A method for controlling stress distribution of a machine tool casting by using a lost foam casting

Abstract

The application discloses a real shape casting method for regulating stress distribution of machine tool castings, and belongs to the technical field of designing and casting composite materials. In view of the problems of uneven stress distribution, easy production of metallurgical defects such as shrinkage cavity, shrinkage porosity and deformation of large machine tool castings in the solidification process, the application adopts a composite scheme of 'pouring system structure optimization + micro-alloying precise regulation': a multi-independent long runner, a ladder type inner runner and a combined structure of riser-cold iron are designed to realize balanced solidification; Cu0.50-0.60% and Sn0.03-0.04% are added to the base component of QT600-3 for micro-alloying to refine pearlite and improve the strength of the base. The volume of shrinkage porosity of the casting is reduced to 423 cm3, the stress of the side wall is controlled below 50 MPa, the proportion of pearlite reaches 90%, the tensile strength reaches 648 MPa, the elongation reaches 5%, the flatness of the guide rail meets the design standard, and the finished product rate and the use performance of the casting are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of lost-mold casting technology, and in particular to a lost-mold casting method for controlling the stress distribution of machine tool castings. Background Technology

[0002] Large machine tool castings (especially machine tool beds) are characterized by large flat surfaces, thick and long guideways, and thin sidewalls, making them core components with extremely high requirements for dimensional accuracy and internal quality. Currently, the industry mostly uses QT600-3 ductile iron to produce these castings. Although the graphitization expansion during solidification can achieve self-compensation, it still faces many challenges: Existing initial gating systems are mostly open stepped gating systems with sprues located at both ends of the casting. Some systems use a wraparound double-layer stepped gating system, which causes the sidewalls of the casting to expand in a drum shape after casting (with the most severe expansion in the middle). The bosses are almost flush with the sidewalls, and the deformation is non-linear and non-uniformly distributed, making it difficult to compensate through anti-deformation design. At the same time, the wall thickness of the casting guide rails varies greatly, which easily leads to defects such as shrinkage cavities and porosity. The stress distribution is uneven during cooling, affecting the straightness of the guide rails and the retention of machining allowances.

[0003] Although the graphitization expansion of QT600-3 material has the potential for shrinkage compensation, the unreasonable design of the gating system in the existing process makes it difficult to achieve uniform solidification, and the single composition control cannot effectively resist the tensile stress during the cooling process, resulting in a need to improve the casting yield and mechanical properties. Therefore, there is an urgent need for a composite solution that combines gating system structure optimization and composition control to solve the problems of uneven stress distribution and metallurgical defects in machine tool castings. Summary of the Invention

[0004] In view of this, the purpose of this invention is to propose a real-form casting method for controlling the stress distribution of machine tool castings, thereby solving the problems in the background art.

[0005] To achieve the above objectives, the present invention provides a method for controlling the stress distribution of machine tool castings using a mold casting process, comprising the following steps: Step 1: Create a lost foam model and place it in a sandbox. Step 2: Construct the gating system using paper tubes. Select a stepped two-end gating system, with the important guide rail surface as the mold surface and the bottom anchor bolt surface as the slag removal surface. Design two independent pouring cups, each corresponding to two independent sprues. The sprues have a diameter of 70mm and a length adapted to the height of the sand box. The horizontal runner is arranged transversely along the casting and connected perpendicularly to the sprues. The ingate is located at the junction of the thick part of the casting and the side wall, prioritizing the filling of the thick area to avoid deformation caused by the molten metal impacting the thin-walled area. Step 3: After setting up the gating system, simulate it on the casting CAE software. During the process, adjust the position of the gate until all the displayed defects are eliminated. Through the simulation of the CAE casting, accurately locate the position where the casting may shrinkage cavities and porosity. Add feeding by placing 8 heat-insulating risers on the bottom anchor bolts. Step 4: Design T-shaped internal chills at the locations of the anchor bolt machining holes on the bottom surface, and design direct chills for the guide rails for rapid cooling. Step 5: Apply paint to the model surface, dry it, and then embed sand to create the desired shape; Step 6: Place a weighting iron on the surface of the sand box. The weight of the weighting iron is about 7 times the weight of the casting. Then, add molten metal into the gating system. The weight percentage composition of the molten metal is: C 3.5-3.6%, Si 2.1-2.2%, Mg 0.04-0.06%, Mn 0.40-0.45%, Cr 0.15-0.18%, Cu 0.50-0.60%, Sn 0.03-0.04%. The casting temperature is 1390℃. The molten metal is injected into the vanishing mold cavity through the gating system for casting and shaping. Step 7: After pouring, measure and record the temperature of the casting in the sand mold using the thermocouples attached during molding. When the temperature drops to about 200℃, remove the casting by dropping the sand. After cooling to room temperature, remove and grind the gating system, risers, etc.

[0006] Preferably, in step one, a foam model is made using a low-density copolymer foam board with a density of less than 20 kg / m³. Simultaneously, the model is machined directly by CNC machining, with the machine bed being scaled up according to the lost foam casting, and the bottom slag discharge surface being scaled up by 15 mm.

[0007] Preferably, in step five, the chilled iron foam and the model are matched and marked, and cloth tape is pasted on all the positions where the chilled iron is laid on the product model so that the tape can be torn off to remove the paint in that position before the chilled iron is placed.

[0008] Preferably, after the model is properly treated with cloth tape in step five, the product model is coated with paint. The water-based paint has a Baume degree of 55-75. After three coats, the model coating is dried to a minimum thickness of 1.0-2.0 mm to prevent the paint from being too thin and causing sand to stick. Preferably, before the molding in step five, the gating system and ingate are pre-set, and an alcohol coating is applied again to the thick and critical areas where sand is easily trapped.

[0009] Preferably, the time for the molten metal to be injected into the vanishing mold cavity through the gating system in step six is ​​140-150 seconds.

[0010] Preferably, in step two, a 500mm pot platform is placed at the sprue position to increase the filling speed of molten iron during pouring, so that the foam can be quickly vaporized and decomposed, ensuring that the casting is filled properly.

[0011] The beneficial effects of this invention: Based on the principle of uniform solidification and considering the limitations of factory travel distance and sand box size, this invention designs an integrated system with multiple independent long straight runners, horizontal runners, and stepped ingates. Two pouring cups are used to introduce independent sprues, ensuring that the molten metal enters the mold cavity independently and quickly, with the time difference between the arrival of molten metal in the mold cavity from each sprue not exceeding 5 seconds. The ingate is set at the junction of the thick part of the casting and the side wall, giving priority to filling the thick area and avoiding deformation caused by the impact of molten metal on the thin-walled area. A combination structure of riser and chill is configured at the anchor bolt location to extend the closing time of the feeding channel, eliminate contact hot spots, and reduce shrinkage porosity and shrinkage defects on the top surface.

[0012] The QT600-3 base composition was optimized by adding Cu and Sn elements to create a synergistic effect: Specific composition (weight percentage): C 3.5-3.6%, Si 2.1-2.2%, Mg 0.04-0.06%, Mn 0.40-0.45%, Cr 0.15-0.18%, Cu 0.50-0.60%, Sn 0.03-0.04%; Cu and Sn synergistically refine the pearlite structure, improve the matrix strength and toughness, and enhance the casting's ability to resist tensile stress during the cooling process; after adjustment, the pearlite proportion of the casting reaches 90%, the tensile strength increases to 648 MPa, and the elongation reaches 5%.

[0013] The resin sand lost foam casting process is adopted. The molten metal is injected into the mold cavity through an optimized gating system. The filling time is controlled at 140-150 seconds to ensure a stable rise in the liquid level. The thin walls around the casting solidify first, and the thick and hot parts are fed by risers to achieve sequential solidification, and finally complete the overall molding.

[0014] Stress distribution is significantly optimized: the sidewall stress is controlled below 50MPa, which does not exceed the material yield limit, the problem of nonlinear and non-uniform deformation is solved, and sufficient allowance is retained after the boss is machined. Metallurgical defects have been significantly reduced: the volume of shrinkage porosity in castings has been reduced to 423 cm³, and defects such as shrinkage cavities and porosity are concentrated at the intersection of ribs and hot joints such as anchor bolts, and the volume of defects has been significantly reduced. Improved mechanical properties: Pearlite content reaches 90%, tensile strength is 648MPa, elongation is 5%, meeting the mechanical requirements of large machine tool castings; Dimensional accuracy meets standards: the straightness of the guide rails meets design standards, the surface after machining has no obvious defects, and the yield of castings is greatly improved; Highly adaptable: It can be adapted to existing factory production conditions without requiring major equipment modifications, making it easy to promote in industry. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a metallographic diagram of the casting obtained in Embodiment 1 of the present invention; Figure 2 This is a metallographic diagram of the casting obtained in Comparative Example 1 of this invention; Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0018] It should be noted that, unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly. Example

[0019] A method for controlling the stress distribution of machine tool castings using a mold casting method, comprising the following steps: S1. To reduce the probability of slag inclusion defects during the casting process of machine tool castings, low-density copolymer foam board with a density of less than 20 kg / m³ is used when making the foam model. Low-density copolymer has low carbon content, contains oxygen atoms in its molecular structure, and has a honeycomb structure inside the foam. It burns more completely and leaves less carbon residue than ordinary EPS foam. At the same time, the machine tool bed is machined according to the normal allowance of the lost foam casting, and the bottom slag discharge surface is allowed to be increased by 15 mm to directly machine the model. The lost foam model is then placed in the sand box. S2, the machine tool casting has dimensions of 3132mm×5000mm×1311mm and a total weight of 25620kg. The main body wall thickness is 40mm, which is relatively thick and large. Paper tubes are used to build the gating system. A stepped two-end gating system is selected. The important guide rail surface is used as the mold surface, and the bottom anchor bolt surface is used as the slag removal surface. Two independent pouring cups are designed, corresponding to two sets of independent sprues with 5 and 7 sprues respectively. The sprue diameter is 70mm and the length is adapted to the height of the sand box. The horizontal runner is arranged in the transverse direction of the casting and is perpendicular to the sprue. The ingate is set at the junction of the thick part of the casting and the side wall, giving priority to filling the thick area and avoiding deformation caused by the molten metal impacting the thin-walled area. S3. After setting up the gating system, simulation was performed on the casting CAE software. During the process, the position of the gate was adjusted until all the displayed defects were eliminated. Through the simulation of the casting in CAE, the location of shrinkage cavities and porosity that may occur in the casting was accurately located. Feeding was carried out by placing 8 heat-insulating risers on the bottom anchor bolts. From the defect + liquid phase CAE analysis, it can be seen that the shrinkage cavities and porosity defects were moved to the upper part of the heat-insulating risers. The lower part of the molten iron was well fed during the solidification process. At the same time, in order to further reduce the risk of shrinkage cavities, the method of using the lower limit of the pouring temperature was adopted. The design pouring temperature was 1390℃ to reduce the probability of shrinkage cavities and porosity in the thick core area. S4, a T-shaped internal chill is designed at the location of the anchor bolt machining hole on the bottom surface, and the guide rail is designed with direct chill treatment; S5. Match the cold iron foam to the model and mark it. Then, stick cloth tape to all the positions where the cold iron will be placed on the product model so that the tape can be torn off to remove the paint in that position before placing the cold iron. S6. After the model is properly glued with cloth tape, apply the paint to the product model. The water-based paint has a Baume degree of 65. After three coats, dry the model. The minimum coating thickness should be 1.5mm to prevent the paint from being too thin and causing sand to stick. S7. Before molding, pre-set the gating system and ingate, and apply another coat of alcohol coating to the thick areas and key areas where sand is easily trapped. S8 employs a flipping molding process. At the start of sand molding, the resin addition ratio of the molding sand is 1.0%, ensuring a molding sand strength greater than 1.5 MPa. Sand is buried layer by layer, with each layer encased in a mesh. After burying, the molding sand is allowed to cure and gain strength. Six hours later, the molding box is flipped over. After flipping, the reinforcing foam is removed, and the sand edges and paint debris around the model outline are cleaned. Damaged areas on the bottom surface are repaired. Pre-coated and dried slag discharge seats are placed at pre-set locations and connected in series to improve slag discharge efficiency. Internal chills are inserted into the bottom anchor bolts, and the model is repaired. Alcohol-based paint is applied, and a riser is placed at the edge of the anchor bolts for shrinkage compensation. After molding, the upper and lower molding boxes are secured with clips. Once fully cured, the model is hoisted to the pouring area.

[0020] S8. A weighting iron is placed on the surface of the sand box, with a weight approximately 7 times that of the casting weight. Then, molten metal is added to the gating system. The molten metal composition by weight percentage is: C 3.55%, Si 2.10%, Mn 0.43%, Mg 0.05%, Cr 0.17%, Cu 0.53%, Sn 0.03%. The casting temperature is 1390℃. The molten metal is injected into the vanishing mold cavity through the gating system for casting. The molten metal reaches the cavity simultaneously at 10 seconds, and the filling is completed in 147 seconds. The temperature of the molten metal at the front end is uniform, and the temperature difference between different parts of the side wall is ≤30℃. The thin walls around the perimeter solidify first, and the thick, hot spots are fed back through risers. When the solidity reaches 100%, there are no obvious shrinkage cavities. S9: After casting, the temperature of the casting within the sand mold is measured and recorded using thermocouples attached during molding. When the temperature drops to approximately 200℃, the casting is removed by sand removal. After cooling to room temperature, the gating system, risers, etc., are removed and the surface is polished smooth. When the temperature drops to 200℃, the sidewall stress is 45MPa, and the stress in the box section is the highest at 80MPa, both lower than the yield strength of QT600-3.

[0021] The metallographic structure of the casting obtained in Example 1 is shown in the figure below. Figure 1 As shown.

[0022] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the latter uses an unmodified gating system with the following composition: C 3.55%, Si 2.53%, Mn 0.43%, Mg 0.05%, Cr 0.17%, Cu 0.53%, and Sn 0.002%. The metallographic structure of the casting obtained in Comparative Example 1 is shown below. Figure 2 As shown.

[0023] Production verification: After actual casting of the improved gating system, there was no obvious bulging expansion on the side wall of the casting. The straightness error of the guide rail after machining met the requirements. The shrinkage volume was 423 cm³, the tensile strength was 648 MPa, and the elongation was 5%, which met the design requirements.

[0024] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and many other variations of different aspects of the invention as described above exist, which are not provided in detail for the sake of brevity. Any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for controlling the stress distribution of machine tool castings using a lost-mold casting technique, characterized in that, It includes the following steps: Step 1: Create a lost foam model and place it in a sandbox. Step 2: Construct the gating system using paper tubes. Select a stepped two-end gating system, with the important guide rail surface as the mold surface and the bottom anchor bolt surface as the slag removal surface. Design two independent pouring cups, each corresponding to two independent sprues. The sprues have a diameter of 70mm and a length adapted to the height of the sand box. The horizontal runner is arranged transversely along the casting and connected perpendicularly to the sprues. The ingate is located at the junction of the thick part of the casting and the side wall, prioritizing the filling of the thick area to avoid deformation caused by the molten metal impacting the thin-walled area. Step 3: After setting up the gating system, simulate it on the casting CAE software. During the process, adjust the position of the gate until all the displayed defects are eliminated. Through the simulation of the CAE casting, accurately locate the position where the casting may shrinkage cavities and porosity. Add feeding by placing 8 heat-insulating risers on the bottom anchor bolts. Step 4: Design T-shaped internal chills at the locations of the anchor bolt machining holes on the bottom surface, and design direct chills for the guide rails for rapid cooling. Step 5: Apply paint to the model surface, dry it, and then embed sand to create the desired shape; Step 6: Place a weighting iron on the surface of the sand box. The weight of the weighting iron is about 7 times the weight of the casting. Then, add molten metal into the gating system. The weight percentage composition of the molten metal is: C 3.5-3.6%, Si 2.1-2.2%, Mg 0.04-0.06%, Mn 0.40-0.45%, Cr 0.15-0.18%, Cu 0.50-0.60%, Sn 0.03-0.04%. The casting temperature is 1390℃. The molten metal is injected into the vanishing mold cavity through the gating system for casting and shaping. Step 7: After pouring, measure and record the temperature of the casting in the sand mold using the thermocouples attached during molding. When the temperature drops to about 200℃, remove the casting by dropping the sand. After cooling to room temperature, remove and grind the gating system, risers, etc.

2. The method for adjusting the stress distribution of machine tool castings according to claim 1, characterized in that, In step one, a foam model is made using low-density copolymer foam board with a density of less than 20 kg / m³. At the same time, the model is machined directly by CNC machining, with the machine bed being scaled up according to the lost foam casting and the bottom slag discharge surface scaled up by 15 mm.

3. The method for adjusting the stress distribution of machine tool castings according to claim 1, characterized in that, In step five, the cold iron foam is matched with the model and marked. Cloth tape is then pasted on all the positions where the cold iron will be placed on the product model so that the tape can be torn off to remove the paint in that position before the cold iron is placed.

4. The method for adjusting the stress distribution of machine tool castings according to claim 3, characterized in that, After the fabric tape is properly applied to the model in step five, the product model is coated with paint. The water-based paint has a Baume degree of 55-75. After three coats, the model coating is dried to a minimum thickness of 1.0-2.0 mm to prevent the paint from being too thin, which could cause sand to stick.

5. The method for adjusting the stress distribution of machine tool castings according to claim 3, characterized in that, Before shaping in step five, the gating system and ingate are pre-set, and an alcohol coating is applied again to the thick and critical areas where sand is easily trapped.

6. The method for adjusting the stress distribution of machine tool castings according to claim 1, characterized in that, The time for the molten metal to be injected into the vanishing mold cavity through the gating system in step six is ​​140-150 seconds.

7. The method for adjusting the stress distribution of machine tool castings according to claim 1, characterized in that, In step two, a 500mm pot platform is placed at the sprue location to increase the filling speed of molten iron during pouring, allowing the foam to quickly vaporize and decompose, ensuring the casting is properly filled.

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