The current development direction of the manufacturing industry is low pollution, light weight and high performance, and traditional manufacturing processes cannot meet the developments of the times. As the representative of high-precision manufacturing technology in special castings, die casting has the advantages of high dimensional accuracy, small machining allowance and high strength of parts. Among the same type of die casting materials, die casting aluminum alloy has better thermoplasticity, lower linear shrinkage, good high temperature thermal resistance and physical and chemical properties, and is the first choice for die casting process materials. The mature die casting process has the characteristics of efficient production and high yield. However, the process cycle of new castings is long and relies on the experience of designers and actual production feedback. Repeated mold testing due to process optimization iterations leads to increased process costs and an extension of the production cycle, which significantly limits development. in the field of die casting. Therefore, CAE simulation is introduced into the field of die casting. By simulating the process of filling and solidifying molten metal, the structure of the casting system is analyzed and optimized to shorten the design cycle.
The hinge bracket studied in this paper has a complex structure and is a non-machining part. It meets strict requirements for dimensional accuracy and subsequent machining area. It is therefore produced using a die casting process. By analyzing the precision requirements of the structure and surface of the hinge bracket, two die casting processes were designed, and Anycasting software was used for numerical simulation. Both schemes were analyzed to predict possible defects such as pores, shrinkage porosity, shrinkage cavities and. cold insulation, location and reasons, select a better solution for process improvement and optimization, and finally perform production verification to provide a reference for the production of such parts.
1 Overall analysis of the parts
The hinge bracket is shown in Figure 1. The material is YL113 aluminum alloy. The chemical composition of YL113 aluminum alloy is shown in Table 1. The average wall thickness of the casting is 2.32mm, the maximum wall thickness is 5.63mm, the dimensions overall are 116mm × 82mm × 43mm and the weight is 131.64g. As shown in Figure 1, the casting structure is complex. The sleeve area and the multi-hole plate area are connected by arc surfaces and oblique straight surfaces. The cross section is half-I shaped and has an internal button shape. A large number of thin-walled transverse ribs are arranged on the outer wall, which requires molding without processing, the dark areas are polished and deburred, the shrinkage rate is 0.5%, and there are no defects molding such as shrinkage cavities and shrinkage. porosity.
Figure 1 Hinge bracket structure
Table 1 Chemical composition of aluminum alloy YL113 wB/%
2 Die casting process design
2.1 Calculation of joint planes
The molded part is a part without machining. Thin-walled ribs with a draft angle of 5° are arranged on the outer wall. The cross section is shaped like a half-I shaped inner button. should be installed in the arc area, sleeve connection area and plate passage. A core pulling slider is arranged in the hole area, and an oblique pin side core pulling mechanism with a corresponding wedge angle of 20° is installed. According to the half-I-shaped cross-section structure of the hinge bracket casting and the principle of selecting the parting surface of the area with the largest projected area of the casting, two parting surfaces were selected (Figure 2). In plane 1, the molded part is partially molded, and the position of the core draw core in the mold core (movable mold insert and fixed mold insert) is evenly distributed, which makes it easier to install and fix the mold. core. In option 2, the separation is carried out on the casting part, the size of the casting part can be guaranteed, and the parting surface is arranged on the grinding surface, so that the generated burr defects can be easily eliminated.
Figure 2 Schematic diagram of the separation surface plan
2.2 Design of the portal system
Two casting systems were designed and the three-dimensional structural diagram is shown in Figure 3.
Figure 3 Structural diagram of the casting system
2.2.1 Design of the interior slide
In scheme 1 of figure 3, in order to avoid direct contact between the channel and the core, the inner channel is arranged on the inner wall in order to shorten the casting process, the inner channel is arranged on the corresponding inner wall at the intersection of ; the ribs, and the casting liquid is filled along the ribs. The filling time is significantly shortened since the distance between the molten metal flowing to all parts of the cavity is as equal as possible, one large and two small inlet distribution patterns are formed. adopted. In the second option in Figure 3, in order to avoid the molten metal directly impacting the core, the casting method is changed to oblique casting, and since the stroke of the molten metal flowing towards the cavity is equal, a separate channel is arranged. The cross section of the portal is calculated as follows.
In the formula: Ag is the cross section of the door, mm㎡; V is the volume of the pouring and overflow tank, mm³ the calculated cross section of the door Ag=138 m㎡;
2.2.2 Runner design
The structural shape of the side runner depends on the shape of the inside runner and the position of the core. The side of this casting is basically an inner loop area, and the separation surface of the two solutions must be pulled to the core. Plan 1: The interior channel is arranged on the interior wall of the molded part. In order to reduce the influence of molten metal on the core during the rapid pressing phase of the channel, the outer channel of the casting is arranged away from the channel. kernel cursor; In order to compensate for the removal of filling pressure, the inner and outer channels of the casting are connected in a non-horizontal linear manner, and only one corner is set to connect. In the second plane, the ingate is placed obliquely on the upper plane of the casting. In order to reduce the impact of the entire filling process on the core, the length of the horizontal channel is extended. The thickness of the runner can be calculated by the following formula.
D=(5~8)T(2)
In the formula: D is the thickness of the side slide, mm; T is the thickness of the inner slide, mm. Taking D = 8 mm, in order to facilitate unmolding of the molded part, adjust the clearance angle of the side slide to 10°.
2.2.3 Core design
The sprue is the channel through which the molten metal enters the mold cavity from the pressure chamber, and its size corresponds to the diameter of the pressure chamber. The castings targeted by this design are small parts, but they are free from machining. In order to ensure that the gas slag can be effectively discharged from the mold cavity, the total volume of the overflow tank is designed to be greater than or equal to 1.2. times the volume of the cast; this casting requires several base draws for the institution. To ensure that the casting can come out smoothly, the volume of each core draw slider should be greater than or equal to 1/3 of the casting volume considering the layout of each mold structure, the die casting machine final selected; was the DCC280 horizontal cold chamber die casting machine, and the diameter of the pressure chamber was chosen to be 50mm, and the remaining thickness of the material is set to 16mm.
2.2.4 Overflow tank design
Design principles of overflow tank: ① The last part filled with molten metal is at the right end of the casting in plane 1, and at both ends of the lower bottom of the casting in plane 2 , therefore an overflow tank is placed at the end of the different planes; ② The place where the molten metal first hits and the wall thickness of the casting, so An overflow groove is set up corresponding to the upper and lower sides of the runner. In plane 1, the wall thickness of the arc branch runner corresponding to the upper and lower sides is too thin, so no overflow groove is provided; currents are easily generated at the confluence of molten metal, so in plane 1 the arc branch runner is An overflow tank is provided above and below the apex.
3Numerical simulation analysis
Save the 3D model of the die casting part with the casting system in stl format and import it into CAE software for meshing. Due to the complex structure of the casting, its minimum wall thickness is different from the minimum wall thickness of the casting system. , a non-uniform mesh is therefore used. Divide the grate size of the pouring and drainage system into 0.8mm and divide the pouring grate size into 10mm to generate a total number of grates. The casting material is YL113 aluminum alloy and the mold material is H13 steel. The casting process parameters are shown in Table 2.
Table 2 Casting process parameters
3.1 Analysis of the filling process
The filling process for option 1 is shown in Figure 4. The molten metal first enters the core. After passing through the arc branching channel at t = 0.160 5 s, it is sprayed down the rib plate and returns to the arc wall along the rib wall at t = 0.162 0 s. , the molten metal passes through the main core channel and the left branch core, pulverized to the bottom of the intersection of the ribs and disperses the filling along the ribs at t=0.168 2; At s, after the filling at the top of the arc is completed, the left side molten metal is filled in the direction of the casting sleeve along the upper and lower planes and ribs at t = 0.176 0 s, the filling of ; the molten metal is complete and the mold cavity is almost completely filled, without any filler gaps. From the perspective of the whole filling process, the flow of molten metal is basically smooth, with a certain degree of splashing, but the splashing area is the area of the ribs and the appearance of the workpiece casting is not affected by the main channel and the left; The branch paths are filled with molten metal in the left branch. The inner wall on the right side of the channel converges and filling occurs. Slag inclusion cannot be effectively removed by the overflow tank; the molten metal converges toward the inner wall of the sleeve connection area, and defects such as air entrainment, cold sealing, shrinkage cavities and shrinkage porosity may occur; the overflow tank is unreasonable. There is no overflow groove provided in the upper and lower sleeve areas.
Figure 4 Simulation results of the filling process of Scheme 1
The filling process of option 2 is shown in Figure 5. The molten metal enters the side channel from the core. At t = 0.451 2 s, it enters the casting cavity through the main channel, after hitting the arc wall, the liquid flows up and down along the arc wall at t = 0.531 8; s, the metal The liquid enters the casting cavity through the two connection sprues. At the main core, molten metal flows slowly to the left side of the core due to core obstruction, as shown in Figure 5, which can lead to casting. generation of eddy currents and air entrainment. At t=0.550 5 s, the filling of the arc surface of the casting is completed, and the molten metal fills the ends of both sides of the casting. At t = 0.551 8 s, the molten metal is completely filled and the mold cavity is completely filled without any filling voids. From the perspective of the whole filling process, plane 2 first fills the sleeve connection area due to the obstruction of the core, and there is a certain degree of splashing during the filling process . The area affected by splashing is the appearance surface of the casting; the flow of molten metal is affected by the curved ribbed plate, and there is air entrainment in certain areas and eddy currents; the molten metal converges to the outer surface of the casting, and an overflow groove cannot be installed in the corresponding area.
Figure 5 Simulation results of the second schematic filling process
There is a certain degree of splashing during the filling process of both pouring methods, but the splashing area in the first option is the side wall of the rib, while the splashing area in the second option is the arc surface of the casting, which may cause burrs on the arc surface of the casting. From the point of view of technical requirements and the absence of the need for processing, the first option is more reasonable.
3.2 Analysis of the solidification process
The solidification process in Scheme 1 is shown in Figure 6. The molten metal first begins to solidify at the edge of the casting, then solidifies from the edge toward the sprue. When t = 1.528 3 s, the ingate begins to solidify, and the main body of the casting is basically solidified, but some wall thickness areas are not yet completely solidified. From the perspective of the entire solidification process, some areas are not solidified in sequence during solidification. The rib walls and the inner walls of the sleeve connection area are solidified first, while the upper and lower planes are the wall thickness areas and are solidified later. Therefore, shrinkage porosity and shrinkage holes are easily formed in this area. The solidification process of Scheme 2 is shown in Figure 7. Compared with Scheme 1, the solidification time of ingate is longer, but the problem of the sleeve connection area is consistent with Scheme 1, and At the cross connection between the rib plate and the inner wall, the rib wall and the inner wall solidify first and there is no ingate to match it. As a result, isolated liquid phase zones will appear in these areas, leading to shrinkage and shrinkage cavities in the castings in these areas.
Figure 6 Simulation results of the solidification process of Scheme 1
Figure 7 Simulation results of the solidification process of Scheme 2
3.3 Defect analysis
Figure 8 shows the shrinkage and shrinkage hole distribution diagrams of the two solutions. Casting defects are concentrated in wall thickness areas such as the sleeve connection platform area, some rib plate intersection areas, and the upper and lower surfaces of the through plate area. This is mainly because the wall thickness in these areas is thicker than in others. During solidification, the temperature of these areas is higher than that of the surrounding thinner walls. Therefore, the molten metal slowly solidifies, creating a gap with the surrounding thin-walled areas, and cannot be replenished by the molten metal when it is. completely solidified, resulting in defects such as shrinkage and shrinkage cavities. Comparing the two planes, the defect locations are approximately the same, but the second plane does not set a corresponding entry at the rib intersection, so there are more defects in the rib intersection area than the first. After removing the overflow tank, the volume of shrinkage porosity and shrinkage porosity in Scheme 1 is 0.056 cm³, while the volume of shrinkage porosity and shrinkage pores in Scheme 2 is 0.083 cm³. Therefore, Scheme 1 is better in terms of reducing shrinkage and porosity. shrinkage porosity.
Figure 8 Distribution diagram of shrinkage porosity and shrinkage cavities
Comprehensive comparison of the two process options, the second option is simpler in terms of removing the remaining material, but in terms of removal and removal holes, the first option produces fewer defects, the second option is more cumbersome when the opening of the mold, with the exception of; the upper and lower parts along the separation surface. In addition to opening the mold, the mold needs to be opened in the tangential direction of the branch core, which complicates the production process. Therefore, Plan 1 was selected as the subsequent process improvement plan.
4. Process improvement
4.1 Optimization plan
In addition to the problems with filling the initial process, the cross section of the main channel in the original plane was too large, making it difficult to remove the subsequent machining allowance from the casting, so adjustments were made. brought to casting. and drainage system. In order to solve the problem that the sleeve connection plane is too thick and the distance between the inner sprues is too large to achieve effective power supply, a cooling system has been added to optimize it.
During the filling process, channel corners are added to reduce turbulence generated when liquid flows from the core into the channel and further compensate for the filling pressure. The main channel is canceled and one channel is added to the left and right; instead. Plug the sprue so that the metal liquid from both sides of the sprue can converge into the overflow groove at the top of the arc. Arrange an overflow groove in the tangential connection platform area of the sleeve to facilitate the removal of slag inclusions and gases before the metal liquid converges to the sleeve wall, reduce the intensity of impact of the liquid flow at the confluence and avoid insulation by the cold at the level of the sleeve wall. At the same time, an exhaust slot is put in place and the corresponding upper and lower overflow slots are connected with the exhaust slot to facilitate the removal of gases and eliminate the existence of pores inside of the molded part. The optimization of the pouring and drainage system is shown in Figure 9.
Figure 9 Improved pouring and drainage system design
During the solidification process, in order to solve the problem of isolated liquid phase areas in the wall thickness of the upper and lower surfaces of the connection area of the sleeve and the through plate, a water pipe with a temperature of 20° was placed above. to optimize the solidification sequence of the casting. In addition, the added door can effectively delay the solidification of the inner wall, ensure that the complete solidification of the inner wall lags behind the wall thickness area, and can also reduce the wall thickness area. The layout of the cooling water path is shown in Figure 10, and the cooling process parameters are shown in Table 3.
Figure 10 Cooling water path distribution diagram
Table 3 Cooling process parameters
4.2 Simulation of the optimization plan
The optimized plan is simulated, and the filling and solidification process is shown in Figure 11. It can be seen that the whole filling process is smooth. The molten metal enters the cavity from the central opening and fills smoothly along the ribs, and converges to the upper overflow groove, which avoids cold insulation caused by the confluence of the molten metal on the wall interior. At the same time, the overflow groove provided in the tangential direction of the sleeve also plays a role in guiding the flow. During the solidification process, the surface of the added cooling system solidifies in advance compared to the thin-walled area, allowing the casting to solidify in an orderly manner.
Figure 11 Changes to the filling process and solidification temperature field after optimization
Figure 12 shows the distribution of shrinkage porosity and shrinkage holes in the casting part after optimization. It can be seen that the surface defects in the connection area of the sleeve have been effectively resolved, and there is no cold insulation phenomenon on the inner wall of the casting.
Figure 12 Distribution diagram of shrinkage and shrinkage holes
4.3 Verification of the optimization plan
In order to verify the feasibility of the optimized scheme, the die casting process parameters in Table 4 were used to make test prototypes of the optimized scheme.
Table 4 Die Casting Trial Production Process Parameters
Based on the calculation of the clamping force, the DCC280 Lijin die casting machine with a clamping force of 2,800 kN was selected for the production of trial molds. The basic information parameters of the die casting machine are shown in Table 5. The actual production hinge bracket is shown. in Figure 13. There are no burrs on the appearance surface. The X-ray inspection is shown in Figure 14. No shrinkage porosity, shrinkage cavities or cold insulation defects were found. It meets technical requirements and can be mass produced.
Table 5 Parameters of Lijin DCC280 Die Casting Machine
Figure 13 Actual image of hinge bracket casting
Figure 14 X-ray defect detection chart
5Conclusion
(1) According to the hinge bracket structure and technical requirements, two die casting processes are designed. Through CAE software, the mold filling and solidification process is simulated and analyzed, and a solution for further optimization is selected by comparison.
(2) By improving the pouring and drainage system and adding cooling water channels, the shrinkage defects, shrinkage holes and cold insulation in the castings of the hinge brackets have been solved , meeting the technical requirements and providing an efficient process production model for the production of such parts. .
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