Patent Application
20180345362
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.
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.
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.
The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:
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.
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.
