermal expansion (CTE), is much less. This provides the ability to cast the gray iron tool in a shape which is much closer to that which is ultimately required by the casting in the DSC process. Gray iron generally exhibits negative shrinkage (expansion) as the mold tool transitions from liquid to solid during solidification. This allows a more intricate exterior and interior cored features that are not possible with an H13 steel mold tool. Therefore, the amount of machining of the cast gray iron mold tool may be significantly reduced in comparison to that of an H13 steel mold tool. This may also enable significantly lower cost and time to manufacture a DSC mold tool. The inventive cast mold tool having cored, contoured internal passages further enables a casting having a near-net-shape. The optimized and improved passages may provide improved venting, gating, and overflow passages for the DSC process which improves the metal yield for the process.
The contoured internal passages may be provided by, for example, a mold tool casting process which relies upon cores to produce the contoured and textured internal passages. The use of cores enables internal passages to be formed within the mold tool which are vastly improved in comparison with the internal passages currently provided within conventional DSC mold tools. As explained above, conventional mold tool internal passages are limited to configurations which are possible by machining techniques. In stark contrast, the configuration of internal passages which are provided by cored casting techniques enable much improved optimization of the shape of those passages. The shape of the core cast internal passages in the mold tool result in greater control over the venting and heating/cooling in terms of thermal management of the casting during the casting process which provides, for example, significantly improved solidification patterns within the casting. The optimal configuration of the internal passages provides ultimate control of the venting, heating and cooling of various regions within the mold for reduced casting cycle times and optimized solidification patterns for improved casting integrity.
The optimization of the shape of the contoured internal passages which are enabled by the core cast mold tools works synergistically with the enhanced quiet non-turbulent mold file and direct pressurization of the DSC process to produce high integrity castings. The cast gray iron mold tools possess sufficient wear and fatigue characteristics to handle the lower metal pressures and reduced thermal fatigue for use with a DSC process in comparison with the HPDC process. The ability to cast mold tools further leads to significant cost reductions and shorter lead times in comparison with H13 steel mold tools which must be fully machined and heat treated to high tolerances.
In an exemplary embodiment, a pressure sensitive die coating may be applied to the cast mold tool to facilitate slow quiescent mold fill of the mold tool cavities when filling and increased heat transfer when hydrostatic metal pressure is applied. A pressure sensitive die coating may provide excellent resistance to heat transfer without pressure and a significant increase in heat transfer when pressure is applied.
In an exemplary embodiment, the cast mold tool having a cored, contoured internal passage may be produced using a sand mold by, for example, additive manufacturing techniques. This permits rapid DSC mold tool production in a near-net-shape which may require only very limited amounts of machining. Traditional green sand mold patterns made from wood, plastic, or metal may also be used to produce sand molds and cores which may then be used to produce an inventive cast mold tool having a cored, contoured internal passage.
This description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims.

Claims

What is claimed is:

1. A casting tool for a direct squeeze casting process, the tool comprising a cast tool including a contoured internal passage.

2. The tool of claim 1, wherein the contoured internal passage forms a thermal management passage for the direct squeeze casting process.

3. The tool of claim 1, wherein the contoured internal passage forms a venting passage for the direct squeeze casting process.

4. The tool of claim 1, wherein the contoured internal passage forms one of a runner and gate passage for the direct squeeze casting process.

5. The tool of claim 1, wherein the tool comprises a gray iron material.

6. The tool of claim 1, further comprising a mold surface having transition layers with a ductile and compacted graphite structure.

7. The tool of claim 1, further comprising a mold surface comprising a pressure sensitive die coating.

8. A casting system comprising:

a pour cup;

a plurality of runners that receive molten metal from the pouring cup;

a top mold and a bottom mold that receives the molten metal from the plurality of runners; and

a plurality of slides positioned within the top mold and the bottom mold, wherein positioning of the plurality of slides applies direct pressure on the molten metal in the top mold and the bottom mold to form a structural component, and wherein one of the top mold and the bottom mold comprises a casting including a contoured internal passage.

9. The system of claim 8, wherein the contoured internal passage forms a thermal management passage.

10. The system of claim 8, wherein the contoured internal passage forms a venting passage.

11. The system of claim 8, wherein the contoured internal passage forms a gate passage.

12. The system of claim 8, wherein the casting comprises a gray iron material.

13. A direct squeeze casting method comprising:

providing a cast mold tool that includes a contoured internal passage;

pouring molten metal into an interior cavity defined by the cast molded tool; and

exerting pressure on the molten metal to form a structural cast component.

14. The method of claim 13, further comprising flowing a thermal management liquid through the contoured internal passage.

15. The method of claim 13, further comprising venting the mold through the contoured internal passage.

16. The method of claim 13, wherein pouring the molten metal comprises pouring the molten metal through a gate passage that is formed by the contoured internal passage.

17. The method of claim 13, wherein the cast mold tool comprises a gray iron material.

18. The method of claim 13, forming a ductile and compacted graphite structure in the structural cast component through contact with transition layers in a surface in the cast mold tool.

19. The method of claim 13, wherein the cast mold tool comprises a pressure sensitive die coating.