According to hinge bracket structure, die-casting process design was carried out. Two die-casting processes were designed according to its structural characteristics. Anycasting analysis software was used to perform numerical simulation on two processes. Location and causes of casting defects in two process schemes were compared and analyzed. Combined with actual production conditions, a better die-casting process was selected for optimization, number and location of inner runner and overflow groove were optimized, air cooling and water cooling were changed. Simulation analysis and mold trial confirmed that optimization measures were effective and met production quality requirements.
Current development direction of manufacturing industry is low pollution, lightweight and high performance, traditional manufacturing processes cannot meet development of times. As a representative of high-precision manufacturing technology in special casting, pressure casting has advantages of high dimensional accuracy, small machining allowance and high part strength. Among same type of die-casting materials, die-casting aluminum alloy has better thermoplasticity, smaller linear shrinkage, good high-temperature thermal strength, physical and chemical properties, and is the first choice for die-casting process materials. Mature die-casting processes are characterized by high production efficiency and high yield rate. However, process cycle of new castings is long and relies on experience of designers and actual production feedback. Repeated mold trials caused by process optimization iterations have led to increased process costs and extended production cycles, greatly limiting development of die-casting field. Therefore, CAE simulation is introduced into die-casting field. By simulating metal liquid filling and solidification process, pouring system structure is analyzed and optimized to shorten design cycle.
Hinge bracket studied in this paper has a complex structure and is a non-processing part. It has strict requirements on dimensional accuracy and subsequent machining areas, so it is produced by die-casting. By analyzing hinge bracket structure and surface accuracy requirements, two die-casting processes were designed, and Anycasting software was used for numerical simulation. Two schemes were analyzed to predict location and causes of possible defects such as pores, shrinkage, shrinkage, cold shut, etc., better scheme was selected for process improvement and optimization. Finally, production verification was carried out to provide a reference for production of such parts.

Hinge bracket is shown in Figure 1. Material is YL113 aluminum alloy. Chemical composition of YL113 aluminum alloy is shown in Table 1. Average wall thickness of casting is 2.32 mm, maximum wall thickness is 5.63 mm, the overall dimensions are 116 mm*82 mm*43 mm, and weight is 131.64 g. As shown in Figure 1, casting has a complex structure. Sleeve area and multi-hole plate area are connected by an arc surface and an inclined flat straight surface. Cross-section is semi-I-shaped and inward-buckling. A large number of cross-thin-walled ribs are arranged on outer wall. Casting is required to be free of processing. Dark area is a polished surface with deburring. Shrinkage rate is 0.5%, there are no casting defects such as shrinkage holes and shrinkage.

 

Figure 1 Hinge bracket structure

Table 1 Chemical composition of YL113 aluminum alloy wB/%

Casting is a non-processing part. Outer wall is arranged with a thin-walled rib with a draft angle of 5°. Cross-section is semi-I-shaped and inward-buckling. It is necessary to set up multiple core pulling mechanisms. Core pulling sliders are arranged in arc surface area, sleeve connection area and plate through-hole area, and oblique pin side core pulling mechanism with a corresponding wedge angle of 20° is set. According to principle of selecting parting surface of semi-I-shaped structure of hinge bracket casting section and the largest projection area of casting, two parting surfaces are selected (Figure 2). In Scheme 1, mold is divided in the middle of casting, position of core-pulling core in mold core (movable mold insert and fixed mold insert) is evenly distributed, so core installation and fixation is more convenient. In Scheme 2, mold is divided in upper part of casting, casting size can be guaranteed, parting surface is arranged on polished surface, and burr defects generated are easy to remove.

 

Figure 2 Schematic diagram of parting surface scheme

Two casting systems are designed, and three-dimensional structure diagram is shown in Figure 3.

 

Figure 3 Schematic diagram of casting system structure

In Scheme 1 of Figure 3, in order to avoid direct contact between runner and core, ingate is arranged on inner wall; in order to shorten casting process, ingate is arranged at inner wall corresponding to intersection of rib plate, and pouring liquid is filled along rib plate, which greatly shortens filling time; considering that distance between metal liquid and each part of cavity is as equal as possible, a large and two small ingate distribution method is adopted. In second scheme of Figure 3, in order to avoid direct impact of molten metal on core, pouring method is changed to oblique pouring, considering that travel of molten metal to cavity is equivalent, gate is arranged. Cross-sectional area of ingrowth is calculated as follows.

 

Where: Ag is cross-sectional area of ingrowth, mm2; V is volume of casting and overflow tank, mm³; cross-sectional area of ingrowth Ag=138 m2 is calculated.

Structural form of runner depends on the form of ingrowth and position of core. Side of this casting is basically an inward buckle area, and parting surface of both schemes needs to be cored. In scheme 1, ingrowth is arranged on inner wall of casting. In order to reduce influence of molten metal on core during fast pressure stage of runner, outer runner of casting is arranged and is far away from core slider; in order to compensate for filling pressure, inner and outer runners of casting are not connected horizontally and linearly, and a single corner is set to connect them. In second scheme, entrail is placed obliquely on upper plane of casting. In order to reduce impact of the entire filling process on core, length of cross runner is extended. Thickness of cross runner can be calculated by following formula.
D = (5~8) T (2)
Where: D is thickness of cross runner, mm; T is thickness of inner runner, mm. Take D=8 mm, and set cross runner demolding slope to 10° for convenience of casting demolding.

Straight runner is channel for molten metal to enter cavity from pressure chamber, and its size is consistent with diameter of pressure chamber. Castings for this design are small parts, but they are machining-free parts. To ensure that slag inclusion gas can be effectively discharged from cavity, the total volume of overflow trough is designed to be greater than or equal to 1.2 times volume of casting; casting needs to be arranged with multiple core pulling mechanisms. To ensure that casting can be smoothly removed, volume of each core pulling slider should be greater than or equal to 1/3 of casting volume; considering arrangement of each mold structure, final die casting machine is DCC280 horizontal cold chamber die casting machine, pressure chamber diameter is selected as 50 mm, and excess material thickness is set to 16 mm.

Overflow trough design principles: ① Last place where molten metal is filled, Scheme 1 is at rightmost end of casting, and Scheme 2 is at both ends of bottom surface of casting, so an overflow trough is arranged at the end of different schemes; ② Place where molten metal first impacts and wall thickness of casting, so an overflow trough is set on upper and lower sides of runner. Wall thickness of arc branch runner in Scheme 1 is too thin, so no overflow trough is set; ③ Molten metal converges easily to generate eddy currents, so Scheme 1 sets an overflow trough on upper and lower sides of arc top.

Three-dimensional model of die casting with pouring system is saved in stl format and imported into CAE software for meshing. Due to complex structure of casting, its minimum wall thickness is different from minimum wall thickness of pouring system, so uneven meshing is used. Mesh size of pouring system is 0.8 mm, and mesh size of casting is 10 mm. Number of meshes generated is 10 mm. Casting material is YL113 aluminum alloy, and mold material is H13 steel. Casting process parameters are shown in Table 2.

Table 2 Casting process parameters

Filling process of Scheme 1 is shown in Figure 4. Molten metal first enters straight runner, and after passing through curved branch runner at t=0.160 5 s, it is sprayed to the bottom of rib plate and flows back to curved wall along rib plate wall; at t=0.162 0s, molten metal passes through main runner and left branch runner, sprays to the bottom of rib plate intersection and disperses along rib plate to fill; at t=0.168 2 s, after filling at the top of arc is completed, left molten metal fills along upper and lower planes and rib plate toward casting sleeve; at t=0.176 0 s, molten metal is filled, and cavity is basically completely filled, with no filling gaps. From whole filling process, metal liquid flow process is basically stable, with a certain degree of splashing, but splashing area is rib plate area, and appearance of casting is not affected; metal liquid of main runner and left branch runner converge on right inner wall of left branch runner, slag generated by filling cannot be effectively removed through overflow groove; metal liquid converges on inner wall of sleeve connection area, defects such as air entrainment, cold shut, shrinkage cavity, and shrinkage may occur; number and position of overflow grooves are unreasonable, overflow grooves are not arranged in upper and lower sleeve areas.

 

Figure 4 Simulation results of filling process of scheme 1
Filling process of scheme 2 is shown in Figure 5. Molten metal enters runner from sprue. At t=0.451 2 s, it enters casting cavity through main sprue. After impacting curved wall, liquid flows up and down along curved wall. At t=0.531 8 s, molten metal enters casting cavity through two branch runners. Molten metal at main runner flows slowly due to obstruction of core on the left side of runner, as shown in circle in Figure 5. This may lead to generation of eddy currents and air entrainment. At t=0.550 5 s, casting curved area is filled, and molten metal fills ends of both sides of casting. At t=0.551 8 s, molten metal is filled, and cavity is completely filled without any filling gaps. From perspective of the entire filling process, second scheme is affected by obstruction of core and fills sleeve connection area first, there is a certain degree of splashing during filling process, and splashing affected area is appearance of casting; flow of molten metal is affected by arc rib plate, there are air curling and eddy currents in some areas; molten metal converges on appearance of casting, and corresponding area cannot be set with an overflow groove.

 

Figure 5 Simulation results of filling process of second scheme
Both pouring methods have a certain degree of splashing during filling process, but splashing area of the first scheme is side wall of rib plate, while splashing area of second scheme is arc appearance of casting, which may cause burrs on arc surface of casting. From perspective of technical requirements and free processing, the first scheme is more reasonable.

Solidification process of the first scheme is shown in Figure 6. Molten metal first solidifies at the edge of casting, then solidifies from edge to runner. When t=1.528 3 s, inner runner begins to solidify, main body of casting is basically solidified, but some wall thickness areas have not yet completely solidified. From perspective of the entire solidification process, some areas did not solidify in sequence during solidification. Rib plate wall and inner wall of sleeve connection area solidified first, while upper and lower planes were wall thickness areas and solidified later, so shrinkage holes were easily formed in this area. Solidification process of Scheme 2 is shown in Figure 7. Compared with Scheme 1, its ingode solidification time is longer, but its sleeve connection area problem is consistent with Scheme 1, at intersection of rib plate and inner wall, rib plate wall and inner wall solidify first and there is no ingode corresponding to it, so isolated liquid phase areas will appear in these areas, resulting in shrinkage holes in 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

Figure 8 is distribution diagram of shrinkage holes of two schemes. Defects of castings are concentrated in thick wall areas of sleeve connection platform area, some rib plate intersection areas, upper and lower surfaces of through-hole plate area. This is mainly because wall thickness of these areas is thicker than other parts. During solidification, temperature of these areas is higher than that of surrounding thin wall areas. Therefore, molten metal solidifies slowly, and a gap is formed with surrounding thin wall areas. When it is completely solidified, it cannot be replenished by molten metal, resulting in shrinkage defects. Comparing two schemes, locations of defects are roughly same, but Scheme 2 does not set corresponding ingate at intersection of rib plates, so defects in rib plate intersection area are more than those in Scheme 1. After removing overflow groove, shrinkage volume of Scheme 1 is 0.056 cm³, while shrinkage volume of Scheme 2 is 0.083 cm³. Therefore, Scheme 1 is better in reducing shrinkage.

 

Figure 8 Distribution of shrinkage holes
Comprehensively comparing two process schemes, Scheme 2 is simpler in terms of excess material removal, but Scheme 1 has fewer defects in terms of shrinkage holes; Scheme 2 is more complicated in mold opening. In addition to opening mold up and down along parting surface, it is also necessary to open mold along tangent direction of branch runner, and production process is complicated. Therefore, Scheme 1 is selected as subsequent process improvement scheme.

In addition to problems existing in initial process filling, main runner cross-sectional area of original scheme is too large, and subsequent machining allowance removal work of casting is more complicated, so pouring system is adjusted. In view of problem that sleeve connection plane is too thick and inner runner distance is too far to achieve effective shrinkage compensation, cooling system is added for optimization.
During filling process, cross runner corner is increased to reduce turbulence generated when liquid flows from straight runner to cross runner and further compensate filling pressure; main runner is cancelled and replaced with a branch runner on each side, so that metal liquid on both sides of runner converges at overflow groove at the top of arc as much as possible. Overflow groove is arranged in tangential connection platform area of sleeve to facilitate removal of slag and gas before metal liquid converges on sleeve wall, reduce impact strength of liquid flow at confluence, and avoid cold shut at sleeve wall. At the same time, an exhaust groove is set to connect corresponding upper and lower overflow grooves with an exhaust groove to facilitate gas removal and eliminate existence of pores inside casting. Optimization of pouring system is shown in Figure 9.

 

Figure 9 Improved pouring system design
During solidification process, in view of problem of isolated liquid phase areas appearing in sleeve connection area, upper and lower surface wall thickness of through-hole plate, a water pipe with a temperature of 20° is arranged above it, thereby optimizing solidification sequence of casting. In addition, added ingates can effectively delay solidification of inner wall, ensure that complete solidification of inner wall lags behind wall thickness area, and compensate for shrinkage of wall thickness area. Cooling water channel layout is shown in Figure 10, and cooling process parameters are shown in Table 3.

 

Figure 10 Cooling water channel distribution diagram

Table 3 Cooling process parameters

Optimized scheme is simulated, filling and solidification process is shown in Figure 11. It can be seen that the entire filling process is smooth, molten metal enters cavity from middle ingates and fills smoothly along rib plate, and converges at overflow groove at the top of arc, avoiding molten metal converging on inner wall and generating cold shut. At the same time, overflow groove set at tangent of sleeve also plays a role in diversion. During solidification process, surface of cooling system is increased to solidify ahead of thin-walled area, so that casting can be solidified in an orderly manner.

 

Figure 11 Changes in filling process and solidification temperature field after optimization

 

Figure 12 shows distribution of shrinkage holes in optimized casting. It can be seen that surface defects of sleeve connection area have been effectively solved, and there is no cold shut phenomenon on inner wall of casting.
Figure 12 Distribution of shrinkage holes

In order to verify feasibility of optimized scheme, die-casting process parameters in Table 4 were used to test optimized scheme.

Table 4 Die-casting trial production process parameters
According to clamping force calculation, DCC280 Lijin die-casting machine with a clamping force of 2800kN was selected for trial mold production. Basic information parameters of die-casting machine are shown in Table 5. Actual hinge bracket produced is shown in Figure 13. There are no burrs on appearance surface. X-ray flaw detection is shown in Figure 14. No shrinkage, shrinkage, or cold shut defects were found. It meets technical requirements and can be mass-produced.

Table 5 DCC280 Lijin die-casting machine parameters

 

Figure 13 Actual picture of hinge bracket casting

 

Figure 14 X-ray flaw detection picture

(1) According to hinge bracket structure and technical requirements, two die-casting processes are designed. CAE software is used to simulate and analyze filling and solidification process, and a solution for further optimization is selected by comparison.
(2) By improving pouring system and adding cooling water channels, defects of shrinkage, shrinkage, cold shut of hinge bracket castings are solved, which meets technical requirements and provides an effective process production template for production of such parts.