In design of aluminum alloy step structures, it is necessary to consider not only safety and lightweight of steps, but also die-casting forming process of steps. By using ABAQUS finite element analysis software, static and dynamic analyzes are performed on steps to optimize step structure. By using MAGMA software, filling and solidification numerical simulation analysis of cascade pouring system is carried out to optimize pouring system and die-casting forming process. Through product trial production verification, cascade meets national quality standards and enterprise production requirements.
Steps are the most important part of escalator. Structure of step components mainly includes: steps, step shafts, main wheels, auxiliary wheels, etc., see Figure 1. Steps are divided into integral and prefabricated types. Integral steps are generally die-cast from aluminum alloy and consist of three parts: pedals, risers and brackets. Treads and risers are cast with ribs, which play an anti-slip role and guide adjacent steps. Integral steps are light in weight, have good stiffness, high precision and appearance quality, and are easy to manufacture, assemble and repair. With development of die-casting technology, integral die-cast aluminum alloy steps gradually replace assembled steps and are widely used in escalators.

 

Figure 1 Structural diagram of integral die-cast aluminum alloy step components

Main features of integral die-cast steps are large size and long length. Domestic escalators generally use 600, 800, and 1000 mm. Generally, step length of double escalators is usually 1000 mm. General mass of step castings is about 13 kg, and some even reach 15 kg. General wall thickness of castings is about 3 mm. Steps require safety, light weight, good stiffness, small elastic deformation, strong corrosion resistance, easy assembly, disassembly and maintenance, low manufacturing cost and energy consumption.

1000 type step material is made of aluminum alloy A380. Its chemical composition and mechanical property requirements are shown in Table 1. This aluminum alloy has high strength and toughness, good machinability and castability. Its yield strength is 160 MPa, elastic modulus is 72 GPa, Poisson's ratio υ is 0.3, and density is 2.7 g/cm³.

Table 1 Chemical composition and mechanical properties of A380

In design of ladder structure, it is necessary to consider not only safety and technical requirements in compliance with national standards, but also lightweight and die-casting process of integral ladder. Maximum external dimensions of steps (length * width * height) are 1 009 mm * 411 mm * 340 mm. Three different step structure designs are analyzed and compared, as shown in Figure 2.
Structure 1: Step wall thickness is 1.8 mm, there are 2 main reinforcing ribs in transverse direction, rib plate thickness is 3.5 mm, height is 45 mm, and length is 810 mm. At the same time, in order to enhance rigidity of pedal, a small reinforcing rib is designed in the middle of pedal. There are 4 main reinforcing ribs in longitudinal direction, and thickness of rib plate is 2 mm, see Figure 2a.
Structure 2: Step wall thickness is 1.8 mm, there is 1 main reinforcing rib in transverse direction, rib thickness is 3 mm, height is 65 mm, and length is 810 mm. There are 5 main reinforcing ribs in longitudinal direction, and thickness of rib plate is 2.5 mm, see Figure 2b.
Structure 3: Step wall thickness is 1.8 mm, wall thickness is uniform, minimum end wall thickness of transverse stiffener is 3 mm, height is 85 mm, and length is 810 mm. There are three main reinforcing ribs in longitudinal direction, and thickness of rib plate is 2mm, see Figure 2c.

 

Figure 2 3 different ladder structure designs
Comb tooth structure of step tread (structure 3) is shown in Figure 3. Wall thickness of tread is 2 mm, height of comb teeth is 11 mm, thickness of small end of a single comb tooth is 2.43 mm. There are 111 comb teeth on pedal, center distance between two adjacent comb teeth is 9.14 mm. Structure of kick plate comb groove is shown in Figure 3b. Wall thickness of comb groove is 1.8 mm, depth of comb groove is 6 mm, and center distance between two adjacent comb grooves is 18.28 mm.

 

Figure 3 Detailed schematic diagram of step treads and kick plates (Structure 3)
Comparative analysis of three different integrated stepped structure designs is shown in Table 2.

Table 2 Comparative analysis of 3 different integrated stepped structure designs

According to GB 16899-2011, steps should be subjected to a bending deformation test. A steel plate is placed in the center of step tread. Size of steel plate is 0.2 m * 0.3 m and thickness is 28 mm. Make its 0.2 m side parallel to the front edge of step, and its 0.3 m side perpendicular to the front edge of step. Passenger load is 3000 N, acting vertically on steel plate. Deformation of step pedal should not be greater than 4 mm, and there should be no permanent deformation.
ABAQUS finite element analysis software was used to conduct static analysis on the steps of three different structures. According to actual working conditions of steps, fixed constraints are imposed on inner surface of bushing where main wheel is located, steel plate is placed in the middle of steps, and a load of 3 000 N is applied. Static analysis is shown in Figures 4 to 6.

1). Structure 1: Equivalent stress of steps is shown in Figure 4a. Maximum stress is 125.90 MPa, which occurs near intersection of inclined brackets on both sides and tread ribs, which is less than step yield strength of 160 MPa. Equivalent displacement of steps is shown in Figure 4b. Maximum displacement is 0.98 mm, which occurs in the middle of tread edge and is less than 4 mm deformation required by national standard.

 

Figure 4 Static analysis of stepped structure 1

2). Structure 2: Equivalent stress of steps is shown in Figure 5a. Maximum stress is 50.18 MPa, which occurs near bushings of driving axles on both sides. Equivalent displacement of steps is shown in Figure 5b. Maximum displacement is 0.41 mm, which occurs in the middle of tread edge.

 

Figure 5 Static analysis of stepped structure 2

3). Structure 3: Equivalent stress of steps is shown in Figure 6a. Maximum stress is 49.20 MPa, which occurs near intersection of oblique brackets on both sides and tread ribs. Equivalent displacement of steps is shown in Figure 6b. Maximum displacement is 0.34 mm, which occurs in the middle of tread edge.

 

Figure 6 Static analysis of stepped structure 3
Through static comparative analysis of above three different structures, maximum stress and maximum displacement of structure 3 are smaller than structure 1 and structure 2, safety factor is 3.25. At the same time, in terms of mass, Structure 3 is 21% and 5% lighter than Structures 1 and 2 respectively, effectively achieving a "lightweight" design. Therefore, Structure 3 has a more reasonable structure, lower cost, higher safety factor, and is optimal design solution.

Use ABAQUS software to perform modal analysis on step (structure 3), impose fixed constraints on bushings where main wheel and auxiliary wheel are located. Vibration shapes and amplitudes of the first six modes of aluminum alloy step are shown in Figure 7. Among them, the first-order mode has a frequency of 101.60 Hz and an amplitude of 1.032 mm; second-order mode has a frequency of 186.95 Hz and an amplitude of 1.028 mm; third-order mode has a frequency of 215.56 Hz and an amplitude of 1.035 mm; fourth-order mode has a frequency of 248.53 Hz and an amplitude of 1.041 mm; fifth-order mode has a frequency of 257.62 Hz and an amplitude of 1.613 mm; sixth-order mode has a frequency of 281.39 Hz and an amplitude of 1.352 mm. It can be seen from modal analysis that natural frequency of steps is greater than 20Hz.

 

Figure 7 Modal analysis of the first 6 orders of 1000 type aluminum alloy steps

According to national standards for steps, in addition to static load tests and modal analysis, steps should also undergo dynamic load tests to simulate actual working conditions and verify actual performance of steps. Test should be carried out with interference-free resonant force waves at any frequency between 5 and 20 Hz, applying a pulsating load between 500 and 3000 N. It is found that steps do not produce cracks and tread surface does not produce 4 mm deformation. Conduct harmonic response analysis on steps (structure 3) in ABAQUS software, as shown in Figure 8. At 20 Hz resonant force wave, maximum equivalent displacement of steps is 0.64 mm, which is less than 4 mm deformation, as shown in Figure 9. Maximum equivalent stress is 51.75MPa, which is less than yield strength of aluminum alloy A380, which is 160 MPa. Through harmonic response analysis, step meets national safety standards under dynamic load.

 

Figure 8 Equivalent displacement cloud diagram (20 Hz resonant force wave)

 

Figure 9 Equivalent stress cloud diagram (20 Hz resonant force wave)

Shape and structure of steps are complex, with large volume, thin wall thickness, many comb-tooth structures and stiffening plates. Requirements for dimensional accuracy and deformation control are high, and casting is difficult to form, which is easy to produce die-casting defects such as pores, shrinkage cavities, cold shuts, mold sticking, undercasting, cracks, etc. Main defect is cold shut. Step shrinkage rate is 0.55%, temperature difference is controlled within 40℃, projected area is 4 336 c㎡, and a 30 000 KN die-casting machine is selected. There is no requirement for step air tightness, and casting pressure should be appropriately reduced. During filling process of aluminum liquid, after aluminum liquid enters mold cavity, switch to high-speed filling with a shorter high-speed stroke. By analyzing die-casting molding process of step castings, die-casting process parameters are determined as shown in Table 3.

Table 3 Casting (A380) die-casting process parameters

There are multiple longitudinal reinforcing ribs on step (structure 3). When designing pouring system, filling direction of aluminum liquid should be consistent with direction of reinforcing ribs so that the entire reinforcing rib can be filled and avoid die-casting defects during mold filling process. There is a reinforcing rib across step on the inside of step. This rib is recessed inside mold cavity. If strength of reinforcing rib is insufficient during die-casting, mold sticking will occur and mold will be extremely difficult to handle. Therefore, aluminum liquid needs to be filled forward, so that more aluminum liquid can fill bottom of stiffener first, thereby draining gas inside stiffener. Figure 10 shows location selection of inner gate. It can be seen from Figure 10a that if inner gate is selected to feed material on long side of step pedal, flow rate of liquid aluminum filling direction in direction of filling reinforcement ribs is too small, it is difficult for aluminum liquid to rush into reinforcement ribs and ensure density of reinforcing ribs by relying solely on pressure flow. It can be seen from Figure 10b that if inner gate is selected to feed material on long side of step kick plate, aluminum liquid filling direction will have a larger flow rate in reinforcing rib filling direction, and a better filling effect can be achieved. Therefore, location of inner gate is selected on long side of kick plate.

 

Figure 10 Inner gate location selection

Cross-sectional area of inner gate is calculated as follows:

 

In formula, A is cross-sectional area of inner gate, c㎡; k is multiple coefficient, generally 1.1 to 1.3; G is product quality, g; ρ is liquid density of aluminum alloy, g/cm³; V is filling speed of aluminum liquid at inner gate, m/s; T is time required for liquid aluminum to fill cavity, s.
Filling speed has an important impact on casting quality and mold life. Generally, filling speed is controlled in the range of 30 to 60 m/s. Filling speed is calculated as follows:
                         A rushes to V rushes = V charges in A (2)
In formula, A punch is cross-sectional area of injection punch; V punch is injection punch speed; A is cross-sectional area of gate; V charge is filling speed.
For step (structure 3), calculate cross-sectional area of inner gate and check filling speed. Step volume is 3888 cm³, mass G is 10.5 kg, general wall thickness is 1.8 mm, rib wall thickness is 2.43 mm, and maximum wall thickness is 25.45 mm.
According to experience, weight of overflow tank generally accounts for about 20% of product weight, and coefficient K is 1.2. According to wall thickness of product, filling speed V is set to 40 m/s, and filling time T is set to 0.06 s. Liquid density of aluminum alloy is ρ=2.4 g/cm³. According to formula (1), cross-sectional area of inner gate is:

 

Through calculation, cross-sectional area of inner gate is 21.88 c㎡.
Using a 30 000 kN die-casting machine, injection punch diameter is selected as Ø180mm. During high-speed injection stage, injection punch speed is selected as 4 m/s. According to formula (2), filling speed is calculated:
               V charge = (A rush/A within) V rush = 46 m/s.
Through calculation, filling speed is 46 m/s, which is less than 60 m/s. Filling speed is reasonable, which also shows that cross-sectional area of inner gate is reasonable.

According to cross-sectional area of inner gate and structural characteristics of steps, three different pouring systems are designed for analysis and comparison, see Figure 11. ① Option 1: Steps belong to a wide range of flat-plate type parts among structural parts. Such products usually adopt plan of flat feeding. Gate layout adopts form of evenly distributed comb gates. Pouring position is selected on the side of kick plate. 10 shunts are designed to facilitate formation of a fracture between pouring system and steps, reduce impact of pouring system on product deformation, as shown in Figure 11a. ②Option 2: According to wall thickness and structural characteristics of steps, pouring system adopts a comb-shaped structure and has 10 shunts. Thickness of inner gate is 3 mm, width is 730 mm, and length is 1.5 mm. Thickness of material is 50 mm, which is convenient for product feeding, see Figure 11b. ③Option 3: Gating system adopts a long tapered tangential flow channel, and gate is opened along parting surface. In width direction of gate, filling range is wide, and aluminum liquid filling flow is relatively uniform. Thickness of inner gate is 2.8 mm, width is 780 mm, and length is 1.5 mm, see Figure 11c.

 

Figure 11 Design of stepped pouring system

MAGMA software was used to conduct mold flow analysis on three cascade pouring system designs.

1). Filling process analysis plan 1: It can be seen from Figure 12 that when filling reaches 46%, aluminum liquid in the middle 6 shunts has reached inner gate, but filling distance of shunts on both sides is long, and aluminum liquid lags obviously . When filling reaches 62%, split channels on both sides dissipate heat quickly and material temperature is low. At the same time, air entrapment occurs in the step shaft area of casting, where there is a risk of air entrapment. When filling reaches 82%, middle area of pedal is completely filled, but the two side areas are not completely filled. Temperature of aluminum liquid is low and cold insulation is easy to occur. When filling reaches 98%, most areas inside casting are below liquidus line, and risk of cold shut-off under-casting is high.

 

Figure 12 Analysis of the filling process of Scheme 1
Option 2: It can be seen from Figure 13 that when filling reaches 46%, filling speed of aluminum liquid in the middle branch channel is fast, and filling speed of the two side branch channels is slow. When filling reaches 62%, there is air entrainment in flow channel, serious air entrainment inside casting, and low material temperature. When filling reaches 82%, there is obvious air entrapment in the corners of product, and there is a risk of air entrapment here. When filling reaches 98%, cold material below liquidus line appears at the end of casting, where risk of cold insulation is high.

 

Figure 13 Analysis of filling process of plan 2
Option 3: It can be seen from Figure 14 that when filling reaches 46%, aluminum liquid is filled evenly and filling range is wide, which is conducive to forming of castings. Although there is trapped gas in flow channel, mold flow analysis shows that gas does not enter production with molten aluminum. When filling reaches 62%, filling temperature is good, temperature of each part is higher than liquidus line, front cold material can be discharged through overflow tank, and risk of product entrapment is low. When filling reaches 82% and 98%, air entrainment appears in some places at the end of casting, but it can be eliminated through overflow groove.

 

Figure 14 Analysis of filling process of plan 3

2). Analysis of solidification process. Analyze solidification process of three pouring system design schemes, see Figure 15. It can be seen that solidification processes of the three pouring systems are basically similar. Solidification temperature analysis shows that initial temperature on both sides is lower and solidification speed is the fastest. Treads and risers are solidified in sequence, and pouring system is solidified last to act as a shrinkage. Step castings have characteristics of thin walls, and solidification sequence has little impact on quality of castings.

 

Figure 15 Analysis of solidification process of three schemes (solidification 45% state)
Through mold flow analysis of filling process and solidification process of above three pouring systems, it was found that scheme 3 is optimal scheme. Therefore, option 3 was selected as final design solution for stepped gating system.

During die-casting process, molten aluminum enters mold cavity through inner gate, and last filled part is bottom edge of pedal, which easily forms slag inclusions and pores. Since surface of casting is required to be smooth and flat, without cold insulation, pores, etc., during design, two overflow grooves with dimensions of 270 mm * 20 mm * 15 mm and one overflow groove of 460 mm * 20 mm * 15 mm are arranged at parting surface of bottom edge of pedal to ensure adequate exhaust and overflow, as shown in figure 16. According to mold flow analysis results, two overflow grooves of 80 mm * 50 mm * 10 mm are reasonably arranged on both sides of pedal. At brackets on both sides, reasonably arrange one overflow tank with dimensions of 60 mm*40 mm*12 mm, 30 mm*30 mm*12 mm and 90 mm*15 mm*3 mm. Thickness of overflow opening is designed to be 1.5 mm. An exhaust slot is provided behind overflow tank. Depth of exhaust slot is designed to be 0.18 mm. Shape of exhaust slot is "Z" shaped to prevent aluminum liquid from being sprayed directly from exhaust slot.

 

Figure 16 Step overflow system and exhaust system design

It is difficult to manufacture large and complex die-casting molds. Step size of 1000-type stepped die-casting mold is large, with a length of 1009 mm. Mold core adopts an inlaid structure, and is assembled after segmented processing. According to ladder structure, movable and fixed mold cores are each composed of 8 inserts. In order to improve smoothness of step surface and reduce die-casting flash, it is necessary to improve processing accuracy and assembly accuracy of inserts. Inserts are processed using a five-axis machining center, and product processing accuracy reaches 0.003~0.005 mm to ensure stability of processing accuracy of inserts. Mold frame adopts an integral type to improve the overall rigidity of mold frame. A 30 000 kN die-casting machine is selected for cascade, mold temperature is 200±10℃, low-speed injection speed is 0.2 m/s, high-speed injection speed is 4.0 m/s, and casting pressure in pressurization stage is 80 MPa. Step castings are shown in Figure 17. Through quality inspection of castings, quality of castings meets national standards for escalators.

 

Figure 17 Type 1000 step die casting

(1) For different step structure designs, maximum deformation and maximum stress of steps are analyzed through finite element analysis, step structure is optimized, step lightweight design is achieved, and step cost is reduced.
(2) Through numerical simulation analysis of filling process and solidification process, gating system was optimized and casting defects such as air holes and cold insulation in steps were easily solved.
(3) Through mold testing and optimization of die-casting process parameters, various indicators of castings reached national standards, and products were successfully mass-produced.