Patent Application
20180236532
The present disclosure relates to tooling for a high pressure die cast process and a method to make the same.
Die casting is a metal casting process that forces molten metal under high pressure into a mold cavity. The mold cavity is typically comprised of multiple hardened tool-steel pieces that are machined into a predetermined shape to produce a desired part. The capital cost and the time associated with producing the tool-steel dies can be relatively expensive and time consuming making prototype parts nearly impossible to produce. In certain applications, a three-dimensional powder sintering process may be used to create the tool-dies in a quicker and more cost-efficient manner.
According to one embodiment of this disclosure, a high pressure casting die is disclosed. The high pressure casting die may include a die half that defines a recessed area and a build plate that may nest within the recessed area of the die half. The high pressure die casting may further include an additive section that is disposed on the build plate. The additive section may include a plurality of metallic powder layers, the thermal conductivity or the thermal expansion coefficient of the build plate and the additive section may be within 10% of each other.
The build plate may be a heat treated build plate having a hardness of at least 50 HRC.
The additive section may be a heat treated additive section having a hardness of at least 50 HRC.
The build plate and the casting surface may each have a thermal conductivity that is approximately equal to one another.
The build plate and the casting surface may each have a thermal conductivity that is approximately equal to one another.
The build plate and the casting surface may each have a thermal expansion coefficient that is approximately equal to one another.
The additive section may include a casting surface and the additive section may further define a plurality of cooling channels surrounding the casting surface. The plurality of cooling channels may facilitate conformal cooling of a cast component.
The casting surface may be configured to define a portion of a part extending above a base of the first die half.
According to another embodiment of this disclosure, a method of producing a high-pressure die casting die is provided. The method may include applying a plurality of powder layers to a build plate and melting at least a portion of each of the powder layers to form an additive section. The method may also include simultaneously heat treating the build plate and the additive section to mechanically couple the additive section to the build plate obtain a hardness of at least 50 HRC.
The method may also include machining the additive section to define a casting surface.
The additive section may further define a plurality of cooling channels surrounding the casting surface. The plurality of cooling channels may facilitate conformal cooling of a cast component.
The build plate and additive section may each define a thermal conductivity. The thermal conductivity of the build plate and additive section may be within 10% of each other.
The build plate and additive section may each define a thermal expansion coefficient. The thermal expansion coefficient of the build plate and additive section may be within 10% of each other.
The method may also include inserting the build plate and casting surface into an outer base ring of a die half.
The melting step may be accomplished by a direct metal laser melting process.
According to yet another embodiment of this disclosure, a method of producing a high-pressure die casting die is provided. The method may include applying a plurality of powder layers to the build plate and directing a laser beam to melt at least a portion of each of the powder layers to form an additive section. The method may also include machining the additive section to define a casting surface.
The method may include heat treating the build plate and casting surface to a provide at least a hardness level of 50 HRC.
The casting surface and the build plate may be mechanically joined by the heat treating step.
The machining step may remove excess flash defined by the additive section.
The build plate may be comprised of H-13 tool steel material and the plurality of powder layers may be comprised of maraging steel powder. In other embodiments the build plate and the plurality of powder layers may be comprised of other ferrous tool steels or ferrous steel powders.
As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.
Die casting is a manufacturing process for producing metal parts that often have complex shape and require dimensional accuracy that is often not achievable by metal stamping operations. High Pressure Die Casting (HPDC) involves injecting liquid metal (often aluminum) at a fast velocity under high pressure into reusable steel dies. Compared with other casting processes, the temperature and velocity of the casting material is very high; temperatures often exceed 700° C. and the fill time, time for the cavity to fill with liquid metal, is often near 40 milliseconds. This high results in a turbulent flow and contact between the molten metal and die surface.
A steel mold, sometimes referred to as a die, contains cavities that form castings, and includes two die halves to permit removal of the castings. Dies are typically capable of producing up to hundreds of thousand parts in rapid succession over the lifetime of the tool. A basic HPDC die includes a cover, which is stationary and is mounted securely in a die casting machine, and an ejector, which is moveable to release the cast part. More complex dies can contain additional slides that form complex surface features on the cast part.
Dies for HPDC typically require a large amount of capital and a long lead time. Because of the relatively high capital expense and the long lead time, dies for HPDC are not generally utilized for prototype components or in situations requiring running design changes, which often require a quick turnaround time. To overcome these disadvantages, the industry has turned to low volume casting processes such as, but not limited to, sand casting, plaster mold casting, shell molding, and investment casting. But these low volume casting processes come with their own disadvantages. A part produced by the low volume casting process may not be fully representative of a part produced by HPDC. The low volume casting processes may cool at a slower rate compared to HPDC and can result in the cast part having different material properties compared to a cast part produced by HPDC. Moreover, because of design constraints associated with the low volume casing processes mentioned above, parting lines often develop in locations different from those produced by HPDC.
One of the main contributors to varying material properties between a part produced by HPDC and a part produced by sand or plaster casting is the difference in thermal conductivity between a HPDC and a sand or plaster casting. Aluminum cools relatively faster in a metal die used in HPDC than in sand or plaster. Additionally, the fill time and the flow of the material vary greatly between a HPDC process and a sand casting process. The time required for a material to cool and subsequently solidify may impact the microstructure of the material. The microstructure, particularly grain size, orientation of the grain, presence and location of pores, may correlate to material properties, e.g., tensile strength, hardness, and etc.
Additive manufacturing also known as three-dimensional (3D) printing, is a process used to create a three-dimensional object applying successive layers of material to create an object. A digital model created through computer aided design (CAD) is typically used to control the process and create the object. The process starts by slicing the 3D CAD file data into layers, normally between 20 to 100 micrometers thick to create a 2D image of each layer. The Metal Powder Bed Melting process applies thin layers of atomized fine metal powder using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table and a piston that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 500 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.
Utilizing additive manufacturing in combination with standard or existing die halves provides a relatively quick and cost efficient method to create at least a portion of a HPDC die. A standard die half, that has been previously used or is intended for multiple uses can be utilized in combination with portion of the die produced by additive manufacturing.
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The heat treatment process may include a relatively simple heat treatment schedule that is conducted at a low enough temperature as to not affect the heat treatment of the existing tool steel in the existing die. In general, maraging steels have good stability at high temperatures and are suitable for die casting applications. The maraging steel may include one of or a range of the following percentages: 14%-22% nickel, 5.0%-15.15% cobalt, 0.5%-8.2% molybdenum, 0.10%-2.5% titanium, and 0.05%-0.25% aluminum by weight with the balance being iron. The maraging steel may have a thermal conductivity within the range of 20.5 W/m K to 30.5 W/m K. The thermal expansion coefficient may be between 6.8×10−6 and 17.3×10−6 inches.
The build plate 18 may be comprised of H-13 tool steel, however other suitable materials may be utilized. The steel of the build plate may include one of or a range of the following percentage of 0.32%-0.48% carbon, 0.32%-0.48% manganese, 0.8%-1.2% silicon, 4.2%-6.3% chromium, 1.08%-1.62% molybdenum, 0.8%-1.20% vanadium, by weight with the balance being iron. The steel may have a thermal conductivity within the range of 16.5 W/m K to 28.5 W/m K. The thermal expansion coefficient may be between 5.0×10−6 and 12.5×10−6 inches.
A high intensity energy beam 30 fully melts the powder in designated areas to create a layer of the additive manufactured portion 24 of the die. The high intensity energy beam, also referred to as a laser 30, applies sufficient power to locally melt the powder to fuse together the material and form a solid cross-section. The build plate 18 and powder delivering piston is then lowered and a roller or recoater 28 rolls over the next layer of the steel powder 16 to a predetermined thickness, generally between the range of 30-150 μm.
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The base plate 18 includes an outer periphery 44 that is sized to fit in an outer base ring 28 (
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While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.
