METHOD AND SYSTEM FOR REDUCING HOT TEARING DURING SOLIDIFICATION OF A CASTING

Abstract

A method and system for reducing hot tearing during solidification of a casting are provided. A mold has a molding surface, and a raised surface feature defined by spaced ribs formed on the molding surface is along a length dimension of the molding surface. The surface feature is configured to reduce hot tearing of the casting along the length dimension of the molding surface by maintaining actual stress of a molten material below an ultimate yield strength of the material during solidification.

Description

BACKGROUND

Foundries use molds, such as sand molds and permanent molds, to produce metal and other material castings having a great variety of sizes, shapes, and complexities. Sand molds, for example, are relatively easy and inexpensive to make, and thus are desired for the mass production of cast parts. However, one limiting factor in the mass production of parts is the cooling rate of the molten material inside the mold, which can cause a casting defect as the molten material cools and solidifies. Hot tears or hot cracking are one of the most frequent failures in the casting that occur during solidification. This happens because the material is weak when it is semi-solid and the residual stresses in the material can cause the casting to fail during solidification often at hot spots where the casting solidifies last. During solidification, stresses occur in a casting due to uneven cooling rates. If these reach the molten material’s ultimate tensile strength before the solidus temperature is reached, hot tears can occur. One way to prevent this type of casting defect is proper mold design.

BRIEF DESCRIPTION

According to one aspect, a method of reducing hot tearing during solidification of a casting is disclosed. The exemplary method includes providing a mold having a molding surface; providing a raised surface feature along a length dimension of the molding surface; and configuring the surface feature to reduce hot tearing of the casting along the length dimension of the molding surface by maintaining actual stress of a molten material provided in a cavity defined by the mold below an ultimate yield strength of the material during solidification.

According to another aspect, a system for reducing hot tearing during solidification of a casting is disclosed. The system comprises a mold configured to mold a molten material arranged in a mold cavity. The mold has a molding surface, and a raised surface feature along a length dimension of the molding surface. The surface feature is defined by spaced ribs formed on the molding surface. The surface feature is configured to reduce hot tearing of the casting along the length dimension of the molding surface by maintaining actual stress of a molten material below an ultimate yield strength of the material during solidification.

According to another aspect, a mold for reducing hot tearing during solidification of a casting is provided. The mold comprises a molding surface. A raised surface feature is along a length dimension of the molding surface. The surface feature is defined by spaced ribs formed on the molding surface. The surface feature is configured to maintain actual stress of a molten material below an ultimate yield strength of the material during solidification to reduce hot tearing of the casting along the length dimension of the molding surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a portion of a known molding system including a mold containing molten material.

FIG. 2 is a view of a known Tatur test device.

FIG. 3 is an enlarged view of a section of the test device of FIG. 2.

FIG. 4 is a graph showing actual stress of a molten material as the material solidifies along a molding surface of the section of the test device of FIG. 3.

FIG. 5 is a view of a modified Tatur test device according to the present disclosure.

FIG. 6 is an enlarged view of a section of the test device of FIG. 5 with a surface feature on a molding surface of the section, the surface feature defined by spaced ribs.

FIG. 7 is a graph showing actual stress of a molten material as the material solidifies along the molding surface of the section of the test device of FIG. 6.

FIG. 8 depicts an alternative configuration of a rib of the surface feature.

FIG. 9 depicts an alternative configuration of a rib of the surface feature.

FIG. 10 is a graph showing a relationship between linear shrinkage gap provided by the exemplary surface feature and the number of hot tears in a casting.

DETAILED DESCRIPTION

It should, of course, be understood that the description and drawings herein are merely illustrative and that various modifications and changes can be made in the structures disclosed without departing from the present disclosure.  Further, any term of degree used herein, such as "substantially," and "approximately" means a reasonable amount of deviation of the modified word is contemplated such that the end result is not significantly changed. For example, such terms can be construed as allowing a deviation of at least 5% of the modified word if this deviation would not negate the meaning of the word the term of degree modifies.

FIG. 1 depicts a known molding system 2 including a mold 4 and a fluid delivery system 14. The mold 4 is made of a fluid permeable material 6 (i.e., sand). The fluid permeable material 6 is permeable to a cooling fluid, and is shaped so as to define a mold cavity 8. The mold 4 defines the mold cavity 8, which may have any size, shape, and dimensions as desired for creating a desired molded object. The mold 4 is configured to mold a molten material 12 arranged in the mold cavity 8, such that the molten material 12 cools while in the mold 4 to become a solid material in the shape of the mold cavity 8.

The fluid delivery system 14 is configured to deliver the cooling fluid to the fluid permeable material 6, and the cooling fluid permeates through the fluid permeable material 6, optionally contacting the molten material 12. This results in the solidification of the molten material 12 arranged in the mold cavity 8, initially forming a solidified outer skin at a surface 18 of the molten material 12, and then further solidifying the molten material 12 in the mold cavity 8 so that eventually all of the molten material 12 becomes a solid material in the shape of the mold cavity 8. The fluid delivery system 14 may include one or more nozzles 22 having tips 26 for ejecting the cooling fluid into the fluid permeable material 6, and fluid conduits 24 for delivering the cooling fluid from a fluid supply to the nozzles 22.

The above molding system 2 is one example for the mass production of cast part using a sand mold 4. As indicated, one limiting factor in the mass production of cast parts is the cooling rate of the molten material 12 inside the mold 4, which can cause a casting defect at, for example, the surface 18 of the molten material 12 as the molten material cools and solidifies. Hot tears or hot cracking are one of the most frequent failures in the casting that occur during solidification of the molten material 12.

FIG. 2 depicts a known hub and spoke Tatur test device 50 to determine hot tear susceptibility during solidification. The test device 50 includes a hub 52, spokes or arms 54 and an outer ring 56. The arms 54 extend between the hub and the outer ring. The hub 52 is off center from the outer ring 56 such that the arms 54 have varying lengths. With the test device 50, molding surfaces 60 along an entire length of each arm 54 are smooth. FIG. 3 depicts solidification of a molten material 62 along a length dimension L of the molding surface 60 of one of the arms 54. The molten material is aluminum or aluminum alloy. In FIG. 3, the solidification of the molten material 62 manifests first near the outer ring 56, with the molten material in a solid state near the outer ring 56 and in a liquid state near the hub 52. FIG. 4 is a graph showing the ultimate yield strength (YS) of the molten material 62 depicted in FIG. 3 as the material solidifies along the length dimension L of the molding surface 60. In FIG. 4, the axis of abscissas (x-axis) shows the length dimension L of the molding surface 60, and the axis of ordinates (y-axis) shows the stress σ of the material during solidification of the casting. The ultimate yield strength (YS) of the molten material 62 at the molding surface 60 increases along the length dimension as the material solidifies, with the solidified material having the highest ultimate yield strength. However, with the molding surface 60 being smooth, during casting an actual stress (σ_actual) of the material along the length dimension L of the molding surface 60 is approximately constant. Because of this, the actual stress of the material exceeds the ultimate yield strength of the material before the solidus temperature is reached, which, in turn, causes hot tearing in the casting during solidification.

FIG. 5 depicts a modified Tatur test device 80 according to the present disclosure. Similar to the test device 50, the test device 80 includes a hub 82, spokes or arms 84 of varying lengths, and an outer ring 86, the arms extended between the hub and the outer ring. As a basis of comparison, with the modified test device 80, a molding surface 90 along a length of the same arm 84 as the arm 54 of FIG. 3 is provided with a raised surface feature 92 along a length dimension L of the molding surface 90 (see FIG. 6). In the depicted aspect, the surface feature is defined by curbs or ribs 94 (hereafter “ribs”) formed on the molding surface 90 and spaced from one another in the length dimension with a portion of the molding surface 90 bridging a gap between immediately adjacent ribs 94. Although, it should be appreciated that the surface feature 92 can include connecting sections located on the molding surface 90 that interconnect adjacent ribs 94. According to one aspect, the ribs 94 are approximately equally spaced along the length dimension of the molding surface 90. In FIG. 6, the ribs 94 are polygonal shaped, with side surfaces 100, 102 and a surface 104 interconnecting the side surfaces. In the depicted aspect, the ribs are approximately trapezoid shaped, the side surfaces 100, 102 extended from the molding surface 90 and angled relative to the molding surface 90 toward each other. The surface 104 is raised from the molding surface 90 and extended approximately parallel to the molding surface. Although, it should be appreciated that alternative polygonal shapes are contemplated.

According to the present disclosure, the surface feature 92 is configured to reduce hot tearing of the casting along the length dimension of the molding surface 90 by maintaining actual stress of a molten material 106 (similar in composition as the molten material 62 for comparison between the test devices 50 and 80, i.e., aluminum or aluminon alloy) below the ultimate yield strength of the material during solidification. To accomplish this, the ribs 94 on the molding surface 90 define fixing points that hold the actual stress of the material along a casting length dimension on the molding surface below the ultimate yield strength of the material during solidification. This is graphically depicted in FIG. 7. Like FIG. 4, FIG. 7 shows the ultimate yield strength (YS) of the molten material 106 depicted in FIG. 6 as the material solidifies along the length dimension L of the molding surface 90. Again, the ultimate yield strength (YS) of the molten material at the molding surface 90 increases along the length dimension as the material solidifies, the solidified material having the highest ultimate yield strength.

In contrast to the approximately constant actual stress of the material 62 during casting as shown in FIG. 4, the surface feature 92 (i.e., ribs 94) are configured such that the actual stress (σ_actual) of the material 106 during solidification is incremental (i.e., stepwise) along the length dimension L of the molding surface 90. The actual stress substantially follows the curvature of the material ultimate yield strength (YS), with the actual stress increasing as the material solidifies. However, the configuration of the surface feature 92 keeps the actual stress below the ultimate yield strength. The ribs 94 are further configured to maintain an approximately constant actual stress of the material 106 in a spacing (𝓁) between immediately adjacent ribs, thereby ensuring that the incremental actual stress does not exceed the ultimate yield strength. Therefore, with the surface feature 92 provided on the molding surface 90, the actual stress of the material does not exceed the ultimate yield strength of the material and hot tearing in the casting during solidification is reduced and/or prevented along the molding surface 90.

The ribs 92 are further configured to minimize a linear shrinkage gap of the casting along the molding surface 90, wherein the smaller the linear shrink gap the less likely of a hot tear during casting. This is shown in FIG. 10, wherein the molding surface 60 of the test device 50 with no curbs or ribs has an increased linear shrinkage gap and increased number of hot tears as compared to the molding surface 90 of the test device 80 with the surface feature 92 having the curbs or ribs 94. It should be appreciated that a configuration of the ribs where the side surfaces are approximately perpendicular to the molding surface 90 (see rectangular shaped rib 110 in FIG. 8 with side surfaces 112, 114) creates a smaller linear shrinkage gap as compared to a configuration of the rib where the side surfaces are angled relative to the molding surface (see triangular shaped rib 120 in FIG. 9 with side surfaces 122, 124). It should also be appreciated that the ribs 94 are configured with a maximum draft angle of approximately two degrees, allowing for ease of removal of the sand core from the surface feature 92. Therefore, in FIG. 10, the depicted trapezoidal shaped ribs 94 with a maximum draft angle of approximately two degrees provides an approximately one percent linear shrink gap along the length dimension of the molding surface, thereby reducing the number of hot tears on the casting.

As indicated, the sand mold 4 of FIG. 1 is one example of a mold that can include the exemplary surface features 92 to reduce hot tearing. It should also be appreciated that the surface features 92 can be applied to a variety of other molds used for the production of cast part. For example, molds created using 3D printing, such as 3D sand printing, can include the surface features 92, and one advantage of the 3D printed sand molds is that the ribs 94 can be configured without a positive draft angle and can be configured with a zero draft and negative draft angles to further reduce the percent linear shrink gap along the length dimension of the molding surface. According to another aspect, the ribs 94 could be placed on a gate, riser, or overflow, and that the ribs can be removed after the casting process so that it does not impact the final product.

As is evident from the foregoing, a method of reducing hot tearing during solidification of a casting is provided. The exemplary method comprises providing a mold having a molding surface; providing a raised surface feature 92 along a length dimension of the molding surface; and configuring the surface feature 92 to reduce hot tearing of the casting along the length dimension of the molding surface by maintaining actual stress of a molten material provided in a cavity defined by the mold below an ultimate yield strength of the material during solidification.

The exemplary method includes configuring the surface feature 92 to anchor sections of the casting along the molding surface as the material solidifies and shrinks. As indicated, the surface feature 92 is defined by spaced ribs 94 formed on the molding surface, and the method includes configuring the ribs to minimize a linear shrinkage gap of the casting along the molding surface. As indicated, the ribs 94 on the molding surface define fixing points that hold the actual stress of the material along a casting length dimension on the molding surface below the ultimate yield strength of the material during solidification. The exemplary method includes configuring the ribs 94 with a maximum draft angle of 2 degrees.

The exemplary method includes configuring the ribs 94 such that the actual stress of the material during solidification is not approximately constant along the length dimension of the molding surface. The exemplary method includes configuring the ribs 94 such that the actual stress of the material during solidification is incremental along the length dimension of the molding surface. As indicated, the ribs 94 are approximately equally spaced along the length dimension of the molding surface, and the exemplary method includes maintaining an approximately constant actual stress of the material in a spacing between immediately adjacent ribs 94.

It will be appreciated that various of the above-disclosed and other features and functions, or alternatives or varieties thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

1. A method of reducing hot tearing during solidification of a casting, the method comprising:

providing a mold having a molding surface;

providing a raised surface feature along a length dimension of the molding surface; and

configuring the surface feature to reduce hot tearing of the casting along the length dimension of the molding surface by maintaining actual stress of a molten material provided in a cavity defined by the mold below an ultimate yield strength of the material during solidification.

2. The method of claim 1, including configuring the surface feature to anchor sections of the casting along the molding surface as the material solidifies and shrinks.

3. The method of claim 1, wherein the surface feature is defined by spaced ribs formed on the molding surface, and the method includes configuring the ribs to minimize a linear shrinkage gap of the casting along the molding surface.

4. The method of claim 3, wherein the ribs on the molding surface define fixing points that hold the actual stress of the material along a casting length dimension on the molding surface below the ultimate yield strength of the material during solidification.

5. The method of claim 3, including configuring the ribs with a maximum draft angle of approximately two degrees.

6. The method of claim 5, including configuring the ribs with a maximum draft angle of approximately zero degrees.

7. The method of claim 5, wherein the ribs are polygonal shaped with a surface approximately parallel to the molding surface.

8. The method of claim 3, including configuring the ribs such that the actual stress of the material during solidification is not approximately constant along the length dimension of the molding surface.

9. The method of claim 8, including configuring the ribs such that the actual stress of the material during solidification is incremental along the length dimension of the molding surface.

10. The method of claim 9, wherein the ribs are approximately equally spaced along the length dimension of the molding surface, and the method including maintaining an approximately constant actual stress of the material in a spacing between immediately adjacent ribs.

11. A system for reducing hot tearing during solidification of a casting, the system comprising:

a mold configured to mold a molten material arranged in a mold cavity, the mold having a molding surface;

a raised surface feature along a length dimension of the molding surface, the surface feature is defined by spaced ribs formed on the molding surface; and

wherein the surface feature is configured to reduce hot tearing of the casting along the length dimension of the molding surface by maintaining actual stress of the molten material below an ultimate yield strength of the material during solidification.

12. The system of claim 11, wherein the ribs on the molding surface define fixing points that hold the actual stress of the material along a casting length dimension on the molding surface below the ultimate yield strength of the material during solidification.

13. The method of claim 12, wherein the ribs are polygonal shaped with a surface approximately parallel to the molding surface and with a maximum draft angle of approximately two degrees.

14. The system of claim 12, wherein the ribs are configured such that the actual stress of the material during solidification is incremental along the length dimension of the molding surface.

15. The system of claim 14, wherein the ribs are approximately equally spaced along the length dimension of the molding surface, and the ribs are configured to maintain an approximately constant actual stress of the material in a spacing between immediately adjacent ribs.

16. The system of claim 11, wherein the ribs are configured to minimize a linear shrinkage gap of the casting along the molding surface.

17. A mold for reducing hot tearing during solidification of a casting, the mold comprising:

a molding surface;

a raised surface feature along a length dimension of the molding surface, the surface feature is defined by spaced ribs formed on the molding surface; and

wherein the surface feature is configured to maintain actual stress of a molten material below an ultimate yield strength of the material during solidification to reduce hot tearing of the casting along the length dimension of the molding surface.

18. The mold of claim 16, wherein the ribs on the molding surface define fixing points that hold the actual stress of the material along a casting length dimension on the molding surface below the ultimate yield strength of the material during solidification, wherein the ribs are configured to maintain an approximately constant actual stress of the material in a spacing between immediately adjacent ribs.

19. The method of claim 18, wherein the ribs are polygonal shaped with a maximum draft angle of approximately two degrees.

20. The method of claim 18, wherein the ribs are polygonal shaped with a maximum draft angle of approximately zero degrees.